Transcriber's note: Text enclosed by underscores is in italics (_italics_).
A single underscore introduces a subscript (CO_2), and a caret a
superscript (B^1).

Page numbers enclosed by curly braces (for example: {25}) have been
incorporated to facilitate the use of the Alphabetical Index and other page
references in the text.

       *       *       *       *       *


THE

CAMBRIDGE NATURAL HISTORY

EDITED BY

S. F. HARMER, Sc.D., F.R.S., Fellow of King's College, Cambridge;
Superintendent of the University Museum of Zoology

AND

A. E. SHIPLEY, M.A., F.R.S., Fellow of Christ's College, Cambridge;
University Lecturer on the Morphology of Invertebrates

VOLUME I

[Illustration]




PROTOZOA

  By MARCUS HARTOG, M.A., Trinity College (D.Sc. Lond.), Professor of
  Natural History in the Queen's College, Cork

PORIFERA (SPONGES)

  By IGERNA B. J. SOLLAS, B.Sc. (Lond.), Lecturer on Zoology at Newnham
  College, Cambridge

COELENTERATA & CTENOPHORA

  By S. J. HICKSON, M.A., F.R.S., formerly Fellow and now Honorary Fellow
  of Downing College, Cambridge; Beyer Professor of Zoology in the Victoria
  University of Manchester

ECHINODERMATA

  By E. W. MACBRIDE, M.A., F.R.S., formerly Fellow of St. John's College,
  Cambridge; Professor of Zoology in McGill University, Montreal



London
MACMILLAN AND CO., LIMITED
NEW YORK: THE MACMILLAN COMPANY
1906


_All rights reserved_



  And pitch down his basket before us,
      All trembling alive
  With pink and grey jellies, your sea-fruit;
      You touch the strange lumps,
  And mouths gape there, eyes open, all manner
      Of horns and of humps.

  BROWNING, _The Englishman in Italy_

CONTENTS


                                                                      PAGE

  SCHEME OF THE CLASSIFICATION ADOPTED IN THIS BOOK                     ix

  PROTOZOA

  CHAPTER I

  PROTOZOA—INTRODUCTION—FUNCTIONS OF PROTOPLASM—CELL-DIVISION—ANIMALS
    AND PLANTS                                                           3

  CHAPTER II

  PROTOZOA (_CONTINUED_): SPONTANEOUS GENERATION—CHARACTERS OF
    PROTOZOA—CLASSIFICATION                                             42

  CHAPTER III

  PROTOZOA (_CONTINUED_): SARCODINA                                     51

  CHAPTER IV

  PROTOZOA (_CONTINUED_): SPOROZOA                                      94

  CHAPTER V

  PROTOZOA (_CONTINUED_): FLAGELLATA                                   109

  CHAPTER VI

  PROTOZOA (_CONTINUED_): INFUSORIA (CILIATA AND SUCTORIA)             136
  PORIFERA (SPONGES)

  CHAPTER VII

  PORIFERA (SPONGES)—INTRODUCTION—HISTORY—DESCRIPTION OF _HALICHONDRIA
    PANICEA_ AS AN  EXAMPLE OF BRITISH MARINE SPONGES AND OF _EPHYDATIA
    FLUVIATILIS_ FROM FRESH WATER—DEFINITION—POSITION IN THE ANIMAL
    KINGDOM                                                            165

  CHAPTER VIII

  PORIFERA (_CONTINUED_): FORMS OF SPICULES—CALCAREA—HOMOCOELA
    —HETEROCOELA—HEXACTINELLIDA—DEMOSPONGIAE—TETRACTINELLIDA
    —MONAXONIDA—CERATOSA—KEY TO BRITISH GENERA OF SPONGES              183

  CHAPTER IX

  PORIFERA (_CONTINUED_): REPRODUCTION, SEXUAL AND ASEXUAL—PHYSIOLOGY
    —DISTRIBUTION—FLINTS                                               226

  COELENTERATA

  CHAPTER X

  COELENTERATA—INTRODUCTION—CLASSIFICATION—HYDROZOA—ELEUTHEROBLASTEA
    —MILLEPORINA—GYMNOBLASTEA—CALYPTOBLASTEA— GRAPTOLITOIDEA
    —STYLASTERINA                                                      245

  CHAPTER XI

  HYDROZOA (_CONTINUED_): TRACHOMEDUSAE—NARCOMEDUSAE—SIPHONOPHORA      288

  CHAPTER XII

  COELENTERATA (_CONTINUED_): SCYPHOZOA = SCYPHOMEDUSAE                310

  CHAPTER XIII

  COELENTERATA (_CONTINUED_): ANTHOZOA = ACTINOZOA—GENERAL CHARACTERS
    —ALCYONARIA                                                        326

  CHAPTER XIV

  ANTHOZOA (_CONTINUED_): ZOANTHARIA                                   365
  CTENOPHORA

  CHAPTER XV

  CTENOPHORA                                                           412

  ECHINODERMATA

  CHAPTER XVI

  ECHINODERMATA—INTRODUCTION—CLASSIFICATION—ANATOMY OF A STARFISH
    —SYSTEMATIC ACCOUNT OF ASTEROIDEA                                  427

  CHAPTER XVII

  ECHINODERMATA (_CONTINUED_): OPHIUROIDEA = BRITTLE STARS             477

  CHAPTER XVIII

  ECHINODERMATA (_CONTINUED_): ECHINOIDEA = SEA-URCHINS                503

  CHAPTER XIX

  ECHINODERMATA (_CONTINUED_): HOLOTHUROIDEA = SEA-CUCUMBERS           560

  CHAPTER XX

  ECHINODERMATA (_CONTINUED_): PELMATOZOA—CRINOIDEA = SEA-LILIES
    —THECOIDEA—CARPOIDEA—CYSTOIDEA—BLASTOIDEA                          579

  CHAPTER XXI

  ECHINODERMATA (_CONTINUED_): DEVELOPMENT AND PHYLOGENY               601

  INDEX                                                                625



SCHEME OF THE CLASSIFICATION ADOPTED IN THIS BOOK

_The names of extinct groups are printed in italics._

PROTOZOA (pp. 1, 48).

SARCODINA (p. 51)
      +-- RHIZOPODA (p. 51)
      |                   +-- Lobosa (p. 51).
      |                   +-- Filosa (p. 52).
      +-- FORAMINIFERA (p. 58)
      |                   +-- Allogromidiaceae (p. 58).
      |                   +-- Astrorhizidaceae (p. 59).
      |                   +-- Lituolidaceae (p. 59).
      |                   +-- Miliolidaceae (p. 59).
      |                   +-- Textulariaceae (p. 59).
      |                   +-- Cheilostomellaceae (p. 59).
      |                   +-- Lagenaceae (p. 59).
      |                   +-- Globigerinidae (p. 59).
      |                   +-- Rotaliaceae (p. 59).
      |                   +-- Nummulitaceae (p. 59).
      +-- HELIOZOA (p. 70)
      |                   +-- Aphrothoraca (p. 70).
      |                   +-- Chlamydophora (p. 71).
      |                   +-- Chalarothoraca (p. 71).
      |                   +-- Desmothoraca (p. 71).
      +-- RADIOLARIA (p. 75)
      |         +-- PORULOSA = HOLOTRYPASTA (p. 76)
      |         |         +-- Spumellaria = Peripylaea (pp. 76, 77)
      |         |         |         +-- Collodaria (p. 77)
      |         |         |         |         +-- Colloidea (p. 77).
      |         |         |         |         +-- Beloidea (p. 77).
      |         |         |         +-- Sphaerellaria (p. 77)
      |         |         |                   +-- Sphaeroidea (p. 77).
      |         |         |                   +-- Prunoidea (p. 77).
      |         |         |                   +-- Discoidea (p. 77).
      |         |         |                   +-- Larcoidea (p. 77).
      |         |         +-- Acantharia = Actipylaea (pp. 76, 78)
      |         |                             +-- Actinelida (p. 78).
      |         |                             +-- Acanthonida (p. 78).
      |         |                             +-- Sphaerophracta (p. 78).
      |         |                             +-- Prunophracta (p. 78).
      |         +-- OSCULOSA = MONOTRYPASTA (p. 76)
      |                   +-- Nassellaria = Monopylaea (pp. 76, 78)
      |                   |                   +-- Nassoidea (p. 78).
      |                   |                   +-- Plectoidea (p. 78).
      |                   |                   +-- Stephoidea (p. 78).
      |                   |                   +-- Spyroidea (p. 78).
      |                   |                   +-- Botryoidea (p. 79).
      |                   |                   +-- Cyrtoidea (p. 79).
      |                   +-- Phaeodaria = Cannopylaea
      |                                  = Tripylaea (pp. 76, 79)
      |                                       +-- Phaeocystina (p. 79).
      |                                       +-- Phaeosphaeria (p. 79).
      |                                       +-- Phaeogromia (p. 79).
      |                                       +-- Phaeoconchia (p. 79).
      +-- PROTEOMYXA (p. 88)
      |                   +-- Myxoidea (p. 89)
      |                   |         +-- Zoosporeae (p. 89).
      |                   |         +-- Azoosporeae (p. 89).
      |                   +-- Catallacta (p. 89).
      +-- MYCETOZOA (p. 90) --------+
                                    +-- Acrasieae (p. 90).
                                    +-- Filoplasmodieae (p. 90).
                                    +-- Myxomycetes (pp. 90, 91).

SPOROZOA (p. 94)
      +-- TELOSPORIDIA (p. 97)
      |                   +-- Gregarinidaceae (pp. 97, 98)
      |                   |         +-- Schizogregarinidae (p. 97).
      |                   |         +-- Acephalinidae (p. 97).
      |                   |         +-- Dicystidae (p. 97).
      |                   +-- Coccidiaceae (pp. 97, 99)
      |                             +-- Coccidiidae (pp. 97, 99).
      |                             +-- Haemosporidae (pp. 97, 102).
      |                             +-- Acystosporidae (pp. 97, 102).
      +-- NEOSPORIDIA (p. 97)
                          +-- Myxosporidiaceae (pp. 98, 106).
                          +-- Actinomyxidiaceae (p. 98).
                          +-- Sarcosporidiaceae (pp. 98, 108).

FLAGELLATA (p. 109) -----+
                          +-- Pantostomata (p. 109).
                          +-- Protomastigaceae (p. 110)
                          |         +-- Distomatidae (p. 110).
                          |         +-- Oikomonadidae (p. 111).
                          |         +-- Bicoecidae (p. 111).
                          |         +-- Craspedomonadidae (pp. 111, 121).
                          |         +-- Phalansteridae (p. 111).
                          |         +-- Monadidae (p. 111).
                          |         +-- Bodonidae (p. 111).
                          |         +-- Amphimonadidae (p. 111).
                          |         +-- Trimastigidae (p. 111).
                          |         +-- Polymastigidae (p. 111).
                          |         +-- Trichonymphidae (pp. 111, 123).
                          |         +-- Opalinidae (pp. 111, 123).
                          +-- Chrysomonadaceae (pp. 110, 125)
                          |         +-- Coccolithophoridae (p. 114).
                          +-- Cryptomonadaceae (p. 110).
                          +-- Volvocaceae (pp. 110, 111)
                          |         +-- Chlamydomonadidae (pp. 111, 125).
                          |         +-- Volvocidae (pp. 111, 126).
                          +-- Chloromonadaceae (p. 110).
                          +-- Euglenaceae (pp. 110, 124).
                          +-- Silicoflagellata (pp. 110, 114).
                          +-- Cystoflagellata (pp. 110, 132).
                          +-- Dinoflagellata (pp. 110, 130).

INFUSORIA (p. 136)
      +-- CILIATA (p. 137)
      |                   +-- Gymnostomaceae (pp. 137, 152).
      |                   +-- Aspirotrichaceae (pp. 137, 153).
      |                   +-- Heterotrichaceae (pp. 137, 153).
      |                   +-- Oligotrichaceae (pp. 137, 155).
      |                   +-- Hypotrichaceae (pp. 137, 138).
      |                   +-- Peritrichaceae (pp. 138, 155).
      +-- SUCTORIA = TENTACULIFERA (p. 158).

PORIFERA (p. 163)

          Class.   Sub-Class.  Order.   Family.  Sub-Family.
MEGAMASTICTORA (pp. 183, 184)
      +-- CALCAREA (p. 184)
                +-- HOMOCOELA (p. 185)
                |                   +-- Leucosoleniidae (p. 185).
                |                   +-- Clathrinidae (p. 185).
                +-- HETEROCOELA (p. 187)
                                    +-- Sycettidae (p. 187).
                                    +-- Grantiidae (p. 192).
                                    +-- Heteropidae (p. 192).
                                    +-- Amphoriscidae (p. 192).
                                    +-- Pharetronidae (p. 192)
                                    |         +-- Dialytinae (p. 192).
                                    |         +-- Lithoninae (p. 193).
                                    +-- Astroscleridae (p. 194).

MICROMASTICTORA (pp. 183, 195)
      +-- MYXOSPONGIAE (p. 196).
      +-- HEXACTINELLIDA (p. 197)
      |         +-- AMPHIDISCOPHORA (p. 203).
      |         +-- HEXASTEROPHORA (p. 203).
      |         +---------------------- _Receptaculitidae_ (p. 207).
      +-- _OCTACTINELLIDA_ (p. 208).
      +-- _HETERACTINELLIDA_ (p. 208).
      +-- DEMOSPONGIAE (p. 209)
                +-- TETRACTINELLIDA (pp. 211, 212)
                |         +-- Choristida (p. 212).
                |         +-- Lithistida (pp. 212, 215).
                +-- MONAXONIDA (pp. 211, 216)
                |         +-- Halichondrina (p. 217).
                |         +-- Spintharophora (p. 217).
                +-- CERATOSA (pp. 211, 220)
                          +-- Dictyoceratina (p. 220)
                          |         +-- Spongidae (p. 220).
                          |         +-- Spongelidae (p. 220).
                          +-- Dendroceratina (pp. 220, 221).

COELENTERATA (p. 243)

Class.    Order.  Sub-Order. Family.  Sub-Family.
HYDROZOA (p. 249)
      +-- ELEUTHEROBLASTEA (p. 253).
      +-- MILLEPORINA (p. 257).
      +-- GYMNOBLASTEA (ANTHOMEDUSAE) (p. 262)
      |                   +-- Bougainvilliidae (p. 269).
      |                   +-- Podocorynidae (p. 270).
      |                   +-- Clavatellidae (p. 270).
      |                   +-- Cladonemidae (p. 270).
      |                   +-- Tubulariidae (p. 271).
      |                   +-- Ceratellidae (p. 271).
      |                   +-- Pennariidae (p. 272).
      |                   +-- Corynidae (p. 272).
      |                   +-- Clavidae (p. 272).
      |                   +-- Tiaridae (p. 273).
      |                   +-- Corymorphidae (p. 273).
      |                   +-- Hydrolaridae (p. 273).
      |                   +-- Monobrachiidae (p. 274).
      |                   +-- Myriothelidae (p. 274).
      |                   +-- Pelagohydridae (p. 274).
      +-- CALYPTOBLASTEA (LEPTOMEDUSAE) (p. 275)
      |                   +-- Aequoreidae (p. 278).
      |                   +-- Thaumantiidae (p. 278).
      |                   +-- Cannotidae (p. 278).
      |                   +-- Sertulariidae (p. 278).
      |                   +-- Plumulariidae (p. 279)
      |                   |         +-- Eleutheroplea (p. 279).
      |                   |         +-- Statoplea (p. 279).
      |                   +-- Hydroceratinidae (p. 279).
      |                   +-- Campanulariidae (p. 280).
      |                   +-- Eucopidae (p. 280).
      |                   +-- _Dendrograptidae_ (p. 281).
      +-- _GRAPTOLITOIDEA_ (p. 281)
      |                   +-- _Monoprionidae_ (p. 282).
      |                   +-- _Diprionidae_ (p. 282).
      |                   +-- _Retiolitidae_ (p. 282).
      |                   +-- _Stromatoporidae_ (p. 283).
      +-- STYLASTERINA (p. 283)
      |                   +-- Stylasteridae (p. 285).
      +-- TRACHOMEDUSAE (p. 288)
      |                   +-- Olindiidae (p. 291).
      |                   +-- Petasidae (p. 294).
      |                   +-- Trachynemidae (p. 294).
      |                   +-- Pectyllidae (p. 294).
      |                   +-- Aglauridae (p. 294).
      |                   +-- Geryoniidae (p. 295).
      +-- NARCOMEDUSAE (p. 295)
      |                   +-- Cunanthidae (p. 296).
      |                   +-- Peganthidae (p. 296).
      |                   +-- Aeginidae (p. 296).
      |                   +-- Solmaridae (p. 296).
      +-- SIPHONOPHORA (p. 297)
                +-- Calycophorae (p. 305)
                |         +-- Monophyidae (p. 306)
                |         |         +-- Sphaeronectinae (p. 306).
                |         |         +-- Cymbonectinae (p. 306).
                |         +-- Diphyidae (p. 306)
                |         |         |                 Oppositae (p. 306).
                |         |         +-- Amphicaryoninae (p. 306) --+
                |         |         +-- Prayinae (p. 306) ---------+
                |         |         +-- Desmophyinae (p. 307) -----+
                |         |         +-- Stephanophyinae (p. 307) --+
                |         |         |              Superpositae (p. 307).
                |         |         +-- Galeolarinae (p. 307) -----+
                |         |         +-- Diphyopsinae (p. 307) -----+
                |         |         +-- Abylinae (p. 307) ---------+
                |         +-- Polyphyidae (p. 307).
                +-- Physophorae (p. 307)
                          +-- Physonectidae (p. 307)
                          |         +-- Agalminae (p. 307).
                          |         +-- Apoleminae (p. 307).
                          |         +-- Physophorinae (p. 308).
                          +-- Auronectidae (p. 308).
                          +-- Rhizophysaliidae (p. 308).
                          +-- Chondrophoridae (p. 308).

SCYPHOZOA = SCYPHOMEDUSAE (pp. 249, 310)
      +-- CUBOMEDUSAE (p. 318)
      |                   +-- Charybdeidae (p. 318).
      |                   +-- Chirodropidae (p. 319).
      |                   +-- Tripedaliidae (p. 319).
      +-- STAUROMEDUSAE (p. 320)
      |                   +-- Lucernariidae (p. 320).
      |                   +-- Depastridae (p. 321).
      |                   +-- Stenoscyphidae (p. 321).
      +-- CORONATA (p. 321)
      |                   +-- Periphyllidae (p. 322).
      |                   +-- Ephyropsidae (p. 322).
      |                   +-- Atollidae (p. 322).
      +-- DISCOPHORA (p. 323)
                +-- Semaeostomata (p. 323)
                |         +-- Pelagiidae (p. 323).
                |         +-- Cyanaeidae (p. 324).
                |         +-- Ulmaridae (p. 324).
                +-- Rhizostomata (p. 324)
                          +-- Cassiopeidae (p. 324)   --+ = Arcadomyaria
                          |                           --+     (p. 324).
                          +-- Cepheidae (p. 324)      --+ = Radiomyaria
                          |                           --+     (p. 324).
                          +-- Rhizostomatidae (p. 325)--+
                          +-- Lychnorhizidae (p. 325)   | = Cyclomyaria
                          +-- Leptobrachiidae (p. 325)  |     (p. 325).
                          +-- Catostylidae (p. 325)   --+

Class.   Sub-Class. Grade.  Order.  Sub-Order.  Family.
ANTHOZOA = ACTINOZOA (pp. 249, 326)
     +-- ALCYONARIA (p. 329)
     |        +-- PROTOALCYONACEA (p. 342)
     |        |                          +-- Haimeidae (p. 342).
     |        +-- SYNALCYONACEA (p. 342)
     |                 +-- Stolonifera (p. 342)
     |                 |                 +-- Cornulariidae (p. 344).
     |                 |                 +-- Clavulariidae (p. 344).
     |                 |                 +-- Tubiporidae (p. 344).
     |                 |                 +-- _Favositidae_ (p. 344).
     |                 +-- Coenothecalia (p. 344)
     |                 |                 +-- _Heliolitidae_ (p. 346).
     |                 |                 +-- Helioporidae (p. 346).
     |                 |                 +-- _Coccoseridae_ (p. 346).
     |                 |                 +-- _Thecidae_ (p. 346).
     |                 |                 +-- _Chaetetidae_ (p. 346).
     |                 +-- Alcyonacea (p. 346)
     |                 |                 +-- Xeniidae (p. 348).
     |                 |                 +-- Telestidae (p. 348).
     |                 |                 +-- Coelogorgiidae (p. 349).
     |                 |                 +-- Alcyoniidae (p. 349).
     |                 |                 +-- Nephthyidae (p. 349).
     |                 |                 +-- Siphonogorgiidae (p. 349).
     |                 +-- Gorgonacea (p. 350)
     |                 |        +-- Pseudaxonia (p. 350)
     |                 |        |        +-- Briareidae (p. 350).
     |                 |        |        +-- Sclerogorgiidae (p. 351).
     |                 |        |        +-- Melitodidae (p. 351).
     |                 |        |        +-- Coralliidae (p. 352).
     |                 |        +-- Axifera (p. 353)
     |                 |                 +-- Isidae (p. 353).
     |                 |                 +-- Primnoidae (p. 354).
     |                 |                 +-- Chrysogorgiidae (p. 355).
     |                 |                 +-- Muriceidae (p. 355).
     |                 |                 +-- Plexauridae (p. 356).
     |                 |                 +-- Gorgoniidae (p. 356).
     |                 |                 +-- Gorgonellidae (p. 357).
     |                 +-- Pennatulacea (p. 358)
     |                          +-- Pennatuleae (p. 361)
     |                          |        +-- Pteroeididae (p. 361).
     |                          |        +-- Pennatulidae (p. 361).
     |                          |        +-- Virgulariidae (p. 362).
     |                          +-- Spicatae (p. 362)
     |                          |        +-- Funiculinidae (p. 362).
     |                          |        +-- Anthoptilidae (p. 362).
     |                          |        +-- Kophobelemnonidae (p. 362).
     |                          |        +-- Umbellulidae (p. 362).
     |                          +-- Verticilladeae (p. 363)
     |                          +-- Renilleae (p. 363)
     |                          |        +-- Renillidae (p. 363).
     |                          +-- Veretilleae (p. 364)
     +-- ZOANTHARIA (pp. 329, 365)
              |            Edwardsiidea (p. 375)
              |                          +-- Edwardsiidae (p. 377).
              |                          +-- Protantheidae (p. 377).
              |            Actiniaria (p. 377)
              |                 +-- Actiniina (p. 380)
              |                 |        +-- Halcampidae (p. 380).
              |                 |        +-- Actiniidae (p. 381).
              |                 |        +-- Sagartiidae (p. 381).
              |                 |        +-- Aliciidae (p. 382).
              |                 |        +-- Phyllactidae (p. 382).
              |                 |        +-- Bunodidae (p. 382).
              |                 |        +-- Minyadidae (p. 383).
              |                 +-- Stichodactylina (p. 383)
              |                          +-- Corallimorphidae (p. 383).
              |                          +-- Discosomatidae (p. 383).
              |                          +-- Rhodactidae (p. 383).
              |                          +-- Thalassianthidae (p. 383).
              +-- Madreporaria (p. 384)
              |                 +----------- _Cyathophyllidae_ (p. 394).
              |                 +----------- _Cyathaxoniidae_ (p. 394).
              |                 +----------- _Cystiphyllidae_ (p. 394).
              |                 +-- Entocnemaria (p. 394)
              |                 |        +-- Madreporidae (p. 395).
              |                 |        +-- Poritidae (p. 396).
              |                 +-- Cyclocnemaria (p. 397)
              |                          |              Aporosa (p. 397).
              |                          +-- Turbinoliidae (p. 398) ----+
              |                          +-- Oculinidae (p. 399) -------+
              |                          +-- Astraeidae (p. 399) -------+
              |                          |    A. Gemmantes (p. 400)     |
              |                          |    A. Fissiparantes (p. 400) |
              |                          +-- Trochosmiliacea -----------+
              |                          |     [Sub-Fam.] (p. 401)      |
              |                          +-- Pocilloporidae (p. 401) ---+
              |                          |             Fungacea (p. 402).
              |                          +-- Plesiofungiidae (p. 403) --+
              |                          +-- Fungiidae (p. 403) --------+
              |                          +-- Cycloseridae (p. 404) -----+
              |                          +-- _Plesioporitidae_ (p. 404) +
              |                          +-- Eupsammiidae (p. 404) -----+
              +-- Zoanthidea (p. 404) ---+
              |                          +-- Zoanthidae (p. 404).
              |                          +-- _Zaphrentidae_ (p. 406).
              +-- Antipathidea = Antipatharia (p. 407)
              |                          +-- Antipathidae (p. 408).
              |                          +-- Leiopathidae (p. 409).
              |                          +-- Dendrobrachiidae (p. 409).
              +-- Cerianthidea (p. 409).

CTENOPHORA (p. 412).

Class.        Order.        Family.
TENTACULATA (p. 417)
          +-- CYDIPPIDEA (p. 417)
          |             +-- Mertensiidae (p. 417).
          |             +-- Callianiridae (p. 417).
          |             +-- Pleurobrachiidae (p. 418).
          +-- LOBATA (p. 418)
          |             +-- Lesueuriidae (p. 419).
          |             +-- Bolinidae (p. 419).
          |             +-- Deiopeidae (p. 419).
          |             +-- Eurhamphaeidae (p. 419).
          |             +-- Eucharidae (p. 420).
          |             +-- Mnemiidae (p. 420).
          |             +-- Calymmidae (p. 420).
          |             +-- Ocyroidae (p. 420).
          +-- CESTOIDEA (p. 420)
          |             +-- Cestidae (p. 420).
          +-- PLATYCTENEA (p. 421)
                        +-- Ctenoplanidae (p. 421).
                        +-- Coeloplanidae (p. 422).

NUDA (p. 423)          Beroidae (p. 423).


ECHINODERMATA (p. 425).

Sub-Phylum. Class.    Order.  Sub-Order.  Family. Sub-Family.
ELEUTHEROZOA (p. 430)
      +-- ASTEROIDEA (pp. 430, 431)
      |         +-- SPINULOSA (pp. 461, 462)
      |         |                   +-- Echinasteridae (p. 462).
      |         |                   +-- Solasteridae (p. 462).
      |         |                   +-- Asterinidae (p. 463).
      |         |                   +-- Poraniidae (p. 464).
      |         |                   +-- Ganeriidae (p. 464).
      |         |                   +-- Mithrodiidae (p. 464).
      |         +-- VELATA (pp. 461, 464)
      |         |                   +-- Pythonasteridae (p. 464).
      |         |                   +-- Myxasteridae (p. 464).
      |         |                   +-- Pterasteridae (p. 466).
      |         +-- PAXILLOSA (pp. 461, 466)
      |         |                   +-- Archasteridae (p. 466).
      |         |                   +-- Astropectinidae (p. 467).
      |         |                   +-- Porcellanasteridae (p. 470).
      |         +-- VALVATA (pp. 461, 471)
      |         |                   +-- Linckiidae (p. 471).
      |         |                   +-- Pentagonasteridae (p. 471).
      |         |                   +-- Gymnasteridae (p. 471).
      |         |                   +-- Antheneidae (p. 471).
      |         |                   +-- Pentacerotidae (p. 471).
      |         +-- FORCIPULATA (pp. 462, 473)
      |                             +-- Asteriidae (p. 473).
      |                             +-- Heliasteridae (p. 474).
      |                             +-- Zoroasteridae (p. 474).
      |                             +-- Stichasteridae (p. 474).
      |                             +-- Pedicellasteridae (p. 474).
      |                             +-- Brisingidae (p. 474).
      +-- OPHIUROIDEA (pp. 431, 477)
      |         +-- STREPTOPHIURAE (p. 494)
      |         +-- ZYGOPHIURAE (pp. 494, 495)
      |         |                   +-- Ophiolepididae (p. 495).
      |         |                   +-- Amphiuridae (p. 497).
      |         |                   +-- Ophiocomidae (p. 499).
      |         |                   +-- Ophiothricidae (p. 499).
      |         +-- CLADOPHIURAE (pp. 494, 500)
      |                             +-- Astroschemidae (p. 501).
      |                             +-- Trichasteridae (p. 501).
      |                             +-- Euryalidae (p. 501).
      +-- ECHINOIDEA (pp. 431, 503)
      |         +-- ENDOCYCLICA (pp. 529, 530)
      |         |                   +-- Cidaridae (p. 533).
      |         |                   +-- Echinothuriidae (p. 535).
      |         |                   +-- Saleniidae (p. 537).
      |         |                   +-- Arbaciidae (p. 538).
      |         |                   +-- Diadematidae (p. 538).
      |         |                   +-- Echinidae (p. 539)
      |         |                            +-- Temnopleurinae (p. 539).
      |         |                            +-- Echininae (p. 539).
      |         +-- CLYPEASTROIDEA (pp. 529, 542)
      |                   +-- Protoclypeastroidea (p. 548).
      |                   +-- Euclypeastroidea (p. 549)
      |                             +-- Fibularidae (p. 549).
      |                             +-- Echinanthidae
      |                             |     = Clypeastridae (p. 549).
      |                             +-- Laganidae (p. 549).
      |                             +-- Scutellidae (p. 549).
      +-- SPATANGOIDEA (pp. 529, 549)
      |                             |                 Asternata (p. 554).
      |                             +-- Echinonidae (p. 553) ------+
      |                             +-- Nucleolidae (p. 554) ------+
      |                             +-- Cassidulidae (p. 554) -----+
      |                             |                  Sternata (p. 554).
      |                             +-- Ananchytidae (p. 554) -----+
      |                             +-- Palaeostomatidae (p. 554) -+
      |                             +-- Spatangidae (p. 554) ------+
      |                             +-- Brissidae (p. 556) --------+
      |                             +-- _Archaeocidaridae_ (p. 557).
      |                             +-- _Melonitidae_ (p. 557).
      |                             +-- _Tiarechinidae_ (p. 557).
      |                             +-- _Holectypoidea_ (p. 558).
      |                             +-- _Echinoconidae_ (p. 558).
      |                             +-- _Collyritidae_ (p. 559).
      +-- HOLOTHUROIDEA (pp. 431, 560)
                +-- ASPIDOCHIROTA (p. 570).
                +-- ELASIPODA (p. 571).
                +-- PELAGOTHURIIDA (p. 572).
                +-- DENDROCHIROTA (p. 572).
                +-- MOLPADIIDA (p. 575).
                +-- SYNAPTIDA (p. 575).

PELMATOZOA (pp. 430, 579)
      +-- CRINOIDEA (p. 580)
      |         +---------------------- Hyocrinidae (p. 590).
      |         +---------------------- Rhizocrinidae (p. 590).
      |         +---------------------- Pentacrinidae (p. 591).
      |         +---------------------- Holopodidae (p. 592).
      |         +---------------------- Comatulidae (p. 594).
      |         +-- _INADUNATA_ (p. 595).
      |         +-- ARTICULATA (p. 595).
      |         +-- _CAMERATA_ (p. 595).
      +-- _THECOIDEA_ = _EDRIOASTEROIDEA_ (pp. 580, 596).
      +-- _CARPOIDEA_ (pp. 580, 596).
      +-- _CYSTOIDEA_ (pp. 580, 597).
      +-- _BLASTOIDEA_ (pp. 580, 599).



PROTOZOA

BY

MARCUS HARTOG, M.A., TRINITY COLLEGE (D.SC. LOND.)

Professor of Natural History in the Queen's College, Cork.

{3}CHAPTER I

PROTOZOA—INTRODUCTION—FUNCTIONS OF PROTOPLASM—CELL-DIVISION—ANIMALS AND
PLANTS


THE FREE AMOEBOID CELL.—If we examine under the microscope a fragment of
one of the higher animals or plants, we find in it a very complex
structure. A careful study shows that it always consists of certain minute
elements of fundamentally the same nature, which are combined or fused into
"tissues." In plants, where these units of structure were first studied,
and where they are easier to recognise, each tiny unit is usually enclosed
in an envelope or wall of woody or papery material, so that the whole plant
is honeycombed. Each separate cavity was at first called a "cell"; and this
term was then applied to the bounding wall, and finally to the unit of
living matter within, the envelope receiving the name of "cell-wall." In
this modern sense the "cell" consists of a viscid substance, called first
in animals "sarcode" by Dujardin (1835), and later in plants
"protoplasm"[1] by Von Mohl (1846). On the recognition of its common nature
in both kingdoms, largely due to Max Schultze, the latter term prevailed;
and it has passed from the vocabulary of biology into the domain of
everyday life. We shall now examine the structure and behaviour of
protoplasm and of the cell as an introduction to the detailed study of the
Protozoa, or better still Protista,[2] the lowest types of living beings,
and of Animals at large.

{4}It is not in detached fragments of the tissues of the higher animals
that we can best carry on this study: for here the cells are in singularly
close connexion with their neighbours during life; the proper appointed
work of each is intimately related to that of the others; and this
co-operation has so trained and specially modified each cell that the
artificial severance and isolation is detrimental to its well-being, if not
necessarily fatal to its very life. Again, in plants the presence of a
cell-wall interferes in many ways with the free behaviour of the cell. But
in the blood and lymph of higher animals there float isolated cells, the
white corpuscles or "leucocytes" of human histology, which, despite their
minuteness (1/3000 in. in diameter), are in many respects suitable objects.
Further, in our waters, fresh or salt, we may find similar free-living
individual cells, in many respects resembling the leucocytes, but even
better suited for our study. For, in the first place, we can far more
readily reproduce under the microscope the normal conditions of their life;
and, moreover, these free organisms are often many times larger than the
leucocyte. Such free organisms are individual Protozoa, and are called by
the general term "Amoebae." A large Amoeba may measure in its most
contracted state 1/100 in. or 250 µ in diameter,[3] and some closely allied
species (_Pelomyxa_, see p. 52) even twelve times this amount. If we place
an Amoeba or a leucocyte under the microscope (Fig. 1), we shall find that
its form, at first spherical, soon begins to alter. To confine our
attention to the external changes, we note that the outline, from circular,
soon becomes "island-shaped" by the outgrowth of a promontory here, the
indenting of a bay there. The promontory may enlarge into a peninsula, and
thus grow until it becomes a new mainland, while the old mainland dwindles
into a mere promontory, and is finally lost. In this way a crawling motion
is effected.[4] The promontories are called "pseudopodia" (=
{5}"false-feet"), and the general character of such motion is called
"amoeboid."[5]

[Illustration: FIG. 1.—_Amoeba_, showing clear ectoplasm, granular
endoplasm, dark nucleus, and lighter contractile vacuole. The changes of
form, _a-f_, are of the _A. limax_ type; _g_, _h_, of the _A. proteus_
type. (From Verworn.)]

The living substance, protoplasm,[6] has been termed a "jelly," a word,
however, that is quite inapplicable to it in its living state. It is
viscid, almost semi-fluid, and may well be compared to very soft dough
which has already begun to rise. It resembles it in often having a number
of spaces, small or large, filled with liquid (not gas). These are termed
"vacuoles" or "alveoles," according to their greater or their lesser
dimensions. In some cases a vacuole is traversed by strands of plasmic
substance, just as we may find such strands stretching across the larger
spaces of a very light loaf; but of course in the living cell these are
constantly undergoing changes. If we "fix" a cell (_i.e._ kill it by
{6}sudden heat or certain chemical coagulants),[7] and examine it under the
microscope, the intermediate substance between the vacuoles that we have
already seen in life is again found either to be finely honeycombed or else
resolved into a network like that of a sponge. The former structure is
called a "foam" or "alveolar" structure, the latter a "reticulate"
structure. The alveoles are about 1 µ in diameter, and spheroidal or
polygonal by mutual contact, elongated, however, radially to any free
surface, whether it be that of the cell itself or that of a larger alveole
or vacuole. The inner layer of protoplasm ("endoplasm," "endosarc")
contains also granules of various nature, reserve matters of various kinds,
oil-globules, and particles of mineral matter[8] which are waste products,
and are called "excretory." In fixed specimens these granules are seen to
occupy the nodes of the network or of the alveoli, that is, the points
where two or three boundaries meet.[9] The outermost layer ("ectoplasm" or
"ectosarc") appears in the live Amoeba structureless and hyaline, even
under conditions the most favourable for observation. The refractive index
of protoplasm, when living, is always well under 1.4, that of the fixed and
dehydrated substance is slightly over 1.6.

Again, within the outer protoplasm is found a body of slightly higher
refractivity and of definite outline, termed the "nucleus" (Figs. 1, 2).
This has a definite "wall" of plasmic nature, and a substance so closely
resembling the outer protoplasm in character, that we call it the
"nucleoplasm" (also "linin"), distinguishing the outer plasm as
"cytoplasm"; the term "protoplasm" including both. Within the nucleoplasm
are granules of a substance that stains well with the commoner dyes,
especially the "basic" ones, and which has hence been called "chromatin."
The linin is {7}usually arranged in a distinct network, confluent into a
"parietal layer" within the nuclear wall; the meshes traversing a cavity
full of liquid, the nuclear sap, and containing in their course the
granules; while in the cavity are usually found one or two droplets of a
denser substance termed "nucleoles." These differ slightly in composition
from the chromatin granules[10] (see p. 24 f.).

The movements of the leucocyte or Amoeba are usually most active at a
temperature of about 40° C. or 100° F., the "optimum." They cease when the
temperature falls to a point, the "minimum," varying with the organism, but
never below freezing-point; they recommence when the temperature rises
again to the same point at which they stopped. If now the temperature be
raised to a certain amount above 40° they stop, but may recommence if the
temperature has not exceeded a certain point, the "maximum" (45° C. is a
common maximum). If it has been raised to a still higher point they will
not recommence under any circumstances whatever.

Again, a slight electric shock will determine the retraction of all
processes, and a period of rest in a spherical condition. A milder shock
will only arrest the movements. But a stronger shock may arrest them
permanently. We may often note a relation of the movements towards a
surface, tending to keep the Amoeba in contact with it, whether it be the
surface of a solid or that of an air-bubble in the liquid (see also p. 20).

[Illustration: FIG. 2.—Ovum of a Sea-Urchin, showing the radially striated
cell-membrane, the cytoplasm containing yolk-granules, the large nucleus
(germinal vesicle), with its network of linin containing chromatin
granules, and a large nucleole (germinal spot). (From Balfour's
_Embryology_, after Hertwig.)]

If a gentle current be set up in the water, we find that the movements of
the Amoeba are so co-ordinated that it moves upstream; this must of course
be of advantage in nature, as keeping the being in its place, against the
streams set up by larger creatures, etc. (see also p. 21).

If substances soluble in water be introduced the Amoeba will, {8}as a rule,
move away from the region of greater concentration for some substances, but
towards it (provided it be not excessive) for others. (See also pp. 22,
23.) We find, indeed, that there is for substances of the latter category a
minimum of concentration, below which no effect is seen, and a maximum
beyond which further concentration repels. The easiest way to make such
observations is to take up a little strong solution in a capillary tube
sealed at the far end, and to introduce its open end into the water, and
let the solution diffuse out, so that this end may be regarded as
surrounded by zones of continuously decreasing strength. In the process of
inflammation (of a Higher Animal) it has been found that the white
corpuscles are so attracted by the source of irritation that they creep out
of the capillaries, and crowd towards it.

We cannot imagine a piece of dough exhibiting any of these reactions, or
the like of them; it can only move passively under the action of some one
or other of the recognised physical forces, and that only in direct
_quantitative_ relation to the work that such forces can effect; in other
words, the dough can have work done on it, but it cannot do work. The
Amoeba or leucocyte on the contrary does work. It moves under the various
circumstances by the transformation of some of its internal energy from the
"potential" into the "kinetic" state, the condition corresponding with this
being essentially a liberation of heat or work, either by the breaking down
of its internal substances, or by the combination of some of them with
oxygen.[11] Such of these changes as involve the excretion of carbonic acid
are termed "respiratory."

This liberation of energy is the "response" to an action of itself
inadequate to produce it; and has been compared not inaptly to the
discharge of a cannon, where foot-tons of energy are liberated in
consequence of the pull of a few inch-grains on the trigger, or to an
indefinitely small push which makes electric contact: the energy set free
is that which was stored up in the charge. This capacity for liberating
energy stored up within, in response to a relatively small impulse from
without, is termed "irritability"; the external impulse is termed the
"stimulus." The responsive act has been termed "contractility," because it
so often means an obvious contraction, but is better termed {9}"motility ";
and irritability evinced by motility is characteristic of all living beings
save when in the temporary condition of "rest."

Again, in the case of the cannon, the gunner after its discharge has to
replenish it for future action with a fresh cartridge; the Amoeba or
leucocyte can replenish itself—it "feeds." When it comes in contact with a
fragment of suitable material, it enwraps it by its pseudopodia (Fig. 3),
and its edges coalesce where they touch on the far side as completely as we
can join up the edges of dough round the apple in a dumpling. It dissolves
all that can be dissolved—_i.e._ it "digests" it, and then absorbs the
dissolved material into its substance, both to replace what it has lost by
its previous activity and to supply fuel for future liberation of energy;
this process is termed "nutrition," and is another characteristic of living
beings.

[Illustration: FIG. 3.—_Amoeba_ devouring a plant cell; four successive
stages of ingestion. (From Verworn.)]

Again, as a second result of the nutrition, part of the food taken in goes
to effect an increase of the living protoplasm, and that of every part, not
merely of the surface—it is "assimilated"; while the rest of the food is
transformed into reserves, or consumed and directly applied to the
liberation of energy. The increase in bulk due to nutrition is thus
twofold: part is the increase of the protoplasm itself—"assimilative
growth," part is the storage of reserves—"accumulative growth": these
reserves being available in turn by digestion, whether for future true
growth or for consumption to liberate energy for the work of the cell.

We can conceive that our cannon might have an automatic feed for the supply
of fresh cartridges after each shot; but not that it could make provision
for an increase of its own bulk, so as to gain in calibre and strength, nor
even for the restoration {10}of its inner surface constantly worn away by
the erosion of its discharges. Growth—and that growth "interstitial,"
operating at every point of the protoplasm, not merely at its surface—is a
character of all living beings at some stage, though they may ultimately
lose the capacity to grow. Nothing at all comparable to interstitial growth
has been recognised in not-living matter.[12]

[Illustration: FIG. 4.—_Amoeba polypodia_ in successive stages of equal
fission; nucleus dark, contractile vacuole clear. (From Verworn, after F.
E. Schulze.)]

Again, when an Amoeba has grown to a certain size, its nucleus divides into
two nuclei, and its cytoplasmic body, as we may term it, elongates, narrows
in the middle so as to assume the shape of a dumb-bell or finger-biscuit,
and the two halves, crawling in opposite directions, separate by the giving
way of the connecting waist, forming two new Amoebas, each with its nucleus
(Fig. 4). This is a process of "reproduction"; the special case is one of
"equal fission" or "binary division." The original cell is termed the
"mother," with respect to the two new ones, and these are of course with
respect to it the "daughters," and {11}"sisters" to one another. We must
bear in mind that in this self-sacrificing maternity the mother is resolved
into her children, and her very existence is lost in their production. The
above phenomena, IRRITABILITY, MOTILITY, DIGESTION, NUTRITION, GROWTH,
REPRODUCTION, are all characteristic of living beings at some stage or
other, though one or more may often be temporarily or permanently absent;
they are therefore called "vital processes."

If, on the other hand, we violently compress the cell, if we pass a very
strong electric shock through it, or a strong continuous current, or expose
it to a temperature much above 45° C., or to the action of certain chemical
substances, such as strong acids or alkalies, or alcohol or corrosive
sublimate, we find that all these vital processes are arrested once and for
all; henceforward the cell is on a par with any not-living substance. Such
a change is called "DEATH," and the "capacity for death" is one of the most
marked characters of living beings. This change is associated with changes
in the mechanical and optical properties of the protoplasm, which loses its
viscidity and becomes opaque, having undergone a process of _de_-solution;
for the water it contained is now held only mechanically in the interstices
of a network, or in cavities of a honeycomb (as we have noted above, p. 5),
while the solid forming the residuum has a refractive index of a little
over 1.6. Therefore, it only regains its full transparency when the water
is replaced by a liquid of high refractive index, such as an essential oil
or phenol. A similar change may be effected by pouring white of egg into
boiling water or absolute alcohol, and is attended with the same optical
results. The study of the behaviour of coagulable colloids has been
recently studied by Fischer and by Hardy, and has been of the utmost
service in our interpretation of the microscopical appearances shown in
biological specimens under the microscope.[13]

{12}The death of the living being finds a certain analogy in the breaking
up or the wearing out of a piece of machinery; but in no piece of machinery
do we find the varied irritabilities, all conducive to the well-being of
the organism (under ordinary conditions), or the so-called "automatic
processes"[14] that enable the living being to go through its
characteristic functions, to grow, and as we shall see, even to turn
conditions unfavourable for active life and growth to the ultimate weal of
the species (see p. 32). At the same time, we fully recognise that for
supplies of matter and energy the organism, like the machine, depends
absolutely on sources from without. The debtor and creditor sheet, in
respect of matter and energy, can be proved to balance between the outside
world and Higher Organisms with the utmost accuracy that our instruments
can attain; and we _infer_ that this holds for the Lower Organisms also.
Many of the changes within the organism can be expressed in terms of
chemistry and physics; but it is far more impossible to state them _all_ in
such terms than it would be to describe a polyphase electrical installation
in terms of dynamics and hydraulics. And so far at least we are justified
in speaking of "vital forces."

The living substance of protoplasm contains a large quantity of water, at
least two-thirds its mass, as we have seen, in a state of physical or loose
chemical combination with solids: these on death yield proteids and
nucleo-proteids.[15] The living protoplasm {13}has an alkaline reaction,
while the liquid in the larger vacuoles, at least, is acid, especially in
Plant-cells.[16]

METABOLISM.—The chemical processes that go on in the organism are termed
metabolic changes, and were roughly divided by Gaskell into (1) "anabolic,"
in which more complex and less stable substances are built up from less
complex and more stable ones with the absorption of energy; and (2)
"catabolic" changes in which the reverse takes place. Anabolic processes,
in all but the cells containing plastids or chromatophores (see p. 36)
under the influence of light, necessarily imply the furnishing of energy by
concurrent catabolic changes in the food or reserves, or in the protoplasm
itself.

Again, we have divided anabolic processes into "accumulative," where the
substances formed are merely reserves for the future use of the cell, and
"assimilative," where the substances go to the building of the protoplasm
itself, whether for the purpose of growth or for that of repair.

Catabolic processes may involve (1) the mere breaking of complex substances
into simpler ones, or (2) their combination with oxygen; in either case
waste products are formed, which may either be of service to the organism
as "secretions" (like the bile in Higher Animals), or of no further use
(like the urine). When nitrogenous substances break down in this way they
give rise to "excretions," containing urea, urates, and allied substances;
other products of catabolism are carbon dioxide, water, and mineral salts,
such as sulphates, phosphates, carbonates, oxalates, etc., which if not
insoluble must needs be removed promptly from the organism, many of them
being injurious or even poisonous. The energy liberated by the protoplasm
being derived through the breakdown of another part of the same or of the
{14}food-materials or stored reserves, must give rise to waste products.
The exchange of oxygen from without for carbonic acid formed within is
termed "respiration," and is distinguished from the mere removal of all
other waste products called "excretion." In the fresh-water Amoeba both
these processes can be studied.

RESPIRATION,[17] or the interchange of gases, must, of course, take place
all over the general surface, but in addition it is combined in most
fresh-water Protista with _excretion_ in an organ termed the "contractile"
or "pulsatile vacuole" (Figs. 1, 4, etc.). This particular vacuole is
exceptional in its size and its constancy of position. At intervals, more
or less regular, it is seen to contract, and to expel its contents through
a pore; at each contraction it completely disappears, and reforms slowly,
sometimes directly, sometimes by the appearance of a variable number of
small "formative" vacuoles that run together, or as in Ciliata, by the
discharge into it of so-called "feeding canals." As this vacuole is filled
by the water that diffuses through the substance, and when distended may
reach one-third the diameter of the being, in the interval between two
contractions an amount of water must have soaked in equal to
one-twenty-seventh the bulk of the animal, to be excreted with whatever
substances it has taken up in solution, including, not only carbon dioxide,
but also, it has been shown, nitrogenised waste matters allied to uric
acid.[18]

That the due interchanges may take place between the cell and the
surrounding medium, it is obvious that certain limits to the ratio between
bulk and surface must exist, which are disturbed by growth, and which we
shall study hereafter (p. 23 f.).

The Protista that live in water undergo a death by "diffluence" or
"granular disintegration" on being wounded, crushed, or sometimes after an
excessive electric stimulation, or contact with alkalies or with acids too
weak to coagulate them. In this process the protoplasm breaks up from the
surface inwards into a mass of granules, the majority of which themselves
finally dissolve. If the injury be a local rupture of the external pellicle
or {15}cuticle, a vacuole forms at the point, grows and distends the
overlying cytoplasm, which finally ruptures: the walls of the vacuole
disintegrate; and this goes on as above described. Ciliate Infusoria are
especially liable to this disintegration process, often termed
"diffluence," which, repeatedly described by early observers, has recently
been studied in detail by Verworn. Here we have death by "solution," while
in the "fixing" of protoplasm for microscopic processes we strive to ensure
death by "desolution," so as to retain as much of the late living matter as
possible. It would seem not improbable that the unusual contact with water
determines the formation of a zymase that acts on the living substance
itself.

We have suggested[19] that one function of the contractile vacuole, in
naked fresh-water Protists, is to afford a regular means of discharge of
the water constantly taken up by the crystalloids in the protoplasm, and so
to check the tendency to form irregular disruptive vacuoles and death by
diffluence. This is supported by the fact that in the holophytic
fresh-water Protista, as well as the Algae and Fungi, a contractile vacuole
is present in the young naked stage (zoospore), but disappears as soon as
an elastic cell-wall is formed to counterbalance by its tension the
internal osmotic pressure.

DIGESTION is always essentially a catabolic process, both as regards the
substance digested and the formation of the digesting substance by the
protoplasm. The digesting substance is termed a "zymase" or "chemical
ferment," and is conjectured to be produced by the partial breakdown of the
protoplasm. In presence of suitable zymases, many substances are resolved
into two or more new substances, often taking up the elements of water at
the same time, and are said to be "dissociated" or "hydrolysed" as the case
may be. Thus proteid substances are converted into the very soluble
substances, "proteoses" and "peptones," often with the concurrent or
ultimate formation of such relatively simple bodies as leucin, tyrosin, and
other amines, etc. Starch and glycogen are converted into dextrins and
sugars; fats are converted into fatty acids and glycerin. It is these
products of digestion, and not the actual food-materials (save certain very
simple sugars), that are really taken up by the protoplasm, {16}whether for
assimilation, for accumulation, or for the direct liberation of energy for
the vital processes of the organism.

Not only food from without, but also reserves formed and stored by the
protoplasm itself, must be digested by some zymase before they can be
utilised by the cell. In all cases of the utilisation of reserve matter
that have been investigated, it has been found that a zymase is formed by
the cell itself (or sometimes, in complex organisms, by its neighbours);
for, after killing the cell in which the process is going on by mechanical
means or by alcohol, the process of digestion can be carried on in the
laboratory.[20] The chief digestion of all the animal-feeding Protista is
of the same type as in our own stomachs, known as "peptic" digestion: this
involves the concurrent presence of an acid, and Le Dantec and Miss
Greenwood have found the contents of food-vacuoles, in which digestion is
going on, to contain acid liquid. The ferment-pepsin itself has been
extracted by Krukenberg from the Myxomycete, "Flowers of tan" (_Fuligo
varians_, p. 92), and by Professor Augustus Dixon and the author from the
gigantic multinucleate Amoeba, _Pelomyxa palustris_ (p. 52).[21] The
details of the prehension of food will be treated of under the several
groups.

The two modes of Anabolism—true "assimilation" in the strictest sense and
"accumulation"—may sometimes go on concurrently, a certain proportion of
the food material going to the protoplasm, and the rest, after allowing for
waste, being converted into reserves.



MOVEMENTS all demand catabolic changes, and we now proceed to consider
these in more detail.

The movements of an Amoeboid[22] cell are of two kinds: "expansion,"
leading to the formation and enlargement of {17}outgrowths, and
"contraction," leading to their diminution and disappearance within the
general surface.[23] Expansion is probably due to the lessening of the
surface-tension at the point of outgrowth, contraction to the increase of
surface-tension. Verworn regards these as due respectively to the
combination of the oxygen in the medium with the protoplasm in diminishing
surface-tension, and the effect of combination with substances from within,
especially from the nucleus in increasing it. Besides these external
movements, there are internal movements revealed by the contained granules,
which stream freely in the more fluid interior. Those Protista that, while
exhibiting amoeboid movements, have no clear external layer, such as the
Radiolaria, Foraminifera, Heliozoa, etc., present this streaming even at
the surface, the granules travelling up and down the pseudopodia at a rate
much greater than the movements of these organs themselves. In this case
the protoplasm is wetted by the medium, which it is not where there is a
clear outer layer: for that behaves like a greasy film.

MOTILE ORGANS.—Protoplasm often exhibits movements much more highly
specialised than the simple expansion or retraction of processes, or the
general change of form seen in Amoeba. If we imagine the activities of a
cell concentrated on particular parts, we may well suppose that they would
be at once more precise and more energetic than we see them in Amoeba or
the leucocyte. In some free-swimming cells, such as the individual cells
known as "Flagellata," the reproductive cells of the lower Plants, or the
male cells ("spermatozoa") of Plants as high as Ferns, and even of the
Highest Animals, there is an extension of the cell into one or more
elongated lash-like processes, termed "flagella," which, by beating the
water in a reciprocating or a spiral rhythm, cause the cell to travel
through it; or, if the cell be attached, they produce currents in the water
that bring food particles to the surface of the cell for ingestion. Such
flagella may, indeed, be seen in some cases to be modified pseudopodia. In
other cases part, or the whole, of the surface of the cell may be covered
with regularly arranged short filaments of similar activity (termed
"cilia," from their resemblance to a diminutive eyelash), which, however,
instead of whirling round, bend sharply {18}down to the surface and slowly
recover; the movement affects the cilia successively in a definite
direction in waves, and produces, like that of flagella, either locomotion
of the cell or currents in the medium. We can best realise their action by
recalling the waves of bending and recovery of the cornstalks in a
wind-swept field; if now the haulms of the corn executed these movements of
themselves, they would determine in the air above a breeze-like motion in
the direction of the waves (Fig. 5).[24] Such cilia are not infrequent on
those cells of even the Highest Animals that, like a mosaic, cover free
surfaces ("epithelium cells"). In ourselves such cells line, for instance,
the windpipe. One group of the Protozoa, the "Ciliata," are, as their name
implies, ciliated cells pure and simple.

[Illustration: FIG. 5.—Motion of a row of cilia, in profile. (From
Verworn.)]

The motions of cilia and of flagella are probably also due to changes of
surface tension—alternately on one side and the other in the cilium, but
passing round in circular succession in the flagellum,[25] giving rise to a
conical rotation like that of a weighted string that is whirled round the
head. This motion is, however, strongest at the thicker basal part, which
assumes a spiral form like a corkscrew of few turns, while the thin lash at
the tip may seem even to be quietly extended like the point of the
corkscrew. If the tip of the flagellum adhere, as it sometimes does, to any
object, the motions induce a jerking motion, which in this case is
reciprocating, not rotatory. When the organism is free, the flagellum is
usually in advance, and the cell follows, rotating at the same time round
its longitudinal axis; such an anterior flagellum, called a "tractellum,"
is the common form in Protista that possess a single one (Figs. 29, 7, 8;
30, C). In the spermatozoa of Higher Animals (and some Sporozoa) the
flagellum is posterior, and is called a "pulsellum."

The cilium or flagellum may often be traced a certain distance into the
substance of the cytoplasm to end in a dot of denser, {19}readily-staining
plasm, which corresponds to a "centrosome" or centre of plasmic forces (see
below, pp. 115, 121, 141); it has been termed a "blepharoplast."[26]

Again, the cytoplasm may have differentiated in it definite streaks of
specially contractile character; such streaks within its substance are
called "myonemes"; they are, in fact, muscular _fibrils_. A "muscle-cell,"
in the Higher Animals, is one whose protoplasm is almost entirely so
modified, with the exception of a small portion of granular cytoplasm
investing the nucleus, and having mainly a nutritive function.

Definite muscular fibrils in action shorten, and at the same time become
thicker. It seems probable that they contain elongated vacuoles, and that
the contents of these vary, so that when they have an increased osmotic
equivalent, the vacuoles absorb water, enlarge, and tend to become more
spherical, _i.e._ shorter and thicker, and so the fibril shortens as a
whole. The relaxation would be due to the diffusion outwards of the
solution of the osmotically active substances which induced expansion.[27]

The MOTILE REACTIONS of the Protozoa[28] require study from another point
of view: they are either (1) "spontaneous" or "arbitrary," as we may say,
or (2) responsive to some stimulus. The latter kind we will take first, as
they are characteristic of all free cells. The stimuli that induce
movements of a responsive character are as follows:—(i.) MECHANICAL: such
as agitation and contact; (ii.) force of GRAVITY, or CENTRIFUGAL FORCE;
(iii.) CURRENTS in the water; (iv.) RADIANT ENERGY (LIGHT); (v.) changes in
the TEMPERATURE of the medium; (vi.) ELECTRIC CURRENTS through the medium;
(vii.) the presence of CHEMICAL SUBSTANCES in the medium.

These, or some of them, may induce one of three different results, or a
combination thereof: (1) a single movement or an arrest of motion; (2) the
assumption of a definite position; (3) movement of a definite character or
direction.

{20}(i.) MECHANICAL STIMULI.—Any sudden touch with another body tends to
arrest all motion; and if the shock be protracted or severe, the retraction
of the pseudopodia follows. It is to this reaction that we must ascribe the
retracted condition of the pseudopodia of most Rhizopods when first placed
on the slide and covered for microscopic examination. Free-swimming
Protista may, after hitting any body, either remain in contact with it, or
else, after a pause, _reverse_ their movement, turn over and swim directly
away. This combination of movements is characteristic as a reaction of what
we may term "repellent" stimuli in general.[29] Another mechanical reaction
is that to continuous contact with a solid; and the surface film of water,
either at the free surface or round an air-bubble, may play the part of a
solid in exciting it; we term it "thigmotaxy" or "stereotaxy." When
positive it determines a movement on to the surface, or a gliding movement
along it, or merely the arrest of motion and prolongation of contact; when
_negative_, a contact is followed by the retreat of the being. Thus
_Paramecium_ (Fig. 55, p. 151) and many other Ciliates are led to aggregate
about solid particles or masses of organic _débris_ in the water, which
indeed serve to supply their food. On contact, the cell ceases to move its
cilia except those of the oral groove; as these lash backwards, they hold
the front end in close contact with the solid, at the same time provoking a
backward stream down the groove, which may bring in minute particles from
the mass.

(ii.) Most living beings are able to maintain their level in water by
floating or crawling against GRAVITY, and they react in virtue of the same
power against centrifugal force. This mode of irritability is termed
(negative) "geotaxy" or "barotaxy." We can estimate the power of resisting
such force by means of a whirling machine, since when the acceleration is
greater than the resistance stimulated thereby in the beings, they are
passively sent to the sides of the vessel. The Flagellates, _Euglena_ and
_Chlamydomonas_, begin to migrate towards the centre when exposed to a
centrifugal force about equal to ½ G (G = 32.2 feet or 982 cm. per second);
they remain at the centre until the centrifugal force is increased to 8 G;
above that they yield to the force, and are driven passively to the sides.
The reaction ceases or is reversed at high temperatures.

{21}(iii.) RHEOTAXY.—This is the tendency to move against the stream in
flowing water. It is shown by most Protists, and can be conveniently
studied in the large amoeboid plasmodia of the Myxomycetes, which crawl
against the stream along wet strips of filter paper, down which water is
caused to flow. Most animals, even of the highest groups, tend to react in
the same way; the energetic swimming of Fishes up-stream being in marked
contrast with their sluggishness the other way; and every student of
pond-life knows how small Crustacea and Rotifers, no less than Ciliates,
swim away from the inrush of liquid into the dipping-tube, and so evade
capture. (See Vol. II. p. 216.)

(iv.) The movements of many Protozoa are affected greatly by LIGHT. These
movements have been distinguished into "photopathic," _i.e._ to or from the
_position_ of greatest luminosity; and "phototactic," along the _direct
path_ of the rays.[30] Those Protozoa that contain a portion of their
cytoplasm, known as a "plastid" or "chromatophore" (see pp. 36, 39),
coloured by a green or yellow pigment are usually "phototactic." They
mostly have at the anterior end a red pigment spot, which serves as an
organ of sight, and is known as an "eye-spot." In diffused light of low
intensity they do not exhibit this reaction, but in bright sunlight they
rise to the surface and form there a green or yellow scum.

Most of the colourless Protista are negatively phototactic or photopathic;
but those which are parasitic on the coloured ones are positively
phototactic, like their hosts.

Here, as in the case of other stimuli,[31] the absolute intensity of the
light is of importance; for as it increases from a low degree, different
organisms in turn cease to be stimulated, and {22}then are repelled instead
of being attracted. The most active part of the spectrum in determining
reactions of movement are the violet and blue rays of wave-length between
40 µ/10 and 49 µ/10, while the warmer and less refractive half of the
spectrum is inert save in so far as it determines changes in the
temperature of the medium.

(v.) The movements of many Protozoa are rendered sluggish by cold, and
active by a rise of TEMPERATURE up to what we may term the "optimum"; the
species becomes sluggish again as the temperature continues to rise to a
certain point when the movements are arrested, and the being is said to be
in a state of "heat-rigor." Most Protozoa, again, tend to move in an
unequally heated medium to the position nearest to their respective optimum
temperature. This is called "thermotaxy." The temperature to which Amoeba
is thermotactic is recorded as 35° C. (95° F.); that of _Paramecium_ is 28°
C. (82° F.).

(vi.) Most active Protozoa tend to take up a definite position in respect
to a current of ELECTRICITY passing through the medium, and in the majority
of cases, including most Ciliates, _Amoeba_, and _Trachelomonas_, they
orient their long diameters in the direction of the lines of force and swim
along these to assemble behind the cathode. The phenomenon is called
"galvanotaxy," and this particular form is "negative." _Opalina_ (Fig. 41,
p. 123), however, and most Flagellates are "positively galvanotactic," and
move towards the anode. H. H. Dale[32] has shown that the phenomenon may be
possibly in reality a case of chemiotaxy, for the direction of motion
varies with the nature and concentration of the medium. It would thus be a
reaction to the "ion" liberated in contact with the one or other extremity
of the being. Induction shocks, as we have seen, if slight, arrest the
movements of Protozoa, or if a little stronger determine movements of
contraction; if of sufficient intensity they kill them. No observation
seems to have been made on the behaviour of Protista in an electric field.
A magnetic field of the highest intensity appears to be indifferent to all
Protista.

(vii.) We have already referred to the effect of dissolved CHEMICAL
SUBSTANCES present in the water. If the substance is in itself not harmful,
and the effect varies with the concentration, we term the reaction one of
"tonotaxy," which combines {23}with that of "chemiotaxy" for substances
that in weak solution are attractive or repellent to the being.
_Paramecium_, which feeds on bacteria, organisms of putrefaction, is
positively chemiotactic to solutions of carbon dioxide, and as it gives
this off in its own respiration, it is attracted to its fellows. The
special case of reaction to gases in solution is termed "aerotaxy," or
"pneumotaxy," according as the gas is oxygen or carbon dioxide. We find
that in this respect there are degrees, so that a mixed culture of
Flagellates in an organic infusion sorts itself out, under the cover of a
microscopic preparation, into zones of distinct species, at different
distances from the freely aerated edge, according to the demands of each
species for oxygen and CO_{2} respectively.

Finally, we must note that the apparently "spontaneous movements" of
Protists can hardly be explained as other than due either to _external_
stimuli, such as we have just studied, or to _internal_ stimuli, the
outcome of internal changes, such as fatigue, hunger, and the like. Of the
latter kind are the movements that result in REPRODUCTION.

REPRODUCTION.—We have noted above that the growth of an organism _which
retains its shape_ alters the ratio of the surface area to the whole
volume, so necessary for the changes involved in life. For the volume of an
organism varies as the cube of any given diameter, whereas the surface
varies with the square only. Without going into the arithmetical details,
we may say that the ratio of surface to volume is lessened to roughly
four-fifths of the original ratio when the cell doubles its bulk. As
Herbert Spencer and others have pointed out, this must reduce the
activities of the cell, and the due ratio is restored by the division of
the cell into two.[33] This accounts for what we must look on as the most
primitive mode of reproduction, as it is the simplest, and which we term
"fission" at Spencer's "limit of {24}growth." Other modes of reproduction
will be studied later (p. 30), after a more detailed inquiry into the
structure of the nucleus and of its behaviour in cell-division. All
cell-division is accompanied by increased waste, and is consequently
_catabolic_ in character, though the _anabolic_ growth of living
protoplasm, at the expense of the internal reserves, may be concurrent
therewith.


CELL-DIVISION

In ordinary cases of fission of an isolated cell the cell elongates, and as
it does so, like other viscid bodies, contracts in the middle, which
becomes drawn out into a thread, and finally gives way. In some cases
(_e.g._ that of the _Amoeba_, Fig. 4) the nucleus previously undergoes a
similar division by simple constriction, which is called direct or
"amitotic" division. But usually the division of the nucleus prior to
cell-division is a more complex process, and involves the co-operation of
the cytoplasm; and we must now study in detail the nucleus and its
structure in "rest" and in fission.[34]

We have noted above (p. 6, Fig. 2) the structure of the so-called "resting
nucleus,"[35] when the cell is discharging the ordinary functions of its
own life, with its wall, network of linin, chromatin-granules, and nucleole
or nucleoles. The chromatin-granules are most abundant at two periods in
the life of the cell, (1) when it is young and fresh from division, and (2)
at the term of its life, when it is itself preparing for division. In the
interim they are fewer, smaller, and stain less intensely. In many Protista
the whole or greater part of the chromatin is densely aggregated into a
central "nuclein-mass" or _karyosome_ {25}suspended in the linin network
(long regarded as a mere nucleole). Such a nucleus is often termed a
"vesicular nucleus".[36]

[Illustration: FIG. 6.—Changes in nucleus and cell in indirect (mitotic)
nuclear division. A, resting nucleus with two centrioles[37] in single
centrosphere (_c_); B, centrosphere divided, spindle and two asters (_a_)
forming; C, centrospheres separated, nuclear wall disappearing; D,
resolution of nucleus into chromosomes; E, mature plasmic spindle, with
longitudinal fission of chromosomes; F, chromosomes forming equatorial
plate (_ep_) of spindle. (From Wilson.)]

{26}When cell-division is about to take place the linin, or at least the
greater part of it, assumes the character of a number of distinct threads,
and the whole of the chromatin granules are distributed at even distances
along these (Fig. 6, A, B, C), so as to appear like so many strings of
beads. Each such thread is called a "chromosome." Then each bead divides
longitudinally into two. The thread flattens into a ribbon, edged by the
two lines of chromatin beads. Finally, the ribbon splits longitudinally
into two single threads of beads (Fig. 6, E). During these changes the
nucleole or nucleoles diminish, or even disappear, as if they had
contributed their matter to the growth of the chromatin proper. In Higher
Animals and Plants the nuclear wall next disappears, and certain structures
become obvious, especially in the cytoplasm of Metazoa. Two minute spheres
of plasm (themselves often showing a concentric structure), the
"centrosomes,"[38] which hitherto lay close together at the side of the
nuclear wall, now separate; but they remain connected by a spindle of clear
plasmic threads (Fig. 6, B-E) which, as the centres diverge, comes to lie
across the spot the nucleus occupied, and now the chromosomes lie about the
equator of this spindle (Fig. 6, F). Moreover, the surrounding cytoplasm
shows a radiating structure, diverging from the centrosome, so that spindle
and external radiations together make up a "strain-figure," like that of
the "lines of force" in relation to the poles of a magnet. Such we can
demonstrate in a plane by spreading or shaking iron filings on a piece of
paper above the poles of a magnet, or in space by suspending finely divided
iron in a thick liquid, such as mucilage or glycerin, and bringing the
vessel with the mixture into a strong magnetic field;[39] the latter mode
has the advantage {27}of enabling us to watch the changes in the
distribution of the lines under changing conditions or continued strain.

[Illustration: FIG. 7.—Completion of mitotic cell-division. G, splitting of
equatorial plate (_ep_); H, recession of daughter chromosomes; I, J,
reconstitution of these into new nuclei, fission of the centrioles and of
the cytoplasm. _if_, Central fibres of spindle; _n_, remains of old
nucleole. (From Wilson.)]

The chromosomes are now completely split, each into its two
daughter-segments, which glide apart (Fig. 7, G, _ep_), and pass each to
its own pole of the spindle, stopping just short of the centrosome (I).
Thus, on the inner side of either centrosome is found an aggregation of
daughter-segments, each of which is sister to one at the opposite pole,
while the number at either pole is identical with that of the segments into
which the old nucleus had resolved itself at the outset. The
daughter-segments shorten and thicken greatly as they diverge to the poles,
and on their arrival crowd close together.

A distinct wall now forms around the aggregated {28}daughter-chromosomes
(J), so as to combine them into a nucleus for the daughter-cell. The
reorganisation of the young nucleus certainly varies in different cases,
and has been ill-studied, probably because of the rapidity of the changes
that take place. The cytoplasm now divides, either tapering into a "waist"
which finally ruptures, or constricting by the deepening of a narrow
annular groove so as to complete the formation and isolation of the
daughter-cells.

We might well compare the cell-division to the halving of a pumpkin or
melon, of which the flesh as a whole is simply divided into two by a
transverse cut, while the seeds and the cords that suspend them are each
singly split to be divided evenly between the two halves of the fruit; the
flesh would represent the cytoplasm, the cords the linin threads of the
nucleus, and the seeds the chromatin granules. In this way the halving of
the nucleus is much more complete and intimate than that of the cytoplasm;
and this is the reason why many biologists have been led to regard the
nuclear segments, and especially their chromatic granules, as the seat of
the hereditary properties of the cell, properties which have to be equally
transmitted on its fission to each daughter-cell.[40] But we must remember
that the linin is also in great part used up in the formation of these
segments, like the cords of our supposed melon; and it is open to us to
regard the halving in this intimate way of the "linin" as the essence of
the process, and that of the chromatin as accessory, or even as only part
of the necessary machinery of the process. The halving or direct splitting
lengthwise of a viscid thread is a most difficult problem from a physical
point of view; and it may well be that the chromatin granules have at least
for a part of their function the facilitation of this process. If such be
the case, we can easily understand the increase in number, and size and
staining power of these granules as cell-division approaches, and their
atrophy or partial disappearance during their long intervening periods of
active cell life. Hence we hesitate to accept the views so commonly
maintained that the chromatin represents a {29}"germ-plasm" or "idioplasm"
of relatively great persistence, which gives the cell its own racial
qualities.[41]

The process we have just examined is called "mitosis," "karyomitosis," or
"karyokinesis"; and the nucleus is said to undergo "indirect" division, as
compared to "direct" division by mere constriction. In an intermediate
mode, common to many Protista, the nuclear wall persists throughout the
whole process, though a spindle is constituted within, and chromosomes are
formed and split: the division of the nucleus takes place, however, by
simple constriction, as seen in the Filose Rhizopod _Euglypha_ (Fig. 8).

[Illustration: FIG. 8.—Fission with modified karyokinesis in the Filose
Rhizopod _Euglypha_. A, outgrowth of half of the cytoplasm, passage of
siliceous plates for young shell outwards; B, completion of shell of second
cell, formation of _intra_-nuclear spindle; C, D, further stages. (From
Wilson, after Schewiakoff.)]

In many Sarcodina and some Sporozoa the nucleus gives off small fragments
into the cytoplasm, or is resolved into them; {30}they have been termed
"chromidia" by E. Hertwig. New nuclei may be formed by their growth and
coalescence, the original nucleus sometimes disappearing more or less
completely.

In certain cases the division of the nucleus is not followed by that of the
cytoplasm, so that a plurinucleate mass of protoplasm results: this is
called an "apocyte"; and we find transitional forms between this and the
uninucleate or true cell. Thus in one species of _Amoeba_ (_A. binucleata_)
there are always two nuclei, which divide simultaneously to provide for the
outfit of the daughter-cells on fission. Again, we find in some cases that
similar multinucleate masses may be formed by the union of two or more
cells by their cytoplasm only: such a union is termed "permanent
plastogamy," and the plurinucleate mass a "plasmodium."[42] Here again we
find intermediate forms between plasmodium and apocyte, for the nuclei of
the former may divide and so increase in number, without division of the
still growing mass. Both kinds of plurinucleate organisms are termed
"coenocytes" without reference to their mode of origin.

The rhythm of cell-life that we have just studied is called the
"Spencerian" rhythm. Each cell in turn grows from half the bulk of its
parent at the time it was formed to the full size of that parent, when it
divides in its own turn. Rest is rare, and assumed only when the cell is
exposed to such unfavourable external conditions as starvation, drought,
etc.; it has no necessary relation to fission.

MULTIPLE FISSION OR BROOD-FORMATION.—We may now turn to a new rhythm, in
strong contrast to the former: a cell after having attained a size, often
notably greater than its parents, divides: without any interval for growth,
the daughter-cells again divide, and this may be repeated as many as ten
times, or even more, so as to give rise to a number of small cells—4, 8,
16—1024,[43] etc., respectively. Such an assemblage of small cells so
formed is called a brood, and well deserves this name, for they never
separate until the whole series of divisions is completed. By this process
the number of individuals is rapidly {31}increased, hence it has received
the name of "sporulation." The term spores is especially applied to the
reproductive bodies of Cryptogams, such as Mosses, Fungi, etc.: the
resulting cells are called "spores," "zoospores" if active ("amoebulae" if
provided with pseudopodia, "flagellulae" if flagellate), "aplanospores," if
motionless. We prefer to call them by the general term "brood-cells," the
original cell the "brood-mother-cell," and the process, "multiple fission"
or "brood-formation." As noted, the brood-mother-cell usually attains an
exceptionally large size, and it in most cases passes into a state of rest
before entering on division: thus brood-formation is frequently the
ultimate term of a long series of Spencerian divisions. Two contrasting
periods of brood-formation may occur in the life cycle of some beings,
notably the Sporozoa.[44]

COLONIAL UNION.—In certain cases, the brood-cells instead of separating
remain together to form a "colony"; and this may enlarge itself again by
binary division of its individual cells at their limit of growth. Here,
certain or all of the cells may (either after separation, or in their
places) undergo brood-formation: such cells are often termed "reproductive
cells" in contrast with the "colonial cells."

Some such colonial Protista must have been the starting-points for the
Higher Animals and Plants; probably apocytial Protista were the
starting-points of the Fungi. In the Higher Animals and Plants, the
spermatozoa and the oospheres (the male and female pairing-cells) are alike
the offspring of brood-formation: and the coupled-cell (fertilised egg)
starts its new life by _segmentation_, which is a brood-formation in which
the cells do not separate, but remain in colonial union, to differentiate
in due course into the tissue-cells of the organism.

RETARDED BROOD-FORMATION.—The nuclear divisions may alternate with
cell-divisions, as above stated, or the former may be {32}completed before
the cytoplasm divides; thus the brood-mother-cell becomes temporarily an
apocyte,[45] which is then _resolved_ simultaneously into the 1-nucleate
brood-cells.

A temporary apocytial condition is often passed through in the formation of
the brood of cells by repeated divisions without any interval for
enlargement; for the nuclear divisions may go on more rapidly than those of
the cytoplasm, or be completed before any cell-division takes place (Figs.
31, 34, 35, pp. 95, 101, 104), the nuclear process being "accelerated" or
the cytoplastic being "retarded," whichever we prefer to say and to hold.
Thus as many as thirty-two nuclei may have been formed by repeated binary
subdivisions before any division of the cytoplasm takes place to resolve
the apocyte into true 1-nucleate cells.

  In many cases of brood-formation the greater part of the food-supply of
  the brood-mother-cell has been stored as reserve-products, which
  accumulate in quantity in the cell; this is notably seen in the ovum or
  egg of the Higher Animals. How great such an accumulation may be is
  indeed well seen in the enormous yolk of a bird's egg, gorged as it were
  to repletion. When a cell has entered on such course of "miserly"
  conduct, it may lose all power of drawing on its own supplies, and
  finally that of accumulating more, and passes into the state of "rest."
  To resume activity there is needed either a change in the internal
  conditions—demanding the lapse of time—or in the external conditions, or
  in both.[46] We may call this resumption "germination."

  Very often in the study of a large and complex organism we are able to
  find processes in action on a large scale which, depending as they must
  do on the protoplasmic activities of its individual cells, reveal the
  nature of similar processes in simple unicellular beings: such a clue to
  the utilisation of reserves by a cell which has gorged itself with them
  so as to pass into a state of rest is to be found in that common
  multicellular organism, the Potato. This stores up reserves in its
  underground stems (tubers); if we plant these immediately on the
  completion of their growth, they will not start at once, even under what
  would outwardly seem to be most appropriate conditions. A certain lapse
  of time is an essential factor for sprouting. It would appear that in the
  Potato the starch can only be digested by a definite ferment, which does
  not exist when it is dug, but which is only formed very slowly, and not
  at all until a certain time has supervened; and that sprouting can only
  {33}take place when soluble material has been provided in this way for
  the growth of the young shoots. We have also reason to believe that these
  ferments are only formed by the degradation of the protoplasm itself. Now
  obviously this degradation must be very slow in a resting organism; and
  any external stimulus that will tend to protoplasmic activity will
  thereby tend to form at the same time the digestive ferments and dissolve
  the stored supplies, to render them available for the life-growth and
  reproduction of the being. We now see why inactive "miserly" cells so
  often pass into a resting state before dividing, and why they go on
  dividing again and again when once they re-enter upon an active life, the
  living protoplasm growing at the expense of the reserves.[47] Resting
  cells of this type occur of course only at relatively rare intervals in
  the animal-feeding Protozoa, that have to take into their substance the
  food they require for their growth and life-work, and cannot therefore
  store up much reserves. For they are constantly producing in the narrow
  compass of their body those very ferments that would dissolve the
  reserves when formed. Internal parasites and "saprophytes," that is,
  beings which live on dead and decayed organic matter, on the other hand,
  live surrounded by a supply of dissolved food; and rarely do we find
  larger cells, richer in reserves, than in the parasitic Sporozoa, which
  owe their name to the importance of brood-formation in their
  life-history. In Radiolaria (p. 75 f.) a central capsule separates off an
  inner layer of protoplasm; the outer layer is the one in which digestion
  is performed, while the inner layer stores up reserves; and here
  brood-formation appears to be the rule. But the largest cells of all are
  the eggs of the Metazoa, which in reality lead a parasitic life, being
  nurtured by the animal as a whole, and contributing nothing to the
  welfare of it as an individual. Their activity is reduced to a minimum,
  and the consequent need for a high ratio of surface to volume is also
  reduced, which accounts for their inordinate size. But directly the
  reserve materials are rendered available by the formation of a digestive
  ferment, then protoplasmic growth takes place, and the need for an
  extended surface is felt; cell-division follows cell-division with
  scarcely an interval in the process of segmentation.

SYNGAMY.[48]—The essence of typical syngamy is, that two cells
("pairing-cells," "gametes") of the same species approach one another, and
fuse, cytoplasm with cytoplasm, and nucleus with nucleus, to form a new
cell ("coupled-cell," "zygote "). This process is called also "conjugation"
or "cytogamy." In the simplest cases the two cells are equal and attract
one another equally ("isogamy"), and have frequently the character of
zoospores.

In an intermediate type, the one cell is larger and more sluggish (female),
"megagamete," "oogamete," "oosphere," "egg"; the other smaller, more active
(male), "microgamete," "spermogamete," "spermatozoon," "sperm"; and in the
most specialised {34}cases which prevail among the Higher Animals and
Plants, the larger cell is motionless, and the smaller is active, ciliate,
flagellate, or amoeboid: the coupled-cell or zygote is here termed the
"oosperm."[49] It encysts immediately in most Protista except Infusoria,
Acystosporidae, Haemosporidae, and Trypanosomatidae.

As the size of the two gametes is so disproportionate in most cases that
the oosphere may be millions of times bigger than the sperm, and the latter
at its entrance into the oosphere entirely escape unaided vision, the term
"egg" is applied in lax speech, both (1) to the female cell, and (2) to the
oosperm, the latter being distinguished as the "fertilised egg," a survival
from the time when the union of _two_ cells, as the essence of the process,
was not understood.

  We know that in _many_ cases, and have a right to infer that in _all_,
  chemiotaxy plays an important part in attracting the pairing-cells to one
  another. In Mammals and Sauropsida there seems also to be a rheotactic
  action of the cilia lining the oviducts, which work downwards, and so
  induce the sperms to swim upwards to meet the ovum, a condition of things
  that was most puzzling until the nature of rheotaxy was understood. A
  remarkable fact is that equal gametes rarely appear to be attracted by
  members of the same brood, though they are attracted by those of any
  other brood of the same species.[50] It may well be that each brood has
  its own characteristic secretion, or "smell," as it were, slightly
  different from that of other broods, just as every dog has his, so easily
  recognisable by other dogs; and that the cells only react to different
  "smells" to their own. Such a secretion from the surface of the female
  cell would lessen its surface tension, and thereby render easier the
  penetration of the sperm into its substance.

  As a rule, one at least of the pair-cells is fresh from division, and it
  would thus appear that the union of the nuclei is facilitated when one at
  least of them is a "young" one. Of the final mechanism of the union of
  the nuclei, we know nothing: they may unite in any of the earlier phases
  of mitosis, or even in the "resting state." A fibrillation of the
  cytoplasm during the process, radiating around a centrosome or two
  centrosomes indicates a strained condition.[51]


{35}REGENERATION.—Finally, experiments have been made by several observers
as to the effects of removing parts of Protozoa, to see how far
regeneration can take place. The chief results are as follows:—

1. A nucleated portion may regenerate _completely_, if of sufficient size.
Consequently, multinucleate forms, such as _Actinosphaerium_ (Heliozoa,
Fig. 19, p. 72), may be cut into a number of fragments, and regenerate
completely. In Ciliata, such as _Stentor_ (Fig. 59, p. 156), each fragment
must possess a portion of the meganucleus, and at least one micronucleus
(p. 145), and, moreover, must possess a certain minimum size. A Radiolarian
"central-capsule" (p. 75) with its endoplasm and nucleus may regenerate its
ectoplasm, but the isolated ectoplasm being non-nucleate is doomed. A
"central capsule" of one species introduced into the ectoplasm of another,
closely allied, did well. All non-nucleate pieces may exhibit
characteristic movements, but appear unable to digest; and they survive
only a short time.[52]


"ANIMALS" AND "PLANTS"

Hitherto we have discussed the cell as if it were everywhere an organism
that takes in food into its substance, the food being invariably "organic"
material, formed by or for other cells; such nutrition is termed
"holozoic." There are, however, limits to the possibilities in this
direction, as there are to the fabled capacities of the Scillonians of
gaining their precarious livelihood by taking in one another's washing. For
part of the food material taken in in this way is applied to the supply of
the energies of the cell, and is consequently split up or oxidised into
simpler, more stable bodies, no longer fitted for food; and of the matter
remaining to be utilised for building up the organism, a certain proportion
is always wasted in by-products. Clearly, then, the supply of food under
such conditions is continually lessening in the universe, and we have to
seek for a manufactory of food-material from inorganic materials: this is
to be found in those cells that are known as "vegetal," in the widest sense
of {36}the word. In this, sense, vegetal nutrition is the utilisation of
nitrogenous substances that are more simple than proteids or peptones,
together with suitable organic carbon compounds, etc., to build up proteids
and protoplasm. The simplest of organisms with a vegetal nutrition are the
Schizomycetes, often spoken of loosely as "bacteria" or "microbes," in
which the differentiation of cytoplasm and nucleus is not clearly
recognisable. Some of these can build up their proteids from the free
uncombined nitrogen of the atmosphere, carbon dioxide, and inorganic salts,
such as sulphates and phosphates. But the majority of vegetal feeders
require the nitrogen to be combined at least in the form of a nitrate or an
ammonium salt—nay, for growth in the dark, they require the carbon also to
be present in some organic combination, such as a tartrate, a carbohydrate,
etc. Acetates and oxalates, "aromatic" compounds[53] and nitriles are
rarely capable of being utilised, and indeed are often prejudicial to life.
In many vegetal feeders certain portions of the protoplasm are specialised,
and have the power of forming a green, yellow, or brown pigment; these are
called "plastids" or "chromatophores." They multiply by constriction within
the cell, displaying thereby a certain independent individuality. These
plastids have in presence of light the extraordinary power of deoxidising
carbon dioxide and water to form carbohydrates (or fats, etc.) and free
oxygen; and from these carbohydrates or fats, together with ammonium salts
or nitrates, etc., the vegetal protoplasm at large can build up all
necessary food matter. So that in presence of light of the right
quality[54] and adequate intensity, such coloured vegetal beings have the
capacity for building up their bodies and reserves from purely inorganic
materials. Coloured vegetal nutrition, then, is a process involving the
absorption of energy; the source from which this is derived in the bacteria
being very obscure at present. Nutrition by means of coloured plastids is
{37}distinguished as "holophytic," and that from lower substances, which,
however, contain organically combined carbon, as "saprophytic," for such
are formed by the death and decomposition of living beings. The third mode
of nutrition (found in some bacteria) from wholly inorganic substances,
including free nitrogen, has received no technical name. All three modes
are included in the term "autotrophic" (self-nourishing).

Vegetal feeders have a great tendency to accumulate reserves in insoluble
forms, such as starch, paramylum, and oil-globules on the one hand, and
pyrenoids, proteid crystals, aleurone granules on the other.

When an animal-feeding cell encysts or surrounds itself with a continuous
membrane, this is always of nitrogenous composition, usually containing the
glucosamide "chitin." The vegetal cell-wall, on the contrary, usually
consists, at least primarily, of the carbohydrate "cellulose"—the vegetal
cell being richly supplied with carbohydrate reserves, and drawing on them
to supply the material for its garment. This substance is what we are all
familiar with in cotton or tissue-paper.

Again, not only is the vegetal cell very ready to surround itself with a
cell-wall, but its food-material, or rather, speaking accurately, the
inorganic materials from which that food is to be manufactured, may diffuse
through this wall with scarcely any difficulty. Such a cell can and does
grow when encysted: it grows even more readily in this state, since none of
its energies are absorbed by the necessities of locomotion, etc. Growth
leads, of course, to division: there is often an economy of wall-material
by the formation of a mere party-wall dividing the cavity of the old
cell-wall at its limit of growth into two new cavities of equal size. Thus
the division tends to form a colonial aggregate, which continues to grow in
a motionless, and, so far, a "resting" state. We may call this "vegetative
rest," to distinguish it from "absolute rest," when all other
life-processes (as well as motion) are reduced to a minimum or absolutely
suspended.

The cells of a plant colony are usually connected by very fine threads of
protoplasm, passing through minute pores where the new party-wall is left
incomplete after cell-division.[55] In a few plants, such as most Fungi,
the cell-partitions are {38}in abeyance for the most part, and there is
formed an apocyte with a continuous investment, sometimes, however,
chambered at intervals by partitions between multinucleate units of
protoplasm. We started with a purely _physiological_ consideration, and we
have now arrived at a _morphological_ distinction, very valid among higher
organisms.

_HIGHER PLANTS consist of cells for the most part each isolated in its own
cell-cavity, save for the few slender threads of communication._

_HIGHER ANIMALS consist of cells that are rarely isolated in this way, but
are mostly in mutual contact over the greater part of their surface._

Again, Plants take in either food or else the material for food in solution
through their surface, and only by diffusion through the cell-wall.
Insectivorous Plants that have the power of capturing and digesting insects
have no real internal cavity. Animal-feeding Protista take in their food
into the interior of their protoplasm and digest it therein, and the
Metazoa have an internal cavity or stomach for the same purpose. Here again
there are exceptions in the case of certain internal parasites, such as the
Tapeworms and Acanthocephala (Vol. II. pp. 74, 174), which have no
stomachs, living as they do in the dissolved food-supplies of their hosts,
but still possessing the general tissues and organs of Metazoa.

Corresponding with the absence of mouth, and the _absorption_ instead of
the _prehension_ of food, we find that the _movements_ of plant-beings are
limited. In the higher Plants, and many lower ones, the colonial organism
is firmly fixed or attached, and the movements of its parts are confined to
flexions. These are produced by inequalities of growth; or by inequalities
of temporary distension of cell-masses, due to the absorption of liquid
into their vacuoles, while relaxation is effected by the cytoplasm and
cell-wall becoming pervious to the liquid. We find no case of a
differentiation of the cytoplasm within the cell into definite muscular
fibrils. In the lower Plants single naked motile cells disseminate the
species; and the pairing-cells, or at least the males, have the same motile
character. In higher Cryptogams, Cycads, and _Ginkgo_ (the Maiden-hair
Tree), the sperms alone are free-swimming; and as we pass to Flowering
Plants, the migratory character of the male cells is restricted to the
smallest limits. {39}though never wholly absent. Intracellular movements of
the protoplasm are, however, found in all Plants.

In Plants we find no distinct nervous system formed of cells and
differentiated from other tissues with centres and branches and
sense-organs. These are more or less obvious in all Metazoa, traces being
even found in the Sponges.

We may then define Plants as beings which have the power of manufacturing
true food-stuffs from lower chemical substances than proteids, often with
the absorption of energy. They have the power of surrounding themselves
with a cell-wall, usually of cellulose, and of growing and dividing freely
in this state, in which animal-like changes of form and locomotion are
impossible; their colonies are almost always fixed or floating; free
locomotion is only possible in the case of their naked reproductive cells,
and is transitory even in these. The movements of motile parts of complex
plant-organisms are due to the changes in the osmotic powers of cells as a
whole, and not to the contraction of differentiated fibrils in the
cytoplasm of individual cells. Plants that can form carbohydrates with
liberation of free oxygen have always definite plastids coloured with a
lipochrome[56] pigment, or else (in the Phycochromaceae) the whole plasma
is so coloured. Solid food is never taken into the free plant-cell nor into
an internal cavity in complex Plants. If, as in insectivorous Plants, it is
digested and absorbed, it is always in contact with the morphological
external surface. In the complex Plants apocytes and syncytes are rare—the
cells being each invested with its own wall, and, at most, only
communicating by minute threads with its neighbours. No trace of a central
nervous system with differentiated connexions can be made out.

Animals all require proteid food; their cyst-walls are never formed of
cellulose; their cells usually divide in the naked condition only, or if
encysted, no secondary party-walls are formed between the daughter-cells to
unite them into a vegetative colony. Their colonies are usually locomotive,
or, if fixed, their parts largely retain their powers of relative motion,
and are often provided on their free surfaces with cilia or flagella; and
many cells have differentiated in their cytoplasm contractile muscular
fibrils. Their food (except in a few parasitic groups) is always taken
{40}into a distinct digestive cavity. A complex nervous system, of many
special cells, with branched prolongations interlacing or anastomosing, and
uniting superficial sense-organs with internal centres, is universally
developed in Metazoa. All Metazoa fulfil the above conditions.

But when we turn to the Protozoa we find that many of the characters evade
us. There are some Dinoflagellates (see p. 130) which have coloured
plastids, but which differ in no other respect (even specific) from others
that lack them: the former may have mouths which are functionless, the
latter have functional mouths. Some colourless Flagellates are saprophytic
and absorb nutritive liquids, such as decomposing infusions of organic
matter, possibly free from all proteid constituents; while others, scarcely
different, take in food after the fashion of Amoeba. Sporozoa in the
persistence of the encysted stage are very plant-like, though they are
often intracellular and are parasitic in living Animals. On the other hand,
the Infusoria for the most part answer to all the physiological characters
of the Animal world, but are single cells, and by the very perfection of
their structure, all due to plasmic not to cellular differentiation, show
that they lie quite off the possible track of the origin of Metazoa from
Protozoa. Indeed, a strong natural line of demarcation lies between Metazoa
and Protista. Of the Protozoa, certain groups, like the Foraminifera and
Radiolaria and the Ciliate and Suctorial Infusoria are distinctly animal in
their chemical activities or metabolism, their mode of nutrition, and their
locomotive powers. When we turn to the Proteomyxa, Mycetozoa, and the
Flagellates we find that the distinction between these and the lower Fungi
is by no means easy, the former passing, indeed, into true Fungi by the
Chytridieae, which it is impossible to separate sharply from those
Flagellates and Proteomyxa which Cienkowsky and Zopf have so accurately
studied under the name of "Monadineae." Again, many of the coloured
Flagellates can only (if at all) be distinguished from Plants by the
relatively greater prominence and duration of the mobile state, though
classifiers are generally agreed in allotting to Plants those coloured
Flagellates which in the resting state assume the form of multicellular or
apocytial filaments or plates.

On these grounds we should agree with Haeckel in distinguishing the living
world into the Metazoa, or Higher Animals, which {41}are sharply marked
off; the Metaphyta, or Higher Plants, which it is not so easy to
characterise, but which unite at least two or more vegetal characters; and
the Protista, or organisms, whose differentiation is limited to that within
the cell (or apocyte), and does not involve the cells as units of tissues.
These Protista, again, it is impossible to separate into animal and vegetal
so sharply as to treat adequately of either group without including some of
the other: thus it is that every text-book on Zoology, like the present
work, treats of certain Protophyta. The most unmistakably animal group of
the Protista, the Ciliata, is, as we have seen, by the complex
differentiation of its protoplasm, widely removed from the Metazoa with
their relatively simple cells but differentiated cell-groups and tissues.
The line of probable origin of the Metazoa is to be sought, for Sponges at
least, among the Choanoflagellates (pp. 121 f. 181 f.).




{42}CHAPTER II

PROTOZOA (_CONTINUED_): SPONTANEOUS GENERATION—CHARACTERS OF
PROTOZOA—CLASSIFICATION


THE QUESTION OF SPONTANEOUS GENERATION

From the first discovery of the Protozoa, their life-history has been the
subject of the highest interest: yet it is only within our own times that
we can say that the questions of their origin and development have been
thoroughly worked out. If animal or vegetable matter of any kind be
macerated in water, filtered, or even distilled, various forms of Protista
make their appearance; and frequently, as putrefaction advances, form after
form makes its appearance, becomes abundant, and then disappears to be
replaced by other species. The questions suggested by such phenomena are
these: (1) Do the Protista arise spontaneously, that is, by the direct
organisation into living beings of the chemical substances present, as a
crystal is organised from a solution: (2) Are the forms of the Protista
constant from one generation to another, as are ordinary birds, beasts, and
fishes?

The question of the "spontaneous generation" of the Protista was readily
answered in the affirmative by men who believed that Lice bred directly
from the filth of human skins and clothes;[57] and that Blow-flies, to say
nothing of Honey-bees, arose in rotten flesh: but the bold aphorism of
Harvey "omne vivum ex ovo" at once gained the ear of the best-inspired men
of science, and set them to work in search of the "eggs" that gave rise to
the organisms of putrefaction. Redi (ob. 1699) showed that Blow-flies never
arise save when other Blow-flies gain access to meat and deposit their very
visible eggs thereon. Leeuwenhoek, his {43}contemporary, in the latter half
of the seventeenth century adduced strong reasons for ascribing the origin
of the organisms of putrefaction to invisible air-borne eggs. L. Joblot and
H. Baker in the succeeding half-century investigated the matter, and showed
that putrefaction was no necessary antecedent of the appearance of these
beings: that, as well as being air-borne, the germs might sometimes have
adhered to the materials used for making the infusion; and that no
organisms were found if the infusions were boiled long enough, and corked
when still boiling. These views were strenuously opposed by Needham in
England, by Wrisberg in Germany, and by Buffon, the great French naturalist
and philosopher, whose genius, unballasted by an adequate knowledge of
facts, often played him sad tricks. Spallanzani made a detailed study of
what we should now term the "bionomical" or "oecological" conditions of
Protistic life and reproduction in a manner worthy of modern scientific
research, and not attained by some who took the opposite side within living
recollection. He showed that infusions kept sufficiently long at the
boiling-point in hermetically sealed vessels developed no Protistic life.
As he had shown that active Protists are killed at much lower temperatures,
he inferred that the germs must have much higher powers of resistance; and,
by modifying the details of his experiments, he was able to meet various
objections of Needham.

Despite this good work, the advocates of spontaneous generation were never
really silenced; and the widespread belief in the inconstancy of species in
Protista added a certain amount of credibility to their cause. In 1845
Pineau put forward these views most strongly; and from 1858 to 1864 they
were supported by the elder Pouchet. Louis Pasteur, who began life as a
chemist, was led from a study of alcoholic fermentation to that of the
organisms of fermentation and of putrefaction and disease. He showed that
in infusions boiled sufficiently long and sealed while boiling, or kept at
the boiling-point in a sealed vessel, no life manifested itself: objections
raised on the score of the lack of access of fresh air were met by the
arrangement, so commonly used in "pure cultures" at the present day, of a
flask with a tube attached plugged with a little cotton-wool, or even
merely bent repeatedly into a zigzag. The former attachment filtered off
all germs or floating solid particles from the air, the latter brought
about the settling of such particles in the elbows {44}or on the sides of
the tube: in neither case did living organisms appear, even after the lapse
of months. Other observers succeeded in showing that the forms and
characters of species were as constant as in Higher Animals and Plants,
allowing, of course, for such regular metamorphoses as occur in Insects, or
alternations of generations paralleled in Tapeworms and Polypes. The
regular sequences of such alternations and metamorphoses constitute,
indeed, a strong support of the "germ-theory"—the view that all Protista
arise from pre-existing germs. It is to the Rev. W. H. Dallinger and the
late Dr. Charles Drysdale that we owe the first complete records of such
complex life-histories in the Protozoa as are presented by the minute
Flagellates which infest putrefying liquids (see below, p. 116 f.). The
still lower Schizomycetes, the "microbes" of common speech, have also been
proved by the labours of Ferdinand Cohn, von Koch, and their numerous
disciples, to have the same specific constancy in consecutive generations,
as we now know, thanks to the methods first devised by De Bary for the
study of Fungi, and improved and elaborated by von Koch and his school of
bacteriologists.

And so to-day the principle "omne vivum ex _vivo_" is universally accepted
by men of science. Of the ultimate origin of organic life from inorganic
life we have not the faintest inkling. If it took place in the remote past,
it has not been accomplished to the knowledge of man in the history of
scientific experience, and does not seem likely to be fulfilled in the
immediate or even in the proximate future.[58]


PROTOZOA

_Organisms of various metabolism, formed of a single cell or apocyte, or of
a colony of scarcely differentiated cells, whose organs are formed by
differentiations of the protoplasm, and its secretions and accretions; not
composed of differentiated multicellular tissues or organs._[59]

{45}This definition, as we have seen, excludes Metazoa (including Mesozoa,
Vol. II. p. 92) sharply from Protozoa, but leaves an uncertain boundary on
the botanical side; and, as systematists share with nations the desire to
extend their sphere of influence, we shall here follow the lead of other
zoologists and include many beings that every botanist would claim for his
own realm. Our present knowledge of the Protozoa has indeed been largely
extended by botanists,[60] while the study of protoplasmic physiology has
only passed from their fostering care into the domain of General Biology
within the last decade. The study of the Protozoa is little more than two
centuries old, dating from the school of microscopists of whom the Dutchman
Leeuwenhoek is the chief representative: and we English may feel a just
pride in the fact that his most important publications are to be found in
the early records of our own Royal Society.

Baker, in the eighteenth century, and the younger Wallich, Carter,
Dallinger and Drysdale, Archer, Saville Kent, Lankester, and Huxley, in the
last half-century, are our most illustrious names. In France, Joblot,
almost as an amateur, like our own Baker, flourished in the early part of
the eighteenth century. Dujardin in the middle of the same century by his
study of protoplasm, or sarcode as he termed it, did a great work in laying
the foundations of our present ideas, while Balbiani, Georges Pouchet,
Fabre-Domergue, Maupas, Léger, and Labbé in France, have worthily continued
and extended the Gallic traditions of exact observation and careful
deduction. Otto Friedrich Müller, the Dane, in the eighteenth century, was
a pioneer in the exact study and description of a large number of forms of
these, as of other microscopic forms of life. The Swiss collaborators,
Claparède and Lachmann, in the middle of the nineteenth century, added many
facts and many descriptions; and illustrated them by most valuable figures
of the highest merit from every point of view. Germany, with her large
population of students and her numerous universities, has given many names
to our list; among these, Ehrenberg and von Stein have added {46}the
largest number of species to the roll. Ehrenberg about 1840 described,
indeed, an enormous number of forms with much care, and in detail far too
elaborate for the powers of the microscope of that date: so that he was led
into errors, many and grave, which he never admitted down to the close of a
long and honoured life. Max Schultze did much good work on the Protozoa, as
well as on the tissues of the Metazoa, and largely advanced our conceptions
of protoplasm. His work was continued in Germany by Ernst Haeckel, who
systematised our knowledge of the Radiolaria, Greeff, Richard Hertwig,
Fritz Schaudinn, and especially Bütschli, who contributed to Bronn's
_Thier-Reich_ a monograph of monumental conception and scope, and of
admirable execution, on which we have freely drawn. Cienkowsky, a Russian,
and James-Clark and Leidy, both Americans, have made contributions of high
quality.

Lankester's article in the _Encyclopædia Britannica_ was of epoch-making
quality in its philosophical breadth of thought.

Delage and Hérouard have given a full account of the Protozoa in their
_Traité de Zoologie Concrète_, vol. i. (1896); and A. Lang's monograph in
his _Vergleichende Anatomie_, 2nd ed. (1901), contains an admirable
analysis of their general structure, habits, and life-cycles, together with
full descriptions of well-selected types. Calkins has monographed "The
Protozoa" in the Columbia University Biological series (1901). These works
of Bütschli, Delage, Lang, and Calkins contain full bibliographies. Doflein
has published a most valuable systematic review of the Protozoa parasitic
on animals under the title _Die Protozoen als Parasiten und
Krankheitserreger_ (1901); and Schaudinn's _Archiv für Protistenkunde_,
commenced only four years ago, already forms an indispensable collection of
facts and views.

The PROTOPLASM of the Protozoa (see p. 5 f.) varies greatly in consistency
and in differentiation. Its outer layer may be granular and scarcely
altered in Proteomyxa, the true Myxomycetes, Filosa, Heliozoa, Radiolaria,
Foraminifera, etc.; it is clear and glassy in the Lobose Rhizopods and the
Acrasieae; it is continuous with a firm but delicate superficial pellicle
of membranous character in most Flagellates and Infusoria; and this
pellicle may again be consolidated and locally thickened in some members of
both groups so as to form a coat of mail, even with definite spines and
hardened polygonal plates (_Coleps_, Fig. 54, {47}p. 150). Again, it may
form transitory or more or less permanent pseudopodia,[61] (1) blunt or
tapering and distinct, with a hyaline outer layer, or (2) granular and
pointed, radiating and more or less permanent, or (3) branching and fusing
where they meet into networks or perforated membranes. Cilia or flagella
are motile thread-like processes of active protoplasm which perforate the
pellicle; they may be combined into flattened platelets or firm rods, or
transformed into coarse bristles or fine motionless sense-hairs. The
skeletons of the Protozoa, foreign to the cytoplasm, will be treated of
under the several groups.

Most of the fresh-water and brackish forms (and some marine ones) possess
one or more contractile vacuoles, often in relation to a more or less
complex system of spaces or canals in Flagellates and Ciliates.

The GEOGRAPHICAL DISTRIBUTION of Protozoa is remarkable for the wide, nay
cosmopolitan, distribution of the terrestrial and fresh-water forms;[62]
they owe this to their power of forming cysts, within which they resist
drought, and can be disseminated as "dust." Very few of them can multiply
at a temperature approaching freezing-point; the Dinoflagellates can,
however, and therefore present Alpine and Arctic forms. The majority breed
most freely at summer temperatures; and, occurring in small pools, can thus
achieve their full development in such as are heated by the sun during the
long Arctic day as readily as in the Tropics. In infusions of decaying
matter, the first to appear are those that feed on bacteria, the essential
organisms of putrefaction. These, again, are quickly followed and preyed
upon by carnivorous species, which prefer liquids less highly charged with
organic matters, and do not appear until the liquid, hitherto cloudy, has
begun to clear. Thus we have rather to do with "habitat" than with
"geographical {48}distribution," just as with the fresh-water Turbellaria
and the Rotifers (vol. ii. pp. 4 f., 226 f.). We can distinguish in
fresh-water, as in marine Protista, "littoral" species living near the
bank, among the weeds; "plankton," floating at or near the surface; "zonal"
species dwelling at various depths; and "bottom-dwellers," mostly
"limicolous" (or "sapropelic," as Lauterborn terms them), and to be found
among the bottom ooze. Many species are "epiphytic" or "epizoic," dwelling
on plants or animals, and sometimes choice enough in their preference of a
single genus or species as host. Others again are "moss-dwellers," living
among the root-hairs of mosses and the like, or even "terrestrial" and
inhabiting damp earth. The Sporozoa are internal parasites of animals, and
so are many Flagellates, while many Proteomyxa are parasitic in
plant-cells. The Foraminifera (with the exception of most Allogromidiaceae)
are marine, and so are the Radiolaria; while most Heliozoa inhabit fresh
water. Concerning the distribution in time we shall speak under the last
two groups, the only ones whose skeletons have left fossil remains.

CLASSIFICATION.—The classification of the Protozoa is no easy task. We omit
here, for obvious reasons, the unmistakable Plant Protists that have a
holophytic or saprophytic nutrition; that are, with the exception of a
short period of locomotion in the young reproductive cells, permanently
surrounded with a wall of cellulose or fungus-cellulose, and that multiply
and grow freely in this encysted state: to these consequently we relegate
the CHYTRIDIEAE, which  so closely allied to the Proteomyxa and the
Phycomycetous Fungi; and the Confervaceae, which in the resting state form
tubular or flattened aggregates and are allied to the green Flagellates;
besides the Schizophyta. At the opposite pole stand the INFUSORIA in the
strict sense, with the most highly differentiated organisation found in our
group, culminating in the possession of a nuclear apparatus with nuclei of
two kinds, and exhibiting a peculiar form of conjugation, in which the
nuclear apparatus is reorganised. The SPOROZOA are clearly marked off as
parasites in living animals, which mostly begin life as sickle-shaped cells
and have always at least two alternating modes of brood-formation, the
first giving rise to aplanospores, wherein is formed the second brood of
sickle-shaped, wriggling zoospores. The remainder, comprising the
SARCODINA, or RHIZOPODA in the old wide sense (including all {49}that move
by pseudopodia during the great part of their active life), and the
FLAGELLATA in the widest sense, are not easy to split up; for many of the
former have flagellate reproductive cells, and many of the latter can emit
pseudopodia with or without the simultaneous retraction of their flagella.
The RADIOLARIA are well defined by the presence in the body plasm of a
central capsule marking it off into a central and a peripheral portion, the
former containing the nucleus, the latter emitting the pseudopodia. Again,
on the other hand, we find that we can separate as FLAGELLATA in the strict
sense the not very natural assemblage of those Protista that have flagella
as their principle organs of movement or nutrition during the greater part
of their active life. The remaining groups (which with the Radiolaria
compose the Sarcodina of Bütschli), are the most difficult to treat. The
RHIZOPODA, as we shall limit them, are naked or possess a simple shell,
never of calcium carbonate, have pseudopodia that never radiate abundantly
nor branch freely, nor coalesce to form plasmatic networks, nor possess an
axial rod of firmer substance. The FORAMINIFERA have a shell, usually of
calcium carbonate, their pseudopodia are freely reticulated, at least
towards the base; and (with the exception of a few simple forms) all are
marine. The MYCETOZOA are clearly united by their tendency to aggregate
more or less completely into complex resting-groups (fructifications), and
by reproducing by unicellular zoospores, flagellate or amoeboid, which are
not known to pair. The HELIOZOA resemble the Radiolaria in their fine
radiating pseudopodia, but have an axial filament always present in each,
and lack the central capsule; and are, for the most part, fresh-water
forms. Finally, the PROTEOMYXA forms a sort of lumber-room for beings which
are intermediate between the Heliozoa, Rhizopoda, and Flagellata, usually
passing through an amoeboid stage, and for the most part reproducing by
brood-formation. Zoospores that possess flagella are certainly known to
occur in some forms of Foraminifera, Rhizopoda, Heliozoa, and Radiolaria,
though not by any means in all of each group.[63]

  A. Pseudopodia the principal means of locomotion and feeding;        {50}
       flagella absent or transitory                           I. SARCODINA

      (1) Plastogamy only leading to an increase in size, never to the
            formation of "fructifications."

          (_a_) Pseudopodia never freely coalescing into a network nor
                  fine to the base                               RHIZOPODA.

             (*) Ectoplasm clear, free from granules; pseudopodia, usually
                   blunt                                   RHIZOPODA LOBOSA

             (**) Ectoplasm finely granular; pseudopodia slender,
                    branching, but not forming a network, passing into the
                    body by basal dilatation               RHIZOPODA FILOSA

          (_b_) Pseudopodia branching freely and coalescing to form
                  networks; ectoplasm granular; test usually calcareous or
                  sandy                                        FORAMINIFERA

          (_c_) Pseudopodia fine to the very base; radiating, rarely
                  coalescing.

             (i.) Pseudopodia with a central filament              HELIOZOA

             (ii.) Pseudopodia without a central filament.

                 (*) Body divided into a central and a peripheral part by a
                       "central capsule"                         RADIOLARIA

                 (**) Body without a central capsule             PROTEOMYXA

      (2) Cells aggregating or fusing into plasmodia before forming a
            complex "fructification"                              MYCETOZOA

  B. Cells usually moving by "euglenoid" wriggling or by excretion of a
       trail of viscid matter; reproduction by alternating modes of
       brood-formation, rarely by Spencerian fission           II. SPOROZOA

  C. Flagella (rarely numerous) the chief or only means of motion and
       feeding                                              III. FLAGELLATA

  D. Cilia the chief organs of motion, in the young state at least; nuclei
       of two kinds                                           IV. INFUSORIA




{51}CHAPTER III

PROTOZOA (_CONTINUED_): SARCODINA


I. SARCODINA.

  _Protozoa performing most of their life-processes by pseudopodia; nucleus
  frequently giving off fragments (chromidia) which may play a part in
  nuclear reconstitution on division; sometimes with brood-cells, which may
  be at first flagellate; but never reproducing in the flagellate
  state.[64]_


1. RHIZOPODA

_Sarcodina of simple form, whose pseudopodia never coalesce into networks
(1),[65] nor contain an axial filament (2), which commonly multiply by
binary fission (3), though a brood-formation may occur; which may
temporarily aggregate, or undergo temporary or permanent plastogamic union,
but never to form large plasmodia or complex fructifications as a prelude
to spore-formation (4); test when present gelatinous, chitinous, sandy, or
siliceous, simple and 1-chambered (5)._


CLASSIFICATION.[66]

  I. Ectoplasm distinct, clear; pseudopodia blunt or tapering, but not
  branching at the apex

    LOBOSA

    _Amoeba_, Auctt.; _Pelomyxa_, Greeff; _Trichosphaerium_, A. Schneid.;
    _Dinamoeba_, Leidy; _Amphizonella_, Greeff; _Centropyxis_, Stein;
    _Arcella_, {52}Ehr.; _Difflugia_, Leclercq; _Lecqueureusia_,
    Schlumberger; _Hyalosphenia_, Stein; _Quadrula_, F. E. Sch.;
    _Heleopera_, Leidy; _Podostoma_, Cl. and L.; _Arcuothrix_, Hallez.

  II. Ectoplasm undifferentiated, containing moving granules; pseudopodia
  branching freely towards the tips

    FILOSA

    _Euglypha_, Duj.; _Paulinella_, Lauterb.; _Cyphoderia_, Schlumb.;
    _Campascus_, Leidy; _Chlamydophrys_, Cienk.; _Gromia_, Duj. =
    _Hyalopus_, M. Sch.

We have defined this group mainly by negative characters, as such are the
only means for their differentiation from the remaining Sarcodina; and
indeed from Flagellata, since in this group zoospores are sometimes formed
which possess flagella. Moreover, indeed, in a few of this group
(_Podostoma_, _Arcuothrix_), as in some Heliozoa, the flagellum or flagella
may persist or be reproduced side by side with the pseudopodia. The
subdivision of the Rhizopoda is again a matter of great difficulty, the
characters presented being so mixed up that it is hard to choose: however,
the character of the outer layer of the cytoplasm is perhaps the most
obvious to select. In LOBOSA there is a clear layer of ectosarc, which
appears to be of a greasy nature at its surface film, so that it is not
wetted. In the FILOSA, as in most other Sarcodina, this film is absent, and
the ectoplasm is not marked off from the endoplasm, and may have a granular
surface. Corresponding to this, the pseudopodia of the Lobosa are usually
blunt, never branching and fraying out, as it were, at the tip, as in the
Filosa; nay, in the normal movements of _Amoeba limax_ (Fig. 1, p. 5) the
front of the cell forms one gigantic pseudopodium, which constantly glides
forward. Apart from this distinction the two groups are parallel in almost
every respect.

There may be a single contractile vacuole, or a plurality; or none,
especially in marine and endoparasitic species. The nucleus may remain
single or multiply without inducing fission, thus leading to apocytial
forms. It often gives off "chromidial" fragments, which may play an
important part in reproduction.[67] In _Amoeba binucleata_ there are
constantly two nuclei, both of which divide as an antecedent to fission,
each giving a separate nucleus to either daughter-cell. _Pelomyxa
palustris_, the giant of the group, attaining a diameter of 1''' (2 mm.),
has very blunt pseudopodia, an enormous number of nuclei, and no
contractile vacuole, though {53}it is a fresh-water dweller, living in the
bottom ooze of ponds, etc., richly charged with organic débris. It is
remarkable also for containing symbiotic bacteria, and brilliant vesicles
with a distinct membranous wall, containing a solution of glycogen.[68]
Few, if any, of the Filosa are recorded as plurinuclear.

The simplest Lobosa have no investment, nor indeed any distinction of front
or back. In some forms of _Amoeba_, however, the hinder part is more
adhesive, and may assume the form of a sucker-like disc, or be drawn into a
tuft of short filaments or villi, to which particles adhere. Other species
of Lobosa and all Filosa have a "test," or "theca," _i.e._ an investment
distinct from the outermost layer of the cell-body. The simplest cases are
those of _Amphizonella_, _Dinamoeba_, and _Trichosphaerium_, where this is
gelatinous, and in the two former allows the passage of food particles
through it into the body by mere sinking in, like the protoplasm itself,
closing again without a trace of perforation over the rupture. In
_Trichosphaerium_ (Fig. 9) the test is perforated by numerous pores of
constant position for the passage of the pseudopodia, closing when these
are retracted; and in the "A" form of the species (see below) it is studded
with radial spicules of magnesium carbonate. Elsewhere the test is more
consistent and possesses at least one aperture for the emission of
pseudopodia and the reception of food; to avoid confusion we call this
opening not the _mouth_ but the "pylome": some Filosa have two
symmetrically placed pylomes. When the test is a mere pellicle, it may be
recognised by the limitation of the pseudopodia to the one pylomic area.
But the shell is often hard. In _Arcella_ (Fig. 10, C), a form common among
Bog-mosses and Confervas, it is chitinous and shagreened, circular, with a
shelf running in like that of a diving-bell around the pylome: there are
two or more contractile vacuoles, and at least two nuclei. Like some other
genera, it has the power of secreting carbonic acid gas in the form of
minute bubbles in its cytoplasm, so as to enable it to float up to the
surface of the water. The chitinous test shows minute hexagonal
sculpturing, the expression of vertical partitions reaching from the inner
to the outer layer.

{54}[Illustration: FIG. 9.—_Trichosphaerium sieboldii._ 1, Adult of "A"
form; 2, its multiplication by fission and gemmation; 3, resolution into
1-nucleate amoeboid zoospores; 4, development (from zoospores of "A") into
"B" form (5); 6, its multiplication by fission and gemmation; 7, its
resolution after nuclear bipartition into minute 2-flagellate zoospores or
(exogametes); 8, liberation of gametes; 9, 10, more highly magnified
pairing of gametes of different origin; 11, 12, zygote developing into "A"
form. (After Schaudinn.)]

Several genera have tests of siliceous or chitinous plates, formed in the
cytoplasm in the neighbourhood of the nucleus, and connected by chitinous
cement. Among these _Quadrula_ (Fig. 10, A) is Lobose, with square plates,
_Euglypha_ (Fig. 8, p. 29), and _Paulinella_[69] are Filose, with hexagonal
plates. In the latter they are in five longitudinal rows, with a pentagonal
oral plate, perforated by the oval pylome. In other genera again, such as
_Cyphoderia_ (Filosa), the plates are merely {55}chitinous. Again, the
shell may be encrusted with sand-grains derived directly from without, or
from ingested particles, as shown in _Centropyxis_, _Difflugia_ (Fig. 10,
D), _Heleopera_, and _Campascus_ when supplied with powdered glass instead
of sand. The cement in _Difflugia_ is a sort of organic mortar, infiltrated
with ferric oxide (more probably ferric hydrate). In _Lecqueureusia
spiralis_ (formerly united with _Difflugia_) the test is formed of minute
sausage-shaped granules, in which may be identified the partly dissolved
valves of Diatoms taken as food; it is spirally twisted at the apex, as if
it had enlarged after its first formation, a very rare occurrence in this
group. The most frequent mode of fission in the testaceous Rhizopods (Figs.
8, 10) is what Schaudinn aptly terms "bud-fission," where half the
protoplasm protrudes and accumulates at the mouth of the shell, and remains
till a test has formed for it, while the other half retains the test of the
original animal. The materials for the shell, whether sand-granules or
plates, pass from the depths of the original shell outwards into the naked
cell, and through its cytoplasm to the surface, where they become connected
by cementing matter into a continuous test. The nucleus now divides into
two, one of which passes into the external animal; after this the two
daughter-cells separate, the one with the old shell, the other, larger,
with the new one.

[Illustration: FIG. 10.—Test-bearing Rhizopods. A, _Quadrula symmetrica_:
B, _Hyalosphenia lata_; C, _Arcella vulgaris_; D, _Difflugia pyriformis_.
(From Lang's _Comparative Anatomy_.)]

If two individuals of the shelled species undergo bud-fission in close
proximity, the offspring may partially coalesce, so that a monstrous shell
is produced having two pylomes.

{56}Reproduction by fission has been clearly made out in most members of
the group; some of the multinucleate species often abstrict a portion,
sometimes at several points simultaneously, so that fission here passes
into budding[70] (Fig. 9, 2, 6).

Brood-division, either by resolution in the multinucleate species, or
preceded by multiple nuclear division in the habitually 1-nucleate, though
presumably a necessary incident in the life-history of every species, has
only been seen, or at least thoroughly worked out, in a few cases, where it
is usually preceded by encystment, and mostly by the extrusion into the
cyst of any undigested matter.[71]

In _Trichosphaerium_ (Fig. 9) the cycle described by Schaudinn is very
complex, and may be divided into two phases, which we may term the A and
the B subcycles. The members of the A cycle are distinguished by the
gelatinous investment being armed with radial spicules, which are absent
from the B form. The close of the A cycle is marked by the large
multinucleate body resolving itself into amoeboid zoospores (3), which
escape from the gelatinous test, and develop into the large multinucleate
adults of the B form. These, like the A form, may reproduce by fission or
budding. At the term of growth, however, they retract their pseudopodia,
expel the excreta, and multiply their nuclei by mitosis (7). Then the body
is resolved into minute 2-flagellate microzoospores (8), which are
_exogamous_ gametes, _i.e._ they will only pair with similar zoospores from
another cyst. The zygote (9-11) resulting from this conjugation is a minute
amoeboid; its nucleus divides repeatedly, a gelatinous test is formed
within which the spicules appear, and so the A form is reconstituted. In
many of the test-bearing forms, whether Lobose or Filose, plastogamic
unions occur, and the two nuclei may remain distinct, leading to
plurinucleate monsters in their offspring by fission, or they may fuse and
form a giant nucleus, a process which has here no relation to normal
syngamy, as it is not associated with any marked change in the alternation
of feeding and fission, etc. In _Trichosphaerium_ also plastogamic unions
between small individuals have for their only result the increase of size,
enabling the produce to deal with {57}larger prey. Temporary encystment in
a "hypnocyst" is not infrequent in both naked and shelled species, and
enables them to tide over drought and other unfavourable conditions.

Schaudinn has discovered and worked out true syngamic processes, some
bisexual, some exogamous, in several other Rhizopods. In _Chlamydophrys
stercorea_ the pairing-cells are equal, and are formed by the aggregation
of the chromidia into minute nuclei around which the greater part of the
cytoplasm aggregates, while the old nucleus (with a little cytoplasm) is
lost. These brood-cells are 2-flagellate pairing-cells, which are
exogamous: the zygote is a brown cyst; if this be swallowed by a mammal,
the original _Chlamydophrys_ appears in its faeces.[72]

_Centropyxis aculeata_, a species very common in mud or moss, allied to
_Difflugia_, also forms a brood by aggregation around nuclei derived from
chromidia. The brood-cells are amoeboid, and secrete hemispherical shells
like those of _Arcella_; some first divide into four smaller ones, before
secreting the shell. Pairing takes place between the large and the small
forms; and the zygote encysts. Weeks or months afterwards the cyst opens
and its contents creep out as a minute _Centropyxis_. Finally, _Amoeba
coli_ produces its zygote in a way recalling that of _Actinosphaerium_ (pp.
73-75, Fig. 21): the cell encysts; its nucleus divides, and each daughter
divides again into two, which fuse reciprocally. Thus the cyst contains two
zygote nuclei. After a time each of these divides twice, so that the mature
cyst contains eight nuclei. Probably when swallowed by another animal they
liberate a brood of eight young amoebae. Thus in different members of this
group we have exogamy, both equal and bisexual, and endogamy.

Most of the Rhizopoda live among filamentous Algae in pools, ponds, and in
shallow seas, etc.; some are "sapropelic" or mud-dwellers (many species of
_Amoeba_, _Pelomyxa_, _Difflugia_, etc.), others frequent the roots of
mosses. _Amoeba coli_ is often found as a harmless denizen of the large
intestine of man. _Amoeba histolytica_, lately distinguished therefrom by
Schaudinn, is the cause of tropical dysentery. It multiplies enormously in
the gut, and is found extending into the tissues, and making its way into
the abscesses that so frequently supervene in the liver and other organs.
_Chlamydophrys stercorea_ is found in the {58}faeces of several mammals.
The best monograph of this group is that of Penard.[73]


2. FORAMINIFERA[74]

_Sarcodina with no central capsule or distinction of ectosarc; the
pseudopodia fine, branching freely, and fusing where they meet to form
protoplasmic networks, or the outermost in the pelagic forms radiating, but
without a central or axial filament: sometimes dimorphic, reproducing by
fission and by rhizopod or flagellate germs in the few cases thoroughly
investigated: all marine (with the exception of some of the
Allogromidiaceae), and usually provided with a test of carbonate of lime
("vitreous" calcite, or "porcellanous" aragonite?), or of cemented
particles of sand ("arenaceous"); test-wall continuous, or with the walls
perforated by minute pores or interstices for the protrusion of
pseudopodia._

The classification of Carpenter (into _Vitreous_ or _Perforate_,
_Porcellanous_ or _Imperforate_, and _Arenaceous_), according to the
structure of the shell, had proved too artificial to be used by Brady in
the great Monograph of the Foraminifera collected by the "Challenger"
Expedition,[75] and has been modified by him and others since then. We
reproduce Lister's account of Brady's classification.[76] We must, however,
warn the tyro that its characterisations are not definitions (a feature of
all other recent systems), for rigid definitions are impossible: here as in
the case, for instance, of many Natural Orders of Plants, transitional
forms making the establishment of absolute boundaries out of the question.
In the following classification we do not think it, therefore, necessary to
complete the characterisations by noting the extremes of variation within
the orders:—

  1. Allogromidiaceae: simple forms, often fresh-water and similar to
  Rhizopoda; test 0, or chitinous, gelatinous, or formed of cemented
  particles, whether secreted platelets or ingested granules. _Biomyxa_,
  Leidy = _Gymnophrys_, Cienk.; {59}_Diaphorodon_, Archer; _Allogromia_,
  Rhumbl. (= _Gromia_, auctt.[77] nec Duj.) (Fig. 14, 1); _Lieberkühnia_,
  Cl. and Lachm. (Fig. 12); _Microgromia_, R. Hertw. (Fig. 11);
  _Pamphagus_, Bailey.

  2. Astrorhizidaceae: test arenaceous, often large, never truly chambered,
  or if so, asymmetrical. _Astrorhiza_, Sandahl; _Haliphysema_, Bowerb.;
  _Saccammina_, M. Sars (Fig. 13, 1); _Loftusia_, Brady.

  3. Lituolidaceae: test arenaceous, often symmetrical or regularly spiral,
  isomorphous with calcareous forms: the chambers when old often
  "labyrinthine" by the ingrowth of wall-material. _Lituola_, Lam.;
  _Reophax_, Montf.; _Ammodiscus_, Reuss; _Trochammina_, Parker and
  Jeffreys.

  4. Miliolidaceae: test porcellanous, imperforate, spirally coiled or
  cyclic, often chambered except in _Cornuspira_: simple in _Squamulina_.
  _Cornuspira_, Max Sch.; _Peneroplis_, Montf.; _Miliolina_, Lam. (incl.
  _Biloculina_ (Fig. 15), _Triloculina_, _Quinqueloculina_ (Figs. 14, 4;
  15, B), _Spiroloculina_ (Fig. 13, 5) of d'Orb.); _Alveolina_, d'Orb.;
  _Hauerina_, d'Orb.; _Calcituba_, Roboz; _Orbitolites_, Lam.;
  _Orbiculina_, Lam.; _Alveolina_, Park. and Jeffr.; _Nubecularia_, Def.;
  Squamulina, Max Sch. (Fig. 14, 3).

  5. Textulariaceae: test calcareous, hyaline, perforated; chambers
  increasing in size in two alternating rows, or three, or passing into a
  spiral. _Textularia_, Def.; _Bulimina_, d'Orb.; _Cassidulina_, d'Orb.

  6. Cheilostomellaceae: test vitreous, delicate, finely perforated,
  chambered, isomorphic with the spiral forms of the Miliolidaceae.
  _Cheilostomella_, Reuss.

  7. Lagenaceae: Test vitreous, very finely perforate, chambers with a
  distinct pylome projecting (ectosolenial), or turned in (entosolenial),
  often succeeding to form a necklace-like shell. _Lagena_, Walker and Boys
  (Fig. 13, 2); _Nodosaria_, Lam. (Fig. 13, 3); _Cristellaria_, Lam.;
  _Frondicularia_, Def. (Fig. 13, 4); _Polymorphina_, Lam.; _Ramulina_,
  Wright.

  8. Globigerinidae: test vitreous, perforate; chambers few, dilated, and
  arranged in a flat or conical spiral, usually with a crescentic pylome to
  the last. _Globigerina_, d'Orb. (Figs. 13, 6; 16, 2); _Hastigerina_, Wyv.
  Thoms.; _Orbulina_, d'Orb. (Fig. 16, 1).

  9. Rotaliaceae; test vitreous, perforate, usually a conical spiral (like
  a snail), chambers often subdivided into chamberlets, and with a proper
  wall, and intermediate skeleton traversed by canals. _Rotalia_, Lam.
  (Fig. 14, 2); _Planorbulina_, d'Orb. (Fig. 13, 9); _Polytrema_, Risso;
  _Spirillina_, Ehr. (non-septate); _Patellina_, Will.; _Discorbina_, P.
  and J. (Fig. 13, 7).

  10. Nummulitaceae: test usually a complex spiral, the turns completely
  investing their predecessors: wall finely tubular, often with a proper
  wall and intermediate skeleton. _Fusulina_, Fisch.; _Polystomella_, Lam.;
  _Nummulites_, d'Orb. (Fig. 13, 11); _Orbitoides_, d'Orb.

The Allogromidiaceae are a well-marked and distinct order, on the whole
resembling the Rhizopoda Filosa, and are often found with them in fresh
water, while all other Foraminifera are marine. The type genus,
_Allogromia_ (Fig. 14, 1), has an oval chitinous shell. _Microgromia
socialis_ (Fig. 11) is often found in aggregates, the pseudopodia of
neighbours fusing where they meet into a {60}common network. This is due to
the fact that one of the two daughter-cells at each fission, that does not
retain the parent shell, remains in connexion with its sister that does:
sometimes, however, it retracts its pseudopodia, except two which become
flagella, wherewith it can swim off. The test of _Pamphagus_ is a mere
pellicle. In _Lieberkühnia_ (Fig. 12) it is hardly that; though the body
does not give off the fine pseudopodia directly, but emits a thick process
or "stylopodium"[78] comparable to the protoplasm protruded through the
pylome of its better protected allies; and from this, which often stretches
back parallel to the elongated body, the reticulum of pseudopodia is
emitted. _Diaphorodon_ has a shell recalling that of _Difflugia_ (Fig. 10,
D, p. 55), formed of sandy fragments, but with interstices between them
through which as well as through the two pylomes the pseudopodia pass. In
all of these the shell is formed as in the Rhizopods once for all, and does
not grow afterwards; and the fresh-water forms, which are the majority,
have one or more contractile vacuoles; in _Allogromia_ they are very
numerous, scattered on the expanded protoplasmic network.

[Illustration: FIG. 11.—_Microgromia socialis._ A, entire colony; B, single
zooid; C, zooid which has undergone binary fission, with one of the
daughter-cells creeping out of the shell; D, flagellula. _c.vac_,
Contractile vacuole; _nu_, nucleus; _sh_, shell. (From Parker and Haswell,
after Hertwig and Lesser.)]

{61}[Illustration: FIG. 12.—_Lieberkühnia_, a fresh-water Rhizopod, from
the egg-shaped shell of which branched pseudopodial filaments protrude.
(From Verworn.)]

The remaining marine families may all be treated of generally, before
noting their special characters.  Their marine habitat is variable, but in
most cases restricted. A few extend up the brackish water of estuaries: a
large number are found between tide-marks, or on the so-called littoral
shelf extending to deep water; they are for the most part adherent to
seaweeds, or lie among sand or on the mud. Other forms, again, are pelagic,
such as _Globigerina_ (Figs. 13, 6, 16, 17) and its allies, and float as
part of the plankton, having the surface of their shells extended by
delicate spines, their pseudopodia long and radiating, and the outer part
of their cytoplasm richly vacuolated ("alveolate"), and probably containing
a liquid lighter than sea water, as in the Radiolaria. Even these, after
their death and the decay of the protoplasm, must sink to the bottom
(losing the fine spines by solution as they fall); and they accumulate
there, to form a light oozy mud, the "Globigerina-ooze" of geographers, at
depths where the carbonic acid under pressure is not adequate to dissolve
the more solid calcareous matter. Grey Chalk is such an ooze, consolidated
by {62}the lapse of time and the pressure of superincumbent layers. Some
Foraminifera live on the sea bottom even at the greatest depths, and of
course their shell is not composed of calcareous matter. Foraminifera may
be obtained for examination by carefully washing sand or mud, collected on
the beach at different levels between tide-marks, or from dredgings, or by
carefully searching the surface of seaweeds, or by washing their roots, or,
again, by the surface or deep-sea tow-net. The sand used to weight sponges
for sale is the ready source of a large number of forms, and may be
obtained for the asking from the sponge-dealers to whom it is a useless
waste product. If this sand is dried in an oven, and then poured into
water, the empty shells, filled with air, will float to the surface, and
may be sorted by fine silk or wire gauze.

From the resemblance of the shells of many of them to the Nautilus they
were at first described as minute Cephalopods, or Cuttlefish, by
d'Orbigny,[79] and their true nature was only elucidated in the last
century by the labours of Williamson, Carpenter, Dujardin, and Max
Schultze. At first they possess only one nucleus, but in the adult stage
may become plurinucleate without dividing, and this is especially the case
in the "microsphaeric" states exhibited by many of those with a complex
shell; the nucleus is apt to give off fragments (chromidia) which lie
scattered in the cytoplasm. At first, too, in all cases, the shell has but
a single chamber, a state that persists through life in some. When the
number of chambers increases, their number has no relation to that of the
nuclei, which remains much smaller till brood-formation sets in.

The shell-substance, if calcareous, has one of the two types, porcellanous
or vitreous, that we have already mentioned, but _Polytrema_, a form of
very irregular shape, though freely perforated, is of a lovely pink colour.
In the calcareous shells sandy particles may be intercalated, forming a
transition to the Arenacea. In these the cement has an organic base
associated with calcareous or ferruginous matter; in some, however, the
cement is a phosphate of iron. The porcellanous shells are often deep brown
by transmitted light.

{63}[Illustration: FIG. 13.—Shells of _Foraminifera_. In 3, 4, and 5, _a_
shows the surface view, and _b_ a section; 8_a_ is a diagram of a coiled
cell without supplemental skeleton; 8_b_ of a similar form with
supplemental skeleton (_s.sk_); and 10 of a form with overlapping whorls;
in 11_a_ half the shell is shown in horizontal section; _b_ is a vertical
section; _a_, aperture of the shell; 1-15, successive chambers, 1 being
always the oldest or initial chamber. (From Parker and Haswell, after other
authors.)]

Despite the apparent uniformity of the protoplasmic body in this group, the
shell is infinitely varied in form. As Carpenter writes, in reference to
the Arenacea, "There is nothing more wonderful in nature than the building
up of these elaborate and symmetrical structures by mere jelly-specks,
presenting no traces {64}whatever of that definite organisation which we
are accustomed to regard as necessary to the manifestations of conscious
life.... The tests (shells) they construct when highly magnified bear
comparison with the most skilful masonry of man. From the same sandy bottom
one species picks up the coarsest quartz grains, unites them together with
a ferruginous cement, and thus constructs a flask-shaped test, having a
short neck and a single large orifice; another picks up the finer grains
and puts them together with the same cement into perfectly spherical tests
of the most extraordinary finish, perforated with numerous small pores
disposed at pretty regular intervals. Another species selects the minutest
sand grains and the terminal portions of sponge-spicules, and works them up
together—apparently with no cement at all, but by the mere laying of the
spicules—into perfect white spheres like homoeopathic globules, each
showing a single-fissured orifice. And another, which makes a straight,
many-chambered test, the conical mouth of each chamber projecting into the
cavity of the next, while forming the walls of its chambers of ordinary
sand grains rather loosely held together, shapes the conical mouths of the
chambers by firmly cementing together the quartz grains which border it."
The structure of the shell is indeed variable. The pylome may be single or
represented by a row of holes (_Peneroplis_, _Orbitolites_), or, again,
there may be several pylomes (_Calcituba_); and, again, there are in
addition numerous scattered pores for the protrusion of pseudopodia
elsewhere than from the stylopodium, in the whole of the "Vitrea" and in
many "Arenacea"; and, as we shall see, this may exercise a marked influence
on the structure of the shell.

In some cases the shell is simple, and in _Cornuspira_ and _Spirillina_
increases so as to have the form of a flat coiled tube. In _Calcituba_ the
shell branches irregularly in a dichotomous way, and the older parts break
away as the seaweed on which they grow is eaten away, and fall to the
bottom, while the younger branches go on growing and branching. The fallen
pieces, if they light on living weed, attach themselves thereto and repeat
the original growth; if not, the protoplasm crawls out and finds a fresh
weed and forms a new tube. In the "Polythalamia" new chambers are formed by
the excess of the protoplasm emerging and surrounding itself with a shell,
organically united with the existing chamber or chambers, and in a
space-relation which follows definite laws characteristic of the species or
of its stage of growth, so as to give rise to circular, spiral, or
irregular complexes (see Fig. 13).

{65}[Illustration: FIG. 14.—Various forms of _Foraminifera_. In 4,
_Miliola_, _a_, shows the living animal; _b_, the same killed and stained;
_a_, aperture of shell; _f_, food particles; _nu_, nucleus; _sh_, shell.
(From Parker and Haswell, after other authors.)]

In most {66}cases the part of the previously existing chamber next the
pylome serves as the hinder part of the new chamber, and the old pylome
becomes the pore of communication. But in some of the "Perforata" each new
chamber forms a complete wall of its own ("proper wall," Fig. 13, 8_b_),
and the space between the two adjacent walls is filled with an intermediate
layer traversed by canals communicating with the cavities of the chambers
("intermediate skeleton"), while an external layer of the same character
may form a continuous covering. The shell of the Perforata may be adorned
with pittings or fine spines, which serve to increase the surface of
support in such floating forms as _Globigerina_, _Hastigerina_, and the
like (Fig. 17). In the "Imperforata" the outer layer is often ornamented
with regular patterns of pits, prominences, etc., which are probably formed
by a thin reflected external layer of protoplasm. In some of the "Arenacea"
a "labyrinthine" complex of laminae is formed.

A very remarkable point which has led to great confusion in the study of
the Foraminifera, is the fact that the shell on which we base our
characters of classification, may vary very much, even within the same
individual. Thus in the genus _Orbitolites_ the first few chambers of the
shell have the character of a Milioline, in _Orbiculina_ of a _Peneroplis_.
The arrangements of the Milioline shell, known as Triloculine,
Quinqueloculine, and Biloculine respectively, may succeed one another in
the same shell (Figs. 14 4, 15). A shell may begin as a spiral and end by a
straight continuation: again, the spherical _Orbulina_ (Fig. 16 1) is
formed as an investment to a shell indistinguishable from _Globigerina_,
which is ultimately absorbed. In some cases, as Rhumbler has pointed out,
the more recent and higher development shows itself in the first formed
chambers, while the later, younger chambers remain at a lowlier stage, as
in the case of the spiral passing into a straight succession; but the other
cases we have cited show that this is not always the case. In _Lagena_
(Fig. 13 2) the pylome is produced into a short tube, which may protrude
from the shell or be turned into it, so that for the latter form the genus
_Entosolenia_ was founded. Shells identical in minute sculpture are,
however, found with either form of neck, and, moreover, the polythalamial
shells (_Nodosaria_, Fig. 13 3), formed of a nearly straight succession of
_Lagena_-like chambers, may have these chambers with their
{67}communications on either type. Rhumbler goes so far as to suggest that
all so-called _Lagena_ shells are either the first formed chamber of a
_Nodosaria_ which has not yet become polythalamian by the formation of
younger ones, or are produced by the separation of an adult _Nodosaria_
into separate chambers.

[Illustration: FIG. 15.—A, Megalospheric; B, microspheric shell of
_Biloculina_. _c_, The initial chamber. The microspheric form begins on the
_Quinqueloculina_ type. (From Calkins' _Protozoa_.)]

Many of the chambered species show a remarkable dimorphism, first noted by
Schlumberger, and finally elucidated by J. J. Lister and Schaudinn. It
reveals itself in the size of the initial chamber; accordingly, the two
forms may be distinguished as "microspheric" and "megalospheric"
respectively (Fig. 15), the latter being much the commoner. The
microspheric form has always a plurality of nuclei, the megalospheric a
single one, except at the approach of reproduction. Chromidial masses are,
however, present in both forms. The life-history has been fully worked out
in _Polystomella_ by Schaudinn, and in great part in _Polystomella_,
_Orbitolites_, etc., by Lister; and the same scheme appears to be general
in the class, at least where the dimorphism noted occurs. The microspheric
form gives birth only to the megalospheric, but the latter may reproduce
megalospheric broods, or give rise to swarmers, which by their (exogamous)
{68}conjugation produce the microspheric young. The microspheric forms
early become multinucleate, and have also numerous chromidia detached from
the nuclei, which they ultimately replace. These collect in the outer part
of the shell and aggregate into new nuclei, around which the cytoplasm
concentrates, to separate into as many amoeboid young "pseudopodiospores"
as there are nuclei. These escape from the shell or are liberated by its
disintegration, and invest themselves with a shell to form the initial
large central chamber or megalosphere.

[Illustration: FIG. 16.—1, _Orbulina universa_. Highly magnified. 2,
_Globigerina bulloides_. Highly magnified. (From Wyville Thomson, after
d'Orbigny.)]

In the ordinary life of the megalospheric form the greater part of the
chromatic matter is aggregated into a nucleus, some still remaining
diffused. At the end of growth the nucleus itself disintegrates, and the
chromidia concentrate into a number of small vesicular nuclei, each of
which appropriates to itself a small surrounding zone of thick plasm and
then divides by mitosis twice; and the 4-nucleate cells so formed are
resolved into as many 1-nucleate, 2-flagellate swarmers, which conjugate
{69}only _exogamously_.[80] The fusion of their nuclei takes place after
some delay: ultimately the zygote nucleus divides into two, a shell is
formed, and we have the microsphere, which is thus pluri-nucleate _ab
initio_. As we have seen, the nuclei of the microsphere are ultimately
replaced by chromidia, and the whole plasmic body divides into
pseudopodiospores, which grow into the megalospheric form.

[Illustration: FIG. 17.—Shell of _Globigerina bulloides_, from tow-net,
showing investment of spines. (From Wyville Thomson.)]

In the Perforate genera, _Patellina_ and _Discorbina_, plastogamy precedes
brood formation, the cytoplasms of the 2-5 pairing individuals contracting
a close union; and then the nuclei proceed to break up _without fusion_,
while the cytoplasm aggregates around the young nuclei to form amoebulae,
which acquire a shell and separate. In both cases it is the forms with a
single nucleus, corresponding to _megalospheric_ forms that so pair, and
the brood-formation is, _mutatis mutandis_, the same as in these forms.
Similar individuals may reproduce in the same way, in both genera, without
this plastogamic pairing, which is therefore, though probably advantageous,
not essential. If pseudopodiospores form their shells while near one
another, they may coalesce to form monsters, as often happens in
_Orbitolites_.[81]

The direct economic uses of the Foraminifera are perhaps greater than those
of any other group of Protozoa. The Chalk is {70}composed largely of
_Textularia_ and allied forms, mixed with the skeletons of
Coccolithophoridae (pp. 113-114), known as Coccoliths, etc. The Calcaire
Grossier of Paris, used as a building stone, is mainly composed of the
shells of Miliolines of Eocene age; the Nummulites of the same age of the
Mediterranean basin are the chief constituent of the stone of which the
Pyramids of Egypt are built. Our own Oolitic limestones are composed of
concretions around a central nucleus, which is often found to be a minute
Foraminiferous shell.

The palaeontology of the individual genera is treated of in Chapman's and
Lister's recent works. They range from the Lower Cambrian characterised by
perforated hyaline genera, such as _Lagena_, to the present day. Gigantic
arenaceous forms, such as _Loftusia_, are among the Tertiary
representatives; but the limestones formed _principally_ of their shells
commence at the Carboniferous. The so-called Greensands contain greenish
granules of "glauconite," containing a ferrous silicate, deposited as a
cast in the chambers of Foraminifera, and often left exposed by the
solution of the calcareous shell itself. Such granules occur in deep-sea
deposits of the present day.[82]


3. HELIOZOA

_Sarcodina with radiate non-anastomosing pseudopodia of granular
protoplasm, each with a stiff axial rod passing into the body plasma; no
central capsule, nor clear ectoplasm; skeleton when present siliceous;
nucleus single or multiple; contractile vacuole (or vacuoles) in
fresh-water species, superficial and prominent at the surface in diastole;
reproduction by fission or budding in the active condition, or by
brood-formation in a cyst, giving rise to resting spores; conjugation
isogamous in the only two species fully studied; habitat floating or among
weeds, mostly fresh water._

  1. Naked or with an investment only when encysted.

    APHROTHORACA.—_Actinolophus_ F.E. Sch.; _Myxastrum_ Haeck.;
    _Gymnosphaera_ Sassaki; _Dimorpha_ (Fig. 37, 5, p. 112) Gruber;
    _Actinomonas_ Kent; _Actinophrys_ Ehrb.; _Actinosphaerium_ St.;
    _Camptonema_ Schaud; _Nuclearia_ Cienk.

  {71}2. Invested with a gelatinous layer, sometimes traversed by a firmer
  elastic network.

    CHLAMYDOPHORA.—_Heterophrys_ Arch.; _Mastigophrys_ Frenzel;
    _Acanthocystis_, Carter.

  3. Ectoplasm with distinct siliceous spicules.

    CHALAROTHORACA.—_Raphidiophrys_ Arch.

  4. Skeleton a continuous, fenestrated shell, sometimes stalked.

    DESMOTHORACA.—_Myriophrys_ Penard; _Clathrulina_ Cienk.; _Orbulinella_
    Entz.

This class were at first regarded and described as fresh-water Radiolaria,
but the differences were too great to escape the greatest living specialist
in this latter group, Ernst Haeckel, who in 1866 created the Heliozoa for
their reception. We owe our knowledge of it mainly to the labours of
Cienkowsky, the late William Archer, F. E. Schulze, R. Hertwig, Lesser, and
latterly to Schaudinn, who has monographed it for the "Tierreich" (1896);
and Penard has published a more recent account.

[Illustration: FIG. 18.—_Actinophrys sol._ _a_, Axial filament of
pseudopod; _c.v_, contractile vacuole; _n_, nucleus. (From Lang's
_Comparative Anatomy_, after Grenacher.)]

_Actinophrys sol_ Ehrb. (Fig. 18) is a good and common type. It owes its
name to its resemblance to a conventional drawing of the sun, with a
spherical body and numerous close-set diverging rays. The cytoplasm shows a
more coarsely vacuolated outer layer, sometimes called the ectosarc, and a
denser internal layer the endosarc. In the centre of the figure is the
large nucleus, to which the continuations of the rays may be seen to
converge; the pseudopodia contain each a stiffish axial filament,[83] which
is covered by the fine granular plasm, showing currents of the granules.
The axial filament disappears when the pseudopodia are retracted or bent,
and is regenerated afterwards. This bending occurs when a living prey
touches and adheres to a ray, all its neighbours bending in like the
tentacles of a Sundew. The prey is carried down to the surface of the
ectoplasm, and {72}sinks into it with a little water, to form a nutritive
vacuole. Fission is the commonest mode of reproduction, and temporary
plastogamic unions are not uncommon. Arising from these true conjugations
occur, two and two, as described by Schaudinn. A gelatinous cyst wall forms
about the two which are scarcely more than in contact with their rays
withdrawn. Then in each the nucleus divides into two, one of which passes
to the surface, and is lost (as a "polar body"), while the other approaches
the corresponding nucleus of the mate, and unites with it, while at the
same time the cytoplasms fuse. Within the gelatinous cyst the zygote so
formed divides to produce two sister resting spores, from each of which,
after a few days, a young _Actinophrys_ escapes, as may take place indeed
after encystment of an ordinary form without conjugation.

[Illustration: FIG. 19.—_Actinosphaerium eichornii._ A, entire animal with
two contractile vacuoles (_c.vac_); B, a portion much magnified, showing
alveolate cytoplasm, pseudopodia with axial rods, non-nucleate cortex
(_cort_), multiple nuclei (_nu_) of endoplasm (_med_), and food-vacuole
(_chr_). (From Parker and Haswell.)]

The axial rods of the pseudopodia may pass either to the circumference of
the nucleus or to a central granule, corresponding, it would appear, to a
centrosome or blepharoplast; or again, {73}in the plurinucleate marine
genus _Camptonema_, each rod abuts on a separate cap on the outer side of
each nucleus. The nucleus is single in all but the genera
_Actinosphaerium_, _Myxastrum_, _Camptonema_, and _Gymnosphaera_. The
movements of this group are very slow, and are not well understood. A slow
rolling over on the points of the rays has been noted, and in _Camptonema_
they move very decidedly to effect locomotion, the whole body also moving
Amoeba-fashion; but of the distinct movements of the species when floating
no explanation can be given. The richly vacuolate ectoplasm undoubtedly
helps to sustain the cell, and the extended rays must subserve the same
purpose by so widely extending the surface. _Dimorpha_ (Fig. 37, 5, p. 112)
has the power of swimming by protruding a pair of long flagella from the
neighbourhood of the eccentric nucleus; and _Myriophrys_ has an investment
of long flagelliform cilia. _Actinomonas_ has a stalk and a single
flagellum in addition to the pseudopodia; these genera form a transition to
the Flagellata.

Several species habitually contain green bodies, which multiply by
bipartition, and are probably Zoochlorellae, Chlamydomonadidae of the same
nature as we shall find in certain Ciliata (pp. 154, 158) in fresh-water
Sponges (see p. 175), in _Hydra viridis_ (p. 256), and the marine
Turbellarian _Convoluta_ (Vol. II. p. 43).

Reproduction by fission is not rare, and in some cases (_Acanthocystis_)
the cell becomes multinuclear, and buds off 1-nucleate cells. In such cases
the buds at first lack a centrosome, and a new one is formed first in the
nucleus, and passes out into the cytoplasm. These buds become 2-flagellate
before settling down. In _Clathrulina_ the formation of 2-flagellate
zoospores has long been known (Fig. 20, 3). In _Actinosphaerium_ (Figs. 19,
21), a large species, differing from _Actinophrys_ only in the presence of
numerous nuclei in its endoplasm, a peculiar process, which we have
characterised as _endogamy_, results in the formation of resting spores.
The animal retracts its rays and encysts; and the number of nuclei is much
reduced by their mutual fusion, or by the solution of many of them, or by a
combination of the two processes. The body then breaks up into cells with a
single nucleus, and each of these surrounds itself with a wall to form a
cyst of the second order.

{74}[Illustration: FIG. 20.—Various forms of Heliozoa. In 3, _a_ is the
entire animal and _b_ the flagellula; _c.vac_, contractile vacuole; _g_,
gelatinous investment; _nu_, nucleus; _psd_, pseudopodia; _sk_, siliceous
skeleton; _sp_, spicules. (From Parker and Haswell, after other authors.)]

Each of these divides, and the two sister cells then conjugate after the
same fashion as in _Actinophrys_, but the nuclear divisions to form the
coupling nucleus are two in number, _i.e._ the nucleus divides into two,
one of which goes to the surface as the first polar body, and the sister of
this again divides to form a second polar body (which also passes to
{75}the surface) and a pairing nucleus.[84] The two cells then fuse
completely, and surround themselves with a second gelatinous cyst wall,
separated from the outer one by a layer of siliceous spicules. The nucleus
appears to divide at least twice before the young creep out, to divide
immediately into as many _Actinophrys_-like cells as there were nuclei;
then each of these multiplies its nuclei, to become apocytial like the
adult form.

[Illustration: FIG. 21.—Diagram illustrating the conjugation of
_Actinosphaerium_. 1, Original cell; 2, nucleus divides to form two,
N_{2}N_{2}; 3, each nucleus again divides to form two, N_{3} and _n__{3},
the latter passing out with a little cytoplasm as an abortive cell; 4,
repetition of the same process as in 3; 5, the two nuclei N_{4} have fused
in syngamy to form the zygote nucleus N_{z}.]

Schaudinn admits 24 genera (and 7 doubtful) and 41 species (and 18
doubtful). None are known fossil. Their geographical distribution is
cosmopolitan, as is the case with most of the minute fresh-water Protista;
8 genera are exclusively marine, and _Orbulinella_ has only been found in a
salt-pond; _Actinophrys sol_ is both fresh-water and marine, and
_Actinolophus_ has 1 species fresh-water, the other marine. One of the 14
species of _Acanthocystis_ is marine; the remaining genera and species are
all inhabitants of fresh water.[85]


4. RADIOLARIA

_Sarcodina with the protoplasm divided by a perforated chitinous central
capsule into a central mass surrounding the nucleus, and an outer layer;
the pseudopodia radiate, never anastomosing enough to form a marked
network; skeleton either siliceous, of spicules, or perforated; or of
definitely arranged spicules of proteid matter (acanthin), sometimes also
coalescing into a latticed shell; reproduction by fission and by zoospores
formed in the central capsule. Habitat marine, suspended at the surface
(plankton), at varying depths (zonarial), or near the bottom (abyssal)._

{76}[Illustration: FIG. 22.—_Collozoum inerme._ A, B, C, three forms of
colony; D, small colony with central capsules (_c.caps_), containing
nuclei, and alveoli (_vac_) in ectoplasm; E, isospores, with crystals
(_c_); F, anisospores; _nu_, nucleus. (From Parker and Haswell.)]

The following is Haeckel's classification of the Radiolaria:—

  I. PORULOSA (HOLOTRYPASTA).—Homaxonic, or nearly so. Central capsule
  spherical in the first instance; pores numerous, minute, scattered;
  mostly pelagic.

    A. SPUMELLARIA (PERIPYLAEA).—Pores evenly scattered; skeleton of solid
    siliceous spicules, or continuous, and reticulate or latticed, rarely
    absent; nucleus dividing late, as an antecedent to reproduction.

    B. ACANTHARIA (ACTIPYLAEA).—Pores aggregated into distinct areas;
    skeleton of usually 20 centrogenous, regularly radiating spines of
    acanthin, whose branches may coalesce into a latticed shell; nucleus
    dividing early.

  II. OSCULOSA (MONOTRYPASTA).—Monaxonic; pores of central capsule limited
  to the basal area (osculum), sometimes accompanied by two (or more)
  smaller oscula at apical pole, mostly zonarial or abyssal.

    C. NASSELLARIA (MONOPYLAEA).—Central capsule ovoid, of a single layer;
    pores numerous on the operculum or basal field; skeleton siliceous,
    usually with a principal tripod or calthrop-shaped spicule passing, by
    branching, into a complex ring or a latticed bell-shaped shell; nucleus
    eccentric, near apical pole.

    D. PHAEODARIA (CANNOPYLAEA, Haeck.; TRIPYLAEA, Hertw.).—Central capsule
    spheroidal, of two layers, in its outer layer an operculum, with
    radiate ribs and a single aperture, beyond which protrudes the outer
    layer; osculum basal, a dependent tube (proboscis); accessory oscula,
    when present, simpler, usually two placed symmetrically about the
    apical pole; skeleton siliceous, with a combination of organic matter,
    often of hollow spicules; nucleus sphaeroidal, eccentric; extracapsular
    protoplasm containing an accumulation of dusky pigment granules
    ("phaeodium").

{77}[Illustration: FIG. 23.—_Actinomma asteracanthion._ A, the shell with
portions of the two outer spheres broken away; B, section showing the
relations of the skeleton to the animal, _cent.caps_, Central capsule;
_ex.caps.pr_, extra-capsular protoplasm: _nu_, nucleus; _sk._1, outer,
_sk._2, middle, _sk._3, inner sphere of skeleton. (From Parker and Haswell,
after Haeckel and Hertwig.)]

A. SPUMELLARIA.

  Sublegion (1). COLLODARIA.[86]—Skeleton absent or of detached spicules;
  colonial or simple.

    Order i. COLLOIDEA.—Skeleton absent. (Families 1, 2.) _Thalassicolla_
    Huxl.; _Thalassophysa_ Haeck.; _Collozoum_ Haeck.; _Collosphaera_ J.
    Müll.; _Actissa_ Haeck.

    Order ii. BELOIDEA.—Skeleton spicular. (Families 3, 4.)

  Sublegion (2). SPHAERELLARIA.—Skeleton continuous, latticed or spongy,
  reticulate.

    Order iii. SPHAEROIDEA.—Skeleton of one or several concentric spherical
    shells; sometimes colonial. (Families 5-10.) _Haliomma_ Ehrb.;
    _Actinomma_ Haeck. (Fig. 23).

    Order iv. PRUNOIDEA.—Skeleton a prolate sphaeroid or cylinder,
    sometimes constricted towards the middle, single or concentric.
    (Families 11-17.)

    Order v. DISCOIDEA.—Shell flattened, of circular plan, simple or
    concentric, rarely spiral. (Families 18-23.)

    Order vi. LARCOIDEA.—Shell ellipsoidal, with all three axes unequal or
    irregular, sometimes becoming spiral. (Families 24-32.)[87]

{78}[Illustration: FIG. 24.—_Xiphacantha_ (Acantharia). From the surface.
The skeleton only, × 100, (From Wyville Thomson.)]

B. ACANTHARIA.

    Order vii. ACTINELIDA.—Radial spines numerous, more than 20, usually
    grouped irregularly. (Families 33-35.) _Xiphacantha_ Haeck.

    Order viii. ACANTHONIDA.—Radial spines equal. (Families 36-38.)

    Order ix. SPHAEROPHRACTA.—Radial spines 20, with a latticed spherical
    shell, independent of, or formed from the reticulations of the spines.
    (Families 39-41.) _Dorataspis_ Haeck. (Fig. 25, A).

    Order x. PRUNOPHRACTA.—Radial spines 20, unequal; latticed shell,
    ellipsoidal, lenticular, or doubly conical. (Families 42-44.)

C. NASSELLARIA.

    Order xi. NASSOIDEA.—Skeleton absent. (Family 45.)

    Order xii. PLECTOIDEA.—Skeleton of a single branching spicule, the
    branches sometimes reticulate, but never forming a latticed shell or a
    sagittal ring. (Families 46-47.)

    Order xiii. STEPHOIDEA.—Skeleton with a sagittal ring continuous with
    the branched spicule, and sometimes other rings or branches. (Families
    48-51.) _Lithocercus_ Théel (Fig. 26, A).

    Order xiv. SPYROIDEA.—Skeleton with a latticed shell developed around
    the sagittal ring (cephalis), and constricted in the sagittal plane,
    with a lower chamber (thorax) sometimes added. (Families 52-55.)

    {79}Order xv. BOTRYOIDEA.—As in Spyroidea, but with the cephalis 3-4
    lobed; lower chambers, one or several successively formed. (Families
    56-58.)

    Order xvi. CYRTOIDEA.—Shell as in the preceding orders, but without
    lobing or constrictions.  (Families 59-70.) _Theoconus_ Haeck. (Fig.
    25, B).

D. PHAEODARIA.

    Order xvii. PHAEOCYSTINA.—Skeleton 0 or of distinct spicules; capsule
    centric. (Families 71-73.) _Aulactinium_ Haeck. (Fig. 26, B).

    Order xviii. PHAEOSPHAERIA.—Skeleton a simple or latticed sphere, with
    no oral opening (pylome); capsule central. (Families 74-77.)

    Order xix. PHAEOGROMIA.—Skeleton a simple latticed shell with a pylome
    at one end of the principal axis; capsule excentric, sub-apical.
    (Families 78-82.) _Pharyngella_ Haeck.; _Tuscarora_ Murr.;
    _Haeckeliana_ Murr. (Fig. 28).

    Order xx. PHAEOCONCHIA.—Shell of two valves, opening in the plane
    ("frontal") of the three openings of the capsule. (Families 83-85.)

We exclude Haeckel's Dictyochida, with a skeleton recalling that of the
Stephoidea, but of the impure hollow substance of the Phaeodaria (p. 84).
They rank now as Silicoflagellates (p. 114).

The Radiolarian is distinguished from all other Protozoa by the chitinous
central capsule, so that its cytoplasm is separated into an outer layer,
the _extracapsular_ protoplasm (ectoplasm), and a central mass, the
_intracapsular_, containing the nucleus.[88]

The _extracapsular_ layer forms in its substance a gelatinous mass, of
variable reaction, through which the plasma itself ramifies as a network of
threads ("sarcodictyum"), uniting at the surface to constitute the
foundation for the pseudopodia. This gelatinous matter constitutes the
"calymma." It is largely vacuolated, the vacuoles ("alveoli"), of
exceptional size, lying in the nodes of the plasmic network, and containing
a liquid probably of lower specific gravity than seawater; and they are
especially abundant towards the surface, where they touch and become
polygonal. On mechanical irritation they disappear, to be formed anew after
an interval, a fact that may explain the sinking from the surface in
disturbed water. This layer may contain minute pigment granules, but the
droplets of oil and of albuminous matter frequent in the central layer are
rare here. {80}The "yellow cells" of a symbiotic Flagellate or Alga,
_Zooxanthella_, are embedded in the jelly of all except Phaeodaria, and the
whole ectosarc has the average consistency of a firm jelly.

The _pseudopodia_ are long and radiating, with a granular external layer,
whose streaming movements are continuous with those of the inner network.
In the Acantharia they contain a firm axial filament, like that of the
Heliozoa, which is traceable to the central capsule; and occasionally a
bundle of pseudopodia may coalesce to form a stout process like a flagellum
("sarcoflagellum"). Here, too, each spine, at its exit from the jelly, is
surrounded by a little cone of contractile filaments, the _myophrisks_,
whose action seems to be to pull up the jelly and increase the volume of
the spherical body so as to diminish its density.

[Illustration: FIG. 25.—Skeletons of _Radiolaria_. A, _Dorataspis_; B,
_Theoconus_. (After Haeckel.)]

The _intracapsular protoplasm_ is free from _Zooxanthella_ except in the
Acantharia. It is less abundantly vacuolated, and is finely granular. In
the Porulosa it shows a radial arrangement, with pyramidal stretches of
hyaline plasma separated by intervals rich in granules. Besides the alveoli
with watery contents, others are present with albuminoid matter in
solution. Oil-drops, often brilliantly coloured, occur either in the plasma
or floating in either kind of vacuole; and they are often luminous at
night. Added to these, the intracapsular plasm contains pigment-granules,
most frequently red or orange, {81}passing into yellow or brown, though
violet, blue, and green also occur. The "phaeodium,"[89] however, that
gives its name to the Phaeodaria, is an aggregate of dark grey, green, or
brown granules which are probably formed in the endoplasm, but accumulate
in the extracapsular plasm of the oral side of the central capsule.
Inorganic concretions and crystals are also found in the contents of the
central capsule, as well as aggregates of unknown composition, resembling
starch-grains in structure.

In the Monopylaea, or Nassellaria (Figs. 25, B, 26, A), the endoplasm is
differentiated above the perforated area of the central capsule into a cone
of radiating filaments termed the "porocone," which may be channels for the
communication between the exoplasm and the endoplasm, or perhaps serve, as
Haeckel suggests, to raise, by their contraction, the perforated area: he
compares them to the myophane striae of Infusoria. In the Phaeodaria (Fig.
26, B), a radiating laminated cone is seen in the outermost layer of the
endoplasm above the principal opening ("astropyle"), and a fibrillar one
around the two accessory ones ("parapyles"); and in some cases, continuous
with these, the whole outer layer of the endoplasm shows a meridional
striation.

The _nucleus_ is contained in the endoplasm, and is always at first single,
though it may divide again and again. The nuclear wall is a firm membrane,
sometimes finely porous. If there are concentric shells it at first
occupies the innermost, which it may actually come to enclose, protruding
lobes which grow through the several perforations of the lattice-work,
finally coalescing outside completely, so as to show no signs of the joins.
In the Nassellaria a similar process usually results in the formation of a
lobed nucleus, contained in an equally lobed central capsule. The chromatin
of the nucleus may be concentrated into a central mass, or distributed into
several "nucleoli," or it may assume the form of a twisted, gut-like
filament, or, again, the nuclear plasm may be reticulated, with the
chromatin deposited at the nodes of the network.

{82}[Illustration: FIG. 26.—A, _Lithocercus annularis_, with sagittal ring
(from Parker and Haswell). B, _Aulactinium actinastrum_. _C_, calymma;
_cent.caps._, _km_, central capsule; _Ext.caps.pr._, Extracapsular, and
_Int.caps.pr._, intracapsular protoplasm; _n_, _nu_, nucleus; _op_,
operculum; _ph_, phaeodium; _psd_, pseudopodium; _Skel._, skeleton; _z_,
Zooxanthella. (From Lang's _Comparative Anatomy_, after Haeckel.)]

The skeleton of this group varies, as shown in our conspectus, in the
several divisions.[90] The Acantharia (Figs. 24, 25, A) have a skeleton of
radiating spines meeting in the centre of figure of the endoplasm, and
forcing the nucleus to one side. The spines are typically 20 in number, and
emerge from the surface of the regular spherical forms (from which the
others may be readily derived) radially, in five sets of four in the
regions corresponding to the equator and the tropics and polar circles of
our world. {83}The four rays of adjacent circles alternate, so that the
"polar" and "equatorial" rays are on one set of meridians 90° apart, and
the "tropical" spines are on the intermediate meridians, as shown in the
figures. By tangential branching, and the meeting or coalescence of the
branches, reticulate (Figs. 23, 24, 25) and latticed shells are formed in
some families, with circles of openings or pylomes round the bases of the
spines. In the Sphaerocapsidae the spines are absent, but their original
sites are inferred from the 20 circles of pylomes.

In the Spumellaria the simplest form of the (siliceous) skeleton is that of
detached spicules, simple or complex, or passing into a latticed shell,
often with one or more larger openings (pylomes). Radiating spines often
traverse the whole of the cavity, becoming continuous with its latticed
wall, and bind firmly the successive zones when present (Fig. 23).

_Calcaromma calcarea_ was described by Wyville Thomson as having a shell of
apposed calcareous discs, and _Myxobrachia_, by Haeckel, as having
collections of the calcareous Coccoliths and Coccospheres. In both cases we
have to do with a Radiolarian not possessing a skeleton, but retaining the
undigested shells of its food, in the former case (_Actissa_) in a
continuous layer, in the latter (_Thalassicolla_) in accumulations that, by
their weight, droop and pull out the lower hemisphere into distinct arms.

The (siliceous) skeleton of the Nassellaria is absent only in the
Nassoidea, and is never represented by distinct spicules. Its simplest form
is a "tripod" with the legs downward, and the central capsule resting on
its apex. The addition of a fourth limb converts the tripod into a
"calthrop," the central capsule in this case resting between the upturned
leg and two of the lower three regarded as the "anterolateral"; the odd
lower leg, like the upturned one, being "posterior." Again, the skeleton
may present a "sagittal ring," often branched and spiny (Fig. 26, A), or
combined with the tripod or calthrop, or complicated by the addition of one
or more horizontal rings. Another type is presented by the "latticed
chamber" surrounding the central capsule, with a wide mouth ("pylome")
below. This is termed the "cephalis"; it may be combined in various ways
with the sagittal ring and the tripod or calthrop; and, again, it may be
prolonged by the addition of one, two, or three chambers below, {84}the
last one opening by a pylome (Fig. 25, B). These are termed "thorax,"
"abdomen," and "post-abdomen" respectively.

In the Phaeodaria the skeleton may be absent, spicular (of loose or
connected spicules) or latticed, continuous or bivalve. It is composed of
silica combined with organic matter, so that it chars when heated, is more
readily dissolved, and is not preserved in fossilisation. The spicules or
lattice-work are hollow, often with a central filament running in the
centre of the gelatinous contents. The latticed structure of the shell of
the Challengeridae (Fig. 28) is so fine as to recall that of the
Diatomaceae. In the Phaeoconchida the shell is in two halves, parted along
the "frontal" plane of the three apertures of the capsule.

[Illustration: FIG. 27.—Scheme of various possible skeletal forms deposited
in the meshes of an alveolar system, most of which are realised in the
Radiolaria. (From Verworn, after Dreyer.)]

The central capsule (rarely inconspicuous and difficult, if not impossible
to demonstrate) is of a substance which resembles chitin, though its
chemical reactions have not been fully studied hitherto, and indeed vary
from species to species. It is composed of a single layer, except in
_Phaeodaria_, where it is double. The operculum in this group, _i.e._ the
area around the aperture, is composed of an outer layer, which is radially
thickened, and a thin inner layer; the former is produced into the
projecting tube ("proboscis").

REPRODUCTION in the Radiolaria may be simple fission due to the binary
fission of the nucleus, the capsule, and the ectoplasm in succession. If
this last feature is omitted we have a colonial organism, composed of the
common ectoplasm containing numerous central capsules; and the genera in
which this occurs, all belonging to the Peripylaea, were formerly separated
(as Polycyttaria) from {85}the remaining Radiolaria (Monocyttaria). They
may either lack a skeleton (Collozoidae, Fig. 22), or have a skeleton of
detached spicules (Sphaerozoidae), or possess latticed shells
(Collosphaeridae) one for each capsule, and would seem therefore to belong,
as only differentiated by their colonial habit, to the several groups
having these respective characters. Fission has been well studied in
_Aulacantha_ (a Phaeodarian) by Borgert.[91] He finds that in this case the
skeleton is divided between the daughter-cells, and the missing part is
regenerated. In cases where this is impossible one of the daughter-cells
retains the old skeleton, and the other escapes as a bud to form a new
skeleton.

[Illustration: FIG. 28.—Shells of Challengeridae: A, _Tuscarora_; B,
_Pharyngella_; C, _Haeckeliana_. (From Wyville Thomson.)]

Two modes of reproduction by flagellate zoospores have been described (Fig.
22). In the one mode all the zoospores are alike—isospores—and frequently
contain a crystal of proteid nature as well as oil-globules. In the
Polycyttaria alone has the second mode of spore-formation been seen, and
that in the same species in which the formation of isospores occurs. Here
"anisospores" are formed, namely, large "mega-," and small
"micro-zoospores." They probably conjugate as male and female respectively;
but neither has the process been observed, nor has any product of such
conjugation (zygote) been recognised. In every case the formation of the
zoospores only involves the {86}endoplasm: the nucleus first undergoes
brood division, and the plasma within the capsule becomes concentrated
about its offspring, and segregates into the spores; the extracapsular
plasm disintegrates.[92]

The YELLOW CELLS (_Zooxanthella_), so frequently found in the Radiolaria
were long thought to be constituents of their body. Cienkowsky found that
when the host died from being kept in unchanged water, the yellow cells
survived and multiplied freely, often escaping from the gelatinised
cell-wall as biflagellate zoospores. The cell-wall is of cellulose. The
cell contains two chloroplastids, or plates coloured with the vegetal
pigment "diatomin." Besides ordinary transverse fission in the ordinary
encysted state in the ectoplasm of the host, when free they may pass into
what is known as a "_Palmella_-state," the cell-walls gelatinising; in this
condition they multiply freely, and constitute a jelly in which the
individual cells are seen as rounded bodies. They contain starch in two
forms—large hollow granules, not doubly refractive, and small solid
granules which polarise light. We may regard them as Chrysomonadaceae (p.
113). Similar organisms occur in many Anthozoa (see pp. 261, 339, 373 f.,
396). Diatomaceae (yellow Algae with silicified cell-walls) sometimes live
in the jelly of certain _Collosphaera_. Both these forms live in the state
known as "symbiosis" with their host; _i.e._ they are in mutually helpful
association, the Radiolarian absorbing salts from the water for the
nutrition of both, and the Alga or Flagellate taking up the CO_{2} due to
the respiration of the host, and building up organic material, the surplus
of which is doubtless utilised, at least in part, for the nutrition of the
host. A similar union between a Fungus and a coloured vegetal
("holophytic") organism is known as a Lichen.

The Suctorian Infusorian _Amoebophrya_ is parasitic in the ectoplasm of
certain Acantharia, and in the peculiar genus _Sticholonche_ which appears
to be intermediate between this group and Heliozoa.

The Silicoflagellate family Dictyochidae are found temporarily {87}embedded
in the ectoplasm of some of the Phaeocystina, and have a skeleton of
similar nature. Their true nature was shown by Borgert.

The Amphipod crustacean _Hyperia_[93] may enter the jelly of the colonial
forms, and feed there at will on the host.[94]

Haeckel, in his Monograph of the Radiolaria of the _Challenger_ enumerated
739 genera, comprising 4318 species; and Dreyer has added 6 new genera,
comprising 39 species, besides 7 belonging to known genera. Possibly, as we
shall see, many of the species may be mere states of growth, for it is
impossible to study the life-histories of this group; on the other hand, it
is pretty certain that new forms are likely to be discovered and described.
The Radiolaria are found living at all depths in the sea, by the
superficial or deep tow-net; and some appear to live near the bottom, where
the durable forms of the whole range also settle and accumulate. They thus
form what is known as Radiolarian ooze, which is distinguished from other
shallower deposits chiefly through the disappearance by solution of all
calcareous skeletons, as they slowly fell through the waters whereon they
originally floated at the same time with the siliceous remains of the
Radiolaria. The greatest wealth of forms is found in tropical seas, though
in some places in cold regions large numbers of individuals of a limited
range of species have been found.

Radiolaria of the groups with a pure siliceous skeleton can alone be
fossilised, even the impure siliceous skeleton of the Phaeodaria readily
dissolving in the depths at which they live: they have been generally
described by Ehrenberg's name _Polycystineae_. Tripolis (_Kieselguhr_) of
Tertiary ages have been found in many parts of the globe, consisting
largely or mainly of Radiolaria, and representing a Radiolarian ooze. That
of the Miocene of Barbados contains at least 400 species; that of Gruppe at
least 130. In Secondary and Palaeozoic rocks such oozes pass into
Radiolarian quartzites (some as recent as the Jurassic). They occur also in
fossilised excrement (coprolites), and in flint or chert concretions, as
far down as the lowest fossiliferous rocks, {88}the Cambrian. The older
forms are simple Sphaerellaria and Nassellaria. From a synopsis of the
history of the order in Haeckel's _Monograph_ (pp. clxxxvi.-clxxxviii.) we
learn that while a large number of skeletal forms had been described by
Ehrenberg, Huxley in 1851 published the first account of the living animal.
Since then our knowledge has been extended by the labours of Haeckel,
Cienkowsky, R. Hertwig, Karl Brandt, and A. Borgert.


5. PROTEOMYXA

_Sarcodina without a clear ectoplasm, whose active forms are amoeboid or
flagellate, or pass from the latter form to the former; multiplying
chiefly, if not exclusively, by brood-formation in a cyst. No complete
cell-pairing (syngamy) known, though the cytoplasms may unite into
plasmodia; pseudopodia of the amoeboid forms usually radiate or filose, but
without axial filaments. Saprophytic or parasitic in living animals or
plants._

This group is a sort of lumber-room for forms which it is hard to place
under Rhizopoda or Flagellata, and which produce simple cysts for
reproduction, not fructifications like the Mycetozoa. The cyst may be
formed for protection under drought ("hypnocyst"), or as a preliminary to
spore-formation ("sporocyst"). The latter may have a simple wall (simple
sporocyst), or else two or three formed in succession ("resting cyst"), so
as to enable it to resist prolonged desiccation, etc.: both differing from
the hypnocyst in that their contents undergo brood formation. On encystment
any indigestible food materials are extruded into the cyst, and in the
"resting cysts," which are usually of at least two layers, this faecal mass
lies in the space between them. The brood-cells escape, either as
flagellate-cells, resembling the simpler Protomastigina, called
"flagellulae," and which often become amoeboid (Fig. 29); or already
furnished with pseudopodia, and called "amoebulae," though they usually
recall _Actinophrys_ rather than _Amoeba_. In _Vampyrella_ and some others
the amoebulae fuse, and so attain a greater size, which is most probably
advantageous for feeding purposes. But usually it is as a uninucleate cell
that the being encysts. They may feed either by ingestion by the
pseudopodia, by the whole surface contained in a living host-cell, or by
passing a pseudopodium into a host-cell (Fig. 29 5). They may be divided as
follows:—

  {89}A. MYXOIDEA.—Flagella 1-3; zoospores separating at once.

    1. ZOOSPOREAE.—Brood-cells escaping as flagellulae, even if they become
    amoeboid later. _Ciliophrys_ Cienk.; _Pseudospora_ Cienk. (Fig. 29).

    2. AZOOSPOREAE.—Cells never flagellate. _Protomyxa_ Haeckel;
    _Plasmodiophora_ Woronin; _Vampyrella_ Cienk.; _Serumsporidium_ L.
    Pfeiffer.

  B. CATALLACTA.—Brood-cells of cyst on liberation adhering at the centre
  to form a spherical colony, multiflagellate; afterwards separating, and
  becoming amoeboid. _Magosphaera_ Haeckel (marine).[95]

[Illustration: FIG. 29.—_Pseudospora lindstedtii._ 1, 2, Flagellate
zoospores; 3, young amoebula, with two contractile vacuoles, one being
reconstituted by three minute formative vacuoles; 4, 5, an amoebula
migrating to a fungus hypha through the wall of which it has sent a long
pseudopodium; 6, amoebula full-grown; 7, 8, mature cells rounded off,
protruding a flagellum, before encysting; 9, young sporocyst; 10, the
nucleus has divided into a brood of eight; 11-14, stages of formation of
zoospores. _cv_, Contractile vacuole; _e_, mass of faecal granules; _fl_,
flagellum; _n_, nucleus, × about 750/1.]

