THE
                       CAMBRIDGE NATURAL HISTORY


                               EDITED BY

  S. F. HARMER, Sc.D., F.R.S., Fellow of King’s College, Cambridge;
      Keeper of the Department of Zoology in the British Museum (Natural
      History)

                                  AND

  A. E. SHIPLEY, M.A., Fellow and Tutor of Christ’s College, Cambridge;
      Reader in Zoology in the University


                               VOLUME IV




[Illustration: Signet]

                       MACMILLAN AND CO., LIMITED
                       LONDON · BOMBAY · CALCUTTA
                               MELBOURNE

                         THE MACMILLAN COMPANY
                      NEW YORK · BOSTON · CHICAGO
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                   THE MACMILLAN CO. OF CANADA, LTD.
                                TORONTO


CRUSTACEA


      By GEOFFREY SMITH, M.A. (Oxon.), Fellow of New College, Oxford;
      and the late W. F. R. WELDON, M.A. (D.Sc., Oxon.), formerly Fellow
      of St. John’s College, Cambridge, and Linacre Professor of Human
      and Comparative Anatomy, Oxford


TRILOBITES


      By HENRY WOODS, M.A., St. John’s College, Cambridge; University
      Lecturer in Palaeozoology


INTRODUCTION TO ARACHNIDA, AND KING-CRABS


      By A. E. SHIPLEY, M.A., F.R.S., Fellow and Tutor of Christ’s
      College, Cambridge; Reader in Zoology


EURYPTERIDA


      By HENRY WOODS, M.A., St. John’s College, Cambridge; University
      Lecturer in Palaeozoology


SCORPIONS, SPIDERS, MITES, TICKS, ETC.


      By CECIL WARBURTON, M.A., Christ’s College, Cambridge; Zoologist
      to the Royal Agricultural Society


TARDIGRADA (WATER-BEARS)


      By A. E. SHIPLEY, M.A., F.R.S., Fellow and Tutor of Christ’s
      College, Cambridge; Reader in Zoology


PENTASTOMIDA


      By A. E. SHIPLEY, M.A., F.R.S., Fellow and Tutor of Christ’s
      College, Cambridge; Reader in Zoology


PYCNOGONIDA


      By D’ARCY W. THOMPSON, C.B., M.A., Trinity College, Cambridge;
      Professor of Natural History in University College, Dundee


                       MACMILLAN AND CO., LIMITED
                      ST. MARTIN’S STREET, LONDON

                                  1909




  All the ingenious men, and all the scientific men, and all the
  fanciful men, in the world, with all the old German bogypainters into
  the bargain, could never invent ... anything so curious, and so
  ridiculous, as a lobster.

                                   CHARLES KINGSLEY, _The Water-Babies_.

        For, Spider, thou art like the poet poor,
            Whom thou hast help’d in song.
        Both busily, our needful food to win,
            We work, as Nature taught, with ceaseless pains,
                Thy bowels thou dost spin,
                    I spin my brains.
                                        SOUTHEY, _To a Spider_.

        Last o’er the field the Mite enormous swims,
        Swells his red heart, and writhes his giant limbs.
                        ERASMUS DARWIN, _The Temple of Nature_.




                                PREFACE


The Editors feel that they owe an apology and some explanation to the
readers of _The Cambridge Natural History_ for the delay which has
occurred in the issue of this, the fourth in proper order, but the last
to appear of the ten volumes which compose the work. The delay has been
due principally to the untimely death of Professor W. F. E. Weldon, who
had undertaken to write the Section on the Crustacea. The Chapter on the
Branchiopoda is all he actually left ready for publication, but it gives
an indication of the thorough way in which he had intended to treat his
subject. He had, however, superintended the preparation of a number of
beautiful illustrations, which show that he had determined to use, in
the main, first-hand knowledge. Many of these figures have been
incorporated in the article by Mr. Geoffrey Smith, to whom the Editors
wish to express their thanks for taking up, almost at a moment’s notice,
the task which had dropped from his teacher’s hand.

A further apology is due to the other contributors to this volume. Their
contributions have been in type for many years, and owing to the
inevitable delays indicated above they have been called upon to make old
articles new, ever an ungrateful labour.

The appearance of this volume completes the work the Editors embarked on
some sixteen years ago. It coincides with the cessation of an almost
daily intercourse since the time when they “came up” to Cambridge as
freshmen in 1880.

                                                          S. F. HARMER.
                                                          A. E. SHIPLEY.


  _March 1909._




                                CONTENTS


                                                                    PAGE

 SCHEME OF THE CLASSIFICATION ADOPTED IN THIS VOLUME                  xi


                                CRUSTACEA

                                CHAPTER I

                                CRUSTACEA

 GENERAL ORGANISATION                                                  3


                               CHAPTER II

                         CRUSTACEA (_continued_)

 ENTOMOSTRACA—BRANCHIOPODA—PHYLLOPODA—CLADOCERA—WATER-FLEAS           18


                               CHAPTER III

                  CRUSTACEA ENTOMOSTRACA (_continued_)

 COPEPODA                                                             55


                               CHAPTER IV

                  CRUSTACEA ENTOMOSTRACA (_continued_)

 CIRRIPEDIA—PHENOMENA OF GROWTH AND SEX—OSTRACODA                     79


                                CHAPTER V

                         CRUSTACEA (_continued_)

 MALACOSTRACA: LEPTOSTRACA—PHYLLOCARIDA: EUMALACOSTRACA: SYNCARIDA—
   ANASPIDACEA: PERACARIDA—MYSIDACEA—CUMACEA—ISOPODA—AMPHIPODA:
   HOPLOCARIDA—STOMATOPODA                                           110


                               CHAPTER VI

                  CRUSTACEA MALACOSTRACA (_continued_)

 EUMALACOSTRACA (_CONTINUED_): EUCARIDA—EUPHAUSIACEA—COMPOUND EYES—
   DECAPODA                                                          144


                               CHAPTER VII

                         CRUSTACEA (_continued_)

 REMARKS ON THE DISTRIBUTION OF MARINE AND FRESH-WATER CRUSTACEA     197


                              CHAPTER VIII

                         CRUSTACEA (_continued_)

 TRILOBITA                                                           221


                                ARACHNIDA

                               CHAPTER IX

 ARACHNIDA—INTRODUCTION                                              255


                                CHAPTER X

                         ARACHNIDA (_continued_)

 DELOBRANCHIATA = MEROSTOMATA—XIPHOSURA                              259


                               CHAPTER XI

                 ARACHNIDA DELOBRANCHIATA (_continued_)

 EURYPTERIDA = GIGANTOSTRACA                                         283


                               CHAPTER XII

                         ARACHNIDA (_continued_)

 EMBOLOBRANCHIATA—SCORPIONIDEA—PEDIPALPI                             297


                              CHAPTER XIII

                ARACHNIDA EMBOLOBRANCHIATA (_continued_)

 ARANEAE—EXTERNAL STRUCTURE—INTERNAL STRUCTURE                       314


                               CHAPTER XIV

                ARACHNIDA EMBOLOBRANCHIATA (_continued_)

 ARANEAE (_CONTINUED_)—HABITS—ECDYSIS—TREATMENT OF YOUNG—MIGRATION—
   WEBS—NESTS—EGG-COCOONS—POISON—FERTILITY—ENEMIES—PROTECTIVE
   COLORATION—MIMICRY—SENSES—INTELLIGENCE—MATING HABITS—FOSSIL
   SPIDERS                                                           338


                               CHAPTER XV

                ARACHNIDA EMBOLOBRANCHIATA (_continued_)

 ARANEAE (_CONTINUED_)—CLASSIFICATION                                384


                               CHAPTER XVI

                ARACHNIDA EMBOLOBRANCHIATA (_continued_)

 PALPIGRADI—SOLIFUGAE = SOLPUGAE—CHERNETIDEA = PSEUDOSCORPIONES      422


                              CHAPTER XVII

                ARACHNIDA EMBOLOBRANCHIATA (_continued_)

 PODOGONA = RICINULEI—PHALANGIDEA = OPILIONES—HABITS—STRUCTURE—
   CLASSIFICATION                                                    439


                              CHAPTER XVIII

                ARACHNIDA EMBOLOBRANCHIATA (_continued_)

 ACARINA—HARVEST-BUGS—PARASITIC MITES—TICKS—SPINNING MITES—
   STRUCTURE—METAMORPHOSIS—CLASSIFICATION                            454


                               CHAPTER XIX

                         ARACHNIDA (APPENDIX I)

 TARDIGRADA—OCCURRENCE—ECDYSIS—STRUCTURE—DEVELOPMENT—AFFINITIES—
   BIOLOGY—DESICCATION—PARASITES—SYSTEMATIC                          477


                               CHAPTER XX

                         ARACHNIDA (APPENDIX II)

 PENTASTOMIDA—OCCURRENCE—ECONOMIC IMPORTANCE—STRUCTURE—DEVELOPMENT
   AND LIFE-HISTORY—SYSTEMATIC                                       488


                               PYCNOGONIDA

                               CHAPTER XXI

 PYCNOGONIDA                                                         501


 INDEX                                                               543




          SCHEME OF THE CLASSIFICATION ADOPTED IN THIS VOLUME

         _The names of extinct groups are printed in italics._


                               =CRUSTACEA= (p. 3).

                             =ENTOMOSTRACA= (p. 18).

 ──────────┬───────────┬───────────┬──────────────────────────┬─────────────────
 Divisions.│  Orders.  │Sub-Orders.│         Tribes.          │    Families.
 ──────────┼───────────┼───────────┴──────────────────────────┼─────────────────
           │=Bran-     │                                      │Branchipodidae
           │  chiopoda=│=Phyllopoda= (pp. 19, 35)             │  (pp. 19, 35).
           │  (p. 18)  │                                      │
           │     „     │                  „                   │Apodidae (pp. 19,
           │           │                                      │  36).
           │     „     │                  „                   │Limnadiidae
           │           │                                      │  (pp. 20, 36).
 ──────────┼───────────┼───────────┬───────────┬──────────────┼─────────────────
           │           │=Cladocera=│Calyptomera│Ctenopoda     │
           │     „     │  (p. 37)  │  (pp. 38, │  (p. 51)     │Sididae (p. 51).
           │           │           │  51)      │              │
           │     „     │     „     │     „     │      „       │Holopediidae
           │           │           │           │              │  (p. 51).
 ──────────┼───────────┼───────────┼───────────┼──────────────┼─────────────────
           │     „     │     „     │     „     │Anomopoda     │Daphniidae
           │           │           │           │  (p. 51)     │  (p. 51).
           │     „     │     „     │     „     │      „       │Bosminidae
           │           │           │           │              │  (p. 53).
           │     „     │     „     │     „     │      „       │Lyncodaphniidae
           │           │           │           │              │  (p. 53).
           │           │           │           │              │Lynceidae =
           │     „     │     „     │     „     │      „       │  Chydoridae
           │           │           │           │              │  (p. 53).
 ──────────┼───────────┼───────────┼───────────┴──────────────┼─────────────────
           │     „     │     „     │Gymnomera (pp. 38, 54)    │Polyphemidae
           │           │           │                          │  (p. 54).
           │     „     │     „     │            „             │Leptodoridae
           │           │           │                          │  (p. 54).

 ──────────┬───────────────────────┬───────────┬──────────────┬─────────────────
 Divisions.│        Orders.        │Sub-Orders.│   Tribes.    │    Families.
 ──────────┼───────────┬───────────┼───────────┼──────────────┼─────────────────
           │=Copepoda= │=Eucope-   │=Gymnoplea=│Amphascan-    │Calanidae
           │  (p. 55)  │  poda=    │  (p. 57)  │  dria (p. 57)│  (p. 57).
           │           │  (p. 57)  │           │              │
 ──────────┼───────────┼───────────┼───────────┼──────────────┼─────────────────
           │           │           │           │Heterar-      │Centropagidae
           │     „     │     „     │     „     │  thrandria   │  (p. 58).
           │           │           │           │  (p. 58)     │
           │     „     │     „     │     „     │      „       │Candacidae
           │           │           │           │              │  (p. 60).
           │     „     │     „     │     „     │      „       │Pontellidae
           │           │           │           │              │  (p. 60).
 ──────────┼───────────┼───────────┼───────────┼──────────────┼─────────────────
           │           │           │=Podoplea= │Amphar-       │Cyclopidae
           │     „     │     „     │  (p. 61)  │  thrandria   │  (pp. 61, 62).
           │           │           │           │  (p. 61)     │
           │     „     │     „     │     „     │      „       │Harpacticidae
           │           │           │           │              │  (pp. 61, 62).
           │     „     │     „     │     „     │      „       │Peltiidae
           │           │           │           │              │  (p. 63).
           │     „     │     „     │     „     │      „       │Monstrillidae
           │           │           │           │              │  (p. 63).
           │     „     │     „     │     „     │      „       │Ascidicolidae
           │           │           │           │              │  (p. 66).
           │     „     │     „     │     „     │      „       │Asterocheridae
           │           │           │           │              │  (p. 67).
           │     „     │     „     │     „     │      „       │Dichelestiidae
           │           │           │           │              │  (p. 68).
 ──────────┼───────────┼───────────┼───────────┼──────────────┼─────────────────
           │     „     │     „     │     „     │Isokerandria  │Oncaeidae
           │           │           │           │  (p. 69)     │  (p. 69).
           │     „     │     „     │     „     │      „       │Corycaeidae
           │           │           │           │              │  (p. 69).
           │     „     │     „     │     „     │      „       │Lichomolgidae
           │           │           │           │              │  (p. 70).
           │     „     │     „     │     „     │      „       │Ergasilidae
           │           │           │           │              │  (p. 71).
           │     „     │     „     │     „     │      „       │Bomolochidae
           │           │           │           │              │  (p. 71).
           │     „     │     „     │     „     │      „       │Chondracanthidae
           │           │           │           │              │  (p. 72).
           │     „     │     „     │     „     │      „       │Philichthyidae
           │           │           │           │              │  (p. 73).
           │     „     │     „     │     „     │      „       │Nereicolidae
           │           │           │           │              │  (p. 73).
           │     „     │     „     │     „     │      „       │Hersiliidae
           │           │           │           │              │  (p. 73).
           │     „     │     „     │     „     │      „       │Caligidae
           │           │           │           │              │  (p. 73).
           │     „     │     „     │     „     │      „       │Lernaeidae
           │           │           │           │              │  (p. 74).
           │     „     │     „     │     „     │      „       │Lernaeopodidae
           │           │           │           │              │  (p. 75).
           │     „     │     „     │     „     │      „       │Choniostomatidae
           │           │           │           │              │  (p. 76).
 ──────────┼───────────┼───────────┴───────────┴──────────────┼─────────────────
           │     „     │=Branchiura= (P. 76)                  │Argulidae
           │           │                                      │  (p. 76).
 ──────────┼───────────┴───────────┬──────────────────────────┼─────────────────
           │=Cirripedia= (p. 79)   │=Pedunculata= (p. 84)     │Polyaspidae
           │                       │                          │  (p. 84).
           │           „           │            „             │Pentaspidae
           │                       │                          │  (p. 87).
           │           „           │            „             │Tetraspidae
           │                       │                          │  (p. 88).
           │           „           │            „             │Anaspidae
           │                       │                          │  (p. 89).
 ──────────┼───────────────────────┼──────────────────────────┼─────────────────
           │           „           │=Operculata= (p. 89)      │Verrucidae
           │                       │                          │  (p. 91).
           │           „           │            „             │Octomeridae
           │                       │                          │  (p. 91).
           │           „           │            „             │Hexameridae
           │                       │                          │  (p. 91).
           │           „           │            „             │Tetrameridae
           │                       │                          │  (p. 92).
 ──────────┼───────────────────────┼──────────────────────────┴─────────────────
           │           „           │=Acrothoracica= (p. 92).
           │           „           │=Ascothoracica= (p. 93).
           │           „           │=Apoda= (p. 94).
           │           „           │=Rhizocephala= (p. 95).
 ──────────┼───────────────────────┴──────────────────────────┬─────────────────
           │=Ostracoda= (p. 107)                              │Cypridae
           │                                                  │  (p. 107).
           │                        „                         │Cytheridae
           │                                                  │  (p. 107).
           │                        „                         │Halocypridae
           │                                                  │  (p. 108).
           │                        „                         │Cypridinidae
           │                                                  │  (p. 108).
           │                        „                         │Polycopidae
           │                                                  │  (p. 109).
           │                        „                         │Cytherellidae
           │                                                  │  (p. 109).

                            =MALACOSTRACA= (p. 110).

                             =LEPTOSTRACA= (p. 111)

 =Phyllocarida= (p. 111).

 ──────────┬───────────┬───────────────────────┬──────────────┬─────────────────
 Divisions.│  Orders.  │      Sub-Orders.      │    Tribes    │    Families.
 ──────────┴───────────┴───────────────────────┴──────────────┴─────────────────
                           =EUMALACOSTRACA= (p. 112).

 =Syncar-  │                                                  │Anaspididae
   ida=    │=Anaspidacea= (p. 115)                            │  (p. 115).
   (p. 114)│                                                  │
     „     │                        „                         │Koonungidae
           │                                                  │  (p. 117).
 ──────────┼──────────────────────────────────────────────────┼─────────────────
 =Per-     │                                                  │Eucopiidae
   acarida=│=Mysidacea= (p. 118)                              │  (p. 118).
   (p. 118)│                                                  │
     „     │                        „                         │Lophogastridae
           │                                                  │  (p. 119).
     „     │                        „                         │Mysidae (p. 119).
 ──────────┼──────────────────────────────────────────────────┼─────────────────
     „     │=Cumacea= (p. 120)                                │Cumidae (p. 121).
     „     │                        „                         │Lampropidae
           │                                                  │  (p. 121).
     „     │                        „                         │Leuconidae
           │                                                  │  (p. 121).
     „     │                        „                         │Diastylidae
           │                                                  │  (p. 121).
     „     │                        „                         │Pseudocumidae
           │                                                  │  (p. 121).
 ──────────┼───────────┬──────────────────────────────────────┼─────────────────
     „     │=Isopoda=  │=Chelifera= (p. 122)                  │Apseudidae
           │  (p. 121) │                                      │  (p. 122).
     „     │     „     │                  „                   │Tanaidae
           │           │                                      │  (p. 122).
 ──────────┼───────────┼──────────────────────────────────────┼─────────────────
     „     │     „     │=Flabellifera= (p. 124)               │Anthuridae
           │           │                                      │  (p. 124).
     „     │     „     │                  „                   │Gnathiidae
           │           │                                      │  (p. 124).
     „     │     „     │                  „                   │Cymothoidae
           │           │                                      │  (p. 126).
     „     │     „     │                  „                   │Cirolanidae
           │           │                                      │  (p. 126).
     „     │     „     │                  „                   │Serolidae
           │           │                                      │  (p. 126).
     „     │     „     │                  „                   │Sphaeromidae
           │           │                                      │  (p. 126).
 ──────────┼───────────┼──────────────────────────────────────┼─────────────────
     „     │     „     │=Valvifera= (p. 127)                  │Idotheidae
           │           │                                      │  (p. 127).
     „     │     „     │                  „                   │Arcturidae
           │           │                                      │  (p. 127).
 ──────────┼───────────┼──────────────────────────────────────┼─────────────────
     „     │     „     │=Asellota= (p. 127)                   │Asellidae
           │           │                                      │  (p. 128).
     „     │     „     │                  „                   │Munnopsidae
           │           │                                      │  (p. 128).
 ──────────┼───────────┼──────────────────────────────────────┴─────────────────
     „     │     „     │=Oniscoida= (p. 128).
 ──────────┼───────────┼───────────┬──────────────────────────┬─────────────────
     „     │     „     │=Epicarida=│=Cryptoniscina= (pp. 129, │Microniscidae
           │           │  (p. 129) │  130)                    │  (p. 130).
     „     │     „     │     „     │            „             │Cryptoniscidae
           │           │           │                          │  (p. 130).
     „     │     „     │     „     │            „             │Liriopsidae
           │           │           │                          │  (p. 130).
     „     │     „     │     „     │            „             │Hemioniscidae
           │           │           │                          │  (p. 130).
     „     │     „     │     „     │            „             │Cabiropsidae
           │           │           │                          │  (p. 130).
     „     │     „     │     „     │            „             │Podasconidae
           │           │           │                          │  (p. 130).
     „     │     „     │     „     │            „             │Asconiscidae
           │           │           │                          │  (p. 130).
 ──────────┼───────────┼───────────┼──────────────────────────┼─────────────────
     „     │     „     │     „     │=Bopyrina= (pp. 129, 130, │Dajidae (p. 130).
           │           │           │  132)                    │
     „     │     „     │     „     │            „             │Phryxidae
           │           │           │                          │  (p. 130).
     „     │     „     │     „     │            „             │Bopyridae
           │           │           │                          │  (pp. 130, 133).
     „     │     „     │     „     │            „             │Entoniscidae
           │           │           │                          │  (pp. 130, 134).
 ──────────┼───────────┼───────────┴──────────────────────────┼─────────────────
     „     │     „     │=Phreatoicidea= (p. 136)              │Phreatoicidae
           │           │                                      │  (p. 136)
 ──────────┼───────────┼──────────────────────────────────────┼─────────────────
     „     │=Amphipoda=│=Crevettina= (p. 137)                 │Lysianassidae
           │  (p. 136) │                                      │  (p. 137).
     „     │     „     │                  „                   │Haustoriidae
           │           │                                      │  (p. 137).
     „     │     „     │                  „                   │Gammaridae
           │           │                                      │  (p. 138).
     „     │     „     │                  „                   │Talitridae
           │           │                                      │  (p. 139).
     „     │     „     │                  „                   │Corophiidae
           │           │                                      │  (p. 139).
 ──────────┼───────────┼──────────────────────────────────────┼─────────────────
     „     │     „     │=Laemodipoda= (p. 139)                │Caprellidae
           │           │                                      │  (p. 139).
     „     │     „     │                  „                   │Cyamidae
           │           │                                      │  (p. 140).
 ──────────┼───────────┼──────────────────────────────────────┴─────────────────
     „     │     „     │=Hyperina= (p. 140).
 ──────────┼───────────┴──────────────────────────────────────┬─────────────────
 =Hoplo-   │                                                  │Squillidae
   carida= │=Stomatopoda= (p. 141)                            │  (p. 143).
   (p. 141)│                                                  │
 ──────────┼──────────────────────────────────────────────────┼─────────────────
 =Eucarida=│=Euphausiacea= (p. 144)                           │Euphausiidae
   (p. 144)│                                                  │  (p. 144).
 ──────────┼───────────┬───────────┬──────────────────────────┼─────────────────
     „     │=Decapoda= │=Macrura=  │Nephropsidea (p. 154)     │Nephropsidae
           │  (p. 152) │  (p. 153) │                          │  (p. 154).
     „     │     „     │     „     │            „             │Astacidae
           │           │           │                          │  (p. 157).
     „     │     „     │     „     │            „             │Parastacidae
           │           │           │                          │  (p. 157).
 ──────────┼───────────┼───────────┼──────────────────────────┼─────────────────
     „     │     „     │     „     │Eryonidea  (p. 157)       │Eryonidae
           │           │           │                          │  (p. 158).
 ──────────┼───────────┼───────────┼──────────────────────────┼─────────────────
     „     │     „     │     „     │Peneidea (pp. 158, 162)   │Peneidae
           │           │           │                          │  (p. 162).
     „     │     „     │     „     │            „             │Sergestidae
           │           │           │                          │  (p. 162).
     „     │     „     │     „     │            „             │Stenopodidae
           │           │           │                          │  (p. 162).
 ──────────┼───────────┼───────────┼──────────────────────────┼─────────────────
     „     │     „     │     „     │Caridea (pp. 158, 163)    │Pasiphaeidae
           │           │           │                          │  (p. 163).
     „     │     „     │     „     │            „             │Acanthephyridae
           │           │           │                          │  (p. 163).
     „     │     „     │     „     │            „             │Atyidae (p. 163).
     „     │     „     │     „     │            „             │Alpheidae
           │           │           │                          │  (p. 163).
     „     │     „     │     „     │            „             │Psalidopodidae
           │           │           │                          │  (p.164).
     „     │     „     │     „     │            „             │Pandalidae
           │           │           │                          │  (p. 164).
     „     │     „     │     „     │            „             │Hippolytidae
           │           │           │                          │  (p. 164).
     „     │     „     │     „     │            „             │Palaemonidae
           │           │           │                          │  (p. 164).
     „     │     „     │     „     │            „             │Glyphocrangonidae
           │           │           │                          │  (p. 164).
     „     │     „     │     „     │            „             │Crangonidae
           │           │           │                          │  (p. 164).
 ──────────┼───────────┼───────────┼──────────────────────────┼─────────────────
     „     │     „     │     „     │Loricata (p. 165)         │Palinuridae
           │           │           │                          │  (p. 167).
     „     │     „     │     „     │            „             │Scyllaridae
           │           │           │                          │  (p. 167).
 ──────────┼───────────┼───────────┼──────────────────────────┼─────────────────
     „     │     „     │     „     │Thalassinidea (p. 167)    │Callianassidae
           │           │           │                          │  (p. 167).
 ──────────┼───────────┼───────────┼──────────────────────────┼─────────────────
     „     │     „     │=Anomura=  │Galatheidea (p. 168)      │Aegleidae
           │           │  (p. 167) │                          │  (p. 169).
     „     │     „     │     „     │            „             │Galatheidae
           │           │           │                          │  (p. 169).
     „     │     „     │     „     │            „             │Porcellanidae
           │           │           │                          │  (p. 170).
 ──────────┼───────────┼───────────┼──────────────────────────┼─────────────────
     „     │     „     │     „     │Hippidea (p. 170)         │Albuneidae
           │           │           │                          │  (p. 171).
     „     │     „     │     „     │            „             │Hippidae
           │           │           │                          │  (p. 171).
 ──────────┼───────────┼───────────┼──────────────────────────┼─────────────────
     „     │     „     │     „     │Paguridea (p. 171)        │Pylochelidae
           │           │           │                          │  (p. 180).
     „     │     „     │     „     │            „             │Paguridae
           │           │           │                          │  (p. 180).
     „     │     „     │     „     │            „             │Eupagurinae
           │           │           │                          │  (p. 180).
     „     │     „     │     „     │            „             │Pagurinae
           │           │           │                          │  (p. 180).
     „     │     „     │     „     │            „             │Coenobitidae
           │           │           │                          │  (p. 181).
     „     │     „     │     „     │            „             │Lithodidae
           │           │           │                          │  (p. 181).
     „     │     „     │     „     │            „             │Hapalogastorinae
           │           │           │                          │  (p. 181).
     „     │     „     │     „     │            „             │Lithodinae
           │           │           │                          │  (p. 181).
 ──────────┼───────────┼───────────┼──────────────────────────┼─────────────────
     „     │     „     │=Brachyura=│Dromiacea (p. 183)        │Dromiidae
           │           │  (p. 181) │                          │  (p. 184).
     „     │     „     │     „     │            „             │Dynomenidae
           │           │           │                          │  (p. 184).
     „     │     „     │     „     │            „             │Homolidae
           │           │           │                          │  (p. 184).
 ──────────┼───────────┼───────────┼──────────────────────────┼─────────────────
     „     │     „     │     „     │Oxystomata (p. 185)       │Calappidae
           │           │           │                          │  (p. 187).
     „     │     „     │     „     │            „             │Leucosiidae
           │           │           │                          │  (p. 188).
     „     │     „     │     „     │            „             │Dorippidae
           │           │           │                          │  (p. 188).
     „     │     „     │     „     │            „             │Raninidae
           │           │           │                          │  (p. 188).
 ──────────┼───────────┼───────────┼──────────────────────────┼─────────────────
     „     │     „     │     „     │Cyclometopa (p. 188)      │Corystidae
           │           │           │                          │  (p. 190).
     „     │     „     │     „     │            „             │Atelecyclidae
           │           │           │                          │  (p. 190).
     „     │     „     │     „     │            „             │Cancridae
           │           │           │                          │  (p. 191).
     „     │     „     │     „     │            „             │Portunidae
           │           │           │                          │  (p. 191).
     „     │     „     │     „     │            „             │Xanthidae
           │           │           │                          │  (p. 191).
           │           │           │                          │Thelphusidae =
     „     │     „     │     „     │            „             │  Potamonidae
           │           │           │                          │  (p. 191).
 ──────────┼───────────┼───────────┼──────────────────────────┼─────────────────
     „     │     „     │     „     │Oxyrhyncha (p. 191)       │Maiidae (p. 193).
     „     │     „     │     „     │            „             │Parthenopidae
           │           │           │                          │  (p. 193).
     „     │     „     │     „     │            „             │Hymenosomatidae
           │           │           │                          │  (p. 193).
 ──────────┼───────────┼───────────┼──────────────────────────┼─────────────────
     „     │     „     │     „     │Catometopa (p. 193)       │Carcinoplacidae
           │           │           │                          │  (p. 195).
     „     │     „     │     „     │            „             │Gonoplacidae
           │           │           │                          │  (p. 195).
     „     │     „     │     „     │            „             │Pinnotheridae
           │           │           │                          │  (p. 195).
     „     │     „     │     „     │            „             │Grapsidae
           │           │           │                          │  (p. 196).
     „     │     „     │     „     │            „             │Gecarcinidae
           │           │           │                          │  (p. 196).
     „     │     „     │     „     │            „             │Ocypodidae
           │           │           │                          │  (p. 196).


                         =TRILOBITA= (p. 221).

                                       Families
                           _Agnostidae_ (p. 244).
                           _Shumardiidae_ (p. 245).
                           _Trinucleidae_ (p. 245).
                           _Harpedidae_ (p. 245).
                           _Paradoxidae_ (p. 246).
                           _Conocephalidae_ = _Conocoryphidae_ (p. 247).
                           _Olenidae_ (p. 247).
                           _Calymenidae_ (p. 247).
                           _Asaphidae_ (p. 249).
                           _Bronteidae_ (p. 249).
                           _Phacopidae_ (p. 249).
                           _Cheiruridae_ (p. 250).
                           _Proëtidae_ (p. 251).
                           _Encrinuridae_ (p. 251).
                           _Acidaspidae_ (p. 251).
                           _Lichadidae_ (p. 252).


                          =ARACHNIDA= (p. 255).

             =DELOBRANCHIATA = MEROSTOMATA= (pp. 258, 259).
 ───────────────────┬────────────────┬────────────────┬─────────────────
       Orders.      │  Sub-Orders.   │   Families.    │  Sub-Families.
 ───────────────────┴────────────────┼────────────────┼─────────────────
 =Xiphosura= (pp. 258, 259, 276)     │Xiphosuridae    │Xiphosurinae
                                     │  (p. 276)      │  (p. 276).
                  „                  │       „        │Tachypleinae
                                     │                │  (p. 276).
 ────────────────────────────────────┼────────────────┴─────────────────
 _Eurypterida = Gigantostraca_       │_Eurypteridae_ (p. 290).
   (pp. 258, 283)                    │

                   =EMBOLOBRANCHIATA= (pp. 258, 297).

 =Scorpionidea= (pp. 258, 297)       │Buthidae        │Buthinae
                                     │  (p. 306)      │  (p. 306).
                  „                  │       „        │Centrurinae
                                     │                │  (p. 306).
 ────────────────────────────────────┼────────────────┼─────────────────
                  „                  │Scorpionidae    │Diplocentrinae
                                     │  (p. 306)      │  (p. 307).
                  „                  │       „        │Urodacinae
                                     │                │  (p. 307).
                  „                  │       „        │Scorpioninae
                                     │                │  (p. 307).
                  „                  │       „        │Hemiscorpioninae
                                     │                │  (p. 307).
                  „                  │       „        │Ischnurinae
                                     │                │  (p. 307).
 ────────────────────────────────────┼────────────────┴─────────────────
                  „                  │Chaerilidae (p. 307).
 ────────────────────────────────────┼────────────────┬─────────────────
                  „                  │Chactidae       │Megacorminae
                                     │  (p. 307)      │  (p. 308).
                  „                  │       „        │Euscorpiinae
                                     │                │  (p. 308).
                  „                  │       „        │Chactinae
                                     │                │  (p. 308).
 ────────────────────────────────────┼────────────────┴─────────────────
                  „                  │Vejovidae (p. 308).
                  „                  │Bothriuridae (p. 308).
 ────────────────────────────────────┼──────────────────────────────────
 =Pedipalpi= (pp. 258, 308)          │Thelyphonidae (p. 312).
                  „                  │Schizonotidae = Tartaridae
                                     │  (p. 312).
 ────────────────────────────────────┼────────────────┬─────────────────
                                     │Tarantulidae =  │Tarantulinae
                  „                  │  Phrynidae     │  (p. 313).
                                     │  (p. 312)      │
                  „                  │       „        │Phrynichinae
                                     │                │  (p. 313).
                  „                  │       „        │Charontinae
                                     │                │  (p. 313).
 ────────────────────────────────────┼────────────────┴─────────────────
 =Araneae= (pp. 258, 314)            │Liphistiidae (p. 386).
 ────────────────────────────────────┼────────────────┬─────────────────
                                     │Aviculariidae = │Paratropidinae
                  „                  │  Mygalidae     │  (p. 387).
                                     │  (p. 386)      │
                  „                  │       „        │Actinopodinae
                                     │                │  (p. 387).
                  „                  │       „        │Miginae (p. 387).
                  „                  │       „        │Ctenizinae
                                     │                │  (p. 388).
                  „                  │       „        │Barychelinae
                                     │                │  (p. 389).
                  „                  │       „        │Aviculariinae
                                     │                │  (p. 389).
                  „                  │       „        │Diplurinae
                                     │                │  (p. 390).
 ────────────────────────────────────┼────────────────┴─────────────────
                  „                  │Atypidae (p. 390).
                  „                  │Filistatidae (p. 391).
                  „                  │Oecobiidae = Urocteidae (p. 392).
                  „                  │Sicariidae = Scytodidae (p. 393).
                  „                  │Hypochilidae (p. 393).
                  „                  │Leptonetidae (p. 393).
                  „                  │Oonopidae (p. 393).
                  „                  │Hadrotarsidae (p. 394).
 ────────────────────────────────────┼────────────────┬─────────────────
                  „                  │Dysderidae      │Dysderinae
                                     │  (p. 394)      │  (p. 394).
                  „                  │       „        │Segestriinae
                                     │                │  (p. 395).
 ────────────────────────────────────┼────────────────┴─────────────────
                  „                  │Caponiidae (p. 395).
                  „                  │Prodidomidae (p. 395).
 ────────────────────────────────────┼────────────────┬─────────────────
                  „                  │Drassidae       │Drassinae
                                     │  (p. 396)      │  (p. 396).
                  „                  │       „        │Clubioninae
                                     │                │  (p. 397).
                  „                  │       „        │Liocraninae
                                     │                │  (p. 397).
                  „                  │       „        │Micariinae
                                     │                │  (p. 397).
 ────────────────────────────────────┼────────────────┴─────────────────
                  „                  │Palpimanidae (p. 398).
                  „                  │Eresidae (p. 398).
                  „                  │Dictynidae (p. 398).
                  „                  │Psechridae (p. 399).
                  „                  │Zodariidae = Enyoidae (p. 399).
                  „                  │Hersiliidae (p. 400).
                  „                  │Pholcidae (p. 401).
 ────────────────────────────────────┼────────────────┬─────────────────
                  „                  │Theridiidae     │Argyrodinae
                                     │  (p. 401)      │  (p. 402).
                  „                  │       „        │Episininae
                                     │                │  (p. 402).
                  „                  │       „        │Theridioninae
                                     │                │  (p. 403).
                  „                  │       „        │Phoroncidiinae
                                     │                │  (p. 404).
                  „                  │       „        │Erigoninae
                                     │                │  (p. 404).
                  „                  │       „        │Formicinae
                                     │                │  (p. 405).
                  „                  │       „        │Linyphiinae
                                     │                │  (p. 405).
 ────────────────────────────────────┼────────────────┼─────────────────
                  „                  │Epeiridae       │Theridiosomatinae
                                     │  (p. 406)      │  (p. 407).
                  „                  │       „        │Tetragnathinae
                                     │                │  (p. 407).
                  „                  │       „        │Argiopinae
                                     │                │  (p. 408).
                  „                  │       „        │Nephilinae
                                     │                │  (p. 408).
                  „                  │       „        │Epeirinae
                                     │                │  (p. 408).
                  „                  │       „        │Gasteracanthinae
                                     │                │  (p. 409).
                  „                  │       „        │Poltyinae
                                     │                │  (p. 410).
                  „                  │       „        │Arcyinae
                                     │                │  (p. 410).
 ────────────────────────────────────┼────────────────┼─────────────────
                  „                  │Uloboridae      │Dinopinae
                                     │  (p. 410)      │  (p. 410).
                  „                  │       „        │Uloborinae
                                     │                │  (p. 410).
                  „                  │       „        │Miagrammopinae
                                     │                │  (p. 411).
 ────────────────────────────────────┼────────────────┴─────────────────
                  „                  │Archeidae (p. 411).
                  „                  │Mimetidae (p. 411).
 ────────────────────────────────────┼────────────────┬─────────────────
                                     │Thomisidae      │Thomisinae =
                  „                  │  (p. 412)      │  Misumeninae
                                     │                │  (p. 412).
                  „                  │       „        │Philodrominae
                                     │                │  (p. 413).
                  „                  │       „        │Sparassinae
                                     │                │  (p. 414).
                  „                  │       „        │Aphantochilinae
                                     │                │  (p. 414).
                  „                  │       „        │Stephanopsinae
                                     │                │  (p. 414).
                  „                  │       „        │Selenopinae
                                     │                │  (p. 414).
 ────────────────────────────────────┼────────────────┴─────────────────
                  „                  │Zoropsidae (p. 415).
                  „                  │Platoridae (p. 415).
 ────────────────────────────────────┼────────────────┬─────────────────
                  „                  │Agelenidae      │Cybaeinae
                                     │  (p. 415)      │  (p. 415).
                  „                  │       „        │Ageleninae
                                     │                │  (p. 416).
                  „                  │       „        │Hahniinae
                                     │                │  (p. 416).
                  „                  │       „        │Nicodaminae
                                     │                │  (p. 416).
 ────────────────────────────────────┼────────────────┴─────────────────
                  „                  │Pisauridae (p.416).
                  „                  │Lycosidae (p. 417).
                  „                  │Ctenidae (p. 418).
                  „                  │Senoculidae (p. 418).
                  „                  │Oxyopidae (p. 419).
                  „                  │Attidae = Salticidae (p. 419).
 ────────────────────────────────────┴──────────────────────────────────
 =Palpigradi= (pp. 258, 422).
 ───────────────────┬────────────────┬──────────────────────────────────
 =Solifugae =       │                │
   Solpugae=        │                │Galeodidae (p. 428).
   (pp. 258, 423)   │                │
 ───────────────────┼────────────────┼────────────────┬─────────────────
          „         │                │Solpugidae      │Rhagodinae
                    │                │  (p. 429)      │  (p. 429).
          „         │                │       „        │Solpuginae
                    │                │                │  (p. 429).
          „         │                │       „        │Daesiinae
                    │                │                │  (p. 429).
          „         │                │       „        │Eremobatinae
                    │                │                │  (p. 429).
          „         │                │       „        │Karshiinae
                    │                │                │  (p. 429).
 ───────────────────┼────────────────┼────────────────┴─────────────────
          „         │                │Hexisopodidae (p. 429).
 ───────────────────┼────────────────┼────────────────┬─────────────────
 =Chernetidea =     │                │                │
   Chernetes =      │                │Cheliferinae    │Cheliferidae
   Pseudoscorpiones=│                │  (p. 436).     │  (p. 436)
   (pp. 258, 430)   │                │                │
          „         │                │       „        │Garypinae
                    │                │                │  (pp. 436, 437).
          „         │                │       „        │Obisiinae
                    │                │                │  (pp. 436, 437).
 ───────────────────┼────────────────┼────────────────┴─────────────────
 =Podogona =        │                │
   Ricinulei=       │                │Cryptosteinmatidae (p.440).
   (pp. 258, 439)   │                │
 ───────────────────┼────────────────┼──────────────────────────────────
 =Phalangidea =     │=Cyphophthalmi= │
   Opiliones=       │  (p. 447)      │Sironidae (p. 448).
   (pp. 258, 440)   │                │
 ───────────────────┼────────────────┼──────────────────────────────────
                    │=Mecostethi =   │
          „         │  Laniatores=   │Phalangodidae (p. 448).
                    │  (p. 448)      │
          „         │       „        │Cosmetidae (p. 449).
          „         │       „        │Gonyleptidae (p. 449).
 ───────────────────┼────────────────┼────────────────┬─────────────────
                    │=Plagiostethi = │Phalangiidae    │Sclerosomatinac
          „         │  Palpatores=   │  (p. 449)      │  (p.449).
                    │  (p. 449)      │                │
          „         │       „        │       „        │Phalangiinae
                    │                │                │  (p. 450).
 ───────────────────┼────────────────┼────────────────┴─────────────────
          „         │       „        │Ischyropsalidae (p. 451).
          „         │       „        │Nemastomatidae (p. 451).
          „         │       „        │Trogulidae (p. 452).
 ───────────────────┼────────────────┼──────────────────────────────────
 =Acarina = Acari = │=Vermiformia=   │Eriophyidae = Phytoptidae
   Acaridea=        │  (p. 464)      │  (p. 464).
   (pp. 258, 454)   │                │
          „         │       „        │Demodicidae (p. 465).
 ───────────────────┼────────────────┼────────────────┬─────────────────
          „         │=Astigmata=     │Sarcoptidae     │Sarcoptinae
                    │  (p. 465)      │  (p. 466)      │  (p. 466).
          „         │       „        │       „        │Analgesinae
                    │                │                │  (p. 466).
          „         │       „        │       „        │Tyroglyphinae
                    │                │                │  (p. 466).
 ───────────────────┼────────────────┼────────────────┴─────────────────
          „         │=Metastigmata=  │Oribatidae (p. 467).
                    │  (p. 467)      │
 ───────────────────┼────────────────┼────────────────┬─────────────────
          „         │       „        │=Ixodoidea=     │Argasidae
                    │                │  (p. 468)      │  (p. 469).
          „         │       „        │       „        │Ixodidae
                    │                │                │  (p. 469).
 ───────────────────┼────────────────┼────────────────┼─────────────────
          „         │       „        │Gamasidae       │Gamasinae
                    │                │  (p. 470)      │  (p. 470).
          „         │       „        │       „        │Dermanyssinae
                    │                │                │  (p. 471).
 ───────────────────┼────────────────┼────────────────┴─────────────────
          „         │=Heterostigmata=│Tarsonemidae (p. 471).
                    │  (p. 471)      │
 ───────────────────┼────────────────┼──────────────────────────────────
          „         │=Prostigmata=   │Bdellidae (p. 471).
                    │  (p. 471)      │
          „         │       „        │Halacaridae (p. 472).
          „         │       „        │Hydrachnidae (p. 472).
 ───────────────────┼────────────────┼────────────────┬─────────────────
          „         │       „        │Trombidiidae    │Limnocharinae
                    │                │  (p. 472)      │  (p. 472).
          „         │       „        │       „        │Caeculinae
                    │                │                │  (p. 472).
          „         │       „        │       „        │Tetranychinae
                    │                │                │  (p. 472).
          „         │       „        │       „        │Cheyletinae
                    │                │                │  (p. 473).
          „         │       „        │       „        │Erythraeinae
                    │                │                │  (p. 473).
          „         │       „        │       „        │Trombidiinae
                    │                │                │  (p. 473).
 ───────────────────┼────────────────┼────────────────┴─────────────────
          „         │=Notostigmata=  │Opilioacaridae (p. 473).
                    │  (p. 473)      │


      Orders.

  =TARDIGRADA= (pp. 258, 477).
  =PENTASTOMIDA= (pp. 258, 488).


            =PYCNOGONIDA = PODOSOMATA = PANTOPODA= (p. 501).

                        Families.

            Decolopodidae (p. 531).
            Colossendeidae = Pasithoidae (p. 532).
            Eurycididae = Ascorhynchidae (p. 533).
            Ammotheidae (p. 534).
            Rhynchothoracidae (p. 535)
            Nymphonidae (p. 536).
            Pallenidae (p. 537).
            Phoxichilidiidae (p. 538).
            Phoxichilidae (p. 539).
            Pycnogonidae (p. 539).




                               CRUSTACEA


                          CHAPTERS I and III-VII

                                    BY

                       GEOFFREY SMITH, M.A. (OXON.)
                      Fellow of New College, Oxford


                                CHAPTER II

                                    BY

               THE LATE W. F. R. WELDON, M.A. (D.SC. OXON.)
 Formerly Fellow of St. John’s College, Cambridge, and Linacre Professor
                 of Human and Comparative Anatomy, Oxford




                               CHAPTER I
                     CRUSTACEA—GENERAL ORGANISATION


The Crustacea are almost exclusively aquatic animals, and they play a
part in the waters of the world closely parallel to that which insects
play on land. The majority are free-living, and gain their sustenance
either as vegetable-feeders or by preying upon other animals, but a
great number are scavengers, picking clean the carcasses and refuse that
litter the ocean, just as maggots and other insects rid the land of its
dead cumber. Similar to insects also is the great abundance of
individuals which represent many of the species, especially in the
colder seas, and the naturalist in the Arctic or Antarctic oceans has
learnt to hang the carcasses of bears and seals over the side of the
boat for a few days in order to have them picked absolutely clean by
shoals of small Amphipods. It is said that these creatures, when crowded
sufficiently, will even attack living fishes, and by sheer press of
numbers impede their escape and devour them alive. Equally surprising
are the shoals of minute Copepods which may discolour the ocean for many
miles, an appearance well known to fishermen, who take profitable toll
of the fishes that follow in their wake. Despite this massing together
we look in vain for any elaborate social economy, or for the development
of complex instincts among Crustacea, such as excite our admiration in
many insects, and though many a crab or lobster is sufficiently uncanny
in appearance to suggest unearthly wisdom, he keeps his intelligence
rigidly to himself, encased in the impenetrable reserve of his armour
and vindicated by the most powerful of pincers. It is chiefly in the
variety of structure and in the multifarious phases of life-history that
the interest of the Crustacea lies. Before entering into an examination
of these matters, it will be well to take a general survey of Crustacean
organisation, to consider the plan on which these animals are built, and
the probable relation of this plan to others met with in the animal
kingdom.

The Crustacea, to begin with, are a Class of the enormous Phylum
Arthropoda, animals with metamerically segmented bodies and usually with
externally jointed limbs. Their bodies are thus composed of a series of
repeated segments, which are on the whole similar to one another, though
particular segments may be differentiated in various respects for the
performance of different functions. This segmentation is apparent
externally, the surface of a Crustacean being divided typically into a
number of hard chitinous rings, some of which may be fused rigidly
together, as in the carapace of the crabs, or else articulated loosely.

Each segment bears typically a pair of jointed limbs, and though they
vary greatly in accordance with the special functions for which they are
employed, and may even be absent from certain segments, they may yet be
reduced to a common plan and were, no doubt, originally present on all
the segments.

Passing from the exterior to the interior of the body we find, generally
speaking, that the chief system of organs which exhibits a similar
repetition, or metameric segmentation, is the nervous system. This
system is composed ideally of a nervous ganglion situated in each
segment and giving off peripheral nerves, the several ganglia being
connected together by a longitudinal cord. This ideal arrangement,
though apparent during the embryonic development, becomes obscured to
some extent in the adult owing to the concentration or fusion of ganglia
in various parts of the body. The other internal organs do not show any
clear signs of segmentation, either in the embryo or in the adult; the
alimentary canal and its various diverticula lie in an unsegmented
body-cavity, and are bathed in the blood which courses through a system
of narrow canals and irregular spaces which surround all the organs of
the body. A single pair, or at most two pairs of kidneys are present.

The type of segmentation exhibited by the Crustacea is thus of a limited
character, concerning merely the external skin with its appendages, and
the nervous system, and not touching any of the other internal
organs.[1] In this respect the Crustacea agree with all the other
Arthropods, in the adults of which the segmentation is confined to the
exterior and to the nervous system, and does not extend to the
body-cavity and its contained organs; and for the same reason they
differ essentially from all other metamerically segmented animals,
_e.g._ Annelids, in which the segmentation not only affects the exterior
and the nervous system, but especially applies to the body-cavity, the
musculature, the renal, and often the generative organs. The Crustacea
also resemble the other Arthropoda in the fact that the body-cavity
contains blood, and is therefore a “haemocoel,” while in the Annelids
and Vertebrates the segmented body-cavity is distinct from the vascular
system, and constitutes a true “coelom.” To this important distinction,
and to its especial application to the Crustacea, we will return, but
first we may consider more narrowly the =segmentation= of the Crustacea
and its main types of variation within the group. In order to determine
the number of segments which compose any particular Crustacean we have
clearly two criteria: first, the rings or somites of which the body is
composed, and to each of which a pair of limbs must be originally
ascribed; and, second, the nervous ganglia.

Around and behind the region of the mouth there is very little
difficulty in determining the segments of the body, if we allow
embryology to assist anatomy, but in front of the mouth the matter is
not so easy.

In the Crustacea the moot point is whether we consider the paired eyes
and first pair of antennae as true appendages belonging to two true
segments, or whether they are structures _sui generis_, not homologous
to the other limbs. With regard to the first antennae we are probably
safe in assigning them to a true body-segment, since in some of the
Entomostraca, _e.g._ _Apus_, the nerves which supply them spring, not
from the brain as in more highly specialised forms, but from the
commissures which pass round the oesophagus to connect the dorsally
lying brain to the ventral nerve-cord. The paired eyes are always
innervated from the brain, but the brain, or at least part of it, is
very probably formed of paired trunk-ganglia which have fused into a
common cerebral mass; and the fact that under certain circumstances the
stalked eye of Decapods when excised with its peripheral ganglion[2] can
regenerate in the form of an antenna, is perhaps evidence that the
lateral eyes are borne on what were once a pair of true appendages.

Now, with regard to the segmentation of the body, the Crustacea fall
into three categories: the Entomostraca, in which the number of segments
is indefinite; the Malacostraca, in which we may count nineteen
segments, exclusive of the terminal piece or telson and omitting the
lateral eyes; and the Leptostraca, including the single recent genus
_Nebalia_, in which the segmentation of head and thorax agrees exactly
with that of the Malacostraca, but in the abdomen there are two
additional segments.

It has been usually held that the indefinite number of segments
characteristic of the Entomostraca, and especially the indefinitely
large number of segments characteristic of such Phyllopods as _Apus_,
preserves the ancestral condition from which the definite number found
in the Malacostraca has been derived; but recently it has been clearly
pointed out by Professor Carpenter[3] that the number of segments found
in the Malacostraca and Leptostraca corresponds with extraordinary
exactitude to the number determined as typical in all the other orders
of Arthropoda. This remarkable correspondence (it can hardly be
coincidence) seems to point to a common Arthropodan plan of
segmentation, lying at the very root of the phyletic tree; and if this
is so, we are forced to the conclusion that the Malacostraca have
retained the primitive type of segmentation in far greater perfection
than the Entomostraca, in some of which many segments have been added,
_e.g._ Phyllopoda, while in others segments have been suppressed, _e.g._
Cladocera, Ostracoda. It may be objected to this view of the primitive
condition of segmentation in the Crustacea that the Trilobites, which
for various reasons are regarded as related to the ancestral
Crustaceans, exhibit an indefinite and often very high number of
segments; but, as Professor Carpenter has pointed out, the oldest and
most primitive of Trilobites, such as _Olenellus_, possessed few
segments which increase as we pass from Cambrian to Carboniferous
genera.

The following table shows the segmentation of the body in the
Malacostraca, as compared with that of _Limulus_ (cf. p. 263), Insecta,
the primitive Myriapod _Scolopendrella_, and _Peripatus_. It will be
seen that the correspondence, though not exact, is very close,
especially in the first four columns, the number of segments in
_Peripatus_ being very variable in the different species.

          TABLE SHOWING THE SEGMENTATION OF VARIOUS ARTHROPODS
 ┌──┬──────────────┬───────────┬────────────┬─────────────┬────────────┐
 │  │Malacostraca. │_Limulus._ │  Insecta.  │ Myriapoda.  │_Peripatus._│
 │  │              │           │            │ (_Scolopen- │            │
 │  │              │           │            │  drella_).  │            │
 ├──┼──────────────┼───────────┼────────────┼─────────────┼────────────┤
 │ 1│Eyes          │Median eyes│Eyes        │             │            │
 │ 2│1st antennae  │Rostrum    │Antennae    │Feelers      │Feelers     │
 │ 3│2nd antennae  │Chelicerae │Intercalary │             │            │
 │  │              │           │segment     │             │            │
 │ 4│Mandibles     │Pedipalpi  │Mandibles   │Mandibles    │Mandibles   │
 │ 5│1st maxillae  │1st walking│Maxillulae  │Maxillulae   │1st jaw-claw│
 │  │              │legs       │            │             │            │
 │ 6│2nd maxillae  │2nd    „   │1st maxillae│1st maxillae │2nd jaw-claw│
 │  │              │  „        │            │             │            │
 │ 7│1st           │3rd    „   │2nd maxillae│2nd maxillae │1st leg     │
 │  │maxillipede   │  „        │            │             │            │
 │ 8│2nd           │4th    „   │1st leg     │1st leg      │2nd  „      │
 │  │maxillipede   │  „        │            │             │            │
 │ 9│3rd           │Chilaria   │2nd  „      │2nd  „       │3rd  „      │
 │  │maxillipede   │           │            │             │            │
 │10│1st ambulatory│Genital    │3rd  „      │3rd  „       │4th  „      │
 │  │              │operculum  │            │             │            │
 │11│2nd     „     │1st        │1st         │4th  „       │5th  „      │
 │  │              │gill-book  │abdominal   │             │            │
 │12│3rd     „     │2nd     „  │2nd     „   │6th  „       │6th  „      │
 │13│4th     „     │3rd     „  │3rd     „   │6th  „       │7th  „      │
 │14│5th     „     │4th     „  │4th     „   │7th  „       │8th  „      │
 │15│1st abdominal │5th     „  │5th     „   │8th  „       │9th  „      │
 │16│2nd     „     │No         │6th     „   │9th  „       │10th „      │
 │  │              │appendages │            │             │            │
 │17│3rd     „     │     „     │7th     „   │10th „       │11th „      │
 │18│4th     „     │     „     │8th     „   │11th „       │12th „      │
 │19│5th     „     │     „     │9th     „   │12th „       │13th „      │
 │20│6th     „     │     „     │10th    „   │Reduced limbs│14th „      │
 │21│     [4]      │     „     │            │Cercopods    │    [5]     │
 │  │Telson        │Telson     │Telson      │Telson       │Telson      │
 └──┴──────────────┴───────────┴────────────┴─────────────┴────────────┘

The =appendages= of the Crustacea exhibit a wonderful variety of
structure, but these variations can be reduced to at most two, and
possibly to one fundamental plan. In a typical Crustacean, besides the
paired eyes, which may be borne on stalks, possibly homologous to highly
modified limbs, there are present, first, two pairs of rod-like or
filamentous antennae, which in the adult are usually specialised for
sensory purposes, but frequently retain their primitive function as
locomotory limbs even in the adult, _e.g._ Ostracoda; while in the
Nauplius larva, found in almost all the chief subdivisions of the
Crustacea, the two pairs of antennae invariably aid in locomotion, and
the base of the second antennae is usually furnished with sharp biting
spines which assist mastication. Following the antennae is a pair of
mandibles which are fashioned for biting the food or for piercing the
prey, and posterior to these are two pairs of maxillae, biting organs
more slightly built than the mandibles, whose function it is to lacerate
the food and prepare it for the more drastic action of the mandibles. So
far, with comparatively few exceptions, the order of specialisation is
invariable; but behind the maxillae the trunk-appendages vary greatly
both in structure and function in the different groups.

As a general rule, the first or first few thoracic limbs are turned
forwards toward the mouth, and are subsidiary to mastication; they are
then called maxillipedes; this happens usually in the Malacostraca, but
to a much less extent in the Entomostraca; and in any case these
appendages immediately behind the maxillae never depart to any great
extent from a limb-like structure, and they may graduate insensibly into
the ordinary trunk-appendages. The latter show great diversity in the
different Crustacean groups, according as the animals lead a natatory,
creeping, or parasitic method of life; they may be foliaceous, as in the
Branchiopoda, or biramous, as in the swimming thoracic and abdominal
appendages of the Mysidae, or simply uniramous, as in the walking legs
of the higher Decapoda, and the clinging legs of various parasitic
forms.

Without going into detailed deviations of structure, many of which will
be described under the headings of special groups, it is clear from the
foregoing description and from Fig. 1 (p. 10), that three main types of
appendage can be distinguished: first, the foliaceous or multiramous;
second, the biramous; and, third, the uniramous.

We may dismiss the uniramous type with a few words: it is obviously
secondarily derived from the biramous type; this can be proved in detail
in nearly every case. Thus, the uniramous second antennae of some adult
forms are during the Nauplius stage invariably biramous, a condition
which is retained in the adult Cladocera. Similarly the uniramous
walking legs of many Decapoda pass through a biramous stage during
development, the outer branches or exopodites of the limbs being
suppressed subsequently, while the primitively biramous condition of the
thoracic limbs is retained in the adults of the Schizopoda, which
doubtless own a common ancestry with the Decapoda. The only Crustacean
limb which appears to be constantly uniramous both in larval and adult
life is the first pair of antennae.

We are reduced, therefore, to two types—the foliaceous and biramous. Sir
E. Ray Lankester,[6] in one of his most incisive morphological essays,
has explained how these two types are really fundamentally the same. He
compares, for instance, the foliaceous first maxillipede (Fig. 1, A), or
the second maxilla (Fig. 1, B) of a Decapod, _e.g._ _Astacus_, with the
foliaceous thoracic limb of _Branchipus_ (Fig. 1, D), and with the
typically biramous first maxillipede of a Schizopod (Fig. 1, F).

In each case there is present, on the outer edge of the limb, one or
more projections or epipodites which are generally specialised for
respiratory purposes, and may carry the gills. The 6th and 5th “endites”
in the foliaceous limb (Fig. 1, D) are compared with the exopodite and
endopodite respectively of the biramous limb, while the endites 4–1 of
the foliaceous limb are found in the basal joints of the biramous limb.
Lankester presumes that the biramous type of limb throughout has been
derived from the foliaceous type by the suppression of the endites 1–4,
as discrete rami, and the exaggerated development of the endites 5 and
6, as above indicated.

[Illustration:

  FIG. 1.—Appendages of Crustacea (=A-G=) and Trilobita (=H=). =A=,
    First maxillipede of _Astacus_; =B=, second maxilla of _Astacus_;
    =C=, second walking leg of _Astacus_; =D=, thoracic limb of
    _Branchipus_; =E=, first maxillipede of _Mysis_; =F=, first
    maxillipede of _Gnathophausia_; =G=, thoracic limb of _Nebalia_;
    =H=, thoracic limb of _Triarthrus_. _bp_, basipodite; _br_, bract;
    _cp_, carpopodite; _cxp_, coxopodite; _cx.s_, coxopoditic setae;
    _dp_, dactylopodite; _end_, endopodite; _ep_, epipodite; _ex_,
    exopodite; _ip_, ischiopodite; _mp_, meropodite; _pp_, propodite;
    1–6, the six endites.
]

The essential fact that the two types of limb are built on the same plan
may be considered as established; but it may be urged that the biramous
type represents this common plan more nearly than the foliaceous. It is,
at any rate, certain that in the maxillipedes of the Decapoda we witness
the conversion of the biramous type into the foliaceous by the expansion
of the basal joints concomitantly with the assumption by the
maxillipedes of masticatory functions. Thus in the Decapoda the first
maxillipede is decidedly foliaceous owing to the expanded “gnathobases”
(Fig. 1, A, _bp_, _cxp_), and the second maxillipedes are flattened,
with their basal joints somewhat expanded and furnished with biting
hairs; but in the “Schizopoda” (_e.g._ _Mysis_) the first maxillipede is
a typical biramous limb, though the expanded gnathobases in some forms
are beginning to project (Fig. 1, E), while the limb following, which
corresponds to the second maxillipede of Decapods, is simply a biramous
swimming leg. Besides this obvious conversion of a biramous into a
foliaceous limb, further evidence of the fundamental character of the
biramous type is found, first, in its invariable occurrence in the
Nauplius stage, which does not necessarily mean that the ancestors of
the Crustacea possessed this type of limb in the adult, but which does
imply that this type of limb was possessed at some period of life by the
common ancestral Crustacean; and, second, the limbs of the Trilobita, a
group which probably stands near the origin of the Crustacea, have been
shown by Beecher to conform to the biramous type (Fig. 1, H).
Furthermore, the thoracic limbs of Nebalia, an animal which combines
many of the characteristics of Entomostraca and Malacostraca, and is
therefore considered as a primitive type, despite their flattened
character, are really built upon a biramous plan (Fig. 1, G).

In conclusion, we may point out that this view of the Crustacean limb,
as essentially a biramous structure, agrees with the conclusion derived
from our consideration of the segmentation of the body, and points less
to the Branchiopoda as primitive Crustacea and more to some generalised
Malacostracan type.

So far we have shortly dealt with those systems of organs which are
clearly affected by the metameric segmentation of the body; we must now
expose the condition of the =body-cavity= to a similar scrutiny. If we
remove the external integument of a Crustacean, we find that the
internal organs do not lie in a spacious and discrete body-cavity, as is
the case in the Annelids and Vertebrates, but that they are packed
together in an irregular system of spaces (“haemocoel”) in communication
with the vascular system and containing blood. In the Entomostraca and
smaller forms generally, a definite vascular system hardly exists,
though a central heart and artery may serve to propel the blood through
the irregular lacunae of the body-cavity; but in the larger Malacostraca
a complicated system of arteries may be present which pour the blood
into fairly definitely arranged spaces surrounding the chief organs.
These spaces return the blood to the pericardium, and so to the heart
again through the apertures or ostia which pierce its walls.

This condition of the body-cavity or haemocoel is reproduced in the
adults of all Arthropods, but in some of them by following the
development we can trace the steps by which the true coelom is replaced
by the haemocoel. In the embryos of all Arthropods except the Crustacea,
a true closed metamerically segmented coelom is formed as a split in the
mesodermal embryonic layer of cells, distinct from the vascular system.
During the course of development the segmented coelomic spaces and their
walls give rise to the reproductive organs and to certain renal organs
in _Peripatus_, Myriapoda, and Arachnida (nephridia and coxal glands),
but the general body-cavity is formed as an extension of the vascular
system, which is laid down outside the coelom by a canaliculisation of
the extra-coelomic mesoderm. In the embryos of the Crustacea, however,
there is never at any time a closed segmented coelom, and in this
respect the Crustacea differ from all other Arthropods. The only clear
instance in which metamerically repeated mesodermal cavities have been
seen in the embryo Crustacean is that of _Astacus_; here Reichenbach[7]
states that in the abdomen segmental cavities are formed which
subsequently break down; but even in this instance no connexion has been
shown to subsist between these embryonic cavities and the reproductive
and excretory organs of the adult.

Since the connexion between the coelom and the excretory organs is
always a very close one throughout the animal kingdom, interest
naturally centres upon the =renal organs= in Crustacea, and it has been
suggested that these organs in Crustacea represent the sole remains,
with the possible exception of the gonads, of the coelom. Since, at any
rate, a part of the kidneys appears to be developed as a closed sac in
the mesoderm, and since they possess a possible segmental value, this
suggestion is plausible; but, on the other hand, since there are never
more than two pairs of kidneys, and since they are totally unconnected
with the gonads or with any other indication of a segmented coelom, the
suggestion remains purely hypothetical.

The renal organs of the Crustacea, excluding the Malpighian tubes
present in some Amphipods which open into the alimentary canal, and
resemble the Malpighian tubes of Insects, consist of two pairs—the
antennary gland, opening at the base of the second antenna, and the
maxillary gland, opening on the second maxilla. These two pairs of
glands rarely subsist together in the adult condition, though this is
said to be the case in _Nebalia_ and possibly _Mysis_; the antennary
glands are characteristic of adult Malacostraca[8] and the larvae of the
Entomostraca, while the maxillary glands (“shell-glands”) are present in
adult Entomostraca and larval Malacostraca, that is to say, the one pair
replaces the other in the two great subdivisions of the Crustacea. The
shell-gland of the Entomostraca is a simple structure consisting of a
coiled tube opening to the exterior on the external branch of the second
maxilla, and ending blindly in a dilated vesicle, the end-sac. The
antennary gland of the Malacostraca is usually more complicated: these
complications have been studied especially by Weldon,[9] Allen, and
Marchal[10] in the Decapoda. In a number of forms we have a tube opening
to the exterior at the base of the second antenna, and expanding within
to form a spacious bladder into which the coiled tubular part of the
kidney opens, while at the extremity of this coiled portion is the
vesicle called the end-sac. This arrangement may be modified; thus in
_Palaemon_ Weldon described the two glands as fusing together above and
below the oesophagus, the dorsal commissure expanding into a huge sac
stretching dorsally down the length of the body. This closed sac with
excretory functions thus comes to resemble a coelomic cavity, and the
view that it is really coelomic has indeed been upheld.

A modified form of this view is that of Vejdovský, who describes a
funnel-apparatus leading from the coiled tube into the end-sac of the
antennary gland of Amphipods; he regards the end-sac alone as
representing the coelom, while the funnel and coiled tube represent the
kidney opening into it.

Not very much is known of the development of these various structures.
Some authors have considered that both antennary and maxillary glands
are developed in the embryo from ectodermal inpushings, but the more
recent observations of Waite[11] on _Homarus americanus_ indicate that
the antennary gland at any rate is a composite structure, formed by an
ectodermal ingrowth which meets a mesodermal strand, and from the latter
are produced the end-sac and perhaps the tubular excretory portions of
the gland with their derivatives.

With regard to the possible metameric repetition of the renal organs, it
is of interest to note that by feeding _Mysis_ and _Nebalia_ on carmine,
excretory glands of a simple character were observed by Metschnikoff
situated at the bases of the thoracic limbs.

The _alimentary canal_ of the Crustacea is a straight tube composed of
three parts—a mid-gut derived from the endoderm of the embryo, and a
fore- and hind-gut formed by ectodermal invaginations in the embryo
which push into and fuse with the endodermal canal. The regions of the
fore- and hind-gut can be recognised in the adult by the fact of their
being lined with the chitinous investment which is continued over the
external surface of the body forming the hard exoskeleton, while the
mid-gut is naked. The chitinous lining of fore- and hind-gut is shed
whenever the animal moults. In the Malacostraca, in which a complicated
“gastric mill” may be present, the chitinous lining of this part of the
gut is thrown into ridges bearing teeth, and this stomach in the crabs
and lobsters reaches a high degree of complication and materially
assists the mastication of the food. The gut is furnished with a number
of secretory and metabolic glands; the so-called liver, which is
probably a hepatopancreas, opening into the anterior end of the mid-gut,
is directed forwards in most Entomostraca and backwards in the
Malacostraca, in the Decapoda developing into a complicated branching
organ which fills a large part of the thorax. In the Decapoda peculiar
vermiform caeca of doubtful function are present, a pair of which open
into the gut anteriorly where fore-passes into mid-gut, and a single
asymmetrically placed caecum opens posteriorly into the alimentary tract
where mid- passes into hind-gut.

The disposition of these caeca, marking as they do the morphological
position of fore-, mid-, and hind-gut, is of peculiar interest owing to
the variations exhibited. From some unpublished drawings of Mr. E. H.
Schuster, which he kindly lent me, it appears that in certain Decapods,
_e.g._ _Callianassa subterranea_, the length of the mid-gut between the
anterior and posterior caeca is very long; in _Carcinus maenas_ it is
considerable; in _Maia squinado_ it is greatly reduced, the caeca being
closely approximated; while in _Galathea strigosa_ the caeca are greatly
reduced, and the mid-gut as a separate entity has almost disappeared.
The relation of these variations to the habits of the different crabs
and to their modes of development is unknown.

The =reproductive organs= usually make their appearance as a small
paired group of mesodermal cells in the thorax comparatively late in
life; and neither in their early development nor in the adult condition
do they show any clear signs of segmentation or any connexion with a
coelomic cavity. The sexes are usually separate, but hermaphroditism
occurs sporadically in many forms, and as a normal condition in some
parasitic groups (see pp. 105–107). The adult gonads are generally
simple paired tubes, from the walls of which the germ-cells are
produced, and as these grow and come to maturity they fill up the
cavities of the tubes; special nutrient cells are rarely differentiated,
though in some cases (_e.g._ Cladocera) a few ova nourish themselves by
devouring their sister-cells (see p. 44). The oviducts and vasa
deferentia are formed as simple outgrowths from the gonadial tubes,
which acquire an opening to the exterior; they are usually poorly
supplied with accessory glands, the epithelium of the canals often
supplying albuminous secretions for cementing the eggs together, while
the lining of the vasa deferentia may be instrumental in the formation
of spermatophores for transferring large packets of spermatozoa to the
female. In the vast majority of Crustacea copulation takes place, the
male passing spermatophores or free spermatozoa into special receptacles
(spermathecae), or into the oviducts of the female. The spermatophores
are hollow chitinous structures in which the spermatozoa are packed;
they are often very large and assume characteristic shapes, especially
in the Decapoda.

The spermatozoa show a great variety of structure, but they conform to
two chief types—the filiform, which are provided with a long whip-like
flagellum; and the amoeboid, which are furnished with radiating
pseudopodia, and are much slower in their movements. The amoeboid
spermatozoa of some of the Decapoda contain in the cell-body a peculiar
chitinous capsule, and Koltzoff[12] has observed that when the
spermatozoon has settled upon the surface of the egg the chitinous
capsule becomes suddenly exceedingly hygroscopic, swells up, and
explodes, driving the head of the spermatozoon into the egg. We cannot
enter here into a description of the embryological changes by which the
egg is converted into the adult form. Crustacean eggs as a whole contain
a large quantity of yolk, but in some forms total segmentation occurs in
the early stages, which is converted later into the pyramidal type,
_i.e._ the blastomeres are arranged round the edge, and the yolk in the
centre is only partly segmented to correspond with them. The eggs during
the early stages of development are in almost all cases (except
Branchiura, p. 77, and _Anaspides_, p. 116) carried about by the female
either in a brood-pouch (Branchiopoda, Ostracoda, Cirripedia,
Phyllocarida, Peracarida), or agglutinated to the hind legs or some
other part of the body (Copepoda, Eucarida), or in a chamber formed from
the maxillipedes (Stomatopoda). Development may be direct, without a
complicated metamorphosis, or indirect, the larva hatching out in a form
totally different to the adult state, and attaining the latter by a
series of transformations and moults. The various larval forms will be
described under the headings of the several orders.

The =respiratory organs= are typically branchiae, _i.e._ branched
filamentous or foliaceous processes of the body-surface through which
the blood circulates, and is brought into close relation with the oxygen
dissolved in the water. In most of the smaller Entomostraca no special
branchiae are present, the interchange of gases taking place over the
whole body-surface; but in the Malacostraca the gills may reach a high
degree of specialisation. They are usually attached to the bases of the
thoracic limbs (“podobranchiae”), to the body-wall at the bases of these
limbs, often in two series (“arthrobranchiae”), and to the body-wall
some way above the limb-articulations (“pleurobranchiae”). In an ideal
scheme each thoracic appendage beginning with the first maxillipede
would possess a podobranch, two arthrobranchs, and a pleurobranch, but
the full complement of gills is never present, various members of the
series being suppressed in the various orders, and thus giving rise to
“branchial formulae” typical of the different groups.

After this brief survey of Crustacean organisation we may be able to
form an opinion upon the position of the Crustacea relative to other
Arthropoda, and upon the question debated some time ago in the pages of
_Natural Science_[13] whether the Arthropoda constitute a natural group.
The Crustacea plainly agree with all the other Arthropoda in the
possession of a rigid exoskeleton segmented into a number of somites, in
the possession of jointed appendages metamerically repeated, some of
which are modified to act as jaws; they further agree in the general
correspondence of the number of segments of which the body is
primitively composed; the condition of the body-cavity or haemocoel is
also similar in the adult state. An apparently fundamental difference is
found in the entire absence during development of a segmented coelom,
but since this organ breaks down and is much reduced in all adult
Arthropods, it is not difficult to believe that its actual formation in
the embryo as a distinct structure might have been secondarily
suppressed in Crustacea.

The method of breathing by gills is paralleled by the respiratory
structures found in _Limulus_ and Scorpions; the transition, if it
occurred, from branchiae to tracheae cannot, it is true, be traced, but
the separation of Arthropods into phyletically distinct groups of
Tracheata and Branchiata on this single characteristic is inadmissible.
On the whole the Crustacea may be considered as Arthropods whose
progenitors are to be sought for among the Trilobita, from whose near
relations also probably sprang _Limulus_ and the Arachnids.




                               CHAPTER II
CRUSTACEA (_CONTINUED_): ENTOMOSTRACA—BRANCHIOPODA—PHYLLOPODA—CLADOCERA—
                              WATER-FLEAS


                       SUB-CLASS I.—ENTOMOSTRACA.

The Entomostraca are mostly small Crustacea in which the segmentation of
the body behind the head is very variable, both in regard to the number
of segments and the kind of differentiation exhibited by those segments
and their appendages. An unpaired simple eye, known as the Nauplius eye
from its universal presence in that larval form, often persists in the
adult, and though lateral compound eyes may be present they are rarely
borne on movable stalks. In the adult the excretory gland
(“shell-gland”) opens on the second maxillary segment, but in the larval
state or early stages of development a second antennary gland may also
be present, which disappears in the adult. The liver usually points
forwards, and is simple and saccular in structure, and the stomach is
not complicated by the formation of a gastric mill. With the exception
of most Cladocera and Ostracoda the young hatch out in the Nauplius
state.


                      =Order I. Branchiopoda.=[14]

The Branchiopods are of small or moderate size, with flattened and
lobate post-cephalic limbs, and with functional gnathobases. Median and
lateral eyes are nearly always present. The labrum is large, and the
second maxillae are small or absent in the adult.

Branchiopods are found in every part of the world; a few are marine, but
the great majority are confined to inland lakes and ponds, or to
slowly-moving streams. The fresh waters, from the smallest pools to the
largest lakes, often swarm with them, as do those streams which flow so
slowly that the creatures can obtain occasional shelter among vegetation
along the sides and bottom without being swept away, while even rivers
of considerable swiftness contain some Cladocera. Several Branchiopods
are found in the brackish waters of estuaries, and some occur in lakes
and pools so salt that no other Crustacea, and few other animals of any
kind, can live in them. The great majority swim about with the back
downwards, collecting food in the ventral groove between their post-oral
limbs, and driving it forwards, towards the mouth, by movements of the
gnathobases (p. 10). The food collected in this way consists largely of
suspended organic mud, together with Diatoms and other Algae, and
Infusoria; the larger kinds, however, are capable of gnawing objects of
considerable size, _Apus_ being said to nibble the softer insect larvae,
and even tadpoles. Many Cladocera (e.g. _Daphnia_, _Simocephalus_) may
be seen to sink to the bottom of an aquarium, with the ventral surface
downwards, and to collect mud, or even to devour the dead bodies of
their fellows, while _Leptodora_ is said to feed upon living Copepods,
which it catches by means of its antennae.

The Branchiopoda fall naturally into two Sub-orders, the PHYLLOPODA
including a series of long-bodied forms, with at least ten pairs of
post-cephalic limbs, and the CLADOCERA with shorter bodies and not more
than six pairs of post-cephalic limbs.


                       =Sub-Order 1. Phyllopoda.=

The Phyllopoda include a series of genera which differ greatly in
appearance, owing to differences in the development of the carapace,
which are curiously correlated with differences in the position of the
eyes. Except in these points, the three families which the sub-order
contains are so much alike that they may conveniently be described
together.

In the BRANCHIPODIDAE the carapace is practically absent, being
represented only by the slight backward projection on each side of the
head which contains the kidney (Fig. 2); the paired eyes are supported
on mobile stalks, and project freely, one on either side of the head.

In the APODIDAE[15] the head is broad and depressed, the ventral side
being nearly flat, the dorsal surface convex; the hinder margin of the
head is indicated dorsally by a transverse cervical ridge, bounded by
two grooves, behind which the carapace projects backwards as a great
shield, covering at least half the body, but attached only to the back
of the head. In _Lepidurus productus_ the head and carapace together
form an oval expansion, deeply emarginate at the hinder, narrower end,
the sides of the emargination being toothed. The carapace has a strong
median keel. The kidneys project into the space between the folds of
skin which form the carapace, and their coils can be seen on each side,
the terminal part of each kidney-tube entering the head to open at the
base of the second maxilla. In all Branchiopoda with a well-developed
carapace the kidney is enclosed in it in this way, whence the older
anatomists speak of it as the “shell-gland.”

[Illustration:

  FIG. 2.—_Chirocephalus diaphanus_, female, × 5, Sussex. _D.O_, Dorsal
    organ; _H_, heart; _Ov_, ovary; _U_, uterus; _V_, external
    generative opening.
]

Associated with the development of the carapace, in this and in the next
family, is a remarkable condition of the lateral eyes, which are sessile
on the dorsal surface of the head, and near the middle line, the median
eye being slightly in front of them. During embryonic life a fold of
skin grows over all three eyes, so that a chamber is formed over them,
which communicates with the exterior by a small pore in front.

In the LIMNADIIDAE the body is laterally compressed, and the carapace is
so large that at least the post-cephalic part of the body, and generally
the head also, can be enclosed within it.

[Illustration:

  FIG. 3.—_Limnetis brachyura_, × 15. (After G. O. Sars.)
]

In _Limnetis_ (Fig. 3) the dorsal surface of the head is bent downwards
and is much compressed, the carapace being attached to it only for a
short distance near the dorsal middle line. The sides of the carapace
are bent downwards, and their margins can be pulled together by a
transverse adductor muscle, so that the whole structure forms an ovoid
or spheroidal case, from which the head projects in front, while the
rest of the body is entirely contained within it. When the adductor
muscle is relaxed the edges of the carapace gape slightly, like the
valves of a Lamellibranch shell, and food-particles are drawn through
the opening thus formed into the ventral groove by the movements of the
thoracic feet, locomotion being chiefly effected by the rowing action of
the second antennae, as in the Cladocera, to which all the Limnadiidae
present strong resemblances in their method of locomotion, in the
condition of the carapace, and in the form of the telson.

In _Limnadia_ and _Estheria_ the carapace projects not only backwards
from the point of attachment to the head, but also forwards, so that the
head can be enclosed by it, together with the rest of the body.

In all these genera the carapace is flexible along the middle dorsal
line; in _Estheria_ especially the softening of the dorsal cuticle goes
so far that a definite hinge-line is formed, and this, together with the
deposition of the lateral cuticle in lines concentrically arranged round
a projecting umbo, gives the carapace a strong superficial likeness to a
Lamellibranch shell, for which it is said to be frequently mistaken by
collectors.

The eyes of the Limnadiidae are enclosed in a chamber formed by a growth
of skin over them, as in Apodidae, but the pore by which this chamber
communicates with the exterior is even more minute than in _Apus_. The
paired eyes are so close together that they may touch (_Limnadia_,
_Estheria_) or fuse (_Limnetis_); they are farther back than in the
Apodidae, while the ventral curvature of the head causes the median eye
to lie below them. In all these points the eyes of the Limnadiidae are
intermediate between those of _Apus_ and those of the Cladocera.

=Dorsal Organ.=—A structure very characteristic of adult Phyllopods is
the “dorsal organ” (Figs. 2, 5, _D.O_), whose function is in many cases
obscure. It is always a patch of modified cephalic ectoderm, supplied by
a nerve from the anterior ventral lobe of the brain on each side; but
its characters, and apparent function, differ in different forms. In the
Branchipodidae the dorsal organ is a circular patch, far forward on the
surface of the head (Figs. 2, 5, _D.O_). Its cells are arranged in
groups, which remind one of the retinulae in a compound eye; each cell
contains a solid concretion, and the concretions of a group may be so
placed as to look like a badly-formed rhabdom. Claus,[16] who first
called attention to this structure in the Branchipodidae, regarded it as
a sense-organ. In Apodidae the dorsal organ is an oval patch of columnar
ectoderm, immediately behind the eyes; it is slightly raised above the
surrounding skin, and is covered by a very delicate cuticle (with an
opening to the exterior?), and below it is a mass of connective tissue
permeated by blood; Bernard has suggested that it is an excretory organ.

Most Limnadiidae resemble the Cladocera in the possession of a “dorsal
organ” quite distinct from the above; in _Limnetis_ and _Estheria_ it
has the form of a small pit, lined by an apparently glandular ectoderm,
and this is its condition in many Cladocera; in _Limnadia lenticularis_
it is a patch of glandular epithelium on a raised papilla. _Limnadia_
has been observed to anchor itself to foreign objects by pressing its
dorsal organ against them, and many Cladocera do the same thing; _Sida
crystallina_, for example, will remain for hours attached by its dorsal
organ to a waterweed or to the side of an aquarium. Structures
resembling a dorsal organ occur in the larvae of many other Crustacea,
but the presence of this organ in the adult is confined to Branchiopods,
and indeed in many Cladocera it disappears before maturity. It is
certain that the sensory and adhesive types of dorsal organ are not
homologous, especially as rudimentary sense-organs may exist on the head
of Cladocera together with the adhesive organ.