_Plasmodiophora_ infests the roots of Crucifers, causing the disease known
as "Hanburies," or "fingers and toes," in turnips, etc. _Serumsporidium_
dwells in the body cavity of small Crustacea. Many of this group were
described by Cienkowsky under the name of "Monadineae" (in _Arch. Mikr.
Anat._ i. 1865, p. 203). Zopf has added more than anyone else since then to
our knowledge. He monographed them under Cienkowsky's name, as a
subordinate group of the Myxomycetes, "_Pilzthiere oder Schleimpilze_," in
Schenk's _Handb. d. Bot._ vol. iii. pt. ii. (1887). To Lankester (_Encycl.
Brit._, reprint 1891) we owe the name here adopted. Zopf has successfully
pursued their study in recent {90}papers in his _Beitr. Nied. Org._ The
Chytridieae, usually ascribed to Fungi, are so closely allied to this group
that Zopf proposes to include at least the Synchytrieae herein.

This group is very closely allied to Sporozoa; for the absence of cytogamy,
and of sickle-germs,[96] and of the complex spores and cysts of the
Neosporidia, are the only absolute distinctions.


6. MYCETOZOA (MYXOMYCETES, MYXOGASTRES)

_Sarcodina moving and feeding by pseudopodia, with no skeleton, aggregating
more or less completely into complex "fructifications" before forming
1-nucleate resting spores; these may in the first instance liberate
flagellate zoospores, which afterwards become amoeboid, or may be amoeboid
from the first; zoospores capable of forming hypnocysts from which the
contents escape in the original form._


  1. Aggregation taking place without plastogamy, zoospores amoeboid, with
  a clear ectosarc

    ACRASIEAE.

    _Copromyxa_ Zopf; _Dictyostelium_ Brefeld.

  2. Aggregation remaining lax, with merely thread-like connexions, except
  when encystment is to take place; cytoplasm finely granular throughout;
  complete fusion of the cytoplasm doubtful

    FILOPLASMODIEAE.

    _Labyrinthula_ Cienk.; _Chlamydomyxa_ Archer; _Leydenia_ (?) Schaud.

  3. Plasmodium formation complete, eventuating in the formation of a
  complex fructification often traversed by elastic, hygroscopic threads,
  which by their contraction scatter the spores; zoospores usually
  flagellate at first

    MYXOMYCETES.

    _Fuligo_ Hall.; _Chondrioderma_ Rostaf.; _Didymium_ Schrad. (Fig. 30).


I. The ACRASIEAE are a small group of saprophytes, often in the most
literal sense, though in some cases it has been proved that the actual food
is the bacteria of putrefaction. In them, since no cell-division takes
place in the fructification, it is certain that the multiplication of the
species must be due to the fissions of the amoeboid zoospores, which often
have the habit of _Amoeba limax_ (Fig. 1, p. 5).

II. FILOPLASMODIEAE.—_Chlamydomyxa_[97] is a not uncommon inhabitant of the
cells of bog-mosses and bog-pools, and its nutrition may be holophytic, as
it contains chromoplasts; but it {91}can also feed amoeba-fashion.
_Labyrinthula_ is marine, and in its fructification each of the component
cells forms four spores. _Leydenia_ has been found in the fluid of ascitic
dropsy, associated with malignant tumour.

III. MYXOMYCETES.—The fructification in this group is not formed by the
mere aggregation of the zoospores, but these fuse by their cytoplasm to
form a multinucleate body, the "plasmodium," which, after moving and
growing (with nuclear division) for some time like a great multinucleate
Reticularian, passes into rest, and develops a fructification by the
formation of a complex outer wall; within this the contents, after
multiplication of the nuclei, resolve themselves into uninucleate spores,
each with its own cyst-wall. The fructifications of this group are often
conspicuous, and resemble those of the Gasteromycetous fungi (_e.g._, the
Puffballs), whence they were at first called _Myxogastres_. De Bary first
discovered their true nature in 1859, and ever since they have been claimed
by botanist and zoologist alike.

The spore on germination liberates its contents as a minute flagellate,
with a single anterior lash and a contractile vacuole (Fig. 30, C). It soon
loses the lash, becomes amoeboid, and feeds on bacteria, etc. (Fig. 30, D,
E). In this state it can pass into hypnocysts, from which, as from the
spores, it emerges as a flagellula. After a time the amoeboids, which may
multiply by fission, fuse on meeting, so as to form the plasmodium (Fig.
30, F). This contains numerous nuclei, which multiply as it grows, and
numerous contractile vacuoles. When it attains full size it becomes
negatively hydrotactic, crawls to a dry place, and resolves itself into the
fructification. The external wall, and sometimes a basal support to the
fruit, are differentiated from the outer layer of protoplasm; while the
nuclei within, after undergoing a final bipartition, concentrate each
around an independent portion of plasma, which again is surrounded as a
spore by a cyst-wall. Often the maturing plasmodium within the wall of the
fruit is traversed by a network of anastomosing tubes filled with liquid,
the walls of which become differentiated into membrane like the fruit-wall,
and are continuous therewith. As the fruit ripens the liquid dries, and the
tubes now form a network of hollow threads, the "capillitium," often with
external spiral ridges (Fig. 30, A, B). These are very hygroscopic, and by
their expansion and contraction {92}determine the rupture of the fruit-wall
and the scattering of the spores.

[Illustration: FIG. 30.—_Didymium difforme._ A, two sporangia (_spg_ 1 and
2) on a fragment of leaf (_l_); B, section of sporangium, with ruptured
outer layer (_a_), and threads of capillitium (_cp_); C, a flagellula with
contractile vacuole (_c.vac_) and nucleus (_nu_); D, the same after loss of
flagellum; _b_, an ingested bacillus; E, an amoebula; F, conjugation of
amoebulae to form a small plasmodium; G, a larger plasmodium accompanied by
numerous amoebulae; _sp_, ingested spores. (After Lister.)]

Again, in some cases the plasmodia themselves aggregate in the same way as
the amoeboids do in the _Acrasieae_, and combine to form a compound fruit
termed an "aethalium,"[98] with the regions of the separate plasmodia more
or less clearly marked off. The species formerly termed _Aethalium
septicum_ is now known as _Fuligo varians_. It is a large and conspicuous
species, common on tan, and is a pest in the tanpits. Its aethalia may
reach a {93}diameter of a foot and more, and a thickness of two inches.
_Chondrioderma diffusum_, often utilised as a convenient "laboratory type,"
is common on the decaying haulms of beans in the late autumn. The interest
of this group is entirely biological, save for the "flowers of tan."[99]




{94}CHAPTER IV

PROTOZOA (_CONTINUED_): SPOROZOA[100]

II. Sporozoa.

  _Protozoa parasitic in Metazoa, usually intracellular for at least part
  of their cycle, rarely possessing pseudopodia, or flagella (save in the
  sperms), never cilia; reproduction by brood-formation, often of
  alternating types; syngamy leading up to resting spores in which minute
  sickle-germs are formed, or unknown (Myxosporidiaceae)._


This group, of which seven years ago no single species was known in its
complete cycle, has recently become the subject of concentrated and
successful study, owing to the fact that it has been recognised to contain
the organisms which induce such scourges to animals as malarial fevers, and
various destructive murrains. Our earliest accurate, if partial knowledge,
was due to von Siebold, Kölliker, and van Beneden. Thirty years ago Ray
Lankester in England commenced the study of species that dwell in the
blood, destined to be of such moment for the well-being of man and the
animals in his service; and since then our knowledge has increased by the
labours of Manson, Ross and Minchin at home, Laveran, Blanchard, Thélohan,
Léger, Cuénot, Mesnil, Aimé Schneider in France, Grassi in Italy,
Schaudinn, Siedlecki, L. and R. Pfeiffer, Doflein in Central Europe, and
many others.

{95}[Illustration: FIG. 31.—_Lankesteria ascidiae_, showing life-cycle.
_a_, _b_, _c_, Sporozoites in digestive epithelium cells of host; _d_, _e_,
growth stages; _f_, free gregarine; _g_, association; _h_, encystment; _i_,
_j_, brood-divisions in associated mates; _k_, pairing-cells; _l_, syngamy;
_m_, zygote; _n_, _o_, _p_, nuclear divisions in spores; _q_, cyst with
adult spores, each containing 8 sickle-germs. (After Luhe, modified from
Siedlecki.)]

As a type we will take a simple form of the highest group, the
Gregarinidaceae, _Monocystis_, which inhabits the seminal vesicles of the
earthworm. In its youngest state, the "sporozoite," it is a naked,
sickle-shaped cell, which probably makes its way from the gut into one of
the large radial cells of the seminal funnel, where it attains its full
size, and then passes out into the vesicles or reservoirs of the semen, to
lie among the sperm morulae and young spermatozoa. The whole interior is
formed of the opaque endosarc, which contains a large central nucleus, and
is full of refractive granules of paramylum or paraglycogen,[101] a
carbohydrate allied to glycogen or animal starch, so common in the liver
and {96}muscles of Metazoa; besides these it contains proteid granules
which stain with carmine, and oil-drops. The ectosarc is formed of three
layers: (1) the outer layer or "cuticle"[102] is, in many cases if not
here, ribbed, with minute pores in the furrows, and is always porous enough
to allow the diffusion of dissolved nutriment; (2) a clear plasmatic layer,
the "sarcocyte"; (3) the "myocyte," formed of "myonemes," muscular fibrils
disposed in a network with transverse meshes, which effect the wriggling
movements of the cell. The endosarc contains the granules and the large
central nucleus. The adult becomes free in the seminal vesicles; here two
approximate, and surround themselves with a common cyst: a process which
has received the name of "association" (Fig. 31, _g-i_). Within this,
however, the protoplasms remain absolutely distinct. The nucleus undergoes
peculiar changes by which its volume is considerably reduced. When this
process of "nuclear reduction" is completed, each of the mates undergoes
brood-divisions (_j_), so as to give rise to a large number of rounded
naked 1-nucleate cells—the true pairing-cells. These unite two and two, and
so form the 1-nucleate spores (_k-m_), which become oat-shaped, form a
dense cyst-wall, and have been termed "pseudonavicellae" from their
likeness to the Diatomaceous genus _Navicella_. Some of the cytoplasm of
the original cells remains over unused, as "epiplasm," and ultimately
degenerates, as do a certain number of the brood-cells which presumably
have failed to pair. It is believed that the brood-cells from the same
parent will not unite together. The contents of each spore have again
undergone brood-division to form eight sickle-shaped zoospores, or
"sporozoites" (_n-q_), and thus the developmental cycle is completed.
Probably the spores, swallowed by birds, pass out in their excrement, and
when eaten by an earthworm open in its gut; the freed sickle-germs can now
migrate through the tissues to the seminal funnels, in the cells of which
they grow, ultimately becoming free in the seminal vesicles.[103]

{97}We may now pass to the classification of the group.

  A. TELOSPORIDIA.—Cells 1-nucleate until the onset of brood-formation,
  which is simultaneous.

    1. GREGARINIDACEAE.—Cells early provided with a firm pellicle and
    possessing a complex ectosarc; at first intracellular, soon becoming
    free in the gut or coelom of Invertebrates. Pairing between adults,
    which simultaneously produce each its brood of gametes, isogamous or
    bisexual, which pair within the common cyst; zygotospores surrounded by
    a firm cyst, and producing within a brood of sickle-shaped zoospores.

      (i.) SCHIZOGREGARINIDAE.—Multiplying by simple fission in the free
      state as well as by brood-formation; the brood-cells conjugating in a
      common cyst, but producing only one pairing nucleus in each mate (the
      rest aborting), and consequently only one spore.

        _Ophryocystis_ A. Schn.

      (ii.) ACEPHALINIDAE.—Cell one-chambered, usually without an epimerite
      for attachment.

        _Monocystis_ F. Stein; _Lankesteria_ Mingazzini.

      (iii.) DICYSTIDAE.—Cell divided by a plasmic partition; epimerite
      usually present.

        _Gregarina_ Dufour; _Stylorhynchus_ A. Schn.; _Pterocephalus_ A.
            Schn.


    2. COCCIDIACEAE.—Cells of simple structure, intracellular in Metazoa.
    Pairing between isolated cells usually sexually differentiated as
    oosphere and sperm, the latter often flagellate. Brood-formation of the
    adult cell giving rise to sickle-shaped zoospores (merozoites), or
    progamic and producing the gametes. Oosperm motile or motionless,
    finally producing a brood of spores, which again give rise to a brood
    of sickle-spores.

      (i.) COCCIDIIDAE.—Cell permanently intracellular, or very rarely
      coelomic, encysting or not before division; zoospores always
      sickle-shaped; oosperm encysting at once, producing spores with a
      dense cell-wall producing sickle-germs.

      (ii.) HAEMOSPORIDAE.—Cells parasitic in the blood corpuscles or free
      in the blood of cold-blooded animals, encysting before
      brood-formation; zoospores sickle-shaped; oosperm at first motile.

        _Lankeresterella_ Labbé; (_Drepanidium_ Lank.;) _Karyolysus_ Labbé;
            _Haemogregarina_ Danilewski.

      (iii.) ACYSTOSPORIDAE.—Cells parasitic in the blood and haematocytes
      of warm-blooded Vertebrates; never forming a cyst-wall before
      dividing; zoospores formed in the corpuscles, amoeboid. Gametocytes
      only forming gametes when taken into the stomach of insects. Oosperm
      at first active, passing into the coelom, producing naked spores
      which again produce a large brood of sickle zoospores, which migrate
      to the salivary gland, and are injected with the saliva into the
      warm-blooded host.

        _Haemamoeba_ Grassi and Feletti; _Laverania_ Grassi and Feletti;
            _Haemoproteus_ Kruse; _Halteridium_ Labbé.[104]


  B. NEOSPORIDIA.—Cells becoming multinucleate apocytes before any
  brood-formation occurs. Brood-formation progressive through the apocyte,
  not simultaneous.

    {98}1. MYXOSPORIDIACEAE.—Naked parasites in cold-blooded animals.
    Spore-formation due to an aggregation of cytoplasm around a single
    nucleus to form an archespore, which then produces a complex of cells
    within which two daughter-cells form the spores and accessory
    nematocysts.

      _Myxidium_ Bütsch.; _Myxobolus_ Bütsch.; _Henneguya_ Thélohan;
          _Nosema_ Nageli (= _Glugea_ Th.).

    2. ACTINOMYXIDIACEAE.[105]—Apocyte resolved into a sporange, containing
    eight secondary sporanges (so-called spores), of ternary symmetry and
    provided with three polar nematocysts.

    3. SARCOSPORIDIACEAE.—Encysted parasites in the muscles of Vertebrates,
    with a double membrane; spores simple.

      _Sarcocystis_ Lankester.


[Illustration: FIG. 32.—_Gregarina blattarum_ Sieb. A, two cephalonts,
embedded by their epimerite (_ep_), in cells of the gut-epithelium; _deu_,
deutomerite; nu, nucleus; _pr_, protomerite; B^1, B^2, two free specimens
of an allied genus; the epimerite is falling off in B^2, which is on its
way to become a sporont; C, cyst (_cy_) of A, with sporoducts (_spd_)
discharging the spores (_sp_), surrounded by an external gelatinous
investment (_g_). (From Parker and Haswell.)]

_Monocystis_ offers us the simplest type of GREGARINIDACEAE. In most
Gregarines (Figs. 31, 32) the sporozoite enters the epithelium-cell of the
gut of an Arthropod, Worm or Mollusc, and as it enlarges protrudes the
greater part of its bulk into the lumen, and may become free therein, or
pass into the coelom. The attached part is often enlarged into a sort of
grapple armed with spines, the "epimerite"; this contains only sarcocyte,
the other layers being absent. The freely projecting body is usually
divided by an ingrowth of the myocyte into a front segment ("protomerite"),
and a rear one ("deutomerite"), with the nucleus usually in the latter. In
this state the cell is termed a "cephalont." Conjugation is frequent, but
apparently is not always connected with {99}syngamy or spore-formation;
sometimes from two to five may be aggregated into a chain or "syzygy." The
number of cases in which a syngamic process between two cells has been
observed is constantly being increased. In _Stylorhynchus_ (Fig. 33) the
conjugation at first resembles that of _Monocystis_, but the actual
pairing-cells are bisexually differentiated into sperms in the one parent,
and oospheres in the other; it is remarkable that here the pear-shaped
sperms are apparently larger than the oospheres. In _Pterocephalus_ the
chief difference is that the sperms are minute.[106] In all cases of
spore-formation the epimerite is lost and the septum disappears; in this
state the cell is termed a sporont. Sometimes the epiplasm of the sporont
forms tubes ("sporoducts"), which project through the cyst-wall and give
exit to the spores, as in _Gregarina_ (Fig. 32, C), a parasite in the
beetle _Blaps_.

Gregarines infest most groups of Invertebrates except Sponges and perhaps
Coelenterates, the only exception cited being that of _Epizoanthus
glacialis_, a Zoantharian (p. 406). They appear to be relatively harmless
and are not known to induce epidemics.

The COCCIDIACEAE never attain so high a degree of cellular differentiation
as the Gregarines, which may be due to their habitat; for in the growing
state they are intracellular parasites. Their life-history shows a double
cycle, which has been most thoroughly worked out in COCCIDIIDAE by
Schaudinn and Siedlecki in parasites of our common Centipedes. We take that
of _Coccidium schubergi_ (in _Lithobius forficatus_[107]), beginning with
the sporozoite, which is liberated from the spores taken in with the food,
in the gut of the Centipede. This active sickle-shaped cell (Fig. 34, _l_)
enters an epithelial cell of the mid-gut, and grows therein till it attains
its full size (_a_), when it is termed a "schizont"; for it segments (Gk.
σχίζω, "I split") superficially into a large number of sickle-shaped
zoospores, the "merozoites" (_c_), resembling the sporozoites. The
segmentation is superficial, so that there may remain a large mass of
residual epiplasm. The merozoites are set free by the destruction of the
epithelium-cell in which they were formed, and which becomes disorganised,
like the residual epiplasm. Each merozoite may repeat the {100}behaviour of
the sporozoite, so that the disease spreads freely, and becomes acute after
several reinfections. After a time the adult parasites, instead of becoming
schizonts and simply forming merozoites by division, differentiate into
cells that undergo a binary sexual differentiation. Some cells, the
"oocytes" (_d_, _e_), escape into the gut, and the nucleus undergoes
changes by which some of its substance (or an abortive daughter-nucleus) is
expelled to the exterior (_f_), such a cell is now an "oogamete" or
oosphere. Others, again, are spermatogones (_h_): each when full grown on
escaping into the gut commences a division (_i_, _j_), like that of the
schizonts. The products of this division or segment-cells are the
flagellate sperms (_s_): they are more numerous and more minute than the
merozoites produced by the schizonts, and are attracted to the oosphere by
chemiotaxy (p. 23), and one enters it and fuses with it (_g_). The oosperm,
zygote or fertilised egg, thus formed invests itself with a dense
cyst-wall, as a "oospore" (_k_), its contents form one or more (2, 4, 8,
etc.) spores; and each spore forms again one, two, or four sickle-shaped
zoospores ("sporozoites"), destined to be liberated for a fresh cycle of
parasitic life when the spores are swallowed by another host.

[Illustration: FIG. 33.—Bisexual pairing of _Stylorhynchus_. _a_,
Spermatozoon; _b-e_, fusion of cytoplasm of spermatozoon and oosphere; _f_,
_g_, fusion of nuclei; _h-j_, development of wall to zygote; _k_, _l_,
formation of four sporoblasts; _l_, side view of spore; _m_, mature
sporozoites in spore. (After Léger.)]

{101}[Illustration: FIG. 34.—Life-history of _Coccidium schubergi_. _a_,
Penetration of epithelium-cell of host by sporozoite; _b-d_, stages of
multiple cell-formation in naked state (schizogony); _e_, _f_, formation of
oogamete; _g_, conjugation; _h-j_, formation of sperms (_s_); _k_,
development of zygote (fertilised ovum) to form four spores; _l_, formation
of two zoospores (or sickle germs) in each spore. (From Calkins's
_Protozoa_, after Schaudinn.)]

In some cases the oogametes are at first oblong, like ordinary merozoites,
and round off in the gut. The microgametocyte, or spermatogone, has the
same character, but is smaller; it applies itself like a cap to one pole of
the oogamete, which has rounded off; it then divides into four sperms,
whose cytoplasm is not sharply separated; one of these then separates from
the common mass, enters the oogamete, and so conjugation is effected, with
an oosperm as its result. This latter mode of conjugation is that of
_Adelea ovata_ and _Coccidium lacazei_: the former is probably the more
primitive and the commoner. The sperms {102}of Coccidiidae, when free,
usually possess two long flagella, either both anterior, or a very long one
in front and a short one behind, both turned backwards.

The genus _Coccidium_ affects many animals, and one species in particular,
_C. cuniculi_ Rivolta, attacks the liver of young rabbits,[108] giving rise
to the disease "coccidiosis." _Coccidium_ may also produce a sort of
dysentery in cattle on the Alpine pastures of Switzerland; and cases of
human coccidiosis are by no means unknown. _Coccidium_-like bodies have
been demonstrated in the human disease, "molluscum contagiosum," and the
"oriental sore" of Asia; similar bodies have also been recorded in smallpox
and vaccinia, malignant tumours and even syphilis, but their nature is not
certainly known; some of these are now referred to Flagellata (see p. 121).

Closely allied to the Coccidiidae are the HAEMOSPORIDAE, dwellers in the
blood of various cold-blooded Vertebrates,[109] and entering the corpuscles
as sporozoites or merozoites to attain the full size, when they divide by
schizogony; they are freed like those of the next family by the breaking up
of the corpuscle. The merozoites were described by Gaule (1879) as
"vermicles" ("Würmchen"), and regarded by him as peculiar
segregation-products of the blood; though Lankester had described the same
species in the Frog's blood as early as 1871, with a full recognition of
its true character. His name, _Drepanidium_, has had to give way, having
been appropriated to another animal, and has been aptly replaced by that of
_Lankesterella_. The sexual process of _Karyolysus_ has been found to take
place in a Tick, that of _Haemogregarina_ in a Leech, thus presenting a
close analogy to the next group, which only differs in its less definite
form in the active state, and in the lack of a cell-wall during
brood-formation.

Laveran was the first to describe a member of the {103}ACYSTOSPORIDAE, in
1880, as an organism always to be found in the blood of patients suffering
from malarial fever; this received the rather inappropriate name of
_Plasmodium_, which, by a pedantic adherence to the laws of priority, has
been used by systematists as a generic name. Golgi demonstrated the
coincidence of the stages of the intermittent fever with those of the
life-cycle of the parasite in the patient, the maturation of the schizont
and liberation of the sporozoites coinciding with the fits of fever.
Manson, who had already shown that the Nematodes of the blood that give
rise to Filarial haematuria (see Vol. II. p. 149) have an alternating life
in the gnats or mosquitos of the common genus _Culex_,[110] in 1896
suggested to Ronald Ross that the same might apply to this parasite, and
thus inspired a most successful work. The hypothesis had old prejudices in
its favour, for in many parts there was a current belief that sleeping
under mosquito-netting at least helped other precautions against malaria.
Ross found early in his investigations that _Culex_ was a good host for the
allied genus _Haemoproteus_ or _Proteosoma_, parasitic in birds, but could
neither inoculate man with fever nor be inoculated from man. He found,
however, that the malaria germs from man underwent further changes in the
stomach of a "dappled-wing mosquito," that is, as we have since learned, a
member of the genus _Anopheles_. Thenceforward the study advanced rapidly,
and a number of inquirers, including Grassi, Koch, MacCallum (who
discovered the true method of sexual union in _Halteridium_[111]), and Ross
himself, completed his discovery by supplying a complete picture of the
life-cycles of the malaria-germs. Unfortunately, there has been a most
unhappy rivalry as to the priority of the share in each fragment of the
discovery, whose history is summarised by Nuttall, we believe, with perfect
fairness.[112]

The merozoite is always amoeboid, and in this state enters the blood
corpuscle; herein it attains its full size, as a schizont, becoming filled
with granules of "melanin" or black pigment, probably a decomposition
product of the red colouring matter (haemoglobin).

{104}[Illustration: FIG. 35.—Life-history of Malarial Parasites. _A-G_,
Amoebula of quartan parasite to sporulation; _H_, its gametocyte; _I-M_,
amoebula of tertian parasite to sporulation; _N_, its gametocyte; _O_, _T_,
"crescents" or gametocytes of _Laverania_; _P-S_, sperm-formation; _U-W_,
maturation of oosphere; _X_, fertilisation; _Y_, zygote. _a_, Zygote
enlarging in gut of Mosquito; _b-e_, passing into the coelom; _f_, the
contents segmented into naked spores; _g_, the spores forming sickle-germs
or sporozoites; _h_, sporozoites passing into the salivary glands. (From
Calkins's _Protozoa_, after Ross and Fielding Ould.)]

The nucleus of the schizont now divides repeatedly, and then the schizont
segments into a flat brood of germs (merozoites), relatively few in the
parasite of quartan fever (_Haemamoeba malariae_, Fig. 35, E-G), many in
that of tertian (_H. vivax_, Fig. 35, M). These brood-cells escape and
behave for the most part as before. But after the disease has persisted for
some time we find that in the genus _Haemamoeba_, {105}which induces the
common malarial fevers of temperate regions, certain of the full-grown
germs, instead of behaving as schizonts, pass, as it were, to rest as round
cells; while in the allied genus _Laverania_, (_Haemomenas_, Ross) these
resting-cells are crescentic, with blunt horns, and are usually termed
half-moons (Fig. 35, O, T), characteristic of the bilious or pernicious
remittent fevers of the tropics and of the warmer temperate regions in
summer. These round or crescent-shaped cells are the gametocytes, which
only develop further in the drawn blood, whether under the microscope,
protected against evaporation, or in the stomach of the _Anopheles_: the
crescents become round, and then they, like the already round ones of
_Haemamoeba_, differentiate in exactly the same way as the corresponding
cells of _Coccidium schubergi_. The female cell only exhibits certain
changes in its nucleus to convert it into an oosphere: the male emits a
small number of sperms, long flagellum-like bodies, each with a nucleus;
and these, by their wriggling, detach themselves from the central core, no
longer nucleated. The male gametogonium with its protruded sperms was
termed the "_Polymitus_ form," and was by some regarded as a
degeneration-form, until MacCallum discovered that a "flagellum" regularly
undergoes sexual fusion with an oosphere in _Halteridium_, as has since
been found in the other genera. The oosperm (Y) so formed is at first
motile ("ookinete"), as it is in Haemosporidae, and passes into the
epithelium of the stomach of the gnat and then through the wall, acquiring
a cyst-wall and finally projecting into the coelom (_a-e_). Here it
segments into a number of spheres ("zygotomeres" of Ross) corresponding to
the Coccidian spores, but which never acquire a proper wall (_f_). These by
segmentation produce at their surface an immense quantity of elongated
sporozoites (the "zygotoblasts" or "blasts" of Ross, Fig. 35, _g_), these
are ultimately freed by the disappearance of the cyst-wall of the oosperm,
pass through the coelom into the salivary gland (_h_), and are discharged
with its secretion into the wound that the gnat inflicts in biting. In the
blood the blasts follow the ordinary development of merozoites in the blood
corpuscle, and the patient shows the corresponding signs of fever. This has
been completely proved by rearing the insect from the egg, feeding it on
the blood of a patient in whose blood there were ascertained to be the
germs of a definite species of {106}_Haemamoeba_, sending it to England,
where it was made to bite Dr. Manson's son, who had never had fever and
whose blood on repeated examination had proved free from any germs. In the
usual time he had a well-defined attack of the fever corresponding to that
germ, and his blood on examination revealed the _Haemamoeba_ of the proper
type. A few doses of quinine relieved him of the consequences of his mild
martyrdom to science. Experiments of similar character but of less rigorous
nature had been previously made in Italy with analogous results. Again, it
has been shown that by mere precautions against the bites of _Anopheles_,
and these only, all residents who adopted them during the malarious season
in the most unhealthy districts of Italy escaped fever during a whole
season; while those who did not adopt the precautions were badly
attacked.[113]

_Anopheles_ flourishes in shallow puddles, or small vessels such as tins,
etc., the pools left by dried-up brooks and torrents, as well as larger
masses of stagnant water, canals, and slow-flowing streams. Sticklebacks
and minnows feed freely on the larvae and keep down the numbers of the
species; where the fish are not found, the larvae may be destroyed by
pouring paraffin oil on the surface of the water and by drainage. A
combination of protective measures in Freetown (Sierra Leone) and other
ports on the west coast of Africa, Ismailia, and elsewhere, has met with
remarkable success during the short time for which it has been tried; and
it seems not improbable, that as the relatively benign intermittent fevers
have within the last century been banished from our own fen and marsh
districts, so the Guinea coast may within the next decade lose its sad
title of "The White Man's Grave."

So closely allied to this group in form, habit, and life-cycle are some
species of the Flagellate genus _Trypanosoma_, that in their less active
states they have been unhesitatingly placed here (see p. 119). Schaudinn
has seen Trypanosomic characters in the "blasts" of this group, which
apparently is the most primitive of the Sporozoa and a direct offshoot of
the Flagellates.

The MYXOSPORIDIACEAE (Fig. 36) are parasitic in various {107}cold-blooded
animals. They are at least binucleate in the youngest free state, and
become large and multinucleate apocytes, which may bud off outgrowths as
well as reproduce by spores. The spores of the apocyte are not produced by
simultaneous breaking up, but by successive differentiation. A single
nucleus aggregates around itself a limited portion of the cytoplasm, and
this again forms a membrane, becoming an archespore or a "pansporoblast,"
destined to produce two spores; within this, nuclear division takes place
so as to form about eight nuclei, two of which are extruded as abortive,
and of the other six, three are used up in the formation of each of the two
spores. Of these three nuclei in each spore, two form nematocysts, like
those of a Coelenterate (p. 246 f.), at the expense of the surrounding
plasm; while the third nucleus divides to form the two final nuclei of the
reproductive body. The whole aggregate of the reproductive body and the two
nematocysts is enveloped in a bivalve shell. In what we may call
germination, the nematocysts eject a thread that serves for attachment, the
valves of the shell open, and the binucleate mass crawls out and grows
afresh. _Nosema bombycis_ Nägeli (the spore of which has a single
nematocyst) is the organism of the "Pébrine" of the silkworm, which was
estimated to have caused a total loss in France of some £40,000,000 before
Pasteur investigated the malady and prescribed the effectual cure, or
rather precaution against its spread. This consisted in crushing each
mother in water after it had laid its eggs and seeking for pébrine germs.
If the mother proved to be infected, her eggs were destroyed, as the eggs
she had laid were certain to be also tainted. Balbiani completed the study
of the organism from a morphological standpoint. Some Myxosporidiaceae
produce destructive epidemics in fish.

[Illustration: FIG. 36.—A, _Myxidium lieberkühnii_, amoeboid phase; B,
_Myxobolus mülleri_, spore with discharged nematocysts (_ntc_); C, spores
(psorosperms) of a Myxosporidian. _ntc_, nematocysts. (From Parker and
Haswell.)]

{108}The DOLICHOSPORIDIA or SARCOSPORIDIACEAE are, in the adult state,
elongated sacs, often found in the substance of the voluntary muscles, and
known as "Rainey's" or "Miescher's Tubes"; they are at first uninucleate,
then multinucleate, and then break up successively into uninucleate cells,
the spores, in each of which, by division, are formed the sickle-shaped
zoospores.[114]




{109}CHAPTER V

PROTOZOA (_CONTINUED_): FLAGELLATA

III. Flagellata.

  _Protozoa moving (and feeding in holozoic forms) by long flagella:
  pseudopodia when developed usually transitory: nucleus single or if
  multiple not biform: reproduction occurring in the active state and
  usually by longitudinal fission, sometimes alternating with
  brood-formation in the cyst or more rarely in the active state: form
  usually definite: a firm pellicle or distinct cell-wall often present._


The Flagellates thus defined correspond to Bütschli's group of the
Mastigophora. The lowest and simplest forms, often loosely called "Monads,"
are only distinguishable from Sarcodina (especially Proteomyxa) and
Sporozoa by the above characters: their artificial nature is obvious when
we remember that many of the Sarcodina have a flagellate stage, and that
the sperms of bisexual Sporozoa are flagellate (as are indeed those of all
Metazoa except Nematodes and most Crustacea). Even as thus limited the
group is of enormous extent, and passes into the Chytridieae and
Phycomycetes Zoosporeae on the one hand, and by its holophytic colonial
members into the Algae, on the other.[115]


CLASSIFICATION.


  A. Fission usually longitudinal (transverse only in a cyst), or if
  multiple, radial and complete: pellicle absent, thin, or if armour-like,
  with not more than two valves.

    I. Food taken in at any part of the body by pseudopodia

      1. PANTOSTOMATA

    _Multicilia_ Cienk.; _Mastigamoeba_ F. E. Sch. (Fig. 37, 4).

    {110}II. Food taken in at a definite point or points, or by absorption,
    or nutrition holophytic.

      1. No reticulate siliceous shell. Diameter under 500 µ (1/50").

      * Contractile vacuole simple (one or more).

        (α) Colourless: reserves usually fat: holozoic, saprophytic or
        parasitic

          2. PROTOMASTIGACEAE

        (β) Plastids yellow or brown: reserves fat or proteid: nutrition
        variable: body naked, often amoeboid in active state (_C. nudae_),
        or with a test, sometimes containing calcareous discs
        ("coccoliths,"  "rhabdoliths") of peculiar form (_C. loricatae_)

          3. CHRYSOMONADACEAE

        _Chromulina_ Cienk.; _Chrysamoeba_ Klebs; _Hydrurus_ Ag.
        _Dinobryon_ Ehrb. (Fig. 37, 11); _Syncrypta_ Ehrb. (Fig. 37, 12);
        _Zooxanthella_ Brandt; _Pontosphaera_ Lohm.; _Coccolithophora_
        Lohm.; _Rhabdosphaera_ Haeck.

        (γ) Green, (more rarely yellow or brown) or colourless: reserves
        starch: fission longitudinal

          4. CRYPTOMONADACEAE

        _Cryptomonas_ Ehrb. (Fig. 37, 9); _Paramoeba_ Greeff.

        (δ) Green (rarely colourless): fission multiple, radial

          5. VOLVOCACEAE

        ** System of contractile vacuoles complex, with accessory formative
        vacuoles or reservoir, or both.

        (ε) Pellicle delicate or absent: pseudopodia often emitted:
        excretory pore distinct from flagellar pit: reserves fat

          6. CHLOROMONADACEAE

        _Chloramoeba_ Lagerheim; _Thaumatomastix_, Lauterborn.

        (ζ) Pellicle dense, tough or hard, often wrinkled or striate:
        contractile vacuole discharging by the flagellar pit. Nutrition
        variable

          7. EUGLENACEAE

        _Euglena_ Ehrb.; _Astasia_ Duj. (Fig. 37, 3); _Anisonema_ Duj.;
        _Eutreptia_ Perty (Fig. 42, p. 124); _Trachelomonas_ Ehrb. (Fig.
        37, 1); _Cryptoglena_ Ehrb.

      2. Skeleton an open network of hollow siliceous spicules. Plastids
      yellow. Diameter under 500 µ.

        8. SILICOFLAGELLATA

      _Dictyocha_ Ehrb.

      3. Diameter over 500 µ. Mouth opening into a large reticulate
      endoplasm: flagella 1, or 2, very unequal.

        9. CYSTOFLAGELLATA

      _Noctiluca_ Suriray (Fig. 48); _Leptodiscus_ R. Hertw.

  B. Fission oblique or transverse: flagella two, dissimilar, the one
  coiled round the base of the other or in a traverse groove; pellicle
  often dense, of numerous armour-like plates

    10. DINOFLAGELLATA

  _Ceratium_ Schrank; _Gymnodinium_ Stein; _Peridinium_ Ehrb. (Fig. 46);
  _Pouchetia_ Schütt; _Pyrocystis_ Murray (Fig. 47); _Polykrikos_ Bütschli.

The Protomastigaceae and Volvocaceae are so extensive as to require further
subdivision.


PROTOMASTIGACEAE

  I. Oral spots 2. Flagella distant in pairs.      DISTOMATIDAE

  II. Oral spot 1 or 0.

  {111}A. Flagellum 1.

    (_a_) No anterior process: often parasitic

      OIKOMONADIDAE

    _Oikomonas_ K. (Figs. 37, 2, 8); _Trypanosoma_ Gruby (Fig. 39, _a-f_);
    _Treponema_ Vuill. (Fig. 39, _g-i_).

    (_b_) Anterior process unilateral or proboscidiform: cell often thecate

      BICOECIDAE

    _Bicoeca_ Clark; _Poteriodendron_ St.

    (_c_) Anterior process a funnel, surrounding the base of the flagellum:
    cells often thecate.

      (i.) Funnel free

        CRASPEDOMONADIDAE

      _Codosiga_ Clark; _Monosiga_ Cl.; _Polyoeca_ Kent; _Proterospongia_
      Kent; _Salpingoeca_ Cl.

      (ii.) Funnel not emerging from the general gelatinous investment

        PHALANSTERIDAE


  B. Flagella 2, unequal or dissimilar in function, the one sometimes short
  and thick.

    (_a_) Both flagella directed forwards

      MONADIDAE

    _Monas_ St.; _Anthophysa_ Bory (Fig. 37, 13).

    (_b_) One flagellum, usually the longer, turned backwards

      BODONIDAE

    _Bodo_ St. (Fig. 38).

  C. Flagella 2, equal and similar

    AMPHIMONADIDAE

  _Amphimonas_ Duj.; _Diplomita_ K. (Fig. 37, 10); _Rhipidodendron_ St.
  (Fig. 37, 14).

  D. Flagella 3

    TRIMASTIGIDAE

  _Dallingeria_ K. (Fig. 37, 6); _Costia_ Leclercq.

  E. Flagella 4 or more: mostly parasitic in Metazoa

    POLYMASTIGIDAE

  _Trichomonas_ Donne; _Tetramitus_ Perty (Fig. 37, 7); _Hexamitus_ Duj.;
  _Lamblia_ Blanchard.

  F. Flagella numerous, sometimes constituting a complete ciliiform
  investment, and occasionally accompanied by an undulating membrane:
  parasitic in Metazoa.

    (_a_) Flagella long: nucleus single: parasitic in insects

      TRICHONYMPHIDAE

    _Dinenympha_ Leidy; _Joenia_ Grassi; _Pyrsonympha_ Leidy;
    _Trichonympha_ Leidy; _Lophomonas_ St.; _Maupasia_ Schew.

    (_b_) Flagella short, ciliiform, uniformly distributed: nuclei very
    numerous, all similar: parasitic in Amphibia

      OPALINIDAE

    _Opalina_ Purkinje and Valentin (Fig. 41).


VOLVOCACEAE

  A. Cells usually isolated, separating after fission or brood-formation.
  Usually green (sometimes red), more rarely colourless saprophytes

    CHLAMYDOMONADIDAE

  _Chlamydomonas_ Ehrb.; _Phacotus_ Perty; _Polytoma_ Ehrb.; _Sphaerella_
  Sommerf. (Fig. 43); _Zoochlorella_.

  B. Cells multiplying in the active state by radial divisions in the same
  plane and usually incurving to form a spherical colony, united in a
  gelatinous investment, sometimes traversed by plasmic threads

    VOLVOCIDAE

  _Gonium_ O.F.M.; _Eudorina_ Ehrb.; _Pandorina_ Bory (Fig. 45);
  _Stephanosphaera_ Cohn; _Volvox_ L. (Fig. 44).

{112}[Illustration: FIG. 37.—Various forms of Flagellata. 2, 6-8, 10, 13,
14, Protomastigaceae; 11, 12, Chrysomonadaceae; 9, Cryptomonadaceae; 1, 3,
Euglenaceae; 4, Pantostomata: note branched stalk in 13; branched tubular
theca in 14; distinct thecae in 11; stalk and theca in 10. In 2, flagellate
(_a_) and amoeboid (_b_) phases are shown; in 5, flagellate (_a_) and
Heliozoan (_b_) phases[116]; in 8 are shown two stages in the ingestion of
a food particle (_f_); _chr_, plastoids; _c.vac_, contractile vacuole; _f_,
food particle; _g_, gullet; _l_, theca; _nu_, nucleus; _p_, protoplasm;
_per_, peristome; _v.i_, vacuole of ingestion. (From Parker and Haswell,
mostly from Bütschli's _Protozoa_.)]

{113}The modes of nutrition are threefold: the simplest forms live in
liquids containing decaying organic matter which they absorb through their
surface ("saprophytic"): others take in food either _Amoeba_ fashion, or
into a vacuole formed for the purpose, or into a definite mouth
("holozoic"): others again have coloured plastids, green or brown or yellow
("holophytic"), having the plant's faculty of manufacturing their own
food-supply. But we meet with species that show chromatophores at one time
and lack them at another; or, again, the same individual (_Euglena_) may
pass from holozoic life to saprophytic (_Paramoeba_, some Dinoflagellates)
as conditions alter.

Many secrete a stalk at the hinder end: by "continuous" formation of this,
without rupture at fission, a branching colony is formed (_Polyoeca_). This
stalk may have a varying consistency. In _Anthophysa_ (Fig. 37, 13) it
appears to be due to the welding of excrementitious particles voided at the
hinder end of the body with a gelatinous excretion; but the division of the
stalk is here occasional or intermittent, so that the cells are found in
tufts at the apex of the branches. A corresponding secretion, gelatinous or
chitinous, around the body of the cell forms a cup or "theca," within which
the cell lies quite free or sticking to it by its surface, or attached to
it by a rigid or contractile thread. The theca, again, may assume the form
of a mere gelatinous mass in which the cell-bodies may be completely
plunged, so that only the flagella protrude, as in Volvocidae,
_Proterospongia_ (Fig. 75, p. 182), and _Rhipidodendron_ (Fig. 37, 14).
Often this jelly assumes the form of a fan (_Phalansterium_), the branching
tubes of which it is composed lying for some way alongside, and ultimately
diverging. In _Hydrurus_, the branching jelly assumes the form of a
branching Confervoid.[117]

The cell-body may be bounded by an ill-defined plasmatic layer in
Chrysomonadaceae and some Protomastigaceae,[118] or it may form a plasmatic
membrane or "pellicle," sometimes very firm and tough, or striated as in
Euglenaceae, or it may have a separate "cuticle" (in the holophytic species
formed of cellulose), or even a bivalve or multivalve shell of distinct
plates, hinged or overlapping (_Cryptoglena_, _Phacotus_, Dinoflagellates).
The wall of the {114}Coccolithophoridae, a family of Chrysomonadaceae, is
strengthened by embedded calcareous spicules ("coccoliths," "cyatholiths,"
"rhabdoliths"), which in the most complex forms (cyatholiths) are like a
shirt-stud, traversed by a tube passing through the stem and opening at
both ends. These organisms[119] constitute a large proportion of the
plankton; the spicules isolated, or in their original state of aggregation
("coccospheres," "rhabdospheres"), enter largely into the composition of
deep-sea calcareous oozes. They occur fossil from Cambrian times (Potsdam
sandstone of Michigan and Canada), and are in some strata extremely
abundant, 800,000 occurring to the mm. cube in an Eocene marl.

The Silicoflagellates have siliceous skeletons resembling that of many
_Radiolaria_, to which they were referred until the living organism was
described (see pp. 79, 86 f.).

The flagellum has been shown by Fischer to have one of two forms: either it
is whip-like, the stick, alone visible in the fresh specimen, being seen
when stained to be continued into a long lash, hitherto invisible; or the
whole length is fringed with fine ciliiform lateral outgrowths. If single
it is almost always protruded as a tugging organ ("tractellum");[120] the
chief exceptions are the Craspedomonads, where it is posterior and acts as
a scull ("pulsellum"), and some Dinoflagellates, where it is reversible in
action or posterior. In addition to the anterior flagellum there may be one
or more posterior ones, which trail behind as sense organs, or may anchor
the cell by their tips. _Dallingeria_ has two of these, and _Bodo saltans_
a single anterior anchoring lash, by which they spring up and down against
the organic débris among which they live, and disintegrate it. The numerous
similar long flagella of the Trichonymphidae afford a transition in the
genus _Pyrsonympha_ to the short abundant cilia of _Opalina_, usually
referred to the Ciliate Infusoria.

{115}An undulating membrane occurs, sometimes passing into the flagellum in
certain genera, all parasitic, such as _Trypanosoma_ (incl.
_Herpetomonas_), _Trichomonas_, _Hexamitus_, and _Dinenympha_.

In some cases the flagellum (or flagella) is inserted into a definite pit,
which in allied forms is the mouth-opening. The contractile vacuole is
present in the fresh-water forms, but not in all the marine ones, nor in
the endoparasites. It may be single or surrounded by a ring of minute
"formative" vacuoles or discharge into a permanently visible "reservoir."
This again may discharge directly to the surface or through the pit or
canal in which the flagellum takes origin (_Euglena_).

The "chromatophore" may be a single or double plate, or multiple.[121] In
the peculiar form _Paramoeba_ the chromatophore may degenerate and be
reproduced anew. It often encloses rounded or polygonal granules of
uncoloured plasma, very refractive, known as "pyrenoids." These, like the
chromatophores, multiply by direct fission. The "reserves" may be (1)
fat-globules; (2) granules of a possibly proteid substance termed
"leucosin"; (3) a carbohydrate termed "paramylum," differing slightly from
starch (see p. 95); (4) true starch, which is usually deposited in minute
granules to form an investment for the pyrenoid when such is present.

A strongly staining granule is usually present in the plasma near the base
of the flagellum. This we may term a "blepharoplast" or a "centrosome" in
the wider sense.

FISSION is usually longitudinal in the active state; a few exceptions are
recorded. Encystment is not uncommon; and in the coloured forms the
cyst-wall is of cellulose. Division in the cyst is usually multiple;[122]
in the coloured forms, however, vegetative growth often alternates with
division, giving rise to plant-like bodies. _Polytoma_ and other
Chlamydomonadidae multiply by "brood-formation" in the active state; the
blepharoplast, as Dangeard suggests, persisting to continue the motion of
the flagella of the parent, while the rest of the plasm divides to form the
brood. CONJUGATION has been observed in many species. In some species of
_Chlamydomonas_ it takes place after one or both of the two {116}cells have
come to rest, but in most cases it occurs between active cells. We find
every transition between equal unions and differentiated sexual unions, as
we shall see in discussing the Volvocaceae.[123] The "coupled-cell" differs
in behaviour in the different groups, but almost always goes to rest and
encysts at once, whatever it may do afterwards.

The LIFE-HISTORY of many Flagellates has been successfully studied by
various observers, and has shed a flood of light on many of the processes
of living beings that were hitherto obscure. The first studies were carried
through by the patient labours of Drysdale and Dallinger. A delicate
mechanical stage enabled the observer to keep in the field of view a single
Flagellate, and, when it divided into two, to follow up one of the
products. A binocular eye-piece saved much fatigue, and enabled the
observers to exchange places without losing sight of the special Flagellate
under observation; for the one who came to relieve would put one eye to the
instrument and recognise the individual Flagellate under view as he passed
his hand round to the mechanism of the stage before the first watcher
finally relinquished his place at the end of the spell of work.
Spoon-feeding by Mrs. Dallinger enabled such shifts to be prolonged, the
longest being one of nine hours by Dr. Dallinger.

{117}[Illustration: FIG. 38.—_Bodo saltans._ A, the positions assumed in
the springing movements of the anchored form; B, longitudinal fission of
anchored forms; C, transverse fission of the same; D, fission of
free-swimming form; E^1-E^4, conjugation of free-swimming with anchored
form; E^5, zygote; E^6, emission of spores from zygote; F, growth of
spores: _c.vac_, contractile vacuole; _fl.1_, anterior; _fl.2_, ventral
flagellum; _nu_, nucleus. (From Parker's _Biology_, after Dallinger.)]

The life-cycles varied considerably in length. It was in every case found
that after a series of fissions the species ultimately underwent
conjugation (more or less unequal or bisexual in character);[124] the
zygote encysted; and within the cyst the protoplasmic body underwent
brood-formation, the outcome of which was a mass of {118}spores discharged
by the rupture of the cyst (Fig. 38). These spores grow from a size too
minute for resolution by our microscopes into the ordinary flagellate form.
They withstand the effects of drying, if this be effected immediately on
their escape from the ruptured cyst; so that it is probable that each spore
has itself a delicate cyst-wall and an aplanospore, from which a single
zoospore escapes. The complex cycle, of course, comprises the whole course
from spore-formation to spore-formation. Such complete and regular
"life-histories," each characteristic of the species, were the final
argument against those who held to the belief that spontaneous generation
of living beings took place in infusions of decomposing organic matter.

Previous to the work of these observers it had been almost universally
believed that the temperature of boiling water was adequate to kill all
living germs, and that any life that appeared in a closed vessel after
boiling must be due to spontaneous change in its contents. But they now
showed that, while none of the species studied resisted exposure _in the
active condition_ to a temperature of 138°-140° F., the spores only
succumbed, in liquid, to temperatures that might even reach 268° F., or
when dry, even 300° F. or more. Such facts explain the constant occurrence
of one or more such minute species in liquids putrefying under ordinary
conditions, the spores doubtless being present in the dust of the air. Very
often several species may co-exist in one infusion; but they separate
themselves into different zones, according to their respective need for
air, when a drop of the liquid is placed on the slide and covered for
examination. Dallinger[125] has made a series of experiments on the
resistance of these organisms in their successive cycles to a gradual rise
of temperature. Starting with a liquid containing three distinct species,
which grew and multiplied normally at 60° F., he placed it under conditions
in which he could slowly raise the temperature. While all the original
inmates would have perished at 142° F., he succeeded in finally producing
races that throve at 158° F., a scalding heat, when an accident put an end
to that series of experiments. In no instance was the temperature raised so
much as to kill off the beings, so that the increased tolerance of their
descendants was due not, as might have been anticipated, to selection of
those that best resisted, but to the inheritance of {119}an increased
toleration and resistance from one generation or cycle to another.

As we noted above (p. 40), the study of the Flagellates has been largely in
the hands of botanists. After the work of Bütschli in Bronn's
_Thier-Reich_, Klebs[126] took up their study; and the principal monographs
during the last decade have appeared in Engler and Prantl's
_Pflanzenfamilien_, where Senn[127] treats the Flagellates generally,
Wille[128] the Volvocaceae, and Schütt the "Peridiniales" or
Dinoflagellata;[129] while only the Cystoflagellata, with but two genera,
have been left to the undisputed sway of the zoologists.[130]

Among this group the majority are saprophytes, found in water containing
putrefying matter or bacteria. The forms so carefully studied by Dallinger
and Drysdale belong to the genera _Bodo_, _Cercomonas_, _Tetramitus_,
_Monas_, and _Dallingeria_. Many others are parasites in the blood or
internal cavities of higher animals, some apparently harmless, such as
_Trichomonas vaginalis_, parasitic in man, others of singular malignity.
_Costia necatrix_, infesting the epithelial scales of fresh-water fish,
often devastates hatcheries. The genus _Trypanosoma_, Gruby, contributes a
number of parasites, giving rise to deadly disease in man and beast.[131]
_T. lewisii_ is common in Rodents, but is relatively harmless. _T. evansii_
is the cause of the Surra disease of Ruminants in India, and is apparently
communicated by the bites of "large brown flies" (almost certainly Breeze
Flies or Tabanidae, Vol. VI. p. 481). _T. brucei_, transferred to cattle by
the Tsetse Fly, _Glossina morsitans_ (see Vol. VI. Fig. 244, p. 513) in
Equatorial Africa, is the cause of the deadly Nagana disease, which renders
whole tracts of country impassable to ox or horse. Other Trypanosomic
diseases of animals are, in Algeria and the Punjab, "dourine," infecting
horses and dogs; in South America, Mal de Caderas (falling-sickness), an
epidemic paralysis of cattle. During the printing of this book, much
additional knowledge has been gained on this genus and the diseases it
engenders. The Trypanosomic {120}fever recently recognised on the West
Coast has been found to be the early stage of the sleeping-sickness, that
well-known and most deadly epidemic of Tropical Africa. Through the
researches of Castellani, Nabarro, and especially Colonel and Mrs. Bruce,
we know now that the parasite _T. gambiense_ is transferred by an
intermediate host, a kind of Tsetse Fly (_Glossina palpalis_). Schaudinn's
full study of a parasite of the blood corpuscles of the Owl has shown that
while in its intracorpuscular state it resembles closely the malarial
parasites in behaviour, and in its schizogenic multiplication, so that it
was considered an Acystosporidian, under the name of _Halteridium_, it is
really a _Trypanosoma_;[132] for the accomplishment of successful sexual
reproduction it requires transference to the gut of a gnat (_Culex_). The
germs may infect the ovary, and give the offspring of the insect the innate
power of infecting Owls. Thus a new light is shed on the origin of the
Coccidiaceae, whose "blasts" in the insect host resemble _Trypanosoma_ in
their morphology.

[Illustration: FIG. 39.—Morphology of _Trypanosoma_. _a-f_, Stages in
development of _Trypanosoma noctuae_ from the active zygote ("ookinete");
_b_, first division of nucleus into larger (trophic) and smaller (kineto-)
nucleus; _c_, _d_, division of smaller nucleus and its transformations to
form "blepharoplast" and myonemes; _f_, adult _Trypanosoma_; _g_, _h_, _i_,
_Treponema zeemannii_ of Owl; _g_, Trypanosome form; _h_, _Spirochaeta_
form; _i_, rosette aggregate. (After Schaudinn.)]

The human Tick fever of the Western United States and the epizootic Texas
fever are known to be due to blood parasites of the genus _Piroplasma_
(_Babesia_), of which the free state is that of a Trypanosome. It appears
certain that Texas fever, though due to Tick bites, is not transferred
directly from one beast to another by the same Tick; but the offspring of a
female Tick that has sucked an infected ox contains Trypanosome germs, and
will by their bites infect other animals. {121}It would seem probable that
the virulence of the Persian Tick (_Argas persica_) is due to similar
causes. The Indian maladies known as "Kala Azar" and "Oriental Sore" are
characterised by blood parasites, at first called after their discoverer
the "Leishman bodies," which have proved to be the effects of a
_Piroplasma_.

_Trypanosoma_ is distinguished by the expansion of its flagellum into an
undulating membrane, that runs down the edge of the body, and may project
behind as a second lash. In this membrane run eight fine muscular
filaments, or myonemes, four on either surface, within the undulating
membrane; at their lower end they are all connected with a rounded body,
the "blepharoplast," which is here in its origin, as well as in its
behaviour in reproductive processes, a true modified nucleus, comparable in
some respects, as was first noted by Plimmer and Rose Bradford,[133] with
the micronucleus of the Infusoria. Part of the segmentation spindle
persists in the form of a filament uniting the blepharoplast with the large
true functional nucleus (Fig. 39, _a-f_).

The blood of patients suffering from relapsing fever contains a fine
wriggling parasite, which was described as a Schizomycete, allied to the
bacteria, and hitherto termed _Spirochaeta obermeieri_. Schaudinn has shown
that this and other similar blood parasites are closely allied to
_Trypanosoma_; and since the original genus was founded on organisms of
putrefaction which are undoubtedly Schizomycetes, Vuillemin has suggested
the name _Treponema_. _T. pallidum_ is found in syphilitic patients, and
appears to be responsible for their illness.[134]

The Craspedomonadidae (often called Choanoflagellates, Fig. 40) are a group
whose true nature was elucidated some forty years ago by the American
zoologist, H. James-Clark. They are attached either to a substratum, by a
stalk produced by the base of the cell, or to other members of the same
colony; they are distinguished by the protrusion of the cytoplasm around
the base of the single flagellum into a pellucid funnel,[135] in which the
plasma is in constant motion, though the funnel retains its shape and size,
except when, as sometimes happens, it is retracted.

{122}[Illustration: FIG. 40.—Various forms of Craspedomonadidae. 2, _a_,
Adult cell; 2, _b_, longitudinal fission; 2, _c_, the production of
flagellulae by brood-formation; _c_, collar; _c.vac_, contractile vacuole;
_fl_, flagellum; _l_, theca; _nu_, nucleus; _s_, stalk. (After Saville
Kent.)]


The agitation of the flagellum determines a stream of water upwards along
the outer walls of the funnel; and the food-particles brought along adhere
to the outside of the funnel, and are carried by its streaming movement to
the basal constriction, where they are swallowed by the plasma, which
appears to form a swallowing vacuole at that point. Longitudinal fission is
the ordinary mode of reproduction, extending up through the funnel. If the
two so formed continue to produce a stalk, the result is the formation of a
tree-like stem, whose twigs bear at the ends the funnelled cells, or
"collar-cells" as they are usually called. In _Salpingoeca_, as in so many
other Flagellates, each cell forms a cup or theca, often of most graceful
vase-like outline, the rim being elegantly turned back. _Proterospongia_
(Fig. 75, p. 182) secretes a gelatinous investment for the colony, which is
attached to solid bodies. In this species, according to Saville Kent, the
central members of the colony retract their collar, lose their flagellum,
become amoeboid, and finally undergo brood-formation to produce minute
zoospores. This is the form which by its differentiation recalls the
Sponges, and has been regarded as a {123}transition towards them; for the
flagellate, nutritive cells of the Sponges are provided with a collar,
which exists in no other group of Metazoa (see pp. 171, 181, and Fig. 70,
p. 176). The most recent monographer of the family is Raoul Francé, but
James-Clark and Saville Kent did the pioneering work.

[Illustration: FIG. 41.—_Opalina ranarum._ A, living specimen; B, stained
specimen showing nuclei; C, stages in nuclear division; D-F, stages in
fission; G, final product of fission; H, encysted form; I, young form
liberated from cyst; K, the same after multiplication of the nucleus has
begun. _nu_, Nucleus. (From Parker's _Biology_, after Saville Kent and
Zeller.)]

Of the life-history of the Trichonymphidae,[136] all of which are parasitic
in the alimentary canal of Insects, especially Termites or White Ants (Vol.
V. p. 356), nothing is known. Some of them have a complete investment of
motile flagella, like enormously long cilia, which in _Dinenympha_ appear
to coalesce into four longitudinal undulating membranes. _Lophomonas_
inhabits the gut of the Cockroach and Mole-cricket. The Opalinidae have
also a complete investment of cilia, which are short, and give the aspect
of a Ciliate to the animal, which is common in the rectum of Amphibia, and
dies when transferred to water. But despite the outward resemblance, the
nuclei, of which there may be as many as 200, are all similar, and
consequently this group cannot be placed among the Infusoria at all.
_Opalina_ has no mouth nor contractile vacuole. It multiplies by dividing
{124}irregularly and at intervals, resolving finally into 1-nucleate
fragments, which encyst and pass into the water. When swallowed the cyst
dissolves, its contents enlarge, and ultimately assume the adult form.[137]

_Maupasia_ has a partial investment of cilia, a single long flagellum and
mouth, a contractile vesicle, and a single simple nucleus. It seems to find
an appropriate place near the two above groups, though it is free, and
possesses a mouth.

[Illustration: FIG. 42.—Longitudinal Fission of _Eutreptia viridis_
(Euglenaceae), showing chloroplasts, nucleus, and flagella arising from
pharynx-tube. (After Steuer.)]

Among the Euglenaceae, _Euglena viridis_ is a very common form, giving the
green colour to stagnant or slow-flowing ditches and puddles in light
places, especially when contaminated by a fair amount of dung, as by the
overflow of a pig-sty, in company with a few hardy Rotifers, such as
_Hydatina senta_ (Vol. II. Fig. 106, p. 199) and _Brachionus_. _Euglena_ is
about 0.1 mm. in length when fully extended, oval, pointed behind,
obliquely truncate in front, with a flagellum arising from the pharyngeal
pit. It shows a peculiar wriggling motion, waves of transverse constriction
passing along the body from end to end, as well as flexures in different
meridians. Such motions are termed "euglenoid." The front part is
colourless, but under a low {125}power the rest of the cell is green, owing
to the numerous chlorophyll bodies or chloroplasts. The outermost layer of
the cytoplasm shows a somewhat spiral longitudinal striation, possibly due
to muscular fibrils. The interior contains many laminated plates of
paramylum, and a large single nucleus. At the front of the body at the base
of the flagellum is a red "eye-spot" on the dorsal side of the pharynx-tube
or pit, from which the flagellum protrudes. Wager has shown that this tube
receives, also on its dorsal side, the opening of a large vacuole,
sometimes called the reservoir, for into it discharges the contractile
vacuole (or vacuoles). The eye-spot is composed of numerous granules,
containing the vegetal colouring matter "haematochrome." It embraces the
lower or posterior side of the communication between the tube and the
reservoir. The flagellum has been traced by Wager through the tube into the
reservoir, branching into two roots where it enters the aperture of
communication, and these are inserted on the wall of the reservoir at the
side opposite the eye-spot. But on one of the roots near the bifurcation is
a dilatation which lies close against the eye-spot, so that it can receive
the light reaction. _Euglena_ is an extremely phototactic organism. It
shows various wrigglings along the longitudinal axis, and transverse waves
of contraction and expansion may pass from pole to pole.[138]

Among the Chrysomonadaceae the genus _Zooxanthella_, Brandt, has already
been described under the Radiolaria (p. 86), in the jelly of which it is
symbiotic. It also occurs in similar union in the marine Ciliates,
_Vorticella sertulariae_ and _Scyphidia scorpaenae_, and in _Millepora_ (p.
261) and many Anthozoa (pp. 373 f., 396).

Of the Chlamydomonadidae, _Sphaerella_ (_Haematococcus_, Ag.) _pluvialis_
(Fig. 43), and _S. nivalis_, in which the green is masked by red pigment,
give rise to the phenomena of "red snow" and "bloody rain." The type genus,
_Chlamydomonas_, is remarkable for the variations from species to species
in the character and behaviour of the gametes. Sometimes they are equal, at
other times of two sizes. In some species they fuse immediately on
approximation, in the naked active state; in others, they encyst on
approaching, and unite by the emission of a fertilising tube, {126}as in
the Algal Conjugatae. _Zoochlorella_ is symbiotic in green Ciliata (pp. 153
f., 158), Sponges (p. 175), _Hydra_ (p. 256), and Turbellaria (Vol. II. p.
43).

[Illustration: FIG. 43.—_Sphaerella pluvialis._ A, motile stage; B, resting
stage; C, D, two modes of fission; E, _Sphaerella lacustris_, motile stage.
_chr_, Chromatophores; _c.vac_, contractile vacuole; _c.w_, cell-wall;
_fl_, flagella; _nu_, nucleus; _nu'_, nucleolus; _pyr_, pyrenoids. (From
Parker's _Biology_.)]

Of the Volvocidae, _Volvox_ (Fig. 44) is the largest and most conspicuous
genus. Its colony forms a globe the size of a pin's head, floating on the
surface of ponds, drains, or even puddles or water-barrels freely open to
the light. It has what may be called a skeleton of gelatinous matter,[139]
condensed towards the surface into a denser layer in which the minute cells
are scattered. These have each an eye-spot, a contractile vacuole, and two
flagella, by the combined action of which the colony is propelled. Delicate
boundary lines in the colonial wall mark out the proper investment of each
cell. The cells give off delicate plasmic threads which meet those of their
neighbours, and form a bond between them. In that half of the hemisphere
which is posterior in swimming, a few (five to eight) larger cells
("macrogonidia" of older writers) are evenly distributed, protruding as
they increase in size into the central jelly. These as they grow segment to
form a new colony.

{127}[Illustration: FIG. 44.—_Volvox globator._ A, entire colony, enclosing
several daughter-colonies; B, the same during sexual maturity; C, four
zooids in optical section; D^1-D^5, development of parthenogonidium; E,
ripe spermogonium; F, sperm; G, ovum; H, oosperm. _a_, Parthenogonidia;
_fl_, flagellum; _ov_, ovum; _ovy_, ovaries; _pg_, pigment spot; _sp_,
sperms; _Spy_, spermogonia dividing to form sperms. (From Parker's
_Biology_, after Cohn and Kirchner.)]

The divisions are only in two planes at right angles, so that the young
colony is at first a plate, but as the cells multiply the plate bends up
(as in the gastrulation of the double cellular plate of the Nematode
_Cucullanus_, Vol. II. p. 136), and finally forms a hollow sphere bounded
by a single layer of cells: the site of the original orifice may be traced
even in the adult as a blank space larger than exists elsewhere. Among the
cells of the young colony some cease to divide, but continue to grow at an
early period, and these are destined to become in turn the mothers
("parthenogonidia") of a new colony; they begin segmenting before the
colony of which they are cells is freed. The young colonies are ultimately
liberated by the rupture of the sphere as small-sized spheres, which
henceforth only grow by enlargement of the sphere as a whole, and the wider
separation of the vegetative cells. Thus the vegetative cells soon cease to
grow; all the supply of food material due to their living activities goes
to the nourishment of the parthenogonidia, or the young colonies, as
{128}the case may be. These vegetative cells have therefore surrendered the
power of fission elsewhere inherent in the Protist cell. Moreover, when the
sphere ruptures for the liberation of the young colonies, it sinks and is
doomed to death, whether because its light-loving cells are submerged in
the ooze of the bottom, or because they have no further capacity for life.
When conjugation is about to take place, it is the cells that otherwise
would be parthenogonidia that either act as oospheres or divide as
"spermogonia" to form a flat brood of minute yellow male cells ("sperms").
These resemble vegetative cells, in the possession of an eye-spot and two
contractile vacuoles, but differ in the enormously enlarged nucleus which
determines a beaked process in front. After one of these has fused with the
female cell ("oosphere") the product ("oosperm") encysts, passes into a
stage of profound rest, and finally gives rise to a new colony. The
oospheres and sperm-broods may arise in the same colony or in distinct
ones, according to the species.

Before we consider the bearings of the syngamic processes of _Volvox_, we
will study those presented by its nearer allies, which have the same
habitat, but are much more minute. Three of these are well known,
_Stephanosphaera_, _Pandorina_, and _Eudorina_, all of which have spherical
colonies of from eight to thirty-two cells embedded at the surface of a
sphere, and no differentiation into vegetative cells and parthenogonidia
(or reproductive cells).

_Stephanosphaera_ has its eight cells spindle shaped, and lying along
equidistant meridians of its sphere; in vegetative reproduction each of
these breaks up in its place to form a young colony, and the eight
daughter-colonies are then freed. In conjugation, each cell of the colony
breaks up into broods of 4, 8, 16, or 32 small gametes, which swim about
within the general envelope, and pair and fuse two and two: this is
"isogamous," "endogamous" conjugation. In _Pandorina_ (Fig. 45) the cells
are rounded, and are from 16 to 32 in each colony. The vegetative
reproduction in this, as in _Eudorina_, is essentially the same as in
_Stephanosphaera_. In conjugation the cells are set free, and are of three
sizes in different colonies, small (S), medium (M), and large (L). The
following fusions may occur: S × S, S × M, S × L, M × M, M × L. Thus the
large are always female, as it were, the medium may play the part of male
to the large, female to the small; the small are males to the medium and to
the large. The medium {129}and small are capable, each with its like, of
equal, undifferentiated conjugation; so that we have a differentiation of
sex far other than that of ordinary, binary sex. _Eudorina_, however, has
attained to "binary sex," for the female cells are the ordinary vegetative
cells, at most a little enlarged, and the male cells are formed by ordinary
cells producing a large flat colony of sixty-four minute males or sperms.
In some cases four cells at the apex of a colony are spermogonia, producing
each a brood of sperms, while the rest are the oospheres. The transition to
_Volvox_ must have arisen through the sterilisation of the majority of
cells of a colony for the better nutrition of the few that are destined
alone for reproduction.

[Illustration: FIG. 45.—_Pandorina morum._ A, entire colony; B, asexual
reproduction, each zooid dividing into a daughter-colony; C, liberation of
gametes; D-F, three stages in conjugation of gametes; G, zygote; H-K,
development of zygote into a new colony. (From Parker's _Biology_, after
Goebel.)]

_Volvox_, as we have seen, has attained a specialisation entirely
comparable to that of a Metazoon, where the segmentation of the fertilised
ovum results in two classes of cells: those destined {130}to form tissues,
and condemned to ultimate death with the body as a whole, and those that
ultimately give rise to the reproductive cells, ova, and sperms. But this
is a mere parallelism, not indicating any sort of relationship: the
oospores of the Volvocaceae show that tendency to an encysted state, in
which fission takes place, that is so characteristic of Algae, and these
again show the way to Cryptogams of a higher status. Thus, _Volvox_,
despite the fact that in its free life and cellular differentiation it is
the most animal of all known Flagellates, is yet, with the rest of the
Volvocaceae, inseparable from the Vegetable Kingdom, and is placed here
only because of the impossibility of cleaving the Flagellates into two.

The Dinoflagellata (Figs. 46, 47) are often of exceptionally large
dimensions in this class, attaining a maximum diameter of 150 µ (1/160")
and even 375 µ (1/67") in _Pyrocystis noctiluca_. The special character of
the group is the presence of two flagella; the one, filiform, arises in a
longitudinal groove, and extending its whole length projects behind the
animal, and is the conspicuous organ of motion: the other, band-like,
arises also in the longitudinal groove, but extends along a somewhat spiral
transverse groove,[140] and never protrudes from it in life, executing
undulating movements that simulate those of a girdle of cilia, or a
continuous undulating membrane (Fig. 46). This appearance led to the old
name "Cilioflagellata," which had of course to be abandoned when Klebs
discovered the true structure.[141] There is a distinct cellulose membrane,
sometimes silicified, to the ectoplasm, only interrupted by a bare space in
the longitudinal groove, whence the flagella take origin. This cuticle is
usually hard, sculptured, and divided into plates of definite form,
bevelled and overlapping at their junction; occasionally the cell has been
seen to moult them.

A large vacuolar space, traversed by plasmic strings, separates the
peripheral cytoplasm from the central, within which is the large nucleus.
There are in most species one or more chromatophores, coloured by a
yellowish or brownish pigment, which is a mixture of lipochromes, distinct
from diatomin. In a few species the presence of these is not constant, and
these species {131}show variability as to their nutrition, which is
sometimes holozoic. Under these conditions the cell can take in
food-particles as bulky as the eggs of Rotifers and Copepods, by the
protrusion of a pseudopod at the junction of the two grooves. As in most
coloured forms an eye-spot is often present, a cup-shaped aggregation of
pigment, with a lenticular refractive body in its hollow. A contractile
vacuole, here termed a "pusule," occurs in many species, communicating with
the longitudinal groove by a canal. Nematocysts (see p. 246 f.) are present
in _Polykrikos_, trichocysts (see p. 142) in several genera.

[Illustration: FIG. 46.—_Peridinium divergens._ _a_, Flagellum of
longitudinal groove; _b_, flagellum of transverse groove; _cr.v_,
contractile vacuole surrounded by formative vacuoles; _n_, nucleus. (After
Schütt.)]

Division is usually oblique, dividing the body into two dissimilar halves,
each of which has to undergo a peculiar growth to reconstitute the missing
portion, and complete the shell. The incomplete separation of the young
cells leads to the formation of chains, notably in _Ceratium_ and
_Polykrikos_, the latter dividing transversely and occurring in chains of
as many as eight. The process of division may take place when the cell is
active, or in a cyst, as in _Pyrocystis_ (Fig. 47). Again, encystment may
precede multiple fission, resulting in the formation of a brood of minute
swarmers. It has been suggested that these are capable of playing the part
of gametes, and conjugating in pairs.[142]

The Dinoflagellates are for the most part pelagic in habit, floating at the
surface, and when abundant tinge the water of fresh-water lakes or even
ponds red or brown. _Peridinium_ (Fig. 46) and _Ceratium_ (the latter
remarkable for the horn-like backward prolongations of the lower end) are
common genera both in the sea {132}and fresh-waters. _Gymnodinium
pulvisculus_ is sometimes parasitic in _Appendicularia_ (Vol. VII. p. 68).
_Polykrikos_[143] has four transverse grooves, each with its flagellum,
besides the terminal one. Many of the marine species are phosphorescent,
and play a large part in the luminosity of the sea, and some give it a red
colour.

Several fossil forms have been described. _Peridinium_ is certainly found
fossil in the firestone of Delitzet, belonging to the Cretaceous. A full
monograph of the group under the name "Peridiniales" was published by
Schütt.[144]

[Illustration: FIG. 47.—_Pyrocystis fusiformis_, Murray. × 100. From the
surface in the Guinea Current. (From Wyville Thomson.)]

The Cystoflagellates contain only two genera,[145] _Noctiluca_, common at
the surface of tranquil seas, to which, as its name implies, it gives
phosphorescence, and _Leptodiscus_, found by R. Hertwig in the
Mediterranean. _Noctiluca_ is enormous for a Flagellate, for with the form
of a miniature melon it measures about 1 mm. (1/25") or more in diameter.
In the depression is the "oral cleft," from one end of which rises, by a
broad base, a large coarse flagellum, as long as the body or longer and
transversely striated. In front of the base of the flagellum are two
lip-like {133}prominences, of which one, a little firmer than the other,
and transversely ridged, is called the tooth; at the junction of the two is
a second, minute, flagellum, usually called the cilium. Behind these the
oral groove has an oval space, the proper mouth; behind this, again, the
oral groove is continued for some way, with a distinct rod-like ridge in
its furrow. The whole body, including the big flagellum, is coated by a
strong cuticular pellicle, except at the oblong mouth, and the lips and rod
are mere thickenings of this. The cytoplasm has a reticulate arrangement:
the mouth opens into a central aggregate, from which strands diverge
branching as they recede to the periphery, where they pass into a
continuous lining for the cuticular wall, liquid filling the interspaces.
The whole arrangement is not unlike that found in many plant-cells, but the
only other Protists in which it occurs are the Ciliata _Trachelius_ (Fig.
56, p. 153) and _Loxodes_. The central mass contains the large nucleus.
_Noctiluca_ is an animal feeder, and expels its excreta through the mouth.
The large flagellum is remarkable for the transverse striation of its
plasma, especially on the ventral side. The cuticle may be moulted as in
the Dinoflagellates. As a prelude to fission the external differentiations
disappear, the nucleus divides in the plane of the oral groove, and a
meridional constriction parts the two halves, the new external organs being
regenerated. Conjugation occurs also, the two organisms fusing by their
oral region; the locomotive organs and pharynx disappear; the conjoined
cytoplasms unite to form a sphere, and the nuclei fuse to form a zygote or
fertilisation nucleus. This conjugation is followed by sporulation or
brood-formation.[146]

[Illustration: FIG. 48.—_Noctiluca miliaris_, a marine Cystoflagellate.
(From Verworn.)]

{134}The nucleus passes towards the surface, undergoes successive fissions,
and as division goes on the numerous daughter-nuclei occupy little
prominences formed by the upgrowth of the cytoplasm of the upper pole. The
rest of the cytoplasm atrophies, and the hillocks formed by the plasmic
outgrowths around the final daughter-nuclei become separate as so many
zoospores (usually 256 or 512); each of these is oblong with a dorsal
cap-like swelling, from the edge of which arises a flagellum pointing
backwards; parallel to this the cap is prolonged on one side into a style
also extending beyond the opposite pole of the animal.[147] In this state
the zoospore is, to all outward view, a naked Dinoflagellate, whence it
seems that the Cystoflagellates are to be regarded as closely allied to
that group. _Leptodiscus_ is concavo-convex, circular, with the mouth
central on the convex face, 1-flagellate, and attains the enormous size of
1.5 mm. (1/16") in diameter.

The remarkable phosphorescence of _Noctiluca_ is not constant. It glows
with a bluish or greenish light on any agitation, but rarely when
undisturbed. A persistent stimulus causes a continuous, but weak, light.
This light is so weak that several teaspoonsful of the organism, collected
on a filter and spread out, barely enable one to read the figures on a
watch a foot away. As in other marine phosphorescence, no rise of
temperature can be detected. The luminosity resides in minute points,
mostly crowded in the central mass, but scattered all through the
cytoplasm. A slight irritation only produces luminosity at the point
touched, a strong one causes the whole to flash. Any form of irritation,
whether of heat, touch, or agitation, electricity or magnetism, is stated
to induce the glow. By day, it is said, _Noctiluca_, when present in
abundance, may give the sea the appearance of tomato soup.

The earliest account of Noctiluca will be read with interest. Henry Baker
writes in _Employment for the Microscope_:[148]—"A curious Enquirer into
Nature, dwelling at Wells upon the Coast of Norfolk, affirms from his own
Observations that the Sparkling of Sea Water is occasioned by Insects. His
Answer to a Letter wrote to him on that Subject runs thus, 'In the Glass of
Sea Water I send with this are some of the Animalcules which cause the
Sparkling Light in Sea Water; they may be seen by holding {135}the Phial up
against the Light, resembling very small Bladders or Air Bubbles, and are
in all Places of it from Top to Bottom, but mostly towards the Top, where
they assemble when the Water has stood still some Time, unless they have
been killed by keeping them too long in the Phial. Placing one of these
Animalcules before a good Microscope, an exceeding minute Worm may be
discovered, hanging with its Tail fixed to an opake Spot in a Kind of
Bladder, which it has certainly a Power of contracting or distending, and
thereby of being suspended at the Surface, or at any Depth it pleases in
the including Water.'"

"The above-mentioned Phial of Sea Water came safe, and some of the
Animalcules were discovered in it, but they did not emit any Light, as my
Friend says they do, upon the least Motion of the Phial when the Water is
newly taken up. He likewise adds, that at certain Times, if a Stone be
thrown into the Sea, near the Shore, the Water will become luminous as far
as the Motion reacheth: this chiefly happens when the Sea hath been greatly
agitated, or after a Storm." Obviously what Mr. Sparshall, Baker's
correspondent, took for a worm was the large flagellum.

The chief investigators of this group have been Huxley, Cienkowski, Allman,
Bütschli, and G. Pouchet, while Ischikawa and Doflein have elucidated the
conjugation.




{136}CHAPTER VI

PROTOZOA (_CONTINUED_): INFUSORIA (CILIATA AND SUCTORIA)

IV. Infusoria.

  _Complex Protozoa, never holophytic save by symbiosis with plant
  commensals, never amoeboid, with at some period numerous short cilia, of
  definite outline, with a double nuclear apparatus consisting of a large
  meganucleus and a small micronucleus (or several),[149] the latter alone
  taking part in conjugation (karyogamy), and giving rise after conjugation
  to the new nuclear apparatus._


The name Infusoria was formerly applied to the majority of the Protozoa,
and included even the Rotifers. For the word signifies organisms found in
"infusions" of organic materials, including macerations. Such were made
with the most varied ingredients, pepper and hay being perhaps the
favourites. They were left for varying periods exposed to the air, to allow
the organisms to develop therein, and were then examined under the
microscope.[150] With the progress of our knowledge, group after group was
split off from the old assemblage until only the ciliate or flagellate
forms were left. The recognition of the claims of the Flagellates to
independent treatment left the group more natural;[151] while it was
enlarged by the admission of the Acinetans (_Suctoria_), which had for some
time been regarded as a division of the Rhizopoda.


{137}I. CILIATA

_Infusoria, with a mouth, and cilia by which they move and feed; usually
with undulating membranes, membranellae, cirrhi, or some of these._ Genera
about 144: 27 exclusively marine, 50 common to both sea and fresh water, 27
parasitic on or in Metazoa, the rest fresh water. Species about 500.

We divide the Ciliata thus:[152]—

  (I.) Mouth habitually closed, opening by retraction of its circular or
  slit-like margin; cilia uniform

    Order 1. GYMNOSTOMACEAE.

  _Lacrymaria_, Ehrb.; _Loxodes_, Ehrb.; _Loxophyllum_, Duj.; _Lionotus_,
  Wrez.; _Trachelius_, Schrank; _Amphileptus_, Ehrb.; _Actinobolus_, St.;
  _Didinium_, St.; _Scaphiodon_, St; _Dysteria_, Huxl.; _Coleps_, Nitzsch.;
  _Dileptus_, Duj.; _Ileonema_, Stokes; _Mesodinium_, St.

  (II.) Mouth permanently open, usually equipped with one or more
  undulating membranes, receiving food by ciliary action (TRICHOSTOMATA,
  Bütschli)

    (_a_) Cilia nearly uniform, usually extending over the whole body,
    without any special adoral wreath of long cilia or membranellae; mouth
    with one or two undulating membranes at its margin or extending into
    the short pharynx.

      Order 2. ASPIROTRICHACEAE.

    _Paramecium_, Hill; _Colpoda_, O. F. Müll.; _Colpidium_, St.;
    _Leucophrys_, Ehrb.; _Cyclidium_, Cl. and L.; _Lembadion_, Perty;
    _Cinetochilum_, Perty; _Pleuronema_, Duj.; _Ancistrum_, Maup.;
    _Glaucoma_, Ehrb.; _Uronema_, Duj.; _Lembus_, Cohn; _Urocentrum_,
    Nitzsch; _Icthyophtheirius_, Fouquet.

    (_b_) Strong cilia or membranellae forming an adoral wreath, and
    bounding a more or less enclosed area, the "peristome," at one point of
    which the mouth lies.

      (i.) Body more or less equally covered with fine cilia; adoral wreath
      an open spiral

        Order 3. HETEROTRICHACEAE

      _Spirostomum_, Ehrb.; _Bursaria_, O. F. Müll.; _Stentor_, Oken;
      _Folliculina_, Lamk.; _Conchophtheirus_, St.; _Balantidium_, Cl. and
      L.; _Nyctotherus_, Leidy; _Metopus_, Cl. and L.; _Caenomorpha_,
      Perty; _Discomorpha_, Levander; _Blepharisma_, Perty.

      (ii.) Body cilia limited in distribution or absent; peristome
      anterior, nearly circular, sinistrorse.

        Order 4. OLIGOTRICHACEAE.

      _Halteria_, Duj.; _Maryna_, Gruber; _Tintinnus_, Schrank;
      _Dictyocystis_, Ehrb.; _Strombidium_, Cl. and L. (= _Torquatella_,
      Lank.).

      (iii.) Peristome extending backwards along the ventral face, which
      alone is provided with motile cirrhi, etc.; dorsal cilia fine,
      motionless.

        Order 5. HYPOTRICHACEAE.

      {138}_Stylonychia_, Ehrb.; _Kerona_, O. F. Müll.; _Oxytricha_, Ehrb.;
      _Euplotes_, Ehrb.; _Stichotricha_, Perty; _Schizotricha_, Gruber.

      (iv.) Body cilia reduced to a posterior girdle, or temporarily or
      permanently absent; peristome anterior, nearly circular, edged by the
      adoral wreath,[153] bounded by a gutter edged by an elevated rim or
      collar.

        Order 6. PERITRICHACEAE.

      _Lichnophora_, Cl.; _Trichodina_, Ehrb.; _Vorticella_, L.;
      _Zoothamnium_, Bory; _Carchesium_, Ehrb.; _Epistylis_, Ehrb.;
      _Opercularia_, Lamk.; _Vaginicola_, Lamk.; _Pyxicola_, Kent;
      _Cothurnia_, Ehrb.; _Scyphidia_, Lachmann; _Ophrydium_, Bory;
      _Spirochona_, St.

The Ciliata have so complex an organisation that, as with the Metazoa, it
is well to begin with the description of a definite type. For this purpose
we select _Stylonychia mytilus_, Ehrb. (Fig. 49), a species common in water
rich in organic matter, and relatively large (1/75" = ⅓ mm.). It is broadly
oval in outline, with the wide end anterior, truncate, and sloping to the
left side behind; the back is convex, thinning greatly in front; the belly
flat. It moves through the water either by continuous swimming or by jerks,
and can either crawl steadily over the surface of a solid or an air surface
such as an air bubble, or advance by springs, which recall those of a
hunting spider. The boundary is everywhere a thin plasmic pellicle, very
tender, and readily undergoing diffluence like the rest of the cell. From
the pellicle pass the cilia, which are organically connected with it,
though they may be traced a little deeper; they are arranged in slanting
longitudinal rows, and are much and variously modified, according to their
place and function. On the edge of the dorsal surface they are fine and
motionless, probably only sensory (_s.h._); except three, which protrude
well over the hinder end (_c.p._), stout, pointed, and frayed out at the
ends, and possibly serving as oars or rudders for the darting movements.
These are distinguished from simple cilia as "cirrhi."

{139}[Illustration: FIG. 49.—Ventral view of _Stylonychia mytilus_. _a.c_,
Abdominal cirrhi; _an_, anus discharging the shell of a Diatom; _c.c_,
caudal cirrhi; _c.p_, dorsal cirrhi; _cv_, contractile vacuole; _e_, part
of its replenishing canal; _f.c_, frontal cirrhi; _f.v_, food vacuoles;
_g_, internal undulating membrane; _l_, lip; _m_, mouth or pharynx; _mc_,
marginal cirrhi; _N_, _N_, lobes of meganucleus; _n_, _n_, micronuclei;
_o_, anterior end; _per_, adoral membranellae; _poc_, preoral cilia;
_p.om_, preoral undulating membrane; _s.h_, sense hairs. (Modified from
Lang.)]

At the right hand of the frontal area there begins, just within the dorsal
edge, a row of strong cilium-like organs (Fig. 49, _per_); these, on
careful examination, prove to be transverse triangular plates, which after
death may fray into cilia.[154] They are the "adoral membranellae." This
row passes to the left blunt angle, and there crosses over the edge of the
body to the ventral aspect, and then curves inwards towards the median
line, which it reaches about half-way back, where it passes into the
pharynx (_m_). It forms the front and left-hand boundary of a wedge-shaped
depression, the "peristomial area," the right-hand boundary being the
"preoral ridge" or lip (_l_), which runs nearly on the median line,
projecting downward and over the depression. This ridge bears on its inner
and upper side a row of fine "preoral cilia" (_poc_) and a wide "preoral
undulating membrane" (_p.om_), which extends horizontally across, below the
peristomial area. The roof of this area bears along its right-hand edge an
"internal undulating membrane" (_g_), and then, as we pass across to the
left, first an "endoral membrane" and then an "endoral" row of cilia. In
some allied genera (not in _Stylonychia_), at the base and on the inner
side of each adoral membranella, is a "paroral" cilium. {140}All these
motile organs, with the exception of the preoral cilia, pass into the
pharynx; but the adoral membranellae soon stop short for want of room.
There are some seventy membranellae in the adoral wreath.

The rest of the ventral surface is marked by longitudinal lines, along
which the remaining appendages are disposed. On either side is a row of
"marginal cirrhi" (_mc._), which, like the membranellae, may fray out into
cilia, but are habitually stiff spine-like, and straight in these rows;
these are the chief swimming organs. Other cirrhi, also arranged along
longitudinal rows, with so many blank spaces that the arrangement has to be
carefully looked for, occur in groups along the ventral surface. On the
right of the peristome are a group which are all curved—the "frontal
cirrhi" (_f.c._). Behind the mouth is a second group—the "abdominal cirrhi"
(_a.c._), also curved hooks; and behind these again the straight spine-like
"caudal" or "anal" cirrhi (_c.c_), which point backwards. These three sets
of ventral cirrhi are the organs by which the animal executes its crawling
and darting movements. Besides the mouth there are two other openings, both
indistinguishable save at the very moment of discharge; the anus (_an_)
which is dorsal, and the pore of the contractile vacuole, which is ventral.

The protoplasm of the body is sharply marked off into a soft, semi-fluid
"endoplasm" or "endosarc," and a firmer "ectoplasm" or "ectosarc." The
former is rich in granules of various kinds, and in food-vacuoles wherein
the food is digested. The mode of ingestion, etc., is described below (p.
145). The ectoplasm is honeycombed with alveoli of definite arrangement,
the majority being radial to the surface or elongated channels running
lengthwise; inside each of these lies a contractile plasmic streak or
myoneme. The contractile vacuole (_cv_) lies in this layer, a little behind
the mouth, and is in connexion with two canals, an anterior (_e_) and a
posterior, from which it is replenished.

The nuclear apparatus lies on the inner boundary of the ectoplasm; it
consists of (1) a large "meganucleus" formed of two ovoid lobes (_N_, _N_),
united by a slender thread; and (2) two minute "micronuclei" (_n_, _n_),
one against either lobe of the meganucleus.

_Stylonychia_ multiplies by transverse fission, the details of which are
considered on pp. 144, 147.

The protoplasm of Ciliata is the most differentiated that we {141}find in
the Protista, and we can speak without exaggeration of the "organs" formed
thereby.

The form of the body, determined by the firm PELLICLE or plasmic membrane,
is fairly constant for each species, though it may be subject to temporary
flexures and contractions. The pellicle varies in rigidity; where the cilia
are abundant it is proportionately delicate, and scarcely differs from the
ectoplasm proper, save for not being alveolate. In the Peritrichaceae it is
especially resistant and proof against decay. In _Coleps_ (Gymnostomaceae)
it is hardened and sculptured into the semblance of plate-armour, and the
prominent points of the plates around the mouth serve as teeth to lacerate
other active Protista, its prey; but, like the rest of the protoplasm, this
disappears by decay soon after the death of the _Coleps_. Where, as in
certain Oligotrichaceae, cilia are absent over part of the body, the
pellicle is hardened; and on the dorsal face and sides of _Dysteria_ it
even assumes the character of a bivalve shell, and forms a tooth-like
armature about the mouth.

From the pellicle protrude the CILIA, each of which is continued inwards by
a slender basal filament to end in a "basal granule" or "blepharoplast."
The body-cilia are fine, and often reversible in action, which is
exceptional in the organic world. They may be modified or combined in
various ways. We have seen that in _Stylonychia_ some are motionless
sensory hairs. The cirrhi and setae sometimes fray out during life, and
often after death, into a brush at the tip, and have a number of
blepharoplasts at their base. The same holds good for the membranellae and
undulating membranes. They are thus comparable to the "vibratile styles" of
Rotifers (Vol. II. p. 202) and the "combs" or "Ctenophoral plates" of the
Ctenophora (p. 412 f.).[155]

{142}[Illustration: FIG. 50.—Ectosarc of Ciliata. _a-f_, from _Stentor
coeruleus_; _g_, _Holophrya discolor_. _a_, Transverse section, showing
cilia, pellicle, canals, and myonemes; _b_, surface view below pellicle,
showing myonemes alternating with blue granular streaks; _c_, more
superficial view, showing rows of cilia adjacent to myonemes; _d_, myoneme,
highly magnified, showing longitudinal and transverse striation; _e_, two
rows of cilia; _f_, _g_, optical sections of ectosarc, showing pellicle,
alveolar layer (_a_), myonemes (_m_), and canals in ectosarc. (From
Calkins, after Metschnikoff, Bütschli, and Johnson.)]

The ECTOSARC has a very complex structure.  Like other protoplasm it has a
honeycombed or alveolate structure, but in this case the alveoli are
permanent in their arrangement and position. Rows of these alveoli run
under the surface; and the cilia are given off from their nodal points
where the vertical walls of several unite, and wherein the basal granule or
blepharoplast is contained. Longitudinal threads running along the inner
walls of the alveoli of the superficial layer are differentiated into
muscular fibrils or "myonemes," to which structures so many owe their
marked longitudinal striation on the one hand, and their power of sudden
contraction on the other. The appearance of transverse striation may be
either due to transverse myonemes, or produced by the folds into which the
contraction of longitudinal fibrils habitually wrinkles the pellicles, when
it is fairly dense (Peritrichaceae); circular muscular fibrils, however,
undoubtedly exist in the peristomial collar of this group. Embedded in the
ectosarc are often found TRICHOCYSTS,[156] analogous {143}to the
nematocysts of the Coelenterata (p. 247), and doubtless fulfilling a
similar purpose, offensive and defensive. A trichocyst is an oblong sac (4
µ long in _Paramecium_) at right angles to the surface, which on
irritation, chemical (by tannin, acids, etc.) or mechanical, emits or is
converted into a thread several times the length of the cilia (33 µ), often
barbed at the tip. In the predaceous Gymnostomaceae, such as _Didinium_,
the trichocysts around (or even within) the mouth are of exceptional size,
and are ejected to paralyse, and ultimately to kill, the active Infusoria
on which they feed. In most of the Peritrichaceae they are, when present,
limited to the rim around the peristome, while in the majority of species
of Ciliata they have not been described. Fibrils, possibly nervous,[157]
have been described in the deepest layer of the ectosarc in
Heterotrichaceae.

The innermost layer of the ectosarc is often channelled by a system of
canals,[158] usually inconspicuous, as they discharge continuously into the
CONTRACTILE VACUOLE; but by inducing partial asphyxia (_e.g._ by not
renewing the limited supply of air dissolved in the drop of water on the
slide under the cover-glass), the action of the vacuole is slackened, and
these canals may be more readily demonstrated. The vacuole, after
disappearance, forms anew either by the coalescence of minute formative
vacuoles, or by the enlargement of the severed end of the canal or canals.
The pore of discharge to the surface is visible in several species, even in
the intervals of contraction.[159] The pore is sometimes near that of the
anus, but is only associated with it in Peritrichaceae, where it opens
beside it into the vestibule or first part of the long pharynx, often
through a rounded reservoir (Fig. 60, _r_) or elongated canal.

The ENDOSARC, in most Ciliates well differentiated from the ectosarc, is
very soft; though it is not in constant rotation like that of a Rhizopod,
it is the seat of circulatory movements alternating with long periods of
rest. Thus it is that the food-vacuoles, after describing a more or less
erratic course, come to discharge their undigested products at the one
point, the ANUS. {144}In a few genera (_Didinium_, for instance) the course
from mouth to anus is a direct straight line, and one may almost speak of a
digestive tract. In _Loxodes_ and _Trachelius_ (Fig. 56) the endosarc, as
in the Flagellate _Noctiluca_ (Fig. 48, p. 133), has a central mass into
which the food is taken, and which sends out lobes, which branch as they
approach and join the ectoplasm. The endosarc contains excretory granules,
probably calcium phosphate, droplets of oil or dissolved glycogen, proteid
spherules, paraglycogen grains, etc.

The NUCLEAR APPARATUS lies at the inner boundary of the ectoplasm. The
"meganucleus" may be ovoid, elongated, or composed of two or more rounded
lobes connected by slender bridges (_Stentor_, _Stylonychia_). The
"micronucleus" may be single; but even when the meganucleus is not lobed it
may be accompanied by more than one micronucleus, and when it is lobed
there is at least one micronucleus to each of its lobes.[160] The
meganucleus often presents distinct granules of more deeply staining
material, varying with the state of nutrition; these are especially visible
in the band-like meganuclei of the Peritrichaceae (Figs. 51, 60). At the
approach of fission it is in many cases distinctly fibrillated.[161] But
all other internal differentiation, as well as any constriction, then
disappears; and the ovoid or rounded figure becomes elongated and
hour-glass shaped, and finally constricts into two ovoid
daughter-meganuclei, which, during and after the fission of the cell,
gradually assume the form characteristic of the species. The micronuclei
(each and all when they are multiple) divide by modification of
karyokinesis (or "mitosis") as a prelude to fission: in this process the
chromatin is resolved into threads which divide longitudinally, but the
nuclear wall {145}remains intact. If an Infusorian be divided into small
parts, only such as possess a micronucleus and a fragment of the
meganucleus are capable of survival. We shall see how important a part the
micronuclei play in conjugation, a process in which the old meganuclei are
completely disorganised and broken up and their débris expelled or
digested.

The MOUTH of the Gymnostomaceae is habitually closed, opening only for the
ingestion of the living Protista that form their prey. It usually opens
into a funnel-shaped PHARYNX, strengthened with a circle of firm
longitudinal bars, recalling the mouth of an eel-trap or lobster-pot
("Reusenapparat" of the Germans); and this is sometimes protrusible. In
_Dysteria_ the rods are replaced by a complicated arrangement of jaw- or
tooth-like thickenings, which are not yet adequately described. We have
above noted the strong adoral trichocysts in this group.

In all other Ciliates[162] the "mouth" is a permanent depression lined by a
prolongation of the pellicle, and containing cilia and one or more
undulating membranes, and when adoral membranellae are present, a
continuation of these. In some species, such as _Pleuronema_ (Fig. 57), one
or two large membranes border the mouth right and left. In Peritrichaceae
the first part of the pharynx is distinguished as the "vestibule," since it
receives the openings of the contractile vacuole or its reservoir and the
anus. The pharynx at its lower end (after a course exceptionally long and
devious in the Peritrichaceae; Figs. 51, 60) ends against the soft
endosarc, where the food-particles accumulate into a rounded pellet; this
grows by accretion of fresh material until it passes into the endosarc,
which closes up behind it with a sort of lurch. Around the pellet liquid is
secreted to form the food-vacuole. If the material supplied be coloured and
insoluble, like indigo or carmine, the vacuoles may be traced in a sort of
irregular, discontinuous circulation through the endosarc until their
remains are finally discharged as faeces through the anus. No prettier
sight can be watched under the microscope than that of a colony of the
social Bell-animalcule (_Carchesium_) in coloured water—all producing
food-currents brilliantly shown up by the wild eddies of the pigment
granules, and the vivid blue or crimson colour of {146}the food-vacuoles,
the whole combining to present a most attractive picture. Ehrenberg fancied
that a continuous tube joined up the vacuoles, and interpreted them as so
many stomachs threaded, as it were, along a slender gut; whence he named
the group "Polygastrica."

[Illustration: FIG. 51.—_Carchesium polypinum._ Scheme of the path taken by
the ingested food in digestion and expulsion of the excreta. The food
enters through the pharynx and is transported downward (small circles),
where it is stored in the concavity of the sausage-shaped meganucleus (the
latter is recognised by its containing darker bodies). It remains here for
some time at rest (small crosses). Then it passes upward upon the other
side (dots) and returns to the middle of the cell, where it undergoes
solution. The excreta are removed to the outside, through the vestibule and
cell mouth. The black line with arrows indicates the direction of the path.
(From Verworn, after Greenwood.)]

We owe to Miss Greenwood[163] a full account of the formation and changes
of the food-vacuoles in _Carchesium polypinum_. The vacuole passes steadily
along the endosarc for a certain time after its sudden admission into it,
and then enters on a phase of quiescence. A little later the contents of
the vacuole aggregate together in the centre of the vacuole, where they are
surrounded by a zone of clear liquid; this takes place in the hollow of the
meganucleus, in this species horseshoe-shaped. The vacuole then slowly
passes on towards the peristome, lying deep in the endosarc, and the fluid
peripheral zone is absorbed. {147}For some time no change is shown in the
food-material itself: this is the stage of "storage." Eventually a fresh
zone of liquid, the true digestive vacuole, forms again round the
food-pellet, and this contains a peptic juice, of acid reaction. The
contents, so far as they are capable of being digested, liquefy and
disappear. Ultimately the solid particles in their vacuole reach the anal
area of the vestibule, and pass into it, to be swept away by the overflow
of the food-current. The anus is seated on a transverse ridge about a third
down the tube, the remaining two-thirds being the true pharynx.

FISSION is usually transverse; but is oblique in the conical
Heterotrichaceae, and longitudinal in the Peritrichaceae. It involves the
peristome, of which one of the two sisters receives the greater, the other
the lesser part; each regenerates what is missing. When there are two
contractile vacuoles, as in _Paramecium_, either sister receives one, and
has to form another; where there is a canal or reservoir divided at
fission, an extension of this serves to give rise to a new vacuole in that
sister which does not retain the old one. In some cases the fission is so
unequal as to have the character of budding (_Spirochona_). We have
described above (p. 144) the relations of the nuclear apparatus in fission.

Several of the Ciliata divide only when encysted, and then the divisions
are in close succession, forming a brood of four, rarely more. This is well
seen in the common _Colpoda cucullus_. In the majority, however, ENCYSTMENT
is resorted to only as a means of protection against drought, etc., or for
quiet rest after a full meal (_Lacrymaria_).

Maupas[164] has made a very full study of the LIFE-CYCLES of the Ciliata.
He cultivated them under the usual conditions for microscopic study, _i.e._
on a slide under a thin glass cover supported by bristles to avoid
pressure, preserved in a special moist chamber; and examined them at
regular intervals.

{148}[Illustration: FIG. 52.—_Paramecium caudatum_, stages in conjugation.
_gul_, Gullet; _mg.nu_, meganucleus; _Mg.nu_, reconstructed meganucleus;
_mi.nu_, micronucleus; _Mi.nu_, reconstructed micronucleus; _o_, mouth.
(From Parker and Haswell, after Hertwig.)]

The animals collect at that zone where the conditions of aeration are most
suitable, usually just within the edge of the cover, and when well supplied
with food are rather sluggish, not swimming far, so that they are easily
studied and counted. When well supplied with appropriate food they undergo
binary fission at frequent intervals, dividing as often as five times in
the twenty-four hours at a temperature of 65-69° F. (_Glaucoma
scintillans)_, so that in this period a single individual has resolved
itself into a posterity of 32; but such a rapid increase is exceptional. At
a minimum and a maximum temperature multiplication is arrested, the optimum
lying midway. If the food-supply is cut off, encystment occurs in those
species capable of the process; but when there is a mixture of members of
different broods of the same species, subject to the limitations that we
shall learn, conjugation ensues. Under the conditions of Maupas'
investigations he found a limit to the possibilities of continuous
fissions, even when interrupted by occasional encystment. The individuals
of a series ultimately dwindle in size, their ciliary apparatus is reduced,
and their nuclear apparatus degenerates. Thus the ultimate members of a
fission-cycle show a progressive decay, notably in the nuclear apparatus,
which Maupas has aptly compared to "senility" or "old age" in the Metazoan.
If by the _timely_ mixture of broods conjugation be induced, these senile
degenerations do not occur.[165] In _Stylonychia {149}mytilus_ the produce
of a being after conjugation died of senility after 336 fissions; in
_Leucophrys_ after 660.

Save in the Peritrichaceae (p. 151) CONJUGATION takes place between similar
mates, either of the general character and size of the species, or reduced
by fissions, in rapid succession, induced by the same conditions as those
of mating. The two mates approach, lying parallel and with their oral faces
or their sides (_Stentor_) together, and partially fuse thereby; though no
passage of cytoplasm is seen it is probable that there is some interchange
or mixture.[166]

[Illustration: FIG. 53.—Diagram of conjugation in _Colpidium colpoda_.
Horizontal line means degeneration; parallel vertical lines, separation of
gametes; broken lines (above), boundary between pairing animals; (below),
first fission; single vertical line, continuity or enlargement. _M_,
Meganucleus; _µ_, micronucleus; _Z_, zygote-nucleus.]

{150}[Illustration: FIG. 54.—Four individuals of _Coleps hirtus_
(Gymnostomaceae) swarming about and ingesting a Vorticella (?) (From
Verworn.)]

The meganucleus lengthens, becomes irregularly constricted, and breaks up
into fragments, which are ultimately extruded or partially digested. The
micronucleus enlarges (Fig. 52, A) and undergoes three successive
divisions, or, strictly speaking, two fissions producing four nuclei, of
which one only undergoes the third. The other three nuclei of the second
fission degenerate like the meganucleus.[167] Of the two micronuclei of
this last division one remains where it is as a "stationary" pairing
nucleus, while its sister passes as a "migratory" pairing-nucleus into the
other mate, and fuses with its stationary pairing-nucleus. Thus in either
mate is formed a "zygote-nucleus," or "fusion-nucleus." All these processes
are simultaneous in the two mates; and the migratory nuclei cross one
another on the bridge of junction of the two mates (Fig. 52, C). Each mate
now has its original cytoplasm (subject to the qualification above),
{151}but its old nuclear apparatus is replaced by the fusion-nucleus. This
new nucleus undergoes repeated fissions; its offspring enlarge unequally,
the larger being differentiated as mega-, the smaller as micro-nuclei. The
mates now separate (Fig. 52, F, G), and by the first (or subsequent)
fission of each, the new mega- and micro-nuclei are distributed to the
offspring. _Colpidium colpoda_ offers the simplest case, on which we have
founded our diagram showing the nuclear relations. During conjugation the
oral apparatus often atrophies, and is regenerated; and in some cases the
pellicle and ciliary apparatus are also "made over."

[Illustration: FIG. 55.—_Paramecium caudatum_ (Aspirotrichaceae). A, The
living animal from the ventral aspect; B, the same in optical section, the
arrows show the course taken by food-particles. _buc.gr_, Buccal groove;
_cort_, cortex; _cu_, cuticle; _c.vac_, contractile vacuole; _f.vac_, food
vacuole; _gul_, gullet; _med_, medulla; _mth_, mouth; _nu_, meganucleus;
_pa.nu_, micronucleus; _trch_, trichocysts discharged. (From Parker's
_Biology_.)]

In the Peritrichaceae the mates are unequal; the larger is the normal cell,
and is fixed; the smaller, mobile, is derived from an ordinary individual
by brood-divisions, which only occur under the conditions that induce
conjugation (Fig. 60). Here, though the two pairs of nuclei are formed, it
is only the migratory {152}nuclei that unite, the stationary ones aborting
in both mates. During the final processes of conjugation the smaller mate
is absorbed into the body of the larger, and so plays the part of male
there. But this process, though one of true binary sex, is clearly derived
from the peculiar type of equal reciprocal conjugation of the other
Infusoria.

The Ciliata are almost all free-swimming animals with the exception of most
of the Peritrichaceae, and of the genera we now cite. _Folliculina_ forms a
sessile tube open at either end; and _Schizotricha socialis_ inhabits the
open mouths of a branching gelatinous tubular stem, obviously secreted by
the hinder end of the animal, and forking at each fission to receive the
produce. A similar habit to the latter characterises _Maryna socialis_; all
three species are marine, and were described by Gruber.[168] _Stentor_
habitually attaches itself by processes recalling pseudopodia, and often
forms a gelatinous sheath.

The majority of the Oligotrichaceous Tintinnidae inhabit free chitinous
tests often beautifully fenestrated, as in _Dictyocystis_.

Many genera are parasitic in the alimentary canal of various Metazoa, but
none appear to be seriously harmful except _Ichthyophtheirius_, which
causes an epidemic in fresh-water fish. Quite a peculiar fauna inhabit the
paunch of Ruminants. _Nyctotherus_ and _Balantidium_ are occasionally found
in the alimentary canal of Man.[169]

The Gymnostomaceae are predaceous, feeding for the most part on smaller
Ciliates. We have described the peculiar character of the mouth and pharynx
in this group, and the mail-like pellicle of _Coleps_ (Fig. 54).
_Loxophyllum_ is remarkable for the absence of cilia from one of the sides
of its flattened body, and the tufts of trichocysts studding its dorsal
edge at regular intervals. _Actinobolus_ has numerous tentacles, exsertile
and retractile, each bearing a terminal tuft of trichocysts, which serve to
paralyse such active prey as _Halteria_. _Ileonema_ has one tentacle
overhanging the mouth; and _Mesodinium_ has four short sucker-like
projections around it.[170] It has only two girdles {153}of cilia, which
are stout and resemble fine-pointed cirrhi. In _Dysteria_ the cilia are
exclusively ventral, and the naked dorsal surface has its pellicle
condensed into a bivalve shell; a posterior motile process ("foot") and a
complex pharyngeal armature add to the exceptional characters of the genus.

The Aspirotrichaceae are well known to every student of "Elementary
Biology" by the "type" _Paramecium_ (Fig. 55), so common in infusions,
especially when containing a little animal matter. _P. bursaria_ often
contains in its endosarc the green symbiotic Flagellate _Zoochlorella_.
_Colpoda cucullus_, very frequent in vegetable infusions, usually only
divides during encystment, and forms a brood of four. _Pleuronema
chrysalis_ (Fig. 57) is remarkable for its habit of lying for long periods
on its side and for its immense undulating membrane, forming a lip on the
left of its mouth; _Glaucoma_ has two, right and left.

[Illustration: FIG. 56.—_Trachelius ovum._ A, general view; B, section
through sucker; C, section through contractile vacuole and its pore of
discharge. _al_, Alveolar layer of ectoplasm; _cil_, cilia; _c.v_,
contractile vacuole; _m_, mouth; _N_, meganucleus; _s_, sucker, from which
pass inwards retractile myonemes. (After Clara Hamburger.)]

{154}[Illustration: FIG. 57.—_Pleuronema chrysalis_ (Aspirotrichaceae). A,
Unstimulated, lying quiet; B, stimulated, in the act of springing by the
stroke of its cilia. (From Verworn.)]

The Heterotrichaceae present very remarkable forms. _Spirostomum_ is nearly
cylindrical, and, a very giant, may attain a length of 4 mm. (1/6").
_Stentor_ can attach itself by its hinder end, which is then finely tapered
and prolonged into a few pseudopodia; its body is trumpet-shaped, with a
spiral peristome forming a coil round its wide end, and leading on the left
side into the mouth. Many species when attached secrete a gelatinous sheath
or tube. _S. polymorphus_ is often coloured green by _Zoochlorella_ (p.
125); _S. coeruleus_[171] and _S. igneus_ owe their names to the brilliant
pigment, blue or scarlet, deposited in granules in lines between the
conspicuous longitudinal myonemes. From their large size and elongated
meganucleus accompanied by numerous micronuclei, these two genera have
frequently been utilised for experiments on regeneration. In _Metopus
sigmoides_ the peristomial area forms a dome above its wreath of
membranellae; and in _M. pyriformis_ this is so great as to form the larger
part of the cell, which is top-shaped, tapering behind to a point.
_Caenomorpha_ (Fig. 58) has the same general form, with a peg-like tail,
and possesses a girdle of cirrhi.[172] The converse occurs in
{155}_Bursaria_; the cell is a half ellipse, something like a common twin
tobacco-pouch when closed: a deep depression thus occupies the whole
ventral surface, and opens by a wide slit extending along the anterior end.
The peristomial area occupies the dorsal side of the pocket so formed, and
the mouth is in the hinder left-hand corner. _Blepharisma_ sp. is parasitic
in the Heliozoon _Raphidiophrys viridis_ (Fig. 20, 1, p. 74).

[Illustration: FIG. 58.—_Caenomorpha uniserialis._ _crh_, Zone of cirrhi;
_c.t_, cilia of tail; _c.v_, contractile vacuole; _c.w_, ciliary wreath;
_g_, granular aggregate; _m_, zone of membranellae; _N_, meganucleus; _n_,
micronucleus; _oe_, pharynx; _t_, tail-spine; _t_^1, accessory spine;
_u.m_, undulating membrane; _v_, vacuole; _z_, precaudal process. (After
Levander.)]

Among Oligotrichaceae, _Halteria_, common among the débris at the bottom of
pools in woods containing dead leaves, is remarkable for an equatorial
girdle of very long fine setae, and for its rapid erratic darting
movements, alternating with a graceful bird-like hover. The Tintinnidae are
mostly marine, pelagic, with the general look of a stalkless _Vorticella_;
some have a latticed chitinous shell.[173]



{156}[Illustration: FIG. 59.—_Stentor polymorphus._ I, Young individual
attached, extended; II, adult in fission, contracted; _cv_ in I, afferent
canal of contractile vacuole; in II, contractile vacuole; N, moniliform
meganucleus (micronuclei omitted); _o_, mouth; the fine lines are the
myoneme fibrils. (From Verworn.)]

Among Peritrichaceae, _Vorticella_ (Fig. 60) and its allies have long been
known as Bell-animalcules to every student of pond-life. The body has
indeed the form of an inverted bell, closed at its mouth by the
"peristome," or oral disc; this is a short, inverted truncate cone set
obliquely so that its wide base hardly projects at one side, but is tilted
high on the other; the edge of the bell is turned out into a rim or
"collar," separated from the disc by a deep gutter. The collar, habitually
everted, or even turned down, contracts over the retracted disc when the
animal is retracted (E^2), which is brought about by any sort of shock, or
when it swims freely backwards. For the latter purpose a posterior ring of
cilia (or rather membranellae) is developed round the hinder end of the
bell (A, _cr_, E^3). The cilia of the adoral wreath are very strong, united
at the base into a continuous membrane, and indeed themselves partake of
the composite nature of membranellae. The wreath forms more than one turn
of a right-handed spiral, the innermost turn ending abruptly on the disc,
the outer leading down into the mouth at the point where the disc is most
tilted and the groove deepest.[174] The pharynx (_p_) is long, and contains
an undulating membrane (_u.m_) on its inner side projecting out through the
mouth, and numerous cilia; it leads deep into the body (_p_). The first
part is distinguished as the "vestibule" (_v_), as into it opens the anus,
and the contractile vacuole (_c.v._), the latter sometimes opening by a
reservoir (_r_). The body contains in the ectoplasm {157}myonema-fibrils
which, by their contraction, withdraw the disc, and at the same time
circular fibrils close the peristome over it. In the type-genus the
pellicle is continued into a long, slender elastic stalk (_s_), of which
the longitudinal myoneme fibrils of the ectoplasm converge to the stalk,
and are prolonged into it as a spirally winding fibre, sometimes
transversely striated.[175] The effect of the contraction of this is to
pull the stalk into a helicoid spiral (like a coil-spring), with the line
of insertion of the muscle along the inner side of the coils, which is, of
course, the shortest path from one end to the other (Fig. 60, B).

[Illustration: FIG. 60.—_Vorticella._ A, expanded; B, stalk in contraction;
_c_, eversible collar below peristome; _cr_, line of posterior ciliary
ring; _c.v_, contractile vacuole; _m_, muscle of stalk; _N_, meganucleus;
_n_, micronucleus; _p_, pharynx; _r_, reservoir of contractile vacuole;
_s_, tubular stalk; _u.m_, undulating membrane in vestibule; _v_, hinder
end of vestibule. E^1, E^2, two stages in binary fission; E^3, free zooid,
with posterior wreath; F^1, F^2 division into mega- and micro-zooids (_m_);
G^1, G^2, conjugation; _m_, microzooid. (Modified from Bütschli, from
Parker and Haswell.)]

The members of the Vorticellidae are very commonly attached to weeds or to
various aquatic Metazoa, each species being more or less restricted in its
haunts. _Vorticella_, the type, is singly {158}attached to a contractile
stalk; fission takes place in the vertical plane, and one of the two so
formed retains the original stalk, while the other swims off (Fig. 60,
E^1-E^3), often to settle close by, so that the individuals are found in
large social aggregates, side by side, fringing water-weeds with a halo
visible to the naked eye, which disappears on agitation by the sudden
contraction of all the stalks. _Carchesium_ and _Zoothamnium_ differ from
_Vorticella_ in the fact that the one daughter-cell remains attached by a
stalk coming off a little below the body of the other, so as to give rise
to large branching colonies.

In _Carchesium_ (Fig. 51) the muscular threads of each cell are separate,
while in _Zoothamnium_ they are continuous throughout the colony.
_Epistylis_ has a solid, rigid stalk, and may give rise to branching
colonies, which often infest the body of the Water-Fleas (Copepoda) of the
genus _Cyclops_. _Opercularia_ is characterised by the depth of the gutter,
the height of the collar, and the tapering downward of the elongated disc.
_Vaginicola_, _Pyxicola_, _Cothurnia_, _Scyphidia_, all inhabit tubes, some
of extreme elegance. _Ophrydium_ is a colonial form, found in ponds and
ditches, resembling _Opercularia_, but inhabiting tubes of jelly[176] that
coalesce by their outer walls into a large floating sphere; it usually
contains the green symbiotic Flagellate _Zoochlorella_. _Trichodina_ is
free, short, and cylindrical, with both wreaths permanently exposed, and is
provided with a circlet of hooks within the aboral wreath. It is often
parasitic, or perhaps rather epizoic, on the surface of _Hydra_ (see p.
254), gliding over its body[177] with a graceful waltzing movement; it
occurs also in the bladder and genito-urinary passages of Newts, and even
in their body-cavity and kidneys.


II. SUCTORIA = TENTACULIFERA

_Infusoria with cilia only in the young state,[178] without mouth or anus,
but absorbing food (usually living Ciliates) by one or more tentacles,
perforated at the apex; mostly attached, frequently epizoic, rarely
parasitic in the interior of other Protozoa._

  {159}_Acineta_, Ehrb. (Fig. 61, 2); _Amoebophrya_, Koppen; _Choanophrya_,
  Hartog (Fig. 62); _Dendrocometes_, St. (Fig. 61, 4); _Dendrosoma_, Ehrb.
  (Fig. 61, 9); _Endosphaera_, Engelm.; _Ephelota_, Str. Wright (Fig. 61,
  5, 8); _Hypocoma_, Gruber; _Ophryodendron_, Cl. and L. (Fig. 61, 7);
  _Podophrya_, Ehrb. (Fig. 61, 1); _Rhyncheta_, Zenker (Fig. 61, 3);
  _Sphaerophrya_, Cl. and L. (Fig. 61, 6), _Suctorella_, Frenzel;
  _Tokophrya_, Bütschli.

This group, despite a superficial resemblance to the Heliozoa, show a close
affinity to the Ciliata; the nuclear apparatus is usually double though a
micronucleus is not always seen; the young are always ciliated, and the
mode of conjugation is identical in all cases hitherto studied. Most of the
genera are attached by a chitinous stalk (Fig. 61), continued in _Acineta_
into a cup or "theca" surrounding the cell. The pellicle is firm, often
minutely shagreened or "milled" in optical section by fine radial
processes, whether superficial rods or the expression of the meeting edges
of radial alveoli is as yet uncertain. The pellicle closely invests the
ectosarc, is continued down into a tubular sheath, from the base of which
the tentacle rises, and upwards to invest the tentacle, and is even
prolonged into its cavity in _Choanophrya_, the only genus where the
tentacles are large enough for satisfactory demonstration. These organs may
be one or more, and vary greatly in character. They may be (1) pointed for
prehension, puncture, and suction (_Ephelota_, Fig. 61, 5); (2) nearly
cylindrical, with a slightly "flared" truncate apex (_Podophrya_, Fig. 61,
1_a_); (3) filiform with a terminal knob; (4) "capitate" (_Acineta_, Fig.
61, 2); (5) bluntly truncate and capable of opening into a wide funnel for
the suction of food[179] (_Choanophrya_, Fig. 62; _Rhyncheta_, Fig. 61, 3).
Their movements, too, are varied, including retraction and protrusion, and
a degree of flexion which reaches a maximum in _Rhyncheta_ (Fig. 61, 3),
whose tentacle is as freely motile as an elephant's trunk might be supposed
to be were it as slender in proportion to its length. They are continued
into the body, and in _Choanophrya_ may extend right across it. In
_Podophrya_ trold the pellicle rises into a conical tube about the base of
the tentacle, which is retracted through it completely with the prey in
deglutition. In _Dendrocometes_, _Dendrosoma_, and _Ophryodendron_ (Fig.
61, 4, 9, 7), the tentacles arise from outgrowths of the cell-body.

{160}[Illustration: FIG. 61.—Various forms of Suctoria. 1, _a_ and _b_, two
species of _Podophrya_; _c_, a tentacle much enlarged; 2, _a_, _Acineta
jolyi_; 2, _b_, _A. tuberosa_, with four ciliated buds; in 6 the animal has
captured several small Ciliata; 8, _a_, a specimen multiplying by budding;
8, _b_, a free ciliated bud; 9, _a_, the entire colony; 9, _b_, a portion
of the stem; 9, _c_, a liberated bud. _a_, Organism captured as food;
_b.c_, brood-cavity; _bd_, bud; _c.vac_, contractile vacuole; _l_, test;
_mg.nu_, meganucleus; _mi.nu_, micronucleus; _nu_, nucleus; _t_, tentacle.
(From Parker and Haswell, after Bütschli and Saville Kent.)]

{161}The mechanism of suction is doubtful; but from the way particles from
a little distance flow into the open funnels of _Choanophrya_, it may be
the result of an increase of osmotic pressure. The external pellicle of the
tentacles is marked by a spiral constriction,[180] which may be prolonged
over the part included in the sheath. The endosarc is rich in oil-drops,
often coloured, and in proteid granules which sometimes absorb stains so
readily as to have been named "tinctin bodies." It usually contains at
least one contractile vacuole.

In _Dendrocometes_ (and perhaps others) the whole cell may become ciliated,
detach itself and swim off; this it does when its host (_Gammarus_) moults
its cuticle.

In fission or budding we have to distinguish many modes. (1) In the
simplest, after the nuclear apparatus has divided, the cell divides
transversely; the distal half acquires cilia and swims off to attach itself
elsewhere, while the proximal remains attached. The tentacles have
previously disappeared and have to be formed afresh in both. (2) More
commonly fission passes into budding on the distal face; a sort of groove
deepens around a central prominence which becomes the ciliated larva (Fig.
62, _em_); the tentacles of the "parent" are retained. This process passes
into (3) "internal budding," where a minute pit leads into a bottle-shaped
cavity.[181] (4) Again, the budding may be multiple, the meganucleus
protruding a branch for each bud, while the micronucleus, by successive
divisions, affords the supply requisite. _Sphaerophrya_ (Fig. 61, 6) and
_Endosphaera_ multiply freely by fission within their Ciliate hosts, and
were indeed described by Stein as stages in their life-cycle. Conjugation
of the same type as in most Ciliates has been fully worked out in
_Dendrocometes_ alone, by Hickson,[182] who has found the meganuclei
(though destined to disorganisation) conjugate for a short time by the
bridge of communication before the reciprocal conjugation of the
micronuclei.

We have referred to the endoparasitism of two genera. _Amoebophrya_ lives
in several Acanthometrids, and in the aberrant Radiolarian _Sticholonche_
(see p. 86). The attached species are {162}some of them indifferent to
their base; others are only found on Algae, or again only epizoic on
special Metazoan hosts, or even on special parts of these. Thus _Rhyncheta_
is only found on the couplers of the thoracic limbs of _Cyclops_, and
_Choanophrya_ on the ventral surface of its head and the adjoining
appendages.

[Illustration: FIG. 62.—_Choanophrya infundibulifera._ A, adult; B-D,
tentacles in action in various stages; E, tentacle at rest; F, young, just
settled down, _a_, _a_, _a_, Tentacles in various stages of activity; _c_,
central cavity; _c.v_, contractile vacuole; _em_, ciliated embryo showing
contractile vacuole and nucleus; _f_, spiral ridge; _m_, muscular wall of
funnel; _n_, nucleus; _tr_, opening of funnel. (A-D, F, modified after
Zenker; E, original.)]

We owe our knowledge of this group to the classical works of Ehrenberg,
Claparède and Lachmann, Stein, R. Hertwig, and Bütschli. Plate has shed
much light on _Dendrocometes_, and Hickson has studied its conjugation.
Ischikawa[183] has utilised modern histological methods for the cytological
study of _Ephelota bütschliana_. René Sand has written a useful, but
unequal, and not always trustworthy monograph of the Order,[184] containing
an elaborate bibliography.



PORIFERA (SPONGES)

BY

IGERNA B. J. SOLLAS, B.SC. (LOND.)

Lecturer on Zoology at Newnham College, Cambridge.

{165}CHAPTER VII

PORIFERA (SPONGES)[185]

INTRODUCTION—HISTORY—DESCRIPTION OF _HALICHONDRIA PANICEA_ AS AN EXAMPLE OF
BRITISH MARINE SPONGES AND OF _EPHYDATIA FLUVIATILIS_ FROM FRESH
WATER—DEFINITION—POSITION IN THE ANIMAL KINGDOM.


Sponges occupy, perhaps, a more isolated position than any other animal
phylum. They are not only the lowest group of multicellular animals, but
they are destitute of multicellular relatives. They are all aquatic
and—with the exclusion of a few genera found in fresh water—marine,
inhabiting all depths from between tide marks to the great abysses of the
ocean. They depend for their existence on a current of water which is
caused to circulate through their bodies by the activity of certain
flagellated cells. This current contains their food, it is their means of
respiration, and it carries away effete matters. Consequently sponges
cannot endure deprivation of oxygenated water except for short periods, and
only the hardiest inhabit regions where the supply is intermittent, as
between tide marks. This also renders useless attempts to keep specimens in
tanks, unless the water is frequently renewed.

The outward appearance of sponges has an exceptionally wide range, so that
it is difficult to give a novice any very definite picture of what he is to
expect when searching for these animals. This diversity is in part due to
the absence of organs of sufficient size to determine the shape of the
whole or limit its variation, {166}that is to say, the separate organs are
of an order of size inferior to that of the entire body. The animals are
fixed or lie loose on the sea bottom; there are in no case organs of
locomotion, and again no sense-organs, no segregated organs of sex, and as
a rule no distinction into axis and lateral members. It is by these
negative characters that the collector may easily recognise a sponge.

HISTORY.—Sponges are, then, in many of their characters unique; and they
present a variety of problems for solution, both of special and general
interest, they are widely distributed in time and space, and they include a
host of forms. It therefore causes no little surprise to learn that they
have suffered from a long neglect, even their animal nature having been but
recently established. Though known to naturalists from the time of
Aristotle, sponges have been left for modern workers as a heritage of
virgin soil: it has yielded to them a rich harvest, and is as yet far from
exhausted.

The familiar bath sponge was naturally the earliest known member of the
phylum. It is dignified by mention in the _Iliad_ and in the _Odyssey_, and
Homer, in his choice of the adjective "full of holes," πολύτρητος, shows at
least as much observation as many a naturalist of the sixteenth and
seventeenth centuries. Aristotle based his ideas of sponges entirely upon
the characters of the bath sponge and its near allies, for these were the
only kinds he knew. With his usual perspicuity he reached the conclusion
that sponges are animals, though showing points of likeness to plants.

The accounts of sponges after Aristotle present little of scientific
interest until the last century. Doubtless this is in part due to the
absence of organs which would admit of dissection, and the consequent
necessity of finer methods of study. Like other attached forms, sponges
were plant or animal as it pleased the imagination of the writer, and
sometimes they were "plant animals" or Zoophyta: those who thought them
animal were frequently divided among themselves as to whether they were
"polypous" or "apolypous." An opinion which it is somewhat difficult to
classify was that of Dr. Nehemiah Grew,[186] who says: "No _Sponge_ hath
any Lignous Fibers, but is wholly composed of those which make the Pith and
all the pithy parts {167}of a Plant, ... So that a _Sponge_, instead of
being a _Zoophyton_, is but the one-half of a Plant."

Sponges figure in herbals beside seaweeds and mushrooms, and Gerarde
says:[187] "There is found growing upon rockes near unto the sea a certaine
matter wrought together of the foame or froth of the sea which we call
Spunges ... whereof to speak at any length would little benefit the reader
... seeing the use thereof is so well known." About the middle of the
eighteenth century, authors, especially Peyssonnel, suggested that sponges
were but the houses of worms, which built them much as a bee or wasp builds
nests and cells. This was confuted by Ellis in 1765,[188] when he pointed
out that the sponge could not be a dead structure, as it gave proof of life
by "sucking and throwing out water." To Ellis, then, is due the credit of
first describing, though imperfectly, a current set up by sponges. He
mentions that Count Marsigli[189] had already made somewhat similar
observations.

It was not till 1825 that attention was again turned to the current, when
Robert Grant approached the group in a truly scientific manner, and was
ably supported by Lieberkühn. It would be impossible to do justice to Grant
in the brief summary to which we must limit ourselves. The most important
of his contributions was the discovery that water enters the sponge by
small apertures scattered over the surface, and leaves it at certain larger
holes, always pursuing a fixed course. He made a few rough experiments to
estimate the approximate strength of the current, and, though he failed to
detect its cause, he supposed that it was probably due to ciliary action.
Grant's suggestion was afterwards substantiated by Dujardin (1838), Carter
(1847), Dobie (1852), and Lieberkühn (1857). These five succeeded in
establishing the claims of sponges to a place in the animal kingdom, claims
which were still further confirmed when James-Clark[190] detected the
presence of the protoplasmic collar of the flagellated cells (see pp. 171,
176). Data were now wanted on which to base an opinion as to the position
of sponges within the animal kingdom. In 1878 Schulze[191] furnished
valuable embryological facts, in a description agreeing with an earlier one
of Metschnikoff's, of the amphiblastula larva (p. 226) and its
metamorphosis. {168}Then Bütschli[192] (1884) and Sollas[193] on combined
morphological and embryological evidence (1884) concluded that sponges were
remote from all the Metazoa, showing bonds only with Choanoflagellate
Protozoa (p. 121). This the exact embryological work of Maas, Minchin, and
Delage has done much to prove, but it has to be admitted that unanimity on
the exact position of the phylum has not yet been attained, some
authorities, such as Haeckel, Schulze, and Maas still wishing to include
sponges in the Metazoa.

In this short history we have been obliged to refer only to work helping
directly to solve the problem of the nature of a sponge, hence many names
are absent which we should have wished to mention.


HALICHONDRIA PANICEA.

One of the commonest of British sponges, which may be picked up on almost
any of our beaches, and which has also a cosmopolitan distribution, is
known by the clumsy popular name of the "crumb of bread sponge," alluding
to its consistency; or by the above technical name, with which even more
serious fault may be found.[194]

In its outward form _H. panicea_ affords an excellent case of a peculiarity
common among sponges. Its appearance varies according to the position in
which it has lived. In fact, Bowerbank remarks that it has no specific
form. It may grow in sheets of varying thickness closely attached to a
rock, when it is "encrusting," or it is frequently massive and lying free
on the sea bottom; again, it may be fistular, consisting of a single long
tube, or it may be ridge-like, apparently in this case consisting of a row
of long tubes fused laterally. In this last form it used to be called the
"cockscomb sponge," having been taken for a distinct species.

Bidder has proposed to call the different forms of the same species
"metamps" of the species. Figures of the metamps of _H. panicea_ will be
found in Bowerbank's useful Monograph.[195]

{169}The colour of the species is as inconstant as its form, ranging from
green to light brown and orange. MacMunn concludes from spectroscopic work
that _H. panicea_, contains at least three pigments, a chlorophyll, a
lipochrome, and a histohaematin.[196] Lipochromes vary from red to yellow,
chlorophyll is always associated with one or more of them. Histohaematin is
a respiratory pigment. Proof has not yet been adduced that the chlorophyll
is proper to the sponge and is not contained in symbiotic algae.

In spite of all this inconstancy _H. panicea_, is one of the most easily
determined species. It is only necessary to dry a small fragment, including
the upper surface; a beautiful honeycomb-like structure is then visible on
this surface, and among British sponges this is a property peculiar to the
species (Bowerbank). Whatever the form of the sponge, one or more large
rounded apertures are always present on the exterior; these are the
"oscula." In the encrusting metamp the oscula are flush with the general
surface, while in the other cases they are raised on conical projections;
fistular specimens carry the osculum at the distal end, and the cockscomb
has a row of them along its upper edge. Much more numerous than the oscula
are smaller apertures scattered over the general surface of the sponge, and
known as "ostia."

[Illustration: FIG. 63.—Portion of the surface of _H. panicea_, from dried
specimen. A, natural size; B, magnified. The large shaded patches are
ostia.]

If the sponge be placed in a shallow glass dish of sea water the function
of the orifices can be made out with the naked eye, especially if a little
powdered chalk or carmine be added to the water. If the specimen has been
gathered after the retreating tide has left it exposed for some time, this
addition is unnecessary, for as soon as it is plunged into water its
current bursts vigorously forth, and is rendered visible by the particles
of detritus that have accumulated in the interior during the period of
{170}exposure and consequent suspended activity. The oscula then serve for
the exit of currents of water carrying particles of solid matter, while the
entrance of water is effected through the ostia.

Sections show that the ostia lead into spaces below the thin superficial
layer or "dermal membrane"; these are continued down into the deeper parts
of the sponge as the "incurrent canals," irregular winding passages of
lumen continually diminishing as they descend. They all sooner or later
open by numerous small pores—"prosopyles"—into certain subspherical sacs
termed flagellated chambers. Each chamber discharges by one wide
aperture—"apopyle"—into an "excurrent canal." This latter is only
distinguishable from an incurrent canal by the difference in its mode of
communication with the chambers.

[Illustration: FIG. 64.—_H. panicea_: the arrows indicate the direction of
the current, which is made visible by coloured particles. (After Grant.)]

The excurrent canals convey to the osculum the water which has passed
through the ostia and chambers. All the peripheral parts of the sponge from
which chambers are absent are termed the "ectosome," while the
chamber-bearing regions are the "choanosome."

The peculiar crumb-of-bread consistency is due to the nature of the
skeleton, which is formed of irregular bundles and strands of minute
needles or spicules composed of silica hydrate, a substance familiar to us
in another form as opal: they are clear and transparent like glass. They
are scattered through the tissues in great abundance.

The classes of cellular elements in the sponge are as follows: Flattened
cells termed "pinacocytes" cover all the free surfaces, that is to say, the
external surface and the walls of the {171}excurrent and incurrent canals.
The flagellated chambers are lined by "choanocytes" (cf. Fig. 70, p. 176);
these are cells provided at their inner end with a flagellum and a collar
surrounding it. They resemble individuals of the Protozoan sub-class
Choanoflagellata, and the likeness is the more remarkable because no other
organisms are known to possess such cells. Taken together the choanocytes
constitute the "gastral layer," and they are the active elements in
producing the current. The tissue surrounding the chambers thus lying
between the excurrent and incurrent canals consists of a gelatinous matrix
colonised by cells drawn from two distinct sources. In the first place, it
contains cells which have a common origin with the pinacocytes, and which
together with them make up the "dermal layer"; these are the "collencytes"
and "scleroblasts"; secondly, it contains "archaeocytes," cells of
independent origin.

Collencytes are cells with clear protoplasm and thread-like pseudopodial
processes; they are distinguished as stellate or bipolar, according as
these processes are many or only two. Scleroblasts or spicule cells are at
first rounded, but become elongated with the growth of the spicule they
secrete, and when fully grown are consequently fusiform.

[Illustration: FIG. 65.—Diagrammatic section of a siliceous Sponge. _a.p_,
Apopyle; _d.o_, dermal ostia; _ex.c_, excurrent, or exhalant canal; _in.c_,
incurrent canal; _o_, osculum. (Modified from Wilson.)]

Each spicule consists of an organic filamentar axis or axial fibre around
which sheaths of silica hydrate are deposited successively by the
scleroblast. Over the greater length of the spicule the sheaths are
cylindrical, but at each end they taper to a point. The axial canal in
which the axial fibre lies is open at both ends, and the fibre is
continuous at these two points with an organic sheath, which invests the
entire spicule. From this structure we may conclude that the spicule grows
at both ends—_i.e._ it grows in two opposite directions along one line—it
has two rays lying in one axis, and is classed among uniaxial diactinal
spicules. Being {172}pointed at both ends it receives the special name
_oxea_. The lamination of the spicule is rendered much more distinct by
heating or treatment with caustic potash.[197]

[Illustration: FIG. 66.—Cut end of a length of a siliceous spicule from
_Hyalonema sieboldii_, with the lamellar structure revealed by solution. ×
104. (After Sollas.)]

The archaeocytes are rounded amoeboid cells early set apart in the larva;
they are practically undifferentiated blastomeres. Some of them become
reproductive elements, and thus afford a good instance of "continuity of
germ plasm," others probably perform excretory functions.[198]

[Illustration: FIG. 67.—Free-swimming larva of _Gellius varius_, in optical
section. _a_, Outer epithelium; _pi_, pigment; _x_, hinder pole. (After
Maas.)]

The reproductive elements are ova and spermatozoa, and are to be found in
all stages in the dermal jelly. Dendy states that the eggs are fertilised
in the inhalant canals, to which position they migrate by amoeboid
movements, and there become suspended by a peduncle.

The larva has unfortunately not been described, but as the course of
development among the near relatives of _H. panicea_ is known to be fairly
constant, it will be convenient to give a description of a "Halichondrine
type" of larva based on Maas' account of the development of _Gellius
varius_.[199] The free-swimming larvae escape by the osculum; they are
minute oval bodies moving rapidly by means of a covering of cilia. The
greater part of the body is a dazzling white, while the hinder pole is of a
brown violet colour. This coloured patch is non-ciliate, the general
covering of cilia ending at its edge in a ring of cilia twice the length of
the others. Forward {173}movement takes place in a screw line; when this
ceases the larva rests on its hinder pole, and the cilia cause it to turn
round on its axis.

Sections show that the larva is built up of two layers:—

1. "The inner mass," consisting of various kinds of cells in a gelatinous
matrix.

2. A high flagellated epithelium, which entirely covers the larva with the
exception of the hinder pole.

[Illustration: FIG. 68.—Longitudinal section through the hinder pole of the
larva of _G. varius_. _a_, Flagellated cells; _ma_^1, undifferentiated
cell; _ma_^2, differentiated cell; _pi_, pigment; _x_, surface of hinder
pole. (After Maas.)]

The cells in the inner mass are classified into (1) undifferentiated cells,
recognised by their nucleus, which possesses a nucleolus; these are the
archaeocytes; (2) differentiated cells, of which the nucleus contains a
chromatin net; these give rise to pinacocytes, collencytes, and
scleroblasts. Some of them form a flat epithelium, which covers the hinder
pole. Some of the scleroblasts already contain spicules. Fixation occurs
very early. The front pole is used for attachment, the pigmented pole
becoming the distal end (Fig. 69). The larva flattens out, the margin of
the attached end is produced into radiating pseudopodial processes. The
flagellated cells retreat to the interior, leaving the inner mass exposed,
and some of its cells thereupon form a flat outer epithelium. This is the
most important process of the metamorphosis; it is followed by a pause in
the outward changes, coinciding in time with rearrangements of the internal
cells to give rise to the canal system; that is to say, lacunae arise in
the inner mass, pinacocytes pass to the surface of the lacunae, and form
their lining; the flagellated cells, which have lain in confusion, become
grouped in small clusters. These become flagellated chambers,
communications are established between the various portions of the canal
system, and its external apertures arise. There is at first only one
osculum. The larvae may be obtained by keeping the parent sponge in a dish
of sea water, shielded from too bright a light, and surrounded by a second
dish of water to keep the temperature constant. They will undergo
{174}metamorphosis in sea water which is constantly changed, and will live
for some days.

We have said that the young sponge has only one osculum. This is the only
organ which is present in unit number, and it is natural to ask whether
perhaps the osculum may not be taken as a mark of the individual; whether
the fistular specimens, for example, of _H. panicea_ may not be solitary
individuals, and the cockscomb and other forms colonies in which the
individuals are merged to different degrees. Into the metaphysics of such a
view we cannot enter here. We must be content to refer to the views of
Huxley and of Spencer on Individuality.

But it is advisable to avoid speaking of a multi-osculate sponge as a
colony of many individuals, even in the sense in which it is usual to speak
of a colony of polyps as formed of individuals. The repetition of oscula is
probably to be regarded as an example of the phenomenon of repetition of
parts, the almost universal occurrence of which has been emphasised by
Bateson.[200] Delage[201] has shown that when two sponge larvae fixed side
by side fuse together, the resulting product has but one osculum. This,
though seeming to bear out our point of view, loses weight in this
connexion, when it is recalled that two Echinoderm larvae fused together
give rise in a later stage to but one individual.

[Illustration: FIG. 69.—Larva of _Gellius varius_ shortly after fixation.
The pigmented pole, originally posterior, is turned towards the reader.
_R_, Marginal membrane with pseudopodia; _x_, hinder pole. (After Maas.)]


EPHYDATIA FLUVIATILIS.

In the fresh water of our rivers, ponds, and lakes, sponges are represented
very commonly by _Ephydatia (Spongilla) fluviatilis_, a cosmopolitan
species. The search for specimens is most likely {175}to be successful if
perpendicular timbers such as lock-gates are examined, or the underside of
floating logs or barges, or overhanging branches of trees which dip beneath
the surface of the water.

The sponge is sessile and massive, seldom forming branches, and is often to
be found in great luxuriance of growth, masses of many pounds weight having
been taken off barges in the Thames. The colour ranges from flesh-tint to
green, according to the exposure to light. This fact is dealt with in a
most interesting paper by Professor Lankester,[202] who has shown not only
that the green colour is due to the presence of chlorophyll, but that the
colouring matter is contained in corpuscles similar to the chlorophyll
corpuscles of green plants, and, further, that the flesh-coloured specimens
contain colourless corpuscles, which, though differing in shape from those
which contain the green pigment, are in all probability converted into
these latter under the influence of sufficient light. The corpuscles, both
green and colourless, are contained in amoeboid cells of the dermal
layer;[203] and in the same cells but not in the corpuscles are to be found
amyloid substances.

The anatomy of _Ephydatia fluviatilis_ is very similar to that of
_Halichondria panicea_, differing only in one or two points of importance.
The ectosome is an aspiculous membrane of dermal tissue covering the whole
exterior of the sponge and forming the roof of a continuous subdermal
space. This dermal membrane is perforated by innumerable ostia, and is
supported above the subdermal cavity by means of skeletal strands, which
traverse the subdermal cavity and raise the dermal membrane into tent-like
elevations, termed conuli. The inhalant canals which arise from the floor
of the subdermal cavity are as irregular as in _H. panicea_, and
interdigitate with equally irregular exhalant canals; these latter
communicate with the oscular tubes. Between the two sets of canals are the
thin folds of the choanosome with its small subspherical chambers provided
with widely open apopyles (Fig. 70). The soft parts are supported on a
siliceous skeleton of oxeas, which may have a quite smooth surface or may
{176}be covered in various degrees with minute conical spines (Fig. 72,
_a_, _b_). These spicules are connected by means of a substance termed
_spongin_ deposited around their overlapping ends, so as to form an
irregular network of strands, of which some may be distinguished as main
strands or fibres, others as connecting fibres. In the main fibres several
spicules lie side by side, while in the connecting fibres fewer or
frequently single spicules form the thickness of the fibre. The fibres are
continuous at the base with a plate or skin of spongin, which is secreted
over the lower surface of the sponge and intervenes between it and the
substratum. Of the chemical composition of spongin we shall speak later
(see p. 237). It is a substance which reaches a great importance in some of
the higher sponges, and forms the entire skeleton of certain kinds of bath
sponge. Lying loose in the soft parts and hence termed flesh spicules, or
microscleres, are minute spicules of peculiar form. These are the
amphidiscs, consisting of a shaft with a many-rayed disc at each end (Fig.
72).

[Illustration: FIG. 70.—_Ephydatia fluviatilis._ Section of flagellated
chamber, showing the choanocytes passing through the apopyle. (After
Vosmaer and Pekelharing.)]

In addition to its habitat the fresh-water sponge is worthy of attention on
account of its methods of reproduction, which have arisen in adaptation to
the habitat. A similar adaptation is widespread among fresh-water members
of most aquatic invertebrates.[204]

{177}_Ephydatia fluviatilis_ normally produces not only free-swimming
larvae of sexual origin, but also internal gemmules arising asexually.
These bodies appear in autumn, distributed throughout the sponge, often
more densely in the deeper layers, and they come into activity only after
the death of the parent, an event which happens in this climate at the
approach of winter.

[Illustration: FIG. 71.—Portion of the skeletal framework of _E.
fluviatilis_. _a_, Main fibres; _b_, connecting fibres. (After Weltner.)]

[Illustration: FIG. 72.—Spicules of _E. fluviatilis_. _a._ _b._ _c._ Oxeas,
spined and smooth; _d._ _e_, amphidiscs, side and end views. (After
Potts.)]

Weltner[205] has shown that on the death and disintegration of the mother
sponge some of the gemmules remain attached to the old skeleton, some sink
and some float. Those which remain attached are well known to reclothe the
dead fibres with living tissue. They inherit, as it were, the advantages of
position, which contributed to the survival of the parent, as one of the
selected fittest. The gemmules which sink are doubtless rolled short
distances along the bottom, while those which float have the opportunity of
widely distributing the species with the risk of being washed out to sea.
But even these floating gemmules are exposed to far less dangers than the
delicate free-swimming larvae, for their soft parts are protected from
shocks by a thick coat armed with amphidiscs.

The gemmules are likewise remarkable for their powers of {178}resistance to
climatic conditions, powers which must contribute in no small way to the
survival of a species exposed to the variable temperatures of fresh water.
Thus, if the floating gemmules or the parent skeleton with its attached and
dormant offspring should chance to be included in the surface layer of ice
during the winter, so far from suffering any evil consequences they appear
to benefit by these conditions. Both Potts and Weltner have confirmed the
truth of this statement by experiments. Weltner succeeded in rearing young
from gemmules which had suffered a total exposure of 17 days to a
temperature "under 0° C."

Of important bearing on the question of the utility of the gemmules are
certain instances in which _E. fluviatilis_ has been recorded as existing
in a perennial condition.[206] The perennial individuals may or may not
bear gemmules, which makes it evident that, with the acquisition of the
power to survive the winter cold, the prime necessity of forming these
bodies vanishes.

The perennial specimens are described as exhibiting a diminished vegetative
activity in winter, the flagellated chambers may be absent (Lieberkühn), or
present in unusually small numbers (Weltner), the entire canal system may
be absent (Metschnikoff), or, on the other hand, it may be complete except
for the osculum.

[Illustration: FIG. 73.—Gemmule of _E. fluviatilis_. _b_, Amphidisc. (After
Potts.)]

In tropical countries gemmulation occurs as a defence against the ravages
caused by the dry season when the waters recede down their banks, exposing
all or most of their sponge inhabitants to the direct rays of the sun. The
sponges are at once killed, but the contained gemmules being thoroughly
dried, become efficient distributing agents of the species; they are light
enough to be carried on the wind. It is probable that those individual
sponges which escape desiccation survive the dry season without forming
gemmules.

It has been shown experimentally that gemmules are not injured by
drying—Zykoff found that gemmules kept dry for a period of two years had
not lost the power of germination.

{179}The mature gemmules consist of a more or less spherical mass of cells,
which we shall refer to as yolk cells, and of a complex coat. The latter is
provided with a pore or pore tube (Fig. 74) which is closed in winter by an
organic membrane.

There are three layers in the coat: an inner chitinous layer surrounded by
an air-chamber layer, which is finely vesicular, showing a structure
recalling plant tissue, and containing amphidiscs arranged along radii
passing through the centre of the gemmule. One of the discs of each
amphidisc lies in the inner chitinous coat, while the other lies in a
similar membrane which envelopes the air-chamber layer and is termed the
outer chitinous coat.

Marshall has suggested that one function of the amphidiscs is to weight the
gemmules and thus protect them against the force of the river current; and
no doubt the sinking or floating of individual gemmules depends on the
relative degree of development of the air-chambers and of the amphidiscs.

A study of the development of _Ephydatia_ gemmules vividly illustrates
various characters of the inner processes of sponges. Specially noteworthy
are the migrations of cells and the slight extent to which division of
labour is carried: one and the same cell will be found to perform various
functions.

[Illustration: FIG. 74.—Part of a longitudinal section of a gemmule of
_Ephydatia_ sp. passing through the pore (_a_). (After Potts.)]

The beginning of a gemmule is first recognisable[207] as a small cluster of
amoeboid archaeocytes in the dermal membrane. These move into the deeper
parts of the sponge to form larger groups. They are the essential part of
the gemmule, the yolk cells, which, when germination takes place, give rise
to a new sponge. They are followed by two distinct troops of actively
moving cells. Those forming the first troop arrange themselves round the
yolk cells and ultimately assume a columnar form so that they make an
epithelioid layer. They then secrete the inner chitinous coat. The cells of
the second troop are entrusted with the nutrition of the gemmule.
Consequently they pass in among the yolk cells, distribute their food
supplies, and make their escape {180}by returning into the tissues of the
mother sponge, before the columnar cells have completed the chitinous coat.
Yet another migration now occurs, the cells—"scleroblasts"—which have been
occupied in secreting amphidiscs at various stations in the sponge, carry
the fully formed spicules to the gemmules and place them radially round the
yolk cells between the radially lying cells of the columnar layer. The
scleroblasts themselves remain with the amphidiscs, and becoming modified,
contribute to the formation of the air-chamber layer. The columnar cells
now creep out between the amphidiscs till their inner ends rest on the
outer ends of these spicules. They then secrete the outer chitinous coat
and return to the mother sponge.

Carter gives directions[208] for obtaining young sponges from the gemmules.
The latter should be removed from the parent, cleaned by rolling in a
handkerchief, and then placed in water in a watch-glass, protected with a
glass cover and exposed to sunlight. In a few days the contents of the
gemmule issue from the foramen and can be seen as a white speck. A few
hours later the young sponge is already active and may be watched producing
aqueous currents. At this age the sponge is an excellent object for
studying in the living condition: being both small and transparent it
affords us an opportunity of watching the movements of particles of carmine
as they are carried by the current through the chambers.

Potts[209] describes how he has followed the transportal of spicules by
dermal cells, the end of each spicule multiplying the motion, swaying like
an oscillating rod.

In _E. fluviatilis_ reproduction also occurs during the warmer months in
this climate by means of sexual larvae. These are interesting for certain
aberrant features in their metamorphosis.[210] While some of the
flagellated chambers are formed in the normal way from the flagellated
cells of the larva, others arise each by division of a single archaeocyte.
This, it is suggested, is correlated with the acquisition of the method of
reproduction by gemmules, the peculiarities (_i.e._ development of organs
from archaeocytes) of which are appearing in the larvae.

DEFINITION.—We may now define sponges as multicellular, {181}two-layered
animals; with pores perforating the body-walls and admitting a current of
water, which is set up by the collared cells of the "gastral" layer.

POSITION IN THE ANIMAL KINGDOM.—Sponges are the only multicellular animals
which possess choanocytes, and their mode of feeding is unique. Since they
are two-layered it has been sought to associate them with the Metazoan
phylum Coelenterata, but they are destitute of nematocysts or any other
form of stinging cell, and their generative cells arise from a class of
embryonic cells set apart from the first, while the generative cells of
Coelenterata are derived from the ectoderm, or in other cases from the
endoderm. These weighty differences between sponges and that group of
Metazoa to which they would, if of Metazoan nature at all, be most likely
to show resemblance, suggest that we should seek a separate origin for
sponges and Metazoa. We naturally turn to the Choanoflagellate Infusorian
stock (see p. 121) as the source of Porifera, leaving the Ciliate stock as
the progenitors of Metazoa.

That both Porifera and Metazoa are reproduced by ova and spermatozoa is no
objection to this view, seeing that the occurrence of similar reproductive
cells has been demonstrated in certain Protozoa (see pp. 100, 128).

Let us now see which view is borne out by facts of embryology. Suppose, for
the moment, we regard sponges as Metazoa, then if the sponge larva be
compared with the Metazoan larva we must assign the large granular cells to
the endoderm; the flagellated cells to the ectoderm; and we are led to the
anomalous statement that the digestive cells in the adult are ectodermal,
the covering, outer cells endodermal; or conversely, if we start our
comparisons with the adults, then it follows that the larval ectoderm has
the characters of an endoderm, and the larval endoderm those of an
ectoderm.

Thus both embryology and morphology lead us to the same point, they both
show that in the absence of any fundamental agreement between Porifera and
Metazoa it is necessary to regard the two stocks as independent from the
very first, and hence the name PARAZOA (Sollas) has been given to the group
which contains the Porifera as its only known phylum.

Interesting in connexion with the phylogeny of Parazoa is the
Choanoflagellate genus _Proterospongia_ (Fig. 75), described by
{182}Saville Kent, and since rediscovered both in England and abroad.[211]
This is a colony of unicellular individuals embedded in a common jelly. The
individuals at the surface are choanoflagellate, while in the interior the
cells are rounded or amoeboid, and some of them undergo multiple fission to
form reproductive cells. This is just such a creature as we might imagine
that ancestral stage to have been of which the free-swimming sponge larva
is a reminiscence: for we have seen that the flagellated cells of the larva
are potential choanocytes.

[Illustration: FIG. 75.—_Proterospongia haeckeli._ _a_, Amoeboid cell; _b_,
a cell dividing; _c_, cell with small collar; _z_, jelly. × 800. (After S.
Kent.)]




{183}CHAPTER VIII

PORIFERA (_CONTINUED_): FORMS OF SPICULES—CALCAREA—HOMOCOELA—HETEROCOELA
—HEXACTINELLIDA—DEMOSPONGIAE—TETRACTINELLIDA—MONAXONIDA—CERATOSA—KEY TO
BRITISH GENERA OF SPONGES


Sponges fall naturally into two branches differing in the size of their
choanocytes: in the MEGAMASTICTORA these cells are relatively large,
varying from 5µ to 9µ in diameter; in MICROMASTICTORA they are about 3µ in
diameter.[212] For further subdivision of the group the SPICULES are such
important weapons in the hands of the systematist that it is convenient to
name them according to a common scheme. This has been arrived at by
considering first the number of axes along which the main branches of the
spicules are distributed, and secondly whether growth has occurred in each
of these axes in one or both directions from a point of origin.[213]

I. _Monaxons._—Spicules of rod-like form, in which growth is directed from
a single origin in one or both directions along a single axis. The axis of
any spicule is not necessarily straight, it may be curved or undulating.
The ray or rays are known as actines.

Biradiate monaxon spicules are termed "rhabdi" (Fig. 76, _a_). A rhabdus
pointed at both ends is an "oxea," rounded at both ends a "strongyle,"
knobbed at both ends a "tylote." By branching a rhabdus may become a
"triaene" (Fig. 110, _k_, _l_).

Uniradiate monaxon spicules are termed "styli."

II. _Tetraxons._—Spicules in which growth proceeds from an {184}origin in
one direction only, along four axes arranged as normals to the faces of a
regular tetrahedron. Forms produced by growth from an origin in one
direction along three axes lying in one plane are classed with tetraxons.

III. _Triaxons._—Spicules in which growth is directed from an origin in
both directions along three rectangular axes. One or more actines or one or
two axes may be suppressed.

IV. _Polyaxons._—Spicules in which radiate growth from a centre proceeds in
several directions.

V. _Spheres._—Spicules in which growth is concentric about the origin.

A distinction more fundamental than that of form is afforded by the
chemical composition: all sponges having spicules composed of calcium
carbonate belong to a single class, CALCAREA, which stands alone in the
branch Megamastictora.

[Illustration: FIG. 76.—Types of megascleres. _a_, Rhabdus (monaxon
diactine); _b_, stylus (monaxon monactine); _c_, triod (tetraxon
triactine); _d_, calthrop (tetraxon tetractine); _e_, triaxon hexactine;
_f_, euaster.]


BRANCH I. MEGAMASTICTORA


CLASS CALCAREA

Calcarea are marine shallow-water forms attached for the most part directly
by the basal part of the body or occasionally by the intervention of a
stalk formed of dermal tissue. They are almost all white or pale grey brown
in colour. Their spicules are either monaxon or tetraxon or both. The
tetraxons are either quadriradiate and then called "calthrops," or
triradiate when the fourth actine is absent. The triradiates always lie
more or less tangentially in the body-wall; similarly three rays of a
calthrop are tangentially placed, the fourth lying across the thickness of
the wall. It is convenient to include the triradiate and the three
tangentially placed rays of a calthrop under the common {185}term
"triradiate system" (Minchin). The three rays of one of these systems may
all be equal in length and meet at equal angles: in this case the system is
"regular." Or one ray or one angle may differ in size from the other rays
or angles respectively, which are equal: in either of these two cases the
system is bilaterally symmetrical and is termed "sagittal." A special name
"alate" is given to those systems which are sagittal in consequence of the
inequality in the angles. Thus all equiangular systems whether sagittal or
not are opposed to those which are alate. This is the natural
classification.[214]


SUB-CLASS I. HOMOCOELA.

The Homocoela or Ascons possess the simplest known type of canal system,
and by this they are defined. The body is a sac, branched in the adult, but
simple in the young; its continuous cavity is everywhere lined with
choanocytes, its wall is traversed by inhalant pores, and its cavity opens
to the exterior at the distal end by an osculum. The simple sac-like young
is the well-known Olynthus of Haeckel—the starting-point from which all
sponges seem to have set out. Two processes are involved in the passage
from the young to the adult, namely, multiplication of oscula and branching
of the original Olynthus tube or sac. If the formation of a new osculum is
accompanied by fission of the sac, and the branching of the latter is
slight, there arises an adult formed of a number of erect, well separated
main tubes, each with one osculum and lateral branches. Such is the case in
the LEUCOSOLENIIDAE. In the CLATHRINIDAE, on the other hand, branching of
the Olynthus is complicated, giving rise to what is termed reticulate body
form, that is, a sponge body consisting of a network of tubules with
several oscula, but with no external indication of the limits between the
portions drained by each osculum. These outward characters form a safe
basis for classification, because they are correlated with other
fundamental differences in structure and development.[215]

As in _Halichondria_, and in fact all sponges, the body-wall is formed of
two layers; the gastral layer, as we have said, forming a continuous lining
to the Ascon tube and its branches. The {186}dermal layer includes a
complete outer covering of pinacocytes, which is reflected over the oscular
rim to meet the gastral layer at the distal end of the tube; a deeper
gelatinous stratum in which lie scleroblasts and their secreted
products—calcareous spicules; and finally porocytes.[216] These last are
cells which traverse the whole thickness of the thin body-wall, and are
perforated by a duct or pore. The porocytes are contractile, and so the
pores may be opened or closed; they are a type of cell which is known only
in Calcarea. It will be noticed that the fusiform or stellate "connective
tissue cells" are absent. The layer of pinacocytes as a whole is highly
contractile, and is capable of diminishing the size of the sponge to such
an extent as quite to obliterate temporarily the gastral cavity.[217]

The choanocytes show certain constant differences in structure in the
families Clathrinidae and Leucosoleniidae respectively. In the former, the
nucleus of the choanocyte is basal; in the latter, it is apical, and the
flagellum can be traced down to it (Fig. 77).

[Illustration: FIG. 77.—The two types of Asconid collar cells. A, of
_Clathrina_, nucleus basal; B, of _Leucosolenia_, nucleus not basal,
flagellum arising from the nuclear membrane. (A, after Minchin; B, after
Bidder.)]

The tetraxon spicules have "equiangular" triradiate systems in the
Clathrinidae, while in Leucosoleniidae they are "alate." Finally, the larva
of Clathrinidae is a "parenchymula" (see p. 226), that of Leucosoleniidae
an "amphiblastula."

The fact that it is possible to classify the Calcarea Homocoela largely by
means of histological characters is in accordance with the importance of
the individual cell as opposed to the cell-layers generally throughout the
Porifera, and is interesting in serving to emphasise the low grade of
organisation of the Phylum. The organs of sponges are often unicellular
(pores), or the products of the activity of a single cell (many skeletal
elements); and even in the gastral layer, which approaches nearly to an
epithelium, comparable with the epithelia of Metazoa, the component cells
{187}still seem to assert their independence, the flagella not lashing in
concert,[218] but each in its own time and direction.


SUB-CLASS II. HETEROCOELA.

[Illustration: FIG. 78.—Transverse section of the body-wall of _Sycon
carteri_, showing articulate tubar skeleton, gastric ostia (_a.p_), tufts
of oxeas at the distal ends of the chambers (_fl.ch_), and pores (_p_).
(After Dendy.)]

[Illustration: FIG. 79.—_Sycon coronatum._ At a a portion of the wall is
removed, exposing the paragaster and the gastric ostia of the chambers
opening into it.]

The Heterocoela present a series of forms of successive grades of
complexity, all derivable from the Ascons, from which they differ in having
a discontinuous gastral layer. The simplest Heterocoela are included in the
family SYCETTIDAE, of which the British representative is _Sycon_ (Fig.
79). In _Sycon_ numerous tubular flagellated chambers are arranged radially
round a central cavity, the "paragaster," into which they open (Figs. 78,
79). The chambers, which are here often called radial tubes, are close set,
leaving more or less quadrangular tubular spaces, the {188}inhalant canals,
between them; and where the walls of adjacent chambers come in contact,
fusion may take place. Pores guarded by porocytes put the inhalant canals
into communication with the flagellated chambers. The paragaster is lined
by pinacocytes; choanocytes are confined to the flagellated chambers.

The skeleton is partly defensive, partly supporting; one set of spicules
strengthens the walls of the radial tubes and forms collectively the "tubar
skeleton." It is characteristic of Sycettidae that the tubar skeleton is of
the type known as "articulate"—_i.e._ it is formed of a number of
successive rings of spicules, instead of consisting of a single ring of
large spicules which run the whole length of the tube.

[Illustration: FIG. 80.—_Sycon setosum._ Young Sponge. × 200. _d_, Dermal
cell; _g_, gastral cell; _o_, osculum; _p_, pore cell; _sp__{1}, monaxon;
_sp__{3}, triradiate spicule. (After Maas.)]

The walls of the paragaster are known as the "gastral cortex"; they contain
quadriradiate spicules, of which the triradiate systems lie tangentially in
the gastral cortex, while the apical ray projects into the paragaster, and
is no doubt defensive. The distal ends of the chambers bristle with tufts
of oxeate spicules, and the separate chambers are distinguishable in
surface view. It is interesting to notice that in some species of _Sycon_,
the gaps between the distal ends of the chambers are covered over by a
delicate perforated membrane, thus leading on, as we shall see presently,
to the next stage of advance.[219] The larva of _Sycon_ is an amphiblastula
(see p. 227). Fig. 80 is a drawing of the young sponge soon after fixation;
it would pass equally well for an ideally simple Ascon or, neglecting the
arrangement of the spicules, for an isolated radial tube of _Sycon_. Figs.
81, 82 show the same sponge, somewhat older. From them it is seen that the
_Sycon_ type is produced from the young individual, in what {189}may be
called its Ascon stage, by a process of outgrowth of tubes from its walls,
followed by restriction of choanocytes to the flagellated chambers. Minute
observation has shown[220] that this latter event is brought about by
immigration of pinacocytes from the exterior. These cells creep through the
jelly of the dermal layer and line the paragaster as fast as its original
covering of choanocytes retreats into the newly formed chambers.

[Illustration: FIG. 81.—_S. setosum._ Young Sponge, with one whorl of
radial tubes. _o_, Osculum; _p_, pore; _sp__{1}, monaxon; _sp__{4},
quadriradiate spicule. (After Maas.)]

With a canal system precisely similar to that of _Sycon_, _Ute_ (Fig. 83)
shows an advance in structure in the thickening of the dermal layers over
the distal ends of the chambers. The dermal thickenings above neighbouring
chambers extend laterally and {190}meet; and there results a sheet of
dermal tissue perforated by dermal ostia, which open into the inhalant
canals, and strengthened by stout spicules running longitudinally. This
layer is termed a cortex; it covers the whole sponge, compacting the radial
tubes so that they form, together with the cortex, a secondary wall to the
sponge, which is once more a simple sac, but with a complex wall. The
cortex may be enormously developed, so as to form more than half the
thickness of the wall (Fig. 84). The chambers taken together are spoken of
as the chamber layer.

[Illustration: FIG. 82.—_Sycon raphanus._ A, Longitudinal section of young
decalcified Sponge at a stage somewhat later than that shown in Fig. 81. B,
Transverse section of the same through a whorl of tubes. _d_, Dermal
membrane; _g_, gastral membrane; _H_, paragaster; _sp__{4}, tetraradiate
spicule; _T_, radial tube. (After Maas.)]

[Illustration: FIG. 83.—Transverse section of the body-wall of _Ute_,
passing longitudinally through two chambers. _a.p_, Apopyle; _d.o_, dermal
ostium; _fl.ch_, flagellated chamber or radial tube; _i.c_, inhalant canal;
_p_, prosopyle. (After Dendy.)]

We have already alluded to the resemblance between a young Ascon person and
a radial tube of _Sycon_—a comparison which calls to mind the somewhat
strange view of certain earlier authors, that the flagellated chambers are
really the sponge individuals. If now we suppose each Ascon-like radial
tube of _Sycon_ to undergo that same process of growth by which the
{191}_Sycon_ itself was derived from the Ascon, we shall then have a sponge
with a canal system of the type seen in _Leucandra_ among British forms,
but more diagrammatically shown in the foreign genus _Leucilla_ (Fig. 85).
The foregoing remarks do not pretend to give an account of the transition
from _Sycon_ to _Leucilla_ as it occurred in phylogeny. For some indication
of this we must await embryological research.

In _Leucandra_ the fundamental structure is obscured by the irregularity of
its canal system. It shows a further and most important difference from
_Leucilla_ in the smaller size and rounded form of its chambers. This
change of form marks an advance in efficiency; for now the flagella
converge to a centre, so that they all act on the same drop of water, while
in the tubular chamber their action is more widely distributed and
proportionately less intense (see p. 236).

[Illustration: FIG. 84.—Transverse section through the body-wall of
_Grantiopsis_. _d.o_, Dermal ostium; _fl.ch_, flagellated chamber; _i.c_,
long incurrent canal traversing the thick cortex to reach the chamber
layer; _p_, apopyle. (After Dendy.)]

[Illustration: FIG. 85.—Transverse section through the body-wall of
_Leucilla_. _d.o_, Dermal ostium; _ex.c_, exhalant canal; _fl.ch_, chamber;
_i.c_, inhalant canal. (After Dendy.)]

Above are described three main types of canal system—that of Homocoela, of
_Sycon_, and of _Leucandra_ and _Leucilla_. These are conveniently termed
the first, second, and third types respectively, and may be briefly
described as related to one another somewhat in the same way as a scape,
umbel, and compound umbel among {192}inflorescences. These types formed the
basis of Haeckel's famous classification.[221] It has, however, been
concluded[222] that the skeleton is a safer guide in taxonomy, at any rate
for the smaller subdivisions; and in modern classifications genera with
canal systems of the third type will be found distributed among various
families; while in the Grantiidae, _Ute_ and _Leucandra_ stand side by
side. This treatment implies a belief that the third type of canal system
has been independently and repeatedly evolved within the Calcarea—an
example of a phenomenon, homoplasy, strikingly displayed throughout the
group. It is, remarkably enough, the case that all the canal systems found
in the remainder of the Porifera are more or less modified forms of one or
other of the second two types of canal system above described.

The families Grantiidae, Heteropidae, and Amphoriscidae, all possessing a
dermal cortex, are distinguished as follows:—The GRANTIIDAE by the absence
of subdermal sagittal triradiate spicules and of conspicuous subgastral
quadriradiates; the HETEROPIDAE by the presence of sagittal triradiates;
the AMPHORISCIDAE by the presence of conspicuous subgastral quadriradiates.

Two families of Calcarea, possibly allied, remain for special mention—the
Pharetronidae, a family rich in genera, and containing almost all the
fossil forms of the group, and the Astroscleridae.

The PHARETRONIDAE are with one, or perhaps two exceptions, fossil forms,
having in common the arrangement of the spicules of their main skeletal
framework in fibres. The family is divided into two sub-families:—

I. DIALYTINAE.—The spicules are not fused to one another; the exact mode of
their union into fibres is unknown, but an organic cement may be present.

_Lelapia australis_, a recent species, should probably be placed here as
the sole living representative. Dendy has shown[223] that this remarkable
species has a skeleton of the same fibrous character as is found in typical
Dialytinae, and that the triradiate spicules in the fibres undergo a
modification into the "tuning-fork" type (Fig. 86, C), to enable them to be
compacted into smooth fibres. {193}"Tuning-forks," though not exclusively
confined to Pharetronids, are yet very characteristic of them.

[Illustration: FIG. 86.—Portions of the skeleton of _Petrostroma schulzei._
A, Framework with ensheathing pellicle; B, quadriradiate spicules with
laterally fused rays; C, a "tuning-fork." (After Doederlein.)]

II. LITHONINAE.—The main skeletal framework is formed of spicules fused
together, and is covered by a cortex containing free spicules.

[Illustration: FIG. 87.—A spicule from the skeleton framework of
_Plectroninia_, showing the terminally expanded rays. (After Hinde.)]

The sub-family contains only one living genus and a few recently described
fossil forms. _Petrostroma schulzei_[224] lives in shallow water near
Japan; _Plectroninia halli_[225] and _Bactronella_ were found in Eocene
beds of Victoria; _Porosphaera_[226] long known from the Chalk of England
and of the Continent, has recently been shown by Hinde[226] to be nearly
allied to _Plectroninia_; finally, _Plectinia_[227] is a genus erected by
Počta for a sponge from Cenomanian beds of Bohemia. Doederlein, in 1896,
expressed his opinion that fossil representatives of Lithoninae would most
surely be discovered. The fused spicules are equiangular quadriradiates;
they are united in _Petrostroma_ by lateral fusion of the rays, in
_Plectroninia_ (Fig. 87) and _Porosphaera_ by {194}fusion of apposed
terminal flat expansions of the rays, and in some, possibly all, genera a
continuous deposit of calcium carbonate ensheaths the spicular reticulum.
Thus they recall the formation of the skeleton on the one hand of the
Lithistida and on the other of the Dictyonine Hexactinellida (see pp. 202,
211). "Tuning-forks" may occur in the dermal membrane.

[Illustration: FIG. 88.—_Astrosclera willeyana_, Lister. A, the Sponge, ×
about 3. _p_, The ostia on its distal surface. B, a portion of the skeleton
showing four polyhedra with radiating crystalline fibres. C, an ostium; the
surrounding tissue contains young stages of polyhedra. (After Lister.)]

The ASTROSCLERIDAE, as known at present, contain a single genus and
species, apparently the most isolated in the phylum. _Astrosclera
willeyana_[228] was brought back from the Loyalty Islands, and from
Funafuti of the Ellice group. Its skeleton is both chemically and
structurally aberrant. In other Calcarea the calcium carbonate of the
skeleton is present as calcite, in _Astrosclera_ as aragonite, and the
elements are solid polyhedra, {195}united by their surfaces to the total
exclusion of soft parts (Fig. 88). Each element consists of crystalline
fibres radially disposed around a few central granules, and terminating
peripherally in contact with the fibres of adjacent elements. Young
polyhedra are to be found free in the soft parts at the surface. The
chambers are exceptionally minute, especially for a calcareous sponge,
comparing with those of other sponges as follows:—

  _Astrosclera_ chambers, 10µ × 8µ to 18µ × 11µ.
  Smallest chambers in Silicea, 15µ × 18µ to 24µ × 31µ.
  Smallest chambers in Calcarea, 60µ × 40µ.

In its outward form _Astrosclera_ resembles certain Pharetronids. The
minute dimensions of the ciliated chambers relegate _Astrosclera_ to the
Micromastictora, and the fortunate fact that the calcium carbonate of its
skeleton possesses the mineral characters not of calcite, but of aragonite,
renders it less difficult to conceive that its relations may be rather with
the non-calcareous than the calcareous sponges.


BRANCH II. MICROMASTICTORA

All sponges which do not possess calcareous skeletons are characterised by
choanocytes, which, when compared with those of Calcarea, are conspicuous
for their smaller size. The great majority (Silicispongiae) of the
non-calcareous sponges either secrete siliceous skeletons or are connected
with siliceous sponges by a nicely graded series of forms. The small
remainder are entirely askeletal. All these non-calcareous sponges are
included, under the title Micromastictora, in a natural group, opposed to
the Megamastictora as of equal value.

The subdivision of the Micromastictora is a matter of some difficulty. The
Hexactinellida alone are a well circumscribed group. After their separation
there remains, besides the askeletal genera, an assemblage of forms, the
Demospongiae, which fall into two main tribes. These betray their
relationship by series of intermediate types, but a clue is wanting which
shall determine decisively the direction in which the series are to be
read. The askeletal genera are the _crux_ of the systematist. It is perhaps
safest, while recognising that many of them bear a likeness of {196}one
kind or another to various Micromastictora, to retain them together in a
temporary class, the Myxospongiae.


CLASS I. MYXOSPONGIAE

The class Myxospongiae is a purely artificial one, containing widely
divergent forms, which possess a common negative character, namely, the
absence of a skeleton. As a result of this absence they are all encrusting
in habit.

One genus, _Hexadella_, has been regarded by its discoverer Topsent[229] as
an Hexactinellid. The same authority places _Oscarella_ with the
Tetractinellida; it is more difficult to suggest the direction in which we
are to seek the relations of the remaining type, _Halisarca_.

_Hexadella_, from the coast of France, is a remarkable little rose-coloured
or bright yellow sponge, with large sac-like flagellated chambers and a
very lacunar ectosome.

_Oscarella_ is a brightly coloured sponge, with a characteristic velvety
surface; it is a British genus, but by no means confined to our shores. Its
canal system has been described by some authors as diplodal, by others as
eurypylous. Topsent[230] has shown, and we can confirm his statement, that
though the chambers have usually the narrow afferent and efferent ductules
of a diplodal system, yet since each one may communicate with two or three
canals, the canal system cannot be described as diplodal. The hypophare
attains a great development, and in it the generative products mature. The
pinacocytes, like those of Plakinidae, and perhaps of _Aplysilla_, are
flagellated.

_Halisarca_, also British, is easily distinguished from _Oscarella_ by the
presence of a mucus-like secretion which oozes from it, and by the absence
of the bright coloration characteristic of _Oscarella_. It naturally
suggests itself that the coloration in the one case and the secretion in
the other are protective, and in this respect perform one of the functions
of the skeleton of other sponges. The chambers are long, tubular, and
branched. There is no hypophare.


{197}CLASS II. HEXACTINELLIDA[231]

_Silicispongiae, defined by their spicules, of which the rays lie along
three rectangular axes. The canal system is simple, with thimble-shaped
chambers. The body-wall is divided into endosome, ectosome, and
choanosome._

Some authors would elevate the Hexactinellida to the position of a third
main sub-group of Porifera, thus separating them from other siliceous
sponges. In considering this view it is important to realise at the outset
that they are deep-water forms. They bear evident traces of the influence
of their habitat, and like others of the colonists of the deep sea, are
impressed with marked archaic features. Yet they are still bound to other
Micromastictora, first by the small size of their choanocytes, and secondly
by the presence of siliceous spicules. This second character is really a
double link, for it involves not merely the presence of silica in the
skeleton, but also the presence in each spicule of a well-marked axial
filament. Now this axial filament is a structure which is gaining in
importance, for purposes of classification, in proportion as its absence in
Calcarea is becoming more probable. The Hexactinellida are the only
sponges, other than the bath sponge, which are at all generally known. They
have won recognition by their beauty, as the bath sponge by its utility,
and, like it, one of their number—the Venus's Flower-Basket—forms an
important article of commerce, the chief fishery being in the Philippine
Islands. This wonderful beauty belongs to the skeleton, and is greatly
concealed when the soft parts are present.

We have said that the Hexactinellids are deep-sea forms; they are either
directly fixed to the bottom or more often moored in the ooze by long tufts
of rooting spicules. In the "glass-rope sponge," the rooting tuft of long
spicules, looking like a bundle of spun glass, is valued by the Japanese,
who export it to us. In _Monorhaphis_ the rooting tuft is replaced by a
single giant spicule,[232] three metres in length, and described as "of the
thickness of a little finger"! Probably it is as a result of their fixed
life in the calm waters of the deep sea[233] that {198}Hexactinellids
contrast with most other sponges by their symmetry. It should not, however,
be forgotten that many of the Calcarea which inhabit shallow water exhibit
almost as perfect a symmetry.

[Illustration: FIG. 89.—Longitudinal section of a young specimen of
_Lanuginella pupa_ O.S., with commencing formation of the oscular area. ×
35. _d.m_, Dermal membrane; _g.m_, gastral membrane; _pg_, paragaster;
_sd.tr_, subdermal trabeculae; _Sg.tr_, subgastral trabeculae. (After F. E.
Schulze.)]

The structure of the body-wall in Hexactinellida is so constant as to make
it possible to give a general description applicable to all members of the
group. It is of considerable thickness, but a large part is occupied by
empty spaces, for the actual tissue is present in minimum quantity. In the
wall the chamber-layer is suspended by trabeculae of soft tissue, between a
dermal membrane on the outside and a similar gastral membrane on the inner
side (Fig. 89). Thus the water entering the chambers through their numerous
pores has first passed through the ostia in the dermal membrane and
traversed the subdermal trabecular space; on leaving the chambers it flows
through the subgastral trabecular space and the ostia in the gastral
membrane, to enter the paragaster and leave the body at the osculum. The
trabeculae and the dermal and gastral membranes together constitute the
dermal layer. This conclusion is based on comparison with adults of the
other groups, for in the absence of embryological knowledge no direct
evidence is available. According to {199}the Japanese investigator, Isao
Ijima,[234] the dermal and gastral membranes are but expansions of the
trabeculae, and the trabeculae themselves are entirely cellular, containing
none of the gelatinous basis met with in the dermal layer of all other
sponges. There is no surface layer of pinacocytes, the cells forming the
trabeculae being all of one type, namely, irregularly branching cells,
connected with one another by their branches to form a syncytium. In the
trabeculae are found scleroblasts and archaeocytes.

The chambers have a characteristic shape: they are variously described as
"thimble-shaped," "tubular," or "Syconate," and they open by wide mouths
into the subgastral trabecular space. Their walls have been named the
_membrana reticularis_ from the fact that, when preserved with only
ordinary precautions, they are seen as a regular network of protoplasmic
strands, with square meshes and nuclei at the nodes. This appearance
recently found an explanation when Schulze, for the first time, succeeded
in preserving the collared cells of Hexactinellids.[235] Schulze was then
able to show that the choanocytes are not in contact with one another at
their bases, where the nuclei are situated, but communicate with one
another by stout protoplasmic strands. The form of the choanocyte can be
seen in Fig. 91.