The =telson= differs considerably in the different genera. In the
Branchipodidae[17] the anus opens directly backwards; and the telson
carries two flattened backwardly directed plates, one on each side of
the anus, the margins of each plate being fringed with plumose setae. In
_Artemia_ the anal plates are rarely as large as in _Branchipus_, and
never have their margins completely fringed with setae; in _A. salina_
from Western Europe, and in _A. fertilis_ (Fig. 4, A) from the Great
Salt Lake of Utah, there is a variable number of setae round the apical
half of each lobe, but in specimens of _A. salina_ from Western Siberia
the number of setae may be very small, or they may be absent; in the
closely allied _A. urmiana_ from Persia the anal lobes are well
developed in the male, each lobe bearing a single terminal hair, but
they are altogether absent in the female. Schmankewitch and Bateson have
shown that there is a certain relation between the salinity of the water
in which _Artemia salina_ occurs and the condition of the anal lobes,
specimens from denser waters having on the whole fewer setae; the
relation is, however, evidently very complex, and further evidence is
wanted before any more definite statements can be made.

[Illustration:

  FIG. 4.—=A=, Ventral view of the anal region in _Artemia fertilis_,
    from the Great Salt Lake; =B=, ventral view of the telson and
    neighbouring parts of _Lepidurus productus_; =C=, side view of the
    telson and left anal lobe of _Estheria_ (sp.?).
]

In the Apodidae the anal lobes have the form of two-jointed cirri, often
of considerable length; in _Apus_ the anus is terminal, but in
_Lepidurus_ (Fig. 4, B) the dorsal part of the telson is prolonged
backwards, so as to form a plate, on the ventral face of which the anus
opens, much as in the Malacostraca.

In the Limnadiidae (Fig. 4, C) the telson is laterally compressed and
produced, on each side of the anus, into a flattened, upwardly curved
process, sharply pointed posteriorly, and often serrate; the anal lobes
are represented by two stout curved spines, while in place of the dorsal
prolongation of _Lepidurus_ we find two long plumose setae above the
anus. In the characters of the telson and anal lobes, as in those of the
head, the Limnadiidae approximate to the Cladocera. In _Limnetis
brachyura_ the ventral face of the telson is produced into a plate
projecting backwards below the anus, in a manner which has no exact
parallel among other Crustacea.

The =appendages= of the Phyllopoda are fairly uniform in character,
except those affected by the sexual dimorphism, which is usually great.

[Illustration:

  FIG. 5.—_Chirocephalus diaphanus_, male. Side view of head, showing
    the large second antenna, _A_{2}_, with its appendage _Ap_, above
    which is seen the filiform first antenna; _D.O_, dorsal organ;
    _E_{1}_, median eye.
]

[Illustration:

  FIG. 6.—_Chirocephalus diaphanus._ Second antenna of male, uncoiled.
]

Of the cephalic appendages, the first antennae are generally small, and
are never biramous; in _Branchipus_ and its allies they are simple
unjointed rods, in some species of _Artemia_ they are three-jointed, in
_Apus_ they are feebly divided into two joints, while in _Estheria_ they
are many-jointed. The second antennae are the principal organs of
locomotion in the Limnadiidae, where they are large and biramous; in all
other Phyllopoda they are uniramous in the female, being either
unjointed triangular plates as in _Chirocephalus_ (Fig. 2), or minute
vestigial filaments as in _Apus_, in which genus Zaddach, Huxley, and
Claus have all failed to find any trace of a second antenna in some
females. In the male Branchipodidae the second antennae are modified to
form claspers, by which the female is seized, the various degrees of
complication which these claspers exhibit affording convenient generic
characters. In _Branchinecta_ each second antenna is a thick,
three-jointed rod, the last joint forming a claw, while the second joint
is serrate on its inner margin; in _Branchipus_ the base is much
thickened, and bears on its inner side a large filament (perhaps
represented by the proximal tubercle of _Branchinecta_ and _Artemia_),
which looks like an extra antenna. In _Streptocephalus_ the terminal
joint of the antenna is bifid, and there is a basal filament like that
of _Branchipus_; in _Chirocephalus diaphanus_ (Figs. 5, 6) the main
branch of the antenna consists of two large joints, the terminal joint
being a strong claw with a serrated process at its base, while the
proximal joint bears two appendages on its inner side; one of these is a
small, subconical tubercle, the second is more complicated, consisting
of a main stem and five outgrowths. The main stem is many-jointed and
flexible, its basal joint being longer than the others, and bearing on
its outer side a large, triangular, membranous appendage, and four soft
cylindrical appendages, the main stem and its appendages being beset
with curious tubercles, ending in short spines, whose structure is not
understood. Except during the act of copulation this remarkable
apparatus is coiled on the inner side of the antennary claw, the jointed
stem being so coiled that it is often compared to the coiled proboscis
of a butterfly, and the triangular membrane folded like a fan beside it,
so that much of the organ is concealed, and the general appearance of
the head is that shown in Fig. 5. During copulation, the whole structure
is widely extended.

[Illustration:

  FIG. 7.—_Artemia fertilis._ Front view of the head of a male, showing
    the large second antennae, _A.2_; _A.1_, first antennae.
]

The males of _Artemia_ (Fig. 7) have the second antenna two-jointed, the
basal joint bearing an inner tubercle, the terminal joint being
flattened and bluntly pointed, its outer margin provided with a
membranous outgrowth. In _A. fertilis_ the breadth of the second joint
varies greatly, the narrower forms presenting a certain remote
resemblance to _Branchinecta_. In the males of _Polyartemia_ the second
antennae have a remarkable branched form not easily comparable with that
found in other Branchipodidae.

The cephalic jaws are fairly uniform throughout the order. The mandibles
have an undivided molar surface, and no palp; the first maxilla is very
generally a triangular plate, with a setose biting edge; mandibles and
maxillae are covered by the labrum. The second maxilla generally lies
outside the chamber formed by the labrum, and is a simple oval plate,
with or without a special process for the duct of the kidney.

The thoracic limbs, in front of the genital segments, are not as a rule
differentiated into anterior maxillipedes and posterior locomotive
appendages, as in higher forms; we have seen, however, that all these
limbs take part in the prehension of food, and except in the Limnadiidae
they all assist in locomotion. One of the middle thoracic legs of
_Artemia_ (Fig. 8, A) has a flattened stem, with seven processes on its
inner, and two on its outer margin. The gnathobase (_gn_) is large, and
fringed with long plumose setae, each of which is jointed; this is
followed by four smaller “endites” (or processes on the median side),
and then by two larger ones, the terminal endite (the sixth, excluding
the gnathobase) being very mobile and attached to the main stem by a
definite joint. On the outer side are two processes; a proximal “bract,”
a flat plate with crenate edges, partly divided by a constriction into
two, and a distal process, cylindrical and vascular, called by Sars and
others the “epipodite.” In other Branchipodidae we have essentially the
same condition, except that the fifth endite often becomes much larger
than in _Artemia_, throwing the terminal endite well over to the outer
edge of the limb; such a shift as this, continued farther, might well
lead to the condition found in the Limnadiidae, or Apodidae, where the
lobe which seems to represent the terminal endite of _Artemia_ is
entirely on the outer border of the limb, forming what most writers have
called the exopodite (Lankester’s “flabellum”).[18] In the two
last-named families the basal exite or bract of the Branchipodidae does
not appear to be represented.

[Illustration:

  FIG. 8.—=A=, Thoracic limb of _Chirocephalus diaphanus_; =B=,
    prehensile thoracic limb of male _Estheria_. _gn_, Gnathobase; 1–6,
    the more distal endites.
]

The limbs of the Apodidae are remarkable in two ways; those in front of
the genital opening (very constantly ten pairs) are not so nearly alike
as in most genera of the sub-order, the first two pairs especially
having the axis definitely jointed, while the endites are elongated and
antenniform; further, while the first eleven segments bear each a single
pair of limbs, as is usual among Crustacea, many of the post-genital
segments bear several pairs; thus in _Apus cancriformis_ there are
thirty-two post-cephalic segments in front of the telson, the first
eleven having each one pair of limbs, while the next seventeen have
fifty-two pairs between them, the last four segments having none.

In all the Phyllopoda some of the post-cephalic limbs are modified for
reproductive purposes; in the Branchipodidae the last two pairs (the
12th and 13th generally, the 20th and 21st in _Polyartemia_) are so
modified in both sexes. In the female these appendages fuse at an early
period of larval life, and surround the median opening of the generative
duct (Fig. 2); in the male the two pairs also fuse, but traces of the
limbs are left as eversible processes round the paired openings of the
vasa deferentia.

In the other families, one or more limbs of the female are adapted for
carrying or supporting the eggs. In the Apodidae the appendages of the
eleventh segment have the exopodite in the form of a rounded,
watchglass-shaped plate, fitting over a similarly shaped process of the
axis of the limb, so that a lens-shaped box is formed, into which the
eggs pass from the oviduct. In Limnadiidae the eggs are carried in
masses between the body and the carapace, and are kept in position by
special elongations of the exopodites of two or three legs, either those
near the middle of the thorax (_Estheria_, _Limnadia_), or at its
posterior end (_Limnetis_). In female _Limnetis_ the last thoracic
segments bear two remarkable lateral plates, which apparently also help
to support the eggs. In the male Limnadiidae, the first (_Limnetis_) or
the first two thoracic feet (_Limnadia_, _Estheria_) are prehensile
(Fig. 8, B).

=Alimentary Canal.=—The mouth of the Phyllopoda is overhung by the large
labrum, so that a kind of atrium is formed, outside the mouth itself, in
which mastication is performed; numerous unicellular glands, opening on
the oral face of the labrum, pour their secretion into the atrial
chamber, and may be called salivary, though the nature of their
secretion is not known. The mouth has commonly two swollen and setose
lips, running longitudinally forwards from the bases of the first
maxillae, and often wrapping round the blades of the mandibles. It leads
into a vertical oesophagus, which opens into a small globular stomach,
lying entirely within the head; the terminal part of the oesophagus is
slightly invaginated into the stomach, so that a valvular ring is formed
at the junction of the two. The stomach opens widely behind into a
straight intestine, which runs backwards to about the level of the
telson, where it joins a short rectum, leading to the terminal or
ventral anus. The stomach and intestine are lined by a columnar
epithelium, and covered by a thin network of circularly arranged
muscle-fibres; the rectum has a flatter epithelium, and radial muscles
pass from it to the body-wall, so that it can be dilated. The only
special digestive glands are two branched glandular tubes, situated
entirely within the head, which open into the stomach by large ducts,
one on each side. In _Chirocephalus_ the gastric glands are fairly small
and simple; in the Apodidae their branches are more complex and form a
considerable mass, filling all that portion of the head which is not
occupied by the nervous system and the muscles. Backwardly directed
gastric glands, like those of the higher Crustacea, are not found in
Branchiopods; both forms occur together in the genus _Nebalia_, but with
this exception the forwardly-directed glands are peculiar to
Branchiopods.

=Heart.=—In _Branchipus_ and its allies, and in _Artemia_, the heart
extends from the first thoracic segment to the penultimate segment of
the body, and is provided with eighteen pairs of lateral openings, one
pair in every segment through which it passes except the last; it is
widely open at its hinder end, and is prolonged in front for a short
distance as a cephalic aorta, the rest of the blood-spaces being
lacunar.

In most, at least, of the other Branchiopods, the heart is closed behind
and is shortened; in _Apus_ and _Lepidurus_ it only extends through the
first eleven post-cephalic segments, while in the Limnadiidae it is
shorter still, the heart of _Limnetis_ passing through four segments
only. In all cases there is a pair of lateral openings in every segment
traversed by the heart.

The blood of the Branchipodidae and Apodidae contains dissolved
haemoglobin, the quantity present being so small as to give but a faint
colour to the blood in _Branchipus_, while _Artemia_ has rather more,
and the blood of _Apus_ is very red. The only other Crustacea in which
the blood contains haemoglobin are the Copepods of the genus
_Lernanthropus_,[19] so that the appearance of this substance is as
irregular and inexplicable in Crustacea as in Chaetopods and Molluscs.

The =nervous system= of _Branchipus_ may be described as an illustration
of the condition prevailing in the group. The brain consists of two
closely united ganglia, in each of which three main regions may be
distinguished; a ventral anterior lobe, a dorsal anterior lobe, and a
posterior lobe. The ventral anterior lobes give off nerves to the median
eye, to the dorsal organ, and to a pair of curious sense-organs,
comparable with the larval sense-knobs of many higher forms, situated
one on each side of the median eye; in late larvae Claus describes the
terminal apparatus of each frontal sense-organ as a single large
hypodermic cell; W. K. Spencer[20] has lately described several terminal
cells, containing peculiar chitinous bodies, in the adult. The
homologous sense-organs of _Limnetis_ are apparently olfactory. The
dorsal anterior lobes give off the large nerves to the lateral eyes,
while the posterior lobes supply the first antennae. The oesophageal
connectives have a coating of ganglion-cells, and some of these form the
ganglion of the second antenna, the nerve to this appendage leaving the
connective just behind the brain. The post-oral nerve-cords are widely
separate, each of them dilating into a ganglion opposite every
appendage, the two ganglia being connected by two transverse
commissures. The ganglia of the three cephalic jaws, so often fused in
the higher Crustacea, are here perfectly distinct. Closely connected
with each thoracic ganglion is a remarkable unicellular gland, opening
to the exterior near the middle ventral line; it is conceivable that
these cells may be properly compared with the larval nephridia of a
Chaetopod,[21] but no evidence in support of such a comparison has yet
been adduced.

Behind the genital segments, where there are no limbs, the nerve-cords
run backwards without dilating into segmental ganglia, except in the
anterior two abdominal segments where small ganglionic enlargements
occur. In Apodidae, on the other hand, those segments which carry more
than one pair of appendages have as many pairs of ganglia, united by
transverse commissures, as they have limbs.

A stomatogastric nervous system exists in _Apus_, where a nerve arises
on each side from the first post-oral commissure, and runs forward to
join its fellow of the opposite side on the anterior wall of the
oesophagus. From the loop so formed a larger median and a series of
smaller lateral nerves pass to the wall of the alimentary canal. A
second nerve to the oesophagus is given off from the mandibular ganglion
of each side.

=Reproductive Organs.=—In _Chirocephalus_ the ovaries (Fig. 2, _Ov_) are
hollow epithelial tubes, lying one on each side of the alimentary canal,
and extending from the sixth abdominal segment forwards to the level of
the genital opening; at this point the two ovaries are continuous with
ducts, which bend sharply downwards and open into the single uterus
contained within the projecting egg-pouch and opening to the exterior at
the apex of that organ. Short diverticula of the walls of the uterus
receive the ducts of groups of unicellular glands, the bodies of which
contain a peculiar opaque secretion, said to form the eggshells. In
Apodidae the ovaries are similar in structure, but they are much larger
and branch in a complex manner, while each ovary opens to the exterior
independently of the other in the eleventh post-cephalic segment;
nothing like the median uterus of the Branchipodidae being formed. The
epithelium of the ovarian tubes proliferates, and groups of cells are
formed; one becoming an ovum, the others being nutrient cells like those
which will be more fully described in the Cladocera.

In _Chirocephalus_ the testes are tubes similar in shape and position to
the ovaries, each communicating in front with a short vas deferens,
which dilates into a vesicula seminalis on its way to the eversible
penis; an essentially similar arrangement is found in all
Branchipodidae, but in Apodidae and Limnadiidae there is no penis.

All the Branchiopoda are dioecious,[22] and many are parthenogenetic.
Among Branchipodidae _Artemia_ is the only genus known to be
parthenogenetic, but parthenogenesis is common in all Apodidae, while
the males of several species of _Limnadia_ are still unknown, although
the females are sometimes exceedingly common. In _Artemia_, generations
in which the males are about as numerous as the females seem to
alternate fairly quickly with others which contain only parthenogenetic
females; in _Apus_ males are rarely abundant, and often absent for long
periods; during five consecutive years von Siebold failed to discover a
male in a locality in Bavaria, though he examined many thousands of
individuals; near Breslau he found on one occasion about 11 per cent of
males (114 in 1026), but in a subsequent year he found less than 1 per
cent; the greatest recorded percentage of males is that observed by
Lubbock in 1863, when he found 33 males among 72 individuals taken near
Rouen.

The eggs of most genera can resist prolonged periods of desiccation, and
indeed it seems necessary for the development of many species that the
eggs should be first dried and afterwards placed in water. Many eggs
(_e.g._ of _Chirocephalus diaphanus_ and _Branchipus stagnalis_) float
when placed in water after desiccation, the development taking place at
the surface of the water.

=Habitat.=—All the Phyllopoda, except _Artemia_, are confined to
stagnant shallow waters, especially to such ponds as are formed during
spring rains, and dry up during the summer. In waters of this kind the
species of _Branchipus_, _Apus_, etc., develop rapidly, and produce
great numbers of eggs, which are left in the dried mud at the bottom
after evaporation of the water, where they remain quiescent until a
fresh rainy season. The mud from the beds of such temporary pools often
contains large numbers of eggs, which may be carried by wind, on the
legs of birds, and by other means, to considerable distances. Many
exotic species have been made known to European naturalists by their
power of hatching out when mud brought home by travellers is placed in
water. The water of stagnant pools quickly dissolves a certain quantity
of solid matter from the soil, and often receives dissolved solids
through surface drainage from the neighbouring land; such salts may
remain as the water evaporates, so that the water which remains after
evaporation has proceeded for some time may be very sensibly denser than
that in which the Branchiopods were hatched; these creatures must
therefore be able to endure a considerable increase in the salinity of
the surrounding waters during the course of their lives. My friend Mr.
W. W. Fisher points out that the plants present in such a pond would
often precipitate the carbonate of lime, so that this might be removed
as evaporation went on, but that chlorides would probably remain in
solution; from analyses which Mr. Fisher has been kind enough to make
for me, it is seen that this happened in a small aquarium in my
laboratory, in which _Chirocephalus diaphanus_ lived for four months. In
April, mud from the dry bed of a pond, known to contain eggs of
_Chirocephalus_, was placed in this aquarium in Oxford, and water was
added from the tap. Oxford tap-water contains about 0·3 grm. salts per
litre, the chlorine being equivalent to 0·023 grm. NaCl. Water was added
from time to time during May and June, but in July evaporation was
allowed to proceed unchecked. At the end of July there was about half
the original volume of water, the _Chirocephalus_ being still active;
the residue contained 0·96 grm. dissolved solids per litre, with
chlorine equal to 0·19 grm. NaCl, so that the percentage of chlorides
was about eight times the initial percentage, but there were only three
and a fifth times the original amount of total solid matter in solution,
the carbonate of lime having precipitated as a visible film.

Some species of _Branchipus_ (e.g. _B. spinosus_, M. Edw.) and of
_Estheria_ (_E. macgillivrayi_, Baird, _E. gubernator_, Klutzinger)
occur in salt pools, but _Artemia_ flourishes in waters beside whose
salinity that endured by any other Branchiopod is insignificant. In the
South of Europe, _Artemia salina_ may be found in swarms, as it used to
be found in Dorsetshire, in the shallow brine-pans from which salt is
commercially prepared; Rathke quotes an analysis showing that a pool in
the Crimea contained living _Artemia_ when the salts in solution were
271 grms. per litre, and the water was said to have the colour and
consistency of beer.

The behaviour of the animals in the water differs a little; in normal
feeding all the species swim with the back downwards, as has already
been said; the Branchipodidae rarely settle on the ground, or on foreign
objects, but the Apodidae occasionally wriggle along the bottom on their
ventral surface, and _Estheria_ burrows in mud.

The greater number of species are found in pools in flat, low-lying
regions, and many appear to be especially abundant near the sea; _Apus
cancriformis_ has, however, been found in Armenia at 10,000 feet above
sea level.

Wells and underground waters do not generally contain Phyllopods; but a
species of _Branchipus_ and one of _Limnetis_, both blind, have been
described from the caves of Carniola.

One of the many puzzles presented by these creatures is the erratic way
in which they are scattered through the regions they inhabit; a single
small pond, a few yards or less in diameter, may be the only place
within many miles in which a given species can be found; in this pond it
may, however, appear regularly season after season for some time, and
then suddenly vanish.

Geographically, the Phyllopoda are cosmopolitan, representatives of
every family and of some genera (e.g. _Streptocephalus_, _Lepidurus_,
_Estheria_) being found in every one of the great zoological regions,
though a few aberrant genera are of limited range, thus _Polyartemia_ is
known only from the northern Palaearctic and Nearctic regions,
_Thamnocephalus_ only from the Central United States. The genus
_Artemia_ is not at present known in Australia.[23] The only recorded
British species are _Chirocephalus diaphanus_, _Artemia salina_, and
_Apus cancriformis_,[24] but other continental islands, for example the
West Indian group, are better supplied. The distribution of the species
is very imperfectly known, but on the whole every main zoological region
seems to have its own peculiar species, which do not pass beyond its
boundaries. _Branchinecta paludosa_ and _Lepidurus glacialis_ are
circumpolar, both occurring in Norway, in Lapland, in Greenland, and in
Arctic North America; but with these exceptions the Palaearctic and
Nearctic species seem to be distinct. The European species _Apus
cancriformis_ occurs in Algiers, but the relations between the species
of Northern Africa as a whole and those of Southern Europe on the one
hand, or of Central and Southern Africa on the other, have yet to be
worked out.

The soft-bodied Branchipodidae are not known in the fossil
condition;[25] an _Apus_, closely related to the modern _A.
cancriformis_, has been found in the Trias, but the most numerous
remains have been left, as might be expected, by the hard-shelled
Limnadiidae; carapaces, closely resembling those of the modern
_Estheria_, are known in beds of all ages from the Devonian period to
recent times; these carapaces are in several cases associated with
fossils of an apparently marine type. None of the fossil species differ
in any important characters from those now living, so that the
Phyllopoda have existed in practically their present form for an
enormously long period; this fact, and the evidence that species of
existing genera were at one time marine, explain the wide distribution
of animals at present restricted to a remarkably limited range of
environmental conditions.


               =Summary of the Characters of the Genera.=

  SUB-ORDER PHYLLOPODA.—Branchiopoda with an elongated body, provided
    with at least ten pairs of post-cephalic limbs, the heart extending
    through four or more thoracic segments, and having at least four
    pairs of ostia.

  =Fam. 1. Branchipodidae.=[26]—Carapace rudimentary, eyes stalked; the
    second antennae flat and unjointed in the female, jointed and
    prehensile in the male; female generative opening single; telson not
    laterally compressed, bearing two flattened lobes, or none. The
    heart extending through the thorax and the greater part of the
    abdomen.

      A. Eleven pairs of praegenital ambulatory limbs.

          _a._ Abdomen of six well-formed segments and a telson; anal
            lobes well formed, their margins setose.

              _Branchinecta_, Verrill—Second antennae of ♂ without
                lateral appendages; ovisac of ♀ elongated. _B.
                paludosa_, O. F. Müll.—Circumpolar.

              _Branchiopodopsis_, G. O. Sars[27]—Second antennae of ♂ as
                in _Branchinecta_; ovisac of ♀ short. _B. hodgsoni_, G.
                O. Sars—Cape of Good Hope.

              _Branchipus_, Schaeffer—Second antennae of ♂ with simple
                internal filamentous appendage. _B. stagnalis_, Linn.—
                Central Europe.

              _Streptocephalus_, Baird—Second antennae of ♂ 3–jointed,
                the last joint bifid; an external filamentous appendage.
                _S. torvicornis_, Wagn., Poland.

              _Chirocephalus_, Prévost—Second antennae of ♂ 3–jointed,
                with a jointed internal appendage, which bears secondary
                processes, four cylindrical and one lamellar. _C.
                diaphanus_, Prévost (Fig. 2, p. 20).—Britain, Central
                Europe.

          _b._ Abdominal segments five or fewer, and a telson. Anal
            lobes small or 0, sparsely or not at all setose.

              _Artemia_, Leach—Second antennae of ♂ without filamentous
                appendage, 2–jointed, the second joint lamellar. _A.
                salina_, Linn.—Brine pools of the Palaearctic region.

          _c._ Hinder abdominal segments united with telson to form a
            fin; anal lobes absent.

              _Thamnocephalus_, Packard—Head with a branched median
                process of unknown nature. Only species _T. platyurus_,
                Packard—Kansas, U.S.A.

      B. Nineteen pairs of praegenital ambulatory limbs.

              _Polyartemia_, Fischer—Second antennae of ♂ forcipate;
                ovisac of ♀ very short. Only species _P. forcipata_,
                Fisch.

    =Fam. 2. Apodidae.=[28]—Carapace well developed as a depressed
    shield, covering at least half the body. Eyes sessile, covered; no
    male clasping organs; anal lobes long, jointed cirri.

              _Apus_, Scopoli—Telson not produced backwards over the
                anus; endites of first thoracic limb very long. _A.
                cancriformis_, Schaeffer—Britain, Europe, Algiers,
                Tunis. _A. australiensis_, Central Australia.

              _Lepidurus_, Leach—Telson produced backwards to form a
                plate above the anus; endites of first thoracic limb
                short. _L. productus_, Bosc.—Central Europe. _L.
                viridis_, Southern Australia, New Zealand, _L.
                patagonicus_, Bergh, Argentines.

    =Fam. 3. Limnadiidae.=—Body compressed; carapace in the form of a
    bivalve shell, the two halves capable of adduction by means of a
    strong transverse muscle; second antennae biramous, alike in both
    sexes; in the male, the first or the first and second thoracic limbs
    prehensile; telson laterally compressed.

      A. Only the first thoracic limbs prehensile in the male; the
        carapace spheroidal, without lines of growth; head not included
        within the carapace-chamber.

              _Limnetis_, Lovén—Compound eyes fused; anal spines absent;
                ambulatory limbs 10–12. _L. brachyura_, O. F. Müll (Fig.
                3, p. 21).—Norway, Central Europe.

      B. The first and second thoracic limbs prehensile in the male;
        carapace distinctly bivalve, enclosing the head, with concentric
        lines of growth round a more or less prominent umbo.

              _Eulimnadia_, Packard—Carapace narrowly ovate, with few
                (4–5) lines of growth. _E. mauritani_, Guérin—Mauritius.
                _E. texana_, Packard—Texas, Kansas.

              _Limnadia_, Brongniart—Carapace broadly ovate, with
                numerous lines of growth, without distinct umbones; _L.
                lenticularis_, Linn.—Northern and Central Europe.

              _Estheria_, Rüppell—Carapace with well-marked umbones and
                numerous lines of growth, oval; _E. tetraceros_,
                Kryneki—Central Europe.

              _Leptestheria_,[29] G. O. Sars—Carapace compressed,
                oblong. Rostrum with a movable spine; thoracic limbs
                with accessory lappet on the exopodite. _L. siliqua_, G.
                O. Sars—Cape Town.

              _Cyclestheria_,[30] G. O. Sars. _C. hislopi_, Baird—
                Queensland, India, East Africa, Brazil.


                       =Sub-Order 2. Cladocera.=

The Cladocera are short-bodied Branchiopods, with not more than six
pairs of thoracic limbs. The second antennae are important organs of
locomotion, and are nearly always biramous; the first antennae are
small, at least in the female; the second maxillae are absent in the
adult. The carapace may extend backwards so as to enclose the whole
post-cephalic portion of the body, or may be reduced to a small dorsal
brood-pouch, leaving the body uncovered.

The Cladocera or “Water-fleas” are never of great size; _Leptodora
hyalina_, the largest, is only about 15 mm. long, while many Lynceidae
are not more than 0·1 or 0·2 mm. in length.

The =head= is bent downwards in all the Cladocera, so that parts which
are morphologically anterior, such as the median eye and the first
antennae, lie ventral to or even behind the compound eyes and the second
antennae (_cf._ Fig. 10).

The compound lateral eyes fuse at an early period of embryonic life, so
that they form a single median mass in the adult, over which a fold of
ectoderm grows, to make a chamber over the eye, like that found in the
Limnadiidae, except that it is completely closed. The fused eyes are
generally large and conspicuous; in some deep-water forms the retinular
elements of the dorsal portion are larger than those of the ventral
(e.g. _Bythotrephes_, Fig. 13). In one or two species which live at very
great depths, or in caves, the eyes are altogether absent.

The appendages of the head are fairly uniform, the most variable being
the first antennae. In the females of many genera the first antennae are
short and immovable, consisting of a single joint, with a terminal bunch
of sensory hairs, and often a long lateral hair, as in _Simocephalus_
(Figs. 9, 10), _Daphnia_, etc. In the female _Moina_ (Fig. 16) they are
movable, as they are in _Ceriodaphnia_ and some others; in _Bosmina_
(Fig. 22) and many Lyncodaphniidae they are elongated and imperfectly
divided into joints by rings of spines, while in _Macrothrix_ they are
flattened plates. In the males the first antennae are elongated and
mobile (_cf._ Figs. 11, 19).

[Illustration:

  FIG. 9.—_Simocephalus vetulus_, female. Ventral view, without the
    carapace; _A_{1}_, _A_{2}_, first and second antennae; _For_, head;
    _Md_, mandible; _Te_, telson; I-IV, first to fourth thoracic
    appendages.
]

The second antennae, the chief organs of locomotion, are biramous in all
genera except _Holopedium_; the number of joints in each ramus, and the
number of the long plumose hairs with which they are provided, are
remarkably constant in whole series of genera, and are therefore useful
for purposes of classification. The creatures row themselves by quick
strokes of these appendages, the movement being slow and irregular in
the rounder forms, such as _Simocephalus_ or _Daphnia_, rapid and well
directed in such elongated lacustrine forms as _Bythotrephes_ or
_Leptodora_.

The mandibles have no palp; the first maxillae are very small, and the
second maxillae are absent (Fig. 9).

The =carapace= varies very much. In most genera (the CALYPTOMERA of
Sars) it is a large, backwardly-projecting fold of skin, bent downwards
at the sides so as to form a bivalve shell, enclosing the whole
post-cephalic portion of the body, as in _Simocephalus_ (Fig. 10). The
eggs are laid into the space between the carapace and the dorsal part of
the thorax, both the carapace and the thorax itself being often modified
for their protection and nutrition. In a few forms, the GYMNOMERA of
Sars, the carapace serves only as a brood-pouch, which is distended when
eggs are laid, but collapses to an inconspicuous appendage at the back
of the head when it is empty (e.g. _Leptodora_, Fig. 24, _Bythotrephes_,
Fig. 13). In the Calyptomera the surface of the carapace is frequently
provided with a series of ridges, which may be parallel, rarely
branching, as in _Simocephalus_; or in two sets which cross nearly at
right angles, as in _Daphnia_; or so arranged as to form a hexagonal
pattern, as in _Ceriodaphnia_. In a few forms the whole surface is
irregularly covered with spines or scales. The hinder edge of the
carapace is often produced into a median dorsal spine (_Daphnia_, Fig.
19), or more rarely there are two spines, one at each ventro-lateral
corner (_Scapholeberis_, Fig. 20).

[Illustration:

  FIG. 10.—_Simocephalus vetulus_, × 30. Side view of female, showing
    the arrangement of the principal organs. _A.2_, Second antenna;
    _C.S_, cervical suture; _E_, fused compound eyes; _H_, heart; _L_,
    forwardly-directed gastric caeca; _N_, dorsal organ.
]

The cuticle of the carapace is often separated from that of the head by
a cervical suture, as in _Simocephalus_ (Fig. 10, _C.S._) and near the
line of demarcation many forms exhibit patches of glandular ectoderm
which seem to be homologous with the dorsal adhesive organs of the
Limnadiidae. The commonest condition is that of a median dorsal pit
(Fig. 10, _N._) by means of which the animal can fix itself to foreign
objects. Certain forms may remain for long periods of time attached by
the dorsal organ to plants, or to the sides of an aquarium, the only
movement being a slow vibration of the feet, by which a current of
water, sufficiently rapid for respiratory purposes, is established round
it.[31] In _Sida crystallina_ (Fig. 11) the dorsal organ is represented
by three structures; in front there is a median raised patch (_N.m_) of
columnar ectoderm, containing concretions like those described in the
Branchipodidae, and behind this is a pair of cup-shaped organs (_N.e_),
with raised margins.

[Illustration:

  FIG. 11.—_Sida crystallina_, male, × 27. Oxford. _A.1_, Elongated
    first antenna; _N.e_, paired element of dorsal organ; _N.m_, median
    element of dorsal organ; _Te_, testes; ♂, opening of vas deferens.
]

The fold of skin which forms the carapace contains the coils of the
single pair of kidneys, and it forms an important organ of respiration,
partly from the great size of the blood-vessels it contains, and partly
from the presence of red, blue, or brown respiratory pigments in the
tissue of the skin itself.

In most Cladocera the cuticle of the carapace is cast at every ecdysis,
with that of other parts of the body; but in _Iliocryptus_ and a few
others it remains after each moult, giving the carapace an appearance of
“lines of growth,” like that seen in many Limnadiidae.

The segmentation of the =body= behind the head is obscure, but we can
generally recognise (1) a thorax, of as many segments as there are pairs
of limbs; (2) an abdomen of three segments; and (3) a telson.

The thoracic limbs of the Calyptomera are flattened, and resemble those
of the Phyllopoda; as a type we may examine the third thoracic limb of
_Simocephalus_ (Fig. 12, C), in which the axis bears a large setose
gnathobase (_Gn_) on its inner edge, followed by two small endites; the
terminal process, or exopodite (_Ex_) is a large flattened plate, with
six long plumose hairs on its edge. The outer margin of the axis bears a
bract (_Br_) and an epipodite.

In _Simocephalus_, as in the other Daphniidae, there are five pairs of
thoracic limbs, of which the third and fourth are alike; in the female
each limb of the first pair consists of a jointed axis, with strong
biting hairs on the inner border, and a rudimentary epipodite (Fig. 12,
A), the second limb being more like the third, but with a more prominent
gnathobase and a narrower exopodite (B), while the limbs of the fifth
pair have the gnathobase and the exopodite filamentous (D).

In the Sididae there are six pairs of thoracic limbs, which are nearly
alike in the female; in the Bosminidae there are six pairs, the first
two modified for prehension, the last much reduced.

[Illustration:

  FIG. 12.—Thoracic limbs of female _Simocephalus vetulus_. =A=, The
    first; =B=, the second; =C=, the third; =D=, the fifth. _Br_, Bract;
    _Ep_, epipodite; _Ex_, exopodite; _Gn_, gnathobase.
]

In the male, the first thoracic limb is usually provided with a long
sensory process and a prehensible hook (Figs. 11, 19).

In the Gymnomera the limbs are cylindrical, jointed rods, with a
gnathobase on the inner side in the Polyphemidae, but not in
_Leptodora_. The number varies from four to six pairs.

The abdomen bears no appendages. The telson is compressed in the
Calyptomera, and is produced into two flattened plates, one on each side
of the anal opening. The backwardly directed margins of these plates are
commonly serrated, and the lower corner of each is produced into a
curved spine, which carries secondary teeth. The number and arrangement
of these teeth, though often extremely variable in the same species, are
used extensively as specific characters. Above the anus the telson
commonly bears two long plumose hairs, which are directed backwards.

[Illustration:

  FIG. 13.—_Bythotrephes cederströmii_, female, × 20, North Wales, from
    a specimen found by A. D. Darbishire. _Car_, carapace.
]

In the Gymnomera the telson is not bilaterally compressed, and it may be
produced into a long spine, dorsal to the anus (e.g. _Bythotrephes_,
Fig. 13).

The =alimentary canal= is extremely simple. The labrum is large, and
forms a chamber above the mouth, into which food is driven by the limbs,
as in the Phyllopoda, food being taken while the animal swims or lies on
its back. The oesophagus runs vertically to join a small stomach, which
bends sharply backwards and passes gradually into an intestine. In the
last segment of the abdomen the intestine joins a short, thin-walled
rectum, provided with radial muscles, by means of which it can be
dilated. The dilatation of the rectum leads to an inhalation of water
through the anus, which may possibly serve as a means of respiration. In
the Daphniidae and Bosminidae there are two forwardly-directed digestive
glands which open into the stomach, and in _Eurycercus_ there is a large
caecum at the junction of the rectum with the intestine. The intestine
is usually straight, but in Lynceidae and in some Lyncodaphniidae it is
coiled (e.g. _Peracantha_, Fig. 14).

In _Leptodora_ the alimentary canal is altogether remarkable; the
oesophagus is a long and very narrow tube, which runs back through the
whole length of the thorax and joins the mid-gut in the third abdominal
segment. The mid-gut is not differentiated into stomach and intestine;
it has no diverticula of any kind, and runs straight backwards to join
the short rectum a little in front of the anus.

[Illustration:

  FIG. 14.—_Peracantha truncata_, female, × 100. Oxford.
]

The =heart= is always short, and never has more than a single pair of
lateral openings; it is longest in the Sididae, which show some
approximation to the Phyllopods in this, as in the slight degree of
difference between their anterior and posterior thoracic limbs. The
pericardium lies in the one or two anterior thoracic segments, dorsal to
the gut. From the heart the blood runs forwards to the dorsal part of
the head, and passes backwards by three main channels, one entering each
side of the carapace, while the third runs down the body, beneath the
alimentary canal to dilate into a large sinus round the rectum. This
ventral blood-channel gives a branch to each limb, which forms a
considerable dilatation in the epipodite, the blood from the limb
returning to the pericardium by a lateral sinus. From the rectum a large
sinus runs forwards to the pericardium along the dorsal wall of the
body. The blood which enters each half of the carapace is collected in a
median vessel and returned through this to the pericardium.

Those spaces between the viscera which are not filled with blood are
occupied by a peculiar connective tissue, consisting of rounded or
polyhedral cells, charged with drops of a fatty material which is often
brightly coloured.

The =reproductive organs= are interesting because of the peculiar
phenomena connected with the nutrition of the two kinds of eggs. The
ovaries or testes are epithelial sacs, one on each side of the body,
each continuous with a duct which opens to the exterior behind the last
thoracic limb. In the female, the opening is dorsal (Fig. 10), in the
male it is ventral (Fig. 11). The external opening is usually simple;
but in the male there is sometimes a penis-like process, on which the
vas deferens opens (_Daphnella_).

The eggs are of two kinds, the so-called “summer-eggs,” with relatively
little yolk, which develop rapidly without fertilisation, and the
so-called “winter-eggs,” containing much yolk, which require to be
fertilised and then develop slowly.

At one end of the ovary, generally that nearest to the oviduct, there is
a mass of protoplasm, containing nuclei which actively divide; this is
the germarium (Fig. 15, A, B, C). As a result of proliferation in the
germarium, nucleated masses are thrown off into the cavity of the ovary;
each such mass contains four nuclei, and its protoplasm soon becomes
divided into four portions, one round each nucleus, so that four cells
are produced. In the simpler ovaries, such as that of _Leptodora_ (Fig.
15, A), these sets of four cells are arranged in a linear series within
the tube of ovarian epithelium; in other cases, as in _Daphnia_, the
arrangement is more irregular. In the normal development of
parthenogenetic eggs, one cell out of each set of four becomes an ovum,
the other three feeding it with yolk and then dying. Weismann[32] has
shown that the ovum is always formed from the third cell of each set,
counting from the germarial end, so that in the ovary of _Leptodora_
drawn in Fig. 15, A, the ova will be formed from the cells marked E_{1},
E_{2}, E_{3}. At certain times, one or two sets of germinal cells fail
to produce ova; the epithelial wall of the ovary thickens round these
cells, so that they become incompletely separated from the rest in a
so-called “nutrient chamber” (Fig. 15, B, _N.C_). Germ-cells enclosed in
a nutrient chamber degenerate and are ultimately devoured by the ovarian
epithelium. The significance of these nutrient chambers is unknown.

[Illustration:

  FIG. 15.—=A=, Ovary of a parthenogenetic _Leptodora hyalina_; =B=,
    base of another ovary of the same species, showing a so-called
    “nutrient chamber”; =C=, ovary of a female _Daphnia_, showing the
    formation of a winter-egg. _E_, _E_{1}–E_{3}_, Parthenogenetic egg;
    _Ep_, ovarian epithelium; _G_, germarium; _N.C_, nutrient chamber;
    _O.D_, oviduct; _W_, winter-egg; 1, 2, 4, the other three cells of
    the same group; II, III, two other groups of germ-cells.
]

The production of a winter-egg is a more complicated process. The
epithelium of the ovarian tube swells up, so that the lumen is nearly
obliterated, and several sets of four germ-cells pass from the germarium
to lie among the swollen epithelial cells. All these groups of
germ-cells, except one, disintegrate and are devoured by the ovarian
epithelium, one cell of the remaining group enlarging to form a
winter-egg, fed during its growth not only by the three cells of its own
set but also by the epithelial cells of the ovarian tube, which have
devoured the germ-cells of other sets. An ovary never contains more than
a single winter-egg at the same time, the number of germ-cells which are
devoured during its formation varying in the different species; the
_Daphnia_ drawn in Fig. 15, C, has produced three groups of germ-cells,
of which two (II, III), will die, while the cell W from the remaining
group will develop into an ovum; in _Moina_, Weismann finds that as many
as a dozen cell-groups may be thrown into the ovary before the
production of a winter-egg, so that only one out of forty-eight
germ-cells survives as an ovum.

[Illustration:

  FIG. 16.—Sketch of a parthenogenetic _Moina rectirostris_, × 45, the
    brood-pouch being emptied and the side of the carapace removed,
    showing the dome of thickened epithelium on the thorax, by which
    nutrient material is thrown into the brood-pouch, and the ridge
    which fits against the carapace in the natural condition so as to
    close the brood-pouch.
]

[Illustration:

  FIG. 17.—_Moina rectirostris_, ♀, × 40, showing the ephippial
    thickening of the carapace which precedes the laying of a
    winter-egg.
]

The summer-eggs are always carried until they are hatched by the
parthenogenetic female which produces them. The brood-pouch is the space
between the dorsal wall of the thorax and the carapace. This space is
always more or less perfectly closed at the sides by the pressure of the
carapace against the body, and behind by vascular processes from the
abdominal segments (Figs. 10, 16, etc.). The presence of a large
blood-sinus beneath the dorsal wall of the thorax and in the middle line
of the carapace suggests the possibility that some special nutrient
substances may pass from the body of the parent into the brood-chamber,
and in some species the thoracic ectoderm is specially modified as a
placenta. In _Moina_ (Fig. 16) the dorsal wall of the thorax is produced
into a dome, covered by a columnar ectoderm, which contains a dilatation
of the dorsal blood-sinus; and in this form it has been shown that the
fluid in the brood-pouch contains dissolved proteids. Associated with
the apparatus for supplying the brood-pouch with nutriment is a special
apparatus for closing it, in the form of a raised ridge, which projects
from the back and sides of the thorax and fits into a groove of the
carapace.

A somewhat similar nutrient apparatus exists in the Polyphemidae, where
the edges of the small carapace are fused with the thorax, so that the
brood-pouch is completely closed, and the young can only escape when the
parent casts her cuticle. In some genera of this family (e.g. _Evadne_)
the young remain in the parental brood-pouch until they are themselves
mature, so that when they are set free they may already bear
parthenogenetic embryos in their own brood-pouches.

[Illustration:

  FIG. 18.—Newly-cast ephippium of _Daphnia_, containing two
    winter-eggs.
]

The winter-eggs are fertilised in the same part of the carapace of the
female in which the parthenogenetic eggs develop, but after
fertilisation they are thrown off from the body of the mother, either
with or without a protective envelope formed from the cuticle of the
carapace. The eggs of _Sida_ are surrounded by a thin layer of a sticky
substance, and when cast out of the maternal carapace they adhere to
foreign objects, such as water-weeds; those of _Polyphemus_ have a
thick, gelatinous coat; in _Leptodora_ and _Bythotrephes_ the egg
secretes a two-layered chitinous shell. In these forms the cuticle of
the parent is not used as a protection for the winter-eggs, although it
is generally, if not invariably, thrown off when the eggs are laid. In
the Lynceidae the cuticle is moulted in such a way that the winter-eggs
remain within it, at least for a time; the cuticle is occasionally
modified before it is thrown off; thus in _Camptocercus macrurus_ the
cuticle of the carapace, in the region of the brood-pouch, becomes
thickened and darkly coloured, forming a fairly strong case round the
eggs. The modification of the cuticle round the brood-pouch is much more
pronounced in the Daphniidae, where it leads to the formation of a
saddle-shaped cuticular box, the “ephippium,” in which the winter-eggs
are enclosed. The ripening of a winter-egg in the ovary of a _Daphnia_
is accompanied by a great thickening of the cuticle of the carapace
(_cf._ Fig. 18), so that a strong case is formed in the position of the
brood-pouch. The winter-eggs are laid between the two valves of this
case, and shortly afterwards the parent moults. The eggs are retained
within the ephippium, from which the rest of the cuticle breaks away
(Fig. 18). After separation, the ephippium, which contains a single egg
(_Moina rectirostris_) or usually two (_Daphnia_, etc.), either sinks to
the bottom, as in _Moina_, or floats.

The winter-eggs usually go through the early stages of segmentation
within a short time after they are laid, but after this a longer or
shorter period of quiescence occurs, during which the eggs may be dried
or frozen without injury. The sides and floor of a dried-up pond are
often crowded with ephippia, containing winter-eggs which develop
quickly when replaced in water; and the resting-stage of winter-eggs
produced in aquaria can often be materially shortened by drying the
ephippia which contain them, though such desiccation does not appear to
be necessary for development. Under normal conditions large numbers of
winter-eggs remain quiescent through the winter and hatch in the
following spring.

The individual developed from a sexually fertilised winter-egg is
invariably a parthenogenetic female: the characters of the succeeding
generations differ in different cases.

In a few forms, of which _Moina_ is the best known, the parthenogenetic
female, produced from a winter-egg, may give rise to males, to sexual
females, and to parthenogenetic females, so that the cycle of forms
which intervene between one winter-egg and the next is short. A sexual
female produces one or two winter-eggs, and if these are fertilised they
are enclosed in an ephippium and cast off; if, however, the eggs when
ripe are not fertilised, they atrophy, and the female produces
parthenogenetic eggs, being thenceforward incapable of forming sexual
“winter” eggs. An accidental absence of males may thus lead to the
occurrence of parthenogenesis in the whole of the second generation. The
regular production of sexual individuals in the second generation from
the winter-egg appears to depend on a variety of circumstances not yet
understood. Mr. G. H. Grosvenor tells me that _Moina_ from the
neighbourhood of Oxford may give rise to several successive generations
of parthenogenetic individuals, when grown in small aquaria.

In the greater number of Daphniidae, the parthenogenetic female,
produced from a winter-egg, gives rise only to parthenogenetic forms,
and it is not until after half a dozen parthenogenetic generations have
been produced that a few sexual forms appear, mixed with the others.
Such sexual forms are fairly common in April or May in this country;
they produce “winter” eggs and then die, the generations which succeed
them through the summer being entirely parthenogenetic. In late autumn
sexual individuals are again produced, giving rise to a plentiful crop
of winter-eggs, but many parthenogenetic females are still found, and
some of these appear to live and to reproduce through the winter.

In _Sida_, in the Polyphemidae and Leptodoridae, and in most of the
Lynceidae, sexual individuals are produced only once in every year,
while in a few forms which inhabit great lakes the sexual condition
occurs so rarely that it is still unknown.

Weismann[33] has pointed out that the sexual forms, with their property
of producing eggs which can endure desiccation, recur most frequently in
species such as _Moina_, which inhabit small pools liable to be dried up
at frequent intervals, while the species which produce sexual forms only
once a year are all inhabitants either of great lakes which are never
dry, or of the sea. Many suggestions have been made as to the
environmental stimulus which induces the production of sexual
individuals, but nothing is definitely known upon the subject.

We have said that even in those generations which contain sexual males
and females there are always some parthenogenetic individuals; there is
therefore nothing in the behaviour of Daphniidae, either under natural
conditions or when observed in aquaria, to suggest that there is any
natural or necessary limit to the number of generations which may be
parthenogenetically produced.

The parthenogenetic Daphniidae are extremely sensitive to changes in
their surroundings; small variations in the character and amount of
substances dissolved in the water are often followed by changes in the
length of the posterior spine, in the shape and size of crests on the
head, and in other characters affecting the appearance of the creatures,
so that the determination of species is often a matter of great
difficulty. It is remarkable that the green light which has passed
through the leaves of water-plants appears to have a prejudicial effect
upon some species. Warren has shown that _Daphnia magna_ reproduces more
slowly when exposed to green light, and that individuals grown in this
way are more readily susceptible to injury from the presence of small
quantities of salt (sodium chloride) in the water than individuals which
have been exposed to white light.

The majority of the Cladocera belong to the floating fauna of the fresh
waters and seas; a few are littoral in their habits, clinging to
water-weeds near the shore, a very few live near the bottom at
considerable depths, but the majority belong to that floating fauna to
which Haeckel gave the name of “plankton.” The Crustacea are an
important element in the plankton, whether in fresh waters or in the
sea, the two great groups which contribute most largely to it being the
Cladocera and the Copepoda. For this reason it will be more convenient
to discuss the habits and distribution of individual Cladocera and
Copepoda together in a chapter specially devoted to the characters of
pelagic faunas (_cf._ Chap. VII.). We will only add to the present
chapter a table of the families with a diagnosis of the British genera.


           =Summary of Characters of the British Genera.=[34]

Tribe I. CALYPTOMERA, Sars.—The post-cephalic portion of the body
enveloped in a free fold or carapace.

  A. Six pairs of thoracic feet, the first pair not prehensile
    (CTENOPODA).

  =Fam. 1. Sididae=: second antennae biramous in both sexes. _Sida_,
    Straus (Fig. 11): second antenna with three joints in the dorsal
    ramus, two in the ventral; the rostrum large, the teeth on the
    telson many. _Latona_, Straus: second antenna with two joints in the
    dorsal ramus, three in the ventral, the proximal joint of the dorsal
    ramus provided with a setose appendage. _Daphnella_, Baird: second
    antenna with the joints as in _Latona_, but with no setose
    appendage.

  =Fam. 2. Holopediidae=: second antennae not biramous in the female; a
    rudimentary second ramus in the male. _Holopedium_, Zaddach.

  B. Four to five or six pairs of thoracic feet, the anterior pair
    prehensile (ANOMOPODA).

    =A.= Ventral ramus of second antenna with three joints, the dorsal
      ramus with four.

  =Fam. 3. Daphniidae=: five pairs of thoracic feet, with a gap between
    the fourth and fifth pairs. The stomach with two forwardly-directed
    diverticula.

[Illustration:

  FIG. 19.—_Daphnia obtusa_, male, × about 50. Oxford. _A._1, First
    antenna; _Th._1, first thoracic appendage.
]

    i. First antennae of female short.

      α A median dorsal spine on posterior margin of carapace.
        _Daphnia_, O. F. Müller (Fig. 19): first antennae of female not
        mobile. The head separated from the thorax only by a slight
        constriction or not at all. Cuticle with a quadrate rhomboid
        pattern. _Ceriodaphnia_, Dana: first antennae of female mobile.
        The head separated by a deep depression from the thorax. Cuticle
        with a polygonal pattern.

      β A pair of ventral spines on posterior margin of carapace.
        _Scapholeberis_, Schoedler (Fig. 20).

[Illustration:

  FIG. 20.—_Scapholeberis mucronata_, female, × 25. Oxford.
]

      γ No spine on posterior margin of carapace. _Simocephalus_,
        Schoedler (Fig. 10, p. 39): the cuticle with a pattern of
        parallel branching ridges.

[Illustration:

  FIG. 21.—_Moina rectirostris_, female, × 24. Oxford.
]

    ii. First antennae of female long, mobile. _Moina_, Baird (Figs. 16,
      17, 21): median eye absent. Posterior margin of carapace without a
      spine.

[Illustration:

  FIG. 22.—_Bosmina_ sp., female, × about 80. Lake Constance.
]

[Illustration:

  FIG. 23.—_Acroperus leucocephalus_, × about 35. Oxford.
]

  =Fam. 4. Bosminidae=: feet equidistant, five or six pairs; the first
    antennae of the female immobile, with sense-hairs arranged in rings,
    not forming an apical tuft. The intestine uncoiled; no caeca.
    _Bosmina_, Baird (Fig. 22).

  =Fam. 5. Lyncodaphniidae=: four, five, or six pairs of equidistant
    thoracic limbs; the first two pairs prehensile. First antennae of
    female mobile, with apical sense-hairs. Intestine coiled or
    straight.

      i. Four pairs of thoracic limbs. _Lathonura_, Lilljeborg.

      ii. Five pairs of thoracic limbs.

        _a._ The four-jointed ramus of the second antenna with four
          swimming hairs. _Macrothrix_, Baird: the first antennae of the
          female flattened, curved. The intestine simple, straight.
          _Streblocerus_, Sars: first antennae of the female very little
          flattened, curved backwards and outwards. The intestine
          coiled, the stomach with two forwardly-directed caeca.

        _b._ The four-jointed ramus of the second antenna with only
          three swimming hairs. _Drepanothrix_, Sars.

      iii. Six pairs of thoracic limbs; the labrum provided with an
        appendage. _Acantholeberis_, Lilljeborg: appendage of labrum
        long, pointed, and setose. Intestine without caecum.
        _Ilyocryptus_, Sars: appendage of the labrum short, truncated.
        Intestine with a caecum.

            =B.= Both rami of second antenna three-jointed.

        =Fam. 6. Lynceidae=[35]: five or six equidistant pairs of
        thoracic feet. Intestine coiled.

          i. Six pairs of thoracic limbs. Head and thorax separated by a
            deep depression. Intestine with one caecum, stomach with
            two. Female carries many summer-eggs. _Eurycercus_, Baird.

          ii. Five pairs of thoracic limbs. Head and thorax separated by
            a slight groove or not at all. Anterior digestive caeca
            absent. Female carries only one or two summer-eggs.

                  A. Body elongate, oval.

            _a._ Head carinate, the eye far from the anterior cephalic
            margin. _Camptocercus_, Baird: body laterally compressed.
            Second antennae with seven swimming hairs. Telson more than
            half as long as the shell. _Acroperus_, Baird (Fig. 23):
            body compressed. Second antennae with eight swimming hairs,
            of which one is very small. Telson less than half as long as
            the shell.

            _b._ Head not carinate, the eye near the anterior cephalic
            margin. _Alonopsis_, Sars: terminal claws of telson with
            _three_ accessory teeth. _Alona_, Baird: terminal claws of
            telson with one accessory tooth (includes sub-genera
            _Leydigia_, _Alona_, _Harporhynchus_, _Graptoleberis_).
            _Peracantha_, Baird (Fig. 14): terminal claws of telson with
            two accessory teeth (includes sub-genera _Alonella_,
            _Pleuroxus_, _Peracantha_).

              B. Body small, spheroidal; the head depressed. _Chydorus_,
              Leach: compound eye present. _Monopsilus_, Sars: compound
              eye absent.

Tribe II. GYMNOMERA, Sars.—The carapace forms a closed brood-pouch,
which does not cover the body; all the thoracic limbs prehensile.

  =Fam. 7. Polyphemidae=: four pairs of thoracic limbs, provided with a
    gnathobase.

    Fresh-water genera.—_Polyphemus_, Müller, with no rudimentary exites
      on first three thoracic limbs. _Bythotrephes_, Leydig (Fig. 13),
      with no trace of processes on the outer sides of the limbs.

    Marine genera.—_Evadne_, Lovén, the head not separated by a
      constriction from the thorax. _Podon_, Lovén, with deep cervical
      constriction.

[Illustration:

  FIG. 24.—_Leptodora hyalina_, × 6. Lake Bassenthwaite. _A._1, First
    antenna; _Car_, carapace; I, VI, first and sixth thoracic
    appendages.
]

  =Fam. 8. Leptodoridae=: six pairs of thoracic limbs, with no
    gnathobase. Only genus, _Leptodora_, Lilljeborg (Fig. 24), from
    fresh water.


  _Note._—For extra-European Cladocera consult Daday, “Microskopische
  Süsswassertiere aus Patagonien und Chili,” _Termés Füzetek_, xxv.,
  1902, p. 201; for Paraguay, _Bibliotheca Zoologica_, Heft 44; for
  Ceylon, _Termés Füzetek_, xxi., 1898; and for Australia, Sars,
  _Christiania Vidensk. Forhand._ 1885, No. 8, and 1888, No. 7; and
  _Arch. f. Math. og Naturvid._ xviii., 1896, No. 3, and xix., 1897, No.
  1.—G. W. S.




                              CHAPTER III
                   CRUSTACEA (_CONTINUED_): COPEPODA


                         =Order II. Copepoda.=

The Copepods are small Crustacea, composed typically of about sixteen
segments, in which the biramous type of limb predominates. They are
devoid of a carapace. Development proceeds gradually by the addition
posteriorly of segments to a Nauplius larval form. Paired compound eyes
are absent, except in Branchiura, the adult retaining the simple eye of
the Nauplius.

In a typical Copepod, such as _Calanus hyperboreus_ (Fig. 25), we can
distinguish the following segments with their appendages: a
cephalothorax, carrying a pair of uniramous first antennae (_1^{st}
Ant._); a pair of biramous second antennae (_2^{nd} Ant._); mandibles
(_Md._) with biting gnathobases and a palp, and a pair of foliaceous
first maxillae (_Mx.^1_). Two pairs of appendages follow, which were
looked upon as the two branches of the second maxillae, but it is now
certain that they represent two pairs of appendages, which may be called
second maxillae (_Mx.^2_) and maxillipedes (_Mxp._) respectively. Behind
these are five pairs of biramous swimming feet, the first pair (_Th.^1_)
attached to the cephalothorax, the succeeding four pairs to four
distinct thoracic somites. Behind the thorax is a clearly delimited
abdomen composed of five segments, the first of which (_Abd.^1_) carries
the genital opening, and the last a caudal furca.

The Copepods exhibit a great variety of structure, and their
classification is attended with great difficulties. Claus[36] based his
attempt at a natural classification on the character of the mouth and
its appendages, dividing the free-living and semi-parasitic forms as
Gnathostomata from the true parasites or Siphonostomata. This division,
although convenient, breaks down in many places, and it is clear that
the parasitic mode of life has been acquired more than once in the
history of Copepod evolution, while the free-living groups do not
constitute a natural assemblage.

[Illustration:

  FIG. 25.—_Calanus hyperboreus_, × 30. _Abd^1_, First abdominal
    segment; _1st Ant_, _2nd Ant_, 1st and 2nd antennae; _Md_, mandible;
    _Mx^1_, _Mx^2_, 1st and 2nd maxillae; _Mxp_, maxillipede; _Th^1_,
    1st thoracic appendage. (After Giesbrecht.)
]

Giesbrecht has more recently[37] founded a classification of the
free-living pelagic Copepods upon the segmentation of the body and
certain secondary sexual characters, and he has hinted[38] that this
scheme of classification applies to the semi-parasitic and parasitic
forms. Although much detail remains to be worked out and the position of
some families is doubtful, Giesbrecht’s scheme is the most satisfactory
that has hitherto been suggested, and will be adopted in this chapter.

The peculiarity in structure of the Argulidae, a small group of
ectoparasites on fresh water fish, necessitates their separation from
the rest of the Copepods (Eucopepoda) as a separate Branch, Branchiura.


                         BRANCH I. EUCOPEPODA.


                       =Sub-Order 1. Gymnoplea.=

The division between the front and hind part of the body falls
immediately in front of the genital openings and behind the fifth
thoracic feet. The latter in the male are modified into an asymmetrical
copulatory organ.


                        TRIBE I. AMPHASCANDRIA.

The first antennae of the male are symmetrical, with highly-developed
sensory hairs.

=Fam. Calanidae.=—The Calanidae are exclusively marine Crustacea, and
form a common feature of the pelagic plankton in all parts of the world.
Some species of the genus _Calanus_ often occur in vast shoals, making
the sea appear blood-red, and they furnish a most important article of
fish food. These swarms appear to consist chiefly of females, the males
being taken rarely, and only at certain seasons of the year. Some of the
Calanidae are animals of delicate and curious form, owing to the
development of plumed iridescent hairs from various parts of their body,
which may often exhibit a marked asymmetry, as in the species figured,
_Calocalanus plumulosus_ (Fig. 26), from the Mediterranean.

[Illustration:

  FIG. 26.—_Calocalanus plumulosus_, × 15. (After Giesbrecht.)
]

Sars makes a curious observation[39] with regard to the distribution of
certain Calanidae. He reports that along the whole route of the “Fram,”
species such as _Calanus hyperboreus_ and _Euchaeta norwegica_ were
taken at the surface, which, in the Norwegian fjords, only occur at
depths of over 100 fathoms. He suggests that the Norwegian individuals,
instead of migrating northwards as the warmer climate supervened, have
sought boreal conditions of temperature by sinking into the deeper
waters.


                               TRIBE II.
                           HETERARTHRANDRIA.

The first antennae of the male are asymmetrical, one, usually the right,
being used as a clasping organ.

The males of the Centropagidae, Candacidae and Pontellidae, besides
possessing the asymmetrically modified thoracic limbs of the fifth pair
also exhibit a modification of one of the first antennae, which is
generally thickened in the middle, and has a peculiar joint in it, or
geniculation, which enables it to be flexed and so used as a clasping
organ for holding the female.

=Fam. 1.—Centropagidae.=—These Copepods are very common in the pelagic
plankton, and some of the species vie with the Calanidae in plumed
ornaments, e.g. _Augaptilus filigerus_, figured by Giesbrecht in his
monograph. The use of these ornaments, which are possessed by so many
pelagic Copepods, is entirely obscure.[40] Certain of the Centropagidae
live in fresh water. Thus _Diaptomus_ is an exclusively fresh-water
genus, and forms a most important constituent of lake-plankton; various
species of _Heterocope_ occur in the great continental lakes, and
certain _Eurytemora_ go up the estuaries of rivers into brackish water.

An excellent work on the fresh-water Copepods of Germany has been
written by Schmeil,[41] who gives analytical tables for distinguishing
various genera and species. The three fresh-water families are the
Centropagidae, Cyclopidae, and Harpacticidae (see p. 62). The
Centropagidae may be sharply distinguished from the other fresh-water
families by the following characters:—The cephalothorax is distinctly
separated from the abdomen; the first antennae are long and composed of
24–25 segments, in the male only a single antenna (generally the right)
being geniculated and used as a clasping organ. The fifth pair of limbs
are not rudimentary; a heart is present, and only one egg-sac is found
in the female. The second antennae are distinctly biramous.

  _Diaptomus._—The furcal processes are short, at most three times as
    long as broad; endopodite of the first swimming appendage 2–jointed,
    endopodites of succeeding legs 3–jointed.

  _Heterocope._—The furcal processes are short, at most twice as long as
    broad; endopodites of all swimming legs 1–jointed.

  _Eurytemora._—The furcal processes are long, at least three and a half
    times as long as broad; the endopodite of the first pair of legs
    1–jointed, those of the other pairs 2–jointed.

[Illustration:

  FIG. 27.—Dorsal view of _Anomalocera pattersoni_, ♂, × 20. (After
    Sars.)
]

It has been known for a long time that some of the marine Copepods are
phosphorescent, and, indeed, owing to their numbers in the plankton,
contribute very largely to bring about that liquid illumination which
will always excite the admiration of seafarers. In northern seas the
chief phosphorescent Copepods belong to _Metridia_, a genus of the
Centropagidae; but in the Bay of Naples Giesbrecht[42] states that the
phosphorescent species are the following Centropagids: _Pleuromma
abdominale_ and _P. gracile_, _Leuckartia flavicornis_ and _Heterochaeta
papilligera_; _Oncaea conifera_ is also phosphorescent. It is often
stated that _Sapphirina_ (p. 69) is phosphorescent, but its wonderful
iridescent blue colour is purely due to interference colours, and has
nothing to do with phosphorescence. Giesbrecht has observed that the
phosphorescence is due to a substance secreted in special skin-glands,
which is jerked into the water, and on coming into contact with it emits
a phosphorescent glow. This substance can be dried up completely in a
desiccated specimen and yet preserve its phosphorescent properties, the
essential condition for the actual emission of light being contact with
water. Similarly, specimens preserved in glycerine for a long period
will phosphoresce when compressed in distilled water. From this last
experiment Giesbrecht concludes that the phosphorescence can hardly be
due to an oxidation process, but the nature of the chemical reaction
remains obscure.

=Fam. 2. Candacidae.=—This family comprises the single genus _Candace_,
with numerous species distributed in the plankton of all seas. Some
species, e.g. _C. pectinata_, Brady, have a practically world-wide
distribution, this species being recorded from the Shetlands and from
the Philippines.

=Fam. 3. Pontellidae.=—This is a larger family also comprising widely
distributed species found in the marine plankton. _Anomalocera
pattersoni_ (Fig. 27) is one of the commonest elements in the plankton
of the North Sea.


                        =Sub-Order 2. Podoplea.=

The boundary between the fore and hind part of the body falls in front
of the fifth thoracic segment. The appendages of the fifth thoracic pair
in the male are never modified as copulatory organs.


                       TRIBE I. AMPHARTHRANDRIA.

The first antennae in the male differ greatly from those in the female,
being often geniculated and acting as prehensile organs.

[Illustration:

  FIG. 28.—_Euterpe acutifrons_, ♀, × 70. _Abd.1_, 1st abdominal
    segment; _Th.5_, 5th thoracic segment. (After Giesbrecht.)
]

[Illustration:

  FIG. 29.—First antenna of _Euterpe acutifrons_, ♂. (After Giesbrecht.)
]

=Fams. 1–2.= =Cyclopidae= and =Harpacticidae=, and other allied
families, are purely free-living forms; they are not usually pelagic in
habit, but prefer creeping among algae in the littoral zone or on the
sea-bottom, or especially in tidal pools. Some genera are, nevertheless,
pelagic; e.g. _Oithona_ among Cyclopidae; _Setella_, _Clytemnestra_, and
_Aegisthus_ among Harpacticidae.

The sketch (Fig. 28) of _Euterpe acutifrons_ ♀, a species widely
distributed in the Mediterranean and northern seas, exhibits the
structure of a typical Harpacticid, while Fig. 29 shows the form of the
first antenna in the male.

Several fresh-water representatives of these free-living families occur.
The genus _Cyclops_ (Cyclopidae) is exclusively fresh-water, while many
Harpacticidae go up into brackish waters: for example on the Norfolk
Broads, Mr. Robert Gurney has taken _Tachidius brevicornis_, Müller, and
_T. littoralis_, Poppe; _Ophiocamptus brevipes_, Sars; _Mesochra
lilljeborgi_, Boeck; _Laophonte littorale_, T. and A. Scott; _L.
mohammed_, Blanchard and Richard; and _Dactylopus tisboides_, Claus.

Schmeil[43] gives the following scheme for identifying the fresh-water
Cyclopidae and Harpacticidae (see diagnosis of Centropagidae on p. 59):—

=Fam. 1. Cyclopidae.=—The cephalothorax is clearly separated from the
abdomen. The first antennae of the female when bent back do not stretch
beyond the cephalothorax; in the male both of them are clasping organs.
The second antennae are without an exopodite. The fifth pair of limbs
are rudimentary, there is no heart, and the female carries two egg-sacs.

  _Cyclops._—Numerous species, split up according to segmentation of
    rudimentary fifth pair of legs, number of joints in antennae, etc.

=Fam. 2. Harpacticidae.=—The cephalothorax is not clearly separated from
the abdomen. The first antennae are short in both sexes, both being
clasping organs in the male. The second antennae have a rudimentary
exopodite. The fifth pair of limbs are rudimentary and plate-shaped; a
heart is absent, and the egg-sacs of the female may be one or two in
number.

  1. _Ophiocamptus_ (_Moraria_).—Body worm-shaped; first antennae of
    female 7–jointed, rostrum forming a broad plate.

  2. Body not worm-shaped; first antennae of female 8–jointed, rostrum
    short and sharp.

      (_a_) Endopodites of all thoracic limbs 3–jointed. The first
        antennae in female distinctly bent after the second joint.
        _Nitocra._

      (_b_) Endopodite of at least the fourth limb 2–jointed; first
        antennae in female not bent. _Canthocamptus._

    3. _Ectinosoma._—Body as in 2, but first antennae are very short,
    and the maxillipede does not carry a terminal hooked seta as in 1
    and 2.

=Fam. 3. Peltiidae.=[44]—This is an interesting family, allied to the
Harpacticidae, and includes species with flattened bodies somewhat
resembling Isopods, and a similar habit of rolling themselves up into
balls. No parasitic forms are known, though _Sunaristes paguri_ on the
French and Scottish coasts is said to live commensally with
hermit-crabs.


We have now enumerated the chief families of free-living Copepods; the
rest are either true parasites or else spend a part of their lives as
such. A number of the semi-parasitic and parasitic Copepods can be
placed in the tribe Ampharthrandria owing to the characters of their
antennae; but it must be remembered that many parasitic forms have given
up using the antennae as clasping organs; however, the sexual
differences in the antennae, and the fact that many of the species which
have lost the prehensile antennae in the male have near relations which
preserve it, enable us to proceed with some certainty. The adoption of
this classification necessitates our separating many families which
superficially may seem to resemble one another, _e.g._ the
semi-parasitic families Lichomolgidae and Ascidicolidae, and the
Dichelestiidae from the other fish-parasites; it also necessitates our
treating the presence of a sucking mouth as of secondary importance.
This characteristic must certainly, however, have been acquired more
than once in the history of the Copepods, for instance in the
Asterocheridae and in the fish-parasites, while it sometimes happens
that genera belonging to a typically Siphonostomatous group possess a
gnathostome, or biting mouth, _e.g._ _Ratania_ among the Asterocheridae.
Again, it is impossible even if we use the character of the mouth as a
criterion to place together all the true parasites on fishes in one
natural group, because the Bomolochidae and Chondracanthidae, which are
otherwise closely similar to the rest of the fish-parasites, possess no
siphon. It seems plain, therefore, that the parasitic habit has been
acquired several times separately by diverging stocks of free-swimming
Copepods, and that it has resulted in the formation of convergent
structures.

[Illustration:

  FIG. 30.—_Haemocera danae_, × 40. =A=, Side view ♀; =B=, ventral view
    ♂. _Ant.1_, 1st antenna; _e_, eye; _ov_, ovary; _ovd_, oviduct;
    _St_, stomach; _Th.1_, 1st thoracic appendage; _Th.5_, 5th thoracic
    segment; _vd_, vas deferens. (After Malaquin.)
]

[Illustration:

  FIG. 31.—Free-swimming Nauplius larva of _Haemocera danae_; _Ant.1_,
    _Ant.2_, 1st and 2nd antennae; _e_, remains of eye; _Md_, mandible.
    (After Malaquin.)
]

=Fam. 4. Monstrillidae.=[45]—These are closely related to the
Harpacticidae. The members of this curious family are parasitic during
larval life and actively free-swimming when adult. There are three
genera, _Monstrilla_, _Haemocera_, and _Thaumaleus_. The best known type
is _Haemocera danae_ (often described as _Monstrilla danae_). In the
adult state (Fig. 30) there are no mouth-parts; the mouth is exceedingly
small and leads into a very small stomach, which ends blindly, while the
whole body contains reserve food-material in the form of brown
oil-drops. The sole appendages on the head are the first antennae; but
on the thorax biramous feet are present by means of which the animal can
swim with great rapidity. This anomalous organisation receives an
explanation from the remarkable development through which the larva
passes. The larva is liberated from the parent as a Nauplius with the
structure shown in Fig. 31; it does not possess an alimentary canal. It
makes its way to a specimen of the Serpulid worm, _Salmacina dysteri_,
into the epidermis of which it penetrates by movements of the antennae,
hanging on all the time by means of the hooks on the mandibles. From the
epidermis it passes through the muscles into the coelom of the worm, and
thence into the blood-vessels, usually coming to rest in the ventral
blood-vessel. As the Nauplius migrates, apparently by amoeboid movements
of the whole body, it loses all its appendages, the eye degenerates, and
the body is reduced to a minute ovoid mass of cells, representing
ectoderm and endo-mesoderm, surrounded by a chitinous membrane (Fig. 32,
A). Arrived in the ventral blood-vessel it begins to grow, and the first
organ formed is a pair of fleshy outgrowths representing the second
antennae (Fig. 32, B), which act as a nutrient organ intermediary
between host and parasite. The adult organs now begin to be
differentiated, as shown in Fig. 32, C, from the undifferentiated
cellular elements of the Nauplius, the future adult organism being
enclosed in a spiny coat from which it escapes. At this stage it
occupies a large part of its host’s body, lying in the distended ventral
blood-vessel, and it escapes to the outside world by rupturing the
body-wall of the worm, leaving behind it the second antennae, which have
performed their function as a kind of placenta. Malaquin, to whom we owe
this account, makes the remarkable statement that if two or three
Monstrillid Nauplii develop together in the same host they are always
males, if only one it may be either male or female. The only parallel to
this extraordinary life-history is found in the Rhizocephala (see pp.
96–99).

[Illustration:

  FIG. 32.—Later stages in the development of _Haemocera danae_. _Abd_,
    Abdomen; _Ant.1_, _Ant.2_, 1st and 2nd antennae; _ch_, chitinous
    investment; _e_, eye; _Ect_, ectoderm; _En_, endoderm; _Mes_,
    mesoderm; _Mes & en_, mesoderm and endoderm; _R_, rostrum; _St_,
    mouth and stomach; _Th_, thoracic appendages. (After Malaquin.)
]

[Illustration:

  FIG. 33.—Side view of _Doropygus pulex_, ♀, × 106. _Abd.1_, 1st
    abdominal segment; _Ant.1_, 1st antenna; _b.p_, brood-pouch; _Th.1_,
    1st thoracic appendage; _Th.4_, 4th thoracic segment. (After Canu.)
]

=Fam. 5. Ascidicolidae.=[46]—Although the members of this family, which
live semiparasitically in the branchial sac or the gut of Ascidians,
betray their Ampharthrandrian nature by the sexual differences of their
first antennae, only two genera, _Notodelphys_ and _Agnathaner_, possess
true prehensile antennae. According as the parasitism is more or less
complete, the buccal appendages either retain their masticatory
structure or else become reduced to mere organs of fixation. In
_Notodelphys_ both sexes can swim actively and retain normal
mouth-parts; they live parasitically, or perhaps commensally, in the
branchial cavities of Simple or Compound Ascidians, feeding on the
particles swept into the respiratory chamber of the host. They leave
their host at will in search of a new home, and are frequently taken in
the plankton.

_Doropygus_ (Fig. 33), a genus widely distributed in the North Sea and
Mediterranean, also inhabiting the branchial sac of Ascidians, is more
completely parasitic, and the female cannot swim actively. Forms still
more degraded by a parasitic habit are _Ascidicola rosea_ (especially
abundant in the stomach of _Ascidiella scabra_ at Concarneau), in which
the female has lost its segmentation, the mouth-parts and thoracic legs
being purely prehensile, and various species of _Enterocola_, parasitic
in the stomach of Compound Ascidians, in which the female is a mere sac
incapable of free motion, while the male preserves its swimming powers
and a general _Cyclops_-form (Fig. 34). We have here the first instance
of the remarkable parallelism between the degree of parasitism and the
degree of sexual dimorphism, a parallelism which holds with great
regularity among the Copepoda, and can be also extended to other classes
of parasitic animals.

[Illustration:

  FIG. 34.—_Enterocola fulgens._ =A=, Ventral view of ♀, × 35; =B=, side
    view of ♂, × 106. _Abd.1_, 1st abdominal segment; _Ant.1_, _Ant.2_,
    1st and 2nd antennae; _c.m_, gland-cells; _n_, ventral nerve-cord;
    _og_, oviducal gland; _ov_, ovary; _po_, vagina; _Th.1_, 1st
    thoracic appendage; _Th.4_, _Th.5_, 4th and 5th thoracic segments.
    (After Canu.)
]

[Illustration:

  FIG. 35.—_Asterocheres violaceus_, ♀, with egg-sacs, × 57. (After
    Giesbrecht.)
]

[Illustration:

  FIG. 36.—Diagrammatic transverse section through the distal part of
    the siphon of _Rhynchomyzon purpurocinctum_ (Asterocheridae). _Md_,
    mandible. (After Giesbrecht.)
]

=Fam. 6. Asterocheridae.=[47]—These forms retain the power of swimming
actively, and are very little modified in outward appearance by their
parasitic mode of life (Fig. 35), though they possess a true siphon in
which the styliform mandibles work. The siphon is formed by the upper
and lower lips, which are produced into a tube with three longitudinal
ridges; in the outer grooves are the mandibles, while the inner groove
forms the sucking siphon (see transverse section, Fig. 36). In
_Ratania_, however, there is no siphon. The first antennae possess a
great number of joints, and may be geniculated in the male
(_Cancerilla_). The members of this family live as ectoparasites on
various species of Echinoderms, Sponges, and Ascidians, but they
frequently change their hosts, and it appears that one and the same
species may indifferently suck the juices of very various animals, and
even of Algae. _Cancerilla tubulata_, however, appears to live only on
the Brittle Starfish, _Amphiura squamata_.

=Fam. 7. Dichelestiidae.=—The males and females are similarly parasitic,
and the body in both is highly deformed, the segmentation being
suppressed and the thoracic limbs being produced into formless fleshy
lobes; they are placed among the Ampharthrandria owing to sexual
differences in the form of the first antennae. There is a well-developed
siphon in which the mandibular stylets work, except in _Lamproglena_,
parasitic on the gills of Cyprinoid fishes; the succeeding mouth-parts
are prehensile.

The majority of the species are parasitic on the gills of various fish
(_Dichelestium_ on the Sturgeon, _Lernanthropus_[48] on _Labrax lupus_,
_Serranus scriba_, etc.), but Steuer[49] has recently described a
Dichelestiid (_Mytilicola_) from the gut of _Mytilus galloprovincialis_
off Trieste. This animal and _Lernanthropus_ are unique among Crustacea
through the possession of a completely closed blood-vascular system
which contains a red fluid; the older observers believed this fluid to
contain haemoglobin, but Steuer, as the result of careful analysis,
denies this. The parasite on the gills of the Lobster, _Nicothoe
astaci_, possibly belongs here.

The inclusion of _Nicothoe_ and the Dichelestiidae among the
Ampharthrandria rests on a somewhat slender basis; this basis is
afforded by the fact that none of the parasitic Isokerandria have more
than seven joints in the first antennae, whereas _Nicothoe_ and some of
the Dichelestiidae[50] have more numerous joints. In most of the
Dichelestiidae, however, the number of joints is less than seven and
practically equal in the two sexes.


                        TRIBE II. ISOKERANDRIA.

The first antennae are short, similar in the two sexes, and are never
used by the male as clasping organs. This function may be subserved by
the second maxillae.

FAMS. ONCAEIDAE, CORYCAEIDAE, LICHOMOLGIDAE, ERGASILIDAE, BOMOLOCHIDAE,
CHONDRACANTHIDAE, PHILICHTHYIDAE, NEREICOLIDAE, HERSILIIDAE, CALIGIDAE,
LERNAEIDAE, LERNAEOPODIDAE, CHONIOSTOMATIDAE.

The families Oncaeidae and Corycaeidae contain pelagic forms of
flattened shape and great swimming powers, but the structure of the
mouth-parts in the Corycaeidae points to a semi-parasitic habit.

=Fam. 1. Oncaeidae.=—This family, including the genera _Oncaea_,
_Pachysoma_, etc., does not possess the elaborate eyes of the next
family, nor is the sexual dimorphism so marked.

=Fam. 2. Corycaeidae.=—These are distinguished from the Oncaeidae, not
only by their greater beauty, but also by the possession of very
elaborate eyes, which are furnished with two lenses, one at each end of
a fairly long tube. The females of _Sapphirina_ are occasionally found
in the branchial cavity of Salps, and their alimentary canal never
contains solid particles, but is filled with a fluid substance perhaps
derived by suction from their prey. _S. opalina_ may occur in large
shoals, when the wonderful iridescent blue colour of the males makes the
water sparkle as it were with a sort of diurnal phosphorescence. The
animal, however, despite the opinion of the older observers, is not
truly phosphorescent. It may be that the ornamental nature of some of
the males is correlated with the presence of the curious visual organs,
which are on the whole better developed in the females than in the
males. As in so many pelagic Copepods, the body and limbs may bear
plumed setae of great elaboration and beautiful colour, _e.g._ _Copilia
vitrea_ (Fig. 37).

We now pass on to the rest of the parasitic Copepods,[51] which probably
belong to the tribe Isokerandria, and we meet with the same variety of
degrees of parasitism as in the Ampharthrandria, often leading to very
similar results.

[Illustration:

  FIG. 37.—_Copilia vitrea_ (Corycaeidae), ♀, × 20. (After Giesbrecht.)
]

In the first seven families mentioned below there is no siphon. The
Lichomolgidae and Ergasilidae have not much departed from the
free-living forms just considered, retaining their segmentation, though
in the Ergasilidae the body may be somewhat distorted (Fig. 39). In both
families the thoracic swimming feet are of normal constitution.

[Illustration:

  FIG. 38.—_Lichomolgus agilis_, × 10. _Abd. 1_, 1st abdominal segment;
    _cpth_, cephalothorax; _Th.1_, 1st thoracic segment; _Th.5_, 5th
    thoracic appendage. (After Canu.)
]

=Fam. 3. Lichomolgidae.=[52]—These are semi-parasitic in a number of
animals living on the sea-bottom, such as Actinians, Echinoderms,
Annelids, Molluscs, and Tunicates. _Lichomolgus agilis_ (Fig. 38) occurs
in the North Sea, Atlantic, and Mediterranean, on the gills of large
species of the Nudibranch, _Doris_, while _L. albeus_ is found in the
peribranchial cavity and cloaca of various Ascidians. _Sabelliphilus_
may infect the gills of Annelids such as _Sabella_, and is common at
Liverpool.