[Illustration: FIG. 90.—Portion of the body-wall of _Walteria_ sp., showing
the thimble-shaped flagellated chambers, above which is seen the dermal
membrane. (After F. E. Schulze.)]

To Schulze's description of the chamber, Ijima has added the important
contributions that every mesh in the reticulum functions as a chamber pore
or prosopyle; and that porocytes, such as are found in Calcarea, are
wanting. This structure of the chamber-walls, the absence of gelatinous
basis in the dermal layer, and the slight degree of histological
differentiation in {200}the same layer, added to the more obvious character
of thimble-shaped chambers, are the chief archaic features of Hexactinellid
morphology.

[Illustration: FIG. 91.—Portion of a section of the membrana reticularis or
chamber-wall of _Schaudinnia arctica_, × 1500. (After F. E. Schulze.)]

The skeleton which supports the soft parts is, like them, simple and
constant in its main features. It is secreted by scleroblasts, which lie in
the trabeculae, and is made up of only one kind of spicule and its
modifications. This is the hexactine, a spicule which possesses six rays
disposed along three rectangular axes. Each ray contains an axial thread,
which meets its fellow at the centre of the spicule, where they together
form the axial cross. Modifications of the hexactine arise either by
reduction or branching, by spinulation or expansion of one or more of the
rays. The forms of spicule arising by reduction are termed pentactines,
tetractines, and so on, according to the number of the remaining rays.
Those rays which are suppressed leave the proximal portion of their axial
thread as a remnant marking their former position (Fig. 94). Octactine
spicules seem to form an exception to the above statements, but Schulze has
shown that they too are but modifications of the hexactine arising by (1)
branching of the rays of a hexactine, followed by (2) recombination of the
secondary rays (Fig. 92).

[Illustration: FIG. 92.—A, discohexaster, in which the four cladi _a_,
_a'_, _b_, _b'_, _c_ of each ray start directly from a central nodule. B,
disco-octaster, resulting from the redistribution of the twenty-four cladi
of A into eight groups of three. (After Schulze, from Delage.)]

The various spicules are named, irrespective of their form, according to
their position and corresponding function. The {201}arrangement of the
spicules is best realised by means of a diagram (Fig. 93).

[Illustration: FIG. 93.—Scheme to show the arrangement of spicules in the
Hexactinellid skeleton. _Canalaria_, microscleres in the walls of the
excurrent canals; _Dermalia Autoderm[alia]_, microscleres in the dermal
membrane; _D. Hypoderm[alia]_, more deeply situated dermalia;
_Dictyonalia_, parenchymalia which become fused to form the skeletal
framework of Dictyonina; _Gastralia Autogastr[alia]_, microscleres in the
gastral membrane; _Gastralia Hypogastr[alia]_, more deeply situated
gastralia; _Parenchymalia Principalia_, main supporting spicules between
the chambers; _P. Comitalia_, slender diactine or triactine spicules
accompanying the last; _P. Intermedia_, microscleres between the P.
principalia; _Prostalia_, projecting spicules; _P. basalia_, rooting
spicules, from the base; _P. marginalia_, defensive spicules, round the
oscular rim; _P. pleuralia_, defensive spicules, from the sides. (From
Delage and Hérouard, after F. E. Schulze.)]

The deviations from this ground-plan of Hexactinellid structure are few and
simple. They are due to folding of the chamber-layer, or to variations in
the shape of the chambers, and to increasing fusion of the spicules to form
rigid skeletons. A simple condition of the chamber-layer, like that of the
young sponge of Fig. 89, {202}occurs also in some adult Hexactinellids,
_e.g._ in _Walteria_ of the Pacific Ocean (Fig. 90). Thus is represented in
this order the second type of canal system described among Calcarea. More
frequently, however, instead of forming a smooth sheet, the chamber-layer
grows out into a number of tubular diverticula, the cavities of which are
excurrent canals; these determine a corresponding number of incurrent
canals which lie between them. In this way there arises a canal system
resembling the third type of Calcarea. By still further pouching so as to
give secondary diverticula, opening into the first, a complicated canal
system is formed, as, for example, in _Euplectella suberea_.

To return to the skeleton, the most complete fusion is attained by the
deposit of a continuous sheath of silica round the apposed parallel rays of
neighbouring spicules. This may be termed the dictyonine type of union, for
it occurs in all those forms originally included under the term Dictyonina,
in which the cement is deposited _pari passu_ with the formation of the
spicules. In other cases connecting bridges of silica unite the spicules,
or there may be a connecting reticulum of siliceous threads, or, again,
rays crossing obliquely may be soldered together at the point of contact.
These more irregular methods occur in species where the spicules are free
at their first formation. Spicules originally free may later be united in a
true Dictyonine fashion. The terms LYSSACINA and DICTYONINA are useful to
denote respectively: the former all those Hexactinellida in which the
spicules are free at their first formation, and the latter those in which
the deposit of the cementing layer goes hand in hand with the formation of
the spicules. But the terms do not indicate separateness of origin of the
groups denoted by them, for there is evidence that Dictyonine types have
been derived repeatedly from Lyssacine types, and that in fact every
Dictyonine was once a Lyssacine.

[Illustration: FIG. 94.—Amphidisc, at _a_ are traces of the four missing
rays.]

The real or natural cleft in the class lies between those genera possessing
amphidiscs (Figs. 94, 97) among their microscleres, and all the remainder
of the Hexactinellida which bear hexasters (Fig. {203}96). The former set
of genera constitute the sub-class Amphidiscophora, the latter the
Hexasterophora.

[Illustration: FIG. 95.—Portion of body-wall of _Hyalonema_, in section,
showing the irregular chambers.]

SUB-CLASS 1. AMPHIDISCOPHORA.—_Amphidiscs are present, hexasters absent. A
tuft of rooting spicules or basalia is always present. The ciliated
chambers deviate more or less from the typical thimble shape, and the
membrana reticularis is continuous from chamber to chamber_ (Figs. 94, 95,
97).

[Illustration: FIG. 96.—Hexasters. A, Graphiohexaster; B, floricome; C,
onychaster.]

SUB-CLASS 2. HEXASTEROPHORA.—_Hexasters are present, amphidiscs absent. The
chambers have the typical regular form, and are sharply marked off from one
another_ (Figs. 90, 96).

All the Amphidiscophora have Lyssacine skeletons; in the Hexasterophora
both types of skeleton occur. The subdivision of the Hexasterophora is
determined by the presence or absence of uncinate spicules. An "uncinatum"
is a diactine spicule, pointed at both ends and bearing barbs all directed
towards one end. This method of classification gives us a wholly Dictyonine
order, UNCINATARIA, and an order consisting partly of Dictyonine, partly of
Lyssacine genera, which may be distinguished as the ANUNCINATARIA. {204}Ova
have rarely been found, and sexually produced larvae never; but Ijima has
found archaeocyte clusters in abundance, and his evidence is in favour of
the view that they give rise asexually to larvae, described by him in this
class for the first time (see p. 231).

Both sub-classes are represented in British waters: the Amphidiscophora by
_Hyalonema thomsoni_ and _Pheronema carpenteri_; the Hexasterophora by
_Euplectella suberea_ and _Asconema setubalense_, and of course possibly by
others.

_Hyalonema thomsoni_, one of the glass-rope sponges, was dredged by the
_Porcupine_ off the Shetland Islands in water of about 550 fathoms. The
spindle-shaped body of the sponge is shown in Fig. 97. Its long rooting
tuft is continued right up its axis, to end in a conical projection, which
is surrounded by four apertures leading into corresponding compartments of
the paragaster.

[Illustration: FIG. 97.—_Hyalonema thomsoni._ A, Whole specimen with
rooting tuft and _Epizoanthus_ crust; B, pinulus, a spicule characteristic
of but not peculiar to the Amphidiscophora, occurring in the dermal and
gastral membranes; C, amphidisc with axial cross; D, distal end of rooting
spicule with grapnel. (After F. E. Schulze.)]

The crust of Anthozoa of the genus _Epizoanthus_ (p. 406) on the rooting
tuft is a constant feature in this as in other species of _Hyalonema_. It
contributed to make the sponge a puzzle, which long defied interpretation.
The earliest diagnosis the genus received was the "Glass Plant." Then the
root tuft was thought to be part of the _Epizoanthus_, which was termed a
"most aberrant Alcyonarian with its base inserted in a sponge"; next we
hear of the sponge as parasitic {205}on the Sea Anemone. Finally, the root
tuft was shown to be proper to the sponge, which was, however, figured
upside down, till some Japanese collectors described the natural position,
or that in which they were accustomed to find it.

_Pheronema carpenteri_ was found by the _Lightning_ off the north of
Scotland in 530 fathoms. The goblet shaped, thick walled body and broad,
ill-defined root tuft are shown in Fig. 98, but no figure can do justice to
the lustre of its luxuriant prostalia and delicate dermal network with
stellate knots at regular intervals. The basalia are two-pronged and
anchor-like.

[Illustration: FIG. 98.—_Pheronema carpenteri._ × ½. (From Wyville
Thomson.)]

Both the Hexasterophoran genera were dredged off the north of Scotland, and
both conform to the Lyssacine type without uncinates. _Euplectella suberea_
is a straight, erect tube, anchored by a tuft {206}of basalia. The upper
end of the tube is closed by a sieve plate, the perforations in which are
oscula, while the beams contain flagellated chambers, so that the sieve is
simply a modified portion of the wall. It is a peculiarity of this as of
one or two other allied genera that the lateral walls are perforated by
oscula. They are termed parietal gaps, and are regularly arranged along
spiral lines encircling the body.

[Illustration: FIG. 99.—Sieve plate of _Euplectella imperialis_. (After
Ijima.)]

Ijima, who has dredged Euplectellids from the waters near Tokyo, finds that
in young specimens oscula are confined to the sieve plate; parietal gaps
are secondary formations. The groundwork of the skeleton is a lattice
similar to that shown in Fig. 100. The chamber-layer is much folded.
Various foreign species of _Euplectella_ afford interesting examples of
association with a Decapod Crustacean, _Spongicola venusta_, of which a
pair lives in the paragaster of each specimen. The Crustacean is light
pink, the female distinguished by a green ovary, which can be seen through
the transparent tissues. It is not altogether clear what the prisoner
gains, nor what fee, if any, the host exacts.

Ijima relates that the skeleton of _Euplectella_ is in great demand in
Japan for marriage ceremonies. He also informs us that the Japanese name
means "Together unto old age and unto the same grave," while by a slight
alteration it becomes "Lobsters in the same cell," and remarks that the
Japanese find this an amusing pun.

[Illustration: FIG. 100.—Skeletal lattice of _Euplectella imperialis_.
(After Ijima.)]

The same _Spongicola_ lives in pairs in _Hyalonema sieboldi_. Another case
of apparently constant association is that of the Hydroid stocks which
inhabit _Walteria_. F. E. Schulze describes _Stephanoscyphus mirabilis_
(see p. 318) in a specimen of _Walteria flemmingi_; the presence of the
polyp causes the sponge to grow out into little dome-shaped elevations,
each of which shelters one polyp; while in _W. leuckarti_ Ijima finds a
similar association in every specimen examined.


{207}FOSSIL HEXACTINELLIDA.

This group has the distinction of including among its Lyssacine members the
oldest known sponge, _Protospongia fenestrata_, of Cambrian age (Salter).
As preserved it consists of a single layer of quadriradiate, or possibly
quinqueradiate spicules, which, arranged as a square meshed lattice,
supported the superficial layer of the sponge (Fig. 101). Whether or not
the fossil represents the whole of the sponge-skeleton does not
appear.[236]

[Illustration: FIG. 101.—Part of the specimen of _Protospongia fenestrata_
in the Sedgwick Museum, Cambridge. Nat. size. (After Sollas.)]

[Illustration: FIG. 102.—A portion of the outer surface of a
Receptaculitid, _Acanthoconia barrandei_, in which the expanded outer rays
of the spicules are partially destroyed, revealing the four tangential rays
beneath, × 3. (After Hinde.)]

The extraordinary RECEPTACULITIDAE are probably early Lyssacine forms: they
are cup- or saucer-shaped fossils, abundant in Silurian and above all in
Devonian strata, and have been "assigned in turn to pine cones,
Foraminifera, Sponges, Corals, Cystideans," and Tunicata. Hinde[237] brings
forward important arguments for retaining them among Hexactinellida. The
only elements in the skeleton of the simpler genera, _e.g._ _Ischadites_,
are structures comparable to Hexactinellid spicules. The surface of the
fossil presents a series of lozenges forming a regular mosaic. Each lozenge
is the expanded end of one of the rays of a spicule; it conceals four rays
in one plane, tangential to the wall of the cup-shaped fossil, while the
sixth ray projects vertically to the wall into the cavity of the cup. In
the genus _Receptaculites_ itself there is an inner layer of plates
abutting against the inner {208}ends of the sixth rays, and at present
problematic. An axial canal is present in each of the rays—the six canals
meeting at the centre of the spicule. Special chinks between the spicules
appear to have provided a passage for the water current.

The beautiful _Ventriculites_, so common in the Chalk and present in the
Cambridge Greensand, are historically interesting, for the fact that they
are fossil Hexactinellida of which the general and skeletal characters were
very minutely described by Toulmin Smith long before recent representatives
of the group were known. In common with a number of fossil Dictyonine
species they are distinguished by the perforation of the nodes, a character
due to the fact that the siliceous investment which unites the spicules
together stops short before reaching the centre of each spicule, and
bridges across the rays so as to form a skeleton octahedron. This character
is rare in recent Hexactinellids, but, as first pointed out by Carter, it
is presented by one or two forms, of which _Aulocystis grayi_ Bwk is best
known. The majority of the fossil Hexactinellida belong to the Dictyonine
section, a fact attributable to the greater coherence of their skeleton.
The "Dictyonina" are to be reckoned among the rock-builders of Jurassic and
Cretaceous times.

[Illustration: FIG. 103.—A node of the skeleton of _Ventriculites_ from the
Cambridge Greensand. (After Sollas.)]

The OCTACTINELLIDA and HETERACTINELLIDA are two classes created by
Hinde[238] to contain certain little-known Devonian and Carboniferous
sponges, possessing in the one case 8-rayed spicules, of which 6 rays lie
in one plane and 2 are perpendicular to this plane; in the other case,
spicules with a number of rays varying from 6 to 30. Bearing in mind the
manner in which octactine spicules are known to arise in recent
Hexactinellida (p. 200), it is clearly possible to derive these 8-rayed
spicules from hexactines by some similar method; while the typical
{209}spicule of the Heteractinellida is a euaster. Hence we may refer the
Octactinellid fossils to the class Hexactinellida, and the Heteractinellid
forms either to the Monaxonida or Tetractinellida.


CLASS III. DEMOSPONGIAE

_Silicispongiae in which triaxonid spicules are absent._

This class has attained the highest level of organisation known among
Porifera; the most efficient current-producing apparatus is met with here,
so, too, are protective coverings, stout coherent skeletons, and the
highest degree of histological differentiation found in the phylum.

Correspondingly it is the most successful group, the majority of existing
sponges coming within its boundaries. A few genera and species are
exceedingly specialised, for example, _Disyringa dissimilis_ (p. 215).
These, however, contribute only a very small contingent to the Demosponge
population, those species which are really prolific and abundant being, as
we should expect, the less exaggerated types.

CANAL SYSTEM.—With a few exceptions the representatives of the Demospongiae
may be said to have taken up the evolution of the canal system at the stage
where it was left in _Leucandra aspera_—a stage which the ancestral
Demosponges must have reached quite independently of the Calcarea. These
commoner members are thus already gifted with the advantages pertaining to
a spherical form of ciliated chamber, and so, too, is the Rhagon (Fig.
105), an immature stage noteworthy as the simplest form of Demosponge, and
thus the starting-point for the higher types of canal system. The
exceptions above alluded to are not without interest: they are the
Dendroceratina, of doubtful affinities, (p. 220), which possess small
tubular Syconate chambers. They may be regarded either as of independent
origin from other Demospongiae, thus making the group polyphyletic, or more
simply as representing the ancestral condition, and in this case we must
look on the possession of spherical chambers by the Rhagon as a secondary
feature. Occupying as it does the important position above indicated, the
Rhagon merits a brief description. It is a small discoid or hemispherical
body attached by a flat base. It contains a central paragaster, with a
single osculum at the free end. Into the paragaster open directly a
{210}few spherical flagellated chambers, which lie in the lateral walls of
the body. The basal wall of the paragaster, the parts of its lateral walls
between the openings of neighbouring chambers, and the entire outer surface
of the body are covered with pinacocytes. It is convenient to call the
basal part of the sponge from which chambers are absent the hypophare, the
upper chamber-bearing part the spongophare. In some of the deeper dermal
cells spicules may be already present. In the Rhagon, then, the canal
system is of the second type, but all the adult Demosponges have advanced
to the third type, and the further evolution in this system is in the
direction of improving the mode of communication of the chambers with the
canal system. The changes involved go hand in hand with increasing bulk of
the dermal layer. A glance at the accompanying figures will show at once
the connexion between the phenomena. The increase in the dermal layer (1)
greatly reduces the extent of the lumen of the excurrent canals; and (2)
results in the intervention of a narrow tube or aphodus between the mouth
of each chamber and the excurrent canal. The chamber system is then
converted from an "eurypylous" to an "aphodal" type. When the incurrent
canal also opens into the chamber by way of narrow tubes, one proper to
each chamber and termed "prosodus," the canal system is of the "diplodal"
type.

[Illustration: FIG. 104.—Diagram of (A) eurypylous and (B) aphodal canal
systems. _a_, Apopyle; _a'_, aphodus; _E_, excurrent canal; _I_, incurrent
canal; _p_, prosopyle; _p'_, short prosodus. (After Sollas.)]

CORTEX.—All the stages in the formation of a cortex are to be seen among
the adult members of the group. Certain species (e.g. _Plakina monolopha_,
F.E.S.) are destitute even of an ectosome, {211}others have a simple dermal
membrane (_Halichondria panicea_, _Tetilla pedifera_) and various others
are provided with a cortex, either of simple structure or showing
elaboration in one or more particulars. Thus a protective armature of
special spicules may be present in the cortex, _e.g._ in _Geodia_, or to a
less extent in _Tethya_, or there may be an abundance of contractile
elements, and these may be arranged in very definite ways, forming
valve-like apparatus that will respond to stimuli.

Everywhere among sponges the goal of the skeleton appears to have been
coherence. We have seen how in Calcarea and in Hexactinellida this has been
attained by the secretion around the separate elements of a continuous
mineral sheath, calcareous in the one case and siliceous in the other. Here
we had an excellent instance of the attainment of one end by similar means
in two different groups, after their separation from the common stock, and
therefore independently. In Demospongiae, on the other hand, the same
end—coherence—has been secured by two new methods, each distinct from the
former: first the spicules may be united in strands by an organic deposit,
spongin; secondly, the spicules may assume irregular shapes and interlock
closely with one another, forming dense and stout skeletons. The latter
method is that characteristic of the Lithistid Tetractinellida.

CLASSIFICATION.—It is not of great moment which scheme of classification we
maintain, seeing that all hitherto proposed are confessedly more or less
artificial, and sufficient data for framing a natural one are not yet
forthcoming. For convenience, we accept three subdivisions and define them
thus:—

  I. TETRACTINELLIDA.—Demospongiae possessing tetraxon or triaene spicules
  or Lithistid desmas.

  II. MONAXONIDA.—Demospongiae possessing monaxon but not tetraxon
  spicules.

  III. CERATOSA.—Demospongiae in which the main skeleton is formed of
  fibres of spongin. The fibres may have a core of sand-grains or of
  foreign spicules, but not of spicules proper to the sponge.

But at the same time we admit that some of the Ceratosa are probably
descended from some of the families of Monaxonida, so that we should
perhaps be justified in separating these families of Monaxonida from the
rest, and associating them with the allied families of Ceratosa—a method of
classification due to {212}Vosmaer. Again, some Monaxonida approximate to
Tetractinellida, and we might, with Vosmaer, unite them under the title
Spiculispongiae. This proceeding, though it has the advantage of being at
least an attempt to secure a natural classification, involves too much
assumption when carried out in detail to be wholly satisfactory.


SUB-CLASS I. TETRACTINELLIDA.[239]

Tetractinellida appear to flourish best in moderate depths from 50 to 200
fathoms, but they are found to be fairly abundant also in shallower water
right up to the coast line, and in deep water up to and beyond the 1000
fathom line. Occasionally they lie free on the bottom, but are far more
commonly attached; fixation may be direct or by means of rooting spicules;
the occurrence of a stalk is rare. There is great variety in the root tuft,
which may be a long loose wisp of grapnel-headed spicules, as in many
species of _Tetilla_, or a massive tangle, as in _Cinachyra barbata_; in
these cases the sponge is merely anchored, so that it rests at the level of
the surface of the ooze; in other cases, _e.g._ _Thenea wyvillei_, the root
tuft consists of a number of pillars of spicules which raise the sponge
above the level of the ooze, into which they descend and there become
continuous with a large dense and confused mass of spicules. The
parachute-like base of _Tetilla casula_ invites comparison with the
"Crinorhiza" forms of some Monaxonids (p. 216).

Two Orders are distinguished thus:—

  I. CHORISTIDA.—Tetractinellida with quadriradiate spicules, which are
  never articulated together into a rigid network.

  II. LITHISTIDA.—Tetractinellida with branching scleres (desmas), which
  may or may not be modified tetrad spicules, articulated together to form
  a rigid network. Triaene spicules may or may not be present in addition.


ORDER I. CHORISTIDA.

_Plakina monolopha_, from the Adriatic and Mediterranean, furnishes a
connecting link between the Rhagon stage and other Tetractinellida. The
choanosome is simply folded; there is no distinct ectosome; the chambers
are eurypylous. The skeleton {213}consists of microcalthrops and their
derivatives. The hypophare is well developed. _Plakina_ thus shows a
certain amount of resemblance to _Oscarella_ (p. 196), with which it shares
the very remarkable possession of flagellated pinacocytes.

One of the species of _Tetilla, T. pedifera_, continues the series. The
folds of its choanosome are more complicated than in _P. monolopha_, and
their outer ends are bridged together by a thin layer of ectosome (cf.
species of _Sycon_ among Calcarea); the chambers are still eurypylous.

The skeleton reaches a high level: it includes oxeas and triaenes radiately
disposed and microscleres (sigmata) scattered throughout the dermal layer.
The British _Poecillastra compressa_ from the north of Scotland and Orkney
and Shetland is at about the same stage of development, being without
cortex and having eurypylous chambers, but it is not so good an example, as
the folds of its choanosome are confused.

[Illustration: FIG. 105.—Diagrammatic vertical sections of A, Rhagon; B,
_Plakina_; C, _Tetilla pedifera_.]

From _T. pedifera_ we pass to the other species of _Tetilla_ and all the
higher genera of Choristida; these possess a cortex not of homologous
origin in the various cases, but probably to be classified under one of two
heads, typified by _Stelletta_ and _Craniella_ respectively (Fig. 106).

[Illustration: FIG. 106.—A, _Craniella_ type; B, Stellettid type. _ch_,
Chone; _co_, collenchyma; _d.o_, dermal ostia; _fb_, fibrous tissue; _i.c_,
intercortical cavity; _sd_, subdermal cavity; _sp_, sphincter. (After
Sollas.)]

{214}In the Stellettids the cortex arises by the centrifugal growth of a
dermal membrane such as that of _Tetilla pedifera_; in _Craniella_ directly
from the dermal tissue of the distal ends of the choanosomal folds.

In both cases the end result, after completion of cell differentiation, is
a cortex either fibrous throughout or collenchymatous in its outer portion
and fibrous in the deeper layers. In the Stellettid type the centrifugal
growth of the dermal membrane involves the addition of secondary distal
portions to the ends of the inhalant passages. These are the intercortical
cavities or canals. Their most specialised form is the "chone." A chone is
a passage through the cortex opening to the exterior by one or more ostia,
and communicating with the deeper parts of the inhalant system by a single
aperture provided with a sphincter (Fig. 106, B).

In the _Craniella_ type the intercortical cavities are parts of the primary
inhalant system. They communicate with its deeper parts by sphinctrate
apertures. Without any knowledge of the development one would certainly
have supposed that the subdermal cavity, pore-sieve and sphinctrate
passages of _Craniella_ represented a number of chones, of which the outer
portions had become fused (Fig. 106, A).

[Illustration: FIG. 107.—_Disyringa dissimilis._ Diagrammatic longitudinal
section of the Sponge. × ½. _a_, _b_, _c_, Transverse sections at the
levels indicated to show subdivision of the lumina of the excurrent and
incurrent tubes; _e.t_, excurrent tube; _i.t_, incurrent tube; _o_,
osculum. (After Sollas.)]

In both _Craniella_ and _Stelletta_ the chamber system is aphodal, and
these genera may fairly be taken as representatives of the average level
reached by Tetractinellida. The skeleton is of the radiate type: the type
which prevails in the Choristida, but which has an erratic distribution,
appearing in some genera of {215}each family but not in others. The genus
_Pachymatisma_, of which we have the species _P. johnstonia_ and _P.
normani_ in these islands, exemplifies this; it belongs to the highly
differentiated family Geodiidae, possesses an elaborate cortex with chones,
but its main skeleton is non-radiate.

_Disyringa dissimilis_ is remarkable for the perfection of its symmetry,
and for the absence of that multiplication of parts which is so common
among sponges. It possesses a single inhalant tube and a single osculum
(Fig. 107). Until quite recently it stood alone in the restriction of its
inhalant apertures to a single area. Kirkpatrick, however, has now
described a sponge—_Spongocardium gilchristi_[240]—from Cape Colony, in
which the dermal ostia are concentrated in one sieve-like patch at the
opposite pole to the single osculum. _Disyringa_ is still without
companions in the possession of an inhalant tube. The concentration of
ostia into sieve areas occurs again in _Cinachyra_, each sponge possessing
in this case several inhalant areas with or without scattered ostia also.


ORDER II. LITHISTIDA.

The characteristic spicule of Lithistida—the desma—may be a modified
calthrop (tetracrepid desma), or it may be produced by the growth of silica
over a uniaxial spicule (rhabdocrepid desma) (Fig. 110, _q_), or it may be
of the polyaxon type. It is probable that the group is polyphyletic,[241]
and that some of its members should remain associated with Tetractinellida,
while others should be removed to Monaxonida. Forms with tetracrepid
desmas, and those forms with rhabdocrepid desmas which possess triaenes,
have Tetractinellid affinities, while forms possessing rhabdocrepid desmas
but lacking triaenes, and again those in which the desmas are polyaxon, are
probably descendants of Monaxonida.

Owing to the consistency of the skeleton Lithistida are frequently found as
fossils. The commonest known example is _Siphonia_.[242] As in the case of
so many other fossil sponges the skeleton is often replaced by carbonate of
lime, a fact which {216}misled some of the earlier investigators but was
established by the researches of Sollas and Zittel.


SUB-CLASS II. MONAXONIDA.[243]

The Monaxonida inhabit for the most part shallow water, but they also
extend through deep water into the abysses, thirteen species having been
dredged from depths of over 2000 fathoms by the "Challenger" Expedition
alone. In some cases, e.g. _Cladorhiza_, _Chondrocladia_, all the species
of a genus may live in deep water, while in others the genus, or in others,
again, the species, may have a wide bathymetrical range. Thus _Axinella_
spp. occur in shallow water and in various depths down to 2385 fathoms,
_Axinella erecta_ ranges from 90 to 1600 fathoms, _Stylocordyla stipitata_
from 7 to 1600, and so on. The symmetry of the deep-water forms contrasts
strikingly with the more irregular shape of their shallow-water
allies.[244] The shallow-water species are almost always directly attached,
some few are stalked; those from deep water have either a long stalk or
some special device to save them from sinking in the soft ooze or mud. Thus
the deep-sea genus _Trichostemma_ has the form of a low inverted cone,
round the base of which a long marginal fringe of spicules projects,
continuing the direction of the somal spicules, and so forming a supporting
rim. The same form has been independently evolved in _Halicnemia patera_,
and an approach to it in _Xenospongia patelliformis_. A similar and more
striking case of homoplasy is afforded by the Crinorhiza form, which has
been attained in certain species of the deep-sea genera _Chondrocladia_,
_Axoniderma_, and _Cladorhiza_; here the sub-globular body is supported by
a vertical axis or root, and by a whorl of stout processes radiating
outwards and downwards from it, and formed of spicular bundles together
with some soft tissue.

There is recognisable in the order Monaxonida a cleft between one set of
genera, typically corticate, and suggesting by their structure a
relationship, whether of descent or parentage, with the Tetractinellida,
and a second set typically non-corticate: these latter are the
Halichondrina, the former are the Spintharophora.


{217}ORDER I. HALICHONDRINA.

We have already seen typical examples of the Halichondrina in _Halichondria
panicea_ and _Ephydatia fluviatilis_. Within the Halichondrina the
development of spongin reaches its maximum among spiculiferous sponges, and
accordingly the Ceratosa take their multiple origin here (p. 220). Among
Halichondrina spongin co-operates with spicules to form a skeleton in
various ways, but always so as to leave some spicules bare or free in the
flesh. It may bind the spicules end to end in delicate networks (as in
_Reniera_ or _Gellius_), or into strands, sometimes reaching a considerable
thickness (as in _Chalina_ and others). In a few cases there appears to be
a kind of division of labour between the spicules and spongin, the latter
forming the bulk of the fibre, _i.e._ fulfilling the functions of support,
while the spicules merely beset its surface as defensive organs, rendering
the sponge unfit for food. Fibres formed on this pattern are called
plumose, and are typical of Axinellidae. The distinctive fibre of the
Ectyoninae is as it were a combination of the Axinellid and Chalinine
types: a horny fibre both cored with spicules and beset with them. Spicules
besetting the surface of a fibre are termed "echinating." Whenever its
origin has been investigated, spongin has proved to be the product of
secretion of cells; in the great majority of cases it is poured out at the
surface of the cell, and Evans showed,[245] at any rate in one species of
_Spongilla_, that the spongin fibres are continuous with a delicate cuticle
at the surface of the sponge. In _Reniera_ spp. occurs a curious case of
formation of spongin as an intracellular secretion. A number of spherical
cells each secrete within themselves a short length of fibre; they then
place themselves in rows, so orientated that their contained rods lie end
to end in one line. The rods then fuse and make up continuous threads; the
cells diminish in breadth, ultimately leaving the fibre free.[246]


ORDER II. SPINTHAROPHORA.

These corticate forms are further characterised by the arrangement of their
megascleres, which is usually, like that of most {218}Tetractinellida,
radial, or approximating to radial. The microscleres are, when present,
some form of aster. The cortex resembles that of Tetractinellida, and v.
Lendenfeld has described chones in _Tethya lyncurium_.[247]

The existence of the above points of resemblance between Spintharophora and
Tetractinellida suggests a relationship between the two groups as its
cause. In judging this possibility the following reflections occur to us. A
cortex exists in various independent branches of Tetractinellida. It has in
all probability had a different phylogenetic history in each—why not then
in these Monaxonida also? Within single genera of Tetractinellida some
species are corticate, others not, witness _Tetilla_. The value of a cortex
for purposes of classification may easily be overestimated. If we are to
uphold the relationship between these two groups, we must base our argument
on the conjunction of similar characters in each.

The genus _Proteleia_[248] is interesting for its slender grapnel-like
spicules, which project beyond the radially disposed cortical spicules, and
simulate true anatriaenes of minute proportions. That they are not
anatriaenes is shown by the absence of an axial thread in their cladi. It
is not surprising that a form of spicule of such obvious utility as the
anatriaene should arise more than once.

Of exceptional interest, on account of their boring habit, are the
Clionidae. How the process of boring is effected is not known; the presence
of an acid in the tissues was suspected, but has been searched for in vain.
The pieces of hard substance removed by the activity of the sponge take
their exit through the osculum and have a fixed shape[249] (Fig. 108).

As borers into oyster shells, Clionidae may be reckoned as pests of
practical importance, and in some coasts they even devastate the rocks,
penetrating to a depth of some feet, and causing them to crumble away.[250]

Sponges, however, as agents in altering the face of the earth do not figure
as destroyers merely. On the contrary, it has {219}been calculated[251]
that sponge skeletons may give rise with considerable rapidity to beds of
flint nodules; in fact, it appears that a period so short as fifty years is
sufficient for the formation of a bed of flints out of the skeletons of
sponges alone.

_Suberites domuncula_ is well known for its constant symbiosis with the
Hermit crab. The young sponge settles on a Whelk or other shell inhabited
by a _Pagurus_, and gradually envelops it, becoming very massive, and
completely concealing the shell, without however closing its mouth. The
aperture of this always remains open to the exterior, however great the
growth of the sponge, a tubular passage being left in front of it, which
continues the lumen of the shell and maintains its spiral direction. When
the crab has grown too big for the shell, it merely advances a little down
this passage. The shell is never absorbed, as was once supposed.[252] The
crab, besides being provided with a continually growing house, and being
thus spared the great dangers attending a shift of lodgings, benefits
continually by the concealment and protection afforded by the massive
sponge; the latter in return is conveyed to new places by the crab.

[Illustration: FIG. 108.—A, calcareous corpuscle detached by _Cliona_; B,
view of the galleries excavated by the Sponge. (After Topsent.)]

_Ficulina ficus_ is sometimes, like _S. domuncula_, found in symbiosis with
_Pagurus_, but the constancy of the association is wanting in this case.
The sponge has several metamps, one of which, from its fig-like shape,
gives it its name.


{220}SUB-CLASS III. CERATOSA.

The Ceratosa are an assemblage of ultimate twigs shorn from the branches of
the Monaxonid tree. They are therefore related forms, but many of them are
more closely connected with their Monaxonid relatives than with their
associates in their own sub-class.

The genera _Aulena_ and _Phoriospongia_, placed by v. Lendenfeld among
Ceratosa, by Minchin among Monaxonida, show each in its own way how close
is the link between these two sub-classes.

_Aulena_ possesses in its deeper parts a skeleton of areniferous spongin
fibres, in fact a typical Ceratose skeleton; but this is continuous with a
skeleton in the more superficial parts, which is composed of spongin fibres
echinated by spicules proper to the sponge, and precisely comparable to the
ectyonine fibres of some Monaxonida.

_Phoriospongia_, as far as its main skeleton is concerned, is a typical
Ceratose sponge, with fibres of the areniferous type, but it possesses
sigmata free in the flesh.

The sub-class is confined to shallow water, no horny sponge having been
dredged from depths greater than 410 fathoms.[253] The greatest number
occur at depths between 10 and 26 fathoms.

In the majority of the Ceratosa the skeletal fibres are homogeneous, formed
of concentric lamellae of spongin, deposited by a sheath of spongoblasts
around a filiform axis. In others, however, the axis attains a considerable
diameter, so as to form a kind of pith to the fibre, which is then
distinguished as heterogeneous. In one or two cases some of the
spongoblasts of a heterogeneous fibre are included in the fibre between the
spongin lamellae. _Ianthella_ is the best-known example in which this
occurs.

Ceratosa are divided into Dictyoceratina and Dendroceratina, distinguished,
as their names express, by the nature of the skeleton—net-like, with many
anastomoses, in the one; tree-like, without anastomoses between its
branches, in the other.

The Dictyoceratina comprise by far the larger number of Ceratosa. They fall
into two main families, the Spongidae and Spongelidae, both represented in
British waters. The Spongidae {221}are characterised by a granular ground
substance and aphodal chamber system; the Spongelidae by a clear ground
substance and sac-like eurypylous chambers.

The bath sponge, _Euspongia officinalis_, belongs to the Spongidae. The
finest varieties come from the Adriatic, the coarser ones from the
Dalmatian and North African coasts of the Mediterranean, from the Grecian
Archipelago, from the West Indies, and from Australian seas. The softer
species of the genus _Hippospongia_ also form a source of somewhat inferior
bath sponges.

Among Dendroceratina, _Darwinella_ is unique and tempts to speculation, in
that it possesses isolated spongin elements, resembling in their forms
triaxon spicules.


KEY TO BRITISH GENERA OF SPONGES.

      {Skeleton calcareous                                                2
   1. {Skeleton siliceous                                                 6
      {Skeleton horny, or without free spicules                          53
      {Skeleton absent                                                   55

   2. {Gastral layer continuous                                           3
      {Gastral layer discontinuous, confined to chambers                  4

   3. {Equiangular triradiate systems present                   _Clathrina_
      {Triradiate systems all alate                          _Leucosolenia_

   4. {Chambers tubular, radially arranged                                5
      {Chambers spherical, irregularly scattered                _Leucandra_

   5. {Tufts of oxeate spicules at the ends of the chambers         _Sycon_
      {Oxeate spicules lying longitudinally in the cortex             _Ute_

      {All the spicules hexradiate or spicules easily derived from
   6. {  hexradiate type                                                  7
      {Some of the spicules calthrops or triaenes                        10
      {Megascleres uniaxial                                              15

   7. {Amphidiscs present                                                 8
      {Amphidiscs absent                                                  9

      {Rooting spicules a well-defined wisp; four apertures lead into
   8. {  the gastric cavity                            _Hyalonema thomsoni_
      {Rooting tuft diffuse; sponge oval; osculum single
      {                                              _Pheronema carpenteri_

      {Sponge tubular, dermal and gastral pinuli absent
   9. {                                               _Euplectella suberea_
      {Sponge a widely open cup; dermal and gastral pinuli present
      {                                              _Asconema setubalense_

      {Tetractine spicule, a calthrop or triaene with short rhabdome;
  10. {  microsclere aspined microxea                  _Dercitus bucklandi_
      {Triaenes with fully developed rhabdome                            11

{222}[Illustration: FIG. 109.—Microscleres of Demospongiae. _a_, _b_,
Sigmaspires viewed in different directions; _c_, _d_, bipocilli viewed in
different directions; _e_, toxaspire; _f_, _f'_, spiraster; _g_,
sanidaster; _h_, amphiaster; _i_, sigma; _j_, diaucistra; _k_, isochela;
_l_, _m_, anisochelae viewed in different directions; _n_, cladotyle; _o_,
toxa; _p_, forceps; _q_, oxyaster; _r_, spheraster; _s_, oxyaster with 6
actines; _t_, another with 4 actines; _u_, another with rays reduced to two
(centrotylote microxea); _v_, tylote microrhabdus; _w_, trichodragmata;
_x_, oxeate microrhabdus or microxea.]

  11. {Microscleres sigmata                             _Craniella cranium_
      {Sigmata absent, asters present                                    12

      {Microscleres include spirasters             _Poecillastra compressa_
  12. {Microscleres include sterrasters                                  14
      {Microscleres include euasters: spirasters and sterrasters absent  13

      {Two kinds of euaster present                             _Stelletta_
  13. {Microscleres include a euaster and a sanidaster or amphiaster
      {                                              _Stryphnus ponderosus_

  14. {Microscleres include microrhabdi           _Pachymatisma johnstonia_
      {Microscleres include many-rayed euasters          _Cydonium milleri_

  15. {Some of the microscleres asters                                   16
      {Microscleres absent, or not asters                                17

      {Skeleton radiate; asters of more than one kind              _Tethya_
  16. {Sponge encrusting; asters of one kind only              _Hymedesmia_
      {Skeleton fibrous                                     _Axinella spp._

      {Megascleres all diactinal; chelae present               _Desmacidon_
  17. {Megascleres all diactinal; chelae absent                          18
      {Some or all of the megascleres monactinal                         19

  18. {Habitat fresh water{223}                                          56
      {Habitat marine                                                    22

      {Megascleres include cladotyles                             _Acarnus_
      {Megascleres include dumb-bell or sausage-shaped spicules forming
      {  the main reticulum                                      _Plocamia_
      {Microscleres include bipocilli                                    20
  19. {Microscleres include diancistra                         _Hamacantha_
      {Megascleres include forceps                               _Forcepia_
      {Skeleton formed of isolated monactines vertically placed
      {                                                        _Hymeraphia_
      {None of the above peculiarities present                           21

  20. {Skeleton fibre not echinated                                _Iophon_
      {Skeleton fibre echinated                                  _Pocillon_

  21. {Skeleton with echinating spicules                                 28
      {Skeleton without echinating spicules                              30

  22. {Spongin abundant                                                  23
      {Spongin scanty                                                    25

  23. {Fibre not echinated                                               24
      {Fibre echinated                                         _Diplodemia_

  24. {Fibre with a single axial series of spicules               _Chalina_
      {Fibres with numerous spicules arranged polyserially   _Pachychalina_

  25. {Microscleres absent                                               26
      {Microscleres sigmata and/or toxa                                  27

  26. {Skeleton confused                                     _Halichondria_
      {Skeleton reticulate                                        _Reniera_

      {Rind and fistulous appendages present; microscleres sigmata
  27. {                                                         _Oceanapia_
      {No rind; skeleton reticulate; microscleres sigmata and/or toxa
      {                                                           _Gellius_

      {Skeleton confused or formed of bundles of spicules with
      {  echinating spined styles                                        29
  28. {Skeleton fibrous or reticulate, or formed of short columns        45
      {Skeleton formed of a dense central axis, and columns radiating
      {  from it to the surface                                          52

      {Spicules of the ectosome styles                            _Pytheas_
  29. {Spicules of the ectosome oxeas or absent                _Clathrissa_
      {Main skeleton confused. Special ectosomal skeleton absent
      {                                                        _Spanioplon_

      {Megascleres of the choanosome not differing from those
  30. {  of the ectosome                                                 31
      {Megascleres of the choanosome differing from those
      {  of the ectosome                                                 32

  31. {Chelae absent.                                                    33
      {Chelae present                                                    44

  32. {Trichodragmata present                                     _Tedania_
      {Trichodragmata absent                                             42

{224}

[Illustration: FIG. 110.—Megascleres. _a-l_ and _q-s_, Modifications of
monaxon type. _a_, Strongyle; _b_, tylote; _c_, oxea; _d_, tylotoxea; _e_,
tylostyle; _f_, style; _g_, spined tylostyle; _h_, sagittal triod (a
triaxon form derived from monaxon); _j_, oxytylote; _k_, anatriaene; _l_,
protriaene; _m_, sterraster (polyaxon); _n_, radial section through the
outer part of _m_, showing two actines soldered together by intervening
silica; _o_, desma of an Anomocladine Lithistid (polyaxon); _q_, crepidial
strongyle, basis of rhabdocrepid Lithistid desma; _r_, young form of
rhabdocrepid desma, showing crepidial strongyle coated with successive
layers of silica; _s_, rhabdocrepid desma.]

      {Skeleton reticulate or fibrous{225}                               34
  33. {Skeleton radiate or diffuse                                       37
      {Skeleton with radiating fibres forming a reticulum with
      {  others crossing them at right angles                  _Quasillina_

      {No microscleres                                                   35
  34. {Microscleres sigmata and/or toxa with or without trichodragmata
      {                                                        _Desmacella_

  35. {Sponge fan- or funnel-shaped                                      36
      {Sponge not fan- or funnel-shaped                      _Hymeniacidon_

  36. {Megascleres slender and twisted                          _Phakellia_
      {Megascleres somewhat stout, not twisted                   _Tragosia_

  37. {Sigmata present, skeleton diffuse                           _Biemma_
      {Sigmata absent                                                    38

  38. {Skeleton more or less radiate                                     39
      {Skeleton diffuse; sponge boring                             _Cliona_

  39. {Sponge discoid with marginal fringe                     _Halicnemia_
      {Sponge massive or stipitate without marginal fringe               40

  40. {Sponge body prolonged into mammiform projections        _Polymastia_
      {Sponge body without mammiform projections                         41

  41. {No microscleres. Megascleres tylostyles with or without styles
      {                                                         _Suberites_
      {Microscleres centrotylote. Megascleres styles or tylostyles
      {                                                          _Ficulina_

  42. {Choanosomal megascleres smooth                                    43
      {Choanosomal megascleres spined                            _Dendoryx_

      {Microscleres chelae and sigmata of about the same size
  43. {                                                     _Lissodendoryx_
      {Chelae, if present, smaller than the sigmata                _Yvesia_

  44. {Isochelae{225}                                         _Esperiopsis_
      {Anisochelae                                              _Esperella_

  45. {Fibres or columns plumose                                         46
      {Fibres or columns ectyonine                                       47

  46. {Microscleres toxa                                   _Ophlitaspongia_
      {Microscleres absent                                       _Axinella_

  47. {Skeleton reticulate                                               48
      {Skeleton not reticulate                                           49

      {Microscleres present. Spicules of the fibre core spined    _Myxilla_
  48. {Microscleres absent. Spicules of the fibre core smooth
      {                                                      _Lissomyxilla_

  49. {Main skeleton formed of plume-like columns                        50
      {Main skeleton formed of horny fibres (ectyonine). Special dermal
      {  skeleton wanting                                        _Clathria_

      {Dermal skeleton contains styles only                    _Microciona_
  50. {Dermal skeleton contains diactine spicules with or without styli
      {                                                                  51

      {Main skeleton columns with a core of smooth oxeas
  51. {                                                 _Plumohalichondria_
      {Main skeleton columns with a core of spined styles    _Stylostichon_

      {Central axis contains much spongin. Echinating spined styli
      {  present                                                _Raspailia_
  52. {Central axis with little or no spongin. Spined styles absent.
      {  Pillars radiating from the axis support dermal skeleton
      {                                                        _Ciocalypta_

  53. {Ground substance between chambers clear; chambers pear-shaped
      {  or oval; eurypylous                                    _Spongelia_
      {Ground substance granular. Chambers spherical with aphodi         54

  54. {Fibres not pithed; sponge fan-shaped                     _Leiosella_
      {Fibres pithed; sponge massive                             _Aplysina_

  55. {Chambers long, tubular, branched                         _Halisarca_
      {Chambers not much longer than broad; not branched        _Oscarella_

  56. {Amphidiscs present                                       _Ephydatia_
      {Amphidiscs absent                                        _Spongilla_


{226}



CHAPTER IX

PORIFERA (_CONTINUED_): REPRODUCTION, SEXUAL AND
ASEXUAL—PHYSIOLOGY—DISTRIBUTION—FLINTS


The reproductive processes of Sponges are of such great importance in
leading us to a true conception of the nature of a sponge that we propose
to treat them here in a special section. Both sexual and asexual methods
are common; the multiplication of oscula we do not regard as an act of
reproduction (p. 174).

[Illustration: FIG. 111.—A, amphiblastula larva of _Sycon raphanus_; B,
later stage, showing invagination of the flagellated cells. _c.s_,
Segmentation cavity; _ec_, ectoderm; _en_, endoderm. (After F. E. Schulze,
from Balfour.)]

A cursory glance at a collection of sponge LARVAE from different groups
would suggest the conclusion that they are divisible into two wholly
distinct types. One of these is the _amphiblastula_, and the other the
_parenchymula_. This was the conclusion accepted by zoologists not long
ago. We are indebted to Delage, Maas, and Minchin for dispelling it, and
showing that {227}these types are but the extreme terms of a continuous
series of forms which have all the same essential constitution and undergo
the same metamorphosis.

The amphiblastula of _Sycon raphanus_ (Fig. 111) consists of an anterior
half, formed of slender flagellated cells, and a posterior half, of which
the cells are large, non-flagellate, and rounded. These two kinds of cell
are arranged around a small internal cavity which is largely filled up with
amoebocytes. The flagellated cells are invaginated into the dome of rounded
cells during metamorphosis, in fact, become the choanocytes or gastral
cells; the rounded cells, on the other hand, become the dermal cells—an
astonishing fact to any one acquainted only with Metazoan larvae.

A typical parenchymula is that of _Clathrina blanca_ (Fig. 112). When
hatched it consists of a wall surrounding a large central cavity and built
up of flagellated cells interrupted at the hinder pole by two cells
(_p.g.c_)—the mother-cells of archaeocytes. Before the metamorphosis,
certain of the flagellated cells leave the wall and sink into the central
cavity, and undergoing certain changes establish an inner mass of future
dermal cells. By subsequent metamorphosis the remaining flagellated cells
become internal, not this time by invagination, but by the included dermal
cells breaking through the wall of the larva, and forming themselves into a
layer at the outside.

[Illustration: FIG. 112.—Median longitudinal section of parenchymula larva
of _Clathrina blanca_. _p.g.c_, Posterior granular cells—archaeocyte
mother-cells. (After Minchin.)]

In the larva of _C. blanca_, after a period of free-swimming existence, the
same three elements are thus recognisable as in that of _Sycon_ at the time
of hatching; in the newly hatched larva of _C. blanca_, however, one set of
elements, the dermal cells, are not distinguishable. The difference, then,
between the two newly hatched larvae is due to the earlier cell
differentiation of the _Sycon_ larva.[254]

Now consider the larva of _Leucosolenia_. It is hatched as a
{228}completely flagellated larva; its archaeocytes are internal (as in
_Sycon_); future dermal cells, recognisable as such, are absent. They
arise, as in _C. blanca_, by transformation of flagellated cells; but (1)
this process is confined to the posterior pole, and (2) the internal cavity
is small and filled up with archaeocytes. Consequently the cells which have
lost their flagella and become converted into dermal cells cannot sink in
as in _C. blanca_: they accumulate at the hinder pole, and thus arises a
larva half flagellated, half not; in fact, an amphiblastula. Or, briefly,
in _Leucosolenia_ the larva at hatching is a parenchymula, and when ready
to fix is an amphiblastula; and, again, the difference between the newly
hatched larva and that of _Sycon_ is due to the earlier occurrence of cell
differentiation in the latter. What completer transitional series could be
desired?

Turning to the Micromastictora, the developmental history already sketched
is fairly typical (p. 172). The differences between Mega- and
Micro-mastictoran larvae are referable mainly to the fact that the dermal
cells in the latter become at once differentiated among themselves to form
the main types of dermal cell of the adult.[255] The metamorphosis is
comparable to that of _C. blanca_. Among Tetractinellida and Hexactinellida
sexually produced larvae have not been certainly identified.

Asexual reproduction takes place according to one of three types, which may
be alluded to as (1) "budding," (2) "gemmulation," (3) formation of
"asexual larvae."

By BUDDING (Fig. 113) is meant the formation of reproductive bodies, each
of which contains differentiated elements of the various classes found in
the parent. A simple example of this is described by Miklucho Maclay in
Ascons, where the bud is merely the end of one of the Ascon tubes which
becomes pinched off and so set free.

In _Leucosolenia botryoides_[256] Vasseur describes a similar process; in
this, however, a strikingly distinctive feature is present (Fig. 114),
namely, the buds have an inverse orientation with respect to that of the
parent, so that the budding sponge presents a contrast to a sponge in which
multiplication of oscula has occurred. In fact, the free distal end of the
bud becomes the base of the young sponge, and the osculum is formed at the
opposite extremity, where the bud is constricted from the parent.

{229}[Illustration: FIG. 113.—_Lophocalyx philippensis._ The specimen bears
several buds attached to it by long tufts of spicules. (After F. E.
Schulze.)]

[Illustration: FIG. 114.—_Leucosolenia botryoides._ A, a piece of the
Sponge laden with buds, _a-f_; _i_, the spicules of the buds directed away
from their free ends; _k_, the spicules of the parent directed towards the
osculum, _j_. B, a bud which has been set free and has become fixed by the
extremity which was free or distal in A. (After Vasseur.)]

Such a reversal of the position of the bud is noteworthy in view of its
rarity, and the case is worth reinvestigating, for in other animal groups a
bud or a regenerated part retains so constantly the same orientation as the
parent that Loeb,[257] after experimenting on the {230}regeneration of
Coelenterata and other forms, concluded that a kind of "polarity" existed
in the tissues of certain animals.

In _Oscarella lobularis_[258] the buds are transparent floating bladders,
derived from little prominences on the surface of the sponge. Scattered in
the walls of the bladders are flagellated chambers, which open into the
central cavity. The vesicular nature of the buds is doubtless an
adaptation, lessening their specific gravity and so enabling them to float
to a distance from the parent.

GEMMULATION.—_Spongilla_ has already afforded us a typical example of this
process. Gemmules very similar to those of _Spongilla_ are known in a few
marine sponges, especially in _Suberites_ and in _Ficulina_. They form a
layer attached to the surface of support of the sponge—a layer which may be
single or double, or even three or four tiers deep. A micropyle is
sometimes present in the spongin coat, sometimes absent; possibly its
absence may be correlated with the piling of one layer of gemmules on
another, as this, by covering up the micropyle, would of course render it
useless. Presumably when a micropyle is present the living contents escape
through it and leave the sponge by way of the canal system (Fig. 115).

[Illustration: FIG. 115.—Gemmules of _Ficulina_. A, vertical section of
gemmules _in situ_; B, vertical section of upper portion of one gemmule.
_m_, Micropyle.]

The only case besides _Spongilla_ in which the details of development from
gemmules have been traced is that of _Tethya_.[259] Mere microscopic
examination of a _Tethya_ in active reproduction would suggest that the
process was simple budding, but Maas has shown that the offspring arise
from groups of archaeocytes in the cortex, that is to say, they are typical
gemmules. As they develop they migrate outwards along the radial
spicule-bundles {231}and are finally freed, like the buds of the
Hexactinellid _Lophocalyx_ (Fig. 113).

The comparison of the process of development on the one hand by gemmules,
and on the other by larval development, is of some interest.[260] In both
cases two cell layers—a dermal and a gastral—are established before the
young sponge has reached a functional state. Differences of detail in the
formation of the chambers occur in the gemmule; these find parallels in the
differences in the same process exhibited by the larvae of various groups
of sponges. On the other hand, the order of tissue differentiation is not
the same in the gemmule as in the larva.

[Illustration: FIG. 116.—Development of the triradiate and quadriradiate
spicules of _Clathrina_. (1) Three scleroblasts; (2) each has divided: the
spicule is seen in their midst; (3) addition of the fourth ray by a
porocyte. _p_, Dermal aperture of pore; _r_, fourth ray. (After Minchin.)]

Of the reproduction of Tetractinellida extremely little is known.
Spermatozoa occur in the tissues in profusion and are doubtless functional,
but larvae have been seldom observed.

[Illustration: FIG. 117.—Three stages in the development of the triradiate
spicules of _Sycon setosum_. × 1200. (After Maas.)]

In Hexactinellida the place of sexually produced larvae is taken by bodies
of similar origin to gemmules but with the appearance of parenchymulae.
Ijima has indeed seen a few egg-cells in Hexactinellids.[261] He finds,
however, that archaeocyte congeries occur in abundance, and there is good
reason to believe with him that these are responsible for the numerous
parenchymula-like ASEXUALLY PRODUCED LARVAE he has observed. The discovery
of "asexual larvae" was first made by Wilson in the Monaxonid _Esperella_;
in this case the asexual larva is, as far as can be detected, identical
with that developed from the fertilised egg. A similar phenomenon, the
production {232}of apparently identical larvae by both sexual and asexual
methods, has been observed in the Coelenterate _Gonionema murbachii_.[262]

Artificially, sponges may be reproduced with great advantage to commerce by
means of cuttings. Cuttings of the bath sponge are fit to gather after a
seven years' growth.

The development of the various forms of SPICULES is a subject about which
little is yet known. Most spicules of which the development has been traced
originate in a single dermal cell. The triradiate and quadriradiate
spicules of Homocoela (Clathrinidae), as Minchin[263] has most beautifully
shown, form an exception. Three cells co-operate to form the triradiate;
these three divide to give six before the growth of the spicule is
complete. A quadriradiate is formed from a triradiate spicule by addition
of the fourth ray, which, again, has a separate origin in an independent
cell, in fact a porocyte. The triradiate spicules of the Sycettidae, on the
other hand, originate in a single cell,[264] but the quadriradiate spicules
are formed from these by the addition of a fourth ray in a manner similar
to that which has just been described for Clathrinidae.

[Illustration: FIG. 118.—Development of monaxon spicules. A, from
_Spongilla lacustris_, showing the single scleroblast. (After Evans.) B, a
very large monaxon, from _Leucosolenia_, on which many scleroblasts are at
work. (After Maas.)]

Monaxon spicules if not of large size undergo their entire development
within a single scleroblast (Fig. 118, A). In some cases if their
dimensions exceed certain limits, several cells take part in their
completion; some of these are derived from the {233}division of the
original scleroblast, others are drawn from the surrounding tissue. In
_Tethya_, for example, and in _Leucosolenia_[265] the scleroblasts round
the large monaxon spicules are so numerous as to have an almost epithelioid
arrangement.

The large oxeas of _Tetilla_, _Stelletta_, and _Geodia_, however, are
formed each within a single scleroblast.[266]

[Illustration: FIG. 119.—Development of spheraster. A, of _Tethya_, from
union of two quadriradiate spicules. (After Maas.) B (_a-e_), of
_Chondrilla_, from a spherical globule. (After Keller.)]

Triaenes have been shown[267] to originate as monaxons with one swollen
termination, from which later the cladi grow out. Information as to the
scleroblasts in this case is needed.

The value of a knowledge of the ontogeny of microscleres might be great.
Maas believes that he has shown that the spherasters of _Tethya_ are formed
by the union of minute tetractine calthrops (Fig. 119, A). If this view
should be confirmed, it would afford a very strong argument for the
Tetractinellid affinities of _Tethya_.

[Illustration: FIG. 120.—Stages in the development of the microscleres of
_Placospongia_. (After Keller.)]

Keller,[268] on the other hand, finds that the spherasters of the
Tetractinellid _Chondrilla_ {234}originate as spheres (Fig. 119, B); and
spheres have been observed in the gemmule of a _Tethya_; no spherasters
were as yet present in the gemmule, and spheres were absent in the
adult.[269]

In the genus _Placospongia_ certain spicules are present which outwardly
closely resemble the sterrasters so characteristic of certain
Tetractinellidae. Their development, however, as will be seen from Fig.
120, shows that they are not polyaxon but spiny monaxon spicules.
_Placospongia_ is consequently transferred to the Monaxonida
Spintharaphora.

Sterrasters originate within an oval cell as a number of hairlike
fibres[270] (trichites), which are united at their inner ends. The outer
ends become thickened and further modified. The position occupied by the
nucleus of the scleroblast is marked in the adult spicule by a hilum.

[Illustration: FIG. 121.—Three stages in the development of an anisochela.
_al_, Ala; _al'_, lower ala; _f_, falx; _f'_, lower falx; _r_, rostrum;
_r'_, lower rostrum. (After Vosmaer and Pekelharing.)]

The anisochela has been shown repeatedly to originate from a C-shaped
spicule.[271]

What little is known of the development of Hexactinellid spicules we owe to
Ijima.[272] Numerous cells are concerned in certain later developmental
stages of the hexaster; a hexaster passes through a hexactin stage, and—a
fact "possibly of importance for the phylogeny of spicules in
Hexactinellida"—in two species the first formed spicules are a kind of
hexactin, known as a "stauractin," and possessing only four rays all in one
plane (cf. _Protospongia_, p. 207).


PHYSIOLOGY

PRODUCTION OF THE CURRENT.—It is not at first sight obvious that the
lashing of flagella in chambers arranged as above {235}described, between
an inhalant and an exhalant system of canals, will necessarily produce a
current passing inwards at the ostia and outwards at the osculum. And the
difficulty seems to be increased when it is found[273] that the flagella in
any one chamber do not vibrate in concert, but that each keeps its own
time. This, however, is of less consequence than might seem to be the case.
Two conditions are essential to produce the observed results: (1) in order
that the water should escape at the mouth of the chamber there must be a
pressure within the chamber higher than that in the exhalant passages; (2)
in order that water may enter the chamber there must be within it a
pressure less than that in the inhalant passages. But the pressure in the
inhalant and exhalant passages is presumably the same, at any rate before
the current is started, therefore there must be a difference of pressure
within the chamber itself, and the less pressure must be round the
periphery. Such a distribution of pressures would be set up if each
flagellum caused a flow of water directed away from its own cell and
towards the centre of the chamber; and this would be true whether the
flagellum beats synchronously with its fellows or not.

The comparative study of the canal systems of sponges[274] acquires a
greater interest in proportion as the hope of correlating modifications
with increase of efficiency seems to be realised. In a few main issues this
hope may be said to have been realised. The points, so to speak, of a good
canal system are (1) high oscular velocity, which ensures rapid removal of
waste products to a wholesome distance; (2) a slow current without eddies
in the flagellated chambers, to allow of the choanocytes picking up food
particles (see below), and moreover to prevent injury to the delicate
collars of those cells; (3) a small area of choanocytes, and consequent
small expenditure of energy in current production.

It is then at once clear at what a disadvantage the Ascons are placed as
compared with other sponges having canal systems of the second or third
types. Their chamber and oscular currents can differ but slightly, the
difference being obtained merely by narrowing the lumen of the distal
extremity of the body to form the oscular rim. Further, the choanocytes are
{236}acting on a volume of water which they can only imperfectly control,
and it is no doubt due to the necessity of limiting the volume of water
which the choanocytes have to set in motion that the members of the Ascon
family are so restricted in size. The oscular rim is only a special case of
a device adopted by sponges at the very outset of their career, and
retained and perfected when they have reached their greatest heights; the
volume of water passing per second over every cross-section of the path of
the current is of course the same, therefore by narrowing the
cross-sectional area of the path at any point, the velocity of the current
is proportionally increased at that point. The lining of the oscular rim is
of pinacocytes; they determine a smooth surface, offering little frictional
resistance to the current, while choanocytes in the same position would
have been a hindrance, not only by setting up friction, but by causing
irregularities in the motion.

Canal systems of the second type show a double advance upon that of the
Ascons, namely, subdivision of the gastral cavity and much greater length
of the smooth walled exhalant passage. The choanocytes have now a task more
equal to their strength, and, further, there is now a very great inequality
between the total sectional areas of the flagellated chambers and that of
the oscular tube.

Canal systems of the third type with tubular chambers are an improvement on
those of the second, in that the area of choanocytes is increased by the
pouching of the chamber-layer without corresponding increase in the size of
the sponge. However, the area of choanocytes represents expenditure of
energy, and the next problem to be solved is how to retain the improved
current and at the same time to cut down expense. The first step is to
change the form of the chamber from tubular to spherical. Now the energy of
all the choanocytes is concentrated on the same small volume of water. The
area of choanocytes is less, but the end result is as good as before. At
the wide mouth of the spherical chamber there is nevertheless still a cause
of loss of energy in the form of eddies, and it is as an obviation of these
that one must regard the aphodi and prosodi with which higher members of
the Demospongiae are provided. The correctness of this view receives
support, apart from mechanical principles, from the fact that the mass of
the body of any one of these sponges is greater relatively to the total
flagellated area than in those sponges with eurypylous chambers; that is to
say, a few {237}aphodal and diplodal chambers are as efficient as many of
the eurypylous type.

It is manifest that the current is the bearer of the supply of FOOD; but it
requires more care to discover (1) what is the nature of the food; (2) by
which of the cells bathed by the current the food is captured and by which
digested. The answer to the latter question has long been sought by
experimenters,[275] who supplied the living sponge with finely powdered
coloured matters, such as carmine, indigo, charcoal, suspended in water.
The results received conflicting interpretations until it became recognised
that it was essential to take into account the length of time during which
the sponge had been fed before its tissues were subjected to microscopic
examination. Vosmaer and Pekelharing obtained the following facts:
_Spongilla lacustris_ and _Sycon ciliatum_, when killed after feeding for
from half an hour to two hours with milk or carmine, contain these
substances in abundance in the bodies of the choanocytes and to a slight
degree in the deeper cells of the dermal tissue; after feeding for
twenty-four hours the proportions are reversed, and if a period of
existence in water uncharged with carmine intervenes between the long feed
and death then the chambers are completely free from carmine. These are
perhaps the most conclusive experiments yet described, and they show that
the choanocytes ingest solid particles and that the amoeboid cells of the
dermal layer receive the ingested matter from them. In all probability it
is fair to argue from these facts that solid particles of matter suitable
to form food for the sponge are similarly dealt with by it and undergo
digestion in the dermal cells.

Choanocytes are the feeding organs _par excellence_; but the pinacocytes
perform a small share of the function of ingestion, and in the higher
sponges where the dermal tissue has acquired a great bulk the share is
perhaps increased.

In the above experiments is implied the tacit assumption that sponges take
their food in the form of finely divided solids. Haeckel[276] states his
opinion that they feed on solid particles derived from decaying organisms,
but that possibly decaying substances in solution may eke out their diet.
Loisel, in 1898,[277] {238}made a new departure in the field of experiment
by feeding sponges with coloured solutions, and obtained valuable results.
Thus solutions, if presented to the sponge in a state of extreme dilution,
are subjected to choice, some being absorbed, some rejected. When absorbed
they are accumulated in vacuoles within both dermal and gastral cells,
mixed solutions are separated into their constituents and collected into
separate vacuoles. In the vacuoles the solutions may undergo change; Congo
red becomes violet, the colour which it assumes when treated with acid, and
similarly blue litmus turns red. The contents of the vacuoles, sometimes
modified, sometimes not, are poured out into the intercellular gelatinous
matrix of the dermal layer, whence they are removed partly by amoeboid
cells, partly, so Loisel thinks, by the action of the matrix itself. It
adds to the value of these observations to learn that Loisel kept a
_Spongilla_ supplied with filtered spring-water, to which was added the
filtered juice obtained from another crushed sponge. This _Spongilla_ lived
and budded, and was in good health at the end of ten days.

MOVEMENT.—Sponges are capable of locomotion only in the young stage; in the
adult the only signs of movement are the exhalant current, and in some
cases movements of contraction sufficiently marked to be visible to the
naked eye. Meresjkowsky was one of the early observers of these movements.
He mentions that he stimulated a certain corticate Monaxonid sponge by
means of a needle point: a definite response to each prick inside the
oscular rim was given by the speedy contraction of the osculum.[278]

PIGMENTS AND SPICULES.—Various reasons lead one to conclude that the
spicules have some function other than that of support and defence,
probably connected with metabolism.  For the spicules are cast off,
sometimes in large numbers, to be replaced rapidly by new ones, a process
for which it is difficult to find an adequate explanation if the spicules
are regarded as merely skeletal and defensive.[279] Potts remarks upon the
striking profusion with which spicules are secreted by developing
Spongillids from water in which the percentage of silica present must have
been exceedingly small. The young sponges climbed {239}up the strands of
spicules as they formed them, leaving the lower parts behind and adding to
the upper ends.

Of the physiology of the pigments of sponges not much is yet known: a
useful summary of facts will be found in Von Fürth's text-book.[280]

SPONGIN.—Von Fürth[280] points out that this term is really a collective
one, seeing that the identity of the organic skeletal substance of all
sponge species is hardly to be assumed. Spongin is remarkable for
containing iodine. The amount of iodine present in different sponges varies
widely, reaching in certain tropical species of the Aplysinidae and
Spongidae the high figure 8 to 14 per cent. Seaweeds which are specially
rich in iodine contain only 1.5 to 1.6 per cent.

In view of the fact that iodine is a specific for croup, it is of interest
to observe that the old herb doctors for many centuries recognised the bath
sponge as a cure for that disease.

[Illustration: FIG. 122.—The ordinals measure (i.) the number of species,
_a-f_, and (ii.) the number of stations, _a'-f'_, at which successful hauls
were made. The abscissae measure the depth: thus at I. the depth is from 0
to 50 fathoms; at II. from 51 to 200; at III. from 201 to 1000; at IV. from
1001 upwards. _a_, _a'_, are the curves for Sponges generally; _b_, _b'_,
for Monaxonida; _c_, _c'_, for Hexactinellida; _d_, _d'_, for
Tetractinellida; _e_, _e'_, for Calcarea; _f_, _f'_, for Ceratosa.]

DISTRIBUTION IN SPACE.—All the larger groups of Sponges are cosmopolitan.
Each group has, however, its characteristic bathymetrical range: the facts
are best displayed by means of curves, as in Fig. 122, which is based
wholly on the results obtained by the "Challenger" Expedition. The
information as to littoral species is consequently inadequate, and we have
not the data requisite for their discussion.

Sponges generally (_a_) and Monaxonida in particular (_b_) are more
generally distributed in water of depths of 51 to 200 fathoms than in
depths of less than 50 fathoms; but localities in shallow water are
{240}richer, for the station curve (_a'_) rises abruptly from I. to II.,
while the species curve (_a_) in the same region is almost horizontal.

The Hexactinellid curve (_c_) culminates on III., showing that the group is
characteristically deep water. That for Tetractinellida (_d_) reaches its
greatest height on II., _i.e._ between 51 and 200 fathoms. Even here, in
their characteristic depths, the Tetractinellida fall below the
Hexactinellida, and far below the Monaxonida in numbers. Again, the
Monaxonida are commoner than Hexactinellida in deep water of 201 to 1000
fathoms, and it is not till depths of 1000 fathoms are passed that
Hexactinellida prevail, finally preponderating over the Monaxonida in the
ratio of 2:1.

The Calcarea and Ceratosa are strictly shallow-water forms. It is a fact
well worth consideration that the stations at which sponges have been found
are situated, quite irrespective of depth, more or less in the
neighbourhood of land. In the case of Calcarea and Ceratosa this is to be
expected, seeing that shallow water is commonest near land, but it is
surprising that it should be true also of the Hexactinellida and of the
deep-water species of Tetractinellida and of Monaxonida.

While the family groups are cosmopolitan, this is not true of genera and
species. The distribution of genera and species makes it possible to define
certain geographical provinces for sponges as for other animals. That this
is so, is due to the existence of ocean tracts bare of islands; for ocean
currents, can act as distributing agents with success only if they flow
along a coast or across an ocean studded with islands. It is, of course,
the larval forms which will be transported; whether they will ever develop
to the adult condition depends on whether the current carrying them passes
over a bottom suitable to their species before metamorphosis occurs and the
young sponge sinks. If such a bottom is passed over, and if the depth is
one which can be supported by the particular species in question, then a
new station may thus be established for that species.

The distance over which a larva may be carried depends on the speed of the
current by which it is borne, and on the length of time occupied by its
metamorphosis. Certain of the ocean currents accomplish 500 miles in six
days; this gives some idea of the distance which may intervene between the
birthplace and {241}the final station of a sponge; for six days is not an
excessive interval to allow for the larval period of at any rate some
species.

DISTRIBUTION IN TIME.—All that space permits us to say on the palaeontology
of sponges has been said under the headings of the respective classes. We
can here merely refer to the chronological table shown in Fig. 123:[281]—

[Illustration: FIG. 123.—Table to indicate distribution of Sponges in
time.]

FLINTS.—The ultimate source of all the silica in the sea and fresh-water
areas is to be found in the decomposition of igneous rocks such as granite.
The quantity of silica present in solution in sea water is exceedingly
small, amounting to about one-and-a-half parts in 100,000; it certainly is
not much more in average fresh water. This is no doubt due to its
extraction by diatoms, which begin to extract it almost as soon as it is
set free from the parent rock. It is from this small quantity that the
siliceous sponges derive the supply from which they form their spicules.
Hence it would appear that for the formation of one {242}ounce of spicules
at least one ton of sea water must pass through the body of the sponge.
Obviously from such a weak solution the deposition of silica will not occur
by ordinary physical agencies; it requires the unexplained action of living
organisms. This may account for the fact that deposits of flint and chert
are always associated with organic remains, such as Sponges and Radiolaria.
By some process, the details of which are not yet understood, the silica of
the skeleton passes into solution. In Calcareous deposits, a replacement of
the carbonate of lime by the silica takes place, so that in the case of
chalk the shells of Foraminifera, such as _Globigerina_ and _Textularia_
and those of Coccoliths, are converted into a siliceous chalk. Thus a
siliceous chalk is the first stage in the formation of a flint.

A further deposition of silica then follows, cementing this pulverulent
material into a hard white porous flint. It is white for the same reason
that snow is white. The deposition of silica continues, and the flint
becomes at first grey and at last apparently black (black as ice is black
on a pond). Frequently flints are found in all stages of formation:
siliceous chalk with the corroded remains of sponge spicules may be found
in the interior, black flint blotched with grey forming the mass of the
nodule, while the exterior is completed by a thin layer of white porous
flint. This layer must not be confused with the white layer which is
frequently met with on the surface of weathered flints, which is due to a
subsequent solution of some of the silica, so that by a process of
unbuilding, the flint is brought back to the incompleted flint in its
second stage. In the chalk adjacent to the flints, hollow casts of large
sponge spicules may sometimes be observed, proving the fact, which is
however unexplained, of the solution of the spicular silica. The formation
of the flints appears to have taken place, to some extent at least, long
after the death of the sponge, and even subsequent to the elevation of the
chalk far above the sea-level, as is shown by the occurrence of layers of
flints in the joints of the solid chalk.[282]



COELENTERATA AND CTENOPHORA

BY

S. J. HICKSON, M.A., F.R.S.

Formerly Fellow and now Honorary Fellow of Downing College, Beyer Professor
of Zoology in the Victoria University of Manchester.

{245}CHAPTER X

COELENTERATA

  INTRODUCTION—CLASSIFICATION—HYDROZOA—ELEUTHEROBLASTEA—MILLEPORINA—
  GYMNOBLASTEA—CALYPTOBLASTEA—GRAPTOLITOIDEA—STYLASTERINA


The great division of the animal kingdom called COELENTERATA was
constituted in 1847 by E. Leuckart for those animals which are commonly
known as polyps and jelly-fishes. Cuvier had previously included these
forms in his division Radiata or Zoophyta, when they were associated with
the Starfishes, Brittle-stars, and the other Echinodermata.

The splitting up of the Cuvierian division was rendered necessary by the
progress of anatomical discovery, for whereas the Echinodermata possess an
alimentary canal distinct from the other cavities of the body, in the
polyps and jelly-fishes there is only one cavity to serve the purposes of
digestion and the circulation of fluids. The name Coelenterata (κοῖλος =
hollow, ἔντερον = the alimentary canal) was therefore introduced, and it
may be taken to signify the important anatomical feature that the
body-cavity (or coelom) and the cavity of the alimentary canal (or enteron)
of these animals are not separate and distinct as they are in Echinoderms
and most other animals.

Many Coelenterata have a pronounced radial symmetry, the body being
star-like, with the organs arranged symmetrically on lines radiating from a
common centre. In this respect they have a superficial resemblance to many
of the Echinodermata, which are also radially symmetrical in the adult
stage. But it cannot be insisted upon too strongly that this superficial
resemblance of the Coelenterata and Echinodermata has no genetic
significance. {246}The radial symmetry has been acquired in the two
divisions along different lines of descent, and has no further significance
than the adaptation of different animals to somewhat similar conditions of
life. It is not only in the animals formerly classed by Cuvier as Radiata,
but in sedentary worms, Polyzoa, Brachiopoda, and even Cephalopoda among
the Mollusca, that we find a radial arrangement of some of the organs. It
is interesting in this connexion to note that the word "polyp," so
frequently applied to the individual Coelenterate animal or zooid, was
originally introduced on a fancied resemblance of a _Hydra_ to a small
Cuttle-fish (_Fr._ Poulpe, _Lat._ Polypus).

The body of the Coelenterate, then, consists of a body-wall enclosing a
single cavity ("coelenteron"). The body-wall consists of an inner and an
outer layer of cells, originally called by Allman the "endoderm" and
"ectoderm" respectively. Between the two layers there is a substance
chemically allied to mucin and usually of a jelly-like consistency, for
which the convenient term "mesogloea," introduced by G. C. Bourne, is used
(Fig. 125).

The mesogloea may be very thin and inconspicuous, as it is in _Hydra_ and
many other sedentary forms, or it may become very thick, as in the
jelly-fishes and some of the sedentary Alcyonaria. When it is very thick it
is penetrated by wandering isolated cells from the ectoderm or endoderm, by
strings of cells or by cell-lined canals; but even when it is cellular it
must not be confounded with the third germinal layer or mesoblast which
characterises the higher groups of animals, from which it differs
essentially in origin and other characters. The Coelenterata are
two-layered animals (DIPLOBLASTICA), in contrast to the Metazoa with three
layers of cells (TRIPLOBLASTICA). The growth of the mesogloea in many
Coelenterata leads to modifications of the shape of the coelenteric cavity
in various directions. In the Anthozoa, for example, the growth of vertical
bands of mesogloea covered by endoderm divides the peripheral parts of the
cavity into a series of intermesenterial compartments in open communication
with the axial part of the cavity; and in the jelly-fishes the growth of
the mesogloea reduces the cavity of the outer regions of the disc to a
series of vessel-like canals.

Another character, of great importance, possessed by all Coelenterata is
the "nematocyst" or "thread-cell" (Fig. 124). {247}This is an organ
produced within the body of a cell called the "cnidoblast," and it consists
of a vesicular wall or capsule, surrounding a cavity filled with fluid
containing a long and usually spirally coiled thread continuous with the
wall of the vesicle. When the nematocyst is fully developed and receives a
stimulus of a certain character, the thread is shot out with great velocity
and causes a sting on any part of an animal that is sufficiently delicate
to be wounded by it.

The morphology and physiology of the nematocysts are subjects of very great
difficulty and complication, and cannot be discussed in these pages. It
may, however, be said that by some authorities the cnidoblast is supposed
to be an extremely modified form of mucous or gland cell, and that the
discharge of the nematocyst is subject to the control of a primitive
nervous system that is continuous through the body of the zooid.

There is a considerable range of structure in the nematocysts of the
Coelenterata. In _Alcyonium_ and in many other Alcyonaria they are very
small (in _Alcyonium_ the nematocyst is 0.0075 mm. in length previous to
discharge), and when discharged exhibit a simple oval capsule with a plain
thread attached to it. In _Hydra_ (Fig. 124) there are at least two kinds
of nematocysts, and in the larger kind (0.02 mm. in length previous to
discharge) the base of the thread is beset with a series of recurved hooks,
which during the act of discharge probably assist in making a wound in the
organism attacked for the injection of the irritant fluid, and possibly
hold the structure in position while the thread is being discharged. In the
large kind of nematocyst of _Millepora_ and of _Cerianthus_ there is a band
of spirally arranged but very minute thorns in the middle of the thread,
but none at the base. In some of the Siphonophora the undischarged
nematocysts reach their maximum size, nearly 0.05 mm. in length.

[Illustration: FIG. 124.—Nematocyst (_Nem_) of _Hydra grisea_, enclosed
within the cnidoblast. _CNC_, Cnidocil; _f_, thread of nematocyst; _Mf_,
myophan threads in cnidoblast; _N_, nucleus of cnidoblast. (After
Schneider.)]

When a nematocyst has once been discharged it is usually {248}rejected from
the body, and its place in the tissue is taken by a new nematocyst formed
by a new cnidoblast; but in the thread of the large kind of nematocyst of
_Millepora_ there is a very delicate band, which appears to be similar to
the myophan thread in the stalk of a _Vorticella_. Dr. Willey[283] has made
the important observation that in this coral the nematocyst threads can be
withdrawn after discharge, the retraction being effected with great
rapidity. The "cnidoblast" is a specially modified cell. It sometimes bears
at its free extremity a delicate process, the "cnidocil," which is supposed
to be adapted to the reception of the special stimuli that determine the
discharge of the nematocyst. In many species delicate contractile fibres
(Fig. 124, _Mf_) can be seen in the substance of the cnidoblast, and in
others its basal part is drawn out into a long and probably contractile
stalk ("cnidopod"), attached to the mesogloea below.

There can be little doubt that new nematocysts are constantly formed during
life to replace those that have been discharged and lost. Each nematocyst
is developed within the cell-substance of a cnidoblast which is derived
from the undifferentiated interstitial cell-groups. During this process the
cnidoblast does not necessarily remain stationary, but may wander some
considerable distance from its place of origin.[284] This habit of
migration of the cnidoblast renders it difficult to determine whether the
ectoderm alone, or both ectoderm and endoderm, can give rise to
nematocysts. In the majority of Coelenterates the nematocysts are confined
to the ectoderm, but in many Anthozoa, Scyphozoa, and Siphonophora they are
found in tissues that are certainly or probably endodermic in origin. It
has not been definitely proved in any case that the cnidoblast cells that
form these nematocysts have originally been formed in the endoderm, and it
is possible that they are always derived from ectoderm cells which migrate
into the endoderm.

It is probably true that all Coelenterata have nematocysts, and that, in
the few cases in which it has been stated that they are absent (e.g.
_Sarcophytum_), they have been overlooked. It cannot, however, be
definitely stated that similar structures do not occur in other animals.
The nematocysts of the Mollusc _Aeolis_ are not the product of its own
tissues, but are introduced {249}into the body with its food.[285] The
nematocysts that occur in the Infusorian _Epistylis umbellaria_ and in the
Dinoflagellate _Polykrikos_ (p. 131) require reinvestigation, but if it
should prove that they are the product of the Protozoa they cannot be
regarded as strictly homologous with those of Coelenterata. In many of the
Turbellaria, however, and in some of the Nemertine worms, nematocysts occur
in the epidermis which appear to be undoubtedly the products of these
animals.



The Coelenterata are divided into three classes:—

1. HYDROZOA.—Without stomodaeum and mesenteries. Sexual cells discharged
directly to the exterior.

2. SCYPHOZOA.—Without stomodaeum and mesenteries. Sexual cells discharged
into the coelenteric cavity.

3. ANTHOZOA = ACTINOZOA.—With stomodaeum and mesenteries. Sexual cells
discharged into the coelenteric cavity.

The full meaning of the brief statements concerning the structure of the
three classes given above cannot be explained until the general anatomy of
the classes has been described. It may be stated, however, in this place
that many authors believe that structures corresponding with the stomodaeum
and mesenteries of Anthozoa do occur in the Scyphozoa, which they therefore
include in the class Anthozoa.

Among the more familiar animals included in the class Hydrozoa may be
mentioned the fresh-water polyp _Hydra_, the Hydroid zoophytes, many of the
smaller Medusae or jelly-fish, the Portuguese Man-of-war (_Physalia_), and
a few of the corals.

Included in the Scyphozoa are the large jelly-fish found floating on the
sea or cast up on the beach on the British shores.

The Anthozoa include the Sea-anemones, nearly all the Stony Corals, the
Sea-fans, the Black Corals, the Dead-men's fingers (_Alcyonium_), the
Sea-pens, and the Precious Coral of commerce.


CLASS I. HYDROZOA

In this Class of Coelenterata two types of body-form may be found. In such
a genus as _Obelia_ there is a fixed branching colony of zooids, and each
zooid consists of a simple tubular body-wall composed of the two layers of
cells, the ectoderm and the {250}endoderm (Fig. 125), terminating distally
in a conical mound—the "hypostome"—which is perforated by the mouth and
surrounded by a crown of tentacles. This fixed colony, the "hydrosome,"
feeds and increases in size by gemmation, but does not produce sexual
cells. The hydrosome produces at a certain season of the year a number of
buds, which develop into small bell-like jelly-fish called the "Medusae,"
which swim away from the parent stock and produce the sexual cells. The
Medusa (Fig. 126) consists of a delicate dome-shaped contractile bell,
perforated by radial canals and fringed with tentacles; and from its centre
there depends, like the clapper of a bell, a tubular process, the
manubrium, which bears the mouth at its extremity. This free-swimming
sexual stage in the life-history of _Obelia_ is called the "medusome."

It is difficult to determine whether, in the evolution of the Hydrozoa, the
hydrosome preceded the medusome or _vice versâ_. By some authors the
medusome is regarded as a specially modified sexual individual of the
hydrosome colony. By others the medusome is regarded as the typical adult
Hydrozoon form, and the zooids of the hydrosome as nutritive individuals
arrested in their development to give support to it. Whatever may be the
right interpretation of the facts, however, it is found that in some forms
the medusome stage is more or less degenerate and the hydrosome is
predominant, whereas in others the hydrosome is degenerate or inconspicuous
and the medusome is predominant. Finally, in some cases there are no
traces, even in development, of a medusome stage, and the life-history is
completed in the hydrosome, while in others the hydrosome stages are lost
and the life-history is completed in the medusome.

If a conspicuous hydrosome stage is represented by H, a conspicuous
medusome stage by M, an inconspicuous or degenerate hydrosome stage by h,
an inconspicuous or degenerate medusome stage by m, and the fertilised ovum
by O, the life-histories of the Hydrozoa may be represented by the
following formulæ:—

  1.    O  —  H  —  O    (_Hydra_)
  2.    O — H — m — O    (_Sertularia_)
  3.    O — H — M — O    (_Obelia_)
  4.    O — h — M — O    (_Liriope_)
  5.    O  —  M  —  O    (_Geryonia_)

The structure of the HYDROSOME is usually very simple. It {251}consists of
a branched tube opening by mouths at the ends of the branches and closed at
the base. The body-wall is built up of ectoderm and endoderm. Between these
layers there is a thin non-cellular lamella, the mesogloea.

In a great many Hydrozoa the ectoderm secretes a chitinous protective tube
called the "perisarc." The mouth is usually a small round aperture situated
on the summit of the hypostome, and at the base of the hypostome there may
be one or two crowns of tentacles or an area bearing irregularly scattered
tentacles. The tentacles may be hollow, containing a cavity continuous with
the coelenteric cavity of the body; or solid, the endoderm cells arranged
in a single row forming an axial support for the ectoderm. The ectoderm of
the tentacles is provided with numerous nematocysts, usually arranged in
groups or clusters on the distal two-thirds of their length, but sometimes
confined to a cap-like swelling at the extremity (capitate tentacles). The
hydrosome may be a single zooid producing others asexually by gemmation (or
more rarely by fission), which become free from the parent, or it may be a
colony of zooids in organic connexion with one another formed by the
continuous gemmation of the original zooid derived from the fertilised ovum
and its asexually produced offspring. When the hydrosome is a colony of
zooids, specialisation of certain individuals for particular functions may
occur, and the colony becomes dimorphic or polymorphic.

[Illustration: FIG. 125.—Diagram of a vertical section through a hydrosome.
_Coel_, Coelenteron; _Ect_, ectoderm; _End_, endoderm. Between the ectoderm
and the endoderm there is a thin mesogloea not represented in the diagram.
_M_, mouth; _T_, tentacle.]

The MEDUSOME is more complicated in structure than the hydrosome, as it is
adapted to the more varied conditions of a free-swimming existence. The
body is expanded to form a disc, "umbrella," or bell, which bears at the
edge or margin a number of tentacles. The mouth is situated on the end of a
hypostome, called the "manubrium," situated in the centre of the radially
symmetrical body. The surface that bears the manubrium is {252}called oral,
and the opposite surface is called aboral. The cavity partly enclosed by
the oral aspect of the body when it is cup- or bell-shaped is called the
"sub-umbrellar cavity."

In the medusome of nearly all Hydrozoa there is a narrow shelf projecting
inwards from the margin of the disc and guarding the opening of the
sub-umbrellar cavity, called the "velum."

The mouth leads through the manubrium into a flattened part of the
coelenteric cavity, which is usually called the gastric cavity, and from
this a number of canals pass radially through the mesogloea to join a
circular canal or ring-canal at the margin of the umbrella.

A special and important feature of the medusome is the presence of
sense-organs called the "ocelli" and "statocysts," situated at the margin
of the umbrella or at the base of the tentacles.

[Illustration: FIG. 126.—Diagram of a vertical section through a medusome.
_coel_, Coelenteron; _M_, mouth; _Man_, manubrium; _R_, radial canal; _r_,
ring or circular canal; _T_, tentacle; _v_, velum.]

The ocelli may usually be recognised as opaque red or blue spots on the
bases of the tentacles, in marked contrast to their transparent
surroundings. The ocellus may consist simply of a cluster of pigmented
cells, or may be further differentiated as a cup of pigmented cells filled
with a spherical thickening of the cuticle to form a lens. The exact
function of the ocelli may not be fully understood, but there can be little
doubt that they are light-perceiving organs.

The function of the sense-organs known as statocysts, however, has not yet
been so satisfactorily determined. They were formerly thought to be
auditory organs, and were called "otocysts," but it appears now that it is
impossible on physical grounds for these organs to be used for the
perception of the waves of sound in water. It is more probable that they
are organs of the static function, that is, the function of the perception
of the position of the body in space, and they are consequently called
statocysts. In the Leptomedusae each statocyst consists of a small vesicle
in the mesogloea at the margin of the umbrella, containing a hard, stony
body called the "statolith." In _Geryonia_ and some other Trachomedusae the
statolith is carried by a short tentacular process, the "statorhab,"
{253}projecting into the vesicle; in other Trachomedusae, however, the
vesicle is open, but forms a hood for the protection of the statorhab; and
in others, but especially in the younger stages of development, the
statorhab is not sunk into the margin of the umbrella, and resembles a
short but loaded tentacle. Recent researches have shown that there is a
complete series of connecting links between the vesiculate statocyst of the
Leptomedusae and the free tentaculate statorhab of the Trachomedusae, and
there can be little doubt of their general homology.

In the free-swimming or "Phanerocodonic" medusome the sexual cells are
borne by the ectoderm of the sub-umbrellar cavity either on the walls of
the manubrium or subjacent to the course of the radial canals.


ORDER I. ELEUTHEROBLASTEA.

This order is constituted mainly for the well-known genus _Hydra_. By some
authors _Hydra_ is regarded as an aberrant member of the order
Gymnoblastea, to which it is undoubtedly in many respects allied, but it
presents so many features of special interest that it is better to keep it
in a distinct group.

_Hydra_ is one of the few examples of exclusively fresh-water
Coelenterates, and like so many of the smaller fresh-water animals its
distribution is almost cosmopolitan. It occurs not only in Europe and North
America, but in New Zealand, Australia, tropical central Africa, and
tropical central America.

_Hydra_ is found in this country in clear, still fresh water attached to
the stalks or leaves of weeds. When fully expanded it may be 25 mm. in
length, but when completely retracted the same individual may be not more
than 3 mm. long. The tubular body-wall is built up of ectoderm and
endoderm, enclosing a simple undivided coelenteric cavity. The mouth is
situated on the summit of the conical hypostome, and at the base of this
there is a crown of long, delicate, but hollow tentacles. The number of
tentacles is usually six in _H. vulgaris_ and _H. oligactis_,[286] and
eight in _H. viridis_, but it is variable in all species.

During the greater part of the summer the number of individuals is rapidly
increased by gemmation. The young Hydras produced by gemmation are usually
detached from their parents {254}before they themselves produce buds, but
in _H. oligactis_ the buds often remain attached to the parent after they
themselves have formed buds, and thus a small colony is produced. Sexual
reproduction usually commences in this country in the summer and autumn,
but as the statements of trustworthy authors are conflicting, it is
probable that the time of appearance of the sexual organs varies according
to the conditions of the environment.

Individual specimens may be male, female, or hermaphrodite. Nussbaum[287]
has published the interesting observation that when the Hydras have been
well fed the majority become female, when the food supply has been greatly
restricted the majority become male, and when the food-supply is moderate
in amount the majority become hermaphrodite. The gonads are simply clusters
of sexual cells situated in the ectoderm. There is no evidence, derived
from either their structure or their development, to show that they
represent reduced medusiform gonophores. The testis produces a number of
minute spermatozoa. In the ovary, however, only one large yolk-laden
egg-cell reaches maturity by the absorption of the other eggs. The ovum is
fertilised while still within the gonad, and undergoes the early stages of
its development in that position. With the differentiation of an outer
layer of cells a chitinous protecting membrane is formed, and the escape
from the parent takes place.[288] It seems probable that at this stage,
namely, that of a protected embryo, there is often a prolonged period of
rest, during which it may be carried by wind and other agencies for long
distances without injury.

The remarkable power that _Hydra_ possesses of recovery from injury and of
regenerating lost parts was first pointed out by Trembley in his classical
memoir.[289]

A _Hydra_ can be cut into a considerable number of pieces, and each piece,
provided both ectoderm and endoderm are represented in it, will give rise
by growth and regeneration to a complete zooid. There is, however, a limit
of size below which fragments of _Hydra_ will not regenerate, even if they
contain {255}cells of both layers. The statement made by Trembley, that
when a _Hydra_ is turned inside out it will continue to live in the
introverted condition has not been confirmed, and it seems probable that
after the experiment has been made the polyp remains in a paralysed
condition for some time, and later reverts, somewhat suddenly, to the
normal condition by a reversal of the process. There is certainly no
substantial reason to believe that under any circumstances the ectoderm can
undertake the function of the endoderm or the endoderm the functions of the
ectoderm.

[Illustration: FIG. 127.—A series of drawings of _Hydra_, showing the
attitudes it assumes during one of the more rapid movements from place to
place. 1, The _Hydra_ bending over to one side; 2, attaching itself to the
support by the mouth and tentacles; 3, drawing the sucker up to the mouth;
4, inverted; 5, refixing the sucker; 6, reassuming the erect posture.
(After Trembley.)]

One of the characteristic features of _Hydra_ is the slightly expanded,
disc-shaped aboral extremity usually called the "foot," an unfortunate term
for which the word "sucker" should be substituted. There are no root-like
tendrils or processes for attachment to the support such as are found in
most of the solitary Gymnoblastea. The attachment of the body to the stem
or weed or surface-film by this sucker enables the animal to change its
position at will. It may either progress slowly by gliding along its
support without the assistance of the tentacles, in a manner similar to
that observed in many Sea-anemones; or more rapidly by a series of
somersaults, as originally described by Trembley. The latter mode of
locomotion has been recently described as follows:—"The body, expanded and
with expanded tentacles, bends over to one side. As soon as the tentacles
touch the bottom they attach themselves and contract. Now one of two things
happens. The foot may loosen its hold on the bottom and the body contract.
In this manner the animal comes to stand on its tentacles with the foot
pointing upward. The body now bends over again until the foot attaches
itself close to the attached tentacles. These loosen in their turn, and so
the _Hydra_ is again {256}in its normal position. In the other case the
foot is not detached, but glides along the support until it stands close to
the tentacles, which now loosen their hold."[290]

_Hydra_ appears to be purely carnivorous. It will seize and swallow
Entomostraca of relatively great size, so that the body-wall bulges to more
than twice its normal diameter. But smaller Crustacea, Annelid worms, and
pieces of flesh are readily seized and swallowed by a hungry _Hydra_. In
_H. viridis_ the chlorophyll corpuscles[291] of the endoderm may possibly
assist in the nourishment of the body by the formation of starch in direct
sunlight.

Three species of _Hydra_ are usually recognised, but others which may be
merely local varieties or are comparatively rare have been named.[292]

_H. viridis._—Colour, grass-green. Average number of tentacles, eight.
Tentacles shorter than the body. Embryonic chitinous membrane spherical and
almost smooth.

_H. vulgaris_, Pallas (_H. grisea_, Linn.).—Colour, orange-brown. Tentacles
rather longer than the body, average number, six. Embryonic chitinous
membrane spherical, and covered with numerous pointed branched spines.

_H. oligactis_, Pallas (_H. fusca_, Linn.).—Colour, brown. Tentacles
capable of great extension; sometimes, when fully expanded, several times
the length of the body. Average number, six. Embryonic chitinous membrane
plano-convex, its convex side only covered with spines.

The genera _Microhydra_ (Ryder) and _Protohydra_ (Greeff) are probably
allied to _Hydra_, but as their sexual organs have not been observed their
real affinities are not yet determined. _Microhydra_ resembles _Hydra_ in
its general form and habits, and in its method of reproduction by
gemmation, but it has no tentacles. It was found in fresh water in North
America.

_Protohydra_[293] was found in the oyster-beds off Ostend, and resembles
_Microhydra_ in the absence of tentacles. It multiplies by transverse
fission, but neither gemmation nor sexual reproduction has been observed.

_Haleremita_ is a minute hydriform zooid which is also marine. {257}It was
found by Schaudinn[294] in the marine aquarium at Berlin in water from
Rovigno, on the Adriatic. It reproduces by gemmation, but sexual organs
have not been found.

Another very remarkable genus usually associated with the Eleutheroblastea
is _Polypodium_. At one stage of its life-history it has the form of a
spiral ribbon or stolon which is parasitic on the eggs of the sturgeon
(_Acipenser ruthenus_) in the river Volga.[295] This stolon gives rise to a
number of small _Hydra_-like zooids with twenty tentacles, of which sixteen
are filamentous and eight club-shaped. These zooids multiply by
longitudinal fission, and feed independently on Infusoria, Rotifers, and
other minute organisms. The stages between these hydriform individuals and
the parasitic stolon have not been discovered.


ORDER II. MILLEPORINA.

_Millepora_ was formerly united with the Stylasterina to form the order
Hydrocorallina; but the increase of our knowledge of these Hydroid corals
tends rather to emphasise than to minimise the distinction of _Millepora_
from the Stylasterina.

_Millepora_ resembles the Stylasterina in the production of a massive
calcareous skeleton and in the dimorphism of the zooids, but in the
characters of the sexual reproduction and in many minor anatomical and
histological peculiarities it is distinct. As there is only one genus,
_Millepora_, the account of its anatomy will serve as a description of the
order.

The skeleton (Fig. 128) consists of large lobate, plicate, ramified, or
encrusting masses of calcium carbonate, reaching a size of one or two or
more feet in height and breadth. The surface is perforated by numerous
pores of two distinct sizes; the larger—"gastropores"—are about 0.25 mm. in
diameter, and the smaller and more numerous "dactylopores" about 0.15 mm.
in diameter. In many specimens the pores are arranged in definite cycles,
each gastropore being surrounded by a circle of 5-7 dactylopores; but more
generally the two kinds appear to be irregularly scattered on the surface.

When a branch or lobe of a Millepore is broken across and examined in
section, it is found that each pore is continued as a {258}vertical tube
divided into sections by horizontal calcareous plates (Fig. 129, _Tab_).
These plates are the "tabulae," and constitute the character upon which
_Millepora_ was formerly placed in the now discarded group of Tabulate
corals.

The coral skeleton is also perforated by a very fine reticulum of canals,
by which the pore-tubes are brought into communication with one another. In
the axis of the larger branches and in the centre of the larger plates a
considerable quantity of the skeleton is of an irregular spongy character,
caused by the disintegrating influence of a boring filamentous Alga.[296]

[Illustration: FIG. 128.—A portion of a dried colony of _Millepora_,
showing the larger pores (gastropores) surrounded by cycles of smaller
pores (dactylopores). At the edges the cycles are not well defined.]

The discovery that _Millepora_ belongs to the Hydrozoa was made by
Agassiz[297] in 1859, but Moseley[298] was the first to give {259}an
adequate account of the general anatomy. The colony consists of two kinds
of zooids—the short, thick gastrozooids (Fig. 129, _G_) provided with a
mouth and digestive endoderm, and the longer and more slender mouthless
dactylozooids (_D_)—united together by a network of canals running in the
porous channels of the superficial layer of the corallum. The living
tissues of the zooids extend down the pore-tubes as far as the first
tabulae, and below this level the canal-system is degenerate and
functionless. It is only a very thin superficial stratum of the coral,
therefore, that contains living tissues.

The zooids of _Millepora_ are very contractile, and can be withdrawn below
the general surface of the coral into the shelter of the pore-tubes. When a
specimen is examined in its natural position on the reef, the zooids are
usually found to be thus contracted; but several observers have seen the
zooids expanded in the living condition. It is probable that, as is the
case with other corals, the expansion occurs principally during the night.

The colony is provided with two kinds of nematocysts—the small kind and the
large. In some colonies they are powerful enough to penetrate the human
skin, and _Millepora_ has therefore received locally the name of "stinging
coral." On each of the dactylozooids there are six or seven short capitate
tentacles (Fig. 129, _t_), each head being packed with nematocysts of the
small kind; similar batteries of these nematocysts are found in the four
short capitate tentacles of the gastrozooids. The nematocysts of the larger
kind are found in the superficial ectoderm, some distributed irregularly on
the surface, others in clusters round the pores. The small nematocysts are
about 0.013 mm. in length before they are exploded, and exhibit four spines
at the base of the thread; the large kind are oval in outline, 0.02 × 0.025
mm. in size, and exhibit no spines at the base, but a spiral band of minute
spines in the middle of the filament. There is some reason to believe that
the filament of the large kind of nematocysts can be retracted.[299]

At certain seasons the colonies of _Millepora_ produce a great number of
male or female Medusae. The genus is probably dioecious, no instances of
hermaphrodite colonies having yet been found. Each Medusa is formed in a
cavity situated above the last-formed tabula in a pore-tube, and this
cavity, the "ampulla," having a greater diameter than that of the
gastrozooid tubes, can be recognised even in the dried skeleton.

{260}[Illustration: FIG. 129.—Diagrammatic sketch to show the structure of
_Millepora_. _Amp_, an ampulla containing a medusa; _Can.1_, canal system
at the surface; _Can.2_, canal system degenerating in the lower layers of
the corallum; _Cor_, corallum; _D_, an expanded dactylozooid with its
capitate tentacles; _Ect_, the continuous sheet of ectoderm covering the
corallum (_Cor_); _G_, a gastrozooid, seen in vertical section; _Med_,
free-swimming Medusae; _t_, tentacle; _Tab_, tabula in the pore-tubes.
(Partly after Moseley.)]

It is not known how frequently the sexual seasons occur, but from the
rarity in the {261}collections of our museums of Millepore skeletons which
exhibit the ampullae, it may be inferred that the intervals between
successive seasons are of considerable duration.

The Medusae of _Millepora_ are extremely simple in character. There is a
short mouthless manubrium bearing the sexual cells, an umbrella without
radial canals, while four or five knobs at the margin, each supporting a
battery of nematocysts, represent all that there is of the marginal
tentacles. The male Medusae have not yet been observed to escape from the
parent, but from the fact that the spermatozoa are not ripe while they are
in the ampullae, it may be assumed that the Medusae are set free. Duerden,
however, has observed the escape of the female Medusae, and it seems
probable from his observations that their independent life is a short one,
the ova being discharged very soon after liberation.

_Millepora_ appears to be essentially a shallow-water reef coral. It may be
found on the coral reefs of the Western Atlantic extending as far north as
Bermuda, in the Red Sea, the Indian and Pacific Oceans. The greatest depth
at which it has hitherto been found is 15 fathoms on the Macclesfield Bank,
and it flourishes at a depth of 7 fathoms off Funafuti in the Pacific
Ocean.

_Millepora_, like many other corals, bears in its canals and zooids a great
number of the symbiotic unicellular "Algae" (Chrysomonadaceae, see pp. 86,
125) known as Zooxanthellae. All specimens that have been examined contain
these organisms in abundance, and it has been suggested that the coral is
largely dependent upon the activity of the "Algae" for its supply of
nourishment. There can be no doubt that the dactylozooids do paralyse and
catch living animals, which are ingested and digested by the gastrozooids,
but this normal food-supply may require to be supplemented by the
carbohydrates formed by the plant-cells. But as the carbohydrates can only
be formed by the "Algae" in sunlight, this supplementary food-supply can
only be provided in corals that live in shallow water. It must not be
supposed that this is the only cause that limits the distribution of
_Millepora_ in depth, but it may be an important one.

The generic name _Millepora_ has been applied to a great many fossils from
different strata, but a critical examination of their structure fails to
show any sufficient reason for including many of them in the genus or even
in the order. Fossils that are {262}undoubtedly _Millepora_ occur in the
raised coral reefs of relatively recent date, but do not extend back into
Tertiary times. There seems to be no doubt, therefore, that the genus is of
comparatively recent origin. Among the extinct fossils the genus that comes
nearest to it is _Axopora_ from the Eocene of France, but this genus
differs from _Millepora_ in having monomorphic, not dimorphic, pores, and
in the presence of a minute spine or columella in the centre of each tube.
The resemblances are to be observed in the general disposition of the canal
system and of the tabulation. Whether _Axopora_ is or is not a true
Milleporine, however, cannot at present be determined, but it is the only
extinct coral that merits consideration in this place.


ORDER III. GYMNOBLASTEA—ANTHOMEDUSAE.

This order was formerly united with the Calyptoblastea to form the order
Hydromedusae, but the differences between the two are sufficiently
pronounced to merit their treatment as distinct orders.

In many of the Gymnoblastea the sexual cells are borne by free Medusae,
which may be recognised as the Medusae of Gymnoblastea by the possession of
certain distinct characters. The name given to such Medusae, whether their
hydrosome stage is known or not, is Anthomedusae. The Gymnoblastea are
solitary or colonial Hydrozoa, in which the free (oral) extremity of the
zooids, including the crown of tentacles, is not protected by a skeletal
cup. The sexual cells may be borne by free Anthomedusae, or by more or less
degenerate Anthomedusae that are never detached from the parent hydrosome.
The Anthomedusae are small or minute Medusae provided with a velum, with
the ovaries or sperm-sacs borne by the manubrium and with sense-organs in
the form of ocelli or pigment-spots situated on the margin of the umbrella.

The solitary Gymnoblastea present so many important differences in
anatomical structure that they cannot be united in a single family. They
are usually fixed to some solid object by root-like processes from the
aboral extremity, the "hydrorhiza," or are partly embedded in the sand
(_Corymorpha_), into which long filamentous processes project for the
support of the zooid. The remarkable species _Hypolytus peregrinus_[300]
from Wood's Holl, {263}however, has no aboral processes, and appears to be
only temporarily attached to foreign objects by the secretion of the
perisarc. Among the solitary Gymnoblastea several species reach a gigantic
size. _Corymorpha_ is 50-75 mm. in length, but _Monocaulus_ from deep water
in the Pacific and Atlantic Oceans is nearly 8 feet in length. Among the
solitary forms attention must be called to the interesting pelagic
_Pelagohydra_ (see p. 274).

The method of colony formation in the Gymnoblastea is very varied. In some
cases (_Clava squamata_) a number of zooids arise from a plexus of canals
which corresponds with the system of root-like processes of the solitary
forms. In _Hydractinia_ this plexus is very dense, and the ectoderm forms a
continuous sheet of tissue both above and below. The colony is increased in
size in these cases by the gemmation of zooids from the hydrorhiza. In
other forms, such as _Tubularia larynx_, new zooids arise not only from the
canals of the hydrorhiza, but also from the body-walls of the upstanding
zooids, and thus a bushy or shrubby colony is formed.

In another group the first-formed zooid produces a hydrorhiza of
considerable proportions, which fixes the colony firmly to a stone or shell
and increases in size with the growth of the colony. This zooid itself by
considerable growth in length forms the axis of the colony, and by
gemmation gives rise to lateral zooids, which in their turn grow to form
the lateral branches and give rise to the secondary branches, and these to
the tertiary branches, and so one; each branch terminating in a mouth,
hypostome and crown of tentacles. Such a method of colony formation is seen
in _Bougainvillia_ (Fig. 130). A still more complicated form of colony
formation is seen in _Ceratella_, in which not a single but a considerable
number of zooids form the axis of the colony and of its branches. As each
axis is covered with a continuous coat of ectoderm, and each zooid of such
an axis secretes a chitinous fenestrated tube, the whole colony is far more
rigid and compact than is usual in the Gymnoblastea, and has a certain
superficial resemblance to a Gorgoniid Alcyonarian (Fig. 133, p. 271).

The branches of the colony and a considerable portion of the body-wall of
each zooid in the Gymnoblastea are usually protected by a thin, unjointed
"perisarc" of chitin secreted by the ectoderm; but this skeletal structure
does not expand distally to {264}form a cup-like receptacle in which the
oral extremity of the zooid can be retracted for protection.

The zooids of the Gymnoblastea present considerable diversity of form and
structure. The tentacles may be reduced to one (in _Monobrachium_) or two
(in _Lar sabellarum_), but usually the number is variable in each
individual colony. In many cases, such as _Cordylophora_, _Clava_, and many
others, the tentacles are irregularly scattered on the sides of the zooids.
In others there may be a single circlet of about ten or twelve tentacles
round the base of the hypostome. In some genera the tentacles are arranged
in two series (_Tubularia_, _Corymorpha_, _Monocaulus_), a distal series
round the margin of the mouth which may be arranged in a single circlet or
scattered irregularly on the hypostome, and a proximal series arranged in a
single circlet some little distance from the mouth. In _Branchiocerianthus
imperator_ the number of tentacles is very great, each of the two circlets
consisting of about two hundred tentacles.

[Illustration: FIG. 130.—Diagrammatic sketch to show the method of
branching of _Bougainvillia_. _gon_, Gonophores; _Hr_, hydrorhiza; _t.z_,
terminal zooid.]

The zooids of the hydrosome are usually monomorphic, but there are cases in
which different forms of zooid occur in the same colony. In _Hydractinia_,
for example, no less than four different kinds of zooids have been
described. These are called gastrozooids, dactylozooids, tentaculozooids,
and blastostyles respectively. The "gastrozooids" are provided with a
conical hypostome bearing the mouth and two closely-set circlets of some
ten to thirty tentacles. The "dactylozooids" are longer than the
gastrozooids and have the habit of actively coiling and {265}uncoiling
themselves; they have a small mouth and a single circlet of rudimentary
tentacles. The "tentaculozooids" are situated at the outskirts of the
colony, and are very long and slender, with rudimentary tentacles and no
mouth. The "blastostyles," usually shorter than the gastrozooids, have two
circlets of rudimentary tentacles and a mouth. They bear on their sides the
spherical or oval gonophores.

The medusome stage in the life-history of these Hydrozoa is produced by
gemmation from the hydrosome, or, in some cases, by gemmation from the
medusome as well as from the hydrosome. In many genera and species the
medusome is set free as a minute jelly-fish or Medusa, which grows and
develops as an independent organism until the time when the sexual cells
are ripe, and then apparently it dies. In other Gymnoblastea the medusome
either in the female or the male or in both sexes does not become detached
from the parent hydrosome, but bears the ripe sexual cells, discharges them
into the water, and degenerates without leading an independent life at all.
In these cases the principal organs of the medusome are almost or entirely
functionless, and they exhibit more or less imperfect development, or they
may be so rudimentary that the medusoid characters are no longer obvious.
Both the free and the undetached medusomes are gonophores, that is to say,
the bearers of the sexual cells, but the former were described by Allman as
the "phanerocodonic" gonophores, _i.e._ "with manifest bells," and the
latter as the "adelocodonic" gonophores. The gonophores may arise either
from an ordinary zooid of the colony (_Syncoryne_), from a specially
modified zooid—the blastostyle—as in _Hydractinia_, or from the hydrorhiza
as in certain species of _Perigonimus_. The free-swimming Medusa may itself
produce Medusae by gemmation from the manubrium (_Sarsia_, _Lizzia_,
_Rathkea_, and others), from the base of the tentacles (_Sarsia_,
_Corymorpha_, _Hybocodon_), or from the margin of the umbrella
(_Eleutheria_).

The free-swimming Medusae or phanerocodonic gonophores of the Gymnoblastea
are usually of small size (1 or 2 mm. in diameter) when first liberated,
and rarely attain a great size even when fully mature. They consist of a
circular, bell-shaped or flattened disc—the umbrella—provided at its margin
with a few or numerous tentacles, and a tubular manubrium bearing the mouth
depending from the exact centre of the under (oral) {266}side of the
umbrella (Fig. 132, A). The mouth leads into a shallow digestive cavity,
from which radial canals pass through the substance of the umbrella to join
a ring-canal at the margin (Fig. 131).

The sense-organs of the Medusae of the Gymnoblastea are in the form of
pigment-spots or very simple eyes (ocelli), situated at the bases of the
tentacles. The orifice of the umbrella is guarded by a thin shelf or
membrane, as in the Calyptoblastea, called the velum. The sexual cells are
borne by the manubrium (Figs. 131 and 132, A).

There are many modifications observed in the different genera as regards
the number of tentacles, the number and character of the radial canals, the
minute structure of the sense-organs, and some other characters, but they
agree in having a velum, ocellar sense-organs, and manubrial sexual organs.
The tentacles are rudimentary in _Amalthea_; in _Corymorpha_ there is only
one tentacle; in _Perigonimus_ there are two; and in _Bougainvillia_ they
are numerous; but the usual number is four or six. The radial canals are
usually simple and four in number, but there are six in _Lar sabellarum_,
which branch twice or three times before reaching the margin of the
umbrella (Fig. 132, B).

[Illustration: FIG. 131.—Medusa of _Cladonema_, from the Bahamas, showing
peculiar tentacular processes on the tentacles, the ocelli at the base of
the tentacles, the swellings on the manubrium that mark the position of the
gonads, and the radial and ring-canals of the umbrella. (After Perkins.)]

There can be no doubt that the Medusae of many Gymnoblastea undergo several
important changes in their anatomical features during the period of the
ripening of the sexual cells. Thus in _Lar sabellarum_ the six radial
canals are simple in the first stage of development (A); but in the second
stage (B) each radial canal bifurcates before reaching the margin, and in
the adult stage shows a double bifurcation. The life-history has, however,
been worked out in very few of the Anthomedusae, and there can be little
doubt that as our knowledge grows several forms which are now known as
distinct species {267}will be found to be different stages of growth of the
same species.

[Illustration: FIG. 132.—Two stages in the development of the Medusa of
_Lar sabellarum_ (_Willsia stellata_). A, first stage with six canals
without branches; B, third stage with six canals each with two lateral
branches. The developing gonads may be seen on the manubrium in A. (After
Browne.)]

The movements of the Medusae are well described by Allman[301] in his
account of _Cladonema radiatum_:—"It is impossible to grow tired of
watching this beautiful medusa; sometimes while dashing through the water
with vigorous diastole and systole, it will all at once attach its grapples
to the side of the vessel, and become suddenly arrested in its career, and
then after a period of repose, during which its branched tentacles are
thrown back over its umbrella and extended into long filaments which float,
like some microscopic sea-weed in the water, it will once more free itself
from its moorings and start off with renewed energy." The Medusa of
_Clavatella_, "in its movements and mode of life, presents a marked
contrast to the medusiform zooid of other Hydrozoa. The latter is active
and mercurial, dancing gaily through the water by means of the vigorous
strokes of its crystalline swimming-bell. The former strides leisurely
along, or, using the adhesive discs as hands, climbs amongst the branches
of the weed. In the latter stage of its existence it becomes stationary,
fixing itself by means of its suckers; and {268}thus it remains, the
capitate arms standing out rigidly, like the rays of a starfish, until the
embryos are ready to escape."[302]

Among the Gymnoblastea there are many examples of a curious association of
the Hydroid with some other living animals. Thus _Hydractinia_ is very
often found on the shells carried by living Hermit crabs, _Dicoryne_ on the
shells of various Molluscs, _Tubularia_ has been found on a Cephalopod, and
_Ectopleura_ (a Corymorphid) on the carapace of a crab. There is but little
evidence, however, that in these cases the association is anything more
than accidental. The occurrence of the curious species, _Lar sabellarum_,
on the tubes of _Sabella_, of _Campaniclava cleodorae_ on the living shells
of the pelagic Mollusc _Cleodora cuspidata_, and of a _Gorgonia_ on the
tubes of _Tubularia parasitica_, appear to be cases in which there is some
mutual relationship between the two comrades. The genus _Stylactis_,
however, affords some of the most interesting examples of mutualism. Thus
_Stylactis vermicola_ is found only on the back of an _Aphrodite_ that
lives at the great depth of 2900 fathoms. _S. spongicola_ and _S.
abyssicola_ are found associated with certain deep-sea Horny Sponges. _S.
minoi_ is spread over the skin of the little rock perch _Minous inermis_,
which is found at depths of from 45 to 150 fathoms in the Indian seas.

In many cases it is difficult to understand what is the advantage of the
Hydroid to the animal that carries it, but in this last case Alcock[303]
suggests that the _Stylactis_ assists in giving the fish a deceitful
resemblance to the incrusted rocks of its environment, in order to allure,
or at any rate not to scare, its prey. Whether this is the real explanation
or not, the fact that in the Bay of Bengal and in the Laccadive and Malabar
seas the fish is never found without this Hydroid, nor the Hydroid without
this species of fish, suggests very strongly that there is a mutual
advantage in the association.

Cases of undoubted parasitism are very rare in this order. The remarkable
form _Hydrichthys mirus_,[304] supposed to be a Gymnoblastic Hydroid, but
of very uncertain position in the system, appears to be somewhat modified
in its structure by its parasitic habits on the fish _Seriola zonata_.
_Corydendrium {269}parasiticum_ is said to be a parasite living at the
expense of _Eudendrium racemosum_. _Mnestra_ is a little Medusa which
attaches itself by its manubrium to the Mollusc _Phyllirhoe_, and may
possibly feed upon the skin or secretions of its host.

Nearly all the species of the order are found in shallow sea water.
_Stylactis vermicola_ and the "Challenger" specimen of _Monocaulus
imperator_ occur at a depth of 2900 fathoms, and some species of the genera
_Eudendrium_ and _Myriothela_ descend in some localities to a depth of a
few hundred fathoms. _Cordylophora_ is the only genus known to occur in
fresh water. From its habit of attaching itself to wooden piers and
probably to the bottom of barges, and from its occurrence in navigable
rivers and canals, it has been suggested that _Cordylophora_ is but a
recent immigrant into our fresh-water system. It has been found in England
in the Victoria docks of London, in the Norfolk Broads, and in the
Bridgewater Canal. It has ascended the Seine in France, and may now be
found in the ponds of the Jardin des Plantes at Paris. It also occurs in
the Elbe and in some of the rivers of Denmark.

The classification of the Gymnoblastea is not yet on a satisfactory basis.
At present the hydrosome stage of some genera alone has been described, of
others the free-swimming Medusa only is known. Until the full life-history
of any one genus has been ascertained its position in the families
mentioned below may be regarded as only provisional. The principal families
are:—

FAM. BOUGAINVILLIIDAE.—The zooids of the hydrosome have a single circlet of
filiform tentacles at the base of the hypostome. In _Bougainvillia_
belonging to this family the gonophores are liberated in the form of
free-swimming Medusae formerly known by the generic name _Hippocrene_. In
the fully grown Medusa there are numerous tentacles arranged in clusters
opposite the terminations of the four radial canals. There are usually in
addition tentacular processes (labial tentacles) on the lips of the
manubrium. _Bougainvillia_ is a common British zoophyte of branching habit,
found in shallow water all round the coast. The medusome of _Bougainvillia
ramosa_ is said to be the common little medusa _Margelis ramosa_.[305] Like
most of the Hydroids it has a wide geographical distribution. Other genera
are _Perigonimus_, which has a Medusa with only two tentacles; and
{270}_Dicoryne_, which forms spreading colonies on Gasteropod shells and
has free gonophores provided with two simple tentacles, while the other
organs of the medusome are remarkably degenerate. In _Garveia_ and
_Eudendrium_ the gonophores are adelocodonic, in the former genus arising
from the body-wall of the axial zooids of the colony, and in the latter
from the hydrorhiza. _Stylactis_ is sometimes epizoic (p. 268). Among the
genera that are usually placed in this family, of which the medusome stage
only is known, are _Lizzia_ (a very common British Medusa) and _Rathkea_.
In _Margelopsis_ the hydrosome stage consists of a single free-swimming
zooid which produces Medusae by gemmation.

FAM. PODOCORYNIDAE.—The zooids have the same general features as those of
the Bougainvilliidae, but the perisarc does not extend beyond the
hydrorhiza.

In _Podocoryne_ and _Hydractinia_ belonging to this family the hydrorhiza
forms an encrusting stolon which is usually found on Gasteropod shells
containing a living Hermit crab. In _Podocoryne_ the gonophores are
free-swimming Medusae with a short manubrium provided with labial
tentacles. _Hydractinia_ differs from _Podocoryne_ in having polymorphic
zooids and adelocodonic gonophores.

A fossil encrusting a _Nassa_ shell from the Pliocene deposit of Italy has
been placed in the genus _Hydractinia_, and four species of the same genus
have been described from the Miocene and Upper Greensand deposits of this
country.[306] These are the only fossils known at present that can be
regarded as Gymnoblastic Hydroids.

The Medusa _Thamnostylus_, which has only two marginal tentacles and four
very long and profusely ramified labial tentacles, is placed in this
family. Its hydrosome stage is not known.

FAM. CLAVATELLIDAE.—This family contains the genus _Clavatella_, in which
the zooids of the hydrosome have a single circlet of capitate tentacles.
The gonophore is a free Medusa provided with six bifurcated capitate
tentacles.

FAM. CLADONEMIDAE.—This family contains the genus _Cladonema_, in which the
zooids have two circlets of four tentacles, the labial tentacles being
capitate and the aboral filiform. The gonophore is a free Medusa with eight
tentacles, each provided with a number of curious capitate tentacular
processes (Fig. 131).

{271}FAM. TUBULARIIDAE.—This important and cosmopolitan family is
represented in the British seas by several common species. The zooids of
the hydrosome of _Tubularia_ have two circlets of numerous filiform
tentacles. The gonophores are adelocodonic, and are situated on long
peduncles attached to the zooid on the upper side of the aboral circlet of
tentacles. The larva escapes from the gonophore and acquires two tentacles,
with which it beats the water and, assisted by the cilia, keeps itself
afloat for some time. In this stage it is known as an "Actinula."[307]

[Illustration: FIG. 133.—_Ceratella fusca._ About nat. size. (After Baldwin
Spencer.[308])]

FAM. CERATELLIDAE.—The colony of _Ceratella_ may be five inches in height.
The stem and main branches are substantial, and consist of a network of
branching anastomosing tubes supported by a thick and fenestrated chitinous
perisarc. The {272}whole branch is enclosed in a common layer of ectoderm.
The zooids have scattered capitate tentacles. The Ceratellidae occur in
shallow water off the coast of New South Wales, extend up the coast of East
Africa as far as Zanzibar, and have also been described from Japan.

FAM. PENNARIIDAE.—In the hydrosome stage the zooids have numerous oral
capitate tentacles scattered on the hypostome, and a single circlet of
basilar filiform tentacles. The medusa of _Pennaria_, a common genus of
wide distribution, is known under the name _Globiceps_.

FAM. CORYNIDAE.—In the hydrosome stage the zooids of this family possess
numerous capitate tentacles arranged in several circlets or scattered.

In _Cladocoryne_ the tentacles are branched. _Syncoryne_ is a common and
widely distributed genus with numerous unbranched capitate tentacles
irregularly distributed over a considerable length of the body-wall of the
zooid. In many of the species the gonophores are liberated as Medusae,
known by the name _Sarsia_, provided with four filiform tentacles and a
very long manubrium. In some species (_S. prolifera_ and _S. siphonophora_)
the Medusae are reproduced asexually by gemmation from the long manubrium.
A common British Anthomedusa of this family is _Dipurena_, but its
hydrosome stage is not known. In the closely related genus _Coryne_ the
gonophores are adelocodonic, and exhibit very rudimentary medusoid
characters.

FAM. CLAVIDAE.—This is a large family containing many genera, some with
free-swimming Medusae, others with adelocodonic gonophores. In the former
group are included a number of oceanic Medusae of which the hydrosome stage
has not yet been discovered. The zooids of the hydrosome have numerous
scattered filiform tentacles. The free-swimming Medusae have hollow
tentacles.

_Clava_ contains a common British species with a creeping hydrorhiza
frequently attached to shells, and with adelocodonic gonophores.
_Cordylophora_ is the genus which has migrated into fresh water in certain
European localities (see p. 269). It forms well-developed branching
colonies attached to wooden gates and piers or to the brickwork banks of
canals. Several Anthomedusae, of which the hydrosome stage is not known,
appear to be related to the Medusae of this family, but are sometimes
separated as {273}the family TIARIDAE. Of these _Tiara_, a very brightly
coloured jelly-fish sometimes attaining a height of 40 mm., is found on the
British coasts, and _Amphinema_ is found in considerable numbers at
Plymouth in September. _Turritopsis_ is a Medusa with a hydrosome stage
like _Dendroclava_. For _Stomatoca_, see p. 415.

FAM. CORYMORPHIDAE.—This family contains the interesting British species
_Corymorpha nutans_. The hydrosome stage consists of a solitary zooid of
great size, 50-75 mm. in length, provided with two circlets of numerous
long filiform tentacles. The free-swimming Medusae are produced in great
numbers on the region between the two circlets of tentacles. These Medusae
were formerly known by the name _Steenstrupia_, and are noteworthy in
having only one long moniliform tentacle, opposite to one of the radial
canals.

The gigantic _Monocaulus imperator_ of Allman was obtained by the
"Challenger" at the great depth of 2900 fathoms off the coast of Japan. It
was nearly eight feet in length. More recently Miyajima[309] has described
a specimen from 250 fathoms in the same seas which was 700 mm. (27.5 in.)
in length. Miyajima's specimen resembles those described by Mark from 300
fathoms off the Pacific coast of North America as _Branchiocerianthus
urceolus_ in the remarkable feature of a distinct bilateral arrangement of
the circlets of tentacles. Owing to the imperfect state of preservation of
the only specimen of Allman's species it is difficult to determine whether
it is also bilaterally symmetrical and belongs to the same species as the
specimens described by Mark and Miyajima. These deep-sea giant species,
however, appear to differ from _Corymorpha_ in having adelocodonic
gonophores.

FAM. HYDROLARIDAE.—This family contains the remarkable genus _Lar_, which
was discovered by Gosse attached to the margin of the tubes of the marine
Polychaete worm _Sabella_. The zooids have only two tentacles, and exhibit
during life curious bowing and bending movements which have been compared
with the exercises of a gymnast. The Medusae (Fig. 132, A and B) have been
known for a long time by the name _Willsia_, but their life-history has
only recently been worked out by Browne.[310]

{274}FAM. MONOBRACHIIDAE.—_Monobrachium_, found in the White Sea by
Mereschkowsky, forms a creeping stolon on the shells of _Tellina_. The
zooids of the hydrosome have only one tentacle.

FAM. MYRIOTHELIDAE.—This family contains the single genus _Myriothela_. The
zooid of the hydrosome stage is solitary and is provided, as in the
Corynidae, with numerous scattered capitate tentacles. The gonophores are
borne by blastostyles situated above the region of the tentacles. In
addition to these blastostyles producing gonophores there are, in _M.
phrygia_, supplementary blastostyles which capture the eggs as they escape
from the gonophores and hold them until the time when the larva is ready to
escape. They were called "claspers" by Allman. In some of the Arctic
species Frl. Bonnevie[311] has shown that they are absent. Each zooid of
_M. phrygia_ is hermaphrodite.

[Illustration: FIG. 134.—_Pelagohydra mirabilis._ _Fl_, The float; _M_,
position of the mouth; _Ten.Fl_, filamentous tentacles of the float. (After
Dendy.)]

FAM. PELAGOHYDRIDAE.—This family was constituted by Dendy[312] for the
reception of _Pelagohydra mirabilis_, a remarkable new species discovered
by him on the east coast of the South Island of New Zealand. The hydrosome
is solitary and free-swimming, the proximal portion of the body being
modified to form a float, the distal portion forming a flexible proboscis
terminated by the mouth and a group of scattered manubrial tentacles. The
tentacles are filiform and scattered over the surface of the float. Medusae
are developed on stolons between the tentacles of the float. They have
tentacles arranged in four radial groups of five each, at the margin of the
umbrella.

As pointed out by Hartlaub,[313] _Pelagohydra_ is not the only genus in
which the hydrosome floats. Three species of the genus _Margelopsis_ have
been found that have pelagic habits, and two {275}of them have been shown
to produce numerous free-swimming Medusae by gemmation; but at present
there is no reason to suppose that in these forms there is any extensive
modification of the aboral extremity of the zooid to form such a highly
specialised organ as the float of _Pelagohydra_.

The affinities of _Pelagohydra_ are not clear, as our knowledge of the
characters of the Medusa is imperfect; but according to Dendy it is most
closely related to the Corymorphidae. _Margelopsis_ belongs to the
Bougainvilliidae.


ORDER IV. CALYPTOBLASTEA—LEPTOMEDUSAE.

The hydrosome stage is characterised by the perisarc, which not only
envelops the stem and branches, as in many of the Gymnoblastea, but is
continued into a trumpet-shaped or tubular cup or collar called the
"hydrotheca," that usually affords an efficient protection for the zooids
when retracted. No solitary Calyptoblastea have been discovered. In the
simpler forms the colony consists of a creeping hydrorhiza, from which the
zooids arise singly (_Clytia johnstoni_), but these zooids may give rise to
a lateral bud which grows longer than the parent zooid.

[Illustration: FIG. 135.—Part of a hydrocladium of a dried specimen of
_Plumularia profunda_. _Gt_, Gonotheca; _Hc_, the stem of the hydrocladium
with joints (_j_); _Ht_, a single hydrotheca; _N_, nematophores. Greatly
enlarged. (After Nutting.)]

The larger colonies are usually formed by alternate right and left budding
from the last-formed zooid, so that in contrast to the Gymnoblast colony
the apical zooid of the stem is the youngest, and not the oldest, zooid of
the colony. In the branching colonies the axis is frequently composed of a
single tube of perisarc, which may be lined internally by the ectoderm and
endoderm tissues formed by the succession of zooids that have given rise to
the branches by gemmation. Such a stem is said to be monosiphonic.

{276}In some of the more complicated colonies, however, the stem is
composed of several tubes, which may or may not be surrounded by a common
sheath of ectoderm and perisarc, as they are in _Ceratella_ among the
Gymnoblastea. Such stems are said to be "polysiphonic" or "fascicled." The
polysiphonic stem may arise in more than one way, and in some cases it is
not quite clear in what manner it has arisen.[314]

In many colonies the zooids are only borne by the terminal monosiphonic
branches, which receive the special name "hydrocladia." The gonophores of
the Calyptoblastea are usually borne by rudimentary zooids, devoid of mouth
and tentacles (the "blastostyles"), protected by a specially dilated cup of
perisarc known as the "gonotheca" or "gonangium." The shape and size of the
gonothecae vary a good deal in the order. They may be simply oval in shape,
or globular (_Schizotricha dichotoma_), or greatly elongated, with the
distal ends produced into slender necks (_Plumularia setacea_). They are
spinulose in _P. echinulata_, and annulated in _P. halecioides_, _Clytia_,
etc.

In some genera there are special modifications of the branches and
hydrocladia, for the protection of the gonothecae. The name "Phylactocarp"
is used to designate structures that are obviously intended to serve this
purpose. The phylactocarp of the genera _Aglaophenia_ and _Thecocarpus_ is
the largest and most remarkable of this group of structures, and has
received the special name "corbula." The corbula consists of an axial stem
or rachis, and of a number of corbula-leaves arising alternately from the
rachis, bending upwards and then inwards to meet those of the other side
above, the whole forming a pod-shaped receptacle. The gonangia are borne at
the base of each of the corbula-leaves. There is some difference of opinion
as to the homologies of the parts of the corbula, but the rachis seems to
be that of a modified hydrocladium, as it usually bears at its base one or
more hydrothecae of the normal type. The corbula-leaves are usually
described as modified nematophores (_vide infra_), but according to
Nutting[315] there is no more reason to regard them as modified
nematophores than as modified hydrothecae, and he regards them as "simply
the modification of a structure originally intended to {277}protect an
indefinite person, an individual that may become either a sarcostyle[316]
or a hydranth."

The other forms of phylactocarps are modified branches as in _Lytocarpus_,
and those which are morphologically appendages to branches as in
_Cladocarpus_, _Aglaophenopsis_, and _Streptocaulus_.

The structures known as "nematophores" in the Calyptoblastea are the thecae
of modified zooids, comparable with the dactylozooids of _Millepora_. They
form a well-marked character of the very large family Plumulariidae, but
they are also found in species of the genera _Ophiodes_, _Lafoëina_,
_Oplorhiza_, _Perisiphonia_, _Diplocyathus_, _Halecium_, and _Clathrozoon_
among the other Calyptoblastea. The dactylozooids are usually capitate or
filiform zooids, without tentacles or a mouth, and with a solid or
occasionally a perforated core of endoderm. They bear either a battery of
nematocysts (_Plumularia_, etc.), or of peculiar adhesive cells
(_Aglaophenia_ and some species of _Plumularia_). The functions of the
dactylozooids are to capture the prey and to serve as a defence to the
colony. In the growth of the corbula of _Aglaophenia_ the dactylozooids
appear to serve another purpose, and that is, as a temporary attachment to
hold the leaves together while the edges themselves are being connected by
trabeculae of coenosarc.

In a very large number of Calyptoblastea the gonophore is a reduced Medusa
which never escapes from the gonotheca, but in the family Eucopidae the
gonophores escape as free-swimming Medusae, exhibiting certain very
definite characters. The gonads are situated not on the manubrium, as in
the Anthomedusae, but on the sub-umbrellar aspect of the radial canals. The
marginal sense-organs may be ocelli or vesiculate statocysts. The bell is
usually more flattened, and the velum smaller than it is in the
Anthomedusae, and the manubrium short and quadrangular. Such Medusae are
called Leptomedusae.

Leptomedusae of many specific forms are found abundantly at the surface of
the sea in nearly all parts of the world, but with the exception of some
genera of the Eucopidae and a few others, their connexion with a definite
Calyptoblastic hydrosome has not been definitely ascertained. It may be an
assumption that time will prove to be unwarranted that all the Leptomedusae
pass through a Calyptoblastic hydrosome stage.

{278}FAM. AEQUOREIDAE.—In this family the hydrosome stage is not known
except in the genus _Polycanna_, in which it resembles a Campanulariid. The
sense-organs of the Medusae are statocysts. The radial canals are very
numerous, and the genital glands are in the form of ropes of cells
extending along the whole of their oral surfaces. _Aequorea_ is a fairly
common genus, with a flattened umbrella and a very rudimentary manubrium,
which may attain a size of 40 mm. in diameter.

FAM. THAUMANTIIDAE.—The Medusae of this family are distinguished from the
Aequoreidae by having marginal ocelli in place of statocysts. The hydrosome
of _Thaumantias_ alone is known, and this is very similar to an _Obelia_.

FAM. CANNOTIDAE.—The hydrosome is quite unknown. The Medusae are ocellate,
but the radial canals, instead of being undivided, as in the Thaumantiidae,
are four in number, and very much ramified before reaching the ring canal.
The tentacles are very numerous. In the genus _Polyorchis_, from the
Pacific coast of North America, the four radial canals give rise to
numerous lateral short blind branches, and have therefore a remarkable
pinnate appearance.

FAM. SERTULARIIDAE.—In this family the hydrothecae are sessile, and
arranged bilaterally on the stem and branches. The general form of the
colony is pinnate, the branches being usually on opposite sides of the main
stem. The gonophores are adelocodonic. _Sertularia_ forms more or less
arborescent colonies, springing from a creeping stolon attached to stones
and shells. There are many species, several of which are very common upon
the British coast. Many specimens are torn from their attachments by storms
or by the trawls of fishermen and cast up on the sand or beach with other
zoophytes. The popular name for one of the commonest species (_S.
abietina_) is the "sea-fir." The genus has a wide geographical and
bathymetrical range. Another common British species frequently thrown up by
the tide in great quantities is _Hydrallmania falcata_. It has slender
spirally-twisted stems and branches, and the hydrothecae are arranged
unilaterally.

The genus _Grammaria_, sometimes placed in a separate family, is
distinguished from _Sertularia_ by several characters. The stem and
branches are composed of a number of tubes which are considerably
compressed. The genus is confined to the southern seas.

{279}FAM. PLUMULARIIDAE.—The hydrothecae are sessile, and arranged in a
single row on the stem and branches. Nematophores are always present.
Gonophores adelocodonic. This family is the largest and most widely
distributed of all the families of the Hydrozoa. Nutting calculates that it
contains more than one-fourth of all the Hydroids of the world. Over 300
species have been described, and more than half of these are found in the
West Indian and Australian regions. Representatives of the family occur in
abundance in depths down to 300 fathoms, and not unfrequently to 500
fathoms. Only a few species have occasionally been found in depths of over
1000 fathoms.

The presence of nematophores may be taken as the most characteristic
feature of the family, but similar structures are also found in some
species belonging to other families (p. 277).

The family is divided into two groups of genera, the ELEUTHEROPLEA and the
STATOPLEA. In the former the nematophores are mounted on a slender pedicel,
which admits of more or less movement, and in the latter the nematophores
are sessile. The genera _Plumularia_ and _Antennularia_ belong to the
Eleutheroplea. The former is a very large genus, with several common
British species, distinguished by the terminal branches being pinnately
disposed, and the latter, represented by _A. antennina_ and _A. ramosa_ on
the British coast, is distinguished by the terminal branches being arranged
in verticils.

The two most important genera of the Statoplea are _Aglaophenia_ and
_Cladocarpus_. The former is represented by a few species in European
waters, the latter is only found in American waters.

FAM. HYDROCERATINIDAE.—The colony consists of a mass of entwined
hydrorhiza, with a skeleton in the form of anastomosing chitinous tubes.
Hydrothecae scattered, tubular, and sessile. Nematophores present.
Gonophores probably adelocodonic.

This family was constituted for a remarkable hydroid, _Clathrozoon
wilsoni_, described by W. B. Spencer from Victoria.[317] The zooids are
sessile, and spring from more than one of the numerous anastomosing tubes
of the stem and branches. The whole of the surface is studded with an
enormous number of small and very simple dactylozooids, protected by
tubular nematophores. Only {280}a few specimens have hitherto been
obtained, the largest being 10 inches in height by 4 inches in width. In
general appearance it has some resemblance to a dark coloured fan-shaped
_Gorgonia_.

FAM. CAMPANULARIIDAE.—The hydrothecae in this family are pedunculate, and
the gonophores adelocodonic.

In the cosmopolitan genus _Campanularia_ the stem is monosiphonic, and the
hydrothecae bell-shaped. Several species of this genus are very common in
the rock pools of our coast between tide marks. _Halecium_ is characterised
by the rudimentary character of its hydrothecae, which are incapable of
receiving the zooids even in their maximum condition of retraction. The
genus _Lafoea_ is remarkable for the development of a large number of
tightly packed gonothecae on the hydrorhiza, each of which contains a
blastostyle, bearing a single gonophore and, in the female, a single ovum.
This group of gonothecae was regarded as a distinct genus of Hydroids, and
was named _Coppinia_.[318] _Lafoea dumosa_ with gonothecae of the type
described as _Coppinia arcta_ occurs on the British coast.

_Perisiphonia_ is an interesting genus from deep water off the Azores,
Australia, and New Zealand, with a stem composed of many distinct tubes.

The genus _Zygophylax_, from 500 fathoms off the Cape Verde, is of
considerable interest in having a nematophore on each side of the
hydrotheca. According to Quelch it should be placed in a distinct family.

_Ophiodes_ has long and very active defensive zooids, protected by
nematophores. It is found in the Laminarian zone on the English coast.

FAM. EUCOPIDAE.—The hydrosome stage of this family is very similar to that
of the Campanulariidae, but the gonophores are free-swimming Medusae of the
Leptomedusan type.

One of the best-known genera is _Obelia_, of which several species are
among the commonest Hydroids of the British coast.

_Clytia johnstoni_ is also a very common Hydroid, growing on red algae or
leaves of the weed _Zostera_. It consists of a number of upright, simple,
or slightly branched stems springing from a creeping hydrorhiza. When
liberated the Medusae are globular in form, with four radial canals and
four marginal tentacles, but {281}this Medusa, like many others of the
order, undergoes considerable changes in form before it reaches the
sexually mature stage.

_Phialidium temporarium_ is one of the commonest Medusae of our coast, and
sometimes occurs in shoals. It seems probable that it is the Medusa of
_Clytia johnstoni_.[319] By some authors the jelly-fish known as
_Epenthesis_ is also believed to be the Medusa of a _Clytia_.

FAM. DENDROGRAPTIDAE.—This family includes a number of fossils which have
certain distinct affinities with the Calyptoblastea. In _Dictyonema_,
common in the Ordovician rocks of Norway, but also found in the Palaeozoic
rocks of North America and elsewhere, the fossil forms fan-shaped colonies
of delicate filaments, united by many transverse commissures, and in
well-preserved specimens the terminal branches bear well-marked uniserial
hydrothecae. In some species thecae of a different character, which have
been interpreted to be gonothecae and nematophores respectively, are found.

Other genera are _Dendrograptus_, _Thamnograptus_, and several others from
Silurian strata.


ORDER V. GRAPTOLITOIDEA.

A large number of fossils, usually called Graptolites, occurring in
Palaeozoic strata, are generally regarded as the skeletal remains of an
ancient group of Hydrozoa.

In the simpler forms the fossil consists of a delicate straight rod bearing
on one side a series of small cups. It is suggested that the cups contained
hydroid zooids, and should therefore be regarded as the equivalent of the
hydrothecae, and that the axis represents the axis of the colony or of a
branch of the Calyptoblastea. In some of the forms with two rows of cups on
the axis (_Diplograptus_), however, it has been shown that the cups are
absent from a considerable portion of one end of the axis, and that the
axes of several radially arranged individuals are fused together and united
to a central circular plate. Moreover, there is found in many specimens a
series of vesicles, a little larger in size than the cups, attached to the
plate and arranged in a circle at the base of the axes. These vesicles are
called the gonothecae.

The discovery of the central plate and of the so-called {282}gonothecae
suggests that the usual comparison of a Graptolite with a Sertularian
Hydroid is erroneous, and that the colony or individual, when alive, was a
more or less radially symmetrical floating form, like a Medusa, of which
only the distal appendages (possibly tentacles) are commonly preserved as
fossils.

The evidence that the Graptolites were Hydrozoa is in reality very slight,
but the proof of their relationship to any other phylum of the animal
kingdom does not exist.[320] It is therefore convenient to consider them in
this place, and to regard them, provisionally, as related to the
Calyptoblastea.

The order is divided into three families.

FAM. 1. MONOPRIONIDAE.—Cups arranged uniserially on one side of the axis.

The principal genera are _Monograptus_, with the axis straight, curved, or
helicoid, from many horizons in the Silurian strata; _Rastrites_, with a
spirally coiled axis, Silurian; _Didymograptus_, Ordovician; and
_Coenograptus_, Ordovician.

FAM. 2. DIPRIONIDAE.—Cups arranged in two or four vertical rows on the
axis.

_Diplograptus_, Ordovician and Silurian; _Climacograptus_, Ordovician and
Silurian; and _Phyllograptus_, in which the axis and cups are arranged in
such a manner that they resemble an ovate leaf.

FAM. 3. RETIOLITIDAE.—Cups arranged biserially on a reticulate axis.

_Retiolites_, Ordovician and Silurian; _Stomatograptus_, _Retiograptus_,
and _Glossograptus_, Ordovician.


FOSSIL CORALS POSSIBLY ALLIED TO HYDROZOA.

Among the many fossil corals that are usually classified with the Hydrozoa
the genus _Porosphaera_ is of interest as it is often supposed to be
related to _Millepora_. It consists of globular masses about 10-20 mm. in
diameter occurring in the Upper Cretaceous strata. In the centre there is
usually a foreign body around which the coral was formed by concentric
encrusting growth. Running radially from pores on the surface to the
centre, there are numerous tubules which have a certain general resemblance
to the pore-tubes of _Millepora_. The monomorphic {283}character of these
tubes, their very minute size, the absence of ampullae, and the general
texture of the corallum, are characters which separate this fossil very
distinctly from any recent Hydroid corals. _Porosphaera_, therefore, was
probably not a Hydrozoon, and certainly not related to the recent
_Millepora_.

Closely related to _Porosphaera_ apparently are other globular,
ellipsoidal, or fusiform corals from various strata, such as _Loftusia_
from the Eocene of Persia, _Parkeria_ from the Cambridge Greensand, and
_Heterastridium_ from the Alpine Trias. In the last named there is
apparently a dimorphism of the radial tubes.

Allied to these genera, again, but occurring in the form of thick,
concentric, calcareous lamellae, are the genera _Ellipsactinia_ and
_Sphaeractinia_ from the Upper Jurassic.

Another important series of fossil corals is that of the family
STROMATOPORIDAE. These fossils are found in great beds of immense extent in
many of the Palaeozoic rocks, and must have played an important part in the
geological processes of that period. They consist of a series of calcareous
lamellae, separated by considerable intervals, encrusting foreign bodies of
various kinds. Sometimes they are flat and plate-like, sometimes globular
or nodular in form. The lamellae are in some cases perforated by tabulate,
vertical, or radial pores, but in many others these pores are absent. The
zoological position of the Stromatoporidae is very uncertain, but there is
not at present any very conclusive evidence that they are Hydrozoa.

_Stromatopora_ is common in Devonian and also occurs in Silurian strata.
_Cannopora_ from the Devonian has well-marked tabulate pores, and is often
found associated commensally with another coral (_Aulopora_ or
_Syringopora_).


ORDER VI. STYLASTERINA.

The genera included in this order resemble _Millepora_ in producing a
massive calcareous skeleton, and in showing a consistent dimorphism of the
zooids, but in many respects they exhibit great divergence from the
characters of the Milleporina.

The colony is arborescent in growth, the branches arising frequently only
in one plane, forming a flabellum. The calcareous skeleton is perforated to
a considerable depth by the gastrozooids, dactylozooids, and nutritive
canals, and the {284}gastropores and dactylopores are not provided with
tabulae except in the genera _Pliobothrus_ and _Sporadopora_. The character
which gives the order its name is a conical, sometimes torch-like
projection at the base of the gastropore, called the "style," which carries
a fold of the ectoderm and endoderm layers of the body-wall, and may serve
to increase the absorptive surface of the digestive cavity. In some genera
a style is also present in the dactylopore, in which case it serves as an
additional surface for the attachment of the retractor muscles. The pores
are scattered on all aspects of the coral in the genera _Sporadopora_,
_Errina_, and _Pliobothrus_; in _Spinipora_ and _Steganopora_ the scattered
dactylopores are situated at the extremities of tubular spines which
project from the general surface of the coral, the gastropores being
situated irregularly between the spines. In _Phalangopora_ the pores are
arranged in regular longitudinal lines, and in _Distichopora_ they are
mainly in rows on the edges of the flattened branches, a single row of
gastropores being flanked by a single row of dactylopores on each side. In
the remaining genera the pores are arranged in definite cycles, which are
frequently separated from one another by considerable intervals, and have,
particularly in the dried skeleton, a certain resemblance to the calices of
some of the Zoantharian corals.

In _Cryptohelia_ the cycles are covered by a lid-like projection from the
neighbouring coenenchym (Fig. 136, _l_ 1, _l_ 2). The gastrozooids are
short, and are usually provided with a variable number of small capitate
tentacles. The dactylozooids are filiform and devoid of tentacles, the
endoderm of their axes being solid and scalariform.

The gonophores of the Stylasterina are situated in large oval or spherical
cavities called the ampullae, and their presence can generally be detected
by the dome-shaped projections they form on the surface of the coral. The
female gonophore consists of a saucer-shaped pad of folded endoderm called
the "trophodisc," which serves the purpose of nourishing the single large
yolk-laden egg it bears; and a thin enveloping membrane composed of at
least two layers of cells. The egg is fertilised while it is still within
the ampulla, and does not escape to the exterior until it has reached the
stage of a solid ciliated larva. All the Stylasterina are therefore
viviparous. The male gonophore has a very much smaller trophodisc, which is
sometimes (_Allopora_) prolonged into a columnar process or spadix,
penetrating the {285}greater part of the gonad. The spermatozoa escape
through a peculiar spout-like duct which perforates the superficial wall of
the ampulla. In some genera (_Distichopora_) there are several male
gonophores in each ampulla.

The gonophores of the Stylasterina have been regarded as much altered
medusiform gonophores, and this view may possibly prove to be correct. At
present, however, the evidence of their derivation from Medusae is not
conclusive, and it is possible that they may have had a totally independent
origin.

_Distichopora_ and some species of _Stylaster_ are found in shallow water
in the tropics, but most of the genera are confined to deep or very deep
water, and have a wide geographical distribution. No species have been
found hitherto within the British area.

[Illustration: FIG. 136.—A portion of a branch of _Cryptohelia ramosa_,
showing the lids _l_ 1 and _l_ 2 covering the cyclosystems, the swellings
produced by the ampullae in the lids _amp_^1, _amp_^2, and the
dactylozooids, _dac._ × 22. (After Hickson and England.)]

A few specimens of a species of _Stylaster_ have been found in Tertiary
deposits and in some raised beaches of more recent origin, but the order is
not represented in the older strata.

FAM. STYLASTERIDAE.—All the genera at present known are included in this
family.

_Sporadopora_ is the only genus that presents a superficial general
resemblance to _Millepora_. It forms massive, branching white coralla, with
the pores scattered irregularly on the surface, and, like many varieties of
_Millepora_, not arranged in cyclosystems. It may, however, be
distinguished at once by the presence of a long, brush-like style in each
of the gastropores. The ampullae are large, but are usually so deep-seated
in the coenenchym that their presence cannot be detected from the surface.
It was found off the Rio de la Plata in 600 fathoms of water by the
"Challenger."

{286}In _Errina_ the pores are sometimes irregularly scattered, but in _E.
glabra_ they are arranged in rows on the sides of the branches, while in
_E. ramosa_ the gastropores occur at the angles of the branches only. The
dactylopores are situated on nariform projections of the corallum. The
ampullae are prominent. There are several gonophores in each ampulla of the
male, but only one in each ampulla of the female. This genus is very widely
distributed in water from 100 to 500 fathoms in depth.

_Phalangopora_ differs from _Errina_ in the absence of a style in the
gastropore; Mauritius.—_Pliobothrus_ has also no style in the gastropore,
and is found in 100-600 fathoms of water off the American Atlantic shores.

_Distichopora_ is an important genus, which is found in nearly all the
shallow seas of the tropical and semi-tropical parts of the world, and may
even flourish in rock pools between tide marks. It is nearly always
brightly coloured—purple, violet, pale brown, or rose red. The colony
usually forms a small flabellum, with anastomosing branches, and the pores
are arranged in three rows, a middle row of gastropores and two lateral
rows of dactylopores on the sides of the branches. There is a long style in
each gastropore. The ampullae are numerous and prominent, situated on the
anterior and posterior faces of the branches. Each ampulla contains a
single gonophore in the female colony and two or three gonophores in the
male colony.

_Spinipora_ is a rare genus from off the Rio de la Plata in 600 fathoms.
The branches are covered with blunt spines. These spines have a short
gutter-like groove at the apex, which leads into a dactylopore. The
gastropores are provided with a style and are situated between the spines.

_Steganopora_[321] from the Djilolo Passage, in about 600 fathoms, is very
similar to _Spinipora_ as regards external features, but differs from it in
the absence of styles in the gastropores, and in the wide communications
between the gastropores and dactylopores.

_Stylaster_ is the largest and most widely distributed genus of the family,
and exhibits a considerable range of structure in the many species it
contains. It is found in all the warmer seas of the world, living between
tide marks at a few fathoms, and extending to depths of 600 fathoms. Many
specimens, but especially those from very shallow water, are of a beautiful
rose {287}or pink colour. The corallum is arborescent and usually
flabelliform. The pores are distributed in regular cyclosystems, sometimes
on one face of the corallum only, sometimes on the sides of the branches,
and sometimes evenly distributed. There are styles in both gastropores and
dactylopores.

_Allopora_ is difficult to separate from _Stylaster_, but the species are
usually more robust in habit, and the ampullae are not so prominent as they
are on the more delicate branches of _Stylaster_. It occurs at depths of
100 fathoms in the Norwegian fjords. A very large red species (_A.
nobilis_) occurs in False Bay, Cape of Good Hope, in 30 fathoms of water.
In this locality the coral occurs in great submarine beds or forests, and
the trawl that is passed over them is torn to pieces by the hard, thick
branches, some of which are an inch or more in diameter.

_Astylus_ is a genus found in the southern Philippine sea in 500 fathoms of
water. It is distinguished from _Stylaster_ by the absence of a style in
the gastropore.

_Cryptohelia_ is an interesting genus found both in the Atlantic and
Pacific Oceans at depths of from 270 to about 600 fathoms. The cyclosystems
are covered by a projecting lid or operculum (Fig. 136, _l_ 1, _l_ 2).
There are no styles in either the gastropores or the dactylopores. The
ampullae are prominent, and are sometimes situated in the lids. There are
several gonophores in each ampulla of the female colony, and a great many
in the ampulla of the male colony.




{288}CHAPTER XI

HYDROZOA (_CONTINUED_): TRACHOMEDUSAE—NARCOMEDUSAE—SIPHONOPHORA


ORDER VII. TRACHOMEDUSAE.

The orders Trachomedusae and Narcomedusae are probably closely related to
one another and to some of the families of Medusae at present included in
the order Calyptoblastea, and it seems probable that when the
life-histories of a few more genera are made known the three orders will be
united into one. Very little is known of the hydrosome stage of the
Trachomedusae, but Brooks[322] has shown that in _Liriope_, and
Murbach[323] that in _Gonionema_, the fertilised ovum gives rise to a
_Hydra_-like form, and in the latter this exhibits a process of
reproduction by gemmation before it gives rise to Medusae. Any general
statement, therefore, to the effect that the development of the
Trachomedusae is direct would be incorrect. The fact that the hydrosomes
already known are epizoic or free-swimming does not afford a character of
importance for distinction from the Leptomedusae, for it is quite possible
that in this order of Medusae the hydrosomes of many genera may be similar
in form and habits to those of _Liriope_ and _Gonionema_.

The free border of the umbrella of the Trachomedusae is entire; that is to
say, it is not lobed or fringed as it is in the Narcomedusae. The
sense-organs are statocysts, each consisting of a vesicle formed by a more
or less complete fold of the surrounding wall of the margin of the
umbrella, containing a reduced clapper-like tentacle loaded at its
extremity with a statolith.

{289}[Illustration: FIG. 137.—_Liriope rosacea_, one of the Geryoniidae,
from the west side of North and Central America. Size, 15-20 mm. Colour,
rose. _cp_, Centripetal canal; _gon_, gonad; _M_, mouth at the end of a
long manubrium; _ot_, statocyst; _t_, tentacle; _to_, tongue. (After
Maas.)]

This statocyst is innervated by the outer nerve ring. There appears to be a
very marked difference between these marginal sense-organs in some of the
best-known examples of Trachomedusae and the corresponding organs of the
Leptomedusae. The absence of a stalk supporting the statolith and the
innervation of the otocyst by the inner instead of by the outer nerve ring
in the Leptomedusae form characters that may be of supplementary value, but
cannot be regarded as absolutely distinguishing the two orders. The
statorhab of the Trachomedusae is probably the more primitive of the two
types, and represents a marginal tentacle of the umbrella reduced in size,
loaded with a statolith and enclosed by the mesogloea. Intermediate stages
between this type and an ordinary tentacle have already been discovered and
described. In the type that is usually found in the Leptomedusae the
modified tentacle is still further reduced, and all that can be recognised
of it is the statolith attached to the wall of the statocyst, but
intermediate stages between the two types are seen in the family
Olindiidae, in which the stalk supporting the statolith passes gradually
into the tissue surrounding the statolith on the one hand and the vesicle
wall on the other. The radial canals are four or eight in number or more
numerous. They communicate at the margin of the umbrella with a ring canal
from which a number of short blind tubes run in the umbrella-wall towards
the centre of the Medusa (Fig. 137, _cp_). These "centripetal canals" are
subject to {290}considerable variation, but are useful characters in
distinguishing the Trachomedusae from the Leptomedusae. The tentacles are
situated on the margin of the umbrella, and are four or eight in number or,
in some cases, more numerous. The gonads are situated as in Leptomedusae on
the sub-umbrella aspect of the radial canals.

In _Gonionema murbachii_ the fertilised eggs give rise to a free-swimming
ciliated larva of an oval shape with one pole longer and narrower than the
other. The mouth appears subsequently at the narrower pole. The larva
settles down upon the broader pole, the mouth appears at the free
extremity, and in a few days two, and later two more, tentacles are formed
(Fig. 138).

At this stage the larva may be said to be _Hydra_-like in character, and as
shown in Fig. 138 it feeds and lives an independent existence. From its
body-wall buds arise which separate from the parent and give rise to
similar _Hydra_-like individuals. An asexual generation thus gives rise to
new individuals by gemmation as in the hydrosome of the Calyptoblastea. The
origin of the Medusae from this _Hydra_-like stage has not been
satisfactorily determined, but it seems probable that by a process of
metamorphosis the hydriform persons are directly changed into the
Medusae.[324]

[Illustration: FIG. 138.—Hydra-like stage in the development of _Gonionema
murbachii_. One of the tentacles is carrying a worm (_W_) to the mouth. The
tentacles are shown very much contracted, but they are capable of extending
to a length of 2 mm. Height of zooid about 1 mm. (After Perkins.)]

In the development of _Liriope_ the free-swimming larva develops into a
hydriform person with four tentacles and an enormously elongated hypostome
or manubrium; and, according to Brooks, it undergoes a metamorphosis which
directly converts it into a Medusa.

There can be very little doubt that in a large number of Trachomedusae the
development is direct, the fertilised ovum giving rise to a medusome
without the intervention of a hydrosome stage. In some cases, however
(_Geryonia_, etc.), the tentacles {291}appear in development before there
is any trace of a sub-umbrella cavity, and this has been interpreted to be
a transitory but definite Hydroid stage. It may be supposed that the
elimination of the hydrosome stage in these Coelenterates may be associated
with their adaptation to a life in the ocean far from the coast.

During the growth of the Medusa from the younger to the adult stages
several changes probably occur of a not unimportant character, and it may
prove that several genera now placed in the same or even different families
are stages in the development, of the same species. In the development of
_Liriantha appendiculata_,[325] for example, four interradial tentacles
appear in the first stage which disappear and are replaced by four radial
tentacles in the second stage.

As with many other groups of free-swimming marine animals the Trachomedusae
have a very wide geographical distribution, and some genera may prove to be
almost cosmopolitan, but the majority of the species appear to be
characteristic of the warmer regions of the high seas. Sometimes they are
found at the surface, but more usually they swim at a depth of a few
fathoms to a hundred or more from the surface. The Pectyllidae appear to be
confined to the bottom of the sea at great depths.

The principal families of the Trachomedusae are:—

FAM. OLINDIIDAE.—This family appears to be structurally and in development
most closely related to the Leptomedusae, and is indeed regarded by
Goto[326] as closely related to the Eucopidae in that order. They have two
sets of tentacles, velar and exumbrellar; the statocysts are numerous, two
on each side of the exumbrellar tentacles. Radial canals four or six.
Manubrium well developed and quadrate, with distinct lips. There is an
adhesive disc on each exumbrellar tentacle.

Genera: _Olindias_, _Olindioides_, _Gonionema_ (Fig. 139), and _Halicalyx_.

As in other families of Medusae the distribution of the genera is very
wide. _Olindias mülleri_ occurs in the Mediterranean, _Olindioides formosa_
off the coast of Japan, _Gonionema murbachii_ is found in abundance in the
eel pond at Wood's Holl, United States of America, and _Halicalyx_ off
Florida.

Two genera may be referred to in this place, although their {292}systematic
position in relation to each other and to other Medusae has not been
satisfactorily determined.

[Illustration: FIG. 139.—_Gonionema murbachii._ Adult Medusa, shown
inverted, and clinging to the bottom. Nat. size. (After Perkins.)]

_Limnocodium sowerbyi_ is a small Medusa that was first discovered in the
_Victoria regia_ tanks in the Botanic Gardens, Regent's Park, London, in
the year 1880. It has lately made its appearance in the _Victoria regia_
tank in the Parc de la Bête d'Or at Lyons.[327] As it was, at the time of
its discovery, the only fresh-water jelly-fish known, it excited
considerable interest, and this interest was not diminished when the
peculiarities of its structure were described by Lankester and others. It
has a rather flattened umbrella, with entire margin and numerous marginal
tentacles, the manubrium is long, quadrate, and has four distinct lips.
There are four radial canals, and the male gonads (all the specimens
discovered were of the male sex) are sac-like bodies on the sub-umbrellar
aspect of the middle points of the four radial canals. In these characters
the genus shows general affinities with the Olindiidae. The difficult
question of the origin of the statoliths from the primary germ layers of
the embryo and some other points in the minute anatomy of the Medusa have
{293}suggested the view that _Limnocodium_ is not properly placed in any of
the other orders. Goto,[328] however, in a recent paper, confirms the view
of the affinities of _Limnocodium_ with the Olindiidae.

The life-history of _Limnocodium_ is not known, but a curious Hydroid form
attached to _Pontederia_ roots was found in the same tank as the Medusae,
and this in all probability represents the hydrosome stage of its
development. The Medusae are formed apparently by a process of transverse
fission of the Hydroid stock[329] similar in some respects to that observed
in the production of certain Acraspedote Medusae. This is quite unlike the
asexual mode of formation of Medusae in any other Craspedote form. The
structure of this hydrosome is, moreover, very different to that of any
other Hydroid, and consequently the relations of the genus with the
Trachomedusae cannot be regarded as very close.

_Limnocodium_ has only been found in the somewhat artificial conditions of
the tanks in botanical gardens, and its native locality is not known, but
its association with the _Victoria regia_ water-lily seems to indicate that
its home is in tropical South America.

_Limnocnida tanganyicae_ is another remarkable fresh-water Medusa, about
seven-eights of an inch in diameter, found in the lakes Tanganyika and
Victoria Nyanza of Central Africa.[330] It differs from _Limnocodium_ in
having a short collar-like manubrium with a large round mouth two-thirds
the diameter of the umbrella, and in several other not unimportant
particulars. It produces in May and June a large number of Medusa-buds by
gemmation on the manubrium, and in August and September the sexual organs
are formed in the same situation.

[Illustration: FIG. 140.—_Limnocnida tanganyicae._ × 2. (After Günther.)]

The fixed hydrosome stage, if such a stage occurs in the life-history, has
not been discovered; but Mr. Moore[331] believes that {294}the development
is direct from ciliated planulae to the Medusae. The occurrence of
_Limnocnida_ in Lake Tanganyika is supposed by the same authority to afford
a strong support to the view that this lake represents the remnants of a
sea which in Jurassic times spread over part of the African continent. This
theory has, however, been adversely criticised from several sides.[332]

The character of the manubrium and the position of the sexual cells suggest
that _Limnocnida_ has affinities with the Narcomedusae or Anthomedusae, but
the marginal sense-organs and the number and position of the tentacles,
showing considerable similarity with those of _Limnocodium_, justify the
more convenient plan of placing the two genera in the same family.

FAM. PETASIDAE.—The genus _Petasus_ is a small Medusa with four radial
canals, four gonads, four tentacles, and four free marginal statorhabs. A
few other genera associated with _Petasus_ show simple characters as
regards the canals and the marginal organs, but as very little is known of
any of the genera the family may be regarded as provisional only. _Petasus_
is found in the Mediterranean and off the Canaries.

FAM. TRACHYNEMIDAE.—In this family there are eight radial canals, and the
statorhabs are sunk into a marginal vesicle. _Trachynema_, characterised by
its very long manubrium, is a not uncommon Medusa of the Mediterranean and
the eastern Atlantic Ocean. Many of the species are small, but _T.
funerarium_ has sometimes a disc two inches in diameter. _Homoconema_ and
_Pentachogon_ have numerous very short tentacles.

FAM. PECTYLLIDAE.—This family contains a few deep-sea species with
characters similar to those of the preceding family, but the tentacles are
provided with terminal suckers. _Pectyllis_ is found in the Atlantic Ocean
at depths of over 1000 fathoms.

FAM. AGLAURIDAE.—The radial canals are eight in number and the statorhabs
are usually free. In the manubrium there is a rod-like projection of the
mesogloea from the aboral wall of the gastric cavity, covered by a thin
epithelium of endoderm, which occupies a considerable portion of the lumen
of the manubrium. This organ may be called the tongue. _Aglaura_ has an
octagonal umbrella, and a manubrium which does not project beyond the
velum. It occurs in the Atlantic Ocean and Mediterranean Sea.

{295}FAM. GERYONIIDAE.—In this family there are four or six radial canals,
the statorhabs are sunk in the mesogloea, and a tongue is present in the
manubrium. _Liriope_ (Fig. 137) is sometimes as much as three inches in
diameter. It has a very long manubrium, and the tongue sometimes projects
beyond the mouth. There are four very long radial tentacles. It is found in
the Atlantic Ocean, the Mediterranean Sea, and the Pacific and Indian
Oceans. _Geryonia_ has a wider geographical distribution than _Liriope_,
and is sometimes four inches in diameter. It differs from _Liriope_ in
having six, or a multiple of six, radial canals. _Carmarina_ of the
Mediterranean and other seas becomes larger even than _Geryonia_, from
which it differs in the arrangement of the centripetal canals.

_Liriantha appendiculata_ sometimes occurs on the south coast of England
during September, October, or at other times.


ORDER VIII. NARCOMEDUSAE.

The Narcomedusae differ from the Trachomedusae in having the margin of the
umbrella divided into a number of lobes, and in bearing the gonads on the
sub-umbrellar wall of the gastral cavity instead of upon the radial canals.
The tentacles are situated at some little distance from the margin of the
umbrella at points on the aboral surface corresponding with the angles
between the umbrella lobes. Between the base of the tentacle and the
marginal angle there is a tract of modified epithelium called the
"peronium." The manubrium is usually short, and the mouth leads into an
expanded gastral chamber which is provided with lobular diverticula
reaching as far as the bases of the tentacles. The marginal sense-organs
are in the form of unprotected statorhabs. Very little is known concerning
the life-history of any of the Narcomedusae. In _Cunoctantha octonaria_ the
peculiar ciliated larva with two tentacles and a very long proboscis soon
develops two more tentacles and creeps into the bell of the Anthomedusan
_Turritopsis_, where, attached by its tentacles, it lives a parasitic life.
Before being converted into a Medusa it gives rise by gemmation to a number
of similar individuals, all of which become, in time, Medusae. The
parasitic stage is often regarded as the representative of the hydrosome
stage reduced and adapted to the oceanic habit of the adult.

{296}In _Cunina proboscidea_, and in some other species, a very remarkable
method of reproduction has been described by Metschnikoff, called by him
"sporogony." In these cases young sexual cells (male or female) wander from
the gonad of the parent into the mesogloea of the umbrella, where they
develop parthenogenetically into ciliated morulae. These escape by the
radial canals into the gastric cavity, and there form a stolon from which
young Medusae are formed by gemmation. In _C. proboscidea_ these young
Medusae are like the genus _Solmaris_, but in _C. rhododactyla_ they have
the form of the parent. In some cases the ciliated larvae leave the parent
altogether and become attached to a _Geryonia_ or some other Medusa, where
they form the stolon.

This very interesting method of reproduction cannot be regarded as a
primitive one, and throws no light on the origin of the order. It might be
regarded as a further stage in the degeneration of the hydrosome stage in
its adaptation to a parasitic existence.

The Narcomedusae have a wide geographical distribution. Species of
_Aeginopsis_ occur in the White Sea and Bering Strait, but the genera are
more characteristic of warmer waters. Some species occur in moderately deep
water, and _Cunarcha_ was found in 1675 fathoms off the Canaries, but they
are more usually found at or near the surface of the sea.

FAM. CUNANTHIDAE.—Narcomedusae with large gastral diverticula corresponding
in position with the bases of the tentacles. _Cunina_ and _Cunoctantha_,
occurring in the Mediterranean and in the Atlantic and Pacific Oceans,
belong to this family. In _Cunina_ the tentacles may be eight in number, or
some multiple of four between eight and twenty-four. In _Cunoctantha_ the
number of tentacles appears to be constantly eight.

FAM. PEGANTHIDAE.—There appear to be no gastral pouches in this family. The
species of _Pegantha_ are found at depths of about 80 fathoms in the Indian
and Pacific Oceans.

FAM. AEGINIDAE.—The large gastral pouches of this family alternate with the
bases of the tentacles. _Aegina_ occurs in the Atlantic and Pacific Oceans.
_Aeginopsis._

FAM. SOLMARIDAE.—In this family the gastral pouches are variable, sometimes
corresponding with, sometimes alternating with, the bases of the tentacles.
The circular canal is represented {297}in some genera by solid cords of
endoderm. _Solmaris_ sometimes appears in the English Channel, but it is
probably a wanderer from the warmer regions of the Atlantic Ocean. It is
found in abundance during November on the west coast of Ireland.


ORDER IX. SIPHONOPHORA.

In this order the naturalist finds collected together a number of very
beautiful, delicate transparent organisms to which the general term
"jelly-fish" may be applied, although their organisation is far more
complicated and difficult to describe than that of any of the Medusae. In
several of the Hydrozoa the phenomenon of dimorphism has already been
noticed. In these cases one set of individuals in a colony performs
functions of stinging and catching food and another the functions of
devouring and digesting it. In many of the Siphonophora there appears to be
a colony of individuals in which the division of labour is carried to a
much further extent than it is in the dimorphic Hydrozoa referred to above.
Not only are there specialised gastrozooids and dactylozooids, but also
gonozooids, zooids for propelling the colony through the water
("nectocalyces"), protective zooids ("hydrophyllia"), and in some cases a
specialised zooid for hydrostatic functions; the whole forming a swimming
or floating polymorphic colony. But this conception of the construction of
the Siphonophora is not the only one that has met with support. By some
zoologists the Siphonophoran body is regarded not as a colony of
individuals, but as a single individual in which the various organs have
become multiplied and dislocated.

The multiplication or repetition of organs that are usually single in each
individual is not unknown in other Hydrozoa. In the Medusa of the
Gymnoblast _Syncoryne_, usually known as _Sarsia_, for example, there is
sometimes a remarkable proliferation of the manubrium, and specimens have
been found with three or four long manubria attached by a tubular stalk to
the centre of the umbrella. Moreover, this complex of manubria may become
detached from the umbrella and live for a considerable time an independent
existence.[333]

If we regard the manubrium of a Medusa as an organ of the {298}animal's
body, it might be thought obvious that the phenomenon observed in the
Medusae of _Syncoryne_ is a case of a simple repetition of the parts of an
individual; but the power that the group of manubria possesses of leading
an independent existence renders its interpretation as a group of organs a
matter of some inconvenience. If we can conceive the idea that an organ may
become detached and lead an independent existence, there is no reason why
we should not regard the Medusa itself of _Syncoryne_ as an organ, and we
should be driven to the paradoxical conclusion that, as regards several
genera and families of Hydrozoa, we know nothing at present of the
individuals, but only of their free-swimming organs, and that in others the
individual has degenerated, although one of its organs remains.

There is, however, no convincing argument to support either the conception
that the Siphonophoran body is a colony of individuals, or that it is an
individual with disjointed organs. These two conceptions are sometimes
called the "Poly-person" and "Poly-organ" theories respectively. The
difficulty is caused by the impossibility of giving any satisfactory
definition in the case of the Hydrozoa of the biological terms "organ" and
"individual." In the higher animals, where the correlation of parts is far
more complex and essential than it is in Coelenterata, a defined limit to
the scope of these terms can be laid down, but in the lower animals the
conception of what is termed an organ merges into that which is called an
individual, and no definite boundary line between the two exists in Nature.
The difficulty is therefore a permanent one, and, in using the expression
"colony" for the Siphonophoran body, it must be understood that it is used
for convenience' sake rather than because it represents the only correct
conception of the organisation of these remarkable Coelenterates.

Regarding the Siphonophora as polymorphic colonies, then, the following
forms of zooids may be found.

_Nectocalyces._—The nectocalyces are in the form of the umbrella of a
medusa attached to the stolon of the colony by the aboral pole. They are
provided with a velum and, usually, four radial canals and a circular
canal. There is no manubrium, and the marginal tentacles and sense-organs
are rudimentary or absent. There may be one or more nectocalyces in each
colony, {299}and their function is, by rhythmic contractions, to propel the
colony through the water (Fig. 142, N).

_Gastrozooids._—These are tubular or saccular zooids provided with a mouth
and attached by their aboral extremity to the stolon (Fig. 142, G). In some
cases the aboral region of the zooid is differentiated as a stomach. It is
dilated and bears the digestive cells, the oral extremity or hypostome
being narrower and more transparent. In some cases the mouth is a simple
round aperture at the extremity of the hypostome, but in others it is
dilated to form a trumpet-like lip.

_Dactylozooids_.—In _Velella_ and _Porpita_ the dactylozooids are similar
in general characters to the tentacles of many Medusae. They are arranged
as a frill round the margin of the colony, and each consists of a simple
tube of ectoderm and endoderm terminating in a knobbed extremity richly
provided with nematocysts.

In many other Siphonophora, however, the dactylozooids are very long and
elaborate filaments, which extend for a great distance from the colony into
the sea. They reach their most elaborate condition in the Calycophorae.

[Illustration: FIG. 141.—A small Crustacean (_Rhinocalanus_) caught by a
terminal filament (_f.t_) of a battery of _Stephanophyes_. _b_, The
proximal end of the battery with the most powerful nematocysts; _e_,
elastic band; _S_, stalk supporting the battery on the dactylozooid. (After
Chun.)]

The dactylozooid in these forms has a hollow axis, and the lumen is
continuous with the cavity of the neighbouring gastrozooid. Arranged at
regular intervals on the axis is a series of tentacles ("tentilla"), and
each of these supports {300}a kidney-shaped swelling, the "cnidosac," or
battery, which is sometimes protected by a hood. Each battery contains an
enormous number of nematocysts. In _Stephanophyes_, for example, there are
about 1700 nematocysts of four different kinds in each battery. At the
extremity of the battery there is a delicate terminal filament. The action
of the battery in _Stephanophyes_ is, according to Chun,[334] a very
complicated one. The terminal filament lassos the prey and discharges its
somewhat feeble nematocysts at it (Fig. 141). If this kills it, the
dactylozooid contracts and passes the prey to a gastrozooid. If the animal
continues its struggles, it is drawn up to the distal end of the battery
and receives the discharge of a large number of nematocysts; and if this
also fails to put an end to its life, a membrane covering the largest and
most powerful nematocysts at the proximal end of the whole battery is
ruptured, and a final broadside of stinging threads is shot at it.

The larger nematocysts of these batteries in the Siphonophora are among the
largest found in Coelenterata, being from 0.5 to 0.1 mm. in length, and
they are frequently capable of inflicting painful stings on the human skin.
The species of _Physalia_, commonly called "Portuguese Men-of-War," have
perhaps the worst reputation in this respect, the pain being not only
intense but lasting a long time.

_Hydrophyllia._—In many Siphonophora a number of short, mouthless,
non-sexual zooids occur, which appear to have no other function than that
of shielding or protecting other and more vital parts of the colony. They
consist of an axis of firm mesogloea, covered by a layer of flattened
ectoderm, and they may be finger-shaped or triangular in form. In _Agalma_
and _Praya_ an endoderm canal perforates the mesogloea and terminates in a
little mouth at the free extremity. In _Athoria_ and _Rhodophysa_ the
hydrophyllium terminates in a little nectocalyx.

_Pneumatophore._—In all the Siphonophora, with the exception of the
Calycophorae, there is found on one side or at one extremity of the colony
a vesicle or bladder containing a gas,[335] which serves as a float to
support the colony in the water. {301}This bladder or pneumatophore is
probably in all cases a much modified nectocalyx. It shows great variations
in size and structure in the group. It is sometimes relatively very large,
as in _Physalia_ and _Velella_, sometimes very small, as in _Physophora_.
It is provided with an apical pore in some genera (_Rhizophysa_), or a
basal pore in others (Auronectidae), but it is generally closed. In the
many chambered pneumatophore of the Chondrophoridae there are several
pores.

In many forms two distinct parts of the pneumatophore can be recognised—a
distal region lined by chitin,[336] probably representing the sub-umbrellar
cavity of the nectocalyx, and a small funnel-shaped region lined by an
epithelium, the homology of which is a matter of dispute. It is believed
that the gas is secreted by this epithelium. In the Auronectidae the region
with secretory epithelium is relatively large and of a more complicated
histological character. It is remarkable also that in this family the pore
communicates, not with the chitin-lined region, but directly with the
epithelium-lined region.

There is no pneumatophore in the Calycophorae, but in this sub-order a
diverticulum of an endoderm canal secretes a globule of oil which may serve
the same hydrostatic function.

The _stolon_ is the common stem which supports the different zooids of the
colony. In the Calycophorae the stolon is a long, delicate, and extremely
contractile thread attached at one end to a nectocalyx, and bearing the
zooids in discontinuous groups. These groups of zooids arranged at
intervals on the stolon are called the "cormidia." The stolon is a tube
with very thick walls. Its lumen is lined by a ciliated endoderm with
circular muscular processes, and the surface is covered with an ectoderm,
also provided with circular muscular processes. Between these two layers
there is a relatively thick mesogloea showing on the outer side deep and
compound folds and grooves supporting an elaborate system of longitudinal
muscular fibres. In many Physonectidae the stolon is long and filamentous,
but not so contractile as it is in Calycophorae, but in others it is much
reduced in length and relatively stouter. The reduction {302}in length of
the stolon is accompanied by a complication of structure, the simple
tubular condition being replaced by a spongy complex of tubes covered by a
common sheath of ectoderm. In the Auronectidae the stolon is represented by
a conical or hemispherical spongy mass bearing the zooids, and in the
Rhizophysaliidae and Chondrophoridae it becomes a disc or ribbon-shaped pad
spreading over the under side of the pneumatophore.