=Fam. 4. Ergasilidae.=—_Thersites_ (Fig. 39) is parasitic on the gills
of various fishes, _e.g._ _T. gasterostei_, which is common on
_Gasterosteus aculeatus_ on the French and North Sea coasts, and may
even be found on specimens of the fish that have run up the River Forth
into fresh water. The animal possesses claw-like second antennae by
which it clings to its host.

[Illustration:

  FIG. 39.—_Thersites gasterostei._ =A=, ♀, × 10; =B=, ♂, × 20. _Abd. 1
    & 2_, Fused 1st and 2nd abdominal segments; _Ant.1_, _Ant.2_, 1st
    and 2nd antennae; _e.s_, egg-sac; _Th_, thoracic appendages. (After
    Gerstaecker.)
]

Similarly characterised by the absence of a siphon are three other
families of fish-parasites, the Bomolochidae, Chondracanthidae, and
Philichthyidae.

=Fam. 5. Bomolochidae.=—_Bomolochus_ (Fig. 40), parasitic on the skin of
the Sole (_Solea_) and in the nostrils of Cod (_Gadus_), is held to be
related to the Ergasilidae. The first thoracic limb is remarkably
modified. Were it not for the absence of a siphon, it would be hard to
separate this family from the Caligidae.

[Illustration:

  FIG. 40.—_Bomolochus_, sp. (Bomolochidae), × 8. _Abd. 1_, 1st
    abdominal segment; _Ant.1_, _Ant.2_, 1st and 2nd antennae; _Mx.1_,
    _Mx.2_, 1st and 2nd maxillae; _Mxp_, maxillipede; _Th.1_, 1st
    thoracic appendage. (After Gerstaecker.)
]

[Illustration:

  FIG. 41.—_Chondracanthus zei_, ♀, × 4.
]

[Illustration:

  FIG. 42.—Dwarf male of _Lernentoma cornuta_ (Chondracanthidae), × 10.
    _Ant.1_, _Ant.2_, 1st and 2nd antennae; _Th.1_, 1st thoracic
    segment. (After Gerstaecker.)
]

=Fam. 6. Chondracanthidae.=—These Copepods infest the gills and even the
mouth of various marine fish, such as the Gurnard, Plaice, Skate, Sole,
and many others. The sexual dimorphism is very marked, the female being
large, indistinctly segmented, and with irregular paired processes
protruding from the sides of the body, giving the animal a monstrous
form (Fig. 41); while the male (Fig. 42) is very small, has a completely
segmented thorax, and lives clinging on to the female by its prehensile
second antennae—_Chondracanthus_, _Lernentoma_.

=Fam. 7. Philichthyidae.=—These parasites, which are hardly known to
occur in British waters,[53] are mucus-feeders and infest the skin of
Teleosts, _e.g._ the Sole; often taking up a position in the lateral
line or in a slime canal. They show a similar sexual dimorphism to the
foregoing family, the adult female being extraordinarily drawn out into
finger-like processes (_e.g._ _Philichthys_)[54] or else long, slender,
and Nematode-like, with much reduced appendages (_Lernaeascus_), while
the male retains a more normal structure. As in all the foregoing forms
there is no siphon.

We now return to two semi-parasitic families, =Fam. 8, Nereicolidae=,
and =Fam. 9, Hersiliidae=, in which there is certainly no well-developed
siphon, but the upper and under lips protrude, forming a hollow between
them in which the mouth-parts work. Both families are ectoparasites
which frequently leave their hosts, and they retain their segmentation
and powers of swimming. Perhaps the best-known form is the Hersiliid,
_Giardella callianassae_, which lives in the adult state in the
galleries excavated in the sand by _Callianassa subterranea_, gaining
its nourishment as an ectoparasite on the Decapod. The larvae are
pelagic, and are said by Thomson[55] to occur in Liverpool Bay.

List[56] describes _Gastrodelphys_, a parasite of doubtful position,
from the gills of tubicolous worms, such as _Myxicola_ and _Sabella_,
which possesses a perfectly siphonostomatous mouth.

The remaining families to be dealt with are those containing all the
fish-parasites which possess a true siphonostome, as well as the
siphonostomatous family Choniostomatidae, which is parasitic on other
Crustacea. In all these forms the mouth is prolonged into a tube in
which the styliform mandibles work.

=Fam. 10. Caligidae.=—Ectoparasites on fish, lodging most frequently in
the gill-chamber. In most of the genera the segmentation and power of
swimming are retained in both sexes, the sexual dimorphism not being
very well marked, though the males are smaller than the females, and
were in some cases originally described as belonging to a special genus
_Nogagus_. The females carry two long egg-sacs; the general structure
may be made out from the ventral view of _Caligus nanus_ (Fig. 43).

Some of the Caligidae are distinguished by the terga of the thoracic
segments being expanded to form large chitinous elytra, e.g. _Cecrops_,
found parasitic on the gills of the Tunny and on the Sun-fish
(_Orthagoriscus mola_). _Caligus rapax_ is parasitic on the skin and in
the gills of Sea-Trout, Pollan, etc.; and _C. lacustris_ is common in
fresh-water lakes and streams on Pike and Carp.

[Illustration:

  FIG. 43.—_Caligus nanus_, × 10. _Abd.1_, 1st abdominal segment;
    _Ant.1_, _Ant.2_, 1st and 2nd antennae; _Mx.1_, _Mx.2_, 1st and 2nd
    maxillae; _Mxp_, maxillipede; _s_, siphon; _Th.1_, _Th.5_ 1st and
    5th thoracic appendages. (After Gerstaecker.)
]

[Illustration:

  FIG. 44.—_Lernaea branchialis_ from the Haddock, ♀, × 1. _Ceph_,
    cephalothorax; _e.s_, egg-sacs. (After Scott.)
]

=Fam. 11. Lernaeidae.=—These parasites burrow with their heads deep into
the skin, or even into the blood-vessels or body-cavity, of various
marine fish. The body of the adult female _Lernaea_ is extraordinarily
deformed, consisting of a mere shapeless sac with irregular branched
processes on the head, and two egg-sacs attached behind (Fig. 44).
_Pennella sagitta_[57] bores so deeply into the flesh of its host,
_Chironectes marmoratus_, that only the egg-sacs and some remarkable
branchial processes attached to its abdomen protrude outside the host to
the exterior. _Peroderma cylindricum_ bores similarly into the flesh of
the Sardine, and where it is common, inflicts considerable damage. The
males of these curious animals are of more normal structure (Fig. 45).
Claus[58] states that fertilisation takes place when both sexes are
free-swimming, and of a more or less similar structure, and that
subsequently the female becomes fixed to her host and degenerates into
the shapeless mass shown in Fig. 44.

[Illustration:

  FIG. 45.—_Lernaea branchialis_, ♂, × 10. _Ant.1_, _Ant.2_, 1st and 2nd
    antennae; _Br_, brain; _e_, eye; _g_, stomach; _t_, testis; _vd_,
    vas deferens; _ves. sem_, vesicula seminalis. (After Claus.)
]

[Illustration:

  FIG. 46.—_Achtheres percarum_. =A=, ♀, × 4; =B=, ♂, × 4. _Ant.2_, 2nd
    antenna; _g_, stomach; _Mx.2_, 2nd maxilla; _Mxp_, maxillipede;
    _ov_, ovary; _ovd_, oviduct. (After Gerstaecker.)
]

=Fam. 12. Lernaeopodidae.=—This family may be illustrated by the common
gill-parasite of Perch and Trout, known as _Achtheres percarum_. The
female (Fig. 46), which is much larger than the male, and is not clearly
segmented, is attached to the host by means of the maxillipedes, which
are fused distally into a pad armed with chitinous hooks. In the male
the maxillipedes are prehensile, but are not so fused. Besides
_Achtheres_ there are other fresh-water forms, _e.g._ _Lernaeopoda
salmonea_ on Salmon, and a number of marine genera. It appears that the
larvae fix themselves to their hosts by means of a long glandular
thread, which proceeds from the middle of the forehead.[59]

[Illustration:

  FIG. 47.—Ventral view of _Stenochotheres egregius_ (Choniostomatidae),
    ♂. _A_, _A′_, 1st and 2nd antennae; _M_, mouth; _Mx_, 2nd maxilla;
    _T_, 1st thoracic leg. (After Hansen.)
]

=Fam. 13. Choniostomatidae.=[60]—The members of this family are all
parasitic on other Crustacea. The majority live parasitically in the
marsupial pouches of female Amphipods, Isopods, Mysidae, and Cumacea,
_e.g._ _Sphaeronella_ and _Stenochotheres_ in the marsupia of Gammarids;
but _Choniostoma_ occurs in the branchial cavity of _Hippolyte_,
_Homoeoscelis_ is common in the branchial cavity of _Diastylis_ and
_Iphinoe_, and _Aspidoecia_ on the outside of the body of the Mysid
_Erythrops_. The males and females live together in the same marsupium,
but the adult males retain the power of roving about, and do not feed so
much as the females, though their mouth-parts are similarly constructed
(Fig. 47). Representatives occur all over the world, but the majority of
species known at present are from the North Sea, the most abundant being
_Stenochotheres egregius_, parasitic on the Gammarid _Metopa bruzelii_,
Goës.

The male bears a median glandular thread on the forehead by which it
attaches itself to the females or to the host. Hansen considers that the
family is most closely allied to the Lernaeopodidae.

[Illustration:

  FIG. 48.—_Argulus foliaceus_, young ♂, × 15. _a^1_, _a^2_, First and
    second antennae; _ab_, abdomen; _E_, compound eye; _l_, liver; _m_,
    mandibles and first maxillae; _mx_, second maxilla (the median eye
    is seen between the two second maxillae); _mxp_, maxillipede; _s.g_,
    shell-gland; _sp_, spine; _t_, testis; 1, 4, first and fourth
    swimming appendages. (After Claus.)
]


                         BRANCH II. BRANCHIURA.

=Fam. Argulidae.=[61]—We have yet to mention this group of
fish-parasites, related to the Copepoda, but occupying an isolated
position. They are ectoparasites upon various species of fish, _Argulus
foliaceus_ being common in the fresh waters of Europe, infesting the
branchial chamber or the skin of fresh-water fish, but being frequently
taken swimming freely in the water. Both males and females can swim with
great agility, and they leave their hosts regularly at the breeding
season in spring and autumn; fertilisation is internal, and the female
deposits the eggs on stones and other objects. After leaving its host,
an _Argulus_, if it cannot find a fish of the same species, can live on
almost any other species, and may even attack Frog tadpoles; while the
kinds that infest migratory fish can change with their hosts from salt
to fresh water, or the reverse. America appears to be the home of the
Argulidae.[62]

The structure of an Argulid is exhibited in Fig. 48. In front of the
siphon, within which the styliform mandibles and first maxillae work,
there is a poison-spine (_sp_); the appendages which correspond to the
second maxillae (_mx_) are modified into sucking discs, but in the genus
_Dolops_ they terminate in normal claws. The next pair of appendages,
usually spoken of as maxillipedes (_mxp_), are clasping organs, and
behind follow four pairs of thoracic swimming feet (1–4). The body is
foliaceous, and they always apply themselves to their hosts with the
long axis pointing forwards and parallel to that of the host, while on
various parts of the under surface of the body are spines pointing
backward which prevent the parasite being brushed off by the passage of
the host through the water. These animals, alone among the Copepoda,
possess compound eyes.

A short sketch has now been given of the variations in Copepod
organisation, but we cannot leave the subject without pointing out the
rich field which still remains for the morphologist, especially in
determining the true relationships of the parasitic families.




                               CHAPTER IV
    CRUSTACEA (_CONTINUED_): CIRRIPEDIA—PHENOMENA OF GROWTH AND SEX—
                               OSTRACODA


                        =Order III. Cirripedia.=

The Cirripedes are medium-sized Crustacea, with the body consisting of
few segments, and enveloped in a mantle formed as a fold of the external
integument, which may be strongly protected by calcified plates. The
abdomen is greatly reduced. The larva, after hatching out as a Nauplius,
and passing through a Cypris stage, when it resembles an Ostracod, fixes
itself to a foreign object by means of the first antennae, and becomes a
pupa, which after profound changes gives rise to the adult.

All the Cirripedes, when adult, live either a fixed or parasitic
existence, and as so frequently happens with animals of this kind, they
have departed widely from the ordinary structure of the class to which
they belong. Their anomalous appearance and the mystery surrounding
their propagation gave rise, probably, to the old legend that the
Barnacles (Lepadidae), which live attached to pieces of floating timber,
hatched out into Barnacle geese[63]; and even so late as 1678, in the
Royal Society’s _Transactions_, Sir Robert Moray describes what he takes
to be little birds enclosed in Barnacle shells, washed ashore on the
coast of Scotland: “The little Bill like that of a Goose, the Eyes
marked, the Head, Neck, Breast, Wings, Tail, and Feet formed, the
Feathers everywhere perfectly shaped and blackish coloured, and the Feet
like those of other Water-fowl, to my best remembrance.” Cuvier in his
classification of the animal kingdom included them in the Mollusca; and
it was not until 1830 that J. V. Thompson described their larval stages,
and showed conclusively that they belonged to the Crustacea. Since the
work of this naturalist a number of observers have securely founded our
knowledge of the group, but we may especially mention the epoch-making
works of Darwin,[64] Hoek,[65] and latterly of Gruvel.[66]

The young Cirripede is hatched out from the maternal mantle-cavity as a
free-swimming Nauplius, a larval form common to most of the Entomostraca
and to some Malacostraca; the Cirripede Nauplius (Fig. 49) is
characterised by the presence of well-developed frontal horns, and
usually by the long spiny processes which spring from various parts of
the body. As an introduction to the study of the group, it will be well
to follow the transformations of this larva in _Lepas_ up to the period
when it begins its sessile existence. The liberated Nauplii swim freely
near the surface of the sea, and remaining in this condition for several
days are dispersed widely from their birthplace; they are then
transformed by the process of moulting into the second larval stage,
known as the Cypris (Fig. 50), from its resemblance to a bivalve
Ostracod. The Cypris larva continues to swim about by means of the six
pairs of biramous thoracic legs until it finds a suitable place on which
to fix; in the case of _Lepas_ fixation usually takes place on loose
floating logs; the Cypris fixes itself by means of the first antennae,
at the bases of which a large cement-gland secretes an adhesive
substance. The biramous swimming legs are cast off, and six pairs of
biramous cirri characteristic of the adult take their place; at this
stage the body has the appearance shown in Fig. 51. The region of the
head at the base of the antennae now becomes greatly swollen and
elongated to form the peduncle or stalk of the adult; the larval bivalve
carapace is cast off and on the external surface of the mantle the
calcifications begin which will give rise to the exoskeletal plates of
the adult. This region is known as the “capitulum” of the adult, as
opposed to the “peduncle.” The young Cirripede is now known as a pupa,
and from this stage the adult form is reached by a gradual transition.

[Illustration:

  FIG. 49.—Nauplius larva of _Lepas fascicularis_, × 12. _A_{1}_,
    _A_{2}_, 1st and 2nd antennae; _B_, brain; _E_, eye; _H_,
    fronto-lateral horn; _M_, mandible; _S_, stomach. (After Groom.)
]

[Illustration:

  FIG. 50.—Cypris stage in the development of _Lepas australis_, × 15.
    _A_, Peduncle; _A.M_, adductor muscle; _C_, caecum of oesophagus;
    _C.g_, cement-glands; _Cr_, cirri (thoracic appendages); _E_,
    compound eye; _E^1_, simple eye; _G_, ventral ganglia; _I_,
    intestine; _M_, mouth; _M.C_, mantle-cavity; _O_, ovary; _S_,
    stomach. (After Hoek.)
]

[Illustration:

  FIG. 51.—Pupa of _Lepas pectinata_, × 8. _A_, Antenna; _C_, carina;
    _M_, adductor muscle; _S_, scutum; _T_, tergum. (After Gruvel.)
]

The body of the adult _Lepas_ is retracted into the mantle, and lies
free in the mantle-cavity, but is continuous anteriorly with the tissues
of the peduncle, into which the mantle does not extend. The thorax, with
its six pairs of legs, can be protruded from the mantle-cavity through
the slit-like opening which separates the two valves of the mantle along
the ventral middle line; and when the animal is feeding, the thoracic
legs are so protruded, and by their concerted waving action they drive
the food-particles in the water along the channel between them, until
the particles reach the oral cone, where they are masticated by the
mandibles and two pairs of maxillae, and so passed into the alimentary
canal. When the animal is disturbed it rapidly retracts its limbs, the
valves of the mantle are closed by means of a strong adductor muscle in
the head, and the animal is protected from all external influences. In
the acorn-barnacles (Operculata), which live in great numbers attached
to rocks and other objects between tide-marks, the body is constructed
on a similar plan, save that there is no stalk, and the body is
completely enclosed in a hard calcareous box formed from the mantle,
which, when the valves are closed, as they always are during low tide,
completely protect the animal inside from desiccation or danger of any
kind. Besides the cement-glands situated in the peduncle, we can
distinguish the generative organs, consisting of a pair of ovaries and
testes, the majority of Cirripedes being hermaphrodite. The testes open
at the end of an elongated median penis behind the thoracic limbs, while
the ovaries, situated in the peduncle, have paired openings into the
mantle-cavity on either side of the head. A pair of maxillary glands or
kidneys are present, and the alimentary canal is provided with various
digestive glands. Special branchial organs are not present in the
Pedunculate Cirripedes, but in the Operculate genera two branchiae are
formed from the plications of the internal surface of the mantle. There
is no contractile heart, and the circulatory system is poorly developed.
The Cirripedes are badly furnished with sensory organs; the remains of a
simple Nauplius eye may persist, situated on the upper part of the
stomach, but the chief sense-organs are the sensory hairs upon the
limbs.

[Illustration:

  FIG. 52.—=A=, Dwarf male of _Scalpellum vulgare_, × 27; =B=, diagram
    of Stalked Barnacle. _a_, Peduncle; _al_, alimentary canal; _b_,
    brain; _c_, carina; _e_, remains of Nauplius eye; _gl_,
    cement-gland; _m_, mantle-cavity; _o_, its opening; _ov_, ovary;
    _p_, penis; _s_, scutum; _t_, testis; _tm_, tergum, seen in =A= as
    the shaded body above the reference-line of _e_ and to the right of
    the carina, on the left of the figure.
]

The recent Cirripedes fall into six clearly defined Sub-orders.


                      =Sub-Order 1. Pedunculata.=

In this division, sometimes combined with the Operculata as THORACICA,
owing to the extremely reduced state of the abdomen, the body is borne
on a distinct stalk, and the bivalve arrangement of the mantle is
clearly retained. The mantle is protected externally by a number of
calcareous plates, the arrangement of which is typical of the various
genera. It appears that in the most primitive and geologically oldest
Cirripedes, the probable ancestors of the Pedunculate and Operculate
sub-orders, the arrangement of the plates was somewhat irregular, and
they were far more numerous than in the modern forms, so that passing
from these older types to modern times we witness a reduction in the
number and a greater precision in the arrangement of the skeletal parts.

[Illustration:

  FIG. 53.—=A=, _Turrilepas wrightianus_ (Silurian), × 1; =B=,
    _Archaeolepas redtenbacheri_ (Jurassic), × 1. _C_, carina; _R_,
    rostrum; _S_, scutum; _T_, tergum. (After Zittel.)
]

One of the most ancient Cirripedes known is _Turrilepas_, which occurs
in the Silurian deposits of England, but it is also known from earlier
deposits, while undoubted Cirripedes have been found in the Cambrian of
North America. The body of _Turrilepas_ is enclosed in imbricating
plates, as shown in Fig. 53, A.

In _Archaeolepas_ of the Upper Jurassic (Lithographic slates of Bavaria)
the arrangement of scutes typical of the Lepadidae is foreshadowed, but
the whole of the peduncle is protected by rows of plates (Fig. 53, B),
as in _Turrilepas_.

The above-mentioned genera did not survive into the Cretaceous period,
their places being taken by the genera _Pollicipes_ and _Scalpellum_,
which first appeared in the Silurian and persist to the present time,
the older and more primitive _Pollicipes_ being represented by about
half a dozen living species, while the species of _Scalpellum_ are
exceedingly numerous.

=Fam. 1. Polyaspidae.=—This family includes the three genera,
_Pollicipes_, _Scalpellum_, and _Lithotrya_.

[Illustration:

  FIG. 54.—_Pollicipes mitella_, × 1. (After Darwin.)
]

_Pollicipes_ is not only very ancient geologically (being found from the
Ordovician upward), but it preserves the primitive characteristic of
numerous skeletal plates, the peduncle being frequently covered with
small calcareous pieces, which graduate into the larger more regularly
placed scutes on the capitulum (Fig. 54). The species of this genus,
many of which are among the largest Cirripedes, are widely distributed
in the temperate and tropical seas, living for the most part attached to
rocks and often in deep water. _P. cornucopia_ occurs off the English
and Scottish coasts.

The members of the genus _Scalpellum_, which is represented by
exceedingly numerous species in the Cretaceous period, also possess a
large number of plates on the capitulum, and often on the peduncle as
well, but never so many as in _Pollicipes_. Although the arrangement of
the plates varies much in the different species, we may describe a
fairly typical case, that of the common _Scalpellum vulgare_ (Fig. 55,
B).

The valves of the capitulum are held together by the median dorsal piece
called the “carina”; the other unpaired skeletal piece is the “rostrum,”
in front, just below the place where the valves gape to allow the
protrusion of the limbs. The paired pieces receive the names “scutum,”
“tergum,” and “laterals,” and the peduncle is covered with rows of small
plates.

The genus _Scalpellum_ is a very large one, and is widely distributed,
though at the time at which Darwin wrote only six species were known.
The reason for this is to be found in the fact that the great majority
of the species live at great depths, so that they remained unknown until
the expeditions of the _Challenger_ and other deep-sea expeditions
brought them to light. They may affix themselves, generally in
considerable numbers together, on branching organisms, such as Corals,
Polyzoa, and Hydroids, but often also on empty shells, rocks, and other
foreign bodies. The body is colourless or of a pale flesh colour, but a
colony of these animals, expanded and drooping in various attitudes from
a piece of coral, gives the appearance of some graceful exotic flower.

[Illustration:

  FIG. 55.—=A=, Complemental male of _Scalpellum peronii_, × 20; =B=,
    hermaphrodite individual of _S. vulgare_, × 2. _a_, Complemental
    males, _in situ_; _b_, rostrum. (=A=, after Gruvel; =B=, after
    Darwin.)
]

Perhaps the most interesting feature of the genus is the remarkable
variation in the sexual constitution of some of the species. The great
majority of the Pedunculata and all the Operculata are hermaphrodites,
which habitually cross-fertilise one another, and this they are well
fitted to do, since they all live gregariously and are provided with a
long exsertile penis for transferring the spermatozoa from one to the
other. In _Pollicipes_, however, the individuals of which often live
solitarily, it appears that self-fertilisation may occur. In
_Scalpellum_ three different kinds of sexual constitution may occur: (1)
According to Hoek in _S. balanoides_, taken by the _Challenger_, the
individuals are ordinary cross-fertilising hermaphrodites. (2) In the
great majority of species, including the common _S. vulgare_, as
originally described by Darwin, and since confirmed by Hoek and
Gruvel,[67] the individuals are hermaphrodite, but there are present
affixed to the adult hermaphrodites, just inside the opening of the
valves in a pocket of the mantle, a varying number of exceedingly minute
males, called by Darwin “complemental males.” These tiny organisms are
really little more than bags of spermatozoa, but they possess to varying
degrees the ordinary organs of the adult in a reduced condition. The
male of _S. peronii_ (Fig. 55, A) retains the shape and skeletal plates
of the ordinary form, and differs chiefly in its reduced size; but the
more common condition is exhibited by the male of _S. vulgare_ (Fig. 52,
A), where the scutes are reduced to vestiges round the mantle-opening,
and almost the whole of the body is occupied by the greatly developed
generative organs. (3) In a few species, e.g. _S. velutinum_ and _S.
ornatum_, the individuals are purely dioecious, being either females of
the ordinary structure resembling the hermaphrodites of the other
Lepadidae, or dwarfed males resembling closely the complemental males
described above for _S. vulgare_.

[Illustration:

  FIG. 56.—_Lithotrya dorsalis_, x 1. _B_, Basal calcareous cup; _C_,
    carina; _R_, rostrum; _S_, scutum; _T_, tergum. (After Darwin.)
]

The nature and derivation of these various conditions will be discussed
when the parallel cases found in _Ibla_ and among the Rhizocephala have
been described.

The remaining genus of the Polyaspidae, also characterised by the
presence of numerous skeletal plates on the capitulum, is _Lithotrya_
(Fig. 56), which bores into rocks and shells, and is an inhabitant of
the warm and tropical seas.

The peduncle of the full-grown animal is completely imbedded in the rock
or shell to which it is attached, and at the basal end of the peduncle
is situated a cup composed of large irregular calcified pieces. This cup
is, however, not formed until the animal has ceased to burrow. The
excavation of the substratum is effected by means of a number of small
rasping plates which cover the peduncle, the whole being set in motion
by the peduncular muscles.

[Illustration:

  FIG. 57.—_Conchoderma virgata_, × 1. _C_, Carina; _S_, scutum; _T_,
    tergum. (After Darwin.)
]

=Fam. 2. Pentaspidae.=—In this family are placed a number of genera, and
among them the common _Lepas_, the species of which possess typically
five skeletal plates, viz., a carina and a pair of scuta and of terga,
the peduncle being naked. These forms are a later development of
Cirripede evolution, and did not come into existence till Tertiary
times. Some of them, e.g. _Oxynaspis_, live at considerable depths
attached to corals, etc., but large numbers float on the surface of the
sea, fixed often on logs and wreckage of various kinds. _Dichelaspis_ is
found attached to the shells of large Crustacea.

_Conchoderma_ is an interesting genus, the species of which live affixed
to various floating objects, the keels of ships, etc.; the mantle is
often brilliantly coloured, as in _C. virgata_, and the skeletal plates
are reduced to the merest vestiges, leaving the greater part of the body
fleshy.

[Illustration:

  FIG. 58.—_Ibla cumingii_, ♀, × 1. _S_, Scutum; _T_, tergum. (After
    Darwin.)
]

[Illustration:

  FIG. 59.—_Ibla cumingii_, dwarf male, × 32. _A_, Antennae; _B_, part
    of male imbedded in the female, to which the torn membrane _M_
    belongs; _E_, eye; _Th_, thoracic appendages or cirri. (After
    Darwin.)
]

=Fam. 3. Tetraspidae.=—This family includes the single genus _Ibla_
(Fig. 58), which possesses only four skeletal plates, a pair of terga
and of scuta, coloured blue, while the peduncle is covered with brown
spines. There are only two very similar species known, _I. cumingii_,
which is found attached to the peduncle of _Pollicipes mitella_, and _I.
quadrivalvis_, living on masses of the Siphonophore _Galeolaria
decumbens_. These two species are quite different in the partition of
the sexes. In _I. cumingii_ the large individuals of normal structure
are females, inside the mantle-cavities of which are attached dwarf
males of the form shown in Fig. 59.

These organisms have the peduncle buried completely in the substance of
the female’s mantle, inside which they live; they exhibit a degenerate
structure, but still retain two pairs of cirri. The large individuals of
_I. quadrivalvis_, on the other hand, are hermaphrodites, but they
harbour within their mantles minute complemental males similar to those
of _I. cumingii_, though they are rather larger.

[Illustration:

  FIG. 60.—Diagram of the shell of an Operculate Cirripede. _a_ “Ala,”
    or overlapped portion of a “compartment”; _B_, basis; _C_, carina;
    _C.L_, carino-lateral; _L_, lateral; _R_, rostrum; _r_, “radius,” or
    overlapping portion of a compartment; _R.L_, rostro-lateral. (After
    Darwin.)
]

=Fam. 4. Anaspidae.=—This includes the remaining pedunculate genera,
characterised by the fleshy nature of the mantle and peduncle, which are
both entirely devoid of calcifications. The species of _Alepas_ live
upon Echinoderms and various other animals; _Chaetolepas_ upon
_Sertularia_, and _Gymnolepas_ upon Medusae. _Anelasma squalicola_ is an
interesting form, living parasitically upon the Elasmobranch fishes,
_Selache maxima_ and _Spinax niger_ in the North Sea. The peduncle is
deeply buried in the flesh of the host, so that only a portion of the
dark blue capitulum protrudes to the surface. From the whole surface of
the peduncle a system of branching processes is given off, which ramify
far into the tissues of the fish, and communicate inside the peduncle
with the lacunar tissue, which is packed round all the organs of the
Cirripede. There can be small doubt that the _Anelasma_ derives its
nutriment parasitically through this root-system, since the cirri are
mere fleshy lobes unadapted to securing food, and the alimentary canal
is always empty. This animal has a suggestive bearing on the
Rhizocephala, which, as will be shown, derive their nutriment from a
system of roots penetrating the host and growing out from what
corresponds morphologically to the peduncle.


                       =Sub-Order 2. Operculata.=

[Illustration:

  FIG. 61.—_Balanus tintinnabulum_, with the right half of the shell and
    of the operculum removed, seen from the right side. _A_, Antennae,
    the size of which is exaggerated; _A.M_, adductor muscle; _B_,
    basis; _C_, carina; _Cr_, cirri or thoracic appendages; _D_,
    oviduct; _G_, ovary; _L_, lateral compartment; _Lb_, labrum or upper
    lip; _M_, _M_, depressor muscles of scutum and tergum; _M.C_,
    mantle-cavity; _O_, orifice of excretory organ; _O.M_, opercular
    membrane; _R_, rostrum; _S_, scutum; _St_, region of stomach; _T_,
    tergum. (After Darwin.)
]

The “acorn-barnacles” appear later in geological time than the earlier
stalked forms. _Verruca_ and _Chthamalus_ are found in the Chalk, and
survive down to the present day, but _Balanus_ does not occur until
middle Tertiary times. Representatives of the last-named genus are
familiar to every one, as the hard sharp objects which cover rocks and
piles near high-water mark on every sea-coast. If we examine the hard
skeleton of one of these animals, we find that, unlike the Pedunculata,
they possess no stalk, the capitulum being fused on to the surface of
attachment by a broad basal disc. Typically, there may be considered to
be eight skeletal pieces forming the outer ring which invests the soft
parts of the animal, an unpaired rostrum and carina, and laterally a
pair of rostro-lateral, lateral, and carino-lateral “compartments,” as
shown in Figs. 60, 63.

The skeletal ring is roofed over by a pair of terga at the carinal end
and a pair of scuta at the rostral end; these four plates make up the
operculum by which the animal can shut itself completely up in its
shell, or between the valves of which it can protrude its limbs for
obtaining food.

[Illustration:

  FIG. 62.—Diagrammatic section of the growing shell of _Balanus
    porcatus_. _C_, Canals; _Ct_, cuticle; _H_, hypodermis (=
    epidermis); _H′_, part of shell secreted by the hypodermis; _Hl_,
    hypodermal lamina; _M_, part of shell secreted by the mantle. (After
    Gruvel.)
]

The relation of the animal to its shell is shown in Fig. 61. The shell
in the Operculata is not merely secreted as a dead structure on the
external surface of the epidermis, but represents a living calciferous
tissue interpenetrated by living laminae (Fig. 62, Hl) derived partly
from the external hypodermis and partly from the lining of the mantle.
The hard parts of the shell usually also contain spaces and canals (C).

[Illustration:

  FIG. 63.—Diagrams of shells of Operculata. =A=, _Catophragmus_
    (Octomeridae); =B=, _Balanus_, _Coronula_, etc. (Hexameridae); =C=,
    _Tetraclita_ (Tetrameridae). _C_, carina; _C.L_, carino-lateral;
    _L_, lateral; _R_, rostrum; _R.L_, rostro-lateral.
]

The various forms of Acorn-barnacle may be classified according to the
number of pieces that go to make up the skeleton; thus starting with the
typical number eight (Fig. 63, A), we find that in various degrees a
fusion between neighbouring pieces has taken place in the different
families.

=Fam. 1. Verrucidae.=—The ancient genus _Verruca_, which is still widely
distributed in all seas, and is found fixed upon foreign objects on the
sea-bottom at various depths, is interesting on account of the asymmetry
of its shell, which bears a different aspect according to which side one
regards it from. This asymmetry is brought about by the skeletal pieces
(carina, rostrum, and paired terga and scuta) shifting their positions
after fixation has taken place.

=Fam. 2. Octomeridae.=—In this family the eight plates composing the
shell are separate and unfused (Fig. 63, A). The majority of the species
come from the Southern hemisphere, _e.g._ the members of the genera
_Catophragmus_ and _Octomeris_, but _Pachylasma giganteum_ occurs in
deep water in the Mediterranean, where it has been found fixed upon
Millepore corals.

=Fam. 3. Hexameridae.=—This family includes by far the greater number of
the Acorn-barnacles, in which only six plates are present, the laterals
having fused with the carino-laterals (Fig. 63, B). The very large genus
_Balanus_ belongs here, the common _B. tintinnabulum_ of our coasts
being found all over the world, and occurring under a number of
inconstant varietal forms. Especial interest attaches to certain other
genera, from their habit of living parasitically on soft-bodied animals,
whose flesh they penetrate.

_Coronula diadema_ and _Tubicinella trachealis_ live embedded in the
skin of whales, the shell of the first-named being of a highly
complicated structure with hollow triangular compartments into which the
mantle is drawn out.

_Xenobalanus globicipitis_ lives attached to various Cetacea, and is
remarkable for the rudimentary condition of its skeleton, the six plates
of which form a mere disc of attachment from which the greatly elongated
naked body rises, resembling one of the naked Stalked Barnacles.

=Fam. 4 Tetrameridae.=—In this family only four skeletal plates are
present (Fig. 63, C). This family is chiefly confined to tropical seas
or those of the Southern hemisphere. The chief genera are _Tetraclita_
and _Pyrgoma_, found in British seas.


                     =Sub-Order 3. Acrothoracica.=

Gruvel includes in this sub-order four genera (_Alcippe_,
_Cryptophialus_, _Kochlorine_, and _Lithoglyptes_) the species of which
live in cavities excavated in the shells of molluscs or in the hard
parts of corals.

[Illustration:

  FIG. 64.—_Alcippe lampas._ =A=, ♀, × about 10, seen from the right
    side, with part of the right half of the animal removed; =B=, dwarf
    male, × about 30. _A.M_, adductor muscle; _An_, antenna; _C_, 1st
    pair of cirri; _Cr_, posterior thoracic appendages; _E_, eye; _G_,
    testis; _M.C_, mantle-cavity; _O_, ovary; _P_, penis; _T_,
    penultimate thoracic segment; _V._ vesicula seminalis. (After
    Darwin.)
]

Darwin discovered and described _Cryptophialus minutus_, and placed it
in a sub-order Abdominalia, believing that it was distinguished from all
the foregoing Cirripedes by the presence of a well-developed abdomen.
Since the discovery of other allied genera, it has been decided that the
abdomen is equally reduced in these forms, and that the terminal
appendages do not belong to this region, but to the thorax.

The sexes are separate. The body of the female (Fig. 64, A) is enclosed
in a chitinous mantle, armed with teeth by which the excavation is
effected, and is attached to the cavity in the host by means of a horny
disc. Upon this disc the dwarf males (B) are found.

_Alcippe lampas_ inhabits holes on the inner surface of dead _Fusus_ and
_Buccinum_ shells; _Cryptophialus minutus_ the shells of _Concholepas
peruviana_; _C. striatus_ [68] the plates of _Chiton_; _Kochlorine
hamata_ the shells of _Haliotis_; and _Lithoglyptes varians_ shells and
corals from the Indian Ocean.


                     =Sub-Order 4. Ascothoracica.=

These are small hermaphrodite animals completely enveloped in a soft
mantle, which live attached to and partly buried in various organisms,
such as the branching Black Corals (_Gerardia_). They retain the
thoracic appendages in a modified state, and the body is segmented into
a number of somites, the last of which probably represents an abdomen.

_Laura gerardiae_, described by Lacaze Duthiers,[69] is parasitic on the
stem of the “Black Coral,” _Gerardia_ (vol. i. p. 406); it has the shape
of a broad bean, the body being entirely enclosed in a soft mantle, with
the orifice in the position corresponding to the hilum of a bean. The
body lying in the mantle is composed of eleven segments, and is curved
into an =S=-shape. Its internal anatomy is entirely on the plan of an
ordinary Cirripede.

_Petrarca bathyactidis_, G. H. Fowler,[70] was found in the mesenteric
chambers of the coral _Bathyactis_, dredged by the _Challenger_ from
4000 metres. The body is nearly spherical, and the mantle-opening forms
a long slit on the ventral surface. The mantle is soft, but is furnished
on the ventral surface with short spines.

The antennae, which form the organs of fixation, remain very much in the
state characteristic of the Cypris larvae of other Cirripedes, being
furnished with two terminal hooks by which attachment is effected. The
thoracic appendages, of which there are the normal number six, are
reduced flabellate structures, and the abdomen forms an indefinitely
segmented lobe of considerable size.

The animal appears to be in an arrested state of development, and so
retains some of the characteristics of the Cypris larvae, but it is very
doubtful how far these characters can be considered primitive.

Other forms are _Dendrogaster astericola_ on Echinoderms, and _Synagoga
mira_ on the “Black Coral,” _Parantipathes larix_, at Naples.


                         =Sub-Order 5. Apoda.=

[Illustration:

  FIG. 65.—_Proteolepas bivincta_, × 26. _A_, Antennae; _a_, _b_, 1st
    and 2nd abdominal segments; _O_, ovary; _P_, penis; _T_, telson;
    1–8, thoracic segments. (After Darwin.)
]

Darwin described a small hermaphrodite parasite in the mantle chamber of
_Alepas cornuta_ from Saint Vincent, West Indies, which he named
_Proteolepas bivincta_.

The body (Fig. 65) is distinctly segmented into eleven somites, the last
three of which are supposed to belong to the abdomen; there are no
appendages except the antennae by which fixation is effected. The
mouth-parts are of normal constitution.

This animal has not been found again since Darwin’s discovery, but
Hansen[71] describes a number of peculiar Nauplius larvae taken in the
plankton of various regions, which he argues probably belong to members
of this group. A wide field of work is offered in attempting to find the
adults into which various larvae grow.


                    =Sub-Order 6. Rhizocephala.=[72]

These remarkable animals are Cirripedes which have taken to living
parasitically on various kinds of Crustacea; the majority infest species
of Decapoda, e.g. _Peltogaster_ on Hermit-Crabs, _Sacculina_ on a number
of Brachyura, _Sylon_ on Shrimps, _Lernaeodiscus_ on _Galathea_; but one
genus, _Duplorbis_, has been found in the marsupium of the Isopod
_Calathura brachiata_ from Greenland. Most of the species are solitary,
but a few, e.g. _Peltogaster sulcatus_, are social. In the adult state
the body consists of two portions: a soft bag-like structure, external
to the host, carrying the reproductive, nervous, and muscular organs,
and attached to some part of the host’s abdomen by means of a chitinous
ring; and a system of branching roots inside the host’s body, which
spring from the ring of attachment and supply the external body with
nourishment.

[Illustration:

  FIG. 66.—Nearly median longitudinal section (diagrammatic) of
    _Peltogaster_. _gn_, Brain; _m_, mantle; _mc_, mantle-cavity; _mes_,
    mesentery; _op_, mantle-opening; _ov_, ovary; _ovd_, oviduct;
    _ring_, ring of attachment; _t_, testis; _vd_, _vas_ deferens.
]

[Illustration:

  FIG. 67.—Diagrammatic median longitudinal section through a normal
    Cirripede, _gn_, Brain; _op_, mantle-opening; _ovd_, oviduct; _vd_,
    vas deferens.
]

The structure of the external bag-like portion is very simple, and
varies only in details, chiefly of symmetry, in the different genera. In
_Peltogaster_, which preserves the simplest symmetrical arrangement of
the organs, a diagrammatic section through the long axis of the body
(Fig. 66) shows that it consists of a muscular mantle (_m_) surrounding
a visceral mass, and enclosing a mantle-cavity (_mc_) or brood-pouch,
which stretches everywhere between mantle and visceral mass, except
along the surface by which the parasite is attached to its host, where a
mesentery (_mes_) is formed. The ring of attachment is situated in the
middle of this mesentery; the mantle-cavity, which is completely lined
externally and internally with chitin, opens anteriorly by means of a
circular aperture (_op_) guarded by a sphincter muscle. The visceral
mass is composed chiefly of the two ovaries (_ov_), which open on either
side of the mesentery by means of a pair of oviducts (_ovd_); the paired
testes (_t_) are small tubes lying posteriorly in the mesentery, and the
nervous ganglion (_gn_) lies in the mesentery between oviducts and
mantle-opening. A comparison with the condition of a normal Cirripede
(Fig. 67) shows us that the mesenterial surface of the parasite by which
it is fixed corresponds to the dorsal surface of an ordinary Pedunculate
Cirripede, and that the ring of attachment corresponds with the stalk or
peduncle of a _Lepas_. The root-system passes out through the ring of
attachment into the body of the host, and ramifies round the organs of
the crab; the roots are covered externally with a thin chitinous
investment, and consist of an epithelium and an internal mass of
branching cells continuous with the lacunar tissue in the visceral mass.

[Illustration:

  FIG. 68.—Development of _Sacculina neglecta_. =A=, Nauplius stage, ×
    about 70; =B=, Cypris stage, × about 70. _A_{1}_, _A_{2}_, 1st and
    2nd antennae of Nauplius; _Ab_, abdomen; _Ant_, antenna of Cypris;
    _E_, undifferentiated cells; _F_, frontal horn; _G_, glands of
    Cypris; _H_, tendon of Cypris; _M_, mandible; _T_, tentacles.
]

The developmental history of the Rhizocephala is one of the most
remarkable that embryology has hitherto revealed. It has been most
accurately followed in the case of _Sacculina_. The young are hatched
out in great numbers from the maternal mantle-cavity as small Nauplii
(Fig. 68, A) of a typical Cirripede nature, but without any alimentary
canal. They swim near the surface of the sea, and become transformed
into Cypris larvae of a typical character (Fig. 68, B). The Cypris
larva, after a certain period of free existence, seeks out a crab and
fixes itself by means of the hooks on its antennae to a hair on any part
of the crab’s body. Various races of _Sacculina_ are known which infest
about fifty different species of crabs in various seas; the best known
are _S. carcini_ parasitic on _Carcinus maenas_ at Plymouth and Roscoff,
and _S. neglecta_ on _Inachus mauritanicus_ at Naples. The antenna, by
which the Cypris is fixed, penetrates the base of the hair; the
appendages are thrown away, and a small mass of undifferentiated cells
is passed down the antenna into the body-cavity of the crab. Arrived in
the body-cavity it appears that this small mass of cells is carried
about in the blood-stream until it reaches the spaces round the
intestine in the thorax. Here it becomes applied to the intestine,
usually at its upper part, immediately beneath the stomach of the crab
(Fig. 69), and from this point it proceeds to throw out roots in all
directions, and as it grows to extend its main bulk, called the central
tumour (_c.t_), towards the lower part of the intestine. As the
posterior border of the central tumour grows down towards the hind-gut,
the future organs of the adult _Sacculina_ become differentiated in its
substance; the mantle-cavity being excavated and surrounding the
rudiment of the visceral mass, while as the central tumour grows
downwards it leaves behind it an ever extending system of roots. When
the central tumour in process of differentiation has reached the
unpaired diverticulum of the crab’s intestine, at the junction between
thorax and abdomen, all the adult organs are laid down in miniature, and
the whole structure is surrounded by an additional sac formed by
invagination known as the perivisceral space (Fig. 70). The young
“_Sacculina_ interna” remains in this position for some time, and being
applied to the ventral abdominal tissues of the crab just at the point
where thorax and abdomen join, or a little below it, it causes the
crab’s epithelium to degenerate, so that when the crab moults, a little
hole is left in this region of the same size as the body of the
_Sacculina_, owing to the failure of the epithelium to form chitin here;
and thus the little parasite is pushed through this hole and comes to
the exterior as the adolescent “_Sacculina_ externa.” From this point
onwards the crab, being inhibited in its growth through the action of
the parasite, never moults again; so that the _Sacculina_ occupies a
safe position protruding from the crab’s abdomen, which laps over it and
protects it. The remarkable features of this development are, firstly,
the difficulty of understanding how the developing embryo is directed in
its complicated wanderings so as always to reach the same spot where it
is destined to come to the exterior; and, secondly, the loss after the
Cypris stage of all the organs and the resumption of an embryonic
undifferentiated state from which the adult is newly evolved. A certain
parallel to this history is found in that of the Monstrillidae,
described on pp. 64–66.

[Illustration:

  FIG. 69.—The mid-gut of _Inachus mauritanicus_ with a young
    _Sacculina_ overlying it, × 2. _c.t_, “Central tumour” of the
    parasite; _d.i_, _d.s_, inferior and superior diverticula of
    alimentary canal of host; _n_, “nucleus,” or body-rudiment of
    _Sacculina_; _r_, its roots; _x_, definitive position of the
    parasite.
]

[Illustration:

  FIG. 70.—Later stage in the development of the “_Sacculina_ interna,”
    × 2. _b_, Body of _Sacculina_; _c.t_, “central tumour”; _d.i_,
    _d.s_, inferior and superior diverticula of alimentary canal of
    host; _o_, opening of perivisceral cavity of _Sacculina_; _r_, its
    roots.
]

[Illustration:

  FIG. 71.—Fourteen Cypris larvae fixed round the mantle-opening (_o_)
    of a young _Sacculina_ externa, × 20.
]

The Rhizocephala are hermaphrodite with the possible exception of
_Sylon_, which appears to be female and perhaps parthenogenetic, no male
having been seen; but unlike most other hermaphrodite Cirripedes, they
reproduce by a continual round of self-fertilisation. This is the more
remarkable in that the vestiges of what appears to be a male sex are
still found in _Sacculina_ and _Peltogaster_; certain of the Cypris
larvae in these genera, instead of fixing on and inoculating other
crabs, become attached round the mantle-openings of young parasites of
the same species as themselves, which have recently attained to the
exterior of their hosts (Fig. 71). These larvae, which remind us of the
complemental males in _Scalpellum_, etc., never produce spermatozoa, but
rapidly degenerate where they are fixed, and appear never to play any
rôle in the reproduction of their species. The nature of this remarkable
phenomenon, together with the sexual condition of the Cirripedes in
general, will be discussed in the next section.

Much remains to be elucidated in the life-histories of these curious
animals, and it seems probable that intermediate stages may exist,
showing us how the extreme discontinuity of development has been
reached. Suggestive in this respect is the newly discovered parasite of
the Isopod, _Calathura_, which the author has named _Duplorbis
calathurae_.[73] This animal does not appear to possess a root-system,
but is attached to its host by a tube which passes right through the
mesentery and opens into the mantle-cavity of the parasite. It may be
suggested that this tube corresponds to the stalk of the normal
Cirripede, but its exact mode of formation would certainly throw much
light on the question of Rhizocephalan development.


            =Phenomena of Growth and Sex in the Crustacea.=

In the foregoing account of the Cirripedia we have met with certain
peculiar sexual relations in which closely allied species exhibit marked
differences in regard to the distribution of the qualities of sex among
their individuals; we have seen that the majority of species are
hermaphrodite, unlike most Crustacea which, with the other exception of
the parasitic Isopoda, are normally dioecious; and that in some species
complemental males exist side by side with the hermaphrodites, while, in
yet others, the individuals are either females or dwarf males.

Before examining the causes of these conditions, it will be opportune to
consider a number of facts which throw light on the question of sex and
hermaphroditism in general. We may then return to the discussion of the
hermaphroditism found in particular in the Cirripedia and Isopoda.

=Parasitic Castration.=—Giard[74] was the first to observe that a number
of parasites exert a remarkable influence on the sexual characters of
their host, such that the generative glands become reduced, or may
completely degenerate, while the secondary sexual characters become
materially altered. This was proved to occur in the most widely
different hosts, affected by the most widely different parasites (_e.g._
Crustacea, Insecta, Worms). Moreover, it was apparent that the affection
does not consist in the parasite merely destroying the generative
organs, with which it often does not come into contact, but rather in
the general disturbance of the metabolism set up by its presence.

The most completely studied cases of parasitic castration are those of
the Rhizocephalous _Sacculina neglecta_, parasitic on the spider-crab,
_Inachus mauritanicus_,[75] and of _Peltogaster curvatus_ on the
Hermit-crab, _Eupagurus excavatus_, var. _meticulosa_.[76] The ordinary
males of _I. mauritanicus_ have the appearance shown in Fig. 72, A. The
abdomen is small and bears a pair of copulatory styles, while the
chelipedes are long and swollen. In the female (B) the abdomen is much
larger and trough-shaped, and carries four pairs of ovigerous
appendages; the chelae are small and narrow.

[Illustration:

  FIG. 72.—Illustrating the effect of parasitic _Sacculina neglecta_ on
    _Inachus mauritanicus_, nat. size. =A=, Normal male; _Inachus_; =B=,
    normal female; =C=, male infested by _Sacculina_ (final stage); =D=,
    abdomen of infested female; =E=, infested male in an early stage of
    its modification.
]

Now it is found that in about 70 per cent of males infected with
_Sacculina_ the body takes on to varying degrees the female characters,
the abdomen becoming broad as in the female, with a tendency to develop
the ovigerous appendages, while the chelae become reduced (Fig. 72, C).
This assumption of the female characteristics by the male under the
influence of the parasite may be so perfect that the abdomen and chelae
become typically female in dimensions, while the abdomen develops not
only the copulatory styles typical of the male, but also the four pairs
of ovigerous appendages typical of the female. The parasitised females,
on the other hand, though they may show a degenerate condition of the
ovigerous appendages (Fig. 72, D), never develop a single positively
male characteristic. On dissecting crabs of these various categories it
is found that the generative organs are in varying conditions of
degeneration and disintegration.

The most remarkable fact in this history is the subsequent behaviour of
males which have assumed perfect female external characters, if the
_Sacculina_ drops off and the crabs recover from the disease. It is
found that under these circumstances these males may regenerate from the
remains of their gonads a perfect hermaphrodite gland, capable of
producing mature ova and spermatozoa. The females appear quite
incapable, on the other hand, of producing the male primary elements of
sex on recovery, any more than they can produce the secondary. Exactly
analogous facts have been observed in the case of the hermit-crabs
parasitised by _Peltogaster_, but here the affected males produce small
ova in their testes before the parasite is got rid of. Here, too, the
females seem incapable of assuming male characters under the influence
of the parasite.

To summarise shortly the conclusions to be deduced from these facts—
certain animals react to the presence of parasites by altering their
sexual condition. This alteration consists in the female sex in an
arrest of reproductive activity, in the male sex in the arrest of
reproductive activity coupled with the assumption of all the external
characters proper to the female. But in these males it is not merely the
external characters that have been altered; their capacity for
subsequently developing hermaphrodite glands shows that their whole
organisation has been converted towards the female state. That this
alteration consists in a reorganisation of the metabolic activities of
the body is clearly suggested, and in the succeeding paragraph we
furnish some further evidence in support of this view.

[Illustration:

  FIG. 73.—_Inachus mauritanicus_, × 1. =A=, Low male; =B=, middle male;
    =C=, high male; the great chela of the right side is the only
    appendage represented.
]


   =Partial and Temporary Hermaphroditism. High and Low Dimorphism.=

The reproductive phases of animals are frequently rhythmic, periods of
growth alternating with periods of reproduction. This is well
exemplified in the case of the ordinary males of _Inachus mauritanicus_,
of some other Oxyrhynchous crabs, and of the Crayfish _Cambarus_.[77]
During the breeding season the males of _I. mauritanicus_ fall into
three chief categories: Small males with swollen chelae (Fig. 73, A),
middle-sized males with flattened chelae (B), and large males with
enormously swollen chelae (C). On dissecting specimens of the first and
third categories it is found that the testes occupy a large part of the
thoracic cavity and are full of spermatozoa, while in the middle-sized
males with female-like chelae the testes appear shrivelled and contain
few spermatozoa. These non-breeding crabs are, in fact, undergoing a
period of active growth and sexual suppression before attaining the
final state of development exhibited by the large breeding males. This
phenomenon is obviously parallel to the “high and low dimorphism”[78] so
common in Lamellicorn beetles, where the males of many species are
divided into two chief categories, viz. “low males” of small size in
which the secondary sexual characters are poorly developed, and “high
males” of large size in which these characters are proportionately much
more highly developed than in the low males. The only difference between
the two cases is that whereas in the beetles growth ceases on the
attainment of maturity in the low degree, in the Crustacea the low male
passes through a period of growth and sexual suppression to reach the
high degree of development.

The condition of the middle-sized males may be looked upon as one of
partial hermaphroditism, indications of the female state being found in
the flattened chelae and in the reduced state of the testes. This
interpretation is greatly strengthened by the state of affairs observed
in the life-history of the male Sand-hoppers, Amphipods of the genus
_Orchestia_.[79] In the young males of several species of this genus, at
the time of year when they are not actively breeding, small ova are
developed in the upper part of the testes of more than half of the male
individuals, these ova being broken down and reabsorbed as the breeding
season reaches its height. Nor is this phenomenon confined to this
genus; in the males of a number of widely different Crustacea these
small ova are found in the testes at certain periods of the life-history
(_e.g._ _Astacus_[80]), when the animal is not breeding.

The foregoing facts indicate unmistakably that the males of a number of
Crustacea under certain metabolic conditions, _i.e._ when a stage of
active growth as opposed to a stage of reproductive activity is
initiated, alter their sexual constitution in such a way that the latent
female characteristics are developed, and the organism appears as a
partial hermaphrodite. In the preceding paragraph we saw that the males
of a number of animals, especially Crustacea, react to the metabolic
disturbance set up by the presence of a parasite in exactly the same
way, _i.e._ by developing into partial or total hermaphrodites. From
these two converging bodies of facts we may conclude, firstly, that sex
and metabolism are two closely connected phenomena; and, secondly, that
the male sex is especially liable to assume hermaphrodite characters
whenever its metabolic requirements are conservative, assimilatory, or
in a preponderating degree anabolic, as when a phase of active growth is
initiated, or the drain on the system, due to the presence of a
parasite, is to be made good.


     =Normal Hermaphroditism in Cirripedia and Isopoda Epicarida.=

The above-mentioned groups contain the only normally hermaphrodite
Crustacea, and since they are in most respects highly specialised, we
may be certain that they have been secondarily derived from dioecious
ancestors. They both lead a sessile or parasitic life, and it is
noteworthy that this habit is often associated with hermaphroditism,
_e.g._ in Tunicates. A sessile or parasitic mode of life is one in which
the metabolic functions are vegetative and assimilatory rather than
actively kinetic or metabolic. It is in this state that we have seen the
males of a number of Crustacea taking on a temporary or partial
hermaphroditism. We may, therefore, inquire, whether in these cases of
normal hermaphroditism there is any evidence to show that here too the
hermaphroditism has been acquired by the male sex as a response to the
change in the metabolic conditions. In the parasitic Isopoda Epicarida
(see pp. 129–136) the hermaphroditism is of a very simple kind; all the
individuals are at first males, whose function it is to fix on and
fertilise the adult parasites. These subsequently develop into females
which are in their turn cross-fertilised by the young larvae derived
from a previous generation. All the individuals being alike, it seems
probable that they have been derived from one sex, and the general
nature of hermaphroditism deduced above may lead us to suppose that that
sex was originally male, the female having been suppressed. In certain
Cirripedia, _e.g._ most species of _Scalpellum_, there exist, besides
the hermaphrodite individuals, complemental males, so that here a
superficial conclusion might be drawn that the hermaphrodites represent
the female sex. But if we can suggest that the complemental males are in
reality similar in derivation to the hermaphrodite individuals, we shall
be in a position to claim that the hermaphrodite Cirripedes are similar
to the Isopoda Epicarida, and have probably also been derived from the
male sex. There is decided evidence pointing to this conclusion. In the
first place, the complemental males of at least one species of
_Scalpellum_, _S. peronii_, do show an incipient hermaphroditism[81] in
the presence of small ova in their generative glands, which, however,
never come to maturity.

The condition of the degenerate males in the Rhizocephala may also be
interpreted in the same manner. These never pass beyond the Cypris stage
of development, in which they resemble in detail the Cypris larvae of
the ordinary hermaphrodite individuals, and they are quite useless in
the propagation of their species.

It is more reasonable to suppose that these Cypris larvae, which fix on
the mantle-openings of adult parasites, are in reality identical with
the ordinary Cypris which infest crabs and develop into the
hermaphrodites, than that they represent a whole male sex doomed
beforehand to uselessness and degeneration. If we suppose that the
Cirripedes have passed through a state of protandric hermaphroditism
similar to that of the Isopoda Epicarida, it is plain that all the
larvae must have originally possessed the instinct of first fixing on
the adult parasites, and we may suppose that this instinct has been
retained in the Rhizocephala, but is now only actually fulfilled by a
certain proportion of the larvae, which, under existing circumstances,
are useless and fail to develop further; while the rest of the larvae,
not finding an adult parasite to fix upon, go straight on to infect
their hosts and develop into the adult hermaphrodites.

The same explanation would apply to the complemental males in
_Scalpellum_, etc., these individuals being also potential
hermaphrodites, which are arrested in development, though not so
completely as in the Rhizocephala, owing to the position they have taken
up.

This theory throws light on another dark feature of Cirripede
life-history, namely, the gregarious instinct. The associations of
Cirripedes are not formed by a number of Cypris larvae fixing together
on the same spot, but rather by the Cypris larvae seeking out adolescent
individuals of their own species and fixing on or near them. Now, if we
suppose that the Cirripedes have passed through a condition of
protandric hermaphroditism similar to that of the Isopoda Epicarida, it
is clear that a slight modification of the sexual instinct of the larvae
would lead to the gregarious habit, while its retention in some
individuals in its original form accounts for their finding their way to
the mantles of adult individuals and developing into the so-called
complemental males.

Certain Cirripedes, viz. certain species of _Scalpellum_ and _Ibla_ and
all the Acrothoracica, are dioecious. It is impossible to decide at
present whether these species retain the primitive dioecious condition
of the ancestral Cirripedes, or whether they too have been derived from
an hermaphrodite state, but in the present state of knowledge they
hardly affect the validity of the theory that has been proposed to
account for the nature of the complemental males and the hermaphrodite
individuals.


                         =Order IV. Ostracoda.=

The Ostracoda are small Crustacea, the body consisting of very few—about
eight—segments, and being completely enclosed in a carapace, which has
the form of a bivalve shell. Development is direct, without a Nauplius
stage.

The Ostracoda[82] are marine and fresh-water animals that can be divided
into several families, differing slightly in habits and in structures
correlated with those habits.

[Illustration:

  FIG. 74.—_Candona reptans._ =A=, Natural size; =B=, X 15. _a_, 1st
    antennae; _b_, 2nd antennae; _c_, walking legs. (After Baird.)
]

The =Cypridae= and =Cytheridae= include all the fresh-water and a vast
majority of marine genera, adapted for a sluggish life among
water-plants, though some can swim with considerable activity. The
common _Cypris_ and _Candona_ of our ponds and streams are familiar
instances. The movements of these animals are effected by means of the
two pairs of uniramous pediform antennae which move together and in a
vertical straight line. In the Cypridae (Fig. 74) there are, besides the
mandibles, two pairs of maxillae, a pair of walking legs, and, lastly, a
pair of appendages, which are doubled up into the carapace, and are used
for cleaning purposes. In the marine Cytheridae there is only one
maxilla, the last three appendages being pediform and used in walking.
The telson in the Cytheridae is rudimentary, but is well developed in
the Cypridae. The heart is altogether absent.

In many of the fresh-water forms, _e.g._ common species of _Candona_ and
_Cypris_, males are never found, and parthenogenetic reproduction by the
females appears to proceed uninterruptedly. Weismann[83] kept females of
_Cypris reptans_ breeding parthenogenetically for eight years. He also
remarks on the fact that these, and indeed all parthenogenetic female
Ostracoda, retain the receptaculum seminis, used normally for storing
the spermatozoa derived from the male, unimpaired.

Some of the Cytheridae occur in deep water. Thus _Cythere dictyon_ was
frequently taken by the _Challenger_ in depths of over 1000 fathoms, but
the majority prefer shallow water.

[Illustration:

  FIG. 75.—_Asterope oblonga_, ♀, removed from its carapace, × 25. _A_,
    Alimentary canal; _A_{1}_, _A_{2}_, 1st and 2nd antennae; _E_, eye;
    _G_, gills; _G.O_, generative opening; _H_, heart; _M_, mandible;
    _T_, 6th appendage; _T′_, last appendage (cleaning foot). (After
    Claus.)
]

The =Halocypridae= and =Cypridinidae= comprise marine genera of a
pelagic habit. The first antennae are chiefly sensory, but the second
antennae are biramous, and they do not merely move up and down, as in
the preceding families, but sideways like oars, the valves of the shells
being excavated to admit of free movements. There are two pairs of
maxillae; the succeeding limbs differ in the two families. In the
Cypridinidae, _e.g._ _Asterope_ (Fig. 75), the first leg (T) is
lamelliform and is used as an accessory maxilla, while the second leg
(T’) is turned upwards into the shell as a cleaning organ. In the
Halocypridae the first leg is pediform, and differs in the two sexes,
while the second leg is rudimentary and points backwards. In _Asterope_
peculiar branchial organs (_G_) are present on the back. Both families
possess a heart; the Halocypridae are blind, while the Cypridinidae
possess eyes.

The =Polycopidae= and =Cytherellidae= are curious marine families of a
pelagic habit, with biramous second antennae well adapted for swimming,
and very broad. The first maxilla in the Polycopidae is also employed in
swimming, while the second is modified into a branchial organ; the
maxillae of the Cytherellidae are more normal in structure, but both
carry branchial lamellae. The posterior limbs are altogether absent in
Polycopidae, and in the Cytherellidae are only represented by the
copulatory organs of the male.




                               CHAPTER V
    CRUSTACEA (_CONTINUED_): MALACOSTRACA: LEPTOSTRACA—PHYLLOCARIDA:
  EUMALACOSTRACA: SYNCARIDA—ANASPIDACEA: PERACARIDA—MYSIDACEA—CUMACEA—
               ISOPODA—AMPHIPODA: HOPLOCARIDA—STOMATOPODA


                      SUB-CLASS II.—MALACOSTRACA.

The Malacostraca are generally large Crustacea, and they are
characterised by the presence of a definite and constant number of
segments composing the body. In addition to the paired eyes we can
distinguish two pairs of antennae, a mandibular segment, and two
maxillary segments composing the head region proper; there then follow
eight thoracic segments, the limbs belonging to the anterior thoracic
segments being often turned forwards towards the mouth, and modified in
structure to act as maxillipedes, while at any rate the last four are
used in locomotion and are termed “pereiopods.”[84] The abdomen is
composed of six segments, which typically carry as many pairs of
biramous “pleopods,” and the body terminates in a telson. Not counting
the paired eyes or the telson, there are present nineteen segments. The
excretory organs in the adult open at the bases of the second antennae,
and are known as “green glands,” but in the larva maxillary glands may
be present homologous to those which persist in the adult Entomostraca.
This is the typical arrangement, but sometimes the maxillary glands
persist in adult Malacostraca, _e.g._ _Nebalia_, _Anaspides_, and some
Isopods.

The hepato-pancreatic diverticula are directed posteriorly, and not
anteriorly as in most Entomostraca, and the stomach is often furnished
with chitinous teeth and ridges forming an elaborate gastric mill,
especially in the larger Decapods.


                         SERIES 1. LEPTOSTRACA.


                       =Division. Phyllocarida.=

[Illustration:

  FIG. 76.—_Nebalia geoffroyi_, ♀, × 20. _A.1_, _A.2_, 1st and 2nd
    antennae; _Ab.1_, _Ab.6_, 1st and 6th abdominal appendages; _A.G._
    antennary gland; _C_, half of caudal fork; _E_, eye; _G_, ventral
    ganglionic chain; _H_, heart; _I_, intestine; _L_, upper
    liver-diverticulum; _M_, adductor muscle of halves of carapace;
    _MX_, palp of 1st maxilla; _O_, ovary; _R_, rostrum. (After Claus.)
]

The small shrimp-like Crustacean _Nebalia_, which is found burrowing in
the superficial layers of sand in the littoral and sometimes the deeper
regions of most seas, has been regarded, ever since its anatomy was made
out by Claus,[85] as a connecting link between Entomostraca and
Malacostraca, and has been placed in a separate group Leptostraca.

The segmentation of the body is Malacostracan, save that two extra
segments are present in the abdomen, and the paired compound eyes are
borne upon stalks. The eight thoracic limbs are all very similar; they
are built on the typical biramous plan, and each carries a bract; they
have been compared, owing to their flattened, expanded shape, to the
foliaceous limbs of the Phyllopods. The abdominal appendages are also
biramous. The heart is greatly elongated, stretching through thorax and
abdomen; there are present both the antennary excretory glands
characteristic of adult Malacostraca and the maxillary glands
characteristic of adult Entomostraca, and both the posterior and
anterior livers characteristic of the two Orders respectively are
present. This combination of characters justifies the belief that
_Nebalia_ represents a primitive form, standing to some extent in an
intermediate position between Entomostraca and Malacostraca, but it may
be doubted if the special relationship to the Phyllopoda, claimed on the
strength of the foliaceous appearance of the thoracic limbs, can be
legitimately pressed.

_Nebalia_ shows the clearest signs of relationship to the other
primitive Malacostraca, and especially to the Mysidae, which it
resembles not only in general form and in the essentially biramous
character of its appendages, but also in many embryological points and
in the similarity in development of the brood-pouch.[86]

A large number of very ancient palaeozoic fossils are known which are
placed provisionally with _Nebalia_ in the Division Phyllocarida, and
some of these are no doubt closely related to the existing isolated
genus. _Hymenocaris_ from the Cambrian.


                       SERIES 2. EUMALACOSTRACA.

Before entering on a description of the members of this Series it is
necessary to introduce and justify a new scheme of classification which
has been proposed by Dr. W. T. Calman. This scheme necessitates the
abandonment of the old Order Schizopoda, and also ignores the
distinction which used to be considered fundamental between the
sessile-eyed Crustacea (Edriophthalmata) and the stalk-eyed forms
(Podophthalmata).

The old group of Schizopoda, to which _Nebalia_ and the isolated form
_Anaspides_, to be considered later, are undoubtedly related, represent
very clearly the stem-forms from which the various branches of the
Malacostracan stock diverge. No doubt they are themselves specialised in
many directions, since they are a dominant group in present day seas,
but their organisation is fundamentally of a primitive type. We see this
especially in the comparative absence of fusion or reduction of the
segments of the body externally and of the nervous system internally,
and in the simple undifferentiated character of the trunk-limbs, all of
which conform to the primitive biramous type. The most anterior thoracic
limbs of the Schizopods are of particular interest. In the higher
Malacostraca three of these limbs are usually turned forwards towards
the mouth to act as maxillipedes, and the most anterior of all, the
first maxillipede, is apt, especially in the Decapoda, to take on a
flattened foliaceous form owing to the expansion of the basal segments
to act as gnathobases (see Fig. 1, A, p. 10). Now this appendage in the
Schizopods preserves its typical biramous character, and resembles the
succeeding thoracic limbs, but in many of the species the basal joints
show a tendency to be produced into biting blades (Fig. 1, E, p. 10),
thus indicating the first step in the evolution of the foliaceous first
maxillipede of the Decapoda. The primitive character of the Schizopods
is also indicated by the fact that most of the Decapoda with uniramous
limbs on the five hinder thoracic segments pass through what is known as
the “Mysis stage” in development, when these limbs are biramous, the
exopodites being subsequently lost in most cases.

The “Schizopoda” include a very large number of pelagic Crustacea of
moderate size, which superficially appear to resemble one another very
closely. The slender, elongated body, the presence of biramous limbs on
all the thoracic and abdominal segments, and the possession of a single
row of gills at the bases of the thoracic limbs, are, generally
speaking, typical of the families Mysidae, Lophogastridae, Eucopiidae,
and Euphausiidae, which go to make up the old Order Schizopoda.

It has, however, been pointed out first by Boas,[87] and subsequently by
Hansen and Calman,[88] that the Euphausiidae are in many respects
distinct from the other three families, and agree with the Decapoda,
while the Eucopiidae, Lophogastridae, and Mysidae agree with the
Cumacea, Isopoda, and Amphipoda.

It has, therefore, been suggested by these authors that the
classification of the Malacostraca should be revised, and Calman (_loc.
cit._) has brought forward the following scheme:—

The division PERACARIDA, including the Eucopiidae, Lophogastridae, and
Mysidae (= Mysidacea), the Cumacea, Isopoda, and Amphipoda, is
characterised by the fact that when a carapace is present it leaves at
least four of the thoracic segments free and uncoalesced: by the
presence of a brood-pouch formed from the oostegites on the thoracic
limbs of the female: by the elongated heart: by the few and simple
hepatic caeca: by the filiform spermatozoa: and by the direct method of
development without a complicated larval metamorphosis. The biting face
of the mandible has a movable joint, the “lacinia mobilis.”[89]

The division EUCARIDA, on the other hand, including the Euphausiidae and
the Decapoda, shows the converse of these characters. The carapace
coalesces with all the thoracic segments, there is never a brood-pouch
formed from oostegites, the hepatic caeca are much ramified, the heart
is short, the spermatozoa are spherical with radiating pseudopodia, the
development is indirect with a complicated metamorphosis, and the
mandible is without a lacinia mobilis.

Corresponding divisions are made by Calman to receive the other
Malacostraca, namely, the PHYLLOCARIDA for _Nebalia_, the SYNCARIDA for
_Anaspides_, and the HOPLOCARIDA for the Stomatopoda or Squillidae.

The important array of characters which separates the Euphausiidae from
the other Schizopods and unites them with the Decapoda can no longer be
neglected, and the consideration of _Anaspides_ and its allies will
further emphasise the extreme difficulty of retaining the Schizopoda as
a natural group. In the sequel Calman’s proposed scheme will be adopted.


                         DIVISION 1. SYNCARIDA.

There is no carapace, and all the eight thoracic segments may be free
and distinct. Eyes may be pedunculate or sessile. The mandible is
without a lacinia mobilis. There is no brood-pouch, the eggs being
deposited and hidden after fertilisation. The spermatozoa are filiform,
the hepatic caeca very numerous, and the heart tubular and elongated,
with ostia only in one place in the anterior thoracic region. The
auditory organ is at the base of the first antennae.


                         =Order. Anaspidacea.=

=Fam. 1. Anaspididae.=—The mountain-shrimp of Tasmania, _Anaspides
tasmaniae_, was first described by Thomson[90] in 1893 from specimens
taken in a little pool near the summit of Mount Wellington; it was
redescribed by Calman,[91] who drew attention to its remarkable
resemblance to certain Carboniferous fossils of Europe and N. America
(_Gampsonyx_, _Palaeocaris_, etc.).

The creature appears to be confined to the deep pools of the rivers and
tarns on the mountains of the southern and western portions of
Tasmania.[92] The waters in which it occurs are always cold and
absolutely clear, and there is no record of its living at altitudes much
below 2000 feet, while it frequently occurs at 4000 feet. The body may
attain upwards of two inches in length; it is deeply pigmented with
black chromatophores, and it is held perfectly horizontal without any
flexure. The animal rarely swims unless disturbed, usually walking about
on stones and water-plants at the bottom of deep pools. In walking the
endopodites of the thoracic limbs are chiefly instrumental, but they are
assisted by the exopodites of the abdominal limbs.

When frightened the shrimp can dart rapidly forwards or sideways by the
strokes of its powerful tail-fan, but it never jumps backwards as do the
other Malacostraca. It appears to browse upon the algal slime covering
the rocks and on the submerged liver-worts and mosses, but it does not
refuse animal food, even feeding on the dead bodies of members of its
own species. The thoracic limbs, which are all biramous except the last
pair, carry a double series of remarkable plate-like gills on their
coxopodites. The slender and setose exopodites of the thoracic limbs are
respiratory in function, being kept in continual motion even when the
animal is at rest, and serving to keep up a current of fresh water round
the gills.

_Anaspides_ shows a remarkable combination of structural characters,
some of which are peculiar, while others are possessed in common with
the Peracarida or Eucarida. The chief peculiar characters are the entire
absence of a carapace, and the freedom of the eight thoracic segments,
with eight free thoracic ganglia in the nerve-cord; the peculiar double
series of plate-like gills; the structure of the alimentary canal; and
the fact that the eggs, instead of being carried in a brood-pouch, or
affixed to the abdominal limbs, are deposited under stones and among
water-plants.[93]

[Illustration:

  FIG. 77.—_Anaspides tasmaniae_ in natural position for walking, × 1.
    The last two pereiopods point backwards and are overlapped by the
    first two pleopods.
]

The Peracaridan features, uniting it especially with the Mysidacea, are
the structure of the elongated heart, the filiform spermatozoa, and the
fact that no complicated metamorphosis is passed through, the young
hatching out in a condition similar to, though possibly not identical
with, the adult form.

The Eucaridan, especially Decapodan, features are the presence of an
auditory sac on the basal joint of the antennules, and the modification
of the endopodites of the first two abdominal appendages in the male to
form a copulatory organ.

A type of a new genus of this family was found by me in the littoral
zone of the Great Lake of Tasmania at an elevation of 3700 feet, and
named _Paranaspides lacustris_.

This little shrimp (Fig. 78), which does not appear to grow to more than
an inch in length, is totally different in appearance from _Anaspides_,
being pale green and transparent, with a very marked dorsal hump as in
_Mysis_, to which it bears a very striking superficial resemblance. It
leads a more active swimming life than _Anaspides_, and with this habit
is correlated the flexure of the body and the greater size of the
tail-fan and the scale of the second antenna. The mandible is peculiar
in being furnished with a four-jointed biramous palp, while that of
_Anaspides_ is three-jointed and uniramous, and the first thoracic
appendage is provided with a setose biting lobe on the antepenultimate
joint, thus more resembling a maxillipede. In other respects it agrees
essentially in structure with _Anaspides_.

[Illustration:

  FIG. 78.—_Paranaspides lacustris_, × 4. _a^1_, _a^2_, First and second
    antennae; _Ab.1_, first abdominal segment; _ep_, epipodites or gills
    on the thoracic legs; _md_, mandible; _Pl.1_, first pleopod; _T_,
    telson; _Th.8_, eighth free thoracic segment; _U_, uropod, or sixth
    pleopod.
]

=Fam. 2. Koonungidae.=—The sole representative of this family, _Koonunga
cursor_, has been recently described by Mr. O. A. Sayce,[94] of
Melbourne University, from a small stream some miles to the west of
Melbourne. Although plainly belonging to the Anaspidacea, this
interesting little animal, which only measures a few millimetres in
length, and follows a similar habit to _Anaspides_, running about with
its body unflexed, differs from all the other members of the Division in
possessing sessile instead of stalked eyes, in the first thoracic
segment being fixed to the head, and in a number of minor anatomical
points.

It is impossible at present to assign the Carboniferous forms
(_Gampsonyx_, _Palaeocaris_, etc.) to their exact position in the
Division, but it seems that they agreed more closely with _Anaspides_
than with the other two genera. From the position in which the fossils
are preserved, it would appear that they followed a similar walking
habit to _Anaspides_, and that the body was unflexed.


                        DIVISION 2. PERACARIDA.

The carapace, when present, leaves at least four of the thoracic somites
distinct; the first thoracic segment is always fused with the head. The
eyes are pedunculate or sessile.

The mandible possesses a lacinia mobilis. A brood-pouch is formed in the
female from oostegites attached to the thoracic limbs. The hepatic caeca
are few and simple; the heart is elongated and tubular; the spermatozoa
are filiform, and development takes place without a complicated
metamorphosis.


                         =Order I. Mysidacea.=

The Mysidacea, although pelagic, are not very often met with in the true
plankton on the surface; they generally swim some way below the surface,
going down in many cases into the abysses. For this reason they thrive
excellently in aquaria, and the common _Mysis vulgaris_ is often present
in such numbers in the tanks at the Zoological station at Naples as to
damage the other inmates by the mere press of numbers. The Mysidacea,
like the majority of the Peracarida, undergo a direct development, and
hatch out with the structure of the adult fully formed.

Many of the Mysidacea bear auditory sacs upon the sixth pair of
pleopods, a characteristic not found in the Euphausiacea.

=Fam. 1. Eucopiidae.=—The curious form _Eucopia australis_ (Fig. 79)
described by Sars,[95] may be chosen as an example of the Mysidacea.

The peculiarity of this form consists chiefly in the immense elongation
of the endopodites of the fifth, sixth, and seventh thoracic appendages.
Characteristic of the Mysidacea is the freedom of the hinder thoracic
segments from fusion with the carapace, otherwise this animal is seen
closely to resemble the _Euphausia_ figured (Fig. 102). _Eucopia
australis_, like so many of the Mysidacea, is a deep-sea animal, being
brought up with the dredge from over 1000 fathoms; it is very widely
distributed over the Atlantic Ocean.

[Illustration:

  FIG. 79.—_Eucopia australis_, young female, × 3. _A_, 1st antenna;
    _Ab.1_, 1st abdominal segment; _Ab.6_, 6th abdominal appendage; _E_,
    eye; _T_, telson; _Th_, 5th thoracic appendage. (After Sars.)
]

=Fam. 2. Lophogastridae.=—The members of this family (_Lophogaster_,
_Gnathophausia_) agree with the Eucopiidae in the possession of branched
gills on some of the thoracic limbs, in the absence of auditory sacs on
the sixth pair of pleopods, in the presence of normally developed
pleopods in both the male and female, and in the brood-lamellae being
developed on all seven of the thoracic limbs. The endopodites of the
posterior thoracic limbs are, however, of a normal size.

[Illustration:

  FIG. 80.—Dorsal view of male _Diastylis stygia_, × 12. _A_, 2nd
    antenna; _Ab.6_, 6th abdominal appendage. (After Sars.)
]

=Fam. 3. Mysidae.=—These differ from both the foregoing families in the
absence of gills, in the presence of an auditory sac on the sixth
pleopods, in the reduction of the other pleopods in the female, and in
the brood-lamellae being developed only on the more posterior pairs of
thoracic limbs. A number of closely related genera compose this family,
of which _Mysis_, _Boreomysis_, and _Siriella_ may be mentioned. _Mysis
oculata_, _var. relicta_, is a fresh-water form from the lakes of
northern and central Europe.


                        =Order II. Cumacea.=[96]

The Cumacea are a group of small marine animals rarely attaining an inch
in length, which agree with the Mysidacea in the characters noted above
as diagnostic of the Division Peracarida; they possess, however, in
addition a number of peculiar properties, and Sars believes them to be
of a primitive nature showing relationship to _Nebalia_, and possibly to
an ancestral Zoaea-like form. They follow a habit similar to that of the
Mysidacea, being caught either in the surface-plankton or in great
depths, many of the deep-sea forms being blind. They are, however, not
true plankton forms, and they appear to attain a greater development
both in point of variety and size in the seas of the northern
hemisphere. The thoracic limbs may be biramous, but there is a tendency
among many of the genera to lose the exopodites of some of the thoracic
legs, an exopodite never being present on the last few thoracic limbs of
the female and on the last in the male. In the Cumidae the four
posterior pairs in both sexes have no exopodites. The first three
thoracic appendages following the maxillae are distinguished as
maxillipedes; they are uniramous, and the first pair carries an
epipodite and a large gill upon the basal joints. Pleopods are only
developed in the male sex.

The flagellum of the second antennae in the male may be enormously
elongated, as in the Atlantic deep-sea species shown in Fig. 80, so as
to exceed in length the rest of the body.

=Fam. 1. Cumidae.=—No sharp demarcation between thorax and abdomen. Four
posterior pairs of legs in both sexes without exopodites. Male with five
well-developed pleopods in addition to the uropods. Telson wanting.
_Cuma_, _Cyclaspis_, etc.

=Fam. 2. Lampropidae.=—Body-form resembles that of Cumidae. All the
thoracic limbs except the last have exopodites. The male has three pairs
of pleopods. Telson present. _Lamprops_, _Platyaspis_, etc.

=Fam. 3. Leuconidae.=—Body-form similar to above. Male has only two
pairs of pleopods. Mouth-parts peculiar, much less setose than in other
families. Telson absent. _Leucon_, _Eudorella_.

=Fam. 4. Diastylidae.=—Anterior part of thorax sharply marked off from
posterior part. Male has two pairs of pleopods. Telson present.
_Diastylis_ (Fig. 80). _D. goodsiri_ from the Arctic ocean measures over
an inch in length.

=Fam. 5. Pseudocumidae.=—Rather similar to Diastylidae, but differ in
reduced size of telson and presence of exopodites on third and fourth
thoracic legs of female. This family is represented by three very
similar marine forms of the genus _Pseudocuma_; but, as Sars has
shown,[97] the Caspian Sea contains thirteen peculiar species, only one
of which can be referred to the genus _Pseudocuma_, while the rest may
be partitioned among four genera, _Pterocuma_, _Stenocuma_,
_Caspiocuma_, _Schizorhynchus_.


                         =Order III. Isopoda.=

The Isopoda and the Amphipoda are frequently classed together as
Arthrostraca or Edriophthalmata, owing to a number of features which
they share in common, as, for instance, the sessile eyes which
distinguish them from the podophthalmatous Schizopoda and Decapoda, the
absence of a carapace, and the thoracic limbs which are uniramous
throughout their whole existence. For the rest, in the presence of
brood-plates and the other diagnostic characters, they are plainly
allied to the other Peracarida, and an easy transition is effected from
the Mysidacea to the Isopoda through the Chelifera or Anisopoda. Only
one thoracic segment is usually fused with the head, the appendage of
this segment being the maxillipede; in the Chelifera among Isopoda, and
the Caprellidae among Amphipoda, two thoracic segments are fused with
the head.

The Isopoda are distinguished from the Amphipoda by the dorso-ventral
flattening of the body, as opposed to the lateral flattening in the
Amphipoda, by the posterior position of the heart, and by the branchial
organs being situated on the abdominal instead of on the thoracic limbs.

The Isopoda, following Sars’[98] classification, fall into six
sub-orders—the Chelifera, Flabellifera, Valvifera, Asellota, Oniscoida,
and Epicarida,—to which must be added the Phreatoicidea.


                       =Sub-Order 1. Chelifera.=

The Chelifera, including the families (1) =Apseudidae= and (2)
=Tanaidae=, are interesting in that they afford a transition between the
ordinary Isopods and the Mysidacea. The important features in which they
resemble the Mysidacea are, first, the fusion of the first two thoracic
segments with the head, with the coincident formation of a kind of
carapace in which the respiratory functions are discharged by a pair of
branchial lamellae attached to the maxillipedes; and, second, the
presence of very small exopodites on the first two thoracic appendages
of the Apseudidae.

The second pair of thoracic limbs, _i.e._ the pair behind the
maxillipedes, are developed both in the Apseudidae and Tanaidae into a
pair of powerful chelae, and these frequently show marked sexual
differences, being much more highly developed in the males than in the
females. The biramous and flattened pleopods are purely natatory in
function, and the uropods or pleopods of the sixth pair are terminal in
position and slender.

[Illustration:

  FIG. 81.—_Apseudes spinosus_, ♂, × 15. _A_, 1st antenna; _Ab_, 6th
    abdominal appendage; _T_, 2nd thoracic appendage. (After Sars.)
]

Both families, of which the Apseudidae contain the larger forms,
sometimes attaining to an inch in length, are littoral in habit, or
occur in sand and ooze at considerable depths, many of the genera being
blind. Many Tanaids (_e.g._ _Leptochelia_, _Tanais_, _Heterotanais_,
etc.) live in the algal growths of the littoral zone, and being highly
heliotropic they are easy to collect if a basinful of algae is placed in
a strong light. The females carry the eggs about with them in a
brood-pouch formed, as is usual in the Peracarida, by lamellae produced
from the bases of the thoracic limbs. The males on coming to maturity do
not appear to grow any more, or to take food, their mouth-parts
frequently degenerating and the alimentary canal being devoid of food.
They are thus in the position of insects which do not moult after coming
to maturity; and, as in Insects, the males are apt to show a kind of
high and low dimorphism—certain of the males being small with secondary
sexual characters little different from those of the females, while
others are large with these characters highly developed. Fritz Müller,
in his _Facts for Darwin_, observes that in a Brazilian species of
_Leptochelia_, apparently identical with the European _L. dubia_, the
males occur under two totally distinct forms—one in which the chelae are
greatly developed, and another in which the chelae resemble those of the
female, but the antennae in this form are provided with far longer and
more numerous sensory hairs than in the first form. Müller suggested
that these two varieties were produced by natural selection, the
characters of the one form compensating for the absence of the
characters of the other. A general consideration of the sexual
dimorphism in the Tanaidae[99] lends some support to this view, since
the smaller species with feeble chelae do appear to be compensated by a
greater development of sensory hairs on the antennae, but the specific
differences are so difficult to appreciate in the Tanaidae that it is
possible that the two forms of the male in Müller’s supposed single
species really belonged to two separate species.


                      =Sub-Order 2. Flabellifera.=

The Flabellifera include a number of rather heterogeneous families which
resemble one another, however, in the uropods being lateral and not
terminal, and being expanded together with the telson to form a caudal
fan for swimming. The pleopods are sometimes natatory and sometimes
branchial in function. Some of the families are parasitic or
semi-parasitic in habit.

=Fam. 1. Anthuridae.=—These are elongated cylindrical creatures found in
mud and among weeds upon the sea-bottom; their mouth-parts are evidently
intended for piercing and sucking, but whether they are parasitic at
certain periods on other animals is not exactly known. _Anthura_,
_Paranthura_, _Cruregens_.

[Illustration:

  FIG. 82.—_Gnathia maxillaris_. =A=, Segmented larva, × 10; =B=,
    Praniza larva, × 5; =C=, gravid female, × 5; =D=, male, × 5.
]

=Fam. 2. Gnathiidae.=[100]—These forms appear to be related to the
Anthuridae; they are ectoparasitic on various kinds of fish during
larval life, but on assuming the adult state they do not feed any more,
subsisting merely on the nourishment amassed during the larval periods.
The larvae themselves are continually leaving their hosts, and can be
taken in great numbers living freely among weeds on the sea-bottom. The
larvae, together with the adults of _Gnathia maxillaris_, are extremely
abundant among the roots of the sea-weed _Poseidonia cavolinii_ in the
Bay of Naples. The young larvae hatch out from the body of the female in
the state shown in Fig. 82, A. This minute larva fixes upon a fish, and
after a time it is transformed into the so-called Praniza larva (B), in
which the gut is so distended with the fluid sucked from the host that
the segmentation in the hind part of the thorax is entirely lost. When
this larva moults it may, however, reacquire temporarily its
segmentation. After a certain period of this parasitic mode of life the
Praniza finally abandons its host, and becomes transformed into the
adult male or female. This may take place at very different stages in
the growth of the larva, the range of variation in size of the adults
being 1–8 mm., and it must be remembered that when once the adult
condition is assumed growth entirely ceases. What it is that determines
the stage of growth in each individual when it shall be transformed into
the adult is not known. The males and females differ from one another so
extraordinarily that it was for long denied that they were both derived
from the Praniza larvae. This is nevertheless the case. The change from
the Praniza to the female (Fig. 82, C) is not very great. The ovary
absorbs all the nourishment in the gut and comes to occupy the whole of
the body, all the other organs degenerating, including the alimentary
canal and mouth-parts. Indeed, only the limbs with their muscles and the
nervous system remain. The change to the male (D) is more radical. The
food is here stored in the liver, which increases in the male just as
the ovary does in the female. The segmentation is reacquired, and the
massive square head is formed from the hinder part of the head in the
Praniza, the anterior portion with its stylet-like appendages being
thrown away. The powerful nippers of the male are not formed inside the
cases of the old styliform mandibles, but are independent and possibly
not homologous organs. The meaning of the marked sexual dimorphism and
the use of the males’ nippers are not in the least known, though the
animals are easy to keep under observation. In captivity the males never
take the slightest notice of either larval or adult females.

=Fam. 3. Cymothoidae.=[101]—This is a group of parasites more completely
parasitic than the foregoing, but their outer organisation does not
differ greatly from an ordinary Isopodan form. A great many very similar
species are known which infest the gill-chambers, mouths, and skin of
various fishes. The chief interest that attaches to them is found in the
fact that a number of them, and perhaps all, are hermaphrodite, each
individual acting as a male when free-swimming and young, and then
subsequently settling down and becoming female. This condition is
exactly the same as that occurring universally in the great group of
parasitic Isopoda, the Epicarida, to be considered later. There is no
evidence that the Cymothoidae are phyletically related to the Epicarida,
so that the similar sexual organisation appears to be due to convergence
resulting from similar conditions of life. The general question of
hermaphroditism in the Crustacea has been shortly discussed on pp.
105–106. _Cymothoa._

=Fam. 4. Cirolanidae.=—In this family is placed the largest Isopod
known—the deep-sea _Bathynomus giganteus_, found in the Gulf of Mexico
and the Indian Ocean, sometimes measuring a foot long by four inches
broad. A common small littoral form is _Cirolana_.

=Fam. 5. Serolidae.=[102]—The genus _Serolis_ comprises flattened forms
bearing a curious resemblance to Trilobites, which Milne Edwards
considered more than superficial. The genus is confined to the littoral
and deep waters of the southern hemisphere.

=Fam. 6. Sphaeromidae.=[103]—These are flattened, broad-bodied forms,
most commonly met with in the Mediterranean and warmer seas. Without
being actually parasitic, they are frequently found as scavengers in
decaying material, and they show some relationship to the parasitic
Cymothoidae. In some of the genera, _e.g._ _Cymodoce_, the ovigerous
female shows a degenerate condition of the mouth-parts, while the
maxillipedes undergo an enlargement, and are used for causing a current
through the brood-chamber.


                       =Sub-Order 3. Valvifera.=

[Illustration:

  FIG. 83.—_Munnopsis typica_ (Munnopsidae), ♂, × 2. _A_, 2nd antenna;
    _Ab_, abdomen; _T_, 5th thoracic appendage or 4th leg. (After Sars.)
]

The Valvifera, illustrated by the =Idotheidae= and =Arcturidae=, are
characterised by the uropods being turned back and expanded to form
folding doors covering up the delicate pleopods, which are mostly
respiratory in function, though the anterior pairs may serve as swimming
organs. _Arcturus_ is a typically deep sea genus, many species,
remarkably furnished with spiny processes, having been taken by the
_Challenger_ in the southern hemisphere. The Idotheidae are more
littoral forms, several species of _Idothea_ being commonly met with off
the British coasts, occasionally penetrating into brackish or even fresh
water.


                        =Sub-Order 4. Asellota.=

In this group the abdominal segments are fused dorsally to form a
shield-like caudal region; the pleopods are respiratory in function and
reduced in numbers, the first pair being often expanded and produced
backwards to form an operculum covering the rest. Several of the
Asellota are fresh-water, _Asellus aquaticus_ (=Asellidae=) being
extremely abundant all over Europe in weed-grown ditches, the mud of
slowly-moving streams, and even on the shores of large lakes. They are
mostly sluggish in habit, but the marine =Munnopsidae= (Fig. 83,
_Munnopsis_) are expert swimmers, the swimming organs being fashioned by
the expansion and elongation of the thoracic legs.


                       =Sub-Order 5. Oniscoida.=

The Oniscoida[104] are terrestrial forms in which the abdomen is fully
segmented, the pleopods are respiratory, their endopodites being
delicate branchiae, while their exopodites are plate-like and form
protective opercula for the gills, and the uropods are biramous and not
expanded. The epimera of the segments are greatly produced. The
terrestrial Isopods, although air-breathers,[105] are dependent on
moisture, and are only found in damp situations. It seems probable that
they have been derived from marine Isopods, since the more generalised
of them, _e.g._, _Ligia_ (Fig. 84), common on the English coasts, are
only found in damp caves and crannies in the rocks.

[Illustration:

  FIG. 84.—_Ligia oceanica_, ventral and dorsal views, × 1. (From
    original drawings prepared for Professor Weldon.)
]

The related _Ligidium_ is found far inland, but always in the
neighbourhood of water. These two genera may be distinguished by the
numerous joints in the flagellum of the second antennae, the flagellum
being in all cases the portion of the antenna succeeding the long fifth
joint. _Philoscia muscorum_ occurs usually near the coast, but it is
also found inland in England under trees in damp moss. This genus and
the common _Oniscus_, found in woods, are distinguished by the presence
of three joints in the flagellum of the second antenna. _Philoscia_ can
be distinguished from _Oniscus_ by its narrower body and the pretty
marbled appearance of its back. The genus _Trichoniscus_ has four joints
in the flagellum; various species are found in woods. In _Porcellio_ and
_Armadillidium_ there are only two joints in the flagellum, while
_Armadillidium_, the common garden wood-louse, can be distinguished from
all others by the flattened shape of the uropods, and the habit of
rolling up into a ball like an Armadillo.

There is also a very peculiar species, _Platyarthrus hoffmannseggii_,
which occurs in England and Northern Europe, and always lives in ants’
nests. It is supposed that they serve as scavengers for the ants, which
tend them carefully, and evidently treat them as domestic animals of
some kind. The small creature is quite white and blind, and has
exceedingly short antennae.


                       =Sub-Order 6. Epicarida.=

The Epicarida include an immense number of Isopods, parasitic upon other
Crustacea. In the adult state they become greatly deformed, and offer
very few characters of classificatory value, but they all pass through
certain highly characteristic larval stages which are essentially
similar in the different families. All the species are protandric
hermaphrodites, each individual being male while in a larval state, and
then losing its male organisation and becoming female as the parasitic
habit is assumed.

Two series of families are recognised according to the larval stages
passed through, the =Cryptoniscina=, in which the adult male
organisation is assumed in the Cryptoniscus stage, and the female
condition is imposed directly upon this form, and the =Bopyrina=, in
which the Cryptoniscus passes into a further larval stage, the Bopyrus,
which performs the function of the male, and upon which the female
organisation is imposed as the parasitic habit is assumed.

The following is a list of the Epicarida with the Crustacea which serve
as their hosts[106]:—

 Cryptoniscina          │Microniscidae          │on  Copepoda.
            „           │Cryptoniscidae         │on  Ostracoda.
            „           │Liriopsidae            │on  Rhizocephala.
            „           │Hemioniscidae          │on  Cirripedia.
            „           │Cabiropsidae           │on  Isopoda.
            „           │Podasconidae           │on  Amphipoda.
            „           │Asconiscidae           │on  Schizopoda.

 Bopyrina               │Dajidae                │on  Decapoda
            „           │Phryxidae              │           „
            „           │Bopyridae              │           „
            „           │Entoniscidae           │           „

[Illustration:

  FIG. 85.—Epicaridian larva, probably belonging to one of the
    Cryptoniscina. _A_, 2nd antenna; _Ab_, abdominal appendages; _T_,
    thoracic appendages. (From Bonnier, after Hansen.)
]

In all cases the first larval form which hatches out from the maternal
brood-pouch is called the Epicaridian larva (Fig. 85).

This little larva has two pairs of antennae, a pair of curious frontal
processes, and a pair of mandibles. The other mouth-parts are missing;
there are only six thoracic limbs, but the full complement of six
biramous pleopods are present, and at the end of the body there may be a
long tube of unknown function.

As a type of the =Cryptoniscina= we may take the =Liriopsidae=,[107]
parasitic on the Rhizocephala, which are, of course, themselves
parasitic on the Decapoda, the whole association forming a very
remarkable study in Carcinology.

Almost every species of the Rhizocephala is subject to the attacks of
Liriopsids, the latter fixing either on the Rhizocephala themselves, or
else on the Decapod host at a point near the fixation of the
Rhizocephalous parasite. An exceedingly common Liriopsid is _Danalia
curvata_, parasitic on _Sacculina neglecta_, which is itself parasitic
on the spider-crab, _Inachus mauritanicus_, at Naples. The adult
_Danalia_ is a mere curved bag full of eggs or developing embryos, and
without any other recognisable organs except two pairs of spermathecae
upon the ventral surface where the spermatozoa derived from the larval
males are stored.

[Illustration:

  FIG. 86.—_Inachus mauritanicus_, ♀, × 1, carrying two _Sacculina
    neglecta_ (_a_, _b_), and a _Danalia curvata_ (_c_), the latter
    bearing two dwarf males.
]

In Fig. 86 is represented a female of _Inachus mauritanicus_ which
carried upon it two Sacculinae and a _Danalia curvata_, and upon the
latter are seen two minute larval males in the act of fertilising the
adult _Danalia_. The eggs develop into the Epicaridian stage, after
which the larva passes into the Cryptoniscus stage (Fig. 87). In this
larval form the segments are clearly delimited; the only mouth-parts
present are the mandibles, but there are seven pairs of thoracic limbs
and the full number of pleopods. This Cryptoniscus stage is found in all
the Epicarida, and only differs in detail in the various families.

[Illustration:

  FIG. 87.—Ventral view of Cryptoniscus larva of _Danalia curvata_, ♂, ×
    25.
]

In the Cryptoniscina the Cryptoniscus larva is the male, and at this
stage possesses a pair of large testes in the thorax. The ovaries are
also present at this stage as very small bodies applied to the anterior
ends of the testes. The larval males in this state seek out adult fixed
Danaliae and fertilise them; and, when this is accomplished, they
themselves become fixed to the host and begin to develop into the adult
female condition. The limbs are all lost, and out of the mouth grows a
long proboscis (Fig. 88, _P_), which penetrates the tissues of the host.
The ovaries begin to grow, and a remarkable process of absorption in the
testes takes place. These organs, when fixation occurs, are never empty
of spermatozoa, and are frequently crammed with them. After fixation
some large cells at the interior borders of the testes begin to feed
upon the remains of these organs and to grow enormously in size and to
multiply by amitosis. These phagocytes, as they really are, attain an
enormous size, but they are doomed to degeneration, the chromatin
becoming dispersed through the cytoplasm, and the nuclei dividing first
by amitosis and then breaking up and disappearing. As the parasite
grows, the heart at the posterior end of the body ceases to beat; the
ovaries increase enormously at the expense of the alimentary canal, and
on the ventral surface two pairs of spermathecae are invaginated ready
to receive the spermatozoa of a larval male. In the adult condition,
after fertilisation has taken place and the ovaries occupy almost the
whole of the body, the remains of the phagocytic cells can be seen on
the dorsal surface in a degenerate state. They evidently are not used as
food, and their sole function is to make away with the male organisation
when it has become useless.[108]

[Illustration:

  FIG. 88.—Side view of _Danalia curvata_, × 15, shortly after fixation
    and loss of larval appendages. _A_, Alimentary canal; _E_, eye; _H_,
    heart; _N_, phagocytic cells; _O_, ovary; _P_, proboscis.
]

[Illustration:

  FIG. 89.—Optical section (dorsal view) of _Danalia curvata_, in the
    same stage as Fig. 88. _A_, Alimentary canal; _Ec_, ectoderm; _H_,
    heart; _N_, phagocytic cells; _O_, ovaries; _P_, proboscis.
]

In the series =Bopyrina=, after the free-living Epicaridian and
Cryptoniscus stages, a further larval state is assumed, called the
Bopyrus, which is the functional male, and, after performing this
function, passes on to the adult female condition.

The family =Bopyridae= is parasitic in the branchial chamber of
Decapoda, especially Macrura and Anomura. When one of these Decapods is
infested with an adult Bopyrid the gill-chamber in which it is situated
is greatly swollen, as shown in Fig. 90. A very common Bopyrid is
_Bopyrus fougerouxi_, parasitic in the gill-chambers of _Palaemon
serratus_. The _Bopyrus_ larva or functional male has the appearance
shown in Fig. 91. It differs from the Cryptoniscus stage in possessing a
rudimentary pair of anterior thoracic limbs and seven pairs normally
developed, while the abdominal limbs are plate-like and branchial in
function. The male can often be found attached to the female beneath the
last pair of incubatory lamellae.

[Illustration:

  FIG. 90.—_Galathea intermedia_, with a _Pleurocrypta microbranchiata_
    under its left branchiostegite (_B_), × 1. (After Sars.)
]

[Illustration:

  FIG. 91.—Ventral view of male _Bopyrus fougerouxi_, × 30. _A_, 1st and
    2nd antennae; _T_, 8th (last) thoracic appendage. (After Bonnier.)
]

The adult female condition, which is assumed after the Bopyrid stage is
passed through, is illustrated in Fig. 92. The body acquires a
remarkable asymmetry, due to the unequal pressure exerted by the walls
of the gill-chamber. The antennae and mandibles (Fig. 92, B) are
entirely covered up by the largely expanded maxillipedes; maxillae are,
as usual, entirely absent. Very large lamellae grow out from the bases
of the thoracic limbs to form a brood-pouch, and in this manner the
adult condition is attained.

The final complication in the life-histories of these Isopoda is reached
by the family =Entoniscidae=, which are parasitic when adult inside the
thoracic cavity of Brachyura and Paguridae. The cephalothorax of a
_Carcinus maenas_, which contains an adult _Portunion maenadis_ (_P_),
is shown in Fig. 93. The parasite is of a reddish colour when alive.

[Illustration:

  FIG. 92.—_Bopyrus fougerouxi._ =A=, Ventral view of female carrying a
    male (_M_) between her abdominal appendages, × 8; =B=, ventral view
    of part of head of female, the maxillipedes and the left mandible
    having been removed. _A.1_, _A.2_, 1st and 2nd antennae; _M_, male;
    _Mn_, right mandible; _Mx_, left maxillipede; _O_, oostegite; _T_,
    left 4th thoracic appendage or 3rd leg. (After Bonnier.)
]

[Illustration:

  FIG. 93.—Cephalothorax of _Carcinus maenas_, seen from the ventral
    side, containing a parasitic _Portunion maenadis_ (_P_), × ½. (After
    Bonnier.)
]

The Entoniscidae pass through a free living Epicaridian and Cryptoniscus
stage, and become adult males in the Bopyrus stage. It is stated,
however, by Giard and Bonnier[109] that these individuals, which
actually function as males, never grow up into adult females, though all
the adult females have passed through a male stage in which the male
genital ducts are not formed. The hermaphroditism, therefore, in these
animals at any rate is absolutely useless from a reproductive point of
view, and this justifies our looking for some other explanation of it,
such as was suggested on p. 105.

[Illustration:

  FIG. 94.—_Portunion maenadis_, ♀:—=A=, Young, × 10; =B=, older, × 5;
    =C=, adult, before the eggs are laid, × 3. _A_, 2nd antenna; _Ab_,
    abdomen; _B_, anterior lobe of brood-pouch; _B′_, its lateral lobe;
    _H_, head; 1, 2, 1st and 2nd incubatory lamellae (oostegites).
    (After Giard and Bonnier.)
]

The Bopyrus fixes in the gill-chamber of the host and becomes converted
into the adult female by a series of transformations. As these changes
take place it invaginates the wall of the gill-chamber and pushes its
way into the thoracic cavity of the crab, though it lies all the time
enveloped in the invaginated wall of the gill-chamber, and not free in
the body-cavity of the crab. The transformations which it undergoes are
shown in Fig. 94. The body first assumes a grub-like appearance (A), and
two pairs of incubatory lamellae (1, 2) grow out from the first and
second thoracic segments. In the next stage (B) these lamellae assume
gigantic proportions, and four pairs of branchiae grow out from the
abdominal segments (_Ab_). In the final stage (C) the incubatory
lamellae have further increased in size, and constitute the main bulk of
the body; the enormous mass of eggs is passed into the incubatory pouch,
and all that remains of the rest of the body is the small head (H) and
the abdomen (_Ab_), furnished with its branchiae. Communication with the
external world is kept up through an aperture which leads from the
brood-pouch into the gill-chamber of the host, and through this aperture
the young are hatched out when they are developed sufficiently.

The presence of these parasites, although they are never in actual
contact with the internal organs of the crab, calls forth the same
phenomenon of parasitic castration as was observed in the Rhizocephala.
A remarkable association is also found to exist between the Entoniscidae
and Rhizocephala, of such a kind that, on the whole, a crab infested
with a Rhizocephalan is more likely to harbour an Entoniscid than one
without. The explanation of this association is probably that a crab
with a _Sacculina_ inside it is prevented from moulting as often as an
uninfected crab, and, in consequence, the larval stages of the
Entoniscid in the crab’s gill-chamber are more safely passed through.


                   =Sub-Order 7. Phreatoicidea.=[110]

The members of this sub-order, although agreeing with the Isopoda in the
essentials of their anatomy, resemble the Amphipoda in being rather
laterally compressed, and in having the hand of the first free thoracic
limb enlarged and subchelate. The abdomen is greatly produced laterally
by expansions of the segments. In fact, the shape of the body and of the
limbs is very Amphipodan.—_Phreatoicus_ from New Zealand, Southern
Australia, and Tasmania. _Phreatoicopsis_,[111] a very large form from
Gippsland, Victoria. Only one family exists, =Phreatoicidae=.


                         =Order IV. Amphipoda.=

In this order the body is flattened laterally, the heart is anterior in
position, and the branchial organs are attached to the thoracic limbs.

There are three well defined sub-orders, (i.) the Crevettina, including
a vast assemblage of very similar animals, of which the common
_Gammarus_ and _Orchestia_ may serve as examples; (ii.) the Laemodipoda
or Caprellids, and (iii.) the Hyperina.

We cannot do more than touch on the organisation of these sub-orders.


                       =Sub-Order 1. Crevettina.=

In this sub-order only one thoracic segment is fused with the head; the
basal joints of the thoracic limbs are expanded to form broad lateral
plates, and the abdomen is well developed, with six pairs of pleopods,
the last three pairs being always turned backwards, and stiffened to act
as uropods.

This group has numerous fresh-water representatives, _e.g._ _Gammarus_
of several species, the blind well-shrimp _Niphargus_, and the S.
American _Hyalella_; but the vast majority of the species are marine,
and are found especially in the littoral zone wherever the rocks are
covered with a rich growth of algae, Polyzoa, etc. The Talitridae or
“Sand-hoppers” have deserted the waters and live entirely in the sand
and under rocks on the shore, and one common European species,
_Orchestia gammarellus_, penetrates far inland, and may be found in
gardens where the soil is moist many miles from the sea.

The Rev. T. R. R. Stebbing, in his standard work[112] on this group,
recognises forty-one families, and more than 1000 species, so that we
can only mention a few of the families, many of which, indeed, differ
from one another in small characters.

=Fam. Lysianassidae.=—The first joint of the first antenna is short,
with an accessory flagellum. Mandible with a palp, and with an almost
smooth cutting edge. The third joint of the second gnathopod is
elongated. This family is entirely marine, comprising forty-eight
genera, with species distributed in all seas. One genus,
_Pseudalibrotus_, inhabits the brackish water of the Caspian Sea.
_Lysianassa_ has several common British and Mediterranean species.

=Fam. Haustoriidae.=—The members of this family are specially adapted
for burrowing, the joints of the hinder thoracic limbs being expanded,
and furnished with spines for digging. Some of the species are common on
the British coasts, _e.g._ _Haustorius arenarius_. _Pontoporeia_ has an
interesting distribution, one species, _P. femorata_, being entirely
marine, in the Arctic and North Atlantic, _P. affinis_ inhabiting the
Atlantic, and also fresh-water lakes in Europe and North America, _P.
microphthalma_ being confined to the Caspian Sea, and _P. loyi_ to Lakes
Superior and Michigan.

=Fam. Gammaridae.=—Includes fifty-two genera. The first antennae are
slender, with the accessory flagellum very variable. The mandibles have
a dentate cutting edge, spine-row, and molar surface, and a
three-jointed palp. The first two thoracic limbs are subchelate. This
family includes a few marine, but mostly brackish and fresh-water
species. _Crangonyx_ is entirely subterranean in habitat, as is
_Niphargus_, _N. forelii_ occurring, however, in the deep waters of Lake
Geneva. Both these genera are blind. _Gammarus_ has thirty species, _G.
locusta_ being the common species on the North Atlantic coasts, and _G.
pulex_ the common fresh-water species of streams and lakes in Europe. A
number of Gammaridae inhabit the Caspian Sea, _e.g._ _Boeckia_,
_Gmelina_, _Niphargoides_, etc., while the enormous Gammarid fauna of
Lake Baikal, constituting numerous genera, showing a great variety of
structure, some of them being blind, belong to this family, _e.g._
_Macrohectopus_ (_Constantia_), _Acanthogammarus_, _Heterogammarus_,
etc.

[Illustration:

  FIG. 95.—_Gammarus locusta_, ♂ (above) and ♀ (below), × 4. _Abd.1_,
    First abdominal segment; _T_, telson; _Th_, seventh free thoracic
    segment (= 8th thoracic segment); _U_, third uropod. (After Della
    Valle.)
]

=Fam. Talitridae.=—This family may be distinguished by the absence of a
palp on the mandible, and by one ramus of the uropods being very small
or wanting. The various kinds of “Sand-hoppers” belong here, familiar
creatures on every sandy coast between tide-marks. The genera _Talitrus_
and _Talorchestia_ always frequent sand, while _Orchestia_ is generally
found under stones and among weed. Some species of _Orchestia_, e.g. _O.
gammarellus_, live inland in moist places at some distance from the sea;
one species of _Talitrus_ (_T. sylvaticus_) occurs at great elevations
in forests in Southern Australia.

_Hyale_ is a coastal genus, and is also found on floating objects in the
Sargasso Sea. _Hyalella_ is confined to Lake Titicaca and the fresh
waters of South America. _Chiltonia_ from S. Australasia.

=Fam. Corophiidae.=—The members of this family have a rather flattened
body and small abdomen, and the side-plates on the thorax are small. The
uropods are also small and weak. Some species of the genus _Corophium_
are characteristic of the Caspian Sea.


                      =Sub-Order 2. Laemodipoda.=

[Illustration:

  FIG. 96.—_Caprella grandimana_, × 4. _a_, Abdomen; _g_, gills; _t_,
    3rd (first free) thoracic segment; _t′_, 8th thoracic segment.
    (After P. Mayer.)
]

=Fam. 1. Caprellidae=[113] are also chiefly littoral forms, swarming
among rocks covered by algae, though they are by no means so easy to
detect as the Gammaridae and Tanaidae which haunt similar situations. In
a basinful of algae or Polyzoa taken from the rocks fringing the Bay of
Naples, the latter are easily collected, the Tanaidae always crawling
out of the weeds in the direction of the light, while the Gammarids dart
about in all directions; but the Caprellidae, with their branching
stick-like forms, harmonise so well with their surroundings that it
requires an experienced eye to detect them. The body is elongated and
thin, resembling that of a stick-insect. The first two thoracic segments
are more or less completely fused with the head; the second and third
thoracic limbs end in claws; the two following thoracic limbs are normal
in the genus _Proto_, rudimentary in _Protella_, and absent in the
remaining genera, though their gills remain as conspicuous flabellate
structures. The three hind legs are normal, and the abdomen is reduced
to a tiny wart at the hind end of the greatly elongated thorax.

P. Mayer has described cases of external hermaphroditism as being fairly
common in certain species, _e.g._ _Caprella acutifrons_, and this is
interesting if we take into consideration the frequent partial
hermaphroditism exhibited by the gonad of _Orchestia_ at certain times
of year (see p. 104).

=Fam. 2. Cyamidae.=—These are closely related to the Caprellidae in the
form of the limbs and the reduced state of the abdomen. _Cyamus ceti_,
which lives ectoparasitically on the skin of whales, has the body
expanded laterally instead of being elongated, as in the Caprellids.


                        =Sub-Order 3. Hyperina.=

[Illustration:

  FIG. 97.—_Phronima sedentaria_, ♀, in a _Pyrosoma_ colony, × 1. (After
    Claus, from Gerstaecker and Ortmann.)
]

These are an equally distinct and curious group of Amphipods,
characterised by the large size of the head and the transparency of the
body. Instead of haunting the littoral zone they are pelagic in habit,
and many of them live inside transparent pelagic Molluscs, Tunicates, or
Jellyfish. A well known form is _Phronima sedentaria_, which inhabits
the glassy barrel-like cases of the Tunicate _Pyrosoma_ in the
Mediterranean. The female is often taken in the plankton together with
her brood in one of these curious glass houses; the zooids of the
_Pyrosoma_ colony are completely eaten away and the external surface of
the case, instead of being rough with the tentacles of the zooids, is
worn to a smooth, glass-like surface. It has been observed that the
female actively navigates her house upon the surface of the sea; she
clings on with her thoracic legs inside, while the abdomen is pushed out
through an opening of the _Pyrosoma_ case behind, and by its alternate
flexion and extension drives the boat forwards, the water being thus
made to enter at the front aperture and supply the female and her brood
with nourishment.


                        DIVISION 3. HOPLOCARIDA.

The carapace leaves at least four of the thoracic somites distinct. The
eyes are pedunculate. The mandibles are without a lacinia mobilis; there
are no oostegites, the eggs being carried in a chamber formed by the
maxillipedes. The hepatic caeca are much ramified, the heart is greatly
elongated, stretching through thorax and abdomen, with a pair of ostia
in each segment. The spermatozoa are spherical, and there is a
complicated and peculiar metamorphosis.


                         =Order. Stomatopoda.=

[Illustration:

  FIG. 98.—Lateral view of _Squilla_ sp., × 1. _A.1_, _A.2_, 1st and 2nd
    antennae; _Ab.1_, 1st abdominal segment; _Ab.6_, 6th abdominal
    appendage; _C_, cephalothorax, consisting of the head fused with the
    first five thoracic segments; _E_, eye; _M_, 2nd maxillipede; _T_,
    telson. (After Gerstaecker and Ortmann.)
]

The Stomatopoda are rather large animals, occasionally reaching a foot
in length, all of which exhibit a very similar structure; _Squilla
mantis_ and _S. desmaresti_ are found on the south coast of England not
very frequently; but they are very common in the Mediterranean, living
in holes or in the sand within the littoral zone of shallow water. They
differ from all the other Malacostraca by a combination of characters,
and Calman proposes the term HOPLOCARIDA for a division equivalent to
the Peracarida, Eucarida, etc.

The abdomen is very broad and well developed, ending in a widely
expanded telson. There is a carapace which covers the four anterior
thoracic segments, leaving the four posterior segments free. The portion
of the head carrying the stalked eyes constitutes an apparently separate
segment articulated to the head. The antennae, mandibles, and maxillae
are normal; there then follow five pairs of uniramous thoracic limbs
turned forwards as maxillipedes and ending in claws; the second pair of
these is modified into a huge raptorial arm, exactly resembling that of
a Praying _Mantis_ (cf. vol. v. p. 242), by means of which the _Squilla_
seizes its prey. The last three thoracic limbs are small and biramous.
The pleopods are powerful, flattened, biramous swimming organs with
small hooks or “retinaculae” upon their endopodites, which link together
each member of a pair in the middle, and with large branching gills upon
the exopodites.

The internal anatomy exhibits several primitive features. The nervous
system is not at all concentrated, there being a separate ganglion for
each segment; and the heart stretches right through thorax and abdomen,
with a pair of ostia in each segment. There are also ten hepatic
diverticula given off segmentally from the intestine.

The female has the curious habit of carrying the developing eggs in a
chamber improvised by the apposition of the maxillipedes, so that it
looks rather as if she were in the act of devouring her own brood.

The metamorphosis of the larvae, despite the work of Claus[114] and
Brooks,[115] is not very accurately known, especially uncertain being
the identification of the different larvae with their adult forms. The
chief interest consists in the fact that certain of the anterior
thoracic limbs develop in their normal order and degenerate, to be
reformed later, just as in the Phyllosoma larva of the Loricata (see pp.
165, 166).

In one series of larvae, probably not of _Squilla_ itself, but of
related genera, the young hatch out as “Erichthoidina” (Fig. 99), with
the thoracic appendages developed as biramous organs as far as the fifth
pair, and with a single abdominal pair of limbs.

[Illustration:

  FIG. 99.—Erichthoidina larva of a Stomatopod, with five pairs of
    maxillipedes, and the first pair of abdominal appendages, × 10.
    (From Balfour, after Claus.)
]

The abdominal series of limbs is next completed; the second thoracic
limb assumes its adult raptorial structure, but the succeeding three
limbs become greatly reduced and may entirely degenerate, leaving the
posterior six thoracic segments without limbs.

Usually the anterior three pairs are only reduced, and then redevelop
side by side with the small posterior limbs as they appear. This larva
is then termed the “Erichthus” (Fig. 100); but when they completely
disappear the larva is called a “Pseudozoaea,” owing to its resemblance
to the Zoaea stage of the Decapoda, which is also characterised by the
suppressed development of the thoracic segments.

[Illustration:

  FIG. 100.—Older Erichthus larva, with six pairs of abdominal
    appendages, × 15. (From Balfour, after Claus.)
]

The so-called “Alima” larva of _Squilla_ is also a Pseudozoaea, but it
is apparently arrived at directly without the previous formation and
degeneration of the anterior thoracic limbs, the larva hatching out from
the egg in the Pseudozoaeal stage.

=Fam. Squillidae.=—Of the six known genera none extend into the cold
subarctic seas; the majority are characteristic of the warm or tropical
seas (_Gonodactylus_), some of the species having very wide ranges,
_e.g._ _G. chiragra_, which is completely circumtropical, and appears to
have entered the Mediterranean at some period, though it is very rare
there.




                               CHAPTER VI
    CRUSTACEA (_CONTINUED_)—EUMALACOSTRACA (_CONTINUED_): EUCARIDA—
                  EUPHAUSIACEA—COMPOUND EYES—DECAPODA


                         DIVISION 4. EUCARIDA.

The carapace fuses with all the thoracic segments. The eyes are
pedunculate. The mandible is without a lacinia mobilis. There are no
oostegites, the eggs being attached to the endopodites of the pleopods.
The hepatic caeca are much ramified, the heart is abbreviated and
saccular, the spermatozoa are spherical with radiating pseudopodia, and
development is typically attended by a complicated larval metamorphosis.


                        =Order I. Euphausiacea.=

[Illustration:

  FIG. 101.—Calyptopis larva of _Euphausia pellucida_, × about 20.
    _A.1_, 1st antenna; _Ab.6_, 6th abdominal segment; _E_, eye; _M_,
    maxillipede. (After Sars.)
]

The =Euphausiidae=[116] agree with the Decapoda in passing through a
complicated larval metamorphosis. The young hatch out as Nauplii, with
uniramous first antennae and biramous second antennae and mandibles. In
the next stage, or “Calyptopis” (Fig. 101), which corresponds exactly to
the Zoaea of the Decapoda, two pairs of maxillae and a pair of biramous
maxillipedes are added; the hinder thoracic segments are
undifferentiated, but the abdomen is fully segmented, and the rudiments
of the sixth pair of pleopods are already visible.

In the next stage (“Furcilia”) the other abdominal pleopods are added,
the whole series being completed before the thoracic appendages number
more than two or three. This stage corresponds to the Metazoaea of the
Decapoda, and the interference in the orderly differentiation of the
segments with their appendages from before backwards is a phenomenon
which we shall meet again when we treat of Decapod metamorphosis. It is
evidently a secondary modification, furnishing the larva precociously
with its most important swimming organs so as to enable it to lead a
pelagic existence. The frequent violation of the law of metameric
segmentation, that the most anterior segments being the first formed
should be the first to be fully differentiated, leads us to suppose that
the larval stages of the Eucarida at any rate do not represent
phylogenetic adult stages through which the Malacostraca have passed.
Nor do they, perhaps, even represent primitive larval stages, but have
been secondarily acquired from an embryonic condition which used to be
passed through within the egg-membranes, as in _Nebalia_ and the
Mysidacea, when the order of differentiation of the segments was normal.
The case is a little different with the Nauplius larva. This larval
form, in an identical condition, is found both in the Entomostraca as a
general rule, and again in certain Malacostraca, viz. the Euphausiidae
and the Peneidea. Whatever its phylogenetic meaning may be, we may be
quite certain that the ancestor of the two great divisions of the
Crustacea had a free-swimming Nauplius larva, and this conclusion is
confirmed by the probable presence of a Nauplius larva in Trilobites.

The Euphausiidae, in contradistinction to the Mysidae, are frequently
met with in the surface-plankton. _Euphausia pellucida_ (Fig. 102) is of
universal distribution, and is frequently taken at the surface as well
as at considerable depths.

Many noteworthy features in Euphausiid organisation are brought out in
Fig. 102. The shrimp-like appearance of the carapace and antennae
indicate the special Decapodan affinities of the family; noteworthy,
also, are the single series of gills and the biramous thoracic and
abdominal limbs, similar to those of the Mysidacea. The Euphausiidae
also possess phosphorescent organs of a highly developed kind, and these
are usually situated, as in the type figured, upon the outer margins of
the stalked eyes, on the bases of the second and seventh thoracic limbs,
and on the ventral median line on the first four abdominal segments.
These organs are lantern-like structures provided with a lens, a
reflector, and a light-producing tissue, and they are under the control
of the nervous system. Their exact use is not known, any more than is
the use of phosphorescence in the majority of organisms which produce
it; but in certain cases it appears that the Euphausiids make use of
their phosphorescent organs as bull’s eye lanterns for illuminating the
dark regions into which they penetrate or in which some of them
permanently dwell. At any rate, associated with the presence of these
organs in some deep-sea Euphausiids are remarkable modifications of the
eyes; and we may perhaps here fittingly introduce a short discussion of
these visual modifications in deep-sea Crustacea, and the conditions
which call them forth.

[Illustration:

  FIG. 102.—_Euphausia pellucida_, female, × 5. _G_, Last gill; _L_,
    luminous organ of first leg; _L′_, luminous organ of 2nd abdominal
    segment; _T_, biramous thoracic appendages. (After Sars.)
]


[Illustration:

  FIG. 103.—=A=, Sections (diagrammatic) of Crustacean compound eye,
    =A=, with pigment in light-position for mosaic vision; =B=, with
    pigment in dark-position for refractive vision. _c_, Corneal lens;
    _c.g_, corneagen cells; _cr_, crystalline cone; _f_, basal membrane,
    or membrana fenestrata; _ip_, irido-pigment; _n_, nerve; _r_,
    retinula; _rh_, rhabdom; _rp_, retino-pigment; _v_, vitrella.
]

The =compound eyes of Crustacea= resemble those of Insects in that they
are composed of a very large number of similar elements or “ommatidia,”
more or less isolated from one another by pigment. Each ommatidium
consists typically of a corneal lens (Fig. 103, _c_), secreted by flat
corneagen cells (_c.g_) below; beneath the corneal lens is a transparent
refractive body called the “crystalline cone” (_cr_), which is produced
by a number of cells surrounding it called the “vitrellae” (_v_). Below
the crystalline cone comes the “rhabdom” (_rh_), produced and nourished
by “retinulacells” (_r_). The rhabdom is a transversely striated rod,
constituting the true sensory part of each ommatidium, and is in
connexion at its lower end with a nerve-fibre (_n_), passing to the
optic ganglion. The rhabdoms rest upon a membrane (_f_) called the
“membrana fenestrata.” Each ommatidium is isolated from its fellows
which surround it by a complete cylinder of pigment, part of which is
especially crowded round the crystalline cone, and is known as
“irido-pigment” (_ip_), while the part which surrounds the rhabdom is
called “retino-pigment” (_rp_).

When the pigment is arranged in this way, as in Fig. A, only those rays
of light which strike an ommatidium approximately at right angles to the
corneal surface can be perceived, since only these can reach the top of
the rhabdom; the others pass through the crystalline cones obliquely,
and are absorbed by the cylinder of pigment surrounding each ommatidium,
so that they neither reach the rhabdom of the ommatidium which they
originally entered, nor can they penetrate to the rhabdom of
neighbouring ommatidia. This gives rise to what is known as “mosaic
vision,” that is to say, each ommatidium only perceives the rays of
light which are parallel to its long axis, and in this way an image is
built up of which the various points are perceived side by side by means
of separate eye-elements. The distinctness and efficiency of this mode
of vision depends chiefly upon the number of ommatidia present, and the
completeness with which they are isolated from one another by the
pigment. Now this form of vision, depending as it does upon the
absorption of a great number of the light-rays by pigment, and the
transmission of only a limited number to the sensory surface, is only
possible when there is a strong light, and there is no need for
economising the light-rays. The most important discovery was made by
Exner,[117] that the majority of animals with compound eyes had the
power of so arranging the pigment in their eyes as to enable them to see
in two ways. In bright light the pigment is situated as in Fig. 103, A,
so as completely to isolate the rhabdoms from one another
(day-position); but in the dusk the pigment actively migrates, the
irido-pigment passing to the surface (B) near the tops of the
crystalline cones, and the retino-pigment passing interiorly to rest on
the membrana fenestrata at the bases of the rhabdoms (night-position).
When this happens the rays of light which strike the ommatidia at all
sorts of angles, instead of being largely absorbed by the pigment, are
refracted by the crystalline cones and distributed over the tops of the
rhabdoms, passing freely from one ommatidium to another. In this way the
eye acts on this occasion, not by mosaic vision, but on the principle of
refraction, as in the Vertebrate eye. Of course the distinctness of
vision is lost, but an immense economy in the use of light-rays is
effected, and the creature can perceive objects and movements dimly in
the dusk which by mosaic vision it could not see at all. The pigment is
contained in living cells or chromatophores, and it is carried about by
the active amoeboid movements of these cells with great rapidity.

Now, besides the active adaptability to different degrees of light
brought about in the individual by these means, we find Crustacea living
under special conditions in which the eyes are permanently modified for
seeing in the dusk, and this naturally occurs in many deep-sea forms.

Doflein[118] has examined the eyes of a great number of deep-sea
Brachyura dredged by the _Valdivia_ Expedition, and as the result of
this investigation he states that the eyes of deep-sea Brachyura are
never composed of so many ommatidia, nor are they so deeply pigmented as
those of littoral or shallow water forms. At the same time an immense
range of variation occurs among deep-sea forms which are apparently
subjected to similar conditions of darkness, a variation stretching from
almost normal eyes to their complete degeneration and the fusion of the
eye-stalks with the carapace; and this variation is very difficult to
account for. A very frequent condition for crabs living at about 100
fathoms, and even more, is for either the irido-pigment or the
retino-pigment to be absent, for the number of ommatidia to be reduced,
and for the corneal lenses to be greatly arched. There can be little
doubt that these crabs use their eyes, not for mosaic vision, but to
obtain the superposition-image characteristic of the Vertebrate eye. In
deeper waters, where no daylight penetrates at all, this type of eye is
also met with, and also further stages in degeneration where all pigment
is absent, and the ommatidia show further signs of reduction and
degeneration, e.g. _Cyclodorippe dromioides_. In a few forms, e.g.
_Cymonomus granulatus_ among Brachyura, and numerous Macrura, the
ommatidia may entirely disappear, and the eye-stalks may become fused
with the carapace or converted into tactile organs.

Progressive stages in degeneration, correlated with the depth in which
the animals are found, are afforded by closely related species, or even
by individuals of apparently the same species. Thus in the large
Serolidae of Antarctic seas, _Serolis schytei_ occurs in 7–128 metres,
and has well-developed eyes; _S. bronleyana_, from 730 to 3600 metres,
has small and semi-degenerate eyes; while _S. antarctica_ in 730–2920
metres is completely blind. _Lispognathus thompsoni_ is a deep-water
spider-crab, and the individuals taken at various depths are said to
exhibit progressive stages in degeneration according to the depth from
which they come.

At the same time many anomalies occur which are difficult to explain. In
the middle depths, _i.e._ at about 100 fathoms, side by side with
species which have semi-degenerate or, at any rate, poorly pigmented
eyes, occur species with intensely pigmented eyes composed of very
numerous ommatidia, _e.g._ the Galatheid _Munidopsis_ and several
shrimps, while in the true abysses many of the species have quite normal
pigmented eyes. This is especially the case with the deep-sea Pagurids,
of which Alcock describes only one species, _Parapylocheles scorpio_, as
having poorly pigmented eyes. An attempt to account for this was made by
Milne Edwards and Bouvier,[119] who pointed out that the truly deep-sea
forms with well-developed eyes were always Crustacea of a roving habit,
which were perhaps capable of penetrating into better lit regions, and
to whom well-developed eyes might be useful, while the degenerate forms
were sluggish. This explanation cannot be held to account for the
phenomenon, as too many deep-sea forms with fairly normal eyes are known
which are never taken outside deep waters. Doflein (_loc. cit._) points
out that in the Brachyura of the deep sea there is a remarkable
correlation between the degree of degeneration of the eye and the size
of the eggs—the large-egged forms having unpigmented and degenerate
eyes, while the species with small eggs have pigmented eyes. He supposes
that the species with large eggs undergo a direct development without
pelagic free-swimming larvae, and that since they never reach the
surface their eyes never meet with the necessary stimulus of light for
the development of pigment; whereas the small-egged species undergo a
pelagic larval existence when this stimulus is present and gives the
necessary initiative for the development of the pigment.

Another factor enters into the question of eye-degeneration in the
Crustacea. The great majority of deep-sea animals, including many
deep-sea Crustacea, are phosphorescent, and it is certain that although
daylight never penetrates into the abysses of the ocean, yet there is
considerable illumination derived from the phosphorescence of the
inhabitants of these regions.

Alcock[120] points out in this connexion that the Pagurids, which are
conspicuous in the great depths as animals with normally developed eyes,
carry about anemones with them, and these organisms are very frequently
phosphorescent to a high degree. It may well be, therefore, that the
Pagurids are enabled to use their eyes in the normal manner owing to the
phosphorescent light which they carry about with them, and this use of
phosphorescent light may apply to a number of deep-sea Crustacea whose
eyes are not at all or only partially degenerate.

An extremely interesting case of the use of phosphorescent light is
given by Chun.[121] In a number of Euphausiids occurring in deep waters
each compound eye is divided into two parts—a frontal and
ventro-lateral—which differ from one another very greatly in the nature
and disposition of their ommatidia.

[Illustration:

  FIG. 104.—Section of eye of _Stylocheiron mastigophorum_. =A=, Frontal
    portion; =B=, ventro-lateral portion; =C=, phosphorescent organ;
    =D=, entrance of optic nerve; _c_, corneal lens; _cr_, crystalline
    cone; _pg_, pigment; _ret_, retinula; _rh_, rhabdom. (After Chun.)
]

In the frontal portion (Fig. 104, A) the ommatidia are few in number and
long, the corneal lenses are highly arched, and the pigment is reduced
to a few clumps in the iris. This part of the eye is evidently adapted
for forming a vague superposition-image in the dusk. The ventro-lateral
part (B), on the other hand, is composed of numerous small ommatidia,
the crystalline cones of which can be completely isolated from one
another by the irido-pigment. Immediately below this part of the eye is
a phosphorescent organ (C) provided with a lens and tapetum. Chun
suggests that the ventro-lateral part of the eye is used for obtaining a
clear mosaic image of objects illuminated by the phosphorescent organ,
while the frontal part of the eye is used for obtaining general visual
impressions in dimly lit regions. This curious differentiation of the
eye into two parts apparently only occurs in predaceous animals, which
capture their prey alive upon the bottom, and to whom a clear vision of
moving organisms is a necessity.

Another instance of Crustaceans making use of their own light is given
by Alcock,[122] who found two deep-sea prawns, _Heterocarpus alphonsi_
and _Aristaeus coruscans_, at about 500 fathoms in the Indian Ocean.
These animals produce a highly phosphorescent substance which they eject
from the antennary glands, and they possess very large, deeply-pigmented
eyes.

The whole subject of the modification of the pigment and structure of
Crustacean eyes is an interesting one, because it presents us with one
of those cases in which the direct response to a stimulus acting within
the lifetime of the individual seems to run parallel to the fixed
adaptations of a whole species, which have become hereditary and
apparently independent of the external stimulus of light or of the
absence of light. As far as is known, however, the direct response of
the individual to the absence of light is limited to the reduction or
disappearance of the pigment, and does not extend to those structural
changes in the ommatidia which are characteristic of so many deep-sea
forms.


                       =Order II. Decapoda.=[123]

The Decapoda, together with the Euphausiidae, make up the Division
Eucarida, the members of which differ from the Orders hitherto described
in a number of characters, _e.g._ the presence of a carapace covering
the whole of the thorax, the absence of a brood-pouch formed of
oostegites, the presence of a short heart, of spermatozoa with radiating
pseudopodia, and of a complicated larval metamorphosis, of which the
Zoaea stages are most prominent.

The Decapoda differ from the Euphausiidae chiefly in the anterior three
thoracic limbs being turned forwards towards the mouth to act as
maxillipedes, and in the five succeeding thoracic limbs being nearly
always uniramous and ambulatory or chelate; there are typically present
three serial rows of gills attached to the thoracic segments, an upper
series (“pleurobranchiae”) attached to the body-wall above the
articulation of the limbs, a middle series (“arthrobranchiae”) attached
at the articulation of the limbs, and a lower series (“podobranchiae”)
attached to the basal joints of the limbs. These gills are enclosed in a
special branchial chamber on each side of the thorax, formed by lateral
wings of the carapace known as “branchiostegites.” The gills of each
series are never all present in the same animal, the anterior and
posterior members showing a special tendency to be reduced and to
disappear. In this manner “branchial formulae” can be constructed for
the various kinds of Decapods, which differ from the ideal formula in a
manner distinctive of each kind. The second maxilla is always provided
with an oar-like appendage on its outer margin (exopodite), known as the
“scaphognathite,” which, by its rhythmical movement, keeps up a constant
current of water through the gill-chamber.

A complicated auditory organ is present on the basal joint of the first
antennae; this is a sac communicating with the exterior and lined
internally with sensory hairs. The animal is said to place small pieces
of sand, etc., in its ears to act as otoliths. _Anaspides_ (see p. 116)
is the only other Crustacean which has an auditory organ in this
position.

The larval histories of the Decapods[124] are of great interest, and
will be given under the headings of the various groups. The first
discoverer of the metamorphosis of the Decapoda was the Irish naturalist
J. V. Thompson, certainly one of the ablest of British zoologists. In
1828, in his _Zoological Researches_, he describes certain Zoaeas of the
Brachyura and proves that these animals are not an adult genus, as
supposed, but larval forms. But Rathke, in 1829, described the direct
development of the Crayfish; and Westwood, after describing the direct
development of _Gecarcinus_, utterly denied Thompson’s assertions
concerning metamorphosis. Thompson replied in the _Royal Society
Transactions_ for 1835, and described the Megalopa stage of _Cancer
pagurus_. Rathke,[125] although previously an opponent of Thompson,
subsequently made confirmatory observations upon the larvae of the
Anomura; and Spence Bate clinched the matter by describing Brachyuran
metamorphosis with great accuracy in the _Philosophical Transactions_
for 1859. Since then a mass of work has been done on the subject, though
much detail still remains to be elucidated.

The Decapoda fall into three sub-orders, which graduate into one
another—(i.) the Macrura, including the Lobsters, Crayfishes, Shrimps,
and Prawns; (ii.) the Anomura, including the Hermit-lobsters and
Hermit-crabs; and (iii.) the Brachyura or true Crabs.


                        =Sub-Order 1. Macrura.=

This sub-order[126] is characterised by the large abdomen, furnished
with five pairs of biramous pleopods, and ending in a powerful tail-fan
composed of the telson and the greatly expanded sixth pair of pleopods,
the whole apparatus being locomotory. The second antennae are furnished
with very large external scales, representing the exopodites of those
appendages. Some of the Shrimps and Prawns closely resemble the
“Schizopods,” but the pereiopods are nearly always uniramous.[127]
Several subdivisions of the Macrura are recognised.


                        =Tribe 1. Nephropsidea.=

This tribe includes the Lobsters and Crayfishes, animals well known from
their serviceableness to man. There are three families, which will be
treated separately.

=Fam. 1. Nephropsidae.= The podobranchs are not united with the
epipodites, and the last thoracic segment is fixed and fused to the
carapace. The chelae are generally asymmetrical. The most important
Lobsters are the European and the American species—_Homarus vulgaris_ (=
_Astacus gammarus_) and _H. americanus_ respectively; these animals
engage a large number of people in the fisheries. It is estimated that
in America about £150,000 are spent every year on Lobsters.

The genus _Nephrops_ contains the small Norwegian lobster and other
forms.

Herrick[128] gives some interesting particulars with regard to the
life-history of the American species. The largest recorded specimen
weighed about twenty-five pounds, and measured twenty inches from
rostrum to tail; similar European specimens have been recorded, but, on
the average, they are not so large as the American forms.

The Lobster, like all Crustacea, undergoes a series of moults as the
result of increase in size, shedding the whole of the external
integument in one piece. This is accomplished by a split taking place on
the dorsal surface at the junction of thorax and abdomen; through the
slit so formed the Lobster retracts first his thorax with all the limbs,
and then his abdomen. When first issuing from the old shell the animal’s
integument is soft and pulpy, but the increase in size of the body is
already manifest; this increase per moult, which is approximately the
same in young and adult animals, varies from 13 to 15 per cent of the
animal’s length. According to this computation, a Lobster 2 inches long
has moulted fourteen times, 5 inches twenty times, and 10 inches
twenty-five times, and it may be roughly estimated that a 10–inch
Lobster is four years old. Young Lobsters probably moult twice a year,
and so do adult males, but females only moult once a year soon after the
young are hatched out.

The process of moulting or ecdysis is an exceedingly dangerous one to
the Lobster and to Crustacea in general, and is very frequently fatal.
There is, first of all, the danger of the act not being accomplished
skilfully, when death always ensues. The Lobster remains soft and
unprotected for about six weeks after the ecdysis, and is very apt to
fall a prey to the predaceous fish, such as Sharks, Skates, Cod, etc.,
which feed upon it. There are, however, some peculiar adaptations
connected with the process which are of interest. In order to facilitate
the ecdysis, areas of absorption are formed upon the dorsal and ventral
surfaces of the carapace, on the narrower parts of the chelipedes, and
at other places; in these areas the calcium carbonate is absorbed, and
the old shell becomes elastic and thin, so as to allow a more easy
escape for the moulting Lobster. It has been noticed that while this is
taking place large concretions of calcium carbonate are formed at the
sides of the stomach, known as “gastroliths,” which perhaps represent
the waste lime that has been abstracted from the areas of absorption.
After moulting the Lobster is in great need of lime for stiffening his
shell, and it has been noticed that on these occasions he is very greedy
of this substance, even devouring his own cast-off skin.

The male Lobster is especially prized on account of his larger chelae,
but in both sexes the chelipedes are differentiated into a smaller
cutting pincer and a larger crushing one. Lobsters may be right or left
handed, with the large crushing claw on the right or left hand, and
sometimes specimens occur with the smaller cutting pincers on both
chelipedes, and very rarely, indeed, with crushing claws on both sides.
Crustacea very commonly have the power of casting off a limb if they are
seized by it or if it is injured, and of regenerating a new one. In the
Lobster a so-called breaking joint is situated on each leg at the suture
between the fused second and third segments; a membrane being pushed
inwards from the skin, which not only serves to form a weak joint where
rupture may easily take place, but also to stop excessive bleeding after
rupture. In the newly-hatched larvae there is a normal joint between the
second and third segments; and autotomy, or the voluntary throwing away
of a limb, never occurs until the fourth larval stage, when the breaking
joint is formed. Autotomy is a reflex act under the control of the
segmental ganglion; if a Crab or Lobster be anæsthetised, and then a
limb be injured or broken off below the breaking joint, the animal
forgets to throw the injured leg or stump off at the breaking joint, a
proceeding which always occurs under normal conditions. The regeneration
of a limb starts from a papilla which grows out of the breaking joint,
and after a number of moults acquires the specific form of the limb that
has been lost. A number of interesting observations have been made upon
the regeneration of the limbs in Crustacea. It was in the Hermit-crab
that Morgan[129] proved that regeneration and the liability to injury do
not always run parallel, as Weismann held they should, since the
rudimentary posterior thoracic limbs, which are never injured in nature,
can regenerate when artificially removed as easily as any others.
Przibram[130] has shown that in the shrimp _Alpheus_, whose chelipedes
are highly asymmetrical, if the large one be cut off, the small one
immediately begins to grow and to take on the form of the large one,
while the regenerated limb is formed as the small variety. This
remarkable inversion in the symmetry of the animal clearly ensures that,
if the large chela is injured and thrown away, the least amount of time
is wasted in providing the shrimp with a new large claw.

To return to the Lobster; for the majority of the individuals there is a
definite breeding season, viz. July and August, but a certain proportion
breed earlier or later. A female begins to “berry” at about eight inches
in length, and to produce more and more eggs up to about eighteen
inches, when as many as 160,000 eggs are produced at a time; after this
there is a decline in numbers. A female normally breeds only once in two
years. Strict laws are enforced forbidding the sale of Lobsters and
Crabs “in berry” in both England and America. The period of incubation,
during which the developing eggs are attached to the swimmerets of the
female, lasts about ten or eleven months, so that the larvae are hatched
out approximately in the following June. On hatching, the larva, which
measures about one-third of an inch, and is in the Mysis stage (_i.e._
it possesses all the thoracic limbs in a biramous condition, but is
without the abdominal limbs), swims at first on the surface. After five
or six months of this life, during which the abdominal pleopods are
added from before backwards, it sinks to the bottom, loses the
exopodites of the thoracic limbs, and is converted into the young
Lobster, measuring about half an inch in length. The little Lobster
starts in deepish water, and gradually crawls towards the shore; here it
passes its adolescence, but on coming to maturity it migrates out again
into the deep water.

=Fam. 2. Astacidae.=—In this family, which includes all the European and
North American Crayfishes, _Astacus_ (_Potamobius_) and _Cambarus_, the
podobranchs are united with the epipodites, the last thoracic segment is
free, there is only one pleurobranch or none at all, the gills have a
central lamina, but the filaments are without terminal hooks, and the
endopodites of the first two pairs of abdominal appendages in the male
serve as copulatory organs. For the distribution, etc., of these forms
see p. 213.

=Fam. 3. Parastacidae.=—This family includes the Crayfishes of the
Southern Hemisphere, viz. _Parastacus_ from South America, _Astacopsis_
and _Engaeus_ from Australia, _Paranephrops_ from New Zealand, and
_Astacoides_ from Madagascar. These genera agree with the Potamobiidae
in the union of the podobranchs with the epipodites, and in the free
condition of the last thoracic segment, but there are generally four
pleurobranchs, the gills are without a lamina, the filaments have
terminal hooks, and there are no sexual appendages in the male. For
distribution, etc., see also p. 213.

The larval development in the Crayfishes is still more abbreviated than
in the Lobsters, the Mysis stage being passed through within the
egg-membranes. The young, when they hatch out, are furnished with hooks
upon the chelipedes, by which they anchor themselves to the pleopods of
the mother.


                         =Tribe 2. Eryonidea.=

[Illustration:

  FIG. 105.—_Willemoesia inornata_, × ⅓. (From a figure prepared for
    Professor Weldon.)
]

These are remarkably archaic animals of great rarity, though they were
common enough in Triassic seas, and have come down to us as fossils from
those times, being thus among the oldest Decapoda known. They only
survive now as deep sea species, and the genus discovered by the
_Challenger_,[131] _Willemoesia_ (Fig. 105), confirmed the expectations
of the _Challenger_ naturalists that the abysses of the ocean would
contain relics from older periods which had managed to survive where the
competition was not so keen. The genus _Willemoesia_ is very widely
distributed, being dredged up from below a thousand fathoms in the
Indian Ocean, the Mediterranean, North and South Atlantic, and the
Pacific oceans. All the walking legs are chelate, and the animal is
quite blind, as are all the Eryonidea, the eye-stalks being fused with
the carapace.

Only a single family =Eryonidae= is recognised.


                 =Tribe 3. Peneidea.—Tribe 4. Caridea.=

We will now consider the Shrimps and Prawns, since in them occurs the
most complete metamorphosis found in the Decapoda. The Peneidea are
distinguished from the ordinary Prawns and Shrimps (Caridea) by having
the first three instead of the first two pereiopods chelate. The genus
_Peneus_ affords several species which are of commercial value as
objects of food; the edible Prawns of the Mediterranean belong to this
genus, while in the North Sea two of the Caridea, viz. the Shrimp,
_Crangon vulgaris_, and the Prawn, _Palaemon serratus_, are the forms
very commonly eaten. Both subdivisions are well represented in the deep
sea fauna from all parts of the world. _Glyphocrangon spinulosa_ (Fig.
110, p. 164) is a deep sea Shrimp with eyes that have lost their
pigment, and with the body covered with spines, while the last abdominal
segment is fused with the telson to form a sharp bayonet-like process at
the hind end of the body. Some of the deep-sea Prawns of the Indian
Ocean are described by Alcock[132] as possessing peculiar secondary
sexual characters. Thus _Parapeneus rectacutus_ ♂ has one lash of the
first pair of antennae peculiarly bent to form a clasping organ, while
_Aristaeus crassipes_ has a hook on the end of the third maxillipede. In
the latter the females have much longer rostra than the males, and are
in general more powerfully built, so that they seem to have usurped the
proper functions of the male, and probably engage in combats with one
another over his person.

[Illustration:

  FIG. 106.—Nauplius larva of _Peneus_, sp. × 25. (From Balfour, after
    F. Müller).
]

As a general rule the Shrimps and Prawns occur in large shoals in the
shallow waters of the littoral zone, and they have a remarkable power of
adapting their colours to the surroundings in which they happen to be at
any particular moment.[133] This is brought about by the variously
coloured chromatophores, which contract and expand in obedience to a
stimulus transmitted through the eyes of the animal. A number of the
Palaemonidae go up rivers into fresh water, while one family, the
_Atyidae_, live in the completely fresh water of rivers and inland
lakes. The Peneidea undergo a very complete metamorphosis which is
primitive in respect to the order of formation of the segments from
before backwards. The larva hatches out as a Nauplius (Fig. 106), which
by the orderly addition of segments behind is converted into the
Protozoaea (Fig. 107), possessing two pairs of biramous maxillipedes. It
should be noted that the maxillae, which are foliaceous in the adult,
are laid down in this condition in the larva, and this principle holds
good throughout Crustacean metamorphosis, viz. that when a limb is
foliaceous in the adult it is foliaceous in the larva, and when biramous
in the adult it is biramous in the larva. Whilst the rest of the
thoracic limbs are still rudimentary, the sixth pair of pleopods are
being precociously developed (Fig. 108), being the only precociously
formed limbs in the Peneidea, though the abdominal segments are fully
marked off before the thoracic segments, and so must be considered as
precocious in development. When the biramous thoracic limbs are
completed the abdominal biramous pleopods are added, beginning from in
front backwards. Thus the Mysis stage (Fig. 109) is reached, which
resembles in all particulars the adult condition of the Schizopoda. The
adult Prawn develops from this stage by the loss of some or all of the
exopodites on the thoracic pereiopods.

[Illustration:

  FIG. 107.—Protozoea larva of _Peneus_, sp. × 25. (From Balfour, after
    F. Müller.)
]

[Illustration:

  FIG. 108.—Zoaea larva of _Peneus_, sp. × 25. _A_, _A′_, 1st and 2nd
    antennae; _Ab.6_, 6th abdominal appendage; _Mxp_, 2nd maxillipede;
    _T_, 4th–8th thoracic appendages (future walking legs). (After F.
    Müller.)
]

Some of the Peneid larvae take on very peculiar forms, _e.g._ the Zoaeae
of the Sergestidae,[134] which often develop the most wonderful spines
all over the body.

[Illustration:

  FIG. 109.—Mysis stage in the development of _Peneus_, sp. _A.2_, 2nd
    antenna; _Ab.6_, 6th abdominal appendage; _T_, telson; _Th_, the
    biramous thoracic appendages. (After Claus.)
]

The Caridea have a greatly abbreviated metamorphosis, the larva hatching
out at a late Zoaea stage with all three pairs of maxillipedes fully
formed and with a fully segmented abdomen. The succeeding thoracic limbs
are added in order from before backwards, though the sixth pair of
pleopods appear precociously as in the Peneidea. The other swimmerets do
not begin to develop until the thoracic limbs are complete. Some Caridea
show a yet more abbreviated metamorphosis, _e.g._ the fresh-water
_Palaemonetes varians_ of S. Europe, which hatches out at the Mysis
stage.

We see, therefore, in the metamorphosis of the Macrura several
apparently primitive features. In the first place, a free swimming
Nauplius stage is preserved in certain forms, identical in all respects
with the Nauplius of the Entomostraca. Secondly, the thoracic limbs when
they are first developed are biramous, thus giving rise to the
characteristic Mysis stage which links the Macrura on to the
“Schizopoda.” Thirdly, the order of differentiation of the segments is
typically from in front backwards, the only precociously developed
appendage being the sixth abdominal. None of these characters are
reproduced in the higher Decapoda in which there is never a free-living
Nauplius, the first larval stage being the Zoaea; a number of the
thoracic pereiopods, and usually all of them, are uniramous from the
start; and the whole of the abdominal segments with their limbs tend to
be precociously developed before the hinder thoracic segments make a
distinct appearance.


                       =Tribe 3. Peneidea.=[135]

The third legs are chelate except in genera in which the legs are much
reduced. The third maxillipedes are seven-jointed, the second
maxillipedes have normal end-joints, and the first maxillipedes are
without a lobe on the base of the exopodite. The pleura of the first
abdominal segment are not overlapped by those of the second. The abdomen
is without a sharp bend. The branchiae are usually not phyllobranchs.

=Fam 1. Peneidae.=—The last two pairs of legs are well developed, and
there is a nearly complete series of gills. _Cerataspis_,[136] a pelagic
form. _Parapeneus_, _Peneus_, _Aristaeus_, etc.

=Fam. 2. Sergestidae.=—The last or last two pairs of legs are reduced or
lost. The gill-series is incomplete or wanting. _Sergestes_ possesses
gills, and the front end of the thorax is not greatly elongated.
_Lucifer_ has no gills, and the front of the thorax is greatly
elongated, giving a very anomalous appearance to the animal. All the
members of this family are pelagic in habit.

=Fam. 3. Stenopodidae.=—One or both legs of the third pair are longer
and much stouter than those of the first two pairs. On a number of small
anatomical points this family, including the littoral genus _Stenopus_
from the Mediterranean and other warmer seas and _Spongicola_ commensal
with Hexactinellid sponges from Japan, is separated by some authors in a
Tribe by itself.


                          =Tribe 4. Caridea.=

The third legs are not chelate. The third maxillipedes are 4–6 jointed,
the end-joint of the second maxillipede nearly always lies as a strip
along the end of the joint before it, and the first maxillipedes have a
lobe on the base of the exopodites. The pleura of the second abdominal
segment overlap those of the first. The abdomen has a sharp bend; the
branchiae are phyllobranchs.

=Fam. 1. Pasiphaeidae.=—In this family the end-joint of the second
maxillipedes is normally formed, and exopodites are usually present on
all the thoracic limbs. Rostrum small or wanting. Rather numerous genera
are known, most of which inhabit the deep sea, though a few come into
the littoral zone. _Pasiphaea_ chiefly in the deep sea, _Leptochela_ in
the tropical littoral zone.

=Fam. 2. Acanthephyridae.=—The end-joint of the second maxillipede is
modified as in other Caridea, and the rostrum is very strong and
serrate, but in the presence of exopodites, and in the form of the
mouth-parts, this family agrees with the preceding. It is also a
characteristic deep-sea family. _Acanthephyra_, _Hymenodora_,
_Nematocarcinus_, etc.

=Fam. 3. Atyidae.=—This is an entirely fresh-water family, especially
characteristic of the rivers and lakes of the tropics, some of the forms
being exceedingly large and taking the place of the Crayfishes in these
waters. Characteristic of this family is the fact that the fingers of
the chelae are spoon-shaped, and carry peculiar tufts of bristles.
Exopodites are present on the thoracic limbs of some of the genera
(_Troglocaris_, _Xiphocaris_ from Australia and the Malay Islands,
_Atyephyra_ from S. and W. Europe), but are absent in others.
_Caridina_, widely spread and common in Indo-Malay and Africa; _Atya_
from West Indies, West Africa, and Pacific Islands.

=Fam. 4. Alpheidae.=[137]—The exopodites are absent, and the rostrum is
absent or very feeble. The chelae are powerful, and usually very
asymmetrically developed. _Alpheus_ has an enormous number of species
which live chiefly in the tropical seas, where they haunt especially the
coral-reefs, making their homes among the coral or in sponges, etc.
Although occurring in the Mediterranean they penetrate very rarely into
colder seas.

=Fam. 5 Psalidopodidae.=—This family, characterised by the absence of
chelae on the second thoracic limbs, which carry instead a terminal
brush of hairs, and by the rudimentary condition of the eyes, is
represented by the genus _Psalidopus_ from the deep waters of the Indian
Ocean.

=Fam. 6. Pandalidae.=—The first thoracic limb is without chelae, only
six-jointed. The rostrum is large and toothed. The genus _Pandalus_ has
numerous representatives in the northern littoral, _P. annulicornis_
being one of the prawns most commonly met with in the fish-markets.

=Fam. 7. Hippolytidae.=—The first and second thoracic limbs bear chelae,
the carpus of the second being divided into two or more segments. The
first pair of chelae are not distinctly stronger than the second.
_Virbius_ has many species in the littoral zone of all seas, and one
species, _V. acuminatus_, is pelagic. _Hippolyte_ also has numerous
littoral forms distributed all over the world, but chiefly in the arctic
or subarctic seas. _H. varians_, common on the English coasts, shows
interesting colour-reactions to its surroundings.[138]

[Illustration:

  FIG. 110.—_Glyphocrangon spinulosa_, from the right side, × 1. (From
    an original drawing prepared for Professor Weldon.)
]

=Fam. 8. Palaemonidae.=—The first two pairs of legs are chelate, the
carpus of the second not being subdivided. _Palaemon serratus_, a very
common prawn in the British littoral. _Palaemonetes_ in the brackish and
fresh waters of Europe and N. America.

=Fam. 9. Glyphocrangonidae.=—The first pair of legs are subchelate, the
carpus of the second pair is subdivided, and the rostrum is long.
_Glyphocrangon_ (Fig. 110) with numerous species entirely confined to
deep water.

=Fam. 10. Crangonidae.=—The first pair of legs are subchelate, the
carpus of the second pair is not subdivided, and the rostrum is short.
_Crangon vulgaris_ is the common Shrimp of the North Sea.


                          =Tribe 5. Loricata.=

[Illustration:

  FIG. 111.—Dorsal view of _Scyllarus arctus_, × ½. (From an original
    figure prepared for Professor Weldon.)
]

[Illustration:

  FIG. 112.—Embryonic area of developing _Palinurus quadricornis._
    _Ab.1_, 1st abdominal segment; _E_, compound eye; _E′_, median
    simple eye; _L_, upper lip; _L′_, lower lip; _M_, mandible; _Mx.1_,
    _Mx.2_, 1st and 2nd maxillae; _Mxp.1_, 1st maxillipede; _T_, 6th
    (antepenultimate) thoracic appendage. (After Claus.)
]

The Loricata include the Langouste (_Palinurus_) of the Mediterranean
coasts, which replaces there the Lobster of the North Sea as an article
of food, and the peculiarly shaped _Scyllarus arctus_ (Fig. 111), which
is also prized in the Mediterranean as a delicacy. The bright red
“Crayfishes,” _Panulirus_ and _Iasus_, of the Australian coasts are also
largely used as food. Besides its peculiarity in shape, _S. arctus_ has
remarkable scales on the second antennae in place of flagella. The larva
hatches out as the so-called Phyllosoma, which must be regarded as a
greatly flattened and modified[139] Mysis stage.

[Illustration:

  FIG. 113.—Phyllosoma larva of _Palinurus_, sp. × 5. _Ab_, Abdomen;
    _Mxp_, 3rd maxillipede; _T_, antepenultimate (6th) thoracic
    appendage. (After Claus.)
]

In the embryo of _Palinurus_ just before hatching (Fig. 112) we can
recognise the limbs of the head and thorax normally developed in order.
There are present three thoracic limbs, besides the maxillipedes. When
the Phyllosoma hatches out the first maxillipedes have become quite
rudimentary, and the second much reduced, while the second antennae and
second maxillae are also reduced in size. The metamorphosis is completed
by the re-development of the limbs and segments that have been
secondarily suppressed during larval life, and by the appearance of the
pleopods.

This process is again met with in the Squillidae (p. 143), but it
resembles the suppression, in so many Decapodan metamorphoses, of
anterior limbs and the precocious development of segments and limbs
lying posteriorly. In the ordinary Decapoda, however, the suppressed
limbs are merely not formed till later; while in the Loricata the limbs
develop in the correct order, and subsequently degenerate. It is natural
to wonder whether the condition of affairs in the Loricata represents
the primitive process, and whether the precocious development of
segments in the other Decapoda owes its origin to these animals having
once had the direct mode of development when the segments were formed in
the proper order, and to their having subsequently acquired the larval
stages first of all by the degeneration, and then by the suppression of
certain segments which were not of use during larval life. The complete
metamorphosis, however, of the Peneidea, in which the segments and limbs
appear in the right order, rather goes to show that this is the
primitive mode of development in the Decapoda, and that the
disarrangement in the order of appearance of the segments, both in the
Squillidae and in the Loricata and other Decapods, has been
independently acquired in the two cases to meet the needs of the larval
existence.

=Fam. 1. Palinuridae.=—The cephalothorax is subcylindrical, the eyes are
not enclosed in separate orbits formed by the edge of the carapace, and
the second antennae possess flagella. _Palinurus_, with _P. elephas_,
the European Rock Lobster or Langouste. _Iasus_ with two species in the
Antarctic littoral; _Panulirus_ in the tropical littoral.

=Fam. 2. Scyllaridae.=—The cephalothorax is depressed, the eyes are
enclosed in separate orbits formed by the edge of the carapace, and the
second antennae have flat scales in the place of flagella. _Scyllarus_
(Fig. 111), with the European _S. arctus_; _Ibacus_ in rather deep water
with several species, chiefly found in the southern hemisphere.


                       =Tribe 6. Thalassinidea.=

This tribe is included by some authors in the Anomura, and held to be
closely related to the Galatheidea, but the unreduced abdomen is carried
straight and unflexed, and gives a very Macrurous appearance to the
animal. The Anomurous characters are the frequent reduction or absence
of the antennal scale, the fact that only the first two pairs of
pereiopods are ever chelate, and the reduced series of gills. The body
is symmetrical, but the first pair of chelae is always highly
asymmetrical. The posterior pairs of pereiopods, although small, are not
characteristically reduced as in the Anomura. The animals belonging to
this Tribe attain two or three inches in length, and generally burrow in
sand or mud either in the littoral zone or in deeper waters; at the same
time they can swim with considerable activity by means of the pleopods.

=Fam. Callianassidae.=—_Callianassa subterranea_ is common at Naples,
_Gebia littoralis_ in the North Sea.


                         =Sub-Order 2. Anomura=

In this division are included the so-called Hermit-lobsters and
Hermit-crabs, in which the condition of the abdomen is roughly
intermediate between that of the Macrura and that of the Brachyura. It
is not much reduced in size, and the pleopods of the sixth pair are
fairly well developed, but it is usually carried flexed towards the
thorax, and is never a powerful locomotory organ as in the Macrura. The
antennal scale, if present at all, is a mere spine, not the large
leaf-like structure of the Macrura; and there is never a partition
between the two first antennae as in the Brachyura.

The last or last two pairs of pereiopods are reduced, and are turned on
to the dorsal surface or carried inside the branchial chamber; but this
curious character is met with again in certain Brachyura (Dromiacea and
Oxystomata).


                      =Tribe 1. Galatheidea.=[140]

[Illustration:

  FIG. 114.—Dorsal view of _Munidopsis hamata_, × ½. (From an original
    figure prepared for Professor Weldon.)
]

These are symmetrical crabs with a long carapace; the abdomen, which is
as broad as the carapace, is always carried flexed under the thorax, and
the sixth pair of pleopods are expanded to form with the telson a
fan-like tail. The most anterior pereiopods are always much elongated
and chelate; while the last pair are much reduced, and either turned up
on to the dorsal surface, or else carried in the branchial chamber. The
exact meaning of this last characteristic in these forms is doubtful;
some of the species are said to carry shells temporarily upon their
backs, a proceeding probably assisted by the last pair of thoracic
limbs, while in others their limbs may be used for cleaning out the
branchial chamber. Most of the Galatheidea, for instance, the common
_Porcellana_ and _Galathea_, are littoral animals, and may be found
hiding under stones and in crevices on the shore; but a number occur in
deep water, e.g. _Munida_ and _Munidopsis_.

[Illustration:

  FIG. 115.—Zoaea of _Porcellana_, × 20. _T_, Telson. (After Claus.)
]

The shallow-water species have ordinarily developed eyes; the various
species of _Munida_, which occur in fairly deep but by no means abyssal
regions, have usually very large and highly pigmented eyes; while in
_Munidopsis_, which is characteristic of very deep water, the eyes are
degenerate and colourless, as shown in Fig. 114.

The Zoaeae, or young larval stages of the Galatheidea, are characterised
by the immense length of the spines upon the carapace (Fig. 115). The
young Zoaea which hatches out from the egg resembles in other respects
that of the Brachyura. The Metazoaea, however, differs from that of the
Brachyura in the fact that the third maxillipede is first present as a
biramous swimming organ, and at its first appearance is not developed in
its definitive form. The other thoracic limbs are not schizopodous when
they appear, and indeed in nearly all respects the development proceeds
as in the Brachyura.

=Fam. 1. Aegleidae.=—The gills are trichobranchiae, and there are eight
arthrobranchs. There are no limbs on the second abdominal segment of the
male. The abdomen is not carried folded on to the thorax. The first two
characteristics separate this family from all the other Galatheidea.
_Aeglea laevis_, a fresh-water species from the rivers of temperate S.
America, is the sole representative.

=Fam. 2. Galatheidae.=—The abdomen is not folded against the thorax. The
members of this family are often littoral in habit (_Galathea_, Fig.
116), but often go down into great depths (_Munidopsis_, Fig. 114).

[Illustration:

  FIG. 116.—Dorsal view of _Galathea strigosa_, × ½. (From an original
    figure prepared for Professor Weldon.)
]

=Fam. 3. Porcellanidae.=—The abdomen is folded against the thorax, and
the body has a crab-like form. These are always littoral in habit, never
descending into the depths. _Pachycheles_ in the tropics, _Porcellana_
with numerous species in all seas, _P. platycheles_ being a common
British species.