_Gonozooids._—The gonozooids are simple tubular processes attached to the
stolon which bear the Medusae or the degenerate medusiform gonophores. In
the Chondrophoridae the gonozooids possess a mouth, but in most
Siphonophora they have neither mouth nor tentacles. In some cases, such as
_Anthophysa_, the colonies are bisexual—the male and female gonophores
being borne by separate gonozooids—but in others (e.g. _Physalia_) the
colonies appear to be unisexual.

As a general rule the gonophores of Siphonophora do not escape from the
parent colony as free-swimming Medusae, but an exception occurs in
_Velella_, which produces a number of small free-swimming Medusae formerly
described by Gegenbaur under the generic name _Chrysomitra_. This Medusa
has a velum, a single tentacle, eight to sixteen radial canals, and it
bears the gonads on the short manubrium. The Medusa of _Velella_ has, in
fact, the essential characters of the Anthomedusae.

Our knowledge of the life-history of the Siphonophora is very incomplete,
but there are indications, from scattered observations, that in some
genera, at least, it may be very complicated.

The fertilised ovum of _Velella_ gives rise to a planula which sinks to the
bottom of the sea, and changes into a remarkable larva known as the
_Conaria_ larva. This larva was discovered by Woltereck[337] at depths of
600-1000 metres in great numbers. It is very delicate and transparent, but
the endoderm is red (the colour so characteristic of animals inhabiting
deep water), and it may be regarded as essentially a deep-sea larva. The
larva rises to the surface and changes into the form known as the
_Ratarula_ larva, which has a simple one-chambered pneumatophore containing
a gas, and a rudiment of the sail. In contrast to the _Conaria_, the
_Ratarula_ is blue in colour. With the development of the zooids on the
under side of this {303}larva (_i.e._ the side opposite to the
pneumatophore), a definite octoradial symmetry is shown, there being for
some time eight dactylozooids and eight definite folds in the wall of the
pneumatophore. This octoradial symmetry, however, is soon lost as the
number of folds in the pneumatophore and the number of tentacles increase.

It is probable that in the Siphonophora, as in many other Coelenterata, the
production of sexual cells by an individual is no sign that its
life-history is completed. There may possibly be two or more phases of life
in which sexual maturity is reached.

An example of a complicated life-history is found in the Calycophoran
species _Muggiaea kochii_. The embryo gives rise to a form with a single
nectocalyx which is like a _Monophyes_, and this by the budding of a second
nectocalyx produces a form that has a remarkable resemblance to a
_Diphyes_, but the primary nectocalyx degenerates and is cast off, while
the secondary one assumes the characters of the single _Muggiaea_
nectocalyx. The stolon of the _Muggiaea_ produces a series of cormidia, and
as the sexual cells of the cormidia develop, a special nectocalyx is formed
at the base of each one of them, and the group of zooids is detached as an
independent colony, formerly known as _Eudoxia eschscholtzii_. In a similar
manner the cormidia of _Doramasia picta_ give rise to the sexual
free-swimming monogastric forms, known by the name _Ersaea picta_ (Fig.
142). In these cases it seems possible that the production of ripe sexual
cells is confined to the _Eudoxia_ and _Ersaea_ stages respectively, but it
is probable that in other species the cormidia do not break off from the
stolon, or may escape only from the older colonies.

[Illustration: FIG. 142.—Free-swimming _Ersaea_ group of _Doramasia picta_.
_B_, _B_, batteries of nematocysts borne by the tentilla; _D_,
dactylozooid; _G_, gastrozooid; _H_, hydrophyllium; _N_, nectocalyx; _O_,
oleocyst; _f.t_, terminal filament of a battery; _t_, _t_, tentilla. The
gonozooid is hidden by the gastrozooid. × 10. (After Chun.)]

The Siphonophora are essentially free-swimming pelagic {304}organisms. Some
of them (Auronectidae) appear to have become adapted to a deep-sea habit,
others are usually found in intermediate waters, but the majority occur
with the pelagic plankton at or very near the surface of the open sea.
Although the order may be said to be cosmopolitan in its distribution, the
Siphonophora are only found in great numbers and variety in the
sub-tropical and tropical zones. In the temperate and arctic zones they are
relatively rare, but _Galeolaria biloba_ and _Physophora borealis_ appear
to be true northern forms. The only British species are _Muggiaea
atlantica_ and _Cupulita sarsii_. _Velella spirans_ occasionally drifts
from the Atlantic on to our western shores, and sometimes great numbers of
the pneumatophores of this species may be found cast up on the beach.
_Diphyes_ sp., _Physalia_ sp., and _Physophora borealis_ are also
occasionally brought to the British shores by the Gulf Stream.

The Calycophorae are usually perfectly colourless and transparent, with the
exception of the oil-globule in the oleocyst, which is yellow or orange in
colour. Many of the other Siphonophora, however, are of a transparent, deep
indigo blue colour, similar to that of many other components of the
plankton.

Most of the Siphonophora, although, strictly speaking, surface animals, are
habitually submerged; the large pneumatophores of _Velella_ and _Physalia_,
however, project above the surface, and these animals are therefore
frequently drifted by the prevailing wind into large shoals, or blown
ashore. At Mentone, on the Mediterranean, _Velella_ is sometimes drifted
into the harbour in countless numbers. Agassiz mentions the lines of deep
blue Velellas drifted ashore on the coast of Florida; and a small species
of blue _Physalia_ may often be seen in long lines on the shore of some of
the islands of the Malay Archipelago.

The food of most of the Siphonophora consists of small Crustacea and other
minute organisms, but some of the larger forms are capable of catching and
devouring fish. It is stated by Bigelow[338] that a big _Physalia_ will
capture and devour a full-grown Mackerel. The manner in which it feeds is
described as follows:—"It floats on the sea, quietly waiting for some
heedless individual to bump its head against one of the tentacles. The
fish, on striking, is stung by the nettle-cells, and fastened probably by
them to the tentacle. Trying to run away the fish pulls on the
{305}tentacle. The tension on its peduncle thus produced acts as a stimulus
on apparently some centre there which causes it to contract. The fish in
this way is drawn up so that it touches the sticky mouths of the squirming
siphons [_i.e._ gastrozooids]. As soon as the mouths, covered as they are
with a gluey substance and provided with nettle-cells, touch the fish they
stick fast, a few at first, and gradually more. The mouths open, and their
lips are spread out over the fish until they touch, so that by the time he
is dead the fish is enclosed in a tight bag composed of the lips of a dozen
or more siphon mouths. Here the fish is digested. As it begins to
disintegrate partially digested fragments are taken into the stomachs of
the attached siphons (gastrozooids). When they have become gorged they
detach themselves from the remains of the fish, the process of digestion is
completed in the stomachs, and the nutrient fluid is distributed...."

In consequence of the very unsatisfactory state of our knowledge of the
life-history of the Siphonophora the classification of the order is a
matter of unusual difficulty.


SUB-ORDER I. CALYCOPHORAE.

The character which distinguishes this sub-order is the absence of a
pneumatophore.

The colony usually consists of a long, slender, contractile stolon,
provided at one end with one, two, or several nectocalyces. Upon the stolon
are arranged several groups ("cormidia") of polymorphic zooids.

The nectocalyces have a well-developed velum, four radial canals, and a
muscular umbrella-wall. A special peculiarity of the nectocalyx of this
sub-order is a diverticulum (oleocyst) from one of the radial canals,
containing a coloured globule of oil. The function of this oil-globule is
probably similar to that of the pneumatophore, and assists the muscular
efforts of the nectocalyces in keeping the colony afloat. One of the
nectocalyces of each colony exhibits on one side a deep ectodermic fold,
which is frequently converted into a pit. At the bottom of this pit is
attached the end of the stolon, the whole of which with its numerous
cormidia can be withdrawn into the shelter of the pit when danger
threatens. The cormidia consist of at least four {306}kinds of zooids: a
gastrozooid with a trumpet-shaped mouth armed with nematocysts, a long
dactylozooid provided with a series of tentilla, and a rudimentary
gonozooid bearing numbers of male or female medusiform gonophores. These
three kinds of zooids are partially covered and protected by a bent
shield-shaped phyllozooid or hydrophyllium.

Each of the cormidia is unisexual, but the colony as a whole is usually
hermaphrodite, the male and female cormidia regularly alternating, or the
male cormidia being arranged on the nectocalycine half and the female
cormidia on the opposite half of the stolon.

The families of the Calycophorae are:—

FAM. 1. MONOPHYIDAE.—In this family there is a single conical or
mitre-shaped nectocalyx. The cormidia become detached as free-swimming
_Eudoxia_ or _Ersaea_ forms.

Sub-Fam. 1. SPHAERONECTINAE.—The primary nectocalyx persists throughout
life—_Monophyes_ and _Sphaeronectes_.

Sub-Fam. 2. CYMBONECTINAE.—The primary nectocalyx is thrown off, and is
replaced by a secondary and permanent nectocalyx—_Cymbonectes_, _Muggiaea_,
and _Doramasia_.

FAM. 2. DIPHYIDAE.—The primary mitre-shaped nectocalyx is thrown off and
replaced by two secondary rounded, prismatic, or pyramidal, heteromorphic
nectocalyces.

This family contains several sub-families, which are arranged in two
groups: the Diphyidae Oppositae, in which the two secondary bells are
opposite one another, and do not exhibit pronounced ridges; and the
Diphyidae Superpositae, in which one of the two secondary nectocalyces is
situated in front of the other, and each nectocalyx is provided externally
with very definite and often wing-like ridges. In all the Diphyidae
Oppositae the cormidia remain attached, whereas in most of the Diphyidae
Superpositae they become free-swimming, as in the Monophyidae.

The sub-families of the DIPHYIDAE OPPOSITAE are:—

Sub-Fam. 1. AMPHICARYONINAE.—One of the two secondary nectocalyces becomes
flattened above to form a shield, and at the same time its sub-umbrellar
cavity is atrophied, and its radial canals reduced. _Mitrophyes_, Atlantic
Ocean.

Sub-Fam. 2. PRAYINAE.—The colony exhibits a pair of large, obtuse
nectocalyces, with a relatively small sub-umbrellar cavity. _Praya_,
Mediterranean and Atlantic.

{307}Sub-Fam. 3. DESMOPHYINAE.—The colony bears a large number of reserve
or tertiary nectocalyces arranged in two rows. _Desmophyes_, Indian Ocean.

Sub-Fam. 4. STEPHANOPHYINAE.—There are four nectocalyces arranged in a
horizontal plane. Each one of the cormidia bears a nectocalyx, which is
periodically replaced. This sub-family is constituted for _Stephanophyes
superba_ from the Canary Islands. It attains a length of 25 cm., and is
probably the largest and most beautiful of all the Calycophoridae.[339]

The group DIPHYIDAE SUPERPOSITAE contains the following:—

Sub-Fam. 1. GALEOLARINAE.—_Galeolaria._

Sub-Fam. 2. DIPHYOPSINAE.—_Diphyes._

Sub-Fam. 3. ABYLINAE.—_Abyla._

These sub-families differ from one another in the character and shape of
the nectocalyces and in other characters. They have a world-wide
distribution, _Diphyes_ and _Galeolaria_ extending north into the Arctic
Seas. _Diphyes_ is British.

FAM. 3. POLYPHYIDAE.—The nectocalyces are numerous, and superposed in two
rows. The cormidia remain attached.

The family contains the genera _Polyphyes_ and _Hippopodius_, both probably
cosmopolitan in warm waters.


SUB-ORDER II. PHYSOPHORAE.

In this sub-order the primary nectocalyx gives rise to a definite
pneumatophore. There are four families.

FAM. 1. PHYSONECTIDAE.—In this, the largest family of the sub-order, there
is a monothalamic pneumatophore supporting a stolon, which in some forms is
of great length, but in others is reduced to a stump or pad, on which there
are usually found several nectocalyces, hydrophyllia, gastrozooids,
gonozooids, and tentilla.

The principal sub-families are:—

AGALMINAE.—With a long stolon, bearing at the upper end (_i.e._ the end
next to the pneumatophore) two rows of nectocalyces. The other zooids are
arranged in cormidia on the stolon, each covered by a hydrophyllium.
Dactylozooids with tentilla. _Agalma_ and _Cupulita_, Mediterranean Sea.

APOLEMINAE.—Similar to the above, but without tentilla.
{308}_Apolemia_—this genus attains a length of two or three metres.
Mediterranean Sea. _Dicymba_, Indian Ocean.

PHYSOPHORINAE.—The pneumatophore larger in proportion than it is in the
preceding families. The stolon is short, and bears rows of nectocalyces at
the upper end. The gastrozooids, dactylozooids, and gonozooids are arranged
in verticils on the lower expanded part of the stolon. Hydrophyllia absent.
_Physophora_, cosmopolitan in the areas of warm sea water.

FAM. 2. AURONECTIDAE.—The pneumatophore is large. The stolon is reduced to
a spongy mass of tissue on the under side of the pneumatophore, and this
bears numerous cormidia arranged in a helicoid spiral. Projecting from the
base of the pneumatophore there is a peculiar organ called the "aurophore,"
provided with an apical pore. This organ has been described as a specially
modified nectocalyx, but it is probably a specialised development of the
epithelium-lined portion of the pneumatophore of other Physophorae. The
Auronectidae are found only at considerable depths, 300 to 1400 fathoms,
and are probably specially adapted to that habitat. _Rhodalia_,
_Stephalia_, Atlantic Ocean.

FAM. 3. RHIZOPHYSALIIDAE.—The pneumatophore is large, or very large, in
this family. The zooids are arranged in horizontal rows on the under side
of the pneumatophore (_Physalia_), or in a helicoid spiral on a short
stolon (_Epibulia_). There are no nectocalyces nor hydrophyllia.

The genus _Physalia_ is the notorious "Portuguese Man-of-War." The
pneumatophore is a large bladder-like vesicle, sometimes attaining a length
of 12 cm. One species described by Haeckel under the generic name
_Caravella_ has a pneumatophore 30 cm. and more in length, and
dactylozooids attaining a length of 20 metres. It is a curious fact that
only the male colonies of _Physalia_ are known, and it is suggested that
the female may have quite a different form.[340] _Epibulia_ has a much
smaller bladder than _Physalia_. Both genera have a cosmopolitan
distribution at the surface of the warm seas.

FAM. 4. CHONDROPHORIDAE.—This family stands quite by itself in the
sub-order Physophorae, and is placed in a separate division of the
sub-order by Chun, who gives it the name TRACHEOPHYSA. The essential
distinguishing characters of the family are {309}the large polythalamic
pneumatophore and the single large central gastrozooid.

The colony is disc-shaped, and has a superficial resemblance to a Medusa.
On the upper side is the flattened pneumatophore, covered by a fold of
tissue continuous with that at the edge of the disc. In _Velella_ a
vertical triangular sail or crest rises from the upper side, but this is
absent in _Porpita_.

The mouth of the gastrozooid opens into a large digestive cavity, and
between this and the under surface of the pneumatophore there is a
glandular spongy tissue called the liver. The liver extends over the whole
of the under side of the pneumatophore, and sends processes round the edge
of the disc into the tissues of its upper surface. Intimately associated
with the liver, and penetrating its interstices, is an organ which appears
to be entirely composed of nematocysts, derived from the ectoderm, and
called the central organ. At the margin of the disc there is a fringe of
simple digitiform dactylozooids, and between the dactylozooids and the
centrally placed gastrozooid are numerous gonozooids. Each of the
gonozooids is provided with a distinct mouth, and bears the gonophores,
which escape before the ripening of the gonads as the free-swimming Medusae
called _Chrysomitra_. The pneumatophore consists of a number of annular
chambers arranged in a concentric manner round the central original chamber
formed from a modified zooid. These annular chambers are in communication
with one another, and have each two pores (pneumatopyles) opening above to
the exterior. The most remarkable feature, however, of the system is a
series of fine branching tubes ("tracheae"), which pass from the annular
chambers of the pneumatophore downwards into the hepatic mass and ramify
there.

There are two well-known genera: _Velella_ with a sail, and _Porpita_
without a sail. They are both found at the surface of the warmer regions of
the great oceans and in the Mediterranean. _Velella_ sometimes drifts on to
British coasts from the Atlantic.

The genus _Discalia_ has a much more simple octoradial structure. It was
found at depths of 2600 and 2750 fathoms in the Pacific Ocean.




{310}CHAPTER XII

COELENTERATA (_CONTINUED_): SCYPHOZOA = SCYPHOMEDUSAE


CLASS II. SCYPHOZOA = SCYPHOMEDUSAE

The Scyphozoa are jelly-fishes, usually found floating at or near the
surface of the sea. A few forms (Stauromedusae) are attached to rocks and
weeds by a stalked prolongation of the aboral region of the umbrella. With
this exception, however, they are all, in the adult stage, of the Medusa
type of structure, having a bell-shaped or discoid umbrella, from the under
surface of which depends a manubrium bearing the mouth or (in Rhizostomata)
the numerous mouths.

Although many of the species do not exceed an inch or a few inches in
diameter, others attain a very great size, and it is among the Scyphozoa
that we find the largest individual zooids of the Coelenterata. Some
Discophora have a disc three or four feet in diameter, and one specimen
obtained by the Antarctic Expedition of 1898-1900 weighed 90 lbs.[341] The
common jelly-fish, _Aurelia_, of our coasts belongs to a species that
appears to be very variable in general characters as well as in size.
Specimens obtained by the "Siboga" in the Malay Archipelago ranged from 6
to 64 cm. in diameter. The colour is very variable, shades of green, blue,
brown, and purple being conspicuous in many species; but a pale milky-blue
tint is perhaps the most prevalent, the tissues being generally less
transparent than they are in the Medusae of the Hydrozoa. The colour of the
Cubomedusae is usually yellow or brown, but _Charybdea xaymacana_ is
colourless and transparent. The deep-sea species, particularly the
Periphyllidae, have usually an opaque brown or dark red colour. The
surface-swimming {311}forms, such as the common _Aurelia_, _Pelagia_,
_Cyanaea_, are usually of a uniform pale milky-blue or green colour.
Generally the colour is uniformly distributed, but sometimes the surface of
the umbrella is freckled with irregular brown or yellow patches, as in
_Dactylometra_ and many others. There is frequently a special colour in the
statorhabs which renders them conspicuous in the living jelly-fish, and the
lips, or parts of the lips, of the manubrium have usually a different
colour or tone to that of the umbrella.

There is no reason to believe that the general colour of any of these
jelly-fishes has either a protective or a warning significance. Nearly all
the larger species, whether blue, green, or brown in colour, can be easily
seen from a considerable distance, and the colours are not sufficiently
bright or alarming to support the belief that they can serve the purpose of
warning either fish or birds of the presence of a dangerous stinging
animal. It is possible, however, that the brighter spots of colour that are
often noticed on the tips of the tentacles and on the lips may act as a
lure or bait in attracting small fish and Crustacea.

Some of the Scyphozoa are phosphorescent, but it is a singular fact that
there are very few recorded observations concerning the phosphorescence or
the absence of it in most of the species. The pale blue light of _Pelagia
noctiluca_ or _P. phosphora_ can be recognised from the deck of a ship in
the open ocean, and they are often the most brilliant and conspicuous of
the phosphorescent organisms.

The food of the Scyphozoa varies a good deal. _Charybdea_ and _Periphylla_,
and probably many others with large mouths, will capture and ingest
relatively large fish and Crustacea; but _Chrysaora isosceles_[342]
apparently makes no attempt to capture either Copepoda or small fish, but
preys voraciously upon Anthomedusae, Leptomedusae, Siphonophora,
Ctenophora, and pelagic worms. Very little is known about the food of the
Rhizostomata, but the small size of the mouths of these forms suggests that
their food must also be of minute size. The frequent association of small
fish with the larger jelly-fish is a matter of some interest that requires
further investigation. In the North Sea young whiting are the constant
guests of _Cyanaea capillata_.[343] Over a {312}hundred young
horse-mackerel (_Caranx trachurus_) may be found sheltering under the
umbrella of _Rhizostoma pulmo_. As the animal floats through the water the
little fishes hover round the margin, but on the slightest alarm dart into
the sub-umbrella cavity, and ultimately seek shelter in the sub-genital
pits.[344]

Two species of fish accompany the American Medusa _Dactylometra lactea_,
one a Clupeoid, the other the young of the Butter-fish (_Stromateus
triacanthus_). According to Agassiz and Mayer[345] this is not an ordinary
case of mutualism, as the fish will tear off and devour fragments of the
tentacles and fringe of the Medusa, whilst the Medusa will in its turn
occasionally capture and devour one of the fish.

A great many of the Scyphozoa, particularly the larger kinds, have the
reputation of being able to sting the human skin, and in consequence the
name Acalephae[346] was formerly used to designate the order. Of the
British species _Aurelia aurita_ is almost harmless, and so is the rarer
_Rhizostoma pulmo_; but the nematocysts on the tentacles of _Cyanaea_,
_Chrysaora_, and _Pelagia_ can inflict stings on the more delicate parts of
the skin which are very painful for several hours, although the pain has
been undoubtedly greatly exaggerated in many popular works.

The soft structure of the Medusae does not favour their preservation in the
rocks, but the impressions left by several genera, all belonging apparently
to the Rhizostomata, have been found in Cambrian, Liassic, and Cretaceous
deposits.

There is reason to believe that many Scyphozoa exhibit a considerable range
of variation in the symmetry of the most important organs of the body. Very
little information is, however, at hand concerning the variation of any
species except _Aurelia aurita_, which has been the subject of several
investigations. Browne[347] has found that in a local race of this species
about 20 per cent exhibit variations from the normal in the number of the
statorhabs, and about 2 per cent in the number of gastric pouches.

The Scyphozoa are not usually regarded as of any commercial or other value,
but in China and Japan two species of Rhizostomata (_Rhopilema esculenta_
and _R. verrucosa_) are used as food. {313}The jelly-fish is preserved with
a mixture of alum and salt or between the steamed leaves of a kind of oak.
To prepare the preserved food for the table it is soaked in water, cut into
small pieces, and flavoured. It is also stated that these Medusae are used
by fishermen as bait for file-fish and sea-bream.[348]

In general structure the Scyphozoa occupy an intermediate position between
the Hydrozoa and the Anthozoa. The very striking resemblance of the
body-form to the Medusa of the Hydrozoa, and the discovery of a fixed
hydriform stage in the life-history of some species, led the older
zoologists to the conclusion that they should be included in the class
Hydrozoa. Recently the finer details of development have been invoked to
support the view that they are Anthozoa specially adapted for a
free-swimming existence, but the evidence for this does not appear to us to
be conclusive.

They differ from the Hydrozoa and resemble the Anthozoa in the character
that the sexual cells are matured in the endoderm, and escape to the
exterior by way of the coelenteric cavity, and not directly to the exterior
by the rupture of the ectoderm as in all Hydrozoa. They differ, on the
other hand, from the Anthozoa in the absence of a stomodaeum and of
mesenteries.

The view that the Scyphozoa are Anthozoa is based on the belief that the
manubrium of the former is lined by ectoderm, and is homologous with the
stomodaeum of the latter; and that the folds of mesogloea between the
gastric pouches are homologous with the septa.[349]

The Scyphozoa, notwithstanding their general resemblance to the Medusae of
Hydrozoa, can be readily distinguished from them by several important
characters. The absence of a velum in all of them (except the Cubomedusae)
is an important and conspicuous character which gave to the class the name
of Acraspeda. The velum of the Cubomedusae can, however, be distinguished
from that of the Craspedote Medusae (_i.e._ the Medusae of the Hydrozoa) by
the fact that it contains endodermal canals.

Sense-organs are present in all Scyphozoa except some of the Stauromedusae,
and they are in the form of statorhabs (tentaculocysts), bearing statoliths
at the extremity, and in many species, {314}at the base or between the base
and the extremity, one or more eyes. These organs differ from the
statorhabs of the Hydrozoa in having, usually, a cavity in the axial
endoderm; but as they are undoubtedly specially modified marginal
tentacles, they are strictly homologous in the two classes. In nearly all
the Scyphozoa these organs are protected by a hood or fold formed from the
free margin of the umbrella, and this character, although not of great
morphological importance, serves to distinguish the common species from the
Craspedote Medusae. It was owing to this character that Forbes gave the
name STEGANOPHTHALMATA, or "covered-eyed Medusae," to the class.

Another character of some importance is the presence in the coelenteric
cavity of all Scyphozoa of clusters or rows of delicate filaments called
the "phacellae." These filaments are covered with a glandular epithelium,
and are usually provided with numerous nematocysts. They have a
considerable resemblance to the acontia of certain Anthozoa, and are
probably mainly digestive in function. These three characters, in addition
to the very important character of the position and method of discharge of
the sexual cells already referred to, justify the separation of the
Scyphozoa from the Medusae of the Hydrozoa as a distinct class of
Coelenterata.

The umbrella of the Scyphozoa varies a good deal in shape. It is usually
flattened and disc-like (Discophora), but it may be almost globular
(_Atorella_), conical (some species of _Periphylla_), or cubical
(Cubomedusae). It is divided into an aboral and a marginal region by a
circular groove in the Coronata. The margin may be almost entire, marked
only by notches where the statorhabs occur, or deeply lobed as in the
Coronata and many Discophora. Marginal tentacles are present in all but the
Rhizostomata, and may be few in number, four in _Charybdea_, eight in
_Ulmaris_ (Fig. 143), or very numerous in _Aurelia_ and many others. The
tentacles may be short (_Aurelia_), or very long as in _Chrysaora
isosceles_, in which they extend for a length of twenty yards from the
disc.

The manubrium of the Scyphozoa is usually quadrangular in section, and in
those forms in which the shape is modified in the adult Medusa the
quadrangular shape can be recognised in the earlier stages of development.
The four angles of the manubrium are of importance in descriptive anatomy,
as the planes drawn {315}through the angles to the centre of the manubrium
are called "perradial," while those bisecting the perradial planes and
passing therefore through the middle line of the flat sides of the
manubrium are called "interradial."

The free extremity of the manubrium in many Scyphozoa is provided with four
triangular perradial lips, which may be simple or may become bifurcated or
branched, and have frequently very elaborate crenate edges beset with
batteries of nematocysts. In _Pelagia_ and _Chrysaora_ and other genera
these lips hang down from the manubrium as long, ribbon-like, folded bands,
and according to the size of the specimen may be a foot or more in length,
or twice the diameter of the disc.

In the Rhizostomata a peculiar modification of structure takes place in the
fusion of the free edges of the lips to form a suture perforated by a row
of small apertures, so that the lips have the appearance of long
cylindrical rods or tubes attached to the manubrium, and then frequently
called the "oral arms." The oral arms may be further provided with
tentacles of varying size and importance. In many Rhizostomata branched or
knobbed processes project from the outer side of the upper part of the oral
arms. These are called the "epaulettes."

[Illustration: FIG. 143.—_Ulmaris prototypus._ _g_, Gonad; _I_, interradial
canal; _M_, the fringed lip of the manubrium; _P_, perradial canal; _S_,
marginal sense-organ; _t_, tentacle. × 1. (After Haeckel.)]

The lumen of the manubrium leads into a large cavity in the disc, which is
usually called the gastric cavity, and this is extended into four or more
interradial or perradial gastric pouches. The number of these pouches is
usually four, but in this, as in {316}other features of their radial
symmetry, the jelly-fish frequently exhibit duplication or irregular
variation of the radii.[350]

The gastric pouches may extend to the margin of the disc, where they are
united to form a large ring sinus, or they may be in communication at the
periphery by only a very narrow passage (Cubomedusae). In the Discophora
the gastric pouches, however, do not extend more than half-way to the
margin, and they may be connected with the marginal ring-canal by a series
of branched interradial canals. Between the gastric pouches in these forms
branched perradial canals pass from the gastric cavity to the marginal ring
canal, and the system of canals is completed by unbranched "adradial"
canals passing between the perradials and interradials from the sides of
the gastric pouches to the ring-canal (Fig. 143).

In the Discophora there are four shallow interradial pits or pouches lined
by ectoderm on the under side of the umbrella-wall. As these pits
correspond with the position of the gonads in the gastric pouches they are
frequently called the "sub-genital pits." In the Stauromedusae and
Cubomedusae they are continued through the interradial gastric septa to the
aboral side of the disc, and they are generally known in these cases by the
name "interradial funnels." The functions and homologies of these
ectodermic pits and funnels are still uncertain.

The Scyphozoa are usually dioecious, but _Chrysaora_ and _Linerges_ are
sometimes hermaphrodite. The female Medusae can usually be distinguished
from the male by the darker or brighter colour of the gonads, which are
band-shaped, horseshoe-shaped, or circular organs, situated on the endoderm
of the interradial gastric pouches. They are, when nearly ripe, conspicuous
and brightly coloured organs, and in nearly all species can be clearly seen
through the transparent or semi-transparent tissues of the disc. The
reproductive cells are discharged into the gastric cavity and escape by the
mouth. The eggs are probably fertilised in the water, and may be retained
in special pouches on the lips of the manubrium until the segmentation is
completed.[351] Asexual reproduction does not occur in the free-swimming or
adult stage of any Scyphozoa. In some cases (probably exceptional) the
development is direct. In _Pelagia_, for example, it is known that the
fertilised egg gives {317}rise to a free-swimming Medusa similar in all
essential features to the parent.

In many species, however, the planula larva sinks to the bottom of the sea,
develops tentacles, and becomes attached by its aboral extremity to a rock
or weed, forming a sedentary asexual stage of development with a
superficial resemblance to a _Hydra_. This stage is the "Scyphistoma," and
notwithstanding its simple external features it is already in all essential
anatomical characters a Scyphozoon.

The Scyphistoma may remain as such for some time, during which it
reproduces by budding, and in some localities it may be found in great
numbers on seaweeds and stones.[352]

In the course of time, however, the Scyphistoma exhibits a ring-like
constriction of the body just below the crown of tentacles, and as this
deepens the general features of a Scyphomedusa are developed in the free
part above the constriction. In time this free part escapes as a small
free-swimming jelly-fish, called an "Ephyra," while the attached part
remains to repeat the process. In many species the first constriction is
followed by a second immediately below it, then a third, a fourth, and so
on, until the Scyphistoma is transformed into a long series of narrow
discs, each one acquiring, as it grows, the Ephyra characters. Such a stage
has been compared in form to a pile of saucers, and is known as the
"Strobila."

The Ephyra differs from the adult in many respects. The disc is thin and
flat, the manubrium short, the margin of the umbrella deeply grooved, while
the statorhabs are mounted on bifid lobes which project outwards from the
margin. The stabilisation of the Scyphistoma is a process of reproduction
by transverse fission, and in some cases this is supplemented by gemmation,
the Scyphistoma giving rise to a number of buds which become detached from
the parent and subsequently undergo the process of strobilisation.

[Illustration: FIG. 144.—The perisarc tubes of a specimen of _Spongicola
fistularis_ (_N_) ramifying in the skeleton of the Sponge _Esperella
bauriana_ (Sp.), as seen in a macerated specimen, × 1. (After Schulze.)]

The Scyphistoma of _Nausithoe_ presents us with the most {318}remarkable
example of this mode of reproduction (Fig. 144), as it forms an elaborate
branching colony in the substance of certain species of sponges. The
ectoderm secretes a chitinous perisarc, similar to that of the hydrosome
stage of many of the Hydrozoa, and consequently _Stephanoscyphus_
(_Spongicola_), as this Scyphistoma was called, was formerly placed among
the Gymnoblastea. It is remarkable that, although the Scyphozoan characters
of _Spongicola_ were proved by Schulze[353] in 1877, a similar Scyphistoma
stage has not been discovered in any other genus.


ORDER I. CUBOMEDUSAE.

Scyphozoa provided with four perradial statorhabs, each of which bears a
statolith and one or several eyes. There are four interradial tentacles or
groups of tentacles. The stomach is a large cavity bearing four tufts of
phacellae (Fig. 145, _Ph_), situated interradially. There are four
flattened perradial gastric pouches in the wall of the umbrella which
communicate with the stomach by the gastric ostia (_Go_). These pouches are
separated from one another by four interradial septa; and the long
leaf-like gonads are attached by one edge to each side of the septa. In
many respects the Cubomedusae appear to be of simple structure, but the
remarkable differentiation of the eyes and the occurrence of a velum (p.
313) suggest that the order is a highly specialised offshoot from a
primitive stock.

[Illustration: FIG. 145.—Vertical section in the interradial plane of
_Tripedalia cystophora_. _Go_, Gastric ostia; _Man_, manubrium; _Ph_, group
of phacellae; _T_, tentacles in four groups of three; _tent_, perradial
sense-organs; _V_, velum. (After Conant.)]

FAM. 1. CHARYBDEIDAE.—Cubomedusae with four interradial tentacles.

{319}_Charybdea_ appears to have a very wide geographical distribution.
Some of the species are usually found in deep water and come to the surface
only occasionally, but others (_C. xaymacana_) are only found at the
surface of shallow water near the shore. The genus can be easily recognised
by the four-sided prismatic shape of the bell and the oral flattened
expansion of the base of the tentacles. The bell varies from 2-6 cm. in
length (or height) in _C. marsupialis_, but a giant form, _C.
grandis_,[354] has recently been discovered off Paumotu Island which is as
much as 23 cm. in height. The colour is usually yellow or brown, but _C.
grandis_ is white and _C. xaymacana_ perfectly transparent.

"_Charybdea_ is a strong and active swimmer, and presents a very beautiful
appearance in its movements through the water; the quick, vigorous
pulsations contrasting sharply with the sluggish contractions seen in most
Scyphomedusae." It appears to be a voracious feeder. "Some of the specimens
taken contained in the stomach small fish, so disproportionately large in
comparison with the stomach that they lay coiled up, head overlapping
tail."[355]

Very little is known of the development, but it is possible that _Tamoya
punctata_, which lacks gonads, phacellae, and canals in the velum, may be a
young form of a species of _Charybdea_.

FAM. 2. CHIRODROPIDAE.—Cubomedusae with four interradial groups of
tentacles.

This family is represented by the genera _Chirodropus_ from the Atlantic
and _Chiropsalmus_ from the Indian Ocean and the coast of North Carolina.

FAM. 3. TRIPEDALIIDAE.—Cubomedusae with four interradial groups of three
tentacles.

The single genus and species _Tripedalia cystophora_ has only been found in
shallow water off the coast of Jamaica. Specimens of this species were kept
for some time by Conant in an aquarium, and produced a number of
free-swimming planulae which settled on the glass, and quickly developed
into small hydras with a mouth and four tentacles. The further development
of this sedentary stage is unfortunately not known.


{320}ORDER II. STAUROMEDUSAE.

This order contains several genera provided with an aboral stalk which
usually terminates in a sucker, by means of which the animal is temporarily
fixed to some foreign object. There can be little doubt that this sedentary
habit is recently acquired, and the wide range of the characteristic
features of the order may be accounted for as a series of adaptations to
the change from a free-swimming to a sedentary habit.

It is difficult to give in a few words the characters of the order, but the
Stauromedusae differ from other Scyphozoa in the absence or profound
modification in structure and function of the statorhabs. They are absent
in _Lucernaria_ and the Depastridae, and very variable in number in
_Haliclystus_.

The statorhab of _Haliclystus_ terminates in a spherical knob, which is
succeeded by a large annular pad or collar bearing a number of glandular
cells which secrete a sticky fluid. At the base of the organ there is a
rudimentary ocellus. The number is very variable, and sometimes they are
abnormal in character, being "crowned with tentacles." There can be little
doubt that the principal function of these organs is not sensory but
adhesive, and hence they have received the names "colletocystophores" and
"marginal anchors," but they are undoubtedly homologous with the statorhabs
of other Scyphozoa.

The tentacles are short and numerous, and are frequently mounted in groups
on the summit of digitate outgrowths from the margin of the umbrella. They
are capitate, except in _Tessera_, the terminal swelling containing a
battery of nematocysts.

Very little is known concerning the life-history and development of the
Stauromedusae.

FAM. 1. LUCERNARIIDAE.—Marginal lobes digitate, bearing the capitate
tentacles in groups. _Haliclystus auricula_ is a common form on the shores
of the Channel Islands, at Plymouth, and other localities on the British
coast. It may be recognised by the prominent statorhabs situated in the
bays between the digitate lobes of the margin of the umbrella. Each of the
marginal lobes bears from 15 to 20 capitate tentacles. It is from 2 to 3
cm. in length. The genus occurs in shallow water {321}off the coasts of
Europe and North America, extending south into the Antarctic region.

_Lucernaria_ differs from _Haliclystus_ in the absence of statorhabs. It
has the same habit as _Haliclystus_, and is often found associated with it.
_L. campanulata_ is British.

_Halicyathus_ is similar in external features to _Haliclystus_, but differs
from it in certain important characters of the coelenteric cavities. It is
found off the coasts of Norway, Greenland, and the Atlantic side of North
America.

In _Capria_, from the Mediterranean, the tentacles are replaced by a
denticulated membrane bearing nematocysts.

The rare genus _Tessera_, from the Antarctic Ocean, differs from all the
other Stauromedusae in having no stalk and in having only a few relatively
long non-capitate tentacles. If _Tessera_ is really an adult form it should
be placed in a separate family, but, notwithstanding the presence of
gonads, it may prove to be but a free-swimming stage in the history of a
normally stalked genus.

FAM. 2. DEPASTRIDAE.—The margin of the umbrella is provided with eight
shallow lobes bearing one or more rows of tentacles. Statorhabs absent.

_Depastrum cyathiforme_ occurs in shallow water at Plymouth, Port Erin, and
in other localities on the coasts of Britain and Norway. The tentacles are
arranged in several rows on the margin of the umbrella. In _Depastrella_
from the Canaries there is only one row of marginal tentacles.

FAM. 3. STENOSCYPHIDAE.[356]—Stauromedusae with simple undivided umbrella
margin. The eight principal tentacles are converted into adhesive anchors.
Secondary tentacles arranged in eight adradial groups. _Stenoscyphus
inabai_, 25 cm., Japan.


ORDER III. CORONATA.[357]

The external surface of the umbrella is divided into two regions, an aboral
region and a marginal region, by a well-marked circular groove (the coronal
groove). The aboral region is usually smooth and undivided, but it is an
elongated dome, {322}thimble- or cone-shaped, in marked contrast to the
flattened umbrella of the Discophora. The margin is divided into a number
of triangular or rounded lobes, and these are continued as far as the
coronal groove as distinct areas delimited by shallow grooves on the
surface of the umbrella. The tentacles arise from the grooves between the
marginal areas, and are provided with expanded bases called the pedalia.
The manubrium may be short or moderately long, but it is never provided
with long lips.

FAM. 1. PERIPHYLLIDAE.[358]—Coronata with four or six statorhabs.

In _Pericolpa_ (Kerguelen) there are only four tentacles and four
statorhabs. In _Periphylla_, a remarkable deep-sea genus from 700 to 2000
fathoms in all seas, but occasionally found at the surface, there are
twelve tentacles and four statorhabs. The specimens from deep water have a
characteristic dark red-brown or violet-brown colour. They are usually
small Medusae, but the umbrella of _P. regina_ is over 21 cm. in diameter.
_Atorella_ has six tentacles and six statorhabs.

FAM. 2. EPHYROPSIDAE.—Coronata with eight or more than eight statorhabs.

_Nausithoe punctata_ is a small, transparent jelly-fish, not exceeding 10
mm. in diameter, of world-wide distribution. Its Scyphistoma stage is
described on p. 317. _N. rubra_, a species of a reddish colour found at a
considerable depth in the South Atlantic and Indian Oceans, is probably an
abysmal form. _Palephyra_ differs from _Nausithoe_ in having elongated
instead of rounded gonads. _Linantha_ and _Linuche_ differ from the others
in having subdivided marginal lobes.

FAM. 3. ATOLLIDAE.—_Atolla_ is a deep-sea jelly-fish of very wide
geographical distribution. It is characterised by the multiplication of the
marginal appendages, but the number is very irregular. There may be double
or quadruple the usual number of marginal lobes, or an indefinite number.
There may be sixteen to thirty-two statorhabs, and the number of tentacles
is quite irregular. Some of the species attain a considerable size, the
diameter of the umbrella of _A. gigantea_ being 150 mm., of _A. valdiviae_
sometimes 130 mm., and of _A. bairdi_ 110 mm.


{323}ORDER IV. DISCOPHORA.

This order contains not only by far the greater number of the species of
Scyphozoa, but those of the largest size, and all those that are familiar
to the seaside visitor and the mariner under the general term jelly-fish.

They may be distinguished from the other Scyphozoa by several well-marked
characters. The umbrella is flattened and disc-shaped or slightly domed,
but not divided by a coronary groove. The perradial angles of the mouth are
prolonged into long lips, which may remain free (Semaeostomata) or fuse to
form an elaborate proboscis (Rhizostomata).


SUB-ORDER I. SEMAEOSTOMATA.

In this sub-order the mouth is a large aperture leading into the cavity of
the manubrium, and is guarded by four long grooved and often tuberculated
lips. The margin of the umbrella is provided with long tentacles.

FAM. 1. PELAGIIDAE.—Semaeostomata with wide gastric pouches, which are not
united by a marginal ring sinus. _Pelagia_, which forms the type of this
family, has eight long marginal tentacles. It develops directly from the
egg, the fixed Scyphistoma stage being eliminated.[359] It is probably in
consequence of this peculiarity of its development and independence of a
shore for fixation that _Pelagia_ has become a common and widespread
inhabitant of the high seas. In the Atlantic and Indian Oceans _P.
phosphora_ occurs in swarms or in long narrow lines many miles in length.
It is remarkable for its power of emitting phosphorescent light. In the
Atlantic it extends from 50° N. to 40° S., but is rare or absent from the
colder regions. _P. perla_ is found occasionally on the west coast of
Ireland. _Chrysaora_ differs from _Pelagia_ in the larger number of
tentacles. There are, in all, 24 tentacles and 8 statorhabs, separated by
32 lobes of the margin of the umbrella. _C. isosceles_ is occasionally
found off the British coast. It passes through a typical Scyphistoma stage
in development. _Dactylometra_, a very {324}common jelly-fish of the
American Atlantic shores, differs from _Chrysaora_ in having sixteen
additional but small tentacles arranged in pairs at the sides of the
statorhabs.

FAM. 2. CYANAEIDAE.—Semaeostomata with eight radial and eight adradial
pouches, which give off ramifying canals to the margin of the umbrella; but
these canals are not united by a ring-canal. The tentacles are arranged in
bundles on the margin of the deeply lobed umbrella.

The yellow _Cyanaea capillata_ and the blue _C. lamarcki_ are commonly
found on the British coasts.

FAM. 3. ULMARIDAE.—The gastric pouches are relatively small, and
communicate with a marginal ring-canal by branching perradial and
interradial canals and unbranched adradial canals.

In _Ulmaris prototypus_ (Fig. 143, p. 315) there are only eight long
adradial tentacles, and the lips of the manubrium are relatively short. It
is found in the South Atlantic.

_Aurelia_ is a well-known and cosmopolitan genus, which may be recognised
by the eight shallow lobes of the umbrella-margin beset with a fringe of
numerous small tentacles.


SUB-ORDER II. RHIZOSTOMATA.

In this sub-order the lips are very much exaggerated in size, and are fused
together by their margin in such a manner that the mouth of the animal is
reduced to a number of small apertures situated along the lines of suture.
Tentacles are absent on the margin of the umbrella. This sub-order contains
some of the largest known jelly-fishes, and exhibits a considerable range
of structure. The families are arranged by Maas[360] in three groups.



Group I. ARCADOMYARIA.—Musculature of the disc arranged in feather-like
arcades. Oral arms pinnate.

FAM. CASSIOPEIDAE.—There are no epaulettes on the arms. Labial tentacles
present. _Cassiopea_ is common in the Indo-Pacific seas, and extends into
the Red Sea. It includes a great many species varying in size from 4 to
about 12 cm. in diameter.



Group II. RADIOMYARIA.—Musculature arranged in radial tracts. Oral arms
bifid.

FAM. CEPHEIDAE.—The genera included in this family differ {325}from the
Cassiopeidae in the characters of the group. _Cephea_ is found in the
Indo-Pacific Oceans and Red Sea. _Cotylorhiza_ is common in the
Mediterranean Sea and extends into the Atlantic Ocean.



Group III. CYCLOMYARIA.—The group contains the majority of the
Rhizostomata. Musculature arranged in circular bands round the disc. Oral
arms primarily trifid, but becoming in some cases very complicated. The
principal families are:—

FAM. RHIZOSTOMATIDAE.—With well-marked epaulettes, and sixteen radial
canals passing to the margin of the umbrella.

_Rhizostoma pulmo_ (= _Pilema octopus_), a widely distributed species, is
often found floating at the surface off the western coasts of Scotland and
Ireland, and sometimes drifts up the English Channel into the German Ocean
in the autumn. The umbrella is about two feet in diameter, and the combined
length of the umbrella and arms is four feet. The colour varies
considerably, but that of a specimen obtained off Valencia in 1895 was
described as follows: "The colour of the umbrella was pale green, with a
deep reddish margin. Arms bright blue."[361]

The family includes _Stomolophus_, of the Pacific and Atlantic coasts of
America, in which the oral arms are united at the base, and _Rhopilema_,
the edible Medusa of Japan and China.

FAM. LYCHNORHIZIDAE.—Here there are only eight radial canals reaching as
far as the margin of the umbrella, and eight terminating in the ring-canal.
There are no epaulettes, and the oral tentacles are often very long. The
family includes _Lychnorhiza_ from the coast of Brazil, _Crambione_ from
the Malay Archipelago, and _Crambessa_ from the Atlantic shores of France
and Spain and from Brazil and Australia. The last-named genus has been
found in brackish water at the mouth of the Loire.

In the families LEPTOBRACHIIDAE and CATOSTYLIDAE there are eight radial
canals reaching the margin of the umbrella, and between them a network of
canals with many openings into the ring-canal. In a few of the
Leptobrachiidae the intermediate canal-network has only eight openings into
the ring-canal, as in the Lychnorhizidae.




{326}CHAPTER XIII

COELENTERATA (_CONTINUED_): ANTHOZOA = ACTINOZOA—GENERAL
CHARACTERS—ALCYONARIA


CLASS III. ANTHOZOA = ACTINOZOA

Among the familiar objects included in this class are the Sea-anemones, the
Stony Corals (Madrepores), the Flexible Corals, the Precious Coral, and the
Sea-pens. With the exception of a few species of Sea-anemone, Anthozoa are
not commonly found on British sea-shores; but in those parts of the
tropical world where coral reefs occur, the shore at low tide is carpeted
with various forms of this class, and the sands and beaches are almost
entirely composed of their broken-down skeletons.

The majority of the Anthozoa are colonial in habit, a large number of
individuals, or zooids as they are called, being organically connected
together by a network of nutritive canals, and forming a communal
gelatinous or stony matrix for their protection and support. Whilst the
individuals are usually small or minute, the colonial masses they form are
frequently large. Single colonies of the stony corals form blocks of stone
which are sometimes five feet in diameter, and reach a height of two or
three feet from the ground. From the tree or shrub-like form assumed by
many of the colonies they were formerly included in a class Zoophyta or
animal-plants.

But whether the individual polyps are large or small, whether they form
colonies in the adult condition or remain independent, they exhibit certain
characters in common which distinguish them not only from the other
Coelenterata, but from all other animals. When an individual zooid is
examined in the living and fully expanded condition, it is seen to possess
a cylindrical {327}body, attached at one end (the aboral end) to the common
colonial matrix or to some foreign object. At the opposite or free
extremity it is provided with a mouth surrounded by a crown of tentacles.
In these respects, however, they resemble in a general way some of the
Hydrozoa. It is only when the internal anatomy is examined that we find the
characters which are absolutely diagnostic of the group.

In the Hydrozoa the mouth leads directly into the coelenteric cavity; in
the Anthozoa, however, the mouth leads into a short tube or throat, called
the "stomodaeum," which opens into the coelenteric cavity. Moreover, this
tube is connected with the body-wall, and is supported by a series of
fleshy vertical bands called the mesenteries (Fig. 146). The mesenteries
not only support the stomodaeum, but extend some distance below it. Where
the mesenteries are free from the stomodaeum their edges are thickened to
form the important digestive organs known as the mesenteric filaments
(_mf_). It is in the possession of a stomodaeum, mesenteries, and
mesenteric filaments that the Anthozoa differ from all the other
Coelenterata. There is one character that the Anthozoa share with the
Scyphozoa, and that is, that the gonads or sexual cells (G) are derived
from the endoderm. They are discharged first into the coelenteric cavity,
and then by way of the mouth to the exterior. In the Anthozoa the gonads
are situated on the mesenteries.

[Illustration: FIG. 146.—Diagram of a vertical section through an Anthozoan
zooid. B, Body-wall; G, gonads; M, mesentery; MF, mesenteric filament; ST,
stomodaeum; T, tentacle.]

Nearly all the Anthozoa are sedentary in habit. They begin life as ciliated
free-swimming larvae, and then, in a few hours or days, they become
attached to some rock or shell at the bottom and immediately (if colonial)
start the process of budding, which gives rise to the colonies of the adult
stage. Many of the Sea-anemones, however, move considerable distances by
gliding {328}over the rocks or seaweeds, others habitually burrow in the
sand (_Edwardsia_, _Cerianthus_), and one family (the Minyadidae) are
supported by a gas bladder, and float at the surface of the sea. The
Sea-pens, too, although usually partly buried in the sand or mud, are
capable of shifting their position by alternate distension and contraction
of the stalk.[362] The Anthozoa are exclusively marine. With the exception
of a few Sea-anemones that are found in brackish or almost fresh water in
river estuaries, they only occur in salt sea water. The presence of a
considerable admixture of fresh water, such as we find at the mouths of
rivers, seems to interfere very materially with the development and growth
of all the reef-forming Corals, as will be noticed again in the chapter on
coral reefs. A few genera descend into the greatest depths of the ocean,
but the home of the Anthozoa is pre-eminently the shallow seas, and they
are usually found in great abundance in depths of 0-40 fathoms from the
shores of the Arctic and Antarctic lands to the equatorial belt.

The only Anthozoa of any commercial importance are the Precious Corals
belonging to the Alcyonarian family Coralliidae. The hard pink axis of
these corals has been used extensively from remote times in the manufacture
of jewellery and ornaments. Until quite recently the only considerable and
systematic fishery for the Precious Corals was carried on in the
Mediterranean Sea, and this practically supplied the markets of the world.
In more recent times, however, an important industry in corals has been
developed in Japan. In 1901 the value of the coral obtained on the coasts
of Japan was over £50,000, the greater part of which was exported to Italy,
a smaller part to China, and a fraction only retained for home consumption.
The history of the coral fishery in Japan is of considerable interest.
Coral was occasionally taken off the coast of Tsukinada in early times. But
in the time of the Daimyos the collection and sale of coral was prohibited,
for fear, it is said, that the Daimyo of Tosa might be compelled to present
such precious treasure to the Shogun. After the Meiji reform, however
(1868), the industry revived, new grounds were discovered, improved methods
employed, and a large export trade developed.

There is evidence, however, in the art of Japan, of another {329}coral
fishery in ancient times, of which the history is lost. Coral was imported
into Japan at least two hundred years ago, and used largely in the
manufacture of those exquisite pieces of handicraft for which that country
is so justly famous. On many of the carved "Netsukes" and other ornaments,
however, the coral branches are represented as the booty of dark-skinned,
curly-headed fishermen, "kurombo," and never of Japanese fishermen. The
coral used in this art-work can hardly be distinguished from Mediterranean
coral, and there are some grounds for believing that Japan imported coral
from the far West in very early times. But this does not account for the
"kurombo." The only coast-dwelling people of the type that is so clearly
carved on these ornaments within the area of the Pacific Ocean at the
present time are the Melanesians and Papuans, and the suggestion occurs
that a coral fishery existed at one time in the Southern Pacific, which has
since been lost.[363]



The class ANTHOZOA is divided into two sub-classes:—I. ALCYONARIA; II.
ZOANTHARIA.

In the Alcyonaria the fully developed zooids have always eight tentacles
and eight mesenteries. In the Zoantharia the number of tentacles and the
number of mesenteries in the fully developed zooids may be six, twelve,
twenty-four, or an indefinite number, but individuals with eight
mesenteries and only eight tentacles are not known to occur.


SUB-CLASS I. ALCYONARIA.

This sub-class includes a large number of genera living in shallow
sea-water and a few genera that extend down into deep water. With a few
doubtful exceptions (Protoalcyonacea) they all form colonies composed of a
large number of zooids. These zooids may be connected together by basal
plates or a network of basal strands (stolons), or by stolons with
additional connecting bars (CLAVULARIA VIRIDIS, SYRINGOPORA) or by plates
(TUBIPORA). In the majority of the genera the individual zooids are for the
greater part of their length, from the base upwards, united together to
form a continuous spongy, colonial mass, which determines the shape of the
colony as a whole.

In this last-named group of genera there may be {330}distinguished the free
distal portions of the zooids bearing the mouths and tentacles (the
"anthocodiae") from the common colonial mass perforated by the coelenteric
cavities of the individual zooids. The coelenteric cavities are separated
by a considerable amount of a substance called the "mesogloea," usually
gelatinous in consistency but chemically more closely related to mucin than
to gelatin, which is traversed by endodermal canals, rods of endoderm cells
and a number of free amoeboid cells. In this substance, moreover, there are
found in nearly all cases numerous spicules of carbonate of lime formed by
the "scleroblasts" (spicule-forming cells) which have wandered from the
superficial ectoderm of the common colonial mass. This common colonial
mesogloea with its spicules, endoderm cells, and superficial covering of
ectoderm is called the "coenenchym." The form assumed by the colonies is
very varied. In some species of _Clavularia_ they form encrusting plates
following the irregularity of the rock or stones on which they grow, in
_Alcyonium_ they construct lobed masses of irregular form, in _Sarcophytum_
they are usually shaped like a mushroom, in _Juncella_ they are long
whip-like rods, in most of the Gorgonacea they are branched in all
directions like shrubs or in one plane to form fan-shaped growths, and in
many of the Pennatulacea they assume that graceful feather form which gives
the order its name.

The consistency and texture of the colonies also varies considerably. In
some cases where the spicules are few or very small, the substance of the
colony is soft to the touch, and frequently slimy at the surface, in other
cases the great number of the spicules makes the colony hard but brittle,
whilst in a few genera (_Sclerophytum_, _Heliopora_) the colony is so hard
that it can only be broken by the hand with difficulty. In some genera
(_Spongodes_ and the Muriceidae) projecting spicules cause the surface to
be rough or thorny, and in the Primnoidae the zooids and the surface of the
general coenenchym are protected by a series of overlapping scales or
plates.

In all the Alcyonaria the nematocysts are very minute, and although they
can undoubtedly paralyse minute organisms they are unable to penetrate the
human skin. None of the Alcyonaria have been described as stinging-corals
except the Pennatulid _Virgularia rumphii_.

ZOOIDS.—The fully formed zooids of the Alcyonaria exhibit {331}a remarkable
uniformity of structure. They have eight intermesenteric tentacles
containing a cavity continuous with the coelenteron. Each of these
tentacles bears at least two rows of simple pinnules, and they are
therefore said to be "pinnate" tentacles. In some species of _Xenia_ the
tentacles may have three or four rows of pinnules, which give them a much
more feathery appearance than is usually the case. In the great majority of
species a single row of from eight to fourteen pinnules is found disposed
laterally on each side of the tentacle. The mouth is usually small and
slit-like with a slight rounded gape at the ventral extremity. The
stomodaeum is usually very short, but in _Xenia_ and in the autozooids of
some Pennatulids it is relatively much longer. It is not known how far the
stomodaeum is of importance in the digestion of the food. In _Xenia_[364]
it has probably some importance, as shown by its unusual length and the
numerous large goblet cells (mucus cells) which it exhibits, associated
with the fact that the mesenteric filaments are relatively very small. In
_Alcyonium_ and other Alcyonaria gland cells also occur in the stomodaeum,
and it is probable that they secrete a fluid capable of digesting to some
extent the food as it passes through. The most important part of the
digestion, however, is performed by the six "ventral" mesenteric filaments.

Attention has already been drawn to the fact (p. 330) that two regions of
the zooids of the colonial Alcyonaria can be recognised. At the oral end
there is a region, which in the fully expanded condition consists of a
crown of eight tentacles surrounding the mouth, and a body-wall free from
its immediate neighbours. This region is called the "anthocodia." The
anthocodia is continuous with a region which forms a part of the common
colonial mass. Some genera seem to have very little power of contracting
the tentacles or of withdrawing the anthocodiae. The zooids of
_Stereosoma_, of _Xenia_, of _Umbellula_, and of a few other genera may be
described as non-retractile. In many cases, however, the tentacles can be
considerably contracted, bent over the mouth, and withdrawn into the
shelter of the subjacent body-wall. In such a condition the surface of the
colony exhibits a number of tubular, conical, or convex protuberances,
called "verrucae," and the colony is said to be partially retractile. In
many genera, however, the whole of the {332}anthocodiae can be withdrawn
below the general surface of the coenenchym, so that the position of the
zooids in the colony is indicated only by star-like holes, or simple
key-hole slits in the superficial coenenchym. Such colonies are said to be
completely retractile (Fig. 147).

It is often very difficult to determine whether a particular species is or
is not completely retractile, unless observations can be made upon the
living colony; and there are many instances of confusion in the work of
systematists due to a species being described as partially retractile in
one instance, and completely retractile in another. The complete retraction
of the anthocodiae may be effected very slowly, and after continuous
irritation only. If the colony is killed too quickly, the anthocodiae
remain in a state of partial retraction. An example of this may be found in
the common British _Alcyonium digitatum_. Specimens of this species which
are put into a bucket of sea water and allowed to roll about with the
movements of a small boat in a rough sea, undergo complete retraction; but
if the same specimens be allowed to expand in the aquarium, and then
plunged into spirit, or allowed to dry in the sun, they will die in a
condition of partial retraction.

[Illustration: FIG. 147.—Diagram of a vertical section of a portion of a
lobe of _Alcyonium_ to show the mode of retraction of the anthocodiae. 1,
Anthocodia of a zooid fully expanded; 2, in the first stage of retraction;
3, in the second stage; 4, in the third stage, leaving a shallow prominence
or "verruca" on the surface; 5, final stage, the verruca flattened down and
the coenenchym closed. _can_, Canal system; _d.m.f_, dorsal mesenteric
filament of a zooid; _si_, siphonoglyph.]

The phenomenon of dimorphism occurs in some Alcyonaria. A certain number of
the zooids of a colony are arrested in their development, and are known as
the "siphonozooids." They may be distinguished from the fully formed
zooids, which, in these {333}cases, are called the "autozooids," by the
absence of tentacles, by the absence of the six ventral and lateral
mesenteric filaments, and by the incomplete development of the muscles on
the mesenteries, and of the mesenteries themselves. They are, moreover,
frequently distinguished by the greater development and extent of the
ciliated groove or siphonoglyph on the ventral side of the stomodaeum.

It is often difficult to distinguish between true siphonozooids and young
autozooids, and consequently dimorphism has been attributed to some genera
in which it almost certainly does not occur. Simple dimorphism undoubtedly
occurs in the genera _Heteroxenia_, _Sarcophytum_, _Anthomastus_,
_Lobophytum_, _Acrophytum_, and _Paragorgia_. It has also been said to
occur in _Corallium_ (Moseley and Kishinouye), _Melitodes_ (Ridley), and
some species of Dasygorgiidae.

The Pennatulacea are trimorphic. The main shaft of these colonies is the
much modified first formed or axial zooid, adapted for the support of all
the other zooids. It usually exhibits no mouth, no tentacles, and only four
of the original eight mesenteries. It has no mesenteric filaments and no
stomodaeum, and bears no sexual cells. The other zooids of the colony are
similar in structure to the autozooids and siphonozooids of the dimorphic
Alcyonaria.

There are eight MESENTERIC FILAMENTS in all Alcyonarian zooids. They have
the appearance of thickenings of the free edges of the mesenteries. Two of
them, called the "dorsal" mesenteric filaments, are straight when the
anthocodia is expanded, and extend from the edge of the stomodaeum for a
long distance down into the coelenteron of the zooid; the other six, called
the "ventral" mesenteric filaments (_i.e._ the ventral and ventro-lateral
and dorso-lateral), are usually short and are almost invariably slightly
convoluted. The dorsal filaments are built up of columnar cells provided
with long cilia, and have usually no gland cells, the others may show a few
cilia but are principally composed of non-ciliated gland cells. When the
bolus of food has passed through the stomodaeum it is seized by these
ventral filaments and rapidly disintegrated by the secretion of its cells.
The function of the dorsal mesenteric filaments is mainly respiratory.
During life their cilia produce a current which flows towards the
stomodaeum. On the ventral side of the {334}stomodaeum itself there is a
groove called the "siphonoglyph" composed of a specialised epithelium
bearing long powerful cilia. But the current produced by the siphonoglyph
flows from the mouth downwards into the coelenteric cavity and is thus in
the opposite direction to that produced by the dorsal mesenteric filaments.
It is very probable that these two currents on the opposite sides of the
zooids maintain the circulation of water in the deep-seated parts of the
colony which is necessary for the respiration of the tissues.

On each of the eight mesenteries there is a longitudinal ridge due to the
presence of a band of retractor muscles. The position of these muscles on
the ventral surfaces of the mesenteries only is one of the characteristic
features of the sub-class (Fig. 148, and p. 329). They vary considerably in
thickness and extent according to the power of retractility possessed by
the zooids, but they never vary in their position on the mesenteries.

[Illustration: FIG. 148.—Diagrammatic transverse sections of an
Alcyonarian. A, through the stomodaeum; B, below the level of the
stomodaeum. _DD_, Dorsal directive; _dlmf_, dorso-lateral mesenteric
filament; _dmf_, dorsal mesenteric filament; _gon_, gonad; _Si_,
siphonoglyph; _V.D_, ventral mesentery; _V.L_, ventro-lateral mesentery.
The upper half of the section in B is taken at a higher level than the
lower half.]

The SKELETON of Alcyonaria may consist of spicules of calcium carbonate, of
a horny substance frequently impregnated with calcium carbonate and
associated with spicules of the same substance, or in _Heliopora_ alone,
among recent forms, of a continuous crystalline corallum of calcium
carbonate.

The spicules constitute one of the most characteristic features of the
Alcyonaria. They are not found in _Cornularia_, _Stereosoma_, in a recently
discovered genus of Gorgoniidae (_Malacogorgia_), in certain Pennatulacea
and in _Heliopora_; and it is probable that they may be absent in some
local varieties of certain species of _Clavularia_.

The spicules of Alcyonaria consist of an organic matrix {335}supporting a
quantity of crystalline calcium carbonate. In some cases (_Xenia_) the
amount of inorganic salt is so small that the spicule retains its shape
after prolonged immersion in an acid; but generally speaking the relative
amount of calcium carbonate is so great that it is only by the careful
decalcification of the spicules in weak acetic acid that the delicate
fibrous organic matrix can be demonstrated.

The spicules vary in size from minute granules to long spindles 9 mm. in
length (_Spongodes_, sp.). They exhibit so many varieties of shape that an
attempt must be made to place them in groups. The most prevalent type
perhaps is that called the spindle. This is a rod-shaped spicule with more
or less pointed extremities. They are usually ornamented with short simple
or compound wart-like tubercles (Fig. 149, 5). Spicules belonging to this
type are found in all the principal subdivisions of the group except the
Pennatulacea.

In the Pennatulacea a very characteristic form of spicule is a long rod or
needle marked with two or three slightly twisted ridges, frequently a
little knobbed or swollen at the extremities. In the same group, in _Xenia_
and _Heteroxenia_ among the Alcyonacea, and in the family Chrysogorgiidae
the spicules are in the form of minute discs or spheres, and in some genera
the discs may be united in couples (twins) or in threes (triplets) by short
connecting bars (Fig. 149, 10). More irregular calcareous corpuscles of
minute size are found in some genera of Pennatulacea.

Other characteristic spicules are the warted clubs of _Juncella_, the
torch-like spicules of _Eunicella_ (Fig. 149, 3), the clubs with irregular
leaf-like expansions at one extremity ("Blattkeulen") of _Eunicea_, and the
flat but very irregular scales of the Primnoidae. There are also many
genera exhibiting spicules of quite irregular form (Fig. 149, 8).

In the greater number of cases the spicules lie loosely in the mesogloea
and readily separate when the soft tissues of the colony decay or are
dissolved in a solution of potash. In a few noteworthy examples the
spicules become in their growth tightly wedged together to form a compact
skeleton, which cannot subsequently be disintegrated into its constituent
elements. In the Precious corals (Coralliidae) the spicules of the axial
region fuse together to form a solid mass of lime almost as hard and
compact as the substance of a pearl.

{336}[Illustration: FIG. 149.—Spicules of Alcyonaria. 1, Club of
_Juncella_; 2, warted cross of _Plexaurella_; 3, torch of _Eunicella_; 4,
needle of _Renilla_; 5, warted spindle of _Gorgonella_; 6, spicule of
_Pennatula_; 7, foliate club of _Eunicea_; 8, irregular spicule of
_Paramuricea_; 9, scale of _Primnoa_; 10, spicules of _Trichogorgia_. (5
and 10 original, the remainder after Kölliker.)]

In _Paragorgia_ and some other closely related genera the spicules of the
axis of the colony also become tightly wedged together, but the core thus
formed is far more porous and brittle than it is in the Coralliidae. In
_Tubipora_ (the organ-pipe coral) and in _Telesto rubra_ the spicules of
the body-walls of the zooids fuse to form perforated calcareous tubes. In
some species of _Sclerophytum_ the large spicules of the coenenchym become
so closely packed that they form dense stony masses, almost as hard as a
Perforate Madreporarian coral. The horny substance, allied chemically to
keratin, plays an {337}important part in the building up of skeletal
structures in many Alcyonaria. In _Clavularia viridis_ and in _Stereosoma_
a change in the chemical character of the mesogloea of the body-walls of
the polyps leads to the formation of a horny tube, which in the former case
is built up of interlacing fibres, and in the latter is formed as a
homogeneous sheath. In many of the Alcyonacea which have a compact axial
skeleton the spicules are cemented together by a horny matrix.

In the Gorgonellidae and some others the hard axis is formed of a horny
substance impregnated with a crystalline form of calcium carbonate; but in
the Gorgoniidae, many of the Pennatulacea and some other genera very little
or no carbonate of lime is found in the horny axis.

The skeleton of the genus _Heliopora_ differs from that of all the other
Alcyonaria in its development, structure, and form. In the words of Dr. G.
C. Bourne,[365] "the calcareous skeleton of _Heliopora_ is not formed from
spicules developed within cells but is a crystalline structure formed by
crystallisation of carbonate of lime, probably in the form of aragonite, in
an organic matrix produced by the disintegration of cells which I have
described as calicoblasts." It is further characterised by its blue colour.
A peculiar form of the axial skeleton (Fig. 155), consisting of alternate
nodes mainly composed of keratin, and internodes mainly composed of calcium
carbonate, is seen in the families Isidae and Melitodidae. In the
Melitodidae the nodes contain a considerable number of loose spicules, and
the internodes are mainly composed of spicules in close contact but firmly
cemented together by a sparse horny matrix. In the Isidae the scanty
calcareous substance of the nodes, and the bulk of the substance of the
internodes, is formed of amorphous crystalline limestone.

The Alcyonaria exhibit a great variety of COLOUR. Very little is known at
present of the chemistry of the various pigments found in the group, but
they may conveniently be arranged in two sections, the soluble pigments and
the insoluble pigments. To the former section belong various green and
brown pigments found in the anthocodiae and superficial coenenchym of many
genera. These are related to chlorophyll, and may be very largely the
product, not of the Alcyonarians themselves, but of the {338}symbiotic
"Algae" (cf. p. 261) they carry. A diffuse salmon-pink colour soluble in
spirit occurs in the living _Primnoa lepadifera_ of the Norwegian fjords,
and a similar but paler pink colour occurs in some varieties of the common
_Alcyonium digitatum_. Gilchrist[366] states that when he was preserving
specimens of _Alcyonium purpureum_ from Cape waters a considerable quantity
of a soluble purple pigment escaped.

But the predominant colour of Alcyonarians is usually due to the insoluble
pigments of the calcareous spicules. These may be of varying shades of
purple, red, orange, and yellow. The colours may be constant for a species
or genus, or they may vary in different specimens of one species, or even
in different parts of a single colony. Thus the skeletons of _Tubipora
musica_ from all parts of the world have a red colour, the species of the
genus _Anthomastus_ have always red spicules. On the other hand, we find in
_Melitodes dichotoma_ red and yellow varieties in the same locality, and in
_M. chamaeleon_ some of the branches of a colony are red and others yellow.
In _Chironephthya variabilis_ the colour of the spicules in any one
specimen varies considerably, but in a collection of several specimens from
a single locality a kaleidoscopic play of colours may be seen, no two
specimens being exactly the same in the arrangement of their colour
pattern. The influences that determine the colour of the spicules is at
present quite unknown, and in view of the great variability that occurs in
this respect, colour must be regarded as a most uncertain guide for the
determination of species. The blue colour of the genus _Heliopora_ is due
to a peculiar pigment which shows characteristic bands in the
spectrum.[367]

PHOSPHORESCENCE.—A great many Alcyonaria are known to be phosphorescent.
Moseley says that "All the Alcyonarians dredged by the 'Challenger' in deep
water were found to be brilliantly phosphorescent when brought to the
surface." The phosphorescence of the common British _Pennatula phosphorea_
has attracted more attention than that of any other species, and has been
well described by Panceri, Forbes, and others. Forbes[368] says, "The pen
is phosphorescent only when irritated by touch; the phosphorescence appears
at the place touched, and {339}proceeds thence in an undulating wave to the
extremity of the rachis, but never in the opposite direction; it is only
the parts at and above the point of stimulation that show phosphorescence,
the light is emitted for a longer time from the point of stimulation than
from the other luminous parts; detached portions may show phosphorescence.
When plunged in fresh water, the _Pennatula_ scatters sparks about in all
directions—a most beautiful sight."

Panceri was of opinion that the mesenteric filaments were the organs of
phosphorescence, but the whole question of the cause and localisation of
the light in these colonies requires further investigation.

FOOD.—Very little is known about the food of Alcyonaria, but it is very
probable that it consists entirely of minute larvae and other living
organisms. When the coelenteric cavities of preserved Alcyonaria are
examined, food is very rarely found in them, although fragments of
Crustacean appendages have occasionally been seen in the neighbourhood of
the mesenteric filaments. Experimenting upon _Alcyonium digitatum_, Miss
Pratt[369] has found that the zooids seize and swallow various small
organisms of a surface-net gathering, and that they will also swallow
finely minced fragments of the muscle of fish, but that they reject many
kinds of fish ova. In many tropical and some extra-tropical species the
superficial canal systems and the inter-mesenterial spaces of the zooids
contain a large number of Zooxanthellae, and their presence seems to be
associated in some cases with a decided degeneration of the digestive
organs. It has been suggested that these symbiotic "Algae" prepare food
materials after the manner of plants, and that these are absorbed by the
hosts, but it appears improbable that in any case this source of food
supply is sufficient. It must probably be supplemented in some degree by
food obtained by the mouth, and digested in the coelenteric cavity.

The question whether the Alcyonaria can form an important part of the
dietary of fish or other carnivorous animals may be economically important.
Fragments of the Pennatulid _Virgularia_ have been found in the stomachs of
cod and other fish, but with this exception there is no evidence that any
genus is systematically or even occasionally preyed upon by any animal.
With a very {340}few exceptions Alcyonaria show no signs of having been
torn, bitten, or wounded by carnivorous animals. It is improbable that the
presence of nematocysts in the tentacles can account for this immunity, as
it is known that some predaceous animals do feed upon Coelenterates
provided with much larger nematocysts than any Alcyonarian possesses. All
Alcyonaria, however, have a characteristic disagreeable odour, and it is
possible, as in many other cases, that this is accompanied by an unpleasant
taste. But if the Alcyonaria themselves are immune, it is possible that
their large yolk-laden eggs may form a not unimportant source of food
supply. In places where large colonies flourish, an immense number of eggs
or embryos must be discharged into the water during the spawning season,
and of these only a minute fraction can survive long enough to found a new
colony.

REPRODUCTION.—The formation of colonies by gemmation has frequently been
mentioned above. The young buds of a colony arise from the endoderm canals
in the body-wall of the zooids, in the general coenenchym, or in the
stolon. They never arise from evagination of the coelenteric cavities of
the zooids. There is no evidence that fission of a colony to form secondary
colonies ever occurs. Gemmation leads to the increase in the number of
zooids forming a colony, but not to an increase in the number of colonies.

Fission of the zooids is of extremely rare occurrence; a single case,
however, has been recorded by Studer in the genus _Gersemia_. Sexual
reproduction usually occurs once in a year; it is doubtful whether it ever
occurs continuously. The colonies appear to be nearly always dioecious,
only one case of hermaphroditism having yet been recorded.[370] The ova and
sperm sacs are usually formed and matured on the six ventral mesenteries,
rarely on the dorsal pair of mesenteries (Fig. 148, B) as well. The
spawning season varies with the locality. _Alcyonium digitatum_ spawns at
Plymouth at the end of December, and somewhat later at Port Erin. The
Pennatulid _Renilla_ and the Gorgonid _Leptogorgia_ spawn in the summer
months on the coast of North America. In the Mediterranean _Alcyonium
palmatum_ spawns in September and October (Lo Bianco), _Gorgonia cavolinii_
in May and June.

{341}It is not known for certain when the fertilisation of the ova is
effected, but in _Alcyonium digitatum_, and in the majority of the
Alcyonarians, it probably takes place after the discharge of the ova from
the zooids. A few forms are, however, certainly viviparous, the larvae of
_Gorgonia capensis_ being retained within the coelenteric cavity of the
parent zooid until they have grown to a considerable size. The other
viviparous Alcyonarians are _Corallium nobile_ (de Lacaze Duthiers), the
"Clavulaires petricoles," and _Sympodium coralloides_ (Marion and
Kowalevsky), and three species of _Nephthya_ found at depths of 269 to 761
fathoms (Koren and Danielssen). The general features of the development are
very similar in all Alcyonarians that have been investigated. The egg
contains a considerable amount of yolk, and undergoes a modified form of
segmentation. The free-swimming larva is called a "sterrula." It consists
of an outer layer of clear ciliated ectoderm cells, surrounding a solid
endodermic plasmodium containing the yolk. As the yolk is consumed a cavity
appears in the endoderm, and the larva is then called a "planula" (Fig.
150). The mouth is subsequently formed by an invagination of the ectoderm
at the anterior pole. The development of the mesenteries has not yet been
fully described.

[Illustration: FIG. 150.—Ciliated "planula" larva of _Alcyonium digitatum_.
_Ec_, Ectoderm; _End_, endoderm.]

CLASSIFICATION.—The sub-class Alcyonaria may conveniently be classified as
follows:—

  Grade A. PROTALCYONACEA.
  Grade B. SYNALCYONACEA.
             Order 1. STOLONIFERA.
             Order 2. COENOTHECALIA.
             Order 3. ALCYONACEA.
             Order 4. GORGONACEA.
             Order 5. PENNATULACEA.


{342}GRADE A. PROTOALCYONACEA.

This Grade includes those genera which, like many sea-anemones, do not
reproduce by continuous gemmation to form colonies.

Several genera have been described, and they have been placed together in
one family called the HAIMEIDAE.

_Haimea funebris_, M. Edwards, was found off the coast of Algeria; _H.
hyalina_, Koren and Danielssen, in Norway; _Hartea elegans_, Wright, from
the Irish coast; _Monoxenia darwinii_, Haeckel, from the Red Sea, and a
large new species found by the "Siboga" Expedition in deep water off Ceram.
All these species, however, are very rare, and there is no satisfactory
evidence at present that they remain solitary throughout life.


GRADE B. SYNALCYONACEA.

The sub-division of the Synalcyonacea into orders presents many
difficulties, and several different classifications have been proposed.
Only two orders of the five that are here recognised are clearly defined,
namely, the Coenothecalia, containing the single living genus _Heliopora_,
and the Pennatulacea or Sea-pens; the others are connected by so many
genera of intermediate characters that the determination of their limits is
a matter of no little difficulty.


ORDER I. STOLONIFERA.

These are colonial Alcyonaria springing from a membranous or ribbon-like
stolon fixed to a stone or some other foreign object. The body-walls of the
individual zooids may be free or connected by a series of horizontal bars
or platforms (autothecalous); never continuously fused as they are in other
orders (coenothecalous).

In the simplest form of this order, _Sarcodictyon catenatum_ Forbes, the
ribbon-like strands of the stolon meander over the surface of stones,
forming a red or yellow network, from the upper surface of which the clear
transparent anthocodiae of the zooids protrude. When retracted the
anthocodiae are drawn down below the surface of the general coenenchym, and
their position is indicated by small cushion-like pads on the stolon.
{343}_Sarcodictyon_ is found in depths of 10 to 22 fathoms in the Irish
Sea, off the west coast of Scotland, the Shetlands, and off the Eddystone
Lighthouse, South Devon.

Another very important genus is _Tubipora_, in which the tubular body-wall
of each zooid is very much longer in proportion to its diameter than it is
in _Sarcodictyon_, and the anthocodia is retracted not into the stolon, but
into the basal part of the body-wall. The zooids are connected together by
horizontal platforms on which new zooids are formed by gemmation. Both
horizontal platforms and the body-walls of the zooids are provided with a
skeleton of fused spicules of a red colour.

This genus is the well-known Organ-pipe coral, and is found sometimes in
immense quantities on the coral reefs of both the old and new world.

It may be seen in pools on the edge of the reefs at low tides in colonies
frequently a foot or more in diameter. The tentacles are often of a bright
emerald green colour, and as the anthocodiae stand expanded in the clear
water they contribute a brilliant patch of colour to the many beauties of
their surroundings. When the coral is disturbed, or the water shallows and
the anthocodiae are retracted, the dull red colour of the skeleton
gradually takes the place of the bright green of the tentacles.

[Illustration: FIG. 151.—_Tubipora musica_, a young colony growing on a
dead Madrepore branch (_M_). _Hp_, The connecting horizontal platforms;
_p_, _p_, the skeletal tubes of the zooids; _St_, the basal stolon.]

It is probable that this order of Alcyonaria was better represented on the
reefs of some of the earlier periods of the world's history than it is at
present. The fossil _Syringopora_, which is found abundantly in the
carboniferous limestone and other strata, was probably an Alcyonarian
belonging to this order. It resembles _Tubipora_ in its mode of growth, but
in place of the horizontal platforms connecting the zooids there are rods
or bars from which new zooids spring (Fig. 152). Similar connecting bars
are found in the recent _Clavularia_ (_Hicksonia_, Delage) {344}_viridis_
of the East Indian reefs (Fig. 153). Other fossil forms belonging to the
order are _Favosites_, a very abundant coral of the Upper Silurian rocks,
and possibly _Columnaria_.

[Illustration: FIG. 152.—_Syringopora_, a fossil, showing autothecalous
tubes (_th_), funnel-shaped tabulae (_tab_), and tubular cross-bars (_t_).]

[Illustration: FIG. 153.—_Clavularia_ (_Hicksonia_) _viridis_, with
creeping stolon and transverse connecting tubes.]

The principal families of the Stolonifera are:—

  Fam. 1. CORNULARIIDAE.—Without spicules; _Cornularia_, Lamarck,
  Mediterranean; _Stereosoma_, Hickson, Celebes.

  Fam. 2. CLAVULARIIDAE.—_Clavularia_, Quoy and Gaimard; _Sarcodictyon_,
  Forbes, British; _Sympodium_, Ehrb.; _Syringopora_, Goldfuss, fossil.

  Fam. 3. TUBIPORIDAE.—_Tubipora_, Linnaeus, tropical shallow water.

  Fam. 4. FAVOSITIDAE.—_Favosites_,  Lamarck; _Syringolites_, Hinde;
  _Stenopora_, King.


ORDER II. COENOTHECALIA.

This order contains the single genus and species _Heliopora coerulea_ among
recent corals, but was probably represented by a large number of genera and
species in earlier periods.

{345}It is found at the present day in many localities in the warm shallow
waters of the tropical Pacific and Indian Oceans. It usually flourishes on
the inside of the reef, and may form masses of stone five or six feet in
diameter. The coral may easily be recognised, as it is the only one that
exhibits a blue colour. This colour usually penetrates the whole skeleton,
but in some forms is absent from the superficial layers.

The skeleton consists of a number of parallel tubes with imperforate walls,
which are fused together in honey-comb fashion. On making a vertical
section through a branch of the coral it is found that the tubes are
divided into a series of chambers by transverse partitions or "tabulae."
The soft living tissues of the coral, the zooids and coenosarc, are
confined to the terminal chambers, all the lower parts being simply dead
calcareous skeleton supporting the living superficial layer. Among the
parallel tubes there may be found a number of larger chambers that seem to
have been formed by the destruction of the adjacent walls of groups of
about nineteen tubes. These chambers are provided with a variable number of
pseudo-septa, and have a remarkable resemblance to the thecae of some
Zoantharian corals. That _Heliopora_ is not a Zoantharian coral was first
definitely proved by Moseley, who showed that each of these larger chambers
contains an Alcyonarian zooid with eight pinnate tentacles and eight
mesenteries. The zooids arise from a sheet of coenosarc that covers the
whole of the living branches of the coral mass, and this sheet of coenosarc
bears a plexus of canals communicating on the one hand with the zooids, and
on the other with a series of blind sacs, each of which occupies the cavity
of one of the skeletal tubes as far down as the first tabula. The zooids of
_Heliopora_ are very rarely expanded during the day-time, and it has been
found very difficult to get them to expand in an aquarium. The coral,
however, is frequently infested with a tubicolous worm allied to the genus
_Leucodora_, which freely expands and projects from the surface. So
constant and so numerous are these worms in some localities that it has
actually been suggested that _Heliopora_ should be regarded as a Polychaete
worm and not as an Alcyonarian. According to Mr. Stanley Gardiner, however,
these worms do not occur in association with the _Heliopora_ found on the
reefs of the Maldive Archipelago.

{346}There is very strong reason to believe that certain fossil corals were
closely related to _Heliopora_; that _Heliopora_ is in fact the solitary
survivor of a group of Alcyonarian corals that in past times was well
represented on the reefs, both in numbers and in species. The evidence is
not so convincing that other fossil corals are closely related to
_Heliopora_, and their true zoological position may remain a matter for
surmise. The order may be classified as follows:—

FAM. 1. HELIOLITIDAE.[371]—Coenothecalia with regular, well-developed
septa, generally twelve in number, in each calicle.

_Heliolites_, Dana, Silurian and Devonian. _Cosmiolithus_, Lindström, Upper
Silurian. _Proheliolites_, Klaer, Lower Silurian. _Plasmopora_, Edwards and
Haime, Upper Silurian. _Propora_, E. and H., Upper Silurian.
_Camptolithus_, Lindström, Upper Silurian. _Diploëpora_, Quenst, Upper
Silurian. _Pycnolithus_, Lindström, Upper Silurian.

FAM. 2. HELIOPORIDAE.[372]—Coenothecalia with small irregularly arranged
coenosarcal caeca, and a variable number of septa or septal ridges.
_Heliopora_, de Blainville, recent, Eocene and Upper Cretaceous.
_Polytremacis_, d'Orbigny, Eocene and Upper Cretaceous. _Octotremacis_,
Gregory, Miocene.

The family COCCOSERIDAE is regarded by Lindström as a sub-family of the
Heliolitidae, and the families THECIDAE and CHAETETIDAE are probably
closely related to the Helioporidae.


ORDER III. ALCYONACEA.

This order contains a large number of genera of great variety of form. The
only characters which unite the different genera are that the body-walls of
some groups of zooids, or of all the zooids, are fused together to form a
common coenenchym penetrated by the coenosarcal canals, and that the
spicules do not fuse to form a solid calcareous, or horny and calcareous,
axial skeletal support.

The affinities with the order Stolonifera are clearly seen in the genera
_Xenia_ and _Telesto_. Some species of _Xenia_ form flattened or domed
colonies attached to stones or corals, with non-retractile anthocodiae and
body-walls united for only a {347}short distance at the base. Young _Xenia_
colonies are in fact Stolonifera in all essential characters. In _Telesto
prolifera_ we find a network of stolons encrusting coral branches and other
objects after the manner of the stolons of many species of _Clavularia_,
although the zooids do not arise from these stolons singly, but in groups,
with their body-walls fused together for a certain distance. In _Telesto
rubra_ the spicules of the body-walls are fused together to form a series
of perforated tubes very similar in some respects to the tubes of
_Tubipora_.

A remarkable genus is _Coelogorgia_. Here we find a branching colony
arising from a basal stolon, and the axis of the main stem and of each
branch consists of a single very much elongated zooid bearing on its
thickened walls the branches of the next series and other zooids. It is
true that in this genus there is very little fusion of neighbouring zooids,
and the amount of true coenenchym is so small that it can hardly be said to
exist at all. Bourne[373] has united this genus with _Telesto_ into a
family Asiphonacea, which he joins with the Pennatulida in the order
Stelechotokea; but their affinities seem to be closer with the Alcyonacea
than with the Pennatulacea, from which they differ in many important
characters.

[Illustration: FIG. 154.—_Alcyonium digitatum_, a single-lobed specimen,
with some of the zooids expanded.]

The genus _Alcyonium_ not only contains the commonest British Alcyonarian
(_A. digitatum_), but it is one of the most widely distributed genera of
all Alcyonaria that occur in shallow water.

The genera _Sarcophytum_ and _Lobophytum_ occur in shallow water in the
tropics of the old world. The former frequently consists of huge toad-stool
shaped masses, soft and spongy in {348}consistency, of a green, brown, or
yellow colour. On some reefs the colonies of _Sarcophytum_ form a very
conspicuous feature, and from their very slimy, slippery surface, add to
the minor dangers of wading in these regions. Both genera are dimorphic.
Some species of the genus _Sclerophytum_,[374] which occur in the Indian
Ocean, are so hard and brittle that they might readily be mistaken for a
Zoantharian coral. This character is due to the enormous number of tightly
packed spicules borne by the coenenchym. Some of these spicules in _S.
querciforme_ are 7 mm. × 1.7 mm.; the largest, though not the longest
(_vide_ p. 335) of any spicules occurring in the order.

Another very important genus occurring on coral reefs, and of very wide
distribution, is _Spongodes_. This genus forms bushy and rather brittle
colonies of an endless variety of beautiful shapes and colours. Arising
from the neck of each anthocodia there are one or two long, sharp,
projecting spicules, which give the surface a very spiny or prickly
character.

The genera _Siphonogorgia_ and _Chironephthya_ form large brittle,
branching colonies which might readily be mistaken for Gorgonians. The
strength of the branches, however, is mainly due to the large, densely
packed, spindle-shaped spicules at the surface of the coenenchym, the long
coelenteric cavities of the zooids penetrating the axis of both stem and
branches. _Siphonogorgia_ is usually uniformly red or yellow in colour.
_Chironephthya_, on the other hand, exhibits a great variety of colour in
specimens from the same reef, and indeed in different branches of the same
colony.

FAM. 1. XENIIDAE.—Alcyonacea with non-retractile zooids. Spicules very
small discs, usually containing a relatively small proportion of lime.

_Xenia_, Savigny; Indian Ocean and Torres Straits. _Heteroxenia_, Kölliker;
Red Sea, Cape of Good Hope, and Torres Straits.

FAM. 2. TELESTIDAE.—Colonies arising from an encrusting membranous or
branching stolon. The erect stem and branches are formed by the body-walls
of two or three zooids only, from which secondary zooids and branches of
the next order arise.

_Telesto_, Lamouroux, widely distributed in warm waters of the Atlantic,
Pacific, and Indian Oceans. The genus _Fascicularia_, Viguier, from the
coast of Algiers, seems to be related to _Telesto_, {349}but the groups of
zooids are short, and do not give rise to branches.

FAM. 3. COELOGORGIIDAE.—The colony arborescent, attached by stolon-like
processes. The stem formed by an axial zooid with thickened body-walls.
Branches formed by axial zooids of the second order, and branchlets by
axial zooids of the third order, borne either on two sides or in spirals by
the main stem. Genus _Coelogorgia_, Zanzibar.

FAM. 4. ALCYONIIDAE.—The colonies of this family are usually soft and
fleshy, and the spicules, evenly distributed throughout the coenenchym, do
not usually fuse or interlock to form a continuous solid skeleton. They may
be unbranched or lobed, never dendritic in form. The principal genera
are:—_Alcyonium_, Linnaeus, cosmopolitan, but principally distributed in
temperate and cold waters. _Alcyonium digitatum_ is the commonest British
Alcyonarian. It is found in shallow water, from the pools left at low
spring tides to depths of 40 or 50 fathoms, at most places on the British
shores. It is stated by Koehler to descend into depths of over 300 fathoms
in the Bay of Biscay. There are two principal varieties; one is white or
pale pink in the living condition, and the other yellow. In some localities
the two varieties may be found in the same pools. Another species,
_Alcyonium glomeratum_, placed in a distinct genus (_Rhodophyton_) by Gray,
and distinguished from the common species by its red colour and long
digitate lobes, is found only off the coast of Cornwall. _Paralcyonium_,
Milne Edwards; Mediterranean. _Sclerophytum_, Pratt; sometimes dimorphic,
Indian Ocean. _Sarcophytum_, Lesson; dimorphic, principally tropical.
_Lolophytum_, Marenzeller; dimorphic, tropical. _Anthomastus_, Verrill;
dimorphic, Atlantic Ocean, deep water. _Acrophytum_, Hickson; dimorphic,
Cape of Good Hope.

FAM. 5. NEPHTHYIDAE.—Colonies dendritic. Usually soft and flexible in
consistency. _Nephthya_, Savigny; Indian and Pacific Oceans. _Spongodes_,
Lesson; widely distributed in the Indian and Pacific Oceans.

FAM. 6. SIPHONOGORGIIDAE.—Colonies often of considerable size. Dendritic.
Spicules usually large and abundant, giving a stiff, brittle consistency to
the stem and branches. _Siphonogorgia_, Kölliker; Red Sea, Indian, and
Pacific tropics. _Chironephthya_, Wright and Studer; Indian and Pacific
Oceans. _Lemnalia_, {350}Gray; Zanzibar.  _Agaricoides_, Simpson;[375]
Indian Ocean, 400 fathoms.


ORDER IV. GORGONACEA.

This order contains a very large number of dendritic and usually flexible
corals occurring in nearly all seas and extending from shallow waters to
the very great depths of the ocean. A large proportion of them are brightly
coloured, and as the principal pigments are fixed in the spicules, and are
therefore preserved when the corals are dead and dried, they afford some of
the most attractive and graceful objects of a natural history museum.

The only character that separates them from the Alcyonacea is that they
possess a skeletal axis that is not perforated by the coelenteric cavities
of the zooids. The coelenteric cavities are usually short. The order may
conveniently be divided into two sub-orders.


SUB-ORDER 1. PSEUDAXONIA.

The axis in this sub-order consists of numerous spicules tightly packed
together, or cemented together by a substance which is probably allied to
horn in its chemical composition. This substance may be considerable in
amount, in which case it remains after decalcification as a spongy, porous
residue; or it may be so small in amount, as in _Corallium_, that the axis
appears to be composed of solid carbonate of lime. The statement is usually
made that the axis is penetrated by nutritive canals in certain genera, but
the evidence upon which this is based is unsatisfactory and in some cases
unfounded. There can be no doubt, however, that in some genera the axis is
porous and in others it is not, and this forms a useful character for the
separation of genera.

FAM. 1. BRIAREIDAE.—The medullary substance consists of closely packed but
separate spicules embedded in a soft horny matrix, which is uniform in
character throughout its course. Nearly all the genera form dendritic
colonies of considerable size.

The principal genera are:—_Solenocaulon_, Gray; Indian Ocean and North
Australia. Many of the specimens of this genus have fistulose stems and
branches. The tubular character of the stem and branches is probably caused
by the activity of a Crustacean, {351}_Alpheus_, and may be regarded as of
the nature of a gall-formation.[376] _Paragorgia_, M. Edwards; Norwegian
fjords, in deep water. This genus forms very large tree-like colonies of a
ruby-red or white colour. It is perhaps the largest of the dendritic
Alcyonarians. It is dimorphic. _Spongioderma_, Kölliker; Cape of Good Hope.
The surface of this form is always covered by an encrusting sponge.
_Iciligorgia_, Ridley; Torres Straits. The stem and branches are compressed
and irregular in section.

FAM. 2. SCLEROGORGIIDAE.—The medullary mass forms a distinct axis
consisting of closely packed elongate spicules with dense horny sheaths.

_Suberogorgia_, Gray, has a wide distribution in the Pacific Ocean, Indian
Ocean, and the West Indies. _Keroeides_, W. and S., comes from Japan.

FAM. 3. MELITODIDAE.—The axis in this family exhibits a series of nodes and
internodes (Fig. 155), the former consisting of pads formed of a horny
substance with embedded spicules, the latter of a calcareous substance with
only traces of a horny matrix. The internodes are quite rigid, the nodes
however give a certain degree of flexibility to the colony as a whole.
Neither the nodes nor the internodes are penetrated by nutritive canals,
but when dried the nodes are porous.

[Illustration: FIG. 155.—_Melitodes dichotoma_, showing the swollen nodes
and the internodes.]

The principal genera are:—_Melitodes_, Verrill; widely distributed in the
Indian and Pacific Oceans, Cape of Good Hope, etc. This genus is in some
localities extremely abundant and exhibits great brilliancy and variety of
colour. The branching is usually dichotomous at the nodes. _Wrightella_,
Gray. This is a delicate dwarf form from Mauritius and the coast of South
Africa. _Parisis_, Verrill; Pacific Ocean from Formosa to Australia but not
very common. One species from Mauritius. The branches arise from the
internodes.

{352}FAM. 4. CORALLIIDAE.—The axis is formed by the fusion of spicules into
a dense, solid, inflexible, calcareous core.

_Corallium_, Lamarck. _Corallium nobile_, Pallas, the "precious coral,"
occurs in the Mediterranean, chiefly off the coast of North Africa, but
also on the coasts of Italy, Corsica, Sardinia, and it extends to the Cape
Verde Islands in the Atlantic Ocean. _C. japonicum_, Kishinouye, called
Akasango by the fishermen, occurs off the coast of Japan, and _C. reginae_,
Hickson, has recently been described from deep water off the coast of
Timor.[377] The genus _Pleurocorallium_, Gray, is regarded by some authors
as distinct, but the characters that are supposed to distinguish it,
namely, the presence of peculiar "opera-glass-shaped spicules," and the
occurrence of the verrucae on one side of the branches only, are not very
satisfactory. The following species are therefore placed by Kishinouye[378]
in the genus _Corallium_:—_C. elatius_, Ridley (Momoirosango); _C.
konojoi_, Kishinouye (Shirosango); _C. boshuensis_, K.; _C. sulcatum_, K.;
_C. inutile_, K.; and _C. pusillum_, K.,—all from the coast of Japan. Of
the coral obtained from these species, the best kinds of Momoirosango vary
in price from £30 per pound downwards according to the quality. The
Shirosango is the least valuable of the kinds that are brought into the
market, and is rarely exported.[379] Three species of _Corallium_
(_Pleurocorallium_) have been described from Madeira,[380] and one of
these, _C. johnsoni_, has recently been found in 388 fathoms off the coast
of Ireland.[381] Other species are _C. stylasteroides_, from Mauritius; _C.
confusum_, Moroff,[382] from Sagami Bay in Japan; and an undescribed
species obtained by the "Siboga," off Djilolo. These corals range from
shallow water to depths of 300-500 fathoms. _Pleurocoralloides_, Moroff,
differs from the others in having very prominent verrucae and in the
character of the large spindle-shaped and scale-like spicules. It was found
in Sagami Bay, Japan. Specimens attributed to the genus _Pleurocorallium_
have been found fossil in the white chalk of France, but Corallium has been
found only in the tertiaries.[383]


{353}SUB-ORDER 2. AXIFERA.

The axis in this sub-order may be horny, or horny with a core of calcium
carbonate, or composed of horn impregnated with calcium carbonate, or of
nodes of horn alternating with internodes of calcium carbonate. It may be
distinguished from the axis of the Pseudaxonia by the fact that in no case
have definite spicules been observed to take part in its formation. It has
been suggested that as the Axifera represent a line of descent distinct
from that of the Pseudaxonia they should be placed in a separate order.
Apart from the character of the axis, however, the two sub-orders show so
many affinities in their general anatomy that it is better to regard the
two lines of descent as united within the Gorgonacean limit. It is very
improbable that the two groups sprang independently from a stoloniferous
ancestor.

FAM. 1. ISIDAE.—This family includes all those Axifera in which the axis is
composed of alternate nodes of horn and internodes of calcareous substance.

There can be little doubt of the close affinities of many of the genera of
this family with the Melitodidae among the Pseudaxonia. In both the
coenenchym is thin and the coelenteric cavities short. No important
differences have been observed between the structure of the zooids of the
two families, and now that we know that the "nutritive canals" of
_Melitodes_ do not perforate the nodes there is no important difference
left between the coenosarcal canal systems. The structure and method of
calcification of the internodes of the two families are very similar. The
main difference between them is that the nodes of the Isidae are purely
horny, whereas in the Melitodidae the horny substance of the nodes contains
calcareous spicules.

The principal genera are:—_Isis_, Linnaeus; Pacific Ocean. This genus forms
substantial fan-shaped colonies with, relatively, a thick coenenchym, short
stout internodes and black horny nodes. _Mopsea_, Lamouroux; Coast of
Australia. The verrucae are club-shaped and are arranged in spiral rows
round the stem. _Acanella_, Gray; principally found in deep water in the
Atlantic Ocean but also in the Pacific. The internodes are long and the
branches arise from the nodes. Most of the species occur in deep water,
some in very deep water (_A. simplex_, 1600 to 1700 fathoms). In this and
the following genera the coenenchym is {354}thin and the zooids imperfectly
or not retractile. _Ceratoisis_, Wright; Atlantic Ocean, extending from
shallow to deep water. The branches arise from the nodes. _Chelidonisis_,
Studer; deep water off the Azores. _Isidella_, Gray; Mediterranean Sea.
_Bathygorgia_, Wright; off Yokohama, 2300 fathoms. This genus is
unbranched, with very long internodes and short nodes. The zooids are
arranged on one side only of the stem.

FAM. 2. PRIMNOIDAE.—This is a well-marked family. The axis of the colonies
is horny and calcareous. The coenenchym and the non-retractile zooids are
protected by scale-like spicules, which usually overlap and form a complete
armour for the protection of the soft parts. On the aboral side of the base
of each tentacle there is a specialised scale, and these fit together, when
the tentacles are folded over the peristome, to form an operculum.

The principal genera are:—_Primnoa_, Lamouroux; Atlantic Ocean, occurring
also in the Norwegian fjords. This genus is usually found in moderately
deep water, 100 to 500 fathoms. _Primnoella_, Gray. This genus seems to be
confined to the temperate seas of the southern hemisphere. It is
unbranched. The zooids are arranged in whorls round the long whip-like
stem. _Plumarella_, Gray; southern hemisphere, in moderately deep water.
This is branched pinnately in one plane. The zooids are small and arise at
considerable intervals alternately on the sides of the branches.
_Stenella_, Gray; widely distributed in deep water. The zooids are large
and are arranged in whorls of three situated at considerable distances
apart. _Stachyodes_, W. and S.; Fiji, Kermadecs, Azores, in deep water.
Colony feebly branched. Zooids in regular whorls of five. Other genera
belonging to this group of Primnoidae are _Thouarella_, Gray, and
_Amphilaphis_, Antarctic seas.

The following genera are placed in separate sub-families:—_Callozostron_,
Wright; Antarctic Sea, 1670 fathoms. The axis is procumbent and the zooids
are thickly set in rows on its upper surface. The zooids are protected by
large imbricate scales, of which those of the last row are continued into
long spine-like processes. _Calyptrophora_, Gray; Pacific Ocean, in deep
water. The base of the zooids is protected by two remarkably large scales.
_Primnoides_, W. and S.; Southern Ocean. The opercular scales are not
distinctly differentiated and the calyx is therefore imperfectly protected.

{355}FAM. 3. CHRYSOGORGIIDAE.[384]—The axis in this family is composed of a
horny fibrous substance with interstratified calcareous particles, and it
springs from a calcareous plate, which sometimes gives off root-like
processes. It may be unbranched or branched in such a way that the branches
of the second, third, and subsequent orders assume in turn the direction of
the base of the main axis. The axis is frequently of a metallic iridescent
appearance. The zooids usually arise in a single straight or spiral row on
the branches, and are not retractile. The coenenchym is thin. The spicules
vary considerably, but in a very large proportion of the species they are
thin, oval, or hour-glass plates (Fig. 149, 10, p. 336).

By some authors this family is considered to be the simplest and most
primitive of the Axifera; but the delicate character of the axis of the
main stem and branches, the thinness of the coenenchym, the position of the
zooids on one side of the branches only, and the tenuity of the calcareous
spicules may be all accounted not as primitive characters, but as special
adaptations to the life in the slow uniform currents of deep water.

The principal genera are:—_Lepidogorgia_, Verrill; Atlantic and Pacific
Oceans, 300 to 1600 fathoms. Axis unbranched. Zooids large and arranged in
a single row. _Trichogorgia_, Hickson; Cape of Good Hope, 56 fathoms.
Colony branching in one plane. Zooids numerous and on all sides of the
branches. _Chrysogorgia_, D. and M.; deep water. Axis branched. Spicules on
the zooids always large. _Metallogorgia_, Versluys; Atlantic Ocean, 400 to
900 fathoms. Basal part of the stem unbranched (monopodial). _Iridogorgia_,
Verrill. Spiral stem and branches. _Pleurogorgia_, Versluys. Axis branched
in one plane. Coenenchym thick. _Riisea_, D. and M. Monopodial stem and
thick coenenchym.

FAM. 4. MURICEIDAE.—This is a large family, exhibiting very great variety
of habit. The spicules are often very spiny, and project beyond the surface
of the ectoderm, giving the colony a rough appearance. A great number of
genera have been described, but none of them are very well known. The
family requires careful revision.

The more important genera are:—_Acanthogorgia_, Gray; principally in deep
water in the Atlantic Ocean. The calices are {356}large, cylindrical, and
spiny. _Villogorgia_, D. and M.; widely distributed. Delicate, graceful
forms, with thin coenenchym. _Echinomuricea_, Verrill; _Muricea_,
Lamouroux; _Paramuricea_, Köll; _Acamptogorgia_, W. and S.; _Bebryce_,
Philippi.

FAM. 5. PLEXAURIDAE.—In this family we find some of the largest and most
substantial Gorgonids. The axis is usually black, but its horny substance
may be impregnated with lime, particularly at the base. The coenenchym is
thick, and the zooids are usually completely retractile, and the surface
smooth. The species of the family are principally found in shallow water in
warm or tropical regions.

The principal genera are:—_Eunicea_, Lamouroux. The calices are prominent,
and not retractile. _Plexaura_, Lamouroux; _Euplexaura_, Verrill.
_Eunicella_, Verrill. With an outer layer of peculiar torch-shaped
spicules. The only British species of this order is _Eunicella cavolini_
(formerly called _Gorgonia verrucosa_). It is found in depths of 10 to 20
fathoms off the coast of the English Channel and west of Scotland.
Occasionally specimens are found in which a gall-like malformation with a
circular aperture is seen, containing a Barnacle. Such gall formations,
common enough in some species of Madreporaria, are rarely found in
Alcyonaria.

[Illustration: FIG. 156.—_Eunicella cavolini._ Some branches of a large
dried specimen, showing a gall formed by a Cirripede.]

FAM. 6. GORGONIIDAE.—This family contains some of the commonest and
best-known genera of the order. They usually form large flexible branched
colonies with delicate horny axes and thin coenenchym. The zooids are
usually completely retractile.

The principal genera are:—_Gorgonia_, Linn.  This genus {357}includes
_Gorgonia_ (_Rhipidogorgia_) _flabellum_, the well-known fan Gorgonia with
intimately anastomosing branches, from the warm waters of the Atlantic
Ocean. The genera _Eugorgia_, Verrill, and _Leptogorgia_, Milne Edwards,
differ from _Gorgonia_ in the character of the spicules. In _Xiphigorgia_,
Milne Edwards, from the West Indies, the branches are much compressed,
forming at the edges wing-like ridges, which bear the zoopores in rows.
_Malacogorgia_, Hickson, has no spicules. Cape of Good Hope.

FAM. 7. GORGONELLIDAE.—In this family the horny axis is impregnated with
lime. The surface of the coenenchym is usually smooth, and the spicules
small. The colonies are sometimes unbranched (_Juncella_). In the branching
forms the axis of the terminal branches is often very fine and thread-like
in dimensions.

[Illustration: FIG. 157.—_Verrucella guadaloupensis_, with an epizoic
Brittle star (_Oph._) of similar colour.]

The principal genera are:—_Gorgonella_, with a ramified flabelliform axis;
_Ctenocella_, with a peculiar double-comb manner of branching; and
_Juncella_, which forms very long unbranched or slightly branched colonies,
with club-shaped spicules. All these genera are found in shallow water in
the tropical or semi-tropical regions of the world. _Verrucella_ is a genus
with delicate anastomosing branches found principally in the shallow
tropical waters of the Atlantic shores. Like many of the Gorgonacea, with
branches disposed in one plane (flabelliform) _Verrucella_ frequently
carries a considerable number of epizoic Brittle stars, which wind their
flexible arms round the branches, and thus obtain a firm attachment to
their host. There is no reason to suppose that these Brittle stars are in
any sense parasitic, as a specimen that bears many such forms shows no sign
of injury or degeneration, and it is possible they may even be of service
to {358}the _Verrucella_ by preying upon other organisms that might be
injurious. An interesting feature of the association is that the Brittle
stars are of the same colour as the host, and the knob-like plates on their
aboral surface have a close resemblance to the verrucae (Fig. 157).


ORDER V. PENNATULACEA.

The Sea-pens form a very distinct order of the Alcyonaria. They are the
only Alcyonarians that are not strictly sedentary in habit, that are
capable of independent movement as a whole, and exhibit a bilateral
symmetry of the colony. No genera have yet been discovered that can be
regarded as connecting links between the Pennatulacea and the other orders
of the Alcyonaria. Their position, therefore, is an isolated one, and their
relationships obscure.

The peculiarities of the order are due to the great growth and modification
in structure of the first formed zooid of the colony. This zooid (Oozooid,
Hauptpolyp, or Axial zooid) increases greatly in length, develops very
thick fleshy walls, usually loses its tentacles, digestive organs, and
frequently its mouth, exhibits profound modification of its system of
mesenteries, and in other ways becomes adapted to its function of
supporting the whole colony.

[Illustration: FIG. 158.—Diagram of a Sea-pen. _L_, leaves composed of a
row of autozooids; _R_, rachis; _St_, stalk; _T_, anthocodia of the axial
zooid, usually suppressed. (After Jungersen.)]

The axial zooid shows from an early stage of development a division into
two regions: a distal region which produces by gemmation on the body-wall
numerous secondary zooids, and becomes the rachis of the colony; and a
proximal region which becomes the stalk or peduncle, and does not produce
buds (Fig. 158). The secondary zooids are of two kinds: {359}the autozooids
and the siphonozooids. The former have the ordinary characters of an
Alcyonarian zooid, and produce sexual cells; the latter have no tentacles,
a reduced mesenteric system, and a stomodaeum provided with a very wide
siphonoglyph.

The arrangement of the autozooids and siphonozooids upon the axial zooid is
subject to great modifications, and affords the principal character for the
classification of the order. In the Pennatuleae the autozooids are arranged
in two bilaterally disposed rows on the rachis, forming the leaves or
pinnae of the colony (Fig. 158). The number in each leaf increases during
the growth of the colony by the addition of new zooids in regular
succession from the dorsal to the ventral side of the rachis[385] (Fig.
159). In other Pennatulacea the autozooids are arranged in rows which do
not unite to form leaves (_Funiculina_), in a tuft at the extremity of a
long peduncle (_Umbellula_), scattered on the dorsal side of the rachis
(_Renilla_, Fig. 160), or scattered on all sides of the rachis
(_Cavernularia_, Fig. 161). In those forms in which the autozooids are
scattered the bilateral symmetry of the colony as a whole becomes obscured.
The siphonozooids may be found on the leaves (_Pteroeides_), but more
frequently between the leaves or rows of autozooids, or scattered
irregularly among the autozooids. Usually the siphonozooids are of one kind
only, but in _Pennatula murrayi_ there is one specially modified
siphonozooid at the base of each leaf,[386] which appears to have some
special but unknown function.

[Illustration: FIG. 159.—Diagram of a portion of a rachis of a Sea-pen,
_aut_, The rows of autozooids; 1-6, the order of age of the autozooids
composing a leaf; _D_, the dorsal side of the rachis; _Si_, the
siphonozooids; _V_, the ventral side of the rachis. (After Jungersen.)]

In _Umbellula gracilis_ each siphonozooid bears a single pinnate tentacle,
and in some other species of the same genus there is a tentacle which is
not pinnate.[387]

{360}The zooids and coenenchym are usually protected by a crust of coloured
or colourless, long, smooth, needle-like, calcareous spicules, situated
principally in the superficial layer, so as to leave the subjacent tissues
soft and spongy in texture. In some cases the spicules are smooth double
clubs, rods, discs, or irregular granules, and in _Sarcophyllum_,
_Chunella_, some species of _Umbellula_ and others, there is no calcareous
skeleton. The tuberculated spindles, so common in other Alcyonaria, are not
found in any species. In most genera a horny, or calcified horny rod is
embedded in the central part of the axial polyp, serving as a backbone or
support for its muscles. It is absent, however, in _Renilla_, and reduced
or absent in _Cavernularia_.

The sexual organs are borne by the mesenteries of the autozooids only, and
each colony is either male or female. There is no record of hermaphroditism
in the order. The eggs contain a considerable amount of yolk, and
fertilisation is effected in the sea-water after their discharge. The
segmentation is irregular, and the free-swimming ciliated larva (of
_Renilla_) shows the rudiments of the first buds from the axial polyp
before it settles down in the mud.

The Sea-pens are usually found on muddy or sandy sea-bottoms, from a depth
of a few fathoms to the greatest depths of the ocean. It is generally
assumed that their normal position is one with the peduncle embedded in the
mud and the rachis erect. Positive evidence of this was given by Rumphius,
writing in 1741, in the case of _Virgularia rumphii_ and _V. juncea_ at
Amboina,[388] and by Darwin in the case of _Stylatula darwinii_ at Bahia
Blanca.[389]

"At low water," writes Darwin, "hundreds of these zoophytes might be seen
projecting like stubble, with the truncate end upwards, a few inches above
the surface of the muddy sand. When touched or pulled they suddenly drew
themselves in with force so as nearly or quite to disappear."

It is not known whether the Pennatulids have the power of moving from place
to place when the local conditions become unfavourable. It is quite
probable that they have this power, but the accounts given of the Sea-pens
lying flat on the sand do not appear to be founded on direct observation.
The fable of {361}_Pennatula_ swimming freely "with all its delicate
transparent polypi expanded, and emitting their usual brilliant
phosphorescent light, sailing through the still and dark abyss by the
regular and synchronous pulsations of the minute fringed arms of the whole
polypi," appears to be based on a statement made by Bohadsch in 1761, and
picturesque though it be, is undoubtedly erroneous.

The brilliant phosphorescence of many species of Pennatulacea has been
observed by many naturalists, and it is very probable that they all exhibit
this property to some degree. The phosphorescence appears to be emitted by
the mesenteric filaments of the autozooids, but it is not yet determined
whether the phenomenon is confined to these organs or is more generally
distributed.

The Pennatulacea are usually devoid of epizoites, but occasionally the
parasitic or semi-parasitic Entomostracan _Lamippe_ is found in the zooids.
A small crab is also frequently found between the large leaves of species
of _Pteroeides_. The most remarkable case of symbiosis, however, has
recently been observed in the form of an encrusting Gymnoblastic
Hydroid[390] living on the free edge of the leaves of a species of
_Ptilosarcus_.



The order Pennatulacea is divided into four sections.

SECT. 1. PENNATULEAE.—In this section the colony is distinctly bilaterally
symmetrical, and the autozooids are arranged in rows with their body-walls
fused to form leaves.

The genus _Pteroeides_, the representative genus of the family
PTEROEIDIDAE, is a fleshy Sea-pen found in shallow sea water in the warm
waters of the Pacific Ocean and in the Mediterranean. It has large leaves
with long spiny, projecting spicules, and the siphonozooids are borne by
the leaves. _Pennatula_, the representative genus of the family
PENNATULIDAE, has a wider distribution in area and in depth. _Pennatula
phosphorea_ is a common British species, found in depths of 10 to 20
fathoms in many localities off our coasts. It is about 5 inches in length.
There are several varieties of this species distributed in Atlantic waters.
_Pennatula grandis_ is a magnificent species found in Norwegian fjords, in
the Faeroe Channel, and off the northern coasts of N. America, in depths of
from 50 to 1255 fathoms. Specimens have been {362}obtained no less than 2½
feet in length. _P. murrayi_ and _P. naresi_ are species of the genus found
at depths of a few hundred fathoms in tropical seas.

The genus _Virgularia_, belonging to the family VIRGULARIIDAE, is
represented in the British seas by _V. mirabilis_, a long slender Sea-pen
found in many localities off the Scottish coasts.

SECT. 2. SPICATAE.—This section includes those Sea-pens in which the
autozooids are arranged bilaterally on the axial zooid in rows or more
irregularly, but do not unite to form leaves. It is a large section and
contains many widely divergent genera.

The family FUNICULINIDAE is represented on our coasts by _Funiculina
quadrangularis_, a long and slender Sea-pen 2 to 3 feet in length. The
autozooids are arranged in oblique rows, and the siphonozooids are on the
ventral side of the rachis. There is one point of special interest in this
genus. The siphonozooids appear to change as the colony grows and to become
autozooids. If this is the case it may be more correct to describe the
genus as devoid of true siphonozooids.

The family ANTHOPTILIDAE contains the species _Anthoptilum grandiflorum_,
which has a wide distribution in depths of 130 to 500 fathoms in the N. and
S. Atlantic Ocean. It is perhaps the largest of all the Pennatulacea,
specimens having been obtained from the Cape of Good Hope over 4 feet long
with expanded autozooids, each more than half an inch in length.

The family KOPHOBELEMNONIDAE contains a number of forms with remarkably
large autozooids arranged in irregular rows on the two sides of the rachis.
The siphonozooids are numerous and scattered, and their position is
indicated by small papilliform calices on the coenenchym. The surface of
these pens is usually rough, owing to the presence of numerous coarse
projecting spicules. _Kophobelemnon_ occurs in the Mediterranean in deep
water, off the coasts of Ireland and Scotland, and in other regions.

The family UMBELLULIDAE contains some of the most remarkable and
interesting examples of the deep-sea fauna. The peduncle is very long and
the rachis stunted and expanded. The autozooids are of great size,
non-retractile, and arranged in a cluster or rosette on the terminal
rachis. There is a wide structural range between the species. Some species
have numerous large spicules, others have none. In some species the
siphonozooids have a single pinnate or digitate tentacle, in others the
siphonozooids {363}are of the usual type. _Umbellula_ appears to be a
somewhat rare but cosmopolitan genus in deep water, extending from the
Arctic to the Antarctic region in water ranging from 200 to 2500 fathoms.

The interesting genus _Chunella_ was discovered by the German "Valdivia"
Expedition at a depth of about 420 fathoms off the coast of E. Africa, and
subsequently by the Dutch "Siboga" Expedition at a depth of about 500
fathoms in the Malay Archipelago. According to Kükenthal,[391] this genus
with another closely allied genus _Amphianthus_ should form a new section
of Pennatulacea, the VERTICILLADEAE. _Chunella_ has a long and very
delicate rachis and peduncle, and the former terminates in a single
autozooid and has five or six whorls of three autozooids, situated at
considerable distances from one another. Spicules are absent. The full
description of this genus has not yet been published, but it is clear that
it occupies a very isolated position in the order.

[Illustration: FIG. 160.—_Renilla reniformis_, a small specimen (34 mm.),
showing the dorsal side of the expanded rachis. _A_, autozooid; _H_, the
mouth of the axial zooid; _s_, siphonozooid; _St_, the short stalk. (After
Kölliker.)]

SECT. 3. RENILLEAE.—This section contains a single family RENILLIDAE and a
single genus _Renilla_ (Fig. 160). The rachis is expanded into a flattened
cordate form set at an angle to the peduncle, and the zooids are confined
to the dorsal surface, which is uppermost in the natural position of the
colony. The peduncle is short and does not contain an axial skeleton. The
colour of {364}this Sea-pen is usually violet when dried or preserved.
Specimens of _Renilla_ are very abundant in shallow water in some
localities on the Atlantic and Pacific coasts of N. America, but the genus
has also been obtained from the Red Sea and the coast of Australia. A
popular name for this genus is "Sea pansy."

SECT. 4. VERETILLEAE.—This section contains a number of genera in which the
bilateral arrangement of the zooids is obscured by their gradual
encroachment on the dorsal side of the axial polyp. The rachis and peduncle
are thick and fleshy, and the autozooids and siphonozooids are irregularly
distributed all round the rachis. The genus _Cavernularia_ is not
uncommonly found in moderate depths of water in the Indian and Pacific
Ocean, and is distinguished from the other genera by the reduction of the
skeletal axis. Other genera are _Veretillum_, Mediterranean and Atlantic
Ocean, and _Lituaria_, Indian Ocean.

[Illustration: FIG. 161.—_Cavernularia obesa._ _Au_, autozooid; _Si_,
siphonozooid; _St_, stalk. (After Kölliker.)]




{365}CHAPTER XIV

ANTHOZOA (_CONTINUED_): ZOANTHARIA


SUB-CLASS II. ZOANTHARIA.

The Zoantharia exhibit a great deal more diversity of form and structure
than the Alcyonaria. The sub-class is consequently difficult to define in a
few words, and it may be taken to include all the Anthozoa which do not
possess the typical Alcyonarian characters.

All the orders, with the exception of the Antipathidea and Zoanthidea,
contain genera of solitary zooids, and the orders Edwardsiidea and
Cerianthidea contain no genera that form colonies. In the Madreporaria,
Zoanthidea, and Antipathidea, on the other hand, colonies are formed
composed of a very large number of individuals which frequently attain to a
very great size. The term "Sea-anemone" is commonly used in writing about
the solitary Zoantharia which do not form any skeletal structures, and the
term "Coral" is applied to all those Zoantharia which do form a skeleton.

In a scientific treatise, however, these popular terms can no longer be
satisfactorily employed. The "Sea-anemones" exhibit so many important
differences in anatomical structure that they must be placed in at least
three distinct orders that are not closely related, and the organisms to
which the term Coral has been applied belong to so many organisms—such as
Alcyonaria, Hydrozoa, Polyzoa, and even Algae—that its use has become
indeterminate.

Whilst these terms must disappear from the systematic part of Zoology, they
may still be employed, however, in the description of a local fauna or
coral reef to signify the soft solitary zooids on {366}the one hand, and
the organisms, animals or plants, which form large, massive skeletons of
carbonate of lime, on the other.

The form of the solitary zooids and of the colony of zooids in the
Zoantharia, then, may be very divergent. In the Actiniaria we find single
soft gelatinous zooids of considerable size adherent to rocks or
half-buried in the sand. Among the Madreporaria we find great branching
colonies of thousands of zooids supported by the copious skeleton of
carbonate of lime that they have secreted. Among the Antipathidea, again,
we find a dendritic skeleton of a dark horny substance, formed by a colony
of small zooids that cover it like a thin bark. The majority of the
Zoantharia are, like other zoophytes, permanently fixed to the floor of the
ocean. Where the embryo settles, there must the adult or colony of adults
remain until death. Some of the common Sea-anemones can, however, glide
slowly over the surface on which they rest, and thus change their position
according to the conditions of their surroundings. Others (the Minyadidae)
float upside down in the sea, and are carried hither and thither by the
currents. Others, again (_Cerianthus_, _Edwardsia_, _Peachia_), burrow in
the sand or mud at the sea-bottom.

The structure of the zooid varies considerably, but in the following
characters differs from the zooid of the Alcyonaria. The TENTACLES are
usually simple finger-like processes, and when they bear secondary pinnae
these can readily be distinguished from the rows of secondary pinnules of
the Alcyonarian tentacle. The number of tentacles is very rarely eight
(young _Halcampa_), and in these cases they are not pinnate. The number of
tentacles may be six (many Antipathidea and some zooids of _Madrepora_),
twelve (_Madrepora_), some multiple of six, or an indefinite number. In the
Thalassianthidae and some other families of Actiniaria the tentacles are
plumose, but do not exhibit the regular pinnate form of the tentacles of
Alcyonaria.

[Illustration: FIG. 162.—Large (A) and small (B) plumose tentacles of
_Actinodendron plumosum_.  Large (C) and small (D) plumose tentacles of _A.
glomeratum_. (After Haddon.)]

As regards the number of MESENTERIES, the Zoantharia exhibit {367}very
great variety. It has been shown that there is frequently a stage in their
development during which there are only eight mesenteries. This stage is
usually called the _Edwardsia_ stage. These eight mesenteries are arranged
in bilateral pairs as follows:—One pair is attached to the body-wall and
reaches to the dorsal side of the stomodaeum, and is called the pair of
dorsal directives; a corresponding pair attached to the ventral side of the
stomodaeum is called the pair of ventral directives. The other two pairs
are the lateral mesenteries. To these four pairs are added, at the close of
the _Edwardsia_ stage, two additional pairs, making in all twelve
mesenteries (cf. Fig. 163).

These six primary pairs of mesenteries, conveniently called the
"protocnemes" by Duerden, may be traced in the development and recognised
in the adult of the majority of Zoantharia. But the number of the
mesenteries is usually increased in the later stages by the addition of
other mesenteries called the "metacnemes." The metacnemes differ from the
protocnemes in that they usually appear in unilateral pairs, that is to
say, in pairs of which both members arise on the same side of the
stomodaeum, and the number is very variable throughout the group. The space
enclosed by a pair of mesenteries is called an "entocoele," and the space
between two pairs of mesenteries is called an "ectocoele."

The twelve protocnemes are usually complete mesenteries, that is to say,
they extend the whole distance from the body-wall to the stomodaeum, while
the metacnemes may be complete or incomplete; in the latter case extending
only a part of the distance from the body-wall towards the stomodaeum.

We find, therefore, in making a general survey of the anatomy of the
Zoantharia that there is no general statement to be made, concerning the
number or arrangement of the mesenteries, which holds good for the whole or
even for a considerable portion of the genera.

The bands of retractor muscles are, as in the Alcyonaria, situated on one
face only of the mesenteries (except in the Antipathidea and Cerianthidea),
but an important character of the Zoantharia is that the muscle bands on
the ventral pair of directives are situated on the dorsal faces of these
mesenteries, and not on the ventral faces as they are in Alcyonaria.

In the Edwardsiidea there are only eight complete mesenteries, {368}but a
variable number of other rudimentary and incomplete mesenteries have
recently been discovered by Faurot.[392] In the Zoanthidea the mesenteries
are numerous, but the order is remarkable for the fact that the dorsal
directives are incomplete, and that, of the pairs of metacnemes that are
added, one mesentery becomes complete and the other remains incomplete. In
most of the genera of the Antipathidea there are only ten mesenteries, but
in _Leiopathes_ there are twelve, and as they bear no bands of retractor
muscles it is difficult to determine accurately their true relation to the
mesenteries of other Zoantharia.

[Illustration: FIG. 163.—Diagrams of transverse sections of 1, Alcyonarian;
2, _Edwardsia_; 3, _Cerianthus_; 4, _Zoanthus_; 5, _Favia_; 6, _Madrepora_.
_DD_, the dorsal directive mesenteries; _VD_, the ventral directives;
_I-VI_, the protocnemes in order of sequence.]

In the Cerianthidea the mesenteries are very numerous, and increase in
numbers by the addition of single mesenteries alternately right and left in
the ventral inter-mesenteric chamber throughout the life of the individual.
These mesenteries do not bear retractor muscles.

In the Actiniaria and Madreporaria, with the exception of the genera
_Madrepora_, _Porites_, and a few others, there are also very many
mesenteries. The two pairs of directives are usually present, but they may
not occur in those zooids that are produced {369}asexually by fission (see
p. 388). The metacnemes are frequently formed in regular cycles, and in
many genera appear to be constantly some multiple of six (Fig. 163, 5).

In _Madrepora_ and _Porites_[393] the two pairs of directives and two pairs
of lateral protocnemes are complete; the other two pairs of protocnemes
are, however, incomplete; and metacnemes are not developed (Fig. 163, 6).

The stomodaeum is usually a flattened tube extending some distance into the
coelenteric cavity and giving support to the inner edges of the complete
mesenteries; in many of the Madreporaria, however, it is oval or circular
in outline. In most of the Actiniaria there are deep grooves on the dorsal
and ventral sides of the stomodaeum, but in Zoanthidea the groove occurs on
the ventral side only and in the Cerianthidea on the dorsal side only. In
the Madreporaria these grooves do not occur or are relatively
inconspicuous.[394] In the Alcyonaria the siphonoglyph exhibits a very
marked differentiation of the epithelium (see Fig. 148, p. 334), and the
cilia it bears are very long and powerful. It has not been shown that the
grooves in the Zoantharia show similar modifications of structure, and they
are called by the writers on Zoantharia the sulci. There is no difference
in structure, and rarely any difference in size, between the dorsal sulcus
and the ventral sulcus in the Actiniaria, and the use of the
word—sulculus—for the former is not to be commended.

The mesenteries bear upon their free edges the mesenteric filaments. These
organs are usually more complicated in structure than the corresponding
organs of the Alcyonaria, and the dorsal pair of filaments is not
specialised for respiratory purposes as it is in that group.

In many genera the mesenteric filaments bear long, thread-like
processes—the "acontia"—armed with gland cells and nematocysts which can be
protruded from the mouth or pushed through special holes (the "cinclides")
in the body-wall.

The gonads in the Zoantharia are borne upon the sides of the mesenteries
and are usually in the form of long lobed ridges instead of being spherical
in form, and situated at the edges of the mesenteries as they are in the
Alcyonaria.

{370}Nearly all the zooids and even the colonies of the Zoantharia are
unisexual, but some species, such as _Manicina areolata_ (Wilson),
_Meandrina labyrinthica_ (Duerden), _Cerianthus membranaceus_, and others,
are hermaphrodite. Mr. J. S. Gardiner has recently given reasons for
believing that the genus _Flabellum_ is protandrous.

SKELETON.—The soft tissues of the Zoantharian zooids may be supported or
protected by hard skeletal structures of various kinds. In the Zoanthidea
and the Actiniaria there are many species that have no skeletal support at
all, and are quite naked. These seem to be sufficiently well protected from
the attacks of carnivorous animals by the numerous nematocysts of the
ectoderm, and perhaps in addition by a disagreeable flavour in their
tissues. Anemones do not seem to be eaten habitually by any fish, but cases
have been described of _Peachia hastata_ being found in the stomach of the
Cod, and of _Edwardsia_ in the stomach of the Flounder.[395] On the
Scottish coasts Anemones are occasionally used with success as a bait for
cod.[396] The body-wall of _Edwardsia_, however, is protected to a certain
extent by the secretion of a mucous coat in which grains of sand and mud
are embedded. Some Anemones, such as _Urticina_, _Peachia_, and others, lie
half-buried in the sand, and others form a cuticle, like that of
_Edwardsia_, to which foreign bodies are attached.

_Cerianthus_ is remarkable for constructing a long tube composed of a
felt-work of discharged nematocysts mixed with mud and mucus, into which it
retires for protection. In the Zoanthidea the body-wall is frequently
strengthened by numerous and relatively large grains of sand, which are
passed through the ectoderm to lie in the thick mesogloea.

In the Madreporaria a very elaborate skeleton of carbonate of lime is
formed. In the solitary forms it consists of a cup-shaped outer covering
for the base and column of the zooid called the "theca," of a series of
radial vertical walls or "septa" projecting into the intermesenteric
chambers carrying the endodermal lining of the coelenteric cavity with
them, and in some cases a pillar, the "columella," or a series of smaller
pillars, the "pali" projecting upwards from the centre of the base of the
{371}theca towards the stomodaeum. In the colonial forms the theca of the
individual zooids is continuous with a common colonial skeleton called the
"coenosteum." This is solid in the Imperforate corals, and it supports at
the surface only a thin lamina of canals and superficial ectoderm. In the
Perforate corals, however, the coenosteum envelopes and surrounds the
canals during its formation, and thereby remains perforated by a network of
fine channels. In the colonial Madreporaria the skeletal cups which support
and protect the zooids are called the "calices."

The skeleton of the Antipathidea is of a different nature. It is composed
of a horny substance allied to keratin. When it is old and thick, it
usually has a polished black appearance, and is commonly termed "black
coral." The surface of this kind of coral is ornamented with thorny or
spiny projections, but it is never perforated by calices or canal systems.
It forms a solid axis for the branches of the corals, and all the soft
parts of the zooids and coenosarc are superficial to it.

It was formerly considered that this type of coral, which shows no trace of
the shape and form of the living organisms that produce it, is of a
different character to the calcareous skeleton which exhibits calices,
septa, pores, and other evidence of the living organism, and it was called
a "sclerobase" to distinguish it from the "scleroderm" of the Madreporaria.

It is now known that both the sclerobasic skeleton and the sclerodermic
skeleton are products of the ectoderm, and consequently these expressions
are no longer in general use.

ASEXUAL REPRODUCTION in the Zoantharia may be effected by continuous or
discontinuous fission or gemmation.

In the Edwardsiidea, Actiniaria, and Cerianthidea, that is to say in the
animals popularly known as Sea-anemones, asexual reproduction does not
commonly occur, but nevertheless a good many instances of it are now known
in individual genera. In _Actinoloba_ (_Metridium_), for example, Parker
has described a case of complete longitudinal fission, and Duerden states
that it occurs in the West Indian Anemones _Actinotryx_ and _Ricordea_. A
still more remarkable form of asexual reproduction known as transverse
fission has been described in the genus _Gonactinia_.[397] In this case,
the body of the Anemone becomes constricted in {372}the middle, a circlet
of tentacles is formed below the constriction, and division takes place.
The upper half floats away with the original tentacles and stomodaeum and
becomes attached by the base in another place; the lower half remains
behind and develops a new stomodaeum, mesenteric filaments, and sexual
organs. In some of the Actiniaria another form of asexual reproduction
occurs, known as "Pedal laceration." In the common British _Actinoloba_,
for example, so often kept in aquaria, the pedal disc sometimes spreads on
the glass or rock upon which the animal rests, in the form of a thin
membrane or film of an irregular circular shape, nearly twice the diameter
of the column. As the Anemone glides along, the film remains behind and
breaks up into a number of hemispherical droplets, which in a few days
develop tentacles, a mouth, mesenteries, and the other organs of a complete
and independent Anemone. A similar method of reproduction has been observed
in several species of _Sagartia_. A true process of discontinuous gemmation
has also been observed in _Gonactinia_, in _Corynactis_, and in
_Actinoloba_.

[Illustration: FIG. 164.—Longitudinal fission of _Actinoloba_. (After
Agassiz and Parker.)]

In the Madreporaria, Zoanthidea and Antipathidea, the usual method of
reproduction to form the colonies is continuous gemmation. The new zooids
that are added to the colony as it grows arise as buds, either from the
superficial canals of the coenenchym, or from the base or body-wall of the
older zooids. In these cases the young zooids acquire the same number of
mesenteries, and the same characters of the stomodaeum as the original
parent. Some further particulars of asexual reproduction in the
Madreporaria are given on p. 387.

The SEXUAL REPRODUCTION of a great many species of Zoantharia has now been
observed. The eggs are, as a general rule, ripened in batches, and
fertilisation is effected before their discharge from the body. In some
cases the sexual condition is seasonal. In temperate climates the
generative organs ripen in the spring and {373}summer months, and remain
small and relatively inconspicuous in the colder weather; but British
Sea-anemones, when kept in an aquarium and regularly fed, will breed nearly
all the year round. The corals of the tropics living in warmer water of a
more regular temperature show considerable variety in their breeding
habits. Thus Duerden found that colonies of _Favia_, _Manicina_,
_Siderastraea_ and _Porites_ are fertile at nearly all times, whereas
colonies of _Madrepora_, _Orbicella_ and _Cladocora_ were rarely so. In
nearly all cases the fertilisation is effected, and segmentation of the
ovum occurs within the body of the parent, the young Zoantharian beginning
its independent life as an oval or pear-shaped ciliated larva.

There are a great many cases among the Actiniaria in which the embryos are
retained within the coelenteron, or in special brood pouches of the parent
(p. 379), until a stage is reached with twelve or more tentacles.

The oval or pear-shaped larva swims about for a few days or hours, and then
settles down on its aboral end. In swimming, the aboral end is always
turned forwards. In the larva of _Lebrunia coralligens_ and _Rhodactis
sancti-thomae_, a distinct sense organ has been observed upon the aboral
extremity, and a similar but less distinct organ on the larva of _Actinia
equina_. These organs are of considerable interest, as they are probably
the only specialised sense organs known to occur in the Zoantharia.

The larvae of Zoantharia present, as a rule, very little variation from the
type described, and live but a short time if they fail to find a suitable
place for fixation. The colour is usually white and opaque, but in some
species the endoderm may be coloured yellow by Zooxanthellae (cf. pp. 86,
125).

The larvae of the Cerianthidea, however, are remarkable and exceptional.
After the larva of these animals has passed through the gastrula stage, a
certain number of mesenteries and tentacles are formed, and it rises in the
water to live a pelagic life of some duration. This larva is known as
_Arachnactis_, and is not unfrequently found in the plankton.

The character of the FOOD of the Zoantharia varies with the size of the
zooids, the occurrence of Zooxanthellae in the endoderm, and local
circumstances; but in general it may be said to consist mainly of small
living animals.

{374}Sea-anemones kept in an aquarium will readily seize and devour pieces
of raw beef or fragments of mussel that are offered to them; but they may
also be observed to kill and swallow the small Crustacea that occur in the
water. When a living animal of a relatively small size comes within range
of the tentacles, it appears to be suddenly paralysed by the action of the
nematocysts and held fast. The tentacles in contact with it, and others in
the neighbourhood but to a lesser extent, then bend inwards, carrying the
prey to the mouth. The passage of the food through the stomodaeum is
effected partly by ciliary, and partly by muscular action, and the food is
then brought to the region of the mesenteric filaments where it is rapidly
disintegrated by the digestive fluids they secrete. Any unsavoury or
undigested portions of the food are ejected by the mouth.

Very little is known concerning the food of the Madreporarian Corals. Many
investigators have noticed that the zooids of preserved specimens very
rarely contain any fragments of animal or plant bodies that could possibly
be regarded as evidence of food. It is possible that many Corals derive a
part, perhaps in some cases a considerable part, of their nourishment from
the symbiotic Zooxanthellae (pp. 86, 125) which flourish in the endoderm;
but it is improbable that in any case this forms the only source of food
supply. The absence of food material in the cavities of the zooids may
perhaps be accounted for by the fact that nearly all the Corals are fully
expanded, and therefore capable of catching their food only at night.
Corals are usually collected during the daytime, and therefore during the
period of rest of the digestive organs.

It is true that nearly all Corals do exhibit Zooxanthellae in their
endoderm, but there are some species from which they are nearly or wholly
absent, such as _Astrangia solitaria_ and _Phyllangia americana_ on the
West Indian reefs,[398] and the Pocilloporidae. The absence of any signs of
degeneration in the tentacles or digestive organs of those corals with
Zooxanthellae as compared with those without them suggests, at any rate,
that the Zooxanthellae do not supply such a large proportion of the food
necessary for the support of the colonies as to warrant any relaxation of
the efforts to obtain food by other means. Mr. Duerden found that when
living Annelids are placed upon the {375}tentacles of a living
_Siderastraea_—a genus with Zooxanthellae, the tentacles at once close upon
them and prevent their escape. The general conclusion seems to be,
therefore, that the Madreporarian Corals feed upon small animals in much
the same way as the Sea-anemones, whether they have Zooxanthellae or not,
but that in general they feed only at night.

AGE.—It is known that Sea-anemones kept in an aquarium and regularly fed
will live for a considerable number of years without showing signs of
weakness or failing health. Dalyell kept in an aquarium a specimen of
_Actinia mesembryanthemum_, which lived for sixty-six years and then died a
natural death; and specimens of _Sagartia_, still living, are known to be
about fifty years old.[399] The unnatural conditions of life in an aquarium
may have favoured the longevity of these specimens, and it would not be
reasonable to conclude from these records that the average life of a
full-grown Anemone on the rocks is more than thirty or thirty-five years,
and perhaps it is a good deal less.

As regards the Madreporarian Corals, we know but little concerning their
duration of life. An examination of any living coral reef is sufficient to
convince an observer that the power of asexual reproduction of the colonial
forms is not unlimited; that colonies, like individuals, have a definite
span of life, and that they grow old, senile, and then die a natural death
if spared in their youth from accident and disease. Mr. Gardiner has
calculated that the duration of life in solitary Corals like _Flabellum_ is
about twenty-four years, in colonial forms such as _Goniastraea_,
_Prionastraea_, _Orbicella_, and _Pocillopora_, from twenty-two to
twenty-eight years.


ORDER I. EDWARDSIIDEA.

This order contains only a few genera and species of small size living in
shallow water in various parts of the world. In external features they
closely resemble several genera of the Actiniaria, particularly those
belonging to the family Halcampidae. The distinguishing character of the
order is to be found in the system of mesenteries. In all the species only
eight mesenteries are complete, namely, the first two pairs of protocnemes,
and the two pairs of directives (Fig. 163, 2), {376}and these usually
support such large and powerful muscle-bands that they appear to be the
only mesenteries present. A careful examination of transverse sections,
however, reveals the fact that other mesenteries are present. The fifth and
sixth pairs of protocnemes seem to be invariably represented, and two or
three pairs of metacnemes can also be traced in some species.

The tentacles are variable in number. In _Edwardsia beautempsii_, for
example, they may be 14-16 in number, arranged in a single row round the
oral disc. In _E. timida_ they vary from 20 to 24. The normal number
appears to be eight tentacles of the first cycle, corresponding to the
eight primary inter-mesenteric chambers, _plus_ 6 or 12 tentacles,
corresponding with the chambers limited by the more rudimentary
mesenteries,—making a total of 14 or 20 tentacles; but by the suppression
of the two primary dorso-lateral tentacles, or by the addition of tentacles
of another cycle, the actual number is found to vary considerably.   The
Edwardsiidea are not fixed to the bottom, but are usually found deeply
embedded in sand, the aboral extremity being pointed and used for burrowing
purposes. The general colour of the body is yellow or yellowish brown, but
it is partly hidden by a short jacket of mud or sand and mucous secretion.
The oral crown frequently shows beautiful colours. De Quatrefages relates
that in _Edwardsia beautempsii_ the oral cone is golden yellow, and the
tentacles, transparent for the greater part of their extent, terminate in
opaque points of a beautiful yellowish red colour.

[Illustration: FIG. 165.—_Edwardsia beautempsii._ Nat. size. (After de
Quatrefages.)]

{377}FAM. 1. EDWARDSIIDAE.—Several species of this family have been found
in the British area. They are very local in their distribution, but
sometimes occur in great numbers.

_Edwardsia beautempsii_ occurs in shallow water near the shores of the
English Channel and has been found in Bantry Bay; and _E. carnea_ and _E.
timida_ have also been found in the Channel. _E. tecta_ is a recently
described species from the S. Irish coast, and _E. allmani_ and _E.
goodsiri_ are found in Scottish waters.

FAM. 2. PROTANTHEIDAE.—This family, constituted for the reception of three
remarkable genera, is now usually included in the order Edwardsiidea on the
ground that not more than eight mesenteries are complete.

The genus _Gonactinia_ exhibits the very exceptional character of having a
thick layer of muscles in the body-wall (cf. Cerianthidea, p. 409), and it
is also remarkable for the frequency with which it reproduces itself
asexually by longitudinal and, more rarely, by transverse fission. It has
been found in Norway, the Mediterranean, and on the reefs of New Caledonia.
The other genera of the family are _Oractis_ from California, and
_Protanthea_ from the coast of Sweden.


ORDER II. ACTINIARIA.

This order contains nearly all the animals popularly known as Sea-anemones.
They are usually found in shallow water, attached by a broad basal disc to
shells, stones, or sea-weeds. In the Halcampidae, however, the aboral
extremity ends in a blunt point as in the Cerianthidea and Edwardsiidea,
and the animals live half-buried in sand or mud. The Minyadidae of the
southern oceans are pelagic in habit, floating near the surface of the sea
with the mouth turned downwards. They are supported in the water by a
bladder, formed by an involution of the pedal disc, and filled with gas.

Many of the Sea-anemones are found in symbiotic association with other
animals. The common _Adamsia_ of the British coasts is found on whelk
shells containing hermit crabs. The crab is probably protected from the
attacks of some of its enemies by the presence of the Anemone, which in its
turn has the advantage of securing some fragments of the food captured and
torn to {378}pieces by the crab. The association, therefore, seems to be
one of mutual advantage to the messmates. It is a noteworthy fact that in
these associations the species of Sea-anemone associated with a particular
hermit crab is nearly always constant. Thus in the English Channel,
_Adamsia palliata_ is almost invariably found associated with _Eupagurus
prideauxii_, and _Adamsia rondeletii_ with _Eupagurus bernhardus_. But,
perhaps, the most remarkable association of this kind is to be seen in the
case of the little shore crab of the Indian Ocean, _Melia tesselata_, which
invariably holds in each of its large claws a small Sea-anemone. Möbius,
who originally described this case, relates that when the crab is robbed of
its Anemone it appears to be greatly agitated, and hunts about on the sand
in the endeavour to find it again, and will even collect the pieces, if the
Anemone is cut up, and arrange them in its claw.[400]

Another very interesting association is that of certain fish and Crustacea
with the large Sea-anemones of the tropical Australian coast.[401] Thus
_Stoichactis kenti_ almost invariably contains two or more specimens of the
Percoid fish _Amphiprion percula_. This fish is remarkable for its
brilliant colour, three pearly white cross-bands interrupt a ground plan of
bright orange-vermilion, and the ends of the cross-bands as well as the
fins are bordered with black. In another species a prawn of similar
striking colours is found. These companions of the giant Anemones swim
about among the tentacles unharmed, and when disturbed seek refuge in the
mouth. It has been suggested that these bright and attractive animals serve
as a lure or bait for other animals, which are enticed into striking
distance of the stinging threads of the Anemone, but how the commensals
escape the fate of the animals they attract has yet to be explained.

In a considerable number of Sea-anemones, such as _Actinoloba marginata_
and _A. dianthus_, some species of _Sagartia_, _Actinia cari_, _Anemonia
sulcata_, and _Calliactis parasitica_, the fertilisation of the eggs and
their subsequent development take place in the sea water.[402] In a great
many others, such as _Bunodes_ (several species), _Cereactis aurantiaca_,
_Sagartia troglodytes_, _Bunodactis {379}gemmacea_, etc., the embryos are
discharged into the water from the body-cavity of the parent, at a stage
with six or twelve tentacles. In the Arctic species of the genera
_Urticina_ and _Actinostola_, however, the embryos are retained within the
body of the parent until several cycles of tentacles are developed, and in
_Urticina crassicornis_ the young have been found with the full number of
tentacles already formed. In _Epiactis prolifera_ from Puget Sound, the
young Anemones attach themselves to the body-wall of the parent after their
discharge, and in _Epiactis marsupialis_, _Pseudophellia arctica_,
_Epigonactis fecunda_, and other species from cold waters, the young are
found in numerous brood sacs opening in rows on the body-wall. It is not
known for certain how these embryos enter the brood sacs, but it is
possible that each sac is formed independently for a young embryo that has
settled down from the outside upon the body-wall of the parent. The most
specialised example of this kind of parental care in the Sea-anemones is
seen in _Marsupifer valdiviae_ from Kerguelen, in which there are only six
brood sacs, but each one contains a great many (50-100) embryos.

The wonderful colours of our British Sea-anemones are familiar to most
persons who have visited the sea-side. The common _Actinia
mesembryanthemum_ of rock pools, for example, is of a purple red colour.
The base is usually green with an azure line. Around the margin of the disc
there are some twenty-five turquoise blue tubercles. On each side of the
mouth there is a small purple spot, and the numerous tentacles forming a
circlet round the mouth are of a pale roseate colour. Nothing could be more
beautiful than the snowy-white _Actinoloba dianthus_ or the variegated
_Urticina crassicornis_.

Similar wonderful variety and beauty of colour are seen in the Sea-anemones
of other parts of the world. Thus Saville Kent[403] in describing a species
of the gigantic _Stoichactis_ of the Australian Barrier Reef says, "the
spheroidal bead-like tentacles occur in irregularly mixed patches of grey,
white, lilac, and emerald green, the disc being shaded with tints of grey,
while the oral orifice is bordered with bright yellow."

The order Actiniaria contains a large number of families, presenting a
great variety of external form and of detail in general anatomy. The
definitions of the families and their {380}arrangement in larger groups
have presented many difficulties, and have led to considerable differences
of opinion; and even now, although our anatomical knowledge has been
greatly extended, the classification cannot be regarded as resting on a
very firm basis. The families may be grouped into two sub-orders:—

SUB-ORDER 1. ACTINIINA.—The tentacles are simple and similar, and there is
one tentacle corresponding to each intermesenteric chamber (endocoel).

SUB-ORDER 2. STICHODACTYLINA.—The tentacles are simple and similar, or
provided with teat-like or ramified pinnules. One or more tentacles may
correspond with an endocoel, and there may be two kinds of tentacles
(marginal and accessory) in the same genus.


SUB-ORDER 1. ACTINIINA.

FAM. 1. HALCAMPIDAE.—This family is clearly most closely related to the
Edwardsiidea. There are, however, twelve complete mesenteries of the first
cycle, and a second cycle of more or less incomplete mesenteries. The
tentacles are usually twelve in number, but may be twenty or twenty-four.
There is no pedal disc, but the base is swollen and rounded or pointed at
the end.

The genus _Halcampa_ includes a considerable number of small species
occurring in the shallow waters of the temperate northern hemisphere, and
of the Kerguelen Islands in the south. Three British species have been
described, of which _Halcampa chrysanthellum_ alone is common. The larva
with eight tentacles and eight mesenteries has been found living on the
Medusa _Thaumantias_.

_Peachia_ is a genus containing Anemones of much larger size (10-25 cm.).
It is remarkable for the very large siphonoglyph on the ventral side of the
stomodaeum, prolonged into a papillate lip projecting from the mouth called
the "conchula." The genera _Scytophorus_ from 150 fathoms off Kerguelen and
_Gyractis_ from Ceylon, although showing some remarkable peculiarities of
their mesenteric system, appear to be closely related to this family.

_Ilyanthus mitchellii_ is a large Anemone with a vesicular base,
forty-eight tentacles and mesenteries, occurring in the English Channel,
but it is not very common. It is usually {381}placed in a separate family,
but is in many respects intermediate in character between the Halcampidae
and the Actiniidae.

FAM. 2. ACTINIIDAE.—This family contains some of the commonest British
Sea-anemones. There is a large flat pedal disc by which the body is
attached to stones and rocks. The body-wall is usually smooth, and not
perforated by cinclides. The edge of the disc is usually provided with
coloured marginal tubercles. There are no acontia.

_Actinia._—This genus contains the widely distributed and very variable
species _Actinia mesembryanthemum_, one of the commonest of the
Sea-anemones found in rock pools on the British coast. The colours of this
species are often very beautiful (see p. 379) but variable.

_Anemonia_ is a genus with remarkably long tentacles which are not
completely retractile. _A. sulcata_ (sometimes called _Anthea cereus_) is
very common in the rock pools of our southern coasts.

_Bolocera tuediae_ is, next to _Actinoloba dianthus_, the largest of the
British Anemones. It has very much the same colour as the common varieties
of _Actinia mesembryanthemum_, but the body-wall is studded with minute,
rounded warts. It is found between tide marks in the Clyde sea-area, but
usually occurs in deeper water.

FAM. 3. SAGARTIIDAE.—This family includes several genera with a contractile
pedal disc, with the body-wall usually perforated by cinclides, and
provided with acontia.

The genera may be arranged in several sub-families distinguished by
well-marked characters. Among the well-known Sea-anemones included in the
family may be mentioned:—

_Sagartia troglodytes_, a very common British species found in hollows in
rocks. It is usually of an olive green or olive brown colour, and the upper
third or two-thirds of the body-wall is beset with numerous pale suckers.
_Adamsia palliata_ has a white body-wall spotted with bright red patches,
and is associated with the hermit crab _Eupagurus prideauxii_.

_Actinoloba_ (frequently called _Metridium_) _dianthus_ is considered the
handsomest of all the British Sea-anemones. It has a lobed disc frilled
with numerous small tentacles, and is uniformly coloured, creamy-white,
yellow, pale pink, or olive brown. It lives well in captivity, and
sometimes reaches a length of 6 inches with a diameter of 3 inches (Fig.
164).

{382}_Aiptasia couchii_ is a trumpet-shaped Anemone, found under stones at
low-water mark in Cornwall and the Channel Islands, with relatively slight
power of retraction.

_Gephyra dohrnii_ is an interesting species with twelve tentacles, which
was supposed at one time to form a connecting link between the Actiniaria
and the Antipathidea. It is found attached to the stems and branches of
various Hydrozoa and Alcyonaria, sometimes in such numbers and so closely
set that it gives the impression of having formed the substance of its
support. Haddon[404] has described specimens found on the stems of
_Tubularia_ from deep water off the south and south-west coasts of Ireland.
It also occurs in the Mediterranean and the Bay of Biscay.

FAM. 4. ALICIIDAE.—The members of this family have a large flat contractile
base and simple tentacles. The body-wall is provided with numerous simple
or compound outgrowths or vesicles, usually arranged in vertical rows.
_Alicia mirabilis_ is a rare Anemone from Madeira with a very broad base,
capable of changing its position with considerable activity, and of
becoming free and floating upside down at the surface of the sea. Other
genera of the family are _Bunodeopsis_ and _Cystiactis_. The genus
_Thaumactis_, described by Fowler,[405] from the Papeete reefs, has many
peculiarities, but is probably capable of crawling rapidly and of floating
at the surface like other members of the family. The remarkable Anemone
_Lebrunia_ from the West Indies may be included in this family.

FAM. 5. PHYLLACTIDAE.—These are distinguished by the presence of a broad
collar of foliaceous or digitate processes outside the circle of tentacles.
The processes have some resemblance to the foliaceous tentacles of the
Stichodactylinae. They are found in the Mediterranean, Red Sea, and on the
shores of the Atlantic Ocean, but have not yet been found in the British
area.

FAM. 6. BUNODIDAE.—This family is characterised by prominent verrucae and
tubercles of the body-wall. It contains several British species, of which
_Bunodes gemmacea_ found between tide marks on our southern shores is
fairly common. The very common British species _Urticina_ (_Tealia_)
_crassicornis_ is usually placed in this family, but exhibits some
peculiarities which seem {383}to warrant its removal to another division of
the Actiniaria. It is found in tide pools attached to rocks, but is usually
partially hidden by adherent sand or small stones.

FAM. 7. MINYADIDAE.—This family contains a number of floating Anemones. The
basal disc is folded over to form a gas bladder lined by a cuticular
secretion. The species are principally found in the seas of the southern
hemisphere.


SUB-ORDER 2. STICHODACTYLINA.

FAM. 1. CORALLIMORPHIDAE.—In this family the marginal cycle of tentacles
and accessory tentacles are all of the same kind. The accessory tentacles
are arranged in radial rows. All the tentacles are knobbed at the
extremity. The musculature is weak. _Capnea sanguinea_, _Corynactis
viridis_, and _Aureliania heterocera_ belong to the British fauna. They are
all small Anemones of exquisite colours, but are not very common. The genus
_Corallimorphus_ is principally found in the southern hemisphere.

FAM. 2. DISCOSOMATIDAE.—The tentacles are all of one kind and are very
numerous. The mesenteries are also very numerous. The sphincter muscle is
strong.

This family includes a rather heterogeneous assembly of forms, and will
probably require some rearrangement as our knowledge increases. Nearly all
the species are found in the shallow waters of the tropics, and among them
are to be found some of the largest Anemones of the world. _Stoichactis
kenti_, from the Barrier Reef, is from one to four feet in diameter across
the disc. In the West Indies these Anemones do not attain to such a great
size, but _Homostichanthus anemone_ from Jamaica is sometimes 8 inches in
diameter.

FAM. 3. RHODACTIDAE.—In this family the body-wall is smooth and the oral
disc greatly expanded. The tentacles are of two kinds. On the margin there
is a single cycle of minute tentacles, while on the disc there are numerous
tuberculate or lobed tentacles. Many of the species of this family are
quite small, but _Actinotryx mussoides_ from Thursday Island has an oral
disc 8 inches in diameter. The genera and species are widely distributed in
the warm, shallow waters of the world.

FAM. 4. THALASSIANTHIDAE.—The tentacles are simple or {384}ramified (Fig.
166), and in some cases very long (_Actinodendron arboreum_). Many of the
specimens of _A. plumosum_ and _Megalactis griffithsi_ are of very large
size, 8 to 12 inches in diameter. Of the former of these two species
Saville Kent remarks: "The colours are lacking in brilliancy, being chiefly
represented by varying shades of light brown and white, which are probably
conducive to its advantage by assimilating it to the tint of its sandy bed.
When fully extended the compound tentacles are elevated to a height of 8 or
10 inches, and bear a remarkable resemblance to certain of the delicately
branching, light brown sea-weeds that abound in its vicinity." The same
author calls attention to their stinging, which is "nearly as powerful as
the ordinary stinging nettle."

[Illustration: FIG. 166.—_Actinodendron plumosum._ _D_, disc of attachment;
_Si_, siphonoglyph; _t_, _t_, lobes of the marginal disc bearing the
tentacles; _W_, body-wall. Height of the column 200 mm. (After Haddon.)]


ORDER III. MADREPORARIA.

The Madreporaria form a heterogeneous group of Zoantharia characterised by
a single common feature, the formation of an extensive skeletal support of
carbonate of lime. In a great many cases the skeleton exhibits cups or
"calices" into which the zooids may be completely or partially retracted,
and these calices usually exhibit a series of radially disposed vertical
laminae, the "septa," corresponding with the inter-mesenteric spaces of the
zooids. Calices and structures simulating septa also occur in _Heliopora_,
which is an Alcyonarian, and in certain fossil corals which are probably
not Zoantharians. The anatomy of the zooids of a great many Madreporaria is
now known, and, {385}although a great deal of work yet remains to be done,
it may be said that the Madreporaria exhibit close affinities in structure
with the Actiniaria. The chief points in the anatomy of the zooids are
described under the different sub-divisions, but a few words are necessary
in this section to explain the principal features exhibited by the
skeleton.

There is no more difficult task than the attempt to explain upon any one
simple plan the various peculiarities of the Madreporarian skeleton.[406]
The authorities upon the group are not agreed upon the use of the terms
employed, nor are the current theories of the evolution of the skeleton
consistent. It is necessary, however, to explain the sense in which certain
terms are employed in the systematic part that follows, and in doing so to
indicate a possible line of evolution of the more complicated compound
skeletons from the simple ones.

[Illustration: FIG. 167.—Series of diagrams to illustrate the structure of
the Madreporarian skeleton. A, young stage of a solitary coral with simple
protheca (_p.t_). B, solitary coral, with theca (_th_), epitheca (_e.t_),
and prototheca (_p.t_). C, young stage of colonial coral, showing
coenosteum (_coe_) and theca (_th_), and the formation of the theca of a
bud (_b_). D, two zooids of a more advanced stage of a colonial coral.
_coe_, Coenosteum; _th_, theca. The black horizontal partitions are the
tabulae. E, transverse section of a calyx. _c_, Costa; _col_, columella;
_d_, dissepiment; _g_, septum; _p_, pali.]

There can be no doubt whatever that the whole of the skeleton of these
animals is formed by the ectoderm, and is external to their bodies. If we
could get rid of the influence of tradition upon our use of popular
expressions we should call this skeleton a shell. There can be little
doubt, moreover, that this skeleton is formed by a single layer of
specialised ectoderm cells called the "calicoblasts."

{386}The calicoblasts form, in the first instance, a skeletal plate at the
aboral end of the coral embryo, which becomes turned up at the edges to
form a shallow saucer or cup. This cup is called the "prototheca."[407] At
this stage the body-wall of the living zooid may or may not overflow the
edge of the prototheca. In the former case the growth of the rim of the
prototheca is brought about by the calicoblasts of an inner and outer layer
of epiblast, and the cup is then called the "theca." In the latter case,
the growth of the rim of the prototheca is continued by the calicoblasts of
one layer of epiblast only, and it is called the "epitheca" (_Flabellum_).
With the continued growth of the theca the tissues that have overflowed—the
"episarc"—retreat from the base, and in doing so the ectoderm of the edge
and, to some extent, the outer side of the episarc secrete a layer of
epitheca which becomes more or less adherent to the theca. Thus the cup may
have a double wall, the theca and the epitheca (_Caryophyllia_).

[Illustration: FIG. 168.—Diagram of a vertical section of a young
_Caryophyllia_, showing the septa (_S_) covered with endoderm projecting
into the coelenteric cavity. _M_, mouth; _St_, stomodaeum. (After G. von
Koch.)]

[Illustration: FIG. 169.—A young _Caryophyllia_, viewed from above, showing
the tentacles (_t_) and the stomodaeum (_St_). The letter _m_ points to a
space between a pair of mesenteries, and the darker shading in this place
shows a septum projecting radially from the wall of the theca. (After G.
von Koch.)]

With the growth of the theca and epitheca a certain number of radially
disposed laminae of lime rise from the walls and grow centripetally. These
are the "septa." Additional ridges on {387}the inner wall of the cup
between the septa are called the "dissepiments." Corresponding with the
septa there may be a circle of columns or bands rising from the basal parts
of the prototheca—the "pali"; and from the actual centre a single column
called the "columella." The longitudinal ridges on the outside of the
theca, corresponding in position with the septa inside, are called the
"costae" (Fig. 167, E, _c_).

We may imagine that in the primitive forms that gave rise to colonies, the
episarc of the primary zooid overflowed on to the substance to which it was
attached, and gave rise to successive layers of epithecal skeleton, which
may be called the "coenosteum." The ectoderm at the base of the original
prototheca is in some corals periodically dragged away from the skeleton,
and forms another cup or platform of lime at a little distance from it—the
"tabula." New zooids are developed at some distance from the primary one by
a process of gemmation in the episarc, and independent thecae, septa, etc.,
are formed in it; the skeleton of the new zooid thus originated being
connected with that of the primary zooid by the coenosteum.

There are many modifications of this simple description of skeleton
formation to be considered before a thorough knowledge of coral structure
can be understood, but sufficient has been said to explain the use of the
terms that it is necessary to employ in the description of the families.
When it is necessary to speak of the cup in which the zooid is situated
without expressing an opinion as to the homology of its wall, it is called
the calyx.

There are many forms of asexual reproduction observed in the Madreporaria.
Of these the most frequent is gemmation. The buds are formed either on the
episarc or on the canals running between zooids at the surface of the
coenenchym. When the young zooids that have been formed by gemmation reach
maturity they have the same characters as their parents. Fission occurs in
the production of a great many colonies of Madreporaria. It occurs
occasionally in such genera as _Madrepora_ and _Porites_, where
reproduction by gemmation prevails, but it is said that gemmation never
occurs in those forms such as the Astraeidae Fissiparantes where fission is
the rule. In fission a division of the zooid takes place in a vertical
plane passing through the stomodaeum and dividing the zooid into two equal
parts. In some cases these two parts become separated during the further
{388}growth of the coral. In other cases, however, further divisions of the
stomodaeum occur before the separation of the zooids, and then elongated,
serpentine polyps are produced (as in _Meandrina_, etc.), which consist of
a number of imperfectly separated zooids, each with a distinct mouth and
stomodaeum but with continuous coelenteric cavities. Two kinds of fission
must be distinguished from each other. In _Madrepora_ and _Porites_ the
plane of fission passes dorso-ventrally through the zooids, that is,
between the dorsal and ventral pairs of directive mesenteries. In these
cases the zooids produced by fission are similar to the parent form. In
most Madreporaria, however, the plane of fission appears to be more or less
at right angles to this, and the resulting zooids are unlike the original
parent form in having either no directive mesenteries at all or only one
pair of them.

[Illustration: FIG. 170.—Diagrammatic transverse sections of _Porites_ to
illustrate the process of fission. A, before division; B, fission nearly
completed. In A four bilateral pairs (_a_, _b_, _c_, _d_) of mesenteries
have appeared in the entocoele of the ventral directives (_VD_). These are
increased to six pairs and then fission commences as seen in B, the plane
of fission passing through the entocoeles of the last pair of secondary
mesenteries (_f_) and of the dorsal directives (_DD_). _I_, _II_, _V_,
_VI_, the protocnemes in the order of their development. (After Duerden.)]

The section Fungacea presents us with some exceptional and remarkable forms
of asexual reproduction. The embryo _Fungia_ gives rise to a conical fixed
coral called a "trophozooid." The upper part of the calyx of this
trophozooid expands and becomes disc-shaped. This is called the
"anthocyathus," and after it has reached a certain size it breaks away from
the rest of the trophozooid as an adult _Fungia_. Several anthocyathi may
be formed in succession from one trophozooid. This may be described as a
process of successive transverse fission. In _Diaseris_ the disc divides
into four quadrants, and each quadrant appears to be capable of acquiring
the shape and size of the undivided parent.

{389}Without doubt a process of sexual reproduction occurs in all
Madreporaria. In some genera sexual reproduction appears to be almost
continuous throughout the year; in others the sexual organs are formed only
at periods separated by considerable intervals of sterility. According to
the researches of Duerden the Madreporaria appear to be usually viviparous,
the early stages of development are passed through within the body of the
parent, and the young coral is discharged into the water as a free-swimming
ciliated larva. The larvae are spheroidal, oval, or pear-shaped, but change
their shape a good deal, and sometimes become elongated, straight, or
spirally twisted rods. The larvae are at first dense and opaque, but
subsequently they become distended by the absorption of water, and more
nearly transparent. They swim about for one or two days, and then settle
down by the aboral pole and become fixed. The tentacles are not formed, in
any species that has yet been observed, during the free-swimming stage of
existence.

[Illustration: FIG. 171.—A fixed stage in the development of _Fungia_. The
trophozooid has become differentiated into a discoid crown, the
anthocyathus (_Cy_) and a pedicle, the anthocaulus (_Ca_). (After G. C.
Bourne.)]

DISTRIBUTION OF REEF CORALS.—The principal reef-forming corals reach their
greatest size and grow with greatest rapidity in the warm, shallow waters
of the world, but they are not confined to this habitat. A species of
_Madrepora_ has been found in the very cold waters of Archangel, and
_Manicina areolata_ occurs in Simon's Bay, Cape of Good Hope, many degrees
south of the region of the East African coral reefs. As regards the
distribution of these corals in depth, very little is known at present. The
face of the growing coral reef that is turned towards the open sea is so
steep that it has been found impossible to determine to what depth the
living reef corals actually extend.

The survey of the Macclesfield bank proved that a considerable number of
reef corals are to be found alive at depths {390}ranging from 30 to 50
fathoms.[408] To give one example:—In the dredging No. 50, depth 32 to 35
fathoms, living examples of the following genera of corals were obtained:
_Madrepora_, _Montipora_, _Psammocora_, _Pavonia_, and _Astraeopora_.

CORAL REEFS AND ATOLLS.—In many regions of the tropical seas, banks and
islands are found which are built up of blocks of coral, coral detritus,
and altered or modified limestone. These are the famous coral reefs of
which so much has been said and written during the last half-century. There
can be little doubt that the superficial strata of these formations are
entirely due to the action of coral-forming animals and plants living in
warm, shallow sea-water.

[Illustration: FIG. 172.—Plan of Minikoi Atoll in Laccadive Archipelago.
_A_, the land elevated above the level of high-water mark; _Ch_, the boat
channel; _5 fm_, the five fathom line; _2 fm_, the two fathom line; _L_,
the lagoon with a maximum depth of 7 fathoms; _R_, the reef continuing the
circle on the east side of the atoll, awash at high tides. (After Stanley
Gardiner.)]

Three classes of coral reefs are usually recognised: the "fringing reefs"
which follow the contour of the coast at a distance of a few hundred yards,
and are separated from the beach at low tide by sand flats or a shallow
lagoon; the "barrier reefs," following the contour of the coast less
regularly than the fringing reefs, but at a much greater distance, and
separated from the beach by a lagoon of sufficient depth to serve as a
harbour for ships of great size; and, finally, the "atolls," which are
ring-shaped, or broken circlets of low islands enclosing a lagoon which is,
in some cases, of considerable depth.

It was observed by the early surveyors that in many cases the sea-bottom
slopes downwards steeply or almost precipitously from the outer edge of the
barrier reefs and atolls to very great {391}depths—to depths, in fact, at
which reef-forming corals do not live.

It seems obvious, therefore, that the atolls and barrier-reefs are resting
upon some stratum which could not possibly have been formed by
reef-building organisms at the same relative position it has now, and the
questions arose, What is the substratum and how was it formed?

If this stratum is a coral rock, it is clear that it must have been formed
at a time when it was nearer to the surface of the sea than it is now, and
that it must have subsided subsequently to greater depths. If, on the other
hand, it is a primitive rock, we must assume that in such regions as the
Indian Ocean and the South Pacific, where the archipelagoes of atolls
extend for hundreds of miles, there are chains of mountain ranges with
peaks reaching to a uniform level beneath the surface of the sea. "But we
cannot believe that a broad mountain summit lies buried at the depth of a
few fathoms beneath every atoll, and nevertheless that throughout the
immense areas above named not one point of rock projects above the level of
the sea. For we may judge of mountains beneath the sea by those on land,
and where can we find a single chain, much less several such chains many
hundred miles in length, and of considerable breadth, with broad summits
attaining the same height from within 120 to 180 feet?"[409]

To account for the observed facts of the atolls and barrier-reefs, Darwin
conceived and expounded the subsidence theory. According to this theory,
the regions where atolls now occur were at one time dry land, or an
archipelago of volcanic islands surrounded by fringing reefs of the
ordinary type. A gradual subsidence of the land took place, and the area of
the land diminished; but the area enclosed by the coral reefs did not
diminish in a corresponding degree, and the young corals growing on the
débris of the older ones as they sank continued the growth of the reef in a
direction nearly vertical to the sea-bottom. The fringing reefs thus became
barrier reefs, and they were separated from the land by a lagoon of
considerable depth. Finally, when the mountain peaks disappeared beneath
the waves, a ring-shaped reef or atoll was all that was left to mark the
position of the former land.

The fundamental assumption in the subsidence-theory is that {392}the
substratum of the coral reefs and islands is coral-formed limestone. To
test the truth of this assumption an expedition was sent out to obtain, by
boring, evidence of the character of the substratum of a typical atoll. The
island of Funafuti in the Ellice group of the Pacific Ocean was selected,
and after several attempts a successful boring was made to a depth of 1114
feet. The material from the boring was found to consist of rocks or sands
entirely derived from the calcareous skeletons of marine Invertebrate
animals and calcareous Algae.[410] Moreover, in the cores from various
depths down to the lowermost the fossilised skeletons of the common genera
of recent corals, and very few or no representatives of genera of corals
now extinct were discovered.

[Illustration: FIG. 173.—Section of the outer edge of one of the Maldive
Atolls. A, foundation of primitive rock cut down by the currents; B,
upgrowth of the rim by the deep-sea-, intermediate depth- and (B')
reef-organisms; C, extension outwards by means of the talus slope; D,
lagoon. Scale in fathoms. (After Stanley Gardiner.)]

These facts, therefore, prove the justice of Darwin's assumption as to the
nature of the substratum—and give support to the subsidence-theory as
applied to this particular island. A strong opinion has, however, been
expressed by several authors of recent years that the subsidence-theory
cannot account for the formation of all the atolls and barrier reefs that
have now been investigated, and alternate hypotheses have been put forward
to account for particular cases. The main chain of the Maldive Archipelago
in the Indian Ocean, for example, presents special difficulties to the
acceptance of the subsidence-theory as one of general application.[411] The
main chain of these islands is more than 300 miles long, and lies at right
angles to the monsoon currents of the {393}Indian Ocean. Here the action of
the currents appears to have cut down a great tract of land to form a
plateau more than 100 fathoms in depth. The outer rim of this plateau may
have grown in height by the deposit of the skeletons of surface-swimming
animals, and the skeletons of deep-sea corals, until it reached a level
where reef-forming corals can thrive. A certain number of channels would be
retained and even deepened as the rim grew up, and thus the coral would
eventually reach the surface not as a single large atoll, but as a series
of coral islands. When the coral reef has thus reached the surface and
cannot grow farther in height, it spreads radially like a fairy ring on the
talus formed by broken corals that have fallen down the slopes. The central
parts, no longer protected by living organisms, are continually subject to
the solvent action of the sea water penetrating the porous substratum, and
sink to form the lagoon.

It is not only in the reefs of the Indian Ocean, however, but in many of
the archipelagoes of the Pacific Ocean, where there is evidence of very
extensive elevation of the land areas in the neighbourhood of atolls and
barrier reefs, that the subsidence-theory does not satisfactorily account
for all the observed facts. It appears probable, therefore, that although a
gradual subsidence of the land may have been the primary cause of coral
reef formation in some areas, similar reefs may have been formed in other
areas by other natural methods.

FOSSIL CORALS.—A great number of the genera of corals found in the newer
Tertiary deposits, and a smaller number of those occurring in the older
Tertiary and Cretaceous strata clearly belong to families now represented
by recent corals. In the earlier strata, however, fossils are found which
cannot be placed in our system with any degree of certainty. Attempts have
been made from time to time to arrange these corals in their proper
positions by the careful study and comparison of their skeletal features,
but the reasons given are not convincing. The genus _Syringopora_, and the
families Favositidae, Heliolitidae, and Coccoseridae have been noticed in
the chapter on Alcyonaria (pp. 343-346). The family Zaphrentidae will be
noticed when dealing with the order Zoanthidea.

Among the families of fossil corals of uncertain position which may still
be included in the order Madreporaria, the more important are:—

{394}CYATHOPHYLLIDAE, a family of solitary and colonial corals with
numerous radially arranged septa, extending from the Silurian to the
Carboniferous limestone. It includes the genera _Cyathophyllum_, which was
very abundant in Devonian times, and _Lithostrotion_, which, in the times
of the formation of the Carboniferous limestone, occurred in continuous
masses extending over great areas of the sea-bottom. The Cyathophyllidae
may possibly be ancestral to the representatives of both Astraeidae and
Fungiidae, which appeared in the Triassic strata.

The CYATHAXONIIDAE form a family of solitary turbinate or horn-shaped
corals, with septa showing a regular, radial arrangement, and may have been
the ancestors of the modern family Turbinoliidae. They have the same
geological range as the Cyathophyllidae.

The CYSTIPHYLLIDAE.—This family consists of solitary corals with very thin
septa; the interseptal spaces are filled with an abundant vesicular
substance called the "stereoplasm." The systematic position of this family
is very doubtful, as the structure is evidently much destroyed, but by some
authors it is supposed to be ancestral to the family Eupsammiidae.

These three families, together with the Zaphrentidae (p. 406), were
formerly grouped together as the Tetracoralla or Rugosa.


SUB-ORDER 1. ENTOCNEMARIA.

Madreporaria forming perforate coralla, with calices that do not project
above, or project only slightly above the surface of the coenosarc. The
zooids of each colony are usually small and crowded. The mesenteries arise
in bilateral pairs, and the increase in their number takes place in the
chamber between the ventral or the dorsal pairs of directives. The corals
included in this order are among the most important of the reef-builders.
On many of the recent coral reefs they occur in enormous numbers and of
great individual size. But although so prevalent upon recent reefs, they
appear to have played a far less important part in the formation of the
reefs of the early Tertiary times, and in the reefs of times antecedent to
the Tertiary they were rare or absent.

Judging from the structure of the skeleton and the palaeontological history
alone it might be thought that the Entocnemaria {395}represent the most
recent types of Madreporarian structure, but the anatomy of the zooids
points to a contrary conclusion. The zooids are of very simple structure;
the mesenteries are found only in bilateral pairs, and all the new
mesenteries formed after the protocnemes originate in one of the directive
chambers. These are characters indicating a very ancient history,
suggesting affinities with the Edwardsiidea on the one hand, and some
ancient type of Cerianthidea on the other. There can be little doubt that
it was owing to the evolution of a porous skeleton of rapid growth that
these corals have caught up and passed the Astraeidae and other more
specialised forms in the struggle for predominance on the coral reefs.

FAM. 1. MADREPORIDAE.—The calices of the corallum are small and contain a
few perfectly distinct septa. The coenosteum is porous and contains a
plexus of the coenosarcal canals, which connects the cavities of
neighbouring zooids. This family is divided into a number of sub-families,
but it is only necessary here to mention the peculiarities of a few of the
well-known genera.

_Madrepora._—This genus is represented by an immense number of forms on the
coral reefs of both the old and new world. Attempts have been made at
various times to divide these forms into specific groups, and a large
number of species have been defined and named. The differences between
these species, however, are such as may be due to varying conditions of
life upon the reefs and not to characters transmitted from generation to
generation by heredity. There can be no doubt that when our knowledge of
the soft tissues of these corals is extended the number of species will be
greatly reduced. There are, however, three principal forms of growth or
_facies_ in the genus.

1. The flabellate or palmate colonies with large flat or concave fronds,
radiating from an encrusting base: _Forma palmata_.

2. Much branched colonies, several branches radiating obliquely from a
common centre: _Forma prolifera_.

3. Large and more erect colonies, less branched except towards the
periphery: _Forma cervicornis_.

On some reefs one of these forms of growth predominates, and for miles the
reef seems to be built up mainly of corals of this shape. On other reefs
two or sometimes all three of these forms may be found within a stone's
throw of one another. {396}Notwithstanding the difficulty of distinguishing
the species, the genus itself is quite well defined. The calices project
slightly from the surface of the branches and contain six septa, of which
the pair that is parallel with the axis of the branch is the strongest.
This strong pair of septa can usually be well seen when a slender branch of
a Madrepore is examined by a lens by transmitted light. At the apex of each
branch there is a terminal zooid and in the skeleton an apical calyx. The
terminal zooid is (in some species at least) different from the lateral or
radial zooids. The former is radially symmetrical and has six long equal
digitiform tentacles, the latter have usually twelve tentacles, of which
six are larger than the others. These tentacles alternate, but they are so
arranged on the disc as to give a distinctly bilateral appearance to the
zooids.

The colour of the West Indian Madrepores appears to be entirely due to
Zooxanthellae (pp. 86, 125). They are lighter or darker shades of brown,
sometimes becoming green, yellow, or orange. On the Australian barrier reef
and other reefs of the eastern seas the growing points of the branches are
variable and often brilliantly coloured, emerald green, violet, or red;
giving some of the most wonderful colour effects for which the reef pools
are famous. The cause of these brilliant apical colours has not yet been
ascertained.

The genus is found in shallow water of all seas of the tropical belt except
on the western side of the continent of America.

_Montipora._—In this genus the calices are small and situated in
depressions in the coenosteum, and there are six, sometimes twelve, septa
of approximately equal size. There is no terminal calyx at the apex of the
branches. This is a genus of very variable form and wide distribution in
all tropical seas except on the shores of the Atlantic Ocean.

_Turbinaria._—This genus is usually cup-shaped or foliaceous and twisted in
form. The septa may be six to thirty in number. Some of the species of this
genus attain to a very great size in favourable localities. There is a
specimen in the British Museum that is 16 feet in circumference and
weighed, when dried, 1500 lbs.