                          =Tribe 2. Hippidea.=

The Mole-crabs have the habit of burrowing in sand, and their limbs are
peculiarly modified into digging organs for this purpose (see Fig. 117).
In other respects they are seen to be closely related to the Galatheidea
by the form of the carapace, the condition of the abdomen, and the
reduced last thoracic limbs.

In _Albunea_, which is found in the Mediterranean, the first
antennae[141] are greatly lengthened and apposed to one another, and by
means of a system of interlocking hairs they form a tube down which the
water is sucked for respiration. The object of this arrangement is to
ensure a supply of clear water, filtered from particles of sand, when
the crab is buried beneath the surface, on these occasions the tip of
the antennal tube being protruded above the surface of the sand. An
exactly similar tube is used by the true Crab _Corystes cassivelaunus_,
which has similar burrowing habits, but here the tube is formed from the
second antennae and not from the first, so that the tubes in the two
cases afford beautiful instances of analogous or homoplastic structures
between which there is no homology (see p. 189).

=Fam. 1. Albuneidae.=—The first legs are subchelate; the carapace is
flattened, without expansions covering the legs. _Albunea_ with several
species in the Mediterranean, West Indies, and Indo-Pacific.

[Illustration:

  FIG. 117.—_Remipes scutellatus_, dorsal and ventral views, × 1. (From
    original drawings prepared for Professor Weldon.)
]

=Fam. 2. Hippidae.=—The first legs are simple, the carapace is
subcylindrical with expansions covering the legs. _Remipes_ (Fig. 117)
and _Hippa_ in tropical or sub-tropical seas.


                       =Tribe 3. Paguridea.=[142]

The ordinary Hermit-crabs, common on the English as on every coast, are
characterised by the fleshy asymmetrical abdomen from which all the hard
matter has disappeared, and which is carried tucked away in an empty
Gasteropod shell. The abdomen is spirally wound in accordance with the
shape of the shell, and a firm attachment is effected by means of the
sixth pair of pleopods, especially that of the left side, which is
fashioned into the form of a hook and is curled round the columella of
the shell; this attachment is so secure that in trying to pull a
Hermit-crab out of its shell the body is torn apart before the hold
gives way. The other pleopods are in a much reduced condition, being
generally altogether absent from the right side of the abdomen, and
often greatly reduced on the left side, especially in the male, though
in the female they are still used for the attachment of the eggs.

The last two pereiopods are much reduced and are concealed inside the
shell, which they help to carry. The great chelae are usually
asymmetrically developed, that on the right side being much larger than
that on the left, and often serving the purpose of shutting the entrance
to the shell when the crab is withdrawn inside.

The constant association of a large group of animals like the
Hermit-crabs with the appropriated empty houses of another group is
sufficiently curious, but it does not stop there. In almost every case
there are present one or more Sea-anemones growing on the outside of the
shell, and each kind of Hermit-crab generally carries a special kind of
Anemone. Thus at Plymouth, _Eupagurus bernhardus_ is generally symbiotic
with _Sagartia parasitica_, or else with a colony of _Hydractinia
echinata_, while _E. prideauxii_ is usually associated with _Adamsia
palliata_. In the latter case the shell is frequently absorbed, so that
the Anemone comes to envelop the crab like a blanket. Instead of
Anemones carried turret-like and imposing aloft, or enveloping the
inmate of the shell like a blanket, some of the Hermits have Sponges, an
unexpected association; and it is a common sight at Naples to find the
little red round Sponge, _Suberites_, running around animated by its
Hermit within. It is held that Anemone and crab mutually assist one
another, that the Anemone stings the crab’s enemies, and that the
Hermit-crab carries the Anemone to new feeding-grounds. It is also said
that when a crab grows too big for its shell, and is forced to seek
another, it persuades the Anemone to loosen its attachment to the
deserted shell and to be transplanted to the new one, and that there is
something mesmeric in its power, because nobody else can pull an Anemone
off a shell without either cutting it off at the base or tearing it to
pieces. Other animals as well sometimes enter into this partnership. At
Plymouth a Polychaet worm, _Nereis fucata_, frequently inhabits the
Whelk’s shell, together with _Eupagurus bernhardus_, and puts out its
head for a share of each meal; and at Naples the Amphipod _Lysianax
punctatus_ is almost always present in the shells of _Eupagurus
prideauxii_.

[Illustration:

  FIG. 118.—_Pylocheles miersii_, × 1. =A=, End view of a piece of
    mangrove or bamboo, the opening of which is closed by the great
    chelae (_c_) of the Pagurid; =B=, the animal removed from its house.
    (After Alcock.)
]

Besides the ordinary twisted Pagurids which inhabit Gasteropod shells,
there are a few which preserve the symmetry of the body. The interesting
_Pylocheles miersii_[143] (Fig. 118), taken by the _Investigator_ in the
Andaman Sea at 185 fathoms, inhabits pieces of bamboo; it is perfectly
symmetrical, with well-developed pleopods and symmetrical chelae, which,
when the animal is withdrawn, completely shut up the entrance to its
house (Fig. 118, A).

It is doubtful whether this animal ever inhabited a spiral shell or not
in its past history; but there is no doubt that a number of peculiar
crabs, which caused the older systematists much trouble, are Pagurids,
derived from asymmetrical shell-haunting ancestors that have secondarily
taken to a different mode of life, and lost, or partially lost those
characteristics of ordinary Hermit-crabs which are associated with life
in a spiral shell. These are the Lithodidae and the “Robber-crab,”
_Birgus latro_, of tropical coral islands.

Although the Robber-crab and the Lithodidae bear a certain superficial
resemblance to one another in that they lead a free existence, and have
reacquired to a great extent their symmetry, yet it is clear that they
have been independently derived from different groups of asymmetrical
Hermit-crabs, and that their resemblance to one another is due to
convergence.

_Birgus latro_ (Fig. 119), a gigantic crab, frequently over a foot in
length, lives on land, and inhabits the coasts of coral islands in the
Indian and Pacific Oceans where cocoa-nut trees grow. It feeds on the
pulp of the cocoa-nut, which it extracts by hammering with its heavy
chela on the “eye-hole” until room is made for the small chela to enter
and extract the pulp. There is not the slightest doubt that the animal
often ascends the cocoa-nut trees for the purpose of picking the nuts, a
fact illustrated by a fine photograph by Dr. Andrews, exhibited in the
Crustacean Gallery in the Natural History Departments of the British
Museum. It uses the husk of the nut to line its burrow, and it is said
to have the habit of putting its abdomen into the nut-shell for
protection and carrying it about with it. Owing to its terrestrial mode
of life, the branchial chamber is highly modified, being divided into
two portions—a dorsal space, the lining of which is thrown into vascular
ridges and folds for aerial respiration, and a lower portion where the
rudimentary branchiae are situated. Although the Robber-crab lives
ordinarily on land, it must be supposed that these branchiae are of some
service; the young are hatched out as ordinary Zoaeas in the sea, and go
through a pelagic existence before seeking the land. At the present time
the Robber-crab is confined to the Pacific and the islands of the Indian
Ocean, wherever the cocoa-nut grows. It seems, however, that its
association with the cocoa-nut is a comparatively modern one. Mr. C.
Hedley, of Sydney, who has had great experience of the Pacific Islands,
informs me that the cocoa-nut is not, as is usually supposed, a native
of these coral islands, but has been introduced, probably from Mexico,
by the Polynesian mariners before the discovery of America by Columbus.
Before the introduction of the cocoa-nut the Robber-crab must have fed
on some other tree, possibly the Screw Pine, _Pandanus_.

The abdomen is full of oil, and is much prized as a delicacy by the
natives, who tell many strange legends about the creature, but the
philosopher may well find its structure more strange than fiction, and
the consideration of its morphology an intellectual feast.

The appearance of the thorax and of the thoracic limbs is thoroughly
Pagurid; the structure of the abdomen is highly peculiar.

From the ventral surface (Fig. 119) we can see at the tip of the tail
three small calcified plates, which represent the fifth and sixth terga
and the telson. Attached to the sixth segment are the much reduced and
rudimentary pleopods of that segment, and on the left hand side of the
body in the female are three well-developed pleopods of the first,
second, and third segments, which are used for carrying the eggs. The
extraordinary asymmetry of these limbs compared with the complete
symmetry of the abdomen itself is only explicable on the hypothesis that
these animals are descended from Hermit-crabs which had lost the
pleopods on the right side.

[Illustration:

  FIG. 119.—_Birgus latro_, ♀, × ⅙, ventral view. _Ab_, First pleopod;
    _T_, last pereiopod.
]

These appendages are entirely absent in the male. The ventral surface of
the abdomen is curiously warty and rugose, and is very soft and pulpy
owing to the immense store of oil which it contains.

[Illustration:

  FIG. 120.—Dorsal view of abdomen, =A=, of _Cenobita_, sp.; =B=, of
    _Birgus latro_. _T_, Telson; 1–6, 1st–6th abdominal segments.
]

If we look at the dorsal surface of the abdomen we find that, unlike
that of the Hermit-crabs, it is completely protected by a number of hard
plates (Fig. 120, B). Beneath the carapace can be seen a number of small
plates belonging to the last thoracic segment; following these there are
four large plates (1–4) representing the terga of the first four
abdominal segments; the fifth, sixth, and the telson are, as has been
stated, carried on the under side of the abdomen, but they are
represented diagrammatically (5, 6, _T_) in the dorsal view. Besides the
large terga, there are a number of small plates laterally, usually two
to each segment, but they show a tendency to subdivide and increase in
the largest specimens. This condition of affairs is very different to
that in the naked fleshy abdomen of an ordinary Pagurid, but it can
easily be deduced from that of the genus _Cenobita_, ordinary
Hermit-crabs found in the Indo-Pacific Oceans, from which the
Robber-crab has evidently descended. In _Cenobita_ (Fig. 120, A) we see
the same system of plates upon the dorsal surface of the abdomen, but
they are much smaller, and the lateral plates are not so numerous;
indeed, the greater part of the abdomen remains fleshy and uncalcified.
The under surface of the abdomen shows the same rugosity as is found in
_Birgus_, and from a number of other anatomical characters it is evident
that the Robber-crab is a highly modified _Cenobita_ that has deserted
its shell and developed a symmetrical abdomen protected by expanded and
hardened plates which represent those found in a reduced condition in
_Cenobita_. The species of _Cenobita_ although they inhabit shells and
have normal branchiae, live on the shore, and have not been seen to
descend actually into the sea.

The Lithodidae, which are found in temperate seas, especially on the
Northern Pacific coasts (though _Lithodes maia_ occurs in the North Sea,
and certain species inhabit deep water in the Indian Ocean), have a
deceptively Brachyuran appearance, the thorax being much shortened and
the abdomen being much reduced and carried tightly flexed on to the
ventral surface of the thorax. They live a free, unprotected existence,
and are highly calcified. They are, however, certainly Pagurids, as is
evidenced by a number of anatomical characters, but most clearly by the
asymmetry of the abdomen, especially in the female, which is not only
markedly asymmetrical in the arrangement of its dorsal plates (Fig.
121), but also in the presence of three pleopods upon the left side
only, as in _Birgus_. The male is without these appendages, and the
sixth pair of pleopods is absent in both sexes. The remarkable calcified
plates upon the abdomen bear a superficial resemblance to those in
_Birgus_, but their evolution is traced, not from a Cenobite, but from
an Eupagurine stock.[144]

[Illustration:

  FIG. 121.—_Lithodes maia_, ♀, in ventral view, × ¼. The abdomen is
    flexed on the thorax, so that its dorsal surface is seen. _l.3_,
    Lateral plates of third abdominal segment; _l.5_, left lateral plate
    of fifth abdominal segment; _m_, marginal plate; _T_, brush-like
    last pereiopod; _Te.6_, telson and sixth abdominal segment.
]

In some of the Eupagurinae, e.g. _Pylopagurus_, feebly calcified plates
are present upon the segments of the abdomen (Fig. 122, A).

In the most primitive of the Lithodidae we witness the reduction (Fig.
122, B) and disappearance (C) of these original plates, their place
being taken first by a number of irregularly situated small spines and
warts, which, however, subsequently fuse up to form definite segmental
plates. In _Lithodes maia_, ♂ (D), there are a series of lateral and
marginal plates, while in _Acantholithus_ (E) a number of median plates
appear, presumably by the fusion of the small spines present in the
median line in _Lithodes maia_; finally, a fusion of the marginal and
lateral plates may take place, so that each abdominal segment is covered
by a median and two paired lateral plates.

[Illustration:

  FIG. 122.—Diagrams of abdomen: =A=, of _Pylopagurus_, sp.; =B=, of
    _Hapalogaster cavicauda_; =C=, of _Dermaturus hispidus_; =D=, of
    _Lithodes maia_, ♂; =E=, of _Acantholithus hystrix_. _c_, Central
    plates; _l_, lateral plates; _m_, marginal plates; _T_, telson; 1–6,
    1st–6th abdominal segments. (After Bouvier.)
]

It is to be noted that the males and females of the various species do
not follow a parallel course of development, the plates in the male
being symmetrical, while those of the female are often highly
asymmetrical (compare Figs. 122, D, and 121), thus giving the strongest
evidence of a Pagurid ancestry.

_Birgus_ and the Lithodidae, then, are Pagurids which have given up
living in shells, and have become adapted to a free existence,
protecting their soft parts by the development of hard plates, and
re-acquiring, to a greater or less degree, a secondary symmetry of form.
But the story of Pagurid evolution does not apparently stop here. The
genus _Paralomis_, from the West Coast of America, superficially
resembles _Porcellana_, and is held to be descended from such forms as
_Pylocheles_, while isolated species are known (though not well known),
such as _Tylaspis_, described in the _Challenger Reports_,[145] which
appear to be Pagurids that have deserted their shells.

[Illustration:

  FIG. 123.—Four stages in the development of _Eupagurus longicarpus_ or
    _E. annulipes_, × 20. =A=, Ventral view of Zoaea; =B=, lateral view
    of Metazoaea; =C=, dorsal view of Glaucothoe; =D=, dorsal view of
    adolescent stage. _Ab.6_, 6th abdominal appendage; _Mxp.1_, _Mxp.3_,
    1st and 3rd maxillipedes. (After M. T. Thompson.)
]

The metamorphosis of the Hermit-crabs has recently been studied by M. T.
Thompson.[146]

The Zoaea (Fig. 123, A) differs from that of the Galatheidea mainly in
the absence of the long spines. It possesses the usual appendages
characteristic of the Zoaea, namely, the first and second antennae,
mandibles, first and second maxillae, and two pairs of biramous swimming
maxillipedes and small third maxillipedes. In the Metazoaea (B), as in
the Anomura generally, the third maxillipedes develop into biramous
swimming organs, a thing they never do in the Brachyura, and the
rudiments of the thoracic segments put in a first appearance. The
abdominal segments are already fully formed in the Zoaea stage, so that
here as in all other Zoaeas, the order of development from in front
backwards is disturbed by the precocious differentiation of the
abdominal segments. The next stage is the “Glaucothoe” (Fig. 123, C),
which corresponds to the Megalopa of Brachyura (Fig. 125, p. 183). It
differs from the adult Hermit-crab in the perfect symmetry of its body,
the segmented abdomen, and the presence of five pairs of normal biramous
pleopods. At this stage, which lasts four or five days, it resembles
closely a little Galatheid. The asymmetry of the adult (Fig. 123, D) is
now imposed upon this larva by the migration of the liver, gonads, and
green glands into the abdomen, and by the shifting of the posterior
lobes of the liver on to the left side of the intestine, which is
displaced dorsally and to the right. The gonad lies entirely on the left
side. The pleopods of the right side now degenerate, more completely in
the male than in the female, and this degeneration is not completed
until the little crab has found a shell and lived in it for some time.
If a shell is withheld from it, the degeneration of the pleopods is much
retarded, so that although the Hermit-crab assumes its asymmetry without
the stimulus of the spiral shell, yet this stimulus is necessary for the
normal completion of the later stages.

=Fam. 1. Pylochelidae.=—The abdomen is macrurous and symmetrical, with
all the limbs present. _Pylocheles_ (Fig. 118, p. 173).

=Fam. 2. Paguridae.=—The abdomen is asymmetrical, with some of the limbs
lost. The antennal scale is well developed, and the flagella of the
first antennae end in a filament.

=Sub-Fam. 1. Eupagurinae.=—The third maxillipedes are wide apart at the
base, and the right chelipedes are much larger than the left.
_Parapagurus_ from deep-sea, _Eupagurus_ from temperate, especially
north temperate seas. _Pylopagurus._

=Sub-Fam. 2. Pagurinae.=—The third maxillipedes are approximated at the
base; the chelipedes are equal or subequal, or the left is much larger.
Chiefly in the warm and tropical seas, but _Clibanarius_ and _Diogenes_
also in the Mediterranean.

=Fam. 3. Cenobitidae.=—The abdomen is as in Paguridae. The antennal
scale is reduced, the flagella of the first antennae end bluntly. The
members of this family are characteristic of tropical beaches, where
they live on the land. _Cenobita_, with about six species, in the West
Indies and Indo-Pacific, living in Mollusc shells; _Birgus_ (Fig. 119)
on Indo-Pacific coral islands.

=Fam. 4. Lithodidae.=—The abdomen is bent under the thorax, and the body
is crab-like and calcified. The rostrum is spiniform, and the sixth
abdominal appendages are lost.

=Sub-Fam. 1. Hapalogasterinae.=—Abdomen not fully calcified, and without
complicated plates. _Hapalogaster_ and _Dermaturus_ in the North Pacific
littoral.

=Sub-Fam. 2. Lithodinae.=—Abdomen fully calcified, with a complicated
arrangement of plates. _Lithodes_ (Fig. 121) practically universal
distribution, littoral and deep sea. _Acantholithus_, deep littoral of
Japan; _Paralomis_, west coast of America. This last genus should
probably be placed in a separate family.


                     =Sub-Order 3. Brachyura.=[147]

The abdomen is much reduced, especially in the male, and is carried
completely flexed on to the ventral face of the thorax so as to be
invisible from the dorsal surface. The pleopods in the male are only
present on the two anterior segments, and are highly modified as
copulatory organs; the pleopods in the female are four in number and are
used simply for carrying the eggs; the pleopods of the sixth pair are
always absent in both sexes. The first antennae and the stalked eyes can
be retracted into special pits excavated in the carapace.

[Illustration:

  FIG. 124.—=A=, Zoaea, × 24, and =B=, Metazoaea, × 13, of _Corystes
    cassivelaunus_. _Ab_, 3rd abdominal segment; _An_, 1st antenna; _E_,
    eye; _G_, gills; _M_, 1st maxillipede; _T.8_, last thoracic
    appendage. (After Gurney.)
]

[Illustration:

  FIG. 125.—Later stage (Megalopa) in the development of _Corystes
    cassivelaunus_, × 10. _A_, Antenna; _Ab_, 3rd abdominal segment;
    _C_, great chela; _T.8_, last thoracic appendage. (After Gurney.)
]

The larva hatches out as a Zoaea[148] (Fig. 124, A) very similar to that
of the Anomura; it is furnished with an anterior and posterior spine on
the carapace. It is characteristic of the Brachyuran Zoaea that the
third maxillipede is fashioned from the beginning in its definitive
expanded form, and is never a biramous swimming organ as in the Anomura.
The only exception to this rule is found in the Dromiacea, the most
primitive of the Brachyura, to be soon considered, in which not only the
third maxillipede, but also the first pair of pereiopods may be
developed as biramous oars, a condition taking one back to the Mysis
stage of the Macrura. The Metazoaea (Fig. 124, B) has the rudiments of
the thoracic limbs developed and crowded together at the back of the
carapace; they are all laid down in their definitive forms, and the
abdomen has the pleopods precociously developed. These Zoaeal stages are
of course pelagic, but the Metazoaea next passes into the Megalopa stage
(Fig. 125), in which the little crab forsakes its pelagic life and
assumes the ground-habits of the adult; the Megalopa, which corresponds
exactly to the Glaucothoe of the Pagurids, resembles a small _Galathea_
or _Porcellana_, the abdomen being still large and unflexed and
furnished with normal pleopods. From this stage the adult structure is
soon achieved, though, owing to the continued growth of the Crustacea
even after maturity is reached, there is often a slight progressive
change in structure, especially in the male, at each successive moult of
the individual. The Megalopa of _Corystes cassivelaunus_ is peculiar in
the immense production of the second antennae, which act as a
respiratory tube (Fig. 125).

The Brachyura must be considered under the following subdivisions:—


                         =Tribe 1. Dromiacea.=

All authorities are agreed that these[149] are the most primitive of the
Brachyura. In them the abdomen is much less reduced in both sexes than
in other Brachyura; there is a common orbitoantennary fossa, into which
eyes and antennae are withdrawn, instead of a separate one on each side
for each organ; the carapace is often much elongated as in the Macrura
and Anomura, and a number of other anatomical characters might be
mentioned which characterise the Dromiacea as intermediate between the
true Brachyura and the lower forms. There are, however, two views as to
the relationship of the Dromiacea; Claus held that they proceeded from a
Galatheid stock, and hence that the development of the Brachyura ran
through an Anomurous strain; but Huxley, and latterly Bouvier,[150]
adopt the view that the Dromiacea are descended, not from the
Galatheidae, but direct from the Macrura, and especially from the
Nephropsidea. Special resemblances are found between the Jurassic
Nephropsidae and certain present day Dromiacea, _e.g._ _Homolodromia
paradoxa_, the detailed form of the carapace in the two cases being very
similar. It is, however, a little strange that in the Dromiacea we meet
with the same reduction and dorsal position of the last, or last two
pairs of thoracic limbs which we saw to be such a characteristic feature
of the Anomura, especially of the Galatheidae. In the Dromiacea these
limbs may be chelate, and they are used for attaching shells and other
bodies temporarily to the back. Must we suppose that this resemblance to
the Anomura is due to convergence, or that the Nephropsidae, which gave
rise to perhaps both Galatheidae and Dromiacea, had this character, and
that it has been subsequently lost in the Macruran stock? We have
already mentioned that the Metazoaea of _Dromia_ has not only a
well-developed swimming third maxillipede, but also a biramous first
pereiopod, a character which speaks strongly for Macruran affinities.

[Illustration:

  FIG. 126.—_Dromia vulgaris_, × 1. (After Milne Edwards and Bouvier.)
]

=Fam. 1. Dromiidae.=—The eyes and antennules are retractile into orbits.
The last two pairs of thoracic limbs are small, and held dorsally. The
sixth pair of pleopods are rudimentary or absent. _Homolodromia_ from
West Indies, deep-sea. _Dromia_, widely dispersed. _D. vulgaris_ (Fig.
126) occurs on the English coasts.

=Fam. 2. Dynomenidae.=—Similar to the preceding family, but only the
last pair of thoracic limbs is small, and held dorsally. The sixth pair
of pleopods are reduced, but always present. _Dynomene_ in the
Indo-Pacific.

=Fam. 3. Homolidae.=—The eyes and antennules are not retractile into
orbits. Only the last pair of thoracic limbs are reduced, the sixth pair
of pleopods altogether absent. _Homola_ and _Latreillia_, widely
distributed, occur in the Mediterranean. _Latreillopsis_ from the
Pacific. _L. petterdi_,[151] a magnificent species, with the carapace
nearly a foot long, and with very long legs like a Spider-crab, has been
dredged from 800 fathoms east of Sydney, New South Wales.


                         =Tribe 2. Oxystomata.=

This group comprises Crabs whose carapace is more or less circular,
while the mouth, instead of being square as in the remaining Brachyura,
is triangular with the apex pointing forward, and the third maxillipedes
are not expanded into the flattened, lid-like structures found in other
Crabs. There is the same tendency in some of the genera for the
posterior thoracic limbs to be reduced and carried dorsally, as in the
Galatheidae and Dromiacea. The well-known _Dorippe_ from the
Mediterranean has this feature, and frequently carries an empty shell
upon its back, and _Cymonomus_[152] presents the same peculiarity.

[Illustration:

  FIG. 127.—_Cymonomus granulatus_, × 1. _A.1_, _A.2_, 1st and 2nd
    antennae; _E_, eye-stalk; _S_, extra-orbital spine of carapace.
    (After Lankester.)
]

_Cymonomus granulatus_ (Fig. 127) is an abyssal form that has been
dredged from the Mediterranean and North Atlantic, in which the
eye-stalks are curiously tuberculated, and the ommatidia of the eye are
entirely unpigmented and degenerate, though a few corneal facets are
still recognisable. This species is replaced by _C. quadratus_ in the
Caribbean Sea and by _C. normani_ on the East African coast, in which
the alteration of the eye-stalks into thorny, beak-like projections
becomes progressively marked, and all traces even of the corneal facets
disappear. This remarkable genus was mentioned in the excursus on
Crustacean eyes on p. 149.

[Illustration:

  FIG. 128.—_Calappa granulata_, from in front, × ½. _C_, Hand of
    chelipede; _T_, walking legs. (After Garstang.)
]

The Oxystomata, like the Cyclometopa, to be considered later, live in
sandy and gravelly regions, and burrow to a greater or less extent, and
we find in both groups admirable adaptations for securing a pure stream
of water, uncontaminated by particles of sand, for flushing the gills.
Perhaps the most remarkable of these adaptations is afforded by
_Calappa_.[153] This animal has the chelipedes wonderfully modified in
structure, and when it is reposing in the sand it holds them apposed to
the front of the carapace, as shown in Fig. 128, so that the spines upon
their edges, together with the hairy margin of the carapace, form a most
efficient filter for straining off sand and grit from the stream of
water which is sucked down between the closely-fitting chelipedes and
carapace, to enter the branchial chambers at their sides. The exhaled
current of water passes out anteriorly through a tube formed by a
prolongation of the endopodites of the first maxillipedes. The exhalant
aperture is shown in Fig. 128 by the two black cavities below the snout
in the middle line.

A similar method is pursued by the related _Matuta banksii_[153] (Fig.
129), a swimming and fossorial Crab found in the Indo-Pacific. In this
Crab the chelipedes also fit against the carapace to form a strainer,
and their function is assisted by the enlargement of the posterior
spine, which acts as a kind of elbow-rest to keep the chelipedes
properly in position. The inhalant openings are situated just in front
of the chelipedes. It is a most remarkable fact that among the
Cyclometopa, _Lupa hastata_ (Fig. 131) has an exactly similar
arrangement. Apparently we have here another instance of convergence,
similar to that of _Corystes_ and _Albunea_, but the case is complicated
by the fact that some of the Oxystomata, and among them _Matuta_, show a
certain amount of relationship to the Cyclometopous Portunids, so that
it is just conceivable that the resemblances in the respiratory
arrangement are due to a common descent and not to convergence.

[Illustration:

  FIG. 129.—Dorsal view of _Matuta banksii_, × 1. (From an original
    drawing prepared for Professor Weldon.)
]

In the Leucosiidae, of which the Mediterranean _Ilia nucleus_ (Fig. 130)
is an example, the inhalant aperture is situated between the orbits, and
leads into gutters excavated in the “pterygostomial plates” flanking the
mouth, which are furnished with filtering hairs and are converted into
closed canals by expansions of the exopodites of the third maxillipedes.
Thus these Crabs possess a filtering apparatus independent of the
chelipedes and of the margin of the carapace.

=Fam. 1. Calappidae.=—Cephalothorax rounded and crab-like. The abdomen
is hidden under the thorax, the antennae are small, and the legs normal
in position. The afferent openings to the gill-chambers lie in front of
the chelipedes. Male openings on coxae of last pair of legs. _Calappa_
(Fig. 128) circumtropical, and extending into the warmer temperate seas.
_Matuta_ (Fig. 129) from the Indo-Pacific.

[Illustration:

  FIG. 130.—Dorsal view of _Ilia nucleus_, × 1. (From an original
    drawing prepared for Professor Weldon.)
]

=Fam. 2. Leucosiidae.=—Similar to the above, but the afferent openings
to the gill-chambers lie at the bases of the third maxillipedes. Male
openings on the sternum. This family contains a great number of forms,
with headquarters in the tropical littoral, but extending into the
temperate seas. _Ilia_ in the European seas. _I. nucleus_ (Fig. 130)
common in the Mediterranean. _Ebalia_ in the Atlantic, North Sea, and
Indo-Pacific. _Leucosia_ in Indo-Pacific.

=Fam. 3. Dorippidae.=—Cephalothorax short and square. The abdomen is not
hidden under the thorax; the antennae are large, and the last two pairs
of legs are held dorsally, and have terminal hooked claws. _Dorippe_,
littoral in Mediterranean and Indo-Pacific. _Cymonomus_ (Fig. 127) from
deep-sea of Atlantic and Mediterranean.

=Fam. 4. Raninidae.=—Similar to Dorippidae, but the cephalothorax is
elongated, and the legs usually have the last two joints very broad.
Several genera, chiefly in the deeper littoral zone. _Ranina dentata_ in
the Indo-Pacific.


                        =Tribe 3. Cyclometopa.=

In these Crabs the carapace is circular rather than square; its frontal
and lateral margins are produced into spines and there is no pointed
rostrum. The mouth is square, and the third maxillipedes are greatly
flattened and form a lid-like expansion over the other oral appendages.
This group includes the common Shore-crab of our coasts (_Carcinus
maenas_), the swimming Crabs with expanded pereiopods (_Portunus_,
_Lupa_, etc.), the Edible Crab (_Cancer pagurus_), and many others.

_Corystes cassivelaunus_ is a Crab of doubtful affinities. It is
sometimes placed among the Oxyrhyncha, but, as Gurney[154] has pointed
out, the Megalopa shows Portunid characters, and the resemblance to the
Oxystomata in the front of the carapace and in the mouth may be
secondary. The respiratory arrangement of this Crab has already been
mentioned in comparing its structure with that of the Mole-crab
_Albunea_. The form of the antennal tube can be gathered from the figure
of the Megalopa stage (Fig. 125, p. 183). It should be noted that when
the Crab is buried in the sand with only the tip of the antennal tube
projecting, the water is sucked down and enters the branchial cavities
anteriorly, the antennal tube being continued by a tube formed from the
third maxillipedes and the forehead; the water is exhaled at the sides
of the branchial cavities beneath the branchiostegites. Thus in
_Corystes_ the normal direction of the current is reversed, but when the
Crab is not buried, and is moving over the surface, it breathes in the
usual manner, taking in the water at the sides of the branchiostegites
and exhaling it anteriorly by the tube. The related _Atelecyclus_, found
like _Corystes_ very commonly at Plymouth, uses two methods of
breathing: when it is in the surface-layers of sand it makes use of its
antennal tube, which is, however, much shorter than in _Corystes_; but
when it burrows deeper, where the antennal tube is no use, it folds its
chelipedes and also its other legs, which are densely covered with
bristles, so as to form a reservoir of pure water underneath it free
from sand, which it passes through the gill-chambers in the usual manner
(see Garstang, _loc. cit._ p. 186).

The respiratory adaptations in _Lupa hastata_ and their convergence
towards those of the Oxystomatous _Matuta_ have been already touched
upon (pp. 186, 187).

In this connexion must be mentioned the interesting experiments of W. F.
R. Weldon[155] upon the respiratory functions of _Carcinus maenas_ at
Plymouth, since these were the first noteworthy observations directed
towards the exact measurement of the action of natural selection upon
any animal, a field of observation in which Weldon will always be looked
upon as a pioneer. An extended series of measurements by Weldon and
Thompson on male specimens of _Carcinus maenas_ of various sizes between
the years 1893 and 1898 showed a steady decrease in the ratio of
carapace breadth to length; the Crabs appeared to be becoming steadily
narrower across the frontal margin, and the same thing, though not to
the same extent, was happening in female Crabs. Weldon supposed that
this change might be correlated with the silting up of Plymouth Sound
and the consequent fouling of the water. To test this hypothesis he kept
a very large number of male Crabs in water to which fine porcelain clay
was added and kept in continual motion. In the course of the experiments
the survivors and the dead were measured, and it was found that the mean
carapace breadth of the survivors was less than that of those that
succumbed. The experiment was repeated with the fine sand that is
deposited and left at low water upon the stones on Plymouth beach, and
the same result was observed. It was also noticed that the individuals
which died had their gills clogged with the sand, while those that
survived had not. As a further confirmation, a great many young male
Crabs were isolated and kept in pure filtered water, and they were
measured before and after moulting; these measurements, when compared
with measurements of the frontal breadth in Crabs of the same size taken
at random upon the beach, were found to show a greater breadth than the
wild Crabs, thus indicating that a selection of narrow Crabs was taking
place in Nature which did not take place when the Crabs were protected
from the effects of fine sand in the water.

The whole chain of evidence goes to show that the carapace breadth in
_Carcinus maenas_ in Plymouth Sound is being influenced by the rapid
change of conditions occurring in the locality. Various objections have
been urged against this conclusion, but, though they merit further
investigation, they do not appear very weighty.

The fresh-water Crab, _Thelphusa fluviatilis_, common in the South of
Europe and on the North coast of Africa, belongs to the Cyclometopa, and
is interesting from its direct mode of development without
metamorphosis.

=Fam. 1. Corystidae.=—The orbits are formed, but, unlike all the other
families of the Cyclometopa, are incomplete. The body is elongate and
oval, and the rostrum and front edge of the mouth rather as in the
Oxyrhyncha, in which Tribe they are sometimes included. _Corystes_, with
a few species in European seas. _C. cassivelaunus_ at Plymouth.

=Fam. 2. Atelecyclidae.=—Perhaps related to the foregoing. The carapace
is sub-circular, and the rostrum short and toothed. _Atelecyclus_,
European seas.

=Fam. 3. Cancridae.=—The carapace is broadly oval or hexagonal, and the
flagella of the second antennae are short and not hairy as in the
foregoing. The first antennae fold lengthwise. _Carcinus maenas_ on
English and North European coasts. This crab has become naturalised in
some unexplained manner in Port Phillip, Melbourne. _Cancer_ in North
Atlantic, North Pacific, and along the west coast of America into the
Antarctic regions. _C. pagurus_ is the British Edible Crab.

[Illustration:

  FIG. 131.—Dorsal view of _Lupa hastata_, × 1. (From an original
    drawing prepared for Professor Weldon.)
]

=Fam. 4. Portunidae.=—The legs are flattened and adapted for swimming.
The first antennae fold back transversely. _Portunus_, Atlantic and
Mediterranean. _Neptunus_, Indo-Pacific. _Callinectes_, _C. sapidus_,
the edible blue Crab of the Atlantic coasts of America. _Lupa_ (Fig.
131).

=Fam. 5. Xanthidae.=—The first antennae fold transversely, but the legs
are not adapted for swimming; the body is usually transversely oval.
This family is especially characteristic of the tropical littoral, where
it is very widely represented. _Xantho_, _Actaea_, _Chlorodius_,
_Pilumnus_, _Eriphia_, with _E. spinifrons_, common in the
Mediterranean.

=Fam. 6. Thelphusidae (Potamonidae).=—Fresh-water crabs, with the
branchial region very much swollen. _Thelphusa_ (or _Potamon_) has
nearly a hundred species distributed from North Australia, through Asia,
Japan, the Mediterranean region, and throughout Africa. _Potamocarcinus_
in tropical America.


                         =Tribe 4. Oxyrhyncha.=

This section includes the Spider-crabs and related genera, in which the
carapace is triangular, with the apex in front formed by a
sharply-pointed rostrum. There are two chief series, the one comprising
the Spider-crabs, with much elongated walking legs, _e.g._ the huge
_Maia squinado_ of European seas, the yet more enormous _Macrocheira
kämpferi_ from Japan, supposed to be the largest Crustacean in
existence, and sometimes spanning from outstretched chela to chela as
much as eleven feet, and the smaller forms, such as _Inachus_, _Hyas_,
and _Stenorhynchus_, which are so common in moderate depths off the
English coasts. The other series is represented by genera like _Lambrus_
(Fig. 133), in which the legs are not much elongated, but the chelipedes
are enormous.

The Spider-crabs do not burrow, and their respiratory mechanism is
simple; but since they are forms that clamber about among weeds, etc.,
upon the sea-bottom, they often show remarkable protective resemblances
to their surroundings, which are not found in the burrowing Cyclometopa.
Alcock[156] gives a good account and figure of _Parthenope
investigatoris_, one of the short-legged Oxyrhyncha, the whole of whose
dorsal surface is wonderfully sculptured to resemble a piece of the old
corroded coral among which it lives.

But besides this, the long-legged forms, such as _Inachus_, _Hyas_,
etc., have the habit of planting out Zoophytes, Sponges, and Algae upon
their spiny carapaces, so that they literally become part and parcel of
the organic surroundings among which they live. It may, perhaps, be
wondered what are the enemies which these armoured Crustacea fear.
Predaceous fish, such as the Cod, devour large quantities of Crabs,
which are often found in their stomachs; and Octopuses of all sorts live
specially upon Crabs, which they first of all paralyse by injecting them
with the secretion of poison-glands situated in their mouth. The poison
has been recently found by Dr. Martin Henze at Naples to be an alkaloid,
minute quantities of which, when injected into a Crab, completely
paralyse it. When the Crab is rendered helpless the Octopus cuts out a
hole in the carapace with its beak, and sucks all the internal organs,
and then leaves the empty shell.

Many of the Oxyrhyncha are found in the abysses; among them are
_Encephaloides armstrongi_ (Fig. 132), dredged by Alcock from below the
100–fathom line in the Indian Ocean, which has the gill-chambers (G)
greatly swollen and enlarged to make up for the scarcity of oxygen in
these deep regions.

[Illustration:

  FIG. 132.—_Encephaloides armstrongi_, × 1. The long walking legs are
    omitted. _C_, Great chela; _G_, one of the greatly swollen
    gill-chambers. (After Alcock.)
]

=Fam. 1. Maiidae.=—The chelipedes are not much larger than the other
legs, but are very mobile. Orbits incomplete. A very large family,
including all the true Spider-crabs, very common in the Atlantic and
Mediterranean littoral. _Inachus_, _Pisa_, _Hyas_, _Stenorhynchus_,
_Maia_, _Encephaloides_ (Fig. 132).

=Fam. 2. Parthenopidae.=—The chelipedes are much larger than the other
legs. Orbits complete. _Lambrus_ (Fig. 133), _Parthenope_.

[Illustration:

  FIG. 133.—_Lambrus miersi_, × 1. (After Milne Edwards and Bouvier.)
]

=Fam. 3. Hymenosomatidae.= The carapace is thin and flat; the chelipedes
are neither very long nor especially mobile. There are no orbits, and
the male openings are on the sternum. Characteristic of the Antarctic
seas. _Hymenosoma_, _Trigonoplax_.


                         =Tribe 5. Catometopa.=

These Crabs resemble the Cyclometopa in general appearance, but the
carapace is very square in outline, and its margins are never so well
provided with spines as in the Cyclometopa. The position of the male
genital openings is peculiar, since they lie upon the sternum, and are
connected with the copulatory appendages upon the abdomen by means of
furrows excavated in the sternum. The Catometopa are either littoral or
shallow water forms, or else they live entirely on land. The Grapsidae
are marine Crabs, _Pachygrapsus marmoratus_ (Fig. 134) at Naples being
exceedingly common on rocks at high-water mark, over which it scuttles
at a great rate; in the Mediterranean it takes the place of our common
_Garcinus maenas_, which is not found there.

[Illustration:

  FIG. 134.—Dorsal view of _Pachygrapsus marmoratus_, × ⅓. (From an
    original drawing prepared for Professor Weldon.)
]

Among the land genera are _Ocypoda_, _Gelasimus_, and _Gecarcinus_ of
tropical lagoons and coastal swamps. _Ocypoda_ often occurs in vast
crowds in these regions, and digs burrows in the sand.

[Illustration:

  FIG. 135.—_Gelasimus annulipes_, × 1. =A=, Female; =B=, male. (After
    Alcock.)
]

_Gelasimus_ (Fig. 135) is remarkable for the enormous size of one of the
chelipedes, generally the right, in the male, which may actually exceed
in size the rest of the body. It is not known what purpose this organ
serves in the various species. In _Gelasimus_ it is supposed that the
male stops up the mouth of the burrow with it when he and the female are
safely inside. It is also used as a weapon in sexual combats with other
males; but Alcock, from observations made in the Indian Ocean, believes
that the males use it for exciting the admiration of the females in
courtship, as the huge chela is bright red in colour, and the males
brandish it about before the females as if displaying its florid beauty.

The species of _Ocypoda_ are exclusively terrestrial, and cannot live
for a day in water. The gills have entirely disappeared, and the
branchial chambers are converted into air-breathing lungs with highly
vascular walls, the entrances into which are situated as round holes
between the bases of the third and fourth pairs of walking legs. As
their name implies, they can run with astonishing rapidity, and they
seem to be always on the alert, directing their eyes, which are placed
on exceedingly long stalks, in all directions.

Some of the Grapsidae, _e.g._ _Aratus pisonii_, are partially adapted
for life on land. Fritz Müller, in his _Facts for Darwin_, alludes to
this creature as “a charming lively crab which ascends mangrove bushes
and gnaws their leaves.” The carapace can be elevated and depressed
posteriorly, apparently by means of a membranous sac, which can be
inflated by the body-fluids. This Crab retains its gills and can breathe
under water in the ordinary way.

A great many other Catometopa are land-crabs; but we may specially
mention the genus _Gecarcinus_, related to the marine Grapsidae, which
has representatives in the West Indies and West Africa. The Crabs of
this genus may live in sheltered situations several miles from the sea,
but in spring the whole adult population rushes down in immense troops
to the shore, where breeding and spawning take place; and when this is
completed they migrate back again to the land. The young pass through
the normal larval stages in the sea and then migrate inland.[157]

=Fam. 1. Carcinoplacidae.=—The carapace is rounded and broader than
long, usually with toothed front margin. The orbits and eyes are normal,
and not much enlarged. _Geryon_, in the deep littoral of the northern
hemisphere. _Euryplax_, _Panoplax_, etc., in the American coastal
waters. _Typhlocarcinus_, etc., in the Indo-Pacific.

=Fam. 2. Gonoplacidae.=—The carapace is square, with the antero-lateral
corners produced into spines. The orbits are transversely widened, and
the eye-stalks long. _Gonoplax_, widely distributed in the littoral
zone. _G. rhomboides_ in British and European seas.

=Fam. 3. Pinnotheridae.=—Carapace round, with indistinct frontal margin.
Orbits and eyes very small, often rudimentary. The members of this
family live symbiotically or parasitically in the shells of living
Bivalve Molluscs, corals, and wormtubes in all seas except the Arctic.
_Pinnotheres pisum_ is fairly commonly met with off the English coasts
in the mantle-cavity of _Cardium norwegicum_.

=Fam. 4. Grapsidae.=[158]—Carapace square, the lateral margins either
strictly parallel or slightly arched. The orbits and eyes are moderately
large, but the eye-stalks are not much lengthened. Littoral,
fresh-water, and land. _Pachygrapsus marmoratus_ (Fig. 134), the common
shore-crab of the Mediterranean. _Sesarma_, with fresh-water and land
representatives in the tropics of both hemispheres. _Cyclograpsus_,
marine in the tropical littoral.

=Fam. 5. Gecarcinidae.=—Carapace square, but much swollen in the
branchial region. Orbits and eyes moderately large. Typically land
forms, which only occasionally visit the sea or fresh water. _Cardisoma_
is a completely circumtropical genus, with species in tropical America,
West and East Africa, and throughout the Indo-Pacific. _Gecarcinus_ in
West Indies and West Africa.

=Fam. 6. Ocypodidae.=—Carapace square or rounded, generally without
teeth on the lateral margins. The orbits transversely lengthened,
eye-stalks usually very long. The members of this family generally
inhabit the mud-flats and sands of tropical coasts; in the southern
hemisphere they extend far into the temperate regions. _Macrophthalmus_,
with numerous species, in Indo-Pacific. _Gelasimus_ (Fig. 135), in the
tropics of both hemispheres. _Ocypoda_, with similar distribution.




                              CHAPTER VII
    REMARKS ON THE DISTRIBUTION OF MARINE AND FRESH-WATER CRUSTACEA


                              =A. Marine.=

The great majority of the Crustacea are inhabitants of the sea. From a
Zoogeographical point of view we divide the sea into three chief
regions, each of which is characterised by a special kind of fauna—the
littoral, the pelagic, and the abyssal regions.

The =littoral= region, which comprises all the shallow coastal waters
down to about 100 fathoms, varies very greatly in its physical character
according to the nature of the coast, its geological constitution,
latitude, etc., but, on the whole, it is characterised by variability of
temperature and salinity, by the presence of sunlight, and by the
continuous motion of its waves. On the shores of the large oceans this
region is also greatly affected by the tides. It is inhabited by a vast
assemblage of Crustacea, all of which are dependent upon a solid
substratum, either of rock or sand, or of vegetable or animal growth,
upon which they may wander in search of food, or in which they may hide
themselves. In consequence, the character of the Crustacea on any shore
is largely determined by its geological nature.

Although a certain number of Entomostraca such as Copepoda
(Harpacticidae and Cyclopidae), Ostracoda (Cypridae and Cytheridae), and
a few Operculata are littoral in habit, it is the Malacostraca, from
their larger size and variety of form, which give the character to
coastal waters.

On rocky coasts, especially those affected by tides, a great many kinds
of Shore-crab are found, which hide at low tide in the rock-pools and
under stones. _Carcinus maenas_ is characteristic of the rocky coasts of
the North Sea, while it is replaced in warmer seas and all round the
tropics by Crabs of the family Grapsidae, which are typical rock-livers,
and exceedingly agile in clambering over tide-washed rocks.
Porcellanidae are also very common under stones at low tide on rocky
beaches. Such typical Shore-crabs as these are remarkably resistant to
desiccation, and can live out of water for an astonishing time; nor do
they require a change of water provided they have access to the air. The
edible crab (_Cancer pagurus_) and the lobsters (_Homarus_ and
_Palinurus_) are dependent on rocks, but they rarely come close
in-shore, preferring depths of a few fathoms.

Sandy coasts are preferred by Shrimps and Prawns, which haunt the
shallow coastal waters in shoals; and in the sand are found all the
Crabs whose respiratory mechanism is specially adapted for life in these
regions, _e.g._ Hippidea or Mole-crabs, _Corystes_, _Matuta_, _Calappa_,
etc.

Characteristic of sandy bottoms are also the Thalassinidea, such as
_Callianassa_, which excavate galleries in the sand. On tropical sandy
shores various species of _Ocypoda_ and _Gelasimus_ are conspicuous,
which have deserted the sea, and live in burrows which they excavate on
the shore. _Gelasimus_ is especially abundant in the muddy sand of
tropical mangrove swamps.

Besides the rocky and sandy coasts we must distinguish the muddy shores
and bottoms which support a large amount of vegetable and animal growth.
These, besides harbouring the greater number of Amphipods and Isopods,
are also the natural home of the Dromiacea and Oxyrhyncha, or
Spider-crabs, among which the habit is common of decking themselves out
with pieces of weed or animal growth in order to harmonise better with
their surroundings. Pagurids are also especially abundant in the deeper
waters of these coasts.

Coral-reefs support a characteristic Crustacean fauna. In the growing
coral at the reef-edge a number of small Cyclometopa are found, e.g.
_Chlorodius_, _Actaea_, _Xantho_, which are finely sculptured and often
coloured so as to harmonise with the coral. Alpheidae also, Shrimp-like
Macrura with highly asymmetrical claws, which can emit a sharp cracking
sound with the larger claw, are commonly found in pools on the reef. In
the coralshingle formed by abrasion from the reef-edge at a few fathoms
depth, Leucosiidae are found, in which, again, respiratory mechanisms
for filtering sand from the gills are present.

Besides the geological nature of the coast, latitude has a very
important bearing upon the distribution of littoral Crustacea. Indeed,
the present distribution of littoral Crustacea appears to be far more
determined by the temperature of the coastal waters than by the presence
of any land-barriers, however formidable. We may distinguish an Arctic,
Antarctic, and Circumtropical zone.

The =Arctic= zone includes the true Arctic seas, and stretches right
down through boreal regions towards the sub-tropical seas. Almost all
the truly Arctic forms penetrate fairly far south, the Arctic seas being
characterised more by the absence of temperate forms than by the
presence of forms peculiar to itself. At the same time it must be noted
that the individuals from the coldest regions often grow to an enormous
size, a characteristic which is physiologically unexplained.

A great many of the Crustacea characteristic of this region are
circumpolar, _i.e._ they are not restricted in range to either the
Atlantic or Pacific. This is especially true of the extremely northern
types, _e.g._ Crangonidae and Hippolytidae, but it is also true of a
number of Crustacea which do not now occur as far north as Greenland or
Bering Strait, so that there is no longer any free communication for
them between Pacific and Atlantic. This gives rise to a discontinuous
distribution in the two oceans, exemplified in the common Shrimp,
_Crangon vulgaris_, which is found on the temperate European coasts and
on the Pacific coasts of Japan and Eastern America. The same is true of
_Eupagurus pubescens_ and _E. bernhardus_.

At the same time the boreal Atlantic and Pacific have their peculiar
forms. Thus the European and American Lobsters are confined to the
Atlantic, while the North Pacific possesses a very rich array of
Lithodinae, which cannot be paralleled in the Atlantic.

We may explain the community of many littoral forms to both the North
Atlantic and Pacific coasts by the continuous coast-line uniting them,
which in former times possibly did not lie so far north, or else was not
subjected to so rigorous a climate as now.

In the =Antarctic= zone we are presented with very different relations,
since the great continents are drawn out to points towards the south,
and are isolated by vast tracts of intervening deep sea. Nevertheless,
certain littoral forms are circumpolar, _e.g._ the Palinurid _Iasus_ and
the Crabs _Cyclograpsus_ and _Hymenosoma_. The genus _Dromidia_ is
common to Australia and South Africa, though it is apparently absent
from South America.

The Isopod genus _Serolis_ is confined to Antarctic seas. The majority
are littoral species, and they are distributed round the coasts of
Patagonia, Australia, and Kerguelen in a manner that certainly suggests
a closer connection between these shores in the past. These facts are,
on the whole, evidence in favour of the former existence of an Antarctic
continent stretching farther north and connecting Australia, Africa, and
S. America—a supposition that has been put forward to account for the
distribution of the Penguins, Struthious birds, Oligochaets, Crayfishes,
etc., in these regions (see pp. 215–217).

In considering the Arctic and Antarctic faunas the supposed phenomenon
of bipolarity must be mentioned, _i.e._ the occurrence of particular
species in Arctic and Antarctic seas, but not in the intermediate
regions. This discontinuous type of distribution was upheld for a
variety of marine animals by Pfeffer, Murray, and others, but it has
been very adversely criticised by Ortmann.[159] As far as the Arctic and
Antarctic Decapod fauna in general are concerned, the north polar forms
are quite distinct from the south polar. Typical of the former are
_Hippolyte_, _Sclerocrangon_, _Hyas_, _Homarus_, etc.; of the latter,
_Hymenosoma_, _Dromidia_, _Iasus_. It appears, however, that in certain
special cases, bipolarity of distribution may be produced owing to the
operation of peculiar causes. Two such cases seem to be fairly well
established. _Crangon antarcticus_ occurs at the two poles, and
apparently not in the intermediate regions; but, as Ortmann points out,
it is represented right down the West American coast by a very closely
related form, _C. franciscorum_. The waters on the tropical western
coasts both of Africa and America are exceedingly cool, and it appears
that in this way the _Crangon_ may have migrated across the tropical
belt, leaving a slightly modified race to represent it in this
intermediate region. The other case of bipolarity is afforded by the
“Schizopod,” _Boreomysis scyphops_, which occurs at both poles, but is
not known from the tropics. This is a pelagic species, and we know that
the Mysidae often descend to considerable depths. We also know that the
Mysidae are dependent on cold water, only occurring in boreal or
temperate waters. We may safely suppose, therefore, that the migration
of this species has taken place by their forsaking the surface-waters as
the tropics were approached, and passing down into the depths where the
temperature is constantly low even in the tropics.

The dependence of Crustacea upon the temperature of the water is also
illustrated by the distribution of the Lithodinae. The headquarters of
this family are in the boreal Pacific, with a few scattered
representatives in the boreal Atlantic. The cool currents on the western
coasts of America, however, have permitted certain forms to migrate as
far south as Patagonia, where they still have a littoral habit. In the
tropical Indo-Pacific, where a few species occur, they are only found in
deep waters. Thus at these various latitudes, by following cool currents
or migrating into deep water, they are always subjected to similar
conditions of temperature. The same kind of thing is observed in Arctic
seas, where deep-sea forms are apt to take on secondarily a littoral
habit owing to the temperature of the depths and of the shore being the
same.

Despite the impassable barriers of land which now sever the tropical
oceans, we can yet speak of a =circumtropical= zone possessing many
species common to its most widely separated parts. Such circumtropical
species, occurring on both the Atlantic and Pacific coasts of tropical
America, on the West African coast, and in the Indo-Pacific, are various
Grapsidae, _Calappa granulata_ and its allies, and certain _Albunea_.
The most striking instance of all is that of the Land-crabs. Of
_Ocypoda_, the greater number of species occur in the Indo-Pacific, but
representatives are also found on the tropical Eastern and Western
American coasts and on the West African coast, and the same is true of
_Gelasimus_. The genus _Cardisoma_, belonging to a different group of
Land-crabs, is also typically circumtropical.

For this community of the circumtropical species we may certainly
advance in explanation the comparatively recent formation of the Isthmus
of Panama. Besides the resemblance of the Crustacea on the east and west
coasts of the isthmus, we have an actual identity of species in several
cases, e.g. _Pachycheles panamensis_ and _Hippa emerita_, and the same
thing has been observed for the marine fish.

Another connexion, at any rate during early tertiary times, which
probably existed between now isolated tropical coasts, was across the
Atlantic from the West Indies to the Mediterranean and West African
coasts. Numerous facts speak for this connexion. Species of _Palinurus_
and _Dromia_ occur in the West Indies and the Mediterranean, which only
differ from one another in detail, and a connexion between these two
regions has been urged from the minute resemblances of the late
Cretaceous Corals of the West Indies with those of the Gosau beds of S.
Europe, and also of the Miocene land-molluscs of S. Europe with those at
the present time found in the West Indies.

To account, then, for the present distribution of littoral Crustacea we
must imagine that great changes have taken place during comparatively
recent times in the coast-lines of the ocean, but the guiding principle
in both the past and present has been temperature, and this factor
enables us, despite the immense changes in the configuration of the
globe that must have taken place, to divide the coasts latitudinally
into Arctic, Antarctic, and Circumtropical zones.

=Pelagic Crustacea= belong chiefly to the Copepoda (Calanidae,
Centropagidae, Candacidae, Pontellidae, Corycaeidae), a few Ostracoda
(Halocypridae and Cypridinae), and among Malacostraca a few Amphipoda
(Hyperina), some “Schizopoda,” and among Decapoda only the Sergestidae,
if we except the few special forms which live on the floating weeds of
the Sargasso Sea, _e.g._ the Prawns _Virbius acuminatus_ and _Latreutes
ensiferus_, and the Brachyura _Neptunus sayi_ and _Planes minutus_.
Besides these Crustacea which are pelagic as adults, there is an
enormous host of larval forms, both among Entomostraca and Malacostraca,
which are taken in the surface-plankton.

In dealing with the Copepoda we have already mentioned the vast pelagic
shoals of these organisms which occur at particular times of the year,
and have an important influence on fishing industries. _Anomalocera
pattersoni_ (Fig. 27, p. 60) is a good instance of this. It is a large
Heterarthrandrian, about 3 mm. long, with the body of a fine bluish
green colour; it has a remarkable power of springing out of the water,
so that a shoal has the appearance of fine rain upon the surface of the
sea. It occurs in the open Atlantic and Mediterranean, but comes into
the coasts during violent storms; the Norwegian fishermen hail its
presence in the fjords as the sign of the approach of the summer
herring.

It was Haeckel[160] who first clearly distinguished between “neritic”
plankton, the species of which have their centres of distribution in
shallow coastal waters and die out gradually as the open ocean is
approached, and “oceanic” plankton which is habitually found in the open
sea, and though it may invade the coasts is not dependent on the
sea-bottom in any way. It appears that although these two kinds of
plankton may get mixed up by currents and storms, they are always
recruited by new generations from the neritic or oceanic stations proper
to each kind.

Common oceanic species, found chiefly in the open Atlantic and in the
North Sea, are _Anomalocera pattersoni_, _Calanus finmarchicus_,
_Centropages typicus_, _Metridia lucens_, _Oithona plumifera_, etc.
Common neritic species in the Channel and other coastal waters are
_Centropages hamatus_, _Euterpe acutifrons_, _Oithona nana_, _Temora
longicornis_, etc. It was found by Gough[161] that although the true
oceanic species invade the Channel from the open Atlantic to the west,
they become rarer and rarer as they advance up the Channel. Thus the
plankton midway between the Lizard and Ushant at all times of year is
about 70 per cent. oceanic, while at the line drawn from Portland to the
Cap de la Hague it is about 35 per cent. Seasonal changes in the
salinity of the Channel water, chiefly due to the influx of oceanic
water from the Atlantic, as observed by Matthews,[162] do not clearly
influence the distribution of oceanic and neritic forms. The influx of
highly saline water from the Atlantic was most marked during the winter
months up to February. From February to May the highly saline water
receded, and during the summer months at the line drawn between Portland
and the Cap de la Hague the salinity was rather low. This was increased
in November by a patch of oceanic water being cut off from the main mass
and passing up Channel, and it is noteworthy that during this month the
highest percentage of oceanic forms was taken in the plankton of this
region.

_Calanus finmarchicus_ affords a clear instance of the way in which the
plankton may be carried about for great distances by means of currents.
This species has its home in the subarctic seas, but is carried down in
the spring by the East Icelandic Polar stream to its spawning-place
south of Iceland; the enormous shoals produced here are carried back,
continually multiplying, along the coasts of Norway during the summer
and autumn.

Besides these great migrations, the plankton organisms perform daily
movements, the majority of the Crustacea avoiding the surface during the
day, and often going down to as much as seventy fathoms or more, and
only coming up to the surface at night. Others, however, e.g. _Calanus
finmarchicus_, behave in the converse manner, preferring the sunlit
surface to swim in.

Owing to their dispersal by means of oceanic currents the pelagic
Crustacea do not offer any very striking features in regard to their
distribution, and the possibility of always finding congenial
temperatures by passing into the upper or under strata of water enables
them to live in almost all seas. The tropical species of Sergestidae are
mostly circumtropical, _i.e._ unhindered by the present barriers of
land.

The =Abyssal= regions of the sea contain many of the most interesting
Crustacea. Families entirely confined to the abyss are the Eryonidae,
Pylochelidae, and certain Caridean Prawns (Psalidopodidae, etc.), but
there are a great number of normally littoral genera which have
representatives in deep water. If we draw the limit between the littoral
and abyssal regions at about 200 metres, we can characterise the latter
as absolutely dark except for the presence of phosphorescent organisms,
with the temperature at a little above zero, and with a comparative lack
of dissolved oxygen in the water. These conditions bring about
remarkable modifications in the structure and life-histories of the
inhabitants of the deep sea; we have already touched on the
modifications of the visual organs and on the presence of
phosphorescence in many of the animals; other points to be noticed are
the usually uniform yellowish or bright red coloration, the frequent
delicacy of the tissues without much calcification, variations in the
structure of the breathing organs, _e.g._ in _Bathynomus giganteus_ and
_Encephaloides armstrongi_, and the loss of the larval development.
Owing to the similarity of conditions in the deep sea all over the globe
most of its inhabitants are universally distributed. It is also a
striking fact that species are found in the deep sea of the tropics
whose nearest allies occur, not in the littoral seas of the tropics, but
in those of the temperate region. This fact has already been alluded to
in dealing with the distribution of the Lithodinae. Alcock[163] remarks
that between 50–500 fathoms in the Indian Ocean are found Crabs such as
_Maia_, _Latreillia_, and _Homola_, regarded as characteristic of the
north temperate seas; the lobster _Nephrops andamanica_, taken at
150–400 fathoms, is closely allied to the Norwegian _N. norwegica_; and
nine species of “Schizopoda,” which are certainly temperate forms, occur
in the Indian Ocean at depths of 500–1750 fathoms.


                         =B. Fresh-Water.=[164]

If we except the Crayfishes and River-crabs, the Crustacean fauna of
running water is exceedingly poor, but in all standing fresh-water, from
the smallest pond to the large lakes and inland seas, Crustacea,
especially Entomostraca, are abundant and characteristic, and form an
important item in the food of fresh-water fishes. In small ponds a vast
assemblage of Cladocera is met with; these animals multiply with great
rapidity by parthenogenesis, especially during spring and summer, but on
the advent of untoward conditions sexual individuals are produced, which
lay fertilised winter-eggs which lie dormant until favourable conditions
again arise. As Weismann first pointed out, the frequency with which
sexual individuals are produced in the various species is closely
correlated with the liability of the water in which they live to dry up;
so that the Cladocera which inhabit small ponds usually have at least
two “epidemics” of sexual individuals, one during early summer and the
other before the onset of winter.

Besides Cladocera, the Phyllopoda (e.g. _Apus_, _Artemia_, etc.) inhabit
small pools; and also a great number of Cyclopidae. Of the other
fresh-water families of Copepoda, viz. Centropagidae and Harpacticidae,
inhabitants of small pieces of water are _Diaptomus castor_, as opposed
to the other species of _Diaptomus_ which are pelagic, and a number of
Harpacticidae (_Canthocamptus_), the members of this family living in
the weed or mud of either small ponds or else on the shores of the
larger lakes. The greater number of Ostracoda are found in similar
situations.

A district like the Broads of Norfolk, which consists partly of
slowly-moving streams and partly of extensive stretches of shallow
water, supports a Crustacean fauna intermediate in character between
that found in small ponds and the truly pelagic fauna characteristic of
deep lakes. A very complete list of the Crustacea of the Norfolk Broads,
with an interesting commentary on their distribution, is given by Mr.
Robert Gurney.[165] We miss here the pelagic Cladocera, such as
_Leptodora_, _Bythotrephes_, _Holopedium_, etc., which form so
characteristic a feature of large lakes; at the same time, besides a
rich development of the Cladocera, Cyclopidae, and Harpacticidae, which
haunt the weeds and mud of shallow waters, we find such species as
_Polyphemus pediculus_ and _Bosmina longirostris_ among Cladocera, which
are otherwise confined to large bodies of water, and a few pelagic
_Diaptomus_, e.g. _D. gracilis_. The fauna is also complicated in this
district by the proximity to the sea and the frequently high salinity of
the water, which allows a number of typically marine Copepods to pass up
the estuaries and intermingle with typically fresh-water species; such
are _Eurytemora affinis_ among the Centropagidae, and several species of
Harpacticidae (see p. 62).

The large lakes of the world, such as the continental lakes of Europe
and America, or of our own Lake District, reproduce on a small scale the
varied conditions which appertain to the ocean—as in the ocean, we can
recognise in these lakes a littoral, a pelagic, and an abyssal region.
Our knowledge of the physiography of lakes is largely due to the
classical work of Forel,[166] and the following account of the physical
conditions in the various regions is condensed from his book.

The =littoral= region is sharply marked off from the others by the
relative instability of its physical conditions, owing to the agitation
of its waters, the affluence of streams and drainage, and the constant
changes of temperature. The water in this region generally contains a
good deal of solid matter in suspension, while the shelving banks of the
lake support a wealth of vegetable growth, both of Algae and of
Phanerogams, down to about 20–25 metres. At this depth the daylight does
not penetrate sufficiently to admit of the growth of green plants, so
that this region marks the limit, both physical and biological, between
the littoral and the abyssal zones. In this littoral region there
flourish a great quantity of Entomostraca, most of which are also found
in small ponds where similar conditions of life prevail—the pelagic
species only penetrating rarely, and by accident, into its waters. At
the beginning of July Mr. H. O. S. Gibson and myself found that the
weedy littoral region of Grasmere contained almost entirely large
quantities of the Cladoceran _Eurycercus lamellatus_, and a number of
_Cyclops fuscus_ and _C. strenuus_. In the littoral zone of large lakes,
Amphipods, Isopods, and fresh-water shrimps may also be met with, but
this applies more to the lakes of the Tropics and of the Southern
Hemisphere.

The =pelagic=[167] region is distinguished from the littoral by the
greater purity and transparency of its waters, and by the relative
stability of the temperature, the annual range, even at the surface, in
Geneva being from 4°–20° C., while at 100 metres the water has a uniform
temperature of 4° or 5° C. The upper strata are, of course, brightly
illuminated, but at 20 metres there is hardly sufficient light for green
plants to grow, and at 100 metres it is completely dark. The inhabitants
of this region, known collectively as plankton, spend their whole life
swimming freely in the water, sometimes at the surface and sometimes in
the deeper strata. They consist chiefly of Diatoms, Protozoa, Rotifera,
and Crustacea. The pelagic Crustacea, especially the Cladocera, are
often the most curiously and delicately built creatures. _Leptodora
hyalina_, which is quite transparent, is the largest of them, attaining
to three-quarters of an inch in length, though _Bythotrephes longimanus_
is nearly as large if we include the immense spine which terminates the
body. _Holopedium gibberum_, which is the commonest of all in Grasmere
lake, but not so frequently met with in the other English lakes, is
peculiar in that its body is enveloped in a spherical mass of
transparent jelly, sometimes a quarter of an inch in diameter, so that
the contents of a tow-net jar full of _Holopedium_ have something of the
consistency of boiled sago. The enormous quantities in which these
animals often occur during summer is very astonishing; but to be truly
appreciated tow-nettings should be taken at the surface of the lake
either during night-time when there is not much moonlight, or else on a
dark still day when there is a quiet drizzle falling on the surface of
the water. In bright sunshine the plankton passes below the surface into
the lower strata, and can be usually taken by sinking the tow-net some
10–20 feet, or to even greater depths in the water. The exact reason of
these periodic migrations out of the light, and their dependence on
other physical conditions, such as temperature and the agitation of the
water, is not clearly understood. It appears, however, that when the
water is rough, plankton always passes into the deeper regions. Besides
the species mentioned, the minute Bosminidae, whose trunked heads are
suggestive under the microscope of elephants, and _Polyphemus pediculus_
are among the commonest pelagic Cladocera, though neither _Polyphemus_
nor _Bythotrephes_ ever form shoals. The above-mentioned genera are
characteristic of the larger lakes in the Northern Hemisphere. Our
knowledge of the Crustacean plankton of tropical lakes and of those of
the Southern Hemisphere is limited (but see p. 216).

A very important constituent of lake-plankton is furnished by the
Copepoda, especially of the genus _Diaptomus_. With the exception of
_Holopedium_, by far the commonest Crustacean in Grasmere during July
was found by Mr. Gibson and myself to be _D. caeruleus_.

At the same time a number of Cyclopidae, e.g. _Cyclops strenuus_, may
occur in the pelagic region in considerable quantities, though they were
never found by us in such numbers as _Diaptomus_.

The life-cycle of the pelagic Entomostraca has been studied in both the
Cladocera and the Copepoda. In some of the Cladocera Weismann at first
supposed that males had altogether disappeared, and that reproduction
was entirely parthenogenetic. It appears, however, that all the pelagic
species have at least one sexual period, namely, in the autumn, when
resting eggs are produced which lie dormant during the winter. The
pelagic Copepods may either produce resting eggs for the winter
(_Diaptomus_), or else the winter is passed through in the Nauplius
stage, the larvae hibernating in the mud until the spring (Cyclopidae).

We have so far only dealt with fresh-water Entomostraca. There are, in
addition, a number of Malacostraca which inhabit fresh water, and some
of these are found in the abyssal region of the great lakes, which must
now be considered.

The physical conditions of the =abyssal= region are still more stable
than those of the pelagic region, since the water is never disturbed,
the bottom is always composed of a fine mud, the temperature is constant
at 4°–5° C., and there is a total absence of light. It was hardly
expected that animals would inhabit this region, until Forel discovered
_Asellus aquaticus_ in a depth of forty metres in the Lake of Geneva,
and subsequently showed that quite a number of animals, including a
_Hydra_, several worms, Molluscs, Crustacea, and larval Insects, may be
found in these or even much greater depths.

The Crustacea of the abyssal region are two in number, and these have
been found in a number of European lakes; _Niphargus puteanus_, a blind
Amphipod closely allied to _Gammarus_; and _Asellus forelii_, allied to
_A. aquaticus_ and _A. cavaticus_, which may be either quite blind or
else retain the rudiments of eyes.

These two Crustacea, under a practically identical form, are also found
in the subterranean waters of Europe, and Forel considers that they have
arrived in the abysses of the lakes from the subterranean channels, and
are not derivatives of the littoral fauna.[168]

Having completed our short review of lacustrine Crustacea, we may deal
with the =subterranean and cave Crustacea=,[169] which, as far as light
and temperature are concerned, are subjected to very similar conditions
to those dwelling at the bottom of deep lakes. The inhabitants of the
subterranean waters have been chiefly brought to light in Artesian
wells, etc., while the cavedwellers have been investigated especially at
Carniola and in the American caves.

A number of species of Cyclopidae and Cypridae, many of which are blind
and colourless, have been brought up in well-water. The Amphipod
_Niphargus puteanus_ has long been known from a similar source in
England[170] and all over Europe, and several other blind Gammarids
inhabit the subterranean waters and caves in various parts of the world.
Among Isopods, _Asellus cavaticus_ is recorded from wells and caves in
various parts of Europe, _Caecidotea stygia_ and _C. nickajackensis_
from the Mammoth and Nickajack Caves in America, and two very remarkable
blind Isopods are described by Chilton from the subterranean waters of
New Zealand, viz. _Cruregens fontanus_, whose nearest allies are the
marine Anthuridae, and the Isopods _Phreatoicus typicus_ and _P.
assimilis_, which bear an extraordinary resemblance superficially to
Amphipods. Besides these, a small number of subterranean Decapoda are
known which retain the eye-stalks but are without functional ommatidia.
These are _Troglocaris schmidtii_, in Hungary, related to the
fresh-water Atyid _Xiphocaris_ of East Indian and East Asiatic fresh
waters rather than to the South European _Atyephyra_; _Palaemonetes
antrorum_, from artesian wells in Texas; and several species of
_Cambarus_ from the Eastern United States. A blind species of
_Cambarus_, _C. stygius_, has been described from the caves of Carniola,
and if this determination is correct, is the sole _Cambarus_ occurring
outside America.

It will be seen from the above account that the subterranean Crustacea
are an exceedingly interesting and, in many respects, archaic group,
many of which have survived in these isolated and probably uncompetitive
districts, while many secular changes were going on in the quick world
overhead.

The remaining fresh water =Malacostraca= may be mentioned under the
headings of the groups to which they belong.

Only one “Schizopod,” apart from _Paranaspides_, is known from
fresh-water lakes, viz. _Mysis relicta_, which was discovered in 1861 by
Lovén in the Scandinavian lakes, and has since been found in the Finnish
lakes, the Caspian Sea, Lake Michigan, and other localities in N.
America, and Lough Erne in Ireland. This species is closely related to
_Mysis oculata_ of Greenlandic seas.

In the Southern Hemisphere we have a species of _Anaspides_, _A.
tasmaniae_, occurring in mountain streams and tarns in Tasmania, a
related form which haunts the littoral zone of the Great Lake in
Tasmania, and a small species, _Koonunga cursor_, occurs in a little
stream near Melbourne.

Of the Isopoda certain genera, viz. _Asellus_ and _Monolistra_, are
confined to fresh water, others, such as _Sphaeroma_, _Idothea_,
_Alitropus_, and _Cymothoa_, have occasional fresh-water
representatives. Packard[171] describes a remarkable blind Isopod,
_Caecidotea_, from the Mammoth Cave of Kentucky, which occupies a very
isolated position, and in the same work gives a very complete exposition
of the cave-fauna of North America and Europe.

The Phreatoicidae are a curious family of Isopods confined to the fresh
waters of Australia and New Zealand, which bear a remarkable resemblance
to Amphipods, being laterally compressed and possessing a subchelate
hand on the anterior thoracic leg. These Isopods are exceedingly common
in small mountain pools and in streams in Tasmania, and in the Great
Lake in that country I have recently found a number of species which,
together with some species of Amphipods, make up the dominant feature in
the Crustacean fauna. One of these species may grow to fully an inch in
length. The family is confined to the temperate regions, and is usually
found on mountains. A number of species are known from the mainland of
Australia, one coming from a high elevation on Mount Kosciusko, and
another (_Phreatoicopsis_) from the forests of Gippsland attaining a
great size, and living among damp leaves, etc.

The fresh-water Amphipoda all belong to the families Talitridae,
Gammaridae, and Haustoriidae (see p. 137).

Among the Talitridae, or Sand-hoppers, _Orchestia_ and _Talitrus_ have
marine as well as fresh-water and land representatives, while the
American _Hyalella_ is entirely from fresh water, most of the species
being peculiar to Lake Titicaca. Many of these animals are partly
emancipated from an aquatic life. Thus _Orchestia gammarellus_, which is
common on the sea-shore of the Mediterranean, frequently penetrates far
inland, and was found in large numbers by Kotschy near a spring 4000
feet up on Mount Olympus.

_Talitrus sylvaticus_ is very common among fallen leaves and decaying
timber in Tasmania and Southern Australia, many miles from the sea, and
often at an elevation of several thousand feet.

Among the Gammaridae, certain genera, e.g. _Macrohectopus_
(_Constantia_), from Lake Baikal, are purely fresh-water. An enormous
development of Gammaridae was discovered by Dybowsky in Lake Baikal,
comprising 116 species, and lately a number more have been found by
Korotneff.[172] The majority of these were originally placed in the
genus _Gammarus_, but Stebbing has rightly created a number of peculiar
genera for them. Certain species are, however, placed in more widely
distributed genera, e.g. _Gammarus_ and _Carinogammarus_, which is also
represented in the Caspian Sea. Korotneff found some remarkable
transparent pelagic forms (_Constantia_) swimming in the abyssal regions
at about 600 metres depth, the majority of them being blind, but some
possessing rudimentary eyes, often on one side only.

Besides various species of _Gammarus_, a number of other Gammaridae are
frequently found in brackish water. Among Haustoriidae _Pontoporeia_ has
representatives in both the oceans and inland lakes of the northern
hemispheres (see p. 137).

Of the Decapoda, seven families are typically fresh-water in habitat—the
Aegleidae, containing the single species _Aeglea laevis_, related to the
Galatheidae, which inhabits streams in temperate S. America; the
Atyidae, a family of Prawns from the tropical rivers and lakes of the
New and Old World, and in the Mediterranean region. A number of
Palaemonidae are found in fresh water, e.g. _Palaemonetes varians_ in
Europe and N. America, while several species of _Palaemon_ occur in
lakes, streams, and estuaries of the tropical Old and New World.

The expeditions of Moore and Cunnington to Lake Tanganyika brought back
an exceedingly rich collection of Prawns, comprising twelve species, all
of which are peculiar to the lake,[173] and this is all the more
surprising since Lakes Nyasa and Victoria Nyanza are only known to
contain one species, _Caridina nilotica_, which ranges all over Africa
and into Queensland and New Caledonia. The Tanganyika species, however,
all belong to purely fresh-water genera, and do not afford any
suggestion that they are part of a relict marine fauna. It would appear
that they have been differentiated in the lake itself during a long
period of isolation.

Two groups of Brachyura, viz. the Thelphusidae and the Sesarminae (a
sub-family of the Grapsidae), are fresh-water. _Thelphusa fluviatilis_
is an inhabitant of North Africa, and penetrates into the temperate
regions of the Mediterranean, and is said to be exceedingly common in
the Alban Lake near Rome. Both these families have representatives on
land, e.g. _Potamocarcinus_ in Central and South America, and certain
species of _Sesarma_, and the closely related Gecarcinidae of the West
Indies.

The remaining families to be dealt with are the two Crayfish families—
the Astacidae and the Parastacidae—which live in rapidly moving rivers
and streams, and occasionally in lakes. A few species of both families
have taken to a subterranean mode of life, and excavate burrows in the
earth, _e.g._ the Tasmanian Crayfish, _Engaeus fossor_. The distribution
of the Crayfishes has long engaged the attention of naturalists. It was
first seriously studied by Huxley,[174] and has subsequently been
followed up, especially in North America, by Faxon[175] and
Ortmann,[176] but our knowledge of the South American and Australian
forms is still very incomplete. The Astacidae inhabit the northern
temperate hemisphere, the Parastacidae the southern temperate
hemisphere, the tropical belt being practically destitute of Crayfishes.
Of the Astacidae the genus _Astacus_ (_Potamobius_), including the
common Crayfishes of Europe, occurs in Europe and in North America west
of the Rockies. The genus _Cambaroides_, which in certain respects
approaches _Cambarus_, is found in Japan and Eastern Asia. The very
large genus _Cambarus_, on the other hand, only occurs in North America
east of the Rockies, so that _Cambaroides_ occupies a very isolated
position. The occurrence of a _Cambarus_, _C. stygius_, in the caves of
Carniola, has been recorded by Joseph, so that it would appear that this
genus had a much wider range formerly than now.

Of the southern temperate Parastacidae, Australia and Tasmania have the
genera _Astacopsis_ and _Engaeus_; New Zealand has _Paranephrops_, South
America _Parastacus_, and Madagascar _Astacoides_. The last named genus
is rather isolated in its characters, possessing a truncated rostrum and
a highly modified branchial system, but it agrees with all the other
Parastacine genera, and differs from the Astacidae in the absence of
sexual appendages on the first abdominal segment, and in the absence of
a distinct lamina on the podobranchiae. The largest crayfish in the
world is _Astacopsis franklinii_, found in quite small streams on the
north and west coast of Tasmania. Specimens have been caught weighing
eight or nine pounds, and rivalling the European Lobster in size.
Crayfishes appear to be entirely absent from Africa.

It seems reasonable to suppose that the two families of Crayfishes
characteristic respectively of the northern and southern hemispheres
have been independently derived from marine ancestors, which have
subsequently become extinct. Their complete absence in the tropics is
striking, and Huxley drew attention to the fact that it is exactly in
those regions where the Crayfishes are absent that the other large
fresh-water Malacostraca are particularly well developed, and _vice
versa_. Thus the large fresh-water Prawns are typically circumtropical
in distribution, while the South African rivers abound with River-crabs,
which, in general, are found wherever Crayfishes do not occur.

A few of the more interesting features in connection with the
distribution of fresh-water Crustacea have now been touched upon. With
regard to the origin of this fauna, we can see that a number of the
species are comparatively recent immigrants from the sea, working their
way up the estuaries of rivers, a proceeding which can be observed to be
taking place to-day in a district like the Broads of Norfolk. Others,
again, but these are few, appear to be true relict marine animals left
stranded in arms of the sea that have been cut off from the main ocean,
and have been gradually converted into fresh-water lakes and seas. Such
are, perhaps, _Mysis relicta_ and the rich Gammarid fauna of Lake
Baikal, a lake that, in the presence of Seals, Sponges, and other marine
forms, has clearly retained some of the characters of the ocean from
which it was derived. The majority of the fresh-water species, however,
have probably been evolved _in situ_, and their origin from marine
ancestors is lost in an obscure past. The Crustacean fauna of the
Caspian Sea[177] shows us in an interesting manner the effects of
isolation and changes in salinity, etc., on the inhabitants of a basin
which once formed part of the ocean. The waters of the Caspian Sea are
not fresh, but they are on the average about one-third as salt as that
of the open ocean. The Crustacea, described by Sars, belong to
undoubtedly marine groups, _e.g._ the Mysidae, Cumacea, and Amphipoda
Crevettina, but the remarkable feature of these Caspian Crustacea is the
great variety of peculiar species representing marine genera which are
very poorly represented in the sea, thus indicating that the variety of
the fauna is not due to a great variety of species having been shut up
in the Caspian Sea to begin with, but rather that, after the separation
from the sea, the isolated species began to vary and branch out in the
most luxuriant way—whether from lack of competition or owing to the
changing conditions of salinity it is difficult to say. As an example,
the Cumacea of the Caspian Sea are ten in number, all belonging to
peculiar genera related to _Pseudocuma_, except one species which is
included in that genus. These Caspian forms make up the Family
Pseudocumidae, which contains in addition only two marine forms of the
genus _Pseudocuma_ (see p. 121). A very similar condition is found in
the numerous Amphipods of the Caspian Sea. Considering the enormous
changes that must have taken place in the distribution of land and water
even during Tertiary times, it is astonishing that the fresh waters of
the world do not contain more species in common with the ocean, but it
must be considered that the limited area and comparatively uniform
conditions of fresh-water lakes and streams would only permit a limited
number of these forms to survive which could most easily adapt
themselves to the changed conditions. And these would in all probability
be the littoral species that were in the habit of passing up into the
brackish waters of estuaries and lagoons, so that the uniform and
limited nature of the fresh-water fauna can be accounted for to a
certain extent by this hypothesis.