FAM. 2. PORITIDAE.—The corallum is usually encrusting, foliaceous, lobed or
tufted, rarely dendritic. The whole skeleton is built up of a system of
trabeculae and stout cross bars, and in {397}section the limits of the
calices are not well defined. The septa are represented by twelve
trabeculae. The zooids are small and are usually provided with twelve
tentacles. The most important genus is _Porites_, which is so abundant on
many reefs that it may be said to rival _Madrepora_ itself in the
luxuriance of its growth. On the Australian barrier reef a species of
_Porites_ builds up coralla over twenty feet in length and as many in
height. According to Saville Kent they are usually found on the outer side
of the reef and form a basis of support for the high-level Madreporas and
other corals.[412]

The colours of _Porites_ are very variable and often beautiful. In
Jamaica[413] the prevailing colours are bright blue, pale yellow, and
yellowish green. In Australia the colours are less brilliant perhaps, but
among the prevailing tints are light or bright lilac, a delicate pink, dark
yellow, and brown. The genus _Porites_ occurs in Eocene and Miocene
deposits, and is now found on all the more important coral reefs of the
world.

The genus _Alveopora_ is usually placed with the Poritidae. According to
Bernard,[414] however, its affinities with this family are remote, and it
is more closely related to the Favositidae (see p. 344). The walls of the
calices are contiguous and the septa are reduced to rows of spines, as in
the Favositidae. It is found in shallow water in the Pacific, the Indian
Ocean, and the Red Sea.


SUB-ORDER 2. CYCLOCNEMARIA.

Madreporaria forming perforate or imperforate coralla. Solitary or
colonial. The zooids have usually a large number of mesenteries arranged in
two or more cycles. The mesenteries beyond the protocnemic pairs arise in
unilateral pairs in chambers other than those between the directives.

SECT. 1. APOROSA.—Cyclocnemaria in which the theca and septa are not
perforated. The zooids of the colonial forms may communicate by means of
superficial canals of the coenosarc, or they may be in contact with one
another only at their edges.

Several families are included in this section, of which the more important
are:—

{398}FAM. 1. TURBINOLIIDAE.—The corals included in this family are mostly
solitary forms attached to foreign objects, or living partly embedded in
sand. In some cases a small colony is formed by gemmation.

The genus _Flabellum_ is a solitary coral of a compressed top shape. It has
a large number of septa arranged radially on the cup-wall. This cup-wall is
not a true theca but an epitheca. In some forms root-like tubes grow out
from the sides of the cup near its base and may serve to support the coral
on solid objects. In some remarkably fine specimens recently obtained from
the Persian Gulf these tubes served to attach the coral to a telegraph
cable. _Flabellum_ seems to be cosmopolitan in its distribution. It is
usually found in deep or moderately deep water, but some specimens have
been dredged in water of 2 to 9 fathoms.

[Illustration: FIG. 174.—Side view of _Trochocyathus hastatus_, with exsert
septa, well-marked costae (_c_), and with three spinous projections (_Sp_)
at the base formed by outgrowths from primary costae. (After G. C.
Bourne.)]

_Caryophyllia_ is a conical coral fixed by a slightly expanded base. The
cup-wall is a true theca covered below by an epitheca. There is a spongy
columella surrounded by a single circle of pali. There is one British
species, _C. smithii_. It is found attached to shells at a depth of about
thirty fathoms near the Eddystone Lighthouse and in other localities in the
English Channel. It also occurs between tide marks in the Scilly Islands,
and is found off the Shetlands, on the west coast of Scotland, and the
south-west of Ireland. The genus is widely distributed and extends from
shallow water to depths of 1500 fathoms. _Caryophyllia_ sometimes occurs in
clusters which have the appearance of an incipient colony. This may be due
to the embryos fixing themselves upon the epitheca of existing individuals
and developing there. It is doubtful whether the species ever reproduce
asexually either by gemmation or by fission. When the zooid is fully
expanded it projects some distance above the corallum and shows a very
transparent body-wall with a crown of some fifty tentacles. Each tentacle
terminates in a globose head (Fig. 169) charged with nematocysts. The
general colour is pale pink, and there is a broad brown circle {399}round
the mouth. Large specimens may be three-quarters of an inch in diameter.

_Turbinolia_ is a common Eocene fossil genus found in England and France,
and is stated to occur in the Caribbean Sea. The columella stands up like a
stylet and the septa are "exsert," _i.e._ project above the rim of the
theca.

_Trochocyathus_ is a genus with well-marked "costae" occurring in tropical
shallow water (Fig. 174).

FAM. 2. OCULINIDAE.—Colonial forms, dendritic or encrusting, with
relatively large and rather prominent calices separated by considerable
stretches of compact coenosteum. The zooids bear a crown of ten to
forty-eight or more capitate tentacles.

_Neohelia_ has a fistulose stem lined internally by a horny membrane. There
seems to be some reason for supposing that this membrane is formed by the
zooids themselves. A similar membrane is found in the fistulose stems of
_Amphihelia_ and perhaps other Oculinidae. If this membrane is really
formed by the activity of the corals it forms an exception to the general
rule that the skeleton of the Madreporaria is entirely calcareous. Others
maintain, however, that this membrane is formed by the Chaetopod worms
which are found in the tubes, and that the fistulose stem of the coral is
formed by folding round and encrusting the horny tubes of the worm.
_Neohelia_ is found in the Pacific Ocean.[415]

_Lophohelia_ is a genus forming dendritic colonies of considerable size.
The calices have thick walls and are very deep. _Lophohelia prolifera_ has
been found in deep water off the island of Skye and in other localities off
the west coast of Scotland. It is also not uncommon in some of the
Norwegian fjords and in other parts of the world.

_Oculina_ is another widely distributed genus found in the shallow tropical
waters of the West Indies, the Indian and Pacific Oceans. It forms
dendritic colonies of considerable size. The calices are usually arranged
in a spiral manner on the branches. The colour of the West Indian species
is stated to be light or dark brown when alive. The tentacles are arranged
in three cycles, and are usually twenty-four in number. Asexual
reproduction takes place by budding at the apex of the branches.

FAM. 3. ASTRAEIDAE.—This is a very large family, and {400}authorities are
not agreed as to its limits or classification. Excluding the simple forms
for the present, the family may be said to be distinguished by having the
calices so closely crowded that there is little or no coenosteum between
them. The corallum is compact and massive, unless bored and perforated by
algae, worms, and other coral-destroying organisms.

The genera of Astraeidae that form colonies may be divided into two groups:
the GEMMANTES and the FISSIPARANTES. In the group GEMMANTES asexual
reproduction is effected by gemmation, and each zooid of a colony is a
distinct individual with two pairs of directive mesenteries. Among the best
known of recent corals included in this group may be mentioned _Galaxea_.
In this genus there is a good deal more coenosteum between the calices than
there is in most of the Astraeidae. The calices are long and project some
distance above the coenosteum. The septa are exsert. In _Galaxea esperi_
examined by Fowler[416] there are twelve septa, twelve pairs of
mesenteries, and twenty-four tentacles, of which twelve are very small and
twelve rather larger. The colour is green or brown. The genus is found in
shallow water in the tropics of the old world.

In _Astrangia solitaria_ the zooids are either isolated or more generally
united by thin strands of perithecal tissue to form encrusting colonies.
The septa are not exsert as in _Galaxea_. Six are prominent and belong to
the first cycle, six smaller ones form a second cycle, and an incomplete
third and fourth cycle may be seen. Corresponding with each septum there is
a tentacle. The tentacles of the innermost cycle are the longest (3 mm. in
length). All the tentacles terminate in a knobbed apex. The living zooids
are colourless throughout, or display only very delicate tints within
restricted areas.[417] This genus occurs principally on the coasts of the
American continent, extending as far south as the Straits of Magellan.
Other well-known genera of Astraeidae Gemmantes are _Orbicella_,
_Cladocora_, _Phyllangia_.

In the group FISSIPARANTES asexual production takes place by fission
without the production of morphologically complete zooids. The tentacles,
mesenteries, and septa, when fission is established, are not arranged in
regular hexameral cycles, and no {401}new directive mesenteries arise. In
some cases very large corals are formed, and, if our conception is correct,
these must be regarded, not as a colony of zooids, but as a single
individual zooid divided into a considerable number of incompletely
separated parts. Among the well-known genera belonging to this group are
_Euphyllia_, _Mussa_, _Meandrina_, _Coeloria_, _Favia_, and _Goniastraea_.

In such genera as _Euphyllia_ the parts of the colony become separated by
deep grooves, and have the superficial appearance of being distinct
individuals; but in the Brain-coral _Coeloria_ and others the surface of
the coral presents a series of more or less bent or curved grooves, each
with a row of slit-shaped mouths and bordered by rows of tentacles.

A number of genera of solitary corals united in the subfamily
Trochosmiliacea are generally included in the family Astraeidae. The study
of their skeletal characters has suggested[418] that they are more closely
allied to the Turbinoliidae. The principal genera thus transferred would be
_Trochosmilia_, _Placosmilia_, _Parasmilia_, and _Asterosmilia_. As these
genera and their allies are nearly all extinct, and nothing is known of the
structure of the living zooids, their removal from the Astraeidae may be
regarded as not fully justified.

FAM. 4. POCILLOPORIDAE.—The general anatomy of the zooids of this family of
corals has some resemblance to that of the Entocnemaria, and it is possible
that they will eventually find a place in our classification near to, if
not actually within that group. The fact, however, that the skeleton is
imperforate is sufficient for the present to justify the inclusion of the
family in the section Aporosa. There are but two genera at present known,
and in both of them the zooids have twelve tentacles, twelve mesenteries,
and only two mesenterial filaments. The zooids are connected together by an
elaborate system of canals running in the superficial coenosarc. The
calices are bilaterally symmetrical, and in _Seriatopora_ the septa which
are parallel with the axis of the branch are united in the centre of the
calyx, and are very much larger than the others, as in _Madrepora_. In all
these characters the family shows affinities with the Entocnemaria. In the
characters of the skeleton, which is imperforate and tabulate, the
affinities are rather with the {402}Cyclocnemaria. The two genera are
widely distributed on the coral reefs of the old world, and in some
localities are very abundant. Neither genus is found in the West Indies.
They are both of recent origin, but _Pocillopora_ occurs in the Miocene. It
is a remarkable feature of the family that both genera may be attacked by
the gall-forming crab _Hapalocarcinus_. From some reefs nearly all the
Pocilloporidae show crab-galls on a large number of their branches, whereas
other Madreporaria are free from them.

[Illustration: FIG. 175.—A portion of a colony of _Pocillopora_ from the
Maldive Archipelago.]

[Illustration: FIG. 176.—A single calyx of _Pocillopora septata_, showing
_Co_, the columella; _S_, _S_, the septa; _Th_, the theca wall. (After
Gardiner.)]

_Pocillopora_ is a coral that forms encrusting masses, rising into lobes or
branches of considerable size, terminating in blunt apices. _Seriatopora_
is much more slender and ramified, the branches terminating in sharp
points.

SECT. 2. FUNGACEA.—This section of Cyclocnemaria contains a number of
solitary and colonial corals of very varied form united in the possession
of a number of cross-bars called "synapticula" connecting the septa, and
thereby giving strength to the calyx apart from any increase in the
thickness of the calyx-wall. The family Fungiidae shows many peculiarities
which separate it very distinctly from both the Cyclocnemaria and the
Aporosa. The Eupsammiidae, however, approach the {403}Cyclocnemaria in many
respects, and the Plesiofungiidae form a connecting link with the
Astraeidae. It is very probable that this section had a dual origin, and
therefore does not represent a single line of descent.

FAM. 5. PLESIOFUNGIIDAE.—This family is related to the Aporosa in the
possession of septa that are generally solid and imperforate, and to the
Astraeidae in particular in the possession of dissepiments. They differ
from them, however, in the presence of synapticula and in certain
peculiarities of the tentacles.

The genus _Siderastraea_ has recently been studied by Duerden.[419] The
colony is usually massive and encrusting in habit. The zooids when expanded
do not rise much above the level of the corallum. The tentacles are short
and are arranged in irregular cycles on the disc. They terminate in knobbed
extremities, and those of the inner cycles are bifurcated. The colour of
_S. sideraea_ is reddish-brown when alive. _Siderastraea_ is found in
shallow water on the coral reefs, and is widely distributed.

In _Agaricia_ the colony is more foliaceous. The tentacles are rudimentary
or small. The colour of the living zooids is very similar to that of
_Siderastraea_. _Epistrelophyllum_ is a solitary coral, from the Jurassic
series, belonging to the family.

FAM. 6. FUNGIIDAE.—_Fungia_ is an unattached solitary coral of a flat
disc-like shape with very numerous exsert imperforate septa. It is
frequently of considerable size (six to twelve inches in diameter). On many
of the coral reefs of the old world it is extremely abundant, and
consequently it is one of the commonest corals of our collections. When
alive the corallum is almost hidden by the disc, which is studded all over
with very numerous long tentacles.[420] The colour varies in different
species, but is usually brown. One species on the Australian barrier reef,
_F. crassitentaculata_, is of a dark olive green colour, the tentacles
terminating in white knobs.

The free adult Fungias are derived from a fixed stock called the
trophozooid, from which the young Fungias are detached by transverse
fission (see p. 388). The thecal wall of the young _Fungia_ when detached
from the trophozooid is perforated, but {404}the pores become largely
filled up during the later growth of the coral.

There are several genera of colonial Fungiidae of less frequent occurrence,
such as _Halomitra_, _Herpetolitha_, and _Cryptabacia_.

FAM. 7. CYCLOSERIDAE.—These are solitary or colonial Fungacea with an
imperforate theca. _Bathyactis_ occurs at great depths. _Diaseris_, shallow
water on coral reefs.

FAM. 8. PLESIOPORITIDAE.—The septa in this family are trabeculate and
perforate, resembling in this respect the septa of Poritidae.
_Leptophyllia_, _Microsolena_, extinct.

FAM. 9. EUPSAMMIIDAE.—This family of perforate corals is usually placed
with the Madreporidae and Poritidae in the old group Perforata. The
researches of Fowler and Gardiner have shown that the arrangement of the
mesenteries is that of the Cyclocnemaria, and the presence of synapticula
connecting the septa suggests affinities with the Fungacea. The synapticula
of the Eupsammiidae, however, are peculiar in being arranged, not in a
vertical series, but alternately with one another or quite irregularly in
position. The members of this family are solitary or colonial in habit.

_Stephanophyllia_ is a flattened disc-shaped coral, with perforate and
dentate septa, found in the Pacific Ocean and as a fossil in various strata
since Cretaceous times.

In _Leptopenus_, from depths of about 1500 fathoms, the perforations are
much larger than in the last-named genus, and the skeleton is reduced to a
system of slender trabeculae.

_Rhodopsammia_ has a conical shape, and gives rise by gemmation to a number
of young zooids, which remain attached for some time to the parent form
before becoming free.

Among the colonial genera are _Dendrophyllia_, _Coenopsammia_, and the
well-known Mediterranean genus _Astroides_.


ORDER IV. ZOANTHIDEA.

This order of Zoantharia consists of a number of solitary or colonial
Anemones that do not form a skeleton of horn or carbonate of lime, and are
distinguished from the Actiniaria by the peculiar arrangement of their
mesenteries.

FAM. 1. ZOANTHIDAE.—_Sphenopus_ is a solitary coral and terminates aborally
in a small sucker-like base, by which it may {405}be attached to foreign
bodies. The genera _Gemmaria_ and _Isaurus_ include solitary forms.

In the majority of the species of Zoanthids, however, a basal encrusting
stolon is formed, which may be thick and fleshy or membranous, or may
consist of a plexus of bands from which several zooids rise and on which
the new buds are formed.

The tentacles are numerous, simple, usually short, and arranged in one or
two circles on the margin of the disc. Most Zoanthidae are encrusted with
sand, shell fragments, or sponge spicules, but _Zoanthus_ and _Isaurus_ are
naked. The foreign particles that form the incrustation are firmly attached
to the ectoderm, and as a rule many of them sink down into the mesogloea to
give additional support to the body-wall. It is the presence of so much
incorporated sand that frequently gives these Zoantharia such a very
brittle character. The stomodaeum usually exhibits a well-marked ventral
siphonoglyph. The mesenteries consist of a pair of complete ventral
directives, a pair of incomplete dorsal directives, while of the remaining
protocnemes the lateral mesenteries which are first and second in the order
of appearance are complete, the sixth is incomplete, whereas the fifth is
complete in the Macrocneminae and incomplete in the Brachycneminae.
Duerden[421] has found in specimens of three species that the arrangement
of the mesenteries is "brachycnemic" (the sixth protocneme imperfect) on
one side and "macrocnemic" (the sixth protocneme perfect) on the other. The
metacnemes appear in the spaces between the sixth protocnemes and the
ventral directives in unilateral pairs, of which one becomes complete and
the other always remains incomplete (Fig. 163, 4, p. 368).

The Zoanthidae are usually dioecious, but hermaphroditism undoubtedly
occurs in the genera _Zoanthus_ and _Isaurus_. Little is known of their
development, but a larval form discovered by Semper off the Cape of Good
Hope, of cylindrical shape, with an opening at each end and distinguished
by a longitudinal band of cilia running from one end to the other, is
probably the larva of a Zoanthid. It is commonly known as Semper's larva.
Other larvae provided with a ring of cilia have also been attributed to
this group.

A great many Zoanthidae are epizoic in habit. Thus several {406}species of
_Epizoanthus_ form colonies on the shells of Gasteropods inhabited by
hermit crabs. _Parazoanthus tunicans_ is found on the stem of a
_Plumularia_; _Parazoanthus separatus_, from Jamaica, is associated with a
sponge. The base of the bundle of long spicules of the Sponge _Hyalonema_
(p. 204) is almost invariably sheathed by a colony of _Epizoanthus
stellaris_.

The only genera occurring within the British area are _Epizoanthus_ (with
six species), _Parazoanthus_ (with four species), and _Zoanthus sulcatus_.

Of the species of _Epizoanthus_, _E. incrustatus_ is fairly common, in
depths of twenty to eighty fathoms on all our coasts, and is frequently
commensal with different species of hermit crabs, while _E. paguriphilus_
is found in much deeper water off the west coast of Ireland and is always
commensal with hermit crabs. _Parazoanthus anguicomus_ is found at depths
of a hundred fathoms off the Shetlands and west of Ireland, and is usually
associated with various species of Sponges.

_Gerardia savalia_ is the largest "black coral" of the Mediterranean. The
colony begins by encrusting the stem of one of the Gorgoniidae, but soon
surpassing its support in growth, it forms a basal horny skeleton of its
own and builds up very large branching colonies. A specimen in the British
Museum,[422] from twenty fathoms off the island Negropont, is two metres
high and two metres wide. The genus appears to be related anatomically to
_Parazoanthus_.

[Illustration: FIG. 177.—_Zoanthus macgillivrayi_, a small colony. The
tentacles are shown somewhat contracted by the preservative. Each zooid is
about 25 mm. in length. (After Haddon.)]

FAM. 2. ZAPHRENTIDAE.—This family of Palaeozoic corals is usually placed
with the Turbinoliidae or in the separate group Tetracoralla. Recently
Duerden[423] has given reasons, based on the method of increase of the
septa in _Lophophyllum_, for believing that their affinities lie rather
with the Zoanthidae than {407}with the Madreporaria. They are solitary
turbinate corals, with numerous septa exhibiting a distinct bilateral
symmetry in arrangement. _Zaphrentis_, _Lophophyllum_.


ORDER V. ANTIPATHIDEA = ANTIPATHARIA.

The members of this order can readily be distinguished from all other
Zoantharia by the presence of a horny axial skeleton (sclerobase) and the
absence of any spicules of calcium carbonate. The skeleton is covered by a
thin bark which consists of a number of simple, naked zooids united at
their edges. The zooids bear six tentacles, or if there are more than six,
six large prominent tentacles. In most genera there are but ten
mesenteries, in others twelve. In _Cladopathes_ only six mesenteries are
found. The skeleton of the Antipathidea is simple in _Stichopathes_ and
_Cirripathes_, but in all other genera it is ramified. The ramification is
usually profuse and irregular. The horny substance of which it is composed
is free from any deposit or infiltration of lime. The surface of the
younger branches is beset with numerous short spines, the number and
arrangement of which are characters largely used in the determination of
species. The basal parts of the main axis and the thicker branches are
frequently bare, the zooids having died and become disintegrated. In these
cases the spines wear away and the skeleton appears to be smooth. The
presence of spines on some of the branches is, however, generally
sufficient to enable the naturalist to distinguish a dried Antipathid from
the axis of a Gorgonid, with which alone it might be confounded.

There are six complete mesenteries in each zooid, but as they bear no
retractor muscles it is not certain that they represent the first six
protocnemes of other Zoantharia. In a great many species the zooids are
oval in shape, the longer diameter being parallel with the axis of the
branch. The mouth and stomodaeum are compressed and at right angles to this
diameter. It is usually assumed that the mesenteries attached to the angles
of the stomodaeum are the directives, and that the remaining pair, which is
axial in direction, corresponds with the first pair of protocnemes. The
axial pair of mesenteries is frequently very well developed and alone bears
the gonads. When other mesenteries are formed they always arise in
bilateral pairs between the axial mesenteries {408}and the directives. The
tentacles correspond with the intermesenteric chambers. In some genera
there is a constriction of the zooid between the pairs of the tentacles on
each side of the axial mesenteries and the directive tentacles. This gives
them the appearance of a division into three zooids with two tentacles
apiece, one with a mouth and two without a mouth; and as the mouthless
parts alone bear the gonads on the single axial mesentery, they have been
called the "gastrozooids" and "gonozooids" respectively. This must not be
regarded, however, as a case of true dimorphism, as the cavities of the
so-called gastrozooid and gonozooids are continuous.

The Antipatharia are widely distributed in nearly all the great seas of the
world. Some species are found in shallow water in the tropics, but most of
them occur in depths of fifty to five hundred fathoms. The genus
_Bathypathes_ is only found at enormous depths ranging from 1070 to 2900
fathoms. Specimens of _Cirripathes spiralis_, _Antipathella gracilis_, and
another species have recently been obtained in deep water off the west
coast of Ireland,[424] but these are the only Antipatharia known to occur
within the British area.

The very simple structure of the Antipatharia is usually attributed to
degeneration. On this view the Antipathidae with only six complete
mesenteries are the most modified, whereas the Leiopathidae with twelve
mesenteries are more closely related to the ancestral forms, and _Gephyra
dohrnii_ (see p. 382) is a link connecting the order with the Actiniaria.

There is no reason, however, for supposing that _Gephyra_ is specially
related to this order, and, as pointed out recently by Roule,[425] the
simple structure of the zooids of the Antipathidea is more easily explained
if they are regarded as primitive forms.

_Gerardia_ (p. 406), from the Mediterranean, forms a horny axial skeleton
like that of the Antipathidea, but this genus is probably a Zoanthid.

FAM. 1. ANTIPATHIDAE.—In this family the zooids have six tentacles and six
or ten mesenteries. It includes nearly all the familiar genera, such as
_Stichopathes_, _Cirripathes_, _Antipathes_, _Antipathella_, _Cladopathes_,
and _Bathypathes_. _Schizopathes_ and {409}its allies occurring in deep
water are the forms regarded by Brook as dimorphic.

[Illustration: FIG. 178.—A portion of a branch of _Antipathes ternatensis_,
showing three zooids and the horny axis beset with thorn-like projections.
(After Schultze.)]

FAM. 2. LEIOPATHIDAE.—This family includes the single genus _Leiopathes_ of
the Mediterranean Sea. It is distinguished from the others by the presence
of twelve mesenteries.

FAM. 3. DENDROBRACHIIDAE.—This family also consists of a single genus,
_Dendrobrachia_, from 400 fathoms in the South Atlantic. It is
distinguished by having pinnate retractile tentacles.


ORDER VI. CERIANTHIDEA.

This order contains the remarkable Sea-anemone called _Cerianthus_. Two of
the species have been placed in separate genera, but they do not appear to
be of more than sub-generic rank. _Cerianthus_ has a long cylindrical body
with a double crown of numerous long tentacles at the oral extremity and
tapering to a blunt point or rounded at the aboral extremity.

There are numerous mesenteries, which increase in number by the addition of
bilateral pairs, arising only in the ventral inter-mesenteric space
throughout the greater part, if not the whole, of the life of the zooid.
The right mesentery of each young pair is always more advanced than the
left, so that the mesenteries have the appearance of arising alternately
right and left. None of the mesenteries bear conspicuous bands of retractor
muscles. The movements of the body are effected by a thick band of
longitudinal fibres lying between the ectoderm and the mesogloea in the
body-wall.

The absence or very slight development of muscles on the mesenteries
renders it difficult to recognise the homologues of the protocnemes of
other Zoantharia in the adult. From the evidence of embryology, however, it
seems certain that the six dorsal pairs of mesenteries represent the
protocnemes (Fig. 163, 3, p. 368) and the others are metacnemes.

{410}The stomodaeum exhibits a single long deep siphonoglyph, which is
probably dorsal in position.

There are two tentacles to each inter-mesenteric space, one being marginal
and the other circumoral. The gonads are borne upon alternate mesenteries,
and both ova and spermatozoa are produced by the same individual.

[Illustration: FIG. 179.—_Cerianthus membranaceus._ Colour pink, with
tentacles annulated pink and brown. About 35 cm. in length. (After
Andres.)]

The ectoderm of _Cerianthus_ is remarkable for the immense number of
nematocysts and gland cells. The latter secrete a quantity of mucus which
binds the threads of the discharged nematocysts into a sticky feltwork and
this secures particles of sand and mud, the whole forming a long tube in
which the animal freely moves. This tube is often of considerable
thickness. It is tough and resistant, smooth inside but ragged and muddy
outside. It is often many times the length of the animal's body.

The embryo of _Cerianthus_ is set free before the completion of
segmentation, and it gives rise to a floating pelagic larva known as
_Arachnactis_. It has a variable number of tentacles and mesenteries
according to its age, but when it reaches a size of {411}about 15 mm. in
length it has developed characters which are sufficient to determine its
position as a Cerianthid.

The genus _Cerianthus_ appears to be widely distributed. _C. membranaceus_
is the common species in the Mediterranean Sea, but a smaller species has
been described from Naples under the name _C. oligopodus_ by Cerfontaine.
_C. americanus_ occurs on the eastern coasts of North America. The British
and North European species is _C. lloydii_, but another species, _C.
vogti_, has been found at a depth of 498 fathoms in the North Sea. _C.
nobilis_ is a gigantic species supposed to be about 1 foot in length when
complete, from Torres Straits.

_C. bathymetricus_ of Moseley, placed by Andres in the genus _Bathyanthus_,
is a species of small size (25 mm.), obtained by the "Challenger" from a
depth of 2750 fathoms in the North Atlantic. It exhibits a remarkable
prolongation of the stomodaeum into the coelenteron in the form of a sack
which contained food. Moseley described a species of _Cerianthus_, 6 inches
long, living on the coral reef at Zebu in the Philippines fully expanded in
the tropical sunshine.

Several species of _Arachnactis_ larvae have been described. Of these
_Arachnactis lloydii_ appears to be undoubtedly the larva of _C. lloydii_.
The adult forms of _Arachnactis albida_ from various stations in the
Atlantic Ocean and of _Arachnactis americana_ are not known. The larva of
_Cerianthus membranaceus_ has been called _Dianthea nobilis_, and is
characterised by the great length of the column, by the general opacity of
all parts of the body, and by the precocious appearance of the median
marginal tentacle. A considerable number of remarkable pelagic larvae have
been described by van Beneden[426] from the Atlantic Ocean, and
provisionally assigned by him to five different genera. The adult forms of
these larvae are not known, but they are probably members of this order.




{412}CHAPTER XV

CTENOPHORA


The Ctenophora are spherical, lobed, thimble-shaped, or band-like animals,
usually very transparent and gelatinous in structure. They are exclusively
marine, and are found floating at or near the surface of the sea.

Although they are generally classified with the Coelenterata, they are
regarded by some authors as having closer affinities with the Polyclad
Turbellaria (cf. Vol. II. p. 7). They agree, however, with neither of these
divisions in their essential characters, and the only way to indicate and
emphasise their unique position is to place them in a separate Phylum.

They differ from all the Coelenterata in the absence of nematocysts, and in
the presence in development of a definite mesoblast. The character from
which they derive their name, Ctenophora, is the presence on the surface of
bands of swimming plates. The plates are called the "combs" (κτείς, gen.
κτενός = a comb) or "ctenophoral plates." They occur in all genera included
in the Phylum except in _Coeloplana_ (Fig. 183, p. 422).

Another peculiarity of all Ctenophora (except the Beroidae) is the
presence, at some stage in the life-history, of two long and extremely
contractile tentacles. There is also a well-developed sense-organ
(statocyst) in the centre of the aboral area of the body.

The Ctenophora differ from the Turbellaria in the presence of the combs and
of the two long tentacles, in the position and relative importance of the
statocyst, and, with the exception of _Coeloplana_, in the general
characters of the alimentary canal.

SHAPE.—Several of the Ctenophora are conical or spherical in shape, but
exhibit at the pole where the mouth is situated {413}(Fig. 180, _M_) a
slight conical projection, and at the opposite pole where the sense-organ
is placed a slight depression (_Ab_). In others, the sides of the body are
drawn out into a pair of wing-like lobes (Lobata), and the body is
considerably flattened or compressed (Fig. 181). The Cestoidea have a long
flattened ribbon- or band-shape (Fig. 182), and the Platyctenea (Fig. 183)
are flattened in the oro-apical axis and exhibit a well-marked distinction
between the dorsal and ventral surfaces. The shape of _Beroe_ is that of a
hollow cone or thimble.

[Illustration: FIG. 180.—_Hormiphora plumosa._ _Ab_, position of the aboral
sense-organ; _Ct_, rib of ctenophoral plates; _M_, mouth; _t_, tentacle,
with two kinds of pinnae. (After Chun.)]

CTENOPHORAL PLATES.—In many Ctenophora eight lines can be traced, like the
lines of longitude on a globe, from the area of the sense-organ to the base
of the mouth-cone or hypostome. In the course of these lines are situated
the ctenophoral plates. In some species they extend along the greater part
of these lines of longitude, but in others they are more restricted. That
part of the line that bears the plates is called the "rib" or "costa."
These plates or combs form the principal organs of locomotion of the
Ctenophores. They consist of a row of cilia fused at the base (cf. p. 141)
to form the plate, but free at the extremity where they form the comb-like
edge. They are alternately raised, by a rapid contractile action, and then
slowly flattened down again. The plates are raised in succession from the
aboral to the oral end of each rib, and the appearance given to the bands
in the living animal is that of a series of waves travelling down the lines
of longitude from the sensory area towards the mouth. The effect of these
rhythmic movements of the combs is to {414}drive the animal slowly through
the water with the oral cone forwards. In some Ctenophores the costæ are
phosphorescent.[427]

TENTACLES.—In all the Ctenophora, except the Beroidae and the adult stages
of Lobata and Cestoidea, there is a single pair of tentacles. They are
attached to the base of a blind funnel-shaped pit which opens to the
exterior near the equator of the animal's body. The pits are on opposite
sides of the body, and the plane which passes through them both vertically
divides the body into approximately equal parts. It is called the
"tentacular" or "transverse" plane (Fig. 180). The plane at right angles to
this, which also passes through the mouth and statocyst, is called the
"sagittal" plane.

The tentacles are solid, and in the Cydippidae, of considerable length.
During life they are usually extended, and trail behind the animal as it
progresses through the water. But they are extremely contractile, and when
the animal is alarmed are suddenly withdrawn into the shelter of the
tentacular pits. Each tentacle usually bears a row of short pinnae. The
surfaces of the tentacles and of their pinnae are crowded with remarkable
cells which carry little globules of an adhesive secretion, and are called
the glue-cells or "colloblasts." These cells stick to any foreign body they
touch, and may be drawn out some distance from the tentacle, but they
remain attached to it by a long spiral thread which unwinds as the cell is
pulled out. Although the colloblasts have the function of catching prey
similar to that of the nematocysts of Coelenterata, they are true animal
cells and are not therefore homologous with nematocysts, which are the cell
products of the cnidoblasts.[428]

The Lobata and Cestoidea pass through a stage in development called the
Cydippiform or _Mertensia_ stage, when they possess a single pair of long
tentacles similar to those described above. In the adult condition,
however, these tentacles are absent, and their functions are performed by
numerous small accessory tentacles or tentilla arranged in rows on definite
lines along the body-wall.

SENSE-ORGAN.—At the aboral pole of the Ctenophore there is a hard
granulated calcareous body, the "statolith." This is {415}supported by four
tufts of fused cilia, and is usually covered by a dome of delicate
protoplasmic texture, which is believed to be formed by a fusion of cilia.
The dome enclosing the statolith is called the "statocyst."

Supporting the statocyst there is a circular or oval area of ciliated
epithelium which is usually supposed, but on insufficient evidence, to be
specially sensory in function. Extending from this area in the sagittal
plane there are two strips of ciliated epithelium called the "polar
fields."

The aboral sense-organ of the Ctenophora is one of the most characteristic
organs of the Phylum. The aboral pole of the Medusae of Coelenterata is
usually devoid of any special modification of the ectoderm of the bell, and
in the Tiarid genus _Stomatoca_ the little tassel at the aboral pole of the
Medusa cannot in any sense be regarded as a homologue of the sense-organ of
the Ctenophore. If the aboral sense-organ of the Ctenophora can be compared
with that of any other group of animals, it would be with the statocyst of
many of the Turbellaria, such as that of _Convoluta_, but it is far more
satisfactory to regard it as an organ peculiar to the Ctenophora and as
having no true relationship with any sense-organ found in other animals.

ALIMENTARY CANAL.—The mouth of the Cydippiform Ctenophores opens into a
sac-like chamber called the "stomodaeum," flattened in the sagittal plane
and stretching from the oral pole as far as the centre of the body. The
stomodaeum passes into a chamber flattened in the transverse plane called
the "infundibulum." From the infundibulum a narrow tube passes in the
direction of the aboral pole called the "intestine," and from the extremity
of this four short tubes pass to the sides of the polar fields at the place
where these fields join the sensory area. Two, or, in some cases, all four
of these tubes open to the exterior; but they do not appear to serve the
purpose of ejecting the undigested portions of the food, which usually pass
to the exterior by the mouth as in Coelenterata and Turbellaria.

From the lateral extremities of the infundibulum four pairs of tubes pass
to the equatorial region of the body, where each one joins a longitudinal
vessel which runs immediately beneath the epithelium supporting the ribs.
These are called the longitudinal or "sub-costal" canals. From the
infundibulum there also {416}passes a single pair of blind canals, the
"paragastric canals," one on each side of the stomodaeum, to end in the
oral cone.

In the Lobata the paragastric canals communicate with the longitudinal
canals under the transverse costae,[429] and send long blind processes into
the lobes. In the Cestoidea the arrangement of the canals is considerably
modified in adaptation to the needs of the ribbon-like body. In the
Beroidae the paragastric and longitudinal canals are in communication by a
peripheral network of canals, and in the Platyctenea there is also a
network of canals but without any definite longitudinal vessels.

SEXUAL ORGANS.—Most of the Ctenophora are undoubtedly hermaphrodite, but
Willey was unable to find ova in some of his specimens of _Ctenoplana_ that
were producing spermatozoa. In the Cydippidea the ova are produced on one
side of the longitudinal canal and the spermatozoa on the other. Each
longitudinal canal therefore performs the functions of a hermaphrodite
gland. When the sexual cells are ripe they escape into the infundibulum and
are discharged by the mouth. In _Ctenoplana_ there are definite and direct
male genital ducts.

The ova are very small when discharged and undergo complete segmentation in
the sea water. The development of the Cydippidea is really direct, but
there is a stage passed through in which the tentacles are relatively very
prominent and situated close to the aboral pole, and this stage is very
different in appearance from the adult. In the Lobata and Cestoidea there
is, however, a definite larval stage, of the general appearance of a
_Mertensia_, and during this stage fertile eggs and spermatozoa are formed
and set free.

DISTRIBUTION.—Ctenophora are found at the surface of nearly all seas, and
many of the genera have a cosmopolitan distribution. Some of the Lobata,
the Cestoidea, and the Platyctenea are more commonly found in the warmer
regions of the world. _Pleurobrachia pileus_, _Bolina infundibulum_, _Beroe
ovata_, and _B. cucumis_ occur off the British coast.

Most of the Ctenophora are from 5 to 20 mm. in diameter, but _Beroe_
reaches the length of 90 mm., _Eucharis multicornis_ {417}a height of 250
mm., and _Cestus veneris_ has been found no less than 1½ metres from one
extremity to the other.

Ctenophores usually go about in shoals, and in the case of _Beroe cucumis_
and _Eucharis multicornis_ the shoals may be of very great extent.
_Pleurobrachia pileus_ of the British coasts is often found at the end of
the season (July) as a series of isolated individuals; but in June they
occur in small shoals, swimming so close together that they will choke a
tow-net in a very short space of time.


CLASS I. TENTACULATA

Ctenophora provided with a pair of tentacles in the larval stages only or
in both larval and adult stages.


ORDER I. CYDIPPIDEA.

This order includes a number of spherical or oval Ctenophores, with a pair
of tentacles retractile into deep tentacular pits in the adult stage.

FAM. 1. MERTENSIIDAE.—The body is compressed in the transverse plane, and
the ribs on the transverse areas are longer than those on the sagittal
areas. The family includes the genus _Euchlora_, which occurs in the
Mediterranean and in the northern part of the Atlantic Ocean. In
_Charistephane_ there are only two enormous ctenophoral plates in each of
the longitudinal tracts. These plates are so broad that they almost meet
laterally to form two continuous circlets round the body of the animal.
This genus is found in the Mediterranean, but a few specimens have also
been obtained in the Atlantic.

In _Tinerfe_ the body is almost cylindrical, and there is a pair of
kidney-shaped swellings at the sides of the aboral pole. It has a pale blue
colour, and is found in the Guinea and south equatorial currents of the
Atlantic Ocean.

The name _Mertensia_ has been given to several forms that are undoubtedly
the young stages of genera belonging to the Lobata, but Chun retains the
name _M. ovum_ for a species which is very abundant in the Arctic currents
of the North Atlantic.

FAM. 2. CALLIANIRIDAE.—Two or four wing-like processes, into {418}which the
longitudinal canals extend, are found at the aboral pole. _Callianira_ has
two of these processes arranged in the transverse plane, and _Lophoctenia_
has four. _Callianira_ is found in the Mediterranean and in the Atlantic
from the Arctic to the Antarctic waters.

FAM. 3. PLEUROBRACHIIDAE.—The body is almost spherical in form, and the
eight ribs are equal in length.

This family includes the genus _Pleurobrachia_, in which the ribs extend
for a considerable distance along the lines of longitude of the spherical
body, but do not reach either the oral or the aboral areas. _P. pileus_ is
the commonest British Ctenophore, and may be found in shoals in May, June,
and July at the surface of the sea or cast up on the sand as the tide ebbs.
It is widely distributed in the North Atlantic waters. _P. rhodopis_ of the
Mediterranean has rather shorter ribs than _P. pileus_. Two new species
have recently been described from the Malay Archipelago.[430] _Hormiphora_
(Fig. 180, p. 413) differs from _Pleurobrachia_ in having much shorter
ribs, and in possessing two kinds of pinnae on the tentacles, those of the
ordinary kind and others much larger and sometimes palmate in character.
This genus has a world-wide distribution.

In _Lampetia_ and _Euplokamis_ the body is more cylindrical in shape than
it is in the other genera, but the ribs and subjacent longitudinal canals
extend up to the margin of the aboral field. Both these genera occur in the
Mediterranean, but _Lampetia_ is also found in the Malay Archipelago.


ORDER II. LOBATA.

The body is considerably flattened in the transverse plane, and the
sagittal areas are extended into the form of two wide peristomial lobes.
The oral ends of the areas between the transverse and sagittal ribs are
extended to form four flaps, called the "auricles." There are no tentacles
nor tentacle-sheaths of the ordinary kind in the adult form; but numerous
tentilla, similar in some respects to the pinnae of the tentacles of other
Ctenophora, form a fringe round the margin of the auricles and the
peristome. A single pair of long, filamentous, non-retractile tentacles
arise from the sides of the peristomium in _Eucharis {419}multicornis_.
These tentacles have no sheaths, and do not bear pinnae. They are probably
not homologous with those of other Ctenophora.

The characters that separate the families of Lobata are chiefly those of
varying size, shape, and position of the peristomial lobes and auricles. In
the Lesueuriidae the peristomial lobes are rudimentary; in the other
families they are moderately or very large. In the Bolinidae the auricles
are short, but in most of the other families they are long and ribbon-like.
In _Eucharis_ they can be spirally twisted in repose.

The modifications of the external form seen in the Lobata are accompanied
by some modifications of the internal structure. Among these, perhaps the
most interesting is a communication between the transverse longitudinal and
the paragastric canals, and the long convoluted tubes given off to the
peristomial lobes by the sagittal longitudinal canals. Very little is known
about the life-history and development of most of the Lobata, but Chun has
shown that in _Eucharis_ and _Bolina_ there is a Cydippiform larval stage
which produces ripe ova and spermatozoa. This is followed by a period of
sterility, but when the adult characters are developed they become again
sexually mature. To this series of sexual phenomena the name "Dissogony" is
given.

[Illustration: FIG. 181.—_Ocyroe crystallina._ _Ab_, aboral sense-organ;
_au_, auricle; _Can_, diverticulum from the paragastric canal passing into
peristomial lobe; _Ct_, costae; _M_, mouth; _Par_, paragastric canal
passing outwards to join one of the transverse subcostal canals; _P.L_,
peristomial lobe; _w_, wart-like tubercles on the lobe. (After Mayer.)]

The order contains only fifteen genera, but they are usually arranged in
the following eight families:—

  1. LESUEURIIDAE.    _Lesueuria._
  2. BOLINIDAE.    _Bolina, Bolinopsis._
  3. DEIOPEIDAE.    _Deiopea._
  4. EURHAMPHAEIDAE.    _Eurhamphaea._
  5. EUCHARIDAE.    _Eucharis._                                     {420}
  6. MNEMIIDAE.    _Mnemia, Mnemiopsis._
  7. CALYMMIDAE.    _Calymma._
  8. OCYROIDAE.    _Ocyroe._

Most of these Ctenophores occur in the warm and tropical seas; but _Bolina_
is found occasionally at Plymouth in the month of May, on the west coast of
Ireland, and at other stations on the British coasts. _Eucharis_ is
regarded as one of the most beautiful of the Phylum. A swarm, some miles in
length, of large specimens of _E. multicornis_ was met by the Plankton
Expedition in the south equatorial current of the Atlantic during the month
of September.


ORDER III. CESTOIDEA.

In this order the body is so much compressed in the transverse plane and
elongated in the sagittal plane that it assumes the shape of a long narrow
band or ribbon. The tentacular sheaths are present but the tentacles are
degenerate in the adult. The tentacular functions are performed by numerous
tentilla situated in long grooves extending along the whole length of the
oral side of the band-like body. The transverse ribs are reduced; the
sagittal ribs extend along the whole of the aboral side.

FAM. CESTIDAE.—This is the only family of the order. _Cestus veneris_, the
Venus's girdle of the Mediterranean Sea, is also found in the Atlantic
Ocean, and specimens belonging to the same genus, but probably to a
different species, occur as far north as the White Sea. Some of the larger
specimens are considerably over 1 metre in length.

[Illustration: FIG. 182.—_Cestus pectenalis._ _Ab_, aboral sense-organ;
_Ct_, the sagittal ribs; _M_, mouth. (After Bigelow.)]

_C. pectenalis_ was found in abundance off one of the Maldive Islands [431]
and differs from _C. veneris_ in having a large and {421}prominent orange
patch at each end of the body. It is said to be extremely graceful in the
water, moving with slow, ribbon-like undulations, and shining in the
sunlight with a violet iridescence. _Vexillum_, from the Mediterranean Sea
and Canary Islands, is rather more pointed at the extremities than
_Cestus_, and differs from it in some important anatomical characters.


ORDER IV. PLATYCTENEA.

This order has been constituted for two remarkable genera, in which the
oro-apical axis is so much reduced that distinct dorsal and ventral
surfaces can be distinguished.

There is a single pair of long milky-white tentacles capable of complete
retraction into tentacular sheaths.

FAM. 1. CTENOPLANIDAE.—_Ctenoplana_ was discovered by Korotneff in 1886
floating with the Plankton off the coast of Sumatra. In 1896 Willey [432]
discovered four specimens on a cuttle-bone floating off the coast of New
Guinea. To these authors we are indebted for the only accounts of this
animal that have been published.

When the _Ctenoplana_ is creeping on the bottom of a dish or with its
dorsal side downwards on the surface film of the water, it has the form of
a flattened disc with a notch on each side. On the upper or dorsal surface
eight short rows of ctenophoral plates may be seen, and in a position
corresponding with the two notches in the margin of the body are situated
the two sheaths from which the long pinnate tentacles protrude. In the
exact centre of the dorsal surface is situated the statolith, supported by
stiff processes from adjacent cells; and forming a circlet round the
statolith there is a row of short ciliated tentacles. These tentacles,
however, when examined carefully in the living animal, are found to be
arranged in two sets of about nine in each, separated by narrow gaps on
each side, the gaps corresponding in position with the axis through the
tentacles.

When the animal is swimming it assumes a helmet-shape by depressing the
sides of the body like a pair of flaps on the tentacular axis, and then the
ctenophoral plates come into play and produce the progressive movements of
the animals. The pinnate tentacles are opaque white in colour, and have
peculiar serpentine {422}movements. Very little is known at present
concerning many details of the internal anatomy, but there is one point of
considerable theoretical interest—namely, the presence of definite male
genital ducts.

Three of Dr. Willey's specimens were mottled with a green pigment, whereas
his fourth specimen and Korotneff's only specimen were mottled with a red
pigment. It has yet to be determined whether the differences which have
been observed in the individual specimens are of specific value.

FAM. 2. COELOPLANIDAE.—_Coeloplana_ was originally discovered by Kowalevsky
in the Red Sea, but has recently been found by Abbott [433] on the coast of
Japan.

[Illustration: FIG. 183.—_Coeloplana mitsukurii_, floating at the surface
of the sea with the dorsal side downwards. _T_, _T_, the tentacles
expanded. (After Abbott.)]

The Japanese species are found principally on encrusting Algae, _Zostera_,
_Melobesia_, etc., which they resemble very closely in colour. The Red Sea
species is, according to Kowalevsky, ciliated all over, but the Japanese
species are ciliated only on the ventral surface. As in _Ctenoplana_, the
body of _Coeloplana_ is a flattened disc with a notch at each end of the
tentacular axis, when creeping; but _Coeloplana_ does not swim, nor at any
time does it assume a helmet-shape. The tentacles are very long and of a
chalky-white colour. They can be retracted into tentacle-sheaths. When the
animal is excited it throws out the whole tentacle in a cloud of white
filaments, "and to watch it at such a time, shooting out and retracting the
tentacles, moving along the side of the aquarium like a battleship in
action is truly a remarkable spectacle."[434] On the dorsal side of the
body there is a series of processes which are called the dorsal tentacles.
The statolith is very small, and is not surrounded by sensory processes as
it is in _Ctenoplana_. There are no ctenophoral plates. The colours of the
Japanese {423}species are scarlet or carmine red and dirty brown or
brownish yellow. They are from 1 to 2 centimetres in diameter.


CLASS II. NUDA

Ctenophora without tentacles.

FAM. BEROIDAE.—_Beroe_, the only genus of this family and class, differs
from other Ctenophora in several important particulars. There are no
tentacles, and the stomodaeum is so large that the body-form assumes that
of a thimble with moderately thick walls. The infundibulum is small. The
paragastric and longitudinal canals give rise to numerous ramifications
which form a network distributed throughout the surface of the body. The
statolith is unprotected by a dome, and the polar fields are bordered by a
number of small branching papillae. The eight ribs extend for nearly the
whole length of the body. _Beroe_ is almost cosmopolitan, and is frequently
found at the surface of the sea in great numbers. _B. ovata_ is found off
the Shetlands, Hebrides, and west coast of Ireland, but is rare on the east
coast of the British Islands and in the English Channel. At Valencia it is
common in August and September, and sometimes reaches the great size of 90
mm. in length by 50 mm. in breadth. It is usually of a pale pink colour.


APPENDIX TO CTENOPHORA

_Hydroctena salenskii_ has recently been discovered by Dawydoff[435]
floating with the Plankton off the island Saparua in the Malay Archipelago.
It is claimed to be a connecting link between the Ctenophora and the
Medusae of the Hydrozoa.

In external features it is like one of the Narcomedusae, having a
transparent jelly-like bell with a wide bell-mouth guarded by a velum (Fig.
184, _V_). There are only two simple but solid tentacles (_t_), provided
with tentacle-sheaths, but inserted on opposite sides of the bell—not on
the margin, but, as in the Ctenophore, at a level not far removed from the
aboral pole. At the aboral pole there is a minute pore surrounded by a high
ciliated epithelium bearing an orange pigment. This leads into {424}a short
blind canal, which terminates in an ampulla bearing two statoliths
supported by elastic processes from the ampullar epithelium.

The sub-umbrellar cavity extends for a distance of about one-half the
height of the bell. The mouth (_M_), which opens into this cavity, leads
into a wide cavity that gives off a short blind canal to the side of each
tentacular sheath, and a straight tube that leads straight to the
statocyst, where it also ends blindly. There are no radial canals and no
ring canal at the margin of the umbrella. There are also no ctenophoral
plates. In the absence of any information concerning the position of the
genital glands, the character of the epithelium of the tentacles and the
development, we are not justified in regarding _Hydroctena_ either as a
Ctenophore or as a connecting link between the Ctenophora and the
Hydromedusae. It may be regarded simply as a Craspedote Medusa, probably
related to the Narcomedusae, with a remarkable aberrant aboral sense-organ.

[Illustration: FIG. 184.—_Hydroctena salenskii._ _ab_, Aboral organ; _M_,
manubrium; _t_, tentacle; _V_, velum. (After Dawydoff.)] ECHINODERMATA

BY

E. W. MACBRIDE, M.A., FRS.

Formerly Fellow of St. John's College
Professor of Zoology in McGill University, Montreal.

{427}CHAPTER XVI

ECHINODERMATA—INTRODUCTION—CLASSIFICATION—ANATOMY OF A STARFISH—SYSTEMATIC
ACCOUNT OF ASTEROIDEA


The name Echinodermata[436] means literally "spiny-skinned," and thus
brings into prominence one very conspicuous feature of most of the animals
belonging to this phylum. All, it is true, do not possess spines; but with
one or two doubtful exceptions, all have calcareous plates embedded in the
skin, and these plates, in many cases, push out projections which raise the
skin into corresponding elevations, which are called the spines. The spines
are, like the other plates, inside the skin, and to speak of an Echinoderm
living in its shell, as we speak of a Snail, is a serious error. The shell
of a Mollusc is fundamentally a secretion poured forth from the skin, and
is thus entirely external to the real living parts; but the plates and
spines of an Echinoderm may be compared to our own bones, which are
embedded deeply in the flesh. Hence the name _ossicle_ (little bone) is
used to designate these organs.

Besides the possession of these spines, Echinoderms are characterised by
having their organisation pervaded by a fundamental radial symmetry. The
principal organs of the body are repeated and are arranged like the spokes
of a wheel round a central axis instead of being, as, for example, in
Chaetopoda, arranged behind one another in longitudinal series.

In addition to these striking peculiarities, Echinoderms possess a most
interesting internal organisation, being in this respect almost exactly
intermediate between the Coelenterata {428}and the higher Invertebrata.
Like so many of the latter, the Echinodermata have an anus, that is, a
second opening to the alimentary canal through which indigestible material
is rejected; like them also, they have a body-cavity or coelom surrounding
the alimentary canal—from the lining of which the genital cells are
developed. On the other hand, there is no definite circulatory system, nor
any specialised excretory organ, and the nervous system exhibits no
concentration which could be called a brain, and is, moreover, in close
connexion with the skin. In all these points the Echinodermata resemble the
Coelenterata.

One of the most characteristic features of the internal anatomy of
Echinodermata is the presence of a peculiar series of organs, known
collectively as the water-vascular system or HYDROCOEL. This is really a
special division of the coelom or body-cavity which takes on the form of a
ring-shaped canal embracing the mouth, from which are given off long radial
canals, usually five in number, running to the more peripheral parts of the
body.[437] Each radial canal carries a double series of lateral branches,
which push out the skin so as to appear as appendages of the body. These
appendages are known as tentacles or tube-feet; they are both sensory and
respiratory in function, and often in addition, as the name TUBE-FOOT
indicates, assist in locomotion. As a general term for these appendages, to
be applied in all cases without reference to their function, the name
PODIUM has been suggested and will be employed here. A system of canals, in
many ways resembling the water-vascular system, is found in Brachiopoda,
Gephyrea and Polyzoa, but the peculiarity of Echinodermata is the way in
which it is kept filled with fluid. From the ring-canal in the interval (or
INTERRADIUS) between two radial canals, a vertical canal, termed the
STONE-CANAL, is given off, which communicates with the exterior by means of
a sieve-like plate, the MADREPORITE, pierced by fine canals. These canals
and the stone-canal itself are lined with powerful cilia, which produce a
strong inward current, and keep the water-vascular system tensely filled
with sea water.

The phylum includes the familiar Starfish and Sea-urchins, which in
sheltered spots are found between tide-marks; the {429}Brittle Stars and
Sea-cucumbers, which can be dredged up from below low-water mark, and
lastly the beautiful Feather-stars, of which there are comparatively few
species still living, although huge beds of limestone are composed of the
remains of fossil Feather-stars.

One species of Sea-cucumber (_Synapta similis_)[438] is said to enter
brackish water in the mangrove swamps of the tropics; but, with this
exception, the whole phylum is marine. A few species can endure partial
exposure to the air when left bare by the receding tide, but the
overwhelming majority are only found beneath low-water mark, and a
considerable number live in the deepest recesses of the ocean.

Their distribution is, no doubt, partly determined by food, a number of
species being strictly confined to the neighbourhood of the shore. On the
other hand, since a very large number of species live on the layer of mud
impregnated with animal remains which forms the superficial layer of the
deposit covering the sea-floor, it is not surprising to learn that many
have an exceedingly wide range, since this deposit is very widely
distributed. Another equally important factor in determining distribution
is wave-disturbance, and it is surprising to learn to what a depth this
extends. Off the west coast of Ireland a large wave literally breaks on a
submerged rock 15 fathoms beneath the surface. Speaking generally, it is
useless to look for Echinoderms on an exposed coast, and the same species,
which in the sheltered waters of the Clyde are exposed at low water, must
be dredged up from 20 to 30 fathoms outside Plymouth Sound.

The ordinary collector is attracted to the group chiefly by the regularity
and beauty of the patterns produced by the radial symmetry, but to the
scientific zoologist they are interesting from many other points of view.
Differing widely nevertheless from the higher Invertebrata in their
symmetry when adult, they have as larvae a marked bilateral symmetry, and
the secondary development of the radial symmetry constitutes one of the
most remarkable life-histories known in the animal kingdom.

Then again, owing to the possession of ossicles, the Echinodermata are one
of the few groups of Invertebrata of which abundant remains occur
fossilised. In attempting, therefore, to {430}decipher the past history of
life from the fossil record, it is necessary to have an exact and detailed
knowledge of Echinoderm skeletons and their relation to the soft parts.
Lastly, the internal organisation of Echinoderms throws valuable light on
the origin of the complicated systems of organs found in the higher
animals.

Echinodermata are divided into two great sub-phyla, which must have very
early diverged from one another. These are:—

  (1) Eleutherozoa,
  (2) Pelmatozoa.[439]

The sub-phylum PELMATOZOA, to which the living Feather-stars (CRINOIDEA)
and the majority of the known fossil species belong, is characterised by
the possession of a fixing organ placed in the centre of the surface
opposite the mouth—the aboral surface as it is called. Ordinarily this
organ takes on the form of a jointed stalk, but in most modern species it
is a little knob with a tuft of rooting processes, termed cirri. In the
other sub-phylum, the ELEUTHEROZOA, no such organ is found, and the animals
wander about freely during their adult life, though for a brief period of
their larval existence they may be fixed by a stalk-like protuberance
arising from the _oral_ surface.


SUB-PHYLUM I. ELEUTHEROZOA

The ELEUTHEROZOA are divided into four main classes, between which no
intermediate forms are found amongst the living species, though
intermediate types have been found fossil.

The four classes into which the Eleutherozoa are divided are defined as
follows:—

(1) ASTEROIDEA (Starfish).—"Star"-shaped or pentagonal Eleutherozoa with
five or more triangular arms, not sharply marked off from the central disc.
The mouth is in the centre of one surface, called from this circumstance
the "oral"; the anus is in the centre of the opposite surface, termed the
"aboral." From the mouth a groove runs out on the under surface of each
{431}arm towards its tip, termed the "ambulacral" groove. Projecting from
the ambulacral groove are found the podia or tube-feet, the organs of
movement and sensation of the animal.

(2) OPHIUROIDEA (Brittle Stars).—Eleutherozoa, in which the body consists
of a round disc with long worm-like arms inserted in grooves on its under
surface. No anus is present, and the ambulacral grooves are represented by
closed canals. The podia are merely sensory and respiratory, locomotion
being effected by muscular jerks of the arms.

(3) ECHINOIDEA (Sea-urchins).—Globular or disc-shaped Eleutherozoa, in
which the skeleton forms a compact cuirass except for a short distance
round the mouth (peristome) and round the anus (periproct).  The ambulacral
grooves are represented by canals which, like meridians of longitude on a
school-globe, run from the neighbourhood of the mouth to near the aboral
pole of the body. The spines are large and movably articulated with the
plates. The animals move by means of podia and spines, or by means of the
latter only. The anus is usually situated at the aboral pole, but is
sometimes displaced towards the side, or even on to the ventral surface.

(4) HOLOTHUROIDEA (Sea-cucumbers).—Sausage-shaped Eleutherozoa, in which
the skeleton is represented only by isolated nodules of calcium carbonate,
and in which the body-wall is highly muscular. The mouth and anus are
situated at opposite ends of the body, and the ambulacral grooves
(represented by closed canals) run from near the mouth to the proximity of
the anus. Movement is accomplished by means of the podia, aided by
worm-like contractions of the body.


CLASS I. ASTEROIDEA[440] (STARFISH)

The Starfish derive their name from their resemblance in shape to the
conventional image of a star. The body consists of broad triangular arms
(generally five in number) which coalesce in the centre to form a disc. The
skin is soft and {432}semi-transparent, permitting the skeleton to be
easily detected; this consists of a mesh-work of rods or plates, leaving
between them intervals of soft skin. In a living Starfish it can be seen
that many of these soft places are raised up into finger-like outgrowths,
which are termed "papulae" or "dermal gills," through the thin walls of
which an active interchange of gases with the surrounding water takes
place, and the animal obtains in this way the oxygen necessary for its
respiration.

Very few and feeble muscle-fibres exist in the body-wall, and the movements
of the arms, as a whole, are very slow and limited in range. There is a
membranous lip surrounding the mouth, from which five broad grooves run
outwards, one on the underside of each arm. These are termed the
"ambulacral grooves." Each groove is Λ-shaped, and its sides are stiffened
by a series of rod-like ossicles called the "ambulacral ossicles."

The animal progresses by the aid of a large number of translucent
tentacles, termed "tube-feet" or "podia," which are attached to the walls
of the ambulacral grooves.

ANATOMY OF A STARFISH.—As an introduction to the study of the anatomy not
only of Starfish but of Echinodermata as a whole, we select _Asterias
rubens_, the common Starfish of the British coasts, which in many places
may be found on the beach near low-water mark.

EXTERNAL FEATURES.—In this species (Fig. 185) the skeleton is a net-work of
rod-like plates, leaving wide meshes between them, through which protrude a
perfect forest of transparent papulae. From the points of junction of the
rods arise short blunt spines surrounded by thick cushions of skin. The
surfaces of these cushions are covered with a multitude of whitish specks,
which, on closer inspection, are seen to have the form of minute pincers,
each consisting of two movable blades crossing each other below and
articulated to a basal piece. These peculiar organs are termed
"pedicellariae" (Fig. 186), and their function is to keep the animal clean
by seizing hold of any minute organisms which would attempt to settle on
the soft and delicate skin. When irritated the blades open and then snap
together violently, and remain closed for a long time.[441] These actions
are brought about by appropriate muscles attaching the blades to the basal
piece.

{433}[Illustration: FIG. 185.—_Asterias rubens_, seen from the aboral
surface, × 1. _mad_, Madreporite.]

The last-named ossicle increases the certainty of the grip by fixing the
lower parts of each blade in the same vertical plane, and preventing
lateral slipping, so that it serves the same purpose as the pivot in a pair
of scissors. Each blade, in fact, fits into a groove on the side of this
piece. The muscles which close the blades arise from the lower ends
(handles) of the blades, and are united below to form a common muscular
string which attaches the whole organ to one of the plates of the skeleton.
An attempt of the victim to tear the pedicellaria out is resisted by the
contraction of this string, which thus brings about a closer grip of the
blades. In order that the blades may open they must first be lifted out of
the grooves on the basal piece—this is effected by special lifting muscles.
The opening is {434}brought about by muscles extending from the "handle" of
one blade to the upper part of the other.

Scattered about amongst the papulae between the cushions are other
pedicellariae of a larger size in which the blades do not cross one another
(Fig. 186, B).

In the space or "interradius" between two arms, on the aboral surface,
there is found a button-shaped ossicle. This is covered with fine grooves,
and from a fancied resemblance between it and some forms of coral it has
received the name "madreporite" (Fig. 185, _mad_). The bottoms of the
grooves are perforated by capillary canals lined by flagella, through the
action of which water is constantly being introduced into the
water-vascular system.

The anus is situated near the centre of the upper surface of the disc, but
it is so minute as to require careful inspection in order to discover its
position (Fig. 185).

[Illustration: FIG. 186.—View of pedicellariae of _A. glacialis_. A,
Crossed form, × 100. 1, Ectoderm covering the whole organ; 2, basal piece;
3, auxiliary muscle closing the blades; 4, muscle lifting right blade out
of the groove; 5, handle of left blade; 6, muscles closing the blades, and
uniting to form 7, the muscular string attaching the pedicellaria to the
skeleton. B, straight form, × 10. 1, Basal piece; 2, blades; 3 and 4,
muscles closing the blades; 5, muscle opening the blades. (From Cuénot.)]

On the under side of the animal the most conspicuous features are the five
ambulacral grooves which radiate out from the "peristome," a thin
membranous area surrounding the central mouth. The grooves are filled with
the tube-feet, which are closely crowded together and apparently arranged
in four rows.

SKELETON.—The sides of the ambulacral grooves are stiffened by the rod-like
"ambulacral ossicles." To the outer ends of these are articulated a set of
shorter rods termed the "adambulacral ossicles" which carry each two or
three rod-like spines, the "adambulacral spines," the skin covering which
bears numerous pedicellariae (Fig. 187, B). When the animal is irritated
the edges of the groove are brought together, and these {435}spines then
form a trellis-work covering and protecting the delicate tube-feet; the
numerous pedicellariae are then in a position to make it unpleasant for any
intruder. The closure of the groove is effected by means of powerful
muscles connecting each ambulacral ossicle with its fellow. There are also
feebler muscles connecting these plates with their successors and
predecessors, which enable the arm to be bent downwards in a vertical
plane. It is raised by a muscular band running along the dorsal wall of the
coelom to the point of the arm.

[Illustration: FIG. 187.—A, _Asterias rubens_, seen from the oral surface,
drawn from a living specimen, × 1. B, an adambulacral spine, showing three
straight pedicellariae; C, a tube-foot expanded and contracted.]

When the series of ambulacral and adambulacral ossicles is followed inwards
towards the mouth it is seen that the first ambulacral ossicle is closely
fixed to the second, but is widely {436}separated from its fellow,
remaining, however, connected with the latter by a powerful adductor
muscle. In consequence of the separation of this pair of ossicles each is
brought into closer contact with the corresponding ossicle in the adjacent
radius, to which it is connected by a muscle called the abductor. The first
adambulacrals in adjacent radii are also brought into closer contact and
carry long spines which, when the ambulacral grooves are contracted,
project like a grating over the mouth. In the order of Asteroidea to which
_Asterias_ belongs, the adambulacrals themselves do not project much, but
in all other cases they form prominent mouth-angles, so that the opening of
the mouth becomes star-shaped (Fig. 211, p. 483).

Except in the case of the ambulacral and adambulacral plates little regular
arrangement is to be detected in the ossicles of the skeleton which, as has
already been mentioned, form a mesh-work. If, however, the arm be cut open
and viewed from the inside it will be seen that the edge is strengthened
above and below by very thick, powerful, rod-like plates. These are called
the "supero-marginal" and "infero-marginal" ossicles; they are not visible
from the outside, since they are covered by a thick layer of the body-wall
containing other smaller plates (Fig. 190, _marg_). In many genera,
however, they are exposed, and form a conspicuous edging to the arm above
and below. In many genera, also, there are three conspicuous series of
plates on the back of each arm, viz. a median row, called "carinals"
(_car._, Fig. 191), and two lateral rows, termed "dorso-laterals"
(_d.lat._, Fig. 191). These three rows, with the two rows of marginals, one
of ambulacrals, and one of adambulacrals on each side (11 rows in all),
constitute the primitive skeleton of the arm, and appear first in
development.

The structure of all these elements of the skeleton is the same. They may
be described as scaffoldings of carbonate of lime, interpenetrated by a
mesh-work of cells fused with one another, by which the carbonate of lime
has been deposited. The matrix in which the ossicles lie is a jelly-like
substance traversed by a few bands of fibres which connect the various rods
with one another. This jelly is almost fluid in the fresh state, but when
heated forms a hard compound, possibly allied to mucin, which will turn the
edge of a razor.

When the covering of the back is dissected off the COELOM is {437}opened.
This is a spacious cavity which apparently surrounds the alimentary canal
and extends into the arms. It has, however, its own proper wall, which is
called the "peritoneum," both on the outer side, where it abuts on the
skin, and on the inner side, where it comes in contact with the wall of the
alimentary canal. The outer wall is called the "somatic peritoneum," and it
is possible to dissect off the rest of the body-wall and leave it intact;
the inner wall, from its close association with the alimentary canal, is
termed the "splanchnic peritoneum." This wall can only be distinguished in
microscopic sections from that of the alimentary canal, to which it is
closely applied.

The coelom is filled with a fluid, which is practically sea water with a
little albuminous matter in solution. Through the thin walls of the papulae
oxygen passes into this fluid, whence it easily reaches the inner organs,
since they are all in contact with some part of the coelomic wall.
Similarly CO_{2} is absorbed by the coelomic fluid from all parts of the
body, and diffuses through the papulae to the surrounding water.

The Starfish possesses no definite KIDNEY for getting rid of nitrogenous
waste. In most of the higher animals with a well-developed coelom it has
been proved that the kidney is simply a specialised portion of the coelom,
and in many cases some parts of the coelomic wall still retain their
excretory functions, which apparently the whole originally possessed. In
the Starfish and in Echinodermata generally this primitive state of affairs
is still retained. From the cells forming the coelomic wall, cells are
budded off into the fluid, where they swim about. These cells from their
movements are called amoebocytes. If a substance such as indigo-carmine,
which when introduced into the tissues of the higher animals is eliminated
by the kidney, is injected into the Starfish, it is found soon after to be
vigorously absorbed by the amoebocytes. These later accumulate in the
dermal branchiae, through the thin walls of which they make their way[442]
to the outside, where they degenerate.

The coelom is indented by five folds, which project inwards from the
interradii. These folds are called the "interradial septa"; they are
stiffened by a calcareous deposit, which is not, however, sufficiently
dense to constitute a plate. In one of the {438}septa the axial sinus and
stone-canal (see below) are embedded. These septa are to be regarded as
areas of lateral adhesion between the arms.

[Illustration: FIG. 188.—View of upper half of a specimen of _Asterias
rubens_, which has been split horizontally into two halves. _ax.c_, Axial
sinus; _g.d_, genital duct; _oe_, cut end of the oesophagus, the narrow
neck of the stomach; _py_, pyloric sac; _py.c_, pyloric caeca; _r_, rectum;
_r.c_, rectal caeca; _sept_, interradial septum; _st.c_, stomach lobe.]

The ALIMENTARY CANAL consists of several distinct portions. The mouth leads
by a narrow neck called the "oesophagus" into a voluminous baggy sac termed
the "stomach," which is produced into ten short pouches, two projecting
into each arm. The stomach leads in turn by a wide opening into a
pentagonal flattened sac, the "pyloric sac," which lies above it. Each
angle of the pyloric sac is prolonged into a tube—the so-called "pyloric
duct"—running out into the arm, where it immediately bifurcates into two
forks, each beset by a large number of small pouches {439}and attached to
the dorsal wall of the coelom by suspensory bands of membrane called
mesenteries. These ten forks are called "pyloric caeca"; they are of a deep
green colour owing to the pigment in their wall. Beyond the pyloric sac the
alimentary canal is continued as the slender "rectum" to the anus. The
rectum gives off two small branched pouches of a brown colour called
"rectal caeca." This comparatively complicated form of alimentary canal is
related to the nature of the food of the animal and the method it employs
to capture its prey.

[Illustration: FIG. 189.—View of a Starfish (_Echinaster_) devouring a
Mussel. 1. The madreporite.]

The favourite FOOD[443] of _Asterias_ consists of the common bivalves of
the coast, notably of the Mussel (_Mytilus edulis_). There is, however, no
animal which it will not attack if it is fortunate enough to be able to
catch it. The Starfish seizes its prey by the tube-feet, and places it
directly under its mouth, folding its arms down over it in umbrella
fashion. The muscles which run around the arms and disc in the body-wall
contract, and the pressure thus brought to bear on the incompressible fluid
contained in the coelom, forces out the thin membranous peristome and
partially turns the stomach inside out. The everted edge of the stomach is
wrapped round the prey.

{440}Soon the bivalve is forced to relax its muscles and allow the valves
to gape. The edge of the stomach is then inserted between the valves and
applied directly to the soft parts of the prey which is thus completely
digested. When the Starfish moves away nothing but the cleaned shell is
left behind. If the bivalve is small it may be completely taken into the
stomach, and the empty shell later rejected through the mouth.

It was for a long time a puzzle in what way the bivalve was forced to open.
Schiemenz[444] has, however, shown that when the Starfish folds itself in
umbrella-like form over the prey it holds on to the substratum by means of
the tube-feet of the distal portions of the arms, whilst, by means of the
tube-feet belonging to the central portions, it drags apart the valves by
main force. He has shown experimentally: (1) that whilst a bivalve may be
able to resist a sudden pull of 4000 grammes it will yield to a pull of 900
grammes long continued; (2) that a Starfish can exert a pull of 1350
grammes; (3) that a Starfish is unable to open a bivalve unless it be
allowed to raise itself into a hump, so that the pull of the central
tube-feet is at right angles to the prey. A Starfish confined between two
glass plates walked about all day carrying with it a bivalve which it was
unable to open.

The lining of the stomach is found to consist very largely of mucus-forming
cells, which are swollen with large drops of mucus or some similar
substance. It used to be supposed that this substance had some poisonous
action on the prey and paralysed it, but the researches of Schiemenz show
that this is incorrect. If when an _Asterias_ is devouring a bivalve
another be offered to it, it will open it, but will not digest it, and the
victim shows no sign of injury but soon recovers. The cells forming the
walls of the pyloric sac and its appendages are tall narrow cylindrical
cells crowded with granules which appear to be of the nature of digestive
ferment. This substance flows into the stomach and digests the captured
prey.

A very small amount of matter passes into the rectum and escapes by the
anus, as the digestive powers of the Starfish are very complete. The rectal
caeca are lined by cells which secrete from the coelomic fluid a brown
material, in all probability an excretion, which is got rid of by the anus.

{441}When the meal is finished the stomach is restored to its former place
by the action of five pairs of retractor muscles, one pair of which
originates from the upper surface of the ambulacral ossicles in each arm
and extends to the wall of the stomach, where they are inserted (Fig. 190,
_ret_).

The tube-feet, which are at once the locomotor and the principal sensory
organs of the Starfish, are appendages of that peculiar system of tubes
known as the WATER-VASCULAR SYSTEM, which is derived from a part of the
coelom cut off from the rest during the development of the animal. This
system, as already mentioned, consists of (1) a narrow "ring-canal,"
encircling the mouth and lying on the inner surface of the membranous
peristome; (2) a radial canal leaving the ring-canal and running along the
under surface of each arm just above the ambulacral groove; (3) a vertical
stone-canal running from the madreporite downwards to open into the
ring-canal in the interspace between two arms. The madreporite is covered
externally by grooves lined with long cilia, and is pierced with narrow
canals of excessively fine calibre, the walls of which are also lined by
powerful cilia. Most of these narrow canals open below into a main
collecting canal, the stone-canal, but some open into a division of the
coelom termed the axial sinus, with which also the stone-canal communicates
by a lateral opening. The cavity of the stone-canal is reduced by the
outgrowth from its walls of a peculiar Y-shaped projection, the ends being
rolled on themselves in a complicated way (Fig. 190, B). The walls of the
canal consist of a layer of very long narrow cells, which carry powerful
flagella, and outside this of a crust of calcareous deposit, which gives
rigidity to the walls and has suggested the name stone-canal.

The tube-feet are covered externally by ectoderm, inside which is a tube in
connexion with the radial water-vascular canal. This latter is lined by
flattened cells, which in the very young Starfish are prolonged into
muscular tails; in the older animal these tails are separated off as a
distinct muscular layer lying between the ectoderm and the cells lining the
cavity of the tube. The tube-foot is prolonged inwards into a bulb termed
the "ampulla," which projects into the coelom of the arm and in consequence
is covered outside by somatic peritoneum. Just where the ampulla passes
into the tube-foot proper the organ passes downwards between two of the
powerful ambulacral ossicles which support {442}the ambulacral groove, and
a little below this spot a short transverse canal connects the tube-foot
with the radial canal which lies beneath these ossicles (Fig. 191).

[Illustration: FIG. 190.—A, view of the under half of a specimen of
_Asterias rubens_, which has been horizontally divided into two halves. B,
enlarged view of the axial sinus, stone-canal and genital stolon cut
across. _amb.oss_, Ambulacral ossicle; _amp._ ampullae of the tube-feet;
_ax.s_, axial sinus; _gon_, gonad; _g.stol_, genital stolon; _marg_,
marginal ossicle; _nerv.circ_, nerve ring; _oe_, cut end of oesophagus;
_pst_, peristome; _ret_, retractor muscle of the stomach; _sept_,
interradial septum; _stone c_, stone-canal; _T_, Tiedemann's body; _w.v.r_,
water-vascular ring-canal.]

The tube-feet are, therefore, really a double row of lateral branches of
the radial canal. The appearance of being arranged in four rows is due to
the fact that the transverse canals connecting them with the radial canal
are alternately longer and shorter so as to give room for more tube-feet in
a given length of the arm. Each tube-foot ends in a round disc with a
slightly thickened edge. The radial canal terminates in a finger-shaped
{443}appendage, called the median tentacle, at the base of which is the
eye.

The manner in which this complicated system acts is as follows:—When the
tube-foot is to be stretched out the ampulla contracts and drives the fluid
downwards. The contraction of the ampulla is brought about by muscles
running circularly around it. The tube-foot is thus distended and its broad
flattened end is brought in contact with the surface of the stone over
which it is moving and is pressed close against it. The muscles of the
tube-foot itself, which are arranged longitudinally, now commence to act,
and the pressure of the water preventing the tearing away of the sucker
from the object to which it adheres, the Starfish is slowly drawn forward,
whilst the fluid in the tube-foot flows back into the ampulla.

[Illustration: FIG. 191.—Diagrammatic cross-section of the arm of a
Starfish. _adamb_, Adambulacral ossicle; _amb_, ambulacral ossicle; _amp_,
ampulla of tube-foot; _branch_, papula; _car_, carinal plate; _d.lat_,
dorso-lateral plate; _inf.marg_, infero-marginal plate; _p.br_,
peribranchial space; _ped_, pedicellaria; _s.marg_, supero-marginal plate.
The nervous ridge between the bases of the tube-feet and the two perihaemal
canals above this ridge are shown in the figure but not lettered.]

If each tube-foot were practically water-tight, then each would be entirely
independent of all the rest, and it would not be easy to suggest a reason
for the presence of the complicated system of radial canals and
stone-canal. Just at the spot, however, where the transverse canal leading
from the radial canal enters the tube-foot there is a pair of valves which
open inwards and allow fluid to pass from the radial canal into the
tube-foot but prevent any passing outwards in the reverse direction. The
presence of these valves renders it probable that the tube-foot is not
quite water-tight; that when it is distended under the pressure produced by
the contraction of the muscles of the ampulla, some fluid escapes through
the permeable walls; and {444}that the loss thus suffered is made up by the
entry of fresh fluid from the radial canal. The radial canal in turn draws
from the ring-canal, and this last is supplied by the stone-canal, the
cilia of which keep up a constant inward current.

In the fluid contained in the water-vascular system, as in the coelomic
fluid, there are amoebocytes floating about. These are produced in short
pouches of the ring-canal, nine in number, which are called after their
discoverer "Tiedemann's bodies" (Fig. 190, T). From the cells lining these
the amoebocytes are budded off.

The NERVOUS SYSTEM of the Starfish is in a very interesting condition. The
essential characteristic of all nervous systems is the presence of the
"neuron," a cell primitively belonging to an epithelium but which generally
has sunk below the level of the others and lies amongst their bases. This
type of cell possesses a round body produced in one direction into a long
straight process, the "axon," whilst in the other it may have several
root-like processes, or "dendrites," which may spring from a common stem,
in which case the neuron is said to be "bipolar." The axon is often
distinguished as a "nerve-fibre" from the round body which is termed the
"nerve-cell." This is due to the fact that for a long time it was not
recognised that these two structures are parts of a whole.

Now at the base of the ectoderm all over the body of the Starfish there is
to be found a very fine tangle of fibrils; these are to be found partly in
connexion with small bipolar neurons lying amongst them and partly with
isolated sense-cells scattered amongst the ordinary ectoderm cells. This
nervous layer is, however, very much thickened in certain places, so as to
cause the ectoderm to project as a ridge. One such ridge is found at the
summit of each ambulacral groove running along the whole under surface of
the arm and terminating in a cushion at the base of the median tentacle of
the water-vascular system. This ridge is called the radial nerve-cord. The
five radial nerve-cords are united by a circular cord, the nerve-ring,
which appears as a thickening on the peristome surrounding the mouth.

The sense-organs of the Starfish are chiefly the discs of the tube-feet.
Round the edges of these there is a special aggregation of sense-cells;
elsewhere, as in the skin of the back, only {445}isolated sense-cells are
found, and it becomes impossible to speak of a sense-organ.

A prolongation of the radial nerve-cord extends outwards along one side of
each tube-foot. This is often spoken of as the "pedal nerve," but the term
nerve is properly retained for a mere bundle of axons such as we find in
the higher animals, whereas the structure referred to contains the bodies
of nerve-cells as well as their outgrowths or cell-fibres and is therefore
a prolongation of the nerve-cord.

[Illustration: FIG. 192.—Diagrammatic longitudinal section through a young
Asteroid passing through the tip of one arm and the middle of the opposite
interradius. This diagram is generalised from a section of _Asterina
gibbosa_. _ab_, Aboral sinus; _ax_, axial sinus; _ax_^1, basal extension of
axial sinus forming the inner perihaemal ring-canal; _br_, branchia = gill
= papula; _g.r_, genital rachis; _mp_, madreporite; _musc.tr_, muscle
uniting a pair of ambulacral ossicles; _nerv.circ_, nerve-ring; _n.r_,
radial nerve-cord; _oc_, eye-pit; _oss_, ossicles in skin; _p.br_,
peribranchial sinus; _p.c_, pore canal; _perih_ (on the right), perihaemal
radial canal, (on the left), outer perihaemal ring-canal; _py_, pyloric
caecum; _rect_, rectum; _rect.caec_, rectal caeca; _sp_, spines; _st.c_,
stone-canal; _t_, median tentacle terminating radial canal; _w.v.r_,
water-vascular radial canal. The genital stolon (not marked by a reference
line) is seen as an irregular band accompanying the stone-canal, its upper
end projects into a small closed sac, also unmarked, which is the _right_
hydrocoele or madreporic vesicle.]

At the base of the terminal tentacle the radial nerve-cord ends in a
cushion. This cushion is called the "eye," for it is beset with a large
number of cup-shaped pockets of the ectoderm. Each pocket is lined partly
by cells containing a bright orange pigment and partly by visual cells each
of which ends in a small clear rod projecting into the cavity of the pit
(Fig. 193, A, _vis.r_). The pit is apparently closed by a thin sheet of
cuticle secreted by the most superficial cells.

An exposed nervous system and simple sense-organs such as the Starfish
possesses lend themselves admirably to the purposes {446}of physiological
experiment, and so Starfish have been favourite "corpora vilia" with many
physiologists.

[Illustration: FIG. 193.—A, longitudinal section of a single eye-pit of
_Asterias_. _s.n_, Nucleus of supporting cell; _vis.n_, nucleus of visual
cell; _vis.r_, visual rod. B, view of the terminal tentacle showing the
eye-pits scattered over it. (After Pfeffer.)]

The light-perceiving function of the eye is easily demonstrated. If a
number of Starfish be put into a dark tank which is illuminated only by a
narrow beam of light they will be found after an interval to have collected
in the space reached by the beam of light.[445] If all the median tentacles
but one be removed this will still be the case; if, however, they are all
removed the Starfish will exhibit indifference to the light.

If the under surface of a Starfish be irritated by an electric shock or a
hot needle, or a drop of acid, the tube-feet of the affected area will be
strongly retracted, and this irritation will be carried by the pedal nerves
to the radial nerve-cord, with the result that finally all the tube-feet in
the groove will be retracted and the groove closed by the action of the
transverse muscle connecting each ambulacral ossicle with its fellow. If,
on the other hand, the back of a Starfish be irritated this may produce a
contraction of the tube-feet if the irritation be strong, but this will be
followed by active alternate expansions and contractions, in a word, by
endeavours to move. Preyer[446] by suspending a Starfish ventral surface
upward, by {447}means of a small zinc plate to which a string was attached
which passed through a hole bored in the back and through the mouth, caused
movements of this description which lasted for hours. Irritation of the
back causes also activity of the local pedicellariae, which open their
valves widely and then close them with a snap in the endeavour to seize the
aggressor.

The uninjured Starfish in moving pursues a definite direction, one arm
being generally directed forwards, but this may be any one of the five. The
tube-feet of this arm are directed forwards when they are stretched out, by
the slightly unequal contraction of the longitudinal muscles of opposite
sides of the foot, which persists even when the circular muscles of the
ampulla are contracting. They thus may be said to swing parallel to the
long axis of the arm. The tube-feet of the other arms assist in the
movement, and hence swing obliquely with reference to the long axis of the
arm to which they belong, although they move parallel to the general
direction in which the Starfish is moving. A change in the direction of the
swing of the tube-feet will bring about a change in the direction of the
movement of the animal as a whole. If now the connexion of each radial
nerve-cord with the nerve-ring be cut through, each arm will act as a
separate Starfish and will move its tube-feet without reference to the
movement of those in the other arms, so that the animal is pulled first one
way and then another according as the influence first of one arm and then
of another predominates. Similarly, when a Starfish is placed on its back,
it rights itself by the combined action of the tube-feet of all the arms,
extending them all as widely as possible, those which first catch hold
being used as the pivot for the turning movement. If, however, the radial
nerve-cords are cut through, each arm tries to right itself and it is only
by chance that the efforts of one so predominate as to turn the whole
animal over. From these experiments it is clear that the nerve-ring acts as
co-ordinator of the movements of the Starfish, that is to say as its brain.

If a section be taken across the arm of a Starfish (Fig. 191), it will be
seen that between the V-shaped ridge constituting the radial nerve-cord and
the radial water-vascular canal there are two canals lying side by side and
separated from one another by a vertical septum. These canals are not mere
splits in the {448}substance of the body-wall, but have a well-defined wall
of flattened cells. They are termed, for reasons which will be explained
subsequently, PERIHAEMAL CANALS, and they open into a circular canal called
the "outer perihaemal ring," situated just beneath the water-vascular
ring-canal (Fig. 192, _perih_). These canals originate as outgrowths from
the coelom. From their upper walls are developed the muscles which connect
the pairs of ambulacral ossicles and close the groove, and also those which
connect each ossicle with its successor and predecessor and help to elevate
or depress the tip of the arm.

In most of the higher animals the processes of many of the ganglion-cells
are connected together in bundles called "motor nerves," which can be
traced into contact with the muscles, and thus the path along which the
stimulus travels in order to evoke movement can clearly be seen. No such
well-defined nerves can be made out in the case of the Starfish, and it is
therefore interesting when exceptionally the paths along which stimuli
travel to the muscles can be traced. This can be done in the case of the
muscles mentioned above. Whereas they originate from the dorsal walls of
the perihaemal canals, ganglion-cells develop from the ventral walls of
these canals, which are in close contact with the nerve-cord, so that the
nervous system of the Starfish is partly ectodermic and partly coelomic in
origin. Stimuli reaching the ectodermic ganglion-cells are transmitted by
them to the nervous part of the wall of the perihaemal canal and from that
to the muscular portion of the same layer of cells.

Besides the radial perihaemal canals and their connecting outer perihaemal
ring there are several other tubular extensions of the coelom found in the
body-wall. These are:—

(1) The "inner perihaemal canal," a circular canal in close contact with
the inner side of the outer perihaemal canal (Fig. 192, _ax_^1).

(2) The "axial sinus" (_ax_) a wide vertical canal embedded in the
body-wall outside the stone-canal. This canal opens into the inner
perihaemal canal below; above it opens into several of the pore-canals and
into the stone-canal. The separation of the axial sinus from the rest of
the coelom is the remains of a feebly marked metamerism in the larva.

(3) The "madreporic vesicle," a closed sac embedded in the dorsal body-wall
just under the madreporite. This sac by its {449}history in the larva
appears to be a rudimentary counterpart of the water-vascular system, since
this organ in correspondence with the general bilateral symmetry of the
larva is at first paired. Into this a special process of the genital stolon
projects.

(4) The "aboral sinus" (Fig. 192, _ab_), a tube embedded in the dorsal
body-wall running horizontally round the disc. The aboral sinus surrounds
the genital rachis (see p. 452) and gives off into each arm two branches,
the ends of which swell so as to surround the genital organs. It has no
connexion with the axial sinus though the contrary has often been stated by
Ludwig.[447]

(5) The "peribranchial spaces," circular spaces which surround the basal
parts of the papulae (Fig. 192, _p.br_).

Besides these, large irregular spaces have been described as existing in
the body-wall by Hamann[448] and other authors, but for various reasons and
especially because they possess no definite wall they appear to be nothing
more than rents caused by the escape of CO_{2} gas during the process of
decalcifying, to which the tissues of the Starfish must be subjected before
it is easy to cut sections of them.

The question as to whether or not there is a BLOOD SYSTEM in the Starfish
has an interesting history. It must be remembered that the examination of
the structure of Echinodermata was first undertaken by human anatomists,
who approached the subject imbued with the idea that representatives of all
the systems of organs found in the human subject would be found in the
lower animals also. So the perihaemal canals were originally described as
blood-vessels. Later, Ludwig[449] discovered a strand of strongly staining
material running in each septum which separates the two perihaemal canals
of the arm. Each of these radial strands could be traced into connexion
with a circular strand interposed between the outer and the inner
perihaemal ring-canals. This circular strand again came into connexion with
a brown, lobed organ, lying in the wall of the axial sinus, and this in
turn {450}joined at its upper end a circular cord of pigmented material
adhering to the dorsal wall of the coelom (lying in fact within the aboral
sinus), from which branches could be traced to the generative organs.
Ludwig concluded that he had at last discovered the true blood-vessels,
though the facts that the radial strands and the oral circular strand
absorbed neutral carmine strongly and that the vertical and aboral strands
were pigmented, constituted a very slender basis on which to found such a
conclusion. The colour apparently appealed to the imagination, and it is
undoubtedly true that the "plasma" or blood-fluid of other animals often
absorbs stain strongly.

The strands were accordingly named "radial blood-vessels," "oral
blood-ring," "aboral blood-ring"; and the brown vertical strand was called
the "heart," although no circulation or pulsations had ever been observed.
When later investigations revealed the fact that the so-called heart was
practically solid, the term "central blood-plexus" was substituted for
heart, although it was still regarded as the central organ of the system.
The name "perihaemal" was given to the spaces so called because they
surrounded the supposed blood-vessels.

In order to come to a satisfactory conclusion on the matter some general
idea as to the fundamental nature and function of the blood-vessels in
general must be arrived at. Investigations made on various groups of
animals, such as Annelida, Mollusca, Crustacea, Vertebrata, show that at an
early period of development a considerable space intervenes between the
alimentary canal and the ectoderm, which is filled with a more or less
fluid jelly. Into this cavity, the so-called "primary body-cavity" or
"archicoel," amoebocytes, budded from the ectoderm or endoderm or both,
penetrate. In this jelly with its contained amoebocytes is to be found the
common rudiment both of the connective tissue and of the blood system. The
resemblance of the archicoele and its contents to the jelly of a Medusa is
too obvious to require special insistence on, and therefore in the
Coelenterata it may be stated that there is to be found a tissue which is
neither blood system nor connective tissue but is the forerunner of both.

In the higher animals as development proceeds the jelly undergoes
differentiation, for some of the amoebocytes become stationary and
connected with their pseudopodia so as to form a protoplasmic network. A
portion of this network becomes {451}altered into tough fibres, but a
portion of each strand remains living, and in this way the connective
tissue is formed. In the interstices of the network of fibres a semi-fluid
substance (the unaltered jelly) is found, and this is traversed by free,
wandering amoebocytes. In other places the jelly becomes more fluid and
forms the plasma, or liquid of the blood, whilst the amoebocytes form the
blood corpuscles. The blood system thus arises from regions of the
archicoel where fibres are not precipitated.

Now in the Starfish the whole substance of the body-wall intervening
between the ectoderm and the coelomic epithelium really represents the
archicoel. The formation of fibres has, it is true, proceeded to a certain
extent, since there are interlacing bundles of these, but there are left
wide meshes in which amoebocytes can still move freely. Apart from the
skeleton, therefore, the tissues of the body-wall of the Starfish do not
exhibit much advance on those of a Jellyfish. If anything is to be compared
to the blood system of the higher animals it must be these meshes in the
connective tissue. From observations made on other Echinoderms it appears
probable that the colour of the skin is due to amoebocytes loaded with
pigment wandering outwards through the jelly of the body-wall and
disintegrating there. The strands regarded as blood-vessels by Ludwig are
specially modified tracts of connective tissue in which fibres are sparse,
and in which there are large quantities of amoebocytes and in which the
"jelly" stains easily. Cuénot[450] suggests that they are placed where new
amoebocytes are formed; this is quite possible, and in this case they ought
to be compared to the spleen and other lymphatic organs of Vertebrates, and
not to the blood-vessels.[451]

The organ regarded as the heart, however, belongs to a different category:
it is really the original seat of the GENITAL CELLS and should be termed
the "genital stolon." Careful sections show that at its upper end it is
continuous with a strand of primitive germ-cells which lies inside the
so-called aboral {452}blood-vessel, and is termed the "genital rachis"
(Fig. 192, _g.r_). The germ-cells are distinguished by their large nuclei
and their granular protoplasm. The genital organs are only local swellings
of the genital rachis, and from the shape of some of the germ-cells it is
regarded as highly probable that the primitive germ-cells wander along the
rachis and accumulate in the genital organs. The genital rachis itself is
an outgrowth from the genital stolon, and this latter originates as a
pocket-like ingrowth of the coelom into the wall separating it from the
axial sinus; when fully formed it projects into and is apparently contained
in this latter space.

Not all the cells forming the genital stolon become sexual cells. Many
degenerate and become pigment-cells, a circumstance to which the organ owes
its brown colour. In very many species of Starfish many of the cells of the
genital rachis undergo a similar degeneration, and hence is produced the
apparent aboral blood-vessel. Further, the rachis is embedded in connective
tissue which has undergone what we may call the "lymphatic" modification,
and this for want of a better name we call the "aboral" blood-ring.

The size of the genital organs varies with the season of the year; they are
feather-shaped, and attached to the genital rachis by their bases, but
project freely into the coelom of the arm. From their great variation in
size and also from the shape of some of the cells in the genital rachis,
Hamann concludes that as each period of maturity approaches fresh
germ-cells are formed in the rachis and wander into the genital organ and
grow there in size. It is probable that the aboral end of the genital
stolon is the seat of the formation of new germ-cells.

In the Starfish, therefore, as in other animals with a well-defined coelom,
the genital cells ultimately originate from the coelomic wall.

The genital ducts are formed by the burrowing outwards of the germ-cells.
When it is remembered that the fundamental substance of the body-wall is
semi-fluid jelly, this process will be better understood.

When the ova and spermatozoa are ripe, they are simply shed out into the
sea and fertilisation occurs there. The development is described in Chapter
XXI. The free-swimming larval period lasts about six weeks.

{453}Having described a single species with some degree of fulness, we must
now give some account of the range of variation of structure met with in
the group.

NUMBER OF ARMS.—In the overwhelming majority of Starfish the number of arms
is 5, but deviations from this rule are met with not only as individual
variations, but as the characteristics of species, genera, and even
families.

The number 5 is rarely diminished, but amongst a large collection of
specimens of _Asterina gibbosa_, belonging to the author, some 4-rayed
individuals are met with. One species of _Culcita_, _C. tetragona_, is
normally 4-rayed.

On the other hand the number 5 is often exceeded. The families
Heliasteridae and Brisingidae are characterised by possessing numerous
(19-25) arms. In the normally 5-rayed family Asteriidae _Pycnopodia_ has 22
arms; and in the Solasteridae the genera _Rhipidaster_ and _Solaster_ are
characterised by possessing 8 and 11-15 arms respectively; whilst
_Korethraster_ and _Peribolaster_ have only 5. The common Starfish of the
Gulf of St. Lawrence, _Asterias polaris_, is 6-rayed, whilst most of the
other species of the same genus are 5-rayed, though 6 rays are often met
with as a variation.

In some species the fact that the number of arms exceeds 5 seems to be
connected with the power of multiplication by transverse fission. Thus
Ludwig[452] has shown that in _Asterias tenuispina_ the number of arms is
usually 7, but sometimes 5, 6, or 8, and that in most cases the arms are
arranged in two groups—one consisting of small arms, the other of large.

SHAPE.—Apart from the varying number of arms, differences in the shape of
the Starfish are due to two circumstances:—

(1) The proportion of breadth to length of arm; and

(2) The amount of adhesion between adjacent arms.

The adhesion can go so far that the animal acquires the shape of a
pentagonal disc. This is the case for instance in _Culcita_. The fact that
the body of this animal is really composed of adherent arms is at once made
clear when the coelom is opened. This space is found to be divided up by
inwardly projecting folds called interradial septa, which are stiffened by
calcareous deposits and represent the conjoined adjacent walls of two arms.

{454}In the family Heliasteridae the mutual adhesion between the arms has
gone on merely to a slight extent, for the interradial septa are still
double.

SKELETON.—Most of the schemes of classification have been founded on the
skeleton, largely because the greater number of species have only been
examined in the dried condition, and little is known of their internal
anatomy or habits. There is, however, this justification for this
procedure, that the habits and food of the species (with the exception of
the Paxillosa) which have been observed in the living condition appear to
be very uniform, and that it is with regard to the skeleton that Asteroidea
seem to have split into divergent groups through adopting different means
of protecting themselves from their foes.

The description of the various elements of the skeleton will be arranged
under the following heads:—(_a_) Main framework; (_b_) Spines; (_c_)
Pedicellariae; (_d_) Ambulacral skeleton.

(_a_) MAIN FRAMEWORK.—The type of skeleton which supports the body-wall of
_Asterias_ is called reticulate. As already indicated it consists of a
series of rods bound together by bundles of connective-tissue fibres so as
to form a mesh-work. This is a very common type of aboral skeleton, but in
a large number of Starfish a different type occurs, consisting of a series
of plates which may fit edge to edge, leaving between them only narrow
interstices, as in the Zoroasteridae, or which may be placed obliquely (as
in _Asterina_) so that they imbricate or overlap one another. In a very
large number of Asteroidea the supero- and infero-marginal ossicles are
represented by squarish plates even when the rest of the skeleton is
reticulate; this is the so-called "phanerozonate" structure, the term
"cryptozonate" being used when the marginals are rod-like and
inconspicuous. In other cases (Ganeriidae) the whole skeleton of the
ventral surface is made of tightly fitting plates, whilst the aboral
skeleton is either reticulate or made of imbricating plates. Lastly, the
skeleton may be represented only by nodules forming the bases of paxillae
(see p. 455), as in the Astropectinidae, or may be entirely absent over
wide areas (Brisingidae).

(_b_) SPINES.—The spines vary more than any other part of the skeleton.
They may be close set and small, or few and large, and often bear spines of
the second order, or spinelets, attached to them. In _Asterias_ and its
allies they are {455}comparatively short, blunt tubercles, covered with
thick skin. In the Echinasteridae and Asterinidae they are short and blunt,
but they are very numerous and thick set. In the Solasteridae they are
long, and arranged in bundles diverging from a common base. Such bundles
may be termed sheaves, and starting from an arrangement like this, two
distinct lines of modification may be traced. Thus (1) the members of a
sheaf become connected by a web of skin, so that the sheaf becomes an
umbrella, and successive umbrellas may adhere, so that a supra-dorsal tent
is formed (a structure characteristic of the Pterasteridae), or (2) the
members of a sheaf may become arranged in a circle round a central vertical
axis so that a structure like a capstan is produced, which is called a
"paxilla" (characteristic of Astropectinidae, Porcellanasteridae, and
Archasteridae). The axis,[453] as shown by its development, represents the
plate which bore the bundle of spines. Again, the skeleton may consist of
plates with a close covering of granules (Pentagonasteridae, etc.). Lastly,
in _Porania_ spines are absent, the plates being deeply embedded in a thick
leathery skin.

[Illustration: FIG. 194.—Views of portions of the aboral surface of
different genera of Asteroidea in order to show the main varieties of
skeleton. A, _Solaster_, showing spines arranged in sheaves; B,
_Pteraster_, showing webs forming supra-dorsal membrane supported by
diverging spines; C, _Astropecten_, showing paxillae; D, _Nardoa_, showing
uniform plating of granules. × 8. (After Sladen.)]

{456}(_c_) PEDICELLARIAE.—These are to be looked on as spines of the second
order. In _Asterina_ and its allies they are not present, but groups of
little spines arranged in twos and threes, each group being attached to a
special small plate, are scattered over the aboral surface; and these on
irritation approach one another, and represent the rudiment out of which
pedicellariae have been developed. The most perfect form, termed
"forcipulate," in which there is a basal ossicle, is found in Asteriidae,
Brisingidae, Heliasteridae, Pedicellasteridae, Zoroasteridae,
Stichasteridae. There are two varieties of forcipulate pedicellariae, the
"crossed" and the "straight," which have been described on p. 432. In all
other cases the pedicellariae are devoid of the basal ossicle, and the two
or more spinelets forming the jaws are directly attached to one of the main
plates of the skeleton.

[Illustration: FIG. 195.—Different forms of pedicellariae (excluding the
forcipulate form, for which see Fig. 186). A, pectinate; B, pectinate; C,
valvate; D, pincer-shaped; E, alveolate, from the side; F, alveolate, from
above. × 10. (After Sladen.)]

The simplest variety is termed "pectinate"; these pedicellariae are
composed of two parallel rows of small spines opposed to each other. They
are found in the Archasteridae, and are hardly more advanced in structure
than the groups of spines found in _Asterina_. In _Leptogonaster_ and its
allies there are pincer-shaped pedicellariae composed of two curved rods
articulating with one of the plates of the skeleton, and also "alveolate"
pedicellariae, composed of two short prongs which are implanted on a
concave tubercle borne on one of the plates of the skeleton. In the
Antheneidae every plate of the ventral surface bears a large "valvate"
pedicellaria consisting of two horizontally elongated ridges, which can
meet one another. It is possible that valvate pedicellariae have been
derived from a pectinate form in which successive spinules of one row have
become adherent.

(_d_) AMBULACRAL SKELETON.—In every case, whether spines are developed
elsewhere or not, the adambulacral plates bear spines. Where the spines are
elsewhere represented by granules (_Nardoa_ and its allies) (Fig. 194, D)
the adambulacral spines are {457}short and blunt. The terms "monacanthid"
and "diplacanthid" are used to express the occurrence of one or two rows of
spines respectively on each adambulacral plate.

In the Zoroasteridae the adambulacral plates are curved, and are
alternately convex and concave towards the ambulacral groove, so that this
groove presents a wavy outline.

In the description of _Asterias_ it was pointed out that the first
adambulacral plates in adjacent radii are closely approximated to one
another, and bear spines which can to some extent form a trellis-work over
the mouth. In very many species not only is this the case, but the plates
themselves project inwards over the mouth so as to form prominent
"mouth-angles." This is not the case in the Asteriidae or the allied
families.

PAPULAE.—In Asteriidae and many allied families these organs are found both
on the upper and under surface of the disc, but in another large group
consisting of Astropectinidae, Pentacerotidae, and allied families, papulae
are only borne on the dorsal surface, and, in some cases, are restricted to
a few groups at the base of the arms. In most Asteroidea the papulae are
arranged singly, that is to say, each occupies one of the interspaces
between the plates of the skeleton, but in _Asterias_ and some other genera
they are arranged in tufts of two or three.

WATER-VASCULAR SYSTEM.—In its general structure this system of organs is
very constant, the two most important variations being found, one, in
Asteriidae and a few allied families, and the other, in the Astropectinidae
and the families allied to them.

The first of the variations alluded to concerns the number of the tube-feet
in a radius. In _Asterias_ and its allies these are so numerous that there
is not room for them one behind the other, but they follow one another in a
zigzag line, the transverse canals connecting them with the radial canals
being alternately longer and shorter. In this way the appearance of four
rows of tube-feet is produced, and the advantage of this increase in number
can be recognised by any one who has compared the quick movements of
_Asterias_ and the slow ones of a _Cribrella_, for instance.

The second important variation referred to is the complete loss of the
sucker of the tube-foot, and, concomitantly, the loss {458}of the power of
climbing. Starfish which have undergone this change live on sandy bottoms
and run over the surface of the sand. They are also incapable of forcing
asunder the valves of Molluscs, and hence are compelled to swallow their
prey whole.

"Polian vesicles," or stalked sac-like outgrowths of the water-vascular
ring, are absent from the Asteriidae, but are found in many families—the
Asterinidae, Solasteridae, Astropectinidae, for example. They project
outwards from the water-vascular ring in the interradii; when there are
several present in one interradius they often arise from a common stalk.
Cuénot believes that their sole function, like that of Tiedemann's bodies,
is to produce amoebocytes, but this appears unlikely. It is more probable
that they act as store-houses of fluid for the water-vascular ring.

[Illustration: FIG. 196.—Dissection of _Ctenodiscus_ to show the Polian
vesicles. _amp_, Ampullae of the tube-feet; _nerv.circ_, nerve-ring; _Pol_,
Polian vesicle; _sept_, interradial septum; _stone c_, stone-canal; _T_,
Tiedemann's body; _w.v.r_, water-vascular ring. × 1.]

The stone-canal is rarely repeated, but this occurs in the aberrant genus
_Acanthaster_, where there may even be several in one interradius, and each
stone-canal has an axial sinus, genital stolon, and madreporite annexed to
it. According to Cuénot, in _Asterias_, when 6-rayed specimens occur in a
species normally 5-rayed, there are two stone-canals, suggesting that the
repetition of stone-canals is a suppressed effort at multiplication by
division. This is also true of _Echinaster_, but in _Ophidiaster_ two
madreporites may occur in an individual with five arms. In the Asterinidae
the Y-shaped fold which projects into the cavity of the stone-canal is
feebly developed, whereas in the Pentacerotidae it meets the opposite side
of the stone-canal, and in _Culcita_ gives out branches which reduce the
cavity of the canal to a series of channels. In Echinasteridae and some
Asterinidae, and in Astropectinidae and Pentacerotidae the ampullae become
so deeply indented as to be almost divided into two, so that each tube-foot
has virtually two ampullae.

The ALIMENTARY CANAL has a remarkably constant structure. {459}The only
important variation from the type, as described in _Asterias_, is found
amongst the Astropectinidae and Porcellanasteridae, where the anus is
wanting. In _Astropecten_ the rectum and the rectal caeca still persist,
but in _Luidia_ even these have disappeared. The rectal caeca are
remarkably variable structures. In _Asterias_ there are two, but in
Pentacerotidae there are five forked caeca, in _Asterina_ five simple
caeca, and in the Echinasteridae and Astropectinidae one large flat
slightly 5-lobed caecum. In the Asterinidae the pyloric caeca are
remarkable for the size of the enlarged basal portion in each radius, which
serves as a reservoir for the juices secreted by the branched forks of the
caecum. In _Porcellanaster pacificus_ the pyloric caeca are vestigial, and
in _Hyphalaster moseri_ they are absent.[454]

The GENITAL ORGANS are, as we have seen, outgrowths from radial branches of
the genital rachis. In most species, as in _Asterias_, they are limited to
a single cluster of tubes on each branch of the rachis, but in the
Astropectinidae and Pentacerotidae each branch gives rise to a large number
of clusters, arranged in longitudinal series, each cluster having its
independent opening to the exterior.

ASEXUAL REPRODUCTION, as a regular occurrence, is not common amongst
Asteroidea. If, however, a Starfish loses some of its arms, it has the
power of regenerating the missing members. Even a single arm will
regenerate the whole Starfish. Now in some cases (Astropectinidae,
Linckiidae) Starfish will readily snap off their arms on irritation. In
_Linckia_ this occurs at regular intervals and the separated arm forms a
new individual. In one of the Asterinidae, _Asterina wega_, a small
Starfish with seven arms, transverse fission regularly occurs, a portion
with three arms separating from one with four. The same is believed to
occur in two species of _Asterias_, and as has already been pointed out,
the repetition of the madreporite and stone-canal is, in many cases,
possibly connected with this tendency to transverse fission.


CLASSIFICATION OF ASTEROIDEA.

Whilst there is considerable agreement amongst the authorities as to the
number of families, or minor divisions of unequivocal {460}relationship, to
be found in the class Asteroidea, there has been great uncertainty both as
to the number and limits of the orders into which the class should be
divided, and also as to the limits of the various species. The difficulty
about the species is by no means confined to the group Echinodermata; in
all cases where the attempt is made to determine species by an examination
of a few specimens of unknown age there is bound to be uncertainty; the
more so, as it becomes increasingly evident that there is no sharp line to
be drawn between local varieties and species. In Echinodermata, however,
there is the additional difficulty that the acquisition of ripe genital
cells does not necessarily mark the termination of growth; the animals can
continue to grow and at the same time slightly alter their characters. For
this reason many of the species described may be merely immature forms. In
proportion, however, as the collections from various localities increase in
number and size, difficulties connected with species will tend to
disappear.

The disputes, however, as to the number of orders included in the
Asteroidea proceed from a different cause. The attempt to construct
detailed phylogenies involves the assumption that one set of structures,
which we take as the mark of the class, has remained constant, whilst
others which are regarded as adaptive, may have been developed twice or
thrice. As the two sets of structures are often of about equal importance
it will be seen to what an enormous extent the personal equation enters in
the determination of these questions.

Where, as in Asteroidea, the internal organisation is very uniform, the
best method of classification is to take as our basis the different methods
in which the demands of the environment have been met. It is in this way,
we hold, that divergence of character has been produced, for whilst species
may differ in trifling details, families and orders differ in points of
functional importance. The fact that one of the orders may have sprung from
several allied species instead of one may be admitted, and at the same time
the hopelessness of trying to push phylogenetic inference into details
asserted.

Sladen, in his Monograph of the Asteroidea collected by the "Challenger"
expedition, took for the basis of his system the presence or absence of
distinct pavement-like marginal plates along the edges of the arms and the
restriction of the papulae to {461}the aboral surface, or their
distribution over the whole surface of the body. What connexion, if any,
the presence of these pavement-like plates has with the habits it is
impossible to say, but it is unlikely to be of the high importance with
which it was regarded by Sladen, for in the same family we have genera with
inconspicuous marginals (_Asterina_) and others with conspicuous marginals
(_Palmipes_). The restriction of the papulae to the back also varies within
the same family (Linckiidae), and whilst, on the whole, it is perhaps a
primitive arrangement, it is in many cases connected with burrowing habits,
which can scarcely be deemed to have been the original mode of life of the
class.

A far better basis is supplied by the system of Perrier,[455] who divides
the Asteroidea into five orders according to the character of the dorsal
skeleton; and this classification really corresponds with the different
habits assumed by groups of Asteroidea in order to meet what must be
regarded as one of their chief dangers, viz. assaults by other animals,
especially parasites, on their soft and delicate skins. Since the food (so
far as is known) of all Asteroidea is more or less similar, the great
differentiating factor in their development must have been the means they
adopt to shelter themselves from their enemies. Perrier's classification,
which we shall adopt, is as follows:—

ORDER 1. SPINULOSA.—Asteroidea in which the plates of the dorsal skeleton
bear spines arranged singly or in groups. The tube-feet have suckers and
there are no pedicellariae. Marginals sometimes conspicuous, sometimes
rod-like.

ORDER 2. VELATA.—Asteroidea in which the dorsal surface of the animal is
concealed from view by a false membrane composed of the webs of skin
stretched between diverging groups of spines united at the base with one
another. No pedicellariae. Tube-feet with suckers.

ORDER 3. PAXILLOSA.—Asteroidea in which the dorsal surface is beset with
paxillae (upright spines bearing two or three circles of horizontal
spinelets). Pedicellariae, when present, few, and never of the forcipulate
variety; often absent. Marginals large. Papulae only on dorsal surface.
Tube-feet mostly devoid of suckers.

ORDER 4. VALVATA.—Asteroidea in which the dorsal surface {462}is protected
by plates covered with a mail of minute granules. Pedicellariae of the
valvate or alveolate type. Marginals large.

ORDER 5. FORCIPULATA.—Asteroidea in which the dorsal surface is beset with
small spines surrounded by numerous forcipulate pedicellariae. Tube-feet
with suckers and arranged in four rows. Marginals rod-like and
inconspicuous.


ORDER I. SPINULOSA.

This is by far the most primitive order of Asteroidea. The tube-feet are
arranged in two rows only, and there is no special means of protecting the
back, other than the small close-set plates bearing spines, with which it
is covered. In some cases, as _Asterina_, these spines have a tendency to
converge when irritated, and thus act somewhat like pedicellariae. This
circumstance suggests strongly the manner in which pedicellariae have been
developed from small groups of spines. The order is divided into six
families, of which four have common representatives on the British coast.

FAM. 1. ECHINASTERIDAE.—Spinulosa in which the aboral skeleton is composed
of close set plates bearing comparatively small spines. This family is
represented on the British coasts by the beautiful scarlet Starfish
_Cribrella_ (_Henricia_) _sanguinolenta_. It is also found on the Norwegian
coast and on the east coast of North America. On the Pacific coast it is
replaced by a larger species, _C. laeviuscula_. The narrow ambulacral
grooves and sluggish movements at once distinguish it from the Starfish
described as the type. Indeed, all the Spinulosa seem to be slow in their
movements in contrast to the comparatively active _Asterias_ and its
allies. _Cribrella_ is remarkable for its large eggs, which have a rapid
development. The larva never swims at the surface but glides only for a
short time over the bottom. _Echinaster_ is an allied genus in which each
plate bears a single somewhat enlarged spine. It possesses on the skin of
the aboral surface numerous pits lined by glandular walls, which probably
secrete a poisonous fluid which defends it. _Acanthaster_ has thorny
spines, more than ten arms, and several stone-canals and madreporites.

FAM. 2. SOLASTERIDAE.—Spinulosa in which the aboral skeleton is a network
of rods. Spines arranged in diverging bundles {463}(sheaves) attached to a
basal button. This family includes the well-known "Sun-stars," with
numerous arms and a wide peristome. There are two species found on both
sides of the Atlantic. _Solaster papposus_, with thirteen or fourteen arms
and long bundles of spines on the dorsal surface, which is of an orange
colour variegated with yellow, and _S. endeca_ with eleven rays and shorter
spines and of a reddish violet colour. _Rhipidaster_ has eight arms. Some
genera have, however, only five arms, as, for instance, _Peribolaster_ and
_Korethraster_ (Fig. 197). In this family there are conspicuous "Polian
vesicles" attached to the water-vascular ring.

[Illustration: FIG. 197.—_Korethraster hispidus._  × 2.  (From Wyville
Thomson.)]

FAM. 3. ASTERINIDAE.—Spinulosa in which the aboral skeleton consists of
overlapping plates, each bearing a few small spines. The common British
representative of this family is the small _Asterina gibbosa_, in which the
arms are short and stout and of somewhat unequal length. This Starfish
differs from most of its allies in being littoral in its habit. At low tide
on the south and west coasts of England it can be found on the underside of
stones feeding on the Sponges and Ascidians with which they are covered.
Like _Cribrella sanguinolenta_ this species has a modified development. The
larva resembles that of _Cribrella_, and the larval stage only lasts about
a week. Owing to the fact that {464}_Asterina_ lays its eggs in accessible
localities, its development has been more thoroughly worked out than that
of any other species. _Palmipes membranaceus_, an animal of extraordinary
thinness and flatness, is sometimes dredged up off the coast of Britain in
deeper water. Its arms are so short that the general form is pentagonal.
The infero-marginal plates are long and rod-like, and form a conspicuous
border to the body when viewed from below.

FAM. 4. PORANIIDAE.—Spinulosa allied to the Asterinidae but possessing a
thick gelatinous body-wall in which the plates and spines are buried, the
marginals forming a conspicuous border to the body. This family is
represented in British waters only by _Porania pulvillus_, a cushion-shaped
Starfish with very short arms and of a magnificent reddish-purple colour.
It is occasionally, but rarely, exposed at low tide.

FAM. 5. GANERIIDAE.—Spinulosa allied to the Asterinidae but distinguished
by the large marginals and by the fact that the skeleton of the oral
surface consists of plates each bearing a few large spines. _Ganeria_,
_Marginaster_.

FAM. 6. MITHRODIIDAE.—Spinulosa with a reticulate aboral skeleton. The
spines are large and blunt, covered with minute spinules. _Mithrodia_, sole
genus.

These last two families are not represented in British waters.


ORDER II. VELATA.

This is a very extraordinary group of Starfish, about the habits of which
nothing is known, since they all live at very considerable depths. Their
nearest allies amongst the Spinulosa must be looked for amongst the
Solasteridae. If the sheaves of spines with which the latter family are
provided were to become adherent at their bases, and connected with webs of
skin so as to form umbrella-like structures, and if then these umbrellas
were to become united at their edges, we should have a supra-dorsal
membrane formed such as is characteristic of the order.

FAM. 1. PYTHONASTERIDAE.—Velata in which each sheaf of spines is enveloped
in a globular expansion of the skin and is not united with the neighbouring
sheaves. _Pythonaster_, sole genus.

FAM. 2. MYXASTERIDAE.—Velata with numerous arms in which the sheaves of
spines are long and form with their connecting "umbrellas" web-like
expansions which do not fuse with one another. _Myxaster_, sole genus.

{465}[Illustration: FIG. 198.—Aboral view of _Pteraster stellifer_. _mars_,
Dorsal brood-pouch, × 1½. (From Sladen.)]

[Illustration: FIG. 199.—Oral view of _Hymenaster pellucidus_.  × 1. (From
Wyville Thomson.)]

{466}FAM. 3. PTERASTERIDAE.—Velata in which the membranes supported by the
sheaves of spines are united so as to form a continuous supra-dorsal tent.
The Pterasteridae are represented in British waters by a single species,
_Pteraster militaris_, which is occasionally dredged in deep water off the
British coast, and is found also in the Norwegian fjords and off the east
coast of Canada. This interesting Starfish has five short, blunt arms, and
its general appearance at first sight recalls that of _Asterina_. Closer
inspection reveals the "false back." The anus is surrounded by five
fan-like valves, supported by spines (Fig. 198), underneath which is a
space in which the young complete their development, _Pteraster_ being one
of the genera in which the normal larval form is not developed. The
tendency towards the union of adjacent spines by webs is deeply rooted in
the organisation of the animal. It is seen on the under side where the
spines borne by the ventral plates are united so as to form transverse
combs. In _Hymenaster_ (Fig. 199) the spines borne by the ventral plates
are long and free.


ORDER III. PAXILLOSA.

This is an exceedingly well-marked order. The armature of the upper surface
consists of paxillae. These organs as already mentioned are probably to be
traced back to sheaves of spines like those of the Solasteridae. The same
end as that striven after in the case of the Velata has been attained, but
in a different way. The horizontal spinelets of the paxillae meet one
another and form a close-fitting mail which is almost as efficient a
protection as the webs and umbrellas of the Velata. Pedicellariae are
occasionally present, but they are always of the pectinate or pincer
variety, never forcipulate.

FAM. 1. ARCHASTERIDAE.—Paxillosa in which the anus is still retained and in
which the tube-feet have suckers.

The Archasteridae are a most interesting family. Thus _Pararchaster_ has no
true paxillae, but only small isolated groups of spines. The pectinate
pedicellariae are composed each of two parallel rows of somewhat smaller
spines. The members of this family are to some extent intermediate in
structure between the {467}Spinulosa, such as Echinasteridae, and the other
families of the Paxillosa—some genera, indeed, might almost be classed as
Spinulosa. At the same time they are apparently closely allied with the
more primitive Valvata such as _Astrogonium_ and its allies, some of which
have paxillae on the upper surface; although the retention of the anus and
of the suckers on the tube-feet (in which characters they agree with the
Archasteridae) distinguishes them from the more typical Paxillosa, in which
both anus and suckers are lost. _Archaster_ (Figs. 200, 201).
_Leptogonaster._

[Illustration: FIG. 200.—Aboral view of _Archaster bifrons_.  × ¾.  (From
Wyville Thomson.)]

FAM. 2. ASTROPECTINIDAE.—Paxillosa which have lost the anus, but which
possess neither aboral protuberance nor interradial grooves. The marginal
plates are thick, covered with spinules and placed horizontally. The
tube-feet have no suckers.

This family is the only one of the order which occurs in British waters,
where it is represented by two genera, _Astropecten_ and _Luidia_. In
_Astropecten_ the inferior marginal plate is in {468}immediate contact with
the adambulacral, whilst in _Luidia_ it is separated from it by a small
intermediate plate.

[Illustration: FIG. 201.—Oral view of _Archaster bifrons_.  × ¾. (From
Wyville Thomson.)]

_Astropecten irregularis_ is a very common species on the coast of Britain,
and a study of its habits when in captivity has thrown a great deal of
light on many obscure points in the anatomy of the Paxillosa. Owing to the
loss of suckers it is unable to climb over rocks and stones like the
ordinary species, but it runs over the surface of the hard sand in which it
lives by means of its pointed tube-feet. The arms are highly muscular, and
the animal when laid on its back rights itself by throwing the arms upwards
and gradually overbalancing itself. The loss of suckers has also rendered
_Astropecten_ and its allies incapable of feeding in the manner described
in the case of _Asterias rubens_. They are unable forcibly to open the
valves of shell-fish, and the only resource left to them is to swallow
their prey whole. The mouth is consequently wide, and the {469}unfortunate
victims, once inside the stomach, are compelled by suffocation to open
sooner or later, when they are digested.[456]

[Illustration: FIG. 202.—Oral view of _Psilaster acuminatus_. × 4/3.
_adamb_, Adambulacral spines; _pax_, paxillae; _pod_, pointed tube-feet
devoid of sucker. (After Sladen.)]

Many interesting experiments have been made on _Astropecten_ by Preyer and
other investigators, but one important fact[457] has escaped their notice,
that _Astropecten_, when at rest, lies buried in the sand, whilst the
centre of the aboral surface is raised into a cone which projects above the
surface. On the sides of this cone the few papulae which this species
possesses are distributed. This raising of the aboral surface is obviously
an expedient to facilitate respiration. It loosens the sand over the region
of the papulae, and thus allows the water to have access to them. We can
thus understand how the restriction of the papulae to the dorsal surface,
so characteristic of the Paxillosa, is not always as Sladen imagined, a
primitive characteristic, but often an adaptation to the burrowing habits
which in all probability are characteristic of the whole order. In both
_Luidia_ and _Astropecten_ Cuénot has described short spines covered with
cilia in {470}the interspaces between the marginal plates, these also
subserve respiration by drawing a current of water over the gills.
_Psilaster_ (Fig. 202).

FAM. 3. PORCELLANASTERIDAE.—Paxillosa which have lost the anus. There is a
conical prominence in the centre of the dorsal surface termed the
epiproctal cone, and in the interradial angles there are vertical grooves
bordered by folds of membrane produced into papillae, the so-called
"cribriform organs." The marginal plates are thin and form the vertical
border of the thick disc. The tube-feet have no suckers.

[Illustration: FIG. 203.—_Porcellanaster caeruleus._ A, aboral view; B,
oral view,  × 1. (From Wyville Thomson.)]

Comparing the Porcellanasteridae with the Astropectinidae we see at once
that the "epiproctal cone" is a permanent representative of the temporary
aboral elevation in _Astropecten_, and we are inclined to suspect that the
cribriform organs are grooves lined with cilia which keep up a respiratory
current like the ciliated {471}spines of _Luidia_.  In all probability the
Porcellanasteridae are more habitual burrowers than even the
Astropectinidae.

_Ctenodiscus_ (Fig. 196), a genus in which there is a short epiproctal cone
and numerous feeble cribriform organs in each interradius, is found in deep
water north of the Shetland Islands. _Porcellanaster_ (Fig. 203) is a more
typical genus, with one large cribriform organ in each interradius.
_Hyphalaster_ has long arms, on which the supero-marginal plates meet
above.


ORDER IV. VALVATA.

The Starfish included in this order are characterised by the absence of
prominent spines and by the superficial covering of minute granules. The
skeleton consists, in most cases, of plates, and these plates with their
covering of granules probably represent the first stage in the evolution of
paxillae.

The tube-feet possess well-developed suckers. No members of this order can
properly be said to be British.

FAM. 1. LINCKIIDAE.—Valvata with long arms, the marginals being developed
equally throughout the whole length. These Starfish are distinguished by
their long narrow arms and small disc. It is possible that these forms, so
different in many respects from the other families of the order, have been
directly derived from the long-armed Echinasteridae. _Ophidiaster_,
_Nardoa_, _Linckia_.

FAM. 2. PENTAGONASTERIDAE.—Valvata with short arms, the marginals being
especially developed at the base and in the interradial angles. The aboral
skeleton consists of close-fitting plates. _Pentagonaster_ (Fig. 204),
_Astrogonium_.

FAM. 3. GYMNASTERIDAE.—Valvata allied to the foregoing but distinguished by
possessing a very thick skin in which the plates are completely buried.
_Dermasterias_, _Asteropsis_.

FAM. 4. ANTHENEIDAE.—Valvata with short arms. The dorsal skeleton is
reticulate and each ventral plate bears one or several large valvular
pedicellariae (Fig. 195, C). _Hippasterias_, _Goniaster_.

FAM. 5. PENTACEROTIDAE.—Valvata with arms of moderate length. The dorsal
skeleton is reticulate but the ventral plates bear only small pedicellariae
or none. The upper marginals are smaller than the ventral ones.

The Pentacerotidae include both short-armed and long-armed {472}forms.
Amongst the former is _Culcita_, in which the body is a pentagonal disc,
all outer trace of the arms being lost; _Pentaceros_ is a long-armed form.

[Illustration: FIG. 204.—_Pentagonaster japonicus._ × ⅔. (After Sladen.)]

The family Pentagonasteridae furnishes the key to the understanding of most
of the forms contained in this order. It contains genera such as
_Astrogonium_ which possess on the back unmistakable paxillae, whilst on
the under surface they have the characteristic covering of granules; these
genera seem to be closely allied to the short-armed species of the
Archasteridae, from which they are distinguished chiefly by the granular
covering of the marginals. From a study of these cases it seems clear that
the plates of the dorsal skeleton of the Valvata correspond to the
supporting knobs of the paxillae much broadened out, and the granules
correspond to the spinelets of the paxillae increased in number and
diminished in size.

{473}As mentioned above, Ludwig has proved that the paxillae develop in the
life-history of the individual out of ordinary plates, the axis of the
paxilla representing the plate.


ORDER V. FORCIPULATA.

This order, which includes the most highly developed members of the class
Asteroidea, is at once distinguished by the possession of forcipulate
pedicellariae which, as we have seen, possess a well-marked basal piece
with which the two plates articulate. The pedicellariae are consequently
sharply marked off from the spinelets, and no intermediate forms occur. The
first conjoined adambulacrals, which in other orders form the "teeth" or
mouth-angles, do not here project beyond the first pairs of ambulacral
plates.

FAM. 1. ASTERIIDAE.—Forcipulata in which the tube-feet are apparently
arranged in four rows. Aboral skeleton a loose reticulum.

The general features of the family Asteriidae have been explained in the
description of _Asterias rubens_ (p. 432). There are five well-marked
species of the genus found on the British coasts. Of these _A. glacialis_
is found chiefly in the south-western parts of the English Channel. It is a
large Starfish of a purplish-grey colour, with large spines surrounded by
cushions of pedicellariae arranged in one or two rows down each arm. _A.
muelleri_ resembles the foregoing species, but is of much smaller size, and
is further distinguished by having straight pedicellariae in the
neighbourhood of the ambulacral groove only. It is found on the east coast
of Scotland, and carries its comparatively large eggs about with it until
development is completed. _A. rubens_ is the commonest species, and is
found on both east and west coasts. Its colour is a bright orange, but
varies to almost a straw colour. It is at once distinguished from the
foregoing species by the spines of the dorsal surface, which are small and
numerous, an irregular line of somewhat larger ones being sometimes seen
down the centre of each arm. _A. murrayi_ is a peculiar species restricted
to the west coast of Scotland and Ireland. It has flattened arms, with
vertical sides, and only three rows of small spines on the dorsal surface.
It is of a violet colour. _A. hispida_ is also a western species. It is a
{474}small Starfish with short stout arms; there are no straight
pedicellariae, and only a few sharp spines on the dorsal surface.

On the eastern coast of North America there are several species of
_Asterias_, of which the most noteworthy is the 6-rayed _A. polaris_ of the
Gulf of St. Lawrence. This species exhibits a marvellous range of
colour-variation, ranging from bluish-violet through purple to red and
straw-coloured. This variation seems to show that colour, as such, is of no
importance to the animal, but probably depends on some compound of slightly
varying composition which is being carried by the amoebocytes towards the
exterior. On the Pacific coast there is a rich fauna of Starfish, among
which we may mention as members of this family _Asterias ochracea_, a large
violet species, so strong that it requires a severe wrench to detach it
from the rock, and _Pycnopodia_ with twenty-two arms.

FAM. 2. HELIASTERIDAE.—Forcipulata allied to the Asteriidae, but with very
numerous arms and double interradial septa. _Heliaster._

FAM. 3. ZOROASTERIDAE.—Forcipulata with the tube-feet in four rows at the
base of the arm, in two rows at the tip. Aboral skeleton of almost
contiguous plates bearing small spines or flattened scales. _Zoroaster_,
_Pholidaster_.

FAM. 4. STICHASTERIDAE.—Forcipulata with the tube-feet in four rows. Aboral
skeleton of almost contiguous plates covered with granules. _Stichaster_,
_Tarsaster_.

The Stichasteridae and Zoroasteridae have acquired a superficial
resemblance to some of the long-armed Valvata, from which they are at once
distinguished by their pedicellariae. It would be exceedingly interesting
if more could be found out concerning the normal environment of these
animals; it might then be possible to discover what is the cause of the
assumption of this uniform mail of plates.

FAM. 5. PEDICELLASTERIDAE.—Forcipulata with two rows of tube-feet. The
aboral skeleton bears projecting spines surrounded by cushions of straight
pedicellariae. _Pedicellaster_, _Coronaster_.

FAM. 6. BRISINGIDAE.—Forcipulata with numerous arms and only two rows of
tube-feet. Aboral skeleton largely rudimentary and confined to the base of
the arms. The small blunt spines are contained in sacs of skin covered with
pedicellariae.

The Brisingidae, including _Brisinga_ and _Odinia_, are a very
{475}remarkable family, chiefly on account of the smallness of the disc and
of the extraordinary length of the arms. The arms have what we must
consider to have been the primitive arrangement, since there is no lateral
adhesion between them, and interbrachial septa are consequently entirely
absent. The reduction of the skeleton is a very marked peculiarity and,
like the tendency to the reduction of the skeleton of deep-sea fish, may
stand in some relation to the great pressure under which the animals live.

[Illustration: FIG. 205.—Aboral view of _Odinia_. × ⅔. (After Perrier.)]


FOSSIL ASTEROIDEA.

The Asteroidea occur somewhat plentifully as fossils. In the Lower Jurassic
_Asterias_, _Astropecten_, _Luidia_, _Solaster_, and _Goniaster_ have
already made their appearance. In the Cretaceous {476}_Pentaceros_ appears.
In the older rocks occur a number of forms of different character from any
now existing. Of these _Aspidosoma_ (Fig. 206), with short lancet-shaped
arms sharply distinguished from the disc and continued along its under
surface, seems to be intermediate between Asteroidea and Ophiuroidea. The
skeleton of the arm is composed of alternating ambulacral ossicles bordered
by adambulacral ossicles, which are at the same time marginals and sharply
distinguished from the marginals forming the edge of the disc.
_Palaeaster_, on the other hand, is a true Asteroid; there are marginals
distinct from the adambulacrals, but the disc is reduced to its smallest
dimensions, there being only one plate on the ventral side of each
interradius. There are a number of genera (_Palaeocoma_, for instance) with
a large disc and very short arms and very shallow ambulacral grooves; all
have alternating ambulacral plates. Some genera appear to have had the
madreporite on the ventral surface of an interradius. On the other hand, in
the Devonian occurs _Xenaster_, which was a fairly normal Asteroid, with
pavement-like marginals, deep ambulacral grooves, and broad arms.

[Illustration: FIG. 206.—Three views of _Aspidosoma_, a fossil Asteroid. A,
oral view; B, aboral view of one arm; C, enlarged view of a portion of the
ambulacral groove. _adamb_, Adambulacral plate; _amb_, ambulacral plate;
_marg_, marginal plate; _pod_, aperture for extension of tube-foot.]

Thus it will be seen that already in Jurassic times the three orders,
Forcipulata, Paxillosa, and Spinulosa were differentiated from each other,
but how these are related to the older Palaeozoic forms it is at present
impossible to say.




{477}CHAPTER XVII

ECHINODERMATA (_CONTINUED_): OPHIUROIDEA = BRITTLE STARS


CLASS II. OPHIUROIDEA

The second class of Eleutherozoa are familiarly known as "Brittle Stars,"
on account of their tendency, when seized, to escape by snapping off an
arm, although this habit is by no means confined to them, but is shared in
a marked degree by many Asteroidea, such as _Luidia_, for instance. Like
the Asteroidea, they are "starfish," that is to say, they consist of a disc
and of arms radiating from it; but the scientific name Ophiuroidea really
expresses the great dominating feature of their organisation. Literally it
signifies "Snake-tail" (ὄφις, snake; οὐρά, tail), and thus vividly
describes the wriggling, writhing movements of the long thin arms, by means
of which the Ophiuroid climbs in and out of the crevices between the stones
and gravel in which it lives. This feature, viz. the effecting of movement
by means of muscular jerks of the arms, instead of by the slow protrusion
and retraction of the tube-feet, is the key to the understanding of most of
the points wherein the Brittle Stars differ from the true Starfish.

Asteroidea and Ophiuroidea agree in the common ground-plan of their
structure, that is, they both possess arms; but the most obvious difference
in their outer appearance is that whereas in Asteroidea the arms merge
insensibly into the disc, in Ophiuroidea the disc is circular in outline
and is sharply marked off from the arms. Closer inspection shows that in
the Ophiuroid the arms are continued inwards along grooves, which run on
the under surface of the disc, and that they finally coalesce to form a
buccal framework surrounding the mouth. In {478}the very young Ophiuroid
the arms melt into a small central disc, as in the Starfish, but the disc
of the adult is made up of a series of interradial dorsal outgrowths which
meet one another above the arms.

[Illustration: FIG. 207.—Aboral view of _Ophiothrix fragilis_. × 1. _r_,
Radial plate.]

{479}[Illustration: FIG. 208.—Oral view of the disc of _Ophiothrix
fragilis_. _g.b_, Opening of the genital bursa; _m.p_, madreporite; _pod_,
podia; _t.p_, tooth-papillae; _v.p_, ventral plates of the arms. × 1.]

One of the commonest British Ophiuroids is _Ophiothrix fragilis_ (Figs.
207, 208), which is found in swarms in shallow water off the west coast of
England and Scotland. We may therefore select it as the type, and, since
the ARM is the most characteristic organ of an Ophiuroid, we may commence
by studying it. Speaking generally, an Ophiuroid either drags itself
forward by two arms and pushes itself by the other three (Fig. 207),[458]
or else it drags itself by one and pushes with the other four (Fig. 217).
The arms during this process are bent into characteristic curves, by the
straightening of which in the posterior arms the animal is pushed onwards,
whilst the intensification of these curves in the anterior arms causes the
animal to be dragged forwards. The grip of the arm on the substratum is
chiefly in the distal portion of the curve. The alteration of the curvature
is due to the contraction of the muscles on one side of the arms. There is
no ambulacral groove such as is found on the under side of the arms of all
Asteroidea, for the arm is completely ensheathed by four series of plates,
an upper row of dorsal plates, an under row of ventral plates, and two
lateral rows of lateral plates. The last named, which in all probability
correspond to the adambulacral plates of Starfish, bear each a transverse
row of seven spines with roughened surfaces; these enable the animal to get
a grip on the substratum over which it moves. The podia in Ophiuroidea are
termed "tentacles"; they are totally devoid of suckers, being simple
conical papillae used as sense-organs, and are of little, if any, service
in locomotion. They issue from openings called "tentacle-pores" situated
between the edges of the {480}ventral and lateral plates, guarded each by a
valve-like plate called the "tentacle-scale." In _Ophiothrix_ they are
covered with sense-organs, each consisting of a hillock-like elevation of
the ectoderm, in which are cells carrying long stiff sense-hairs. In most
Ophiuroids such organs are not present, though abundant scattered
sense-cells occur, and the outer surface of the tube-feet and the lining of
certain pockets called "genital bursae" (Fig. 208, _g.b_) are the only
portions of the surface where the ectoderm persists. Everywhere else,
although present in the young, it disappears, leaving as remnants a few
nuclei here and there attached to the under side of the cuticle.[459]

[Illustration: FIG. 209.—Diagrammatic transverse section of the arm of an
Ophiuroid. _coe_, Dorsal coelomic canal; _ect_, ectoderm covering the
tube-foot; _ep_, epineural canal; _gang.p_, pedal ganglion; _L_,
nerve-cord; _musc_, longitudinal muscles attaching one vertebra to the
next; _nerv.rad_, radial nerve-cord; _perih_, radial perihaemal canal;
_pod_, podium (tube-foot); _sp_, lateral spines; _w.v.r_, radial
water-vascular canal.]

The greater part of the section of the arm is occupied by a disc-like
ossicle called the "vertebra." Each vertebra articulates with its
predecessor and successor by cup-and-ball joints, and it is connected to
each of them by four powerful longitudinal muscles. Above, its outline is
notched by a groove, in which lies an extension of the coelom of the disc
(Fig. 209, _coe_), but contains no outgrowth of the alimentary canal, as is
the case in Asteroidea. The vertebra is also grooved below, and in this
lower groove are contained the radial water-vascular canal {481}(Fig. 209,
_w.v.r_), and below it perihaemal canals as in Asteroidea; below this again
the radial nerve-cord (_L_), and beneath this again a canal called the
"epineural canal" (_ep_), which represents the missing ambulacral groove.
This canal in the very young Brittle Star is an open groove, but becomes
closed by the approximation of its edges. The vertebra, which has a double
origin, represents a pair of fused ambulacral ossicles. In _Ophiohelus_
these are only slightly adherent to one another (Fig. 216).

[Illustration: FIG. 210.—Proximal and distal views of the three types of
vertebra found amongst Ophiuroidea. A, _Ophioteresis_, a type of the
Streptophiurae (after Bell), × 24; B, _Astroschema_, a type of the
Cladophiurae (after Lyman), × 10; C, _Ophiarachna_, a type of the
Zygophiurae (after Ludwig), × 3. The upper figure in all cases represents
the distal aspect, the lower the proximal aspect of the vertebra. _v.g_,
Ventral groove.]

When the surface of a vertebra is examined it is found that it can be
divided into a thin border, to which are attached the four muscles by which
it is connected to its successor and predecessor, and a central portion, on
which are situated the knobs and pits, by means of which it articulates
with the next vertebra.

The simultaneous contraction of the two upper muscles causes the arm to
bend upwards. The contraction of the two lower bend it downwards, whilst a
sideward movement is effected by the contraction of the upper and lower
muscle of the same side. On the proximal surface of the central portion of
the vertebra there is a central knob and two ventro-lateral knobs, {482}a
median ventral pit and two dorso-lateral pits, and on the distal surface
there are pits corresponding to the knobs on the proximal side and _vice
versa_ (Fig. 210, C). These knobs and pits restrict the movement of one
vertebra on the next, so that although the arms can undergo an unlimited
amount of flexion from side to side, they cannot be rolled up in the
vertical plane. When the under surface of the vertebra is examined there is
seen on each side of the central groove two round holes, a distal and a
proximal. The distal pair are for the passage of the canals connecting the
radial water-vessel with the tentacles, these canals traversing the
substance of the vertebra for a part of their course; the proximal pair are
for nerves going to the longitudinal muscles, which likewise perforate part
of the ventral border of the vertebra.

In order to understand the anomalous circumstance that the canals going to
the tentacles actually perforate the vertebrae, it must be clearly borne in
mind that the basis of the body-wall in all Echinoderms is a mass of jelly
with amoebocytes in it, to which we must assign the power of secreting
carbonate of lime, and all we have to assume in the case of Ophiuroids is
that calcification spread outwards from the original ambulacral ossicles
into the surrounding jelly, enclosing any organs that happened to traverse
it.

When the ossicles of the arm are followed inwards towards the MOUTH, they
are seen to undergo a profound modification, so as to form, by union with
the corresponding ossicles of adjacent arms, a structure called the
mouth-frame. The general character of this modification is similar to that
affecting the first ambulacral and adambulacral ossicles in the arms of an
Asteroid, but in the Ophiuroid the change is much more profound. The first
apparent vertebra consists of two separated halves, and each is fused with
the first adambulacral (lateral) plate, which in turn is firmly united with
the corresponding plate in the adjoining arm. Thus is formed the "jaw," as
the projection is called. The extensions of the mouth-cavity between
adjacent jaws are termed "mouth-angles." To the apex of each jaw is
attached a plate bearing a vertical row of seven short blunt spines called
"teeth" (Fig. 212, _p_). The plate is called the "torus angularis" (Fig.
211, _T_), and on its ventral edge there is a tuft of spines which are
termed "tooth-papillae" (Fig. 208, _t.p_). On the upper aspect of the jaw
{483}are a pair of plates termed "peristomial plates." These discs—of which
there are two in each radius, one on each jaw which flanks the
radius—possibly represent the separated halves of the first vertebra, the
apparent first vertebra being really the second. On the flank of the jaw
there is dorsally a groove for the water-vascular ring and nerve-ring (Fig.
212, _n.r_), and beneath this a groove for the first tentacle and a pore
for the second, both of which spring directly from the ring-canal; below
these, in most Ophiuroidea, but not in _Ophiothrix_, there is a row of
blunt triangular spines called "mouth-papillae" (Fig. 212, _p^1_).

[Illustration: FIG. 211.—Diagrams to show the modification of the
ambulacral and adambulacral ossicles to form the armature of the mouth. A,
Asteroid; B, Ophiuroid. _A_{1}-A_{4}_, the first four ambulacra ossicles;
_Ad_{1}-Ad_{4}_, the first four adambulacral ossicles; _J_{1}_, the first
plate of the interradius (in the Ophiuroid the _scutum buccale_); _P_, the
spines borne by the jaw (in the Ophiuroid the teeth); _T_, the torus
angularis; _W_, the water-vascular ring; _Wr_, the radial water-vessel;
_I_, _II_, the first two pairs of tube-feet. (After Ludwig.)]

The words "jaw" and "tooth" are misleading. There is no evidence that the
jaws of a Brittle Star are ever used for crushing food, but by means of the
muscles attaching them to the first {484}complete vertebra in the arm they
can be rotated downwards so as greatly to enlarge the mouth, and again
rotated upwards and inwards, when they form an excellent strainer to
prevent the entrance of coarse particles. To permit this extensive movement
the articulatory facets on the proximal surface of the first vertebra have
been much modified; the median knob and pit have disappeared, and the
dorso-lateral pits are raised on to the surface of processes, so that there
are in all four processes, two of which articulate with one half of a jaw.

[Illustration: FIG. 212.—Lateral view of mouth-frame of _Ophiarachna
incrassata_. × 4. _A^1_?, peristomial plate, possibly the half of the first
vertebra; _A^2_, the half of the second vertebra; _A^3_, the third
vertebra; _F^1_, pores for pair of tentacles; _gen_, genital scale lying
beside opening of genital bursa; _musc_, longitudinal muscles connecting
vertebrae; _n.r_, groove for nerve-ring; _p_, tooth; _p^1_, mouth-papilla;
_t_, torus angularis. (After Ludwig.)]

The mouth can be narrowed and the jaws forced inwards towards the centre by
the simultaneous contraction of five muscles (_musc. tr_, Fig. 213) each,
which unite the two halves of a jaw.

Turning now to the skeleton of the DISC, we notice that dorsally it
consists of a closely-fitting mosaic of small plates, which are usually
concealed from view by a covering of minute spines. Opposite the insertion
of each arm there are, however, a pair of large triangular plates
("radials"), which extend outwards to the periphery and strengthen it, much
as the ribs do in an umbrella. These radial plates are always exposed, in
_Ophiothrix_, even when the rest of the dorsal plates are concealed by
spines. On the under surface there is a similar plating; but adjoining the
jaws are five large, more or less rhomboidal, plates {485}termed "scuta
buccalia" (Fig. 211, _J_{1}_), on one of which open the few madreporic
pores which the animal possesses. Attached to the sides of the scuta
buccalia are the "lateral mouth shields," which are in fact the
adambulacral plates belonging to the second pair of ambulacral plates which
form the main mass of the jaws. Further out, on the under side of the disc,
there is, on each side of each arm, a long narrow slit—the opening of the
genital bursa (Fig. 208, _g.b_), so that there are ten genital bursae. The
"genital bursa" (Fig. 214) is a sac lined by ciliated ectoderm projecting
into the interior of the disc. It is called genital because the openings of
the genital organs are situated on its surface; its main function, however,
is respiratory, the cilia bringing about a constant inward current of fresh
sea-water, the oxygen contained in which diffuses through the thin wall of
the sac into the coelomic fluid. The opening of the bursa is strengthened
on its radial side by a rod-like ossicle, the "genital plate," and on its
interradial side by an ossicle called the "genital scale" (Fig. 212,
_gen_), and in _Ophiothrix_ the outer end of the radial plate articulates
with the outer end of the genital plate. Muscles connect the two plates
running on either side of the articulation.

Observations on _Ophiothrix_[460] show that in this species at any rate the
radial plates can be raised or lowered. When they are raised the centre of
the disc is lifted into a cone and water is sucked into the genital bursae,
whereas when they are lowered the bursae are compressed and water is
expelled. This forced respiration appears to come into play when the supply
of oxygen is getting scanty.

The ALIMENTARY CANAL of _Ophiothrix_ is a simple flattened sac (Fig. 213).
It is devoid of an anus and cannot be everted through the mouth. There is a
horizontal pouch given off into each interradial lobe of the disc. The sac
is attached to the dorsal wall of the coelom by numerous mesenteries,
fibrous cords traversing the coelomic cavity and clothed on the outer side
by coelomic epithelium. To the mouth-frame it is attached by a circular
membrane, which we have reason for believing is a {486}functionless remnant
of the retractor muscles of the stomach of Asteroidea. In the young
Asteroid there is a similar sheet of membrane, which later becomes resolved
into the ten retractor bands.

The simple structure of the alimentary canal appears to be correlated with
the exceedingly simple character of the food. _Ophiothrix_ feeds on the
most superficial layer of mud at the bottom of the sea. This deposit
consists partly of microscopic Algae and partly of decaying organic matter,
and is much more easily disposed of than the living animals on which the
Starfish preys. The food is shovelled into the mouth by the first two or
"buccal" pairs of tube-feet in each ray.

[Illustration: FIG. 213.—Longitudinal section through the disc of a young
Ophiuroid passing along one arm and the middle of the opposite interradius.
(Diagrammatised from an actual section of _Amphiura squamata_.) _ab_,
Aboral sinus (dorsal in the radius, ventral in the interradius); _ax_,
axial sinus; _coe_, dorsal coelomic canal of the arm; _ep_, epineural
canal; _gang.rad_, ganglion of the radial nerve; _gen.r_, genital rachis
contained in the aboral sinus; _gen.st_, genital stolon; _mp_, madreporic
pore; _musc.long_, longitudinal muscle of the arm; _musc.tr_, transverse
muscle uniting the two halves of each jaw; _mv_, madreporic vesicle;
_nerv.r_, nerve-ring; _p.c_, pore-canal; _perih_, perihaemal canal; _vert_,
vertebra; _w.vr_, radial water-vessel.]

The WATER-VASCULAR SYSTEM has undergone a most interesting set of
modifications, which can be explained by noticing the fact that the
tube-feet have almost, if not quite, lost their locomotor function and are
now used as tactile organs. The ampulla, or swollen inner end of the
tube-foot, has disappeared, and the upper end of the organ is directly
connected with the radial canal by means of a curved canal, which traverses
the outermost flange of the vertebra, appearing on its {487}surface in a
groove on the outer side of the dorsal lateral knob on the distal side of
the ossicle. As in Asteroidea there are valves, which regulate the entrance
of fluid into the tube-foot. The stone-canal is a curved tube of simple
circular section and excessively narrow bore which extends from the
water-vascular ring _downwards_ to the madreporite (Fig. 213, _mp_)
situated on one of the scuta buccalia. The madreporite, in _Ophiothrix_ as
in most Brittle Stars, is an exceedingly rudimentary structure, consisting
of one or two pores leading into as many pore-canals. From each
interradius, except that in which the stone-canal lies, a large Polian
vesicle hangs down from the water-vascular ring into the coelom.

We saw that in the Asteroid the ampulla was used like the bulb of a pipette
to force the fluid in the tube-foot down into the tip, so as to press the
sucker against the substratum. But when the tube-foot is used as a
sense-organ, a few circular fibres around its upper end suffice to bring
about all the extension that is needed. Since the extension is no longer a
very vigorous act, the loss of fluid by transudation has probably been
rendered insignificant, and hence the stone-canal and madreporite, whose
function it is to repair the loss, have been reduced in size. The curious
ventral curvature of the stone-canal is, however, due to another cause. In
the very young Ophiuroid the madreporite is on the edge of the disc, and
the stone-canal extends horizontally outwards; and in some Asteroidea there
is a similar outward direction in its course. As development proceeds the
dorsal interradial areas of the disc of the young Ophiuroid grow out into
lobes, building up the conspicuous adult disc and forcing the madreporite,
and with it the stone-canal, downwards towards the ventral surface.

The pores of the madreporite in _Ophiothrix_, like some of those in the
Asteroid, open not directly into the stone-canal but into the AXIAL SINUS
(Fig. 213, _ax_). This is a large ovoid sac, lined with thin epithelium,
lying between the stone-canal and the mouth-frame, since of course it has
shared in the ventral rotation of the stone-canal. Its open connexion with
the stone-canal was easily recognised by Ludwig, who termed it, on this
account, the "ampulla."[461] The name "axial sinus" was bestowed
{488}mistakenly on another cavity, which will be mentioned in connexion
with the genital organs.

The radial PERIHAEMAL SPACES of the arms open into a "perihaemal ring"
representing the outer perihaemal ring of Asteroids; but the axial sinus
does not have any such extension as constitutes the inner perihaemal ring
in Starfish. So-called oral circular and radial BLOOD STRANDS are to be
found in similar positions to the corresponding structures in _Asteroidea_.

The NERVOUS SYSTEM might have been expected to have become very much
modified, since the activities of the Brittle Stars are so different from
those of the Starfish. It is indeed a universal rule in the Animal Kingdom
that, concomitantly with the increase in size and activity of a muscle,
there is a corresponding increase in the number of ganglion-cells which
control it. An accurate radial section of an arm shows that there is,
corresponding to the interspaces between the two vertebrae, a ganglionic
swelling of the nerve-cord. As in Asteroids, there are not only ectodermic
ganglion-cells on the under surface of the cord abutting on the epineural
canal, but also coelomic ganglion-cells derived from the floor of the
radial perihaemal canal. Both these categories of cells are largely
increased in number in the ganglion. From the dorsal-cells arise a pair of
large nerves which pass directly up and supply the great intervertebral
muscles. From the interspace between the ganglia a direct prolongation of
the ventral part of the nerve-cord, the so-called pedal nerve, extends out
along the side of the tentacle, as in Asteroids. In Ophiuroids it swells
out into a ganglion, completely surrounding the tentacle and giving off
nerves to the surfaces of the arm which terminate in the cuticle.

There is a large ganglion where the radial cord joins the nerve-ring, and,
owing to the more specialised condition of the nervous system, a severed
arm in an Ophiuroid is much more helpless than an arm of an Asteroid. It
will not carry out "escape movements," and is for a long time rigid under
the shock of section; at last it simply gives reflex movements on
stimulation.

Preyer[462] endeavoured to test the "intelligence" of Ophiuroids by
observing how they would adapt themselves to circumstances which it might
be fairly assumed they had never encountered {489}in their ordinary
experience. To this end he passed over the arm of a specimen a piece of
indiarubber tubing, which clung to it tightly. He found that the animal
first tried walking off, pressing the encumbered arm against the ground, so
that the piece of tubing was rubbed off. It was then replaced more tightly
than before; the animal, having tried the first method without result,
waved the arm to and fro in the water till the rubber floated off. In a
third experiment the animal held the rubber against the ground by a
neighbouring arm, and drew the encumbered arm out. When the rubber was
replaced a fourth time, the animal kicked it off by alternately pressing
neighbouring arms against it. Finally, when the rubber was put on so firmly
that all the above-mentioned methods failed, the arm was broken off. Preyer
concludes from this that Ophiuroids have a high degree of intelligence; but
this may be doubted, and the reader is referred to the account of Uexküll's
experiments given in the next chapter. There is, however, no doubt at all
that Ophiuroidea are by far the most active of all Echinoderms, and one
would naturally correlate this with higher psychic development.

The radial nerve ends in a terminal tentacle sheltered by a median plate at
the end of the arm; but eyes, such as are found in Asteroids, are wanting,
and the animal does not appear to be sensitive to light.

The REPRODUCTIVE SYSTEM in Ophiuroids consists of a genital stolon giving
rise at its distal end to a genital rachis, which extends in a circular
course round the disc, ensheathed in an "aboral sinus" (Fig. 213, _ab_) and
swelling out so as to form the gonads (testes or ovaries), where it passes
over the inner side of the genital bursae. The genital stolon (Fig. 213,
_gen.st_) is a compact ovoid organ, often termed on account of its shape
the "ovoid gland." It is situated close to the stone-canal, and, as in
Starfish, it indents the outer wall of the axial sinus; but, unlike the
stolon of the Asteroid, it is separated from the general coelom by a space,
of which it forms the inner wall, but whose outer wall is formed by a sheet
of membrane. This cavity must be carefully distinguished from the axial
sinus of Asteroidea, to which it was supposed at one time to correspond; it
is really formed by a pocket-like ingrowth of the general coelom into the
septum dividing it from the axial sinus. The cells forming the inner side
of this pocket form the primitive germ-cells, which {490}constitute the
main mass of the ovoid gland; those of the outer side remain thin. The
cavity of the ingrowth is shut off from the general coelom, but persists
throughout life. In Asteroids a similar ingrowth takes place, but both
walls thicken and become converted into germ cells, and the cavity
disappears, and, as in Asteroidea, a considerable number of the germ-cells
in the stolon degenerate.

[Illustration: FIG. 214.—Diagram of a tangential section through the edge
of the disc of an Ophiuroid to show the relations of the disc, _arm_, and
genital bursae. _ep_, Epineural canal; _musc_, longitudinal muscle of the
arm; _nerv.rad_, radial nerve cord; _ov_, ovary; _perih_, radial perihaemal
canal; _w.v.r_, radial water-vessel.]

The genital rachis (Fig. 213, _gen.r_) is an outgrowth of the distal end of
the genital stolon, which extends in a complete circle round the disc. The
rachis does not, however, lie everywhere in the same plane, but by its
undulating course bears witness to the distortion which the disc has
undergone. In the radii it is, as in the Asteroid, dorsal; but in the
interradii it is ventral, this ventral portion having, like stone-canal and
axial sinus, been carried down by the preponderant growth of the dorsal
parts of the disc. It is everywhere ensheathed by the aboral sinus, which,
as in Asteroids, is an outgrowth of the coelom. The rachis is embedded in a
strand of modified connective tissue, to which we may (as in the case of
_Asterias_) apply the name "aboral blood-ring." Both on the central and
peripheral sides of this sinus are vertical muscles connecting the genital
and the radial plates, which bring about the respiratory movements already
referred to. Just above the madreporite, at the end of the genital stolon,
is a small, completely closed space, which by its position corresponds with
the madreporic vesicle of Asteroids and represents the right hydrocoel
(Fig. 213, _mv_). As the rachis passes over the genital bursa it gives off
branches, which swell up to form the genital organs. In _Ophiothrix_ there
is {491}one such organ on each side of each bursa, but in other genera (cf.
_Ophiarachna_) a large number of small ones. The genital products are shed
into the water through the bursae.


CLASSIFICATION OF OPHIUROIDEA.

Before proceeding to study the classification of Brittle Stars, it is
necessary to give some account of the range of structure met with in the
group.

NUMBER OF RADII.—The number of arms is rarely increased, and hardly ever
exceeds six; a few species (each an isolated one in its genus) have six
arms, and in one case (_Ophiactis virens_), at any rate, this is associated
with the power of transverse fission. In many Cladophiurae the arms fork
repeatedly, so that although there are only five radii, there is quite a
crowd of terminal branches.

VERTEBRAE.—The vertebrae differ in the manner in which they articulate with
one another. In _Ophiothrix fragilis_ taken as the type, which in this
respect resembles the vast majority of species (Zygophiurae), the knobs and
pits on the faces of the vertebrae prevent the arms from being coiled in
the vertical plane. In _Ophioteresis_ (Fig. 210, A) and some allied genera
(Streptophiurae) the knobs are almost obsolete, and the arms are free to
coil in the vertical plane; whilst in _Gorgonocephalus_ and _Astrophyton_
(Cladophiurae) the arms are repeatedly branched and the vertebrae have
saddle-shaped articulating surfaces, so that they have quite a snake-like
capacity for coiling themselves round external objects. In _Ophiohelus_
(Fig. 216) each vertebra consists of two rod-like plates placed parallel
with the long axis of the arm and fused at both ends, but divergent in the
middle, leaving a hole between them.

COVERING PLATES OF THE ARMS.—The upper arm-plates are the most variable.
They may be surrounded by small supplementary plates (_Ophiopholis_) or
double (_Ophioteresis_). In all (?) Cladophiurae and most Streptophiurae
they are absent, being replaced by minute calcareous granules. Under
arm-plates are absent in _Ophioteresis_ and in the distal portion of the
arms in many Cladophiurae. Side arm-plates are constantly present, and in
most Cladophiurae meet in the middle line below.

ARM-SPINES.—The spines borne by the lateral covering plates of the arms
vary greatly in character. In _Ophiura_ and its {492}allies they are short
and smooth, and are borne by the hinder edge of the arm and directed
backwards; but in the larger number of genera they are borne nearer the
centre of the plate, and are directed outwards at right angles to the arm.
They may be covered by small asperities, as in _Ophiothrix_ (Fig. 215, C),
when they are said to be rough; or these asperities may become secondary
spines, as in _Ophiacantha_ (Fig. 215, B), when they are said to be thorny.
In _Ophiopteron_ all the spines borne by a single plate are united by a web
of skin so as to constitute a swimming organ. The small plates guarding the
ends of the tentacles (tentacle-scales) may be absent, or more rarely
double. In Cladophiurae there is a regular transition from tentacle-scale
to arm-spine; the tentacle-scale being merely the smallest of the series of
lateral spines.

True PEDICELLARIAE are unknown amongst Ophiuroidea, since there is no
longer a soft ectoderm to protect, but in some cases, as for instance in
_Ophiohelus_, small hooks movable on a basal piece attached to the arms are
found which may represent the vestiges of such organs (Fig. 216). Similar
hooks are found in the young _Ophiothrix fragilis_ just after metamorphosis
and in all Cladophiurae, replacing in the latter case the arm-spines in the
distal portion of the arm.

[Illustration: FIG. 215.—Three types of mouth-frame found in Zygophiurae.
A, _Ophioscolex_, × 10; B, _Ophiacantha_, × 6; C, _Ophiothrix_, × 6. (After
Lyman.)]

MOUTH-FRAME.—In its broad outlines there is practically no variation in
this organ throughout the group, but in respect of {493}the spines, which
are borne on the flanks of the jaws (mouth-papillae) and on their apices
(teeth and tooth-papillae) there is very great variation. Teeth are always
present. Mouth-papillae are very frequently present, tooth-papillae are
rarer, and it is only in a restricted number of genera (_Ophiocoma_ and its
allies) that both mouth-papillae and tooth-papillae are present at the same
time.

[Illustration: FIG. 216.—A portion of an arm of _Ophiohelus umbella_, near
the distal extremity, treated with potash to show the skeleton, × 55. The
vertebrae are seen to consist of two curved rods united at their ends. The
triangular side-plates bear a row of movable hooks which articulate with
basal outgrowths of the plate. (After Lyman.)]

SKELETON OF THE DISC.—This is typically composed of a mosaic of plates of
different sizes, but in some cases (_Ophiomyxa_, most Streptophiurae, and
Cladophiurae) these, with the exception of the radials and genitals, are
entirely absent, and the disc is then quite soft and covered with a
columnar epithelium, the persistent ectoderm. Even the scuta buccalia may
disappear. Radial shields are absent in _Ophiohelus_. In many cases
(_Ophiothrix_ and _Ophiocoma_) all the dorsal plates except the radials are
concealed from view by a covering of small spines. In some genera
(_Ophiopyrgus_) there are five large plates in the centre of the upper part
of the disc, which have been termed "calycinals" from a mistaken comparison
with the plates forming the cup or calyx of the Pelmatozoa, but there is no
connexion between the two sets of structures.

The MADREPORITE is usually quite rudimentary, but in Cladophiurae there may
be five madreporites, each with about 200 pores, and, of course, five
stone-canals.

{494}The number of GENITAL ORGANS varies very much. In the small _Amphiura
squamata_ there are two gonads, an ovary and a testis, attached to each
bursa, but in the larger species there may be very many more.

We follow Bell's classification,[463] according to which the Ophiuroidea
are divided, according to the manner in which the vertebrae move on one
another (cf. Fig. 210), into three main orders, since these movements are
of prime importance in their lives.

(1) STREPTOPHIURAE, in which the faces of the vertebrae have rudimentary
knobs and corresponding depressions, so that the arms can be coiled in the
vertical plane. These are regarded as the most primitive of Ophiuroidea.

(2) ZYGOPHIURAE, in which the vertebral faces have knobs and pits which
prevent their coiling in a vertical plane.

(3) CLADOPHIURAE, in which the arms can be coiled as in (1) and are in most
cases forked. No teeth; the arm-spines are papillae, the covering plates of
the arms are reduced to granules.


ORDER I. STREPTOPHIURAE.

This is not a very well defined order; it includes a few genera
intermediate in character between the Cladophiurae and the Zygophiurae, and
believed to be the most primitive Ophiuroids living. It is not divided into
families. The vertebrae have rudimentary articulating surfaces, there being
two low bosses and corresponding hollows on each side, and so they are
capable of being moved in a vertical plane, as in the Cladophiurae; the
arms never branch, and further, they always bear arm-spines and lateral
arm-plates at least. No species of this order are found on the British
coast, but _Ophiomyxa pentagona_, in which the dorsal part of the disc is
represented only by soft skin, is common in the Mediterranean.

_Ophioteresis_ is devoid of ventral plates on the arms, and appears to
possess an open ambulacral groove, though this point has not been tested in
sections. _Ophiohelus_ and _Ophiogeron_ have vertebrae in which traces of
the double origin persist (see p. 491).


{495}ORDER II. ZYGOPHIURAE.

This group includes all the common and better-known British forms. They are
divided into five families, all of which are represented in British waters.

[Illustration: FIG. 217.—Aboral view of _Ophioglypha_ (_Ophiura_)
_bullata_. × 3. (From Wyville Thomson.)]

FAM. 1. OPHIOLEPIDIDAE.[464]—Arm inserted in a definite cleft {496}in the
disc, or (expressing the same fact in another way) the interradial lobes
out of which the disc is composed are not completely united. Radial shields
and dorsal plates naked. Arm-spines smooth and inserted on the posterior
border of the lateral arm-plates.

[Illustration: FIG. 218.—Oral view of _Ophioglypha_ (_Ophiura_) _bullata_.
× 5. (From Wyville Thomson.)]

This family includes all the Brittle Stars of smooth porcelanous aspect and
provided with only short spines. Forbes[465] called them Sand-stars, since
their short spines render these animals incapable of burrowing or of
climbing well, and hence they appear to move comparatively rapidly over
firm ground, sand, gravel, or muddy sand, and they are active enough to be
able to capture small worms and Crustacea. The prey is seized by coiling
one of the arms around it.

One genus, _Ophiura_, is fairly common round the British coast, {497}being
represented by _O. ciliaris_ and _O. albida_; the former is the commoner.
An allied species dredged by H.M.S. "Challenger" is represented in Figs.
217 and 218.

_Ophiomusium_ (Fig. 219) is a very peculiar genus. The mouth-papillae on
each side of each mouth-angle are confluent, forming a razor-like
projection on each side of each mouth-angle (Fig. 220). The arms are short,
and the podia are only developed at the bases of the arms. _Ophiopyrgus_
has the dorsal surface raised into a conical elevation protected by a
central plate surrounded by five large plates.

[Illustration: FIG. 219.—Aboral view of _Ophiomusium pulchellum_. × 7.
(From Wyville Thomson.)]



In the remaining four families the arms are inserted on the under surface
of the disc; in other words, the interradial lobes which make up the disc
have completely coalesced dorsally; and the spines stand out at right
angles to the arm.

FAM. 2. AMPHIURIDAE.—Mouth-papillae present, but no tooth-papillae; radial
shields naked; small scuta buccalia.

The most interesting Brittle Star belonging to this family is _Amphiura
squamata_ (_elegans_), a small form, with a disc about ¼ inch in diameter
covered with naked plates. It is hermaphrodite and viviparous, the young
completing their development inside the bursae of the mother. Occasionally
the whole disc, with the exception of the mouth-frame, is thrown off and
regenerated. This appears to be a device to enable the young to escape.
Three other species of _Amphiura_ are found in British waters.

{498}[Illustration: FIG. 220.—Oral view of _Ophiomusium pulchellum_. × 7.
(From Wyville Thomson.)]

_Ophiactis_ is another genus belonging to this family, distinguished from
_Amphiura_ by its shorter arms and smoother arm-spines. It lives in the
interstices of hard gravel. The British species, _O. balli_, presents no
special features of interest, but the Neapolitan _O. virens_ is an
extraordinary form. It has six arms, three of which are usually larger than
the other three, for it is always undergoing a process of transverse
division, each half regenerating the missing part. It has from 1 to 5
stone-canals, the number increasing with age; numerous long-stalked
Polian-vesicles in each interradius, and in addition a number of long
tubular canals which spring from the ring-canal, and entwine themselves
amongst the viscera.[466] All the canals of the water-vascular system,
except the stone-canals, contain non-nucleated {499}corpuscles, carrying
haemoglobin,[467] the respiratory value of which compensates for the loss
of the genital bursae, which have entirely disappeared.

_Ophiopholis_ is distinguished from the foregoing genera by the granular
covering of its dorsal plates; whilst in _Ophiacantha_ these granules
develop into prominent spinelets, and the arm-spines are also thorny.
_Ophiopholis aculeata_ occurs in swarms in the branches of the Firth of
Clyde, and presents a most remarkable series of variations in colour.
_Ophiopsila_ is a closely allied form, distinguished by its large
peristomial plates.

[Illustration: FIG. 221.—Oral view of _Ophiacantha chelys_. × 4. (From
Wyville Thomson.)]

FAM. 3. OPHIOCOMIDAE.—Both mouth-papillae and tooth-papillae are
present;[468] the arm-spines are smooth, and the disc is covered with
granules.

_Ophiocoma nigra_ is the only common British representative of this family.
In this species the plates of the dorsal surface are completely hidden from
view by a covering of granules. _Ophiarachna_.

FAM. 4. OPHIOTHRICIDAE.—Tooth-papillae alone present, mouth-papillae
absent; arm-spines roughened or thorny.

This family is represented only by _Ophiothrix fragilis_, which is perhaps
the most abundant of all British Ophiuroids, and has been selected as the
type for special description.

The back is covered with spinules, having, however, the triangular radial
plates bare. This produces a contrast-effect, which suggested the name
_pentaphyllum_, formerly used by some naturalists for the species. It
occurs in swarms, and presents variations in colour nearly as marked as
those of _Ophiopholis_. {500}_Ophiopteron_ is probably a swimming
Ophiuroid, as the lateral spines of each segment of the arm are connected
by a web of skin.


ORDER III. CLADOPHIURAE.

[Illustration: FIG. 222.—Aboral view of young _Astrophyton linckii_,
slightly enlarged. (From Wyville Thomson.)]

These, like the Streptophiurae, have the power of rolling the arms in a
vertical plane, but the articulating surfaces of the vertebrae are
well-developed and saddle-shaped. The dorsal surface of the disc and arms
is covered with a thick skin with minute calcifications. Upper-arm plates
wanting. Radial plates always present, though occasionally represented by
lines of scales. {501}The order is divided into three families, two of
which are represented in British waters.

FAM. 1. ASTROSCHEMIDAE.—Arms unbranched. _Astronyx_ is comparatively common
in the sea-lochs of Scotland. There are a series of pad-like ridges on the
arms, representing the side-plates and bearing the spines. _Astroschema_.

FAM. 2. TRICHASTERIDAE.—Arms forked only at the distal ends. _Trichaster_,
_Astrocnida_.

FAM. 3. EURYALIDAE.—Arms forked to their bases. _Gorgonocephalus_ is
occasionally taken in deep water off the north coast of Scotland. In it the
arms repeatedly fork, so that a regular crown of interlacing arms is
formed. The animal obviously clings to external objects with these, for it
is often taken in fishermen's nets with its arms coiled around the meshes.
The genital bursae are said to be represented by slits which open directly
into the coelom. (Lyman describes the coelom as divided into ten
compartments by radiating septa; it is possible—even probable—that these
are really the bursae.) An allied species is common in the Bay of Fundy,
being found in comparatively shallow water. _Astrophyton_ (Fig. 222) is
closely allied to _Gorgonocephalus_, differing only in trifling points. It
is doubtful whether the separation of these two genera is justified.

FOSSIL OPHIUROIDEA.—The Ophiuroidea are rather sparsely represented among
fossils, but in the Silurian and Devonian a series of very interesting
forms occur which are intermediate in character between Starfish and
Brittle Stars, and which were therefore in all probability closely allied
to the common ancestors of modern Ophiuroids and Asteroids. Jaekel[469] has
recently added largely to our knowledge of these primitive forms, and has
described a number of new genera. Thus _Eophiura_ from the Lower Silurian
has an open ambulacral groove, and the vertebrae are represented by an
alternating series of quadrate ossicles, each deeply grooved on its under
surface for the reception of the tentacle, which was not yet (as in modern
forms) enclosed in the vertebra. The lateral or adambulacral plates
extended horizontally outwards, and each bore a series of spines at its
outer edge.

A remarkable fact is that where the halves of the vertebrae (_i.e._ the
ambulacral ossicles) diverge in order to form the {502}mouth-angles, no
less than five or six vertebrae are thus affected, instead of only two as
in modern forms. The actual "jaw," however, seems, as in modern forms, to
consist only of the first adambulacral fused to the second ambulacral, so
that instead of concluding with Jaekel that the "jaws" of modern forms
result from the fusion of five or six vertebrae, a conclusion which would
require that a number of tentacles had disappeared, we may suppose that the
gaping "angles" of these old forms have, so to speak, healed up, except at
their innermost portions.

In _Bohemura_, which belongs to a somewhat younger stratum, the structure
is much the same, but the groove in the ambulacral ossicle for the tentacle
has become converted into a canal, and the ambulacral groove itself has
begun to be closed at the tip of the arm by the meeting of the
adambulacrals.

In _Sympterura_, a Devonian form described by Bather,[470] the two
ambulacral plates of each pair have thoroughly coalesced to form a
vertebra, but there is still an open ventral groove, and no ventral plates.

In the Trias occurs the remarkable form _Aspidura_, which had short
triangular arms, in which the tentacle pores were enormous and the ventral
plates very small. The radial plates formed a continuous ring round the
edge of the disc. _Geocoma_ from the Jurassic is a still more typical
Ophiuroid; it has long whip-like arms, and the dorsal skeleton of the disc
is made of fifteen plates, ten radials, and five interradials. In the
Jurassic the living genus _Ophioglypha_, appears.

The Cladophiurae are represented already in the Upper Silurian by
_Eucladia_, in which, however, the arms branch not dichotomously, as they
do in modern forms, but monopodially. There is a large single madreporite.

_Onychaster_, with unbranched arms, which occurs in the Carboniferous, is a
representative of the Streptophiurae.

It will therefore be seen that the evolution of Ophiuroidea must have begun
in the Lower Silurian epoch. The Streptophiurae are a few slightly modified
survivors of the first Ophiuroids. By the time the Devonian period had
commenced, the division of the group into Zygophiurae and Cladophiurae had
been accomplished.




{503}CHAPTER XVIII

ECHINODERMATA (_CONTINUED_): ECHINOIDEA = SEA-URCHINS


CLASS III. ECHINOIDEA

The Sea-urchins or Echinoidea (Gr. ἐχῖνος, Hedgehog or Sea-urchin), which
constitute the third class of the Eleutherozoa, have derived both their
popular and scientific names from the covering of long spines with which
they are provided. At first sight but little resemblance is to be discerned
between them and the Starfish and Brittle Stars. They are devoid of any
outgrowths that could be called arms; their outline is generally either
circular or that of an equilateral pentagon, but as their height is almost
always smaller than their diameter, they are never quite spherical;
sometimes it is so small that the animals have the form of flattened discs.

All doubt as to the relationship of the Echinoidea to the Starfish is at
once dispelled in the mind of any one who sees one of the common species
alive. The surface is beset with delicate translucent tube-feet, terminated
by suckers resembling those of Starfish, although capable of much more
extension. The animal throws out these organs, which attach themselves by
their suckers to the substratum and so pull the body along, whilst the
spines are used to steady it and prevent it from overturning under the
unbalanced pull of the tube-feet. When moving quickly the animal walks on
its spines, the tube-feet being little used. The tube-feet are distributed
over five bands, which run like meridians from one pole of the animal to
the other. These bands are termed "radii," and they extend from the mouth,
which is situated in the centre of the lower surface, up to the
neighbourhood of the aboral pole. The radii must be compared to the
ambulacral grooves on the {504}oral surface of the arms of Starfish, and
hence in Urchins the aboral surfaces of the arms have, so to speak, been
absorbed into the disc, so that the oral surfaces have become bent in the
form of a semicircle. The radii are separated from one another by
meridional bands called "interradii," which correspond to the interradial
angles of the disc of a Starfish and to the sides of its arms. The small
area enclosed between the upper terminations of the radii is called the
"periproct," and this corresponds to the entire dorsal surface of the
Starfish, including that of the arms.

One of the commonest species of British Sea-urchin is _Echinus esculentus_.
In sheltered inlets, such as the Clyde, it is often left exposed by the
receding tide, whilst everywhere on the coast in suitable localities it may
be obtained by dredging at moderate depths on suitable ground. In the Clyde
it is easy to observe the habits of the animal through the clear still
water. It is then seen to frequent chiefly rocky ground, and to exhibit a
liking for hiding itself in crevices. Often specimens will be seen clinging
to the rock by some of their tube-feet, and, as it were, pawing the under
surface of the water with the others. In the Clyde it feeds chiefly on the
brown fronds of _Laminaria_, with which the rocks are covered. In more
exposed situations, such as Plymouth Sound, it does not occur in shallower
water than 18 to 20 fathoms. At this depth it occurs on a rocky ridge; but
in 1899, after a south-west gale, all the specimens had disappeared from
this ridge, showing at what a depth wave disturbance is felt.

A full-grown specimen is as large as a very large orange; its under surface
is flattened, and it tapers somewhat towards the aboral pole. The outline
is that of a pentagon with rounded angles. The spines in _Echinus
esculentus_ are short in comparison to the diameter of the body, and this
is one of the characteristics of the species.

The animal is provided with a well-developed skeleton, consisting of a mail
of plates fitting closely edge to edge, and carrying the spines. This
cuirass bears the name "corona" (Fig. 227). It has two openings, an upper
and a lower, which are both covered with flexible skin. The upper area is
known as the "periproct" (Fig. 227, 2); it has in it small isolated plates,
and the anus, situated at the end of a {505}small papilla, projects from it
on one side of the centre. The lower area of flexible skin surrounds the
mouth, and is called the "peristome" (Fig. 229), though it corresponds to
considerably more than the peristome of Asteroidea. In the mouth the tips
of the five white chisel-like teeth can be seen.

The plates forming the corona are, like all the elements of the skeleton of
Echinodermata, products of the connective tissue which underlies the
ectoderm, which in Echinoidea remains in a fully developed condition
covering the plates, and does not, as in Ophiuroidea, dry up so as to form
a mere cuticle. The ectoderm consists of the same elements as that of
Asteroidea, viz. delicate tapering sense-cells with short sense-hairs,
somewhat stouter supporting cells and glandular cells. It is everywhere
underlaid by a plexus of nerve fibrils, which, in part, are to be regarded
as the basal outgrowths of the sense-cells and partly as the outgrowths of
a number of small bipolar ganglion-cells, found intermixed with the fibres.

[Illustration: FIG. 223.—Aboral view of _Echinus esculentus_. × ½. (After
Mortensen.)]

Just as the MUSCULAR ARM has been the determining factor in the structure
of the Ophiuroidea, so the MOVABLE SPINE has been {506}the leading factor
in the evolution of Echinoidea. The SPINES have cup-shaped basal ends,
which are inserted on special projections of the plates of the skeleton
called tubercles. The tubercle is much larger than the cup, and hence the
spine has a great range of possible motion. The spines differ from those of
Starfish and Brittle Stars in being connected with their tubercles by means
of cylindrical sheaths of muscle fibres, by the contraction of which they
can be moved in any direction. The muscles composing the sheath consist of
an outer translucent and an inner white layer. The former are easily
stimulated and soon relax; they cause the movements of the spines. The
latter require stronger stimulation, but when aroused respond with a
prolonged tetanus-like contraction, which causes the spines to stand up
stiffly in one position; these muscles can be torn across sooner than
forced to relax. Uexküll[471] has appropriately named them "block
musculature." These sheaths, like everything else, are covered with
ectoderm, which is, however, specially nervous, so that we may say that the
muscular ring is covered by a nerve-ring from which stimuli are given off
to the muscles.

The spines are, speaking generally, of two sizes, the larger being known as
"primary spines" and the smaller as "secondary." In many Echinoidea these
two varieties are very sharply contrasted, but in _Echinus esculentus_
there is not such a great difference in length, and intermediate kinds
occur. The forest of spines has an undergrowth of PEDICELLARIAE. All
Echinoidea possess pedicellariae, which are much more highly developed than
those of any Asteroid. With few exceptions all the pedicellariae of
Echinoidea possess three jaws and a basal piece. This latter is, however,
drawn out so as to form a slender rod, which articulates with a minute boss
on a plate of the skeleton.