We have seen in dealing with the marine Crustacea of the littoral zone
that the chief condition determining their distribution is temperature,
and that the world may be divided into three chief areas of distribution
for these animals, viz. the north temperate hemisphere, the tropics, and
the south temperate hemisphere. It seems that the same division holds
good for fresh-water Crustacea. We have already seen that the Crayfishes
follow this rule, being practically absent from the tropics, and
represented in the two temperate hemispheres by two distinct families,
the Astacidae in the north and the Parastacidae in the south.
Characteristic of the tropical belt are the absence of Crayfishes and
the great development of Prawns and River-crabs. In the case of
Entomostraca the great majority of the genera are cosmopolitan,
especially those which live in small bodies of water liable to dry up,
because these forms always have special means of dissemination in the
shape of resting eggs which can be transported in a dry state by
water-birds and other agencies to great distances; but those genera
which inhabit large lakes are more confined in their distribution. The
Copepod genus _Diaptomus_, characteristic of lake-plankton, ranges all
over the northern hemisphere and into the tropics, but it is almost
entirely replaced in the southern hemisphere by the related but distinct
genus _Boeckella_,[178] which occurs in temperate South America, New
Zealand, and southern Australia, and was found by the author to be the
chief inhabitant in the highland lakes and tarns of Tasmania,
_Diaptomus_ being entirely absent. Of the Cladocera there are a number
of pelagic genera (e.g. _Leptodora_, _Holopedium_, _Bythotrephes_)
entirely confined to the lakes of the northern hemisphere. The
distribution of _Bosmina_ is interesting. This genus is distributed all
over the north temperate hemisphere in lakes and ponds of considerable
size, not liable to desiccation; in the New World it passes right
through the tropics into Patagonia,[179] the chain of the Andes
doubtless permitting its migration. In the tropics of the Old World it
is unknown, but it turns up again, as the author recently found, as a
common constituent in the plankton of the Tasmanian lakes. There is
another instance of a group of Crustacea, characteristic of the north
temperate hemisphere, being entirely absent from the tropics, at any
rate of the Old World, but reappearing in the temperate regions of
Australasia. The commonest fresh-water Amphipods in this region belong
to the genus _Neoniphargus_, intermediate in its characters between the
northern _Niphargus_ and _Gammarus_, but grading almost completely into
the latter. Both _Niphargus_ and _Gammarus_ are absolutely unknown from
the tropics, but whether, like _Bosmina_, they occur in the Andes and
temperate South America is not known; it seems, however, probable that
they have reached Southern Australia by way of South America rather than
through the tropics of Asia and Australia, where there is no range of
mountains to permit the migration of a group of animals apparently
dependent on a temperate climate. The other common fresh-water Amphipod
in temperate Australia and New Zealand is _Chiltonia_, whose nearest
ally is _Hyalella_ from Lake Titicaca on the Andes, and temperate South
America.

The Anaspidacea and Phreatoicidae, which are so characteristic of
temperate Australia, and are generally of an Alpine habit, have never
been found in South America, but the Anaspidacea are represented by
numerous marine forms in the Permian and Carboniferous strata of the
northern hemisphere, so that it is probable that this group reached the
southern hemisphere from the north through America.

The distribution of the fresh-water Crustacea, therefore, in the
temperate southern hemisphere affords strong evidence in favour of the
view that the chief land-masses of this hemisphere, which are at present
separated by such vast stretches of deep ocean, were at no very remote
epoch connected in such a way as to permit of an intermixture of the
temperate fauna of New Zealand, Australia, and South America. While this
connexion existed, a certain number of forms characteristic of the
northern hemisphere, which had worked through the tropics by means of
the Andes, were enabled to reach temperate Australia and New Zealand.
The existence of a coast-line connecting the various isolated parts of
the southern hemisphere would, of course, also account for the community
which exists between their littoral marine fauna. It is impossible to
enter here into the nature of this land-connexion which is becoming more
and more a necessary hypothesis for the student of geographical
distribution, whatever group of animals he may choose, but it may be
remarked that the connexion was probably by means of rays of land
passing up from an Antarctic continent to join the southernmost
projections of Tierra del Fuego, Tasmania, and New Zealand.




                               TRILOBITA


                                   BY

                           HENRY WOODS, M.A.

  St. John’s College, Cambridge, University Lecturer in Palaeozoology




                              CHAPTER VIII
                               TRILOBITA


Among the many interesting groups of fossils found in the Palaeozoic
deposits there is none which has attracted more attention than the
Trilobites. As early as 1698, Edward Lhwyd, Curator of the Ashmolean
Museum in Oxford, recorded in the _Philosophical Transactions_ the
discovery of Trilobites in the neighbourhood of Llandeilo in South
Wales; and of one of his specimens he remarked that “it must be the
Sceleton of a flat Fish.” In the following year the same writer gave in
his _Lithophylacii Britannici Ichnographia_ descriptions and figures of
two Trilobites which are evidently examples of the species now known as
_Ogygia buchi_ and _Trinucleus fimbriatus_.

Although Trilobites differ so much from living Arthropods that it was
difficult to determine even whether they belonged to the Crustacea or
the Arachnida, yet one of the earliest writers, Dr. Cromwell Mortimer,
Secretary of the Royal Society (1753), recognised their resemblance to
_Apus_ (see pp. 19–36). This view of their affinities was adopted by
Linnaeus, and has been supported by many later writers. Another early
author, Emanuel Mendez da Costa, thought that the Trilobites were
related to the Isopods, an opinion which has been held by some few
zoologists of more recent times.

The Trilobites form the only known Order of the Crustacea which has no
living representatives. They are found in the oldest known fossiliferous
deposits—the Lower Cambrian or _Olenellus_ beds, where they are
represented by 19 genera belonging to the families Agnostidae,
Paradoxidae, Olenidae, and Conocephalidae. From the variety of forms
found and the state of development which they have reached, it is
evident that even at that remote period the group must have been of
considerable antiquity; but of its pre-Cambrian ancestors nothing is yet
known; consequently there is no direct evidence of the origin of the
group.

Trilobites form an important part of all the faunas of the Cambrian
system; they attain their greatest development in the Ordovician period,
after which they become less numerous; their decline is very marked in
the Devonian, in which nearly all the genera are but survivals from the
Silurian period; in the Carboniferous, evidence of approaching
extinction is seen in the small number of genera represented, all of
which belong to one family—the Proëtidae, in the relatively few species
in each genus and in the small size of the individuals of those species.
In Europe no representatives of the group appear to have survived the
Carboniferous period, but in America one form has been recorded from
deposits of Permian age.

Trilobites seem to have been exclusively marine, since they are found
only in association with the remains of marine animals. Their range in
depth was evidently considerable, for they occur in many different kinds
of sediment, and were apparently able to live regardless of the nature
of the sea-floor—whether muddy, sandy, calcareous, or rocky. In some
cases they occur in deposits containing reef-building corals and other
shallow water animals; in others they are associated with organisms
which lived at greater depths. The group appears to have had a
world-wide distribution, for the remains of Trilobites are found in the
Palaeozoic rocks of all countries. Their range in size is considerable;
for whilst a large proportion of the species are about two or three
inches in length, some, like _Agnostus_, are only a quarter of an inch
long, others are from ten to twenty inches long, the largest forms
including species of _Paradoxides_, _Asaphus_, _Megalaspis_, _Lichas_,
and _Homalonotus_.

The feature in a Trilobite which first attracts attention is the marked
division of the dorso-ventrally flattened body into a median or axial
part, and a lateral or pleural part on each side. It was this character
that led Walch, in 1771, to give the name by which the group is now
known. The axial part of the body contained the alimentary canal, as is
shown by the position of the mouth and anus, as well as by casts in mud
of the canal which are found in some specimens. The trilobation of the
body is quite distinct in the majority of Trilobites, but in a few
genera belonging to the Asaphidae and Calymenidae (Fig. 136) it becomes
more or less completely obsolete.

[Illustration:

  FIG. 136.—_Homalonotus delphinocephalus_, Green, × 1. Silurian. (After
    Zittel.)
]

In most cases the only part of the Trilobite which is preserved is the
exoskeleton which covered the dorsal surface of the body. That skeleton
consists largely of calcareous material, and shows in sections a finely
perforated structure. Generally it is arched above, but in some cases is
only slightly convex; in outline it is more or less oval. Three regions
can always be distinguished in the body of a Trilobite—the head, the
thorax, and the abdomen or pygidium.

The carapace which covers the =head= is known as the cephalic shield
(Fig. 137, A, 1), and is commonly more or less semicircular in outline,
but varies considerably in different genera. Only in a few cases, as in
some species of _Agnostus_ (Fig. 146), is its length greater than its
breadth. The axial part of the cephalic shield, called the “glabella”
(Fig. 137, A, _a_), is usually more convex than the lateral parts
(“cheeks” or “genae”), and is separated from them by longitudinal or
axial furrows (_b_). The shape of the glabella varies greatly; it may be
oblong, circular, semi-cylindrical, pyriform, spherical, etc. Its
relative size likewise varies; thus in _Phacops cephalotes_ it expands
in front and forms the larger part of the head, whilst in _Arethusina_
(Fig. 151, B) it is narrow and short, being only about one-half of the
length of the head.

[Illustration:

  FIG. 137.—_Calymene tuberculata_, Brünn. × 1. Silurian, Dudley. =A=,
    Dorsal surface: 1, head; 2, thorax; 3, pygidium or abdomen. _a_,
    Glabella; _b_, axial furrow; _c_, glabella-furrow; _d_, neck-furrow;
    _e_, fixed cheek; _f_, free cheek; _g_, facial suture; _h_, eye;
    _i_, genal angle; _k_, axis of thorax; _l_, pleura. =B=, Ventral
    surface of head (after Barrande): _a_, hypostome; _b_, doublure;
    _c_, _c′_, facial sutures; _d_, rostral suture; _e_, rostral plate.
    =C=, One segment of the thorax: _a_, ring of axis; _b_, groove; _c_,
    articular portion; _d_, axial furrow; _d-f_, pleura; _d-e_, internal
    part of pleura; _e-f_, external part of pleura; _e_, fulcrum; _g_,
    groove. =D=, Coiled specimen: _a_, glabella; _b_, eye; _c_, facial
    suture; _d_, pygidium; _e_, rostral suture; _f_, continuation of
    facial suture.
]

The segmentation of the head is indicated by transverse furrows on the
glabella (Fig. 137, A, _c_, _d_). In some cases these furrows extend
quite across the glabella (Fig. 147), but commonly they are found on the
sides only and divide the glabella into lateral lobes. Only the
posterior or “neck-furrow” (Fig. 137, A, _d_) is continued on to the
cheeks, and the segment which it limits anteriorly on the glabella[180]
is known as the occipital or neck-ring. In front of the neck-furrow
there may be three other furrows, so that altogether five cephalic
segments are indicated by the furrows of the glabella. Commonly all the
furrows are distinct in the primitive types; but in the more modified
forms some, especially the anterior, become either reduced in size or
obsolete. The actual number of furrows present consequently varies in
different genera, and may even differ in different species of the same
genus. In a few genera all the furrows are either indistinct or absent,
as for example in _Ellipsocephalus_ (Fig. 150, B). In some cases four
furrows are present in addition to the neck-furrow; this is due to the
division of the anterior lobe of the glabella by fulcra which are
developed for the attachment of muscles.

When the glabella reaches the front border of the head the two cheeks
are separated (Fig. 150, I); but in other cases they unite in front of
the glabella (Fig. 150, C). The outer posterior angle of the cheeks or
genae (“genal angle,” Fig. 137, A, _i_) may be rounded, pointed, or
produced into backwardly directed spines (Fig. 140). The marginal part
of the cephalic shield is often flattened or concave; this border may be
quite a narrow rim as in _Calymene_ (Fig. 137, A), but in some genera
(e.g. _Trinucleus_, Fig. 140, B; _Harpes_, Fig. 150, A; _Asaphus_) it
attains a great development. Each cheek is usually divided by a suture—
the “facial suture” (Fig. 137, A, _g_)—into an inner and an outer part;
the former is the “fixed cheek” (_e_), and the latter the “free cheek”
(_f_). The course of the facial suture varies in different genera: on
the posterior part of the head it begins either at the posterior margin
(Fig. 150, C) or at the posterior part of the lateral margin (Fig. 151,
C, D); at first it is directed inwards, and then bends forward, forming
an angle. In front it may (_a_) end at the front margin (Fig. 147), or
(_b_) be united beneath the front margin by a rostral suture (Fig. 137,
B, _d_, D, _e_), or (_c_) unite with the other suture on the dorsal
surface in front of the glabella (Fig. 151, C). In the last case the
free cheeks also unite in front of the glabella.

The facial suture is one of the distinguishing features of the
Trilobites, and may have been of some use in ecdysis. In only a few
forms is it absent, as for example in _Agnostus_ (Fig. 146) and
_Microdiscus_. In the former, however, Beecher states that a suture is
really present, but, unlike that of most other Trilobites, it is
situated at the margin of the cephalic shield, and consequently the free
cheek, if present, must be on the ventral surface. Lindström and Holm,
after a re-examination of well-preserved specimens, deny the existence
of a suture in _Agnostus_. By most authors _Olenellus_ is said to be
without a suture, but Beecher maintains that although the fixed and free
cheeks have coalesced, yet a raised line passing from the eye-lobe to
the posterior margin marks the position of the suture; this view is not
accepted by Lindström.

The existence of a facial suture in _Trinucleus_ has likewise been
disputed. But Emmerich, Salter, and M‘Coy[181] have maintained that a
suture is present in a normal position on the dorsal surface, extending
from the posterior margin just within the genal angle to the eye (when
present), and from thence bending forward and ending on the front margin
near the glabella. It must be admitted that no indications of the suture
are seen in the majority of specimens, perhaps owing to the fact that
most examples of _Trinucleus_ are in the form of internal casts; perhaps
also to the more or less complete coalescence of the fixed and free
cheeks, since in no specimen has the free cheek been found separated
from the rest of the head, as occurs not uncommonly in many other
Trilobites. The probability of the existence of a suture receives some
support from the fact that one is found in the allied genera
_Orometopus_ and _Ampyx_ (Fig. 140). Barrande and Oehlert deny its
existence in _Trinucleus_. There is, however, in that genus a suture
running close to the margin of the cephalic border,[182] and joining the
genal angle so as to cut off the genal spine. Lovén and Oehlert claim
that this suture represents the facial suture, but in an abnormal
position; this view, however, is not accepted by Beyrich. In this
connection it should be noted that in _Acidaspis_, whilst the majority
of the species possess a facial suture, there are two in which it has
disappeared owing to the fusion of the fixed and free cheeks. Such being
the case, it seems not improbable that the curved line passing backwards
from the eye in _Harpes_ may mark the position of the suture; but it is
stated that the only suture present in that form runs at the margin of
the cephalic border, and is similar to that of _Trinucleus_. This matter
will be referred to again when discussing the nature of the eyes in
_Trinucleus_ and _Harpes_.

The relative sizes of the fixed and free cheeks obviously depend on the
position of the facial suture; when this starts on the lateral margin of
the cephalic shield and passes forward to the outer part of the front
margin, the free cheek will be a narrow strip; when, on the other hand,
the suture starts from the posterior margin and runs close to the
glabella, the free cheek will be relatively large and the fixed cheek
narrow. The fixed cheek is small in _Phacops_, _Cheirurus_, and
_Illaenus_; relatively large in _Remopleurides_, _Phillipsia_, and
_Stygina_. It was suggested by M‘Coy[183] that the free cheek represents
the pleura of an anterior segment which has not become fused with the
other cephalic pleurae. The fixed cheek appears to be formed of the
coalesced pleurae of the other cephalic segments, but of those pleurae
the only indication seen in adult specimens is in the neck-ring; in
young specimens of _Olenellus_, however, the presence of other pleurae
is indicated by furrows on the cheeks in front of the neck-furrow.

[Illustration:

  FIG. 138.—_Phacops latifrons_, Bronn, × 1. Devonian. Showing large
    compound eye. (After Zittel.)
]

A pair of compound =eyes= are present in the majority of Trilobites.
Each eye is situated on the free cheek, at that part of its inner margin
where the facial suture bends to form an angle (Figs. 137, A, _h_, 138).
The position of the eye is consequently determined by the position of
the facial suture; it may be near the glabella or near the lateral
margin of the head, and either as far forward as the first segment of
the glabella or nearly as far back as the neck-furrow. In many
Trilobites the eye is more or less conical, with its summit truncated or
rounded, but in some genera it is ovoid, or crescentic. In _Aeglina_
(Fig. 150, H) the eye is flattened and scarcely raised above the general
level of the cheek. The eye of a Trilobite is oriented so that its
longer axis is parallel or nearly parallel to the axis of the body (Fig.
150, G); but in one case (_Encrinurus intercostatus_) it is placed at
right angles to this axis. The size of the eye varies considerably; it
is largest in _Aeglina_, in which it covers nearly the whole of the free
cheek; it is small in _Acidaspis_ and _Encrinurus_.

Though the eye is always entirely on the free cheek, the adjoining part
of the fixed cheek is raised to form a buttress on which the eye rests;
this buttress, which is known as the “palpebral lobe,” is seen clearly
when the fixed cheek is removed. The eyes of Trilobites are always
sessile; for although in some species, such as _Asaphus cornigerus_, _A.
kowalewskii_, and _Encrinurus punctatus_, they are on the summits of
prominent stalks, yet those stalks are immovable.

Three types of compound eye have been recognised in Trilobites[184]—
holochroal, prismatic, and schizochroal.

[Illustration:

  FIG. 139.—Eyes of Trilobites. (After Lindström.) =A=, =B=,
    _Sphaerophthalmus alatus_, Ang. Upper Cambrian. Vertical and
    horizontal sections, × 100. =C=, _Asaphus fallax_, Dalm. Horizontal
    section, × 60. =D=, _Nileus armadillo_, Dalm. Vertical section, ×
    60, _a_, prismatic lenses; _b_, cuticle; _c_, part of free cheek.
    =E=, _Dalmanites vulgaris_, Salt. Part of eye, × 30. =F=,
    _Dalmanites imbricatulus_, Ang. Vertical section of eye, with a part
    of the free cheek on the left, × 60. =G=, =H=, _Harpes vittatus_,
    Barr. =G=, The two lenses of one eye, × 8; =H=, vertical section of
    the same, × 60.
]

1. In the holochroal eye (Fig. 139, A, B) the lenses are globular or
biconvex and close together, so that the cornea is continuous over the
entire eye. Examples of this are seen in _Bronteus_ and
_Sphaerophthalmus_.

2. In the prismatic type (Fig. 139, C, D) the lenses are prismatic and
plano-convex, and the entire surface of the eye is covered by a smooth
cuticle. The lenses are close together and usually hexagonal, but
occasionally rhombic or square. Near the margin of the eye the lenses
may become irregular, giving rise to a border in which the prismatic
structure is more or less indistinct. The prismatic type of eye is found
in the genera _Asaphus_, _Nileus_, _Illaenus_, etc.

3. The schizochroal eye occurs only in the family Phacopidae (Fig. 139,
E, F). The lenses are biconvex and are separated by portions of the
cephalic shield, so that each lens appears to rest in a separate socket,
and the cornea is not continuous for the entire eye. The lenses are
circular in outline, but owing to the upward and inward growth of the
interstitial test they may appear, on the surface, to be hexagonal. The
diameter of a lens may be as much as 0·5 mm. The crystalline cones have
not been preserved. In specimens of _Phacops rana_, in which the inner
face of the lens is more convex than the outer, J. M. Clarke[185] has
obtained evidence of a posterior spheroidal cavity in addition to the
anterior corneal cavity. The complete separation of the lenses in this
type of eye has led to the suggestion that the schizochroal eye is an
aggregate rather than a compound eye. But the difference between this
and the holochroal eye is probably less than appears at first sight if
the statement made by Clarke is confirmed, namely, that in young
specimens of _Calymene senaria_ the lenses are relatively large and
similar to those of _Phacops_, whereas in the adult the eye is
holochroal.

These three types of eye, according to Lindström, have appeared
successively in chronological order: the prismatic occurring first in
the _Olenus_ beds (Upper Cambrian), the holochroal first in the
_Ceratopyge_ Limestone (Uppermost Cambrian), and the schizochroal first
in the Ordovician. The number of lenses in the eye varies greatly. For
example, in _Trimerocephalus volborthi_ there are 14 only, whilst in
_Remopleurides radians_ there are as many as 15,000. Even in different
species of the same genus there may be considerable differences. Thus
_Bronteus brongniarti_ possesses 1000, _B. palifer_ 4000, lenses in each
eye. The number increases from the young up to the adult, but decreases
in old age. The lenses are usually arranged in alternating rows. In
Trilobites with a conical eye the outer segment of the cone bears the
visual surface. It has been stated that the eyes of Trilobites resemble
those of Isopods,[186] but close comparison is difficult to make, since
in Trilobites no part of the eye beneath the lenses is preserved. In
some genera a threadlike ridge, called the “eye-line,” passes from the
glabella, generally from the front segment, to the eye, where it often
ends in the palpebral lobe; this eye-line is found in nearly all genera
which are confined to the Cambrian period, and persists in a few of
later date, as for example in _Triarthrus_, _Euloma_, and some species
of _Calymene_ from the Ordovician; in _Arethusina_ and _Acidaspis_ from
the Silurian; and in _Harpes_ from the Devonian (Fig. 150, A).

[Illustration:

  FIG. 140.—Trinucleidae. =A=, _Orometopus elatifrons_, Ang. × 5.
    Restoration based on specimens from the Upper Cambrian (Tremadoc) of
    Shineton, Shropshire. =B=, _Trinucleus bucklandi_, Barr. Ordovician,
    Bohemia. A complete but not fully-grown individual showing eyes.
    Natural size. (After Barrande.) =C=, _Ampyx rouaulti_, Barr. × 3.
    Ordovician, Bohemia. (After Barrande.)
]

In _Harpes_ and in some species of _Trinucleus_ eyes are present, but
have been stated to be of a different type. They are described as simple
eyes, and have been compared with ocelli; they are never found in
Trilobites which possess the compound eyes described above. In _Harpes_
(Fig. 150, A) the eye is near the middle of the cheek, in the position
where compound eyes occur in other genera; it appears to consist of two
or three granules or tubercles which are really lenses, and is connected
with the front of the glabella by an eye-line. No facial suture can be
seen, consequently the whole of the cheek is stated to be the fixed
cheek.[187] In _Trinucleus_ (Fig. 140, B) a single tubercle is found on
the middle of the cheek in the young of some species, and is sometimes
connected with the glabella by an eye-line; the latter disappears before
the adult state is reached, and in some species the tubercle also
disappears, but in others (such as _T. seticornis_, _T. bucklandi_) it
persists in the adult individuals.

From the lateral position of these eyes they can hardly be compared with
the median simple eye of other Crustacea. In _Harpes_ it is more
probable that, as suggested by J. M. Clarke, they are schizochroal eyes
imperfectly developed. Their structure (Fig. 139, G, H) is somewhat
similar to that of schizochroal eyes, and moreover, in one species, _H.
macrocephalus_,[188] there are, in addition to the three main tubercles,
other smaller tubercles in regular rows. Further, the eye-line occupies
the same position as in other Trilobites which have undoubted compound
eyes. The absence of a facial suture cannot be taken as evidence against
these eyes being of the ordinary type, since in some species of
_Acidaspis_ (e.g. _A. verneuili_, _A. vesiculosa_) which possess
compound eyes there is, in consequence of the coalescence of the fixed
and free cheeks, no suture.

In some species of _Trinucleus_ (Fig. 140, B) the simple eye is found in
the same position as the eye in _Harpes_, and if, as some writers have
maintained, there is evidence of the existence of a suture in that
genus, then there is no reason for regarding the eye as other than a
degenerate form of compound eye. The probability of its being such is
supported by the existence of a compound eye in a similar position in
the allied form _Orometopus_ (Fig. 140, A) which possesses a facial
suture.

In some species of _Trinucleus_ (Fig. 140, B) and _Ampyx_ there is a
small median tubercle on the front part of the glabella, which from its
position may be a simple unpaired eye, but its structure appears to be
unknown.

Some Trilobites possess no eyes. Well-known examples of such are
_Agnostus_, _Microdiscus_, _Ampyx_, _Conocoryphe_, and some species of
_Illaenus_ and _Trinucleus_; such blind Trilobites are almost confined
to the Cambrian and Ordovician periods. All the forms of later periods,
with the exception of a species of _Ampyx_, and possibly one or two
other species, possess eyes. In addition to those undoubtedly blind
forms Lindström considers that most of the Olenidae and Paradoxidae were
without eyes. Many of the members of these families possess a lobe
closely resembling a palpebral lobe, and a corresponding excavation in
the free cheek; such forms have been generally regarded as possessing
eyes; and the absence of any indication of lenses in those cases, on
which Lindström lays stress, has been explained by the comparatively
imperfect preservation of these early Trilobites. The development of the
supposed eye-lobe in some of the Paradoxidae and Olenidae differs from
that of the eyes in other families of Trilobites. In the latter the eye
appears first at the margin of the head and always in connexion with the
facial suture. But in _Olenellus_, in which there is said to be no
facial suture, development shows that the crescentic eye-like lobe (Fig.
145, E) is really of the nature of a pleura coming off from the base of
the first segment of the glabella. In _Paradoxides_, which resembles
_Olenellus_ in many respects, a facial suture is present and forms the
outer boundary of the eye-like lobe, but it is developed subsequently to
the appearance of the latter, which seems to be similar to that of
_Olenellus_. In some genera of the Olenidae the eye-line, which comes
off from the first segment of the glabella, ends in some cases in a
swelling or knob which has hitherto been regarded as a palpebral lobe,
but according to Lindström’s view no trace of an eye has been found in
connexion with that lobe, nor is there any space between the lobe and
the free cheek in which the eye could have occurred. If this view is
correct it follows that the majority of the Cambrian Trilobites were
blind. The earliest genus with eyes would then be _Eurycare_ found in
the _Olenus_ beds of the Upper Cambrian. _Sphaerophthalmus_ and
_Ctenopyge_, found in the higher beds of the Cambrian, also possessed
eyes, but _Olenus_ and _Parabolina_ were probably blind.

On the ventral surface of the head there is a flat rim around the margin
(Fig. 137, B, _b_); this rim or “doublure” is the reflexed border of the
cephalic shield. In many Trilobites its median part in front is cut off
by sutures so as to form a separate plate (_e_); such is the case when
the two facial sutures (_c_, _c′_) cut the anterior margin of the
cephalic shield and are continued across the doublure, where they are
joined by a transverse or rostral suture (_d_) just below the margin.
When, however, as in _Phacops_ and _Remopleurides_, the two facial
sutures unite on the dorsal surface, in front of the glabella, the
median part of the doublure is not separated from the lateral parts, or
from the dorsal part of the cephalic shield.

[Illustration:

  FIG. 141.—=A=, Hypostome of _Bronteus polyactin_, Ang. showing
    maculae, × 4. =B=, Left macula of _Bronteus irradians_, Lindst. ×
    12. (After Lindström.)
]

The “labrum” or “hypostome” is attached to the doublure in front (Fig.
137, B, _a_); it is commonly an oval or shield-shaped plate, but is
occasionally nearly square. Its surface is sometimes divided into two or
three areas by shallow transverse grooves (Fig. 141, A), Just behind the
middle of the hypostome, or when transverse grooves are present either
in or near the anterior groove, there are often found a pair of small
patches or “maculae” which are more or less oval or elliptical in
outline (Fig. 141). The maculae may be (1) surrounded by a raised
border, or (2) in the form of pits, or (3) raised like tubercles. In
some cases the entire surface of a macula is smooth and glossy; in
others either the whole or a part is covered with granules, and in the
latter case the granules may be limited to the internal third (Fig. 141,
B) or to the central portion. Sections of a macula show that the
granules are really globular lenses similar to those of the compound
eyes on the dorsal surface of the head. Some of the maculae which are
without lenses show no structure, but in others there is a spongy or
irregularly polyhedric structure with prisms, resembling the marginal
zone of the prismatic eyes of some genera. There seems no doubt that the
maculae with lenses are visual organs, and those without are degenerate
eyes. They occur in some genera which, according to Lindström, are
without eyes on the dorsal surface. Maculae do not appear to be present
in other Crustacea, but they have been compared with a median organ,
found just in front of the hypostome in _Branchipus_.[189] Maculae, have
so far been found in 136 species of Trilobites belonging to 39 genera
ranging from Lower Cambrian to Carboniferous.

A “metastoma” or lower lip plate (Fig. 142, _Ep_) is found just behind
the hypostome in _Triarthrus_, but has not been noticed in any other
genus. Between the hypostome and the metastoma lies the mouth.

The segments of the =thorax= are free, and their number varies from two
in _Agnostus_ (Fig. 146) to twenty-six in _Harpes_ (Fig. 150, A). In the
Trilobites confined to the Cambrian period the number (except in the
Agnostidae) is usually larger than in the genera found in the Ordovician
and later periods. Owing to the free thoracic segments many Trilobites
were able to curl up somewhat after the manner of a Wood-louse (Figs.
137, D, 138). The axial part of each thoracic segment is more or less
considerably arched. Usually it consists of three parts: (i.) the
largest part (Fig. 137, C, _a_), called the ring, is band-like in form,
and is always visible whether the Trilobite is extended or coiled up;
(ii.) in front of the ring is a depressed, groove-like part (Fig. 137,
C, _b_) separating it from (iii.) the articular portion (_c_) which is
convex in front and extends beneath the ring of the preceding segment;
this part is only visible when the Trilobite is coiled up or when the
segments are separated. In some few genera the axial part consists of a
simple arched band without either a groove or a specially modified
articular portion. The pleurae (Fig. 137, A, _l_, C, _d-f_) are fixed
firmly to the axis, and have the form of narrow bands with the ends
rounded, obtuse, pointed, or spinose. In a few cases the pleurae have a
plain surface; but usually they possess either a ridge or a groove (Fig.
137, C, _g_); the former is generally parallel to the margins of the
pleura, the latter is generally oblique, being inclined backwards from
the axis. Sometimes in front of the ridge there is a small groove. On
the ventral surface each pleura shows, at its outer extremity, a
reflexed margin or doublure. At some distance from the axis the pleurae
are bent downwards and backwards. The point where this bend occurs is
called the “fulcrum” (_e_); it divides the pleura into an internal and
an external part: the internal part (_d-e_) is flat or slightly convex,
and just touches the front and back margins of the adjacent pleurae; the
external part (_e-f_) may be (i.) narrower than the internal part, so
that it is separated from the previous and succeeding pleurae; such
occurs principally in pleurae with ridges, as in _Cheirurus_ and
_Bronteus_; or (ii.) it may be in the form of a long cylindrical
process, as in many species of _Acidaspis_; or (iii.) the external part
may be of the same width, either throughout or in part, as the internal
part, and may overlap the next pleura behind; this type is found
principally in pleurae with a groove such as in _Phacops_, _Calymene_,
_Sao_, _Asaphus_, _Ellipsocephalus_.

In some Trilobites there is beyond the fulcrum a smooth, flat,
triangular part at the front margin of the pleura; this part is known as
the “facet,” and forms a surface articulating with the preceding segment
which overlaps it.

In the remarkable form _Deiphon_ (Fig. 151, E) the pleurae are separate
throughout their entire length.

In some Trilobites broad and narrow forms of the same species occur—the
difference being seen especially in the axis. The former are regarded as
females, the latter as males.[190]

The segments of the =abdomen= or =pygidium= (Fig. 137, A, 3) are similar
to those of the thorax, except that they are fused together. In a few
forms, such as _Illaenus_ (Fig. 150, F) and _Bumastus_, the fusion is so
complete that no trace of segmentation can be seen on the dorsal
surface. Usually, however, the segments are easily distinguishable; the
number seen on the axis is commonly greater than on the lateral parts of
the pygidium; this difference is particularly well shown in
_Encrinurus_. In Trilobites which have grooved pleurae the conspicuous
grooves seen on the lateral parts of the pygidium are the grooves of the
pleurae, the sutures _between_ the pleurae being less distinct. The
shape of the pygidium may be semicircular, a segment of a circle,
trapezoidal, triangular, semi-parabolic, etc.; its size varies
considerably; in the Cambrian forms it is usually small, but in the
Trilobites of later periods it becomes relatively larger. The number of
segments in the pygidium varies from two to twenty-eight. The axis of
the pygidium tapers more rapidly than that of the thorax; sometimes it
reaches quite to the posterior end of the body, but is commonly shorter
than the pygidium; in _Bronteus_ it is extremely short, and the grooves
on the lateral parts of the pygidium radiate from it in a fan-like
manner. Occasionally, as in _Bumastus_, the axis cannot be distinguished
from the lateral parts. In a few early Trilobites (_Olenellus_,
_Holmia_, Fig. 148, _Paradoxides_, Fig. 147) the lateral parts of the
pygidium are very small. In some genera, such as _Asaphus_, the marginal
part of the pygidium forms a flattened or concave border. The margin may
be entire or produced into spines, and sometimes (Fig. 151, C) a caudal
spine comes off from the end of the axis. On the ventral surface of the
pygidium there is a marginal rim similar to the doublure of the cephalic
shield. The anus is on the ventral surface of the last segment of the
pygidium.

Although Trilobites are often found in abundance and in an excellent
state of preservation, it is only in very rare cases that anything is
seen of the ventral surface except the hypostome and the reflexed
borders of the cephalic shield, of the thoracic segments, and of the
pygidium. The usual absence of =appendages= is probably due to their
tenuity. Billings, in 1870, first obtained clear evidence of the
presence of pairs of appendages, in _Asaphus platycephalus_. Soon
afterwards Walcott[191] showed their existence in American specimens of
_Asaphus megistos_, _Calymene senaria_, and _Cheirurus pleurexacanthus_.
In the two latter species the appendages were found by cutting sections
of curled-up specimens obtained from the Trenton Limestone; 2200
examples were sliced, of which 270 showed evidence of the existence of
appendages. They were seen to be present on the head, thorax, and
pygidium; a ventral uncalcified cuticle with transverse arches was also
found. By means of sections of curled-up specimens it was difficult to
determine satisfactorily the form and position of the appendages.
Subsequently extended specimens of _Triarthrus_ (Fig. 142) and
_Trinucleus_, showing the ventral surface and appendages clearly, were
discovered in the Utica Slate (Ordovician) near Rome, New York. A full
account of the appendages in those specimens has been given by
Beecher.[192]

[Illustration:

  FIG. 142.—_Triarthrus becki_, Green, × 2½. Utica Slate (Ordovician),
    near Rome, New York. =A=, Ventral surface with appendages; _Ep_,
    metastome; _Hy_, hypostome. =B=, second thoracic appendage; _en_,
    endopodite; _ex_, exopodite, × 12. (After Beecher.)
]

In _Triarthrus_ each segment, except the anal, bears a pair of
appendages, all of which, except the first, are biramous. There are five
pairs of cephalic appendages; the first pair are attached at each side
of the hypostome, and have the structure of antennae, each consisting of
a single flagellum formed of short conical joints. The other cephalic
appendages increase in size successively. At present the second and
third pairs are not satisfactorily known, but appear to have been
similar to the fourth and fifth pairs. The second pair is attached at
the level of the posterior end of the hypostome. The fourth and fifth
pairs have large, triangular coxopodites which served as gnathobases,
their inner edges being denticulate; the endopodites consist of stout
joints; the exopodites are slender, and bear setae which are often
arranged in a fan-like manner.

The first pair of appendages appear to be antennules, whilst the second
pair probably represent the antennae, the third pair the mandibles, and
the fourth and fifth pairs the maxillae of other Crustacea. The
appendages of the thorax and pygidium do not differ essentially from the
two posterior cephalic appendages. Those on the anterior part of the
thorax are the longest; the others gradually decrease in size in passing
posteriorly. Each thoracic leg (Fig. 142, B) consists of a short
coxopodite with an inward cylindrical prolongation forming a gnathobase
which is best developed on the anterior legs; the endopodite and
exopodite are long and nearly equal; the former consists of six joints
tapering gradually to the end; the latter, of a long proximal joint with
a denticulate edge and a distal part of ten or more joints, and it bears
a row of setae along the whole of the posterior edge.

The anterior appendages of the pygidium differ but little from the
posterior thoracic legs; but the phyllopodous character, which appears
in the latter, becomes more distinct in the appendages of the pygidium,
especially those near its posterior end, and is due to the broad, flat,
laminar joints of the endopodite.

The more striking features of the appendages of _Triarthrus_ are the
small amount of differentiation which they show in different parts of
the body, especially the want of specialisation in the cephalic region;
the distinctly biramous character of all except the first pair; and the
presence of one pair of functional antennae only, and the occurrence of
thoracic gnathobases.

In _Trinucleus_ the appendages are not so well known, but they are
considerably shorter than in _Triarthrus_.

In the Palaeozoic rocks of Bohemia, where Trilobites are very perfectly
preserved, Barrande[193] discovered the larval forms of several species,
and in some cases was able to trace out the =development= very
completely, but in others the earliest stages were not found. In the
strata in which Trilobites occur Barrande found minute spheroidal
bodies, usually of a black colour, and only a little smaller than the
youngest larval stages; those bodies are probably the eggs of
Trilobites. Since the publication of Barrande’s work the development of
some species found in North America has been studied by Ford, Matthew,
Walcott, and Beecher. But even now the development is known in only a
very small proportion of the total number of genera of Trilobites. The
early larval form (Fig. 143, A) is similar in general character in the
various species in which it has been found. It is circular or ovoid in
outline, with a length of from 0·4 to 1 mm., and consists of a large
cephalic and a small pygidial portion; the axis is distinct and usually
shows more or less clear indications of five cephalic segments; the
eyes, when present, are found at or near the front margin, and the free
cheeks, if visible at all on the dorsal surface, are narrow. For this
early larval form Beecher has proposed the name “protaspis”; he regards
it as the representative of the Nauplius of other Crustacea, but that
view is not accepted by Professor J. S. Kingsley.[194]

The general changes which occur in the course of development are:
modifications in the shape and relative size of the glabella, and of the
number and depth of the glabella-furrows; the growth of the free cheeks
and the consequent inward movement of the facial sutures and eyes; the
introduction of and gradual increase in number of the thoracic segments,
and the relative decrease in size of the head.

[Illustration:

  FIG. 143.—Development of _Sao hirsuta_, Barr. Cambrian. =A=,
    Protaspis; =B-F=, later stages; =G=, adult. The small outlines below
    each figure show the actual size of each specimen. (After Barrande.)
]

_Sao hirsuta_ is a species found in the Cambrian, the development of
which was fully described by Barrande. Its earliest protaspis stage
(Fig. 143, A) is circular in outline; the glabella expands in front and
reaches the anterior margin; the pygidial region is not distinctly
separated from the cephalic region; segmentation is indicated in the
former, and the neck-ring is present in the latter; the eye-line is seen
on each side of the glabella near the anterior margin. In a later stage
(Fig. 143, C) the segmentation of the glabella becomes more distinct,
indicating the existence of five cephalic segments, and the facial
suture appears near the margin limiting a very narrow free cheek.
Subsequently (Fig. 143, D-F) the thoracic segments develop, and increase
in number until the adult stage (G) is reached; also the eyes appear at
the margin of the cephalic shield, and gradually move inwards, and the
glabella becomes narrower and rounded in front, and ceases to reach the
anterior margin. In this species the eye-line is present in the adult.

[Illustration:

  FIG. 144.—_Triarthrus becki_, Green. Ordovician. =A=, =B=, Two
    successive stages of the protaspis, × 45. (After Beecher.)
]

In the protaspis of _Triarthrus_ (Fig. 144), found in the Ordovician,
the glabella does not reach the front margin nor expand in front as it
does in _Sao_; an eye-line is present, but disappears before the adult
stage is reached.

[Illustration:

  FIG. 145.—Larval stages of Trilobites. =A-D=, _Dalmanites socialis_,
    Barr. Ordovician, Bohemia. The small figures below show the natural
    size of each specimen. (After Barrande.) =E=, _Mesonacis
    asaphoides_, Emmons, × 10. Lower Cambrian, North America. (After
    Walcott.) =F=, _Acidaspis tuberculata_, Conrad, × 20. Lower
    Helderberg Group (Lower Devonian or Upper Silurian), Albany County.
    (After Beecher.)
]

_Dalmanites_ (Fig. 151, C) is a more advanced type than _Sao_ and
_Triarthrus_, and is found in later deposits. In the earliest stage
(Fig. 145, A) the head and pygidium are quite distinct, and there is no
eye-line present at this or any stage in development, but large ovoid
eyes are found on the front margin, and have their long axes placed
transversely to the axis of the body; the glabella is strongly segmented
and is rounded in front. In later stages (C, D) the pygidium increases
in size relatively, and the thoracic segments are successively
introduced; the facial sutures and free cheeks appear on the dorsal
surface, and as the free cheeks grow the eyes move inwards and
backwards, and gradually swing round until their long axes become
parallel with the axis of the body.

The larval form of _Acidaspis_ (Fig. 145, F) is of interest since even
in the earliest stage it shows the spiny character which forms such a
striking feature of the adult (Fig. 151, F).

Before the discovery of the ventral surface of Trilobites it was thought
by some zoologists that their =affinities= were with the Xiphosura
rather than with the Crustacea. But the presence of antennae, and of
five pairs of cephalic appendages; the biramous thoracic and pygidial
appendages, the hypostome, and the character of the larval form, as well
as the absence of a genital operculum, separate the Trilobites from the
Xiphosura and connect them with the Crustacea.

The position of the Trilobites in the Crustacea is, however, difficult
to determine. Already in the Cambrian period, at least five main groups
of the Crustacea were clearly differentiated, namely, the Phyllopoda,
Ostracoda, Cirripedia, Trilobita, and Leptostraca (Phyllocarida), and
probably also the Copepoda, but of the last no remains have been
preserved as fossils. Palaeontology, therefore, furnishes no connecting
links between any two of these orders.

The Crustacea to which the Trilobites show some resemblance are the
families Apodidae and Branchipodidae of the Order Phyllopoda (see pp.
19–36). The Trilobita agree with those families in having a large but
variable number of trunk-segments, in the possession of a large labrum
(hypostome), and in the occurrence of gnathobases on the thoracic
appendages; also the foliation of some of the trunk-appendages is
somewhat similar. The points of difference, however, are considerable;
thus the cephalic appendages are much more specialised in the Apodidae
and Branchipodidae than in the Trilobita; in the latter all, with the
exception of the antennae, are distinctly biramous, and whilst the basal
joints were masticatory the distal parts appear to have been locomotor
organs. The appendages of the trunk also differ considerably; in the
Trilobita all are clearly biramous, those of the thorax having a
schizopodal form. In the possession of a single pair of antennae the
Trilobita differ from other Crustacea; but in some forms of _Apus_ the
second pair of antennae may be rudimentary or even absent.

There are still other features which characterise the Trilobita: thus
the eyes are borne on free cheeks, and differ in structure from those of
Phyllopods. The broad pygidium formed of fused segments and without
terminal fulcra is quite unlike the slender-jointed abdomen of _Apus_
and _Branchipus_; and whilst in the Trilobites all the segments bear
appendages, in the Phyllopods some, at any rate, of the posterior
segments are devoid of appendages. The distinct division of the body
into an axial and pleural region is not seen in Phyllopods, and is
probably a character of some importance, since it occurs in the great
majority of Trilobites, including all the early forms.

The existence of some relationship between the Trilobita and the
Leptostraca (Phyllocarida) has been maintained by Professor G. H.
Carpenter.[195] He points out that some of the earliest Trilobites, such
as _Holmia kjerulfi_ (Fig. 148), possess nearly the same number of
segments as _Nebalia_ (Fig. 76, p. 111), and that in the latter genus
the cephalic appendages, especially the mandibles and maxillae, are less
specialised than in _Apus_, and consequently differ less from those of
Trilobites than do the appendages of the Apodidae. Further, in another
genus of the Leptostraca, _Paranebalia_, the biramous thoracic legs, in
which both endopodite and exopodite are elongate, approach those of
Trilobites more nearly than do the thoracic legs of _Apus_.

The view[196] that some connexion may exist between the Isopoda and the
Trilobita seems to have been based on the similar dorso-ventral
flattening of the body, its division into three regions—head, thorax,
and abdomen—and the presence of sessile eyes. Beyond this it is
difficult to find any resemblance; whilst the differences, such as the
variable number of thoracic segments and their biramous appendages in
Trilobites, are important.

At present, then, we can only conclude that the Trilobita are more
primitive than any other Crustacea, and that their resemblance to some
of the Phyllopoda is sufficient to make it probable that they had some
ancestral connexion;[197] the possibility of such a relationship
receives some support from the presence in the Lower Cambrian rocks of
_Protocaris_, a genus of the Phyllopoda which resembles _Apus_.[198] The
primitive characters of Trilobites are the variable and often large
number of segments in the thorax and pygidium; the presence of a pair of
appendages on every segment except the anal; the biramous form of all
except the first pair of appendages; and the lack of specialisation
shown by the appendages, especially those of the head.

The =classification= of Trilobites is due largely to the work of
Barrande and Salter, and the families defined by those authors have
been, in the main, generally adopted. But the phylogenetic relationship
of the families has still, to a large extent, to be established.
Salter[199] arranged the families in four groups, but did not claim that
that classification was entirely natural. His groups with the families
included in each are:—

1. _Agnostini._ Without eyes or facial suture. Agnostidae.

2. _Ampycini._ Facial sutures obscure, or submarginal, or absent. Eyes
often absent. Trinucleidae.

3. _Asaphini._ Facial sutures ending on the posterior margin.
Acidaspidae, Lichadidae, Harpedidae, Calymenidae, Paradoxidae,
Conocephalidae, Olenidae, Asaphidae, Bronteidae, and Proëtidae.

4. _Phacopini._ Facial sutures ending on the lateral margins. Eyes well
developed. Phacopidae, Cheiruridae, and Encrinuridae.

A modification of Salter’s classification has been brought forward by
Beecher[200] who divides the Trilobita into three main groups:—

1. _Hypoparia._ Facial sutures at or near the margin, or ventral.
Compound eyes absent. This is equivalent to Salter’s Agnostini and
Ampycini with the addition of the Harpedidae.

2. _Opisthoparia._ Facial sutures extending from the posterior margin to
the front margin, but occasionally uniting in front of the glabella.
Eyes holochroal or prismatic, but sometimes absent. This comprises the
same families as Salter’s Asaphini with the exclusion of the Harpedidae
and Calymenidae.

3. _Proparia._ Facial sutures extending from the lateral margins, and
either cutting the anterior margin or uniting in front of the glabella.
Eyes holochroal or schizochroal; occasionally absent. This is equivalent
to Salter’s Phacopini with the addition of the Calymenidae.

In each of the groups proposed Beecher regards as the more primitive
forms those which possess characters similar to those of the early
larval stages, such as narrow free cheeks, the absence of compound eyes,
and a glabella which is broad in front and reaches the anterior margin
of the head.

The modifications introduced by Beecher can scarcely be regarded as
making Salter’s classification more natural. For instance, the
Agnostidae differ so much from all other families that, at present,
there is no evidence to show that they have any close phylogenetic
relationship with the Trinucleidae and Harpedidae. Further, the
Calymenidae, which Salter recognised as related to the Olenidae, have
been shown by the careful work of Professor Pompeckj[201] to have
descended from the latter family, and to have no genetic connexion with
the Phacopidae with which they are grouped by Beecher. Then in the
Trinucleidae the earliest genus, _Orometopus_[202] (Fig. 140, A),
possesses compound eyes and facial sutures which begin at the posterior
margin and unite in front of the glabella; so that, according to
Beecher’s classification, that genus would belong to the Opisthoparia,
whereas the later genera (_Trinucleus_, etc.) of the same family would
be placed in the Hypoparia. At present, therefore, the only
classification of Trilobites which can be adopted is a division into
families, of which a short account is given below.

[Illustration:

  FIG. 146.—_Agnostus integer_, Beyr., × 8. Cambrian. (After Barrande.)
]

=Fam. 1. Agnostidae= (Fig. 146).—Small Trilobites, in which the head and
pygidium are of nearly the same size and shape. The thorax is shorter
than the head or pygidium, and consists of from two to four segments
with grooved pleurae. The length and width of the head are commonly
nearly equal, but sometimes the length is greater. Eyes are absent.
Facial sutures appear to be absent, but are stated by Beecher to be at
the margin of the cephalic shield. From the absence of eyes, the
probable absence of facial sutures, the few or indistinct furrows on the
glabella, and the smaller number of thoracic segments, the Agnostidae
appear to be degenerate forms. _Microdiscus_ is apparently less modified
than _Agnostus_, on account of the larger number of thoracic segments,
the more distinct segmentation of the pygidium, and, in some species,
the larger number of furrows on the glabella. Cambrian and Ordovician.
Genera: _Agnostus_, _Microdiscus_.

=Fam. 2. Shumardiidae.=—The body is very small and oval. The cephalic
shield is nearly semicircular and very convex, with a broad glabella
which expands in front, and in which the furrows, except the
neck-furrow, are indistinct. The facial suture is marginal and eyes are
absent. There are six thoracic segments with ridged pleurae; the axis is
broader than the pleurae. The pygidium is large, and is formed of about
four segments similar to those of the thorax. Upper Cambrian and
Ordovician. Genus: _Shumardia_.

=Fam. 3. Trinucleidae= (Fig. 140).—The head is large and has a flat
border (except in _Ampyx_), and long genal spines. In the earliest genus
(_Orometopus_) the facial sutures start from the posterior margin (near
the genal angle) and pass obliquely inwards to the compound eye, from
whence they continue forward and unite in front of the glabella. In
_Ampyx_ the suture starts from just within the genal angle and passes to
the front border, cutting off a narrow free cheek; eyes are absent. In
most specimens of _Trinucleus_ no sutures are seen, but some examples
show indications of what may be a facial suture (see p. 226), and a
suture is sometimes found at the margin of the cephalic border; eyes may
occur (see p. 230). The thorax consists of from five to eight segments,
with grooved pleurae. The pygidium is triangular. Principally
Ordovician. Genera: _Orometopus_ (Upper Cambrian), _Ampyx_,
_Trinucleus_, _Dionide_.

=Fam. 4. Harpedidae= (Figs. 139, G, H; 150, A).—The head is large and
has a broad, flat border which is finely punctate, and extends backwards
on each side in the form of a horn-like projection nearly as far as the
posterior end of the thorax. The glabella is convex and does not reach
the front margin. The cheeks are less convex than the glabella, and bear
eyes which usually consist of two or three lenses. An eye-line connects
the eye with the anterior part of the glabella. A suture is stated to
occur at the external margin of the flat border. The thorax consists of
from twenty-five to twenty-nine segments; its axis is narrow, and the
pleurae are long and grooved. The pygidium is very small, and consists
of three or four segments. Ordovician to Devonian. Genus: _Harpes_.

[Illustration:

  FIG. 147.—_Paradoxides bohemicus_, Barr. × ½. Middle Cambrian. (After
    Zittel.)
]

[Illustration:

  FIG. 148.—_Holmia kjerulfi_, Linnars. × 1. Lower Cambrian. (After
    Holm.)
]

=Fam. 5. Paradoxidae= (Figs. 147, 148, 149).—The cephalic shield is
large, and bears long genal spines. The glabella is more or less swollen
in front. The facial sutures appear to be absent in some genera, and
when present extend from the posterior to the anterior margin. The
palpebral lobes are long, and often more or less semicircular or
kidney-shaped. The thorax is long, and consists of from sixteen to
twenty segments with their pleurae produced into spines. The pygidium is
very small, and plate-like, or sometimes in the form of a long spine.
Cambrian. Genera: _Olenellus_, _Holmia_, _Mesonacis_, _Olenelloides_,
_Paradoxides_, _Zacanthoides_, _Centropleura_ (_Anopolenus_).
_Remopleurides_ (Fig. 150, D) from the Ordovician is usually included in
the Paradoxidae, but probably belongs to a separate family.

[Illustration:

  FIG. 149.—_Clenelloides armatus_, Peach. Lower Cambrian, × 3. (After
    Peach.)
]

=Fam. 6. Conocephalidae (Conocoryphidae)= (Fig. 150, E).—The cephalic
shield is semicircular, and larger than the pygidium. The glabella
narrows in front. The facial suture passes from near the genal angle on
the posterior border to the antero-lateral margin, and limits a large
fixed cheek and a narrow free cheek. Eyes are absent or rudimentary, but
an eye-line is usually present. The thorax consists of from fourteen to
seventeen segments with grooved pleurae, which may be pointed, but are
not usually produced into spines. The pygidium is small, and formed of
few segments. Cambrian. Genera: _Conocoryphe_, _Atops_, _Ctenocephalus_,
_Bathynotus_.

=Fam. 7. Olenidae= (Figs. 142, 143; 150, B, C).—The cephalic shield is
larger than the pygidium. The glabella is either rectangular or
parabolic. The facial suture passes from the posterior to the anterior
margin. The palpebral lobes are of moderate or rather large size, and
are connected by an eye-line with the front part of the glabella. The
thorax includes from eleven (occasionally fewer) to eighteen segments
with grooved pleurae. The pygidium is usually small, with from two to
eight segments. Principally Cambrian. Genera: _Ptychoparia_, _Angelina_,
_Solenopleura_, _Sao_, _Agraulos_ (_Arionellus_), _Ellipsocephalus_,
_Protolenus_, _Olenus_, _Peltura_, _Acerocare_, _Eurycare_, _Ctenopyge_,
_Leptoplastus_, _Triarthrus_, _Parabolina_, _Sphaerophthalmus_,
_Parabolinella_, _Ceratopyge_ (position doubtful). _Dikelocephalus_ is
usually placed in the Olenidae, but perhaps belongs to a distinct
family.

=Fam. 8. Calymenidae= (Figs. 136, 137).—The glabella is broadest behind.
The facial suture starts at or near the genal angle—sometimes on the
posterior border just inside the angle, sometimes on the lateral border
just in front of the angle; the suture may be continuous with the other
suture in front of the glabella, or may cut the anterior margin, beneath
which it is connected with the other suture by means of a transverse
suture (Fig. 137, B, D). The eyes are rather small. The thorax consists
of thirteen segments with grooved pleurae; the pygidium of from six to
fourteen segments. Ordovician to Devonian. Genera: _Calymene_,
_Synhomalonotus_, _Homalonotus_.

[Illustration:

  FIG. 150.—=A=, _Harpes ungula_, Sternb., Ordovician. =B=,
    _Ellipsocephalus hoffi_, Scloth., Cambrian. =C=, _Olenus truncatus_,
    Brünn., Cambrian. (After Angelin.) =D=, _Remopleurides radians_,
    Barr., Ordovician. =E=, _Conocoryphe sulzeri_, Barr., Cambrian. =F=,
    _Illaenus dalmanni_, Volb., Ordovician. =G=, _Proëtus bohemicus_,
    Corda, Silurian, × 1½. =H=, _Aeglina prisca_, Barr., Ordovician, ×
    3. =I=, _Phacops sternbergi_, Barr., Devonian. (=A=, =D=, =E=, =G=,
    =H=, =I=, after Barrande; =B=, =F=, from Zittel; natural size except
    =G=, =H=.)
]

=Fam. 9. Asaphidae= (Fig. 150, F).—The body is oval and commonly rather
large. The cephalic shield is large, with its glabella often
indistinctly limited and the glabella-furrows often obscure. The facial
suture starts from the posterior margin and usually cuts the anterior
margin, but is sometimes continued in front of the glabella. The
relative size of the fixed and free cheeks varies greatly. The eyes are
of variable size. The thorax consists of eight or ten (sometimes fewer)
segments; the pleurae are generally grooved, but sometimes plane. The
pygidium is large, often being similar in form and size to the head; it
consists of numerous segments which, however, may be indistinctly shown;
the axis in some forms is obsolete. Upper Cambrian (Tremadoc) to
Silurian; common in the Ordovician. Genera: _Asaphus_ (sub-genera,
_Megalaspis_, _Asaphellus_, _Symphysurus_, etc.), _Ogygia_, _Barrandia_,
_Niobe_, _Nileus_, _Illaenus_, _Bumastus_, _Stygina_. _Aeglina_ (Fig.
150, H) is usually placed in this family, but its systematic position is
doubtful.

=Fam. 10. Bronteidae.=—The general form is similar to that of the
Asaphidae. The glabella broadens rapidly in front, and is marked with
furrows on each side, which are usually short, and may be indistinct.
The facial suture passes from the posterior margin to the crescentic eye
which is situated rather near the posterior border, and from thence to
the anterior margin. There are ten thoracic segments with ridged
pleurae. The pygidium is longer than the head, and has a very short
axis, from which the furrows on the pleural part radiate. Ordovician to
Devonian. Genus: _Bronteus_.

=Fam. 11. Phacopidae= (Figs. 138; 150, I; 151, C).—The head and pygidium
are of about the same size. The glabella is distinctly limited, and
wider in front than behind, with a neck-furrow and three other furrows,
of which some of the anterior may be indistinct or obsolete. The eyes
are schizochroal and usually large. The facial suture begins at the
lateral margin and unites with the suture of the other side in front of
the glabella. There are eleven thoracic segments with grooved pleurae.
The pygidium is usually large, with a distinct axis and many segments.
Ordovician to Devonian. Genera: _Phacops_, _Trimerocephalus_, _Acaste_,
_Pterygometopus_, _Chasmops_, _Dalmanites_, _Cryphaeus_.

[Illustration:

  FIG. 151.—=A=, _Phillipsia gemmulifera_, Phill., Carboniferous. =B=,
    _Arethusina konincki_, Barr., Ordovician. =C=, _Dalmanites
    limulurus_, Green, Silurian. (After Hall.) =D=, _Cheirurus
    insignis_, Beyr., Silurian. =E=, _Deiphon forbesi_, Barr., Silurian.
    =F=, _Acidaspis dufrenoyi_, Barr., Silurian. (=A=, =B=, from Zittel;
    =D=, =E=, =F=, after Barrande; natural size.)
]

=Fam. 12. Cheiruridae= (Fig. 151, D, E).—The glabella is convex or
inflated, and distinctly defined. The facial suture passes from the
lateral to the front margin. The free cheeks are small, and the eyes
usually rather small. There are from nine to eighteen (usually eleven)
thoracic segments; the pleurae have ridges or grooves and free ends. The
pygidium is small, consisting of from three to five segments often
produced into spines. Upper Cambrian to Devonian. Genera: _Cheirurus_,
_Deiphon_, _Placoparia_, _Sphaerexochus_, _Amphion_, _Staurocephalus_.

=Fam. 13. Proëtidae= (Figs. 150, G; 151, A, B).—The body is rather
small, and the head forms about a third of its entire length. The
glabella is sharply defined, and its furrows are sometimes indistinct;
the posterior furrow curves backward to the neck-furrow, thus limiting a
basal lobe on each side of the glabella. The eyes are often large (Fig.
150, G); but in _Arethusina_ (Fig. 151, B), in which an eye-line is
present, they are small. The facial sutures pass from the posterior to
the anterior margin. The free cheeks are large. There are from eight to
twenty-two thoracic segments with grooved pleurae. The pygidium is
usually formed of numerous segments, and its margin is usually entire.
Ordovician to Permian. Genera: _Proëtus_, _Arethusina_, _Cyphaspis_,
_Phillipsia_, _Griffithides_, _Brachymetopus_, _Dechenella_.[203]

=Fam. 14. Encrinuridae.=—The cephalic shield is ornamented with
tubercles. The free cheeks are narrow, and the eyes very small. The
facial suture extends from the lateral margin (or from the genal angle)
to the anterior margin. There are from ten to twelve thoracic segments
with ridged pleurae. On the axis of the pygidium numerous segments are
seen, but usually fewer are indicated on the lateral parts. Ordovician
and Silurian. Genera: _Encrinurus_, _Cybele_, _Dindymene_.

=Fam. 15. Acidaspidae= (Fig. 151, F).—The cephalic shield is broad, with
a spinose margin, genal spines, and sometimes spines on the neck-ring.
The glabella has a longitudinal furrow on each side, due to the backward
bending of the lateral furrows. The facial suture passes from the
posterior border (near the genal angle) to the anterior border. The free
cheeks are large; the eyes small. There are from eight to ten thoracic
segments with ridged pleurae, which are produced into long backwardly
directed spines. The pygidium is short, and is formed of two or three
segments with long spines at the margin. Ordovician to Devonian. Genus:
_Acidaspis_.

=Fam. 16. Lichadidae.=—The body is broad, with a granular dorsal
surface. The cephalic shield is small and short, with spinose genal
angles. The glabella is broad, and its anterior furrows are directed
backwards, limiting a convex median lobe and some lateral lobes. The
facial suture extends from the posterior to the anterior margin. There
are nine or ten thoracic segments with grooved pleurae, which have
pointed ends. The pygidium is large and triangular, with a short axis
and a toothed margin. Ordovician to Devonian. Genus: _Lichas_
(sub-genera, _Arges_, _Dicranogmus_, _Conolichas_, _Ceratolichas_).




                       INTRODUCTION TO ARACHNIDA,
                                  AND
                               XIPHOSURA




                                   BY

                      A. E. SHIPLEY, M.A., F.R.S.

  Fellow of Christ’s College, Cambridge, and Reader in Zoology in the
                               University




                               CHAPTER IX
                         ARACHNIDA—INTRODUCTION


The Arachnida, together with the Crustacea, Insecta, Myriapoda, and
_Peripatus_, make up the great phylum Arthropoda, a phylum which, from
the point of view of numbers of species and of individuals, is the
dominant one on this planet, and from the point of view of intelligence
and power of co-operating in the formation of social communities is
surpassed but by the Vertebrata. The Arachnida form a more diverse class
than the Insecta; they differ, perhaps, as much _inter se_ as do the
Crustacea, and in structure as in size and habit they cover a wide
range.

Lankester in his article upon the Arthropoda, in the tenth edition of
the _Encyclopaedia Britannica_, dwells upon the fact that whereas the
adult _Peripatus_ has but one persisting segment in front of the head,
and its mouth is between the second persisting appendages, in Arachnids
the mouth has receded and lies between the bases of the appendages
(pedipalpi) of the third persisting segment, while two of the persisting
segments, those of the eyes and chelicerae, have passed in front of the
mouth. This process has continued in the Crustacea and in the Insecta;
in both of these classes there are three embryonic segments in front of
the adult mouth, which lies between the appendages of the fourth
segment.

In the larger and more complex Arachnida the number of segments is fixed
and constant, and though possibly no adult member of the group, owing to
the suppression of one or more segments during the ontogeny, ever shows
the full number at any one time, the body can be analysed into
twenty-one segments. It is interesting to note that the same number of
segments occurs in Insecta and in the higher Crustacea.[204] The
significance of this fact is not perhaps apparent, but it seems to
indicate “a sort of general oneness, if I may be allowed to use so
strong an expression,” as Mr. Curdle said when discussing the unities of
the drama with Nicholas Nickleby.

These segments are arranged in higher categories or “tagmata,” of which
we can recognise three: (i.) the prosoma, (ii.) the mesosoma, and (iii.)
the metasoma. The prosoma, sometimes termed the “cephalothorax,”
includes all the segments in front of the genital pore. According to
this definition the prosoma includes the segment which bears the
chilaria in _Limulus_ (the King-crab) and the pregenital but evanescent
segment in Scorpions. The mesosoma begins with the segment bearing the
genital pore, and ends with the last segment which bears free
appendages, six segments in all. The metasoma also consists of six
segments which have no appendages; together with the mesosoma it forms
the abdomen of some writers. The anus lies posteriorly on the last
segment, and behind it comes in the higher forms a post-anal “telson,”
taking in Scorpions the form of the sting, in King-crabs that of the
spine.

As we have seen, it is only in the more typical and perhaps higher forms
that we can find our twenty-one segments, and then they are never
present all at once. In many groups of Arachnids the number is reduced
at the hinder end, and obscured by the fusion of neighbouring segments.
Also segments are dropped as a stitch is dropped when knitting; for
instance, in the rostral segment which has a neuromere, and in the
Spider _Trochosa_ vestigial antennae, or in Scorpions the pregenital
segment.

Primitive Arachnids appear to have lived in the sea and to have breathed
by gill-books borne on appendages; when their descendants took to living
on land and to breathing air instead of water, the gill-books sank into
the body and became lung-books, to which the air was admitted by
slit-like stigmata. In other terrestrial forms the lung-books are
replaced by tracheae which in their structure and arrangement resemble
those of _Peripatus_ rather than those of the Insecta. The circulation,
as is usual in Arthropods, is largely lacunar, but in Scorpions and
_Limulus_ the arteries form definite channels, and are in fact better
developed than in any other Arthropod.

As a rule the alimentary canal in Arachnids is no longer than the
distance between the mouth and the anus; but in the King-crab, where the
mouth is pushed back almost to the centre of the body, there is a
flexure in the median vertical plane. Paired glands, usually called the
liver, open into the mesenteron; food passes into the lumen of these
glands, and is probably digested there. In many Arachnids these glands
extend into the limbs. In those members of the group that have become
terrestrial the nitrogenous excreta are separated out by Malpighian
tubules which open into the proctodaeum; but coxal glands, homologous
with the green gland and shell-glands of Crustacea, may coexist, and in
the aquatic _Limulus_ these alone are found. They usually open on the
base of one or more pairs of walking legs.

The endosternite, or internal skeletal plate to which muscles are
attached, reaches a higher development in the Arachnida than in the
Crustacea. In Scorpions it forms a kind of diaphragm incompletely
separating the cavities of the pro- and meso-soma.

The supra-oesophageal ganglion supplies the two existing segments which
have slipped before the mouth, _i.e._ those of the eyes and of the
chelicerae. The post-oral ganglia in the Acarina, the Pedipalpi, the
Solifugae, and the Araneae have fused into a central nerve-mass, from
which nerves radiate; but in _Limulus_ the prosomatic appendages are all
supplied from the nerve-ring. The chief sense-organs are eyes of the
characteristic Arthropod type, and sensory hairs of a great variety of
complexity. Scorpions and Spiders have stridulating organs, and we may
assume that they have also some auditory apparatus; perhaps some of the
hairs just mentioned act as hearing organs.

Arachnids are male and female; they do not reproduce asexually, and
there is no satisfactory proof that they ever reproduce
parthenogenetically. As a rule there is little external difference
between the two sexes, except in Spiders, where the male is as a rule
smaller than the female, and when adult has the pedipalpi modified for
use in depositing the spermatophores. The ovaries and testes are
annular, and with their ducts encircle the alimentary canal in Mites and
Phalangids; in Scorpions and King-crabs they have become retiform.
Mites, Scorpions, and Pedipalps are viviparous, their eggs developing in
the ovary or in a uterus. Other Arachnids lay eggs, and many Spiders and
Pseudoscorpions carry their eggs about with them. As a rule the young is
but a miniature of the parent, and the Arachnid, unlike the Crustacean
or Insect, undergoes little or no metamorphosis.

A certain number of Mites are parasitic in plants and in animals, and a
few, together with a few Spiders, have resumed the aquatic life of their
remote ancestors. The members of some Orders, such as the Solifugae and
Opiliones, are nocturnal, and many are provided with silk-glands and
weave webs which reach their highest pitch of perfection amongst the
Spiders. At times—especially is this the case with the Mites—enormous
numbers of individuals live together, but they never show the least
adaptation to communal life, and no individuals are ever specialised to
perform certain functions, as is the rule in communities of social
Insects.

With the two exceptions that we regard the Trilobites as more nearly
allied to the Crustacea, and have therefore considered them apart, and
have treated the Pycnogonida independently of the Arachnida, we have
followed Lankester in his classification, though not always in his
nomenclature:—

        Sub-class 1. Delobranchiata[205] (Merostomata).
          Order    (i.) Xiphosura.
          Order   (ii.) Eurypterida (= Gigantostraca, Extinct).

        Sub-class 2. Embolobranchiata.
          Order    (i.) Scorpionidea.
          Order   (ii.) Pedipalpi.
          Order  (iii.) Araneae.
          Order   (iv.) Palpigradi.
          Order    (v.) Solifugae.
          Order   (vi.) Chernetidea (= Pseudoscorpiones).
          Order  (vii.) Podogona.
          Order (viii.) Phalangidea (= Opiliones).
          Order   (ix.) Acarina.

                        APPENDICES

                   (i.) Tardigrada.
                  (ii.) Pentastomida.




                               CHAPTER X
     ARACHNIDA (_CONTINUED_)—DELOBRANCHIATA = MEROSTOMATA—XIPHOSURA


               SUB-CLASS I.—DELOBRANCHIATA = MEROSTOMATA.


                       =Order I. Xiphosura.=[206]

In his recent classification of the Arachnida, Lankester[207] has
grouped the Xiphosura or King-crabs with the extinct Eurypterids or
Gigantostraca under the name of Delobranchiata, better known under the
name Merostomata[208] of Dana. The chief character of this group, and
one which differentiates it from all the animals placed together by
Lankester in the group Embolobranchiata, is that they have gills patent
and exposed. The Xiphosura are, in fact, with the exception of a few
marine Mites, the only Arachnids which now live in the sea as did their
allies the Eurypterids in Palaeozoic times. With a few fresh-water
exceptions, all other Arachnids have taken to life on land, and with a
change from water-breathing to air-breathing came a change in the
respiratory system, the gills becoming “lung-books,” or possibly
tracheae, or disappearing altogether.

A few years ago Pocock re-classified the Xiphosura, and his
classification will be found on pp. 276, 277. It will be noticed that in
his classification the generic name _Limulus_ has disappeared. I have,
however, retained it in this article, partly because I regard the name
as so well established that every one knows what it denotes, and partly
because in a group which contains confessedly very few species,
differing _inter se_ comparatively slightly, it seems unnecessary to
complicate matters with sub-families and new names.

Looked at from above a _Limulus_ presents a horse-shoe-like outline,
from the posterior end of which projects a long spine. It is often
called in America the Horsefoot-crab, but its common or vulgar name is
the King-crab. Across the middle of the body is a joint, and this joint
separates the prosoma from the meso- and metasoma which are in
King-crabs fused together. The prosoma comprises all the segments up to
and including the segment which carries the chilaria;[209] the mesosoma
begins with the segment bearing the genital pores, and ends with the
last segment which bears appendages; the metasoma comprises all the
segments posterior to the last segment which carries appendages. The
prosoma corresponds with the “cephalothorax” of some authors, and the
meso-_plus_ the metasoma are equivalent to their “abdomen.”

Dorsally, then, the prosoma is a vaulted structure with a smooth,
horse-shoe-shaped anterior and lateral margin. Its posterior edge, the
line where the meso-_plus_ the metasoma are hinged, is a re-entrant bay
with three sides. The meso- and metasoma are in the King-crabs fused
together and form a hexagon. Three sides of this hexagonal double region
form the hinge, two form the lateral margins and slope inwards; these
bear six fused and six-jointed spines which have a segmental value. The
sixth or posterior side is indented, and its concavity forms the area to
which the large post-anal, unsegmented tail or spine is hinged.

The whole body is covered by a smooth chitinous sheath varying from
sage-green to black in colour, and it is kept very clean, probably by
some excretion which hinders various sessile animals attaching
themselves to it as they do, for instance, on many Copepods. Burrowing
animals like _Limulus_ are usually free from these messmates. King-crabs
have a self-respecting, well-groomed appearance. On the rounded dorsal
surface the chitinous covering is produced into a certain number of
spines arranged in a median and two lateral rows. The anterior median
spine overhangs the median eyes, and the anterior lateral spine on each
side overshadows the large lateral eyes.

[Illustration:

  FIG. 152.—Dorsal view of the King-crab, _Limulus polyphemus_, × ½.
    From Shipley and MacBride. 1, Carapace covering prosoma; 2, meso-
    and metasoma; 3, telson; 4, median eye; 5, lateral eye.
]

The vaulted carapace is turned in on the under side, where there is a
flat rim which widens anteriorly, and on the inner edge this rim borders
a sunken area, into the concavity of which the numerous appendages
project. Thus, although when viewed from above a _Limulus_ looks as
though it had a solid body shaped something like half a pear, when
viewed from below, especially if the appendages be removed, it will be
found that the body is thin and hollowed, and almost leaf-like, as if
most of the edible part of the half-pear had been scooped out. Within
the hollow thus formed the appendages lie, and here they move about,
seldom or never protruding beyond the edge of the carapace,—in fact, all
except the pedipalps and ambulatory legs are too short to project beyond
this limit.

[Illustration:

  FIG. 153.—Ventral view of the King-crab, _Limulus polyphemus_, × ½.
    From Shipley and MacBride. 1, Carapace covering prosoma; 2, meso-
    and metasoma; 3, telson; 4, chelicera; 5, pedipalp; 6, 7, 8, 9, 3rd
    to 6th appendages, ambulatory limbs; 10, genital operculum turned
    forward to show the genital apertures; 11, 12, 13, 14, 15,
    appendages bearing gill-books; 16, anus; 17, mouth; 18, chilaria.
]

The body of a King-crab can be analysed into twenty-one segments, but
these do not all persist to the adult stage. They are grouped together
in higher aggregates, or “tagmata” as Lankester calls them, and most of
the segments bear paired appendages.

The segments with their respective appendages and their grouping into
tagmata are shown in the following scheme:—

                               Appendages.

      I.     Segment    │Median eyes    │Preoral        │Prosoma
     II.        „       │Rostrum        │       „       │       „
    III.        „       │Chelicerae     │       „       │       „
     IV.        „       │Pedipalpi      │Lateral to     │       „
                        │               │  mouth        │
      V.        „       │1st Walking    │Postoral       │       „
                        │  Legs         │               │
     VI.        „       │2nd Walking    │       „       │       „
                        │  Legs         │               │
    VII.        „       │3rd Walking    │       „       │       „
                        │  Legs         │               │
   VIII.        „       │4th Walking    │       „       │       „
                        │  Legs         │               │
     IX.        „       │Chilaria       │       „       │       „

      X.        „       │Genital        │       „       │Mesosoma
                        │  operculum    │               │
     XI.        „       │1st Gill-books │       „       │       „
    XII.        „       │2nd Gill-books │       „       │       „
   XIII.        „       │3rd Gill-books │       „       │       „
    XIV.        „       │4th Gill-books │       „       │       „
     XV.        „       │5th Gill-books │       „       │       „

    XVI.        „       │No appendages  │       „       │Metasoma
   XVII.        „       │       „       │       „       │       „
  XVIII.        „       │       „       │       „       │       „
    XIX.        „       │       „       │       „       │       „
     XX.        „       │       „       │       „       │       „
    XXI.        „       │       „       │       „       │       „

We have followed Carpenter[210] in inserting the rostral segment. This
corresponds with the segment that in Insects and Crustacea bears the
antennae or first antennae respectively, the absence of these organs
being one of the characteristic but negative features of all Arachnids.
The evidence for the existence of this evanescent segment rests partly
upon the observation of von Jaworowski[211] on the vestigial feelers in
an embryo Spider, _Trochosa_, and perhaps more securely on the fact
that, according to Korschelt and Heider, there is a distinct neuromere
for this segment, between the proto-cerebral neuromere which supplies
the eyes and the trito-cerebral neuromere which supplies the chelicerae.
According to Brauer[212] the chelicerae of Scorpions are also supplied
by the third neuromere.

The bases of the chelicerae do not limit the mouth, but between and
behind them is a ridge or tubercle which has the same relationship to
the mouth of _Limulus_ that the labrum has in Insects and some
Crustacea. Posteriorly the mouth is bounded by the “promesosternite,” a
large median plate which lies between the bases of the ambulatory limbs.
The pedipalps and all the ambulatory limbs have their bases directed
towards the mouth, their gnathobases or sterno-coxal processes are
cushion-like structures covered with spines—all pointing inwards—and
with crushing teeth. They form a very efficient manducatory apparatus.
The boundary of the mouth is finally completed by the chilaria.

Certain of the appendages which persist will be described with the
functions they subserve, the eyes with the sense-organs, the genital
operculum with the generative organs, the gill-books with the
respiratory system, but the chelicerae, pedipalpi, and walking limbs,
which have retained the functions of prehension and locomotion usual to
limbs, merit a little attention.[213] The chelicerae are short and
composed of but three joints. They are, like the succeeding segments,
chelate, and the chelae of all are fine and delicate like a pair of
forceps rather than like a Lobster’s claw. In the female _L. polyphemus_
the pedipalp is remarkably like the three ambulatory legs which succeed
it, and all four are chelate, but in the adult male the penultimate
joint of the pedipalp is not prolonged to form one limb of the chela,
which is therefore absent, and the appendage is thicker and heavier than
in the other sex. In _L. longispina_ and _L. moluccanus_ the first
walking leg, as well as the pedipalp, ends in a claw and not in a chela;
the immature males resemble the females. The first three walking legs in
both sexes of _L. polyphemus_ resemble the pedipalpi of the female, and
like them have six joints. The fourth and last pair of ambulatory
appendages is not chelate, but its distal joints carry a number of
somewhat flattened structures, which are capable of being alternately
divaricated and approximated or bunched together. This enables them to
act as organs for clearing away sand or mud from beneath the carapace as
the creature lies prone on the bottom of the sea. To quote Mr.
Lloyd,[214] the “two limbs are, sometimes alternately and sometimes
simultaneously, thrust backward below the carapace, quite beyond the
hinder edge of the shell; and in the act of thrusting, the lobes or
plates on each leg encounter the sand, the resistance or pressure of
which causes them to open and fill with sand, a load of which at every
thrusting operation is pushed away from under the king-crab, and
deposited outside the carapace. The four plates then close and are
withdrawn closed, previous to being opened and charged with another load
of sand; and at the deposit of every load the whole animal sinks deeper
into its bed, till it is hidden all except the eyes.” There seems little
doubt that the action of these appendages in removing the sand from
under the carapace is reinforced by the fanning action of the
respiratory appendages, which set up a current that helps to wash the
particles away. But the posterior walking legs are not the only organs
used in burrowing. The Rev. Dr. Lockwood,[215] who observed the habits
of _L. polyphemus_ off the New Jersey coast, says, “The king-crab
delights in moderately deep water, say from two to six fathoms. It is
emphatically a burrowing animal, living literally in the mud, into which
it scoops or gouges its way with great facility. In the burrowing
operation the forward edge of the anterior shield is pressed downward
and shoved forward, the two shields being inflected, and the sharp point
of the tail presenting the fulcrum as it pierces the mud, whilst
underneath the feet are incessantly active scratching up and pushing out
the earth on both sides. There is a singular economy of force in this
excavating action; for the doubling up or inflecting and straightening
out of the two carapaces, with the pushing purchase exerted by the tail,
accomplish both digging and subterranean progression.”

At night-time _Limulus_ is apt to leave the sand and progress by a
series of short swimming hops, the respiratory appendages giving the
necessary impetus, whilst between each two short flights the animal
balances itself for a moment on the tip of its tail. During this method
of progressing the carapace is slanting, forming an angle of about 45°
with the ground. The unsegmented tail is also used when a King-crab
falls on its back. “The spine is then bent, _i.e._ its point is planted
in the sand so that it makes an acute angle with the carapace, which is
then so far raised that some of the feet are enabled to grasp a
projecting surface, either longitudinal or vertical, or at some
combination of the two; and the crab then turns over.”

[Illustration:

  FIG. 154.—A sagittal section of _Limulus_, seen from the right side,
    somewhat smaller than natural size. After Patten and Redenbaugh.

  All the prosomatic appendages, except the chelicera (4) and chilarium
    (33) of the right side, are omitted. The genital operculum (32) and
    the five gills (28) are represented.

  The muscles are omitted except the fibres running from the occipital
    ring to the posterior side of the oesophagus, the chilarial muscles,
    the sphincter ani (27), and the levator ani (24).

  The endosternite (34), with the occipital ring and the capsuliginous
    bar, is seen from the side, and the positions of the abdominal
    endochondrites (31) are indicated.

  The mouth (1) leads into the oesophagus, which passes through the
    brain to the proventriculus (12). A constriction, which marks the
    position of the pyloric valve, separates the proventriculus from the
    intestine (23) which passes posteriorly to the anus (26). A pair of
    hepatic ducts (15) enter the intestine opposite the endocranium.

  The heart (16) surrounded by the pericardial sinus lies above the
    intestine. The pericardium is shown between the heart and the
    intestine. The ostia (17) of the heart and the origins of the four
    lateral arteries (19) are indicated; the frontal artery (13) and the
    aortic arches (14) curving down to the brain, arise from the
    anterior end of the heart; the superior abdominal artery and the
    opening of the collateral artery into it are shown.

  The brain surrounding the oesophagus is seen in side view upon the
    neural side of the endosternite (34). The ventral cord (35) passes
    through the occipital ring into the abdominal region. The anterior
    commissure (3), with the three rostral nerves (2) innervating the
    rostrum, or labrum, and four of the post-oral commissures, are
    represented.

  The cheliceral nerve with the small external pedal branch is shown
    entire, but the next five neural nerves are cut off. The chilarial
    nerve, the opercular nerve, and the five branchial nerves, enter
    their respective appendages, the two former passing through the
    occipital ring.

  From the fore-brain the three olfactory nerves (5) pass anteriorly to
    the olfactory organ; the median eye-nerve (10) passes to the right
    of the proventriculus (12) to the median eyes (11); the lateral
    eye-nerve (7) passes forward and is represented as cut off opposite
    the proventriculus. The lateral nerve (9) or first haemal nerve is
    also cut off just beyond the point where it fuses with the second
    haemal nerve (8). The stomodaeal nerve (6) ramifies over the
    oesophagus and proventriculus.

  The second haemal nerve (8) passes to the anterior extremity of the
    carapace; its haemal branch is cut off opposite the proventriculus.
    An intestinal branch arises from near its base and disappears behind
    the anterior cornu of the endosternite.

  The next three haemal nerves (36) are cut off close to the brain, and
    the following nine haemal nerves are cut off beyond the cardiac
    branches. The fifteenth haemal nerve (29) is cut off beyond its
    branch to the telson muscles. Both branches of the haemal nerve are
    represented extending into the telson (25).

  The intestinal nerves are shown arising from the haemal nerves and
    entering the intestine. Those from the sixth and seventh neuromeres
    pass through foramina in the endosternite, and communicate with a
    plexus in the longitudinal abdominal muscles before entering the
    intestine. The eighth passes just posterior to the endosternite and
    joins the same plexus. Those from the first four branchial
    neuromeres arise very near the abdominal ganglia, and are double in
    their origins, the anterior branches joining the above-mentioned
    plexus, and the posterior branches entering the intestine. The
    fifteenth extends far back towards the rectum and anastomoses with
    the sixteenth, which arises from the caudal branch of the sixteenth
    haemal nerve, and innervates the rectum and anal muscles.

  The segmental cardiac nerves (18) arise from the haemal nerves of the
    sixth to the thirteenth neuromeres respectively. The most anterior
    one passes to the inter-tergal muscles and the epidermis in the
    median line, but the connections with the cardiac plexus have not
    been made out. The next two (18) fuse to form a large nerve, which
    passes to the inter-tergal muscles and epidermis, but has not been
    observed to connect directly with the cardiac plexus. It, however,
    sends posteriorly a branch, the pericardial nerve (20), which in
    turn gives a branch to each of the cardiac nerves of the branchial
    neuromeres, and then continues onward to the posterior margin of the
    abdomen. This nerve lies in the epidermis. The median and lateral
    cardiac nerves (22 and 21) are seen upon the walls of the heart. The
    five cardiac nerves from the branchial neuromeres pass, in the
    epidermis, to the median line, and dip down to the median nerve (22)
    of the heart opposite the last five pairs of ostia (17). They
    communicate with the pericardial nerve (20) and also with the
    lateral sympathetic nerve (30).

  Two post-cardiac nerves pass from the first and second post-branchial
    nerves to the epidermis posterior to the heart.

  The last cardiac nerve and the two post-cardiac nerves give off
    branches which anastomose with each other and innervate the
    extensors of the telson.

  The lateral sympathetic nerve (30) receives branches from all the
    neuromeres from the eighth to the fourteenth, either through the
    cardiac nerves or the haemal nerves, and innervates the
    branchio-thoracic muscles, extending with these far into the
    cephalothorax.

  1, Mouth; 2, rostral nerve in labrum; 3, anterior commissure; 4,
    chelicera; 5, olfactory nerves; 6, stomodaeal nerve; 7, lateral
    eye-nerve; 8, 2nd haemal nerve; 9, lateral nerve; 10, median
    eye-nerve; 11, median eye; 12, proventriculus; 13, frontal artery;
    14, aortic arch; 15, anterior hepatic duct of liver; 16, heart; 17,
    2nd ostium; 18, 7th and 8th segmental cardiac nerves; 19, one of the
    lateral arteries; 20, pericardial nerve; 21, lateral cardiac nerve;
    22, median cardiac nerve; 23, intestine; 24, levator ani muscle; 25,
    telson; 26, anus; 27, sphincter ani muscle; 28, last branchial
    appendage; 29, 15th haemal nerve; 30, lateral sympathetic nerve; 31,
    8th abdominal endochondrite; 32, genital operculum; 33, chilarium;
    34, endosternite; 35, ventral nerve-cord; 36, 6th haemal nerve; 37,
    origin of 6th neural nerve.
]

_Limulus_ feeds partly on bivalves, but mainly on worms, especially
Nereids, which it catches with its chelate limbs as it burrows through
the sand. The food is held immediately under the mouth by the
chelicerae, aided at times by the succeeding appendages; it is thus
brought within range of the gnathobases of the walking legs, and these
by an alternate motion “card” the food into fragments, which when
sufficiently comminuted pass into the mouth. At times its appendages are
caught between the valves of _Venus mercenaria_, a burrowing bivalve
known in America as the “quahog” or “round clam.” The _Limulus_ has
seized with its chelate claws the protruding siphon of this mollusc,
which, being rapidly drawn in, drags with it the limb of the king-crab,
and the valves of the clam are swiftly snapped to.

As a rule in Arachnids the alimentary canal is no longer than the body,
and runs straight from mouth to anus, but in _Limulus_, the mouth being
pushed far backward, there is a median loop, and the narrow oesophagus
which leads from the mouth, having traversed the nerve-ring, passes
forward towards the anterior end of the carapace. Here it enters into a
somewhat ⸧ shaped and spacious proventriculus; posteriorly the
proventriculus opens by a funnel-shaped valve into the anterior end of
the narrow intestine. All these structures are derived from the
stomodaeum, are lined with chitin and are provided with very muscular
walls whose internal surface is thrown into longitudinal ridges. The
intestine runs straight backward, diminishing in its diameter, and ends
in a short, chitin-lined, and muscular rectum which is derived from the
proctodaeum; the anus is a longitudinal slit. A large gland, usually
called the liver, consisting of innumerable tubules, pours its
secretions into the broader anterior end of the intestine by two ducts
upon each side; it extends into the meso- and metasoma, and, together
with the reproductive organs, forms a “packing” in which the other
organs are embedded. The contents of the alimentary canal are described
as “pulpy and scanty,” and probably much of the actual digestion goes on
inside the lumen of the above-mentioned gland.

The vascular system of _Limulus_, like that of the Scorpions, is more
completely developed than is usually the case in Arthropods. For the
most part the blood runs in definite arteries, and when it passes as it
does into venous lacunae these are more definite in position and in
their retaining walls than in other members of the phylum.

The heart lies in a pericardial space with which it communicates by
eight[216] pairs of ostia. Eight paired bands of connective tissue, the
“alary muscles” of authors, sling the heart to the pericardial membrane.
Posteriorly the pericardial chamber receives five paired veins on each
side coming from the gills and returning the purified blood to the
heart.

Eleven arteries arise from the heart. These are (i.) a median _frontal_
artery which, passing forward, divides into a right and left _marginal_
artery. These run round the edge of the carapace to its posterior angle,
where each receives a branch of the _collateral_ artery mentioned below.
(ii.) and (iii.) are the _aortic_ arches (Fig. 154), paired vessels
running round and supplying the proventriculus and oesophagus. These
unite ventrally in a vascular ring which encloses the nerve-ring, and is
continued along the ventral nerve-cord as the _ventral_ artery and along
some of the chief nerves. This vascular ring supplies the lateral eyes
and all the appendages mentioned on p. 263 up to and including the
genital operculum. The _ventral_ artery supplies the respiratory
appendages, and gives branches to the rectum, caudal spine, etc. Two of
its branches encircle the rectum, and uniting open into the _superior
abdominal artery_. iv.–xi. are paired _lateral_ arteries which leave the
heart beneath the anterior four ostia, and soon enter a longitudinal
pair of _collateral_ arteries which unite behind in the just mentioned
superior abdominal artery; they also give off branches to the muscles
and to the intestine, and a stout branch mentioned above which passes
into the marginal artery posteriorly. The venous system is lacunar, and
the blood is collected from the irregular spaces between the various
organs into a pair of longitudinal sinuses, whence it passes into the
operculum and the five pairs of gills. A large branchio-cardiac canal
returns the blood from each gill to the cavity of the pericardium, and
so through the ostia to the heart. Eight _veno-pericardiac_ muscles run
from the under surface of the pericardium to be inserted into the upper
surface of the longitudinal sinus; they occur opposite the ostia, and
play an important part in the mechanism of the circulation. The blood is
coloured blue by haemocyanin; amoeboid corpuscles float in the plasma.

The respiratory organs are external gills borne on the posterior face of
the exopodite of the lamella-like posterior five mesosomatic limbs. Each
gill consists of a series of leaves like the leaves of a book, and some
150–200 in number. Within the substance of each leaf the blood flows,
while without the oxygen-carrying water circulates between the leaves.
These gillbearing appendages can be flapped to and fro, and they seem to
be at times held apart by the _flabellum_, a spatulate process which
Patten and Redenbaugh regard as a development of the median sensory knob
on the outer side of the coxopodite of the last pair of walking limbs.

[Illustration:

  FIG. 155.—Diagram of the first gill of _Limulus_, from the posterior
    side, showing the distribution of the gill-nerve to the gill-book
    (about natural size). After Patten and Redenbaugh. 1, Inner lobe of
    the appendage; 2, outer lobe of appendage; 3, median lobe of
    appendage; 4, gill-book; 5, neural nerve of the ninth neuromere; 6,
    internal branchial nerve; 7, gill-nerve; 8, median branchial nerve;
    9, external branchial nerve.
]

_Limulus_ has no trace of Malpighian tubules, structures which seem
often to develop only when animals cease to live in water and come to
live in air. The Xiphosura have retained as organs of nitrogenous
excretion the more primitive nephridia, or coxal glands as they are
called, in the Arachnida. They are redbrick in colour, and consist of a
longitudinal portion on each side of the body, which gives off a lobe
opposite the base of the pedipalps and each of the first three walking
legs—in the embryo also of the chelicerae and last walking legs, but
these latter disappear during development. A duct leads from the
interior of the gland and opens upon the posterior face of the last pair
of walking legs but one.

The nervous system has been very fully described by Patten and
Redenbaugh, and its complex nature plays a large part in the ingenious
speculations of Dr. Gaskell as to the origin of Vertebrates. It consists
of a stout ring surrounding the oesophagus and a ventral nerve-cord,
composed—if we omit the so-called fore-brain—of sixteen neuromeres. The
_fore-brain_ supplies the median and the lateral eyes, and gives off a
median nerve which runs to an organ, described as olfactory by Patten,
situated in front of the chelicerae on the ventral face of the carapace.
Patten distinguishes behind the fore-brain a _mid-brain_, which consists
solely of the cheliceral neuromere, a _hind-brain_ which supplies the
pedipalps and four pair of walking legs, and an _accessory brain_ which
supplies the chilaria and the genital operculum. This is continued
backward into a ventral nerve-cord which bears five paired ganglia
supplying the five pairs of gills and three pairs of post-branchial
ganglia; the latter are ill-defined and closely fused together. As was
mentioned above, the whole of the central nervous system is bathed in
the blood of the ventral sinus.

The sense-organs consist of the olfactory organ of Patten, the median
and lateral eyes, and possibly of certain gustatory hairs upon the
gnathobases. The lateral eyes in their histology are not so
differentiated as the median eyes, but both fall well within the limits
of Arachnid eye-structure, and their minute anatomy has been advanced as
one piece of evidence amongst many which tend to demonstrate that
_Limulus_ is an Arachnid.

Both ovaries and testes take the form of a tubular network which is
almost inextricably entangled with the liver. From each side a duct
collects the reproductive cells which are formed from cells lining the
walls of the tubes, and discharges them by a pore one on each side of
the hinder surface of the genital operculum. As is frequently the case
in Arachnids the males are smaller than the females, and after their
last ecdysis the pedipalps and first two pairs of walking legs, or some
of these appendages, end in slightly bent claws and not in chelae. Off
the New Jersey coast the king-crabs (_L. polyphemus_) spawn during the
months of May, June, and July, Lockwood states at the periods of highest
tides, but Kingsley[217] was never “able to notice any connexion between
the hours when they frequent the shore and the state of the tide.” “When
first seen they come from the deeper water, the male, which is almost
always the smaller, grasping the hinder half of the carapace of the
female with the modified pincer of the second pair of feet. Thus
fastened together the male rides to shallow water. The couples will stop
at intervals and then move on. Usually a nest of eggs can be found at
each of the stopping-places, and as each nest is usually buried from one
to two inches beneath the surface of the sand, it appears probable that
the female thrusts the genital plate into the sand, while at the same
time the male discharges the milt into the water. I have not been able
to watch the process more closely because the animals lie so close to
the sand, and all the appendages are concealed beneath the carapace. If
touched during the oviposition, they cease the operation and wander to
another spot or separate and return to deep water. I have never seen the
couples come entirely out of the water, although they frequently come so
close to the shore that portions of the carapace are uncovered.”[218]

[Illustration:

  FIG. 156.—A view of the nervous system of _Limulus_ from below. (About
    natural size.) After Patten and Redenbaugh.


  The carapace is represented as transparent. The appendages have been
    removed, but the outlines of the left entocoxites (6) have been
    sketched in. The positions of the abdominal appendages are indicated
    by the external branchial muscles (17), the branchial cartilages
    (19), the tendinous stigmata (18), and the abdominal endochondrites
    (21). In the cephalothorax (1) all the tergo-coxal and plastro-coxal
    muscles have been dissected away, leaving the endosternite (11) with
    the occipital ring exposed. One of the left tergo-proplastral
    muscles (4) and the left branchio-thoracic muscles (16) are
    represented. The longitudinal abdominal muscles are also seen. All
    the muscles of the right side have been omitted except the
    haemo-neural muscles (23), of which the last two are represented
    upon the left side also. At the base of the telson the flexors (29)
    and extensors (27) of the caudal spine are represented as cut off
    near their insertions. The sphincter ani (26), levator ani, and
    occludor ani (25), and their relations to the anus (28), are shown.

  The oesophagus runs forward to the proventriculus (3). From this the
    intestine (20) passes posteriorly.

  The brain lies upon the neural side of the endosternite, and the
    ventral cord (22) passes back through the occipital ring. The neural
    nerves are cut off, but the left haemal nerves and those from the
    fore-brain (12) are represented entire.

  The first pair of neural nerves go to the chelicerae. The second to
    sixth pairs go to the next five cephalothoracic appendages, which
    are represented by the entocoxites (6). The seventh pair of neural
    nerves go to the chilaria, and the eighth pair to the operculum. The
    neural nerves from the ninth to the thirteenth arise from the
    abdominal ganglia and innervate the five pairs of gills.

  From the fore-brain a median olfactory nerve (9) and two lateral ones
    (8) pass forward to the olfactory organ; a median eye-nerve (2)
    passes anteriorly and haemally upon the right of the proventriculus
    (3) to the median eyes; and a pair of lateral eye-nerves pass to the
    lateral eyes (15).

  The first haemal nerve, or lateral nerve, follows the general course
    of the lateral eye-nerve, but continues posteriorly far back on to
    the neural side of the abdomen.

  The haemal nerves of the hind-brain radiate from the brain to the
    margins of the carapace, and each one passes anterior to the
    appendage of its own metamere. The integumentary portions divide
    into haemal and neural branches, of which the haemal branches (5)
    are cut off. Each haemal branch gives off a small nerve which turns
    back toward the median line upon the haemal side of the body.

  The haemal nerves of the accessory brain pass through the occipital
    ring to the sides of the body between the operculum and the sixth
    cephalothoracic appendage. The seventh innervates the posterior
    angles of the cephalothorax, the eighth the opercular portion of the
    abdomen. The next five haemal nerves arise from the five branchial
    neuromeres, pass out anterior to the gills to the sides of the
    abdominal carapace, and innervate the first five spines upon the
    sides of the abdomen.

  The first post-branchial nerve innervates the last abdominal spine;
    the second post-branchial nerve and one branch of the third
    post-branchial innervate the posterior angles of the abdomen and the
    muscles of the telson; and the caudal branch of the third
    post-branchial nerve innervates the telson.

  Intestinal branches arise from all the haemal nerves from the sixth to
    the sixteenth, and pass to the longitudinal abdominal muscles and to
    the intestine.

  Cardiac nerves arise from all the haemal nerves from the sixth to the
    thirteenth. Six of the cardiac nerves communicate with the lateral
    sympathetic nerve (24), which innervates the branchio-thoracic
    muscles (16).

  Two post-cardiac nerves arise from the first two post-branchial
    nerves, and passing to the haemal side anastomose with a branch from
    the last cardiac nerve, and innervate the extensors (27) of the
    telson and the epidermis behind the heart.

  1, Cephalothorax; 2, median eye-nerve; 3, proventriculus; 4,
    tergo-proplastral muscles; 5, haemal branch of integumentary nerve;
    6, entocoxites; 7, 2nd haemal nerve; 8, right olfactory nerve; 9,
    median olfactory nerve; 10, intestine; 11, endosternite; 12,
    fore-brain; 13, origin of 4th neural nerve; 14, lateral nerve; 15,
    lateral eye; 16, branchio-thoracic muscles; 17, external branchial
    muscles; 18, tendinous stigmata; 19, branchial cartilages; 20,
    intestine; 21, abdominal endochondrites; 22, ventral cord; 23,
    haemo-neural muscles; 24, lateral sympathetic nerve; 25, occludor
    ani; 26, sphincter ani; 27, extensors of telson; 28, anus; 29,
    flexors of telson; 30, lateral projections of abdomen; 31, nerves of
    spines; 32, external branchial muscles.
]

[Illustration:

  FIG. 157.—The markings on the sand made by the female _Limulus_ when
    depositing eggs. Towards the lower end the round “nests” cease to be
    apparent, the king-crab being apparently exhausted. (From
    Kishinouye.) About natural size.
]

The developing ova and young larvae are very hardy, and in a little
sea-water, or still better packed in sea-weed, will survive long
journeys. In this way they have been transported from the Atlantic to
the Pacific coasts of the United States, and for a time at any rate
flourished in the western waters. Three barrels full of them consigned
from Woods Holl to Sir E. Ray Lankester arrived in England with a large
proportion of larvae alive and apparently well.

According to Kishinouye, _L. longispina_ spawns chiefly in August and
between tide-marks. “The female excavates a hole about 15 cm. deep, and
deposits eggs in it while the male fertilises them. The female
afterwards buries them, and begins to excavate the next hole.”[219] A
line of nests (Fig. 157) is thus established which is always at right
angles to the shore-line. After a certain number of nests have been
formed the female tires, and the heaped up sand is not so prominent. In
each “nest” there are about a thousand eggs, placed first to the left
side of the nest and then to the right, from which Kishinouye concludes
that the left ovary deposits its ova first and then the right. _Limulus
rotundicauda_ and _L. moluccanus_ do not bury their eggs, but carry them
about attached to their swimmerets.

The egg is covered by a leathery egg-shell which bursts after a certain
time, and leaves the larva surrounded only by the blastodermic cuticle;
when ripe it emerges in the condition known as the “Trilobite larva”
(Fig. 158), so-called from a superficial and misleading resemblance to a
Trilobite. They are active little larvae, burrowing in the sand like
their parents, and swimming vigorously about by aid of their leaf-like
posterior limbs. Sometimes they are taken in tow-nets. After the first
moult the segments of the meso- and metasoma, which at first had been
free, showing affinities with _Prestwichia_ and _Belinurus_ of
Palaeozoic times, become more solidified, while the post-anal
tail-spine—absent in the Trilobite larva—makes its first appearance.
This increases in size with successive moults. We have already noted the
late appearance of the external sexual characters, the chelate walking
appendages only being replaced by hooks at the last moult.

[Illustration:

  FIG. 158.—Dorsal and ventral view of the last larval stage (the
    so-called Trilobite stage) of _Limulus polyphemus_ before the
    appearance of the telson. 1, Liver; 2, median eye; 3, lateral eye;
    4, last walking leg; 5, chilaria. (From Kingsley and Takano.)
]

_Limulus_ casts its cuticle several times during the first year—Lockwood
estimates five or six times between hatching out in June and the onset
of the cold weather. The cuticle splits along a “thin narrow rim” which
“runs round the under side of the anterior portion of the cephalic
shield.”[220] This extends until it reaches that level where the animal
is widest. Through this slit the body of the king-crab emerges, coming
out, not as that of a beetle anteriorly and dorsally, but anteriorly and
ventrally, in such a way as to induce the unobservant to exclaim “it is
spewing itself out of its mouth.” In one nearly full-sized animal the
increase in the shorter diameter of the cephalic shield after a moult
was from 8 inches to 9½ inches, which is an indication of very rapid
growth. If after their first year they moult annually Lockwood estimates
it would take them eight years to attain their full size.

The only economic use I know to which _Limulus_ is put is that of
feeding both poultry and pigs. The females are preferred on account of
the eggs, of which half-a-pint may be crowded into the cephalic shield.
The king-crab is opened by running a knife round the thin line mentioned
on p. 275. There is a belief in New Jersey that this diet makes the
poultry lay; undoubtedly it fattens both fowls and pigs, but it gives a
“shocking” flavour to the flesh of both.


                           =CLASSIFICATION.=

But five species of existing King-crabs are known, and these are grouped
by Pocock into two sub-families: (i.) the Xiphosurinae, and (ii.) the
Tachypleinae. These together make up the single family Xiphosuridae
which is co-extensive with the Order. The following is Pocock’s
classification.[221] The names used in this article are printed in
italic capitals.


                            ORDER XIPHOSURA.


                       =Family 1. Xiphosuridae.=


                      =Sub-Fam. 1. Xiphosurinae.=

This includes the single species _Xiphosura polyphemus_ (Linn.) (=
_LIMULUS POLYPHEMUS_, Latreille), “which is said to range from the coast
of Maine to Yucatan.”


                      =Sub-Fam. 2. Tachypleinae.=

Genus A. _Tachypleus_ includes three species: (i.) _T. gigas_, Müll. (=
_Limulus gigas_, Müll., and _L. MOLUCCANUS_, Latreille), widely
distributed in Malaysia; (ii.) _T. tridentatus_, Leach (= _L.
tridentatus_, Leach, and _L. LONGISPINA_, Van der Hoeven), extending
from British North Borneo to China and Southern Japan; and (iii.) _T.
hoeveni_, Pocock (= _L. MOLUCCANUS_, Van der Hoeven), found in the
Moluccas.

Genus B. _Carcinoscorpius_ with one species, _C. rotundicauda_
(Latreille) (= _L. ROTUNDICAUDA_, Latreille). It occupies a more
westerly area than _T. gigas_ or than _T. tridentatus_, having been
recorded from India and Bengal, the Gulf of Siam, Penang, the Moluccas,
and the Philippines.

With regard to the =affinities= of the group it is now almost
universally accepted that they are Arachnids. The chief features in
which they differ from other Arachnids are the presence of gills and the
absence of Malpighian tubules, both being features associated with
aquatic life. As long ago as 1829 Straus-Dürckheim emphasised the points
of resemblance between the two groups, and although the view was during
the middle of the last century by no means universally accepted, towards
the end of that epoch the painstaking researches of Lankester and his
pupils, who compared the King-crab and the Scorpion, segment with
segment, organ with organ, tissue with tissue, almost cell with cell,
established the connexion beyond doubt. Lankester would put the
Trilobites in the same phylum, but in this we do not follow him. With
regard to the brilliant but, to our mind, unconvincing speculations as
to the connexion of some _Limulus_-like ancestor with the Vertebrates,
we must refer the reader to the ingenious writings of Dr. Gaskell,[222]
recently summarised in his volume on “The Origin of Vertebrates,” and to
those of Dr. Patten in his article “On the Origin of Vertebrates from
Arachnids.”[223]


                        =Fossil Xiphosura.=[224]

[Illustration:

  FIG. 159.—=A.=, _Hemiaspis limuloides_, Woodw., Upper Silurian,
    Leintwardine, Shropshire. Natural size. (After Woodward.) =B.=,
    _Prestwichia_ (_Euroöps_) _danae_ (Meek), Carboniferous, Illinois, ×
    ⅔. (After Packard.)
]

_Limulus_ is an example of a persistent type. It appears first in
deposits of Triassic age, and is found again in the Jurassic,
Cretaceous, and Oligocene. In the lithographic limestone of Solenhofen
in Bavaria, which is of Upper Jurassic age, _Limulus_ is common and is
represented by several species. One species is known from the Chalk of
Lebanon, and another occurs in the Oligocene of Saxony. No other genus
of the Xiphosura appears to be represented in the Mesozoic and Tertiary
deposits, but in the Palaeozoic formations (principally in the Upper
Silurian, the Old Red Sandstone, and the Coal Measures) several genera
have been found, most of which differ from _Limulus_ in having some or
all of the segments of the abdomen free; in this respect they resemble
the Eurypterida, but differ from them in the number of segments. In
_Hemiaspis_ (Fig. 159, A), from the Silurian, the segments of the
abdomen are divisible into two groups (mesosoma and metasoma) in the
same way that they are in Eurypterids; the first six segments have
broad, short terga, the lateral margins of the sixth being divided into
two lobes, probably indicating the presence of two fused segments; the
last three segments are narrower and longer than the preceding, and at
the end is a pointed tail-spine. In _Belinurus_ (Fig. 160) from the
Carboniferous, the two regions of the abdomen are much less distinct;
there are eight segments, the last three of which are fused together,
and a long tail-spine. In _Neolimulus_, from the Silurian, there seems
to be no division of the abdomen into two regions, and apparently all
the segments were free. On the other hand, in _Prestwichia_
(Carboniferous), all the segments of the abdomen, of which there appear
to be seven only, were fused together (Fig. 159, B).

[Illustration:

  FIG. 160.—_Belinurus reginae_, Baily, Coal Measures, Queen’s Co.,
    Ireland, × 1. (After Woodward).
]

In the Palaeozoic genera the median or axial part of the dorsal surface
is raised and distinctly limited on each side, so presenting a trilobed
appearance similar to that of Trilobites. In _Neolimulus_, _Belinurus_,
and _Prestwichia_, lateral eyes are present on the sides of the axial
parts of the carapace, and near its front margin median eyes have been
found in the two last-named genera.

In nearly all the specimens of Palaeozoic Xiphosura[225] which have been
found nothing is seen but the dorsal surface of the body; in only a very
few cases have any traces of the appendages been seen,[226] but, so far
as known, they appear to have the same general character as in
_Limulus_.

_Aglaspis_, found in the Upper Cambrian of Wisconsin, has been regarded
as a Xiphosuran. If that view of its position is correct, then
_Aglaspis_ will be the earliest representative of the group at present
known. Other genera of Palaeozoic Xiphosura are _Bunodes_, _Bunodella_,
and _Pseudoniscus_ in the Silurian; _Protolimulus_ in the Upper
Devonian; and _Prolimulus_ in the Permian.




                              EURYPTERIDA


                                   BY

                           HENRY WOODS, M.A.

  St. John’s College, Cambridge, University Lecturer in Palaeozoology.




                               CHAPTER XI
  ARACHNIDA (_CONTINUED_)—DELOBRANCHIATA = MEROSTOMATA (_CONTINUED_)—
                              EURYPTERIDA


                        =Order II. Eurypterida.=

The Eurypterida or Gigantostraca are found only in the Palaeozoic
formations. Some species of _Pterygotus_, _Slimonia_, and _Stylonurus_
have a length of from five to six feet, and are not only the largest
Invertebrates which have been found fossil but do not seem to be
surpassed in size at the present day except by some of the Dibranchiate
Cephalopods. All the Eurypterids were aquatic, and, with the possible
exception of forms found in the Coal Measures, all were marine. The
earliest examples occur in the Cambrian deposits, and the latest in the
Permian; but although the Eurypterids have thus a considerable
geological range, yet it is mainly in the Silurian and the Old Red
Sandstone that they are found, the principal genera represented in those
deposits being _Eurypterus_, _Stylonurus_, _Slimonia_, _Pterygotus_,
_Hughmilleria_, _Dolichopterus_, and _Eusarcus_. From the Cambrian rocks
the only form recorded is _Strabops_;[227] in the Ordovician the
imperfectly known _Echinognathus_[228] and some indeterminable fragments
have alone been found. In the Carboniferous deposits _Eurypterus_ and
_Glyptoscorpius_ occur, and the former survived into the Permian.[229]

[Illustration:

  FIG. 161.—_Eurypterus fischeri_, Eichw. Upper Silurian, Rootziküll,
    Oesel. Dorsal surface. _a_, Ocellus; _b_, lateral eye; 2–6,
    appendages of prosoma; 7–12, segments of mesosoma; 13–18, segments
    of metasoma; 19, tail-spine. (After Holm.)
]

The Eurypterid which is best known is _Eurypterus fischeri_ (Figs. 161,
162), which is found in the Upper Silurian rocks at Rootziküll in the
Island of Oesel (Gulf of Riga). In the Eurypterids from other deposits
the chitinous exoskeleton has been altered into a carbonaceous
substance, but in the specimens from Oesel the chitin is perfectly
preserved in its original condition; and since these specimens are found
in a dolomitic rock which is soluble in acid, it has been possible to
separate the fossil completely from the rock in which it is embedded,
with the result that the structure can be studied more easily and more
thoroughly than in the case of specimens from other localities.
Consequently _Eurypterus fischeri_[230] may, with advantage, be taken as
a type of the Eurypterida.

The general form of the body (Fig. 161) is somewhat like that of a
Scorpion, but is relatively broader and shorter. On the surface of many
parts of the exoskeleton numerous scale-like markings are found (Figs.
162, 163).[231] The =prosoma= or cephalothorax consists of six fused
segments covered by a quadrate carapace with its front angles rounded.
This bears on its dorsal surface two pairs of eyes—large kidney-shaped
lateral eyes and median ocelli (Fig. 161, _b_, _a_). The margin of the
dorsal part of the carapace is bent underneath to form a rim which joins
the ventral part of the carapace.

On the ventral surface of the prosoma (Fig. 162) six pairs of appendages
are seen, of which only the first pair (the chelicerae) are in front of
the mouth. The chelicerae are small, and each consists of a basal joint
and a chela, the latter being found parallel to the axis of the body;
they closely resemble the chelicerae of _Limulus_. The remaining five
pairs of appendages are found at the sides of the elongate mouth, and in
all these the gnathobases of the coxae are provided with teeth at their
inner margins and were able to function in mastication, whilst the
distal part of each appendage served as an organ of locomotion. The
posterior part of each coxa is plate-like and is covered (except in the
case of the sixth appendage) by the coxa of the next appendage behind. A
small process or “epicoxite” is found at the posterior end of the
toothed part of the coxae of the second, third, fourth, and fifth pairs
of appendages. The second appendage consists of seven joints, whilst the
remaining four consist of eight joints; none of these appendages end in
chelae. The second, third,[232] and fourth pairs of appendages are
similar to one another in structure, but become successively larger from
before backwards. These three pairs are directed radially outwards; each
consists of short joints tapering to the end of the limb, and bearing
spines at the sides and on the under surface, and also a spine at the
end of the last joint.

[Illustration:

  FIG. 162.—_Eurypterus fischeri_, Eichw. Upper Silurian, Rootziküll,
    Oesel. Restoration of ventral surface; 1–6, appendages of prosoma;
    _m_, metastoma. Immediately posterior to the metastoma is the
    “median process” of the genital operculum. (After Holm.)
]

The fifth appendage is longer than the fourth and is directed backwards;
its second and third joints are short and ring-like; the others (fourth
to eighth) are long and similar to one another, each being of uniform
width throughout; the last joint is produced into a spine on each side,
and between these two is the movable end-spine; the other joints do not
bear long spines as is the case in the three preceding pairs of
appendages.

The sixth appendage is much larger and stronger than the others, and
like the fifth, is without long spines. The coxa is large and quadrate;
the second and third joints are short, like those of the fifth
appendage; the fourth, fifth, and sixth joints are longer and more or
less bell-shaped; the seventh and eighth joints are much larger than the
others and are flattened.

The metastoma (Fig. 162, _m_) is an oval plate immediately behind the
mouth; it covers the inner parts of the coxae of the sixth pair of
appendages, and represents the chilaria of _Limulus_. But, unlike the
latter, it is not a paired structure; nevertheless the presence of a
longitudinal groove on its anterior part renders probable the view that
it is derived from a paired organ.[233] The front margin of the
metastoma is indented and toothed. On its inner side in front is a
transverse plate, the endostoma, which is not seen from the exterior,
since the front margin of the metastoma extends a little beyond it.

Behind the prosoma are twelve free segments, of which the first six form
the =mesosoma= (Fig. 161, 7–12). The tergum on the dorsal surface of
each segment is broad and short, the middle part being slightly convex
and the lateral parts slightly concave; the external margin is bent
under, thus forming a narrow rim on the ventral surface. The tergum of
each segment overlaps the one next behind. The segments increase in
breadth slightly up to the fourth segment, posterior to which they
gradually become narrower.

On the ventral surface the segments of the mesosoma bear pairs of
plate-like appendages, each of which overlaps the one behind like the
tiles on a roof. On the posterior (or inner) surfaces of these
appendages are found the lamellar branchiae, which are oval in outline
(Fig. 165, _d_). Between the two appendages of the first pair is a
median process which is genital in function; this pair are larger than
the other appendages, and cover both first and second segments, the
latter being without any appendages, and they represent the genital
operculum of _Limulus_ (Fig. 153, 10). The form of the operculum, more
particularly of the median process, differs in the male and female. In
that which is believed to be the female (Fig. 162) the median process is
long, and extends beyond the posterior margin of the operculum; it is
formed of two small five-sided parts at the base which are united at the
sides to the two plates of the operculum; behind this is a long,
unpaired part, which is pointed in front; this, together with the
remaining parts, is not joined to the side-plates of the operculum, so
that the latter are here separated from one another. The third part of
the median process is shorter than the second, and bears at its end a
pair of small pointed and diverging plates, the tips of which reach to
the middle of the third plate-like appendages. On the inner side of the
operculum there are, in the female, a pair of curved, tubular organs,
attached to the anterior end of the median process, where they open, the
free ends being closed; the function of these organs is not known, but
was probably sexual.

In the male (Fig. 163, A, _a_) the median process is formed of two parts
only, and is very short, so that the two plates of the operculum unite
behind the process.

In the female a median process (Fig. 163, B) is also present between the
second pair of appendages (belonging to the third segment of the
mesosoma); it consists of a basal unpaired part, and of a pair of long
pointed pieces which project on to the next segment. Just as in the case
of the genital operculum the basal part is united in front to the
appendages, the remainder being free, and separating the greater part of
the two plate-like appendages. In the complete animal the median process
of this segment is covered by the median process of the genital
operculum. The remaining appendages of the female, and all the
appendages behind the operculum in the male, are without any median
process, and the plates of each pair unite by a suture in the middle
line.

[Illustration:

  FIG. 163.—_Eurypterus fischeri_, Eichw. Upper Silurian. (After Holm.)
    =A=, Genital operculum of male; _a_, median process. =B=, Middle
    part of second appendage of the mesosoma in the female, showing the
    median process.
]

The =metasoma= (Fig. 161, 13–18) consists of six segments which become
longer and narrower from before backwards. Each segment is covered by a
ring-like sheath or sclerite, and bears no appendages. The posterior end
of the last segment is produced into a lobe on each side, and between
these lobes the long, narrow tail-spine arises (Fig. 161, 19).

The other genera of the Eurypterida do not differ in any important
morphological respects from the form just described, All the genera, of
which about thirteen have been recognised, are placed in one family.

[Illustration:

  FIG. 164.—_Pterygotus osiliensis_, Schmidt, Upper Silurian,
    Rootziküll, Oesel. Ventral surface. Reduced. (After Schmidt.) 1–6,
    Appendages of the prosoma; 7–12, mesosoma; 7, 8, genital operculum;
    13–18, metasoma; 19, tail-plate; _a_, epistome; _b_, metastoma; _c_,
    coxae of sixth pair of appendages.
]

=Fam. Eurypteridae.=—The carapace varies somewhat in outline; in
_Slimonia_ it is more distinctly quadrate than in _Eurypterus_, whilst
in _Pterygotus_ (Fig. 164) and _Hughmilleria_[234] it is semi-ovoid. The
lateral eyes are at the margin of the carapace in _Pterygotus_,
_Slimonia_ (Fig. 165, _a_), and _Hughmilleria_, but in the other genera,
including the earliest form, _Strabops_,[235] they are on the dorsal
surface at a greater or less distance from the margin.

The pre-oral appendages of _Pterygotus_ (Fig. 164, 1) differ from those
of other genera in their much greater length and in the large size of
the chelae; they probably consist of a proximal joint and chelae only,
although, commonly, they are represented as having a larger number of
joints. Unlike _Eurypterus_ and _Pterygotus_, the second pair of
appendages in _Slimonia_ (Fig. 165, 2) differ from the third, fourth,
and fifth pairs in being distinctly smaller and more slender, and it is
probable that they were tactile. Whilst in _Eurypterus_ the fifth pair
of appendages are larger than the three preceding pairs, and also differ
from them in structure, in the genus _Pterygotus_ (Fig. 164, 5) they
agree closely with the second, third, and fourth pairs, and in
_Slimonia_ (Fig. 165, 5) they are nearly the same as the third and
fourth pairs. The sixth pair of appendages are much larger and more
powerful than the fifth pair in nearly all genera; in _Stylonurus_ (Fig.
166), however, the sixth pair are similar to the fifth, both being
greatly elongated and slender; also in _Eusarcus_ (_Drepanopterus_) the
sixth pair differ less from the preceding pair of appendages than is
usually the case.

[Illustration:

  FIG. 165.—_Slimonia acuminata_, Salter. Upper Silurian. Restoration of
    ventral surface, × ⅑. 1–6, Appendages of prosoma; 7, 8, genital
    operculum; 7–12, mesosoma; 13–18, segments of metasoma; 19,
    tail-spine; _a_, lateral eye; _b_, metastoma, covering the inner
    parts of the coxae of the last pair of appendages; _c_, median
    process of genital operculum; _d_, branchial lamellae seen through
    the plate-like appendages. (After Laurie.)
]

In _Pterygotus_ there is a well-developed epistome (Fig. 164, _a_)
between the mouth and the front margin of the carapace, thus occupying
the same position as the hypostome of Trilobites (p. 233). The metastoma
is always well developed and forms one of the distinguishing features of
the Eurypterids; in form it varies from oval in _Eurypterus_, to cordate
in _Slimonia_, and lyrate in _Dolichopterus_.

The principal modifications seen in the genital operculum are in the
form of its median process; in _Slimonia_ this either ends in three
sharp points posteriorly (Fig. 165, _c_), or has the form of a truncated
cone; its form in _Eurypterus_ has already been described.
_Glyptoscorpius_ differs from other Eurypterids in the possession of
comb-like organs closely resembling the pectines of Scorpions.
_Slimonia_ apparently differs from other genera in that the plate-like
appendages on the posterior three segments of the mesosoma do not meet
in the middle line (Fig. 165, 10–12). In some forms, such as
_Pterygotus_ (Fig. 164), there is a nearly gradual decrease in the width
of the segments in passing from the mesosoma to the metasoma; but in
some others, which in this respect are less primitive, such as
_Slimonia_ (Fig. 165), the posterior five segments of the body (like
those of Scorpions) are distinctly narrower and longer than the
preceding segments. The long tail-spine of _Eurypterus_ is represented
in _Slimonia_ by an oval plate produced into a spine at the end (Fig.
165, 19); whilst in some species of _Pterygotus_ the plate is bi-lobed
at the posterior end (Fig. 164, 19). In _Hughmilleria_ the tail-spine is
lanceolate.

The Eurypterids resemble the Xiphosura in many respects. In both groups
the prosoma consists of at least six fused segments, and bears two pairs
of eyes—one pair simple, the other grouped eyes—on the dorsal surface of
the carapace. The number and position of the appendages of the prosoma
in Eurypterids agree with those of _Limulus_. The chelicerae are closely
similar in both cases. The coxae of all five pairs of legs in
Eurypterids are toothed and function in mastication; similarly in
_Limulus_ all are spiny except the coxae of the last pair of legs. In
both a similar epicoxite is present on the coxae. The number of joints
in the legs is somewhat greater in the Eurypterids than in _Limulus_,
and in the former none of the legs end in chelae, whereas in the latter
all the walking legs, except the last, and also the first in the male,
may be chelate. The metastoma of Eurypterids differs in being a large
unpaired plate, but is represented in _Limulus_ by the pair of
relatively small chilaria. On the mesosoma the genital operculum and
plate-like appendages with branchial lamellae are similar in both
groups, but in the Eurypterids the genital operculum shows a greater
development and covers the second segment, which is without plate-like
appendages. A striking difference between the two groups is seen in the
segments of the mesosoma and metasoma; in Eurypterids these are all
free, whilst in _Limulus_ they are fused together, but this difference
is bridged over by some of the Palaeozoic Xiphosura (Fig. 159, A) in
which those segments are free.

[Illustration:

  FIG. 166.—_Stylonurus lacoanus_, Claypole. Upper Devonian,
    Pennsylvania. Restoration of dorsal surface. Length nearly five
    feet. (After Beecher.)
]

The Eurypterids present a striking resemblance to Scorpions. In both
groups the segments in the three regions of the body are the same in
number, and the appendages of the prosoma also agree in number and
position. The pre-oral appendages are chelate in both, but the second
pair of appendages are chelate in the Scorpions only. In Eurypterids the
coxae of the five pairs of legs are toothed and meet in the middle line,
but in the Scorpions the coxae of the last two pairs do not meet; this
difference, however, appears to be bridged over in the earliest known
Scorpion—_Palaeophonus_,[236] from the Silurian rocks. The Eurypterids
are distinguished from the Scorpions by the much greater development of
the last pair of legs. The large metastoma of the former is homologous
with the sternum of the Scorpion. The genital operculum is much smaller
in Scorpions than in Eurypterids, and in this respect the latter agree
with _Thelyphonus_ (one of the Pedipalpi) more than with the Scorpions.
The pectines are absent in the Eurypterids except in _Glyptoscorpius_.
Instead of the lung-books of the Scorpions the Eurypterids possess
branchial lamellae on the plate-like appendages; but this difference
between the two groups appears to be bridged over by _Palaeophonus_,
which was marine, and may have possessed branchial lamellae since
stigmata seem to be absent.

_Glyptoscorpius_,[237] which is found in the Lower Carboniferous of the
south of Scotland, is a form of considerable interest. It is about a
foot in length, and agrees in many respects with Eurypterida, but it may
be necessary to separate it from that group since it possesses pectines,
and the legs end in a double claw; it cannot, however, be regarded as a
link between Eurypterids and Scorpions, but must rather be considered as
an offshoot from the former, since the latter group was already in
existence at a much earlier period.




                       ARACHNIDA EMBOLOBRANCHIATA
                   (SCORPIONS, SPIDERS, MITES, ETC.)


                                    BY

                          CECIL WARBURTON, M.A.

 Christ’s College, Cambridge; Zoologist to the Royal Agricultural Society




                              CHAPTER XII
    ARACHNIDA (_CONTINUED_)—EMBOLOBRANCHIATA—SCORPIONIDEA—PEDIPALPI


                  SUB-CLASS II.—EMBOLOBRANCHIATA.[238]


                        =Order I. Scorpionidea.=

_Segmented Arachnids with chelate chelicerae and pedipalpi. The abdomen,
which is broadly attached to the cephalothorax or prosoma, is divided
into two regions, a six-jointed mesosoma and a six-jointed tail-like
metasoma, ending in a poison-sting. There are four pairs of lung-books,
and the second mesosomatic segment bears a pair of comb-like organs, the
pectines._


The Scorpions include the largest tracheate Arachnid forms, and show in
some respects a high grade of organisation. It is impossible, however,
to arrange the Arachnida satisfactorily in an ascending series, for
certain primitive characteristics are often most marked in those Orders
which on other grounds would seem entitled to rank at the head of the
group. Such a primitive characteristic is the very complete segmentation
exhibited by the Scorpions. They are nocturnal animals of rapacious
habit. In size they range from scarcely more than half an inch to eight
inches in length. In the northern hemisphere they are not found above
the fortieth parallel of latitude in the Old World, though in the New
World they extend as high as the forty-fifth. A corresponding southward
limit would practically include all the land in the southern hemisphere,
and here the Order is universally represented except in New Zealand,
South Patagonia, and the Antarctic islands.

Fossil scorpions are rarely found. The earliest examples known occur in
the Silurian rocks, and belong to the genus _Palaeophonus_. In the
Carboniferous _Eoscorpius_ is found, and in the Oligocene _Tityus_.

Much remains to be discovered with regard to the habits of scorpions,
and most of the isolated observations which have been recorded lose much
of their value through the uncertainty as to the species concerned. The
brief accounts given by Lankester and by Pocock,[239] and the more
recent and elaborate studies of Fabre,[240] are free from this defect
and contain almost the only trustworthy information we possess.

All are viviparous, and the females carry the newly-hatched young on
their backs. They are predaceous, feeding for the most part on insects
and spiders. These are seized by the chelate pedipalps, and, if small,
are simply picked to pieces by the chelicerae and devoured, but if large
the tail-sting is brought into play and the victim quickly paralysed.
The process of eating is a slow one, and a Cape scorpion in captivity
took two hours to devour a cockroach.

In walking, scorpions carry their pedipalps horizontally in front, using
them partly as feelers and partly as raptorial organs. As regards the
body the attitude varies considerably. In some cases (_Parabuthus_,
_Prionurus_, etc.) it is raised high upon the legs, and the “tail” or
metasoma is curved forward over the back, but in others (_Euscorpius_)
the body is held low, and the “tail” is dragged along behind, the end
only being slightly curled. In the daytime they hide away under wood or
stone, or in pits which they dig in the sand. _Parabuthus capensis_ was
observed to dig a shallow pit by means of its second and third
ambulatory legs, resting on its first and fourth legs aided by the
chelae and the metasoma. Those that hide under wood are usually found
clinging to their shelter ventral side uppermost. In captivity the
creatures, though supplied with water, were never observed to drink;
indeed, they are characteristic inhabitants of arid steppes and parched
wastes. Like most Arachnids they can endure prolonged abstinence from
food.

The only sense well developed seems to be that of touch. Notwithstanding
the possession of several eyes their sight is poor. A moving object
within the range of a few inches is certainly perceived, but it has to
be touched before its nature is recognised. Some writers have attributed
to scorpions a keen sense of hearing, and so-called “auditory hairs” are
described on the tibia of the pedipalp, but Pocock came to the
conclusion that _Parabuthus capensis_ and _Euscorpius carpathicus_ were
entirely deaf, and Lankester could obtain no indication of auditory
powers in the case of _Prionurus_. The sense of touch is extremely
delicate, and seems to reside in the hairs with which the body and
appendages are more or less thickly clothed. The pectines are special
tactile organs. That they are in some way related to sex seems probable
from the fact that they are larger in the male and sometimes curiously
modified in the female, but they appear to be of use also in determining
the nature of the ground traversed by the animal, being long in such
species as raise the body high on the legs, and short in those that
adopt a more grovelling posture. Pocock noticed that a scorpion which
had walked over a portion of a cockroach far enough for the pectines to
come in contact with it immediately backed and ate it.

[Illustration:

  FIG. 167.—_Buthus occitanus_ in the mating period. (After Fabre.)
]

As is the case with most poisonous animals, their ferocity has been much
exaggerated; they never sting unless molested, and their chief anxiety
is to slink off unobserved. The fables that they kill their young, and
that when hard pressed they commit suicide by stinging themselves to
death, perhaps hardly deserve serious consideration. The latter
accusation is disproved by the fact that a scorpion’s poison has no
effect upon itself, or even upon a closely allied species. Some writers
think that in the frantic waving of the “tail,” which is generally
induced by strong excitement, a scorpion may sometimes inadvertently
wound itself with the sharp point of its telson.

Fabre gives a fascinating account of the habits of _Buthus occitanus_,
which occurs in the south of France. He found these scorpions
plentifully in arid, stony spots exposed to the sun. They were always
solitary, and if two were found under the same stone, one was engaged in
eating the other. Their sight is so poor that they do not recognise each
other without absolute contact.

Fabre established colonies in his garden and study, providing them with
suitable soil and sheltering stones. They dug holes by reducing the
earth to powder by means of the three anterior pairs of legs—never using
their pedipalpi in the operation—and sweeping away the débris with the
tail. From October to March they ate nothing, rejecting all food offered
to them, though always awake and ready to resent disturbance. In April
appetite seemed to awaken, though a very trifling amount of food seemed
to suffice. At that time, too, they began to wander, and apparently
without any intention of returning, and they continued daily to escape
from the garden enclosure until the most stringent measures were taken
to keep them in. Not till they were surrounded by glass and the
framework of their cages covered with varnished paper were their
attempts to climb out of their prison frustrated. Fabre came to the
conclusion that they took at least five years to attain their full size.

[Illustration:

  FIG. 168.—The “_promenade à deux_” of _Buthus occitanus_. (After
    Fabre.)
]

His most interesting observations were concerned with their mating
habits, in connection with which he noted some extraordinary phenomena.
After some very curious antics, in which the animals stood face to face
(Fig. 167) with raised tails, which they intertwined—evidently with no
hostile intention—they always indulged in what Fabre calls a “promenade
à deux,” hand in hand, so to speak, the male seizing the chelae of the
female with its own, and walking backwards, while the female followed,
usually without any reluctance. This promenade occupied an hour or more,
during which the animals turned several times. At length, if in the
neighbourhood of a suitable stone, the male would dig a hole, without
for a moment entirely quitting its hold of the female, and presently
both would disappear into the newly-formed retreat.

After mating, the male was often devoured by the female. Moreover, after
any combat with an enemy, such as a _Lycosa_ or a _Scolopendra_, it
appeared to be _de rigueur_ to eat the vanquished, and on such occasions
only was any considerable amount of food consumed.

The scorpions were not, however, anxious to fight, greatly preferring to
retire if possible; but when incited to combat, their sting was quickly
fatal to any mature insect, to spiders and to centipedes. Curiously
enough, however, insect larvae, though badly wounded, did not succumb to
the poison. Newly-hatched scorpions mounted on the mother’s back, where
they remained motionless for a week, entirely unfed. They then underwent
a moult, after which they were able to forage for themselves.


                         =External Structure.=

The chitinous plates of the prosoma are fused to form a carapace. Six
segments are clearly indicated by the six pairs of appendages, but,
though the development of _Scorpio_ affords little direct evidence of
the fact, there is reason to believe that there once existed a
pre-cheliceral segment,[241] as has been clearly proved in the case of
the spiders. An embryonic pregenital segment has also been recognised.
The six prosomatic appendages are those proper to the Arachnida, being
the chelicerae, pedipalpi, and four pairs of ambulatory legs. The
mesosoma, which is broadly attached to the prosoma, comprises six
segments, indicated ventrally by the genital operculum, the pectines,
and the four pairs of pulmonary stigmata. The last of the broad
abdominal segments, which tapers abruptly, belongs to the metasoma,
which also comprises six segments, and is succeeded by the post-anal
spine or sting.

=Prosoma.=—Near the middle of the carapace are two median eyes, and on
its antero-lateral borders are usually to be found groups of smaller
eyes, numbering from two to five. All the eyes are simple. There is a
difference, however, in their development, the median eyes being
_diplostichous_, or involving two layers of hypoderm, while the lateral
eyes are _monostichous_, and pass through a stage strikingly like the
permanent condition of the eyes of _Limulus_. The arrangement of various
slight longitudinal ridges on the dorsal surface of the carapace is of
systematic importance. On the ventral surface, just in front of the
genital operculum, is a sternum, never large, and sometimes barely
visible. Its shape and size constitute one of the principal family
characteristics.

[Illustration:

  FIG. 169.—_Buthus occitanus_. =A=, Dorsal view; =B=, ventral view.
    (After Kraepelin.)
]

=Mesosoma.=—The dorsal plates or terga are distinct, and are connected
by soft chitin with their corresponding sterna.

Beneath the second abdominal segments are borne the “pectines” or
comb-like organs. In their structure four portions are distinguishable,
an anterior lamella or shaft attaching them to the body, a middle
lamella, the teeth, and the fulcra, a series of small chitinous pieces
intercalated between the bases of the movable teeth.

Beneath the third, fourth, fifth, and sixth segments are the paired
openings of the lung-sacs.

=Metasoma.=—The first segment is usually and the remainder are
invariably enclosed in complete chitinous rings and show considerable
variations in their comparative size and shape, and in the arrangement
of the ridges and keels with which they are usually furnished. The
post-anal segment is more or less globular at its base, constituting a
“vesicle,” and terminates in a fine curved point, the “aculeus,”
perforated for the passage of the delicate poison-duct. With the abdomen
fully extended the point is directed downward, but in the attitude of
attack or defence, when the “tail” is carried horizontally over the
back, the sting points forward in the neighbourhood of the animal’s
head.

[Illustration:

  FIG. 170.—=A=, Diagram of a Scorpion’s leg; 1, coxa; 2, trochanter; 3,
    femur; 4, patella; 5, tibia; 6, protarsus; 7, tarsus; _p.s_, pedal
    spur; _t.s_, tibial spur. =B=, Fourth tarsus of _Palamnaeus
    swammerdami_; _l_, lateral lobe. (After Pocock.)
]

=Appendages.=—The three-jointed chelicerae are powerful and chelate. The
first joint is small, but the second is strongly developed and bears at
its anterior end on the inner side a projection which forms the
immovable finger of the chela. The third joint, or movable finger, is
articulated on the outer side, and both fingers are armed with teeth
whose arrangement is useful in distinguishing the species. The pedipalpi
consist of six joints. The coxa is small and has an inwardly directed
lamella which assists in feeding. The trochanter is also a small joint,
bearing, normally at right angles to the longitudinal axis, the powerful
humerus or femur. Then follows the brachium or tibia, again directed
forward, and the last two joints form the chela or “hand,” the terminal
joint or movable finger being on the outer side as in the chelicerae. In
systematic determination special attention is given to the “hand.” In
some forms the upper surface is uniformly rounded, while in others a
“finger-keel” divides it into two flattish surfaces almost at right
angles. The biting edges of the fingers are usually furnished with rows
of minute teeth arranged characteristically in the different genera. The
ambulatory legs are seven-jointed, though, unfortunately, authors are
not agreed upon the nomenclature of the joints. Kraepelin[242] names
them coxa, trochanter, femur, tibia, and three-jointed tarsus, and
Simon[243] agrees with him. Pocock’s names[244] are coxa, trochanter,
femur, patella, tibia, protarsus, and tarsus, and it is certainly
convenient that each joint should have a separate name, but it must be
borne in mind that the tibia of different authors is not always the same
joint. Special attention must be directed to the three terminal joints,
which furnish highly important characteristics. The tibia (in Pocock’s
sense) is sometimes provided with a “tibial spur” at its lower distal
extremity. From the soft arthrodial membrane between the protarsus and
tarsus may proceed one or more dark-tipped claw-like spurs, the “pedal
spurs.” The terminal joint (tarsus of Pocock) is variously furnished
with hairs and teeth, and always ends in a pair of well-developed
movable claws beneath which a much reduced and sometimes almost obsolete
third claw is distinguishable. The tarsus generally projects in a
“claw-lobe” over the base of the superior claws, and sometimes lateral
lobes are present. The first and second coxae have triangular maxillary
lobes directed towards the mouth. The third and fourth coxae are fused
together on each side, and those on one side are separated from those on
the other by the sternum. In other respects the four pairs of legs are
usually similar.


                          =Internal Anatomy.=

The =alimentary canal= is a fairly uniform tube, nowhere greatly
dilated. The very small mouth leads into a small suctorial chamber, and
this is connected by a narrow oesophagus, which pierces the cerebral
nerve-mass, with a slightly dilated portion which receives the ducts of
the first pair of gastric glands, often called salivary glands. The
succeeding portion in the prosoma receives four or five more pairs of
ducts from the well-developed gastric glands. In the rapidly narrowing
first metasomatic segment the intestine receives one or two pairs of
Malpighian tubes, and thence proceeds to the anus, situated ventrally in
the last segment.

The =vascular system= is of the usual Arachnid type, the heart being a
seven-chambered dorsal longitudinal vessel lying in a pericardium, with
which it communicates by seven pairs of valvular ostia. Lankester[245]
has demonstrated several pairs of superficial lateral veins connecting
two deep-seated ventral venous trunks with the pericardium. The
lung-books are, so to speak, pushed in to dilatations of these trunks,
so that some of the lateral veins carry blood newly aerated by the
lung-books directly to the pericardium.

The =nervous system= is not greatly concentrated except in the prosoma,
where there is a single ganglionic mass which innervates not only the
whole prosoma but the mesosoma as far as the first and sometimes the
second pair of lung-books. There are two mesosomatic ganglia, variously
situated in different genera, and each metasomatic segment has its
ganglion.

The =generative organs= are more or less embedded in the gastric glands.
There are two testes, each composed of a pair of intercommunicating
tubules, and connected by a common vas deferens with the generative
aperture, which is furnished with a double protrusible intromittent
organ. A pair of vesiculae seminales and a pair of accessory glands are
also present. The female possesses a single ovary, consisting of a
median and two lateral tubules, all connected by cross branches.

In addition to the external sclerites a free =internal skeletal plate=,
situated in the prosoma between the alimentary canal and the nerve-cord,
furnishes convenient fulcra for muscular attachment. It is known as the
“endosternite.”

Brauer[246] has made the most complete study of the development of
_Scorpio_, and two of the most interesting of his conclusions may be
mentioned here. He has shown the lung-books to be derived from gills
borne on mesosomatic appendages. Moreover he found in the embryo five
pairs of segmental ducts—in segments 3–6 and 8—and demonstrated that
those of segment 5 persisted, though without external aperture, as coxal
glands, and those of segment 8 as the genital ducts.


                           =Classification.=

More than 350 species of scorpions have been described, but many of
these are “doubtful,” and probably the number of known forms may be put
at about 300. These are divided by Kraepelin[247] into six families and
fifty-six genera. The best indications of the family of a scorpion are
to be found in the shape of the sternum, the armature of the tarsi, and
the number of the lateral eyes, while assistance is also to be derived
from the shape of the stigmata and of the pectines, and from the absence
or presence of a spine beneath the aculeus.

The six families are: Buthidae, Scorpionidae, Chaerilidae, Chactidae,
Vejovidae, and Bothriuridae.

=Fam. 1. Buthidae.=—_Sternum small and generally triangular. Tibial
spurs in the third and fourth legs. Generally a spur beneath the
aculeus. Lateral eyes three to five in number._

There are two sub-families: BUTHINAE and CENTRURINAE.

The BUTHINAE, which possess a tibial spur, comprise fourteen genera,
most of them Old World forms. The principal genera are _Buthus_, which
contains about 25 species, and _Archisometrus_ with 20 species. One
genus only, _Ananteris_, is South American, and it includes only a
single species. The genus _Uroplectes_, with 16 species, is almost
entirely African.

The CENTRURINAE, without tibial spur, are New World scorpions, though
_Isometrus europaeus_ (_maculatus_) is cosmopolitan. The principal
genera are _Tityus_ with 30 species, _Centrurus_ with 13, and
_Isometrus_ with 6.

=Fam. 2. Scorpionidae.=—_Sternum broad and pentagonal, with sides
approximately parallel. No tibial spur, but a single pedal spur.
Generally three lateral eyes._

Nearly a hundred species of Scorpionidae have been described,
distributed among fifteen genera. The following sub-families are
recognised: Diplocentrinae, Urodacinae, Scorpioninae, Hemiscorpioninae,
and Ischnurinae.

The DIPLOCENTRINAE have a spur under the aculeus. They form a small
group of only eight species. The principal genus, _Diplocentrus_, is
entirely Neotropical, but _Nebo_ has a single Old World representative
in Syria.

The URODACINAE, with the single genus _Urodacus_, are Australian
scorpions. As in the next sub-family, there are rounded lobes on the
tarsi, but there is only a single keel on the “tail,” and the lateral
eyes are two in number. Six good and three doubtful species are
recognised.

The SCORPIONINAE are Asiatic and African forms, and are recognised by
the tarsi having a large lobe on each side, by the convex upper surface
of the “hand,” by the presence of two median keels on the “tail,” and by
the possession of three lateral eyes. _Palamnaeus_ (_Heterometrus_) has
sixteen species in the Indian region. There are about thirty species of
_Opisthophthalmus_, all natives of South Africa. _Pandinus_ includes
about ten species, but there are only two species of the type genus
_Scorpio_, _S. maurus_ and _S. boehmei_.

The sub-family HEMISCORPIONINAE was formed for the reception of the
single Arabian species _Hemiscorpion lepturus_. Its most striking
characteristic is the cylindrical vesicle of the tail in the male.

The ISCHNURINAE differ from the Scorpioninae chiefly in the absence of
the tarsal lobes, the presence of a well-marked finger-keel, and the
generally more depressed form of the body and hand. In the opinion of
some authors they should be separated from the Scorpionidae as a
distinct family, the Ischnuridae. There are more than twenty species,
divided among six genera. The type genus _Ischnurus_ has only the single
species _I. ochropus_. There are eight species of _Opisthacanthus_,
which has representatives in Africa and America.

=Fam. 3. Chaerilidae.=—_Sternum pentagonal with median depression or
“sulcus” rounded posteriorly. Two pedal spurs. Stigmata circular. Two
lateral eyes with a yellow spot behind the second. Pectines very short._

This small family has the single genus _Chaerilus_ with but seven
species, natives of the Oriental region.

=Fam. 4. Chactidae.=—_Two pedal spurs. Two lateral eyes (or, rarely, no
eyes) but without yellow spot. Characteristic dentition on movable
finger of “hand.”_

There are three sub-families, Megacorminae, Euscorpiinae, and Chactinae.

The MEGACORMINAE include but a single Mexican form, _Megacormus
granosus_. There is a single toothed keel under the “tail,” and all the
under surface is spiny. There is a row of long bristles under the
tarsus.

In the EUSCORPIINAE the upper surface of the hand is divided into two
surfaces almost at right angles by a strong finger-keel. This is a small
group of about six species found in the Mediterranean region. The two
genera are _Euscorpius_ and _Belisarius_.

The CHACTINAE are without any marked keel on the hand. The scorpions of
this sub-family are found in equatorial South America and the West
Indies, where there are more than twenty species divided about equally
between the four genera _Chactas_, _Broteas_, _Broteochactas_, and
_Teuthraustes_.

=Fam. 5. Vejovidae.=—_No tibial, but two pedal spurs. A single row of
hairs or papillae under the tarsus. Sternum generally broader than long.
Elongate stigmata, and three lateral eyes._

Seven of the eight genera of this family include only American forms,
the principal genus being _Vejovis_, with about ten species. The genus
_Scorpiops_, however, belongs to the Indian region and numbers more than
ten species.[248]

=Fam. 6. Bothriuridae.=—_Sternum much reduced and sometimes hardly
visible, consisting of two slight, nearly transverse bars._

Of the seven genera of this family one, _Cercophonius_, is Australian.
The other six genera include some dozen South American forms,
_Bothriurus_ having four species.


                         =Order II. Pedipalpi.=

_Arachnids with non-chelate, two-jointed chelicerae, powerful pedipalpi,
and four pairs of legs, of which only the last three are ambulatory, the
first being used as tactile organs. The cephalothorax is usually covered
by an undivided carapace, but the pedunculated abdomen is segmented.
Respiration is by lung-books._


The Pedipalpi are a little-known group of animals of nocturnal habits.
Though rarely seen they are widely distributed, being found in India,
Arabia, the greater part of Africa, and Central and South America. They
are of ancient origin, a fossil genus, _Graeophonus_, of the
Tarantulidae (Phrynidae, see p. 312), occurring in the Carboniferous
strata in North America. They live under stones and bark, and in caves,
where, when disturbed, they seek safety in crannies in the rock.

Little is known of their habits, but they are believed to feed chiefly
upon insects. The female _Tarantula_ carries the developing eggs,
somewhat after the manner of the Chernetidea (see p. 434), in a bag
beneath the abdomen, the under surface of which becomes concave and
dome-like during the period of gestation.[249]

[Illustration:

  FIG. 171.—_Thelyphonus_, diagrammatic ventral view; about natural
    size. _c_, Coxal joint of pedipalp; _g_, generative opening; _p_,
    pedipalp; _sp_, spiracles; _st_, sternal plates; 1, 2, 3, 4,
    ambulatory legs. (After Pickard-Cambridge.)
]

=External Structure.=—The external features which the members of this
Order have in common are the segmented pediculate abdomen (9 to 12
segments), the two-jointed non-chelate chelicerae, the antenniform first
pair of legs, and the presence of two pairs of lung-book stigmata
beneath the abdomen. The constituent families differ so much in outward
form that they must be dealt with separately.

The Thelyphonidae or “Whip Scorpions” (see p. 312) have a long-oval
carapace bearing well-developed eyes, two in front, and a group of three
or five on either side some distance behind. The pedipalpi are chelate,
and have their basal joints fused beneath the mouth, being thus
incapable of any masticating motion.

The first legs are six-jointed, and have multi-articulate tarsi; the
others are seven-jointed, and their tarsi, in some species at least, are
tri-articulate. The abdomen consists of two portions, a wide
nine-jointed pre-abdomen and a short narrow three-jointed post-abdomen,
to which a filiform tail is articulated. Beneath the cephalothorax,
between the coxae of the legs, is a distinct sternal plate in two
portions (Fig. 171). The first abdominal ventral plate is largely
developed, and covers two segments. Behind it are the median genital
opening and two pulmonary stigmata, while the other stigmata are behind
the second ventral plate, which corresponds to the third abdominal
segment. On the last abdominal segment there are often two or four
light-coloured spots called “ommatoids,” and considered by some authors
to be organs of sight. Laurie, however (_vide infra_), thinks it more
probable that they are olfactory in function.

The Schizonotidae (see p. 312) have a two-jointed carapace, and do not
possess more than two eyes. There is a short unjointed tail-piece.

In the Tarantulidae (Phrynidae) the whole body is much flattened and
extended laterally, the undivided carapace being reniform, and broader
than long. The long non-chelate pedipalps have their basal joints free
and movable, and there are several sternal plates. There are nine
abdominal tergal plates, the last three diminishing rapidly in size, and
the last plate covering a button-like terminal portion of the abdomen.
The first abdominal ventral plate is largely developed, as in the
Thelyphonidae, and the genital orifice and pulmonary stigmata are in the
same situation as in that group. The Tarantulidae have glutinous glands
in the first abdominal segment which are capable of spinning a few
irregular threads.

In the whole group paired circular depressions are conspicuous dorsally
on all the abdominal segments. These indicate the points of attachment
of the dorso-ventral muscles.

=Internal Structure.=—The anatomy of the Pedipalpi has been very
inadequately studied. Disconnected notes on various points of structure
have been published by various morphologists, but no complete
investigation has yet been made of the internal organs. This is largely
due to the difficulty of obtaining material, and the bad state of
preservation of the internal parts of such specimens as have been
available for dissection.

The following points have been made out in the anatomy of
_Thelyphonus_.[250]

The alimentary canal commences after the mouth with a pharynx which,
though not dilated, is furnished with sucking muscles. It then narrows
into an oesophagus which passes through the nerve-mass, and afterwards
dilates to form the mid-gut, which immediately gives off two large
lateral diverticula which extend backwards, each having five lobes.
There are also two median diverticula which proceed from the ventral
surface and pass through the endosternite. The abdominal portion of the
canal is entirely concealed by the great “liver” mass which communicates
with it by four paired ducts in the anterior part of the abdomen. Behind
the fourth abdominal segment the gut is narrow till it expands in the
seventh segment into an hour-glass-shaped stercoral pocket which,
according to Laurie, is a portion of the mesenteron.

The excretory organs are the Malpighian tubes and the coxal glands. The
former are generally described as entering the anterior portion of the
stercoral pocket, but according to Laurie they pass along its ventral
surface, attached to it by connective tissue, and really enter at the
posterior end. The coxal glands are well developed, and lie beneath the
endosternite, opening near the first coxae.

The nervous system is much concentrated and of the usual Arachnid type.
The median abdominal nerve has a ganglion towards its extremity,
supplying, according to Bernard,[251] the muscles which move the tail.
The heart is extremely long, and varies little in width. It has nine
pairs of ostia[252]—two in the thorax and seven in the abdomen. The
generative glands are paired, and in the male there are large seminal
vesicles. In the most ventral portion of the abdominal cavity lies a
remarkable asymmetrically-situated gland, the “stink-gland.” It consists
of a number of secretory tubules communicating with two elongated sacs,
one of which lies beneath the nerve-cord, and therefore medially, while
the other lies far to the left. Their ducts proceed to the anus or its
vicinity.

The caudal organs, or white spots which, as already mentioned, are
usually found on the last of the three post-abdominal segments of
_Thelyphonus_, are of doubtful function. They have been variously
explained as the stink-gland orifices, and as organs sensitive to light
(“ommatoids”). Laurie[253] was unable to find any pore in this region,
nor was there any of the pigment so characteristic of organs of sight.
The histological structure indicated a sense-organ rather than a gland,
but the use of these organs is entirely conjectural.

=Classification.=—The order Pedipalpi is divided into three families—
Thelyphonidae, Schizonotidae and Tarantulidae. The first two are
considered by some authors to form a sub-order, UROPYGI, or tailed
Pedipalpi, while the Tarantulidae constitute the remaining sub-order
AMBLYPYGI, the members of which are tailless.

=Fam. 1. Thelyphonidae.=[254]—This family comprises nine or more genera,
differing chiefly in the position of the eyes, the structure of the
genital operculum, the armature of the pedipalps, and the presence or
absence of “ommatoids” in the anal segment.

The three following genera are among those most likely to be met with.
Two ommatoids are present in each.

_Thelyphonus_ has a spine on the second ventral plate, and a deep median
impression on the male genital operculum, which is, however, absent from
that of the female. There are about fifteen known species of this genus,
inhabiting Southern Asia and the East Indies.

_Typopeltis_ has ridges running forward from the lateral eyes. The
middle third of the female operculum is raised and deeply impressed in
the middle. This genus is represented in China and Japan.
_Mastigoproctus_ has a short and stout coxal apophysis of the pedipalp,
without a tooth on its inner side. It is found in Mexico, Brazil, and
the West Indies. Other genera are _Thelyphonellus_ (Demerara),
_Labochirus_ (Ceylon), _Hypoctonus_ (Burma), _Mimoscorpius_
(Philippines), _Uroproctus_ (Assam), _Abalius_ (New Guinea), without
ommatoids, and _Tetrabalius_ (Borneo), with two pairs of ommatoids.

=Fam. 2. Schizonotidae (= Tartaridae).=—This family contains only two
genera, _Schizonotus_ (= _Nyctalops_, Pickard-Cambridge, nom. preocc.
Aves) and Trithyreus[255] (= _Tripeltis_, Thorell, nom. preocc.
Reptilia). They are very small, pale-coloured forms (about 5 mm. in
length), found in Burma and Ceylon.

=Fam. 3. Tarantulidae=, better known as =Phrynidae=. Pocock has shown
that Fabricius established the genus _Tarantula_ from the species _T.
reniformis_ in 1793, while there is no earlier record of Olivier’s
_Phrynus_, established for the same species, than Lamarck’s citation of
it in 1801. The family is divided into three sub-families, Tarantulinae,
Phrynichinae, and Charontinae.

(i.) The TARANTULINAE are new-world forms, represented by three genera,
_Tarantula_, _Acanthophrynus_ (_Phrynopsis_), and _Admetus_
(_Heterophrynus_), in Central and South America and the West Indies.

(ii.) The PHRYNICHINAE belong to the Old World, being found in Africa,
India, and Ceylon. _Phrynichus_, _Titanodamon_ and _Nanodamon_ are
genera of this sub-family.

(iii.) The CHARONTINAE are natives of South-East Asia and the Pacific
Islands. There are five genera and eight species.




                              CHAPTER XIII
  ARACHNIDA EMBOLOBRANCHIATA (_CONTINUED_)—ARANEAE—EXTERNAL STRUCTURE—
                          INTERNAL STRUCTURE.


                         =Order III. Araneae.=


                       (ARANEIDA,[256] ARANEINA.)

_Arachnida breathing by tracheae and “lung-books.” Cephalothorax and
pedicellate abdomen, the latter usually soft, and only very rarely
showing any traces of segmentation. Two-jointed non-chelate chelicerae,
the distal joint bearing the orifice of a poison-gland. The tarsal joint
of the male pedipalp develops a sexual organ. The abdomen is furnished
with spinning mammillae._


The true Spiders can readily be distinguished from allied Arachnid
groups, with which they are often popularly confounded, by the presence
of a narrow constriction or “waist” between the cephalothorax and
abdomen, and of a group of “spinnerets” or external spinning organs
beneath the hind portion of the body. Thus the so-called
“Harvest-spider” or “Harvestman” is clearly not a Spider, for there is
no constriction of its body into two parts, nor does it possess any
spinnerets. It belongs to the Phalangidea. The same considerations will
exclude the “Red Spider” of popular nomenclature, which must be referred
to the Acarina or Mites.

The Araneae, even as at present known, form a very extensive and
widely-distributed order of animals. Compared with certain insect
orders, they have received little attention from the collector, and the
number of known forms is certain to be very largely increased. They form
an extremely compact and natural group, for though, within the order,
there is an infinite variety of detail, their uniformity in essential
points of structure is remarkable, and they are sharply marked off from
the neighbouring groups of Arachnida.

[Illustration:

  FIG. 172.—_Epeira angulata._ ♀.
]

It is perhaps unfortunate that the obtrusiveness of particularly
unattractive specimens of the race has always caused spiders to be
regarded with more or less aversion. This prejudice can hardly fail to
be modified by a wider acquaintance with these animals. There are
certainly few groups which present points of greater interest in respect
to their adaptation to special modes of life and the ingenuity displayed
in the construction of their nests and the ensnaring of their prey.

Spiders are wingless, yet they may often be observed travelling through
the air. They are air-breathing, yet many are amphibious in their
habits, and one species at least spends the greater part of its
existence beneath the surface of the water. On land they may be found in
all imaginable localities which admit of the existence of that insect
life on which they depend for food.

=External Structure.=—The spider’s body consists of two portions, the
cephalothorax and the abdomen.

=Cephalothorax.=—Looked at dorsally (Fig. 173), the cephalothorax is
generally seen to have a depression near the middle, the “median fovea,”
and from this certain lines, the “radial striae,” radiate towards the
sides. These depressions indicate the attachment of internal muscles.

The head region or “caput” lies in front of the foremost of the radial
striae, and is often clearly marked off from the thorax, and different
from it in elevation. It bears the eyes, which, in the great majority of
spiders, are eight in number. Many, however, are six-eyed, while in rare
cases the number is reduced to four (_Tetrablemma_, see p. 404), or even
to two (_Nops_, see p. 395). The number, relative size, and particular
arrangement of these eyes are of considerable systematic importance.
Their disposition varies very greatly, but it is generally possible to
regard them as forming two transverse rows, an anterior and a posterior,
each possessing a pair of median and a pair of lateral eyes.

[Illustration:

  FIG. 173.—Diagrammatic dorsal view of a Spider. _ch_, Chelicera; _f_,
    median fovea; _n_, normal marking; _o_, ocular area; _p_, pedipalp;
    _st_, stria. (The dotted line should reach the radial marking on the
    cephalothorax.)
]

In many spiders all the eyes have a dorsal aspect, but in some groups
(Attidae, Lycosidae) the prevailing arrangement is to have the anterior
eyes directed forwards and the posterior upwards. In other spiders,
again, a dorsal view may only show the eyes in profile, all having their
axes directed forwards or sideways, or they may be mounted on turrets,
and thus command a wide range of view. The rows are described as
straight, “procurved” (with the convexity backwards), or “recurved”
(with the convexity forwards). Thus, in Fig. 177, the anterior row is
slightly, and the posterior row considerably “recurved.”

Sometimes there is a marked difference in the colour of the eyes, two or
more being black, while the remainder are pearly white. In other cases
they are homogeneous, either of the black or the white type. Simon
considers the black eyes to be diurnal and the white nocturnal, but the
evidence for this is indirect and not altogether satisfactory. The
portion of the caput occupied by the eyes is often alluded to as the
“ocular area.” The space between the ocular area and the chelicerae,
well shown in Fig. 177, is known as the “clypeus.” It is usually more or
less vertical, but in the Aviculariidae (see p. 386) it is horizontal
and dorsal.

The under surface of the cephalothorax is protected by the “sternum” or
“plastron,” a large plate of variable shape, usually notched at either
side for the reception of the legs, and having in front a small plate,
generally hinged, but sometimes soldered to it, known as the “labium.”
This has no homology with the labium of insects, but is a true sternite,
more correctly described as “pars labialis sterni.”

The labium and the maxillary lobes of the palpi more or less conceal the
under surface of the caput. The shape of the sternum and of the labium,
and the contour and degree of inclination towards one another of the
maxillae, are important considerations in the taxonomy of Spiders.

[Illustration:

  FIG. 174.—Diagrammatic ventral view of a Spider. Cephalothorax—_l_,
    Labium; _m_, maxilla; _p_, paturon of chelicera; _st_, sternum; _u_,
    unguis of chelicera. Abdomen—_a.t_, Anal tubercle; _c_, colulus;
    _ep_, epigyne; _s_, stigma; _sp_, spinnerets; _tr_, tracheal
    opening.
]

The appendages of the cephalothorax, which are the chelicerae or jaws,
the pedipalpi or feelers, and the four pairs of ambulatory legs, will be
treated separately.

=Pedicle.=—The chitinous investment of the narrow stalk which unites the
thorax with the abdomen is for the most part thin and flexible, with
only slight indurations of various patterns on the dorsal surface, where
it is in most cases more or less protected by the forwardly-projecting
abdomen. Beneath, it is usually quite membranous, guarded only by a sort
of collar formed by the raised border of the anterior portion of the
abdomen at the point of insertion. In some Spiders, however
(Dysderidae), there is a posterior sternal plate, the “plagula,” closely
corresponding with the labium in front, which partly embraces the
pedicle. In _Hermippus_ (Zodariidae) the plagula is detached from the
sternum, and is succeeded posteriorly by two smaller paired plates.

=Abdomen.=—The abdomen differs remarkably in shape in the different
groups of Spiders. In some families the prevailing shape is more or less
globular, and in others cylindrical, while it may be diversified to
almost any extent by prominences or spines. Ordinarily no sign of
segmentation is observable, but in _Liphistius_ it is covered dorsally
by seven well-marked chitinous plates.

In most Spiders the integument of the abdomen is uniformly soft and
flexible all over, but it is not rare to find portions of it thickened
and hardened to form “scuta.” In the Gasteracanthinae and the
Phoroncidinae there is a great dorsal scutum armed with spines, while in
several families there are species characterised by the possession of a
smooth dorsal scutum; and in some a ventral scutum is present.

[Illustration:

  FIG. 175.—Spider profiles. 1, _Poltys ideae_; 2, _Phoroncidia
    7–aculeata_; 3, _Ariamnes flagellum_; 4, _Stegosoma testudo_; 5,
    _Formicinoides brasiliana_.
]

That these scuta are sometimes indicative of an obsolete segmentation
would seem likely from the study of the remarkable species, _Tetrablemma
medioculatum_ (Fig. 176), described by Pickard-Cambridge, from Ceylon.
In addition to large dorsal and ventral scuta, the sides and posterior
extremity are guarded by smaller scuta, the disposition of which is well
seen in the figure.

[Illustration:

  FIG. 176.—_Tetrablemma medioculatum_, much enlarged. =A=, Posterior
    view; =B=, profile, showing the scuta. (After Cambridge.)
]

The normal smooth abdomen presents dorsally no very striking features.
In species of variegated coloration there is very generally noticeable a
median dentated band (Fig. 173), the “normal marking” of some writers,
which would appear to have some correlation with the underlying dorsal
vessel. Beneath the abdomen are to be seen the orifices of the breathing
and genital organs, the spinnerets, and the anal aperture upon its
tubercle.

The breathing organs are, as will be explained later, of two kinds,
lung-books and tracheae. The great majority of Spiders possess only two
lung-books, and their transverse, slit-like openings (“stigmata” or
“spiracles”) may be seen on either side of the anterior part of the
abdomen. Where, as in the Theraphosae, there are four lung-books, the
second pair open by similar slits a short distance behind the first.
According to Bertkau, pulmonary sacs are entirely lacking in the genus
_Nops_.

The tracheae generally debouch by a single median stigma towards the
posterior end of the abdomen, just in front of the spinnerets. This
opening clearly results from the fusion of two stigmata, which in some
species retain their paired arrangement.

On a level with the openings of the anterior lung-books or pulmonary
sacs there is usually observable a slight transverse ridge, the
epigastric fold (Fig. 174), and in the centre of this is the genital
opening. This is never visible until after the last moult, and in the
male is always a simple inconspicuous aperture. This is also the case
with the females of some groups (Theraphosae, Filistatidae, Dysderidae,
etc.), but in most cases there is a more or less complicated armature,
the “epigyne,” the special design of which is of great specific value.
In its simplest form it is merely a plate, usually of dark colour, with
one or two apertures (Fig. 174, _ep_), but in some families, notably the
Epeiridae, it is more complicated, and is furnished with a hooked median
projection, the “ovipositor” (“clavus” of Menge), which is often
absurdly like a petrified elephant’s trunk in miniature.

The abdomen also presents on its under surface, usually towards the
posterior end or apex, a group of finger-like mammillae or spinnerets.
They are normally six in number, two superior (or posterior), two
median, and two inferior (or anterior). The number is reduced, in most
of the Theraphosae, to four, while a few spiders possess only a single
pair of spinnerets. These organs are described more fully on p. 325.

A small papilla, the “colulus” (Fig. 174, _c_), is often observable,
projecting between the anterior spinnerets. The “anal tubercle” (Fig.
174, _a.t_), on which the vent is situated, terminates the abdomen, and
is generally in close juxtaposition with the posterior spinnerets.

=Appendages.=—The cephalothoracic appendages are the chelicerae, the
pedipalpi, and the four pairs of ambulatory legs. Those of the abdomen
are the mammillae or spinnerets.