Of these pedicellariae there are in _E. esculentus_ four varieties, viz.
(1) "tridactyle" (Fig. 225, C; Fig. 226, B): large conspicuous
pedicellariae with three pointed jaws, each armed with two rows of teeth on
the edges. There is a flexible stalk, the basal rod reaching only half way
up. These are scattered over the whole surface of the animal.

(2) "Gemmiform" (Fig. 225, A, B; Fig. 226, A), so called from the
translucent, almost globular head. The appearance of {507}the head is due
to the fact that there is on the outer surface of each jaw a sac-like gland
developed as a pouch of the ectoderm. From it are given off two ducts which
cross to the inner side of the blades and, uniting into one, run in a
groove to near the tip. The gland secretes a poisonous fluid. The basal rod
reaches up to the jaws, so that this form of pedicellaria has a stiff
stalk. On the inner side of each blade, near the base, there is a slight
elevation (Fig. 225, B, _s_), consisting of cells bearing long cilia; this
is a sense-organ for perceiving mechanical stimuli. The gemmiform
pedicellariae are particularly abundant on the upper surface of the animal.

[Illustration: FIG. 224.—View of the apical region of _Echinus esculentus_,
showing spines and pedicellariae; drawn from the living specimen, × 3. _a_,
Anus; _g.p_, genital pore; _i_, interradius; _mp_, madreporite; _per_,
periproct; _p.gemm_, gemmiform pedicellaria; _pod_, podia; _p.trid_,
tridactyle pedicellaria; _p.trif_, trifoliate pedicellaria; _r_, radius;
_t.t_, pore for terminal tentacle of the radial water-vascular canal.]

{508}(3) "Trifoliate" (Fig. 225, E; Fig. 226, D): these are very small
pedicellariae, in which the jaws are shaped like leaves with the broad end
projecting outwards. They are scattered over the whole surface of the body.

(4) "Ophicephalous" (Fig. 225, D; Fig. 226, C): pedicellariae in which the
jaws have broad rounded distal ends fringed with teeth; these ends bear a
resemblance to a snake's head, whence the name. The bases are also broad
and thin, with a strong median rib and a peculiar semicircular hoop beneath
the spot where they articulate with one another. The three hoops of the
three jaws work inside each other in such a way as to cause the jaws to
have a strong grip and to be very difficult to dislocate from their mutual
articulation.

The ophicephalous pedicellariae are in _Echinus_ the most abundant of all;
and they alone extend on to the peristome, where a special small variety of
them is found.

A thorough investigation of the functions and reactions of the
pedicellariae has quite recently been made by von Uexküll.[472] He showed,
first of all, that there is a nervous centre in the stalk of each
pedicellaria (see below), which causes the organ to incline towards a weak
stimulus, but to bend away from a stronger stimulus. In the head there is
an independent nervous centre, which regulates the opening and closing of
the valves, and causes these to open on slight stimulus and close when a
stronger one is applied. The amount of stimulus necessary to cause the
pedicellariae to retreat varies with the kind of pedicellariae, being least
with the tridactyle and most with the gemmiform, so that when a chemical
stimulus, such as a drop of dilute ammonia, is applied to the skin, the
tridactyle pedicellariae may be seen to flee from and the gemmiform to
approach the point of stimulation. In a living Sea-urchin, if the attempt
is made to seize the tridactyle pedicellariae they will evade the forceps,
but the ophiocephalous are easy to catch.

The tridactyle pedicellariae open with the very slightest mechanical
stimulus and close with rather greater mechanical stimuli or with
exceedingly slight chemical ones. Uexküll calls them "Snap-pedicellariae,"
and their function is to seize and destroy the minute swimming larvae of
various sessile parasitic {509}animals, which would otherwise settle on the
delicate exposed ectoderm of the Sea-urchin.

The gemmiform pedicellariae are brought into action when a more serious
danger threatens the Sea-urchin, such as an attack of a Starfish. The
corrosive chemical influence, which it can be proved exudes not only from
the stomach but even from the tube-feet of the Starfish, causes the
gemmiform pedicellariae to approach and open widely. When the foe
approaches so closely as to touch the sense-organs (Fig. 225, B, _s_)
situated on the inner side of the valves of these pedicellariae, the blades
close violently, wounding the aggressor and causing its juice to exude,
thus producing a renewed and severe chemical stimulation which irritates
the poison glands and causes the poison to exude. The virulence of the
poison may be gauged from the fact that the bite of a single gemmiform
pedicellaria caused a frog's heart to stop beating.

[Illustration: FIG. 225.—The pedicellariae of _Echinus acutus_, drawn from
a living specimen. A, gemmiform pedicellaria, closed. B, gemmiform
pedicellaria, open; _g_, poison gland; _s_, sense-organ, × 3. C, tridactyle
pedicellaria, × 6. D, ophicephalous pedicellaria, × 9. E, trifoliate
pedicellaria, × 12; _a_ (in all figures), axial rod of the stalk. (After
Uexküll.)]

Prouho[473] has described a combat between a Sea-urchin and a Starfish.
When the latter approached, the spines of the {510}Sea-urchin diverged
widely (strong form of reaction to chemical stimulus), exposing the
gemmiform pedicellariae. These at once seized the tube-feet of the enemy
and the Starfish retreated, wrenching off the heads of these pedicellariae;
then the Starfish returned to the attack and the same result followed, and
this was repeated till all the pedicellariae were wrenched off, when the
Starfish enwrapped its helpless victim with its stomach.

The minute trifoliate pedicellariae are brought into play by any prolonged
general irritation of the skin, such as bright light or a rain of particles
of grit or mud. They have the peculiarity that not all the blades close at
once, so that an object may be held by two blades and smashed by the third.
They may be seen in action if a shower of powdered chalk is poured on the
animal, when they seize the particles and by breaking up any incipient
lumps reduce the whole to an impalpable powder, which the cilia covering
the skin speedily remove. In thus assisting in the removal of mechanical
"dirt" they earn the name which Uexküll has bestowed on them, of "cleaning
pedicellariae."

[Illustration: FIG. 226.—Views of a single blade of each kind of
pedicellaria. A, blade of gemmiform pedicellaria of _Echinus elegans_; _g_,
groove for duct of poison gland; B, blade of tridactyle pedicellaria of the
same species; C, blade of ophicephalous pedicellaria of the same species;
_r_, ring for clamping this blade to the other blades; D, blade of
trifoliate pedicellaria of _E. alexandri_. (After Mortensen.)]

The ophicephalous pedicellariae, with their powerful bull-dog grip, assist
in holding small animals, such as Crustacea, till the tube-feet can reach
them and convey them to the mouth.

{511}The number and variety of the pedicellariae, then, is an eloquent
testimony to the dangers to which the soft sensitive skins of the
Sea-urchin and other Echinodermata are exposed, and afford confirmatory
evidence in support of the view expressed above, that the method adopted to
defend the skin was one of the great determining features which led to the
division of the Asteroidea into different races.

[Illustration: FIG. 227.—Dried shell of _Echinus esculentus_, showing the
arrangement of the plates of the corona. × 1. 1, The anus; 2, periproct,
with irregular plates; 3, the madreporite; 4, one of the other genital
plates; 5, an ocular plate; 6, an interambulacral plate; 7, an ambulacral
plate; 8, pores for protrusion of the tube-feet; 9, tubercles of the
primary spines, _i.e._ primary tubercles.]

The CORONA consists of five radial or "ambulacral" bands of plates and five
interradial, or as they are usually termed, "interambulacral" bands of
plates—ten in all. Each of the ten consists of two vertical rows of plates
throughout most of its extent, and each plate is studded with large bosses,
or "primary tubercles" for the primary spines, smaller bosses called
"secondary tubercles" for the secondary spines, and finally, minute
elevations called "miliary tubercles" for the pedicellariae.

{512}[Illustration: FIG. 228.—The so-called calyx and the periproct of
_Echinus esculentus_. × 4. 1, Genital plates with genital pores; 2, ocular
plates with pores for terminal tentacles of the radial water-vascular
canals; 3, madreporite; 4, periproct with irregular plates; 5, anus. (After
Chadwick.)]

Even in the dried skeleton, however, the ambulacral plates can be
discriminated from the interambulacral by the presence of pores to permit
the passage of the tube-feet. These pores are arranged in pairs, and each
pair corresponds to a single tube-foot, since the canal connecting the
ampulla with the external portion of the tube-foot is double in the
Echinoidea. In _Echinus esculentus_ there are three pairs of such pores in
each plate, in _Strongylocentrotus droëbachiensis_ four pairs. The
ambulacral plate is really made up of a series of "pore-plates," each
carrying a single pair of pores, and these become united in threes in
_Echinus_ and fours in _Strongylocentrotus_, while in primitive forms like
the Cidaridae they remain separate. Each ambulacral and interambulacral
area ends at the edge of the periproct with a single plate. The plate
terminating the ambulacral band is pierced by a single pore for the exit of
the median tentacle, which, as in Asteroids, terminates the radial
water-vascular canal. Thus the aboral end of the radius in an Echinoid
corresponds to the tip of the arm in an Asteroid. The plate is termed
"ocular," because the terminal tentacle has a mass of pigmented cells at
its base; but no eye-cups can be seen, and there is no evidence that this
spot is specially sensitive to light. Species which show special
sensitiveness to light have often a large number of what we may perhaps
term secondary eyes. The plate terminating the interambulacral {513}series
is termed the "genital plate," because it is pierced by the duct of one of
the five genital organs. One of the genital plates is also pierced by the
madreporic pores. Some zoologists have separated the ocular and the genital
plates under the name of "calyx" from the rest of the corona, under a
mistaken idea that they are homologous with the plates of the body or calyx
of a Crinoid.

[Illustration: FIG. 229.—The peristome of _Echinus esculentus_. × 2. 1,
Tube-feet of the lower ends of the radii; 2, gill; 3, teeth; 4, buccal
tube-foot; 5, smooth peristomial membrane. (After Kükenthal.)]

The PERIPROCT (Fig. 228, 4) is covered with small plates and bears a few
pedicellariae. The PERISTOME (Fig. 229) is covered by flexible skin with
abundant pedicellariae; it terminates in a thick lip surrounding the mouth,
from which the tips of five white teeth are just seen projecting. There are
ten short tube-feet projecting from the peristome—one pair in each
radius—and each tube-foot terminates in an oval disc and is capable of
little extension, and each has around its base a little plate. The presence
of these tube-feet shows that in _Echinus_ the {514}peristome extends
outwards beyond the water-vascular ring, whereas in Asteroidea it is
contained entirely within the ring. In the primitive Cidaridae (Fig. 235)
the whole peristome down to the lip surrounding the mouth is covered with a
series of ambulacral and interambulacral plates similar to those forming
the corona, though smaller and not immovably united, and the series of
tube-feet is continued on to it. It is thus evident that the peristome is
merely part of the corona, which has become movable so as to permit of the
extension of the teeth. In _Echinus_ the peristome is continued in each
interradius into two branched outgrowths called gills, the relation of
which to the respiratory function will be described later. These gills
(Fig. 229, 2) are situated in indentations of the edge of the corona called
"gill-clefts" (Fig. 230, _g_).

[Illustration: FIG. 230.—The dried peristome of _Echinus esculentus_ and
the surrounding portions of the corona. × 1. _amb_, Ambulacral plate;
_b.t_, buccal tube-foot; _g_, gill-cleft; _inter_, interambulacrum; _per_,
peristome.]

The most conspicuous plates in the peristome are those {515}surrounding the
buccal tube-feet; besides these, however, there are in _Echinus
esculentus_, and probably in most species, a large number of thinner
irregularly-scattered plates (Fig. 230).

The term ambulacral plate, applied to the plate pierced by the pores for
the tube-feet, conveys a misleading comparison with the ambulacral plate of
an Asteroid. In Echinoids the ambulacral groove has become converted into a
canal called the "epineural canal," and the ambulacral plates form the
floor, not the roof, of this canal; they may perhaps correspond with the
adambulacral plates of the Starfish, which one may imagine to have become
continually approximated as the groove became narrower until they met.

[Illustration: FIG. 231.—Dissection of _Echinus esculentus_. × 1. The
animal has been opened by a circumferential cut separating a small piece of
the skeleton at the aboral end, which is turned outwards exposing the
viscera on its inner surface. The other viscera are seen through the hole
thus made. _amp_, Ampullae of the tube-feet; _aur_, auricle; _b.v_,
so-called "dorsal blood-vessel"; _comp_, "compasses" of Aristotle's
lantern, often termed "radii" by English authors; _comp.elv_, elevator
muscles of the compasses; _comp.ret_, retractor muscles of the compasses;
_eph_, epiphyses of the jaws in Aristotle's lantern; _gon_, gonad;
_g.rach_, genital rachis; _int_, intestine; _oe_, oesophagus; _prot_,
protractor of Aristotle's lantern; _rect_, rectum; _ret_, retractor of
Aristotle's lantern; _siph_, siphon; _st_, stomach; _stone.c_,
stone-canal.]

{516}The internal organs of the Urchin can best be examined by making a
horizontal incision about one-third the distance from the mouth and pulling
the two parts gently asunder. A large amount of fluid escapes from the
exceedingly spacious coelomic cavity, the alimentary canal being
comparatively narrow.

The ALIMENTARY CANAL commences with a short vertical tube which has been
shown to be a stomodaeum; this is surrounded by the upper ends of the teeth
and their supporting ossicles, which are collectively termed "Aristotle's
lantern." The oesophagus leads into a baggy, flattened tube, the stomach,
which runs horizontally round the animal, supported by strings of tissue
from the coelomic wall, so that it hangs down in a series of festoons.
Having encircled the animal, it bends directly back on itself and
immediately opens into the intestine, which is also a flattened tube, which
runs round the circumference of the animal, but in the opposite direction,
the festoons of the second circle alternating with those of the first. The
intestine opens into a short rectum which ascends vertically to open by the
anus. The stomach is accompanied by a small cylindrical tube called the
"siphon" (Fig. 231, _siph_), which opens into it at both ends; this
represents merely a gutter which has been completely grooved off from the
main intestine; it is lined by cilia, and its function is believed to be
that of keeping a stream of fresh water flowing through the gut, so as to
subserve respiration.

_Echinus esculentus_ seems to feed chiefly on the brown fronds of
_Laminaria_ and the small animals found thereon, which it chews up with its
teeth, but it may regale itself on the same diet as Brittle Stars, as
Allen[474] has shown to be the case in Plymouth Sound. Dohrn[475] has
described the Neapolitan _Sphaerechinus granularis_ attacking and capturing
Crustacea such as _Squilla_.

The WATER-VASCULAR SYSTEM presents several features of great interest. The
ring-canal is situated at a considerable distance above the nerve-ring, and
is separated from it by the whole of the jaws and teeth. It has five small
interradial pouches on it, which apparently correspond to Tiedemann's
bodies in an Asteroid. The stone-canal (Fig. 231) opens as {517}usual into
the ring-canal, and is accompanied by the axial sinus and genital stolon.
The name "stone-canal" is very unsuitable in this order, for there are no
calcifications in its walls; it is a simple membranous tube of circular
section. On reaching the upper wall of the test it expands into an ampulla,
into which the numerous ciliated pore-canals traversing the madreporite
open. The radial canals, starting from the ring-canal, pursue a downward
course till they come into contact with the radial nerve-cords, and they
then bend upwards and run along the centre of the ambulacral region,
finally terminating in the small terminal tentacles. In the just
metamorphosed Echinoid these are well-developed tube-feet, each with a
well-developed sucker, in the centre of which is a conical sensory
prominence, but as development proceeds they become enclosed in a circular
outgrowth of the test, so that only the tip projects in the adult.

The long extensible tube-feet are connected by transverse canals with the
radial canal. Instead of the pair of valves which in Asteroids prevent the
reflux of liquid into the canal, there is a perforated diaphragm[476] with
circular muscles, which by contraction close the opening in the diaphragm,
while when they are relaxed fluid can return from the tube-foot. The
ampulla is flattened, and is contracted by muscular fibres called
"trabeculae" stretching across its cavity. These muscular strands are
developed by the cells lining the ampulla. The external portion of the
tube-foot, as in Asteroids, is provided with powerful longitudinal muscles,
and there is the same alternate filling and emptying of the ampulla as the
tube-foot is contracted and expanded. The tube-foot is connected by a
double canal with the ampulla, the object of which is to assist in
respiration. The cells lining it are ciliated, and produce a current up one
side of the tube-foot and down the other, and the double canal leading to
the ampulla separates these two currents and prevents them interfering with
one another. Thus water is continually transported from the ampulla to the
tube-foot, through the thin walls of which it absorbs oxygen, and it is
then carried back to the ampulla, and transfers its oxygen to the fluid of
the general body-cavity through the walls of the ampulla. The disc of the
tube-foot is supported by a calcareous plate {518}(Fig. 232, _oss_), a
circumstance which enabled Johannes Müller to recognise the Echinoid larva
when the form of the adult was as yet unrecognisable. Below the edge of the
disc there is a well-marked nerve-ring, from which two bundles of
nerve-fibres go to the disc itself, in the edge of which there is an
abundance of sense-cells.

The buccal tube-feet (Fig. 229, 4) are much shorter than the rest, and are
provided with oval discs which are highly sensory. These feet are not used
for seizing, but for tasting food; when a piece of food is placed near them
they are thrown into the most violent agitation.

[Illustration: FIG. 232.—Diagrammatic transverse section of the radius of
an Echinoid. _amb.oss_, Ambulacral ossicle; _amp_, ampulla of the
tube-foot; _ep_, epineural canal; _musc_, muscles attaching spine to its
boss; _nerv_, nervous ring in base of spine; _n.r_, radial nerve-cord;
_oss_, ossicle in sucker of tube-foot; _ped_, tridactyle pedicellaria;
_perih_, radial perihaemal canal; _pod_, tube-foot; _wv.r_, radial
water-vascular canal.]

The NERVOUS SYSTEM has the same form as in an Asteroid, viz. that of a ring
surrounding the mouth and giving off radial nerve-cords (Fig. 232, _n.r_),
one of which accompanies each water-vascular canal to the terminal
tentacle, where it forms a nervous cushion in which pigmented cells are
embedded.

A large band-like nerve is given off from the radial nerve-cord to each
tube-foot. This pedal nerve, as it is called, contains bipolar neurons, and
is really an extension of the nerve-cord itself. Beneath the sucker it
branches out to form a sensory ring. From the base of the pedal nerve,
branches are given off {519}which run to the ectoderm and enter into
connexion with the plexus there. Romanes[477] scraped away the radial cords
and found that the spines still converged when a point on the ectoderm was
stimulated, but that, on the other hand, if definite locomotor movements
were to be carried out, the presence of these cords was a necessity; hence
he concluded that the superficial plexus sufficed for ordinary reflexes,
but that for purposeful movements the central nervous system was necessary.

Von Uexküll[478] has made an exhaustive study of the physiology of the
nervous system in the Echinoidea. He points out that all the organs
controlled by the nervous system, spines, pedicellariae, tube-feet, and
(see below) Aristotle's lantern, give two opposite reactions in response to
the same stimulus according as it is strong or weak, bending away from the
point of stimulation when it is strong and towards it when it is weak. This
reversal of reaction can only be due to the action of the neuron in
altering the effect of the stimulus on the muscles, and this Uexküll
regards as its fundamental property. Thus in Preyer's[479] experiments with
Starfish the strong form of stimulation is obtained by directly applying
the stimulus to the radial cord or to the tube-feet, the weak form by
stimulating the back, when of course the stimulus has to traverse a longer
path before affecting the tube-feet, and is consequently weakened. Von
Uexküll also introduces the conception of "tone" with regard to the nervous
system. This term has been used to denote the amount of chronic contraction
in a muscle, and it is to be distinguished from the fleeting contractions
which cause movement. The more tone there is in a muscle the less
responsive it is to stimuli tending to bring about movement. As applied to
the nervous system "tone" denotes a condition when it is not receptive to
small stimuli, but when it is maintaining a condition of tone in a muscle
by which of course its own tone is measured. Tone in a neuron can therefore
be measured by the produced tone in the muscle, and the one is to be
discriminated from the other only by using stimulants, such as caffeine,
which have no direct action on muscle. Tone can also be measured by the
amount of stimulus necessary to irritate the neuron. When {520}muscles are
stretched the tone is lowered, and this loss of tone extends to the neuron
controlling the muscle, and _vice versa_. When the spines on being gently
stimulated bend towards the point of stimulation, this is due to the
contraction of the muscles on the side towards the point of stimulus, for
if the superficial plexus of nerve-fibres be cut through so that the
stimulus has to pursue a round-about course the spine will bend towards the
direction from which the stimulus comes. The bending of the spines away
from the stronger stimulus is likewise due to the muscles on the side
towards the stimulus. It is caused by a sudden fall of tone in these
muscles, which causes them to yield to the tone of the muscles on the
opposite side, and this fall of tone is due to a fall of tone in the
neurons, for it can be produced by chemicals, and the direct action of all
chemicals applied to muscle is to raise tone.

In _Arbacia_ this form of reaction cannot be produced; the spines respond
to stimuli of all degrees of intensity by convergence towards the point of
stimulation.

When a general skin-irritant like dilute acetic acid, or even strong light,
is applied to the skin of a Sea-urchin the spines bend alternately to all
points of the compass, or, in a word, rotate. This is due to the fact that
the weight of the inclined spine stretches the muscles of one side and so
renders them more open to the general stimulus; these muscles in
consequence, contract, and so move the spine to a new position in which
other muscles are stretched, and a similar result follows. A continuation
of this process brings about rotation.

When a piece of glass rod or other light object is laid on the spines of a
Sea-urchin, it naturally, by its weight, presses asunder the spines and
stretches their muscles on one side, thus lowering the tone. If now the
skin be stimulated at any point the piece of rod will be rolled by the
spines towards the point of stimulation. This is caused by the fact that
the muscles of the spines holding the rod are made more receptive by being
stretched, and therefore they contract more than do the others in response
to the stimulation, and so the rod is rolled onwards on to the next spines,
which then act in the same manner. This passage of stimulus is entirely
independent of direct nervous connexion between the bases of the spines,
for it will traverse at right angles a crack going clean through the shell;
it is merely the {521}result of the mechanical weight of the object and of
the juxtaposition of the spines.

If the stimulation be too violent the first spines affected diverge wildly
and strike their neighbours with vehemence, so arousing into activity the
block musculature of these. This causes them to stand rigidly up, and so
the path of the stimulus is barred.

Now the escape movements of the animal under strong stimulation which
Romanes[480] alludes to are just an example of this handing on of
stimulation from spine to spine, not by nervous connexion but by mechanical
touch only; the object in this case is the substratum on which the animal
lies, which is, so to speak, rolled towards the point of stimulation, or
putting it otherwise, the animal is rolled away from it. Righting when
upset is another example of the same phenomenon; the aboral spines are
stretched by the weight of the animal, and the animal acts as if it were
stimulated in the region of the periproct. When a Sea-urchin is in its
normal position and is stimulated in the periproct (as for instance by a
strong light), it would, according to this rule, tend to move downwards,
which is of course impossible; but as the stimulus never affects all sides
quite alike the result is that the Urchin rotates, turning itself ever away
from the point of strongest stimulation. In the case of _Strongylocentrotus
lividus_ when living on limestone, as on the west coast of Ireland, this
results in the animal excavating for itself holes in the rock, where it is
safe from the action of the breakers.[481]

But it may be objected that no account is taken in the above description of
the action of the "central nervous system," _i.e._ of the ring and the
radial cords, and yet Romanes found that when they were removed the escape
movements could not be carried out. The answer is that the central nervous
system is a store-house of _tone_, not, as in higher animals, a controlling
centre for co-ordinating the movements of the spines. When it is removed at
first the escape movements can be carried out, but in a day or two all tone
in the spine-muscles is lost, and then, since the tone of all is equally
low, there is no tendency in those that are stretched to be more responsive
than others, and hence the escape movements cannot be carried out.
Sea-urchins kept in the tanks {522}of an aquarium are apt to lose the tone
of their spines owing to the poisoning of the nervous system.

The central nervous system is, however, the system which controls the
movements of the tube-feet. As we have seen, extensions of the radial
nerves run to the tip of each podium. Tube-feet are chiefly used in
ordinary progression; when this is quickened the spines come into play
exclusively. The extent to which these two organs of locomotion are used
varies from genus to genus. Thus _Centrostephanus_ uses its spines a good
deal, _Echinus_ and _Strongylocentrotus_ very little. The last-named genus
sometimes walks on its tube-feet entirely without touching the ground with
its spines.

The faculty of vision in its simplest form may be defined as sensitiveness
to light and shade. Now strong light acts on all Sea-urchins as a general
skin irritant. They fly from it towards the darkest corner, and then if it
continues the spines rotate. A number of little violet spines on the aboral
pole of _Centrostephanus longispinosus_ are especially sensitive to light,
and hence are almost constantly in rotation. This is due, according to
Uexküll,[482] to a pigment of a purple colour, which can be extracted by
means of alcohol and which is decomposed by light, the products of
decomposition being supposed to irritate the nerves. _Centrostephanus_ when
exposed to light becomes darker in colour. This is due to the migration
outwards of amoebocytes, which carry a pigment which acts as a screen in
order to prevent the valuable visual purple being too rapidly decomposed.
Not all Sea-urchins, in fact very few of those living in northern waters,
give a reaction to shadow. _C. longispinosus_ is one of the few; it reacts
to a shadow by converging its spines towards it. A much larger number of
species inhabiting tropical waters show this reaction. It is entirely
stopped if the radial nerve-cords be removed, whereas the reaction to
strong light continues. The reaction to shade is strongest after a long
previous exposure to light, hence Uexküll has given the following
explanation of it. The continued irritation due to light, having spread to
all the spines, eventually reaches the radial cords and is there stored in
the bipolar nerve-cells as _tone_. When the light-stimulus is interrupted
some of {523}the stored tone spreads upwards to the spines, causing the
weak form of spine reaction, and the spines converge.

[Illustration: FIG. 233.—To show character and distribution of the
sphaeridia in _Strongylocentrotus droëbachiensis_. A, a portion of a
radius, with sphaeridia, and the adjoining edge of the peristome. _p_, Pair
of pores for a tube-foot; per, peristome; _t_, primary tubercle. B, an
isolated sphaeridium. (After Lovén.)]

It will be seen therefore that the so-called central nervous system of
_Echinus_ does not act in any sense as a brain, as indeed might have been
guessed from the absence of any differentiation in it. As Uexküll points
out, when an animal is covered all over with similar organs, such as spines
and pedicellariae, capable of acting automatically, a brain is not needed.
The object of a brain is to direct organs which are in a certain place to a
danger which may come from any quarter, but in the Sea-urchin any spine is
as good as any other spine, and such orientation is not needed. "In a dog
the animal moves its legs, in a Sea-urchin the legs move the animal." What
the Sea-urchin does need is a means to prevent its pedicellariae attacking
its own organs with which they may come into contact. Thus it possesses an
"autodermin," a chemical contained in the ectoderm which paralyses the
muscles of the pedicellariae, as may be seen by offering to them a spine of
the same animal. If, however, the spine be treated with boiling water, and
then offered, it is viciously seized, showing that this substance can be
dissolved out.

Just as in the case of the Starfish, when the nerve-ring is {524}cut
through, the tube-feet in the various radii are no longer co-ordinated with
one another.

Besides the tips of the tube-feet the Urchin possesses another kind of
sense-organ, the SPHAERIDIA (Fig. 233). These are minute glassy spheres of
calcareous matter attached by connective tissue to equally minute bosses on
the plates of the ambulacra, generally near the middle line. They are in
fact diminutive spines, and like the latter are covered with a thick layer
of ectoderm, beneath which is a particularly well-developed cushion of
nerve-fibrils. Only the layer of muscles which connects a normal spine with
its boss is wanting. Although definite experimental proof is lacking, the
whole structure of the sphaeridia shows that they belong to the category of
"balancing organs." As the animal sways from side to side climbing over
uneven ground, the heavier head of the sphaeridia will incline more to one
side or to another, and thus exercise a strain on different parts of the
sheath, and in this way the animal learns its position with regard to the
vertical.

Intervening between the radial nerve-cord and the radial vessel is a single
RADIAL PERIHAEMAL CANAL (Fig. 232, _perih_), representing the two parallel
canals found in the same position in the Asteroid. The five perihaemal
canals lead downwards to a space called the LANTERN-COELOM, surrounding the
oesophagus.[483] Since the skeleton of the corona is composed of plates
immovably connected together, muscles corresponding to the ambulacral
muscles of the Asteroids would be useless, and so the wall of the
perihaemal canal remains thin and the side of it turned towards the general
coelom develops no muscles, and that turned towards the nerve-cord no
nerve-cells. Where, however, the radial nerve enters the nerve-ring, and on
the ring itself, an inner layer of nerve-cells is developed from the
lantern-coelom which represents the lower or oral portions of the radial
perihaemal canals. These cells control the muscles moving the teeth. These
canals are originally parts of the lantern-coelom, but in the adult they
become closed off from it.

{525}[Illustration: FIG. 234.—_Echinus esculentus_ dissected in order to
display Aristotle's lantern, × 2. The whole upper part of the shell has
been cut away. 1, Upper growing end of tooth; 2, outer forked end of one
"compass"; 3, muscle joining adjacent compasses and acting as elevator of
these ossicles; 4, depressor of the compasses; 5, lower end of jaw; 6,
retractor of the whole lantern; 7, protractor of the whole lantern; 8,
auricle; 9, ampullae of the tube-feet; 10, interambulacral plate; 11, lower
part of tooth; 12, water-vascular ring; 13, meeting-point of a pair of
epiphyses; 14, so-called Polian vesicle, really equivalent to Tiedemann's
body in an Asteroid; 15, oesophagus; 16, so-called ventral blood-vessel;
17, genital stolon; 18, stone-canal; 19, rectum; 20, aboral sinus. (Partly
after Chadwick.)]

In the outer wall of this space are developed the calcareous rods forming
ARISTOTLE'S LANTERN. These are first: five teeth (Fig. 234, 11),
chisel-shaped ossicles of peculiarly hard and close-set calcareous matter,
the upper ends (1) pushing out projections of the upper wall of the
lantern-coelom. These projections are the growing points of the TEETH,
whose lower ends pierce the {526}ectoderm and project into the lower end of
the oesophagus. Each tooth is firmly fixed by a pair of ossicles inclined
towards one another like the limbs of a V and meeting below. Each ossicle
is called an "alveolus," and taken together they form a "jaw." Their upper
ends are connected by a pair of ossicles called "epiphyses" (13). These two
epiphyses meet in an arch above. The jaws and their contained teeth are
situated interradially. Intervening between successive alveoli are radial
pieces called "rotulae," which extend directly inwards towards the
oesophagus. Above the rotulae are pieces termed "radii" or "compasses" (2),
which are not firmly attached to the other pieces but lie loosely in the
flexible roof of the lantern-coelom.

The uses of the various components of this structure can be made out from
an inspection of the muscles which connect them together.

Overarching each radial perihaemal canal where it leaves the lantern is a
bridge of calcareous matter called the "auricula" (Fig. 234, 8). This
arises as two rods which meet each other in a pent-house over the canal. It
is the only part of the skeleton which can be compared to the ambulacral
ossicles of the Asteroidea, and like them it serves as the point of
insertion for important muscles. Thus we find (1) protractor (Fig. 234, 7)
muscles which arise from the upper ends of the alveoli and are inserted in
the auricula; when these contract they tend to push the whole "lantern"
outwards so as to expose the tips of the teeth. (2) The retractor muscles
(Fig. 234, 6) extend from the auriculae to the lower ends of the jaws and
restore the lantern when it has been extruded to its original position. (3)
The comminator muscles connect adjacent jaws with one another: these on
contraction approximate the pair of jaws into which they are inserted, and
it will easily be seen that by the successive contraction of the five
comminator muscles a rotating movement of the teeth would be produced which
would cause them to exert an action something like that of an auger; by
their simultaneous contraction the teeth are brought to a point. (4) The
internal and external rotula muscles: these are small muscles which connect
the outer side of the epiphysis with the rotula. There are two facets on
the epiphysis, which permit it to rock to and fro on the rotula under the
action of these muscles. This rocking action must greatly increase the
cutting power of the {527}tooth. These muscles are controlled by the
nerve-ring and the incipient portions of the radial nerves, which, as we
have seen, have an inner layer of nerve-cells. If the nerve-ring be gently
stimulated on one side the upper end of the lantern bends away from the
spot, causing the lower end, _i.e._, the teeth, to move towards it; but a
stronger stimulation produces the opposite effect, just as is the case with
spines. But besides these masticatory muscles there are others which have
nothing to do with moving the teeth. These muscles are attached to the rods
called radii or compasses (Fig. 234, 2),[484] which lie in the upper wall
of the lantern-coelom, and may be termed the compass muscles. There are two
sets:—(1) The elevator muscles (Fig. 234, 3), which connect the inner ends
of the compasses with one another. When these contract, the radii tend to
bend upwards at the inner ends and thus raise the roof of the coelom. (2)
The depressor muscles (Fig. 234, 4), which run downwards from the forked
outer ends of the compasses to the auriculae. Uexküll[485] has shown that
the function of these muscles and of the rods to which they are attached is
respiratory. These muscles are also controlled by the nerve-ring. If this
be stimulated by passing a pin-head into the oesophagus, the roof of the
lantern cavity is raised by the contraction of the elevator muscles. This
is followed by contraction of the depressor muscles lowering it; the same
result may be brought about by placing the animal in water with excess of
carbonic acid. The ten branched gills described on p. 514 are outgrowths of
the lantern-coelom. When the roof of this cavity is depressed the fluid
contents are driven out into the gills, which are thus expanded and then
absorb oxygen from the surrounding sea water. When, on the other hand, the
roof is raised the aerated water is sucked back into the lantern cavity,
and the oxygen passes easily through the thin walls of the lantern into the
fluid filling the main coelomic cavity. There are thus two independent
respiratory mechanisms in the Sea-urchin, the one being the compass
muscles, the other the cilia lining the interior of the tube-feet.

The function of EXCRETION is performed, as in Asteroidea, by {528}the
amoebocytes floating in the general coelomic cavity. These in part escape
through the thin bases of the gills. In other parts of the body they seem
not to succeed in reaching the exterior at all, but to degenerate and to
form masses of pigment; the colour of the animal is largely due to these
excrementitious substances.

The REPRODUCTIVE SYSTEM, as in the two preceding orders, consists of a
vertical pillar, the "genital stolon," and a circular "genital rachis"
giving off interradial branches from which the genital organs bud. The
genital stolon is developed from the wall of the general coelom near the
upper end of the axial sinus; it attains a great development and ultimately
completely surrounds the axial sinus, which then appears like the cavity of
a glandular tube, the walls of which are constituted by the genital stolon.
The compound structure consisting of stolon and axial sinus was actually
described as a nephridium by the Sarasins[486] in the case of
_Asthenosoma_. Its true nature, however, is shown when the upper end is
examined; it is then seen to open into the stone-canal and to be in
communication with the ampulla, into which the pore-canals open. Lying
alongside the upper end of the axial sinus is the somewhat elongated
"madreporic vesicle," or right hydrocoele, which was described by Sarasin
as the accessory kidney (_Nebenniere_), since like the axial sinus it is
partly enveloped by the genital stolon. Leipoldt,[487] however, showed
clearly that it is a completely closed space.

The genital rachis springs from the upper end of the stolon, and as in
Asteroids, it lies in the outer wall of a space called the "aboral sinus"
(Fig. 234, 20) intervening between it and the test. In adult specimens it
seems to degenerate. The genital organs are situated at the ends of five
interradial branches of the rachis (Fig. 231, _gon_). Each is an immense
tree-like structure consisting of branching tubes, which are lined by the
sexual cells. So enormous do they become in the breeding season that they
form an article of food among fishermen. The term _esculentus_ is derived
from this circumstance. Other species are regularly sold for food as
_Frutta di Mare_ (Fruit of the Sea) at Naples, and {529}as "sea eggs" in
the West Indian Islands.  One female _Echinus esculentus_ will produce
20,000,000 eggs in a season.

The so-called BLOOD SYSTEM is more distinctly developed in Echinoidea than
in Asteroidea and Ophiuroidea. There is an oral ring of lymphoid tissue
surrounding the oesophagus below the water-vascular ring. From this are
given off two strands, the so-called "dorsal" (Fig. 231, _b.v_), and
"ventral" vessels (Fig. 234, 16), which run along the two opposite sides of
the stomach or first coil of the alimentary canal. The position of these
strands suggests that like the lacteals of the human intestine they are
channels along which the products of digestion exude from the stomach. The
dorsal strand is situated on the same side as the genital stolon, and from
it branches are given off which ramify on the surface of the stolon, on
account of which this organ, as in Asteroidea, was at one time regarded as
a "heart," but the distinction of the stolon from the strands is easily
made out. An aboral ring enclosing the genital rachis lies embedded in the
septum dividing the aboral sinus (Fig. 234, 20) from the general coelom.


CLASSIFICATION OF ECHINOIDEA.

The Echinoidea are sharply divided into three main orders, which differ
from each other profoundly in their habits and structure. These are: (1)
The ENDOCYCLICA or Regular Urchins, of which the species just described may
be taken as the type. (2) The CLYPEASTROIDEA or Cake-urchins, which are of
extremely flattened form, and in which the periproct is shifted from the
apical pole so that it is no longer surrounded by the genital plates, while
some of the tube-feet of the dorsal surface are flattened so as to serve as
gills. (3) The SPATANGOIDEA or Heart-urchins, in which the outline is oval:
the periproct is shifted, as in the Cake-urchins, and the dorsal tube-feet
are similarly modified; but the Heart-urchins have totally lost Aristotle's
lantern, whilst the Cake-urchins have retained it. This strongly-marked
cleavage of the group was primarily due, as in all such cases, to the
adoption of different habits by different members of the same group. Were
we to term the three orders Rock-urchins, Sand-urchins, and
Burrowing-urchins, it would not be entirely true, for secondary invasions
of the other's territory on {530}the part of each order have undoubtedly
taken place; but still the statement would remain roughly true, and would
give a fair idea of the differences in habitat which have led to the
differentiation of the group.


ORDER I. ENDOCYCLICA (_Regular Urchins_).

The principal variations concern (1) the peristome, (2) the periproct, (3)
the corona, (4) Aristotle's lantern and its appendages, (5) the spines, (6)
the pedicellariae, and lastly, (7) the tube-feet. We shall consider these
points in order.

PERISTOME.—In the vast majority of species this region is covered only with
flexible skin in which ten small plates are embedded, pierced by pores for
the buccal tube-feet; besides these there are irregularly arranged thin
plates. In the Cidaridae both the ambulacral and the interambulacral series
of plates are continued on it; these plates differ from those of the corona
in being movable on one another. In Echinothuriidae only the ambulacral
series of plates is continued on to the peristome. In the case of both
these families there are a considerable number of tube-feet within the
region of the peristome which may be classed as buccal.

PERIPROCT.—This area, which represents the whole dorsal surface of
Asteroidea, is very large in the Cidaridae, where, as in _Echinus_, it is
covered with leathery skin in which small plates are embedded. In the
Saleniidae it is covered with a single large sur-anal plate, in the edge of
which the anus is excavated; in the Arbaciidae it is covered with four
valve-like plates; whilst in the remaining species its condition is similar
to that described in the case of _Echinus esculentus_.

CORONA.—In Echinothuriidae all the plates are separated by slips of
membranous skin, so that the test is flexible. In all other families it is
an unyielding cuirass. In the Cidaridae the pore-plates remain separate
throughout life, and are therefore identical with the ambulacral plates.
These are small and placed in two vertical rows, and so the ambulacra are
exceedingly narrow. In Echinothuriidae there is some tendency to adhesion
amongst the pore-plates; these are of different sizes, and usually one
larger and one smaller adhere to one another. In all other species regular
ambulacral plates are formed at least {531}in the lower part of the radii
near the peristome by the adhesion of the pore-plates in groups of two,
three, or more. Sometimes as many as nine pore-plates may thus adhere.

When adhesion takes place between the pore-plates it is of course preceded
by crowding, and this interferes with their equal development. Some which
extend so far horizontally as to meet their fellows of the opposite side of
the radius are called primary plates; others which are small and wedged in
between the larger ones are called demi-plates. Systems of classification
have been built up (chiefly by palaeontologists) in which great stress has
been laid on how the primaries and secondaries enter into the constitution
of the compound plate, but it does not seem to the present author as if
this were at all a satisfactory basis for classification. All the
pore-plates are primarily equivalent, and the question as to which are
interfered with in their growth so as to become secondary is trivial. The
so-called Arbacioid type consists of one primary with a secondary on each
side; the Diadematoid type of three primaries, with occasionally a
secondary between the aboral and the middle primary; and finally the
Triplechinoid type of two primaries, with one or more secondaries between
them.

ARISTOTLE'S LANTERN.—Under this head we may consider the auriculae and
gills as well as the jaws and teeth. In Cidaridae external gills appear to
be absent, but from the lantern coelom large radial pouches project upwards
into the general coelom cavity. These pouches are supposed to be
respiratory, and are termed INTERNAL GILLS or STEWART'S ORGANS.[488] They
co-exist with external gills in Echinothuriidae and in Diadematidae, though
in the last family they are present only in a vestigial form, two being
found in each radius. The auricular arch both in Cidaridae and in
Arbaciidae is composed of two pillars which do not meet, but in the
last-named family they are based, as in Echinidae, generally on the
ambulacral plates, whereas in Cidaridae they arise from the interambulacral
plates (the ambulacral plates being here very narrow). The epiphyses are
absent in Cidaridae and Arbaciidae, and are imperfect in Diadematidae.

SPINES.—These organs are extraordinarily variable, and {532}usually differ
very much in species of the same genus. In the vast majority of species
there is a limited number of long spines called "primaries," amongst the
bases of which a large number of much shorter "secondaries" are
distributed. In Cidaridae the primaries are very long and thick and blunt
at the ends, and the secondaries form small circles around their bases. The
primaries in Cidaridae and the tips of the primaries in Arbaciidae and
Echinothuriidae are covered with a special investment of extremely close,
hard, calcareous matter very different from the loosely fenestrated
material out of which the bodies of the spines of all species are composed.
In _Colobocentrotus_ and _Heterocentrotus_ the primaries are very thick and
triangular in section, whilst the secondaries on the aboral surface have
expanded outer ends, which form a close-set pavement protecting the
ectoderm from the shocks of the breakers. In Echinothuriidae the primaries
are short and so delicate as to be termed silky.

PEDICELLARIAE.—In Cidaridae only gemmiform and tridactyle pedicellariae are
found. In the gemmiform the glands lie inside the grooved blades instead of
outside as normally, and they are covered internally by ingrowths of
calcareous matter from the edges. In Echinothuriidae only tridactyle and
trifoliate are found in most species, but rudimentary gemmiform are found
in one species and well-developed ophicephalous in another. In some species
(_Centrostephanus longispinosus_) there are found gemmiform pedicellariae
which have lost the jaws but retained the glands. These are termed
"globiferae." Mortensen[489] uses minute details in the structure of the
pedicellariae to discriminate species and even genera, but in this the
present author is not prepared to follow him.

TUBE-FEET.—The tube-feet belonging to the aboral surface are pointed and
devoid of a sucker in Diadematidae, Echinothuriidae, Arbaciidae, and
Cidaridae;[490] in the last-named family those belonging to the oral
surface have suckers, in the centre of which a pointed (sensory) prominence
is to be noted.

The CLASSIFICATION of the Endocyclica is by no means in a satisfactory
condition, and different authorities have arrived at {533}widely different
results. Agassiz,[491] for instance, places the genera _Echinus_ (the
common British form) and _Strongylocentrotus_ (the commonest American form)
in different families. Bell,[492] on the other hand, considers them to be
closely allied. Bell's system, based as it is on the development of the
peristome, seems to the present author the most justifiable, for the
peristome is undoubtedly a differentiation of the corona, which has been
brought about by the manner in which the animal breathes and masticates,
two functions of prime importance. The periproct is also of importance,
representing as it does the whole aboral surface of the Starfish, and so
are to a less extent the arrangements of the spines and of the tube-feet.
Proceeding in this way, living Endocyclica can be divided into six
families, which are briefly described below.

[Illustration: FIG. 235.—Oral view of dried and cleaned test of _Cidaris_.
_p_, Pores for tube-feet arranged in single series; _per_, peristome with
both ambulacral and interambulacral plates; _t_, tubercle of a large
interambulacral spine.]

FAM. 1. CIDARIDAE.—Endocyclica with a large peristome and a large
periproct. The peristome is covered with a regular series of both
ambulacral and interambulacral plates, the former pierced by tube-feet. No
special buccal tube-feet and no external gills. The periproct is large, and
is covered with irregular plates (Fig. 236, A). The lantern coelom is
provided with large Stewart's organs.

{534}The auriculae are incomplete and consist only of pillars arising from
the interambulacral plates. The ambulacral pore-plates remain disunited,
and the pores are arranged in a single vertical series; hence the ambulacra
are very narrow. The interambulacral plates each bear one large primary
spine surrounded by several circles of secondaries. No ophicephalous or
trifoliate pedicellariae are to be found, and the gland of the gemmiform
pedicellaria is placed inside the concavity of the blade.

[Illustration: FIG. 236.—Figure showing periprocts of A, Cidaris; B,
Echinus, × 1. _amb_, Ambulacral plate; _g.o_, genital opening; _g.p_,
genital plate; _inter_, interambulacral plate; _m.p_, madreporite; _oc_,
ocular pore.]

The Cidaridae are in many respects the most primitive of the six families
living. They are distributed all over the world, and chiefly inhabit deep
water. No two naturalists agree as to how they are to be divided into
genera. Mortensen,[493] who takes the structure of the pedicellariae as his
principal guide, recognises fourteen genera. Others (as for instance Bell)
have been inclined to attribute nearly all the living species to one
polymorphic genus, _Cidaris_, finding all attempts to divide the genera
from one another frustrated by the discovery of transitional forms.
_Goniocidaris_ (Fig. 237), however, distinguished by its comparatively
broad poriferous zones, by bare places in the middle line of both radii and
interradii, and by deep pits on the lines of suture of the plates, is by
general consent distinct. This genus is confined to the Eastern Pacific,
but from British waters three species of _Cidaris_ have been recorded, only
one of which, _C._ (_Dorocidaris_) _papillata_, is at all common. It is
found in water from 100 to 500 fathoms in depth {535}off the western coast
of Ireland and Scotland. It also occurs in the Mediterranean, and has been
carefully examined and described when living by Prouho.[494] From his
description it appears that locomotion is effected almost entirely by
spines, and that the tube-feet of the lower parts of the radii have each in
the centre of the disc a pointed sense-organ like those in the centre of
the first tube-feet of the just metamorphosed _Echinus_, whilst those of
the aboral surface have no suckers.

[Illustration: FIG. 237.—_Goniocidaris canaliculata._ × 2. (From Wyville
Thomson.)]

FAM. 2. ECHINOTHURIIDAE.—Endocyclica with a large peristome and
comparatively small periproct. The peristome has a regular series of
ambulacral plates bearing pores for tube-feet, but no interambulacral
plates. No specially modified buccal tube-feet, but external gills are
present, and internal gills (Stewart's organs) also occur. The periproct is
covered with numerous small plates. All the plates of the corona are
separated by thin slips of flexible body wall. Numerous comparatively short
primary spines on both ambulacral and interambulacral plates; these spines
are covered on the tips with a layer of hard dense material.

{536}[Illustration: FIG. 238.—Oral view of _Asthenosoma hystrix_. × ⅔.
(From Wyville Thomson.)]

This remarkable family is divided by Mortensen into ten genera, based as
usual on the pedicellariae, but taking into account also the shape of the
tip of hard material on the spines. Most authors refer the majority of the
species to two genera, _Phormosoma_ and _Asthenosoma_ (Fig. 238),
recognising also a genus _Sperosoma_ for one or two aberrant species.
_Asthenosoma_ is distinguished by having wide interspaces of membrane
between the plates, and by having ten longitudinal folds of the body-wall,
two in each radius, in which powerful longitudinal muscles are developed
projecting inwards in the radii. The organs of Stewart are very large. In
_Phormosoma_, on the contrary, the interspaces of membrane are very narrow,
and the longitudinal folds are thin and membranous and the organs of
Stewart are vestigial. _Asthenosoma hystrix_ and _Phormosoma placenta_ have
both been dredged in deep water off the Irish coast. _A. urens_, in which
there are ectodermic poison-sacs at the bases of the spines, inhabits the
Indian Ocean near Ceylon, and was thoroughly {537}described by the
Sarasins,[495] who regarded its structure as a proof that Echinoidea were
derived from Holothuroidea. Both palaeontology and embryology have,
however, yielded strong evidence that Echinoidea were derived from
Asteroidea, and hence there is ground for believing that Holothuroidea are
descended from primitive Echinoidea and not _vice versa_. The
Echinothuriidae may perhaps be regarded as showing the first steps in the
change, and though possibly not closely related to the actual ancestors of
the Holothuroidea, they at any rate show parallel modifications.

[Illustration: FIG. 239.—View of peristome of _Asthenosoma hystrix_. _amb_,
Ambulacral plates on the lower edge of the corona; _inter_, lower plates of
the interambulacral area. (From Wyville Thomson.)]

FAM. 3. SALENIIDAE.—Endocyclica with a large peristome and periproct. The
peristome is covered with thin, scattered, irregular plates. There are five
pairs of special buccal tube-feet, each supported by a special plate, and
there are external gills. The periproct is excavated in the side of a large
central pentagonal plate. It is covered with fifteen or twenty plates.[496]
The ambulacral plates are separate as in the Cidaridae, but occasionally
adhere in pairs near the peristome. The interambulacral plates also, as in
Cidaridae, each bear one large primary spine surrounded by a {538}circle of
secondaries. A few deep-water forms belong to this family, the type genus
_Salenia_ (Fig. 240) being the best known. None occur in the British area.
Superficially they resemble the Cidaridae, but in reality they are widely
separated by the essentially modern character of the peristome.

[Illustration: FIG. 240.—Dried and cleaned shell of _Salenia varispina_,
showing periproct covered by one large plate. × 4. (From Wyville Thomson.)]

FAM. 4. ARBACIIDAE.—Endocyclica with a peristome on which, as in
Saleniidae, there are only ten prominent plates perforated by the buccal
tube-feet, and besides these thin irregular plates; external gills are
present, and the auricles consist of incomplete arches springing from the
ambulacral plates. The periproct is covered by four valve-like plates. The
ambulacral pore-plates are separate near the periproct, but near the
peristome unite on the "Arbacioid" pattern (_v._ p. 531) to form secondary
plates. The interambulacral plates each carry several spines. No
representatives of this remarkable family are known in British waters, but
_Arbacia_ is found both on the east coast of North America and in the
Mediterranean. It is distinguished by its conical test. All the upper
tube-feet are devoid of a sucker; only those on the oral surface are used
for locomotion.

Uexküll has studied the Mediterranean species, and has shown that the
spines converge no matter how strong the stimulus may be, and so are
incapable of aiding in locomotion; also that the ectoderm is devoid of
ciliation, and hence the faecal matter which falls on the surface of the
animal is not, as in other genera, allowed to fall off by the divergence of
the spines nor swept off by the action of the cilia. In its natural habitat
the wash of the ripples on the shore cleanses the animal. In captivity it
is liable to suffocate itself.

FAM. 5. DIADEMATIDAE.—Endocyclica with a peristome similar to that of the
Arbaciidae and the Saleniidae. External gills present and ten buccal
tube-feet. Periproct small, covered with numerous small plates. The
auricles form complete arches arising from the ambulacral region.
Aristotle's lantern is provided with rudimentary Stewart's organs. The
ambulacral pore-plates are separated at the apex, but unite orally in
{539}"Diadematoid" fashion (p. 531) to form compound plates. The
interambulacral plates bear numerous primaries. The aboral tube-feet are
pointed, having lost their suckers.

This family is represented (according to Agassiz) at the present day by
seven genera, none of which are found in British waters, though one
(_Centrostephanus_) enters the Mediterranean. _C. longispinus_[497] was
investigated by Uexküll and found to be distinguished by its sensitiveness
to light and shade, and by the quickness of its movements, which were
mainly carried out by its long spines. The family resembles the Arbaciidae
in the pointed aboral tube-feet, but in the complete auriculae it resembles
the next family.

FAM. 6. ECHINIDAE.—Endocyclica with peristome and periproct as in the
preceding family. External gills and buccal tube-feet present, but
Stewart's organs totally absent. Ambulacral plates combined on the
"Triplechinoid" plan (p. 531) to form secondary plates. Interambulacral
plates with numerous tubercles. All the tube-feet have suckers.

This family contains by far the larger number of living genera. It is
divided into two sub-families, viz.:—

(_a_) TEMNOPLEURINAE.—Echinidae in which the plates of the corona dovetail
into each other by means of pits and knobs along the line of suture. This
sub-family does not occur in British waters; almost all the species are
confined to the Indian and Pacific Oceans, but on the east coast of America
it is represented by several genera, which however inhabit deep water,
_e.g._ _Trigonocidaris arbacina_.

(_b_) ECHININAE.—Echinidae in which the plates meet each other in straight,
simple sutures.

This sub-family is represented in British waters by three genera, viz.
_Echinus_, _Sphaerechinus_, and _Strongylocentrotus_. _Echinus_ is
distinguished by having its pores arranged in arcs of three, owing to the
fact that its pore-plates are united in threes to form secondary plates,
whilst in the other two genera the ambulacral plates are composed of four
or more pore-plates. Six species of _Echinus_ have been recorded from
British waters, viz. _E. esculentus_, _E. acutus_, _E. miliaris_, _E.
norvegicus_, _E. microstoma_, and _E. elegans_. The validity of the last
three is very doubtful. {540}Mortensen[498] regards _E. norvegicus_ and _E.
microstoma_ as mere variations of _E. acutus_, and this is probably
correct. _E. esculentus_ has already been described; its most marked
character is the forest of comparatively short, close-packed, reddish or
white primary spines with which it is covered, between the bases of which
the delicate secondaries are hard to detect. It is essentially a
shallow-water species. _E. acutus_ is distinguished by having much fewer
and longer primaries and numerous delicate secondaries. It is an inhabitant
of deeper water, being abundant at 100 fathoms, though stragglers are found
in shallower water. At the depths at which it lives wave-disturbance can
scarcely be felt, and hence the long primaries are not irritated.

_E. elegans_ has spines intermediate in character between those of _E.
esculentus_ and those of _E. acutus_. Like the latter it is an inhabitant
of the deeper water. It seems to the present author not at all improbable
that further research might show that _E. acutus_, _E. elegans_, and _E.
esculentus_ are all members of continuous series of forms; certainly the
larvae and early development of _E. acutus_ and _E. esculentus_, the
extreme members of the series, are strikingly similar.

_E. miliaris_ differs somewhat widely from the other species and is closely
allied to _E. microtuberculatus_ of the Mediterranean, from which it is
distinguished mainly by the greater thickness of the scattered plates on
the peristome of the latter species. From the other British species it
differs in its much smaller size and in the greenish hue of its primary
spines, which are short and thick and possess purple tips. Its larva is
markedly distinct from the larva of _E. esculentus_. _E. miliaris_ is a
littoral species, and is found in great numbers in some of the Scottish
sea-lochs; when the tide recedes, under every stone of the gravelly beach
several specimens will be found. It has a curious habit of "dressing"
itself, _i.e._ of covering itself with fragments of dead shell, sea-weed,
etc., which are held in position by the aboral tube-feet. This habit aids
in concealing the animal, and has probably been developed on account of the
dangers to which _E. miliaris_ is exposed owing to its littoral habit of
life.

_Sphaerechinus_ differs from _Echinus_ in the structure of the ambulacral
plates, in which it agrees with _Strongylocentrotus_, but it is
distinguished from this genus by the very deep {541}gill-clefts, or
indentations of the edge of the corona from which the gills are extruded.
Its most marked peculiarity, however, as shown by both Mortensen and
Uexküll, consists in the highly developed character of its gemmiform
pedicellariae, on the stalks of which are situated glands. When the head
with its poison-glands is torn off, the secretion of these stalk-glands can
envelop an enemy with a glutinous secretion, which impedes its movements.
The blades on a slight mechanical stimulus divaricate very widely and
become locked in this position, so that the enemy's body gets in well
within their reach. The muscles of the poison-glands contract, but their
ducts are bent by the act of opening, so that the secretion cannot escape.
The sense-organs have stiff hairs, which penetrate the surface of the enemy
and cause its juices to exude and so stimulate the blades to close, and at
the same time permit the poison to be expelled. It will be remembered that
the gemmiform pedicellariae of _Echinus_ open in response to a chemical
stimulus and close on a mechanical one being superadded; so that their
responses are the direct opposite of what occurs in _Sphaerechinus_. _S.
granularis_, a Mediterranean species with short red spines, just reaches
the Channel Islands.

_Strongylocentrotus_ has shallow gill-clefts and gemmiform pedicellariae,
like those of _Echinus_, except that they have a muscular stalk. In the
British area it is represented by two species, _S. lividus_, in which the
primary spines are markedly longer than the secondaries and are of a
brownish purple colour, and _S. droëbachiensis_, in which the primaries are
little longer than the secondaries and are of a greenish brown colour. _S.
lividus_ occurs abundantly in the Mediterranean, and reaches the English
Channel and the west coast of Ireland. In the last-named locality, where it
is exposed to the full sweep of the Atlantic, it is said to excavate holes
for itself in the limestone rocks, about ten inches in depth.[499] _S.
droëbachiensis_, which has been recorded in the British area, chiefly from
the west coast of Scotland, is one of the most abundant members of the
fauna of the east coast of America. In the Gulf of St. Lawrence and in the
branches of the Bay of Fundy it is found in thousands, and {542}is
frequently left bare at low tide. It thus takes the place of _E. miliaris_
in the British fauna. An allied if not identical species, _S. purpuratus_,
is found in Puget Sound on the Pacific coast.

Other interesting genera of the Echininae are _Echinometra_,
_Colobocentrotus_, and _Heterocentrotus_. All possess large, thick
primaries, and all are elliptical in outline. In _Echinometra_ the
primaries are pointed, and the long axis of the body makes an oblique angle
with the axis passing through mouth and madreporite. In _Colobocentrotus_
and _Heterocentrotus_ the axis passing through mouth and madreporite is the
short axis of the ellipse, and the primary spines are very thick and
triangular in section, whilst the expanded ends of the secondaries form a
closely set armour between the bases of these. In _Colobocentrotus_ the
test is markedly flattened on the under side, and this flattened area is
fringed with a circle of primaries; but in _Heterocentrotus_ there are a
few rows of primaries all over the test. These are tropical genera and are
found on the outer side of coral reefs, and they require the cuirass of
expanded secondaries to protect them against the waves.


ORDER II. CLYPEASTROIDEA (_Cake-urchins_).

The "Cake-urchins" have only one representative in the British area, and
this is unsuitable for dissection on account of its small size. We shall
therefore select as type the "Sand-dollar" _Echinarachnius parma_ (Figs.
241, 242), which occurs abundantly in shallow water on the east coast of
North America. As its popular name implies, this is an extremely flattened
Sea-urchin of nearly circular outline, so as to suggest a resemblance to
the silver dollar of North American currency. The peristome is exceedingly
small, and is placed in the centre of the lower surface (Fig. 241), whilst
the periproct is placed on one edge. The outline is not quite circular, for
the periproct lies in a slight indentation of the edge; and this side is
broader and of a lesser degree of curvature than the opposite one, so that
a secondary bilateral symmetry is superimposed on the fundamental radial
symmetry common to all Echinoderms. A line drawn so as to pass through the
anus and the centre of the disc will divide the animal into two similar
halves; the periproct of course lies in {543}an interradius and the axis of
symmetry passes through the centre of one radius. We can thus distinguish
an anterior group of three radii, or "trivium," from a posterior pair or
"bivium." The madreporite lies in the left anterior interradius. The five
genitals and five oculars surround a dorso-central plate, which covers the
spot which in Endocyclica is occupied by the periproct.

[Illustration: FIG. 241.—Oral view of "Sand-dollar" (_Echinarachnius
parma_), with spines. _amb_, Ambulacral furrow, × 1.]

The whole test is covered with extremely short delicate spines, which form
a velvety felt-work, and are all of approximately the same length; they are
of a brownish purple colour. The spines on the dorsal surface are all
ciliated, and these cilia cause a current of fresh sea-water to flow
continually over the modified tube-feet. Pedicellariae are scattered
amongst the bases of the {544}spines; they are of the tridactyle, the
gemmiform, and the ophicephalous types, but they have only two jaws.

[Illustration: FIG. 242.—Aboral view of the "Sand-dollar" (_Echinarachnius
parma_), with its spines. _m.p_, Madreporite; _pod_, small tube-foot with
sucker; _pod'_, flattened respiratory tube-foot. × 1.]

The ambulacral areas on the upper surface of the test can be distinguished
only by the flattened respiratory tube-feet (Fig. 242, _pod'_), which can
be seen protruding from between the spines. Below these areas are clearly
marked, for in the centre of each is a well-marked groove proceeding
inwards to the peristome. This groove receives lateral branches on its
course which traverse the adjacent interambulacral regions. The purpose of
these grooves will be explained later. The interambulacral regions do not
reach the peristome, which is entirely surrounded by the ambulacral areas.
The ambulacral and interambulacral areas both consist of somewhat large
hexagonal plates, except in the region of the respiratory tube-feet. Here
the pore-plates are not united with one another. This region in each radius
is termed {545}a "petal" (Fig. 243, A, _p_), for the respiratory tube-feet
are arranged in two rows which diverge from their commencement at the
"calyx" and slightly converge again towards the outer margin of the disc,
and thus in a dried specimen the two rows of double pores outline an area
having some resemblance in shape to the petal of a flower. Besides these
double pores for the larger tube-feet there are numerous small single pores
for the smaller tube-feet; these are found in all the plates, ambulacral
and interambulacral, of the dorsal surface, but in the neighbourhood of the
grooves only on the ventral side.

[Illustration: FIG. 243.—A, aboral, B, oral view of _Echinarachnius parma_
after spines have been removed, _amb_ (in A), Ambulacral plates, (in B),
ambulacral furrows; _g.p_, genital pore; _inter_, interambulacral plate;
_p_, petal; _t.t_, pore for terminal tentacle, × ½.]

The sphaeridia are only present to the number of one in each radius. Each
sphaeridium is enclosed in a pit situated near the edge of the peristome.

A remarkable feature in the skeleton of _Echinarachnius_ which is
characteristic in greater or lesser degree of all Clypeastroidea is the
presence of vertical partitions of calcareous matter traversing the coelom
and stretching from the upper to the lower surface of the test. These are
found principally in the peripheral region of the animal; and there can be
no doubt that they have originated as cellular bands traversing the coelom,
for the formation of similar structures can be followed step by step in the
Crinoidea. In the axis of these trabeculae, or folds of the coelomic wall,
jelly is secreted, and into this the lime-producing {546}amoebocytes
wander. In _Echinarachnius_ these partitions are arranged in groups, each
group radiating from a common centre.

The main peculiarities in the structure of _Echinarachnius_ are
comprehensible when the species is viewed from above in its normal
environment. It is found in comparatively shallow water on a sandy bottom,
and normally is nearly but not quite buried in the sand. It might thus be
overturned by the force of the waves and currents, and it is protected
against this fate by its flattened shape. This shape, however, necessitates
some kind of support for the upper part of the test, and this is provided
by the internal partitions.

In order to view the internal anatomy of the "Sand-dollar," it is necessary
carefully to pick away the dorsal surface of the shell piece by piece. In
this way the whole course of the alimentary canal is exposed; as in
_Echinus esculentus_ it can be seen to issue from the upper surface of
Aristotle's lantern. It then bends sharply to the left, and makes a
complete circle round the edge of the disc; this portion is the stomach,
and is considerably inflated and accompanied by a "siphon." It then bends
sharply back on itself, but only goes half way round; when it reaches the
posterior interradius it ends in the anus (Fig. 244).

Aristotle's lantern is greatly simplified as compared with its condition in
the Regular Urchins. Both rotulae and compasses are absent; the jaws are
sharply bent on themselves, and their appearance gives one the impression
that they have shared in the process of compression which the test as a
whole has undergone, and have thus become bent. The teeth are nearly
horizontal, and they actually articulate with the auriculae, which, as in
Cidaridae, consist of disconnected pillars and spring from the plates of
the interradius. Each pillar is fused with the adjacent one belonging to
the next radius, so that the system which in _Echinus_ consists of five
radial arches here consists of five interradial pillars. Aristotle's
lantern has lost its respiratory function and apparently its masticatory
function as well, for the teeth are used as spades to shovel into the mouth
the sand mixed with organic detritus and small organisms on which the
animal lives.

The water-vascular system is highly modified. There are two sharply marked
kinds of tube-feet—(_a_) the respiratory tube-feet, (_b_) the locomotor
tube-feet. Both kinds are terminated by {547}suckers, but the first variety
are much larger than the second; they possess a flattened lobed base, and
are connected with the ampulla by a double canal. They issue only from the
double pores which form the petal. The locomotor tube-feet are small and
cylindrical; they are, as already mentioned, scattered over the whole upper
surface of the test, penetrating both ambulacral and interambulacral
plates, but all are connected by transverse canals with the radial canals
of the water-vascular system. On the under surface they are confined to the
neighbourhood of the ambulacral grooves, which have nothing to do with the
ambulacral grooves of an Asteroid, but are due to secondary localisations
of the tube-feet, which are here also connected in each radius with a
single radial canal. The appearance of a living _Echinarachnius_ covered
with a veritable forest of short brown tube-feet is very striking.[500]

[Illustration: FIG. 244.—Dissection of _Echinarachnius parma_. × 1. The
oesophagus has been cut through and moved to one side so as to expose
Aristotle's lantern. The aboral part of the test has been removed. _gon_,
Genital organ; _int_, intestine; _musc_, transverse muscle connecting jaws
of adjacent interradii; _rect_, rectum; _siph_, siphon; _st_, stomach.]

The condition of the water-vascular system is to be explained entirely by
the peculiar environment of the animal. The demand for specialised
respiratory organs is brought about by the habit of living half buried in
the sand. Under these circumstances the strain of supplying the needful
oxygen is thrown on the {548}dorsal tube-feet, and they become modified in
order to fit them for this function. The locomotor tube-feet are very small
and feeble compared with those of _Echinus esculentus_, but this is
comprehensible when it is recollected how little resistance the yielding
sand would offer to the pull of a powerful tube-foot like that of the
Regular Urchins, for in order to move the creature through the sand a
multitude of feeble pulls distributed all over its surface is necessary,
and the locomotor tube-feet are exactly fitted, both as to size and number,
for this object.

The principal points in which Clypeastroidea vary amongst themselves are
(1) the nature of the internal skeleton, (2) the shape, and (3) the spines.

INTERNAL SKELETON.—In _Echinocyamus_ and its allies this consists in each
interradius of two simple partitions radiating out towards the edge of the
disc; in _Laganum_ it consists of walls parallel to the edge of the disc;
in _Clypeaster_, of isolated pillars.

SHAPE.—In _Echinocyamus_ the outline is oval and the test comparatively
high. In _Clypeaster_ and its allies the outline is pentagonal, and the
test is swollen up into a blunt elevation in the centre. In a large number
of genera, however, the test is, as in _Echinarachnius_, extremely thin and
flat, and the outline may be variously indented. A first indication of this
process is seen in _Echinarachnius_ itself, but in _Rotula_ the edge is
drawn out into finger-like processes which are all interradial. In
_Mellita_ these processes unite with one another distally so as to surround
spaces called "lunules," which appear as perforations of the test.

The CLASSIFICATION of the Clypeastroidea adopted by Agassiz is based
chiefly on the degree of development of the internal skeleton, and as this
is of great physiological importance to the animals we shall follow it
here; but since it was published the remarkable discovery has been made of
_Pygastrides_, a type previously known only from fossils. We must therefore
recognise two sub-orders:—


SUB-ORDER I. PROTOCLYPEASTROIDEA.

Anus on dorsal surface near apical pole. One species, _Pygastrides
relictus_,[501] with no "petals," from deep water in the Caribbean Sea.


{549}SUB-ORDER II. EUCLYPEASTROIDEA.

Anus on under surface.

FAM. 1. FIBULARIDAE.—The "petals" are short and imperfect, and the internal
skeleton consists of two short outwardly-directed septa in each
interradius. To this family the only British Clypeastroid, _Echinocyamus
pusillus_, belongs. This animal never exceeds an inch in length, and has an
oval outline. It inhabits shallow water, and is often found in the same
ground as _Echinus miliaris_, but like all Clypeastroids it prefers a sandy
bottom.

FAM. 2. ECHINANTHIDAE or CLYPEASTRIDAE.—"Petals" well marked, internal
skeleton consisting of isolated pillars. The largest Cake-urchins belong to
this family, which is found chiefly in tropical waters. _Clypeaster_, the
great Cake-urchin, with a deeply sunken peristome, belongs to this family.

FAM. 3. LAGANIDAE.—Closely allied to the foregoing, but distinguished by
the fact that the internal skeleton consists of walls parallel to the edge
of the test. (_Laganum_, _Arachnoides_, _Peronella_.)

FAM. 4. SCUTELLIDAE.—This family includes about half the genera, and is
sharply distinguished from all the rest by (1) the extremely flattened
shape, (2) the indentation of the outline in the anal interradius and often
elsewhere, (3) the branching of ambulacral furrows on the under surface.
_Echinarachnius_, taken as the type in describing the anatomy of the
Cake-urchins, is the best-known genus. Others are _Mellita_, with five
perforations in the edge of the test; and _Rotula_, with the edge produced
into a number of finger-like processes.


ORDER III. SPATANGOIDEA (_Heart-urchins_).

As the type we may select _Echinocardium cordatum_, which occurs abundantly
in the Clyde and on the west coast of Ireland. The animal is found buried
in sand at a depth of about 8-10 inches from the surface. At this depth it
lies in a burrow, the walls of which are kept from collapsing by the
somewhat broadened tips to the spines. This burrow communicates with the
surface by a narrow cylindrical opening similar to the opening of the
burrows made by the Clams and other bivalves. A little practice, however,
enables one to distinguish the burrow of the Heart-urchin from these.

{550}The animal is about the size of a small potato, and is of light straw
colour. Its outline is oval, and the test is about two-thirds as high as
the shorter diameter. It is thus higher in proportion to its width than is
the case with any living Cake-urchin. The highest point is behind the
centre. The narrower end of the animal terminates in a vertical edge, in
the upper part of which is a large periproct covered with a number of thin
movable plates. The mouth is situated on the under surface, considerably
nearer the front end of the test than the hinder end. It is entirely devoid
of jaws or of teeth, and also of gills or of a movable peristome.

Aristotle's lantern has entirely disappeared, leaving as the only trace of
its former presence a canal with membranous walls encircling the mouth,
which has the form of a transverse slit, the posterior lip projecting
considerably forward.

The ambulacral areas are easily distinguishable from the interambulacral
areas by being comparatively bare of spines. On the upper surface they are
distinctly grooved, the groove being especially deep in the case of the
anterior one. On the lower surface they coalesce round the mouth, shutting
out the interambulacral regions, and are here perforated by the large pores
of the buccal tube-feet. Between the two posterior radii on the oral
surface there is a space with specially arranged spines called the plastron
or sternum. The interambulacral plates composing this region are very much
lengthened, and interdigitate with one another at the sutures. To this
lengthening is due the apparent forward shift of the mouth. The spines are
very characteristic, and are very different from any which have as yet been
described. They are the sole organs of locomotion. The primaries are long
and curved, with flattened tips, admirably adapted to plough through the
sand in which the animal lives. On the upper surface, mingled with the
tube-feet, are a large number of small secondary spines. Between the two
posterior petals there is a hoop-shaped band of very small black spines.
These spines are ciliated, and draw a current of fresh sea-water over the
respiratory tube-feet. Beneath the periproct there is a similar band called
the "sub-anal fasciole"; this probably produces a current of water which
sweeps away the material ejected from the anus.

The pedicellariae are of the trifoliate and gemmiform varieties. {551}The
sphaeridia are situated in open pits, one or two in each, situated at the
bases of the tube-feet nearest the mouth.

When the upper part of the test is picked away, the course of the
alimentary canal is exposed (Fig. 247). It is very similar to the
alimentary canal of _Echinarachnius_, except that from the first coil a
large blind pouch, called the caecum, is given off.

The water-vascular system shows many characteristic features. The tube-feet
are confined to two rows in each ambulacrum, the scattered smaller feet
found in such abundance in _Echinarachnius_ being entirely absent. There
are four distinct varieties of tube-feet in _Echinocardium_, which are as
follows:—(_a_) The respiratory tube-feet of the petals. These have, as in
_Echinarachnius_, broad flat bases, but they have lost the sucker. (_b_)
The prehensile tube-feet of the anterior ambulacrum. These are enormously
long structures, measuring when expanded several times the length of the
body. They end in discs, which are frayed out into fingers, so as to look
like miniature sea-anemones. These tube-feet are comparatively few in
number and are confined to the apical portion of the anterior ambulacrum.
(_c_) The buccal tube-feet. These are short, thick, and pointed, and
covered with a multitude of club-shaped processes. They are found on all
the ambulacra in the neighbourhood of the mouth. (_d_) The degenerate
tube-feet found in the portions of the ambulacra between the "floscelle"
(see p. 553) and the petals. These are single and pointed, few in number,
and issue from single pores in the test.

[Illustration: FIG. 245.—_Echinocardium cordatum_. A, aboral view; B, oral
view,  × 1.]

This extraordinary diversity in the tube-feet is fully explained {552}when
the habits of the animal are known. The function of the respiratory
tube-feet requires, of course, no special elucidation, but the peculiar
anterior ambulacrum was a mystery till the feeding habits of the animal
were observed by the late Dr. Robertson[502] of Cumbrae. He found that the
animal protruded the long prehensile tube-feet through the opening of the
burrow up to the surface of the sand. With their finger-like processes they
then collected the surface film of the sand, which was impregnated with
Diatoms and other small organisms. When a "handful," so to speak, of this
nutritive material has been collected, the long tube-foot is withdrawn down
the burrow and passed over the deeply grooved part of the ambulacrum to the
buccal tube-feet, to which the food is given up. These last then push it
into the mouth. Only one prehensile tube-foot is extended at a time.

[Illustration: FIG. 246.—Interior of test of _Hemiaster philippi_, showing
the genital organs and their ducts (only three are developed in this
species). Slightly enlarged. (From Wyville Thomson.)]

[Illustration: FIG. 247.—Dissection of _Echinocardium cordatum_. × 1. The
oral part of the test has been removed. _caec_, Blind pouch of the stomach;
_gon_, genital organ; _int_, intestine; _oe_, oesophagus; _rect_, rectum;
_siph_, siphon; _st_, stomach; _w.v.r_, water-vascular ring.]

The stone-canal is very short and soon opens into the axial sinus; it is
widely separated from the pore-canals which traverse the madreporite.
Communication between the two is effected by the long axial sinus. There
are only four genital organs.

Heart-urchins vary amongst themselves chiefly in the {553}following points,
viz.:—(1) the shape and position of the peristome, (2) the characters of
the "petals," and (3) the number and position of the fascioles.

PERISTOME.—In many genera this is pentagonal and central, and in these
cases the interradii commence at the peristome with a single plate, which
is often covered with a thick crowd of small spines and is termed a
"bourrelet." The oral ends of the radii also often consist of a crowded
series of narrow plates, looking something like a "petal," and termed a
"phyllode." The five bourrelets and five phyllodes constitute a flower-like
figure termed a "floscelle."

AMBULACRA.—In _Echinoneus_ all five are alike and are provided with similar
tube-feet, which are respiratory but possess suckers. The ambulacra are not
grooved, and the petaloid arrangement of the pores is hardly marked; but in
_Cassidulus_, _Pourtalesia_, and many other genera the five petals are well
marked, though they are all similar to one another.

FASCIOLES.—These structures are often entirely absent; the sub-anal one
alone is present in _Spatangus_. In _Eupatagus_ a peripetalous one is
added. This surrounds all the "petals," and has obviously the function of
sweeping fresh water over the respiratory tube-feet. In _Echinocardium_, as
we have seen, there is an "internal fasciole" between the two anterior
petals which has a similar function. In addition, this genus possesses an
anal fasciole which surrounds the anus and sweeps away the faeces.

[Illustration: FIG. 248.—Young _Echinoneus_ to show five equal radii
scarcely petaloid. _amb_, Ambulacral area; _g.p_, genital pore; _mad_,
madreporite. (After Agassiz.)]

The CLASSIFICATION of the _Spatangoidea_ is based mainly on the degree of
development of the petals, that is to say, on the extent to which the
burrowing habit has been developed. But weight is also laid on the shape of
the peristome, the pentagonal form being more primitive. Seven families are
recognised, which are as follows:—

FAM. 1. ECHINONIDAE.—"Petals" hardly marked at all; peristome in the centre
of the lower surface and pentagonal. Floscelle not developed.

One genus, _Echinoneus_ (Fig. 248).

{554}FAM. 2. NUCLEOLIDAE.—"Petals" distinct; peristome as in the foregoing
family. No floscelle. _Nucleolites_, with the anus in a furrow.
_Anochanus_, with a concave apical system serving as brood-pouch.

FAM. 3. CASSIDULIDAE.—"Petals" usually distinct; peristome eccentric, but
provided with a well-marked floscelle.

_Echinolampas_, with the anus on the under surface.

_Neolampas_, with the anus on a projecting papilla. One specimen of this
genus has been dredged in the British area.

The three foregoing families probably use their tube-feet to walk with, and
bury themselves only to a slight extent. They are often united as a
sub-order, the ASTERNATA, and distinguished from all the rest which possess
an eccentric mouth and well-marked plastron. These families are then
grouped together as STERNATA. They are as follows:—

FAM. 4. ANANCHYTIDAE.[503]—Spatangoidea with elongated apical system,
ambulacra all similar and not grooved. Petals feebly marked. _Pourtalesia_,
with bottle-shaped posterior prolongation of the test. _Platybrissus_, with
flattened test.

[Illustration: FIG. 249.—_Pourtalesia jeffreysi_, slightly enlarged. (From
Wyville Thomson.)]

FAM. 5. PALAEOSTOMATIDAE.—An aberrant family consisting of one genus,
_Palaeostoma_. Petals grooved, with a peripetalous fasciole, but peristome
central and pentagonal.

{555}[Illustration: FIG. 250.—_Hemiaster philippi_. × 2. (From Wyville
Thomson.)]

FAM. 6. SPATANGIDAE.—Spatangoidea of more or less flattened shape, with
well-marked petals and a sub-anal plastron as well as the ventral one. One
fascicle at least, but a peripetalous one never present. The anterior
ambulacrum grooved and different from the rest. This family is represented
in British waters by two genera, _Spatangus_ and _Echinocardium_. The
former possesses only a sub-anal fasciole, and has specially long curved
spines on the ventral plastron. It is represented by two species, _S.
purpureus_ and _S. raschi_, the latter being distinguished by a pointed
lower lip. It is a deep-water species, found in 100 fathoms and over on the
west coast. _S. purpureus_ is fairly common in rather shallow water. From
observations made on specimens kept in confinement it appears to burrow
only so far as to leave the petals uncovered; hence there is no need of a
peripetalous fasciole. _Echinocardium_ is devoid of the thicker spines on
the plastron, and has an internal fasciole and a perianal one as well as
the sub-anal. As already mentioned, it is a deep burrower. It is
represented by three species, _E. cordatum_, _E. pennatifidum_, and _E.
flavescens_. The first, described as the type of the Spatangoidea, has a
deeply grooved anterior ambulacrum. In the remaining two species this
ambulacrum is not grooved. _E. flavescens_ has only six or seven pairs of
pores in the posterior {556}petals, _E. pennatifidum_ twelve to fourteen.
Both come from deeper water than _E. cordatum_.

FAM. 7. BRISSIDAE.—Allied to the Spatangidae, but distinguished by sunken
petals and a peripetalous fasciole.

Two genera are recorded from the British area, _Schizaster_ and
_Brissopsis_, but the first has only been found once in deep water; the
second is common. _Schizaster_ has the front petals three times as long as
the hind ones, and no sub-anal fasciole. _Brissopsis_ has the front and
hind petals of about the same length, and a sub-anal fasciole. The only
British species is called _B. lyrifera_, on account of the fiddle-shaped
outline of the peripetalous fasciole.

_Hemiaster_ (Fig. 250) in general resembles _Schizaster_, but the petals
are equal in length, and the two posterior serve as brood-pouches for the
young. This genus is mainly Antarctic.

[Illustration: FIG. 251.—Dried shell of _Schizaster_, showing peripetalous
fasciole. _ant.amb_, Anterior ambulacrum; _fasc_, peripetalous fasciole;
_g.p_, genital pores, × 1. (After Agassiz.)]

FOSSIL ECHINOIDEA.—Echinoidea are well represented in the geological
record, and form a characteristic element in many fossil faunas. They
appear in the Ordovician formation, but the first representatives of an
existing family (Cidaridae) only appear in the Permian.

Space will only permit us to treat of the extinct members of the group very
briefly. Leaving out of sight the representatives of families still living,
the fossil Echinoidea may be divided into two great groups, viz.:—

(_a_) Palaeozoic forms, which in some points serve to connect the
Endocyclica with the primitive Asteroidea.

(_b_) Mesozoic forms, which serve to connect the Clypeastroidea and
Spatangoidea with the Endocyclica.

{557}[Illustration: FIG. 252.—A, _Bothriocidaris_. × 1. B, _Palaeoechinus_.
× 1. _amb_, Ambulacral plates; _inter_, interambulacral plates. (After
Zittel.)]

The Palaeozoic forms are often called Palaeoechinoidea, and they are above
all distinguished by the fact that the number of vertical bands of plates
composing the corona is variable, in a word, that the corona has not yet
acquired a fixed definite constitution. One genus (_Echinocystites_) has
the anus outside the apical system. It has four rows of pore-plates in each
radius, and numerous rows of plates each with a single spine in the
interradii. Another (_Palaeodiscus_) has been shown by Sollas[504] to be in
many respects the missing link between Asteroidea and Echinoidea. Inside
the plates of the corona there is a series of ambulacral plates like those
of Asteroidea. The tube-feet in the oral portion of the radii seem to have
issued between the (outer) ambulacral plates. No anus has been detected.
All the rest are Endocyclic. The oldest known form, _Bothriocidaris_ (Fig.
252, A), from the Ordovician, has only one row of interambulacral plates
and two of ambulacral; no peristome is distinguishable from the corona. The
ARCHAEOCIDARIDAE appear in the Devonian. They have narrow ambulacra of two
rows of pore-plates as in the Cidaridae, but the interambulacra consist of
many rows, the members of which overlap, and therefore were probably
slightly movable, as in the Echinothuriidae; the primary tubercles are
large, and there is only one on each plate. The MELONITIDAE (Fig. 252, B)
appear in the Carboniferous. Each interambulacral plate, of which there may
be five rows in each interradius, bears numerous small tubercles, and there
may be four or more vertical rows of pore-plates, though in the genus
figured, _Palaeoechinus_, there are only two. The TIARECHINIDAE are
represented by one genus, _Tiarechinus_, with an enormous apical system,
from the Triassic of the Tyrol. The interambulacra consist of one plate
bordering the mouth, three, {558}side by side, forming the interradial area
of the corona, and one large genital plate; the ambulacra, of two rows of
pore-plates. This family consists of dwarfed forms which probably inhabited
the land-locked seas and salt lagoons of the Triassic epoch.

When we recollect that some of the oldest Asteroidea known to us had very
narrow arms and interradial areas edged by large square marginals, it does
not require a very great effort to imagine how these marginals could be
converted into the vertical rows of the interambulacra, and the pointed
narrow arms becoming recurved, could have formed the ambulacra. The
physiological advantage of this will be discussed in the chapter on
development.

True Cidaridae occur in the Permian, and are abundant in all the younger
formations. One Cretaceous genus, _Tetracidaris_, has four rows of
interambulacral plates near the mouth, diminishing to two at the apex. This
circumstance renders it probable that the Cidaridae are the direct
descendants of the Archaeocidaridae. The Saleniidae, Echinothuriidae, and
Diadematidae appear in the Jurassic, the Echinidae in the Cretaceous, and
the Arbaciidae only in the Tertiary epoch.

[Illustration: FIG. 253.—_Hyboclypus gibberulus_. × 1. _a_, Anus. (After
Zittel.)]

Turning now to the Mesozoic forms with an excentric anus, there were a
number of forms which have been grouped together as HOLECTYPOIDEA which had
auricles and teeth and gills, although these were only feebly developed,
and in which the pore-plates remained separate. The periproct was a
comparatively large area, and in _Pygaster_, as in the surviving form
_Pygastrides_, it was in contact with the apical system, although outside
it. Many of the genera were of considerable height in proportion to their
length. In _Conoclypeus_ and _Discoidea_ the jaws and auricles were very
weak. The ECHINOCONIDAE have only vestigial auricles, and on this account
are often definitely grouped with the Spatangoidea, but they are closely
allied to the Holectypoidea. They are Cretaceous forms of high conical
shape (_Galerites_). In _Hyboclypus_ (Fig. 253) all trace of the teeth has
disappeared, {559}but the periproct is large and in contact with the apical
system; these forms appeared in the Jurassic. The COLLYRITIDAE, also
Jurassic, had a marginal anus. The apical system was so much elongated that
two of the ocular plates are widely separated from the other three, two
opposite interambulacra meeting between them. Unmistakable Spatangoidea
(Spatangidae and Ananchytidae) appear in the Cretaceous, true
Clypeastroidea (_Fibularites_) in the Cretaceous, the other families in the
Tertiary.

Reviewing these facts, we see that from the Holectypoidea we can pass by
insensible steps on the one hand into true Clypeastroidea, and on the other
hand into true Spatangoidea. The Holectypoidea differed from Endocyclica
only in the position of the anus, and the initial step in the backward
shift of this organ is seen in _Pygaster_. One result follows from this
conclusion, that the modification of the dorsal tube-feet into breathing
organs, and the consequent appearance of petals which accompany the taking
on of burrowing habits, were independently developed in the Clypeastroidea
and Spatangoidea, since these features were absent in the more primitive
members of both groups.




{560}CHAPTER XIX

ECHINODERMATA (_CONTINUED_): HOLOTHUROIDEA = SEA-CUCUMBERS


CLASS IV. HOLOTHUROIDEA

This class of the Eleutherozoa comprises those sausage-shaped, leathery
Echinodermata familiarly known as Sea-cucumbers. They are named
Holothuroidea from ὁλοθούριον, an animal described by Aristotle, and
believed to belong to this class.

The Holothuroidea resemble Echinoidea in the fact that the radial canals of
the water-vascular system run backwards and upwards from the ring-canal
over the surface of the body, terminating in small papillae near the anus,
which, as in the Echinoidea Endocyclica, is situated at the upper pole of
the body. There are, of course, no arms; and a further resemblance to
Echinoidea is shown by the fact that the ambulacral grooves are represented
by closed epineural canals, and that the ectoderm consists of long,
slender, flagellated cells interspersed with gland-cells, underneath which
is a plexus consisting of nerve-fibres and small bi-polar ganglion cells.
There are, however, no spines or pedicellariae; and Holothuroidea differ
not only from Echinoidea, but from all other Echinodermata, in the
vestigial character of their skeleton, which consists merely of isolated
nodules of calcium carbonate embedded in the skin. The body-wall is
provided with transverse muscles running across the interradii, and also
with powerful longitudinal muscles, running along the radii, by means of
which worm-like contractions are carried out. Similar muscles, though much
less developed, occur in the Echinothuriidae, and must have been present in
many extinct Echinoidea in which the plates of the corona overlapped; and
hence it is exceedingly probable that from some of these {561}early forms,
as, for instance, _Bothriocidaris_, Holothuroidea may have been evolved.
The MUSCULAR BODY-WALL has indeed been as important a factor in the
evolution and differentiation of the Holothuroidea as the MUSCULAR ARM in
that of Ophiuroidea, or the MOVABLE SPINE in the case of Echinoidea.

[Illustration: FIG. 254.—_Holothuria nigra_. _t_, Buccal tentacle or
"feeler." × ½.]

There are about 520 species of living Holothuroidea, and of these about
twenty-one have been recorded from British waters. One of the best-known of
the British species is _Holothuria nigra_ (Fig. 254), commonly known as the
"Cotton-spinner"; and this we shall take as a type for special description.
The animal may attain a length of a foot when fully extended, and has a
diameter of from 3 to 4 inches. It is of a very dark brown colour on one
side, which in crawling it keeps uppermost, whilst on the lower side it is
of a tawny yellow hue. Three of the radii (often termed the "trivium") are
situated on the lower surface; two (termed the "bivium") on the upper
surface. The podia are scattered fairly evenly over the whole surface
without reference to the radii; below they are regular tube-feet provided
with suckers, whilst on the upper surface they are pointed tentacles,
employed only for sensory purposes.

If the animal be observed alive and in its natural surroundings, a ring of
twenty large tentacles can be seen surrounding the mouth. These buccal
tentacles are in every respect comparable with the buccal tube-feet of
Ophiuroidea and Spatangoidea, and, like them, are employed in shovelling
the muddy substratum on which the animal lies into the mouth.

Ludwig employs the term "feeler" for these buccal tentacles, {562}in order
to distinguish them from the pointed podia scattered over the bivium. This
procedure will be adopted here. In the Cotton-spinner the feelers, when
extended, show a short smooth stem, from the apex of which springs a circle
of short branches, which are in turn beset with a double row of branchlets,
themselves branched. Such feelers are said to be shield-shaped.

A transverse section through the RADIUS of a Sea-cucumber is, in general,
like one through the radius of a Sea-urchin; the points of difference to be
noted are: (_a_) In the Sea-cucumber, beneath the ectoderm, is a thick
dermis with small plates scattered in it, instead of the whole dermis being
calcified, as is the case in the Sea-urchin; (_b_) the ampulla of each
podium is connected with the peripheral portion by one canal, not two, as
in the case of the Sea-urchin; (_c_) there is a development of coelomic
nervous tissue from the outer side of the perihaemal canal; (_d_) internal
to the radial water-vascular canal are to be seen cross-sections of two
great bands of longitudinal muscles, by the contraction of which the body
is shortened. Lengthening is brought about by the contraction of transverse
muscles, which are found on the inner side of the body-wall in each
interradius; the five sets taken together act like circular muscles, or a
rubber band, on the incompressible fluid in the body-cavity.

When the Sea-cucumber is opened by a cut along the left dorsal interradius,
the spacious coelom is laid open, and lying in it is seen the ALIMENTARY
CANAL. This tube is bent on itself, so that it has a form like ~ (Fig. 255,
B) running backwards to the posterior end of the body, then running
forwards to near the anterior end, before it finally turns to run backwards
to the anus. By taking cross-sections of the body at different levels, it
can be shown that the alimentary canal makes a half-turn round the
longitudinal axis (Fig. 255, A). It is suspended by bands of membrane,
termed "mesenteries," to the body-wall, and of these there are three, the
first of which (_i.e._ the one nearest the mouth) is attached to the
mid-dorsal interradius (Fig. 255, A, M^1), the next to the left dorsal
interradius (M^2), and the last to the right ventral interradius (M^3).

The alimentary tube shows four regions, which are distinguished as
follows:—(1) A short oesophagus with strongly-marked longitudinal folds in
its walls; this is separated by a constriction from (2) the stomach, a very
short region, {563}characterised by its strong musculature. Next follows
(3) the intestine, a thin-walled tube comprising the middle limb and most
of the descending and ascending limbs. This finally passes into (4) the
wide terminal "rectum," or "cloaca," which is connected to the body-wall by
muscular bands which traverse the coelom (Fig. 256, 10).

The cells lining the oesophagus resemble ectodermal cells; those lining the
stomach are nearly all gland-cells, and obviously secrete the digestive
juice. The powerful muscles of this portion of the gut produce a strong
peristalsis which thoroughly mixes the juice with the food, and in the
thin-walled intestine absorption of the digested material takes place. The
extreme thinness of the intestinal wall is common to many animals (e.g.
_Sipunculus_, Vol. II. p. 412) which swallow mud and sand for the sake of
the organic matter which they contain.

[Illustration: FIG. 255.—A, diagrammatic cross-section of a Holothurian; B,
diagrammatic longitudinal section. I-V, radii; _Int.1-Int.3_, the three
limbs of the alimentary canal; _M^1-M^3_, the three mesenteries attaching
the same to the body-wall. (After Ludwig.)]

The rectum, or cloaca, is one of the most characteristic features in this
and most other Sea-cucumbers. In addition to the passing of faeces, it is
used to pump water in and out, and it thus serves as a breathing organ.
This pumping is effected by alternate contractions of the radiating muscles
attaching the cloaca to the body-wall, and of the circular muscles which
immediately surround it. Two long branched tubes termed RESPIRATORY TREES
(Fig. 256, 11) open into the cloaca, and into these the inspired water
penetrates. The finer branches of these gills end in rounded thin-walled
swellings termed "ampullae"; and when water is forced into these they
become tense, and a considerable quantity diffuses through their walls,
carrying {564}oxygen into the fluid which fills the coelom. If a
Sea-cucumber be left in a limited quantity of water, it will sometimes
direct the posterior end upwards until it reaches the surface of the
liquid, and will pump air into the trees. Besides the trees, other much
shorter tubes open into the cloaca, termed the CUVIERIAN ORGANS. These
tubes are really the modified basal branches of the trees. They are
unbranched, and their peritoneum consists of cells which secrete a slime
which swells up enormously on the addition of sea water. When the
Cotton-spinner is strongly irritated, it contracts all the muscles of the
body-wall, and these, acting on the incompressible fluid in the
body-cavity, transmit the pressure to the thin rectum, which tears, and
allows a portion of the viscera to be forced out. The first parts to be
rejected are the Cuvierian organs, and the cells covering these absorb
water, and their contained mucus splits up into a tangle of white threads,
in which an enemy may be completely ensnared. A large lobster has been seen
so enveloped with this "cotton" as to be completely incapable of motion.
The origin of the name "Cotton-spinner" requires no further elucidation.
Such self-mutilation, even when it involves not only the Cuvierian organs,
but the trees and the whole of the intestine, is not necessarily fatal. If
the animal be left alone, it can regenerate the whole of these organs.

The WATER-VASCULAR SYSTEM in its general features resembles that of the
Echinoidea. We notice as its first striking peculiarity the modification of
the stone-canal. This is often multiplied, as in the species (_H.
tubulosa_) represented in Fig. 256, where there are five; but whether there
is one or many, they do not reach the body-wall, but end each in a swelling
projecting into and bathed by the coelomic fluid. These swellings are
termed "internal madreporites." They are pierced by numerous fine ciliated
canals, which lead into a space from which the stone-canal takes its
origin. Both stone-canal and madreporite (especially the latter) are
stiffened by the deposition of carbonate of lime. In the young Holothurian
there is a single ciliated pore-canal opening to the exterior and leading
into a thin-walled axial sinus, which, as Bury[505] has shown, is later
converted into the internal madreporite; the pore-canal, which represents
the external madreporite of other Echinoderms, disappearing at the same
time.

{565}[Illustration: FIG. 256.—Dissection of _Holothuria tubulosa_. × ½. 1,
Feelers; 2, feeler-ampullae; 3, ring-canal; 4, Polian vesicle; 5,
stone-canals; 6, radial canal; 7, one of a pair of longitudinal muscles; 8,
genital tubes; 9, intestine; 10, radiating muscles of cloaca; 11, base of
respiratory tree; 12, ventral blood-vessel; 13, plexus of dorsal
blood-vessel. (After Ludwig.)]

{566}This extraordinary modification is the consequence of the habit of
forcing water into the respiratory trees. The body-cavity is by this means
kept tensely filled with fluid, and the stone-canal is enabled to draw on
it for the supply to the water-vascular system, thus rendering the external
madreporite supererogatory. A large-stalked sac—the Polian vesicle (Fig.
256, 4)—multiplied in many species, hangs down from the water-vascular ring
and serves as a reservoir of fluid.

All the podia, including the feelers, have ampullae. In the feelers a
semicircular valve is situated just where the external part passes into its
long ampulla. When this valve is expanded, the feeler is moved about by the
contraction of its muscles, but when it is contracted, the contents of the
feeler can flow back into the ampulla, so that the feeler is reduced to an
insignificant papilla (as in Fig. 254). The interior of the feeler is
ciliated, and a current seems to flow up one side and down the other, so
that this organ, like the dorsal tube-foot of a Cake-urchin or
Heart-urchin, seems to assist in respiration.

The NERVOUS SYSTEM differs from that of Echinoidea in the absence of the
pigment spot (or so-called eye) on the terminal podium of the radial
water-vascular canal. Each podium receives a so-called nerve—really an
extension of the radial nerve-cord with its ganglion-cells—and this ends in
a plate of sensory epithelium in the sucker of the tube-foot or tip of the
tentacle, or of each of its branches in the case of the feeler.

There is a coelomic nervous system developed from the radial perihaemal
canals. The PERIHAEMAL RING is represented in Echinoidea by the lantern
coelom, in Holothuroidea in all probability by the "buccal sinus," a space
intervening between the water-vascular ring and the oesophagus. In the
outer wall of this are developed ossicles, which constitute the CALCAREOUS
RING found in all[506] Sea-cucumbers (Fig. 257, A and B). In this ring
(Fig. 257, B) are to be distinguished radial and interradial pieces. The
former are notched at their upper ends, and in all probability represent
the auriculae of Echinoidea, as the radial nerve-cords pass out over the
notches, whilst the interradial pieces probably represent a coalesced pair
of jaws and their included tooth, since these ossicles develop from a
single rudiment in the larval Echinoid.

{567}The so-called BLOOD SYSTEM is in its main features similar to that of
Echinoidea. It consists of a blood-ring surrounding the oesophagus inside
the water-vascular ring, and sending branches along the stone-canal, and of
dorsal and ventral strands accompanying the gut in its course. These are
best marked in the region of the intestine, where absorption principally
takes place; in the wall of the stomach they are represented by a delicate
plexus which can hardly be traced into connexion with the blood-ring. The
dorsal "vessel" is situated in a fold of peritoneum projecting from the
intestinal wall; it gives off branches to the intestine, which unite on its
surface to form a plexus. In the middle limb of the intestine these
branches are grouped into tufts, and the fold of peritoneum between
successive tufts becomes absorbed; through the holes so formed branches of
the respiratory tree penetrate, so that the trees cannot be separated from
the intestine without tearing the dorsal vessel (Fig. 256, 13).

[Illustration: FIG. 257.—Types of calcareous rings and of ossicles. A,
calcareous ring of _Phyllophorus rugosus_, × 2; B, calcareous ring of
_Holothuria cinerascens_; C, ossicle of _Holothuria atra_; D, ossicle of
_Holothuria fusco-rubra_. _r_, Radial piece. (After Ludwig.)]

The GENITAL ORGANS consist of a single group of branched tubes situated on
the left side of the dorsal mesentery, which converge to open into a short
genital duct, which leads to a pore situated in the mid-dorsal line, a
short distance behind the feelers. From the common point of origin of the
tubes, the "genital base," as it is called, a worm-shaped genital
stolon[507] extends back along the genital duct towards the body-wall.
There is no genital rachis.


CLASSIFICATION OF HOLOTHUROIDEA.

The class is in many points of structure exceedingly variable, but many
striking variations in important organs occur in allied {568}species, and
even in the same species, and hence are probably not of physiological
importance. We shall therefore confine our attention mainly to those
differences in structure which are correlated with differences in habits,
and therefore of systematic importance. We shall consider in order (1) the
feelers; (2) the method of protecting these; (3) the rest of the
water-vascular system; (4) the gills; and (5) the skeleton.

FEELERS.—These organs have been made the basis of the division of the
Holothuroidea into orders, and as they are the means by which food is
obtained, and are thus of first-class physiological importance, this
procedure is fully justified. In three orders they have the shield-shaped
ends described in the case of _Holothuria nigra_, but in another large
order (Dendrochirota) they are much branched, and end in a mass of delicate
twigs. In another order (Synaptida) they are feather-shaped, with two rows
only of branches, whilst finally in Molpadiida they are simple
finger-shaped processes with one or two lateral branches. The number of the
feelers varies from ten to thirty.

In the Dendrochirota the entire anterior portion of the body can be
introverted into the interior, so that in this way the crown of feelers can
be effectively protected. The retractor muscles are modified portions of
the longitudinal muscles of the body-wall, which traverse the body-cavity,
and are inserted into the radial pieces of the calcareous ring. Similar
muscles are found in the genus _Molpadia_ and in many of the Synaptida. In
Aspidochirota and _Pelagothuria_ they are totally wanting, and here the
feelers possess long ampullae which allow of the tentacles being
individually contracted to very small dimensions. These ampullae seem to be
present in nearly all cases in Molpadiida, and in Synaptida, although in
the last-named order they are very feebly developed, and must be looked on
as vestigial. In Dendrochirota, owing to the strongly developed retractors,
they would be useless, and so are absent.

WATER-VASCULAR SYSTEM.—In Synaptida the radial canals are totally absent in
the adult, and the only podia are the feelers, which spring directly from
the ring-canal. The radial canals are present in _Pelagothuria_, but the
feelers are still the only podia; in the Molpadiida there are only five
small terminal tentacles round the anus in addition to the feelers. In the
Elasipoda all the podia have pointed ends, but the dorsal podia {569}are
few, long, and stiff, and often coalescent in places to form grotesque or
remarkable appendages. In the remaining forms the podia of the trivium have
always suckers, whilst those of the bivium may or may not be pointed. In
_Psolus_ the two dorsal radial canals and their podia are totally absent.

RESPIRATORY TREES.—These are present in Aspidochirota, Dendrochirota, and
Molpadiida, totally absent in the Synaptida and _Pelagothuria_, and
doubtfully represented in a few Elasipoda by a single unbranched outgrowth
of the gut.

SKELETON.—This consists, as explained above, of the scattered deposits in
the skin and of the calcareous ring. As regards the first, their shape
varies immensely, and yet one or two principal types characteristic of each
of the main divisions can be defined. Thus the Synaptida are characterised
by wheels, with spokes ending in a hub, and by anchors attached to a plate.
The Elasipoda have simple St. Andrew's crosses, whilst the Aspidochirota
are mainly characterised by "stools" (Fig. 257, C) and buckles (Fig. 257,
D). The Dendrochirota have a bewildering variety of forms; the most
characteristic, however, are a right-angled cross and a grating, very
similar to the buckles of the Aspidochirota, except that in the former
there are usually four holes placed cross-wise, whilst the buckle has
generally two parallel rows of three holes. Since these ossicles are the
only records we possess of the existence of fossil Holothuroidea, they have
been studied with great care. The calcareous ring varies very much. The
radials are always five (except in individuals where there are more than
five radii), but the interradials are increased in the Synaptida, and in
the other orders are in some cases diminished or occasionally suppressed
altogether. The last is the case in nearly all Elasipoda; here the radials
consist of a central horizontal piece with two diverging arms at each side.
These arms, which can branch repeatedly, traverse the adjacent interradii,
meeting those of the next radii, so that interradials are in most cases
entirely absent. The Aspidochirota have usually a ring consisting of small
squarish ossicles (Fig. 257, B). In the Molpadiida and Dendrochirota the
radials are prolonged backwards into forked tails, which in some
Dendrochirota are broken into a number of small pieces (Fig. 257, A), the
lower parts of the interradialia being similarly divided.

The CLASSIFICATION of the Holothuroidea is comparatively easy. {570}All
authors recognise six divisions, and the only dispute is as to whether they
are to be regarded as families or orders. Ludwig[508] divides the group
into two orders, PARACTINOPODA and ACTINOPODA, but the first includes only
those forms which have lost the radial canals, and this is only one step
farther in a degeneration, intermediate stages of which can be traced in
the other divisions. There is really no ground for placing the
Paractinopoda in contrast to all the other divisions, and the only
alternative is to regard the six main divisions as orders, since a class
must be divided into orders. In the case of only one, however, is a further
division into families practicable, and therefore each of the others will
contain a single family.

ORDER I. ASPIDOCHIROTA.

Holothuroidea with shield-shaped feelers provided with ampullae; with
radial canals and numerous podia and with respiratory trees. Retractor
muscles absent. Nearly a third (158) of the species of Holothuroidea belong
to this order, but there are only six genera, and of these _Holothuria_
includes no less than 109 species. The Aspidochirota seem for the most part
to live on somewhat firm ground, the surface of which they are continually
sweeping with their shield-shaped feelers, which brush the adherent
organisms into the capacious mouth. Four species of _Holothuria_—viz. _H.
intestinalis_, _H. tremula_, _H. aspera_, and _H. nigra_ are recorded from
British waters. The first-named is a northern form, distinguished by the
fact that all its podia have suckers; it is found in the north of Scotland.
_H. tremula_ is intermediate in structure between _H. intestinalis_ and _H.
nigra_, and is found in deep water off our western coasts. _H. aspera_,
remarkable for the radiating spines growing out from its ossicles, has been
recorded only once from deep water. Of the other genera it is only
necessary to mention _Stichopus_, remarkable for the square outline of its
transverse section, and for the restriction of the ventral tube-feet to the
radii; there is also a well-marked tapering of the anterior end, so that
this genus may be said to have a neck. _Stichopus_ is almost entirely
confined to tropical waters, and some of its species, as also species of
the ubiquitous genus _Holothuria_, as well as many other undetermined
species, constitute the valuable "Trepang," which is a delicacy much prized
{571}by the Chinese. The Trepang are caught in various parts of the Malay
Archipelago. They are cooked in sea water to preserve them, dried in the
sun, and boiled in fresh water repeatedly, till all the salt is extracted.
They are then dried and sent to market, where they are used in making soup.


ORDER II. ELASIPODA.

Holothuroidea with shield-shaped feelers, destitute of retractor muscles;
all the podia have more or less pointed ends,[509] but there is a marked
contrast between dorsal and ventral podia, and the ventral surface is
flattened so as to constitute a creeping sole. No respiratory trees, at
most a simple diverticulum of the intestine; frequently the primitive
external madreporite is retained, and contains several pores.

A number of spherical sacs containing little spherical calcifications
(otocysts) are attached to the nerve-ring in some genera. Can these be
metamorphosed sphaeridia of Echinoid ancestors?

[Illustration: FIG. 258.—_Ilyodaemon maculatus._ × ⅔. (After Théel.)]

The first member of this remarkable order to be discovered was _Elpidia_,
which was dredged in 1875 by the Swedish Arctic Expedition, and described
by Théel.[510] The majority of the known members of the order were
discovered by the dredging expedition of H.M.S. "Challenger." The species
composing it are, with one exception, inhabitants of what may be termed the
abysmal depths of the sea. The exception alluded to (_Ilyodaemon
maculatus_) is confined to the belt between 100 and 150 fathoms in depth.
The well-marked sole and the absence of suckers point to a life consisting
of constant peregrinations over {572}the soft ooze forming the ocean floor.
The ooze forms their food, and as their weight must to a certain extent
immerse them in it, we can understand why the stiff, long dorsal podia have
been specialised as respiratory organs, since there are no respiratory
trees. These respiratory podia sometimes undergo extraordinary development;
thus in _Peniagone_ several very long ones cohere to form a huge vertical
sail, whilst in _Psychropotes_ one or two cohere to form a backwardly
projecting tail. On the other hand, in _Ilyodaemon_ (Fig. 258) the dorsal
podia are numerous and slender.


ORDER III. PELAGOTHURIIDA.

Holothuroidea with shield-shaped feelers provided with long ampullae which
project outwards, pushing the skin before them so as to form external
appendages, connected at the base by a web. Calcifications absent. No
retractor muscles. No respiratory trees. The external madreporite is
retained, but all podia other than the feelers have disappeared, although
the radial canals have been retained.

This order contains one species, _Pelagothuria natans_, which is the only
free-swimming Holothuroid known, the muscular web connecting the freely
projecting ampullae being the organ of locomotion.


ORDER IV. DENDROCHIROTA.

Holothuroidea with long repeatedly branched feelers terminating in fine
pointed twigs. No feeler-ampullae; but retractor muscles are present, which
can introvert the anterior end of the body. Respiratory trees well
developed. This order includes twelve genera and over 180 species, and,
like the Aspidochirota, is of worldwide distribution. So far as can be
safely generalised from the few species whose habits have been closely
observed, it seems that this order is adapted to catch swimming prey—it is
an order of fishers. The long branched tentacles are extended like the
lines of an angler. Their surface is coated with adhesive slime, and before
long becomes covered with small organisms which have come in contact with
it. When a feeler has captured in this way a large enough haul, it is
turned round and pushed into the mouth, which is closed on it. It is then
forcibly pulled out, during which process the prey is, so to speak,
stripped off it. {573}Four genera (_Cucumaria_, _Thyone_, _Phyllophorus_,
and _Psolus_) and sixteen species have been recorded from British waters.

[Illustration: FIG. 259.—_Cucumaria crocea_, carrying its young, × 1. (From
Wyville Thomson.)]

_Cucumaria_ is remarkable for being the only genus of Holothuroidea in
which the body is pentagonal in cross-section. In the majority of its
species the tube-feet are confined to two rows along each radius, but in a
few there are some scattered tube-feet in addition. There are only ten
buccal tentacles. The species figured (_C. crocea_) is an Antarctic one
which carries the young on the back. _Thyone_ differs in being circular in
cross-section and in having the tube-feet scattered evenly over the whole
surface. In _Phyllophorus_ (Fig. 260) the tentacles are more than fifteen,
and are disposed in two circles, an inner of smaller and an outer of larger
tentacles. The other podia are, as in _Thyone_, scattered.

{574}[Illustration: FIG. 260.—_Phyllophorus urna._ × 1.]

[Illustration: FIG. 261.—_Psolus ephippifer._ × 3. A, with feelers
retracted and brood-pouch closed; B, with feelers extended and some of the
plates of the brood-pouch removed. (From Wyville Thomson.)]

_Psolus_ is a most extraordinary genus. There is a well-marked sole, to
which the tube-feet are confined, whilst the dorsal radial canals, and
consequently all the dorsal tube-feet, are absent. The dorsal ossicles are
enlarged to form a complete mail of plates, recalling the corona of a
Sea-urchin. The two British species are small, and found in comparatively
deep water, but a fine large species is found in the Gulf of St. Lawrence,
and {575}extends into brackish water up the estuary. The species figured
(_P. ephippifer_) is an Antarctic one, which carries the eggs until
development is complete in a dorsal brood-pouch.


ORDER V. MOLPADIIDA.

Holothuroidea with simple, finger-shaped feelers, provided with ampullae;
retractor muscles occasionally present; respiratory trees present. Besides
the feelers, the only podia are five minute papillae terminating the radial
canals in the neighbourhood of the anus.

[Illustration: FIG. 262.—_Trochostoma violaceum._ × 1. _m_, Mouth.]

This order includes six genera and about thirty species. Its peculiarities
seem to be due to the fact that its members are burrowers, leading a life
like an earthworm. Hence the absence of the tube-feet, and the small,
almost vestigial character of the feelers. _Trochostoma_ (Fig. 262) and
_Caudina_ are remarkable for the presence of a tail. This appendage is in
reality only the narrow posterior end of the body, and is especially long
in _Caudina_; and observations on a species found off the coast of Maine,
U.S.A.,[511] have shown that the tail, like the siphon of a Mollusc,
projects up from the burrow to the surface in order to maintain the
respiratory current of water.


ORDER VI. SYNAPTIDA.

Holothuroidea with short bipinnate (_i.e._ feather-shaped) feelers,
provided with only vestigial ampullae, and with well-developed retractor
muscles. No other podia; radial canals absent in the {576}adult.
Respiratory trees absent, and transverse muscles of adjacent interradii
continuous, so as to form circular muscles. Otocysts attached to the
nerve-ring as in Elasipoda.

[Illustration: FIG. 263.—_Synapta digitata._ A, animal viewed from the
side, × 7/13; B, anterior end, with tentacles extended, × 2.]

The members of this remarkable order, like those of the preceding one, are
burrowers; but though their feelers are larger, the rest of their anatomy
has undergone much more profound modification than that experienced by the
Molpadiida. The loss of the radial canals, which must be practically
functionless in Molpadiida, is not a great step, but the change in the mode
of respiration is a greater modification. Respiration appears to be
effected by diffusion through the body-wall, which is always comparatively
thin. The circulation of the body-cavity fluid is assisted by a number of
stalked, ciliated cups placed on the mesenteries near the line of their
insertion on the body-wall. In dealing with Asteroidea it was pointed out
that the ends of the tube-feet are the only places where numerous
sense-hairs are to be found, and which, therefore, can be called
sense-organs. This is true generally throughout Echinodermata. Now in
Synaptida, where the tube-feet are lost, the surface of the body has
scattered over it little sense-organs consisting of hillocks of ectoderm
with an aggregation of sense-cells. These may be regarded as representing
the discs of the missing tube-feet. One is involuntarily reminded by the
ciliated cups and scattered sense-organs of the ciliated urns and
sense-organs of the Sipunculidae, which lead a similar life; and taking
into consideration the general superficial likeness in {577}appearance of
the two groups, the epigram is almost justified that "if the Synaptida were
not extremely careful they would become Gephyrea."

This order is represented in British waters by three species of the genus
_Synapta_, which is remarkable for possessing, as ossicles, only the
peculiar anchors attached to anchor plates. The present author has dug up
the commonest species (_S. inhaerens_) from its burrows in the sand at low
water in the Clyde. These animals seem to seek their food at the surface;
the feather-shaped feelers are used to seize small algae and zoophytes, of
which the food apparently consists. If seized, _S. inhaerens_ readily
amputates the posterior part of the body, whilst the head with its feelers
immediately buries itself. The other genera of the order (except _Anapta_)
are characterised by the possession of wheels with spokes as their
characteristic ossicle, as the names _Trochodota_, _Trochoderma_,
_Acanthotrochus_ bear witness.

The only fossil remains of Holothuroidea consist of isolated
ossicles—wheels, gratings, anchors, etc.—which first make their appearance
in the Carboniferous limestone and tell us practically nothing of the
evolution of the group. From a comparison with one another of the living
families, certain conclusions can be drawn. The Aspidochirote feeler and
the method of using it recall forcibly the shape and function of the buccal
tube-feet of Spatangoidea. It is probably safe to assume that it is the
primitive form from which the other forms of feeler have been derived.
Secondly, the anal respiration and the curious internal madreporite have
been developed in correlation with one another, and are like nothing found
elsewhere among the Eleutherozoa. Hence we may with high probability assume
a Protoholothuroid stock with shield-shaped feelers but devoid of
respiratory trees, and with an external madreporite. From this stock the
Elasipoda developed by migrating into deeper water, whilst the
Pelagothuriida sprang from the same root by taking to swimming; the
Aspidochirota constituting the main line. The Dendrochirota were developed
from a stock with respiratory trees and internal madreporite—in a word,
from Aspidochirota. From them the Synaptida and the Molpadiida have
developed as offshoots at different periods through taking to a burrowing
life. These relationships are shown by the following diagram:—

       Protoholothuroidea                                             {578}
            / |  \
           /  |   \
  Elasipoda   |   Pelagothuriida
              |
          Protaspidochirota
            /         \
  Aspidochirota        Protodendrochirota
                           |\
                           |  Synaptida
                           |\
                           |  Molpadiida
                           |
                        Dendrochirota




{579}CHAPTER XX

ECHINODERMATA (_CONTINUED_): PELMATOZOA—CRINOIDEA =
SEA-LILIES—THECOIDEA—CARPOIDEA—CYSTOIDEA—BLASTOIDEA


SUB-PHYLUM II. PELMATOZOA

The PELMATOZOA differ from the ELEUTHEROZOA in several important respects.
They are fixed (at any rate in the young stage) by the centre of the aboral
surface, and this portion of the body usually takes on the form of a stem
supported by a definite series of ossicles, so that we can discriminate a
"calyx"—the main part of the body—from the "stem." Further, the podia and
the ambulacral grooves seem to be always covered with powerful cilia, which
are employed in producing a current which sweeps small organisms to the
mouth. The podia are never locomotor in function; their use is similar to
that of the tentacles on the lophophore of Polyzoa and Brachiopoda.

The living Pelmatozoa are very few in number compared with the extinct
forms. It may with justice be said that the group is nearly extinct;
indeed, out of its five classes one alone, and that the most highly
specialised class, survives till the present day. Now we have already seen
that, in the case of the Eleutherozoa, if the annectant fossil types were
taken into consideration, the definition of the classes would be difficult,
so that it is not to be wondered at if the classes of the Pelmatozoa are
also somewhat difficult to define; and it must be added that this
difficulty is not only due to the fact that intermediate types occasionally
occur, but also to our ignorance of the functions of many structures found
in fossil types, speculations regarding which are to be received with
caution. Bearing in mind, then, the provisional nature {580}of the
classification, we may give the diagnoses of the principal divisions as
follows:—

Class I. CRINOIDEA.—Pelmatozoa provided typically with a well-marked stem;
calyx consisting of an aboral "patina" of two or three circles of plates,
and a flexible "tegmen" or oral surface with small plates or none; radial
canals supported by long branched arms, which are developed as direct
prolongations of the uppermost circle of plates in the patina.

Class II. THECOIDEA (Jaekel) = EDRIOASTEROIDEA (Bather).—Pelmatozoa without
a stalk, fixed to the substratum by the whole aboral surface. The radial
canals run out over the oral surface in grooves, which are closed by
specially modified plates; but there are no arms of any kind.

Class III. CARPOIDEA (Jaekel).—Pelmatozoa with a well-developed stalk. The
radial canals and their branches are devoid of a skeleton, and either
produce no modifications at all on the skeleton of the calyx, or at most
are supported by short horn-like outgrowths of some of its plates.

Class IV. CYSTOIDEA.—Pelmatozoa which typically possess a well-developed
stalk, a sac-like calyx contracted at the mouth and covered with plates,
some of which are pierced with pores or slits; the radial canals, though
they may for part of their course run over the surface in grooves, have
their terminal portions supported by free unbranched arms ("fingers").

Class V. BLASTOIDEA.—Pelmatozoa provided with a well-developed stalk and
ovoid bud-like calyx. From the mouth the radial canals run backwards over
the calyx, as in Echinoidea, but they give rise to numerous lateral
branches, which are supported by free unbranched arms ("fingers"). Special
respiratory organs occur on the interradial areas in the form of parallel
folds called "hydrospires."


CLASS I. CRINOIDEA

This is the only class which has living representatives. There are twelve
recent genera, of which eight retain the stalk throughout life; the
remaining four lose it when adult, retaining only a stump, termed the
"centro-dorsal," covered with fixing organs ("cirri"). The stalked forms
are confined to considerable depths, and can only be obtained by deep
dredging, whereas many of the {581}stalkless forms are comparatively
common. We shall select as type for special description the common
Feather-star, _Antedon rosacea_ (_bifida_), which can be dredged in depths
of ten fathoms off the south-west coast of England.

[Illustration: FIG. 264.—Oral view of _Antedon rosacea_. × 3. _a_, Arm;
_a.g_, ambulacral groove; _an_, anus; _c_, calyx; _m_, mouth; _p_,
pinnules.]

The animal consists of a small flattened calyx, from which radiate out ten
long delicate arms, each fringed with a double series of short branches
called "pinnules." In the centre of the aboral surface can be seen the
centre-dorsal plate (Fig. 265, _c_), a knob-like stump of the broken-off
stem, covered with small whip-like outgrowths called "cirri," by means of
which the animal is anchored to the substratum (Fig. 265, _cir_). When
_Antedon_ is disturbed it relaxes its hold, and swims by graceful muscular
movements of the arms. These are arranged in five pairs, and the
corresponding members (right and left) of all the pairs are bent and
relaxed together. On coming to rest the animal reattaches itself by means
of the cirri. These are composed of cylindrical ossicles joined to one
another by muscles, and they can thus act as efficient grasping organs. In
the centre of the oral surface, which is termed the "tegmen," and is soft,
flexible, and without visible calcifications, is situated the mouth,
surrounded by five short triangular flaps called "oral valves." In the
intervals between these valves, grooves radiate from the mouth which
bifurcate at the points of origin of each pair of arms, and are continued
over their surfaces. These grooves correspond to the ambulacral grooves of
Asteroidea, and to the epineural canals of the other classes of
Eleutherozoa. At each side of each groove {582}are to be found a series of
podia in the form of delicate finger-like processes, which serve only for
respiration and for producing a current of water, their surfaces, like that
of the grooves between them, being covered with powerful cilia. The anus is
at the extremity of a little knob called the anal papilla, situated in one
of the interradii (Fig. 264, _an_).

As in Ophiuroidea, the ectoderm cells have disappeared over the whole
surface of the body, except the grooves and the podia, the only trace of
their former existence being a cuticle with adherent nuclei. Pedicellariae
are unknown in all Pelmatozoa; and spines have only been described from one
fossil species of Crinoid. Beneath the cuticle is the dermis, having the
composition described in the case of _Asterias rubens_; this on the aboral
side of the calyx gives rise to the "patina," consisting of plates, in part
movable on one another, in part immovably fused together. Those visible
from the outside are (1) the centro-dorsal ossicle, from which the cirri
spring; (2) five columns of ossicles termed radials (Fig. 266, _R^1_,
_R^2_, _R^3_); each column consists of three radials, extending from the
centro-dorsal to the origin of a pair of arms. The uppermost radial in each
column bears two facets for the articulation of these arms. Each arm is
supported by a series of "brachial ossicles" (_Br_).

[Illustration: FIG. 265.—View of _Antedon rosacea_ from aboral surface, ×
4. _c_, Centro-dorsal; _cir_, cirrus; _R^1_, _R^2_, _R^3_, the three radial
plates of one column; _syz_, syzygy.]

It is evident, both from the number of ambulacral grooves and of the
columns of radials, that _Antedon_ has only five radii, and each pair of
arms must be regarded as having arisen by the bifurcation of a primitive
arm. This is proved to be true by {583}a study of the development, and it
can further be shown that the arms fork repeatedly; but in these further
bifurcations one fork remains short, and forms a pinnule, whilst the other
continues the arm. Thus the arm, instead of being a single axis, is really
a series of axes—in a word, it is a "sympodium."

If in the case of any bifurcation the two forks were to develop equally,
the number of arms in that ray would be doubled, and this actually happens
in the case of other species of _Antedon_.

DIGESTIVE SYSTEM.—The mouth leads through a short vertical oesophagus into
an enlarged stomach, which lies horizontally curved around the axis of the
calyx. The stomach is succeeded by a short intestine, which leads into the
anal papilla. Both oesophagus and stomach are ciliated, and the food
consists of minute organisms, swept into the mouth by the current produced
by the cilia covering the ambulacral grooves and podia; the ten arms may
indeed be compared to a net spread out in the water to catch swimming prey.

The WATER-VASCULAR SYSTEM consists of a ring closely surrounding the mouth,
from which radial canals are given off which underlie the ambulacral
grooves and bifurcate with them. The podia have no ampullae, but muscular
strands traverse the cavities of the radial canals, and that of the
ring-canal, and by their action water can be forced into the podia, which
are thus extended. Numerous stone-canals hang down from the ring-canal, and
open freely into the coelom; they do not, as in Holothuroidea (where the
same arrangement occurs), end in sieve-like madreporites. The tegmen,
_i.e._ the ventral surface of the calyx, is pierced by a number of isolated
pores lined by ciliated cells, which suck in water. In the oldest
Pelmatozoa there seems to have been a regular madreporite. In the larva of
_Antedon_ there is but one pore-canal, which, as in most Eleutherozoa,
leads into a special section of the coelom, the "axial sinus," embedded in
the body-wall, with which also the single stone-canal communicates; but
later the division between the axial sinus and the rest of the coelom
breaks down, and then the pore-canals and stone-canals become multiplied
independently of each other (Fig. 266, _m.p_, _p.c_, and _st.c_).

NERVOUS SYSTEM.—In the young stalked form the nervous system, as in other
Echinoderms, consists of a ring round the {584}mouth, from which radial
cords are given off which run under the ambulacral grooves (Fig. 266,
_nerv.rad.v_).

The fibres of this nervous system are, as in Asteroidea, immediately
beneath the bases of the ectoderm cells. A large band of fibres is given
off to each podium, which is covered with minute elevations, each with
pointed sense-hairs in the centre. As the animal grows, another nervous
system makes its appearance, which is developed from the coelomic wall, the
cells of certain tracts of which multiply and bud off ganglion cells from
which the fibres grow out.

[Illustration: FIG. 266.—Diagrammatic longitudinal section through one arm
and the opposite interradius of _Antedon_. _ax_, Central canal of
centro-dorsal, with prolongation of genital stolon; _B_, rosette,
consisting of coalesced basals; _Br^1_, _Br^2_, _Br^3_, _Br^4_, the first
four brachial ossicles; _chamb_, chambered organ; _coel.coe_, coelomic
canal of arm; _gen.coe_, genital canal of arm; _gen.r_, genital rachis;
_gen.st_, genital stolon; _m.p_, madreporic pores; _musc.long_,
longitudinal muscle; _nerv.rad.d_, dorsal radial nerve; _nerv.rad.v_,
ventral radial nerve; _p.c_, pore-canal; _pod_, podium; _R^1-R^3_, 1st to
3rd radials; _st.c_, stone-canal; _sub.coe_, sub-tentacular canal of arm;
_w.v.r_, radial water-vessel.]

This "aboral nervous system," as it is called, has its centre in the
"chambered organ" (Fig. 266, _chamb_), which is embedded in the
centro-dorsal ossicle, and is roofed over by a plate called the "rosette."
This represents the five coalesced "basals," a ring of plates which in
other forms alternate with the lowest radials, and it intervenes between
these and the centro-dorsal. The chambered organ consists of a ring of five
vesicles, which have originated as pouches of the aboral coelom (Fig. 266,
_chamb_). The walls of these vesicles develop nervous matter; from them
radiate out five great cords, deeply embedded in the {585}plates of the
patina. These cords rapidly fork, and one division of each of two adjacent
cords enters the lowest radial. In the third radial all the cords are
connected by a commissure which runs completely round the calyx. Each of
the cords in the third radial forks again, and one branch of each cord
enters each of the two arms connected with it, and the two branches
entering an arm coalesce to form a single cord. In the arms, as in the
calyx, the cords are deeply embedded in the ossicles, but branches extend
to the ventral surface of the arms and here unite to form two longitudinal
cords, one on each side of the groove. In the tegmen these cords are
connected by an outer nerve-ring, branches from which join the ectodermal
nerve-ring already described.

The researches first of W. B. Carpenter[512] and then of Marshall[513] have
proved that it is the aboral nervous system which really controls the
movements of the animal. If the chambered organ is destroyed by cautery,
the whole movements of the animal are paralysed; but it will carry out its
characteristic swimming movements just as well if the whole tegmen with the
ambulacral nerve-ring and the whole of the alimentary canal are torn away.
The commissure in the third radials co-ordinates the movements of the arms.
If it is cut they move independently of one another. The position of the
radial cords inside the ossicles is gradually acquired. At first they are
gutter-like evaginations of the coelom; by upgrowth of their sides the
gutters become canals, and are then surrounded by calcified tissue. The
cirri have each a cord traversing them which originates from the chambered
organ.

COELOM.—In the young stalked form the coelom consists of the water-vascular
system ("hydrocoel"), and underlying it an oral coelom, separated from an
aboral coelom by a horizontal mesentery. As the animal grows, this
horizontal mesentery becomes largely absorbed, and the coelom becomes
everywhere traversed by cellular cords (trabeculae), which are later
calcified.

Both oral and aboral coelom become, like the hydrocoel, bent into hoops,
and along the axis of the aboral coelom a cord of germ-cells is developed,
which constitutes the "genital stolon." The chambered organ is developed
from the aboral coelom, and {586}in the centre of its five chambers a
median pocket grows down into the centro-dorsal, along the side of which is
an extension of the genital stolon. In the arms the mesentery separating
the extensions of the oral and aboral coelom persists; the oral extension
consists of two parallel canals called "subtentacular" (Fig. 267, _s.c_),
whilst the aboral space is termed the "coeliac" canal (Fig. 267, _c.c_). In
the tip of the pinnule, that is to say at the extremity of a ramification
of the arm, the coeliac and subtentacular canals communicate. As portions
of the lining of both canals are ciliated, a circulation of the coelomic
fluid is thus kept up. The GENITAL STOLON gives rise at the level of the
remnant of the horizontal mesentery in the disc to a circular genital
rachis, whence cords pass down the arms in the tissue separating
subtentacular and coeliac canals (Fig. 267, _g.r_). Each cord is contained
in a special tube, the "genital canal," which is probably developed in the
same way as the aboral sinus of the Eleutherozoa, _i.e._ as a special
sheltering outgrowth of the coelom (Fig. 267, _g.c_). In the pinnule the
rachis swells out into a genital organ, from which a short duct is
developed when the organ is mature. The eggs are large (3 mm. in diameter),
and adhere for a considerable period of their development to the pinnules.

[Illustration: FIG. 267.—Diagrammatic transverse section of arm of
_Antedon_. To compare it with the section of an Ophiuroid arm it is
inverted from its natural position. _br_, Brachial ossicle; _c.c_,  coeliac
 canal; _c.p_, covering plate; _g.c_, genital  canal; _g.r_, genital
rachis; _n.r.d_, dorsal nerve-cord; _n.r.v_, ventral nerve-cord; _pod_,
podium; _s.c_, subtentacular canal; _w.v.r_, radial water-vessel.]

{587}The MUSCLES of _Antedon_ are of two kinds. Those of the water-vascular
system are, as in Eleutherozoa, basal outgrowths of the cells forming the
walls of the system. The muscles moving the joints of the arms appear to be
modifications of connective-tissue cells. When the brachials are isolated
their terminal faces, strikingly long, recall those of Ophiuroid vertebrae.
There is a ventral groove for the coelomic canal. Above this groove the
face is divided by ridges into four areas for attachment of the muscles.
Dorsal to this is the pit for the strong ligament which binds the ossicles
together; then comes the canal for the aboral nerve-cord, whilst dorsal to
this is the pit for what is called the "dorsal elastic ligament." The
theory underlying this name is that the muscles bend the arms ventrally,
and the ligament by its elasticity restores them to their places; but there
seems reason to believe that the "ligament" is really a dorsal muscle. It
is particularly to be noted that similar muscles occur between the first
and second radials, proving that the primary arm really begins with the
first radial. The second and third radials, as also the first two ossicles
and certain others of each arm, are closely united by calcified fibres, and
this kind of union is called a "syzygy" (Fig. 265, _syz_). The cirri have
all their ossicles united by muscular attachment, and can move rapidly.

The BLOOD SYSTEM (see pp. 449-451) forms a ring consisting of a network of
strings round the oesophagus. This is termed the "labial plexus." From this
cords can be traced to the wall of the stomach and to the surface of the
genital stolon. The assertion that radial strands intervene between the
ectodermic nerve-cord and the radial water-vascular canal, though usually
made, does not appear to be justified, since what is termed the vessel
appears to be a crevice formed by shrinkage in preservation.

The process of respiration is doubtless largely carried out by the podia,
but it must be assisted by the constant instreaming of fresh sea-water
through the pore-canals. The process of EXCRETION has not been directly
observed in _Antedon_, but structures called "sacculi" may be connected
with this function. These are spherical masses of amoebocytes embedded in
the tegmen. During life they are colourless, but after death they become
coloured, showing that they secrete a peculiar compound. These sacculi
abound in the disc, and a row of them is to be found at each side of the
ambulacral groove in the arms. When, as in {588}tropical species, the
groove is supported by side-plates, these are notched for the reception of
the sacculi.

Turning now to survey the group Crinoidea as a whole, lack of space forces
us to confine our attention mainly to the living forms. These differ
amongst themselves chiefly in the following points: (1) the condition of
the stem; (2) the structure of the calyx; (3) what is intimately connected
with this, the method of branching of the arms; and (4) the length of the
alimentary canal.

CONDITION OF THE STEM.—This is represented by a centro-dorsal stump in
_Antedon_ and most of its allies, but in _Actinometra_ it becomes a flat
plate, and in some species in old age all the cirri drop off. In
_Uintacrinus_ and _Marsupites_ (fossil genera) there is no trace of cirri.
In Pentacrinidae there is a long stem, pentagonal in cross-section, in
which alternate ossicles carry whorls of cirri; in Rhizocrinidae the stem
consists of compressed ossicles, elliptical in section, bearing cirri only
at the rooting tip, whilst in _Hyocrinus_ the stem is made up of
cylindrical ossicles, cirri being apparently absent. Finally, in _Holopus_
the stem is represented by an uncalcified leathery outgrowth from the
calyx.

SKELETON OF CALYX AND ARM.—In living Crinoidea, with the doubtful
exceptions of _Holopus_ and _Hyocrinus_, the calyx is supposed to be built
up originally of four whorls of plates, viz. "infra-basals," "basals,"
"radials," and "orals," the last named forming the skeleton of the oral
valves round the mouth. In the two exceptions named there is no certain
evidence of the existence of infra-basals. In living forms the infra-basals
coalesce with the uppermost joint of the stem; the basals remain large and
conspicuous, though they are fused into a ring in Rhizocrinidae,
_Atelecrinus_, and _Thaumatocrinus_, whilst in Pentacrinidae this ring is
nearly, and in _Antedon_ and its allies completely, hidden when the calyx
is viewed from the outside. In _Hyocrinus_ the basals are represented by
three ossicles. The lowest radials are an important element in the patina
in every case, but the upper radials, the incipient portions of the arms,
may be incorporated in the calyx (_Pentacrinus_, _Antedon_) or may be free
(Rhizocrinidae and _Hyocrinus_); in _Metacrinus_ there are five to eight
radials in each column, all incorporated.

The oral plates are very large in _Hyocrinus_, _Holopus_, and
{589}_Thaumatocrinus_, small in _Rhizocrinus_, vestigial or absent in
_Bathycrinus_, and completely absorbed in _Antedon_. In addition to these
main elements, in many species small accessory plates are developed (_a_)
at the sides of the ambulacral groove, over which they can close down (many
species of _Antedon_, _Hyocrinus_, _Holopus_, Rhizocrinidae, some species
of Pentacrinidae); these are "covering plates," and correspond in function
to the adambulacrals of Asteroidea; (_b_) supporting the sides of the
groove and corresponding to the ambulacrals of Asteroidea;[514] these are
"side-plates," and the covering plates articulate with them; (_c_) on the
surface of the tegmen; these are the interradial plates, which in
_Thaumatocrinus_ alone among recent forms, but in many fossil forms, are
continued into the patina, where they separate the radial plates.

MODE OF BRANCHING OF THE ARMS.—All modern Crinoids have pinnules, and this,
as has already been explained, is due to a suppressed dichotomy. The extent
of the suppression determines the number of arms, which varies within the
same genus.

ALIMENTARY CANAL.—In _Hyocrinus_ there is no dilatation which could be
called a stomach; in _Actinometra_ the mouth is excentric, and the anal
papilla occupies the centre of the tegmen. The intestine is elongated, and
describes several turns round the papilla before ending in the anus.

The CLASSIFICATION of Crinoidea cannot properly be considered without
taking account of fossil forms, but to do so at all adequately is
impossible on account of limitations of space. Less regret may be felt
because the three specialists in this branch, viz. Bather in England,
Springer in America, and Jaekel in Germany, come to fundamentally different
conclusions on the subject. If we confine our attention to living forms we
may, with P. H. Carpenter,[515] select the stem as the basis of
classification. As the method of gaining food is the same in all cases, the
Crinoidea have probably split on the method of attachment to the
substratum. These families—it is impossible, in view of the greater range
of variety in fossils, to dignify them with the name of orders—are as
follow:—

{590}FAM. 1. HYOCRINIDAE.—Stem long and persistent; cirri absent; stem
ossicles cylindrical—ligaments uniting them not specialised. Arms (five)
short, but with extremely long pinnules. Patina composed of long exposed
basals and a ring of five spade-shaped radials. Five large persistent
orals. Interradials and covering plates present. One species (_Hyocrinus
bethellianus_, Figs. 268, 269) dredged up in the Southern Pacific Ocean.

[Illustration: FIG. 268.—_Hyocrinus bethellianus._ × 2. (From Wyville
Thomson.)]

[Illustration: FIG. 269.—Tegmen of _Hyocrinus_, viewed from above after
removal of the arms, × 8. (From Wyville Thomson.)]

FAM. 2. RHIZOCRINIDAE.—Stem long and persistent; cirri confined to a few
near the root or replaced by rooting branches of the stem. Stem-ossicles
thin and pentagonal at the summit, but lower down compressed, elliptical in
section, and united by two ligaments separated by a transverse ridge for
articulation. Patina composed of exposed basals and a ring of five short
radials. Orals large {591}or vestigial. Covering plates and interradials
present. Two genera, both from great depths in the Atlantic—_Rhizocrinus_
(Fig. 270), with five arms and well developed orals (attachment by
branching root-cirri); and _Bathycrinus_, with ten arms and vestigial
orals; attachment by root-like branches of stem (this is essentially the
same as root-cirri).

[Illustration: FIG. 270.—_Rhizocrinus lofotensis._ × 1½. (From Wyville
Thomson.)]

FAM. 3. PENTACRINIDAE.—Stem consisting of ossicles which are pentagonal in
section, united in pairs by syzygy, the upper one of each pair bearing a
whorl of cirri and united by five bundles of fibres of petal-like section
with the lower one of the pair above it. No rooting processes. Patina
consists almost entirely of columns of radials, the basals being almost or
completely hidden. Orals absent, but side-plates in the ambulacral grooves.
Two recent genera, _Pentacrinus_ (_Isocrinus_), with three radials in each
column; _Metacrinus_, with five to eight radials in a column, but the third
{592}radial bears a pinnule. _Pentacrinus_ is found in both the Caribbean
Sea and the Pacific Ocean; _Metacrinus_ in the Pacific. It appears that the
Pentacrinidae when young are attached by a foot-plate at the apex of the
stem; but when adult, the stem is broken in two and the animals, like
_Antedon_, swim by movements of the arms, dragging a large part of the stem
after them, by which they effect temporary attachment. As in other stalked
forms, the cavities of the chambered organ are prolonged into canals which
traverse the stalk; but in this family there is the peculiarity that a
repetition of the chambered organ is found opposite every whorl of cirri.

[Illustration: FIG. 271.—_Pentacrinus asteria._ × ¼. (From Wyville
Thomson.)]

FAM. 4. HOLOPODIDAE.—Stem represented by a leathery noncalcified outgrowth
from the base of the calyx; one circle of radials indistinguishably fused
with the basals and with each other to form the walls of the calyx. Large
oral plates, ten short arms. One genus, _Holopus_, in shallow water in the
Caribbean Sea.

{593}[Illustration: FIG. 272.—Arms and portion of stem of _Pentacrinus
maclearanus_, slightly enlarged. In this species the basals can be seen.
(From Wyville Thomson.)]

{594}[Illustration: FIG. 273.—Calyx of _Actinocrinus_, one of the Camerata,
broken open to show structure. _amb_, Ambulacral groove enclosed in
covering plates; _B_, basal; _R^1_, _R^2_, _R^3_, the three radials of a
column. (After Zittel.).]

FAM. 5. COMATULIDAE.—Stem in the adult broken off, leaving only a stump,
the centro-dorsal, covered with cirri. Six genera. _Antedon_ (= _Comatula_)
has already been described; many tropical species have numerous arms and
often side-plates and covering plates. _Actinometra_ is distinguished by
its excentric mouth, and by the fact that the centro-dorsal is flat and has
cirri only round its edges; _Atelecrinus_ has an acorn-shaped
centro-dorsal, and the basals are externally visible; _Eudiocrinus_ differs
from _Antedon_ only in having five arms; _Promachocrinus_ is a remarkable
form, having ten radii (this is a unique feature in Crinoidea); finally,
_Thaumatocrinus_ has basals externally visible, large persistent orals and
interradial plates, and in addition a short free appendage of several
plates on the anal interradius. _Antedon_ and _Actinometra_ are almost
world-wide. Six species of the first have been recorded from British
waters, of which the commonest is _Antedon rosacea_; four others are
distinguished by having longer cirri, and do not seem to be well defined;
but _A. eschrichtii_, a northern form, is larger, and is distinguished by
having long proximal pinnules. The other genera are rare, and occur in deep
water.

When we turn to survey FOSSIL CRINOIDEA, we are met with a bewildering
variety of forms ranging from the Lower Cambrian to the present day. As
already mentioned, there is no agreement amongst experts as to how they
should be classified. Bather makes the fundamental cleavage depend on the
possession of two whorls of plates in the base (Fig. 274), or of only one
whorl. These two divisions he calls DICYCLICA and MONOCYCLICA respectively.
He admits that in many forms allied to Dicyclica the infra-basals have
disappeared; these he terms "pseudomonocyclic" forms, and believes that he
is able to discriminate them from true Monocyclica.

{595}[Illustration: FIG. 274.—_Crotalocrinus pulcher._ × 1. _B_, basal;
_Br_, arm-fan of adhering branches; _col_, ossicle of stem; _IB_,
infra-basal; _R_, radial. (After Zittel.)]

The present author is utterly unable to believe that the Crinoidea diverged
into two groups on what is a trifling point of meristic variation
comparable to the varying number of rows of plates in the interradial areas
of the older Echinoidea; and he is equally sceptical as to the validity of
Jaekel's division of the group into CLADOCRINOIDEA and PENTACRINOIDEA,
leading to the view that organs like pinnules represent totally different
structures in different groups. Wachsmuth and Springer adopt as bases of
classification the extent to which the arms and their branches are
incorporated in the disc, and they recognise three main divisions:
INADUNATA, in which the arms are completely free from the calyx;
ARTICULATA, in which the arms are partly incorporated but the tegmen
remains flexible; and finally CAMERATA, in which the arms and their first
branches are largely incorporated in the cup; the tegmen is converted into
a rigid dome and the ambulacral grooves on it become closed, as does the
mouth, by the meeting of overarching folds; the grooves remaining, of
course, open in the distal portions of the arms (Fig. 273). This
classification, founded as it is on physiological factors, seems to the
present author more satisfactory. Speaking generally, the points in which
fossil Crinoids may differ from living genera are: (1) the total absence or
irregular nature of the branching in the arms, so that pinnules may be said
to be absent; (2) the closure of the ambulacral grooves and mouth already
alluded to, and (3) the adhesion of the arms in the same ray to produce
net-like structures (_Crotalocrinus_, Fig. 274), or a fan-shaped structure
(_Petalocrinus_); (4) the frequent presence of two rows of brachials in one
arm (biserial structure); (5) the {596}development of an enormous anal
tube, so large that in extreme cases (_Eucalyptocrinus_) the arms may be
lodged in grooves of it.


CLASS II. THECOIDEA (EDRIOASTEROIDEA, BATHER)

These remarkable Pelmatozoa are the most primitive known. They have
sac-like or sometimes cushion-shaped or even disc-shaped bodies, covered
with numerous irregular plates without any symmetry in their arrangement.
There is no stem, but when they are fixed this is effected by an adhesion
of the aboral pole. There are no arms, but on the upper surface is to be
seen the impression of five ambulacral grooves radiating from a central
mouth. These grooves are bordered by covering plates, which in the earliest
form (_Stromacystis_) are seen to be slight modifications of the plates
covering the upper surface of the body, but in the later genera (Fig. 275,
_Thecocystis_) become specialised. The anus is situated on the side, as is
also the madreporite. It has been suggested that Eleutherozoa were derived
from this group; that individuals were occasionally overturned by the waves
or currents, and in this way compelled to use their podia for locomotion.
When Eleutherozoa, however, have a fixed stage in their development, they
are fixed by the oral, not the aboral, surface, and hence can have no close
affinity to Thecoidea. Thecoidea begin in the Middle Cambrian, but
according to Jaekel impressions in the Lower Cambrian, referred to Medusae,
may be casts of this group.

[Illustration: FIG. 275.—_Thecocystis saeculus._ × 6. _a._ Anus; _m_,
mouth; _p_, madreporite(?) (After Jaekel.)]


CLASS III. CARPOIDEA

Pelmatozoa with a well-developed stem; body bilaterally compressed; only
two rays apparently developed. These are indicated only by grooves
radiating from the mouth; but in {597}some cases slight horn-like
outgrowths of some of the plates of the calyx may support prolongations of
the grooves.

This group, which, like the foregoing, commences in the Cambrian, is
perhaps more primitive than the Thecoidea in showing less influence of the
water-vascular system on the skeleton; but in the presence of a
differentiated stem and the development of only two rays, it is more
differentiated. The anus is on one of the flat sides, covered with a flat
plate acting as a valve. The members of this group were formerly confounded
with Cystoidea, from which they differ in the absence of the characteristic
pores. _Trochocystis_, the genus figured, is devoid of any horn-like
outgrowths of the calyx.

[Illustration: FIG. 276.—_Trochocystis bohemicus_, viewed from two sides.
_o_, Mouth. (After Jaekel.)]


CLASS IV. CYSTOIDEA

Pelmatozoa with respiratory organs in the form of "diplopores" or
"pore-rhombs." In a great many cases there is a stalk, but in other cases
this is atrophied, and the animal is attached by the base of the calyx. The
radial canals run for a shorter or longer distance over the calyx, but the
plates of the calyx themselves are not modified for them. Either they run
in simple grooves, or they are protected by a special series of plates
lying above the plates of the calyx. The terminal portions of the radial
canals are in all cases free, supported by unbranched arms consisting
usually of a double row of ossicles. These arms are termed "fingers."

It will be gathered from the description just given that the fingers and
the respiratory organs distinguish Cystoidea from {598}the two foregoing
classes. Formerly this class was a lumber-room in which were placed all
primitive irregular Pelmatozoa. The labours of Jaekel[516] have, however,
dispelled the mist which enveloped this group, and in his monograph all
that can be extracted both from superficial examination and dissection of
these fossils is contained. It seems possible to the present author that
the class may eventually require to be divided into two, corresponding to
the two main divisions which Jaekel recognises, viz. DICHOPORITA, with
pectinated rhombs, and DIPLOPORITA, with diplopores.

[Illustration: FIG. 277.—_Echinosphaerites aurantium._ A, from above; B,
from the side; C, neighbourhood of mouth, enlarged. _amb_, Ambulacral
groove with side-plates and covering plate; _mad_, madreporite. The short
parallel lines across the sutures are the "pore-rhombs." (After Zittel.)]

The pore-rhombs of the Dichoporita (indicated in Fig. 277 by the small
parallel lines crossing the boundaries of the plates) were, according to
Jaekel, nothing but a series of folds of thin integument projecting into
the interior, the outer opening of which in most cases adhered in the
middle, leaving two pores connected by a groove. The inner boundaries of
the folds are sometimes preserved, but in many cases they were entirely
devoid of calcification, and so were lost. The radial vessels either
branched a great deal, giving rise to a multitude of fingers, or, as in
_Echinosphaerites_ (Fig. 277), there were a few long fingers supporting a
reduced number of radial canals. In some cases the calyx can be analysed
into a regular series of cycles of plates, consisting of basals, orals, and
three intervening whorls, thus including one more ring than the calyx of
Crinoidea. Jaekel regards this as a primitive arrangement, believing that
the irregularity seen in _Echinosphaerites_ secondary. This is a doubtful
hypothesis.

{599}The diplopores of the Diploporita appear to consist of two canals
traversing the body-wall, opening close together into a common pit
externally, but diverging internally. Since in some cases, as in
_Aristocystis_ (Fig. 278), this common pit is proved to have been closed
externally by a very delicate layer of calcification, it is probable that
the pores represent in other cases the points of origin of finger-like
gills similar to those of Asteroidea. Where they were closed by
calcification this was so thin and porous that the diffusion through it
sufficed for respiration. Jaekel regards the Diploporita as a group derived
from Dichoporita, but this seems to be extremely doubtful.

[Illustration: FIG. 278.—_Aristocystis._ In the upper part of the calyx the
heavy dots are "diplopores," seen owing to removal of the superficial
layer. (After Zittel.)]


CLASS V. BLASTOIDEA

Pelmatozoa with respiratory organs in the form of longitudinal calcified
folds, termed "hydrospires," radiating from the mouth. Stem well developed;
calyx regular, consisting of a whorl of basals surmounted by a whorl of
forked radiais, in the clefts of which lay the recumbent radial
water-vascular vessels, supported each on a special plate ("lancet plate"),
and giving off two rows of branches supported by short fingers (Fig. 279).
Side-plates and covering plates were also developed; five orals
("deltoids") completed the calyx. The anus was at the side, just beneath
one of the orals.

The hydrospires, which are the great characteristic of the class, are seen
in section in Fig. 279, B (_hyd_). They consist of a varying number of
parallel folds on each side of each "pseudambulacrum," as the lancet plate
with its adhering side-plates and covering plates has been termed. In the
most primitive genus, _Codaster_, they appear to have opened directly to
the exterior, and to have been placed at right angles to the lines of union
of the radial and oral plates, just like the grooves of a pectinated rhomb.
In more modified forms, such as _Pentremites_ and _Granatocrinus_ (Fig.
279), the outer openings were overarched {600}by the extension of the
side-plates of the radial vessel, and the whole group of folds has a common
opening near the mouth; indeed, in the highest form there is one common
"spiracle" for the two groups of folds in an interradius, which in one
interradius is confluent with the anus. The hydrospires, when they reach
this form, irresistibly recall the genital bursae of Ophiuroidea (Fig. 214,
p. 490), and very possibly served the same purpose.

[Illustration: FIG. 279.—_Granatocrinus norwoodi._ A, view of whole animal;
B, section of radius; C, an isolated finger. _hyd_, Hydrospire; _l_, lancet
plate; _pinn_, finger; _p.p_, covering plate; _R_ and _D_ both signify
radial plate. (After Zittel.)]

Reviewing the whole group of the Pelmatozoa, we see that in the Cambrian
they begin with the extremely primitive Thecoidea and Carpoidea, together
with some obscure forms which, combining a stem with pentamerous symmetry
in the calyx, are supposed to be the forerunners of the Crinoidea. In the
Lower Silurian or Ordovician the two groups of the Cystoidea make their
appearance, possibly independently developed from either Carpoidea or
primitive Crinoidea, which in this period are present in unmistakable form.
In the Upper Silurian the Blastoidea appear, distinguishable from the most
regular Cystoidea only by their hydrospires. It seems practically certain
that they were developed from Cystoidea, and we follow Jaekel in believing
that they arose from Dichoporita. The Carpoidea do not extend beyond the
Ordovician, and by the end of the Carboniferous period Cystoidea and
Blastoidea die out, leaving only the Crinoidea, which at that period were
at their maximum development. From the Carboniferous to the present day the
Crinoidea have continually decreased, leaving in recent seas, as sole
representatives of the Pelmatozoa, only the few forms described at the
beginning of this chapter.




{601}CHAPTER XXI

ECHINODERMATA (_CONTINUED_): DEVELOPMENT AND PHYLOGENY


In Chapter XVI. it was stated that whilst a more or less perfectly
developed radial symmetry was one of the characteristic features of the
phylum Echinodermata when in the adult condition, yet in the immature or
larval condition the members of the group have a strongly marked bilateral
symmetry. In this feature larval Echinodermata resemble the other Phyla of
the animal kingdom which have a well-developed coelom, such as Annelida,
Mollusca, Vertebrata, etc. Since, then, the peculiar radial symmetry is
gradually acquired during the growth of the Echinoderm, we may possibly
discover by a close scrutiny of the life-history what is the nature and
meaning of this departure from the ordinary type of structure among
coelomate animals.

There are two kinds of development met with amongst Echinodermata, which
may be roughly characterised as the "embryonic" and the "larval" type
respectively, although neither description is exact. In developmental
histories of the first type so much reserve material is laid up in the egg
in the form of food-yolk that the young animal whilst in the bilateral
stage requires little or no food. In some cases, however, as in _Amphiura
squamata_, the mother pours out a nourishing exudation; but whether this is
so or not, the parent in nearly every case carries the young about with her
until they have reached the adult condition. In some Asteroidea, as for
instance in the Antarctic species _Asterias spirabilis_ (Fig. 280), the
young become fixed to the everted lips of the mother; in _Amphiura
squamata_, and some other Ophiuroidea the eggs remain in the genital
bursae, which serve as nurseries; in some Spatangoidea, {602}as for
instance in _Hemiaster philippi_ (Figs. 250, 281), the eggs are carried in
some of the deeply grooved petaloid ambulacra; whilst in Holothuroidea they
may develop in the body-cavity (_Phyllophorus urna_), or they may adhere to
the back of the mother (_Cucumaria crocea_, Fig. 259, p. 573), or they may
be protected in special brood-pouches either on the ventral side of the
parent (_Cucumaria laevigata_) or on the dorsal surface (_Psolus
ephippifer_, Fig. 261).

The majority of these cases of embryonic development have been recorded
from Arctic or Antarctic waters; it appears as if conditions there were not
favourable to the larval type of development. In Pelmatozoa the development
of _Antedon rosacea_ alone is known, and that is of the embryonic type.

[Illustration: FIG. 280.—Oral view of _Asterias spirabilis_, slightly
enlarged, showing embryos attached to the everted lips. _emb_, Embryos.
(After Perrier.)]

So far, however, as their mode of propagation is known, it may confidently
be affirmed that the development of the majority of the species of
Eleutherozoa is of the second or larval type. In this type there is little
food-yolk in the egg, and the young animal or larva is forced from a very
early period of development to seek its own living, and hence it is usually
a considerable time (from a fortnight to two months) before the adult form
is attained. When the _embryos_ of different groups of Eleutherozoa are
compared, there is no obvious agreement in structure between them; but the
_larvae_ of the four classes of Eleutherozoa exhibit with differences in
detail a most remarkable fundamental similarity in type, and we are
accordingly justified in regarding the larval development as primitive, and
the embryonic type as derived from it and differently modified in each
case.

{603}[Illustration: FIG. 281.—_Hemiaster philippi._ Enlarged view of a
single petal, showing the embryos _in situ_. (From Wyville Thomson.) The
whole animal is shown in Fig. 250, p. 555.]

In the typical larval development the eggs are fertilised after being laid,
and they then undergo segmentation into a number of equal, or nearly equal,
segments or "blastomeres." These arrange themselves in the form of a hollow
sphere or "blastula," the cavity of which is called the "blastocoel" and
afterwards becomes the primary body-cavity of the larva. This cavity
contains an albuminous fluid, at the expense of which development appears
to be carried on (Fig. 282, B). The cells forming the blastula acquire
cilia, and the embryo begins to rotate within the egg-membrane, which it
soon bursts, and, rising to the surface of the sea, begins its larval life.
The blastula is therefore the first well-marked larval stage, and it is
found in a more or less recognisable form in life-histories of members of
every large group in the animal kingdom. Only in the case of Echinodermata
and of forms still lower in the scale, however, does it appear as a larval
stage. The free-swimming blastula stage is reached in from twelve to
twenty-four hours. Soon the spherical form of the blastula is lost; one
side becomes flattened and thickened, owing to a multiplication of cells,
so that they become taller and narrower in shape. Shortly afterwards this
thickened plate becomes buckled inwards, encroaching on the cavity of the
blastocoel. The larva has now reached the second stage of its development;
it has become a "gastrula" (Fig. 282, C). The plate of thickened cells has
become {604}converted into a tube called the "archenteron" (Fig. 282, C,
_arch_), which is the rudiment of both the alimentary canal and the coelom
of the adult. This tube communicates with the exterior, in virtue of its
mode of formation, by a single opening which is called the "blastopore,"
which becomes the anus of the later larva and adult. Whilst the gastrula
stage is being acquired, the blastocoel or primary body-cavity is invaded
by wandering cells budded from the wall of the archenteron (Fig. 282, A, B,
C, _mes_). These cells, which are called "mesenchyme," are the formative
cells of the skeleton, connective tissue, and wandering cells of the adult.
When the larva has a skeleton they are formed very early, arising in the
young blastula stage (Ophiuroidea) or in the stage immediately before the
formation of the archenteron (Echinoidea, Fig. 282, A, B) and secreting the
skeleton. When the larva is devoid of a skeleton (Asteroidea and
Holothuroidea), the mesenchyme usually does not appear till the gastrula is
fully formed.

[Illustration: FIG. 282.—_Echinus esculentus._ A, optical section of living
blastula. B, section of preserved blastula. The network of strings in the
interior is the result of the coagulation of the albuminous fluid. C,
section of gastrula. _arch_, Archenteron; _mes_, mesenchyme cells, attached
by protoplasmic strands to the wall of the embryo. × 150.]

The gastrula stage is reached in twenty to thirty-six hours. Then one side
of the larva becomes concave, and the cilia become restricted to a thick
band surrounding this area. In this way is formed the rudiment of the
_longitudinal band of cilia_, which is the organ of locomotion throughout
the larval life. At the apex of the archenteron a thin-walled vesicle is
formed, which soon becomes divided off from the rest. This vesicle, which
almost immediately divides into two sacs, right and left, is the
{605}rudiment of the "coelom" or secondary body-cavity of the larva; the
remainder of the archenteron forms the definitive gut, and becomes divided
by constriction into an oesophagus, a stomach, and an intestine, and at the
same time bent into a shallower or deeper V-shape, the concavity of which
is towards the concave side of the body. Within this area of the surface a
new funnel-shaped depression makes its appearance. This is the
"stomodaeum," the rudiment of the mouth of the larva, and it soon joins the
apex of the larval oesophagus; the conjoined tubes henceforth bearing the
name oesophagus since the ectodermal and endodermal parts become
indistinguishably fused. Along the sides and floor of the oesophagus is
formed a V-shaped ridge bearing strong cilia; this is the "adoral band of
cilia" which sweeps the food (consisting of Diatoms, Infusoria, etc.) into
the mouth. The larva is now known as a Dipleurula and appears in four
modifications, each characteristic of a Class of Eleutherozoa. These differ
from one another principally in the following points:—(_a_) The folding of
the ciliated band; (_b_) the divisions of the coelomic sacs; (_c_) the
development and fate of the praeoral lobe (_i.e._ the part of the body in
front of the mouth); (_d_) the fate of the larval mouth. The types of
Dipleurula are as follows:—

[Illustration: FIG. 283.—Bipinnaria of _Luidia_. _a_, Anus; _a.b_, adoral
ciliated band; _a.c.o.b_, anterior median process; _a.d.a_, anterior dorsal
process; _a.v.a_, prae-oral process; _m_, mouth; _p.c.o.b_, median dorsal
process; _p.d.a_, posterior dorsal process; _p.l.a_, posterior lateral
process; _p.v.a_, post-oral process. (After Garstang.)]

(1) The _Bipinnaria_, the larva of Asteroidea. In this type there is a very
long prae-oral lobe. The ciliated band runs along its edges, and is
produced into a backwardly directed loop on its under surface. This loop
soon becomes separated from the rest of the band as a distinct prae-oral
loop, the rest forming a post-oral loop. Both loops are drawn {606}out into
short tag-like processes, in which we may distinguish (following
Mortensen's[517] notation) in the prae-oral loop an anterior median process
(Fig. 283, _a.c.o.b_), and a pair of prae-oral processes (_a.v.a_). In the
post-oral loop there is a median dorsal process (_p.c.o.b_) and paired
anterior dorsal (_a.d.a_), posterior dorsal (_p.d.a_), posterior lateral
(_p.l.a_), and post-oral (_p.v.a_) processes. At the apex of the prae-oral
lobe between prae-oral and post-oral ciliated rings there is an ectodermic
thickening, recalling the so-called apical plate of Annelid larvae.

[Illustration: FIG. 284.—A, Ophiopluteus of _Ophiothrix fragilis_. _hy_,
Hydrocoel; _l.p.c_, left posterior coelom; _oes_, oesophagus; _r.p.c_,
right posterior coelom; _st_, stomach. B, metamorphosis of Ophiopluteus of
_Ophiura_ sp. (After Johannes Müller.)]

(2) The _Ophiopluteus_, the larva of the Ophiuroidea. In this type the
prae-oral lobe remains small, and the primitive ciliated band is undivided.
The processes into which it is drawn out are very long, and are supported
by calcareous rods. Of these processes we may distinguish prae-oral,
postero-dorsal, postero-lateral, and post-oral. The postero-lateral are
always much longer than the rest, so that the larva when swimming appears
to the naked eye as a tiny V. In the case of _Ophiothrix fragilis_ (Fig.
284, A) the postero-lateral processes are many times longer than the rest
of the body. The Ophiopluteus was the first Echinoderm larva to be
recognised. It was discovered by Johannes Müller,[518] who also discovered
the other three types of {607}Dipleurula. He named this one _Pluteus_
(easel), from a fancied resemblance, when turned upside down, to a
painter's easel. The same name was bestowed on the next type, to which it
presents a superficial resemblance, and hence the distinguishing prefix
"Ophio-" was added to the original name by Mortensen.

(3) The _Echinopluteus_, the larva of the Echinoidea. This type is
strikingly like the preceding one in possessing a very small prae-oral lobe
and in having the processes of the ciliated ring supported by calcareous
rods, but a close inspection of these shows that they do not exactly
correspond to those of the Ophiopluteus. Thus we have prae-oral,
postero-dorsal, and post-oral processes (Fig. 285), but usually no
postero-lateral process, and when it does occur it remains short. On the
other hand, an antero-lateral process unrepresented in the Ophiopluteus is
constantly present, and in its later stage the Echinopluteus develops, out
of parts of the ciliated ring, horizontally-placed crescentic ridges of
cilia, which are termed ciliated epaulettes (Fig. 285, _a.cil.ep_). There
may even be, as in the larva of _Echinus esculentus_, a second posterior
set of these (Fig. 285, _p.cil.ep_). In the older larva at the apex of the
prae-oral lobe there is an ectodermic thickening, at the base of which are
developed nerve-cells and nerve fibres constituting a larval brain (Fig.
285, _ap_).

[Illustration: FIG. 285.—Dorsal view of larva of _Echinus esculentus._ ×
45. _a.cil.ep_, Anterior ciliated epaulette; _ap_, apical plate or larval
brain; _ech_, rudiment of Sea-urchin; _l.a.c_, left anterior coelom;
_l.oes_, larval oesophagus; _l.p.c_, _r.p.c_, as in Fig. 284; _p.cil.ep_,
posterior ciliated epaulette; _r.a.c_, right anterior coelom.]

(4) The _Auricularia_, the larva of the Holothuroidea. This type strikingly
resembles the Bipinnaria in its external features. The prae-oral lobe is
well developed, and has on its under surface a backwardly projecting loop
of the ciliated band, which is not, {608}however, as in the Bipinnaria,
separated from the rest of the band. The processes of the band are much
more faintly marked than in the Bipinnaria, the anterior median, prae-oral,
and median dorsal processes being absent; but a pair of intermediate dorsal
processes are developed in the interspace between anterior and posterior
dorsal.

[Illustration: FIG. 286.—Three views of metamorphosis of Auricularia of
_Synapta digitata_. A, fully grown _Auricularia_; B and C, stages in the
metamorphosis. _hy_, Hydrocoel; _Int_, intestine; _l.p.c_, left posterior
coelom; _O_, fragments of ciliated band which are invaginated into the
stomodaeum, and coalesce to form a ring round the mouth; _oss_, ossicle;
_pod_, rudiment of feelers which here spring directly from the hydrocoel;
_r.p.c_, right posterior coelom; _st_, stomach; _w.v.r_, rudiment of
water-vascular radial canals; 1-5, corresponding pieces in the three
figures of the longitudinal ciliated band. (After Bury.) × 40.]

In the Bipinnaria, Ophiopluteus, and Echinopluteus the coelomic vesicle,
after separation from the archenteron, divides into right and left halves.
The left then sends out a short dorsal process, which, fusing with the
ectoderm, acquires an opening to the exterior. This opening is the primary
madreporic pore, and the process of the left coelomic sac, which is
ciliated, is the pore-canal. In the Auricularia the pore and pore-canal are
formed _before_ the division of the coelom. In the Bipinnaria the right and
left sacs subsequently fuse in the front part of the prae-oral lobe. In the
first three types of larva the coelomic sac on each side then undergoes a
segmentation into anterior and posterior portions. At the hinder end of the
anterior sac on each side a swelling occurs. That on the left side is the
"hydrocoel," or rudiment of the water-vascular system (Fig. 287, A^3,
_l.hy_); it {609}quickly assumes a crescentic form, and gives off five
blunt outgrowths, which are the rudiments of the radial canals, and the
terminal tentacles. It remains in connexion with the anterior coelom by a
narrow neck, which later becomes the stone-canal. That on the right side
separates completely from the right anterior coelom; it remains small, and
forms the madreporic vesicle (Fig. 287, A^3, _r.hy_) of the adult. In the
Ophiopluteus and in the larva of _Asterina gibbosa_ (_v. infra_) it
occasionally takes on a form similar to that of the hydrocoel; from which
circumstance, as well as from the similarity in its mode of origin, it is
here regarded as a right hydrocoel, _i.e._ a rudimentary fellow of the
organ which develops into the water-vascular system.

[Illustration: FIG. 287.—Diagrams of the mode of formation and division of
the coelom in Echinodermata. _a.c_, Anterior coelom; _coe_, primitive
coelomic rudiment; _int_, intestine; _l.a.c_, left anterior coelom; _l.hy_,
left hydrocoel; _l.p.c_, left posterior coelom; _oes_, oesophagus; _p.c_,
posterior coelom; _r.a.c_, right anterior coelom; _r.hy_, right hydrocoel;
_r.p.c_, right posterior coelom; _st_, stomach; _stom_, stomodaeum.]


In Auricularia (Fig. 287, D) the coelomic vesicle, after the pore-canal is
formed, divides into an anterior and a posterior half. The posterior part
then divides into right and left halves, {610}whilst the anterior sac
divides into dorsal and ventral halves, connected by a narrow neck. The
ventral half soon assumes the familiar feature of the hydrocoel (Fig. 287,
D^3, _l.hy_), whilst the dorsal half forms an insignificant swelling on the
course of the conjoined stone- and pore-canals, which represents the left
anterior coelom of the other types; neither right anterior coelom nor right
hydrocoel being developed. The neck of communication between dorsal and
ventral halves is, of course, the stone-canal.

The Dipleurula larva leads a free-swimming life for a period varying from
two weeks to two months, and then undergoes METAMORPHOSIS into the adult
form. The details of this process have been worked out in comparatively few
cases; and the species in which they are most thoroughly known is the
Asteroid _Asterina gibbosa_. The development of this species is
intermediate in character between the embryonic and larval types. The eggs
are larger than is usual among Asteroidea, and are filled with a bright
orange yolk. The larva differs from the Bipinnaria in the absence of the
characteristic ciliated bands and in the very early occlusion of the anus.
There is, however, a band of cilia round the edge of the prae-oral lobe,
which corresponds to portions of the prae-oral and post-oral bands combined
of the Bipinnaria.[519]

The larva has a form which may be described as boot-shaped (Figs. 288,
289). The sole of the boot is the great prae-oral lobe, behind which is the
mouth. The larva takes little or no food, and completes its metamorphosis
in ten to twelve days. It does not swim at the surface, but creeps slowly
over the bottom by the aid of the ciliated band mentioned above, while it
can also attach itself, using the edges of the prae-oral lobe as a sucker.

After leading an existence of this kind for seven or eight days it fixes
itself permanently by a disc-like prominence, which appears on the anterior
surface of the prae-oral lobe within the area surrounded by the thickened
rim which, as explained above, forms a margin to the prae-oral lobe. The
larva then becomes divided by a constriction into a disc and a stalk, and
the former is gradually converted into the body of the young Starfish,
whilst {611}the latter continually diminishes in size, and eventually
entirely disappears, when the young Starfish commences to walk about on its
podia. The disc becomes bent downwards and to the left, so as to make
nearly a right angle with the stalk, and the last vestige of the latter
springs from the peristome of the Starfish inside the water-vascular ring
(Figs. 289, B, C).

[Illustration: FIG. 288.—Fully grown larval stages of _Asterina gibbosa_.
A, fully grown larva; B, left, and C, right view of a larva seven days old
in the beginning of the metamorphosis. _m_, Mouth; 1-5, the five lobes of
the hydrocoel; I.-V., the rudiments of the arms. (After Ludwig.) × 45.]

The form of the Starfish is attained principally by the preponderant growth
of the left hydrocoel and of the left posterior coelom. Both these sacs
take on the form of hoops, which, by the meeting of their ends, are
converted into rings. The hydrocoel has already grown out into five lobes,
which are the rudiments of the radial water-vascular canals, and the tips
of which become the terminal sensory tentacles (Figs. 288, 289, 1-5); but
now the left posterior coelom grows out into five lobes also, forming a
parallel but outer ring. These lobes (Figs. 288, 289, I.-V.) are the
rudiments of the arms, which are at first quite independent of those of the
radial canals, but gradually, when the larva has attained the age of nine
days (Fig. 289, B, C), they become applied to the outgrowths of the
hydrocoel. These by this time have developed each two pairs of branches,
the rudiments of the first two pairs of tube-feet in each radius. The
larval mouth vanishes, and a new mouth is formed on the left side in the
centre of the hydrocoel ring, when the metamorphosis is complete. The adult
anus is {612}formed about the same time. The primary pore-canal in
_Asterina_ as in all Dipleurulae opens into the anterior coelom; the
stone-canal is formed from a ciliated groove running along the neck of
communication between this and the hydrocoel. The constriction dividing the
body into disc and stalk divides the anterior coelom (single in _Asterina_
as in the older Bipinnaria) into two parts; the portion included in the
disc forms the axial sinus of the adult. The lower end of the axial sinus
expands and surrounds the adult mouth, forming the inner perihaemal ring;
the outer perihaemal ring is formed by the juxtaposition of four
wedge-shaped outgrowths from the left posterior coelom and one from the
anterior coelom. From these the radial perihaemal canals subsequently grow
out into the arms. The metamorphosis of Bipinnaria has been well worked out
by Goto,[520] and it agrees in essential features with that of _Asterina
gibbosa_; in fact, the differences which Goto maintains between the two
types may be reasonably explained on the supposition of some stages having
escaped the notice of this observer. The larva develops on the apex of the
prae-oral lobe three papillae for occasional attachment,[521] and in the
centre of these a cup-shaped disc for permanent fixation when the prae-oral
lobe is converted into a stalk. When these papillae (Fig. 290, _fix_) have
been developed the larva is known as a Brachiolaria.

[Illustration: FIG. 289.—Views of larvae of _Asterina gibbosa_ in the
course of metamorphosis. A, larva of eight days, from the right; B, left,
and C, right view of larva of nine days; 1-5, lobes of hydrocoel; I-V,
rudiments of arms. (After Ludwig.) × 45.]

{613}[Illustration: FIG. 290.—_Brachiolaria_ fixing itself, × 60. _Ast_,
rudiment of the body of the Starfish; _fix_, fixing processes. (After
Johannes Müller.)]

The metamorphoses of the other types of Dipleurula contain no fixed stage.
They are what might be called "cataclysmal metamorphoses." That is to say,
the outer form and habits of the larva are preserved till the last moment,
whilst the organs of the adult are being gradually perfected; then in an
hour or two all trace of larval structures disappears. The Ophiopluteus
preserves the larval mouth, round which the hydrocoel grows; the long
lateral ciliated processes are preserved till the animal has attained all
the adult characters. Before this, however, it passes through what may be
called an "Asteroid" stage in development, in which the ambulacral grooves
are open. The Echinopluteus loses both larval mouth and anus. It develops
the adult organs on the floor of a sac-like invagination of the ectoderm,
situated on the left side within a loop of the ciliated band (Fig. 291, B,
C). This invagination becomes completely closed. It is termed the "amniotic
cavity," and its roof is termed the "amnion." On its floor are developed
the primary tentacles, terminating the radial canals, as well as a number
of spines. After taking on a creeping life and losing its larval
appendages, the young Sea-urchin passes through an "Asteroid" condition, in
which the arched dorsal surface, the future periproct, is greater in extent
than the ventral, and the radial canals run horizontally out from the
water-vascular ring and terminate in free movable podia (Fig. 291, C and D,
_pod_), ending in suckers, in the centre of which are pointed sense-organs.
These {614}podia become later enclosed in grooves in the corona, and are
reduced to vestiges in the adult.

[Illustration: FIG. 291.—Four views of Echinopluteus from the left side, to
show the metamorphosis. A, B, and C are taken from the development of
_Echinus miliaris_. D is a young _Echinus esculentus_. The rudiment of the
oral disc of the _Echinus_ is seen beginning in B and larger in C.
_ad.stom_, Adult stomodaeum; _cil.ad_, adoral ciliated band; _cil.ep_,
ciliated epaulette; _coe_, coelom; _d.sp_, prismatic spine of dorsal
surface (periproct) of adult; _ech_, rudiment of _Echinus_; _int_,
intestine; _l.oes_, larval oesophagus; _mp_, madreporic pore; _nerve circ_,
nerve-ring of adult; _pod_, first paired tube-feet; _st_, stomach; _t_,
terminal tentacle of the radial band; _v.sp_, pointed ventral spine of
adult. A, B, and D magnified 45 diameters; C, 60 diameters.]

The Auricularia is the only type of Dipleurula in which larval mouth and
anus are retained. For this reason it has been supposed that its median
plane of symmetry remains the median plane of the adult. The researches of
Bury[522] have shown that this is not so. As in other types of Dipleurula
(with the possible exception of the Ophiopluteus) the adult position of the
{615}mouth is on the left side of the larva, and in the commencement of the
metamorphosis the mouth migrates into this position (Fig. 286, C). Then the
rudimentary prae-oral lobe is rapidly absorbed, so that the mouth again
acquires a terminal position. The hydrocoel (Fig. 286, A, _hy_) has by this
time completely encircled the oesophagus, and from it grow out the radial
canals which bud off the feelers[523] (buccal tentacles) into the larval
stomodaeum. This, although it later flattens out to form the adult
peristome, forms in these stages an almost closed sac, reminding us of the
amniotic cavity in the Echinopluteus. The ciliated band breaks up into a
number of pieces, which rearrange themselves so as to form a series of
transverse rings of cilia; so that the free-swimming life can be carried on
somewhat longer. The animal in this stage is called a "pupa" (Fig. 292); it
eventually loses the rings, drops to the bottom, and develops tube-feet.
From specimens which the author has seen, he has little doubt that in some
cases the young animal passes through an "Echinoid" stage, for it
possesses, besides the feelers, only median tube-feet, terminating the
radial canals, and it is covered by a cuirass of plates, which recalls the
Echinoid corona.[524]

[Illustration: FIG. 292.—"Pupa" of _Synapta digitata_. × 50. _circ.cil_,
Ciliated rings; _oss_, calcareous ossicle; _ot_, otocysts; _pod_, feelers;
_w.v.r_, radial water-vessel. (After Semon.)]

Reviewing the development of the Eleutherozoa in the light of the facts so
far presented, and using the same method of reasoning which is employed in
the case of other groups of animals, we seem to be justified in concluding
that the Echinodermata are descended from a simple free-swimming ancestor
possessing the fundamental characters of the Dipleurula. These would
include a longitudinal folded band of cilia as the principal organ of
locomotion; a thickened plate of nervous epithelium at {616}the anterior
end serving as combined sense-organ and brain; a V-shaped band of cilia
projecting into the oesophagus as the organ of nutrition; a wide, shallow
stomodaeum and an alimentary canal consisting of three well-marked
divisions, viz. oesophagus, stomach, and intestine; and finally a secondary
body-cavity or coelom, consisting of three divisions on each side, though
possibly the most anterior pair were confluent in the prae-oral lobe. On
the left side the anterior coelom opened to the exterior by a short
ciliated canal. To the hypothetical group so defined which were certainly
not Echinodermata the name PROTOCOELOMATA may be given.

[Illustration: FIG. 293.—_Tornaria_ larva. _a_, Anus; _a.c_, anterior
coelom; _a.p_, apical plate; _g.s_, rudiments of gill-sacs; _m_, mouth;
_m.c_, middle or "collar" coelom; _p_, posterior ciliated band; _p.c_,
posterior coelom; _pr_, longitudinal ciliated band. (After Morgan.)]

Now amongst the lowest types of animal in which traces of Vertebrate
structure can be detected, there is one group, the HEMICHORDATA (Vol. VII.
p. 3), in which there is a larva which strikingly recalls the Dipleurula.
This larval form belongs to Balanoglossus and is called the Tornaria. It
possesses a well-marked prae-oral lobe and a folded longitudinal ciliated
band, which resembles that of Auricularia. Its peculiarity is that in
addition there is a posterior ring of cilia (Fig. 293, _p_). The coelom is
in five divisions:—a median anterior sac (_a.c_) opening to the exterior by
a short ciliated canal on the left side; and paired middle divisions
(_m.c_) and posterior divisions (_p.c_). At the apex of the prae-oral lobe
there is a plate consisting of sensory epithelium, with nerve-fibres at its
base, which acts as a brain. Tornaria undergoes metamorphosis, assumes a
worm-like form, and takes on a burrowing life. The five divisions of
{617}the coelom are retained, and it can be proved that the pore-canal,
like the madreporite of Echinodermata, is used for taking in water.
Further, there are two aberrant sessile members of the group
(_Cephalodiscus_ and _Rhabdopleura_), in which the middle divisions of the
coelom which would correspond to the hydrocoels are produced into long
arms, each with a double row of ciliated tentacles, which strikingly recall
the radial canals and podia of the Pelmatozoa. Taking all these facts into
consideration, it seems probable that Vertebrata and Echinodermata both
arose from PROTOCOELOMATA.

When we turn to the developmental history of Echinodermata for light on the
question as to how the bilaterally symmetrical ancestor became converted
into the radially symmetrical Echinoderm, it seems probable that only in
the development of the Asteroidea can we hope to find the solution of the
problem. The abrupt changes of habits shown in the metamorphoses of the
other types are clearly secondary phenomena. No species of animal could
suddenly change its habits from swimming by means of cilia to walking with
tube-feet. In the development, however, of _Asterina gibbosa_ we get a hint
of the way in which a free-swimming life could alternate with periods of
temporary fixation, gradually passing into a condition in which the
fixation was permanent. This period in the history of the race when
ancestral Echinodermata were sessile would mark the point at which
Eleutherozoa diverged from Pelmatozoa, and the former existence of a fixed
ancestor explains the tendency first to asymmetry and later to radial
symmetry. Bilateral symmetry is characteristic of most free-swimming
animals which have to pursue a straight course through the water, but in
fixed forms no disadvantage arises from want of symmetry. A radial
disposition of organs is, however, valuable to them, since food must be
sought and danger avoided from all points of the compass; and hence we can
understand, when fixation became permanent, how one hydrocoel could grow
larger than the other, and finally assume the form of a ring.

The last question which arises is the vexed one of the mutual relationships
of the various Classes constituting the Phylum. Before attempting to seek
for light on this problem from development, it will be necessary to sketch
the life-history of _Antedon rosacea_, the only Pelmatozoon whose
development is known.

{618}[Illustration: FIG. 294.—Three views of the development of _Antedon
rosacea_. A, free-swimming larva; B, longitudinal section of free-swimming
larva; C, oral view of young fixed form. _a.c_, Anterior coelom; _amb_,
ambulacral groove; _ap_, apical plate of sensory and nervous tissue; _cil_,
ciliated ring; _hy_, hydrocoel; _l.p.c_, left posterior coelom; _mad_,
primary pore-canal; _pod_, podia; _r.p.c_, right posterior coelom; _stom_,
larval stomodaeum. (A and B after Bury; C after Perrier.)]

The eggs are comparatively large and full of food-yolk, and they adhere for
a considerable period to the pinnules. They pass through a large portion of
the development within the egg-membrane. The blastula and gastrula are
formed in the usual way, but the formation of the coelom is most remarkable
(Fig. 287, E^1, E^2). The archenteron divides into anterior and posterior
divisions. The posterior divides into right and left, posterior coelomic
sacs, but before the division is complete a dorsal and a ventral tongue
grow out from the anterior division and unite posteriorly, encircling the
band of connexion between right and left posterior coelomic sacs like a
ring. This band of connexion becomes solid and is absorbed, and _pari
passu_ the ring becomes converted, by the disappearance of its central
opening, into a sac, which is the definitive gut (Fig. 287, E). The rest of
the {619}anterior division divides into a thick-walled sac, the hydrocoel,
on the left, and a median thin-walled anterior coelom, which sends a long
extension into the anterior portion of the larva, which we may compare to
the prae-oral lobe of the Bipinnaria. The anterior coelom communicates with
the exterior by a short pore-canal, and later forms a connexion, the
stone-canal, with the hydrocoel. At the apex of the prae-oral lobe there is
formed a thickened patch of ectoderm, bearing stiff sensory hairs, and
having at their bases nerve-fibres and ganglion cells. This larval brain
corresponds to that of the Tornaria and Echinopluteus. Behind the brain
there is a glandular pit, which is used for fixation, and recalls the
similar organ in the Bipinnaria. A series of ciliated rings is then formed,
and between the second and third of them an oval depression appears. This
is the stomodaeum; but as the larva takes no food it does not communicate
with the gut (Fig. 295, A, _stom_).

The larva next escapes from the egg-membrane and swims freely for a day or
two, and then, like the Bipinnaria, fixes itself by the apex of the
prae-oral lobe, which is converted into a stalk. The larval stomodaeum
closes, and the oesophagus of the adult appears as a solid peg of cells
abutting against it; round this peg the hydrocoel grows like a ring.

The closed stomodaeum and the underlying hydrocoel are now rotated
backwards until they come to be at the end of the animal opposite the stalk
(Fig. 295, C). The left posterior coelom, which has also, as in the
Asteroid larva, assumed a hoop-like form, is carried along with them; but
the right posterior coelom becomes shifted forwards and sends out five
outgrowths into the stalk, which form the rudiments of the chambered organ,
and a central one as a continuation of the genital stolon (Fig. 295, D,
_gen.st_), the extension of the anterior coelom (Fig. 294, B) having
disappeared.

Then the outer wall of the stomodaeum splits into five valves—the future
oral valves. The radial canals appear as freely projecting tentacles, which
issue in the intervals of these valves and soon acquire two pairs of
lateral branches. The skeleton consists of five oral plates in the oral
valves, of a ring of five basals, of three small under-basals, and of a
series of "columnals," _i.e._ stem-ossicles, as rings embracing the stalk.
The area of attachment is supported by a "foot-plate." The radial
{620}plates next appear as a ring of small ossicles between the orals and
basals, and simultaneously the arms make their appearance as five
outgrowths supported by the first radials, and by the other radials when
these appear. The free radial canals now become adherent to the arms, but
these canals soon give off paired branches of unlimited growth, which are
supported by bifurcations of the primitive arms, and in this way the ten
arms of the adult are established. So far, then, as the water-vascular
system is concerned, the apparent forking is not a true dichotomy, but
results from the production of two opposite branches, whilst the main axis
ceases to grow. The appearance of cirri marks the fusion of the uppermost
stem-ossicles to form a centro-dorsal, and shortly afterwards the young
_Antedon_ snaps off its stem and swims away.

[Illustration: FIG. 295.—Four diagrams to explain the metamorphosis of the
larva of _Antedon rosacea_. _a.c_, Anterior coelom; _gen.st_, genital
stolon; _l.hy_, left hydrocoel; _l.pc_, left posterior coelom; _mp_,
madreporic pore; _r.p.c._ right posterior coelom; _stom_, stomodaeum. (A,
B, and C after Korschelt and Heider; D after Perrier.)]

Now in reviewing this life-history we cannot fail to be struck with
resemblances to the development of Asteroidea, and especially to that of
_Asterina gibbosa_. The absence of a connexion between stomodaeum and gut
is due to the embryonic mode of life. On the other hand, the presence of a
long prae-oral lobe, containing an extension of the anterior coelom and
having a fixing organ at its apex, can only be paralleled among Asteroidea.
In broad outlines, then, up to the period of fixation the two developments
are parallel, but after this point a {621}divergence takes place, which
points clearly to the splitting of the Echinoderm stem into two main
branches, corresponding with two different sets of habits. In the
Eleutherozoan stock, represented by the development of the Asteroidea, the
disc became flexed ventrally on the stalk, so that the mouth and podia were
brought within reach of material drifting along the bottom, which the podia
were employed to seize. As a consequence the base of the stalk was brought
near the mouth, and so it came about that the hydrocoel, when it became a
ring, encircled both. In the Pelmatozoan stock, on the other hand, the
podia and mouth are rotated upwards and backwards from the stalk, which
thus came to have an aboral position (Fig. 296, B). The podia are thus
placed in a favourable position for capturing free-swimming organisms,
which their cilia sweep toward them. It is worthy of note that a similar
change of position of the mouth occurs in other groups of animals which
have similar habits (Polyzoa Entoprocta, Tunicata).

[Illustration: FIG. 296—Figures to show the supposed connexion of
Eleutherozoa and Pelmatozoa. A, common fixed ancestor of the two stocks,
still bilaterally symmetrical; B, primitive Pelmatozoon; C, primitive
Eleutherozoon. _a_, Anus; _a.c_, anterior coelom; _l.p.c_, _l.p.c^1_, left
posterior coelom; _m_, mouth; _p_, primary pore-canal; _r.p.c_, right
posterior coelom.]

The division therefore of the phylum must have occurred at an extremely
remote epoch, before the hydrocoel was a closed ring, and before,
therefore, radial symmetry was completely attained.

Turning now to the question of the origin of the classes of Eleutherozoa,
we find that the study of development strongly reinforces the views gained
from the study of adult anatomy. The Asteroidea are the most primitive
group; only in their case {622}is the fixed stage retained, and both
Ophiuroidea and Echinoidea pass through an Asteroid stage in development.
The only serious competitors for the position are the Holothuroidea, which
many have imagined to have been directly derived from Cystoidea (in the old
sense; better Thecoidea). This view, though adopted by Semon,[525]
Haeckel,[526] and Bather,[527] is open to many objections. The type of
Holothuroid development referred to in these discussions is that of the
extremely aberrant _Synapta digitata_, in which the radial canals are
vestigial structures which disappear in the adult. In this species, where
the feelers are multiplied, some originate in the larva directly from the
water-vascular ring, and thus alternate with the canals. From this
circumstance Semon drew the conclusion that the radial canals of
Holothuroidea are not homologous with those of other Echinoderms, but this
conclusion is contradicted by the development of more normal species, in
which all the feelers spring from the radial canals. The meridional course
of these canals, the closure of the ambulacral grooves, coupled with the
retention of a nervous ectoderm, are all features found in Echinoidea. So
is also a reduction in the number of the genital organs, on which Bell[528]
laid such stress that he separated Holothuroidea from all other
Echinoderms. But if in Spatangoidea a reduction to four and even three can
take place (Fig. 246, p. 552), why should a reduction to two or one excite
surprise? The primitive outer appearance of the Auricularia is
counterbalanced by the development of the coelom, which is much modified,
so that the primitive bilateral arrangement is obscured. If, then,
Asteroidea are the most primitive Eleutherozoa, we may imagine that
primitive Echinoidea were derived from Asteroidea through adaptation to
life in crevices, where an upward bending of the radii was of advantage, in
order to enable the animal to attach its podia above as well as below
itself; and that Holothuroidea arose from primitive Echinoidea in which the
plates of the corona were still movable, through a further adaptation to
narrow crevices, where worm-like wriggling would be the most successful
method of adapting themselves to their environment.

{623}The final result, then, of all our inquiries leads us to a view of the
mutual affinities of the classes of Echinoderms, which may be indicated in
the following table:—

            Protocoelomata
                  |
           First fixed ancestor
           /                 \
      Asteroidea        Protopelmatozoa (Thecoidea?)
      /      \                    \
  Ophiuroidea Protechinoidea       \
                /    \           Carpoidea
         Echinoidea Holothuroidea    /  \
                                    /    \
                              Cystoidea   \
                                  /      Crinoidea
                           Blastoidea

We shall hazard the prophecy that if ever pre-Cambrian Echinoderms are
found, there will be amongst them small stalked forms which may be
superficially classed with "Cystids," but which are in reality the fixed
ancestors of Asteroidea. They should have an irregular skeleton, and be
devoid of arms, which are secondary formations; but they should indicate,
by the proximity of the mouth to the stalk and by the relation to the stalk
of the grooves for the podia, that they have diverged from the Pelmatozoan
stock, and are the ancestors of Eleutherozoa.

INDEX

  Every reference is to the page: words in italics are names of genera or
  species; figures in italics indicate that the reference relates to
  systematic position; figures in thick type refer to an illustration; f. =
  and in following page or pages; n. = note.


  Abbott, 422
  Abdomen (= third chamber of Monaxonic Radiolarian shell), 84
  Abdominal cirrhi, 139 f.
  Aboral, blood-ring, of _Asterias rubens_, 452;
    of _Ophiothrix fragilis_, 490;
    of _Echinus esculentus_, 529;
    coelom, of _Antedon rosacea_, 585;
    nervous system, of _Antedon rosacea_, 584;
    sinus, of _Asterias rubens_, 449;
    of _Ophiothrix fragilis_, 490;
    of _Echinus esculentus_, 528, 529
  Abortive nuclei, of Myxosporidiaceae, 107
  _Abyla_, _307_
  Abylinae, _307_
  Abyssal, Radiolaria, 76;
    Sponges, 239 f.
  _Acamptogorgia_, _356_
  _Acanella_, _353_;
    _A. simplex_, 353
  Acantharia (Actipylaea), _76_, 78, _78_, 80;
    myophrisks in, 80;
    _Zooxanthella_ in, 80;
    reproduction of, 86 n.;
    skeleton of, 82
  _Acanthaster_, 458, _462_
  Acanthin, 71 n., 75, 76
  Acanthocephala, nutrition of, 38
  _Acanthoconia barrandei_, _207_, 207
  _Acanthocystis_, _71_;
    budding of, 73;
    habitat, 75
  _Acanthogorgia_, _355_
  Acanthometrids (= Acantharia, _q.v._), parasitic _Amoebophrya_ in, 161
  Acanthonida, _78_
  _Acanthotrochus_, _577_
  _Acarnus_, 223
  Accumulative anabolism, growth, 9, 13, 15 f.;
    in relation to brood-formation, 32 f.
  Acephalinidae, _97_
  _Acineta_, _159_, 160
  Acinetina = Suctoria, 158 f.
  Acontia, 369
  Acrasieae, _90_ f.
  _Acrophytum_, 333, _349_
  Actine, 183
  Actinelida, _78_
  _Actinia_, _381_;
    _A. cari_, 378;
    _A. equina_, sense organ of larva of, 373;
    _A. mesembryanthemum_, 379, 381;
    longevity of, 375
  Actiniaria, _377_ f.;
    viviparity of, 373;
    symbiosis of, 377;
    colours of, 379
  Actiniidae, _381_
  Actiniina, _380_
  _Actinobolus_, _137_;
    tentacles and trichocysts of, 152
  _Actinocrinus_, 594
  _Actinodendron_, _384_;
    _A. arboreum_, 384;
    _A. glomeratum_, 366;
    _A. plumosum_, 366, 384
  _Actinoloba_ (= _Metridium_), 371, 372, _381_;
    _A. dianthus_, 379, 381;
    _A. marginata_, 378
  _Actinolophus_, _70_;
    habitat of, 75
  _Actinometra_, 588, 589, _594_
  _Actinomma_, _77_;
    _A. asteracanthion_, skeleton of, 77
  _Actinomonas_, _70_, 73, 75 n.
  Actinomyxidiaceae, _98_
  _Actinophrys_, _70_ f., 96 n.;
    _A. sol_, 71;
    habitat of, _75_
  Actinopoda, _570_
  _Actinosphaerium_, _70_, 72 f.;
    endogamy of, 73 f., 75;
    regeneration of, 35;
    _A. eichornii_, 72
  _Actinostola_ (usually placed in Hertwig's family Paractidae, allied to
      Bunodidae, _382_), 379
  _Actinotryx_, 371, _383_;
    _A. mussoides_, 383
  Actinozoa = Anthozoa, _326_ f.
  Actipylaea (= Acantharia), _76_
  _Actissa_, _77_, 83
  Acystosporidae, _97_, 102 f.;
    active zygote of, 34
  Adambulacral, ossicle, of _Asterias rubens_, 434;
    of _Ophiothrix fragilis_, 479;
    of Asteroidea compared to covering-plates of Crinoidea, 589;
    spine, of _Asterias rubens_, 434
  _Adamsia_, _377_;
    _A. palliata_, 378, 381;
    _A. rondeletii_, 378
  Adaptation to fresh water, 176 f.
  _Adelea ovata_, syngamy of, 101
  Adelocodonic, 265
  Adhesion of species, 174 n.
  Adinidae, 130 n.
  Adoral, band of cilia in Dipleurula larva, 605;
    membranellae, 145;
    trichocysts, 145;
    wreath, 137 f.;
    wreath of _Vorticella_, 156, 157
  _Aegina_, _296_
  Aeginidae, _296_
  _Aeginopsis_, _296_
  _Aeolis_, nematocysts of, 248
  Aequorea, _278_
  Aequoreidae, _278_
  Aerotaxy, 23
  _Aethalium septicum_ (= _Fuligo varians_), 92 f.
  Afferent canal of contractile vacuole, of _Stylonychia_, 139, 140;
    of _Stentor_, 156
  Africa, Tropical, antimalarial measures in, 106;
    Trypanosomic diseases of, 120 f.
  _Agalma_, 300, _307_
  Agalminae, _307_
  _Agaricia_, _403_
  _Agaricoides_, _350_
  Agassiz, 258, 312, 319;
    on classification of Endocyclica, 533;
    of Clypeastroidea, 548
  Aggregate, formation, in _Microgromia_, 59;
    social, of _Vorticella_, 158;
    (rosette) of _Treponema_, 120
  Aggregation of plasmodia into aethalium, 92 f.
  Agitation, stimulus of, 19 f.
  _Aglaophenia_, 276, _279_
  _Aglaophenopsis_ (Plumulariidae, _279_), 277
  _Aglaura_, _294_
  Aglauridae, _294_
  _Aiptasia couchii_, _382_
  Air, presence of Flagellate spores in dust of, 118
  Air-borne germs in relation to putrefaction, etc., 43
  Alate, 185
  Alcock, 268
  Alcyonacea, _346_ f.
  Alcyonaria, _329_ f., 340;
    colour, 337;
    dimorphism, 333;
    food, 339;
    mesenteric filaments, 333;
    nematocysts, 247;
    phosphorescence, 338;
    reproduction, 340;
    skeleton, 334;
    spicules, 334;
    zooids, 330
  Alcyoniidae, _349_
  _Alcyonium_, 330 f., 332, _349_;
    nematocyst of, 247;
    _A. digitatum_, 332, 338 f., 347, 349;
    larva, 341;
    _A. glomeratum_, 349;
    _A. palmatum_, 340;
    _A. purpureum_, 338
  Aleurone, 37
  Algae, related to holophytic colonial Flagellata, 109, 130
  Algeria, dourine disease in, 119
  _Alicia mirabilis_, _382_
  Aliciidae, _382_
  Alimentary canal, of _Asterias rubens_, 438;
    of _Ophiothrix fragilis_, 485;
    of _Echinus esculentus_, 516;
    of _Echinarachnius parma_, 546 f.;
    of _Echinocardium cordatum_, 551;
    of _Antedon rosacea_, 583;
    of _Hyocrinus_, 589;
    of _Actinometra_, 589;
    development of, in Eleutherozoa, 604, 605;
    in _Antedon rosacea_, 618, 619
  Allen, on food of _Echinus esculentus_ in Plymouth Sound, 516
  Allman, 246, 265, 267, 273, 274;
    on Cystoflagellates, 135
  _Allogromia_, _59_ f., 65
  Allogromidiaceae, _58_;
    habitat of, 48, 59
  _Allopora_, _287_;
    _A. nobilis_, 287
  _Alpheus_, 351
  Alternating modes of brood-formation in Sporozoa, 48
  Alveolar, structure (fine) of cytoplasm, 6;
    system (coarse) in relation to skeleton of Radiolaria, Dreyer's scheme
      of, 84
  Alveolate ectoplasm, of pelagic Foraminifera, 61;
    of Heliozoa, 71 f.;
    of _Actinosphaerium_, 72 f.;
    of Radiolaria, 79;
    pedicellaria of _Leptogonaster_, 456
  Alveole (= minute cavity in cytoplasm), 5 f.;
    in Ciliata, 142;
    in _Stylonychia_, 140;
    (= large vacuole of Radiolaria, etc.), 76, 79, 84
  _Alveolina_, _59_
  Alveolus, of _Echinus esculentus_, 526
  _Alveopora_, _397_
  _Amalthea_ (Fam. Corymorphidae, _273_), 266
  Ambulacral area, of _Echinarachnius parma_, 544;
    of _Echinocardium cordatum_, 550
  Ambulacral groove, of Asteroidea, 432;
    representative in Ophiuroidea, 481;
    representative in Echinoidea, 515;
    of Pelmatozoa, 579;
    of _Antedon rosacea_, 581, 582, 587;
    of fossil Crinoidea, 595;
    of Thecoidea, 596;
    of Carpoidea, 596;
    of young Ophiuroid, 613
  Ambulacral ossicle, of Asteroidea, 432;
    of _Asterias rubens_, 434;
    of _Ophiothrix fragilis_, 481;
    compared to auricula of _Echinus esculentus_, 526;
    to inner plates of _Palaeodiscus_, 557;
    to side-plates of Crinoidea, 589
  Ambulacral plate, of _Echinus esculentus_, 511;
    of Cidaridae, 533;
    of _Sphaerechinus_, 539;
    of _Strongylocentrotus_, 539;
    of _Echinarachnius parma_, 544
  Amines, 15
  _Ammodiscus_, _59_
  Amnion, 613
  Amniotic cavity, 613
  Amoeba, 4 f.;
    reactions of, 7 f.;
    devouring
    a plant cell, 9;
    excretion of, 14 f.;
    motion of, 17 f.;
    respiration of, 17 f.;
    taxies of, 22
  _Amoeba_, _51_;
    habitat of, 57;
    posterior disc or sucker, 53;
    _A. binucleata_, 30, 52;
    _A. coli_, habitat, 57;
    _A. limax_, 5;
    form of amoebulae of Acrasieae, 90;
    motions of, 47 n., 52;
    _A. polypodia_ in fission, 10;
    _A. proteus_, 5;
    brood-divisions of, 56 n.
  Amoeboid, motion, 5;
    stages of Acystosporidae, 97, 103 f.;
    zoospores of _Trichosphaerium_, 54, 56;
    gametes of Flagellata, 116 n.
  _Amoebophrya_, 86, _159_, 161
  Amoebula, 31, 51;
    of Myxomycetes, 91, 92;
    of _Didymium_, 92;
    of fever parasites, 104
  Amphiaster (an aster in which the actines form a whorl at each extremity
      of the axis, which is straight), 222
  Amphibia, hosts of Opalinidae, 111, 123
  Amphiblastula, 226
  Amphicaryoninae, _306_
  Amphidisc, 176, 177 f., 202
  Amphidiscophora, _203_ f.
  _Amphihelia_, _399_
  _Amphileptus_, _137_
  Amphimonadidae, _111_
  _Amphimonas_, _111_
  _Amphinema_, _273_
  _Amphiprion percula_, 378
  _Amphiura_, _497_;
    _A. squamata_ (= _elegans_), 485 n., 497, 601
  Amphiuridae, _497_
  _Amphizonella_, _51_;
    test of, 53
  Amphoriscidae, _192_
  Ampulla, of _Millepora_, 259, 260;
    of Stylasterina, 284, 285;
    of tube-foot of _Asterias rubens_, 441, 443;
    synonym of axial sinus of _Ophiothrix fragilis_, 487;
    of stone-canal of _Echinus esculentus_, 517;
    of tube-foot of _Echinus esculentus_, 517;
    of respiratory trees of _Holothuria nigra_, 563;
    of podia of _Holothuria nigra_, 566;
    of feelers of Aspidochirota, 568, 570;
    of _Pelagothuria_, 568, 570;
    of Molpadiida, 568;
    of Synaptida, 568;
    of Elasipoda, 571
  Anabolic, 12 f.
  Anabolism, 12 f.;
    modes of, 15 f.
  Anal, cirrhi, 139 f.;
    papilla (tube) of _Antedon rosacea_, 581, 583;
    of _Eucalyptocrinus_, 596
  Ananchytidae, _554_, 559
  _Anapta_, _577_
  Anatomy, of a starfish (_Asterias rubens_), 432 f.;
    of _Ophiothrix fragilis_, 479 f.;
    of _Echinus esculentus_, 504 f.;
    of _Echinarachnius parma_, 542 f.;
    of _Echinocardium cordatum_, 549 f.;
    of _Holothuria nigra_, 561 f.;
    of _Antedon rosacea_, 581 f.
  Anatriaene (a triaene of which the cladi or branches point backwards, in
      the same direction as the rhabdome or shaft), 224
  Anchoring flagella, 114;
    of _Dallingeria_, 112;
    of _Bodo saltans_, 117
  _Ancistrum_, _137_
  _Anemonia_, _381_;
    _A. sulcata_, 381
  Animal-feeding Protista, 38
  Animals, defined, 39 f.;
    and plants, discussion on, 35 f.;
    Higher, 31, 38
  Anisochela (a chela of which the two ends are unequally developed), 222,
      234
  _Anisonema, 110_
  Anisospores of Radiolaria, 76, 85;
    of _Collozoum inerme_, 76
  _Anochanus_, _554_
  _Anopheles_, intermediate host of malarial parasite, 103 f.;
    enemies of, 106;
    precautions against, 106
  _Antedon_, _594_;
    _A. rosacea_, 581, 582;
    external features, 581;
    skeleton, 582;
    alimentary canal, 583;
    water-vascular system, 583;
    nervous system, 583 f.;
    coelom, 585;
    genital organs, 586;
    muscles, 587;
    blood-system, 587;
    development of, 617 f.;
    _A. eschrichtii_, 594
  _Antennularia_, _279_;
    _A. antennina_, 279;
    _A. ramosa_, 279
  Anterior, dorsal process, of ciliated band of Bipinnaria, 606;
    of Auricularia, 608;
    median process, of ciliated band of Bipinnaria, 606
  Antero-lateral process, of ciliated band of Echinopluteus, 607
  _Anthea cereus_, _381_
  Antheneidae, _471_
  Anthocaulus, 389
  Anthocodia, 330
  Anthocyathus, 388, 389
  _Anthomastus_, 333, _349_
  Anthomedusae, _262_ f.
  _Anthophysa_ (Flagellata), _111_, 112 f.;
    (Siphonophora, Physophorinae), _308_, 302
  Anthoptilidae, _362_
  _Anthoptilum grandiflorum_, _362_
  Anthozoa, _326_ f.;
    commercial importance, 328
  Antipatharia, _407_ f.
  _Antipathella_, _408_;
    _A. gracilis_, 408
  _Antipathes_, _408_;
    _A. ternatensis_, 409
  Antipathidea, 367, 371, _407_ f.
  Antiseptic properties of "aromatic" compounds, 36 n.
  Anuncinataria, _203_
  Anus, of Ciliates, 143 f.;
    of _Stylonychia_, 139 f.;
    of _Carchesium_, 147;
    of _Vorticella_, 156;
    of _Asterias rubens_, 434;
    of _Echinus esculentus_, 516;
    of _Echinarachnius parma_, 546;
    of _Pygastrides relictus_, 548;
    of Euclypeastroidea, 549;
    of _Echinolampas_, 554;
    of _Neolampas_, 554;
    of _Holothuria nigra_, 560;
    of _Antedon rosacea_, 582;
    of Dipleurula larva, 604;
    of _Asterina gibbosa_, 611
  Aphodal, 210
  Aphrothoraca, _70_
  Aplanospore, 31
  _Aplysilla_, 196
  _Aplysina_, 225
  Apocyte, 30, 32
  Apocytial, condition, 32;
    forms among Myxosporidiaceae, 107 f.;
    Rhizopoda, 52
  _Apolemia_, _308_
  Apoleminae, _307_
  Apopyle, 170
  Aporosa, _397_
  _Appendicularia_, host of _Gymnodinium pulvisculus_, 132
  Aquatic organisms, minute, distribution of, 47 n.
  _Arachnactis_, 373, _411_;
    _A. albida_, 411;
    _A. americana_, 411;
    _A. lloydii_, 411
  _Arachnoides_, _549_
  _Arbacia_, 520, _538_
  Arbaciidae, 530, 531, 532, _538_, 558
  Arbacioid type of ambulacral plate, 531, 538
  Arcadomyaria, _324_
  _Arcella_, _51_, 53;
    _A. vulgaris_, 55
  Archaeocidaridae, _557_, 558
  Archaeocyte, 171
  _Archaster_, _467_;
    _A. bifrons_, 467
  Archasteridae, 456, _466_
  Archenteron, definition of, 604
  Archer, on Protozoa, 45;
    on Heliozoa, 71
  Archicoel, 450
  _Arcuothrix_, _52_;
    transition between pseudopodium and flagellum in, 47 n.
  Arenaceous Foraminifera, _58_ f.;
    Carpenter on, 63 f.;
    labyrinthine structure in, 66
  _Argas persica_, 121
  _Aristocystis_, _599_, 599
  Aristotle, 166
  Aristotle's lantern, of _Echinus esculentus_, 515, 516, 524, 525;
    variations of, in Endocyclica, 531;
    of _Echinarachnius parma_, 546, 547;
    absent in _Echinocardium cordatum_, 550
  Arm, of Asteroidea, 431, 432, 453;
    of _Ophiothrix fragilis_, 479;
    of Ophiuroidea, 481, 491;
    of Crinoidea, 580, 589;
    of _Antedon rosacea_, 581;
    of _Antedon_ (other species), 594;
    of _Hyocrinus_, 590;
    of _Rhizocrinus_, 591;
    of _Bathycrinus_, 591;
    of Pentacrinidae, 592;
    of _Holopus_, 594;
    of _Eudiocrinus_, 594;
    of Inadunata, 595;
    of Articulata, 595;
    of Camerata, 595;
    development of, in _Asterina gibbosa_, 611;
    in _Antedon rosacea_, 620
  Arm-spines, of _Ophiothrix fragilis_, 479;
    of Ophiuroidea, 491;
    of _Ophiothrix_, 492;
    of _Ophiacantha_, 492;
    of _Ophiopteron_ 492
  Aromatic compounds in relation to nutrition and antisepsis, 36
  Arthropods, hosts of Gregarines, 97 f.
  Articulata, _595_
  Ascitic dropsy, _Leydenia_ associated with, 91
  Ascon, 185
  _Asconema setubalense_, 221
  Asexual reproduction of Asteroidea, 459;
    of _Linckia_, 459;
    of _Asterina wega_, 459
  Ashworth, 331 n.
  Asiphonacea, _347_
  Asphyxia, its effect on contractile vacuole, 143
  Aspidochirota, 568, 569, _570_ f., 577, 578
  _Aspidosoma_, _476_
  Aspirotrichaceae, _137_, 148, 151, 153, 154
  Assimilation, assimilative anabolism, growth, 9, 13, 15 f.
  Association, in Gregarines, 95, 98 f.;
    in _Lankesteria ascidiae_, 95 f.
  _Astasia_, _110_, 112
  Aster (= centrosome of mitotically dividing cell and peripheral rays),
      25, 27;
    (a polyaxonid spicule), 184
  _Asterias_, _473_, 475;
    _A. rubens_, 432;
    external features, 432;
    pedicellariae, 433;
    skeleton, 434;
    coelom, 437;
    alimentary canal, 438;
    food, 439;
    water-vascular system, 441;
    nervous system, 444;
    perihaemal spaces, 448;
    blood-system, 449;
    genital organs, 451;
    _A. glacialis_, 473;
    pedicellariae, 434;
    _A. hispida_, 474;
    _A. muelleri_, 473;
    _A. murrayi_, 473;
    _A. ochracea_, 474;
    _A. polaris_, 474;
    _A. spirabilis_, 601, 602;
    _A. tenuispina_, 453
  Asteriidae, 453, 458, _473_
  _Asterina_, 454, 456, 459, 461, _463_;
    _A. gibbosa_, number of arms, 453;
    eggs of, 463;
    development of, 609, 610 f., 617;
    _A. wega_, 459
  Asterinidae, 455, 458, _463_
  Asternata, _554_
  Asteroid stage in the development of Ophiuroidea and Echinoidea, 613, 622
  Asteroidea, _430_, _431_ f.;
    compared with Ophiuroidea, 477 f.;
    compared with Echinoidea, 503, 558;
    mesenchyme of larva of, 602;
    development of, 605, 608, 609, 610 f.;
    phylogeny of, 621
  _Asteropsis_, _471_
  _Asterosmilia_, _401_
  _Asthenosoma_, _536_;
    _A. hystrix_, 536, 537;
    _A. urens_, _536_
  Astraeidae, 387, _399_
  _Astraeopora_, 390
  _Astrangia_, _400_;
    _A. solitaria_, 374, 400
  _Astrocnida_, _501
  Astrogonium_, _472_
  _Astroides_, _404_
  _Astronyx_, _501_
  _Astropecten_, 455, 459, _467_;
    fossil, 475;
    _A. irregularis_, _468_;
    movements, 468;
    burrowing habits, 469
  Astropectinidae, 454, 458, 459, _467_, 470
  _Astrophyton_, 491, _501_
  Astropyle, 81
  _Astrorhiza_, _59_
  Astrorhizidaceae, _59_
  _Astroschema_, _501_;
    vertebra, 481
  Astroschemidae, _501_
  _Astrosclera willeyana_, _194_, 194
  Astroscleridae, _194_
  _Astylus_, _287_
  _Atelecrinus_, 588, _594_
  _Athoria_ (usually placed in a subfamily Athoriinae of the Physonectidae,
      _307_), 300
  _Atolla_, _322_;
    _A. bairdi_, 322;
    _A. gigantea_, 322;
    _A. valdiviae_, 322
  Atollidae, _322_
  Atoll, 390 f.
  _Atorella_, 314, _322_
  Atrophy, of oral apparatus of Ciliata during conjugation, 151
  Attached, Foraminifera, 64;
    Flagellata, 112 f.;
    Ciliata, 152;
    Suctoria, 160 f., 162
  Attachment, temporary, of _Stentor_, 155;
    permanent, of _Rhizocrinus_, 591;
    of _Bathycrinus_, 591;
    of young Pentacrinidae, 592;
    of Thecoidea, 596;
    of Cystoidea, 597;
  — temporary, of larva of _sterina gibbosa_, 610;
    of Brachiolaria larva, 612;
    of larva of _Antedon rosacea_, 619
  _Aulactinium_, _79_;
    _A. actinastrum_, 82
  _Aulena_, _220_
  _Aulocystis grayi_, _208_
  _Aurelia_, 310, 314, _324_;
    _A. aurita_, 312
  _Aureliania heterocera_, _383_
  Auricula, of _Echinus esculentus_, 526;
    of Cidaridae, 531;
    of Arbaciidae, 531;
    of _Echinarachnius parma_, 546;
    represented by radial pieces of calcareous ring of Holothuroidea, 566
  Auricularia, 607;
    metamorphosis of, 614, 615
  Auronectidae, 301, _308_
  Aurophore, 308
  Autodermin, 523
  Automatic processes, so-called, 12
  Autotrophic, 37
  Autozooids, 333
  Axial filament, of Heliozoan pseudopodia, 49, 71, 72, 74;
    of _Actinophrys sol_, 71;
    of _Actinosphaerium eichornii_, 72;
    of Radiolaria Acantharia, 80
  Axial sinus, of _Asterias rubens_, 448;
    of _Ophiothrix fragilis_, 487;
    of _Echinus esculentus_, 517, 528;
    of _Echinocardium cordatum_, 552;
    of Holothuroid larva, 564;
    of larva of _Antedon rosacea_, 583;
    development of, in _Asterina gibbosa_, 609
  Axifera, _353_
  _Axinella_, _216_, 222, 225;
    _A. erecta_, 216
  Axinellidae, _217_
  Axon, 444
  _Axoniderma_, _216_
  _Axopora_, _262_
  Azoosporeae, _89_

  _Babesia_ (= _Piroplasma_), 120
  Bacteria, food of Myxomycetes, 92 f.;
    nutrition of, 36
  _Bactronella_, _193_
  Baker, H., on organisms of putrefaction, 43;
    on Protozoa, 45;
    on _Noctiluca_, 134 f.
  Balanoglossus, larva and affinities of, 616
  _Balantidium_, _137_;
    habitat of, 152
  Balbiani, on Protozoa, 45;
    on regeneration, 35 n.;
    on "Pébrine" (_Nosema bombycis_), 107
  Barbados, Miocene deposit of Radiolaria, 87
  Barotaxy, 20
  Barrier reef, 390 f.
  Bary, A. de, on methods of culture of lower organisms, 44;
    on nature of Myxomycetes, 91
  Basal granules, of cilia, etc., 138 n., 141
  —see also Blepharoplast
  Basal plates, of _Antedon rosacea_, 584;
    of Crinoidea, 588;
    _of Hyocrinus_, 588, 590;
    of _Atelecrinus_, 588, 594;
    of Rhizocrinidae, 588, 590;
    of _Thaumatocrinus_, 588, 594;
    of Pentacrinidae, 591;
    of _Holopus_, 592;
    of Cystoidea, 598;
    of Blastoidea, 599
  Bastian, H. Charlton, on spontaneous generation, 44 n.
  Bateson, 174
  Bath sponge, 221
  Bather, on _Sympterura_, 502;
    on classification of Crinoidea, 589;
    on phylogeny of Echinodermata, 622
  _Bathyactis_, _404_
  _Bathyanthus_, _411_
  _Bathycrinus_, 589, _591_
  _Bathypathes_, _408_
  _Bebryce_, _356_
  Bell, F. J., 406 n.;
    on classification of Ophiuroidea, 494;
    of Endocyclica, 533
    on relationships of Holothuroidea, 430 n., 622
  Bell-animalcules, 155 f.;
    feeding of, 145
  —see also _Carchesium_, _Epistylis_, Peritrichaceae, _Vorticella_,
      _Zoothamnium_
  Beloidea, 77
  Beneden, E. van, on Sporozoa, 94;
    on pelagic Anthozoa, 411
  Bernard, F., 241 n.
  Bernard, H. M., 386 n., 397
  _Beroe_, 413, _423_;
    _B. cucumis_, 416, 417;
    _B. ovata_, 416, 423
  Beroidae, 414, _423_
  Berthold, on protoplasmic movements, 16 n.
  Bezzenberger, list of species of _Opalina_, 124 n.;
    on parasitic Ciliata, 152 n.
  _Bicoeca_, _111_
  Bicoecidae, _111_
  Bidder, 168, 172 n., 186 n., 235 n.
  _Biemma_, 224
  Bigelow, 304, 420 n.
  Bile, 13
  _Biloculina_, _59_, 67
  Biloculine, 66
  Binary sex (= syngamy with marked inequality between the pairing cells),
      33 f.;
    in _Centropyxis_, 57;
    in _Stylorhynchus_, 99;
    in _Pterocephalus_, 99;
    in Coccidiaceae, 100 f.;
    in _Sarcocystis tenella_, 108 n.;
    in Volvocidae, 127 f.;
    in _Eudorina_, 129;
    in Peritrichaceae, 151 f.
  _Biomyxa_, _58_
  Bionomics of Protistic life, 43
  Bipinnaria, 605 f., 608;
    metamorphoses of, 612
  Bipocillus (a microsclere consisting of a curved shaft, terminated by a
      cup-shaped expansion at each end, characteristic of the genus
      _Iophon_), 222
  Birds, hosts of Acystosporidae, 103
  Birth-pore, of brood-cavity of Suctoria, 161
  Bisexual differentiation—see Binary sex
  Bivium, of _Echinarachnius parma_, 543;
    of _Holothuria nigra_, 561
  Black Corals (= _Gerardia_, 406, and Antipatharia, 407)
  Bladder, of Rotifers and Platyhelminthes, 14 n.
  Blanchard, on Sporozoa, 94
  Blastocoel, 603
  Blastoidea, _580_, 599 f.
  Blastomere, 603
  Blastostyles, 265
  Blasts of Coccidiaceae, morphology of, 120;
    of Acystosporidae, 104 f.
  Blastula, definition of, 603
  Blepharoplast, 19;
    of Heliozoa, 72;
    of Flagellata, 109, 115;
    of _Trypanosoma_, 121;
    of nuclear origin in _Trypanosoma_, 109 n.;
    of _T. noctuae_, 120 f.;
    of Ciliata, 141
  Block-musculature, of spines of _Echinus esculentus_, 506
  Blood, Acystosporidae and Haemosporidae parasitic in, 97, 102 f.;
    Protomastigaceae parasitic in, in fever, 119 f.
  Blood-corpuscle, entered by Haemosporidae and Acystosporidae, 102 f., 104
      f.;
    by _Treponema_ and _Trypanosoma_, 120
  Blood-system, of _Asterias rubens_, 449 f.;
    of _Echinus esculentus_, 529;
    of _Holothuria nigra_, 567;
    of _Antedon rosacea_, 587
  Bloody rain, 125
  Blowflies, alleged spontaneous generation of, 42
  _Bodo_, _111_, 119;
    _B. caudatus_, 116 n.;
    _B. saltans_, 114, 116 n., 117 f.
  Bodonidae, _111_;
    movements of, 114 n.
  Body-cavity—see Coelom
  _Bohemura_, 502
  _Bolina_, _419_;
    _B. infundibulum_, 416
  Bolinidae, _419_
  _Bolinopsis_, _419_
  _Bolocera tuediae_, _381_
  Borchgrevink, 310 n.
  Borgert, on Dictyochidae, 87;
    on fission in _Aulacantha_, 85;
    on phaeodium in Radiolaria, 81 n.
  Botanists, contributors to knowledge of Flagellates, 119
  _Bothriocidaris_, _557_, 561
  Botryoidea, _79_
  _Bougainvillia_, 263, 264, 266, _269_;
    _B. ramosa_, 269
  Bougainvilliidae, _269_
  Boulenger, 294 n.
  Bourne, G. C., 246, 337, 338 n., 347
  Bourrelet, 553
  Boveri, on functions of chromatin, 28 n.
  Bowerbank, 168, 169, 234 n.
  Brachial ossicle, of _Antedon rosacea_, 582;
    of fossil Crinoidea, 595
  Brachiolaria, 612, 613
  _Brachionus_, often found with _Euglena viridis_, 124
  Brachiopoda, 428, 579
  Brachyenemic, 405
  Brachyeneminae, _405_
  Brady, classification of Foraminifera, 58 f.
  Brain-Coral, 401
  Branched, endoplasm, of _Noctiluca_, _Trachelius_, and _Loxodes_, 133,
      153;
    meganucleus, of _Dendrosoma_, 160;
    theca, of _Dinobryon_, 112;
    of _Schizotricha_, 152
  Branching colony, of _Carchesium_, _Epistylis_, _Zoothamnium_, 158
  Brandt, Karl, on commensals and parasites of Radiolaria, 87 n.;
    on Radiolaria, 88
  Breeding temperature of Protozoa, 47
  Breeze Flies, intermediate hosts of _Trypanosoma evansii_, 119
  Briareidae, _350_
  _Brisinga_, _474_
  Brisingidae, 453, 454, _474_
  Brissidae, _556_
  _Brissopsis_, _556_;
    _B. lyrifera_, 556
  Brittle Star—see Ophiuroidea
  Brood-cavity, in Suctoria, 160 f., 162
  Brood-cell, 31
  Brood-cyst, of Proteomyxa, 88 f.
  Brood-division, brood-formation, 30 f.;
    in Rhizopoda, 56 f.;
    in Foraminifera, 67 f.;
    in Radiolaria, 85 f.;
    in Proteomyxa, 49, 88 f.;
    in Sporozoa, 48, 94 f.;
    in Flagellata, 109, 115, 117 f.;
    in Polytomeae, 115;
    in Chlamydomonadidae, 115;
    in _Paramoeba eilhardii_, 116 n.;
    in Choanoflagellates, 122;
    in _Proterospongia_, 122;
    in Volvocidae, 127 f.;
    in Ciliata, 147;
    in _Colpoda cucullus_, 147, 153;
    of male Peritrichaceae, 151 f., 157;
    retarded, 31 f.
  Brood-mother-cell, 31
  Brood-pouch, of _Pteraster_, 465, 466;
    of _Hemiaster philippi_, 556, 602;
    of _Anochanus_, 554;
    of _Cucumaria laevigata_, 602;
    of _Psolus ephippifer_, 602
  Brooks, 288, 290, 308 n.
  Brown colour of lakes or ponds often due to Dinoflagellates, 131
  Browne, 273, 281 n., 291 n., 312
  Bruce, Col. and Mrs., on Trypanosomic fever and sleeping sickness, 120
  Bubbles of carbon dioxide formed in _Arcella_, etc., 53
  Buccal sinus of _Holothuria nigra_, 566
  Buccal tentacle—see Buccal tube-foot
  Buccal tube-foot (and tentacle), of _Ophiothrix fragilis_, 486;
    of _Echinus esculentus_, 518;
    of _Echinocardium cordatum_, 551, 561;
    of _Holothuria nigra_, 561;
    of Spatangoidea, 577;
    development of, in Holothuroid pupa, 615
  Budding, in _Trichosphaerium_, 54;
    in Rhizopoda, 56;
    in _Acanthocystis_, 73;
    in Acantharia, 86 n.;
    in _Spirochona_, 147;
    in Suctoria, 160, 162;
    in Sponges, 177 f., 228, 229, 230;
    in Hydrozoa, 250 f., 263, 275;
    in Medusae, 270, 296;
    in Scyphistoma, 317;
    in Alcyonaria, 340;
    in Zoantharia, 371
  Bud-fission in testaceous Rhizopoda, 55;
    in _Euglypha_, 29
  Buffon, on organisms of putrefaction, 43
  _Bulimina_, _59_
  Bulk and surface of organism, ratio between, 14
  _Bunodactis gemmacea_, 378
  _Bunodeopsis_, _382_
  _Bunodes_, 378, _382_;
    _B. gemmacea_, 382
  Bunodidae, _382_
  Burrowing habits, of certain Asteroidea, 461;
    of _Astropecten_, 469;
    of Paxillosa, 469;
    of Porcellanasteridae, 471;
    of _Strongylocentrotus lividus_, 541;
    of _Echinarachnius parma_, 546;
    of _Echinocardium cordatum_, 549;
    of _Spatangus purpureus_, 555;
    of Molpadiida, 575;
    of Synaptida, 576;
    of _Synapta inhaerens_, 577
  Burrowing Urchins, 529
  _Bursaria_, _137_, 155
  Bury, on primary axial sinus of Holothuroid larva, 564;
    on change in position of mouth during metamorphosis of Auricularia, 614
  Bütschli, on protoplasm, 16 n.;
    on paramylum, 95 n.;
    on Protozoa, 46;
    on Cystoflagellates, 135;
    on classification of Ciliata, 137;
    on _Strombidium_ and _Torquatella_, 155 n.;
    on Sponges, 168, 172 n.

  Caecum (diverticulum), of alimentary canal of _Echinocardium cordatum_,
      551;
    of alimentary canal of Elasipoda, 569, 571
  _Caenomorpha_, _137_, 141 n., 154;
    _C. uniserialis_, 155
  Cake-urchins = Clypeastroidea, _q.v._
  Calcaire Grossier, 70
  Calcarea, _184_ f.
  Calcareous ooze, 114
  Calcareous ring, of _Holothuria nigra_, 566;
    of _H. cinerascens_, 567;
    of _Phyllophorus rugosus_, 567
  _Calcaromma calcarea_, 83
  _Calcituba_, _59_;
    growth of, 64;
    pylomes of, 64
  Calices of Madreporaria, 371
  Calicoblasts, 385
  Calkins, on nucleus in Protozoa, 25 n.;
    on Protozoa, 46;
    on rhythm in life-cycle of Ciliata, 148 n.
  _Calliactis_ (family Sagartiidae, _381_);
    _C. parasitica_, 378
  _Callianira_, _418_
  Callianiridae, _417_
  Calthrop, 184;
    of Radiolaria, 83
  Calycophorae, 300 f. _305_
  Calymma, 79, 82
  _Calymma_, _420_
  Calymmidae, _420_
  Calyptoblastea, _275_ f.
  Calyx, of _Echinus esculentus_, 513;
    of _Echinarachnius parma_, 545;
    of Pelmatozoa, 579;
    of Crinoidea, 580, 588 f.;
    of Carpoidea, 580;
    of Cystoidea, 580, 598;
    of Blastoidea, 580, 599;
    of _Holopus_, 592;
    of fossil Crinoidea, 595
  Camerata, _595_
  _Campanularia_, _280_
  Campanulariidae, _280_
  _Campascus_, _52_;
    test of, 55
  _Camptolithus_, _346_
  _Camptonema_, _70_, 73
  Canalaria, 201
  Canals, "feeding," afferent, or replenishing of contractile vacuole
      system in Ciliata, 14, 143, 146;
    of _Stylonychia_, 139 f.;
    of _Stentor_, 156
  _Cannopora_, _283_
  Cannopylaea (= Phaeodaria), _76_
  Cannotidae, _278_
  Capillitium of Myxomycetes, 90 f., 92
  _Capnea sanguinea_, _383_
  _Capria_, _321
  Caravella_, _308_
  Carbohydrates, formation of, 33
  Carbon dioxide, attracts _Paramecium_, 23;
    excretion of, 8, 13 f.;
    secreted by _Arcella_, etc., 53
  _Carchesium_, _138_;
    feeding of, 145, 158
  Carinal ossicle of Asteroidea, 436
  Carlgren, 378 n.
  _Carmarina_, _295_
  Carpenter, P. H., on the classification of the Crinoidea, 589
  Carpenter, W. B., classification of Foraminifera, 58;
    on their true nature, 62;
    on their structure, 63 f.;
    on Arenacea, 65 f.;
    on the nervous system of _Antedon rosacea_, 585
  Carpoidea, 580, _596_ f.
  Carter, on Protozoa, 45;
     on Sponges, 167, 180, 208, 237 n.;
     on fossil Hydrozoa, 270 n.
  _Caryophyllia_, 386, _398_
  Cash, on Rhizopoda, 58 n.
  Cassidulidae, _554_
  _Cassidulina_, _59_
  _Cassiopea_, _324_
  Cassiopeidae, _324_
  Castellani, on Trypanosomic fever and sleeping sickness, 120
  Catabolic, catabolism, 13 f., 24
  Cataclysmal metamorphosis of Dipleurula, 613
  Catallacta, _89_
  Catostylidae, _325_
  Cattle, Trypanosomic diseases of, 119 f.
  Caudal cirrhi, 139 f.
  _Caudina_, _575_
  Caullery and Mesnil, on Actinomyxidiaceae, 98 n.
  _Cavernularia_, 359, _364_;
    _C. obesa_, 364
  Cell, 3 f.;
    definition of, 3;
    nutrition of, 15 f., 35 f.;
    -membrane of ovum of Sea-urchin, 7;
    -wall, 3;
    in Flagellates, 109, 113;
    in Dinoflagellates, 130;
    -boundary in Flagellates, 113;
    -division, 24 f., 25, 27;
    Spencerian division, 31 f.;
    -unions in _Volvox_, 126, 127;
    collar-, of Choanoflagellates, 121, 122;
    of Sponges (= choanocytes), 171, 176, 186
  Cellular relationship explained, 10
  Cellulose, 37;
    cell-wall of holophytic Flagellates, 113;
    in Dinoflagellates, 130
  Central blood plexus—see Heart
  Central capsule, 49, 76, 77, 82, 84;
    its functions in regeneration, 35;
    of _Collozoum inerme_, 76
  Centrifugal force, stimulus of, 19 f.
  Centriole, 25, 27
  Centripetal canals, 289
  Centro-dorsal ossicle, of Crinoidea, 580;
    of _Antedon rosacea_, 582;
    of _Actinometra_, 588, 594;
    of _Atelecrinus_, 594
  Centrogenous (used of spicules = meeting in a common centre and growing
      outwards), 76
  _Centropyxis_, _51_;
    test of, 55;
    _C. aculeata_, reproduction of, 57
  Centrosome, 19, 26 f.;
    of Heliozoa, 72;
    (= blepharoplast) in Flagellates, 115
  Centrosphere, 25 f., 27
  _Centrostephanus_, 522, _539_;
    _C. longispinosus_, 522, 532, 539
  Cephalis (= uppermost chamber of monaxonic Radiolarian shells), 83
  _Cephalodiscus_, 617
  Cephalont of Gregarines, 98
  Cephalopoda, erroneous reference of Foraminifera to, 62
  _Cephea_, _325_
  Cepheidae, _324_
  _Ceratella_, 263, _271_;
    _C. fusca_, 271
  Ceratellidae, _271_
  _Ceratium_, _110_;
    habitat of, 131
  Ceratosa, _211_, _220_
  _Cercomonas_, 116 n., 119;
    _C. dujardini_, gametes of, 116 n.
  _Cereactis_ (family Actiniidae, _381_);
    _C. aurantiaca_, 378
  Cerianthidea, 367, 373, 377, _409_
  _Cerianthus_, 328, 366, _409_;
    nematocyst of, 247;
    _C. americanus_, 411;
    _C. bathymetricus_, 411;
    _C. lloydii_, 411;
    _C. membranaceus_, 370, 410, _411_;
    _C. oligopodus_, 411;
    _C. vogti_, 411
  Cestidae, _420_
  Cestoidea, 413, 414, 416, _420_
  _Cestus_, _420_;
    _C. pectenalis_, 420;
    _C. veneris_, 417, 420
  Chaetetidae, _346_
  Chalarothoraca, _71_
  _Chalina_, _217_, 223
  Chalk, Foraminifera, etc., in, 69 f.
  Challengeridae (a family of Phaeogromia, _79_);
    shells, skeleton of, 84, 85
  Chambered organ, of _Antedon rosacea_, 584;
    of Pentacrinidae, 592
  Chambers, of Foraminiferal shell, 62
  Chapman, on Foraminifera, 58 n., 70
  _Charistephane_, _417_
  _Charybdea_, 311, 314, _319_;
    _C. xaymacana_, 310, 319;
    _C. marsupialis_, 319;
    _C. grandis_, 319
  Charybdeidae, _318_
  _Cheilostomella_, _59_
  Cheilostomellaceae, _59_
  Chela (a complex microsclere derived from a sigma and consisting of a
      curved shaft bearing recurved processes), 234
  Chemical, reactions, of protoplasm and of vacuoles, 13;
    substances in solution, 19, 22 f.;
    rays of spectrum in relation to plant pigments, 36 n.
  Chemiotaxy, 23;
    its rôle in syngamy, 34;
    of Coccidians, 100
  Chirodropidae, _319
  Chirodropus_, _319_
  _Chironephthya_, _349_;
    _C. variabilis_, 338
  _Chiropsalmus_, _319_
  Chitin, 37
  Chlamydomonadidae, _111_, 125, 126;
    brood-division of active, 115
  _Chlamydomonas_, _111_, 125 f.;
    barotaxy of, 20;
    conjugation of, 115 f.;
    Dill on, 119 n.
  Chlamydophora, _71_
  _Chlamydophrys_, _52_;
    _C. stercorea_, reproduction of, 57;
    habitat of, 57 f.
  _Chloramoeba_, _110_
  Chloromonadaceae, _110_;
    trichocysts in, 113 n.
  Chlorophyll, 36 n.;
    in Flagellates, 115 n.;
    bodies of Euglenaceae, 124 f.
  Chloroplasts (= chlorophyll bodies), of _Eutreptia viridis_, 124 f.
  Choanocytes, 171, 176, 186, 200, 237
  —see also Collar-cells
  Choanoflagellata,  Choanoflagellates (= Craspedomonadidae, _111_), 121,
      122 f.;
    in relation to Sponges, 41, 171, 181
  _Choanophrya_, _159_ f., 162;
    _C. infundibulifera_, 162
  Choanosome, 170
  _Chondrilla_, spicules of, 233
  _Chondrioderma_, _90_;
    _C. diffusum_, 93
  _Chondrocladia_, _216_
  Chondrophoridae, 301, _308_
  Chone, 213, 214
  Choristida, _212_
  Chromatin, 6 f.;
    function of, in cell-division, 24 f.;
    of ovum of Sea-urchin, 7;
    of Radiolaria, 81;
    -granules, 7, 24
  Chromatophore, 13, 21, 36 f., 113, 115;
    of _Sphaerella_, 126
  —see also Chromoplastid, Chlorophyll, Plastid
  Chromidia, 30;
    of Rhizopoda, 51;
    of Foraminifera, 67 f.
  Chromoplastid, 21, 36 f.;
    of _Zooxanthella_, 86
  —see also Chromatophore
  Chromosomes, 25 f., 27;
    functions of, 28 f.
  _Chromulina_, _110_
  _Chrysamoeba_, _110_
  _Chrysaora_, 312, 315, 316, _323_;
    _C. isosceles_, 311, 314, 323
  _Chrysogorgia_, _355_
  Chrysogorgiidae, _355_ (= Dasygorgiidae, 333)
  _Chrysomitra_, 302, _309_
  Chrysomonadaceae, _110_;
    external plasmic layer of, 113;
    symbiotic, 86, 125
  Chun, 197 n., 300, 307 n., 308, 414 n.
  _Chunella_, 360, _363_
  Chytridieae, movements and affinities of, 114 n.;
    relations of, 40, 48, 119
  Cidaridae, 530, 531, 532, _533_, 558
  _Cidaris_, 533, _534_;
    _C. (Dorocidaris) papillata_, 534
  Cienkowsky, on Monadineae (= Flagellates and Proteomyxa), 40, 89;
    on Radiolaria, 88;
    on _Zooxanthella_, 86;
    on Cystoflagellates, 135
  Cilia, 17, 18;
    of Protozoa, 47;
    paroral, 156 n.;
    preoral, 139;
    of Trichonymphidae, 123;
    of _Opalina_, 123;
    of _Maupasia_, 124;
    of Ciliata, 141;
    organs formed of combined, 138, 141, 413;
    sensory, of _Stylonychia_, 138;
    Schuberg, A., on, 141 n.
  Ciliary motion, 18;
    mechanism of, 18 n.
  Ciliata, 18, 41, _137_ f., 181;
    animal nutrition, 40;
    conjugation, 149 f.;
    contractile vacuole, 14 f., 143;
    encystment, 147 f.;
    feeding, 145;
    fission, 147 f.;
    form of body, 141;
    galvanotaxy, 22;
    infested by Suctorian parasites, 160 f.;
    gut, 146;
    mouth, 145;
    nuclear apparatus, 144 f.;
    parasitic, 152;
    pharynx, 145;
    pellicle, 141;
    regeneration, 35, 145;
    relations to Metazoa, 41;
    rheotaxy, 21;
    Suctoria allied to, 159;
    thigmotaxy of, 20;
    tubicolous, 152;
    _Zooxanthella_ symbiotic with, 125
  Ciliated, buds of Suctoria, 159, 160 f., 162;
    epaulette, 607
  Cilioflagellata (= Dinoflagellata, given by misinterpretation of
      transverse flagellum), 130.
  _Ciliophrys_, _75_ n., _89_
  Cilium of _Noctiluca_, 133
  _Cinachyra_, _212_, 215;
    _C. barbata_, 212
  Cinclides, 369
  _Cinetochilum_, _137_
  _Ciocalypta_, 225
  _Cirripathes_, _408_;
    _C. spiralis_, 408
  Cirrus, of Crinoidea, 430, 580;
    of _Antedon rosacea_, 581, 585;
    of Rhizocrinidae, 588, 590;
    of Pentacrinidae, 588, 591, 592;
    of _Rhizocrinus_, 591;
    of Comatulidae, 594;
    of _Actinometra_, 594;
    of _Antedon_, 594;
    development of, in _A. rosacea_, 620;
    of fossil Crinoidea, 595
  _Cladocarpus_, _279_
  _Cladocora_, 373, 400
  _Cladocoryne_, _272_
  Cladocrinoidea, 595
  _Cladonema_, 266, _270_;
    _C. radiatum_, 267
  Cladonemidae, _270_
  _Cladopathes_, _408_
  Cladophiurae, 491, _494_, 502
  _Cladorhiza_, _216_
  Cladotyle (a rhabdus on which one actine is branched, the other tylote or
      knobbed at the extremity), 222
  Claparède and Lachmann on Protozoa, 45;
    on Suctoria, 162
  Clark—see James-Clark
  Classification, of Protozoa, 48 f., 50;
    of Rhizopoda, 51 f.;
    of Foraminifera, 58 f.;
    of Heliozoa, 70 f.;
    of Radiolaria, 76 f.;
    of Proteomyxa, 90;
    of Sporozoa, 97;
    of Flagellata, 109 f.;
    of Protomastigaceae, 111;
    of Volvocaceae, 111;
    of Infusoria, 136;
    of Ciliata, 137;
    of Suctoria, 159;
    of Sponges, 183 f.;
    of Coelenterata, 249 f.;
    of Ctenophora, 417 f.;
    of Eleutherozoa, 430 f.;
    of Asteroidea, 459 f.;
    of Ophiuroidea, 491 f.;
    of Echinoidea, 529 f.;
    of Endocyclica, 532;
    of Clypeastroidea, 548 f.;
    of Spatangoidea, 552;
    of Holothuroidea, 567 f.;
    of Pelmatozoa, 580;
    of Crinoidea, 589 f.
  _Clathria_, 225
  _Clathrina_, _186_, 221, 231;
    _C. blanca_, larva of, 227
  Clathrinidae, _185_ f.
  _Clathrissa_, 223
  _Clathrozoon_, 277, _279_;
    _C. wilsoni_, 279
  _Clathrulina_, _71_, 73, 74
  _Clava_, _272_;
    _C. squamata_, 263
  _Clavatella_, 267, _270_
  Clavatellidae, _270_
  Clavidae, _272_
  _Clavularia_, 330, 334, _344_;
    _C. viridis_, 329, 337, 343 f., 344
  Clavulariidae, _344_
  Clearing of tissues, physical explanation of, 11
  _Climacograptus_, _282_
  Cliona, 219, 224
  Clionidae, _218_
  Cloaca of _Holothuria nigra_, 563
  _Clypeaster_, 548, _549_
  Clypeastridae (= Echinanthidae), _549_
  Clypeastroidea, 529, _542_ f., 556, 559, 566
  _Clytia_, _280_;
    _C. johnstoni_, 275, 280 f.
  Cnidoblast, 247, 248
  Cnidocil, 248
  Cnidopod, 248
  Cnidosac, 300
  Coalescence of individual Rhizopods during bud-fission, 55
  Coccidiaceae, _97_, 99 f.;
    relations to _Trypanosoma_, 120
  Coccidiidae, _97_, 99 f., 101
  Coccidiosis, 102
  _Coccidium_, _99_ f., 101 f.;
    _C. cuniculi_, 102;
    _C. lacazei_, syngamy of, 101;
    _C. schubergi_, 99 f., 101
  _Coccolithophora_, _110_
  Coccolithophoridae, in Chalk, 70;
    wall of, 114
  Coccoliths, 83, 110, 114, 242
  Coccoseridae, _346_
  Coccospheres, 83, 114
  Cockroach, _Lophomonas_ parasitic in gut of, 123
  _Codaster_, _599_
  _Codosiga_, _111_
  Coelenterata, 243 f.;
    definition, 245;
    almost all immune from Gregarines, 99
  Coeliac canal of _Antedon rosacea_, 586
  _Coelogorgia_, _349_
  Coelogorgiidae, _349_
  Coelom (including body-cavity), 428;
    of _Asterias rubens_, 437;
    of arm of _Ophiothrix fragilis_, 480;
    of _Echinus esculentus_, 516;
    of _Holothuria nigra_, 562;
    of _Antedon rosacea_, 585;
    development of first rudiment in larva, 605;
    subsequent development in Dipleurula, 608, 609;
    in _Asterina gibbosa_, 611;
    in _Antedon rosacea_, 618, 619
  Coelomic nervous system, of _Asterias rubens_, 448;
    of _Ophiothrix fragilis_, 488;
    of _Echinus esculentus_, 524;
    of _Holothuria nigra_, 566;
    of _Antedon rosacea_, 584, 585
  _Coeloplana_, 412, _422_;
    _C. mitsukurii_, 422
  Coeloplanidae, _422_
  Coenocyte, 30
  _Coenograptus_, _282_
  _Coenopsammia_, _404_
  Coenosteum, 371, 387
  Coenothecalia, _344_
  Cohn, Ferdinand, on cultures of Schizomycetes, etc., 44
  Cold-blooded Vertebrates, as hosts of Haemosporidae, 102
  _Coleps_, _137_;
    mail-like pellicle of, 141, 152;
    _C. hirtus_, group feeding, 150
  Collar, of Choanoflagellates, 121 f., 122;
    of peristome of _Vorticella_, etc., 156
  Collar-cells, in Choanoflagellates, 121 f., 122, 171, 237;
    of Calcarea, 186;
    of Non-Calcarea, 176, 200
  —see also Choanocytes
  Collencyte, 171
  Colletocystophores, 320
  Collida, 77 n.
  Colloblasts, 414
  Collodaria, 77
  Colloidea, 77
  _Collosphaera_, _77_;
    symbiotic Diatoms in, 86
  Collosphaeridae, 85
  Collozoidae, 85
  _Collozoum_, _77_;
    _C. inerme_, 76
  Collyritidae, _559_
  _Colobocentrotus_, 532, _542_
  Colonial, cells, 31;
    Protista, 31
  Colony, 31;
    of _Collozoum inerme_, 76;
    -formation in Polycyttarian Radiolaria, 84 f.;
    in Flagellata, 113;
    of Choanoflagellates, 121, 122;
    in Vorticellidae, 158;
    of Volvocidae, 126 f.;
    of _Pandorina_, 128 f.;
    of _Eudorina_, 129
  Colour, red, of lakes and ponds, often due to Dinoflagellata, 131
  Coloured vegetal nutrition, 36 f.
  Colouring matter of chromatophores of Flagellates, 115 n.
  _Colpidium_, _137_;
    _C. colpoda_, diagram of conjugation, 149;
    nuclear relations in conjugation, 151
  _Colpoda_, _137_;
    _C. cucullus_, 153;
    brood-fission in cyst, 147
  Columella, 370, 385
  Columnals, 619 (= Stem-ossicles, _q.v._)
  _Columnaria_, _344_
  _Comatula_ = _Antedon_, _q.v._
  Comatulidae, _594_
  Combs of Ctenophora, 141, 412
  Comitalia, 201
  Commensals, of Heliozoa, 73;
    of Radiolaria, 80, 86 f.;
    of Infusoria, 153 f.;
  —see also _Zoochlorella_, _Zooxanthella_, and Symbiosis
  Comminator muscles of Aristotle's lantern, 526
  Commissure of radial cords of aboral nervous system of _Antedon rosacea_,
      585
  Compasses (or radii) of Aristotle's lantern, 526
  Conant, 319
  Conaria larva, 302
  _Conchophtheirus_, _137_
  Conchula, 380
  Confervaceae, related to green Flagellates, 48
  Confervoid form of _Hydrurus_, 113
  Conjugatae, syngamy of, compared to certain Chlamydomonads, 126
  Conjugation, 33 f.;
    of Rhizopoda, 54, 56 f.;
    of _Trichosphaerium_, 54, 56 f.;
    exogamous, in Foraminifera, 68 f.;
    of Heliozoa, 72, 73 f.;
    of Sporozoa, 95 f.;
    of _Lankesteria_, 95 f.;
    of _Monocystis_, 96;
    of Gregarines, 97, 100;
    of _Stylorhynchus_, 99;
    bisexual, of _Sarcocystis tenella_, 108 n.;
    of Flagellates, 115;
    of _Bodo saltans_, 117;
    of _Trypanosoma_, 120;
    by a fertilising tube in _Chlamydomonas_, 125;
    of Volvocaceae, 127 f.;
    of _Volvox_, 127 f.;
    isogamous and endogamous, of _Stephanosphaera_, 128;
    in Dinoflagellates, 131 n.;
    of _Noctiluca_, 133;
    of Ciliata, 148 f.;
    of _Paramecium caudatum_, 148;
    of _Colpidium colpoda_, diagram, 149;
    of Peritrichaceae, 151 f., 157;
    of _Vorticella_, 157;
    of Suctoria, 161;
    of meganucleus in _Dendrocometes_, 161, 162
  —see also Syngamy, Fertilisation
  _Conoclypeus_, _558_
  Constancy of type in Protista, 42 f.
  Conte, 292 n.
  Contractile vacuole, 5, 10, 14 f.;
    of _Amoeba polypodia_, 5, 10;
    of fresh-water and brackish Protozoa, accessory spaces and canals, 47;
    of Rhizopods, 52;
    of fresh-water Allogromidiaceae, 60;
    of _Microgromia socialis_, 60;
    of Heliozoa, 71, 72, 74;
    of zoospore of _Clathrulina_, 74;
    of Myxomycetes, 92;
    of Flagellata, 110, 112, 115;
    of _Cryptomonas_, 112;
    of _Diplomita_, 112;
    of _Oikomonas_, 112;
    of _Tetramitus_, 112;
    of _Trachelomonas_, 112;
    of _Bodo saltans_, 117;
    of Choanoflagellates, 122;
    absent from Opalinidae, 123;
    of Euglenaceae, 125;
    of _Volvox_, 126;
    of Ciliata, 143 f.;
    in fission, 147;
    of _Stylonychia_, 139 f.
    of _Stentor_, 156;
    of _Vorticella_, 157;
    of Suctoria, 160 f., 162
  Contractility, 8, 9;
    muscular mechanism of, 14 f.
  Contraction, of Amoeboid cell, 16 f.
  Copepoda, infested by _Epistylis_, 158
  —see also _Cyclops_
  _Coppinia_, _280_;
    _C. arcta_, 280
  Coprolites, Radiolaria in, 87
  _Copromyxa_, _90_
  Coral, 326, 365;
    Organ-pipe, 343;
    Precious (= Red), 326, 352;
    Flexible (= various Alcyonaria), 326;
    Stony (= Madreporaria), 326, 384 f.;
    Brain-, 401;
    Black (= _Gerardia_, 406, and Antipatharia, 407);
    -Reefs, 390 f.;
    Reef-, 389 f.
  Coralliidae, 335, _352_;
    commercial importance, 328
  Corallimorphidae, _383_
  _Corallimorphus_, _383_
  _Corallium_, 333, 350, _352_;
    _C. boshuensis_ 352;
    _C. confusum_, 352;
    _C. elatius_, 352;
    _C. inutile_, 352;
    _C. japonicum_, 352;
    _C. johnsoni_, 352;
    _C. konojoi_, 352;
    _C. nobile_, 340 n., 341, 352;
    _C. pusillum_, 352;
    _C. reginae_, 352;
    _C. stylasteroides_ 352;
    _C. sulcatum_, 352
  Corbula, 276
  _Cordylophora_, 269, _272_
  Cormidia, 301, 305
  _Cornularia_, 334, _344_
  Cornulariidae, _344_
  _Cornuspira_, _59_;
    shell of, 64
  Corona, of _Echinus esculentus_, 504, 511;
    of Endocyclica, 530;
    of Cidaridae, 530;
    of Echinothuriidae, 530, 535;
    of Temnopleurinae, 539
  _Coronaster_, _474_
  Coronata, 314, _321_
  Cortex, 190, 191, 213;
    gastral cortex, 188
  Corticata, 49 n.
  _Corydendrium_ (family Tiaridae, _273_);
    _C. parasiticum_, 269
  _Corymorpha_, 263, 265, 266, _273_;
    _C. nutans_, 273
  Corymorphidae, _273_
  _Corynactis_, 372, _383_;
    _C. viridis_, 383
  _Coryne_, _272_
  Corynidae, _272_
  _Cosmiolithus_, _346_
  Costae, 385, 387;
    of Ctenophora, 413, 416 n.
  _Costia_, _111_;
    _C. necatrix_, produces epidemics in fresh-water fish, 119
  _Cothurnia_, _138_, 158
  Cotte, 218 n.
  Cotton-spinner, 564
  _Cotylorhiza_, _325_
  Coupled cell, 31, 33 f.;
    in Flagellates, 116 f.
  —see also Zygote
  Covering-plates, of arms of Ophiuroidea, 491;
    of arms of Crinoidea, 589;
    of _Hyocrinus_, 589, 590;
    of Rhizocrinidae, 589, 591;
    of Pentacrinidae, 589;
    of _Antedon_, 589, 594;
    of Thecoidea, 596;
    of Blastoidea, 599
  _Crambessa_, _325_
  _Crambione_, _325_
  _Craniella_, _213_, 213, 214;
    _C. cranium_, 222
  Craspedomonadidae, _111_, 115 n., 121 f., 122;
    transverse division in, 115 n.
  —see also Choanoflagellata
  Crescent (gametocyte of _Laverania_), 104 f.
  Cretaceous firestone of Delitzet contains _Peridinium_, 132
  _Cribrella_, 457, _462_;
    _C. (Henricia) sanguinolenta_, 462, 463;
    _C. laeviuscula_, 462
  Cribriform organs, 470
  Cricket, Mole-, _Lophomonas_ parasitic in gut of, 123
  Crinoidea, 430, _580_ f.;
    development of, 617 f.
  Crinorhiza, 212, 216
  _Cristellaria_, _59_
  _Crotalocrinus_, 595;
    _C. pulcher_, 595
  Crustacea, small, rheotaxy of, 21
  _Cryptabacia_, _404_
  Cryptogams, Higher, spermatozoa of, 38
  _Cryptoglena_, shell of, 113
  _Cryptohelia_, 284, _287_;
    _C. ramosa_, 285
  Cryptomonadaceae, _110_
  _Cryptomonas_, _110_
  Cryptozonate, 454
  Crystals, in isospores of _Collozoum inerme_, 76;
    proteid, 37
  _Ctenocella_, _357_
  _Ctenodiscus_, 458, _471_
  Ctenophora, _412_ f.;
    comb-plates of, 141
  Ctenophoral plates, 141, 412
  _Ctenoplana_, 416, _421_
  Ctenoplanidae, _421_
  Cubomedusae, 310, 316, _318_ f.
  _Cucumaria_, _573_;
    _C. crocea_, 573, 602;
    _C. laevigata_, 602
  Cuénot, on Sporozoa, 94;
    on reproduction of _Monocystis_, 96 n.
  _Culcita_, 453, _472_;
    _C. tetragona_, 453
  _Culex_, host of _Haemoproteus_ or _Proteosoma_, 103;
    intermediate host of a _Trypanosoma_, 120
  Cultures, pure, 43
  Cunanthidae, _296_
  _Cunarcha_, 296
  _Cunina_, _296_;
    _C. proboscidea_, 296;
    _C. rhododactyla_, 296
  _Cunoctantha_, _296_;
    _C. octonaria_, 295
  Cup (= theca), of Flagellates, 113;
    of _Salpingoeca_, 122;
    of _Acineta_, 159, 160
  —see also Theca, Tube
  _Cupulita_, _307_;
    _C. sarsii_, 304
  Current, 169, 171, 234 f.;
    electric, stimulus of, 19, 22;
    in liquid, relation of protoplasmic movements to, 7, 19, 21
  Cuticle, of Dinoflagellata, 130;
    of Gregarines, 96;
    of _Noctiluca_, 133
  —see also Membrane, Pellicle
  Cuticular shell of Flagellates, 113
  Cuvier, 245, 246
  Cuvierian organs of _Holothuria nigra_, 564
  _Cyanaea_, 311, 312, _324_;
    _C. capillata_, 311, 324;
    _C. lamarcki_, 324
  Cyanaeidae, _324_
  Cyathaxoniidae, _394_
  Cyatholiths, 114
  Cyathophyllidae, _394_
  _Cyathophyllum_, _394_
  Cycads, spermatozoa of, 38
  _Cyclidium_, _137_
  Cyclocnemaria, _397_
  Cyclomyaria, _325_
  _Cyclops_, host of _Choanophrya_, 159;
    of _Rhyncheta_ and other Suctoria, 159 f., 162;
    of Vorticellidæ, 158
  Cycloseridae, _404_
  Cydippidea, _417_
  Cydippiform stage of Lobata and Cestoidea, 414
  _Cydonium milleri_, 222
  _Cymbonectes_, _306_
  Cymbonectinae, _306_
  _Cyphoderia_, _52_
  Cyrtoidea, _79_
  Cyst (a closed membrane distinct from the cytoplasm around a resting-cell
      or apocyte), 37, 39;
    cellulose-, 37;
    chitinous, 37;
    growth of vegetal cell in, 37;
    of Protozoa present in dust, 47;
    of _Centropyxis aculeata_, 57;
    of _Chlamydophrys stercorea_, 57;
    of _Amoeba coli_, 57;
    of _Actinophrys sol_, 72;
    of _Actinosphaerium_, 73 f.;
    of Flagellata, 109, 117 f.;
    brood-, of _Paramoeba eilhardii_, 116 n.;
    of _Bodo saltans_, 117;
    of _Opalina_, 123 f.;
    of Volvocaceae, 128;
    of Dinoflagellates, 131;
    of _Pyrocystis_, 131, 132;
    of Ciliata, 147;
    of _Colpoda cucullus_, 147, 153;
    temporary (hypnocyst) of Rhizopoda, 57;
    of Proteomyxa, 88;
    of Myxomycetes, 91;
    -wall, of Acystosporidae, 104 f.
  _Cystiactis_, _382_
  Cystid—see Cystoidea
  Cystiphyllidae, _394_
  Cystoflagellata, _110_, 132 f.
  Cystoidea, 580, _597_ f.
  Cytogamy, 33 f.
  Cytoplasm, 6;
    of ovum of Sea-urchin, 7;
    granular, nutritive, of muscle cell, 19;
    in cell-division by mitosis, 26 f.;
    during syngamy, 34

  _Dactylometra_, 311, _323_;
    _D. lactea_, 312
  Dactylopores, 257
  Dactylozooids, 264;
    of _Hydractinia_, 264;
    of _Millepora_, 259;
    of Siphonophora, 299
  Dale, on chemiotaxy, 22
  Dallinger, W. H., and Drysdale, C., on Protozoa, 44, 45;
    on organisms of putrefaction, 44, 116 f.;
    on life-histories of Flagellates, 116 f.
  _Dallingeria_, 111, 112, 119;
    anchoring flagella of, 114;
    _D. drysdali_, gametes of, 116 n.
  Dalyell, 317 n., 375
  Dangeard, on brood-division in active Chlamydomonadidae, 115;
    on Flagellata, 119 n.
  Dantec, Le, on protoplasmic movements, 16 n.;
    on peptic digestion in Protozoa, 16
  Darwin, 328 n., 360, 391
  _Darwinella_, _221_
  Dasygorgiidae, 333 (= Chrysogorgiidae, _355_)
  Davenport on protoplasmic movements, 16 n., 19 n.
  Dawydoff, 423
  Dead men's fingers (= _Alcyonium digitatum_, 349)
  Death, 11;
    by diffluence, granular disintegration or solution, 14 f.;
    by desolution, 15;
    necessary, of colonial cells of _Volvox_, 128;
    in _Volvox_ and in Metazoa, compared, 130
  Deep-sea deposits (Foraminifera), 70
  Degen, on functions of contractile vacuole, 15 n.
  Degeneration, senile, among Ciliata, 148
  Deglutition in _Podophrya trold_, 159
  —see also Ingestion of food
  _Deiopea_, _419_
  Deiopeidae, _419_
  Delage, on protoplasm, 3 n.;
    on syngamy, 34 n.;
    on motion of flagella, 114 n.;
    on Sponges, 168, 174, 226;
    and Hérouard, on Protozoa, 46
  Delap, M. J., 311 n.
  Deltoid plate of Blastoidea, 599
  Demospongiae, 195, _209_ f.
  _Dendoryx_, 224
  Dendrite, 444
  _Dendrobrachia_, _409_
  Dendrobrachiidae, _409_
  Dendroceratina, 209, _220_, 221
  Dendrochirota, 568, 569, _572_, 577, 578
  _Dendrocometes_, _159_, 160, 161 f.
  Dendrograptidae, _281_
  _Dendrograptus_, _281_
  _Dendrophyllia_, _404_
  Dendy, 188 n., 192, 274, 275
  _Depastrella_, _321_
  Depastridae, 320, _321_
  _Depastrum cyathiforme_, _321_
  Depressor muscles of compasses of _Echinus esculentus_, 527
  _Dercitus bucklandi_, 221
  Dermal, gill—see Papula;
    membrane, 170
  Dermalia, 201
  _Dermasterias_, _471_
  Desma (the megasclere which forms the characteristic skeletal network of
      the Lithistida, an irregular branched spicule), 215, 224
  _Desmacella_, 224
  _Desmacidon_, 222
  _Desmophyes_, _307_
  Desmophyinae, _307_
  Desmothoraca, _71_
  Desolution of protoplasm, 11 f.
  Deutomerite, 98
  Development, of  Sponges, 226;
    of Scyphozoa, 316;
    of Alcyonaria, 341;
    of Zoantharia, 373;
    of Echinodermata, 601 f.
  Dextrin, 15
  Diadematidae, 531, 532, _538_ f., 558
  Diadematoid type of ambulacral plate, 531, 539
  Dialytinae, _192_
  Diancistra (a spicule resembling a stout sigma, but the inner margin of
      both hook and shaft thins out to a knife edge and is notched), 222
  _Diaphorodon_, _59_;
    shell of, 60
  _Diaseris_, _404_;
    asexual reproduction, 388
  Diatomaceae, skeleton of, 84;
    symbiotic with _Collosphaera_, 86
  Diatomin, 86;
    (?) in coloured Flagellates, 115 n.
  Dichoporita, _598_, 599
  _Dicoryne_, 268, _270_
  Dictyoceratina, _220_
  _Dictyocha_, _110_
  Dictyochida (= Silicoflagellata, _110_), 79;
    in Phaeocystina, 86 f.
  _Dictyocystis_, _137_;
    test of, 152
  Dictyonalia, 201
  _Dictyonema_, _281_
  Dictyonina, 202
  _Dictyostelium_, _90_
  Dicyclica, 594
  _Dicymba_, _308_
  Dicystidae, _97_
  _Didinium_, _137_;
    trichocysts of, 143
  _Didymium_, _90_;
    _D. difforme_, 92
  _Didymograptus_, _282_
  Diffluence, 14 f.
  _Difflugia_, _52_;
    _D. pyriformis_, 55;
    test of, 55
  Digestion, 9, 15 f.;
    of reserves in brood-formation, 33;
    in _Carchesium_, 147;
    in Starfish, 440
  Digestive system—see Alimentary Canal
  _Dileptus_, _137_, 152 n.
  Dill, on _Chlamydomonas_, etc., 119 n.
  _Dimorpha_, _70_, 73, 75 n., 112
  Dimorphism of chambered Foraminifera, 67 f.
  _Dinamoeba_, _51_;
    test of, 53
  _Dinenympha_, _111_, 115;
    undulating membranes of, 123
  _Dinobryon_, _110_, 112
  Dinoflagellata, _110_, 113, 130, 131, 132;
    plastids of, 40;
    nutrition of, 113
  Dinoflagellate condition of young _Noctiluca_, 134
  _Diphyes_, 303, _307_
  Diphyidae, _306_
  Diphyopsinae, _307_
  Diplacanthid, 457
  Dipleurula, definition, 605;
    forms of, 605-608
  _Diplocyathus_, 277
  Diplodal, 210
  _Diplodemia_, 223
  _Diploëpora_, _346_
  _Diplograptus_, 281, _282_
  _Diplomita_, _111_
  Diplopore, 597, 599
  Diploporita, _598_, 599
  Diprionidae, _282_
  Directives, 367
  Disc, of Vorticellidae, 155, 158;
    of _Ophiothrix fragilis_, 484
  _Discalia_, _309_
  Discohexaster, 200
  Discoidea, _77_
  _Discoidea_, _558_
  Discomedusae (= Ephyropsidae, 322 + Atollidae, 322 + Discophora, 323)
  _Discomorpha_, _137_
  Discooctaster, 200
  Discophora, 310, 316, _323_ f.
  _Discorbina_, _59_, 63;
    reproduction of, 69
  Discosomatidae, _383_
  Diseases, produced by Coccidiidae, 102;
    by Acystosporidae, 103 f.;
    by Flagellates, 119 f.;
    by Trypanosomes, 119 f.;
    Protozoic organisms of, 43 f.
  Dissepiments, 385, 387
  Dissogony, 419
  _Distichopora_, 284, _286_
  Distomatidae, _110_
  Distribution of Protozoa, 47;
    of Sponges, in space, 239 f.;
    in time, 241
  _Disyringa dissimilis_, 209, 214, 215
  Diverticulum—see Caecum
  Division, binary, 10;
    reduction-, 75 n.
  Dixon and Hartog on pepsin in _Pelomyxa_, 16
  Dobie, 167
  Doederlein, 193 n.
  Doflein, 46;
    on parasitic and morbitic Protozoa, 94 n.;
    on syngamy of Cystoflagellates, 135
  Dohrn, on carnivorous habits of _Sphaerechinus_, 516
  Dolichosporidia (= Sarcosporidiaceae), _98_, 108
  _Doramasia, 306_;
    _D. picta_, 303
  _Dorataspis_, _78_;
    skeleton, 80
  _Dorocidaris_—see _Cidaris_
  Dorsal elastic ligament of _Antedon rosacea_, 587
  Dorso-central plate of _Echinarachnius parma_, 543
  Dourine, disease of horses and dogs, 119
  _Drepanidium_ (= _Lankesterella_), _97_, 102
  Dreyer, on genera and species of Radiolaria, 87 f.;
    on skeleton of Radiolaria, 82 n.
  Dropsy, ascitic, associated with _Leydenia_, 91
  Drysdale and Dallinger, on organisms of putrefaction, 44 f., 116 f.
  Dual force of dividing cell, 26 f.
  Duboscq, Léger and, sexual process in _Sarcocystis tenella_, 108 n.
  Duerden, 261, 369 n., 371, 373, 374, 389, 397 n., 400 n., 403, 405, 406
  Dujardin, on sarcode (= protoplasm), 3;
    on Protozoa, 45;
    on true nature of Foraminifera, 62 f.;
    on Sponges, 167
  Dust, containing cysts of Protozoa, 47;
    of Flagellata, 118
  Dysentery, in Swiss cattle, caused by _Coccidium_, 102;
    tropical, caused by _Amoeba coli_, 57
  _Dysteria_, _137_, 153;
    oral apparatus, 145;
    shell, 141

  Earthworm, _Monocystis_ parasitic in, 95
  Echinanthidae, _549_
  _Echinarachnius_, 548, _549_;
    _E. parma_, 542 f., 543, 544, 545, 547;
    shape, 542;
    sphaeridia, 545;
    internal skeleton, 545;
    habits, 546;
    alimentary canal, 546;
    Aristotle's lantern, 546;
    tube-feet, 547
  _Echinaster_, 439, _462_
  Echinasteridae, 455, 458, _462_
  Echinating, 217
  Echinidae, _539_, 558
  Echininae, _539_
  _Echinocardium_, _549_;
    _E. cordatum_, 549 f., 551, 552;
    habitat, 549;
    shape, 550;
    spines, 550;
    sphaeridia, 551;
    alimentary canal, 551;
    tube-feet, 551;
    habits, 552;
    stone-canal, 552;
    _E. flavescens_, 555;
    _E. pennatifidum_, 555
  Echinoconidae, _558_
  _Echinocyamus_, 548, _549_;
    _E. pusillus_, 549
  _Echinocystites_, 557
  Echinodermata, 425 f.
  Echinoid stage in the development of a Holothuroid, 615
  Echinoidea, 431, _503_ f.;
    compared with Holothuroidea, 560;
    with Blastoidea, 580;
    mesenchyme of larva, 604;
    development of, 607, 608, 609, 613, 614;
    phylogeny of, 622
  _Echinolampas_, _554_
  _Echinometra_, _542_
  _Echinomuricea_, _356_
  _Echinoneus_, _553_, 553
  Echinonidae, _553_
  Echinopluteus, 607, 608;
    metamorphosis of, 613, 614
  _Echinosphaerites_, 598;
    _E. aurantium_, 598
  Echinothuriidae, 530, 531, 532, _535_, 558, 560
  _Echinus_, 533, _539_;
    _E. esculentus_, 504 f. 505, 507, 511-515;
    locality, 504;
    spines, 506;
    pedicellariae, 506;
    corona, 511;
    periproct, 513;
    peristome, 513;
    alimentary canal, 516;
    water-vascular system, 516 f.;
    nervous system, 518 f.;
    sphaeridia, 524;
    perihaemal spaces, 524 f.;
    genital system, 528;
    blood-system, 529;
    larva, 507;
    _E. acutus_, 540;
    pedicellariae, 509;
    _E. alexandri_, pedicellaria, 510;
    _E. elegans_, 539, 540;
    pedicellariae, 510;
    _E. microtuberculatus_, 540;
    _E. miliaris_, 540, 542, 549;
    _E. norvegicus_, 539, 540
  Economic uses of Foraminifera, 69 f.
  Ectocoele, 367
  Ectoderm, 246
  Ectoplasm (= ectosarc), 6, 46 f., 50;
    of _Amoeba_, 5;
    of Rhizopoda, 51 f.;
    of Heliozoa, 71 f.;
    of Radiolaria, 79 f. (see also Extracapsular protoplasm);
    regeneration of, in Radiolaria, 35;
    of _Collozoum inerme_, 76;
    of Gregarines, 96 f.;
    of Ciliata, 141 f.;
    of _Stylonychia_, 140;
    of Suctoria, 159;
    of _Trachelius ovum_, 153;
    of _Vorticella_, 156
  _Ectopleura_, 268
  Ectosarc—see Ectoplasm
  Ectosome, 170
  Ectyoninae, _217_
  Edrioasteroidea, _580_, _596_
  _Edwardsia_, 328, 366, 368, _376_;
    _E. allmani_, 377;
    _E. beautempsii_, 376, 377;
    _E. carnea_, 377;
    _E. goodsiri_, 377;
    _E. tecta_, 377;
    _E. timida_, 376, 377
  Edwardsia stage of Zoantharia, 367
  Edwardsiidae, _377_
  Edwardsiidea, 367, 371, _375_, 395
  Egg, fertilised, 31;
    of Metazoa, 32 f.;
    of bird, 32;
    various meanings of, 34;
    of affected Silkworm moths transmitters of pébrine, 107
  Ehrenberg, on Protozoa, 45 f.;
    on skeletons of Radiolaria, 87 f.;
    on Ciliata, 146;
    on Suctoria, 162
  Eimer and Fickert, on classification of Foraminifera, 58 n.
  Elasipoda, 569, _571_, 577, 578
  Electric, currents, stimulus of, 19, 22;
    shock, action on Amoeba, etc., 7
  _Eleutheria_, the medusa of _Clavatella_, 265
  Eleutheroblastea, _253_
  Eleutheroplea, _279_
  Eleutherozoa, _430_, 560, 577, 579, 583;
    development of, 602 f.;
    larva of, 605;
    phylogeny of, 621, 622
  Elevator muscles of compasses of _Echinus esculentus_, 527
  _Ellipsactinia_, 283
  Ellis, 167
  Embryonic type of development, 601
  Encystment, 37, 39;
    of animal cells, 37;
    of vegetal cells, 37, 39;
    growth during, 37;
    of zygote, general in Protista, 34;
    of Rhizopoda, 57;
    temporary, of Rhizopoda, 57 (see also Hypnocyst);
    of Heliozoa, 72 f.;
    of _Actinophrys_, 72;
    of _Actinosphaerium_, 73 f.;
    of Proteomyxa, 88, 89
    of Myxomycete zoospores, 90 f.;
    of Sporozoa, 96 f.;
    of zygote of Sporozoa, 95 f.;
    of Gregarines, 95 f., 98;
    of _Lankesteria_, 95;
    of _Monocystis_, 96;
    of Coccidiidae, 97 f.;
    of _Coccidium_, 100, 101;
    of archespore or pansporoblast of Myxosporidiaceae, 107;
    of Flagellates, 115, 117 f.;
    of zygote of _Bodo saltans_, 117;
    of _Opalina_, 123 f.;
    of oosperm of Volvocaceae, 127 f., 129 f.;
    of Dinoflagellates, 131;
    of Ciliata, 147;
    of _Colpoda cucullus_, 147, 153
  Endocyclica, 529, _530_ f., 556, 559
  Endoderm, 246
  Endogamy, in _Amoeba coli_, 57;
    in _Actinosphaerium_, 73 f., 75 (diagram);
    in _Stephanosphaera_, 128
  Endogenous budding in Suctoria, 160 f., 162
  Endoparasitic Suctoria, 86, 160 f.
  Endoplasm (= endosarc, _q.v._), 6
  —see also Intracapsular protoplasm (Radiolaria)
  Endoral, cilia, 139;
    undulating membrane, 139
  Endosarc (= endoplasm), 6;
    of Gregarines, 95 f.;
    branching, of _Noctiluca_, 110, 133;
    of _Loxodes_ and _Trachelius_, 144, 153;
    of Ciliata, 143 f.;
    of _Stylonychia_, 140;
    of Suctoria, 161
  _Endosphaera_, _159_, 161
  Energy, changes of, in living organism, 8, 13;
    sources of, 13 f.
  Entocnemaria, _394_
  Entocoele, 367
  _Entosolenia_, 66
  Entz, Geza, on Choanoflagellates, 121 n.;
    on structure of _Vorticella_, 157 n.
  Eocene Foraminifera, 70
  _Eolis_ (= _Aeolis_), 248
  _Eophiura_, 501
  _Eozoon_, 70 n.
  Epaulettes, 315
  _Epenthesis_, _281_
  _Ephelota_, _159_, 160;
    _E. bütschliana_, cytological study of, 162
  _Ephydatia_, _217_, 225;
    _E. fluviatilis_, structure, etc., 174 f., 176, 177, 178, 179
  Ephyra, 317
  Ephyropsidae, _322_
  _Epiactis_ (usually placed in the order Zoanthidea, _404_);
    _E. marsupialis_, 379;
    _E. prolifera_, 379
  _Epibulia_, _308_
  Epidemic, of pébrine among Silkworms, 107;
    among Fish, due to _Costia necatrix_, 119;
    to Myxosporidiaceae, 107;
    to _Icthyophtheirius_, 152
  —see also Diseases, Fever
  _Epigonactis fecunda_, 379
  Epimerite of Gregarines, 97, 98 f.
  Epineural canal, of _Ophiothrix fragilis_, 481;
    of _Echinus esculentus_, 515
  Epiphysis, of jaw, of _Echinus esculentus_, 526;
    of jaws of Diadematidae, 531;
    absent in Cidaridae and Arbaciidae, 531
  Epiphytic Protozoa, 48
  Epiplasm (= cytoplasm of a brood-mother-cell remaining over unused in
      brood-formation), 96
  _Epistrelophyllum_, _403_
  _Epistylis_, _138_, 158;
    _E. umbellaria_, nematocysts of, 249
  Epitheca, 386
  _Epizoanthus_, _406_;
    on _Hyalonema_, 204;
    _E. glacialis_, infested by Gregarines, 99;
    _E. incrustatus_, 406;
    _E. paguriphilus_, 406;
    _E. stellaris_, 406
  Epizoic, Protozoa, 48;
    Ciliata, 158;
    Suctoria, 158, 162
  —see also Symbiosis
  Equatorial plate (= the collective chromosomes at the equator of the
      spindle in mitosis), 25, 27
  Equiangular, 185
  _Errina_, 284, _286_;
    _E. glabra_, 286;
    _E. ramosa_, 286
  _Ersaea_, _306_;
    _E. picta_, 303
  _Esperella_, 225, 231
  _Esperiopsis_, 225
  Euaster (a true aster in which the actines proceed from a centre,
      contrasting with the streptaster), 184
  _Eucalyptocrinus_, 596
  Eucharidae, _420_
  _Eucharis_, _420_;
    _E. multicornis_, 416, 418 f., 420
  _Enchlora_, _417_
  _Eucladia_, _502_
  Euclypeastroidea, _549_
  Eucopidae, 277, _280_
  _Eudendrium_, 269, _270_
  _Eudiocrinus_, _594_
  _Eudorina_, _111_, 128 f.
  _Eudoxia_, _306_;
    _E. eschscholtzii_, 303
  _Euglena_, _110_;
    barotaxy of, 20;
    nutrition of, 113;
    _E. viridis_, 124
  Euglenaceae, _110_, 124;
    pellicle of, 113
  Euglenoid motion, 124;
    of Sporozoa, 50
  _Euglypha_, _52_;
    in fission, 29;
    test, 29, 54
  _Eunicea_, _356_;
    spicules, 335, 336
  _Eunicella_, _356_;
    spicules, 335, 336;
    _E. cavolini_, 356
  _Eupagurus prideauxii_, 378, 381;
    _E. bernhardus_, 378
  _Eupatagus_, 553
  _Euphyllia_, _401_
  _Euplectella_, _204_;
    _E. aspergillum_ (Venus's Flower-Basket, 197);
    _E. imperialis_, 206;
    _E. suberea_, 202, 204, 205, 221
  _Euplexaura_, _356_
  _Euplokamis_, _418_
  _Euplotes_, _138_
  Eupsammiidae, 402, _404_
  _Eurhamphaea_, _419_
  Eurhamphaeidae, _419_
  Euryalidae, _501_
  Eurypylous, 210
  _Euspongia_, _221_
  _Eutreptia_, _110_;
    _E. viridis_, 124
  Evacuation of faeces by mouth in _Noctiluca_, 133
  Evans, 179 n., 217
  Excretion, 13 f.;
    in Sponges, 172;
    in _Asterias rubens_, 437;
    in _Echinus esculentus_, 527, 528;
    in _Antedon rosacea_, 587
  Excretory, granules, 6;
    of Ciliata, 144;
    pore of contractile vacuole of Flagellates, 110;
    of _Trachelius ovum_, 153
  Exogametes of _Trichosphaerium_, 54
  Exogamy, 34 n.;
    in Rhizopoda, 56 f.;
    in Foraminifera, 68 f.
  Expansion of Amoeboid cell, 16 f.
  Exsert, septa, 398, 399
  Extracapsular protoplasm, of Phaeodaria, 76;
    of Radiolaria, 79 f. (see also Ectoplasm)
  Eye of _Asterias_, 445 f., 446;
    of Echinoidea, 512
  Eye-spot of coloured Protista, 21, 125 f.

  _Fascicularia_, _348_
  Fasciole, of _Echinocardium_, 550, 555;
    of Spatangoidea, 553;
    of _Spatangus_, 553;
    of _Eupatagus_, 553;
    of Spatangidae, 555
  Fats, fatty acids, 15;
    in Flagellates, 110, 115;
    formation of, 36
  Fauré-Fremiet, on attachment of Peritrichaceae, 141 n.
  Faurot, 368
  _Favia_, 373, _401
  Favosites_, _344_
  Favositidae, _344_
  Feather-star, 581
  Feeding, of _Noctiluca_, 133, 144;
    of Peritrichaceae, 145
  —see also Food
  Feeler, of _Holothuria nigra_, 561 f., 566;
    of Holothuroidea, 568;
    of Dendrochirota, 568, 572;
    of Synaptida, 568, 575;
    of Molpadiida, 568, 575
  Female gamete, 33;
    of _Pandorina_, 128 f.;
    of Acystosporidae, 104 f.;
    of Peritrichaceae, 151, 157
  —see also Megagamete, Oosphere
  Ferment, required for germination, brood-formation, etc., 32 f.
  —see also Zymase
  Fermentation, organisms of, 43 f.
  Fertilisation, 33 f.;
    "chemical," 32 n.
  Fertilised egg, 31
  —see also Oosperm, Zygote
  Fertilising tube of _Chlamydomonas_, 125
  Fever, intermittent, malarial, 103 f.;
    relapsing, 121;
    remittent, 105;
    Texas-, Tick, 120;
    Trypanosomic, 119 f.
  Fewkes, 268 n.
  Fibularidae, _549_
  _Fibularites_, 559
  Fickert, Eimer and, on classification of Foraminifera, 58 n.
  _Ficulina_, _219_, 224, 230;
    _F. ficus_, 219
  Filoplasmodieae, _90_ f.
  Filopodia, 47 n.
  Filosa, 29, _50_, _52_ f.;
    resemblance to Allogromidiaceae, 59
  Finger, 580;
    of Cystoidea, 597;
    of Blastoidea, 599, 600
  Firestone of Delitzet contains fossil _Peridinium_, 132
  Fischer, on fixing reagents, 11;
    on structure of flagellum, 114
  Fish, rheotaxy of, 21;
    epidemics of, due to Myxosporidiaceae, 107;
    to _Costia necatrix_, 119;
    to _Ichthyophtheirius_, 152
  Fission, 10, 23 f.;
    equal, 10;
    Spencerian, 23;
    multiple, 30 f. (see also Brood-division);
    of Heliozoa, 72 f.;
    of Radiolaria, 84 f.;
    radial, in Volvocaceae, 110;
    transverse, in Craspedomonadidae, 115 n.;
    longitudinal and transverse, of _Bodo saltans_, 117;
    of _Opalina_, 123;
    of Euglenaceae, 124;
    of _Eutreptia viridis_, 124;
    of _Noctiluca_, 133;
    of Ciliata, 147 f.;
    of _Stentor polymorphus_, 156;
    of Vorticellidae, 157 f.
  —see also Bud-fission
  Fissiparantes, 387, _400_
  Fixing protoplasm, 15
  _Flabellum_, 375, 386, _398_;
    protandry of, 370
  Flagella, flagellum, 17 f., 47;
    of Protozoa, 47;
    formed by altered pseudopodia in _Microgromia_, 60;
    of Heliozoa, 73;
    of sperms of Coccidiidae, 102;
    of Acystosporidae, 105;
    of Flagellata, 109, 114 f.;
    of Trichonymphidae, 114;
    Delage on mechanism of, 114 n.;
    of _Bodo saltans_, 117;
    of _Trypanosoma_, 121;
    of Euglenaceae, 124 f.;
    of _Maupasia_, 124;
    of _Eutreptia viridis_, 124;
    of _Sphaerella_, 126;
    of Dinoflagellata, 130, 131;
    of _Peridinium_, 131;
    of _Polykrikos_, 132;
    of _Noctiluca_, 132, 133 f.
  —see also Sarcoflagellum
  Flagellar pit, in Flagellates, 110, 124 f.
  Flagellata, 17 f., 40, 48 f., _50_, _109_ f.;
    barotaxy of, 20;
    galvanotaxy of, 22;
    chemiotaxy of, 23;
    nutrition of, 40, 113;
    of putrefying liquids, 44, 116 f.;
    studied by botanists, 45;
    as internal parasites, 48, 119 f.;
    relations with Acystosporidae, 106;
    shell of, 113;
    stalk of, 113;
    life-history of, 116 f.;
    literature of, 119;
    saprophytic, 119 f.
  Flagellate stage, of Sarcodina, 56 f., 60, 109;
    of Heliozoa, 74;
    of Radiolaria, 85 f.
  —see also Flagellula
  Flagellated chamber, 170
  Flagellula, 31;
    of Proteomyxa, 88, 89;
    of Myxomycetes, 91, 92;
    of _Didymium_, 92
  —see also Zoospores
  Flagellum—see Flagella
  Fleming, 168 n.
  Flexible Corals, 326
  Flint, 219, 241
  Floricome, 203
  Floscelle, of _Echinocardium cordatum_, 551;
    of Cassidulidae, 554
  Flowering plants, male cells of, 38
  Flowers of tan (= _Fuligo varians_), 92 f.;
    peptic ferment in, 16
  Foam structure, 6
  _Folliculina_, _137_;
    tube of, 152
  Food, 35 f. (see also Ingestion);
    of Higher Animals, 38;
    absorption of, by Plants, 38;
    in relation to life-cycle of Ciliata, 147 f.;
    of Sponges, 237;
    of _Hydra_, 256 and n.;
    of _Millepora_, 261;
    of Siphonophora, 304;
    of _Charybdea_, 319;
    of _Alcyonium_, 339;
    of Zoantharia, 373;
    of _Asterias rubens_, 439;
    of _Ophiothrix fragilis_, 486;
    of Ophiolepididae, 496;
    of _Echinus esculentus_, 516;
    of _Echinarachnius parma_, 546;
    of _Echinocardium cordatum_, 552;
    of _Holothuria nigra_, 561;
    of Dendrochirota, 572;
    of _Synapta inhaerens_, 577;
    of _Antedon rosacea_, 583
  Food-vacuole, of _Actinosphaerium eichornii_, 72;
    of Ciliata, 145 f.;
    of _Carchesium_, 146
  Foot-plate, of young Pentacrinidae, 592;
    of larva of _Antedon rosacea_, 619
  Foraminifera, 40, 49, _50_, _58_ f.;
    relations of, 49;
    shell of, 49, 59 f., 60, 61, 63, 65;
    habitat of, 59 f.;
    literature of, 58 n.;
    marine, 60 f.;
    nuclei of, 62, 67 f.;
    nutrition of, 40, 62;
    streaming of granules in, 17;
    chromidia of, 62, 68 f.;
    collection of, 62;
    dimorphism of, 66, 67;
    reproduction of, 67 f.;
    economic uses of, 69 f.;
    palaeontology of, 69 f.
  Forbes, 338
  Force, dual, of dividing-cell, 26 n.
  _Forcepia_, 223
  Forceps, 222
  Forcipulata, _462_, _473_ f.
  Forcipulate pedicellaria, 456, 473
  Formative vacuole of contractile vacuole, in Flagellata, 110, 115;
    in Ciliata, 143
  Fossil, Foraminifera, 69 f.;
    Radiolaria, 87 f.;
    Dinoflagellata, 132;
    _Peridinium_, 132;
    Sponges, 192 f., 207 f., 215, 241;
    Coelenterates, 270, 281 f., 343 f., 346, 393 f., 406;
    Asteroidea, 475 f.;
    Ophiuroidea, 501 f.;
    Echinoidea, 556 f.;
    Crinoidea, 594 f.;
    Thecoidea, 596;
    Carpoidea, 596 f.;
    Cystoidea, 597 f.;
    Blastoidea, 599 f.
  Fowler, 293 n., 382, 400, 404
  Framboesia, 121 n.
  France, epidemic of pébrine in, 107
  Francé, on structure of funnel of Choanoflagellates, 115 n., 121 n.;
    monograph of Choanoflagellates, 123, 182 n.;
    on Polytomeae, 119 n.
  Freetown, prophylaxis of malaria at, 106
  Fringing reef, 390 f.
  Frog's blood, _Lankesterella_ in, 102
  _Frondicularia_, _59_, 63
  Fructification, of Mycetozoa, 90 f.;
    of Acrasieae, 90;
    of Myxomycetes, 49, 91 f.
  Fry, E. and A., on Myxomycetes, 93 n.
  _Fuligo_, _90_;
    _F. varians_, 92 f.;
    pepsin in, 16
  Fungacea, _402_
  Fungi, cell connexions in, 37 f.;
    in relation to Protista, 40;
    Gasteromycetous, 91
  _Fungia_, _403_;
    asexual reproduction of, 388, 389;
    _F. crassitentaculata_, 403
  Fungiidae, _403_
  _Funiculina_, 359, _362_;
    _F. quadrangularis_, 362
  Funiculinidae, _362_
  Funnel, of Craspedomonadidae or Choanoflagellates, 111, 121, 122, 182;
    of Phalausteridae, 111;
    of choanocytes of Sponges, 171
  Fusion of larval Sponges, 174
  Fusion-nucleus of Ciliata, 150
  —see also Reproduction, Syngamy, Zygotonucleus
  _Fusulina_, _59_

  _Galaxea_, _400_;
    _G. esperi_, 400
  _Galeolaria_, _307_;
    _G. biloba_, 304
  Galeolarinae, _307_
  _Galerites_, _558_
  Gamble, 312 n.;
    and Keeble, 175 n.
  Gametes, 33 f.;
    of _Trichosphaerium_, 54;
    of certain Protomastigaceae, 116 n.;
    of Volvocidae, 127 f.;
    of _Pandorina_ (of three sizes), 128, 129
  Gametocyte of Acystosporidae, 104 f.
  Gametogonium (= parent-cell of gametes), male, of Acystosporidae, 105
  Gametonuclei (= nuclei capable of syngamous fusion), 34
  _Ganeria_, _464_
  Ganeriidae, 454, _464_
  Gardiner, 345, 370, 375, 392 n., 404
  _Garveia_, _270_
  Gasteromycetous fungi, 91
  Gastral layer, 171
  Gastralia, 201
  Gastropores, 257, 258
  Gastrozooids, of _Millepora_, 259, 260;
    of _Hydractinia_, 264;
    of Siphonophora, 299;
    of Antipatharia, 408
  Gastrula, definition of, 603
  Gaule, misinterpretation of nature of Haemosporidae, 102
  Gegenbaur, 302
  _Gellius_ _217_, 223;
    _G. varius_, development, 172 f., 173, 174
  Gemmantes, _400_
  _Gemmaria_, _405_
  Gemination = Budding, _q.v._
  Gemmiform, pedicellariae, of _Echinus esculentus_, 506;
    of _E. acutus_, 509;
    of _E. elegans_, 510;
    of Cidaridae, 534;
    of _Echinarachnius parma_, 544;
    of _Echinocardium cordatum_, 550
  Gemmule, 177, 178, 179, 230
  Generation, spontaneous, 42 f.
  Generations, alternation of, 44, 250
  Genital base of _Holothuria nigra_, 567
  Genital bursa, of _Ophiothrix fragilis_, 485
    compared with hydrospires of Blastoidea, 600
  Genital canal of _Antedon rosacea_, 586
  Genital organs (including ducts), of _Asterias rubens_, 451 f.;
    of _Ophiothrix fragilis_, 490;
    of _Ophiarachna_, 491;
    of Ophiuroidea, 494;
    of _Amphiura squamata_, 494;
    of _Echinus esculentus_, 528;
    of _Echinocardium cordatum_, 552;
    of _Hemiaster philippi_, 552;
    of _Holothuria nigra_, 567;
    of _Antedon rosacea_, 586
  Genital plate, of _Ophiothrix fragilis_, 485;
    of _Echinus esculentus_, 512, 513
  Genital rachis, of _Asterias rubens_, 452;
    of _Ophiothrix fragilis_, 490;
    of _Echinus esculentus_, 528;
    of _Antedon rosacea_, 586
  Genital scale of _Ophiothrix fragilis_, 485
  Genital stolon, of _Asterias rubens_, 451;
    of _Ophiothrix fragilis_, 489;
    of _Echinus esculentus_, 528;
    of _Holothuria nigra_, 567;
    of _Antedon rosacea_, 585, 586;
    of larva of _A. rosacea_, 619
  _Geodia_, 211
  Geographical distribution of Protozoa, 47
  Geotaxy (= barotaxy), 20
  _Gephyra dohrnii_, _382_, 408
  Gephyrea, 577
  Gerarde, 167
  _Gerardia savalia_, _406_
  _Gerbillus indicus_ infested by a Haemosporidian, 102 n.
  Germinal spot (= nucleole of ovum), 7
  Germinal vesicle (= nucleus of ovum), 7
  Germination, 32;
    of Myxosporidian spores, 107
  Germ-plasm, 28 f.;
    continuity of, 172
  Germ theory, 44
  Germs, invisible air-borne, 43
  _Geryonia_, 290, _295_
  Gilchrist, 338
  Gill of _Echinus_, 514, 527
  Gill-cleft, of _Echinus_, 514;
    of _Sphaerechinus_, 540 f.;
    of _Strongylocentrotus_, 541
  _Ginkgo_, spermatozoa of, 38
  _Glaucoma_, _137_, 153;
    _G. scintillans_, rate of fission of, 147 f.
  Glauconite, 70
  _Globiceps_, 272
  Globiferae of _Centrostephanus longispinosus_, 532
  _Globigerina_, 59, 63, 242;
    spines of, 61, 66;
    -ooze, 61 f.;
    _G. bulloides_, 68, 69
  _Glossina morsitans_, intermediate host of _Trypanosoma brucei_, 119;
    _G. palpalis_, intermediate host of _T. gambiense_, 120
  _Glossograptus_, _282_
  Glycerin, 15
  Glycogen, of Ciliata, 144;
    -vesicles of _Pelomyxa palustris_, 53
  Gnat (_Anopheles_), intermediate host of _Haemamoeba_ and _Laverania_,
      103 f.;
    (_Culex_) intermediate host of _Haemoproteus_, 103;
    of _Trypanosoma_, 120
  Golgi, on relation of Acystosporidian life-cycle and stages of
      intermittent fever, 103
  _Gonactinia_, 371, 372, _377_
  Gonangium, 276
  _Goniaster_, _471_;
    fossil, 475
  _Goniastraea_, 375, _401_
  _Goniocidaris_, _534_;
    _G. canaliculata_, 535
  _Gonionema_, 288, _291_;
    _G. murbachii_, 232, 290, 291, 292
  _Gonium_, _111_
  Gonophore, of Gymnoblastea, 265;
    of Calyptoblastea, 277;
    of Stylasterina, 284;
    of Siphonophora, 302
  Gonotheca, 276, 281
  Gonozooids, of Siphonophora, 302;
    of Antipatharia, 408
  Gorgonacea, _350_ f.
  _Gorgonella_, _357_;
    spicule, 336
  Gorgonellidae, 337, _357_
  _Gorgonia_, _356_;
    _G. cavolinii_, 340;
    _G. flabellum_, 357;
    _G. verrucosa_, 356
  Gorgoniidae, 334, 337, _356_
  _Gorgonocephalus_, 491, _501_
  Gosse, 273
  Goto, 291, 293;
    on development of Bipinnaria, 612
  _Grammaria_, _278_
  _Granatocrinus_, _599_;
    _G. norwoodi_, 600
  Grant, 167
  Grantiidae, _192_
  _Grantiopsis_, 191
  Granular disintegration of Protista, 14 f.
  Granules, in protoplasm, 6;
    excretory, 6, 144;
    aleurone, 37;
    basal, of cilia, etc., 138 n., 141 (see also Blepharoplast);
    proteid, of Suctoria, 161
  Graphiohexaster, 203
  Graptolitoidea, _281_
  Grassi, on malarial parasites, 103
  Gravity, stimulus of, 19 f.
  Greasy film, outer clear layer of protoplasm behaves like, 17
  Greeff, on Protozoa, 46
  Green Flagellates, relations of, 48
  Greensand, 70;
    Cambridge, 208
  Green water often due to _Euglena viridis_, 124
  Greenwood, M., on peptic digestion in Protozoa, 16;
    on feeding of _Carchesium polypinum_, 45 f., 146 f.
  _Gregarina_, _97_, 98, 99;
    _G. blattarum_, 98
  Gregarines, habitat, 99;
    syngamy, 99
  Gregarinidaceae, 95 f., _97_ f.
  Gregory, 346
  Grew, 166
  Grey chalk, 61
  _Gromia_, _52_;
    _G. oviformis_, 59 n.
  —see also _Allogromia_
  Grooves, longitudinal and transverse, of Dinoflagellata, 110, 130, 131,
      132;
    of _Peridinium_, 131;
    of _Polykrikos_, 132;
    oral, of _Noctiluca_, 133
  Grosvenor, 249 n.
  Growth, 19 f.;
    Spencer's limit of, 23, 31
  Gruber, on regeneration in Protozoa, 35 n.;
    on diffused nucleus in marine Ciliata, 144 n.;
    on tubicolous marine Ciliata, 152
  Gruppe, deposit of Radiolaria, 87
  Guinea Coast, 106
  Gullet (= pharynx) of _Paramecium caudatum_, 151
  Gut, supposed, of Ciliata, 145
  —see also Alimentary canal
  Gutter, oral, of Vorticellidae, 156, 158
  Gymnamoebae, 51 n.
  Gymnasteridae, _471_
  Gymnoblastea, _262_ f.
  _Gymnodinium_, _110_;
    _G. pulvisculus_, parasitic in _Appendicularia_, 132
  Gymnomyxa, 49 n.
  _Gymnophrys_, _58_
  _Gymnosphaera_, _70_, 73
  Gymnostomaceae, _137_;
    predaceous, trichocysts of, 143;
    mouth and pharynx of, 145;
    noteworthy members of, 152
  _Gyractis_, _380_

  Häcker, on skeleton of Radiolaria, 82 n.
  Haddon, 382
  Haeckel, 168, 185, 192, 237, 308;
    on Monera, 4 n., 89 n.;
    on Protista, 40 f.;
    on Protozoa, 46;
    on Heliozoa, 71;
    on classification of Radiolaria, 76;
    on functions of porocone in Radiolaria, 81;
    on enumeration of Radiolaria, 87 f.;
    on _Myxobrachia_, 83;
    on Sponges, 168, 192;
    on phylogeny of Echinodermata, 622
  _Haeckeliana_, _79_, 85
  _Haemamoeba_, _97_, 103 f.;
    _H. malariae_, parasite of quartan fever, 104 f.;
    _H. vivax_, parasite of tertian fever, 104 f.
  Haematochrome, 125
  _Haematococcus_ (= _Sphaerella_, _111_), 125, 126
  Haemoflagellates (= _Trypanosoma_, etc., _q.v._), 119 n.
  Haemoglobin, 103;
    in water-vascular system of _Ophiactis virens_, 499
  _Haemogregarina_, _97_
  _Haemomenas_ (Ross's name for parasite of pernicious fever = _Laverania_,
      _97_), 105
  _Haemoproteus_, _97_;
    parasitic in birds, 103
  Haemosporidae, _97_, 102
  _Haimea hyalina_, _342_;
    _H. funebris_, 342.
  Haimeidae, _342_
  _Halcampa_, 366, _380_;
    _H. chrysanthellum_, 380
  Halcampidae, 375, _380_
  _Halecium_, 277, _280_
  _Haleremita_, _256_
  _Halicalyx_, _291_
  _Halichondria_, _217_, 223;
    _H. panicea_, structure, etc., 168 f., 169, 170, 211
  Halichondrina, 216, _217_
  _Haliclystus_, 320, _321_;
    _H. auricula_, 320
  _Halicnemia_, _216_, 224
  _Haliomma_, _77_
  _Haliphysema_, _59_
  _Halisarca_, _196_, 225
  _Halomitra_, _404_
  _Halteria_, _137_, 152, 155
  _Halteridium_, _97_;
    sexual fusion in, 103, 105;
    regarded by Schaudinn as a state of _Trypanosoma_, 103 n., 120
  _Hamacantha_, 223
  Hamann, on supposed cavities in the body-wall of Asteroidea, 449;
    on classification of Zygophiurae, 495 n.
  Hanitsch, 168 n.
  _Hapalocarcinus_, 402
  Hardy, on structure of protoplasm and clearing, 11, 12 n.
  _Hartea elegans_, _342_
  Hartlaub, 269 n., 274, 297 n.
  Hartog, on Protozoa, 1 f.;
    on structure of protoplasm, living and dead, 11;
    on function of contractile vacuole, 15 n.;
    on intracellular digestion, 16;
    on brood-division (multiple cell-division), 16 n.;
    on dual force of dividing cell, 26 n.;
    on syngamy, etc., 34 n.;
    and Dixon, on pepsin in _Pelomyxa_, 16
  Harvey, "omne vivum ex ovo," 42
  _Hastigerina_, _59_, 66
  _Hauerina_, _59_
  Heart, of _Asterias rubens_, 450
  Heart-urchins = Spatangoidea, _q.v._
  Heat—see Temperature
  Heat-rigor, 22
  _Heleopera_, _52_;
    test of, 55
  _Heliaster_, _474_
  Heliasteridae, 453, 454, _474_
  _Heliolites_, _346_
  Heliolitidae, _346_
  _Heliopora_, 330, 334, 337, _345_ f.
  Helioporidae, _346_
  Heliozoa, _50_, _70_ f.;
    streaming of granules, 17;
    regeneration, 35;
    habitat, 48;
    locomotion, 73;
    various forms of, 74;
    marine, 75;
    distribution of, 75;
    resemblance of Suctoria to, 159
  _Hemiaster_, _556_;
    _H. philippi_, 552, 555, 602, 603
  Hemichordata, 616
  Henneguy, on protoplasm, 3 n.;
    on syngamy, 34 n.
  _Henneguya_, _98_
  _Henricia_—see _Cribrella_
  Hérouard, Delage and, on Protozoa, 46
  _Herpetolitha_, _404_
  _Herpetomonas_, 115
  Hertwig, R., on Protozoa, 46;
    on chromidia in Sarcodina, 52 n.;
    on Heliozoa, 71;
    on Radiolaria, 88;
    on Suctoria, 161 n., 162
  Heteractinellida, _208_
  _Heterastridium_, _283_
  _Heterocentrotus_, 532, _542_
  Heterocoela, _187_ f.
  _Heterophrys_, _71_
  Heteropidae, _192_
  Heterotrichaceae, _137_, 153 f.;
    fission of, 147
  _Heteroxenia_, 333, 335, _348_
  Hexactine (a triaxon in which all six actines are developed), 184
  Hexactinellida, 194, 195, _197_ f., 228, 240
  _Hexadella_, _196_
  _Hexamitus_, 115
  Hexaster (a hexactine with secondary or terminal rays = Carter's
      "rosette"), 203
  Hexasterophora, _203_ f.
  Hickson, on interchange of cytoplasm in conjugating Infusoria, 149 n.;
    on conjugation in Suctoria, 161 f.;
    on Coelenterata, 243 f.;
    on _Millepora_, 259 n.;
    on Stylasterina, 286 n.;
    on Alcyonaria, 329 n., 351 n., 352 n., 359 n.;
    on Antipatharia, 408 n.;
    on Ctenophora, 412 f.
  Hieronymus on _Chlamydomyxa_, 90 n.
  Hincks, 268 n.
  Hinde, 193, 207
  _Hippasterias_, _471_
  _Hippopodius_, _307_
  _Hippospongia_, _221_
  Holectypoidea, _558_
  Holophytic, Algae and Fungi, zoospores of, 5;
    nutrition, 37;
    Flagellates, 113
  Holopodidae, _592_
  _Holopus_, 588, 589, _594_
  _Holothuria_, _570_;
    _H. nigra_, 561 f.;
    shape, 561;
    feelers, 561;
    body-wall, 562;
    alimentary canal, 562;
    respiratory trees, 563;
    water-vascular system, 564;
    nervous system, 566;
    calcareous ring, 566;
    blood system, 567;
    genital organs, 567;
    _H. cinerascens_, 567;
    _H. fusco-rubra_, 567;
    _H. aspera_, 570;
    _H. intestinalis_, 570;
    _H. tremula_, 570
  Holothuroidea, 431, 537, _560_ f., 583;
    mesenchyme of larva, 604;
    development of, 609, 614, 615;
    phylogeny of, 622
  Holotrypasta (= Porulosa), _76_
  Holozoic, 35 f.;
    Flagellates, 113;
    Dinoflagellates, 131
  Holt, 311 n.;
    on burrowing habits of _Strongylocentrotus lividus_, 541 n.
  Homaxonic (= symmetrical about a centre along an indefinite number of
      equivalent axes), 76
  Homocoela, _185_ f.
  _Homoeonema_, _294_
  _Homostichanthus anemone_, _383_
  Honey-bees, alleged spontaneous generation of, 42
  _Hormiphora_, _418_;
    _H. plumosa_, 413
  Human diseases, produced by Coccidiaceae, 102 f.;
    by _Amoeba histolytica_, 57;
    by Trypanosomids, 119 f.
  Huxley, on Protozoa, 45;
    first description of a living Radiolarian, 88;
    on Cystoflagellates, 135
  _Hyalonema_, 203, _204_;
    _H. sieboldi_, 206;
    _H. thomsoni_, 204, 221
  _Hyalopus_, _52_;
    _H. dujardini_, 59 n.
  _Hyalosphenia_, _52_;
    _H. lata_, 55
  _Hyboclypus_, _558_;
    _H. gibberulus_, 558
  _Hybocodon_ (Corymorphidae, _273_), 265
  _Hydatina senta_, often found with _Euglena viridis_, 124
  _Hydra_, _253_ f., 255;
    nematocysts of, 247;
    species of, 256;
    specific gravity of, 13 n.;
    host of _Kerona_ and _Trichodina_, 158;
    _Zoochlorella_ in, 126, 256;
    _H. oligactis_ (= _fusca_), 253, 256;
    _H. pallida_, 256 n.;
    _H. viridis_, 253, 256;
    _H. vulgaris_, (= _grisea_), 253, 256;
    nematocyst, 247
  _Hydractinia_, 263, 265, 268, _270_
  _Hydrallmania falcata_, _278_
  _Hydrichthys mirus_, 268
  Hydroceratinidae, _279_
  Hydrocladia, 276
  Hydrocoel (including left hydrocoel), 428;
    of _Antedon rosacea_, 585;
    development of, 608 f., 609;
    development in _Asterina gibbosa_, 611;
    in Auricularia, 615;
    in _Antedon rosacea_, 619
  _Hydroctena salenskii_, _423_, 424
  Hydrolaridae, _273_
  Hydrophyllium, 297, 300
  Hydrorhiza, 262
  Hydrosome, 250, 251
  Hydrospire, of Blastoidea, 580, 599;
    of _Codaster_, 599;
    of _Pentremites_, 599;
    of _Granatocrinus_, 599 f.
  Hydrotheca, 275
  Hydrozoa, _249_ f.;
    hydrosome, 250;
    life-history, 250;
    medusome, 250, 251 f.;
    sense-organs, 252
  _Hydrurus_, _110_;
    theca of, 113
  _Hymedesmia_, 222
  _Hymenaster_, _466_;
    _H. pellucidus_, 465
  _Hymeniacidon_, 224
  _Hymeraphia_, 223
  Hyocrinidae, _590_
  _Hyocrinus_, 588, 589, _590_, 590;
    _H. bethellianus_, 590
  _Hyperia_ (Amphipod), parasitic in Radiolaria, 87
  _Hyphalaster_, _471_;
    _H. moseri_, 459
  Hypnocyst, of Rhizopoda, 57;
    of Proteomyxa, 88 f.;
    of _Pseudospora_, 89;
    of Myxomycetes, 90 f.
  _Hypolytus peregrinus_, 262, _271_ n.
  Hypophare, 210
  Hypostome, 250
  Hypotrichaceae, _137_, 138 f., 158 n.

  _Ianthella_, _220_
  _Ichthyophtheirius_, _137_;
    noxious parasite of fish, 152
  _Iciligorgia_, _351_
  Idioplasm, 29
  Ijima, 199, 206, 231, 234
  _Ileonema_, _137_, 152
  _Ilyanthus mitchellii_, _380_
  _Ilyodaemon_, _571_, 572;
    _I. maculatus_, 571
  Imperforate, Foraminifera, _58_ f.;
    Corals, 371
  Inadunata, _595_
  Incurrent canal, 170
  India, diseases of Trypanosomic origin, 119 f.
  Induction shocks, action on Protozoa, 7, 22
  Infero-marginal ossicle of Asteroidea, 436
  Inflammation, 8
  Infra-basal plate, of Crinoidea, 588;
    of fossil Crinoidea, 594;
    of larval _Antedon rosacea_, 619
  Infundibulum, 415
  Infusions, appearance of organisms in, 42 f.;
    organisms of, 136
  Infusoria, 40, 48, _50_, 136 f.;
    specific gravity of, 13 n.;
    zygote does not encyst, 34.
  Ingestion, of food, by _Amoeba limax_, 9;
    by Choanoflagellates, 122;
    by Dinoflagellates, 131;
    by _Carchesium_, 146;
    by _Coleps_, 150
  —vacuole of, in Flagellates, 113;
    in _Oikomonas_, 112;
    in Choanoflagellates, 122
  Inner perihaemal ring-canal, of _Asterias rubens_, 448;
    development of, in _Asterina gibbosa_, 612
  Inoculation of malarial fever in man through a mosquito, 105 f.
  Insectivorous plants, 38
  Insects, metamorphoses of, 44;
    as hosts of Trichonymphidae, 123
  Interambulacral area, of _Echinarachnius parma_, 544;
    of _Echinocardium cordatum_, 550
  Interambulacral plate, of _Echinus esculentus_, 511;
    of Cidaridae, 533 f.;
    of _Echinarachnius parma_, 544 f.
  Interbrachial septa—see Interradial septa
  Interchanges between cell and medium, 14
  Intermediate dorsal process of ciliated band of Auricularia, 608
  Intermediate (= supplemental) skeleton of Perforate Foraminiferal shell,
      63, 66
  Intermittent fever, malarial, produced by Acystosporidae, 103 f.
  Internal budding of Suctoria, 160 f., 162;
    of _Ephydatia_, 177
  Internal gills—see Stewart's organs
  Internal movements of protoplasm, 17
  Interradial plates, of calcareous ring of _Holothuria nigra_, 566;
    of Holothuroidea, 569;
    of Synaptida, 569;
    of Dendrochirota,  569;
    of calyx of Crinoidea, 589;
    of _Thaumatocrinus_, 589;
    of _Hyocrinus_, 590;
    of Rhizocrinidae, 591;
    of corona of Echinoidea—see Interambulacral plate
  Interradial septa, of _Asterias rubens_, 437;
    of Heliasteridae, 474;
    absent in Brisingidae, 475
  Interradius, 428;
    of _Asterias rubens_, 434;
    of _Echinus esculentus_, 504;
    of _Holothuria nigra_, 562
  Interstitial growth, 10
  Intestine, 415;
    of _Echinus esculentus_, 516;
    of _Holothuria nigra_, 563;
    of _Antedon rosacea_, 583;
    of _Actinometra_, 589;
    of  Dipleurula, 605;
    of Protocoelomata, 616
  Intracapsular protoplasm of Radiolaria, 80 f.
  Intramolecular respiration, 14 n.
  Intranuclear spindle of _Euylypha_, 29
  Invertebrata, hosts of Gregarines, 97 f.
  Iodine, 239
  _Iophon_, 223
  _Iridogorgia_, _355_
  _Isaurus_, _405_
  _Ischadites_, _207_
  Ischikawa, on syngamy of Cystoflagellates, 135;
    on structure of _Ephelota_, 162
  Isidae, 337, _353_
  _Isidella_, _354_
  _Isis_, _353_
  Ismailia, prophylaxis of malaria at, 106
  Isochela (a chela divisible by each of two planes into two equal parts,
      the two ends being equally developed), 222
  _Isocrinus_—see _Pentacrinus_
  Isogamy, 33 f.;
    of Rhizopoda, 56 f.;
    of _Stephanosphaera_, 128
  —see also Syngamy
  Isospores, 85;
    of Radiolaria, 76;
    of _Collozoum inerme_, 76

  Jaekel, on Silurian Asteroidea and Ophiuroidea, 501;
    on classification of Crinoidea, 589, 595;
    on classification of Cystoidea, 598
  James-Clark, on Protozoa, 46;
    on Choanoflagellates, 121, 123;
    on Sponges, 167
  Jaw, of _Ophiothrix fragilis_, 482;
    of fossil Ophiuroidea, 502;
    of _Echinus esculentus_, 526
  Jelly, forming theca in Flagellates, 113
  Jelly-fish, 249, 297, 323
  Jennings, on protoplasmic movements, 4 n., 16 n.;
    reaction of Protista to repellent stimuli, 20 n., 21 n.
  Jensen, on density of living protoplasm, 13 n.;
    on protoplasmic movements, 16 n.
  Joblot, on organisms of putrefaction, 43;
    on Protozoa, 45
  _Joenia_, _111_
  Johnson, 352 n.
  _Juncella_, 330, 335, _357_
  Jung, 253 n.
  Jungersen, 359 n.

  Karyogamy, 34 n.
  —see also Syngamy of Ciliata
  Karyokinesis, 25, 26, 27;
    function of, 28 f.;
    of micronuclei of Ciliata, 144 f.
  —see also Mitosis
  _Karyolysus_, _97_
  Karyosome, 24
  Keeble, 175 n.
  Keller, 233
  Kemna, on stylopodium of Foraminifera, 60
  Kent, Saville, on Choanoflagellates, 122 f., 182;
    on Infusoria and Flagellates, 136 n.
  _Keroeides_, _351_
  _Kerona_, _138_;
    _K. polyporum_, 158 n.
  Kieselguhr, 87
  Kirkpatrick, 215
  Kishinouye, 313 n., 321 n., 333, 352
  Klebs, on Flagellates, 119;
    on Dinoflagellates, 130
  Koch, von, on methods of cultivation of lower organisms, 44;
    on malarial parasites, 103
  Kölliker, on Sporozoa, 94 f.
  _Kophobelemnon_, _362_
  Kophobelemnonidae, _362_
  Köppen, on _Sticholonche_ and its parasite, _Amoebophrya_, 87 n.
  _Korethraster_, 453, _463_
  Kowalevsky, 341, 422
  Krukenberg, on pepsin in a Myxomycete, 16
  Kükenthal, 363

  Labbé, on Protozoa, 45;
    monograph of Sporozoa, 102 n.
  Labial plexus of _Antedon rosacea_, 587
  Labyrinthine shell-wall of arenaceous Foraminifera, 59, 66
  _Labyrinthula_, _90_ f.
  Lachmann, Claparède and, on Protozoa, 45;
    on Suctoria, 162
  _Lacrymaria_, _137_, 152 n.;
    encystment, 147
  _Lafoea_, _280_;
    _L. dumosa_, 280
  _Lafoëina_ (Campanulariidae, _280_), 277
  _Laganum_, 548, _549_
  _Lagena_, _59_, 63, 66 f.;
    fossil, 70
  Lagenaceae, _59_
  Lagoon, 390 f.
  _Lamblia_, _111_;
    _L. intestinalis_, conjugation, 116 n.
  _Lampetia_, _418_
  Lancet-plate, 599
  Lang, on syngamy, etc., 34 n.;
    on Protozoa, 46;
    on distinctions of pseudopodia, 47 n.
  Lankester, on Protozoa, 45 f.;
    on classification of Protozoa, 49 n.;
    on Proteomyxa, 89;
    on Sporozoa, 94;
    on Haemosporidae, 102;
    on pigment of _Stentor coeruleus_, 154 n.;
    on _Torquatella_, 155 n.;
    on chlorophyll of _Ephydatia_, 175;
    on _Limnocodium_, 292
  _Lankesterella_, _97_, 102
  _Lankesteria_, _97_;
    _L. ascidiae_, life-cycle of, 95
  Lantern-coelom of _Echinus esculentus_, 524;
    represented by buccal sinus of _Holothuria nigra_, 566
  _Lanuginella pupa_, 198
  _Lar_, _273_;
    _L. sabellarum_, 266, 267, 268
  Larcoidea, _77_
  Larva, of sponges, 180, 226, 227;
    "asexual," 228;
    of _Tubularia_, 271;
    of Stylasterina, 284;
    of Trachomedusae, 290;
    of Narcomedusae, 295;
    of _Velella_, 302;
    of Scyphozoa, 317;
    of Alcyonaria, 341;
    of _Renilla_, 360;
    of Zoantharia, 373;
    of Zoanthidae, 405;
    of Cerianthidea, 411;
    of Ctenophora, 416, 419;
    of _Cribrella_, 462;
    of _Luidia_, 605;
    of _Asterina gibbosa_, 463, 610, 611, 612;
    of Ophiuroidea, 606;
    of _Echinus_, 607;
    of _Synapta_, 608;
    of _Antedon rosacea_, 618, 619, 620
  Larval brain, of Echinopluteus, 607;
    of _Antedon rosacea_, 619
  Larval type of development, 601
  Lateral mouth-shields of _Ophiothrix fragilis_, 485
  Lauterborn, on sapropelic organisms, 48;
    on budding in Rhizopods, 56 n.
  Laveran, on Sporozoa, 94;
    on Acystosporidae, 102
  _Laverania_, _97_;
    parasite of bilious or pernicious fever, 104 f.
  _Lebrunia_, _382_;
    _L. coralligens_, 373
  _Lecqueureusia_, _52_;
    _L. spiralis_, test of, 55
  Lee, A. Bolles, on action of clearing reagents, 11 n.
  Leech, host of _Haemogregarina_, for sexual process, 102
  Leeuwenhoek, on organisms of putrefaction, 42 f.
  Léger, on Protozoa, 45;
    on Sporozoa, 94;
    on sperms of _Pterocephalus_, 99 n.;
    and Duboscq, on _Sarcocystis tenella_, 108 n.
  Leidy, on Protozoa, 46
  _Leiopathes_, 368, _409_
  Leiopathidae, _409_
  _Leiosella_, 225
  Leipoldt, on the madreporic vesicle of Echinoidea, 528
  _Lelapia australis_, _192_
  _Lembadion_, _137_;
    caudal cilia of, 141 n.
  _Lembus_, _137_;
    caudal cilia of, 141 n.
  _Lemnalia_, _349_
  Lendenfeld, von, 218, 220, 220 n.
  _Lepidogorgia_, _355_
  Leptobrachiidae, _325_
  _Leptodiscus_, _110_, 132, 134
  _Leptogonaster_, 456, _467_
  _Leptogorgia_, 340, _357_
  _Leptopenus_, _404_
  _Leptophyllia_, _404_
  Lesser, Hertwig and, on Heliozoa, 71
  _Lesueuria_, _419_
  Lesueuriidae, _419_
  _Leucandra_, 191, _192_, 209, 221
  _Leucilla_, _191_, 191
  Leucin, 15
  Leuckart, 245
  Leucocyte, 4 f.;
    movements of, 7 f.
  _Leucophrys_, _137_
  Leucosin, 115
  _Leucosolenia_, 221;
    collar-cell, 186;
    larva, 227 f.;
    spicule, 232;
    _L. botryoides_, buds, 228, 229
  Leucosoleniidae, _185_ f.
  Levander, on _Caenomorpha_, _Metopus_, etc., 154 n.
  _Leydenia_, _90_ f.
  Liberation of sporozoites of Acystosporidian parasite in relation to fits
      of fever, 103
  Lice, supposed spontaneous generation of, 42
  Lichen compared to Radiolarian with symbiotic holophytic organisms, 86
  _Lichnophora_, _138_;
    adoral wreath, 138 n.
  Lieb, Calkins and, on rhythm in life-cycle of Ciliata, 148 n.
  Lieberkühn, 167, 178, 237 n.
  _Lieberkühnia_, _59_, 61
  Life-cycle, life-history, of _Trichosphaerium sieboldi_, 54, 56;
    of Polythalamic Foraminifera, 67 f.;
    of _Lankesteria ascidiae_, 95 f.;
    of _Coccidium schubergi_, 99 f., 101;
    of malarial parasites, 103, 104 f.;
    of Flagellata, 116 f.;
    of Ciliata, 147 f.
  Light, stimulus of, 19, 21 f.;
    function of, in carbohydrate formation, 36;
    effect on Euglena, 125
  Lillie, on regeneration in Protozoa, 35 n.
  Limicolous Protozoa, 48
  Limit of growth, Herbert Spencer's, 23 f., 31
  _Limnocnida_, _293_;
    _L. tanganyicae_, 293
  _Limnocodium_, _293_;
    _L. sowerbyi_, 292
  _Linantha_, _322_
  _Linckia_, 459, _471_
  Linckiidae, 461, _471_
  Lindström, 346
  _Linerges_ (allied to Atollidae, _322_), 316
  Linin, 6, 24 f.;
    of ovum of Sea-urchin, 7
  _Linuche_, _322_
  _Lionotus_, _137_, 152 n.
  Lipochrome, 39
  _Liriantha appendiculata_, 291, _295_
  _Liriope_, 288, 290, _295_;
    _L. rosacea_, 289
  _Lissodendoryx_, 224
  _Lissomyxilla_, 225
  Lister, A., on Myxomycetes, 93 n.
  Lister, J. J., on Foraminifera—reproduction, 67 f.;
    dimorphism, 67;
    palaeontology, 70;
    classification, 58 f.;
    on _Astrosclera_, 194 n.
  Lithistida, 194, _212_, _215_
  _Lithobius forficatus_, host of _Coccidium schubergi_, 99
  _Lithocercus_, _78_;
    _L. annularis_, 82
  Lithoninae, _193_ f.
  _Lithostrotion_, _394_
  Littoral Protozoa, 48
  _Lituaria_, _364_
  _Lituola_, _59_
  Lituolidaceae, _59_
  Living beings, characters of, 16 f.;
    criterion of, 11
  _Lizzia_, 265, _270_
  Lobata, 414, 416, _418_ f.
  _Lobophytum_, 333, 347, _349_
  Lobopodia, 47 n.
  Locomotion, in Heliozoa, 73
  Loeb, Jacques, on "chemical" fertilisation, 32 n.;
    on polarity in regeneration, 229 f.
  _Loftusia_, _59_, 70, _283_
  Lohmann, on Silicoflagellates, 114 n.
  Loisel, 237, 238
  Longitudinal band of cilia of Dipleurula, 604;
    of Tornaria, 616
  Longitudinal fission of _Eutreptia viridis_, 124;
    of Flagellates, 109, 115;
    of _Bodo saltans_, 117 f.;
    of Craspedomonadidae, 122
  Longitudinal flagellum and groove in Dinoflagellata, 130, 131
  Longitudinal section, of a young Asteroid, 445;
    of a young Ophiuroid, 486;
    of a Holothuroid, 563;
    of _Antedon_, 584;
    of free-swimming larva of _Antedon_, 618
  _Lophocalyx philippensis_, 229
  _Lophoctenia_, _418_
  _Lophohelia_, _399_;
    _L. prolifera_, _399_
  _Lophomonas_, _111_, 123
  Lophophore, 579
  _Lophophyllum_, _406_ f.
  Louse, host for sexual process, etc., of Haemosporidian, 102 n.
  _Loxodes_, _137_, 144, 152 n.
  _Loxophyllum_, _137_, 152
  _Lucernaria_, 320, _321_;
    _L. campanulata_, 321
  Lucernariidae, _320_
  Ludwig, on the blood-system of Asteroidea, 449;
    on the axial sinus of Ophiuroidea, 487;
    on the classification of Holothuroidea, 570
  Lühe, figures of _Lankesteria_, 95
  _Luidia_, _467_ f., 471, 477;
    fossil, 475;
    larva of _L. ciliaris_, 605
  Luminosity or phosphorescence of sea, due to Cystoflagellata, 132, 134
      f.;
    to Dinoflagellata, 132
  Lunule, 548
  _Lychnorhiza_, _325_
  Lychnorhizidae, _325_
  _Lytocarpus_ (Plumulariidae, _279_), 277

  Maas, 168, 189 n., 228 n., 230, 231 n., 232 n., 233, 324
  MacBride, E. W., on Echinodermata, 425 f.
  MacBride, Massee, on Myxomycetes, 93 n.
  MacCallum, on malarial parasites, 103
  M'Dougall, on motile reaction of Protozoa, 19 n.
  M'Intosh, 370 n.
  MacMunn, 169
  Macrocnemic, 405
  Macrocneminae, _405_
  Macrogonidia of _Volvox_, 126, 127
  Macro-, prefix misused to mean "large," usually replaced here by "mega,"
      _q.v._
  _Madrepora_, 368, 373, 387, 389, _395_;
    _M._ forma _cervicornis_, 395;
    _M._ forma _palmata_, 395;
    _M._ forma _prolifera_, 395
  Madreporaria, 369, 371, _384_ f.
  Madrepores, 326 = Madreporaria, _q.v._
  Madreporic vesicle (or right hydrocoel) of _Asterias rubens_, 448;
    of _Ophiothrix fragilis_, 490;
    of _Echinus esculentus_, 528;
    development in Dipleurula, 609
  Madreporidae, _395_
  Madreporite, 428;
    of _Asterias rubens_, 434;
    of _Ophiothrix fragilis_, 487;
    of Ophiuroidea, 493;
    of Cladophiurae, 493;
    of _Echinus esculentus_, 512, 517;
    of _Echinocardium cordatum_, 562;
    of _Holothuria tubulosa_, 564;
    of Elasipoda, 571;
    of Pelagothuriida, 572;
    in older fossil Pelmatozoa, 583;
    in _Thecocystis sacculus_, 596
  _Magosphaera_, _89_
  Maidenhair tree, spermatozoa of, 38
  _Malacogorgia_, 334, _357_
  Malarial fever produced by Acystosporidae, 103 f.
  Mal de Caderas (= falling sickness of cattle), 119
  Male gamete, 33;
    motile in Lower Plants, Higher Cryptogams, Cycads, and _Ginkgo_, 38;
    of _Pandorina_, 128 f.;
    of _Eudorina_, 129;
    of Peritrichaceae, 151
  —see also Sperm, Spermatozoon
  Malignant tumour, associated with _Leydenia_, 91
  Mammals, syngamy in, 34;
    contain Sarcosporidiaceae in muscles, 108
  Man, host of _Amoeba_, 57;
    of Coccidiaceae, 102 f.;
    of _Sarcocystis tenella_, 108 n.;
    of _Trichomonas vaginalis_, 119;
    of Trypanosomes, 119 f.;
    of the Ciliata _Nyctotherus_ and _Balantidium_, 152
  _Manicina_ (Fam. Astraeidae, _399_), 373;
    _M. areolata_, 370, 389
  Mann, on function of nucleus, 24 n.
  Manson, on relation of Filarial disease to gnats or mosquitos, 103
  Manson, the subject of inoculation experiments with malarial parasites,
      106
  Manubrium, 251
  _Margelis ramosa_, _269_
  _Margelopsis_, _270_, 274 f.
  Marginal, anchors, 320;
    cirrhi, 139 f.
  _Marginaster_, _464_
  Marine, Foraminifera, 60 f.;
    Heliozoa, 75
  Marrow, red, of bones, habitat of resting states of malarial parasites,
      106 n.
  Marshall, on amphidiscs, 179
  Marshall, on Pennatulacea, 359 n.;
    on the physiology of the nervous system of _Antedon rosacea_, 585
  Marsigli, 167
  _Marsupifer valdiviae_, 379
  _Marsupites_, 588
  _Maryna_, _137_;
    _M. socialis_, tube, 152
  Massee, on Myxomycetes, 93 n.
  _Mastigamoeba_, _109_, 112
  Mastigophora (Bütschli's name for Flagellata), 109
  _Mastigophrys_, _71_, _75_ n.
  Maturation of schizont of Acystosporidian parasite in relation to
      fever-fit, 103
  Maupas, on Protozoa, 45;
    on life-cycle of Ciliata, 147 f.
  _Maupasia_, _111_, 124
  Mayer, 312
  _Meandrina_, 388, _401_;
    _M. labyrinthica_, 370
  Mechanical stimuli, 19 f.
  Median dorsal process of ciliated band of Bipinnaria, 606
  Medium gametes in _Pandorina_, 128 f.
  Medusa, 250;
    of _Millepora_, 259 f.;
    of Gymnoblastea, 262 f.;
    of Calyptoblastea, 277 f.;
    in Trachomedusae, 288 f.;
    in Narcomedusae, 295 f.;
    fresh-water, 292 f.;
    of Siphonophora, 302, 309;
    in Scyphozoa, 310 f.
  Medusome, 250, 251, 252
  Megagamete, 33;
    see also Female gamete, Oosphere
  _Megalactis griffithsi_, _384_
  Megalosphere, megalospheric, 67 f.
  Megamastictora, _183_, 184 f.
  Meganucleus, 136, 139 f., 144, 149 f.;
    degeneration of, in conjugation, 148 f.;
    new formation of, in conjugation, 148, 151;
    of _Stylonychia mytilus_, 139 f.;
    of _Carchesium_, 146;
    of _Paramecium caudatum_, 148, 151;
    of _Trachelius ovum_, 153;
    of _Stentor_, 154;
    of _S. polymorphus_, 156;
    of Suctoria, 160 f., 162;
    conjugation of, in _Dendrocometes_, 161
  Megazooid of _Vorticella_, 157
  Megazoospores, 85
  Meissner, on classification of Spatangoidea, 554 n.
  Melanin, 103
  _Melitodes_, 333, _351_, 353;
    _M. chamaeleon_, 338;
    _M. dichotoma_, 338, 351
  Melitodidae, 337, _351_
  _Mellita_, 548, _549_
  _Melobesia_, 422
  Melonitidae, _557_
  Membrana reticularis, 199, 200
  Membrane, undulating, of Flagellata, 110, 115, 123;
    of _Trypanosoma_, 115;
    of Trichonymphidae, 123;
    of Ciliata, 137, 139 f., 145, 156 f.;
    of _Stylonychia mytilus_, 139;
    of _Pleuronema_, 145;
    of _P. chrysalis_, 153, 154;
    of _Caenomorpha uniserialis_, 155;
    of _Vorticella_, 156, 157
  Membranella, 137, 139 f., 145;
    of _Stylonychia mytilus_, 139 f.;
    of _Metopus sigmoides_, 154;
    of _Caenomorpha uniserialis_, 155;
    of _Vorticella_, 156
  Meresjkowsky, 238, 274
  _Merozoite_, 97;
    of _Coccidium schubergi_, 99 f., 101;
    of Haemosporidae, 102;
    of Acystosporidae, 103, 104 f.
  _Mertensia_, _417_;
    _M. ovum_, 417;
    stage of Lobata and Cestoidea, 414
  Mertensiidae, _417_
  Mesenchyme, 604
  Mesenteric filaments, Alcyonaria, 331, 333;
    Zoantharia, 369
  Mesenteries, of Alcyonaria, 329, 334;
    of Zoantharia, 329, 366 f., 368;
    of _Asterias rubens_, 439;
    of _Holothuria nigra_, 562;
    of _Antedon rosacea_, 585, 586
  Mesnil, on Sporozoa, 94;
    Caullery and, on Actinomyxidiaceae, 98 n.
  _Mesodinium_, _137_, 152
  Mesogloea, 246;
    of Alcyonaria, 330
  Metabolic, metabolism, 13
  Metacnemes, 367
  _Metacrinus_, 588, _591_
  _Metallogorgia_, _355_
  Metamorphosis, of Insects, 44;
    of Dipleurula, 610 f.
  Metamp, 168, 169
  Metaphytes, 41
  —see also Plants, Higher
  Metazoa, 40 f.;
    rheotaxy of, 21;
    origin of, from Protozoa, 40 f.;
    flagellate sperms of, 109;
    hosts of Polymastigidae, 111
  —see also Animals, Higher
  Method of study of the life-cycle, of organisms of putrefaction, etc.,
      44;
    of Flagellata, 116;
    of Ciliata, 147
  _Metopus_, _137_;
    _M. pyriformis_, 154;
    _M. sigmoides_, 154
  _Metridium_, _381_ (= _Actinoloba_, _q.v._)
  Metschnikoff, 167, 178, 237 n., 296
  Microbes, 44
  _Microciona_, 225
  Microgamete, 33;
    of certain Coccidiaceae, 101
  —see also Sperm, Spermatozoon
  _Microgromia socialis_, _59_ f., 60
  _Microhydra_, _256_
  Micromastictora, _183_, _195_ f.
  Micronuclei, micronucleus, 136, 139, 144 f., 148 f., 151 f., 155, 157,
      159, 160 f.;
    of certain Flagellata, a blepharoplast, 109 n.;
    relations of Trypanosomic blepharoplast to, 121;
    of _Stylonychia mytilus_, 139;
    of _Paramecium caudatum_, 148, 151;
    in conjugation, 148 f.;
    numerous, of _Stentor_, 154;
    of _Vorticella_, 157;
    of Suctoria, 160 f.;
    of _Podophrya_, 160;
    of _Acineta jolyi_, 160
  Micropyle, 230
  Microscleres, 176
  _Microsolena_, _404_
  Microsphere, microspheric, 67 f.
  Microzooid of _Vorticella_, 157
  Microzoospores, 85
  Miescher's tubes, 108
  Migratory pairing nucleus, 149 f.;
    of Peritrichaceae, 151 f.
  _Miliola (Quinqueloculina)_, 65
  Miliolidaceae, _59_
  _Miliolina_, _59_, 66, 70
  _Milleporina_, _257_ f., 258, 260, 282;
    nematocysts of, 247, 259
  Milleporina, _257_
  Mimicry among Gymnostomaceous Ciliata, 152 n.
  Minchin, on Sporozoa, 94 f.;
    on Sponges, 168, 172 n., 185, 186 n., 227 n., 232, 316 n.
  Minnows prey on _Anopheles_, 106
  _Minous inermis_, 268
  Minyadidae, 328, 366, 377, _383_
  Miserly cells, 32 f.
  _Mithrodia_, _464_
  Mithrodiidae, _464_
  Mitosis, 25, 26, 27 f.;
    functions of, 28 f.;
    of micronuclei in Ciliata, 144
  Mitrophanow, on trichocysts, 142 n.
  _Mitrophyes_, _306_
  Miyajima, 273
  _Mnemia_, _420_
  Mnemiidae, _420_
  _Mnemiopsis_, _420_
  _Mnestra_ (position undetermined), 269
  Mohl, von, on protoplasm, 3
  Mole-cricket, host of _Lophomonas_, 123
  Molluscs, hosts of Gregarines, 98
  Molluscum contagiosum, 102
  Molpadiida, 568, 569, _575_, 576, 577, 578
  Monacanthid, 457
  Monadidae, _111_
  Monadineae, applied to Proteomyxa by Cienkowsky and Zopf, 89
  Monads, a name for the lowest, simplest Flagellata, 109, 116 n.
  _Monas_, _111_, 119;
    _M. dallingeri_, gametes of, 116 n.
  Monaxon, 183, 184, 232
  Monaxonic (= symmetrical about one single axis), 76
  Monaxonida, _211_, _216_ f.
  Moniliform meganucleus of _Stentor_, 156
  Monobrachiidae, _274_
  _Monobrachium_, _274_
  _Monocaulus_, 263, _273_;
    _M. imperator_, 269, 273
  Monocyclica, 594
  _Monocystis_, _97_ f.
  _Monograptus_, _282_
  _Monophyes_, 303, _306_
  Monophyidae, _306_
  Monoprionidae, _282_
  Monopylaea (= Nassellaria), _76_
  _Monorhaphis_, _197_
  _Monosiga_, _111_, 122
  Monosiphonic, 275
  Monotrypasta (= Osculosa), _76_
  _Monoxenia darwinii_, _342_
  Monstrous Foraminiferal shells, possible formation of, 69
  _Montipora_, 390, 396
  Moore, 293
  _Mopsea_, _353_
  Morgan, on regeneration, 35 n.
  Morphological contrast of Animals and Plants, 38 f.
  Mortensen, on classificatory value of pedicellariae, 532;
    on classification of Cidaridae, 534;
    of Echinothuriidae, 536
  Moseley, 258, 333, 338, 345, 411
  Mosquito (= gnat), 103 f.;
    dappled-wing-, intermediate hosts of Acystosporidae, 103
  Mosquito-netting, a prophylactic against malarial fever, 103
  Moss-dwelling Protozoa, 48
  Mosser, F., 418 n.
  Motile organs, 17 f.
  Motile reactions of Protozoa, 19 f.
  Motility, 9
  Motion, ciliary, 18;
    gliding, of protoplasm, 47 n.
  Moulting of cuticle or cell-wall in Dinoflagellata, 130;
    of _Dendrocometes_, 161
  Mouth, of Flagellata, 113;
    absent from Opalinidae, 123;
    of _Maupasia_, 124;
    excreta expelled by, in _Noctiluca_, 133;
    of Ciliata, 137, 145 f.;
    of Gymnostomaceae, 137, 143, 145, 152;
    of _Stylonychia mytilus_, 139 f.;
    of _Dysteria_, 145;
    of _Pleuronema_, 145;
    of _P. chrysalis_, 153;
    of _Paramecium caudatum_, 148, 151;
    trichocysts of, in Gymnostomaceae, 143;
    of _Trachelius ovum_, 153
  Mouth-angle of _Ophiothrix fragilis_, 482
  Mouth-frame, of Asteroidea, 436, 483;
    of _Ophiothrix fragilis_, 482;
    of Ophiuroidea, 483, 492 f.;
    of _Ophiarachna incrassata_, 484;
    of _Ophiacantha_, 492;
    of _Ophioscolex_, 492;
    of _Ophiothrix_, 492
  Mouth-papilla, of Ophiuroidea, 483, 492;
    of _Ophiocoma_, 493
  Movements, amoeboid, 5 f., 125 n.;
    of Protista, 16 f.;
    of Higher Plants, how produced, 38;
    springing, of _Bodo saltans_, 114;
    of _Euglena_, 124 f.;
    euglenoid, 125 f.;
    metabolic, 125 n.;
    of Sporozoa, 125 n.;
    of _Stylonychia_, 138;
    springing, of tailed Ciliata, 141 n.;
    of _Halteria_, 155;
    of Suctorian tentacles, 159 f.
  _Muggiaea_, _306_;
    _M. atlantica_, 304;
    _M. kochii_, 303
  Müller, J., on recognition of Echinoid larva, 518;
    on the name Pluteus, 607
  Müller, O. F., on Protozoa, 45
  _Multicilia_, _109_
  Multinucleate Amoeba (_Pelomyxa_), 16;
    Protozoa, regeneration of, 35
  Multiple budding, in Suctoria, 160 f.
  Multiple fission, 30 f.
  —see also Brood-division
  Murbach, 248 n., 288
  _Muricea_, _356_
  Muriceidae, 330, _355_
  Muscle of _Vorticella_, 157
  Muscle-cell, 19
  Muscular contraction, physical explanation of, 19
  —see also Myonemes
  _Mussa_, _401_
  Mycetozoa (= Myxomycetes, _q.v._), _50_, _90_ f.;
    in relation to Fungi, 40;
    studied by botanists, 45;
    relations of, 49
  Myocyte, 96, 98
  Myonemes, 19, 96;
    of _Trypanosoma_, 120 f.
    of _Stylonychia_, 140;
    of Ciliata, 142;
    of _Vorticella_, 157
  Myophrisks, 80
  _Myriophrys_, _71_
  _Myriothela_, 269, _274_
  Myriothelidae, _274_
  _Myxaster_, _466_
  Myxasteridae, _464_
  _Myxastrum_, _70_, 73
  _Myxidium_, _98_;
    _M. lieberkühnii_, 107
  _Myxilla_, 225
  _Myxobolus_, _98_;
    spores of, 107
  _Myxobrachia_, 83
  Myxogasteres, Myxogastres, _90_ f.
  Myxoidea, _89_
  Myxomycetes, _90_ f.;
    rheotaxy of plasmodium in, 21
  —see also Mycetozoa
  Myxospongiae, _196_
  Myxosporidiaceae, _98_, 106 f.;
    spores, 107

  Nagana disease of hoofed quadrupeds, 119
  Naked Protozoa, 51 n.
  Narcomedusae, 288, _295_ f.
  _Nardoa_, 455, 456, _471_
  Nassellaria, _76_, _78_;
    skeleton of, 83;
    geological occurrence of, 88
  Nassoidea, _78_
  _Nausithoe_, _322_;
    Scyphistoma of (= _Spongicola fistularis_), 317;
    _N. punctate_, 322;
    _N. rubra_, 322
  Nectocalyces, 297, 298, 305
  Needham on spontaneous generation, 43
  Nematocyst, 246 f., 247;
    of Actinomyxidiaceae, 98;
    of Myxosporidiaceae, 98, 107;
    of _Myxobolus mülleri_, 107;
    of _Polykrikos_, 131, 249;
    of _Epistylis_, 249;
    of _Aeolis_, 248;
    of _Hydra_, 247;
    of _Millepora_, 247, 259;
    of Siphonophora, 300;
    of Scyphozoa, 312;
    of _Alcyonium_, 247;
    of _Sarcophytum_, 248;
    of _Cerianthus_, 247
  Nematodes parasitic in blood, 103
  Nematophores, 277
  Nemocera (= gnats or mosquitos), 103 n.
  _Neohelia_, _399_
  _Neolampas_, _554_
  Neosporidia, _97_, 106
  _Nephthya_, 341, _349_
  Nephthyidae, _349_
  Neresheimer, on neurophane fibrils in Ciliata, 143 n.
  Nerve-ring, of _Asterias rubens_, 444, 447;
    of _Ophiothrix fragilis_, 488;
    of _Echinus esculentus_, 518, 521, 527;
    of _Antedon rosacea_, 583;
    outer, of _A. rosacea_, 585
  Nervous fibrils in Ciliata, 143
  Nervous system, in Animals, not in Plants, 39 f.;
    of _Asterias rubens_, 444 f.;
    of _Ophiothrix fragilis_, 488;
    of _Echinus esculentus_, 518 f.;
    of _Holothuria nigra_, 566;
    of _Antedon rosacea_, 583 f.
  Neuron, 444
  Neurophane (= supposed nervous fibrils in Ciliata), 143 n.
  Newts, _Trichodina_ parasitic in, 158
  Nitriles in relation to nutrition, 36
  _Noctiluca_, _110_, 132 f.;
    endosarc, 144;
    _N. miliaris_, 133
  _Nodosaria_, _59_, 63, 66 f.
  _Nosema_, _98_;
    _N. bombycis_, 107;
    organism of pébrine, 107
  _Nubecularia_, _59_
  Nuclear apparatus, of Infusoria, 48, 136;
    of Ciliata, 139 f., 144 f.;
    of diffused granules, in marine Ciliata, 144 n.;
    of Suctoria, 159
  —bipartition in _Trichosphaerium_, 54
  —reduction of _Actinosphaerium_, 75 n.;
    of _Monocystis_, 96;
    of Coccidiaceae, 100, 104 f.;
    of Acystosporidae, 104 f.;
    of Myxosporidiaceae, 107;
    of Flagellates, 116 n.
  —divisions, in spores of _Lankesteria_, 95
  —see also Mitosis, Karyokinesis
  _Nuclearia_, _70_
  Nuclein mass (= karyosome), 24
  Nucleinic acid, 7 n.
  Nucleole, nucleolus, 7, 24, 25 f., 27;
    of Sea-urchin ovum, 7;
    of _Sphaerella_, 126
  Nucleolidae, _554_
  _Nudeolites_, _554_
  Nucleoplasm, 6
  Nucleoproteids, 12
  Nucleus, 6;
    of cell, 6 f.;
    of _Amoeba_, 5 f.;
    of _A. polypodia_, 10;
    resting, function of, 24 n
  —in mitosis, 25 f., 27 f.;
  —of _Euglypha_, 29;
    of _Paramecium caudatum_, 148
  —pairing state of, 34;
    of Ciliata, 150 f.;
    of _Paramecium caudatum_, 148
  —of Rhizopods, 52;
    of _Pelomyxa_, 52;
    of _Microgromia socialis_, 60;
    of Foraminifera, 62;
    of mega- and microspheric forms of Foraminifera, 68 f.;
    of Heliozoa, 71, 72, 74;
    of _Clathrulina_, 74;
    of Radiolaria, 76;
    of _Collozoum inerme_, 76;
    of Myxomycetes, 92;
    of Sporozoa, 95 f.;
    of Gregarines, 95 f., 98 f.;
    of _Bodo saltans_, 117;
    kineto-, of _Trypanosoma noctuae_, 120, 121;
    trophic, of _T. noctuae_, 120;
    of Choanoflagellates, 122;
    of _Opalina_, 123;
    of _Maupasia_, 124;
    of _Sphaerella_, 126;
    of _Volvox_, 126;
    of _Noctiluca_, 133
  —fusion- or zygote-, 150;
    of _Paramecium caudatum_, 148
  —see also Meganucleus, Micronucleus, Gametonuclei, Nuclear apparatus
  Nuda, _423_
  Nummulitaceae, _59_
  _Nummulites_, _59_, 63, 70
  Nussbaum, on regeneration in Protozoa, 35 n.;
    on _Hydra_, 254
  Nutrition, 9;
    animal and vegetal, 35 f.;
    of Flagellates, 110 f., 113;
    of Dinoflagellates, 130 f.—see Holozoic, Holophytic, Saprophytic
  —of Alcyonaria, 339
  Nutritive function of granular cytoplasm of muscle-cell, 19
  Nuttall, history of discoveries on Acystosporidae, 103
  Nutting, 276, 280 n.
  _Nyctotherus_, _137_;
    habitat of, 152

  _Obelia_, _280_
  _Oceanapia_, 223
  Ocellus, 252
  Octactine, 200
  Octactinellida, _208_
  _Octotremacis_, _346_
  Ocular plate of _Echinus esculentus_, 512
  _Oculina_, _399_
  Oculinidae, _399_
  _Ocyroe_, _420_;
    _O. crystallina_, 419
  Ocyroidae, _420_
  _Odinia_, _474_, 475
  Oecology of Protista, 43
  Oesophagus, of _Asterias rubens_, 438;
    of _Echinus esculentus_, 516;
    of _Holothuria nigra_, 562;
    of _Antedon rosacea_, 583;
    of Dipleurula, 605
  Ogilvie, M., 401 n.
  Oikomonadidae, _111_
  _Oikomonas_, _111_, 112
  Oil-drops, of Radiolaria, 79 f.;
    luminous, 80
  Oil-globules, 37;
    of Ciliata, 144
  Oleocyst, 305
  Oligotrichaceae, _137_, 155
  _Olindias_, _291_;
    _O. mülleri_, 291
  Olindiidae, 289, _291_
  _Olindioides_, _291_;
    _O. formosa_, 291
  Olynthus, 185
  Omne vivum, ex ovo, 42;
    ex vivo, 44
  Onychaster, 203
  _Onychaster_, 502
  Oocyte (a cell which by unequal divisions or mere nuclear divisions
      becomes converted into an oosphere), 100
  Oogamete of Coccidiaceae, 100 f.
  Ookinete (_active zygote_) or oosperm, of Acystosporidae, 104 f.;
    of _Trypanosoma noctuae_, 120
  Oolitic limestones, nucleus of concretions of, 70
  Oosperm, 34;
    of Sporozoa, 96 f.;
    of _Volvox globator_, 127 f.
  Oosphere, 31;
    formation of, in Metazoa, 75 n.;
    of Gregarinidaceae, 99, 100;
    of Coccidiaceae, 100 f.;
    of Acystosporidae, 104 f.
  Oospore (= zygotospore), from bisexual syngamy, 100;
    of _Volvox globator_, 127 f.
  Ooze, _Globigerina_, 61;
    Radiolarian, 87
  Oozooid, 358
  _Opalina_, _111_;
    galvanotaxy, 22;
    ciliiform flagella, 114;
    species, 124 n.;
    nuclei, 144 n.;
    systematic position of, 144 n., 145 n.;
    _O. ranarum_, 123
  Opalinidae, _111_, 123 f.
  Opalinopsidae, 145 n.
  _Opercularia_, _138_, 158
  Operculum of central capsule of Phaeodaria, 76, 82
  _Ophiacantha_, 492, _499_;
    mouth-frame, 492;
    _O. chelys_, 499
  _Ophiactis_, _498_;
    _O. balli_, 498;
    _O. virens_, 491, 498
  _Ophiarachna_, 481, _499_
  Ophicephalous pedicellariae, of _Echinus esculentus_, 508;
    of _E. acutus_, 509;
    of _E. elegans_, 510;
    absent in Cidaridae, 534;
    of _Echinarachnius parma_, 544
  _Ophidiaster_, _471_
  _Ophiocoma_, 493, _499_;
    _O. nigra_, 499
  Ophiocomidae, _499_
  Ophiodermatidae, 495 n.
  _Ophiodes_, 277, _280_
  _Ophiogeron_, _494_
  _Ophioglypha_—see _Ophiura_
  _Ophiohelus_, 481, 491, _494_;
    _O. umbella_, skeleton, 493
  Ophiolepididae, _495_
  _Ophiomusium_, _497_;
    _O. pulchellum_, 497, 498
  _Ophiomyxa_, 493, _494_;
    _O. pentagona_, 494
  _Ophiopholis_, 491, _499_;
    _O. aculeata_, 499
  Ophiopluteus, 606, 608;
    metamorphosis of, 613
  _Ophiopsila_, _499_
  _Ophiopteron_, 492, _500_
  _Ophiopyrgus_, 493, _497_
  _Ophioscolex_, mouth-frame, 492
  _Ophioteresis_, 481, 491, _494_
  _Ophiothrix_, 493, _499_;
    mouth-frame, 492;
    _O. fragilis (pentaphyllum)_, 478, 479;
    arm of, 479;
    vertebra, 480, 491;
    podia, 479;
    mouth-frame, 482;
    disc, 484;
    genital bursa and respiratory movements, 485;
    alimentary canal, 485;
    water-vascular system, 486;
    axial sinus, 487;
    perihaemal spaces, 488;
    nervous system, 488;
    genital organs, 489
  _Ophiura (Ophioglypha)_, _496_;
    _O. albida_, 497;
    _O. bullata_, 495, 496;
    _O. ciliaris_, 485 n., 497
  Ophiuroidea (Brittle Stars), _431_, _477_ f., 561;
    mesenchyme of larva, 604;
    development of, 606, 606, 608, 613;
    phylogeny, 622
  _Ophlitaspongia_, 225
  _Ophrydium_, _138_, 158
  _Ophryocystis_, _97_
  _Oplorhiza_ (Campanulariidae, _280_), 277
  _Oractis_, _377_
  Oral apparatus, of Ciliata, its atrophy and regeneration during
      conjugation, 151
  Oral blood-ring, of _Asterias rubens_, 450;
    of _Ophiothrix fragilis_, 488;
    of _Holothuria nigra_, 567
  Oral cleft or groove of _Noctiluca_, 132 f.
  Oral coelom of _Antedon rosacea_, 585
  Oral plates, of Crinoidea, 588;
    of _Thaumatocrinus_, 589;
    of _Hyocrinus_, 588, 590;
    of _Holopus_, 588, 592;
    of _Rhizocrinus_, 589, 591;
    absent in adult _Antedon_, 589;
    of Rhizocrinidae, 590, 591;
    of _Bathycrinus_, 591;
    of Cystoidea, 598;
    of Blastoidea, 599;
    of young _Antedon rosacea_, 619
  Oral spots of Protomastigaceae, 110
  Oral valves, of _Antedon rosacea_, 581;
    of young _A. rosacea_, 619
  _Orbicella_, 373, 375, _400_
  _Orbiculina_, _59_, 66
  Orbigny, A. d', on Foraminifera, 62
  _Orbitoides_, _59_
  _Orbitolites_, _59_, 66;
    pylomes, 64;
    dimorphism, 67;
    monstrous shell, 69
  _Orbulina_, _59_, 66;
    _O. universa_, 68
  _Orbulinella_, habitat, 75
  Organ-pipe Coral, 343
  Organella, 44 n.
  Organic compounds, their function in nutrition, 35 f.
  Organoid, 44 n.
  Oriental sore, 121
  Ornamentation of shell-wall in Foraminifera, 66
  _Oscarella_, _196_, 213, 225;
    _A. lobularis_, 230
  Osculosa (Monotrypasta), _76_
  Osculum, of Radiolaria, 76;
    of _Phaeodaria_, 76, 82;
    of Sponges, 169, 171, 174, 188, 189
  Ossicles of Holothuroidea, 569
  Ostium, 169
  Otocysts, of Elasipoda, 571;
    of Synaptida, 576
  Outer perihaemal ring, of _Asterias rubens_, 448;
    development of, in _Asterina gibbosa_, 612;
    represented by lantern-coelom of _Echinus esculentus_, 524;
    by buccal sinus of _Holothuria nigra_, 566
  Ovary of gnat infected by _Trypanosoma_ germs, 120
  Ovoid gland of _Ophiothrix fragilis_, 489
  Ovum of Sea-urchin, 7;
    of _Sarcocystis tenella_, 108 n.;
    of _Volvox globator_, 127 f.
  —see also Oosphere, Oosperm, Egg
  Owl, blood parasites of, 120
  Oxea, 183, 224
  Oxyaster (an aster with a small centrum and oxeate actines), 222
  _Oxytricha_, _138_
  Oxytylote (a rhabdus of which one actine is oxeate, the other tylote or
      knobbed, the latter directed towards the surface of the Sponge), 224

  _Pachychalina_, 223
  _Pachymatisma_, _215_;
    _P. johnstonia_, 215, 222;
    _P. normani_, 215
  Pairing in _Trichosphaerium_, 54
  Pairing-cells, 31, 33 f.;
    in _Lankesteria_, 95
  —see also Gametes, Syngamy
  Pairing nuclei, state of, 34;
    of Ciliata, 150;
    of _Paramecium_, 148
  _Palaeaster_, 476
  _Palaeocoma_, 476
  _Palaeodiscus_, 557
  Palaeoechinoidea, 556
  _Palaeoechinus_, _557_
  _Palephyra_, _322_
  Pali, 370, 385, 387
  _Palmella_ state of _Zooxanthella_, 86
  _Palmipes_, 461, _464_;
    _P. membranaceus_, 464
  _Pamphagus_, test of, 59 f.
  Panceri, 339
  _Pandorina morum_, _111_, 129
  Pansporoblast of Myxosporidiaceae, 107
  Pantostomata, _109_
  Papula (including dermal gill), 432, 457;
    compared to diplopore of Cystoidea, 599
  Paractinopoda, 570
  Paragaster, 187
  Paragastric canals, 416
  Paraglycogen (= paramylum), 95
  _Paragorgia_, 333, 336, _351_
  _Paralcyonium_, _349_
  _Paramecium_, _137_, 143 n., 151, 153;
    specific gravity of, 13 n.;
    thigmotaxy of, 20;
    reaction to repellent stimuli, 21 n.;
    thermotaxy of, 22;
    galvanotaxy of, 22;
    chemiotaxy of, 23;
    trichocysts of, 143, 151;
    contractile vacuoles of, 143 n., 151;
    _P. bursaria_, 153;
    _P. caudatum_, 151;
    in conjugation, 148
  _Paramoeba_, 51 n., _110_;
    nutrition of, 113;
    formation and regeneration of chromatophores in, 115;
    _P. eilhardii_, 116 n.;
    reproduction of, 116 n.
  _Paramuricea_, _356_;
    spicule, 336
  Paramylum (= paraglycogen), 37, 95, 115;
    in Gregarines, 95;
    in Flagellates, 115;
    in Ciliata, 144
  Parangi, 121 n.
  Parapyle, 81
  _Pararchaster_, _466_
  Parasites, in relation to brood-formation, 33;
    internal, belonging to Metazoa, nutrition of, 38;
    of man, 57, 103 f., 108 n., 119 f., 152;
    of Radiolaria, 86 f.;
    of _Sticholonche_, 86, 161;
    of plants, 88 f.;
    of Acantharia, 86, 161;
    of Crucifers, 89;
    of Crustacea, 89;
    of earthworm, 95;
    of centipedes, 99;
    of _Epizoanthus glacialis_, 99;
    of _Lithobius forficatus_, 99;
    of cold-blooded Vertebrates, 102;
    of rabbit, 102;
    of cattle, 102, 119 f.;
    of frog, 102, 124;
    of birds, 103;
    of fish, 107;
    of silkworm, 107;
    of fish, 107, 119, 152;
    of sheep, 108 n.;
    of insects, 111, 123;
    of Amphibia, 111, 123 f., 158;
    of dog, 119;
    of horse, 119;
    of ox, 119;
    of Rodents, 119;
    of owl, 120 f.;
    of cockroach, 123;
    of mole-cricket, 123;
    of Termites, 123;
    of Metazoa, 152;
    of Ruminants, 152;
    of Heliozoa, 155;
    of _Raphidiophrys_, 155;
    of _Hydra_, 158;
    of newts, 158;
    of Ciliata, 159
  Parasitic, Proteomyxa, 48, 88 f.;
    Sporozoa, 48, 94 f.;
    Flagellata, 48, 111, 119 f., 123;
    Rhizopoda, 57;
    Ciliata, 152, 158;
    Suctoria, 161;
    Hydrozoa, 268 f.
  _Parasmilia_, _401_
  Parazoa, _181_
  _Parazoanthus_, _406_;
    _P. anguicomus_, 406;
    _P. separatus_, 406;
    _P. tunicans_, 406
  Parenchymalia, 201
  Parenchymula, 227
  _Parisis_, _351_
  Parker, 371
  _Parkeria_, _283_
  Paroral cilia, 139;
    of _Vorticella_, 156 n.
  Parthenogenesis, of malarial parasites, conjectured, 106 n.
  Pasteur, on organisms of fermentation and putrefaction, 43;
    on nature of pébrine (_Nosema bombycis_), 107
  _Patellina_, _59_;
    reproduction of, 69
  Patina, 580;
    of _Antedon rosacea_, 582;
    of _Thaumatocrinus_, 589;
    of _Hyocrinus_, 590;
    of Rhizocrinidae, 590;
    of Pentacrinidae, 591
  _Paulinella_, _52_;
    test of, 54;
    pylome of, 54
  _Pavonia_ (Fungiidae, _403_), 390
  Paxilla, 455;
    evolution of, 466;
    relation to granules of Valvata, 471
  Paxillosa, 461, _466_ f.;
    restriction of papulae to dorsal surface, 469
  _Peachia_, 366, _380_;
    _P. hastata_, 370
  Pébrine, 107
  Pectinate pedicellariae, 456, 466
  Pectyllidae, _294_
  _Pectyllis_, _294_
  Pedal laceration, 372
  Pedal nerve, of _Asterias rubens_, 455;
    of _Ophiothrix fragilis_, 488;
    of _Echinus esculentus_, 518;
    of _Holothuria nigra_, 566
  Pedicellaria, of _Asterias rubens_, 432;
    of _A. glacialis_, 434;
    of Asteroidea, 456;
    alveolate, 456;
    pectinate, 456;
    pincer-shaped, 456;
    valvate, 456;
    forcipulate, 433, 434, 456, 462;
    representatives in Ophiuroidea, 492;
    in _Ophiothrix fragilis_, 492;
    in _Ophiohelus_, 492, 493;
    of _Echinus acutus_, 509;
    of _E. esculentus_, 506 f., 507;
    tridactyle, 506, 509, 510;
    gemmiform, 506, 509, 510;
    trifoliate, 508, 509, 510;
    ophicephalous, 508, 509, 510;
    function of, 508 f.;
    of Endocyclica, 532;
    of Cidaridae, 532;
    of Echinothuriidae, 532, 535;
    of _Centrostephanus longispinosus_, 532;
    of _Echinarachnius parma_, 544, 545;
    of _Echinocardium cordatum_, 550;
    absent in Pelmatozoa, 582
  _Pedicellaster_, _474_
  Pedicellasteridae, 456, _474_
  _Pegantha_, _296_
  Peganthidae, _296_
  Pekelharing, 187 n., 234 n., 237
  _Pelagia_, 311, 312, 315, 316, _323_;
    _P. noctiluca_, 311;
    _P. perla_, 323;
    _P. phosphora_, 311, 323
  Pelagic, Foraminifera, 61, 66, 69;
    Radiolaria, 76;
    Dinoflagellates, 131
  Pelagiidae, _323_
  _Pelagohydra_, 263, _274_;
    _P. mirabilis_, 274
  Pelagohydridae, _274_
  _Pelagothuria_, 568, 569, _572_;
    _P. natans_, 572
  Pelagothuriida, _572_, 577, 578
  Pellicle, of Protozoa, 46;
    of Flagellates, 109 f., 113;
    of _Noctiluca_, 133;
    of Ciliata, 139, 141, 142;
    of _Coleps_, 141, 152;
    of _Dysteria_, 153;
    of _Vorticella_, 157;
    of Suctoria, 159;
    of tentacles of Suctoria, 161
  —see also Cuticle
  Pelmatozoa, _430_, 579 f.;
    phylogeny of, 621
  _Pelomyxa_, _51_;
    _P. palustris_, 52 f.;
    pepsin in, 16
  Penard, on Heliozoa, 71;
    on Rhizopoda, 58 n.
  _Peneroplis_, _59_, 66;
    pylomes of, 64
  _Peniagone_, _572_
  _Pennaria_, _272_
  Pennariidae, _272_
  _Pennatula_, _361_;
    _P. grandis_, 361;
    _P. murrayi_, 359, 362;
    _P. naresi_, 362;
    _P. phosphorea_, 338, 361
  Pennatulacea, 333, 335, 337, _358_, 358, 359, 363 f.
  _Pentaceros_, _472_
  Pentacerotidae, 457, 458, 459, _471_
  _Pentachogon_, _294_
  Pentacrinidae, 588, 589, _591_ f.
  Pentacrinoidea, 595
  _Pentacrinus (Isocrinus)_, _591_ f.;
    _P. asteria_, 592;
    _P. maclearanus_, 593
  _Pentagonaster_, _471_;
    _P. japonicus_, 472
  Pentagonasteridae, 455, _471_
  _Pentremites_, _599_
  Pepsin, 16
  Peptic, digestion, 16;
    juice in _Carchesium_, 147
  Peptones, 15
  Perforate, Corals, 371;
    Foraminifera, _58_ f.
  _Peribolaster_, 453, _463_
  Peribranchial spaces of _Asterias rubens_, 449
  _Pericolpa_, _322_
  Peridiniales, synonym of Dinoflagellata used by Schütt, 119, 132
  _Peridinium_, _110_;
    fossil, 132;
    _P. divergens_, 131
  _Perigonimus_, 265, 266, _269_
  Perihaemal spaces (or canals), of _Asterias rubens_, 448;
    of _Ophiothrix fragilis_, 481, 488;
    of _Echinus esculentus_, 524;
    of _Holothuria nigra_, 566
  _Peripatus_, segmentation of, 32 n.
  _Periphylla_, 311, 314, _322_;
    _P. regina_, 322
  Periphyllidae, 310, _322_
  Periproct, of _Echinus esculentus_, 504, 507, 512, 513, 534;
    of Endocyclica, 530, 534, 538;
    of Cidaridae, 530, 533, 534;
    of Arbaciidae, 530;
    of Echinothuriidae, 535;
    development of, in young Echinoid, 613
  Peripylaea (= Spumellaria), _76_
  Perisarc, 251, 263
  _Perisiphonia_, 277, _280_
  Peristalsis of intestine of _Holothuria nigra_, 563
  Peristome, of Ciliata, 137 f.;
    in fission, 147;
    of Peritrichaceae, 155 f.;
    of Vorticellidae, 155 f., 157;
    of _Asterias rubens_, 434;
    of _Echinus esculentus_, 505, 513, 514;
    of Asteroidea, 514;
    of Cidaridae, 514, 530, 533;
    of Endocyclica, 530;
    of Echinothuriidae, 535;
    of _Asthenosoma hystrix_, 537;
    of Saleniidae, 537;
    of Arbaciidae, 538;
    of Diadematidae, 538;
    of Echinidae, 539;
    of _Echinocardium cordatum_, 550;
    of Spatangoidea, 553;
    of Palaeostomatidae, 554
  Peristomial area, 139;
    of _Caenomorpha_ and _Metopus_, 154;
    of _Bursaria_, 155
  Peristomial collar, 142, 156, 157
  Peristomial plate, of _Ophiothrix fragilis_, 483;
    of Endocyclica, 530;
    of Cidaridae, 530, 533;
    of Echinothuriidae, 530, 535;
    of Saleniidae, 537;
    of Arbaciidae, 538;
    of _Echinus microtuberculatus_, 540
  Peritoneum, of _Asterias rubens_, 437;
    of _Holothuria nigra_, 567
  Peritrichaceae, _138_;
    pellicle, 141;
    myonemes, 142;
    trichocysts, 143;
    contractile vacuole and reservoir, 145;
    pharynx, vestibule, mode of feeding, 145;
    fission, 147;
    conjugation, 151;
    peristomial collar, 156, 157
  Perkins, 232 n., 290
  _Peronella_, _549_
  Peronium, 295
  Perrier, on classification of Asteroidea, 461
  Persian Tick, 121
  Petal, of _Echinarachnius parma_, 545;
    of Fibularidae, 549;
    of Echinanthidae, 549;
    of _Echinocardium cordatum_, 551
  _Petalocrinus_, 595
  Petasidae, _294_
  _Petasus_, _294_
  Peters, 414 n.
  _Petrostroma_, _193_;
    _P. schulzei_, 193
  Pfeiffer, on Sporozoa, 94
  Phacella, 314
  _Phacotus_, _111_;
    shell, 113
  Phaeoconchia, _79_, 85
  Phaeocystina, _79_, 82, 87
  Phaeodaria (Cannopylaea, Tripylaea), _76_, _79_;
    skeleton, 82, 84, 85;
    central capsule, 82, 84;
    fission, 85
  Phaeodium, 76, 81, 82
  Phaeogromia, _79_
  Phaeosphaeria, _79_
  _Phakellia_, 224
  _Phalangopora_, 284, _286_
  Phalansteridae, _111_
  _Phalansterium_, 113
  Phanerocodonic, 253, 265
  Phanerozonate, 454
  Pharetronidae, _192_
  _Pharyngella_, _79_, 85
  Pharynx, of _Stylonychia mytilus_, 139;
    of Ciliata, 145;
    of Gymnostomaceae, 145;
    of _Paramecium caudatum_, 151;
    of _Caenomorpha uniserialis_, 155;
    of Peritrichaceae, 145;
    of _Carchesium polypinum_, 146 f.;
    of _Vorticella_, 156, 157
  Pharynx-tube of Euglenaceae, 124 f.
  —see also Flagellar Pit
  _Pheronema_, _204_;
    _P. carpenteri_, 204, 205, 221
  _Phialidium temporarium_, _281_
  _Pholidaster_, _474_
  _Phoriospongia_, _220_
  Phosphorescence, of Dinoflagellata, 132;
    of Cystoflagellata, 132, 134;
    of _Noctiluca_, 132, 134;
    of Scyphozoa, 311;
    of Alcyonaria, 338;
    of Pennatulids, 361;
    of Ctenophores, 414
  Phosphorescent oil-drops in Radiolaria, 80
  Photopathy, 21
  Phototaxy, 21;
    of _Euglena_, 125
  Phycochromaceae, 39
  Phycomycetes Zoosporeae related to Flagellata, 109
  Phycomycetous Fungi, relations of, 48
  Phylactocarp, 276
  Phyllactidae, _382_
  _Phyllangia_, _400_;
    _P. americana_, 374
  Phyllode, 553
  _Phyllograptus_, _282_
  _Phyllophorus_, _573_;
    _P. rugosus_, 567;
    _P. urna_, 574
  _Physalia_, 300, 302, 304, _308_
  Physiological contrast of Animals and Plants, 38
  Physiology, of cell and protoplasm, 3 f.;
    of Sponges, 234 f.;
    of nervous system of Echinoidea, 519 f.
  Physonectidae, 301, _307_
  _Physophora_, 301, _308_;
    _P. borealis_, 304
  Physophorae, _307_
  Physophorinae, _308_
  Pigment, of _Stentor_, 154;
    of Sponges, 169, 238;
    -granules, of Phaeodaria, 76, 80 f.
  Pinacocyte, 170, 213, 237
  Pincer-shaped pedicellariae, 456
  Pineau, on spontaneous generation, 43
  Pinnule, of _Antedon rosacea_, 581, 583;
    of _A. eschrichtii_, 594;
    of _Hyocrinus_, 590;
    of _Metacrinus_, 592;
    of fossil Crinoidea, 595
  Pinulus (a pentactine triaxon in which the unpaired actine bears lateral
      spines and projects beyond the bounding surface), 204
  _Piroplasma_, 120 f.
  _Placosmilia_, _401_
  _Placospongia_, spicules of, 233
  _Plakina_, _212_, 213;
    _P. monolopha_, 210, 212, 213
  Plankton, Protozoa of, 48;
    Foraminifera of, 61;
    Radiolaria of, 75
  _Planorbulina_, _59_, 63
  Plant(s), definition, 39;
    Animals and, discussion on, 35 f.;
    Higher, movements of, 38;
    insectivorous, 38;
    -Protists, relations of, 48;
    -cells, wall of, 37;
    protoplasmic connexions of, 37
  —see also Metaphytes
  Planula, 341
  _Plasmodiophora_, _89_
  Plasmodium, 30;
    rheotaxy, 21;
    of Myxomycetes, 50, 90 f.;
    of Proteomyxa, 88;
    of _Didymium_, 92
  _Plasmodium_, a generic name given to Acystosporid Coccidiaceae producing
      malarial fever, 103 f.—see _Haemamoeba_, _Laverania_
  Plastid, 21, 36 f.;
    of _Paulinella_, 54 n.;
    of Flagellates, 110, 112 f.;
    of _Trachelomonas_, 112
  —see also Chromoplasts, Chromatophore, Chromoplastid
  Plastogamy, 30, 34 n.;
    of Rhizopods, 56;
    of Foraminifera, _Discorbina_ and _Patellina_, 69;
    temporary, in _Actinophrys sol_, 72;
    of Myxomycetes, 90 f.
  Plastron, of _Echinocardium cordatum_, 550;
    of Sternata, 554
  Plate, on _Dendrocometes_, 162
  Plates, siliceous, of shell of Rhizopods, 29, 53 f.;
    ctenophoral, 141, 412 f.
  Platt, Julia B., on density of living protoplasm, 13 n.
  _Platybrissus_, _554_
  Platyctenea, 413, 416, _421_
  Platyhelminthes, bladder of, 14 n.
  _Plectinia_, _193_
  Plectoidea, _78_
  _Plectroninia_, _193_, 193;
    _P. halli_, 193
  Plesiofungiidae, _403_
  Plesioporitidae, _404_
  _Pleurobrachia_, _418_;
    _P. pileus_, 416, 417, 418;
    _P. rhodopis_, 418
  Pleurobrachiidae, _418_
  _Pleurocorallium_, _352_
  _Pleurocoralloides_, _352_
  _Pleurogorgia_, _355_
  _Pleuronema_, _137_;
    _P. chrysalis_, 153, 154
  _Plexaura_, _356_
  Plexauridae, _356_
  Plimmer and Rose Bradford, on _Trypanosoma_, 121
  _Pliobothrus_, 284, _286_
  _Plocamia_, 223
  _Plumohalichondria_, 225
  _Plumularia_, _279_;
    _P. echinulata_, 276;
    _P. halecioides_, 276;
    _P. profunda_, 275;
    _P. setacea_, 276
  Pluteus, 607
  Pneumatophore, 300 f., 307 f.
  Pneumatopyles, 309
  Pneumotaxy, 23
  _Pocillon_, 223
  _Pocillopora_, 375, _402_, 402;
    _P. septata_, 402
  Pocilloporidae, _401_
  Podium, 428;
    of _Ophiothrix fragilis_, 479 f., 487;
    of _Holothuria nigra_, 561;
    of Elasipoda, 571 f.;
    of Molpadiida, 575;
    of _Antedon rosacea_, 582
  _Podocoryne_, _270_
  Podocorynidae, _270_
  Podoplast, 19 n.
  —see also Blepharoplast
  _Podostoma_, _52_;
    transition between pseudopodium and flagellum in, 47 n.
  _Poecillastra compressa_, _213_, 222
  Polar body, in Heliozoan syngamy, 72, 74
  Polar fields, 415
  Polian vesicle, of Asteroidea, 458;
    of _Ctenodiscus_, 458;
    of Solasteridae, 463;
    of _Ophiothrix fragilis_, 487;
    of _Echinus esculentus_, 525;
    of _Holothuria nigra_, 566
  Polyaxon, 184
  _Polycanna_, _278_
  Polycystineae (= Radiolarian skeletons), 87
  Polycyttaria (= colonial Radiolaria), 76, 84 f.
  Polygastrica (Ehrenberg's name for Ciliata), 146
  _Polykrikos_, _110_, 113 n., 131 f.
  _Polymastia_, 224
  Polymastigidae, _111_
  _Polymitus_ form of male gametogonium of Acystosporidae liberating
      sperms, 104 f.
  _Polymorphina_, _59_
  _Polyoeca_, _111_;
    stalk of, 113
  _Polyorchis_, _278_
  _Polyphyes_, _307_
  Polyphyidae, _307_
  _Polypodium_, _257_
  Polyps, 245, 246;
    alternation of generations in, 44, 250
  Polysiphonic, 276
  _Polystomella_, _59_;
    dimorphism and life-history of, 67 f.
  Polythalamia (= Foraminifera with more than one chamber to shell), 64
  _Polytoma_, _111_;
    brood-division of active, 115;
    _P. uvella_, gametes of, 116 n.
  Polytomeae, Francé on, 119 n.
  _Polytrema_, _59_;
    shell substance of, 62
  _Polytremacis_, _346_
  Polyzoa, 428, 579;
    Entoprocta, 621
  _Pontosphaera_, _110_
  _Porania_, 455, _464_;
    _P. pulvillus_, 464
  Poraniidae, 464
  _Porcellanaster_, _471_;
    _P. pacificus_, 459;
    _P. caeruleus_, 470
  Porcellanasteridae, 455, 459, _470_
  Porcellanous Foraminifera, _58_, 59, 62
  Pore(s), in test of Foraminifera, 64;
    in central capsule of Radiolaria, 76;
    of contractile vacuole, 14;
    of Ciliata, 143;
    of _Trachelius ovum_, 153;
    of embryonic cavity of Suctoria, not seen in _Choanophrya_, 161 n.;
    of Sponges, 186, 187, 188, 189, 198
  Pore-canal, of _Asterias rubens_, 441;
    of _Ophiothrix fragilis_, 486, 487;
    of _Echinus esculentus_, 517;
    of _Echinocardium cordatum_, 552;
    of a larval Holothuroid, 564;
    of Elasipoda, 571;
    of _Antedon rosacea_, 583;
    of its larva, 619;
    of Dipleurula, 608;
    of larva of _Asterina gibbosa_, 612;
    of Balanoglossus, 617
  Pore-plate, of _Echinus esculentus_, 512;
    of _Strongylocentrotus droëbachiensis_, 512;
    of Endocyclica, 530 f.;
    of petals of _Echinarachnius parma_, 544
  Pore-rhomb, 597, 598
  Porifera, 163 f.;
    definition, 180;
    systematic position, 181
  —see also Sponges
  _Porites_, 368 f., 373, 387, 388, _397_
  Poritidae, _396_
  Porocone, 81
  Porocyte, 186, 199, 231
  _Porosphaera, 193_, _282_
  _Porpita_, _309_
  Porta, on reproduction of Radiolaria Acantharia, 86 n.
  Porulosa (Holotrypasta), _76_ f.
  Portuguese Man-of-War, 300, 308
  Post-abdomen = fourth chamber of Monaxonic Radiolarian shell, 84
  Posterior dorsal process, of ciliated band, of Bipinnaria and
      Ophiopluteus, 606;
    of Echinopluteus, 607;
    of Auricularia, 608
  Posterior lateral process, of ciliated band, of Bipinnaria, Ophiopluteus,
      and _Ophiothrix fragilis_, 606;
    of Echinopluteus, 607
  Posterior wreath, of Peritrichaceae, 138;
    of _Vorticella_, 156, 157;
    of _Trichodina_, 158
  Post-oral process, of ciliated band, of Bipinnaria and Ophiopluteus, 606;
    of Echinopluteus, 607
  Potato, rest and germination of, 32
  _Poteriodendron_, _111_
  Potts, 180
  Pouchet (the elder), on spontaneous generation, 43
  Pouchet, Georges, on Protozoa, 45
  _Pouchetia_, _110_
  _Pourtalesia_, _554_;
    _P. jeffreysi_, 554
  Pourtalesiidae, _554_ n.
  Prae-oral process, of ciliated band, of Bipinnaria and Ophiopluteus, 606;
    of Echinopluteus, 607
  —see also Pre-oral
  Pratt, E. M., 339, 348 n., 399 n.
  _Praya_, 300, _306_.
  Prayinae, _306_
  Pre-Cambrian Echinodermata, 623
  Precious Corals, 326, 352
  Pre-oral, cilia, 139;
    ridge, 139;
    undulating membrane, 139
  —see also Prae-oral
  Preyer, on the response of Asteroidea to stimuli, 446 f.;
    on the intelligence of Ophiuroidea, 488 f.
  Primary spine, of _Echinus esculentus_, 506;
    of Cidaridae, 532;
    of Arbaciidae, 532;
    of Echinothuriidae, 532, 535;
    of _Colobocentrotus_, 532;
    of _Heterocentrotus_, 532;
    of _Echinocardium cordatum_, 550
  _Primnoa_, _354_;
    _P. lepadifera_, 338
  Primnoidae, 330, 335, _354_
  Pringsheim, on exogamy in _Pandorina_, 34 n.
  _Prionastraea_ (Astraeidae, _399_), 375
  Proboscis, of central capsule of Phaeodaria, 76
  Progamic brood-division (= a brood-division to produce gametes), 96, 100
  _Proheliolites_, _346_
  _Promachocrinus_, _594_
  Proper wall of Perforate Foraminiferal shell, 59, 63, 66
  Prophylaxis, against mosquitos and malarial fever, 103, 106;
    against pébrine in silkworms, 107
  Prosodus, 210
  Prosopyle, 170
  Prostalia, 201
  _Protanthea_, _377_
  Protantheidae, _377_
  Protaspidochirota, 578
  Protechinoidea, 623
  Proteid(s), 12;
    digestion of, 15;
    formation of, 36;
    crystals, 37;
    reserves in Flagellates, 110;
    granules or spherules of Ciliata, 144;
    of Suctoria, 161
  _Proteleia_, _218_
  Proteomyxa, 48 f., _50_, 88, 89 f.;
    relations of, 40;
    studied by botanists, 45;
    parasitic in plant-cells, 48;
    distinctions from Flagellates, 109
  Proteoses, 15
  _Proterospongia_, _111_, 113, 122, 181 f.;
    _P. haeckeli_, 182
  Protista, 3 f.—see Protozoa
  Protoalcyonacea, 329, _342_
  Protoclypeastroidea, _548_
  Protocnemes, 367
  Protocoelomata, 616, 617
  Protodendrochirota, 578
  Protoholothuroid, 577
  Protoholothuroidea, 578
  _Protohydra_, _256_
  Protomastigaceae, _110_ f., 112;
    external plasmatic layer of, 113
  Protomerite, of Gregarines, 97, 98
  _Protomyxa_, _89_
  Protopelmatozoa, 623
  Protophytes, 3 n.
  Protoplasm, 3 f.;
    structure of, 6;
    refractive index of, 6, 11 n.;
    movements of, 7;
    specific gravity of, 13 n.;
    of Protozoa, 46 f.;
    of Radiolaria, 79 f.;
    of _Stylonychia_, 140
  _Protospongia fenestrata_, _207_, 207
  Prototheca, 385, 386
  Protozoa, 1 f., 44 f.;
    characters, 40;
    parasitic, 40, 46;
    literature, 45 f.;
    geographical distribution, 47 f.;
    habitat, 47;
    classification, _50_
  Protractor muscles of Aristotle's lantern, 526
  Protriaene (a triaene in which the cladi [branches] point forwards or in
      the opposite direction to the rhabdome or shaft), 224
  Prouho, on transverse fission in _Gonactinia_, 371 n.;
    on gemmiform pedicellariae of Echinoidea, 509;
    on habits of _Dorocidaris papillata_, 535
  Prunoidea, _77_
  Prunophracta, _78_
  _Psammocora_ (Fungiidae, _403_), 390
  Pseudambulacrum, 599
  Pseudaxonia, _350_, 353
  Pseudomonocyclic, 594
  Pseudonavicella, 96
  _Pseudophellia arctica_, 379
  Pseudopodia (um), 4 f., 17, 47, 49, 50 f.;
    streaming of granules in, 17;
    Lang's classification of, 47 n.;
    transition to flagella, 47 n.;
    of Rhizopoda, 49, 50 f., 52;
    of Foraminifera, 49, 50, 60, 61, 65;
    of _Microgromia_, 59, 60;
    of _Lieberkühnia_, 61;
    of _Allogromia_, 65;
    of _Miliola_, 65;
    of _Rotalia_, 65;
    of _Squamulina_, 65;
    of Heliozoa, 49, 50, 71, 72 f.;
    of Radiolaria, 49, 50, 79, 80;
    of _Lankesteria_, 96 n.;
    of Flagellates, 109, 110;
    transitory, in Flagellates, 109;
    of _Stentor_, 152, 154;
    of young _Gellius varius_, 173, 174
  Pseudopodiospores, 68 f.
  _Pseudospora_, _89_;
    _P. lindstedtii_, 89
  _Psilaster_, _470_;
    _P. acuminatus_, 469
  _Psolus_, 569, _573_;
    _P. ephippifer_, 574, 575, 602
  _Psychropotes_, _572_
  _Pteraster_, 455, _466_;
    _P. stellifer_, 465;
    _P. militaris_, 466
  Pterasteridae, 455, _466_
  _Pterocephalus_, _97_
  _Pteroeides_, 359, _361_
  Pteroeididae, _361_
  Puffballs, 91
  Pulsatile vacuole, 14;
  = Contractile vacuole, _q.v._
  Pulsellum, 18, 114
  Punjab, dourine disease in, 119
  Pupa, of Holothuroidea, 615
  Pure cultures, 43
  Putrefaction, organisms of, 42 f., 116 f.
  _Pycnolithus_, _346_
  _Pycnopodia_, 453, _474_
  _Pygaster_, _558_, 559
  _Pygastrides_, _548_, 558;
    _P. relictus_, 548
  Pylome, 53;
    of Rhizopods, 53 f.;
    double, in monstrous Rhizopods, 55;
    of _Diaphorodon_, 60;
    of Foraminifera, 64;
    of Radiolaria, 83;
    of Sphaeropylida, 77 n.;
    of Nassellaria, 83
  Pyloric caeca (and duct), of _Asterias rubens_, 439;
    of _Porcellanaster pacificus_, 459;
    absent in _Hyphalaster moseri_, 459
  Pyloric sac of _Asterias rubens_, 438
  Pyramids, building-stone of, 70
  Pyrenoid, 37, 115;
    of _Sphaerella_, 126
  _Pyrocystis_, _110_;
    _P. fusiformis_, 132
  _Pyrsonympha_, _111_;
    flagella of, 114
  _Pytheas_, 223
  _Pythonaster_, _464_
  Pythonasteridae, _464_
  _Pyxicola_, _138_, 158

  _Quadrula_, _52_;
    _Q. symmetrica_, 55;
    test of, 54
  Quartan fever, a parasitic disease, 104 f.
  Quartzites, Radiolarian, 87
  _Quasillina_, 224
  Quatrefages, de, 376
  Quelch, 280
  _Quinqueloculina_, _59_, 65 f.
  Quinqueloculine type, 67

  Radial blood-strand (or vessel), of _Asterias rubens_, 450;
    of _Ophiothrix fragilis_, 488
  Radial canal, of water-vascular system, 428;
    of _Asterias rubens_, 441;
    of _Ophiothrix fragilis_, 480, 486;
    of _Echinus esculentus_, 517;
    of _Echinarachnius parma_, 547;
    of _Holothuria nigra_, 560;
    of _Pelagothuria_, 572;
    of Cystoidea, 580, 597;
    of Carpoidea, 580;
    of Thecoidea, 580;
    of Crinoidea, 580;
    of Blastoidea, 580, 599;
    of _Antedon rosacea_, 583;
    development of, in _Asterina gibbosa_, 611;
    in Holothuroidea, 615;
    in _Antedon rosacea_, 619
  Radial fission in Volvocaceae, 110 f.
  Radial nerve-cord, of _Asterias rubens_, 444, 445, 446, 447;
    of _Ophiothrix fragilis_, 488;
    of _Echinus esculentus_, 518, 519, 522;
    of _Holothuria nigra_, 566;
    of oral system of _Antedon rosacea_, 584;
    of coelomic system of _A. rosacea_, 584, 585
  Radial ossicle—see Radial Plate
  Radial perihaemal canal, of _Asterias rubens_, 448;
    of _Ophiothrix fragilis_, 488;
    of _Echinus esculentus_, 518, 524;
    of _Holothuria nigra_, 566
  Radial plate, of _Ophiothrix fragilis_, 484;
    of Cladophiurae, 500;
    of calcareous ring in _Holothuria nigra_, 566;
    in Holothuroidea, 567, 569;
    in Elasipoda, 569;
    in Molpadiida, 569;
    in Dendrochirota, 569;
    of calyx in _Antedon rosacea_, 582, 584, 585;
    in Crinoidea, 588;
    in _Hyocrinus_, 588, 590;
    in Rhizocrinidae, 588, 590;
    in _Metacrinus_, 588, 591;
    in _Pentacrinus_, 588, 591;
    in _Holopus_, 592;
    in Blastoidea, 599;
    development, in _Antedon rosacea_, 619 f.
  Radiolaria, 49, _50_, _75_ f.;
    streaming of granules, 17;
    brood-formation, 33, 85 f.;
    regeneration, 35;
    animal nutrition of, 40;
    relations of, 49;
    freshwater (= Heliozoa), 71;
    shells and skeletons of, 77, 78, 80, 82 f., 84, 85;
    symbiosis of Diatoms and yellow cells (= _Zooxanthella_) in, 82, 86 f.,
      125;
    parasites in, 86 f., 161;
    Dreyer's scheme of skeletal forms in meshes of an alveolar system, 84;
    habitat, 87;
    census of, 87;
    palaeontology, 87;
    students of, 88
  Radiolarian ooze, 87
  Radiomyaria, _324_
  Radius, of _Echinus esculentus_, 503;
    synonym of compass, 526;
    of a Holothuroid, 562;
    of Spatangoidea, 553;
    of _Antedon rosacea_, 582;
    of _Promachocrinus_, 594
  Rain, bloody, 125
  Rainey's tubes, 108
  _Ramulina_, _59_
  _Raphidiophrys_, _71_, 74
  _Raspailia_, 225
  _Rastrites_, _282_
  Ratarula larva, 302
  _Rathkea_, 265, _270_
  Ratio (Spencer's) between bulk and surface of organism, 14, 23 f., 31 f.
  Reactions, chemical, of protoplasm and of vacuoles, 13;
    motile, of Protozoa, 19 f.
  _Receptaculites_, _207_
  Receptaculitidae, _207_
  Rectal caeca, of _Asterias rubens_, 439;
    of Asteroidea, 459;
    of _Astropecten_, 459;
    of _Asterias_, 459;
    of _Asterina_, 459;
    of Echinasteridae, 459;
    of Astropectinidae, 459
  Rectum, of _Asterias rubens_, 439;
    of _Holothuria nigra_, 563
  Red colour of sea, due to Dinoflagellata, 132
  Red Corals (= Coralliidae, 352)
  Red marrow of bones, habitat of resting states of malarial parasites, 106
      n.
  Red snow, 125
  Redi, on reproduction of Blowflies, 42
  Reduction divisions, 75 n.;
    of nucleus, 116 n.;
    in _Monocystis_, 96;
    in _Coccidium_, 100;
    in Acystosporidae, 104 f.
  Reef Corals, distribution of, 389
  Reefs, Coral-, 390;
    barrier-, 390 f.;
    fringing, 390 f.;
    atolls, 390 f.
  Regeneration, 35;
    of _Stentor_, 35;
    of _Thalassicolla nucleata_, 79 n.;
    of Infusoria after division, 145;
    of oral apparatus of Ciliata during conjugation, 151
  Regular Urchins, 529, 530 = Endocyclica, _q.v._
  Relapses of malarial fevers explained, 106
  Relapsing fever, inoculated by Zambezian Tick, 121 n.
  Relationship, cellular, explained, 10
  _Reniera_, _217_, 223
  _Renilla_, 340, 359, 360, _363_;
    _R. reniformis_, 363
  Renilleae, _363_
  Renillidae, _363_
  _Reophax_, _59_
  Repellent stimuli, response to, 20 f.
  Reproduction, 10, 23 f.;
    of _Actinophrys sol_, 72;
    of Radiolaria, 84 f.;
    of Flagellata, 107;
    of Choanoflagellates, 122;
    of _Opalina_, 123 f.;
    of Volvocidae, 126, 127 f., 129 f.
  —see also Brood-division, Spore, Syngamy
  Reproductive cells, 31
  —see also Gametes, Brood-cell
  Reproductive organs—see Genital organs
  Reserve(s), 16, 37;
    accumulation of, 9, 13, 32 f.;
    digestion of, 16;
    consumption of, in brood-formation, 24, 32 f.;
    of Flagellates, 110, 115;
    -products, of brood-mother-cell, 32
  Reservoir of contractile vacuole, of Flagellates, 110, 115;
    in Euglenaceae, 125;
    of _Vorticella_, 157
  Respiration, 8, 14;
    intramolecular, 14 n.;
    in Asteroidea, 432;
    in _Asterias rubens_, 437;
    in _Astropecten_, 469;
    in _Luidia_, 469, 470;
    in Porcellanasteridae, 470;
    in _Ophiothrix fragilis_, 485;
    in _Echinus esculentus_, 517, 527;
    in _Echinarachnius parma_, 544, 547;
    in _Echinocardium cordatum_, 551;
    in _Holothuria nigra_, 563, 564, 566;
    in Molpadiida, 575;
    in Synaptida, 576;
    in _Antedon rosacea_, 582;
    in Cystoidea, 597 f.;
    in Blastoidea, 600
  Respiratory tree, of _Holothuria nigra_, 563;
    of Holothuroidea, 569;
    of Aspidochirota, 569;
    of Dendrochirota, 569, 572;
    of Molpadiida, 569;
    represented by caecum of Elasipoda, 569;
    absent in Synaptida, 576;
    in _Pelagothuria_, 572
  Response, 8
  Rest, of cell, 32;
    vegetative and absolute contrasted, 37
  —see also Cyst, Encystment, Hypnocyst
  Resting state of malarial parasites, 106 n.
  Reticulate, structure of protoplasm, 6;
    endoplasm of Cystoflagellates, 110;
    of _Loxodes_ and _Trachelius_, 144, 153 f.
  _Retiograptus_, _282_
  _Retiolites_, _282_
  Retiolitidae, _282_
  Retractor muscles, of Aristotle's lantern, 526;
    of anterior part of body of Holothuroidea, 568;
    of Dendrochirota, 568, 572
  Reusenapparat, 145
  Rhabdocrepid, 215, 224 (Crepis is the term applied to the fundamental
      spicule by deposition of silica upon which a desma is formed. A desma
      of which the crepis is uniaxial is called a rhabdocrepid desma)
  Rhabdoliths, 110, 114
  _Rhabdopleura_, 617
  _Rhabdosphaera_, _110_
  Rhabdospheres, 114
  Rhabdus, 183, 184
  Rhagon, 209, 213
  Rheotaxy, 20 f.;
    its rôle in syngamy of Mammals and Sauropsida, 34
  _Rhipidaster_, 453, _463_
  _Rhipidodendron_, _111_, 112 f.
  Rhizocrinidae, 588, 589, _590_ f.
  _Rhizocrinus_, 588, _591_
  Rhizophysaliidae, 302, _308_
  Rhizopoda, _50_, _51_ f.;
    formation of chromidia, 29 f.;
    relations, 48 f.;
    shell, 49, 51 f.;
    Filosa, _50_, _52_ f.;
    Lobosa, _50_, _51_ f.;
    test-bearing, 53 f., 55;
    literature, 58
  _Rhizostoma_, _325_;
    _R. pulmo_, 312, 325
  Rhizostomata, 310, 323, _324_
  Rhizostomatidae, _325_
  Rhodactidae, _383_
  _Rhodactis_ (Rhodactidae, _383_);
    _R. sancti-thomae_, 373
  _Rhodalia_, _308_
  _Rhodophysa_ (sub-family Physophorinae, _308_), 300
  _Rhodophyton_, _349_
  _Rhodopsammia_, _404_
  _Rhopilema_, _325_;
    _R. esculenta_, 312;
    _R. verrucosa_, 312
  Rhumbler, on Foraminifera, classification, 58 n.;
    shell, 66 f.;
    monstrous shells, 69 n.
  _Rhyncheta_, _159_ f., 160, 162
  Rhythm, of cell-life and reproduction, 30 f.;
    of contractile vacuoles in _Paramecium_, 143 n.;
    in life-cycle of Infusoria, 148 n.
  _Ricordea_, 371
  Ridley, 216 n., 352 n.
  _Riisea_, _355_
  Ring-canal, of water-vascular system, 428;
    of _Asterias rubens_, 441;
    of _Echinus esculentus_, 517;
    of _Holothuria nigra_, 566
  Robertson, on the habits of _Echinocardium cordatum_, 552
  Rock-urchins, 529
  Rodents, hosts of _Trypanosoma lewisii_, 119
  Roemer, 199 n.
  Romanes, on physiology of Echinoidea, 519, 521
  Rompel, on _Spirochona_, 144 n.
  Rosette, -aggregate, of _Treponema_, 120;
    of _Antedon rosacea_, 584
  Ross, on intermediate host of malarial parasites, 103
  _Rotalia_, _59_, 65
  Rotaliaceae, _59_
  Rotifers, bladder of, 14 n.;
    rheotaxy of, 21;
    distribution of, 48;
    associated with _Euglena viridis_, 124;
    formerly included under Infusoria, 136;
    vibratile styles of, 141
  _Rotula_, 548, _549_
  Rotula of _Echinus esculentus_, 526
  Rotula muscles of Aristotle's lantern, 526
  Roule, 408
  Royal Society, early publications on Protozoa, 45
  Ruminants, infested by _Trypanosoma evansii_, 119;
    parasitic Ciliata in paunch of, 152
  Rumphius, 360

  _Saccammina_, _59_, 63
  Sacculus, of _Antedon rosacea_, 587;
    of other species of _Antedon_, 588
  _Sagartia_, 372, 375, _381_;
    _S. troglodytes_, 378
  Sagartiidae, _381_
  Sagittal, 185
  Sagittal ring, 78, 83;
    plane, 414;
    costae, 416 n.
  _Salenia_, _538_;
    _S. varispina_, 538
  Saleniidae, 530, _537_, 558
  Salivary gland of gnat in relation to malarial parasites, 105
  _Salpingoeca_, _111_, 122
  Sand from sponges, a source of Foraminiferal tests, 62
  Sand, René, on Suctoria, 162
  Sand-dollar, 542
  Sand-urchin, 529
  Sanidaster (a modified euaster in which a slender rod-like axis bears
      spines at intervals along its length), 222
  Sapropelic Protozoa, 48
  Saprophyte, 33, 37, 90, 113, 119;
    relation to brood-formation, 33
  Saprophytic, nutrition, 33, 37;
    Flagellata, 113, 119
  Sarasin, C. F. and P. B., on the madreporic vesicle and axial sinus of
      Echinoidea, 528;
    on the relationships of the Echinothuriidae and Holothuroidea, 537
  _Sarcocystis_, _98_;
    _S. tenella_, 108 n.
  Sarcocyte, 96, 98
  Sarcode (Dujardin's term for protoplasm), 3 f.
  _Sarcodictyon_, _344_;
    _S. catenatum_, 342
  Sarcodictyum, 79
  Sarcodina, relations of, 48 f., _49_, _50_ f.;
    distinction from Flagellata, 109
  Sarcoflagellum, 80
  Sarcolemma of stalk-muscle of _Vorticella_, 157 n.
  _Sarcophyllum_ (Pennatulidae, 361), 360
  _Sarcophytum_, 248, 330, 333, 347, _349_
  Sarcosporidiaceae, _98_, 108
  _Sarsia_, 265, _272_;
    _S. prolifera_, 272;
    _S. siphonophora_, 272
  Sauropsida, egg of, 34
  _Scaphiodon_, _137_, 141 n.
  Schäfer, on mechanism of ciliary action, 18 n.
  Schaudinn, on exogamy in Foraminifera and _Trichosphaerium_, 34 n.;
    on Protozoa, 46;
    _Archiv für Protistenkunde_, 46;
    on chromidia in Sarcodina, 52 n.;
    on _Trichosphaerium sieboldi_, 54, 56 f.;
    on bud-fission in Rhizopoda, 55;
    on syngamic processes of Rhizopoda, 57;
    on reproduction in Foraminifera, 67, 69 n.;
    on Heliozoa, 71;
    on Sporozoa, 94;
    on life-cycle of Coccidiidae, 99, 101;
    on relations of _Halteridium_ and _Trypanosoma_, 103 n., 116 n., 120;
    on relations of Acystosporidae, 106;
    on conjugation in Flagellates, 116 n.;
    on _Trypanosoma_, 120;
    on _Treponema_, 120;
    _Fauna Arctica_, 199 n.;
    on _Haleremita_, 257
  _Schaudinnia arctica_, 200
  Scheel, on brood-formation in _Amoeba proteus_, 56 n.
  Schewiakoff, on protoplasmic granules, 6 n.;
    on geographical distribution of fresh-water Protozoa, 47 n.
  Schiemenz, on the way in which Starfish open bivalves, 440
  _Schizaster_, _556_
  Schizogony, of Coccidiaceae, 99 f., 101 f.;
    of Haemosporidae, 102;
    of Acystosporidae, 104 f.
  Schizogregarinidae, _97_
  Schizomycetes, 36 f., 44
  Schizont, 99;
    of Acystosporidae, 103, 104 f.
  _Schizopathes_, _408_
  Schizophytes, relations of, 48
  _Schizotricha_ (Ciliata), _138_;
    _S. socialis_, branched tube of, 152;
    _S. dichotoma_ (Plumulariidae, _279_), 276
  Schlumberger, on dimorphism of Foraminifera, 67
  Schneider, on Sporozoa, 94
  Schrammen, 215 n.
  Schröter, on Myxomycetes, 93 n.
  Schuberg, on cilia and ciliary motion, 18 n., 141 n.
  Schultze, Max, on Protozoa and on protoplasm, 46;
    on structure of Foraminifera, 62
  Schulze, F. E., on Heliozoa, 71;
    on Sponges, 167, 197 n., 199, 200;
    on _Spongicola_, 318
  Schütt, on Dinoflagellata ("Peridiniales"), 119, 132
  Sclerobase, 371, 407
  Scleroblast, 171, 330
  Scleroderm, 371
  Sclerogorgiidae, _351_
  _Sclerophytum_, 330, 336, 348, _349_;
    _S. querciforme_, 348
  Scopula, 141 n.
  Scuta buccalia, of _Ophiothrix fragilis_, 485
  Scutellidae, _549_
  _Scyphidia_, _138_, 158;
    _S. scorpaenae_, _Zooxanthella_ symbiotic in, 125
  Scyphistoma, 317
  Scyphomedusae, _310_ f.
  Scyphozoa, _310_ f.;
    colour, 310;
    food, 311;
    phosphorescence, 311;
    reproduction, 316;
    size, 310;
    structure, 312;
    symbiosis, 311
  _Scytophorus_, _380_
  Sea, luminosity or phosphorescence, produced by Cystoflagellata, 132,
      134;
    by Dinoflagellata, 132;
    red colour of, due to Dinoflagellata, 132
  Sea-anemones, 326, 365, 377
  Sea-cucumbers (= Holothuroidea), 561
  Sea-fans (= species of Gorgonacea), 350
  Sea-lilies (= Crinoidea), 580 f.
  Sea-pens, 326, 358
  Sea-pansy, 364
  Sea-urchins(= Echinoidea), 503;
    ovum of, 7
  Secondary body-cavity (= Coelom, _q.v._)
  Secondary spines, of _Echinus esculentus_, 506;
    of Cidaridae, 532;
    of _Colobocentrotus_, 532;
    of _Heterocentrotus_, 532;
    of _Echinocardium cordatum_, 550
  Secretion, 13
  Segmentation, 32 n.;
    of schizont of Acystosporidae, 104;
    of oosperm, 104 f.;
    of zygotomeres, 104 f.;
    of reproductive cells of _Volvox_, 126, 127;
    telolecithal, 133 n.
  Semaeostomata, _323_
  Semon, on the phylogeny of Echinodermata, 622
  Semper's larva, _405_
  Senility in life-cycle of Ciliata, 148
  Senn, on Flagellates, 119
  Sense-organs of Metazoa, 40
  Sensory cilia, 141
  Septum, protoplasmic, in Dicystic Gregarines, 97, 98 f.;
    calcareous, of Madreporaria, 370
  _Seriatopora_, _401_
  _Sertularia_, _278_;
    _S. abietina_, 278
  Sertulariidae, _278_
  _Serumsporidium_, _89_
  Sex, binary (= syngamy with marked inequality between the pairing-cells),
      33 f.;
    of _Pterocephalus_, 99;
    of _Stylorhynchus_, 99, 100;
    of Coccidiaceae, _97_ f., 99 f.;
    of Haemosporidae, 102 f.;
    of _Sarcocystis tenella_, 108 n.;
    of Volvocaceae, 128 f.;
    of _Eudorina_, 129;
    of Peritrichaceae, 151 f.
  Sex, ternary, of _Pandorina_, 128 f.
  Sexual fusion of _Halteridium_, 103, 105
  Sheath, tentacular, of Suctoria, 159
  Sheep, host of _Sarcocystis tenella_, 108 n.
  Shell, of Diatomaceae, 84;
    of Foraminifera and of Rhizopoda—see Test;
    of Radiolaria—see Skeleton;
    cuticular, of Flagellates, 113—see Theca;
    siliceous reticulate, of Silicoflagellates, 110;
    -substance of Foraminifera, 62;
    of _Polytrema_, 62
  Shipley, 197 n.
  Sickle-cells, sickle-germs, 48, 94 f., 97, 101;
    of _Lankesteria_, 95;
    of Sarcosporidiaceae, 108
  Side-plates, of arms of Crinoidea, 589;
    of Pentacrinidae, 591;
    of Comatulidae, 594
  _Siderastraea_, 373, 375, _403_;
    _S. sideraea_, 403
  Siebold, v., on Sporozoa, 94
  Siedlecki, on Sporozoa, 94;
    on reproduction of _Lankesteria_, 96 n.;
    on life-cycle of Coccidiidae, 99
  Sierra Leone, prophylaxis of malaria at, 106
  Sigma (a slender rod-like spicule curved in the shape of the letter C),
      220, 222
  Sigmaspire (a slender rod-like spicule twisted through about a single
      revolution of a spiral, and consequently having the form of a C or an
      S according to the direction in which it is viewed), 222
  Siliceous plates, of test of Rhizopods, 29, 53 f.
  Siliceous skeleton of Heliozoa, 71, 74;
    of Radiolaria, 76 f.;
    of Silicoflagellata, 110;
    of Sponges, 171 f., 175 f., 195 f.
  Silicified cell-wall of some Dinoflagellates, 130
  Silicispongiae, 195, 197
  Silicoflagellata, 86, _110_, 114
  Silkworm infested by disease pébrine, due to _Nosema bombycis_, 107
  Simpson, on life-cycle of Ciliata, 148 n.
  Siphon, of _Echinus esculentus_, 516;
    of _Echinarachnius parma_, 546, 547
  _Siphonia_, _215_
  Siphonoglyph, 334, 334, 369, 410
  _Siphonogorgia_, _349_
  Siphonogorgiidae, _349_
  Siphonophora, _297_ f.;
    dactylozooids, 299;
    food, 304;
    gastrozooids, 299;
    gonozooids, 302;
    hydrophyllia, 300;
    life-history, 302;
    nectocalyces, 298;
    pneumatophore, 300, 307 f.;
    stolon, 301
  Siphonozooids, 332;
    of Pennatulacea, 359
  Sipunculidae, 576
  _Sipunculus_, 563
  Skeleton, intermediate or supplemental of Perforate Foraminifera, 59, 63,
      66;
    of Heliozoa, 71, 74;
    of _Raphidiophrys_, 74;
    of Radiolaria, 76 f., 77 f., 81 f., 84;
    of Spumellaria, 77, 77, 83;
    of _Actinomma asteracanthion_, 77;
    of Acantharia, _76_ f., 78, 80, 82;
    of _Xiphacantha_, 78;
    of _Dorataspis_, 80;
    of Nassellaria, 76, 78, 82, 83;
    of _Lithocercus annularis_, 82;
    of _Theoconus_, 80;
    of Phaeodaria, 76, 79, 82, 84, 85;
    of _Aulactinium actinastrum_, 82;
    of Challengeridae, 85;
    of _Pharyngella_, 85;
    of _Haeckeliana_, 85;
    of _Tuscarora_, 85;
    of Diatomaceae, 84;
    gelatinous, of _Volvox_, 126;
  —see also Shell, Test, Theca
  —of _Asterias rubens_, 434 f.;
    of disc of Ophiuroidea, 493;
    internal, of _Echinarachnius parma_, 545, 548;
    of Clypeastroidea, 548;
    of _Laganum_, 548;
    of _Clypeaster_, 548;
    of _Echinocyamus_, 548;
    of Echinanthidae, 549;
    of Laganidae, 549;
    of _Holothuria nigra_, 560;
    of Holothuroidea, 569;
    of Aspidochirota, 569;
    of Dendrochirota, 569;
    of Elasipoda, 569;
    of Molpadiida, 569;
    of Synaptida, 569;
    calyx and arm in Crinoidea, 588
  Sladen, on classification of Asteroidea, 460
  Sleeping-sickness, 120
  Snow, red, 125
  _Solaster_, 453, 455, _463_;
    fossil, 475;
    _S. endeca_, 463;
    _S. papposus_, 463
  Solasteridae, 453, 455, 458, _462_, 466
  _Solenocaulon_, _350_
  Sollas, I. B. J., on Sponges, 163 f.
  Sollas, W. J., on Sponges, 165 n., 168, 172 n., 176 n., 183 n., 207 n.,
      208, 212 n., 215 n., 216, 219 n., 233 n., 234 n., 238 n.;
    on _Palaeodiscus_, 557
  Solmaridae, _296_
  _Solmaris_, _296_, 297
  Soluble substances in greater or less concentration, effect on
      protoplasm, 7 f., 22 f.
  Sore, Oriental, 121
  Spallanzani, on origin of organisms of putrefaction, 43
  _Spanioplon_, 223
  Sparshall's discovery of _Noctiluca_, 135
  Spatangidae, _554_, 559
  Spatangoidea, 529, _549_ f., 556, 559, 561, 577
  _Spatangus_, 553, _555_;
    _S. purpureus_, 555;
    _S. raschi_, 555
  Specialisation in Metazoa and _Volvox_ compared, 129 f.
  Specific gravity of living protoplasm, 13 n.
  Spencer, Herbert, on limit of growth, 23
  Spencer, W. B., on Hydroids, 271 n., 279
  Spencerian, fission at limit of growth, 23;
    rhythm, 30 f.
  Sperm, Spermatozoon, 17, 31, 33 f.;
  = spermogametes, 33;
    penetration of ovum by, 34;
    rheotaxy of, in Mammals and Sauropsida, 34;
    of Sporozoa, 18;
    of _Pterocephalus_, 99, 108 n.;
    of _Stylorhynchus_, 99, 100;
    of _Coccidium_, 100, 101 f.;
    of Acystosporidae, 104 f.;
    of _Sarcocystis tenella_, 108 n.;
    of bisexual Protozoa and most Metazoa comparable with Flagellata, 109;
    of _Volvox globator_, 127 f.;
    of _Eudorina_, 129
  Spermatogone (= a brood-mother-cell, whose offspring are sperms) of
      Coccidiaceae, 100 f.;
    of _Volvox globator_, 127 f.;
    of _Eudorina_, 129;
    of Acystosporidae, 104 f.
  Spermatozoon, 17, 18
  —see also Sperm
  _Sperosoma_, _536_
  _Sphaeractinia_, _283_
  _Sphaerechinus_, _539_, 540, 541;
    _S. granularis_, 541
  _Sphaerella_, _111_;
    _S. lacustris_, 126;
    _S. nivalis_, 125;
    _S. pluvialis_, 125 f., 126
  Sphaerellaria, _77_, 77;
    geological occurrence of, 88
  Sphaeridium, of _Strongylocentrotus_, 523;
    of _Echinus esculentus_, 524;
    of _Echinarachnius parma_, 545;
    of _Echinocardium cordatum_, 551
  Sphaeroidea, _77_, 77 f.
  _Sphaeronectes_, _306_
  Sphaeronectinae, _306_
  Sphaerophracta, _78_, 80
  Sphaerozoea, _77_ n.
  Sphaerozoidae, 85
  _Sphenopus_, _404_
  Spheraster (an aster in which the centrum is large, with a diameter equal
      to or greater than one-third the length of the actines), 233
  Sphere, 184
  Spicatae, _362_
  Spicules, calcareous, of Coccolithophoridae, 114;
    siliceous, of Heliozoa, 71, 74;
    of Radiolaria, 83;
    of Silicoflagellata, 110;
    of Sponges, 170 f.;
    composition, 170;
    structure, 171, 172;
    classification, 183;
    forms of, 184, 222, 224;
    development, 232;
    of Alcyonaria, 334 f., 336
  Spindle, in cell-division by mitosis, 25 f.;
    intranuclear, of _Euglypha_, 29
  Spines, of pelagic Foraminiferal shell, 66, 69;
    of _Globigerina bulloides_, 69;
    of Asteroidea, 454;
    of arms of _Ophiothrix fragilis_, 479;
    of arms of Ophiuroidea, 491;
    of _Echinus esculentus_, 505 f.;
    of Endocyclica, 531 f.;
    of Cidaridae, 532;
    of Arbaciidae, 532;
    of Echinothuriidae, 532;
    of _Colobocentrotus_, 532;
    of _Heterocentrotus_, 532;
    of _Echinarachnius parma_, 543;
    of _Echinocardium cordatum_, 550
  _Spinipora_, 284, _286_
  Spintharophora, 216, _217_ f.
  Spinulosa, _461_, 462 f.
  Spiral, of stalk and stalk-muscle in _Vorticella_, 156 f.;
    ridge on tentacles of Suctoria, 160 f., 162
  Spiraster (a spire of one or more turns, produced on the outside into
      several spines), 222
  _Spirillina_, _59_, 64
  _Spirochaeta_ (= _Treponema_, _111_), 120 f.;
    _"S." zeemannii_, 120 f.;
    _S. obermeieri_, 121
  _Spirochona_, _138_;
    adoral wreath of, 138 n.;
    bud-fission of, 147
  _Spiroloculina_, _59_, 63
  _Spirostomum_, _137_, 153 f.;
    supposed nervous fibrils in, 143
  _Spongelia_, 225
  Spongelidae, _220_
  Sponges (= Porifera), 163 f.;
    spicules, 170, 171, 172, 177 f., 183, 184, 187 f., 198 f., 222, 224,
      231 f., 232;
    canal system, 170, 171, 191, 198, 210, 235 f.;
    physiology, 234 f.;
    distribution, in space, 239;
    in time, 241;
    history, 166;
    reproduction, 172, 226 f.;
    nervous system, 39;
    immune from Gregarines, 99;
    relations to Protista, 41;
    to Choanoflagellates, 122 f., 168, 181
  Sponge-sand, a source of Foraminiferal tests, 62
  Sponge-spicules, in arenaceous shell of Foraminifera, 64
  —see also Spicules
  _Spongicola_ (= _Nausithoe_), 206, _318_;
    _S. fistularis_, 317;
    (a Decapod Crustacean), 206;
    _S. venusta_, 206
  Spongidae, _220_
  _Spongilla_, _217_, 225, 230, 232, 237, 238;
    _S. lacustris_, spicule, 232
  Spongin, 176, 217, 239
  _Spongioderma_, _351_
  _Spongocardium gilchristi_, 215
  _Spongodes_, 330, 335, 348, _349_
  Spongophare, 210
  Spontaneous, generation, 42 f.;
    rendered improbable by life-histories of Flagellates, 118;
  —movements of Protista, 23
  _Sporadopora_, 284, _285_
  Sporange (= a sac containing spores), of Myxomycetes, 91 f.;
    of _Didymium_, 92;
    of Actinomyxidiaceae, 98
  Spore, 31;
    of _Actinophrys_, 72;
    of _Actinosphaerium_, 73 f.;
    of Acantharia, 86 n.;
    of Myxomycetes, 90 f.;
    of _Didymium difforme_, 92;
    of Sporozoa, 94 f.;
    of Gregarinidaceae, 97 f.;
    of _Lankesteria_, 95;
    of _Gregarina blattarum_, 98;
    of _Stylorhynchus_, 100;
    of Coccidiaceae, 97;
    of Coccidiidae, 97;
    of _Coccidium schubergi_, 101;
    of malarial parasites, 104;
    of Acystosporidae, 97;
    of Myxosporidiaceae, 98, 107;
    of _Myxobolus mülleri_, 107;
    of Actinomyxidiaceae, 98;
    of Sarcosporidiaceae, 98, 108;
    of _Bodo saltans_, 117;
    of Flagellates highly resistant to heat, 118
  —see also Oospore, Zoospores, Zygotospore
  Sporocyst, 88, 89;
    of Proteomyxa, 88;
    of _Bodo saltans_, 117 f.
  Sporoducts of _Gregarina blattarum_, 98 f.
  Sporogony, 296
  Sporont of Gregarines, 98 f.
  Sporozoa, 31, 33, 40, 48, _50_, _94_ f.;
    formation of chromidia, 29 f.;
    habitat, 48;
    relations, 48 f.;
    movements, 50, 125 n.;
    Acystosporidae most primitive group of, 106;
    distinction from Flagellata, 109
  Sporozoite, 95 f.;
    of _Lankesteria_, 95;
    of Coccidiaceae, 99 f., 101;
    of Haemosporidae, 102;
    of Acystosporidae, 104 f.
  Sporulation, 31;
    in _Noctiluca_, 133 f.;
  = Brood-formation, _q.v._
  Springer, on classification of Crinoidea, 589
  Springing movements, of Flagellates _Dallingeria_, 114, and _Bodo
      saltans_, 114, 117;
    of tailed Ciliata, 141 n.;
    of _Pleuronema_, 154
  Spumellaria (Peripylaea), _76_, 76 f., _77_;
    skeleton, 77, 83
  Spyroidea, _78_
  _Squamulina_, _59_, 65
  Stalk or stem, of _Clathrulina_, 74;
    of Flagellates, 112 f.;
    of _Anthophysa_, 112;
    of _Diplomita_, 112;
    of Choanoflagellates, 121, 122;
    of _Monosiga_, 122;
    of _Polyoeca_, 113, 122;
    of Peritrichaceae, 141 n.;
    of _Schizotricha socialis_, 152;
    of _Vorticella_, 157;
    of _Carchesium_, 158;
    of _Epistylis_, 158;
    of _Zoothamnium_, 158;
    of Suctoria, 159, 160, 162;
    of Pelmatozoa, 430, 579;
    of Crinoidea, 580;
    of Carpoidea, 580, 596;
    of Cystoidea, 580, 597;
    of Blastoidea, 580, 599;
    of _Antedon rosacea_, 581, 585;
    of _Actinometra_, 588;
    of Rhizocrinidae, 588, 590;
    of _Holopus_, 588, 592;
    of _Hyocrinus_, 588, 590;
    of Pentacrinidae, 588, 591, 592;
    of larva of _Asterina gibbosa_, 610;
    of Brachiolaria, 612;
    of larva of _Antedon rosacea_, 619;
    of ancestral Crinoidea, 600;
    of ancestral Echinodermata, 621
  Starch, 15, 37;
    in _Zooxanthella_, 86;
    in Flagellata, 110, 115
  Starfish = Asteroidea, _q.v._
  Stationary pairing-nucleus, 150
  Statocysts, 252, 288
  Statolith, 252, 289
  Statoplea, _279_
  Statorhab in _Geryonia_, 252
  Stauractin, 234
  Stauromedusae, 310, 316, _320_
  Steganophthalmata, 314
  _Steganopora_, 284, _286_
  Stein, von, on Protozoa, 45 f.;
    misinterpretation of parasitic Suctoria in Ciliata, 161;
    on Suctoria, 162
  Stelechotokea, _347_
  _Stelletta_, _213_, 214, 222
  Stem—see Stalk
  Stem-ossicles, of Pentacrinidae, 588, 591;
    of _Hyocrinus_, 588, 590;
    of Rhizocrinidae, 588, 590;
    of larva of _Antedon rosacea_, 619
  Stenoscyphidae, _321_
  _Stenoscyphus inabai_, _321_
  Stentor, _137_, 154;
    regeneration, 35;
    supposed nervous fibrils, 143;
    meganucleus, 144;
    conjugation, 149;
    attachment, 152;
    _S. coeruleus_, 154;
    _S. igneus_, 154;
    _S. polymorphus_, young, and adult in fission, 156
  Stentorin, blue, 154 n.
  _Stephalia_, _308_
  _Stephanophyes_, 300, _307_;
    _S. superba_, 307
  Stephanophyinae, _307_
  _Stephanophyllia_, _404_
  _Stephanoscyphus_, _318_;
    _S. mirabilis_, 206
  _Stephanosphaera_, _111_, 128
  Stephoidea, _78_ f.
  Stereoplasm, 394
  _Stereosoma_, 331, 334, 337, _344_
  Stereotaxy, 20
  Sterilisation of colonial cells in _Volvox_, 129
  Sternata, _554_
  Sternum = Plastron, _q.v._
  Sterraster (an aster with very numerous actines soldered together by
      subsequently deposited silica, which extends almost to their
      extremities), 224
  Sterrula, 341
  Stewart's organs, of Cidaridae, 531;
    of Echinothuriidae, 531;
    of Arbaciidae, 531, 539;
    of Diadematidae, 538
  _Stichaster_, _474_
  Stichasteridae, 456, _474_
  Stichodactylina, _380_, _383_
  _Sticholonche_, 86;
    host of _Amoebophrya_, 161
  _Stichopathes_, 407, _408_
  _Stichopus_, _570_
  _Stichotricha_, _138_
  Sticklebacks, prey on _Anopheles_, 106
  Stimuli, 8 f.;
    inducing responsive movements, 19 f.
  _Stoichactis kenti_, 378, _383_
  Stolč, on _Pelomyxa_, 53 n.
  Stolon, 301
  Stolonifera, _342_
  Stomach, present in Metazoa, 38;
    of gnats, seat of syngamy of Acystosporidae, 103, 104 f.;
    of _Asterias rubens_, 438;
    of _Echinus esculentus_, 516;
    of _Holothuria nigra_, 562;
    of _Antedon rosacea_, 583;
    of Dipleurula, 605;
    absent in _Hyocrinus_, 589
  _Stomatoca_ (Tiaridae, _273_), 415
  _Stomatograptus_, _282_
  Stomodaeum, of Anthozoa, 327;
    of Ctenophora, 415;
    of _Echinus esculentus_, 516;
    of Dipleurula, 605;
    of metamorphosing Auricularia, 615;
    of larva of _Antedon_, 619
  _Stomolophus_, _325_
  Stony Corals, 326
  Strain-figure of cell dividing by mitosis, 25, 26, 27
  Strained condition of cytoplasm during syngamy, 34
  Streaming of granules in protoplasm, 17
  _Streptocaulus_, 277
  Streptophiurae, 491, _494_, 500, 502
  Strobila, 317
  _Stromacystis_, _596_
  _Stromatopora_, _283_
  Stromatoporidae, _283_
  _Strombidium (Torquatella)_, _137_, 155 n.
  Strongyle, 183, 224
  _Strongylocentrotus_, 512, 522, 533, _540_;
    _S. droëbachiensis_, 512, 523, 541;
    _S. lividus_, 541;
    _S. purpuratus_, 542
  _Stryphnus ponderosus_, 222
  Studer, 340
  _Stylactis_, 268, _270_;
    _S. abyssicola_, 268;
    _S. minoi_, 268;
    _S. vermicola_, 268, 269;
    _S. spongicola_, 268
  _Stylaster_, 285, _286_
  Stylasteridae, _285_
  Stylasterina, _283_
  _Stylatula darwinii_ (Virgulariidae, _362_), 360
  Style, 284
  _Stylocordyla stipitata_, _216_
  _Stylonychia_, _138_ f.;
    meganucleus of, 144;
    _S. mytilus_, 138, 139 f.
  Stylopodium, 60, 61
  _Stylorhynchus_, _97_, 99;
    syngamy of, 100
  _Stylostichon_, 225
  Stylus, 183, 184
  Sub-costal canals, 415
  Sub-tentacular canal of _Antedon rosacea_, 586
  _Suberites_, _219_, 224, 230;
    _S. domuncula_, 219
  _Suberogorgia_, _351_
  Sucker(s), of _Mesodinium_, 152;
    of _Trachelius_, 153 n.
  Suction mechanism in _Choanophrya_, 159, 161
  Suctoria, _158_ f., 160, 162;
    animal nutrition of, 40
  Sugar, 15
  Sulculus, 369
  Sulcus, 369
  Supero-marginal ossicle of Asteroidea, 436
  Supplemental skeleton of Foraminifera, 63, 66
  Surface, protoplasmic movements in relation to a, 20—see Thigmotaxy,
      Stereotaxy;
    ratio of mass to, 14, 23 f.;
    -tension in relation to protoplasmic movements, 17;
    to penetration of ovum by sperm, 34
  Surra disease, 119
  Swarmers of Foraminifera, 67 f.;
    of Dinoflagellates, 131
  —see also Zoospores
  Sycettidae, _187_
  _Sycon_, _187_, 221;
    _S. carteri_, 187;
    _S. coronatum_, 187;
    _S. setosum_, development, 188, 189, 231;
    _S. raphanus_, development, 190, 226
  Symbiosis, 86, 125, 219;
    _Pelomyxa_, 53;
    Heliozoa, 73;
    Radiolaria, 80, 86;
    Acantharia, 80;
    _Ophrydium_, 158;
    _Paramecium bursaria_, 153;
    _Scyphidia scorpaenae_, 125;
    _Stentor polymorphus_, 154;
    _Vorticella sertulariae_, 125;
    _Ephydatia fluviatilis_, 175;
    _Hydra viridis_, 126, 256;
    Turbellaria, 73, 126;
    _Convoluta_, 73;
    _Millepora_ and Zooxanthellae, 261;
    in Gymnoblastea, 268;
    _Lar_ and _Sabella_, 273;
    _Cannopora_ and _Aulopora_, 283;
    in Scyphozoa, 311;
    Alcyonaria and Zooxanthellae, 339;
    _Solenocaulon_ and _Alpheus_, 350;
    _Eunicella_ and Cirripede, 356;
    _Verrucella_ and Ophiurid, 357;
    _Pteroeides_ and Crab, 361;
    _Ptilosarcus_ and Hydroid, 361;
    Zoantharia and Zooxanthellae, 373 f.;
    _Adamsia_ and hermit crabs, 377, 381;
    _Melia_ and Sea-anemone, 378;
    _Stoichactis_ and _Amphiprion_, 378;
    Pocilloporidae and _Hapalocarcinus_, 402;
    in Zoanthidae, 405
  —see also _Zoochlorella_, _Zooxanthella_
  Sympodium, 583
  _Sympodium_, _344_;
    _S. coralloides_, 341
  _Sympterura_, 502
  Synalcyonacea, _342_
  _Synapta_, _577_;
    _S. similis_, 429;
    _S. digitata_, 576;
    its larva, 608;
    its pupa, 615;
    _S. inhaerens_, 577
  Synapticula, 402;
    of Eupsammiidae, 404
  Synaptida, 568, 569, _577_, 578
  _Syncoryne_, 265, _272_, 297
  _Syncrypta_, _110_, 112
  Syngamy (= Conjugation), 33 f.;
    Rhizopoda, 56 f.;
    Radiolaria, 85;
    Flagellates, 115 f.;
    Chlamydomonadidae, 115 f., 125;
    between resting-cells, 115 f.;
    by a fertilising tube, 125;
    Volvocaceae, 128 f.;
    Suctoria, 161
  —Equal, exogamous, _Trichosphaerium sieboldii_, 54, 56;
    _Chlamydophrys stercorea_, 57;
    Foraminifera, 68 f.;
    _Actinophrys sol_, 72;
    Gregarines, 97 f.;
    _Lankesteria_, 95;
    _Monocystis_, 96;
    _Cercomonas dujardinii_, 116 n.;
    _Lamblia_, 116 n.;
    _Polytoma uvella_, 116 n.;
    _Tetramitus rostratus_, 116 n.;
    _Trichomonas_, 116 n.;
    Chlamydomonadidae, 125;
    Dinoflagellates, 131;
    _Noctiluca_, 133;
    Ciliata, 148 f.
  —Equal, endogamous, _Amoeba coli_, 57;
    _Actinosphaerium_, 73 f.;
    _Stephanosphaera_, 128
  —Unequal (binary, bisexual), Centropyxis aculeata, 57;
    Radiolaria (?), 85;
    _Stylorhynchus_, 99, 100;
    _Pterocephalus_, 99;
    Coccidiaceae, 97, 100 f.;
    _Coccidium_, 100, 101 f.;
    _Adelea ovata_, 101;
    Acystosporidae, 97, 104 f.;
    Sarcosporidiaceae, 108;
    _Dallingeria drysdali_, 116 n.;
    _Monas dallingeri_, 116 n.;
    _Bodo saltans_, 116 n., 117 f.;
    _B. caudatus_, 116 n.;
    _Halteridium_, 120;
    Chlamydomonadidae, 125;
    _Volvox_, 127 f.;
    _Pandorina morum_ (ternary), 128, 129;
    _Eudorina_, 129;
    Peritrichaceae, 151, 157;
    _Vorticella_, 157
  Syphilis in relation to _Treponema pallidum_, 121
  _Syringolites_, _344_
  _Syringopora_, 283, 329, _343_, 344
  Syzygy (= association) of Gregarines, 99;
    in Crinoids, 582, 587

  Tabulae, 257, 345, 385, 387
  Tail-like appendages, of certain Ciliata, 141 n.;
    (spine) of _Caenomorpha uniserialis_, 154, 155
  _Tamoya punctata_, _319_
  Tan-pits infested by _Fuligo varians_, 92
  Tapeworms, nutrition of, 38;
    alternation of generations in, 44
  _Tarsaster_, _474_
  _Tealia_ = _Urticina_, _q.v._
  _Tedania_, 223
  Tegmen, of Crinoidea, 580;
    of _Antedon rosacea_, 581, 585;
    of _Hyocrinus_, 590;
    of Articulata, 595;
    of Camerata, 595
  Telestidae, _348_
  _Telesto_, 346, _348_;
    _T. prolifera_, 347;
    _T. rubra_, 336, 347
  Telolecithal (= segmentation limited to one region of the oosperm owing
      to excess of yolk), 133 n.
  Telosporidia, _97_ f.
  Temnopleurinae, _539_
  Temperature, in relation to protoplasmic movements, 7;
    to breeding, 47;
    to fission of Ciliates, 148;
    changes of, stimulus of, 19, 22;
    maximum (Dallinger and Drysdale's experiments), 118
  Tension, surface-, 17, 34
  Tentacles, of _Actinobolus_ and _Ileonema_, 152;
    of Suctoria, 158 f., 160, 162;
    of Hydrozoa, 251 f.;
    of Scyphozoa, 311 f.;
    of Anthozoa, 327;
    of Alcyonaria, 331 f.;
    of Zoantharia, 366 f.;
    of Ctenophora, 414 f.;
    of Holothuroidea, 561 f.;
    of Pelmatozoa (= podia), 579 f.
  Tentacle-scale, of _Ophiothrix fragilis_, 480
  Tentacular (= transverse) plane, 414
  Tentaculata, _417_
  Tentaculifera (= Suctoria), _159_ f., 160, 162
  Tentaculozooids, 265
  Tentilla, 299
  Termites, Trichonymphidae parasitic in, 123
  Terrestrial Protozoa, 48
  Tertian fever, a parasitic disease, 104 f.
  _Tessera_, 320, _321_
  Test, classificatory value of, 51 n.;
    of _Euglypha_, 29;
    of Rhizopoda, 29, 53, 54, 55;
    of _Trichosphaerium sieboldii_, 54;
    of _Arcella vulgaris_, 55;
    of _Diffugia pyriformis_, 55;
    of _Hyalosphenia lata_, 55;
    of _Quadrula symmetrica_, 55;
    of Foraminifera, 58 f., 61, 63, 65, 67, 68, 69;
    of Allogromidiaceae, 60;
    of _Microgromia socialis_, 60;
    of _Lieberkühnia_, 61;
    of marine Foraminifera, 62 f.;
    of _Discorbina_, 63;
    of _Frondicularia_, 63;
    of _Globigerina_, 63;
    of _Lagena_, 63;
    of _Nodosaria_, 63;
    of _Nummulites_, 63;
    of _Planorbulina_, 63;
    of _Saccammina_, 63;
    of _Spiroloculina_, 63;
    of _Allogromia_, 65;
    of _Miliola_, 65;
    of _Quinqueloculina_, 65, 67;
    of _Rotalia_, 65;
    of _Squamulina_, 65;
    of _Biloculina_, 67;
    (gelatinous) of _Nuclearia_, 74;
    of Chrysomonadaceae, 110, 113 f.;
    (chitinous) of Tintinnidae, 152, 155
  —see also Shell, Skeleton, Theca
  _Tethya_, 211, _218_, 222;
    gemmule, 230;
    scleroblasts, 233, 233;
    _T. lyncurium_, 218
  _Tetilla_, 212, _213_, 218, 233;
    _T. casula_, 212;
    _T. pedifera_, 211, 213
  Tetracoralla, 394
  Tetracrepid, 215—see Rhabdocrepid
  Tetractinellida, _211_ f., 218, 231
  _Tetramitus_, _111_, 119;
    _T. rostratus_, gametes of, 116 n.
  Texas fever, 120 f.
  _Textularia_, _59_;
    in chalk, 70, 242
  Textulariaceae, _59_
  Thalassianthidae, _383_
  _Thalassicolla_, _77_, 83;
    _T. nucleata_, regeneration, 79 n.
  _Thalassophysa_, _77_;
    reproduction, 86 n.
  _Thamnograptus_, _281_
  _Thamnostylus_, _270_
  _Thaumactis_, _382_
  _Thaumantias_, _278_
  Thaumantiidae, _278_
  _Thaumatocrinus_, 588, 589, _594_
  _Thaumatomastix_, _110_
  Theca, of Flagellata, 112 f.;
    of _Dinobryon_, 112;
    (branched) of _Rhipidodendron_, 112;
    (stalked) of _Diplomita_, 112;
    of _Salpingoeca_, 122;
    of _Acineta_, 159, 160;
    of Corals, 370
  —see also Shell, Skeleton, Test
  Thecamoebae, 51 n.
  Thecidae, _346_
  _Thecocarpus_ (Plumulariidae, _279_), 276
  _Thecocystis_, _596_, 596
  Thecoidea, _580_, _596_
  Thélohan, on Sporozoa, 94
  _Thenea wyvillei_, _212_
  _Theoconus_, _79_, 80
  Thermotaxy, 22
  Thigmotaxy, 20
  Thomson, Wyville, on _Calcaromma_, 83
  Thorax = second chamber of Monaxonic Radiolarian shell, 84
  _Thyone_, _573_
  _Tiara_, _273_
  Tiarechinidae, _557_
  _Tiarechinus_, _557_
  Tiaridae, _273_
  Tick, intermediate host of _Karyolysus_, 102;
    of _Piroplasma_, 120;
    of _Treponema_, 121 n.;
    Persian, 120;
    Zambezian, 121 n.
  Tick fever, 120
  Tiedemann's bodies, 442, 444, 458;
    represented by so-called Polian vesicles of _Echinus esculentus_, 516
  Tinctin bodies, 161
  _Tinerfe_, _417_
  Tintinnidae, _155_;
    tests of, 152
  _Tintinnus_, _137_
  Tissues (definition), 3
  Tolerance, induced, of a higher temperature, 118
  Tone, 519
  Tooth, of _Ophiothrix fragilis_, 482;
    of _Echinus esculentus_, 505, 524, 525;
    of _Echinarachnius parma_, 546
  Tooth-papilla, of _Ophiothrix fragilis_, 482;
    of _Ophiocoma_, 493;
    of Ophiocomidae, 499
  Topsent, 196, 218 n.
  Tornaria, 616
  _Torquatella typica_ (= _Strombidium_), 155 n.
  Torus angularis, of _Ophiothrix fragilis_, 482
  Toxa (= a bow-shaped spicule without spiral twist), 222
  Toxaspire (a spiral rod in which the twist a little exceeds a single
      revolution. The pitch of the spiral being great the spicule appears
      bow-shaped), 222
  Trabeculae, traversing ampullae of tube-feet of _Echinus esculentus_,
      517;
    traversing coelom of _Echinarachnius parma_, 545;
    traversing coelom of _Antedon rosacea_, 585
  Tracheae of Chondrophoridae, 309
  _Trachelius_, _137_, 152 n.;
    _T. ovum_, 153;
    endosarc of, 144
  _Trachelomonas_, _110_, 112;
    galvanotaxy of, 22
  Trachomedusae, _288_ f.
  _Trachynema_, _294_;
    _T. funerarium_, 294
  Trachynemidae, _294_
  Tractellum, 18, 114
  _Tragosia_, 224
  Transverse fission, of Flagellata, 109 f.;
    of Dinoflagellata, 110, 131;
    of _Polykrikos_, 131;
    of Ciliata, 147;
    of Suctoria, 161
  Transverse flagellum and groove, in Dinoflagellata, 110, 130 f.;
    multiple, in _Polykrikos_, 132
  Transverse (= tentacular) plane, 414
  Transverse section, of the arm of an Asteroid, 443;
    of the arm of an Ophiuroid, 480;
    of the radius of an Echinoid, 518;
    of the radius of a Holothuroid, 562;
    of body of a Holothuroid, 563;
    of arm of _Antedon_, 586
  Trembley, 254, 255
  Trepang, 571
  _Treponema_, _111_, 121;
    _T. obermeieri_, 121;
    _T. pallidum_, 121;
    _T. zeemannii_, 120
  Triaene, 183, 224, 233
  Triaxon, 184
  _Trichaster_, _501_
  Trichasteridae, _501_
  Trichites (hair-like spicules often occurring in sheaves or clusters),
      234
  Trichocysts, 142 f.;
    of Chloromonadaceae, 113 n.;
    Mitrophanow on, 142 n.;
    adoral, of Gymnostomaceae, 145
  _Trichodina_, _138_, 158
  Trichodragmata (a sheaf of straight spicules of hair-like fineness), 222
  _Trichogorgia_, _355_
  _Trichomonas_, _111_, 115;
    conjugation, 116 n.;
    _T. vaginalis_, 119
  _Trichonympha_, _111_
  Trichonymphidae, _111_, 123 f.;
    flagella, 114
  _Trichosphaerium_, _51_, 53 f.;
    test, 53;
    life-cycle, 54, 56
  _Trichostemma_, _216_
  Trichostomata, _137_
  Tridactyle pedicellariae, of _Echinus esculentus_, 506, 507;
    of _E. acutus_, 509;
    of _E. elegans_, 510;
    of _Echinarachnius parma_, 544
  Trifoliate pedicellariae, of _Echinus esculentus_, 507, 508;
    of _E. acutus_, 509;
    of _E. alexandri_, 510;
    of _Echinocardium cordatum_, 550;
    absent in Cidaridae, 534
  _Trigonocidaris arbacina_, _539_
  _Triloculina_, _59_, 66
  Trimastigidae, _111_, 112
  _Tripedalia cystophora_, _319_
  Tripedaliidae, _319_
  Triplechinoid type of ambulacral plate, 531, 539
  Tripod, 83;
    -shaped spicule of Radiolaria, 76
  Tripolis, 87
  Tripylaea, _76_
  Trivium, of _Echinarachnius parma_, 543;
    of _Holothuria nigra_, 561
  _Trochammina_, _59_
  _Trochocyathus_, _399_;
    _T. hastatus_, 398
  _Trochocystis_, _597_;
    _T. bohemicus_, 597
  _Trochoderma_, _577_
  _Trochodota_, _577_
  _Trochosmilia_, _401_
  _Trochostoma_, _575_;
    _T. violaceum_, 575
  Trophodisc, 284
  Trophozooid, 388
  Tropical Africa, Trypanosomic diseases of, 119 f.
  Trout, black-spotted, destroyed by _Hydra_, 256 n.
  _Trypanosoma_, _111_, 115 f., 119 f., 120;
    podoplast or blepharoplast of, 19 n., 109 n.;
    undulating membrane of, 115;
    _Halteridium_, a supposed state of, 103 n., 120;
    affinities to Acystosporidae, 106;
    morphology of, 120, 121;
    _T. brucei_ infests hoofed quadrupeds, 119;
    _T. evansii_ causes Surra disease in Ruminants, 119;
    _T. gambiense_, cause of sleeping-sickness, 120;
    _T. lewisii_, infests Rodents, 119;
    _T. noctuae_, 120;
    conjugation in, 116 n.
  Trypanosomoid character of blasts of Acystosporidae, 106
  Tsetse Flies, intermediate hosts of Trypanosomes of Nagana and
      sleeping-sickness, 119 f.
  Tube, of _Phalansterium_, 113;
    of certain Ciliates, 152;
    of _Maryna socialis_, 152;
    of _Schizotricha socialis_, 152;
    of _Stentor_, 154;
    of Vorticellidae, 158;
    fertilising in _Chlamydomonas_, 125
  Tube-foot, 428;
    of _Asterias rubens_, 441 f.;
    of _Echinus esculentus_, 517 f.;
    of Endocyclica, 532;
    of Arbaciidae, 532;
    of Cidaridae, 532;
    of Diadematidae, 532;
    of Echinothuriidae, 532;
    of _Echinocardium cordatum_, 551;
    of _Echinarachnius parma_, 545, 546, 547;
    of _Palaeodiscus_, 557;
    of _Holothuria nigra_, 561
  _Tubipora_, 329, 336, 343, _344_;
    _T. musica_, 338, 343
  Tubiporidae, _344_
  _Tubularia_, 268, _271_;
    _T. larynx_, 263;
    _T. parasitica_, 268
  Tubulariidae, _271_
  Tumour, malignant, associated with _Leydenia_, 91
  Tunicata, 621
  Tuning-fork, 192, 193
  Turbellaria, fresh-water, distribution of, 48;
    symbiotic with _Zoochlorella_, 126
  _Turbinaria_, _396_
  _Turbinolia_, _399_
  Turbinoliidae, _398_
  _Turritopsis_, _273_, 295
  _Tuscarora_, _79_, 85
  Tylostyle (a style in which a knob surrounds the origin), 224
  Tylote, 183, 224
  Tylotoxea (a rhabdus of which one actine is tylote or knobbed, the other
      oxeate, the latter directed towards the surface of the Sponge), 224
  Tyrosin, 15

  Uexküll, on function of pedicellariae of _Echinus_, 508;
    on physiology of nervous system of Echinoidea, 519;
    on vision of Echinoidea, 522;
    on respiratory function of Aristotle's lantern, 527;
    on pedicellariae of _Sphaerechinus_, 541
  _Uintacrinus_, 588
  Ulmaridae, _324_
  _Ulmaris_, 314, _324_;
    _U. prototypus_, 315, 324
  _Umbellula_, 331, 359, 360, _363_;
    _U. gracilis_, 359
  Umbellulidae, _362_
  Umbrella of Medusae, 251
  Uncinataria, _203_
  Under arm-plate of Ophiuroidea, 491
  Under basal-plate of Crinoidea—see Infra-basal plate
  Undulating membrane, of Flagellata, 111, 115, 123;
    of _Trypanosoma_, 121;
    of _Dinenympha_, 123;
    of _Stylonychia mytilus_, 139 f.;
    of Ciliata Trichostomata, 137 f., 145;
    of _Glaucoma_, 153;
    of _Pleuronema chrysalis_, 153, 154;
    of _Caenomorpha uniserialis_, 155;
    of _Vorticella_, 156, 157
  Unequal fission in _Spirochona_, 147
  Upper arm-plate, of Ophiuroidea, 491;
    of _Ophioteresis_, 491;
    absent in Cladophiurae, 491, 500
  _Urechinidae_, _554_ n.
  Urine, 13
  _Urocentrum_, _137_, 141 n.
  _Uronema_, _137_, 141 n.
  _Urticina_, 379, _382_;
    _U. crassicornis_, 379, 382
  Ussov, 257 n.
  _Ute_, 189, 190, _192_, 221

  Vacuole, 5 f.;
    of _Collozoum inerme_, 76;
    of _Oikomonas_, 112;
    contractile or pulsatile, 14 f.—see Contractile vacuole;
    formative, 14 f.—see Alveole, Food-vacuole, Formative vacuole,
      Ingestion, vacuole of
  _Vaginicola_, _138_, 158
  Valvata, _461_, _471_ f.
  Valvate, pedicellariae, of Antheneidae, 456, 471
  _Vampyrella_, _89_
  Vaney, 292 n.
  Variation in character of Foraminiferal shell at different stages of
      growth, 66
  Vegetative, growth, in coloured Flagellates, 115;
    rest, 37
  Velata, _461_, _464_ f., 466
  _Velella_, 301, 302, _309_;
    _V. spirans_, 304
  Veley, Lilian, on _Pelomyxa_, 53 n.
  _Ventriculites_, 208, 208
  Venus's Flower-basket (= _Euplectella aspergillum_), 197
  Venus's girdle, 420
  Veretilleae, _364_
  _Veretillum_, _364_
  Vermicles, of Gaule, a name for _Lankesterella_, 102
  Verrucae, 331
  _Verrucella_, _357_;
    _V. guadaloupensis_, 357
  Vertebra, of Ophiuroidea, 481, 491;
    of Streptophiurae, 491, 494;
    of _Ophioteresis_, 481, 491;
    of _Ophiohelus_, 491, 493;
    of Zygophiurae, 491;
    of _Ophiothrix fragilis_, 480;
    of _Ophiarachna_, 481;
    of Cladophiurae, 491, 500;
    of _Astroschema_, 481;
    of _Gorgonocephalus_, 491;
    of _Astrophyton_, 491;
    of fossil Ophiuroidea, 501, 502
  Vertebrates, cold-blooded, hosts of Haemosporidae, 102
  Verticilladeae, _363_
  Verworn, on general physiology and protoplasm, 3 n.;
    on protoplasmic movements, 16 n., 17;
    on regeneration, 35 n.;
    of _Thalassicolla nucleata_, 79 n.
  Vesicular nucleus, 25
  Vestibule, of Peritrichaceae, 145;
    of _Carchesium_, 146;
    of _Vorticella_, 156, 157
  _Vexillum_, _421_
  Vibratile styles of Rotifers, 141
  _Villogorgia_, _356_
  _Virgularia_, 339, _362_;
    _V. juncea_, 360;
    _V. mirabilis_, 362;
    _V. rumphii_, 330, 360
  Virgulariidae, _362_
  Vision, of Asteroidea, 446;
    of Echinoidea, 522
  Vital forces, 12 f.;
    processes, 11 f.
  Vitreous Foraminifera, _58_, 62
  Voluntary muscles of Mammals infested by Sarcosporidiaceae, 108
  Volvocaceae, _110_, _111_, 125 f.;
    literature of, 119
  Volvocidae, _111_, 126 f., 127, 129;
    theca, 113
  _Volvox_, plasmic cell connexions of, 37 n.;
    a true vegetable Protist, 130;
    _V. globator_, 127 f.
  _Vorticella_, _138_, 155 f., 157;
    _V. sertulariae_, symbiotic _Zooxanthella_ in, 125
  Vorticellidae, 157 f.;
    fission, 158;
    colonies, 158
  Vosmaer, 187 n., 212, 234 n., 237

  Wager, on Euglenaceae, 125
  Wagner, 256 n.
  Wallich, on Protozoa, 45
  _Walteria_, 199, _202_;
    _W. flemmingi_, 206;
    _W. leuckarti_, 206
  Wasielewski, on Sporozoa, 94 n.
  Water in protoplasm, 12
  Water-Fleas, Vorticellidae found on, 158;
    rheotaxy of (small Crustacea), 21
  Water-vascular system, 428;
    of _Asterias rubens_, 441;
    of Asteroidea, 457;
    of _Ophiothrix fragilis_, 486;
    of _Echinus esculentus_, 516;
    of _Echinarachnius parma_, 546;
    of _Echinocardium cordatum_, 551;
    of _Holothuria nigra_, 564;
    of Holothuroidea, 568;
    of Synaptida, 568;
    of Molpadiida, 568;
    of Elasipoda, 568;
    of _Psolus_, 569;
    of _Antedon rosacea_, 583;
    of Carpoidea, 597
  Weltner, 177, 178
  White Ants, hosts of Trichonymphidae, 123
  White Man's Grave, 106
  Wille, on Volvocaceae, 119
  Willey, 194 n., 248, 421, 422
  Williams, on density of living protoplasm, 13 n.
  Williamson, on structure of Foraminifera, 62
  Wilson, on protoplasm, 3 n.;
    on syngamy, 34 n.
  Wilson, on Sponges, 171, 231
  Woltereck, 302
  Wolters, on reproduction of _Monocystis_, 96 n.
  Woodcock, on association and conjugation in Gregarines, 99 n.;
    on Haemoflagellates (= Trypanosomidae), 119 n.
  Worms, Earth-, hosts of _Monocystis_, 95
  Wreath, adoral, peristomial, of cilia or membranellae of Ciliata
      Trichostomata, 137 f.;
    of _Stylonychia mytilus_, 139 f.;
    of _Metopus_, 154;
    of _Caenomorpha uniserialis_, 155;
    of Vorticella, 156, 157;
    posterior, of _Vorticella_, 156, 157
  _Wrightella_, _351_
  Wrisburg, on organisms of putrefaction, 43
  Würmchen, of Gaule, a name for _Lankesterella_, 102

  _Xenaster_, _476_
  _Xenia_, 331, 335, 346, _348_
  Xeniidae, _348_
  _Xenospongia patelliformis_, _216_
  _Xiphacantha_, _78_, 78
  _Xiphigorgia_, _357_

  Yaws, 121 n.
  Yellow-cells (= _Zooxanthella_), 80, 86, 125, 261, 373, 396
  Yolk-granules of ovum of Sea-urchin, 7
  Young state of _one_ pairing-nucleus essential, 34
  _Yvesia_, 224

  Zambezian Tick, infects man with _Treponema_, 121 n.
  Zaphrentidae, _406_
  _Zaphrentis_, _407_
  Zederbauer, on syngamy in Dinoflagellates, 131 n.
  Zittel, 241 n.
  Zoantharia, _329_, _365_ f.;
    age, 375;
    food, 373;
    form, 366;
    gonads, 369;
    mesenteries, 366 f., 368;
    mesenteric filaments, 369;
    reproduction, 371;
    skeleton, 370;
    stomodaeum, 369;
    tentacles, 366
  Zoanthidae, _404_
  Zoanthidea, 370, _404_ f.
  _Zoanthus_, _405_;
    _Z. macgillivrayi_, 406;
    _Z. sulcatus_, 406
  Zonarial Radiolaria, 75
  _Zoochlorella_, _111_;
    a Chlamydomonad, 126;
    symbiotic, in Heliozoa, 73;
    in _Paramecium bursaria_, 153;
    in _Stentor polymorphus_, 154;
    in _Ophrydium_, 158;
    in _Ephydatia_, 175;
    in _Hydra viridis_ (= chlorophyll corpuscles), 256
  Zooids of _Volvox globator_, 127
  Zoosporeae, _89_
  Zoospores, of Algae and Fungi, possess contractile vacuole, 15;
    of Lower Plants, 17 f.;
    of Sarcodina, 49;
    of _Trichosphaerium_, 54;
    of _Microgromia socialis_, 60;
    of Foraminifera, 67 f.;
    of _Clathrulina_, 73;
    of Radiolaria, 85 f.;
    of _Zooxanthella_, 86;
    of Acrasieae, 90;
    of _Didymium_, 92;
    of _Paramoeba eilhardii_, 116 n.
  _Zoothamnium_, _138_, 158
  _Zooxanthella_, _110_;
    a Chrysomonad, 125;
    in Radiolaria, 80, 86;
    in _Vorticella sertulariae_, 125;
    in _Millepora_, 261;
    in Zoantharia, 373 f.;
    in Madrepores, 396
  Zopf, on Monadineae (Flagellates and Proteomyxa), 40
  _Zoroaster_, _474_
  Zoroasteridae, 454, _474_
  _Zostera_, 422
  Zygophiurae, 491, _494_, _495_ f., 502
  _Zygophylax_, _280_
  Zygote, 37 f.;
    _Amoeba coli_, 57;
    _Centropyxis aculeata_, 57;
    _Chlamydophrys stercorea_, 57;
    Foraminifera, 69;
    _Actinophrys sol_, 72;
    _Actinosphaerium_, 75;
    Gregarinidaceae, 95 f., 97;
    Coccidiaceae, 97, 100 f.;
    _Coccidium_, 100, 101 f.;
    Acystosporidae, 97, 104 f.;
    Flagellata, 116 n., 117 f.;
    _Bodo saltans_, 117 f.;
    Dinoflagellata, 131 n.;
    Ciliata, 148 f.
  —see also Coupled cell, Fertilised egg, Ookinete, Oosperm, Oospore,
      Zygotospore
  Zygotoblasts of Acystosporidae, 104 f.
  Zygotomeres (= naked spores of Acystosporidae), 104 f.
  Zygotonucleus (= Fusion-nucleus, a nucleus formed by fusion of two
      gametonuclei), 33 f.
  Zygotospore (= resting zygote), 97
  Zykoff, 178
  Zymase (= chemical ferment), 15;
    in relation to brood-division, 32 f.



END OF VOL. I


_Printed by_ R. & R. CLARK, LIMITED, _Edinburgh_.



  [1] For detailed studies of protoplasm see Delage, _Hérédité_, 2nd ed.
      1903; Henneguy, _Leçons sur la Cellule_, 1896; Verworn, _General
      Physiology_, English ed. 1899; Wilson, _The Cell in Development and
      Inheritance_, 2nd ed. 1900. All these books contain full
      bibliographies.

  [2] As we shall see later, it is by no means easy to separate sharply
      Protozoa and Protophyta, the lowest animals and the lowest plants;
      and therefore in our preliminary survey to designate lowly forms of
      life, not formed of the aggregation of differentiated cells, we shall
      employ the useful term "Protista," introduced by Haeckel to designate
      such beings at large, without reference to this difficult problem of
      separation into animals and plants (see also p. 35 f.).

  [3] The "micron," represented by the Greek letter µ, is 1/1000 mm., very
      nearly 1/25,000 of an inch, and is the unit of length commonly
      adopted for microscopic measurements.

  [4] A solid substratum is required, to which the lower surface adheres
      slightly: that movement is complicated by a sort of rolling over of
      the upper surface, constantly prolonging the front of the
      pseudopodium, while the material of the lower surface is brought up
      behind. H. S. Jennings, _Contr. to the Study and Behaviour of the
      Lower Organisms_, 1904, pt. vi. p. 129 f., "The Movements and
      Reactions of Amoeba."

  [5] If the protoplasm contains visible granules, as it usually does,
      within a clear external layer, we see that these stream constantly
      forwards along the central axis of each process as it forms, and
      backwards just within the clear layer all round, like a fountain
      playing in a bell-jar. This motion is most marked when a new
      pseudopodium is put forth, and ceases when it has attained full
      dimensions.

  [6] We use as a corresponding adjective the term "plasmic."

  [7] For the study of the structure of protoplasm under the microscope it
      is necessary to examine it in very thin layers, such as can for the
      most part be obtained only by mechanical methods (section-cutting,
      etc.). These methods, again, can only be applied to fixed specimens,
      for natural death is followed by rapid changes, and notably by
      softening, which makes the tissue less suitable for our methods. We
      further bring out and make obvious pre-existing differentiations of
      our specimens by various methods of staining with such dyes as
      logwood and cochineal and their derivatives, and coal-tar pigments
      (see also p. 11 n.).

  [8] In many Protista these granules have been shown by Schewiakoff, in
      _Z. wiss. Zool._ lvii. 1893, p. 32, to consist of a calcium
      phosphate, probably Ca_{3}P_{2}O_{8}.

  [9] It is not always possible to tell how much of these structures
      represents what existed in life (see p. 11).

 [10] The chromatin and nucleoles are especially rich in phosphorus,
      probably in the combination nucleinic acid.

 [11] In chemical phrase the process is "exothermic."

 [12] The growth of crystals is a mere superficial deposit, and cannot at
      all be identified with protoplasmic growth.

 [13] A. Bolles Lee, in his _Microtomist's Vade Mecum_, 1st ed. (1885),
      pointed out that "Clearing reagents are liquids whose primary
      function is to make microscopic preparations transparent by
      penetrating amongst the highly refractive elements of which the
      tissues are composed, having an index of refraction not greatly
      inferior to that of the tissues to be cleared" (p. 213). We showed
      later ("The State in which Water exists in Live Protoplasm," in _Rep.
      Brit. Ass._ 1889, p. 645, and _Journ. Roy. Micr. Soc._ 1890, p. 441)
      that since the refractivity of living protoplasm is only 1.363-1.368,
      it follows that the water in the living protoplasm is in a state of
      perfect physical combination, like the water of a solution of gum
      [read a "mucilage"] or of a jelly. Now the phenomena of protoplasmic
      motions as studied in the Rhizopoda and in the vegetable cell, seem
      absolutely to preclude the jelly supposition, and for these cases we
      must admit that living protoplasm is a viscid liquid whose
      refractivity is probably the mean of the two constituents separated
      by death, the one solid, the other a watery solution: and death is
      for us essentially a process of precipitation (or better,
      "desolution"). For further work on these lines see Hardy in _Journ.
      Physiol._ vol. xxiv. 1899, p. 158, and Fischer, _Fixirung u.
      Färbung_, 1900.

 [14] In its original use "automatism" designates the continuous sequence
      and combination of actions, without external interference, performed
      by complex machines designed and made for specific ends by
      intelligent beings: thus we speak correctly of "automatic ball
      bearings" that tighten of themselves when they become loose; but even
      these cannot take up fresh steel and redeposit it, either to replace
      the worn parts or to strengthen a tube that is bending under a
      stress.

 [15] Proteids are organic compounds containing carbon, hydrogen, nitrogen,
      and oxygen, of which white of egg (albumen) is a familiar type.
      Nucleo-proteids are compounds of proteids with nucleinic acid, which
      in addition to the above elements contain phosphorus.

 [16] The specific gravity of living protoplasm has been estimated by
      determining the density of a solution of gum in which certain
      Infusoria float freely at any depth. It was found by the concurrent
      results of Julia B. Platt and Stephen R. Williams (see _Amer.
      Natural._ xxxiii. 1899, p. 31, xxxiv. 1900, p. 95) to be from 1.014
      to 1.019, while the Metazoon _Hydra_ was found to give a density of
      only 1.0095 to 1.0115. The difference of about 0.006, it is easy to
      show, is of the correct "order of magnitude," if we admit that the
      actual substance of the Hydra has about the same specific gravity as
      the Infusorian, while the density of the whole is lightened by the
      watery contents of the internal cavity, etc. Jensen obtained a much
      higher result for _Paramecium_, using a solution of the crystalloid
      substance, potassium carbonate; but it is almost certain that this
      would be readily absorbed by the organism, and so raise its density
      in the course of the experiment.

 [17] Energy may be derived from the mere _splitting up_ of complex
      substances within the cell: when such a splitting involves the
      liberation of CO_{2} the process is (mis-)called "intramolecular
      respiration."

 [18] A similar organ, but with _cellular_ walls, is the bladder of the
      Rotifers and certain Platyhelminthes, in connexion with their renal
      system (vol. ii. pp. 53, 199, and especially pp. 213-5).

 [19] In _Rep. Brit. Ass._ 1888, p. 714; _Ann. Mag. Nat. Hist._ (6), iii.
      1889, p. 64. This view has been fully worked out, mainly on Ciliates,
      by Degen in _Bot. Zeit._ lxiii. Abt. 1. 1905.

 [20] See Hartog, "On Multiple Cell-division, as compared with Bi-partition
      as Herbert Spencer's limit of growth," in _Rep. Brit. Ass._ 1896, p.
      833; "On a Peptic Zymase in Young Embryos," _ibid._ 1900, p. 786;
      "Some Problems of Reproduction," ii. _Quart. Journ. Micr. Sci._
      xlvii. 1904, p. 583.

 [21] "On the Digestive Ferment of a large Protozoon." _Rep. Brit. Ass._
      1893, p. 801.

 [22] See for studies of the movements of Protoplasm, Berthold,
      _Protoplasmamechanik_ (1886); Bütschli, _Investigations on
      Microscopic Foams and on Protoplasm_, English ed. 1894; Verworn,
      _General Physiology_, 1899; Le Dantec, _La Matière Vivante_, 1893?;
      and Jensen, "Unters. ueb. Protoplasmamechanik," in _Arch. Ges. Phys._
      lxxxvii. 1901, p. 361; Davenport, _Experimental Morphology_, i. 1897;
      H. S. Jennings, _Contr._ etc. 1904.

 [23] The terms "expansion" and "contraction" refer only to the
      _superficial area_: it is very doubtful whether the _volume_ alters
      during these changes.

 [24] For discussions on the mechanism of ciliary action, see Schäfer,
      _Anat. Anz._ xxiv. 1904, p. 497, xxvi. 1905, p. 517; Schuberg, _Arch.
      Protist._ vi. 1905, p. 85.

 [25] Like the line of most rapid growth in a circumnutating plant-stem.

 [26] A similar body lies at the centre to which the axial filaments of the
      radiating pseudopodia of the Heliozoa converge, and might be termed
      by parity a "podoplast"; but "centrosome" is a convenient general
      term to include all such bodies. It is clearly of nuclear origin in
      _Trypanosoma_ (Fig. 39, p. 120).

 [27] See for development of this view W. M'Dougall in _Journ. Anal.
      Physiol._ xxxi. 1897, pp. 410, 539. I put it forward in the first
      draft of this essay in 1894.

 [28] The best general account is to be found in Davenport, _Experimental
      Morphology_, 1897.

 [29] See Jennings in _Woods Holl. Biol. Lect._ 1899, p. 93.

 [30] It is not always easy to distinguish these two classes of phenomena.

 [31] Jennings, in his studies on _Reactions to Stimuli in Unicellular
      Organisms_ (1899-1900), has shown that whatever be the nature of the
      repellent stimulus, chemical or mechanical or thermal, the reaction
      of _Paramecium_ and many other Protista is always the same. It swims
      backward a short distance, turns towards the aboral surface, and then
      having thus reversed swims on again in the new direction, front
      foremost as before. Apparent "positive taxies" are often really
      negative ones; for if the _Paramecium_ be placed in water containing
      CO_{2} it shows the reaction not on entering the part charged with
      this acid, but on passing away from it into purer water, so that it
      continually tends to turn back into the acid part, while within it or
      in the water at a distance not yet charged it swims about
      irregularly.  It appears due to this that the individuals become
      aggregated together, as they excrete this gas into the water. If a
      repellent substance diffuse towards the hinder end of a _Paramecium_,
      the response, instead of carrying it away, brings it into the region
      of greater concentration, and may thus kill it.

 [32] "Galvanotaxis and Chemotaxis," _Journ. of Physiol._ vol. xxvi.
      1900-1901, p. 291.

 [33] Let us take the case of a 1-centimetre cube, growing to the size of a
      2-centimetre cube. The superficial area of the 1 cm. cube measures 6
      square centimetres, and its bulk is 1 cubic centimetre. The
      superficial area of the 2-centimetre cube measures 24 square
      centimetres, while its volume measures 8 cubic centimetres. Thus the
      larger cube has only 3 cm. sq. of surface to every cubic cm. of
      volume, instead of 6; in other words, the ratio of surface to volume
      has been halved by growth. Three successive bipartitions of the
      larger cube will divide it into eight separate 1-centimetre cubes,
      each now possessing the original ratio of surface to volume.

 [34] The nucleus is regarded by some as equivalent to a central nervous
      organ for the cell; by others, such as G. Mann and Verworn, as the
      chief chemical centre of the cell, and notably the seat of the
      secretion of the zymases or ferments that play so important a part in
      its life-work; for it is found that a Protist deprived of its nucleus
      can execute its wonted movements, but can neither digest nor grow.
      This conclusion may appear to be rather sweeping and premature, but
      we have seen that the changes of surface tension are the direct
      antecedents of the motions of the cytoplasm, we know that such
      changes are induced by chemical changes; and thus the nucleus—if it
      be the central laboratory to which such changes are ultimately
      due—would really in a certain sense be a directive centre.

 [35] The term "resting" is very ill-chosen, for even superficial
      observation shows that the relative position and characters of the
      internal structures of such a nucleus are constantly changing with
      the vital activities and functions of the cell.

 [36] For a detailed study of the nucleus in Protista, see Calkins in
      _Arch. Protistenk._ vol. ii. 1903.

 [37] The "centriole" is a minute granule sometimes recognisable in the
      centre of the centrosphere, and undergoing fission in advance. But
      centrosomes are often found without a distinction into centrosphere
      and centriole, and there is much confusion in the use of the terms.

 [38] The origin of the centrosomes is a problem not yet certainly solved,
      if indeed it be susceptible of any universal solution. They are
      certainly absent in many plants; and, on the other hand, structures
      which correspond to them often appear in mitotic divisions of
      Protista. In some cases the centrosomes are undoubtedly of nuclear
      origin, and pass out through the nuclear wall into the cytoplasm.

 [39] Though the forces at work in the dividing cell are similar in their
      effects to such physical forces as magnetism, static electricity, and
      even capillarity, and models utilising such physical forces have been
      devised to represent the strain-figures of the cell, the cell forces
      are distinct from any known physical force. For discussions of the
      nature of the forces at work, with bibliographies, see Angel
      Gallardo, _Interpretatión Dinámica de la División Celular_, 1902;
      Rhumbler, in _Arch. Entw._ xvi. 1903, p. 476; Hartog, _C.R._
      cxxxviii. 1904, p. 1525, and "On the Dual Force of the
      Dividing-cell," pt. i. _Proc. Roy. Soc._ 1905 B, lxxvi. p. 548.

 [40] See Th. Boveri, _Ergebnisse ueb. d. Konstitution d. chromatischen
      Substanz des Zellkerns_ (1903), for the most recent defence of this
      view. He lays, however (p. 2), far more stress on the individuality
      of the segments themselves than on the actual chromatin material they
      contain.

 [41] The fact that it is by mitotic division that the undifferentiated
      germ-cells produce the "differentiated" tissue-cells of the body of
      the highest animals, is again irreconcilable with such theories,
      whose chief advocates have been A. Weismann and his disciples.

 [42] Temporary plastogamy is a process found in some Foraminifera, where
      two organisms unite by their cytoplasms so that there can be complete
      blending of these, while the nuclei remain distinct: they ultimately
      separate again. In the conjugation of the Infusoria, the union of the
      cytoplasms is a temporary plastogamy (see p. 148 f.).

 [43] See Figs. 9, 29, 31, 34, etc., pp. 54, 89, 95, 101.

 [44] One obvious effect of brood-formation is to augment rapidly the ratio
      of superficial area to bulk: after only three divisions (p. 23, note)
      the ratio is doubled; if the divisions be nine in succession so as to
      produce a brood of 512, the ratio is increased eightfold, on the
      supposition that the figure is preserved. However, the
      brood-mother-cell is usually spherical, while zoospores are mostly
      elongated, thus giving an additional increase to the surface, which
      we may correlate with that increased activity; so that they
      disseminate the species, spreading far and wide, and justifying the
      name of "spore" in its primitive sense (from the Greek σπείρω—I
      scatter [seed]).

 [45] This condition may be protracted in the segmentation of the egg of
      certain Higher Animals, such as _Peripatus_ (Vol. V. p. 20). It is
      clearly only a secondary and derived condition.

 [46] The usual antecedent of change in the condition of the egg is
      "fertilisation"—its conjugation with the sperm; but this is not
      invariable; and a transitory sojourn of certain marine eggs in a
      liquid containing other substances than sea-water may induce the egg
      on its return to its native habitat to segment and develop. This has
      been mistermed "Chemical fertilisation," discovered within the last
      six years by Jacques Loeb, and already the subject of an enormous
      literature.

 [47] See Hartog in _Rep. Brit. Ass._ 1896, p. 933, 1900, p. 786.

 [48] Commonly called "fertilisation," or "sexual union," inadequate and
      misleading terms.

 [49] For details see Hartog, "Some Problems of Reproduction," _Quart.
      Journ. Micr. Sci._ xxxiii. p. 1, xlvii. p. 583; and _Ann. Biol._ vol.
      iv. (1895) 1897; E. B. Wilson, Yves Delage, and Henneguy (references
      on p. 3, note); and for a singularly clear and full treatment of the
      processes in Protozoa, Arnold Lang, _Lehrb. d. Vergl. Anat._ 2nd ed.
      Lief. 2, "Protozoa," 1900.

 [50] This phenomenon, which we have termed "exogamy," is common in
      Protophyta; it has been clearly demonstrated by Schaudinn in
      Foraminifera and the Lobose Rhizopod _Trichosphaerium_ (p. 53 f. Fig.
      9), and by Pringsheim in the Volvocine _Pandorina_ (p. 128 f. Fig.
      45). It is quite independent of the differentiation of _binary_ sex.

 [51] Other modes of syngamy, such as karyogamy and plastogamy, we shall
      discuss below, pp. 69, 148; see also p. 30.

 [52] See Gruber in _Biol. Centralb._ iv. p. 710, v. p. 137 (1884-6), in
      _Ber. Ges. Freiburg_, i. ii. 1886-7; Verworn (reference on p. 16); F.
      R. Lillie in _Journ. Morph._ xii. 1896, p. 239; Nussbaum in _Arch.
      mikr. Anat._ xxvi. 1886, p. 485; Balbiani in _Recueil Zool. Suisse_,
      v. 1888, in _Zool. Anz._ 1891, pp. 312, 323, in _Arch. Microgr._ iv.
      v. 1892-3. For Higher Organisms especially see T. H. Morgan,
      _Regeneration_, 1901.

 [53] Whence the antiseptic powers of such aromatic alcohols as phenol and
      thymol, and acids as salicylic acid, etc., and their salts and
      esters.

 [54] The portion of the spectrum that is operative in "holophytic"
      nutrition is the red or less refrangible half, and notably those rays
      in the true red, which are absorbed by the green pigment chlorophyll,
      and so give a dark band in the red of its absorption spectrum. The
      more refrangible half of the spectrum, so active on silver salts,
      that it is usually said to consist of "chemical rays," is not only
      inoperative, but has a destructive action on the pigments themselves,
      and even on the protoplasm. Chlorophyll is present in all cases even
      when more or less modified or masked by the accompaniment of other
      pigments.

 [55] Similarly, threads unite the cells of the colonial plant—Flagellate
      _Volvox_, passing through the thick gelatinous cell-wall (pp.
      126-127, Fig. 44).

 [56] Pigments soluble in the ordinary solvents of fats, such as ether,
      benzol, chloroform, etc.

 [57] We have ourselves had hard work to persuade intelligent men of fair
      general education, even belonging to a learned profession, that this
      is not the case.

 [58] Dr. H. Charlton Bastian has recently maintained a contrary thesis
      (_The Nature and Origin of Living Matter_, 1905), but has adduced no
      evidence likely to convince any one familiar with the continuous
      life-study of the lower organisms.

 [59] The terms "organoid," "organella," have been introduced to designate
      a definite portion of a Protist specialised for a definite function;
      the term "organ" being reserved for a similarly specialised group of
      cells or tissues in a Metazoon or Metaphyte. We do not consider that
      this distinction warrants the introduction of new words into the
      terminology of general Zoology, however convenient these may be in an
      essay on the particular question involved.

 [60] This has been especially the case with the Flagellata, the
      Proteomyxa, and the Mycetozoa.

 [61] Lang distinguishes "lobopodia," "filopodia," and "pseudopodia"
      according to their form,—blunt, thread-like, or anastomosing. In some
      cases the protoplasm shows a gliding motion as a whole without any
      distinct pseudopodium, as in _Amoeba limax_ (Fig. 1, p. 5), and a
      pseudopodium may pass into a thin, active flagellum, which is,
      however, glutinous and serves for the capture of prey: such often
      occurs in the Lobosa _Podostoma_ and _Arcuothrix_, which are possibly
      two names for one species or at least one genus; and in many cases a
      slender pseudopodium may be waved freely.

 [62] See Schewiakoff, "Ueb. d. Geograph. Verbreitung d.
      Süsswasserprotozoen," in _Mém. Acad. St. Pétersb._ ser. 7, xli. 1893,
      No. 8. His views apply to most minute aquatic organisms—Animal,
      Vegetable, or Protistic.

 [63] See E. R. Lankester, art. "Protozoa" in _Encycl. Brit._ 9th ed.
      (1885), reprinted with additions in "Zoological Articles." We cannot
      accept his primary division into Corticata and Gymnomyxa, which would
      split up the Flagellata and mark off the Gregarines from the other
      Sporozoa.

 [64] On this ground I have referred _Paramoeba_, Greeff, to the
      Cryptomonadineae.

 [65] Differences (1) from _Foraminifera_; (2) from _Heliozoa_; (3) from
      _Proteomyxa_ and _Sporozoa_; (4) from _Myxomycetes_; (5) from many
      _Foraminifera_.

 [66] I have not followed the usual classification into Gymnamoebae and
      Thecamoebae, according to the absence or presence of a test
      (perforated by one or more openings) in the active state, as such a
      test occurs in isolated genera of Flagellata and Infusoria, and does
      not appear to have any great systematic importance.

 [67] The significance of chromidia in Sarcodina (first noted by Schaudinn
      in Foraminifera) was fully recognised and generalised by R. Hertwig
      in _Arch. Protist._ i. 1902, p. 1.

 [68] Stolč in _Z. wiss. Zool._ lxviii. 1900, p. 625. Lilian Veley,
      however, gives reasons for regarding them as of proteid composition,
      _J. Linn. Soc._ (_Zool._) xxix. 1905, p. 374 f. They disappear when
      the _Pelomyxa_ is starved or supplied with only proteid food.

 [69] This genus contains two sausage-shaped, blueish-green plastids,
      possibly symbiotic Cyanophyceous Algae.

 [70] See Lauterborn in _Z. wiss. Zool._ lix. 1895, pp. 167, 537.

 [71] C. Scheel has seen _Amoeba proteus_ produce a brood of 500-600 young
      amoebulae, which he reared to full size (in _Festschr. f. Kupffer_,
      1899).

 [72] _Arb. Kais. Gesundheitsamte Berlin_, xix. 1903.

 [73] _Faune Rhizopodique du Bassin du Léman_, 1902. See also Cash, _The
      British Freshwater Rhizopoda and Heliozoa_, vol. i., Ray Society,
      1905.

 [74] Chapman, _The Foraminifera_, London, 1902; Lister, "Foraminifera" in
      Lankester's _Treatise on Zoology_, pt. i. fasc. 2, 1903.

 [75] _Challenger Reports_ (_Zool_.), vol. ix. 1884.

 [76] In Lankester's _Treat. Zool._ pt. i. fasc. 1. For other
      classifications see Eimer and Fickert in _Z. wiss. Zool._ lxv. 1899;
      Rhumbler in Lang's _Protozoa_, 1901; and for a full synopsis of
      genera and species, "Systematische Zusammenstellung der recenten
      Reticulosae" (pt. i. only), in _Arch. Prot._ iii. 1903-4, p. 181.

 [77] The type of Dujardin's genus _Gromia_ is _G. oviformis_ = _Hyalopus
      dujardinii_, M. Sch., which is one of the Filosa.

 [78] This convenient name is due to my friend Dr. A. Kemna of Antwerp.

 [79] The name Foraminifera was used to express the fact that the chambers
      communicated by pores, not by a tubular siphon as in Nautiloidea and
      Ammonoidea (Vol. III. pp. 393, 396).

 [80] Which probably accounts for the earlier failure of Lister and of
      Schaudinn himself to note their conjugation.

 [81] Rhumbler, "Die Doppelschalen v. _Orbitolites_ u. and. Foraminiferen,"
      in _Arch. Protist._ i. 1902, p. 193.

 [82] The alleged Archaean genus _Eozoon_, founded by Carpenter and Dawson
      on structures found in the Lower Laurentian serpentines
      (ophicalcites), and referred to the close proximity of Nummulites,
      has been claimed as of purely mineral structure by the petrologists;
      and recent biologists have admitted this claim.

 [83] Possibly composed of the same proteid, "acanthin," that forms
      spicules of greater permanence in the Acantharia among the Radiolaria
      (p. 75 f. Figs. 24, 25, A).

 [84] Such divisions into functional and abortive sister nuclei are termed
      "reducing divisions," and are not infrequent in the formation of
      pairing-cells, especially oospheres of Metazoa, where the process is
      termed the maturation of the ovum.

 [85] Besides these genera enumerated by Schaudinn, we include _Dimorpha_
      Gruber (Fig. 37 5, p. 112), _Mastigophrys_ Frenzel, _Ciliophrys_
      Cienk., and _Actinomonas_ usually referred to Flagellates.

 [86] K. Brandt, in _Arch. Prot._ i. 1902, p. 59, regards the presence of
      spicules as not even of generic moment, and subdivides the Collodaria
      into two families—_Collida_ (solitary), and _Sphaerozoea_, colonial,
      _i.e._ with numerous central capsules.

 [87] Dreyer adds an additional order—Sphaeropylida, distinguished by a
      basal (or a basal and an apical) pylome.

 [88] Verworn has shown that _Thalassicolla nucleata_ can, when the
      exoplasm is removed from the central capsule, regenerate it
      completely. First a delicate exoplasm gives off numerous fine
      radiating pseudopodia, and the jelly is re-formed at their bases, and
      carries them farther out from the central capsule. See _General
      Physiology_ (Engl. ed. 1899), p. 379.

 [89] The pigment is singularly resistant and insoluble, and shows no
      proteid reaction. Borgert states that it appears to be formed in the
      oral part of the endoplasm, and to pass through the astropyle into
      the ectoplasm, where it accumulates. It is probably a product of
      excretion, and may serve, by its retention, indirectly to augment the
      surface. See Borgert, "Ueb. die Fortpflanzung der tripyleen
      Radiolarien" in _Zool. Jahrb. Anat._ xiv. 1900, p. 203.

 [90] Dreyer has shown that in many cases it may be explained by
      geometrical considerations. V. Häcker has written a most valuable
      account of the Biological relations of the skeleton of Radiolaria in
      _Jen. Zeitschr._ xxxix. 1904, p. 297.

 [91] _Zool. Jahrb. Anat._ xiv. 1900, p. 203.

 [92] Porta has described reproduction by spores and by budding in
      Acantharia, _Rend. R. Ist. Lomb._ xxxiv. 1901 (ex _Journ. R. Micr.
      Soc._ 1903, p. 45). In _Thalassophysa_ and its allies zoospore
      reproduction appears to be replaced by a process in which the central
      capsule loses its membrane, elongates, becomes multinuclear, and
      ultimately breaks up into the nucleate portions, each annexing an
      envelope of ectoplasm to become a new individual (see _Arch. Prot._
      vol. i. 1902).

 [93] Brandt, "Die Koloniebildenden Radiolarien," in _Fauna u. Flora des
      Golfes v. Neapel_, xiii. 1885, gives a full account of the
      Zooxanthellae and Diatoms, and notes the parasitism of _Hyperia_.

 [94] See Köppen in _Zool. Anz._ xvii. 1894, p. 417. For _Sticholonche_,
      see R. Hertwig in _Jena. Zeitsch._ xi. 1877, p. 324; and Korotneff in
      _Zeitsch. wiss. Zool._ li. 1891, p. 613. Borgert's paper on
      Dictyochidae is in the same volume, p. 629.

 [95] Most of Haeckel's Monera, described as non-nucleate, belong here.
      Several have been proved to be nucleate, and to be rightly placed
      here; and all require renewed study.

 [96] Even the Acystosporidiae have sickle-germs (blasts) in the insect
      host.

 [97] See Zopf, _Beitr. Nied. Org._ ii. 1892, p. 36, iv. 1894, p. 60, for
      the doubtful genus _Chlamydomyxa_; Hieronymus, abstracted by
      Jenkinson, in _Quart. J. Micr. Sci._ xiii. 1899; Penard, _Arch.
      Protist._ iv. 1904, p. 296.

 [98] The name "aethalium" is now always used in this sense.

 [99] The group was monographed by Schröter in Engler and Prantl's
      _Pflanzenfamilien_, I. Teil, Abt. 1, 1897. See also A. Lister's
      Monograph of the Mycetozoa, 1894; Massee, Monog. of the Myxogastres,
      1893; Sir Edward and Agnes Fry, _The Mycetozoa_, 1899; and Massee
      MacBride, _The North American Slime Moulds_, 1899.

[100] Several monographs of the group have been published recently dealing
      with the group from a systematic point of view, including their
      relation to their hosts. Wasielewski, "Sporozoenkunde" (1896); Labbé,
      "Sporozoa" (in _Tierreich_, 1899). Doflein's "Protozoen als Parasiten
      und Krankheitserreger" (1901) contains most valuable information of
      the diseases produced by these and other Protozoic hosts. Minchin's
      Monograph in Lankester's _Treatise on Zoology_, pt. i. fasc. 2
      (1903), is a full account of the class, and admirable in every way.

[101] For its reactions see Bütschli, _Arch. Protist._ vii. 1906, p. 197.

[102] The cuticle in the allied genus _Lankesteria_, which is the form we
      figure on p. 95, is perforated by a terminal pore, through which the
      clear plasma of the sarcocyte may protrude as a pseudopodium.

[103] This account is taken from Cuénot (in _Arch. de Biol._ 1900, p. 49),
      which confirms Siedlecki's account of the process in the allied genus
      _Lankesteria_ in _Bull. Acad. Cracow_, 1899.  Wolters's previous
      description, assimilating the processes to those of _Actinophrys_, is
      by these authors explained as the result of imperfect preservation of
      his material.

[104] See p. 120.

[105] See Caullery and Mesnil, "Rech. sur les Actinomyxidies," _Arch. Bot._
      vi. 1905, p. 272 f.

[106] Léger, _Arch. Zool. Exp._ sér. 3, x. and sér. 4, v. (1902-3); for a
      full discussion of the relations of association and conjugation in
      Gregarines, see Woodcock in _Quart. Journ. Micr. Sci._ l. 1906, p. 61
      f.

[107] A _Lithobius_ is figured in Vol. V. p. 45.

[108] The schizont forms of some species, before the invariable alternation
      of schizogony and sporogony had been made out clearly, were regarded
      as "monogenic" genera, under the names of _Eimeria_, A. Schn., and
      _Pfeifferella_, Labbé; while those in which the formation of spores
      containing sickles had been clearly seen were termed "digenic."
      Labbé's monograph, "Die Sporozoen," in the _Tierreich_, is
      unfortunately written from this point of view, which had already
      become doubtful, and is now demonstrated to be erroneous, chiefly by
      the labours of Schaudinn and Siedlecki.

[109] A species has been described, however, in the blood of the Indian
      Gerbille (_Gerbillus indicus_), completing the sexual process in the
      Louse of its host. A figure of _G. aegyptius_ will be found in Vol.
      X. (1902) p. 475.

[110] There is no difference between a mosquito (little fly) and a gnat,
      both names are applied indiscriminately to thin-bodied Diptera of the
      group Nemocera which attack man; only the females bite (see Vol. VI.
      pp. 466-468).

[111] Regarded by Schaudinn as a state of the Flagellate _Trypanosoma_ (p.
      119 f.).

[112] In _Quart. Journ. Micr. Sci._ xliv. 1901, p. 429.

[113] It would seem that resting-cells, _i.e._ the crescents and
      corresponding spheres, of _Laverania_ and _Haemamoeba_ may linger
      during months of apparent health in the spleen and red marrow of the
      bones; and that these by parthenogenesis produce sporozoites and
      determine relapses when, owing to a lowering of the general health,
      conditions favourable to new sporulation occur.

[114] Léger and Duboscq have found that _Sarcocystis tenella_, a parasite
      common in the muscles of the sheep (and rarely found in man), has a
      conjugation and sexual process recalling that of _Stylorhynchus_,
      save that the sperms are much smaller than the ova (_C.R._ 1902, i.
      p. 1148).

[115] The alleged micronucleus of certain forms appears to be merely a
      "blepharoplast" (see p. 19); even when of nuclear origin, as in
      _Trypanosoma_, it has no function in reproduction like the
      micronucleus of Infusoria (see pp. 115, 120 f.).

[116] _Dimorpha_ is now referred to Heliozoa (p. 70).

[117] _I.e._ resembling the thread-like water Algae.

[118] Trichocysts (see p. 142) occur in some Chloromonadaceae; and the
      Dinoflagellate _Polykrikos_ possesses true nematocysts (see p. 131).

[119] For a full monograph of this family see H. Lohmann, in _Arch. f.
      Protistenkunde_, vol. i. 1902, p. 89.

[120] Delage has well explained the action of the single anterior flagellum
      which waves in a continuous spiral like a loaded string whirled round
      one's head; it thus induces a movement of the water, beyond its
      actual range, backwards and outwards, maintained by a constant influx
      from behind, which carries the cell onward at the same time that it
      necessarily rotates round its axis. If there is a pair of
      symmetrically placed flagella they co-operate like the arms of a
      swimmer; when the second flagellum is unilateral the motion is most
      erratic, as seen in the Bodonidae (and the zoospores of many
      Chytridieae, which have most of the characters of the Flagellates,
      though habitually removed to the Fungi).

[121] The colouring matter is chlorophyll or some allied colouring matter.
      In the yellow and brown forms the additional pigment is termed
      loosely "diatomin," but its identity with that of Diatoms is in no
      case proved.

[122] Notably in the Craspedomonadidae, where transverse division also
      occurs. See Raoul Francé, _Die Craspedomonadineen_ (Buda-Pesth,
      1897).

[123] And also in the "Monads," described by Dallinger and Drysdale, see
      above.

[124] In _Cercomonas dujardinii_, _Polytoma uvella_, and _Tetramitus
      rostratus_ the gametes resemble the ordinary forms and are isogamous.
      In _Monas dallingeri_ and _Bodo caudatus_ conjugation takes place
      between one of the ordinary form and size and another similar but
      smaller. In _Dallingeria drysdali_ the one has the ordinary size and
      form, the other is equal in size, but has only one flagellum, not
      three; in _Bodo saltans_ they are unequal, the larger gamete arising
      in the ordinary way by longitudinal fission, the smaller by
      transverse division. Doubt has been thrown on the validity of our
      authors' results by subsequent observers abroad; but I can find no
      evidence that these have even attempted to repeat the English
      observations under the same severely critical conditions, and
      therefore consider the attacks so far unjustified. Schaudinn has
      observed conjugation between _Trichomonas_ individuals which have
      lost their flagella and become amoeboid; also in _Lamblia
      intestinalis_ and in _Trypanosoma_ (_Halteridium_?) _noctuae_ (Fig.
      39) "Reduction-divisions" (see p. 75, note 1) of the nuclei take
      place before fusion, and the nuclear phenomena are described as
      "complicated" (_Arb. Kais. Gesundheitsamte_, xx. 1904, p. 387).
      _Paramoeba eilhardii_ in its adult state is colourless, amoeboid,
      multiciliate. It forms a brood cyst, from which are liberated
      flagellate zoospores, with a chromatophore, which reproduce by
      longitudinal fission in this state. They may also conjugate.

[125] In _P.R.S._ xxvii. 1878, p. 332.

[126] In _Z. wiss. Zool._ lv. 1893, p. 353.

[127] 1. Teil, Abt. 1. a, 1900.

[128] In the _Chlorophyceae_, 1. Teil, Abt. 2, 1897.

[129] 1. Teil, Abt. 1. b, 1896.

[130] Besides the above, Dangeard, in various papers in his periodical _Le
      Botaniste_, has treated of most of the groups, and Raoul Francé has
      monographed the _Polytomeae_ in the _Jahrb. wiss. Bot._ xxvi. 1894,
      p. 295, and Dill the genus _Chlamydomonas_, etc., its closest allies,
      in _op. cit._ xxviii. 1895, p. 323.

[131] For a detailed abstract of our knowledge of _Trypanosoma_ and its
      allies up to Feb. 1, 1906, see Woodcock, "The Haemoflagellates," in
      _Quart. Journ. Micr. Sci._ 1. 1906, p. 151.

[132] Doubts still subsist as to the interpretation of Schaudinn's
      observations.

[133] _Quart. Journ. Micr. Sci._ xlvi. 1902.

[134] A Zambezian Tick infects man with a _Treponema_, producing
      relapsing-fever; another species is found in the tropical disease
      "framboesia" ("yaws" or "parangi").

[135] Stated by Geza Entz and Raoul Francé to be due to the spiral twisting
      of a plasmic membrane, and to be like a cone formed by twisting
      paper, with the free edges overlapping.

[136] Discovered by Leidy. For the most recent description of this group
      see Grassi and Sandias in _Quart. Journ. Micr. Sci._ xxxix. (figures)
      and xl. p. 1 (text), 1897.

[137] Bezzenberger has given an analytical table of the eleven known
      species of the genus _Opalina_ in _Arch. Protist._ iii. 1903, p. 138.

[138] Such movements, permissible by the perfectly flexible but firm
      pellicle, are termed "metabolic" or "euglenoid" in contradistinction
      to "amoeboid." They also occur in many Sporozoa.

[139] Within which is often harboured the Rotifer, _Proales parasita_, Vol.
      II. p. 227.

[140] In the Adinidae there is no groove; the two lashes arise close
      together, and the one is coiled round the base of the other.

[141] In _Unt. Inst. Tübingen_, i. 1883, p. 233.

[142] Conjugation of adults has been observed by Zederbauer (_Ber. Deutsch.
      Ges._ xxii. 1904). A short connecting tube is formed by the meeting
      of outgrowths from either mate; their protoplasmic contents meet and
      fuse herein to form a spherical resting-spore, as in the Conjugate
      Algae.

[143] According to Bergh, _Polykrikos_ has as many nuclei as grooves, each
      accompanied by one or more "micronuclei." Possibly these latter
      bodies are merely blepharoplasts, in connexion with the transverse
      flagella.

[144] Engler and Prantl's _Pflanzenfamilien_, 1. Teil, Abt. 1, 1896.

[145] The luminous genus, _Pyrocystis_ (Fig. 47), regarded as a
      Cystoflagellate by Wyville Thomson, has a cellulose wall, no mouth,
      and in the zoospore state has the two flagella in longitudinal and
      transverse grooves of the Dinoflagellata.

[146] This process has the character of telolecithal segmentation in a
      Metazoan egg.

[147] See Doflein, in _Zool. Jahrb. Anat._ xiv. 1900, p. 1.

[148] London, 1753, 402-403.

[149] On this account Hickson has termed the group "Heterokaryota" in
      Lankester's _Treat. Zool._ i. fasc. 1, 1903.

[150] See Baker, _Employment for the Microscope_, ed. 2, 1758.

[151] Saville Kent's valuable _Manual of the Infusoria_ (1880-1882), which
      gives figures of every genus and descriptions of every species known
      at that date, includes the Flagellates in its scope.

[152] Orders 1 and 2 constitute together the _Holotricha_ of Stein;
      Bütschli regards 3 to 6 as sections of _Spirotrocha_.

[153] Dextrorse in all but _Lichnophora_ and _Spirochona_.

[154] Each membranella is a transversely elongated oval in reality, and
      below it is a double row of basal granules, corresponding to the
      individual cilia that constitute it. Similarly, the undulating
      membranes have a single row of basal granules.

[155] Tail-like appendages are found in _Scaphiodon_ and in _Dysteria_ and
      its allies (Gymnostomaceae), _Urocentrum_ (Aspirotrichaceae),
      _Discomorpha_ and _Caenomorpha_ (Heterotrichaceae). In the first two
      and last two cases they are prolongations of the body; in the third
      an aggregate of cilia. One or more long caudal setiform cilia are
      present in the genera _Lembadion_, _Pleuronema_, _Cyclidium_,
      _Lembus_, _Cinetochilum_, _Ancistrum_, and _Uronema_; all these are
      addicted to making springing darts. Tufts of cilia of exceptional
      character often serve for temporary attachment. The stalk (or at
      least its external tube) of the Peritrichaceae appears to be the
      chitinous excretion of a zone of such cilia. Fauré-Fremiet terms such
      a zone or annular brush a "scopula" ("Struct. de l'app. fixateur chez
      les Vorticellides," _Arch. Protist._ vi. 1905, p. 207). For a
      discussion of the finer structure of the cilia in Ciliata, and the
      mechanism of their action, see Schuberg, _Arch. Protist._ vi. 1905,
      p. 61.

[156] See Mitrophanow "Sur les Trichocystes ... du _Paramoecium_," _Arch.
      Protist._ v. 1904, p. 78.

[157] The "neurophane" fibrils of Neresheimer, _Arch. Protist._ ii. 1903,
      p. 305 f.

[158] Sometimes the number of afferent canals is limited to five
      (_Paramecium_), or even one. There may be one or more contractile
      vacuoles, and in the latter case the different ones have an
      independent rhythm.

[159] It is from such conclusive cases that the universal character of a
      discharge to the surface has been inferred in the rest of Protista
      possessing this organ.

[160] _Gruber_ (_Ber. Ges. Freib._ 1888) has shown that in several marine
      Ciliata the meganucleus is represented by an enormous number of
      minute granules disseminated through the endosarc, which, on the
      approach of fission, unite into a single meganucleus. As an adjacent
      micronucleus makes its appearance at this stage, he infers that the
      micronucleus must be also resolved in the intermediate life of the
      cell into granules too small for recognition under the highest
      magnification attainable, and that they must then coalesce.

[161] In the peculiar Peritrichan _Spirochona_ the division of the
      meganucleus is a much more complex process than usual, and recalls
      that of the undifferentiated nuclei of many Rhizopods (see Rompel in
      _Z. wiss. Zool._ lviii. 1894, p. 618). _Opalina_ has neither mouth
      nor anus, nor contractile vacuole, but a large number of similar
      nuclei, that divide by a true mitotic process, like micronuclei.  We
      have referred it (pp. 114, 123) to the Flagellates, next to the
      Trichonymphidae.

[162] Save the Opalinopsidae, which are usually termed "Opalinidae"; but
      which cannot retain the latter name on the removal of the genus
      _Opalina_ to the Flagellates.

[163] _Phil. Trans._ clxxxv. 1895, pp. 355 f.

[164] _Arch. Zool. Exp._ (2) vi. vii. 1888-1889.

[165] Calkins has recently found that the vitality within a cycle is
      rhythmical, with alternations of more and of less frequent fissions,
      under the same set of conditions; and that minute doses of beef-tea
      or various mineral salts will not only keep up the higher rate, but
      even stave off senescence. Minute doses of alcohol will keep up the
      higher rate, but not avert senescence. He considers that Maupas'
      generalisations are in most respects too sweeping (_Arch. Entw._ xv.
      1902, p. 139). But Dr. James Y. Simpson informs me that the
      possibility of stimulative regeneration has been found to be limited.
      See also Calkins and Lieb, _Arch. Prot._ i. 1902, p. 355.

[166] As inferred by Hickson from the prolongation of the union.

[167] When there are at the outset two or more micronuclei all undergo the
      first two fissions, but only one undergoes the third.

[168] _Zeitschr. wiss. Zool._ xxxiii. 1880, p. 439.

[169] Bezzenberger has given a key to the species of these two genera in
      _Arch. Prot._ iii. 1903, pp. 149, 157.

[170] We note that _Lacrymaria_ is prolonged in front into a long, slender
      flexible "neck," with the mouth terminal. This swan-like conformation
      is "mimicked" by _Dileptus_ and _Lionotus_, where the neck, like the
      prostomium of worms, is a mere extension of the front of the body
      above and beyond the mouth; all three swim with peculiar grace.
      _Trachelius_ (Fig. 56) has a distinct cup-shaped sucker behind the
      mouth, and is remarkable, like _Loxodes_, for the branching
      disposition of its endosarc.

[171] The pigment of this species has been examined and described by
      Lankester under the name of "blue stentorin" (_Quart. Journ. Micr.
      Sci._ xii. 1873).

[172] For a full account of _Caenomorpha_, _Metopus_, and allied forms, see
      Levander, _Beitr. z. Kenntn. einiger Ciliaten_, Dissert. Helsingfors,
      1894.

[173] _Torquatella typica_, described by Lankester as possessing a
      continuous undulating membrane for its peristomial wreath, is
      identified by Bütschli as a _Strombidium_, possessing exceptionally
      large membranellae.

[174] Outside the principal wreath is another of fine cilia ("paroral"),
      standing out at an angle.

[175] Covered with a rather lax structureless membrane (sarcolemma), which
      is spirally wrinkled when the muscle contracts. I am unable to verify
      Geza Entz's observations, adopted by Calkins and Delage.

[176] Of the composition of cellulose (Halliburton, in _Quart. Journ. Micr.
      Sci._ xxv. 1885, p. 445).

[177] As does the Hypotrichan _Kerona polyporum_.

[178] Permanently ciliate in _Hypocoma_ and _Suctorella_.

[179] In this case the débris of the live prey torn up by the _Cyclops_ on
      which they live.

[180] The spiral ridge figured by Hertwig (Fig. 61, 1. _c_) is probably an
      incorrect representation of this structure, exceedingly minute in all
      genera but _Choanophrya_.

[181] In _Choanophrya_ I have failed to find any pore, and believe the
      bud-formation to be strictly endogenous.

[182] See _Quart. Journ. Micr. Sc._ xlv. 1902, p. 325.

[183] In _Journ. Coll. Sc. Japan_, x. 1896.

[184] _Étude monographique sur le groupe des Tentaculifères, Ann. Soc.
      Belge Micr._ xxiv.-xxvi. 1901.

[185] To Professor W. J. Sollas, Sc.D., F.R.S., who undertook to write the
      chapters on Porifera when the work was first planned, the Author and
      the Editors are indebted for his kind assistance in reading and
      criticising this article.

[186] _Rarities belonging to the Royal Society preserved at Gresham
      College_, 1686.

[187] _Gerarde's Herbal, enlarged and revised by Thomas Johnson_, 1636, p.
      1587.

[188] _Phil. Trans._ lv. p. 280.

[189] _Histoire Phys. de la Mer_, 1725.

[190] _Mem. Boston Soc._ i. 1867, p. 305.

[191] _Zeitschr. wiss. Zool._ xxxi. 1878, p. 262.

[192] _Ann. Mag. Nat. Hist._ (5) xiii. 1884, p. 381.

[193] _Quart. Journ. Micr. Sci._ xxiv. 1884, p. 612.

[194] The name was coined by Dr. Fleming from χάλιξ "silex" and χόνδρος
      "cartilage," and as these roots could only give _Chalic-chondria_ it
      is not surprising that those who have not referred to Dr. Fleming's
      statements give the derivation as ἅλς "sea" and χόνδρος.

[195] _Monograph of British Sponges_, vol. iii. pl. xxxix.-xl. For revision
      of nomenclature in this Monograph, see Hanitsch, _Tr. Liverp. Biol.
      Soc._ viii. 1894, p. 173.

[196] _Journ. Physiol._ ix. 1888, p. 1.

[197] Sollas, _Ann. Mag. Nat. Hist._ (4) xx. 1877, p. 285; Bütschli,
      _Zeitschr. f. wiss. Zool._ xix. 1901, p. 236.

[198] Minchin, "Sponges" in _Treatise on Zoology_, edited by E. Ray
      Lankester, p. 87. See also Bidder, _Proc. Roy. Soc._ li. 1892, p.
      474.

[199] _Zool. Jahrb. Anat._ vii. 1894.

[200] _Materials for the Study of Variation_, 1894, p. 30.

[201] _Arch. de Zool. Exp._ (2) x. 1892, pp. 345-498.  On the general
      subject of adhesion of species, see Bowerbank, _Brit. Ass. Rep._
      1857, p. 11, who quotes Grant as the first to observe the phenomenon.

[202] _Quart. Journ. Micr. Sci._ xxii. 1882, p. 229.

[203] But see Gamble and Keeble, _Quart. Journ. Micr. Sci._ xlvii. 1904, p.
      363, who show that various green animals really owe their colour to
      "algae," though the infection with the "alga" is difficult to detect
      because it takes place by means of a colourless cell. See also
      _Zoochlorella_, on p. 126.

[204] Sollas, _Tr. Dublin Soc._ (2) iii. 1884, p. 87.

[205] _Arch. Naturg._ lix. 1893, p. 246.

[206] Weltner, _Blatt. Aquar. Fr._ vii. 1896, p. 277, and
      "Spongillidenstudien," _Arch. Naturg._ ii. 1893, p. 271.

[207] Evans, _Quart. Journ. Micr. Sci._ xliv. 1900, p. 72.

[208] _Ann. Mag. Nat. Hist._ (2), x. 1882, p. 365.

[209] _P. Ac. Philad._ 1887, pp. 158-278.

[210] Evans, _Quart. Journ. Micr. Sci._ xlii. 1899, p. 363.

[211] Francé, _Organismus der Craspedomonaden_, Budapest, 1897, p. 217.

[212] Sollas, _Encyclopædia Britannica_, art. "Sponges," 1887.

[213] Sollas, _Ann. Mag. Nat. Hist._ (5) iii. 1879, p. 23; _Challenger
      Report_, vol. xxv. pt. lxiii. 1888, p. lii.

[214] Minchin, Lankester's _Treatise on Zoology_, pt. ii. 1900.

[215] Minchin, _loc. cit._ p. 110.

[216] Bidder, _Quart. Journ. Micr. Sci._ xxxii. 1891, p. 631, and Minchin,
      _Quart. Journ. Micr. Sci._ xxxiii. 1892, p. 266.

[217] Minchin, Lankester's _Treatise on Zoology_, p. 30.

[218] Vosmaer and Pekelharing, _Verh. Ak. Amsterdam_, (2) vi. 3, 1898, p.
      1.

[219] Dendy, _Quart. Journ. Micr. Sci._ xxxv. 1894, p. 230.

[220] Maas, _Zeitschr. wiss. Zool._ lxvii. 1899-1900, p. 215.

[221] "Die Kalkschwämme," 1871.

[222] Dendy. _loc. cit._ p. 159.

[223] _Quart. Journ. Micr. Sci._ xxxvi. 1894, p. 127.

[224] Doederlein, _Zool. Jahrb. Abth. Anat._ x. 1896, p. 15, pl. ii. and
      iii.

[225] Hinde, _Quart. Journ. Geol. Soc._ lvi. 1900, p. 50.

[226] Hinde, _Tr. R. Micr. Soc._ 1904, p. 3.

[227] Počta, _Bull. Acad. Bohème_, 1903.

[228] J. J. Lister in Willey's _Zoological Results_, pt. iv. 1900, p. 459.

[229] _Mém. Soc. Zool. France_, 1896, p. 119.

[230] _Arch. Zool. Exp._ (3) iii. 1895, p. 561, pl. xxiii.

[231] F. E. Schulze, _Challenger Monograph_, xxi.

[232] Chun, "Aus den Tiefen des Weltmeeres," 1900, p. 481.

[233] Shipley, "Fauna of the Antarctic Regions." See also p. 216.

[234] _J. Coll. Japan_, xv. 1901, pp. 128, 147, 190.

[235] _Fauna Arctica_ (Roemer and Schaudinn), i. 1900, p. 84; and _Sitzb.
      Akad. Berlin_, 1899, p. 98.

[236] Sollas, _Quart. Journ. Geol. Soc._ 1880, p. 362.

[237] _Quart. Journ. Geol. Soc._ xl. 1884, p. 795.

[238] "Monograph British Fossil Sponges," _Palaeont. Soc._ xl. and xli.
      1887 and 1888.

[239] Sollas, _Challenger Monograph_, xxv. 1888.

[240] _Marine Investigations in South Africa_, i. 1902, p. 224.

[241] Cf. Sollas, _Encyclopædia Britannica_, 1887, art. "Sponges," and
      Schrammen, _Mitth. Mus. Hildesheim_, 14, 1901.

[242] Sollas, _Quart. Journ. Geol. Soc._ xxxiii. 1877, p. 790.

[243] Ridley and Dendy, _Challenger Monograph_, lix. 1887.

[244] _Ibid._ p. 262; cf. also p. 197.

[245] _Quart. J. Micr. Sci._ xli. 1901, p. 477.

[246] Loisel, _J. de l'Anat. et Phys._ xxxiv. 1898, p. 1.

[247] R. v. Lendenfeld, _Acta Ac. German._ lxix. 1896, p. 22.

[248] _Challenger Report_, lix. 1887, p. 214.

[249] Topsent, _Zoologie Descriptive_, i.; also Cotte, _C. R. Soc. Biol.
      Paris_, 1902, pp. 638-639.

[250] Topsent, _Arch. Zool. Exp._ (3) viii. 1900, p. 36.

[251] Sollas, _Challenger Monograph_, xxv. pt. lxiii. 1888, p. lxxxix.

[252] Topsent, _Arch. Zool. Exp._ (3) viii. 1900, p. 226. For an account of
      certain very remarkable structures termed diaphragms in _Cliona
      mucronata_ and _C. ensifera_, see Sollas, _Ann. Mag. Nat. Hist._ (5)
      i. 1878, p. 54.

[253] R. von Lendenfeld, _Monograph of Horny Sponges_, 1889, p. 831.

[254] Cf. Minchin in E. Ray Lankester's _Treatise_, p. 77.

[255] Maas, _Zool. Centralbl._ v. 1898, p. 581.

[256] _Arch. Zool. Exp._ viii. 1879, p. 59.

[257] "Biological Lectures, Wood's Holl," 1894, p. 43.

[258] F. E. Schulze, _Zool. Anz._ ii. 1879, p. 636.

[259] Maas, _Zeitschr. wiss. Zool._ lxx. 1901, p. 263.

[260] Maas, _loc. cit._ p. 284.

[261] _J. Coll. Japan_, xv. 1901, p. 180.

[262] Perkins, _Johns Hopkins Univ. Circ._ xxi. 1902, p. 87.

[263] For details of this interesting process see Minchin, _Quart. J. Micr.
      Sci._ xl. 1898, p. 469.

[264] Maas, _Zeitschr. wiss. Zool._ lxvii. 1900, p. 225.

[265] Maas, _SB. Ak. München_, xxx. 1900, p. 553, and _Zeitschr. wiss.
      Zool._ lxx. 1901, p. 265; see also Sollas, _Ann. Mag. Nat. Hist._ (5)
      ix. 1880, p. 401.

[266] Sollas, _Challenger Monograph_, xxv. 1888, p. xlv.

[267] Sollas, _ibid._ pp. 13 and 34, pl. v.

[268] _Zeitschr. wiss. Zool._ lii. 1891, p. 294.

[269] I. Sollas, _P. Zool. Soc. London_, ii. 1902, p. 215.

[270] Sollas, _Ann. Mag. Nat. Hist._ (5) ix. 1880, p. 402.

[271] Bowerbank, and also Vosmaer and Pekelharing, _Verh. Ak. Amsterdam_
      (2) vi. 3, 1898.

[272] _J. Coll. Japan_, xv. 1901, p. 193.

[273] Vosmaer and Pekelharing, _Verh. Ak. Amsterdam_, 1898.

[274] See Bidder, _P. Camb. Soc._ vi. 1888, p. 183; Sollas, _Challenger
      Monograph_, xxv. 1883, pp. xviii.-xxi.; and Vosmaer and Pekelharing,
      _loc. cit._

[275] Carter and Lieberkühn in 1856, Haeckel in 1872, Metschnikoff in 1879,
      and many later workers.

[276] _Die Kalkschwämme_, 1872, i. p. 372.

[277] _J. Anat. Physiol._ 1898, pp. 1, 6, 234.

[278] _Mém. Ac. St. Pétersb._ (7) xxvi. 1878, p. 10.

[279] Sollas, _Challenger Report_, xxv. pt. lxiii. p. lxxxviii.

[280] _Vergl. Physiologie d. niederen Thiere_, 1903, p. 441.

[281] For further details see Zittel, _Lehrbuch der Palaeontologie_, and
      Felix Bernard, _Eléments de Palaeontologie_, 1894.

[282] For further details see Sollas, "The Formation of Flints," in _The
      Age of the Earth_, 1905, p. 131.

[283] _Willey's Zool. Results_, pt. ii. 1899, p. 127.

[284] Murbach, _Archiv f. Naturg._ lx. Bd. i. 1894, p. 217.

[285] G. H. Grosvenor, _Proc. Roy. Soc._ lxxii. 1903, p. 462.

[286] H. Jung, _Morph. Jahrb._ viii. 1881, p. 339.

[287] _Verh. Ver. Rheinland_, xlix. 1893, pp. 13, 14, 40, 41.

[288] For an account of the development and of the chitinous membrane see
      A. Brauer, _Zeitschr. f. wiss. Zool._ lii. 1891, p. 9.

[289] Trembley, _Mémoires pour servir à l'Histoire d'un genre de Polypes
      d'eau douce_, 1744.

[290] G. Wagner, _Quart. Journ. Micr. Sci._ xlviii. 1905, p. 589.

[291] See p. 126.

[292] _Hydra pallida_, Beardsley, has been found to be very destructive to
      the fry of the Black-spotted Trout in Colorado, _U.S. Fish. Rep.
      Bull._ 1902, p. 158.

[293] For figures of _Protohydra_ see Chun, Bronn's _Thier-Reich_,
      "Coelenterata," 1894, Bd. ii. pl. ii.

[294] _Sitzber. Ges. naturf. Freunde Berlin_, ix. 1894, p. 226.

[295] M. Ussov, _Morph. Jahrb._ xii. 1887, p. 137.

[296] This organism is usually described as a fungus (_Achlya_), but it is
      probably a green Alga. See J. E. Duerden, _Bull. Amer. Mus. Nat.
      Hist._ xvi. 1902, p. 323.

[297] _Bibl. Univ. de Genève, Arch. des Sciences_, v. 1859, p. 80.

[298] _Phil. Trans._ cxlvii. 1876, p. 117.

[299] S. J. Hickson, _Willey's Zool. Results_, pt. ii. 1899, p. 127.

[300] _Quart. Journ. Micr. Sci._ xlii. 1899, p. 341.

[301] "Gymnoblastic Hydroids," _Ray Society_, 1871, p. 359.

[302] Hincks, _British Hydroid Zoophytes_, 1868, p. 74.

[303] _Ann. Mag. Nat. Hist._ (6) x. 1892, p. 207.

[304] Fewkes, _Bull. Mus. Comp. Zool._ xiii. 1887, p. 224.

[305] Hartlaub, _Wiss. Meeresunt. deutsch. Meere in Kiel_ N.F.I. 1894, p.
      1.

[306] Carter, _Ann. Mag. Nat. Hist._ (4) xix. 1877, p. 44; (5) i. 1878, p.
      298.

[307] The aberrant genus _Hypolytus_ (p. 262) may belong to this family.

[308] Spencer, _Trans. Roy. Soc. Vict._ 1892, p. 8.

[309] _Journ. Coll. Sci. Tokyo_, xiii. 1900, p. 235 (with a beautiful
      coloured illustration).

[310] _Proc. Zool. Soc._ 1897, p. 818.

[311] _Zeitschr. f. wiss. Zool._ lxiii. 1898, p. 489.

[312] _Quart. Journ. Micr. Sci._ xlvi. 1902, p. 1.

[313] _Zool. Zentralbl._ x. 1903, p. 27.

[314] For a discussion of the origin of the polysiphonic stem in
      Calyptoblastea see Nutting, "American Hydroids," _Smithsonian
      Institution Special Bulletin_, pt. i. 1900, p. 4.

[315] _Loc. cit._ p. 33.

[316] The term "sarcostyle" is usually applied to the dactylozooid of the
      Calyptoblastea.

[317] _Trans. Roy. Soc. Victoria_, 1890, p. 121.

[318] See C. C. Nutting, _Proc. U.S. National Museum_, xxi. 1899, p. 747.

[319] E. T. Browne, _Bergens Museums Aarbog_, 1903, iv. p. 18.

[320] Cf. Schepotieff, _Neues Jahrb. f. Mineralogie_, 1905, ii. pp. 79-98.

[321] S. J. Hickson and H. England, _Siboga Exped._ viii. 1904, p. 26.

[322] "Life-History of the Hydromedusae," _Mem. Boston Soc._ iii. 1885, p.
      359.

[323] _Journ. Morph._ xi. 1895, p. 493.

[324] H. F. Perkins, _Proc. Acad. Nat. Sci. Phil._ Nov. 1902, p. 773.

[325] E. T. Browne, _Proc. Zool. Soc._ 1896, p. 495.

[326] _Mark Anniversary Volume_, New York, 1903, p. 1.

[327] C. Vaney et A. Conte, _Zool. Anz._ xxiv. 1901, p. 533.

[328] S. Goto, _l.c._

[329] G. H. Fowler, _Quart. Journ. Micr. Sci._ xxx. 1890, p. 507.

[330] _Limnocnida_ has recently been discovered by Budgett in the river
      Niger. See Browne, _Ann. Nat. Hist._ xvii. 1906, p. 304.

[331] "The Tanganyika Problem," 1903, p. 298.

[332] Cf. Boulenger, Presidential Address to Section D of the British
      Association (Cape Town, 1905).

[333] C. Hartlaub, _Verhandl. Deutsch. Zool. Ges_. 1896, p. 3.

[334] _Abh. Senckenb. Ges._ xvi. 1891, p. 44.

[335] This gas is frequently called air. The gas contained in the
      pneumatophore of _Physalia_ was analysed by Schloessing and Richard,
      _C. R._ cxxii. 1896, p. 615, and found to consist of CO_{2}, 1.7
      parts, O 15.1, nitrogen and argon, 83.2.

[336] The chemical composition of the substance here called "chitin" has
      not been accurately determined. An analysis of two specimens of
      _Velella_ bladders gave 9.71 and 10.35 per cent of nitrogen, which is
      higher than that of chitin and nearer to that of mucin.

[337] _Zool. Jahrb. Suppl._ 1904, p. 347.

[338] _Johns Hopkins Univ. Circ._ x. 1891, p. 91.

[339] C. Chun, _Abh. Senck. Nat. Ges. Frankfort_, xvi. 1891.

[340] Brooks and Conklin, _Johns Hopkins Univ. Circ._ x. 1891, No. 88.

[341] C. E. Borchgrevink, "First on the Antarctic Continent," 1901, p. 227.

[342] M. J. Delap, _Irish Naturalist_, x. 1901, p. 27.

[343] E. W. L. Holt, _Report on the Sea and Inland Fisheries of Ireland for
      1902_, pt. ii. 1903, p. xvi.

[344] F. W. Gamble. See E. T. Browne, _Proc. Roy. Irish Acad._ 1900, p.
      735.

[345] _Bull. Mus. Comp. Zool._ xxxii. 1, 1898.

[346] ἀκαλήφη = a nettle.

[347] _Biometrika_, i. 1901, p. 90.

[348] K. Kishinouye, _Zool. Jahrb. Syst._ xii. 1899, p. 206.

[349] For the discussion of this relationship the reader is referred to
      Goette, _Zeitschr. wiss. Zool._ lxiii. 1897, p. 360, and Carlgren,
      _Zool. Anz._ xxii. 1899, p. 31.

[350] See note ^{[347]}, above.

[351] E. A. Minchin, _Proc. Zool. Soc._ 1889, p. 583.

[352] For good illustrations of this see Sir J. Dalyell, "Rare and
      Remarkable Animals of Scotland," vol. i. 1847, pll. 13, 14, 18, 19,
      20.

[353] _Archiv. Mikr. Anat._ xiii. 1877, p. 795.

[354] Agassiz and Mayer, _Mem. Mus. Comp. Zool._ xxvi. 3, 1902, p. 153.

[355] F. S. Conant, _Mem. Johns Hopkins Univ._ iv. 1, 1898.

[356] Kishinouye, _Journ. Coll. Sci. Tokyo_, xvii. 7, 1902.

[357] A discussion of the classification of this order occurs in Vanhöffen,
      "Acrasped. Med. d. deutschen Tiefsee Expedition," iii. 1902, p. 49.

[358] The Periphyllidae constitute Haeckel's order Peromedusae.

[359] A stage in development before the formation of the sub-umbrellar
      cavity, but subsequent to the formation of the first tentacles, is
      regarded as homologous with the Scyphistoma stage of other Scyphozoa.

[360] _"Siboga" Exped._ Mon. xi. 1903.

[361] _Proc. Roy. Irish Acad._ 3rd ser. v. 1900, p. 735.

[362] Cf. Darwin, _Voyage of the Beagle_, chap. v.

[363] Hickson, _K. Akad. Wet. Amsterdam_, 1905.

[364] J. H. Ashworth, _Proc. Roy. Soc._ lxiii. 1898, p. 443.

[365] _Quart. Journ. Micr. Sci._ xli. 1899, p. 521.

[366] Quoted by Hickson, _Marine Investigations, S. Africa_, iii. 1904, p.
      215.

[367] G. C. Bourne, _Phil. Trans. Roy. Soc._ clxxxvi. 1895, B. p. 464.

[368] Quoted by Marshall, _Oban Pennatulida_, 1882, p. 49.

[369] _Quart. Journ. Micr. Sci._ xlix. 1905, p. 327.

[370] _Corallium nobile_ appears to be the exception to this rule, as it is
      stated that colonies and even individual zooids are occasionally
      hermaphrodite. Lacaze Duthiers, "Hist. Nat. du Corail," 1864, p. 127.

[371] G. Lindström, _Handl. k. Svensk. Vet. Akad._ xxxii. 1899.

[372] J. W. Gregory, _Proc. Roy. Soc._ lxvi. 1899, p. 291.

[373] G. C. Bourne, Lankester's _Treatise on Zoology_, pt. ii. 1900,
      "Anthozoa," p. 26.

[374] E. M. Pratt, _Fauna and Geogr. Maldive Archip._ ii. pt. i. 1903, p.
      516.

[375] _Zool. Anz._ xxix. 1905, p. 263.

[376] S. J. Hickson, _Fauna and Geog. Maldive Archip._ ii. pt. i. 1903, p.
      495.

[377] Hickson, _K. Akad. Wet._ Amsterdam, 1905.

[378] _Journ. Imp. Fish. Bureau_, Tokyo, xiv. 1, 1904.

[379] Kitahara, _Journ. Imp. Fish. Bureau_, Tokyo, xiii. 3, 1904.

[380] Johnson, _Proc. Zool. Soc._ 1899, p. 57.

[381] Hickson, _Nature_, lxxiii. 1905, p. 5.

[382] Moroff, _Zool. Jahrb. Syst._ xvii. 1902, p. 404.

[383] Ridley, _Proc. Zool. Soc._ 1882, p. 231.

[384] For a revision of this family, see Versluys, _Siboga Expeditie_, xii.
      1902.

[385] Jungersen (_Danish Ingolf Expedition_, Pennatulida, 1904) has shown
      that this is the correct nomenclature of the regions of the rachis.
      Nearly all other authors describe the dorsal side as ventral and the
      ventral as dorsal.

[386] S. J. Hickson, _Report British Association_ (Southport Meeting),
      1903, p. 688.

[387] Marshall, _Trans. Roy. Soc. Edinb._ xxxii. 1883, p. 143.

[388] Rumphius, _Amboinsche Rariteitkamer_, 1741, p. 64.

[389] Darwin, _Naturalist's Voyage round the World_, 1845, p. 99.

[390] To be described in the forthcoming Report on the Pennatulidae of the
      "Siboga" Expedition.

[391] _Zool. Anz._ xxv. 1902, p. 302.

[392] Faurot, _Arch. Zool. Expér._ 3rd ser. iii. 1895, p. 71.

[393] Duerden, _Mem. Acad. Washington_, 3rd Ser. viii. 1902.

[394] Duerden, _l.c._ p. 436.

[395] M'Intosh, "The Marine Invertebrates and Fishes of St. Andrews," 1875,
      pp. 37, 38.

[396] M'Intosh, "The Resources of the Sea," 1899, pp. 10, 129.

[397] H. Prouho, _Arch. Zool. Expér._ 2nd ser. ix. 1891, p. 247.

[398] Duerden, _Mem. Acad. Washington_, viii. 1902, p. 437.

[399] Ashworth and Annandale, _Proc. Roy. Soc. Edinb._ xxv. 1904, p. 11.

[400] For recent experiments on this case, see a forthcoming paper by J. E.
      Duerden (_P.Z.S._).

[401] Saville Kent, "Great Barrier Reef," London, 1893, p. 145.

[402] O. Carlgren, _Biolog. Centralbl._ xxi. 1901, p. 480.

[403] Saville Kent, "The Great Barrier Reef," 1893, p. 144.

[404] A. C. Haddon, _Trans. Roy. Dubl. Soc._ iv. 1889, p. 325.

[405] G. H. Fowler, _Quart. Journ. Micr. Sci._ xxix. 1888, p. 143.

[406] For a general account of the Madreporarian skeleton, cf. Ogilvie,
      _Phil. Trans. Roy. Soc._ clxxxvii. B. 1896.

[407] H. M. Bernard, _Ann. Mag. Nat. Hist._ (7) xiii. 1904, p. 1.

[408] "Report on the Results of Dredging on the Macclesfield Bank,"
      _Admiralty Report_, 1894.

[409] C. Darwin, _Coral Reefs_, 3rd edition, 1889, p. 125.

[410] For the details of these borings, see "The Atoll of Funafuti," _Royal
      Society of London_, 1904.

[411] For further information, see J. Stanley Gardiner, _The Fauna and
      Geography of the Maldive and Laccadive-Archipelagoes_, vol. i. pt.
      ii. 1902, p. 172.

[412] Saville Kent, "Great Barrier Reef," 1893, p. 185.

[413] Duerden, _Mem. Ac. Washington_, viii. 1902, p. 550.

[414] H. M. Bernard, _Journ. Linn. Soc. Zool._ xxvi. 1897, p. 495.

[415] E. M. Pratt, _Willey's Zoological Results_, pt. v. 1900, p. 591.

[416] G. H. Fowler, _Quart. Journ. Micr. Sci._ xxx. 1890, p. 410.

[417] J. E. Duerden, _Mem. Ac. Washington_, viii. 1902, p. 553.

[418] M. Ogilvie, _Trans. Roy. Soc._ clxxxvii. B. 1896.

[419] "The Coral _Siderastraea_," _Carnegie Inst._ No. 20, Washington,
      1904.

[420] The reader is referred to the excellent photographs of living Fungias
      in Saville Kent's "Great Barrier Reef," 1893, pl. xxiv. p. 160.

[421] _Trans. Roy. Soc. Dubl._ (2) vi. 1898, p. 331.

[422] F. J. Bell, _Trans. Zool. Soc._ xiii. pt. ii. 1891, p. 87.

[423] J. E. Duerden, _Ann. Mag. Nat. Hist._ (7) ix. 1902, p. 381.

[424] Hickson, _Nature_, lxxiii. 1905, p. 5.

[425] L. Roule, _Bull. Mus. Océanogr. Monaco_, 1904, p. 3.

[426] E. van Beneden, _Les Anthozoaires de la Plankton Expédition_, Kiel,
      1898.

[427] A. W. Peters, _Journ. Exper. Zool._ ii. (1) 1905, p. 103.

[428] Cnidoblasts are stated by Chun to occur on the tentacles of
      _Euchlora_; and batteries of "nettle cells" by Abbott on the
      tentacles of _Coeloplana_.

[429] The two costae that are seen in the middle when the Ctenophore is
      viewed in the transverse plane, as in Figs. 180 and 181, and the
      corresponding costae on the opposite side are called the "transverse"
      costae; the other four are called the "sagittal" costae.

[430] F. Mosser, "Ctenophoren de Siboga Expedition," Leiden, 1903.

[431] H. B. Bigelow, _Bull. Mus. Comp. Zool._ xxxix. 1904, p. 267.

[432] _Quart. Journ. Micr. Sci._ xxxix. 1897, p. 323.

[433] _Annot. Zoolog. Japon._ iv. pt. iv. 1902, p. 103.

[434] Abbott, _l.c._ p. 106.

[435] _Zool. Anz._ xxvii. 1904, p. 223.

[436] The name seems first to have been used by Klein in 1734, "Naturalis
      dispositio Echinodermatum" (Danzig). Leuckart about 1850 first
      established Echinodermata as a primary division of the animal
      kingdom.

[437] In the Synaptidae the radial canals although present in the young are
      lost in the adult (Ludwig, 1892, in Bronn's _Thier-Reich_, Bd. ii.
      Abt. 3, Buch i. p. 460).

[438] Ludwig, _loc. cit._ p. 357.

[439] This classification is substantially that suggested by Jeffrey Bell,
      _Catalogue of British Echinoderms in the British Museum_, 1892,
      except that Bell separates Holothuroidea from all others. Reasons
      will be given later for regarding Holothuroidea as modified
      Echinoidea.

[440] Gr. ἀστήρ, a "star"; εἶδος, "form." Linnaeus established the genus
      _Asterias_ in 1766. Johannes Müller in 1842 used the name
      "Asteriden," and in _System der Asteriden_, 1842, by Müller and
      Troschel, the foundation of our knowledge of the group was laid.

[441] Uexküll, "Die Physiologie der Pedicellarien," _Zeitschr. f. Biol._
      xxxvii. 1899, p. 356.

[442] Durham, "Wandering Cells in Echinodermata," _Quart. J. Micr. Sci._
      xxxiii. 1891, pp. 81 _et seq._

[443] Starfish are most destructive on oyster-beds, and hence possess
      considerable negative economic value.

[444] _Mitth. des deutschen Seefischervereins_, xii. 1896, p. 102, and _J.
      Mar. Biol. Ass._ iv. 1895-97, p. 266.

[445] Romanes, "Jellyfish, Starfish, and Sea Urchins," _Intern. Scientific
      Series_, 1885, pp. 320, 321; Preyer, "Bewegungen von Stelleriden,"
      _Mitth. Zool. Stat. Neapel_, vii. 1886-87, p. 22.

[446] Preyer, _loc. cit._ p. 49.

[447] Bronn's _Thier-Reich_, Bd. ii. Abt. 3, Buch ii. Seesterne, p. 617.

[448] _Beiträge zur Histologie der Echinodermen_, Jena, 1889. Such spaces
      are always to be seen in _Asterina gibbosa_ when preserved with
      corrosive sublimate or other acid reagents, but are absent when it is
      preserved with osmic acid and Mueller's fluid. Though corrosive
      sublimate is usually regarded as a neutral salt, its aqueous solution
      decomposes with the production of a certain amount of free
      hydrochloric acid.

[449] "Beiträge zur Anatomie der Asteriden," _Zeitschr. wiss. Zool._ xxx.
      1877, pp. 122 _et seq._

[450] "Cont. à l'Étude anat. des Astérides," _Arch. Zool. Exp._ (2) v.
      _bis_, 1887, p. 104.

[451] The analogy of Echinoidea (see p. 529) might suggest that, like the
      lacteals in man, these strands were channels along which the products
      of digestion diffused outward. No connexion, however, between the
      oral ring and the alimentary canal has been made out, nor do there
      appear to be such strands developed in the proximity of the wall of
      the digestive tube. A connexion between the aboral ring and the
      rectum through a mesenteric cord has been asserted, but this is
      doubtful.

[452] "Die Echinodermen des Golfes von Neapel," _Fauna u. Flora G. von
      Neapel_, xxiv. Monogr. 1897, pp. 349-351.

[453] Ludwig, "Die Echinodermen des Golfes von Neapel," pp. 68, 69.

[454] Ludwig, "Scientific Results of the Expedition of the 'Albatross' to
      the Tropical Pacific"—"Asteroidea," 1905, pp. 91, 103.

[455] _Rés. sci. Expéd. Travailleur et Talisman_, "Échinodermes," 1894, pp.
      10-15.

[456] Schiemenz (reference on p. 440 n.).

[457] This fact was discovered by Dr. E. J. Allen, Director of the Plymouth
      Biological Station, who pointed it out to the author during the
      latter's sojourn at the station in 1899.

[458] This figure does not show the animal's attitude during forward
      progression quite correctly. The tips of the two anterior arms should
      be bent outwards, not inwards as in the figure.

[459] In the more primitive Ophiuroidea (Streptophiurae) it persists all
      over the body; in Cladophiurae it is found on the central part of the
      disc.

[460] How far this form of respiratory mechanism is distributed amongst
      Ophiurids it is impossible to say. It was first observed by me in the
      case of _Ophiothrix fragilis_ at Plymouth in 1905, but since then I
      have found it in _Ophiura ciliaris_ and in _Amphiura squamata_.

[461] "Neue Beitr. zur Anat. d. Ophiuriden," _Zeitschr. wiss. Zool._ xxxiv.
      1880, p. 340.

[462] "Bewegungen d. Seesternen," _Mitth. Zool. Stat. Neapel_, vii.
      1886-87, p. 123.

[463] Bell, "Contribution to the Classification of Ophiuroids," _Proc.
      Zool. Soc._ 1892, p. 175.

[464] Hamann, _Bronn's Thier-Reich_, Bd. ii. Abt. 3, Ophiuroidea, 1900, p.
      910 f., discriminates a family Ophiodermatidae, but gives no
      character by which it can be distinguished from Ophiolepididae.

[465] Forbes, "A History of British Starfishes and other animals of the
      class Echinodermata," 1841, p. 23.

[466] Simroth, "Anatomie und Schizogonie der _Ophiactis virens_,"
      _Zeitschr. wiss. Zool._ xxvii. 1876, p. 452.

[467] Cuénot, "Études Morphologiques sur les Echinodermes," _Arch.
      Biologie_, xi. 1891, pp. 568 _et seq._

[468] This type of mouth-frame is represented in Fig. 215, A, by a figure
      of _Ophioscolex_, which belongs to the Streptophiurae.

[469] "Asteriden und Ophiuriden aus dem Silur Böhmens," _Zeitschr. der
      deutschen geol. Ges._ lv. 1903, pp. 106-113 (Protokolle).

[470] _Geol. Magazine_, No. 490, April 1905, pp. 161-168.

[471] "Die Physiologie des Seeigelstachels," _Zeitschr. für Biol._ xxxix.
      1900, pp. 73 _et seq._

[472] Uexküll, "Die Physiologie der Pedicellarien," _Zeitschr. für Biol._
      xxxvii. 1899, p. 334.

[473] "Du rôle des pédicellaires gemmiformes des Oursins," _Compt. Rend.
      Acad. de Paris_, cxi. 1890, pp. 62-64.

[474] "The Fauna and Bottom Deposits near the thirty-fathom line from the
      Eddystone grounds to Startpoint," _Journ. Marine Biol. Ass._ v. 1899,
      p. 472.

[475] "Mitth. über die zool. Stat. v. Neapel," _Zeitschr. wiss. Zool._ xxv.
      1875, p. 471.

[476] Cuénot, "Études Morphologiques sur les Échinodermes," _Arch. Biol._
      xi. 1891, p. 544.

[477] "Jellyfish, Starfish, and Sea-urchins," _Intern. Sci. Series_, 1885,
      p. 302 _et seq._

[478] "Die Physiologie des Seeigelstachels," _Zeitschr. für Biol._ xxxix.
      p. 73.

[479] "Bewegungen von Stelleriden," _Mitth. Zool. Stat. Neapel_, vii.
      1886-7, p. 22.

[480] _Loc. cit._

[481] See note on p. 541.

[482] "Die Wirkung von Licht und Schatten auf die Seeigel," _Zeitschr. für
      Biol._ xl. 1900, p. 447.

[483] In the aberrant genus _Asthenosoma_, where there are internal radial
      muscles, there is also an internal series of nerve-cells on the
      radial cord.

[484] We prefer the term "compasses," to avoid confusion with the other
      meanings of the word "radius."

[485] "Ueber die Function der Polischen Blasen am Kauapparat der regulären
      Seeigel," _Mitth. Zool. Stat. Neapel_, xii. 1897, p. 464.

[486] _Ergebnisse naturwissenschaftlicher Forschungen auf Ceylon_,
      1887-1888, Bd. i. Heft 3, pp. 105 _et seq._

[487] "Das angebliche Excretionsorgan der Seeigel," _Zeitschr. wiss. Zool._
      lv. 1893, p. 585.

[488] In this case the fluid flows from the lantern coelom into Stewart's
      organs and _vice versa_. Oxygen must be absorbed through the
      peristome. The Cidaridae are not as sensitive to want of oxygen as
      the other families (Uexküll, _loc. cit._).

[489] _Danish Ingolf Expedition_, "Echinoidea," pt. i. 1903.

[490] Prouho, "Recherches sur le _Dorocidaris papillata_ et quelques autres
      Échinides de la Méditerranée," _Arch. Zool. Exp._ (2) v. 1887, p.
      308.

[491] "Revision of the Echini," _Illustrated Catalogue of Museum of Comp.
      Zool. Harvard_, No. 7, 1874, p. 423.

[492] _British Museum Catalogue_, "British Echinoderms," 1892, p. 30.

[493] _Loc. cit._

[494] _Loc. cit._

[495] Reference on p. 528 n.

[496] This account of the periproct is different from that ordinarily
      given. It is based on the most recent examination of this
      family—Agassiz, "Panamic Deep-sea Echini," _Mem. Mus. Comp. Zool._
      xxxi. 1904, p. 36.

[497] "Der Schatten als Reiz für _Centrostephanus longispinus_," _Zeitschr.
      für Biol._ xxxiv. 1896, p. 319.

[498] Reference on p. 532, note [489].

[499] Mr. E. W. L. Holt, Scientific Adviser to the Irish Board of
      Fisheries, casts doubt (_in litt._) on much of this supposed
      excavation. While disclaiming any novelty in this observation, he
      points out that in many cases one side of the cavity is formed by
      calcareous algae, and it seems as if the animal wanders into a
      crevice, in which it is imprisoned by the growth of this plant.

[500] These statements are based on the author's observations of the animal
      in the Bay of Fundy in 1900.

[501] Lovén, "On a recent Form of Echinoconidae," _Bih. Svenska Akad.
      Hand._ xiii. Af. 4, No. 10, 1889.

[502] Details were given to the author in conversation with Dr. Robertson
      in 1896.

[503] This family includes three families discriminated by Meissner
      (Bronn's _Thier-Reich_, vol. ii. Abt. 3, Buch iv. "Die Seeigel,"
      1904, p. 402), viz.: Ananchytidae, Pourtalesiidae, and Urechinidae.
      They only differ in the pores for the tube-feet, which are paired in
      the first, slit-like and single in the second, and single in the
      third.

[504] "On Silurian Echinoidea and Ophiuroidea," _Quart. Journ. Geol. Soc._
      lv. 1899, pp. 701 _et seq._

[505] "The Metamorphosis of Echinoderms," _Quart. J. Micr. Sci._ xxxviii.
      1896, p. 53.

[506] _Pelagothuria_ is said to have no calcifications.

[507] Hjalmar Théel, "On a singular Case of Hermaphroditism in
      Holothurids," _Bih. Svenska Vet. Akad. Hand._ xxvii. Af. 4, No. 6,
      1901.

[508] Bronn's _Thier-Reich_, vol. ii. Abt. 3, Buch i. "Die Seewalzen,"
      1891, pp. 327 _et seq._

[509] Genuine suckers appear never to be developed; but the ends of the
      ventral podia are sometimes rounded, sometimes slightly flattened.

[510] "Mémoire sur l'_Elpidia_ nouveau genre d'Holothuries," _K. Sv. Vet.
      Akad._ xiv. No. 8, 1877.

[511] Gerould, "The Anatomy and Histology of _Caudina armata_," _Bull. Mus.
      Comp. Zool._ No. 56, 1895, p. 124.

[512] "Structure, Physiology, and Development of _Antedon rosaceus_,"
      _Phil. Trans. Roy. Soc._ 1866, pp. 671-756.

[513] "On the Nervous System of _Antedon rosaceus_," _Q.J.M.S._ xxiv. 1884,
      p. 507.

[514] Bather calls the side-plates "adambulacral." The name is unfortunate,
      as it suggests that the side-plates correspond to the adambulacrals
      of Asteroidea; but when the groove is closed the position and
      function of adambulacrals and covering plates is the same
      (Lankester's _Treatise on Zoology_, iii. "Echinodermata," 1900).

[515] _Challenger Reports_ (_Stalked Crinoids_), vol. xi. pt. xxxii. 1884.

[516] _Stammesgeschichte der Pelmatozoen_, pt. i. 1899.

[517] "Die Echinodermenlarven der Plankton Expedition," _Ergebn. Plankton
      Exp._ Bd. ii. J, 1898.

[518] "Über die Larvenzustande und die Metamorphose der Ophiuren und
      Seeigel," _Abh. K. Akad. wiss. Berlin_, 1846, and other papers in the
      same publication in subsequent years.

[519] This is clearly seen by comparing the larva of _Asterina gibbosa_
      with a young Bipinnaria in which the longitudinal band is as yet
      undivided. The shape of the prae-oral lobe is practically the same in
      both.

[520] "The Fate of the Body-cavities in the Metamorphosis of _Asterias
      pallida_," J. _Coll. Japan_, x. 1898, p. 239.

[521] Yves Delage, "Elevage des Larves parthénogénétiques _d'Asterias
      glacialis_," _Arch. Zool. Exp._ (4) ii. 1904, p. 27.

[522] "The Metamorphosis of Echinoderms," _Quart. Journ. Micr. Sc._
      xxxviii. 1895, p. 45.

[523] In the type figured (larva of _Synapta digitata_) the feelers are
      budded off directly from the ring-canal and alternate with the
      rudiments of the radial canal.

[524] Observed in Plymouth, 1905.

[525] "Die Entwicklung der _Synapta digitata_," _Jen. Zeitschr._ xxii.
      1888, p. 175.

[526] "Die Cambrische Stammgruppe der Echinodermen," _Jen. Zeitschr._ xxx.
      1895.

[527] Lankester's _Treatise on Zoology_, "Echinodermata," pt. iii. 1900, p.
      33.

[528] _Brit. Mus. Cat._ "British Echinodermata," 1892, p. 14.

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Corrections made to printed text