=Chelicerae.=—These are two-jointed appendages, articulated immediately
below or in front of the clypeus. They are the “mandibles” of many
authors, but there is good reason for believing that they are not
homologous with the mandibles of Insects. There is little agreement,
moreover, with regard to the names given to the two joints of which they
consist. The term “falx,” often applied to the basal joint, is much more
appropriate to the sickle-like distal joint. Base and fang are tolerably
satisfactory, or we may avoid ambiguity by adopting the terms “paturon”
and “unguis” suggested by Lyonnet.[257]

The paturon is a stout joint of more or less cylindrical or conical
shape. The unguis (the “crochet” of Simon) is hook-like, and can
generally be folded back upon the paturon, which often presents a groove
for its reception. The Theraphosid spiders are distinguished from all
others by the fact that the plane of action of the chelicerae is
vertical and longitudinal. The paturon projects forward in a line
parallel with the axis of the body, and its distal end can be raised or
depressed, but not moved laterally; while the unguis in action has the
point directed downwards, and, at rest, is applied to the under surface
of the paturon.

In other spiders the patura hang more or less vertically, and while to
some extent mobile in all directions, their principal motion is
_lateral_, and the ungues have their points directed towards each other
in action, and are applied to the inner surfaces of the patura in
repose. The plane of action in this case is also more or less vertical,
but _transverse_.

[Illustration:

  FIG. 177.—Front view of _Textrix denticulata_. × about 10. 1, Caput;
    2, eyes; 3, paturon; and 4, unguis of chelicera.
]

The paturon is always extremely hard and strong. In Theraphosae of
burrowing habits the distal end is furnished with a group of powerful
teeth, the “rastellus.” The groove for the reception of the unguis is
often guarded on one side or on both by rows of teeth, the arrangement
of which is frequently an important specific character. The inner
anterior border is also often furnished with a group of stiff hairs or
bristles. This powerful joint is of use in crushing and expressing the
fluids of insects pierced by the ungues.

The crescent-shaped unguis is tapering and smooth, except for the
presence, on the posterior surface, of one or two feebly dentated
ridges. Near its free extremity there is a small orifice leading to the
poison reservoir and gland.

In the genus _Pholcus_ (see p. 401) the chelicerae may almost be
regarded as chelate, the unguis being met by a spiny projection from the
inner anterior border of the paturon.

=Rostrum.=—On examining a spider, even under a dissecting microscope, it
will not be easy at first to discover the mouth. Indeed, Lyonnet had
almost come to the conclusion that Spiders, like some Myrmelionid
larvae, imbibed the juices of their prey by way of the mandibles, before
he found the orifice and gave a remarkably accurate description of the
adjacent parts.

If a specimen be placed on its back, and the labium raised while the
chelicerae are pushed forward, no orifice is visible, but on careful
examination it will be found that what appears to be a thick and fleshy
labium is, in reality, two organs. The labium is thin and flat, and
closely opposed to its upper surface is a somewhat flattened cone. This
is the “rostrum,” and when it is separated from the labium the buccal
orifice is disclosed. In a few spiders (Archeidae) in which the
chelicerae are far removed from the mouth, the rostrum is tolerably
conspicuous, but in most it is so hidden as to have escaped the
observation of the great majority of observers. Schimkewitsch considers
it homologous with the labrum of insects, but Simon thinks that it
represents all the insect mouth-parts reduced to an exceedingly simple
form. It is more probable that a beak consisting of a simple labrum and
labium was a primitive Arachnid characteristic. If the rostrum be
removed and its inner (or posterior) surface examined, a lance-shaped
chitinous plate, the “palate,” becomes visible. It is furrowed down the
middle by a narrow groove, which is converted into a tube for the
passage of fluids when the rostrum is opposed to the labium.

[Illustration:

  FIG. 178.—Pedipalp of _Tegenaria domestica_ ♂, × 5. 1, Coxa; 2,
    maxilla; 3, trochanter; 4, femur; 5, patella; 6, tibia; 7, tarsus;
    8, palpal organ.
]

=Pedipalpi.=—The pedipalpi are extremely leg-like feelers, and are
six-jointed, the metatarsal joint of the ambulatory legs being absent.
The joints, therefore, are the coxa, trochanter, femur, patella, tibia,
and tarsus (Fig. 178).[258]

In the Theraphosae the coxa resembles that of the ambulatory leg, but in
other spiders it is furnished, on the inner side, with a blade-like
projection, the “maxilla” (Fig. 178). The shape of the maxillae and the
degree of their inclination towards the labium are of considerable
taxonomic importance. The inner border of the maxilla is furnished with
a tuft of hairs, which assist in retaining the juices expressed by the
chelicerae, and its anterior border presents a cutting edge with a
finely dentated ridge called the “serrula.”

In the female, and in the immature male, the remaining joints differ
little from those of the legs, except that the tarsal joint is either
clawless or has a single claw, which is generally smooth, and is never
much dentated.

At the last moult but one the male pedipalp appears tumid at the end,
and after the last moult the tarsus is seen to have developed a
remarkable copulatory apparatus, the “palpal organ,” comparatively
simple in some families, but in others presenting an extraordinary
complexity of structure.

[Illustration:

  FIG. 179.—Diagram of palpal organ. 1, Tarsus; 2, bulb; 3, receptaculum
    seminis; 4, its aperture; 5, style; 6, haematodocha; 7, alveolus; 8,
    tibia.
]

=Palpal Organs.=—Externally the essential parts of the palpal organ are
three, the “haematodocha,” the “bulb,” and the “style.” The spines and
projections, or “apophyses,” which often accompany the palpal organ
proper, are of secondary importance, and in many spiders are entirely
absent; nor is their function when present at all clear; but the
infinite variety of design which they exhibit, and their singular
uniformity in all the males of a species, render them of the utmost
value as specific characteristics.

The “haematodocha” is the portion of the palpal organ attached to the
tarsus, and often received into an excavation, the “alveolus,” on its
under surface. It is a fibro-elastic bag, in its normal collapsed state
usually somewhat spirally disposed round the base of the following
portion, the “bulb.”

The bulb is generally the most conspicuous portion of the organ, and is
a sub-globular sac with firm, though often semi-transparent, integument.
Its base rests upon the haematodocha, and its apex is produced, often
spirally, to a point which bears the seminal orifice. This external
opening leads into a coiled tube within the bulb, ending in a blind sac,
the “receptaculum seminis,” which projects into the haematodocha; and it
is the aperture by which the sperm both enters and leaves the organ. How
the sperm is conveyed to the receptaculum was long a matter for
speculation, after the belief in a direct communication between the
generative glands and the pedipalpi had been abandoned. The process has
been actually observed in the case of a few spiders, which have been
seen to deposit their sperm on a small web woven for the purpose, and
then, inserting the styles of their palpal organs into the fluid, to
suck it up into the receptacula seminis. This is probably the usual
method of procedure, though it may be true, as some have asserted, that
the palp is sometimes applied directly to the genital orifice.

The receptaculum and its tube being thus charged with sperm, it is the
function of the haematodocha to eject it by exerting pressure on its
base. For this purpose the haematodocha is in communication with the
cavity of the tarsus, from which, in copulation, it receives a great
flow of blood, and becomes greatly distended. Bertkau believes that he
has detected very minute pores (meatus sanguinis) communicating between
the haematodocha and the receptaculum, and allowing some of the
blood-plasma from the former to mingle with the semen, but this appears
to be very doubtful.

The =Legs= are uniformly eight in number, and are seven-jointed, the
joints, counting from the body, being the _coxa_, _trochanter_, _femur_,
_patella_, _tibia_, _metatarsus_, and _tarsus_.[259] In a few cases,
through the presence of false articulations, _i.e._ rings of softer
chitin, this number appears to be exceeded. Some of the Palpimanidae
(see p. 398) were at first thought to have only six joints on their
anterior legs, but the tarsus is present, though very small.

In the case of most spiders, the legs take a general fore and aft
direction, the first pair being directed forwards, the second forwards
or laterally, and the third and fourth backwards. In the large group of
“Crab-spiders” (Thomisidae), and in many of the Sparassinae, all the
legs have a more or less lateral direction, and the spider moves with
equal ease forwards, backwards, or sideways. The legs are usually more
or less thickly clothed with hairs, but in some genera the clothing is
so sparse that they appear glossy, while in others they have a
positively shaggy appearance. Stouter hairs or “bristles” are often
present, and some of the joints are also often furnished with “spines,”
which in many cases are erectile.

The tarsi of all spiders are furnished with terminal claws, usually
three in number, though in some families (Drassidae, Thomisidae, etc.)
there are only two. The two principal claws are paired and usually
dentated, though the number of their teeth may be unequal. The third
claw, when present, is always smaller, median, and inferior.

[Illustration:

  FIG. 180.—Spider tarsi. 1, Tarsus of _Epeira_ showing three claws and
    supplemental serrate hairs (_a_); 2, tarsus of a Thomisid Spider,
    with two claws; 3, 3_a_, lateral and dorsal view of tarsus of an
    Attid Spider, showing scopula at _b_.
]

In many spiders of climbing habits the place of the third claw is taken
by a remarkable tuft of club-like hairs termed a “scopula” (Fig. 180,
_b_), by means of which they are able to cling to smooth surfaces where
claws would be able to obtain no hold. In some species there is a
special false articulation—the “onychium”—at the end of the tarsus to
bear the claws.

In the Cribellatae the metatarsus is always furnished with a comb-like
organ, the “calamistrum,” correlated with an extra spinning apparatus,
the “cribellum,” but this will be dealt with when we reach the
systematic portion of the subject.

The general direction taken by the legs, the comparative length of the
different joints, their armature of hairs, bristles, and spines, and the
number and conformation of the tarsal claws, are points of great
importance in the classification of Spiders.

Under considerable magnification the legs of all Spiders exhibit a
number of minute organs, arranged with absolute uniformity throughout
the Araneae, and known as the “lyriform organs.” They consist of little
parallel ridges of thickened chitin, the slit between them being covered
by thinner chitin. They are eleven on each leg, and are distributed near
the distal extremities of each of the first six joints. Their function
is unknown, though some authors consider them to be organs of hearing.

[Illustration:

  FIG. 181.—Spinnerets of _Epeira diademata_. =A=, Ventral view of
    _Epeira_; =B=, spinnerets magnified; =C=, profile.
]

The =Spinnerets= are normally six in number, and, except in rare
instances, are placed beneath the abdomen, near its apex and immediately
in front of the anal tubercle. Their arrangement varies greatly, but
they can generally be recognised as comprising three pairs, a posterior
(or superior) pair, a median pair, and an anterior (or inferior) pair.

In nearly all the Theraphosae the anterior pair are absent, while the
posterior spinnerets are largely developed. In the Palpimanidae only the
anterior spinnerets are present. When all six are found, the usual
arrangement is in the form of a rosette, the median spinnerets being
hidden by the others in repose, but this disposition is widely departed
from. In _Hahnia_ (Agelenidae), for instance, they are ranged in a
transverse row at the end of the abdomen, the posterior spinnerets
occupying the extremities of the row, and the median ones the centre.

These spinnerets are highly mobile appendages, and additional play is
given to their action by the presence of articulations, much resembling
the “false” joints sometimes found on the legs, on the posterior and
anterior pairs. They are always at least bi-articulate, and sometimes
present three or four joints. They are movable turrets on which are
mounted the “fusulae” or projections where the tubes from the spinning
glands open. These are often very numerous, especially in the
orb-weaving spiders, where the spinning powers are most highly
developed. They consist of two portions, a cylindrical or conical basal
part, succeeded by a very fine, generally tapering tube.

In some spiders the fusulae are all much alike, but usually a few very
much larger than the rest are noticeable under the microscope, and these
are often alluded to as “spigots.” The smaller ones are also divisible
into two kinds, a few short conical fusulae being noticeable amongst the
much more numerous cylindrical tubes. We shall treat of the functions of
the various fusulae later (see pp. 335 and 349).

Simon remarks that though the battery of fusulae is most complicated in
those spiders which possess the greatest spinning powers, it is by no
means among them that extremely long spinnerets are developed. The
posterior spinnerets of some of the Hersiliidae are of great length, but
these spiders spin very little except in forming their egg-cocoons.

[Illustration:

  FIG. 182.—=A=, Spinnerets of _Amaurobius similis_ ♀. Much enlarged.
    _a_, Anus; _cr_, cribellum; _i.s_, inferior spinneret; _m.s_, median
    spinneret; _s.s_, superior spinneret. =B=, Part of the 4th leg of
    the same Spider, showing the calamistrum (_ca_) on the metatarsus.
]

In addition to the six spinnerets, and just in front of them, there is
to be found in some spiders an extra spinning organ in the form of a
double sieve-like plate, the “cribellum.” This is always correlated with
a comb of curved bristles on the metatarsi of the fourth pair of legs,
the “calamistrum.” Such importance is assigned to these organs by Simon,
that the Araneae Veraeare divided by him according to whether they are
present or absent, into CRIBELLATAE and ECRIBELLATAE. This is probably
an exaggerated view of the importance of these organs, and the spiders
possessing them certainly do not seem to form a natural group.

=Stridulating Organs.=—When Arthropod animals are capable of producing a
sound, the result is nearly always obtained by “stridulation,” that is,
by the friction of two rough surfaces against each other. The surfaces
which are modified for this purpose form what is called a “stridulating
organ.” Such organs have been found in three very distinct Spider
families, the Theridiidae, the Sicariidae, and the Aviculariidae.
Hitherto they have only been observed in three positions—either between
the thorax and abdomen, or between the chelicerae and the pedipalpi, or
between the pedipalpi and the first legs.

In the Sicariidae and the Aviculariidae, the sounds have been distinctly
heard and described. Those produced by the Theridiidae would appear to
be inaudible to human ears.

[Illustration:

  FIG. 183.—Stridulating apparatus of _Steatoda bipunctata_, ♂. Much
    enlarged. =A=, Ridged and toothed abdominal socket; =B=, striated
    area on the cephalothorax; =C=, profile of the Spider, × 5.
]

Westring[260] was the first to discover (1843) a stridulating organ in
the small Theridiid spider _Asagena phalerata_. The abdomen, where the
pedicle enters it, gives off a chitinous collar, which projects over the
cephalothorax, and has the inner surface of the dorsal part finely
toothed. When the abdomen is raised and depressed, these teeth scrape
against a number of fine striae on the back of the posterior part of the
cephalothorax. A similar organ has been since found in various allied
spiders, of which the commonest English species is _Steatoda
bipunctata_. In this group it is generally possessed by the male alone,
being merely rudimentary, if present at all, in the female.

In 1880 Campbell[261] observed that in some of the Theridiid Spiders of
the genus _Lephthyphantes_, the outer surface of the chelicera and the
inner surface of the femur of the pedipalp were finely striated at the
point, where they were rubbed together when the palps were agitated, but
though the appropriate motion was frequently given, he could hear no
sound.

[Illustration:

  FIG. 184.—_Chilobrachys stridulans_ in stridulating attitude. After
    Wood-Mason. Natural size.
]

Meanwhile the noise produced by a large Theraphosid spider in Assam
(_Chilobrachys stridulans_) had attracted attention, and its
stridulating apparatus was described in 1875 by Wood-Mason.[262] The
sound resembled that obtained by “drawing the back of a knife along the
edge of a strong comb.”

Subsequently certain Sicariid spiders of a genus confined to the
southern hemisphere were heard to produce a sound like the buzzing of a
bee by the agitation of their palps, and both sexes were found to
possess a very perfect stridulating organ, consisting of a row of short
teeth on the femur of the pedipalp, and a striated area on the paturon
of the chelicera.

Pocock has recently discovered that all the large kinds of Theraphosidae
in the countries between India and New Zealand are, like _Chilobrachys_,
provided with a stridulating organ. In these spiders also it is between
the palp and the chelicera, and consists of a row of teeth or spines
constituting a “pecten,” and a series of vibratile spines or “lyra,” but
whereas in _Chilobrachys_ and its near relations the lyra is on the palp
and the pecten on the paturon, in other spiders the positions are
reversed. The lyra is a very remarkable organ, consisting of
club-shaped, often feathery bristles or spines, which lie parallel to
the surface to which they are attached, and which is slightly excavated
for their reception.

Lastly, many African Theraphosids possess a similar organ, not between
the palp and the chelicera, but between the palp and the first leg.

Various suggestions have been hazarded as to the use of these organs,
but they partake largely of the nature of conjecture, especially in
connexion with the doubt as to the possession of a true auditory organ
by the Araneae. They may be summarised as follows. The Theridiid spiders
are among those which show most indication of auditory powers, and the
stridulating organs, being practically confined to the male, may have a
sexual significance. _Chilobrachys_ stridulates when attacked, assuming
at the same time a “terrifying attitude,” and its stridulating organ may
serve the purpose attributed to the rattle of the rattlesnake, and warn
its enemies that it is best let alone. If this be the case, there is no
need that it should itself hear the sound, and, indeed, there is no
evidence that the Aviculariidae possess the power of hearing. In the
inoffensive stridulating Sicariid spiders the sounds could hardly serve
this purpose, and the presence of the organ in both sexes, and in
immature examples, precludes the idea that its function is to utter a
sexual call. Instead of trying to escape when disturbed, the spider
starts stridulating, and Pocock suggests that the similarity of the
sound produced to the buzzing of a bee may be calculated to induce its
enemies to leave it in peace.


                          =Internal Anatomy.=

=Alimentary System.=—The alimentary canal of the Spider is divided into
three regions, the “stomodaeum,” the mid-gut or “mesenteron,” and the
hind-gut or “proctodaeum.”

=The Stomodaeum= consists of the pharynx, the oesophagus, and the
sucking stomach. As we have said, the mouth is to be found between the
rostrum and the labium. It opens into the pharynx, the anterior wall of
which is formed by a chitinous plate on the inner surface of the
rostrum, sometimes called the palate. As the inner surfaces of the
rostrum and labium are practically flat, the cavity of the pharynx would
be obliterated when they are pressed together, were it not for a groove
running down the centre of the palate, which the apposed labium converts
into a tube, up which the fluids of the prey are sucked. In the
Theraphosidae there is a corresponding groove on the inner surface of
the labium.

At the top of the pharynx, which is nearly perpendicular, the canal
continues backwards and upwards as a narrow tube, the oesophagus,
passing right through the nerve-mass, which embraces it closely on all
sides, to the sucking stomach. At the commencement of the oesophagus is
the opening of a gland, probably salivary, which is situated in the
rostrum.

[Illustration:

  FIG. 185.—Diagram showing the anatomy of the cephalothorax of a
    Spider. The right alimentary diverticulum has been removed. _a_,
    Aorta; _c_, left diverticulum with secondary caeca; _e_,
    endosternite; _oes_, oesophagus, descending to the mouth; _s_,
    sucking stomach; _sh_, dorsal shield of sucking stomach.
]

We now reach the sucking stomach, which occupies the centre of the
cephalothorax. It is placed directly over a skeletal plate, the
“endosternite” (Fig. 185, _e_), to which its lower surface is connected
by powerful muscles, while its upper wall is protected by a hard plate
or “buckler,” which is similarly attached to the roof of the
cephalothorax in the region of the “fovea media.” The walls of the
stomach are not themselves muscular, but by the contraction of the
muscles above mentioned its cavity is enlarged, and fluids from the
pharynx are pumped up into it.

The canal thus far is lined by chitin, like the exterior of the body,
and forms a sort of complicated mouth-apparatus.

The =Mesenteron= lies partly in the cephalothorax and partly in the
abdomen. The thoracic portion, shortly behind the sucking stomach, sends
forward on either side a large branch or “diverticulum,” from each of
which five secondary branches or “caeca” are given off (Fig. 185). Of
these the anterior pair sometimes join, thus forming a complete ring;
but usually, though adjacent, they remain distinct. The other four pairs
of caeca curve downwards, protruding into the coxae of the legs, where
they often terminate, but sometimes (_Epeira_) they continue their curve
until they meet, though they never fuse, under the nerve-mass. Behind
the origin of the diverticula the mesenteron continues as a widish tube,
and shortly passes through the pedicle and enters the abdomen, where,
curving slightly upwards, it proceeds along the middle line till it ends
in the proctodaeum.

In the abdomen it is surrounded by a large gland, the so-called liver,
and is dilated at one spot (Fig. 186) to receive the ducts from this
gland. The fluid elaborated by this large abdominal gland has been shown
to have more affinity with pancreatic juice than with bile.

The =Proctodaeum= consists of a short rectum, from the dorsal side of
which protrudes a large sac, the “stercoral pocket.” At its origin, the
rectum receives the openings of two lateral tubes which reach it after
ramifying in the substance of the liver. These have been called
“Malpighian tubules,” but their function is unknown. Loman[263] has
shown that they open into the mid-gut and not into the rectum, and there
is reason to believe that true Malpighian tubules homologous to those of
Insecta are absent in Arachnida, where their place seems to be taken by
the coxal glands, which are considered to be the true excretory organs.
In most spiders they open near the third coxae. Like the stomodaeum, the
proctodaeum has a chitinous lining.

=Vascular System.=—The earlier investigations on the circulation of the
blood in Spiders were made by direct observations of the movements of
the blood corpuscles through the more or less transparent integuments of
the newly-hatched young. Claparède’s[264] results were arrived at by
this method. It is invaluable for demonstrating roughly the course taken
by the blood, but in these immature spiders the blood-system has not
attained its full complexity, and other methods of research have shown
the spider to possess a much more elaborate vascular system than was at
first suspected.

The tubular heart lies along the middle line in the anterior two-thirds
of the abdomen, sometimes close up against the dorsal wall, but
occasionally at some little distance from it, buried in the substance of
the liver. It is a muscular tube with three pairs of lateral openings or
“ostia,” each furnished with a simple valve which allows the entrance,
but prevents the exit, of the blood. It is contained in a bag, the
“pericardium,” into which the ostia open. Both heart and pericardium are
kept in place by a complicated system of connective tissue strands, by
which they are anchored to the dorsal wall of the abdomen. Eight
arteries leave the heart, the principal one, or “aorta,” plunging
downward and passing through the pedicle to supply the cephalothorax.
Besides this, there is a caudal artery at the posterior end, and three
pairs of abdominal arteries, which proceed from the under surface of the
heart, and the ramifications of which supply, in a very complete manner,
the various organs of the abdomen. The heart is not divided up into
compartments. The anterior aorta passes through the pedicle, above the
intestine, and presently forks into two main branches, which run along
either side of the sucking stomach, near the front of which they bend
suddenly downwards and end in a “patte d’oie,” as Causard[265] expresses
it—a bundle of arteries which proceed to the limbs (Fig. 185). Where the
downward curve begins, a considerable artery, the mandibulo-cephalic,
runs forward to supply the chelicerae and the head region. We have
omitted certain minor branches from the main trunks which supply the
thoracic muscles. The nerve-mass receives fine vessels from the “patte
d’oie.”

[Illustration:

  FIG. 186.—Diagram of a Spider, _Epeira diademata_, showing the
    arrangement of the internal organs, × about 8. 1, Mouth; 2, sucking
    stomach; 3, ducts of liver; 4, so-called Malpighian tubules; 5,
    stercoral pocket; 6, anus; 7, dorsal muscle of sucking stomach; 8,
    caecal prolongation of stomach; 9, cerebral ganglion giving off
    nerves to eyes; 10, sub-oesophageal ganglionic mass; 11, heart with
    three lateral openings or ostia; 12, lung-sac; 13, ovary; 14,
    acinate and pyriform silk-glands; 15, tubuliform silk-gland; 16,
    ampulliform silk-gland; 17, aggregate or dendriform silk-glands; 18,
    spinnerets or mammillae; 19, distal joint of chelicera; 20,
    poison-gland; 21, eye; 22, pericardium; 23, vessel bringing blood
    from lung-sac to pericardium; 24, artery.
]

There are no capillaries, but the blood is delivered into the tissues
and finds its way, by irregular spaces or “lacunae,” into certain main
venous channels or “sinuses.” There are three such in the cephalothorax,
one median and the others lateral, considerably dilated in front, in the
region of the eyes, and connected by transverse passages. By these the
blood is brought back through the pedicle to the lung-books. In the
abdomen also there are three main sinuses, two parallel to one another
near the lower surface, and one beneath the pericardium. These likewise
bring the blood to the lung-books, whence it is conducted finally by
pulmonary veins (Fig. 186) back to the pericardial chamber, and thus, by
the ostia, to the heart.

The Spider’s blood is colourless, and the majority of the corpuscles are
“amoeboid,” or capable of changing their shape.

=Generative System.=—The internal generative organs present no great
complexity, consisting, in the male, of a pair of testes lying beneath
the liver, and connected by convoluted tubes, the “vasa deferentia,”
with a simple aperture under the abdomen, between the anterior stigmata.

The ovaries are hollow sacs with short oviducts which presently dilate
to form chambers called “spermathecae,” which open to the exterior by
distinct ducts, thus forming a double orifice, fortified by an external
structure already alluded to as the “epigyne.” The eggs project from the
outer surface of the ovary like beads, connected with the gland by
narrow stalks, and it was not at first clear how they found their way
into the interior cavity, but it has been ascertained that, when ripe,
they pass through these stalks, the empty capsules never presenting any
external rupture.

The palpal organs have already been described. The spermatozoa, when
received by them, are not perfectly elaborated, but are contained in
little globular packets known as “spermatophores.”

=Nervous System.=—The Spider’s central nervous system is entirely
concentrated in the cephalothorax, near its floor, and presents the
appearance of a single mass, penetrated by the oesophagus. It may,
however, be divided into a pre-oesophageal portion or brain, and a
post-oesophageal or thoracic portion.

The brain supplies nerves to the eyes and chelicerae, while from the
thoracic mass nerves proceed to the other appendages, and through the
pedicle to the abdomen. The walls of the oesophagus are closely invested
on all sides by the nerve-sheath or neurilemma.

=Sense-Organs.=—Spiders possess the senses of sight, smell, and touch.
Whether or not they have a true auditory sense is still a matter of
doubt. Since sounds are conveyed by vibrations of the air, it is never
very easy to determine whether responses to sounds produced near the
animal experimented upon are proofs of the existence of an auditory
organ, or whether they are only perceived through the ordinary channels
of touch. In any case, the organs of hearing and of smell have not yet
been located in the Spider. M‘Cook considers various hairs scattered
over the body of the spider to be olfactory, but from Gaskell’s
researches upon allied Arachnid groups it would seem that the true
smelling organ is to be sought for in the rostrum.

=Eyes.=—Spiders possess from two to eight simple eyes, the external
appearance and arrangement of which have already been briefly explained.
They are sessile and immovable, though often so placed as to command a
view in several directions. In structure they are essentially like the
ocelli of Insects. Externally there is a lens, succeeded by a mass of
transparent cells, behind which is a layer of pigment. Then come the
rods and cones of the retina, to which the optic nerve is distributed. A
comparison of this with the arrangement in the Vertebrate eye will show
a reversal of the positions of the retina and the pigment-layer. The
lens is part of the outside covering of the animal, and is cast at the
time of moulting, when the spider is temporarily blind. It is stated,
however, that the eyes do not all moult simultaneously. There is often a
considerable difference between the various eyes of the same spider,
especially with regard to the convexity of the lens and the number of
rods and cones.

Though most spiders possess eight eyes, the number is sometimes smaller,
and in some groups of eight-eyed spiders two of the eyes are sometimes
so reduced and degenerate as to be practically rudimentary. As might be
expected, Cave-spiders (e.g. _Anthrobia mammouthia_) may be entirely
sightless.

=Touch.=—The sense of touch would appear to be extremely well developed
in some spiders, and there is reason for believing that the Orb-weavers,
at all events, depend far more upon it than upon that of sight.

Among the hairs which are distributed over the spider’s body and limbs,
several different forms may be distinguished, and some of them are
undoubtedly very delicate sense-organs of probably tactile function.

=Spinning Glands.=—Spiders vary greatly in their spinning powers. Some
only use their silk for spinning a cocoon to protect their eggs, while
others employ it to make snares and retreats, to bind up their prey, and
to anchor themselves to spots to which they may wish to return, and
whence they “drag at each remove a lengthening chain.”

All these functions are performed by the silk-glands of the Orb-weavers,
and hence it is with them that the organs have attained their greatest
perfection. We may conveniently take the case of the common large
Garden-spider, _Epeira diademata_. The glands occupy the entire floor of
the abdomen. They have been very thoroughly investigated by
Apstein,[266] and may be divided into five kinds.

[Illustration:

  FIG. 187.—Spinning glands. =A=, Aciniform; =B=, tubuliform; =C=,
    piriform gland.
]

On either side of the abdomen there are two large “ampullaceal” glands
debouching on “spigots,” one on the anterior, and one on the middle
spinneret; there are three large “aggregate” glands which all terminate
on spigots on the posterior spinneret; and three “tubuliform” glands,
two of which have their orifices on the posterior, and one on the middle
spinneret. Thus, in the entire abdomen there are sixteen large glands,
terminating in the large fusulae known as spigots. In addition to this
there are about 200 “piriform” glands whose openings are on the short
conical fusulae of the posterior and anterior spinnerets, and about 400
“aciniform” glands which debouch, by cylindrical fusulae, on the middle
and posterior spinnerets. Thus there are, in all, about 600 glands with
their separate fusulae in the case of _Epeira diademata_.

The great number of orifices from which silk may be emitted has given
rise to the widespread belief that, fine as the Spider’s line is, it is
woven of hundreds of strands. This is an entire misconception, as we
shall have occasion to show when we deal with the various spinning
operations.

A few families are, as has already been stated, characterised by the
possession of an extra spinning organ, the cribellum, and the orifices
on this sieve-like plate lead to a large number of small glands, the
“cribellum glands.”

=Respiratory Organs.=—Spiders possess two kinds of breathing organs,
very different in form, though essentially much alike. They are called
respectively “lung-books” and “tracheae.” The Theraphosae (and
_Hypochilus_) have four lung-books, while all other spiders, except
_Nops_, have two. Tracheae appear to be present almost universally, but
they have not been found in the Pholcidae.

The pulmonary stigmata lead into chambers which extend forwards, and
which are practically filled with horizontal shelves, so to speak,
attached at the front and sides, but having their posterior edges free.
These shelves are the leaves of the lung-book. Each leaf is hollow, and
its cavity is continuous, anteriorly and laterally, with the blood-sinus
into which the blood from the various parts of the Spider’s body is
poured.

The minute structure of the leaf is curious. Its under surface is
covered with smooth chitin, but from its upper surface rise vast numbers
of minute chitinous points whose summits are connected to form a kind of
trellis-work. The roof and floor of the flattened chamber within are
connected at intervals by columns. The pulmonary chamber usually
contains from fifteen to twenty of these leaves, and the two chambers
are always connected internally between the stigmata.

The tracheae are either two or four (Dysderidae, Oonopidae,
Filistatidae) in number, and their stigmata may be separate or fused in
the middle line. Each consists of a large trunk, projecting forwards,
and giving off tufts of small tubes which lose themselves among the
organs of the abdomen, but do not ramify. In the tracheae of
_Argyroneta_[267] a lateral tuft is given off immediately after leaving
the stigma, and another tuft proceeds from the anterior end.
Histologically the main trunk of the trachea is precisely like the
general chamber of the pulmonary sac, and differs greatly from the
trachea of an insect.

=Cephalothoracic Glands.=—In addition to the generative glands and the
so-called “liver” which occupy so large a portion of the abdomen, there
are, in Spiders, certain glandular organs situated in the cephalothorax
which call for some notice. These are the coxal glands and the
poison-glands.

The COXAL GLANDS are two elongated brownish-yellow bodies, situated
beneath the lateral diverticula of the stomach, and between it and the
endosternite. They present four slight protuberances which project a
short distance into the coxae of the legs. The glands appear to be
ductless, but their function is thought to be excretory. They were first
observed in the Theraphosae.

All Spiders possess a pair of POISON-GLANDS, connected by a narrow duct
with a small opening near the extremity of the fang of the chelicerae.
The glands are sac-like bodies, usually situated in the cephalothorax,
but sometimes partially (_Clubiona_) or even entirely (_Mygale_) in the
patura, or basal joints of the chelicerae. Each sac has a thin outer
layer of spirally-arranged muscular and connective tissue fibres, and a
deep inner epithelial layer of glandular cells. The cavity of the gland
acts as a reservoir for the fluid it secretes. The virulence of the
poison secreted by these glands has been the subject of much discussion,
and the most diverse opinions have been held with regard to it. The
matter is again referred to on p. 360.




                              CHAPTER XIV
     ARACHNIDA EMBOLOBRANCHIATA (_CONTINUED_)—ARANEAE (_CONTINUED_)


  HABITS—ECDYSIS—TREATMENT OF YOUNG—MIGRATION—WEBS—NESTS—EGG-COCOONS—
    POISON—FERTILITY—ENEMIES—PROTECTIVE COLORATION—MIMICRY—SENSES—
    INTELLIGENCE—MATING HABITS—FOSSIL SPIDERS


                         EARLY LIFE OF SPIDERS.

=Ecdysis or Moulting.=—Spiders undergo no metamorphosis—that is to say,
no marked change of form takes place, as is so often the case among
Insects, in the period subsequent to the hatching of the egg. This fact,
by the by, is a great trouble to collectors, as it is generally
extremely difficult, and sometimes quite impossible, to identify
immature specimens with certainty.

But although unmistakably a spider as soon as it leaves the egg, the
animal is, at first, in many respects incomplete, and it is only after a
series of moults, usually about nine in number, that it attains its full
perfection of form.

Until the occurrence of its first moult it is incapable of feeding or
spinning, mouth and spinning tubes being clogged by the membrane it then
throws off. It is at first pale-coloured and less thickly clothed with
hairs and spines than it eventually becomes, and the general proportions
of the body and the arrangement of the eyes are by no means those of the
adult in miniature, but will be greatly modified by unequal growth in
various directions. It speedily, however, attains its characteristic
shape and markings, and after one or two ecdyses little alteration is to
be noticed, except increase in size, until the final moult, when the
spider at length becomes sexually mature.

The first moult takes place while the newly-hatched spider is still with
the rest of the brood either in or close to the “cocoon” or egg-bag.
M‘Cook[268] thus describes the conclusion of the operation in the case
of _Agelena naevia_:—

“While it held on to the flossy nest with the two front and third pairs
of legs, the hind pair was drawn up and forward, and the feet grasped
the upper margin of the sac-like shell, which, when first seen, was
about half-way removed from the abdomen. The feet pushed downwards, and
at the same time the abdomen appeared to be pulled upward until the
white pouch was gradually worked off.”

The later moults are generally accomplished by the spider collecting all
its legs together and attaching them with silk to the web above, while
the body, also attached, hangs below. The old skin then splits along the
sides of the body, and the animal, by a series of violent efforts,
wriggles itself free, leaving a complete cast of itself, including the
legs, suspended above it. For a day or two before the operation the
spider eats nothing, and immediately upon its completion it hangs in a
limp and helpless condition for a quarter of an hour or so, until the
new integument has had time to harden. It is not unlikely that the
reader has mistaken these casts for the shrivelled forms of unlucky
spiders, and has had his sympathies aroused, or has experienced a grim
satisfaction, in consequence—an expenditure of emotion which this
account may enable him to economise in future.

Limbs which the animal has accidentally lost are renewed at the time of
moulting, though their substitutes are at first smaller than those they
replace. On the other hand, the struggle to get rid of the old skin
sometimes results in the loss of a limb, and the spider is doomed to
remain short-handed until the next ecdysis.

Until the last moult the generative apertures, which are situated under
the anterior part of the abdomen, are completely sealed up. Their
disclosure is accompanied, in the case of the male, by a remarkable
development of the last joint of each pedipalp, which becomes swollen
and often extremely complicated with bulbs, spines, and bristles. A
mature male spider may at once be distinguished by the consequent
knobbed appearance of its palps; and the particular form they assume is
highly characteristic of the species to which the spider belongs.

The number of moults, and the intervals at which they occur, no doubt
vary with different species. In the case of _Argiope aurelia_,
Pollock[269] has found that the female moults nine times after leaving
the cocoon, the first ecdysis occurring after an interval of from one to
two months, according to the abundance or scarcity of food. The
subsequent intervals gradually increase from about a fortnight to
something over three weeks.

=Behaviour of the Newly-hatched Spider.=—The mode of life of a spider
just freed from the cocoon will of course vary greatly according to the
Family to which it belongs.

The EPEIRIDAE are the builders of the familiar wheel or orb-web. Spiders
of this Family usually remain together on friendly terms for a week or
more after leaving the nest. Most of the time they are congregated in a
ball-like mass, perhaps for the sake of warmth, but upon being touched
or shaken they immediately disperse along the multitudinous fine lines
which they have spun in all directions, to reassemble as soon as the
panic has subsided. Such a ball of the yellow and black offspring of the
large Garden-spider, _Epeira diademata_, is no uncommon sight in the
early autumn, and the shower of “golden rain” that results from their
disturbance is not likely to be forgotten if it has ever been observed
by the reader. This harmonious family life only continues as long as the
young spiders are unable to feed—a period which, in some of the larger
species, is said to extend to ten days or a fortnight.

Individual life then commences, and each member of the dispersed group
sets up housekeeping on its own account, constructing at the first
attempt a snare in all respects similar, except in size, to those of its
parent.

Of course the young Spiders have not migrated far, and a bush may
frequently be seen covered by the often contiguous nets of the members
of a single brood. This, as Dr. M‘Cook thinks, is the true explanation
of some of the cases of “gregarious spiders” which Darwin[270] and other
naturalists have occasionally described, though social spiders exist
(see _Uloborus_, p. 411).

Very similar habits obtain among the THERIDIIDAE, or line-weaving
spiders, a familiar example of which is the pretty little _Theridion
sisyphium_, whose highly-irregular snare may be found on any holly bush
during the summer months.

[Illustration:

  FIG. 188.—=A=, _Pardosa_ sp. ♀, with young on the abdomen; =B=, young
    _Pardosa_ detached; =C=, outline of the Spider with young removed.
    (From the living specimen.)
]

The LYCOSIDAE, or Wolf-spiders, which chase their prey instead of lying
quietly in ambush to ensnare it, are exceedingly interesting in their
treatment of their young. The cocoon, or bag of eggs, is carried about
on all their expeditions, attached beneath the abdomen, or held by the
jaws, and the young spiders, on escaping from it, mount on the mother’s
back, and indulge vicariously in the pleasures of the chase from this
point of vantage. The empty egg-bag is soon discarded, but the brood
continues to ride on the mother’s back for about a week, dismounting
only to follow her as she enters her little silk-lined retreat in the
ground.

During this time they appear to require no food, but they at length
begin to disperse, the mother gently but firmly removing such
individuals as are disposed to trespass upon her maternal solicitude
longer than she considers desirable.

Many young spiders of various Families proceed immediately to seek new
hunting-grounds by the aid of the wind, and become for the time being
diminutive aeronauts. This habit was observed by the earliest British
araneologist, Martin Lister,[271] as long ago as 1670, and has been
alluded to by many writers since his time.

The topmost bar of an iron railing in spring or early autumn will
generally be found peopled with minute spiders, and if the day be fair
and the wind light, the patient observer may be rewarded by a curious
and interesting sight.

The spider seeks the highest spot available, faces the wind, and
straightens its legs and body, standing, so to speak, upon its toes, its
abdomen with its spinning tubes being elevated as much as possible.
Streamers of silk presently appear from the spinnerets and float gently
to leeward on the light current of air. The spider has no power to shoot
out a thread of silk to a distance, but it accomplishes the same result
indirectly by spinning a little sheet or flocculent mass which is borne
away by the breeze.

[Illustration:

  FIG. 189.—Young Spider preparing for an aerial voyage. (After
    Emerton.)
]

When the streaming threads pull with sufficient force the animal casts
off, seizes them with its legs, and entrusts itself to the air, whose
currents determine the height to which it is carried and the direction
of its journey. The duration, however, is not quite beyond the spider’s
control, at all events in calm weather, for it can furl its sail at
will, hauling in the threads “hand-over-hand,” and rolling them up into
a ball with jaws and palps.

This curious ballooning habit of young Spiders is independent of the
particular family to which they belong, and it is remarkable that
newly-hatched Lycosidae and Aviculariidae, whose adult existence is
spent entirely on or under the ground, should manifest a disposition to
climb any elevated object which is at hand.

The “Gossamer,” which so puzzled our forefathers, is probably no mystery
to the reader. It is, of course, entirely the product of Spider
industry, though not altogether attributable to the habit of ballooning
above described. Only a small proportion of gossamer flakes are found to
contain spiders, though minute insects are constantly to be seen
entangled in them. They are not formed in the air, as was supposed long
after their true origin was known, but the threads emitted by multitudes
of spiders in their various spinning operations have been intermingled
and carried away by light currents of air, and on a still, warm day in
spring or autumn, when the newly-hatched spider-broods swarm, the
atmosphere is often full of them.

They rise to great heights, and may be carried to immense distances.
Martin Lister relates how he one day ascended to the highest accessible
point of York Minster, when the October air teemed with gossamer flakes,
and “could thence discern them yet exceeding high” above him. Gilbert
White describes a shower, at least eight miles in length, in which “on
every side, as the observer turned his eyes, he might behold a continual
succession of fresh flakes falling into his sight, and twinkling like
stars as they turned their sides toward the sun.” The ascent of a hill
300 feet in height did not in the least enable him to escape the shower,
which showed no sign of diminution.

The mortality among very young spiders must be exceedingly great;
indeed, this is indicated by the large number of eggs laid by many
species, an unfailing sign of a small proportion of ultimate survivors.
We shall have, by and by, to speak of some of their natural enemies, but
apart from these their numbers are sadly reduced by the rigours of the
weather, and appreciably also by their tendency to cannibalism. A
thunderstorm will often destroy a whole brood, or they may perish from
hunger in the absence of an adequate supply of insects minute enough for
their small snares and feeble jaws. In the latter case they sometimes
feed for a time on one another, and it is even said that two or three of
a brood may be reared on no other food than their unfortunate
companions.

The large and handsome Garden-spider, _Epeira diademata_, has been
known, when well fed, to construct six cocoons, each containing some
hundreds of eggs, and some species are even more fertile, while their
adult representatives remain stationary, or even diminish in number.

=Spider-Webs.=—Some account has already been given of the external and
internal spinning organs of Spiders. Within the body of the animal the
silk is in the form of a gummy fluid; and this, being emitted in
exceedingly fine streams, solidifies as it meets the air. It cannot be
shot out to any distance, but the animal usually draws it out by its
hind legs, or attaches it to a spot, and moves away by walking or
allowing itself to drop. It has some power of checking the output, and
can stop at will at any point of its descent; but the sphincter muscles
of the apertures are but weak, and by steady winding the writer has
reeled out a hundred yards of the silk, the flow of which was only then
interrupted by the spider rubbing its spinnerets together and breaking
the thread.

There is, of course, no true spinning or interweaving of threads in the
process, but parallel silken lines are produced, varying in number
according to the special purpose for which they are designed, and
sometimes adhering more or less to one another on account of their
viscidity and closeness.

The silk is utilised in many ways, serving for the construction of
snares, nests, and cocoons, as well as for enwrapping the captured prey,
and for anchoring the spider to a spot to which it may wish to return.

Spiders may be roughly distinguished as sedentary or vagabond, the
former constructing snares, and the latter chasing their prey in the
open. We will first consider the various forms of snare, beginning with
that characteristic of the Epeiridae.

=The Circular Snare.=—This familiar object, sometimes spoken of as the
orb-web or wheel-web, is always the work of some spider of the Family
Epeiridae.

The accuracy and regularity of form exhibited by these snares has caused
their architects to be sometimes called the _geometric_ spiders. The
ingenuity displayed by them has always excited the admiration of the
naturalist, and this is increased on closer observation, for the snares
are in reality even more complex than they appear at first sight.

The first care of the spider is to lay down the foundation threads which
are to form the boundary lines of its net. If the animal can reach the
necessary points of attachment by walking along intervening surfaces the
matter is comparatively simple. The spinnerets are separated and rubbed
against one of the points selected, and the spider walks away, trailing
behind it a thread which it keeps free from neighbouring objects by the
action of one of its hind legs. On reaching another desirable point of
attachment the line is made taut and fixed by again rubbing the
spinnerets against it. By a repetition of this proceeding a framework is
presently constructed, within which the wheel or orb will ultimately be
formed.

The process of fixing and drawing out a line can be conveniently watched
in the case of a Spider imprisoned in a glass vessel, and it will be
seen, by the aid of a lens, that a large number of very fine lines
starting from the point of attachment seem to merge into a single line
as the Spider moves away. This has given rise to the prevalent and very
natural idea that the ordinary spider’s line is formed or “woven” of
many strands. This, however, is not the case,[272] for the fine
attachment-lines are not continued into the main thread, but only serve
to anchor it to the starting-point.

As has been said, the spider can throw into play a varying number of
spinning tubes at will, and in point of fact those used in laying down
these foundation-lines are either two or four in number. The spider,
however, often finds it necessary to strengthen such a line by going
over it afresh.

Every one must have noticed that orb-webs frequently bridge over gulfs
that are clearly quite impassable to the spider in the ordinary way.
They often span streams—and Epeirid spiders cannot swim—or they are
stretched between objects unattainable from each other on foot except by
a very long and roundabout journey. When this is the case, the animal
has had recourse to the aid of the wind. A spider of this family placed
on a stick standing upright in a vessel of water is helpless to escape
if the experiment be tried in a room free from draughts. With
air-currents to aid it, silken streamers will at length find their way
across the water and become accidentally entangled in some neighbouring
object. When this has happened, the spider hauls the new line taut, and
tests its strength by gently pulling at it, and if the result is
satisfactory, it proceeds to walk across, hand-over-hand, in an inverted
position, carrying with it a second line to strengthen the first. This
is exactly what happens in nature when a snare is constructed across
chasms otherwise impassable, and it may be imagined that the spider
regards as very valuable landed property the foundation lines of such a
web, for, if destroyed, the direction or absence of the wind might
prevent their renewal for days. They are accordingly made strong by
repeated journeys, and are used as the framework of successive snares,
till accident at length destroys them.

A single line which finds anchorage in this way is sufficient for the
purposes of the spider. It has only to cross over to the new object,
attach a thread to some other point of it, and carry it back across the
bridge to fix it at a convenient spot on the surface which formed the
base of its operations. Between two such bridge-lines the circular snare
is constructed in a manner to be presently described. Sometimes the
tentative threads emitted by the spider travel far before finding
attachment. In the case of the English _Epeira diademata_ the writer has
measured bridge-lines of eleven feet in length; and with the great
Orb-weavers of tropical countries they frequently span streams several
yards in width.

Two stout bridge-lines thus constructed will form the upper and lower
boundaries of the net. The lateral limits are easily formed by cross
lines between them at a convenient distance apart. The spider chooses a
point, say, on the upper bridge-line, fixes its thread there, and
carries it round to the lower line, where it is hauled taut and firmly
attached. Two such cross lines give, with the bridge-lines, an irregular
four-sided figure within which to stretch the snare, and now the work is
perfectly straightforward, and can proceed without interruption.

Attention is first paid to the radii of the circular web. The first
radii are formed by drawing cross lines within the framework in the same
manner as before, but the spider carefully attaches these where they
intersect by a small flossy mass of silk, and this central point or hub
becomes the basis of its subsequent operations. It is a simple matter to
add new radial lines by walking from the centre along one of those
already formed and fixing the thread to some new point of the
circumference. They are not laid down in any invariable order, but with
a kind of alternation which has the general effect of keeping the strain
on every side fairly equal. Almost every time the spider reaches the
centre it slowly revolves, uniting the radii afresh at their point of
junction, and increasing the strength and complexity of the hub. It also
occasionally digresses so far as to stretch the whole structure by
bracing the framework at additional points, so that it loses its
four-sided form and becomes polygonal. We have now a number of spokes
connecting a central hub with an irregular circumference.

The hub is next surrounded by what Dr. M‘Cook calls a “notched zone,”
consisting of a few turns of spiral thread which serve to bind more
firmly the spokes of the wheel. The most important part of the work is
still to be performed. The lines hitherto laid down are perfectly dry
and free from viscidity, so that an entangled insect would easily be
able to free itself. A viscid spiral line remains to be spun, and the
snare will be complete. The precise method of laying this down will vary
somewhat according to the species, but, to refer again to the large
Garden-spider, the proceeding is as follows:—Commencing at a point
somewhat outside the notched zone, the creature rapidly works in a
spiral thread of ordinary silk with the successive turns rather far
apart. This forms a kind of scaffolding, by clinging to which the spider
can put in the viscid spiral, which it commences at the _circumference_.

Its action now becomes exceedingly careful and deliberate, though by no
means slow, and so great is its absorption in the work that it may be
observed quite closely with a hand-lens without fear of interrupting it.
The proceeding consists in drawing out from its spinnerets with one (or
both) of its hind legs successive lengths of a highly elastic line,
which it stretches just at the moment of fixing it to a radius, and then
lets go with a snap. There is no hesitation or pause for consideration,
but there is a peculiar deliberateness in drawing out each length of the
thread which, together with stretching and sudden release, require
explanation. Now, it has already been mentioned that the framework and
radii of the snare are not at all moist or adhesive. This, however, is
not the case with the spiral, upon which the spider chiefly relies in
capturing its prey. A close examination of it—even with the naked eye—
will show it to be beaded over with little viscid globules which, under
a low magnifying power, are seen to be arranged with remarkable
regularity.

[Illustration:

  FIG. 190.—=A=, =B=, =C=, =D=, Stages in the formation of the viscid
    globules of the web.
]

A very convenient method of investigation is to carry off a
newly-constructed web—or, better still, one not quite finished—on a
piece of plate glass, to which it will adhere by reason of the viscid
spiral, and on which it may be examined at leisure. Immersion in a
staining fluid will colour the viscid spiral, and show its structure in
a striking manner. It will appear to consist of a thread strung with
beads of two sizes, occurring with pretty uniform alternation, though
two of the larger beads are often separated by two or more of the
smaller.

Until recently it was supposed that the deposition of these beads upon
the spiral line was a subsequent operation, and, in view of their vast
numbers and regularity, the circumstance naturally excited great wonder
and admiration. Blackwall[273] estimated that, in a fourteen-inch net of
_Epeira cornuta_, there were at least 120,000 viscid globules, and yet
he found that its construction occupied only about forty minutes! The
feat, from his point of view, must be allowed to be rather startling.

As a matter of fact, the thread, on being slowly drawn out, is uniformly
coated with viscid matter which _afterwards_ arranges itself into beads,
the change being assisted by the sudden liberation of the stretched
line.

Boys[274] has shown their formation to be quite mechanical, and has
obtained an exact imitation of them by smearing with oil a fine thread
ingeniously drawn out from molten quartz. The oil arranged itself into
globules exactly resembling the viscid “beads” on the spider’s line. If
the web be carried bodily away on a sheet of glass, as above described,
while the spider is engaged upon the spiral line, the experimenter will
have permanent evidence of the manner in which the globules are formed.
The last part of the line will be quite free from them, but uniformly
viscid. Tracing it backwards, however, the beads are soon found, at
first irregularly, but soon with their usual uniformity. The thread
which the spider has thus “limed” for the capture of prey is really
two-stranded—the strands not being twisted, but lying side by side, and
glued together by their viscid envelope.

The snare is now practically complete, and the proprietor takes up her
position either in the centre thereof, or in some retreat close at hand,
and connected with the hub by special lines diverging somewhat from the
plane of the web. Notwithstanding the possession of eight eyes—which, in
sedentary spiders, are by no means sharp-sighted—it is mainly by the
sense of touch that the spider presently becomes aware that an insect is
struggling in the net. She immediately rushes to the spot, and suits her
action to the emergency.

If the intruder is small, it is at once seized, enveloped in a band of
silken threads drawn out from the spinnerets, and carried off to the
retreat, to be feasted on at leisure. If it seems formidable it is
approached carefully—especially if armed with a sting—and silk is deftly
thrown over it from a safe distance till it is thoroughly entangled, and
can be seized in safety by the venomous jaws of its captor. Sometimes
the insect is so powerful, or the spider so sated with food, that the
latter hastens to set free the intruder by biting away the threads which
entangle it before much havoc has been wrought with the net.

The viscid matter on the spiral line dries up after some hours, so that,
even if the web has not been destroyed by insects and stress of weather,
this portion of it must be frequently renewed. Commencing a new web is,
as has been seen, a troublesome matter, and it will readily be
understood that the spider prefers, where practicable, to patch up the
old one. This is done by biting away torn and ragged portions and
inserting new lines in their place.

The part played by the various spinning glands in the construction of
the orb-web may be briefly stated.[275] The ampullaceal glands furnish
the silk for the foundation lines and radii. The spiral has a double
ground-line proceeding from the middle spinnerets, but it is not quite
certain whether it proceeds from the ampullaceal or the tubuliform
glands. The chief function of the latter, in the female, is to furnish
silk for the egg-cocoon. The viscid globules are the products of the
aggregate glands. The aciniform and piriform glands provide the
multitudinous threads by which the spider anchors its various lines and
enwraps its prey.

Some Orb-weavers always decorate their snares with patches or tufts of
flossy silk. In the snare of the North American _Argiope cophinaria_ the
hub is sheeted, and from it extends downwards a zigzag ribbon of silk
stretched between two consecutive radii. Vinson[276] discovered a
remarkable use for similar zigzag bands in the web of the Mauritian
spider, _Epeira mauritia_. It furnished a reserve supply of silk for
enveloping partly entangled insects whose struggles were too vigorous to
succumb to the rather feeble threads which the spider was able to emit
at the moment of capture. The spider was able to overcome a grasshopper
much more powerful than itself by dexterously throwing over it with one
of its hind legs a portion of the ribbon of silk which it had thus
stored up for emergencies.

[Illustration:

  FIG. 191.—=A=, Snare of _Hyptiotes cavatus_; =B=, enlarged view of the
    Spider, showing the “slack” of the hauled-in line. (After Emerton.)
]

Many orb-webs are defective, a sector of the circle being uniformly
omitted in the structure. The genus _Hyptiotes_ does not belong to the
Epeiridae but to the cribellate Uloboridae, but its defective orb-web is
so curious that it deserves a special mention. A single foundation-line
is laid down, and from it four radii are drawn and are connected with
cross lines, the snare constituting about one-sixth of a circle. From
the centre of the incomplete circle a thread proceeds to some more or
less distant object, and on this the spider takes up its position,
inverted, and hauls in the line till the snare is taut. When the
trembling of the line shows the spider that an insect has struck the
net, it lets go with its fore-legs, and the web, springing back to its
normal position, entangles the intruder more thoroughly by its
vibrations. When large insects are in question the spider has been
observed to “spring” the net several times in succession. _H. cavatus_
is common in the pine woods of Pennsylvania, but the only English
species, _H. paradoxus_, is extremely rare.

A remarkable spider has been discovered in Texas by M‘Cook, which, after
building a horizontal orb-web, converts it subsequently into a dome
(Fig. 192) of exceedingly perfect form. It is named _Epeira basilica_,
and has been the object of careful study by Dr. Marx, who observed the
whole process of web-construction. Threads are attached at various
points on the upper surface of the horizontal wheel, the central portion
of which is gradually pulled up until the height of the dome is nearly
equal to the diameter of its base. But the snare of this spider does not
consist of the dome alone. A sheet of irregular lines is stretched
below, while above there is a maze of threads in the form of a pyramid.
Several other Orb-weavers, as, for instance, _E. labyrinthea_ and _E.
triaranea_, supplement their typical webs by an irregular structure of
silk, and thus form connecting links, as regards habit, between the
group of which we have been speaking and the Theridiidae or
Line-weavers, which may now briefly be dealt with.

[Illustration:

  FIG. 192.—Snare of _Epeira basilica_. (After M‘Cook.)
]

=The Irregular Snare.=—The great majority of British Spiders belong to
the family of the Theridiidae, or Line-weavers. Some of these are among
the handsomest of our native species, and are in other respects highly
interesting, but their snares lack the definiteness of structure
exhibited by the orb-web, and little need be said about them.

For the most part they consist of fine irregular lines running in all
directions between the twigs of bushes or among the stems of grass and
herbage. One large and important genus, _Linyphia_, always constructs a
horizontal sheet of irregular threads with a maze of silk above it. Such
snares may be seen in myriads in the wayside hedges during the summer,
and they are especially notable objects when heavily laden with dew.
Insects impeded in their flight by the maze of threads drop into the
underlying sheet, and are soon completely entangled. The spider usually
runs _beneath_ the sheet in an inverted position.

The sheet or hammock of silk is absent in the case of most of the other
genera of this family, their snares being innocent of any definite
method in their structure. They are frequently quite contiguous, and it
is no uncommon thing to find a holly bush completely covered with a
continuous network of threads, the work of a whole colony of the pretty
little spider _Theridion sisyphium_.

As might be imagined from the simplicity or absence of design in the
structure of the net, there appears to be very little complexity in the
nature of the silk used. It is interesting, however, to find that viscid
globules, not unlike those which stud the “spiral line” of the
Epeiridae, are sometimes present in the snares of the Line-weavers,[277]
and in these, too, aggregate glands are present. There is a large spider
of this family, _Theridion tepidariorum_, which may be found to a
certainty in almost any hothouse in this country. In its snare, which is
of the ordinary irregular type, F. Pickard-Cambridge has observed little
patches of flocculent silk, calculated to render more certain the
entanglement of prey, and he has further described a curious comb-like
structure on the hind leg of the animal which is probably used in the
production of this phenomenon. It is by no means unlikely that a more
careful study of these apparently simple snares will lead to the
discovery of further complexity of structure.

[Illustration:

  FIG. 193.—Snare of _Uloborus_ sp., some of the lines being thickened
    with threads from the cribellum. (After M‘Cook.)
]

_Uloborus_, a cribellate genus which has an Epeirid-like, orbicular
snare, decorates some of the lines with the produce of the cribellate
glands, but viscid globules are absent.

=Sheet-Webs.=—The webs which are such familiar—and, by association,
unpleasant—objects in unused rooms and outhouses are usually the work of
spiders belonging to the Agelenidae and the Dictynidae. To the first
belongs the common House-spider, _Tegenaria civilis_, and its larger
congener, _T. parietina_. These spiders are not attractive in
appearance, and the last-named species especially, with the four-inch
span of its outstretched legs, is a formidable object, and a terror to
domestic servants. An obscure tradition connecting it with Cardinal
Wolsey and Hampton Court has caused it to be known as the Cardinal
Spider. An out-door example of the Agelenidae is the very abundant
_Agelena labyrinthica_, whose sheet-web, with its tubular retreat, is to
be sought on the banks of ditches, or in the hedges of our country
lanes.

The snares of these spiders are exceedingly closely woven of very fine
silk, and take a long time to complete. The process of their
construction may be watched by keeping an _Agelena labyrinthica_
confined in a box with a glass front, and the web, kept free from dust,
is a beautiful object, as its fine texture gradually becomes visible as
a delicate transparent film which develops by imperceptible stages into
an opaque white sheet. The excessive fineness of the silk makes it
difficult at first to see what is taking place. The animal is seen to be
busily moving about, but the result of its labours only gradually
becomes visible. A few delicate foundation-lines are first stretched
across the compartment in which it is confined, and upon these the
spider walks to and fro incessantly with a serpentine motion, and by and
by a muslin-like floor of silk comes into view.

An examination of the spinnerets throws some light upon the operation.
The posterior pair are very long and mobile, and the hair-like
spinning-tubes are distributed on their under surface. The cephalothorax
and abdomen are far more rigidly connected in _Agelena_ than in the
Orb-weavers, but its length of leg and the length and mobility of its
posterior spinnerets enable it to give a wide lateral sweep as it walks
along, strewing fine silken threads upon the foundation-lines already
laid down. Some hours elapse before even a moderately stout web is
constructed, and for long afterwards the spider devotes odd moments of
leisure in going over the ground again and strewing new silk upon the
gradually thickening web. At one corner a silken tube of similar
structure is formed, and in this the spider awaits the advent of any
insect which may alight upon the sheet, when it immediately rushes forth
and seizes it.

The webs of the Dictynidae are very similar in general appearance to
those of the Agelenidae, consisting of a closely-woven sheet with a
tubular nest. They are to be found, moreover, in similar situations,
stretching across the angles of walls in cellars or outhouses, though
some species prefer an out-door existence. Crannies in rock form
convenient sites for such snares, but the family is not without its
representatives in still more open situations. The web, though so
similar to that of _Agelena_, is, however, constructed in a different
manner. In the Dictynidae neither the legs nor the spinnerets are
unusually long, and they do not strew the foundation-lines by a swinging
motion of the body, but the operation is effected by a special
apparatus. These spiders are _cribellate_, and in front of the six
ordinary spinnerets there are a pair of perforated plates connected with
a large number of additional minute spinning glands (see Fig. 182, p.
326). In conjunction with this, the female possesses on the last joint
but one of each hind leg a curious comb-like arrangement of spines, the
“calamistrum.” The animal constructs a sort of skeleton web by means of
its ordinary spinnerets, and when this is completed it combs out silk
from the cribellum by means of the calamistrum, using each hind leg
alternately, and distributes it with a curling motion upon the
scaffolding prepared for it, a nearly opaque web being the result. The
silk from the cribellum is of an adhesive nature, and renders escape
from the web very difficult.

=Spiders’ Nests and Retreats.=—All Spiders construct some description of
nest, and often display great ingenuity in building them. Perhaps none
are more curious than those of the burrowing Aviculariidae, a family
which includes the interesting “Trap-door Spiders.” They are nocturnal
in their habits, about which, consequently, little is known, but their
nests have been carefully studied, especially by Moggridge, who found
them in considerable abundance in various districts of the Riviera.

The jaws of these spiders are especially adapted for digging, and with
them a hole is excavated in the ground to the depth of several inches,
and wide enough to allow the animal to turn. This is carefully lined
with silk which the spider throws against the sides from its long and
upturned posterior spinnerets. But the _chef d’œuvre_ of the whole
structure is a lid or door which protects the entrance to the tube.
There are two types of door which find favour with different species—the
wafer and the cork type, as Moggridge has named them. The former
consists of a thin circular or oval sheet of silk which flaps down
loosely over the tube-entrance, with which it is connected by a
hinge-like attachment. A trap-door of the cork type is a more
complicated structure, being of considerable thickness and having a
bevelled edge, so that it fits into the tube like a plug. Like the wafer
door, it possesses a silken hinge.

To form the wafer door, the spider covers the entrance to the tube with
a closely-woven layer of silk, which it afterwards bites away at the
edge, except at the point where the hinge is to be. Doors of the cork
type consist of alternate layers of silk and earth. After weaving a
covering of silk, the creature brings earth in its jaws and lays it on
the top, binding it down with a second layer of silk, and the process is
repeated until the requisite thickness is attained.

The nests are exceedingly difficult to detect, as the spiders take the
precaution of attaching leaves, moss, or small twigs to the outer
surface of the doors. This does not appear to be the result of
intelligence, but a mere instinctive habit; for if a door be removed and
the surrounding earth denuded of moss, the spider will render the new
door conspicuous by bringing moss from a distance, and thus making a
green spot in the bare patch of earth.[278]

The cork doors fit with great exactness, and there is always to be found
on their under surface a notch by which they are held down by the
fore-legs of the spider against any attempt to open them from without.

Many nests with trap-doors of the wafer type are found to have a second
and more solid door within the tube. This serves to shut off the lower
part of the nest as a still more secure retreat. This second door opens
downwards, and the Spider, getting beneath it, is effectually shielded
from an enemy which may have mastered the secret of the outer barrier.
The nests of some species present still further complications in the way
of lateral branches from the main tube. In one case (_Nemesia congener_)
the burrow becomes =Y=-shaped, and the second door hangs at the fork of
the =Y= in such a manner as to connect the bottom chamber either with
the entrance or with the branch, which does not reach the surface, but
ends blindly.

Trap-door Spiders are greatly attached to their tubes, which they
enlarge and repair at need. They begin burrowing very early in life, and
their tiny tubes resemble in all respects those of their parents. Their
habits are nocturnal, and little is known of them; an observation,
however, on a species inhabiting the island of Tinos in the Grecian
Archipelago (_Cteniza ariana_), by Erber,[279] must not be omitted. This
spider leaves its tube at night and spins a web near at hand and close
to the ground. It carries captured insects into its tube, and in the
morning clears away the net, adding the material of it, M. Erber
believes, to the trap-door.

No true trap-door Spider has as yet been found in this country, but the
allied Atypidae are represented by at least one species, _Atypus
affinis_, which has been discovered in colonies in some localities in
the south of England, notably near Ventnor in the Isle of Wight, and on
Bloxworth Heath in Dorsetshire. This spider, like its continental
cousins, excavates a hole in the earth, generally near the edge of a
heathery bank, and lines it with a tube of silk of such firm texture
that it may be removed intact from the earth in which it is embedded.
The silken tube projects some two inches above the ground, either erect
among the roots of the heather, or lying loosely upon the surface. Its
extremity is always found to be closed, whether from its own elasticity
or by the deliberate act of the proprietor is uncertain, and it seems
probable that the animal spends almost the whole of its existence in the
tube. Simon believes that it feeds almost entirely upon earth-worms
which burrow into its vicinity, and which it, therefore, need not leave
its nest to catch; but the remains of beetles and earwigs have been
found in the tubes at Ventnor.

[Illustration:

  FIG. 194.—Funnel of _Cyrtauchenius elongatus_. (After M‘Cook.)
]

This description of nest seems common to all species of the genus
_Atypus_. The American “Purse-web Spider,” _A. abboti_, burrows at the
foot of a tree, against the trunk of which it rears the projecting
portion of its silken tube. At the bottom of the nest the cavity is
enlarged, and blind processes project in different directions.

Another burrowing spider, _Cyrtauchenius elongatus_, surmounts its
silk-lined burrow by a funnel-shaped structure of pure white silk, about
three inches in height and two or three inches in width. There is no
attempt at concealment, and the white funnels are conspicuous among the
thin grass, presenting the appearance of fungi.

The burrowing habit is also common to the Wolf-spiders or Lycosidae, but
beyond a very slight lining of silk there is usually little spinning
work about their nests. Occasionally there is a certain amount of
superstructure in the shape of a silken funnel (_Lycosa tigrina_,
M‘Cook), or of an agglomeration of twigs and pebbles, as in the case of
the “Turret-spider” (_Lycosa arenicola_, Scudder).

[Illustration:

  FIG. 195.—Turret of _Lycosa carolinensis_. (After M‘Cook.)
]

A colony of our handsome species, _Lycosa picta_, is an interesting
sight to watch. Their favourite habitat is a sandy soil, variegated with
many-coloured patches of moss and lichen, among which their own markings
are calculated to render them inconspicuous. The observer, by lying
perfectly still, may see them silently stealing forth from their burrows
in the bright sunshine, and hunting diligently in the neighbourhood,
ready to dart back on the faintest alarm, or if the sun should be
temporarily obscured by a passing cloud. So closely do they resemble
their surroundings, that it is only when in motion that they can readily
be detected. It is very curious to see them popping out their heads to
ascertain that the coast is clear before venturing forth, and the utter
silence of their operations adds to the eeriness of the effect. The
tubes of these spiders, though without a trap-door, and only slightly
lined with silk, are =Y=-shaped like those of _Nemesia congener_, the
main tunnel giving off a blind branch about half-way down.

The nest of the Water-spider, _Argyroneta aquatica_, must not be passed
over without mention. This spider, though strictly an air-breathing
animal, spends almost the whole of its existence beneath the water. That
it can live in such a medium is due to the fact that the long hairs
which clothe its abdomen retain a bubble of air as it swims beneath the
water, so that it carries with it its own atmosphere. The air-bubble
which invests its body gives it a strong resemblance to a globule of
quicksilver, and renders it a pretty object in an aquarium as it swims
about in search of food or in prosecution of its spinning operations.

Of these the most interesting is the building of its nest. Working
upon a water plant some distance below the surface, it forms a silken
dome of closely-woven threads, which it next proceeds to fill with
air. To do this the spider rises in the water, raises its abdomen
above the surface, and jerks it down again quickly, so as to carry
with it a bubble of air which it helps to retain with its hind legs.
With this it swims back to its tent, into which it allows the
imprisoned air-globules to escape. By degrees the dome or bell is
filled, and the creature has a dry and snug retreat beneath the water.
In this it passes the winter in a torpid condition. The young of this
species appear to be fond of utilising the empty shells of
water-snails, which they float by filling them with air, and thus save
themselves the trouble of nest-construction.

[Illustration:

  FIG. 196.—Egg-cocoons. =A=, _Epeira diademata_, nat. size. =B=,
    _Theridion pallens_ × 4, attached to a leaf. =C=, _Agroeca brunnea_,
    nat. size, attached to a weed, and not yet coated with mud. =D=,
    _Ero furcata_ × 4, attached to a log.
]

=Cocoon.=—The last important spinning operation which remains to be
described is the building of the so-called cocoon. This must be
distinguished from the cocoon of insects, which is a protective covering
of silk within which the larva assumes the pupa form. In the case of the
Spider, the term is applied to the structure which serves to protect and
conceal the eggs. It is often of considerable complexity, and is highly
characteristic of the particular species which constructs it.

All egg-bags are commenced in very much the same way. A small sheet of
silk is woven, and against this, sometimes upon the upper and sometimes
on the under surface, the eggs are deposited, and then covered in with a
second silken layer. The compact silk-covered ball of eggs is then, in
many cases, enclosed in a small compartment which the spider builds with
infinite care and unfailing uniformity, after the pattern peculiar to
its kind. A considerable number of the Orb-weavers are content with a
simple silken case closely investing the eggs, and by its thickness and
the non-conducting quality of the material, sufficient protection is
afforded against inclement weather.

The egg-bag of the large Garden-spider (_E. diademata_) may be
recognised by its great size and its yellow colour, which is deepened by
the still more yellow tint of the eggs within. Those of _Zilla x-notata_
and of many other English Epeirids are of similar structure, but of
white silk. The mother generally avails herself of some natural shelter,
hiding her cocoon beneath loose bark, in the crannies of masonry, or
under the copings of walls.

Many species, on the contrary, boldly expose their cocoons in their
snares, sometimes as many as fourteen being constructed in succession
and strung in a chain. The American species _Epeira caudata_ and _E.
bifurca_ are good examples of this habit, stringing a chain of
characteristic cocoons upon the line connecting the retreat with the
web.

The sedentary Theridiid spiders usually suspend their cocoons in the
neighbourhood of their irregular snares. The green cocoon of _Theridion
sisyphium_ is generally more or less concealed by an accumulation of
débris. The minute species _T. pallens_ constructs a cocoon of peculiar
shape on the under surface of a leaf (Fig. 196, B). It is a conical
structure of white silk, considerably larger than the spider itself,
attached at its broad end,[280] and having several curious lateral
projections near the middle.

Among the Lycosidae or “Wolf-spiders” the prevailing habit of the mother
is to carry the egg-bag attached beneath her abdomen upon all her
hunting excursions. It is spheroidal in shape, made up of an upper and a
lower half, with a seam-like junction at the equator, so to speak. The
lower half is first woven, and the eggs are deposited within it. The
upper hemisphere is then spun, and the edges gathered in and finished
off, the seam or suture being always discernible. The bag is now
attached by silken threads to the spinnerets, and bumps merrily over the
ground as the animal hurries along in search of prey. If deprived of it
she evinces the greatest distress, and frequently will not try to escape
without it.

=Attempts to utilise Spider Silk.=—It is long since the web of the
House-spider, taken internally, was considered a specific for the ague,
though its value as a styptic has been recognised in quite recent times.
It is, however, with other uses of Spider silk that we are here
concerned.

Spider silk has been extensively used in the micrometer eyepieces of
telescopes where very fine intersecting lines are required. For this
purpose the radial or scaffolding lines of the circular snare were
selected, the spiral being unsuited on account of its row of viscid
beads. Professor C. V. Boys has, however, discovered in his quartz
fibres a material better adapted for this purpose.

Several attempts have been made to weave the silk of Spiders as a
substitute for that of the silk-worm. Web silk is, of course, far too
fine to furnish a durable material, but the cocoons are usually formed
of coarser silk, and it is with them that the experiment has been tried.
About the beginning of the eighteenth century certain stockings and
mittens made of Spider silk from the cocoons of _Epeira diademata_, by
M. Bon of Languedoc, attracted so much attention that the Academy
desired M. Réaumur to investigate the matter. His report was
unfavourable to the commercial utility of Spider silk. The cocoon
threads, though eighteen times stronger than those of the web, were but
one-fifth of the strength of those obtained from the silk-worm, and the
lustre was inferior. A still more fatal objection, however, was founded
upon the cannibalistic habits of the spider, and the difficulty of
furnishing it with acceptable food.

M. Vinson has recorded that some of the spiders of Madagascar,
especially _Epeira madagascarensis_, are far better adapted than any of
our English species to a commercial use. They furnish silk of a
beautiful clear yellow colour; they are accustomed to live harmoniously
together in families; and the range of climate in which they can thrive
is very considerable. The Creole ladies of this island, under the
administration of General Decaen, wove a magnificent pair of gloves from
spider silk, with their own hands, for presentation to the French
Empress.

=Poison of Spiders.=—All spiders possess poison-glands, which have their
openings on the fangs of the chelicerae. The action of the chelicera in
striking does not express the venom, but the poison-bag itself is
covered with a muscular coat by which the contained fluid is expelled.
It is highly probable, therefore, that the venom is under the control of
the animal’s will, and is economised when the simple wound is sufficient
for the purpose—a supposition which may partially explain the very
divergent opinions held with regard to the effect of the spider’s bite.
The reputation of the “Tarantula” Spider is well known, but what
particular species, if any, was intended by the name is quite uncertain.
The name is derived from the town Tarentum, and was certainly applied to
a Lycosid spider. Probably the common south European species, _Lycosa
narbonensis_, has as good a claim to the honour as any. The confusion
has been increased by extending the name to spiders of quite a different
family. _Eurypelma hentzii_, one of the Aviculariidae, is commonly known
as the Tarantula in America.

The superstition of the tarantula dance is well known. The bite of the
spider was supposed to induce a species of madness which found its
expression—and its cure—in frantic and extravagant contortions of the
body. If the dance was not sufficiently frenzied, death ensued. In the
case of survivors, the symptoms were said to recur on the anniversary of
the bite. Particular descriptions of music were supposed to incite the
patient to the excessive exertion necessary for his relief; hence the
“Tarantella.”

In the Middle Ages epidemics of “tarantism” were of frequent occurrence,
and spread with alarming rapidity. They were seizures of an hysterical
character, analogous to the ancient Bacchic dances, and quite
unconnected with the venom of the spider from which they took their
name. The condition of exaltation and frenzy was contagious, and would
run through whole districts, with its subsequent relapse to a state of
utter prostration and exhaustion. The evil reputation of the Tarantula
appears to have exceedingly little basis in fact.

Baglivi relates how the country people capture the Tarantula by
imitating the buzzing of an insect at the mouth of its hole. “_Quo
audito, ferox exit Tarentula ut muscas, quorum murmur esse putat,
captet; captatur tamen a rustico insidiatore._”

Fabre[281] acted the part of the “insidious rustic” with slight success;
but by other stratagems he enticed the creatures from their holes, and
made some interesting observations upon the effects of their bite. He
found that bees and wasps were instantaneously killed by them. This
immediately fatal effect he found to be due to the fact that the spider
invariably struck the insect in a particular spot, at the junction of
the head with the thorax. Bees must often wander into Tarantula’s holes,
and a prolonged contest, though it might end in the death of the insect,
would be certain also to result fatally for the spider. It has,
therefore, acquired the habit of striking its foe in the one spot which
causes instant death. When Fabre presented a bee to a Tarantula in such
a manner that it was bitten in some other region, the insect survived
several hours.

A young sparrow, just ready to leave the nest, was bitten in the leg.
The wound became inflamed, and the limb appeared to be paralysed, but
the victim did not at first suffer in general health, and fed heartily;
death resulted, however, on the third day. A mole died in thirty-six
hours after the bite.

From these experiments, Fabre came to the conclusion that the venom of
the Tarantula was at all events too powerful to be entirely negligible
by man.

[Illustration:

  FIG. 197.—_Latrodectus mactans_, ♂, natural size.
]

Trifling causes may have a fatal effect upon a man in ill health, and it
is quite possible that death has sometimes resulted from the Tarantula’s
bite. Its effect upon a healthy subject, however, is certainly not
serious. Goldsmith, in his _Animated Nature_, entirely discredits the
current stories about this animal, saying that the Italian peasants
impose upon credulous travellers by allowing themselves, for money, to
be bitten by the Tarantula, and then feigning all the symptoms which are
traditionally supposed to ensue.

There is a genus of the Theridiidae, by name _Latrodectus_, whose
poisonous reputation almost rivals that of the Tarantula. It is
remarkable, moreover, that it is regarded as particularly dangerous in
such widely separated portions of the world as Madagascar, New Zealand,
Algeria, the West Indies, and North America. These spiders, strangely
enough, are by no means particularly large or formidable in appearance.

There are two species in Madagascar, known to the natives by the names
of _Mena-vodi_ and _Vancoho_. Vinson[282] describes the terror which is
locally inspired by the first-named species, whose bite is believed to
be fatal unless measures are promptly taken to counteract the poison.
They sometimes cauterise the wound, but the usual treatment consists in
inducing profuse perspiration—a method of cure which recalls the
Tarantula dance of Southern Europe. Flacourt[283] mentions the _Vancoho_
as the most dangerous animal of Madagascar, and more formidable than the
scorpion. He relates cases among his own negroes where the bite was
followed by a condition of syncope which lasted two days.

A New Zealand species is known by the natives as the _Katipo_. It is of
about the size of a pea, and almost black in colour. Mr. Meek of Waiwera
gives a most circumstantial account of the effect of its bite upon his
son.[284] During the four days which followed the bite he suffered
excruciating pain, which spread from his leg to the spine, arms, and
chest, and he lost twelve pounds in weight. Relief was obtained by
frequent doses of brandy and the use of a liniment.

The natives of New Zealand have a great horror of this spider, but hold
the curious belief that its death will ensure the cure of any one it may
have bitten. If unable to find it, they will burn the house down rather
than allow it to escape. Their dread, however, is confined to a variety
which lives among the sedge of the sea-beach, and they carefully avoid
sleeping in such places.

Two of the best authenticated cases of serious results ensuing from the
bite of a spider of this genus come from North Carolina.[285]

A farm labourer in the employ of Mr. John Dick of Greensborough was
bitten by _Latrodectus mactans_ about half-past eight in the morning,
and died between ten and eleven o’clock at night. Small pimples were
raised in the neighbourhood of the bite, but no puncture was
discernible. Intermittent pains and spasms ended in a comatose condition
from which he did not rally. The man appeared previously to be in
perfect health.

Another man on Mr. Dick’s farm was bitten by the same species of spider.
He resumed work, but a spasm of pain caused him to mount his horse and
endeavour to ride home, but he fell off, and lay in a state of
unconsciousness. He was found in this condition by a fellow-workman, and
taken home. Large quantities of whisky were administered without any
intoxicating effect, and this afforded some relief from the
constantly-recurring spasms. The paroxysms continued for three weeks,
and two months elapsed before he was able to resume work. On the ankle
where he was bitten pimples appeared as in the previous case, and these
broke out again, long after the occurrence, whenever he became
overheated in his work.

These accounts are sufficiently circumstantial and well authenticated,
but the fact of the actual bite depends upon the statement of the
victims alone, and they may possibly have mistaken the cause of their
trouble.

Southern Europe possesses a congener of this spider in _Latrodectus
13–guttatus_, the well-known “Malmignatte,” which is also considered
extremely poisonous. The Royal Academy of Medicine and Surgery at
Barcelona appointed Dr. Graells, in 1833, to inquire into the effects of
the bite of this spider, cases of which had become exceedingly frequent.
He found a curious correspondence between the frequency of these cases
and the advent of migratory locusts, which the spider successfully
attacked. In his report[286] he details the symptoms in certain
unquestionably authentic cases. There was a double puncture, surrounded
by red circles, the region of the wound afterwards swelling greatly. The
pain and swelling extended over the whole limb, and often to the body,
and convulsions occurred, followed by great prostration and collapse.
All the patients eventually recovered, their cure being heralded by
profuse perspiration.

It must be mentioned, however, that the eminent Arachnologist M. Lucas
states that he has several times allowed himself to be bitten by this
identical spider without any ill effects.

The testimony is thus conflicting in this case also. It is impossible,
however, to believe that there is no basis in fact for the poisonous
reputation of a comparatively insignificant-looking spider in so many
widely separated parts of the world, supported as it is by certain
well-substantiated cases. The variable effects of its bite may find a
partial explanation in a variation in the strength of its venom at
different seasons, and it has already been mentioned that the injection
of poison into its victim is a voluntary act, and does not necessarily
accompany its bite. Among the species regarded as especially venomous
must be mentioned _Phidippus morsitans_, one of the larger of the
Attidae.

It is exceedingly likely that the bite of the large tropical
Aviculariidae is really formidable. They appear, however, more anxious
to escape than to show fight, and we possess little reliable information
with regard to them. Doleschall shut up small birds with two West Indian
species, and death followed their bite almost immediately. Ten days’
starvation appeared to weaken the venom, for a bird bitten by a spider
fasting for that period recovered after an indisposition of six hours.

Most Arachnologists have recorded experiments with regard to the venom
of the commoner European species, with equally conflicting results.
Blackwall came to the conclusion that loss of blood, and not poison,
caused the death of spider-bitten insects. He could not himself
distinguish a spider bite from the prick of a needle inflicted upon his
hand at the same time. Bees, wasps, and grasshoppers survived the bite
about as long as other insects of the same species outlived a
needle-prick in the same part of the body. Walckenaer’s experience was
of the same nature. Bertkau, however, when bitten in the hand, felt
clear indications of an irritant poison in the wound. The hairs of some
of the large hairy species of the Aviculariidae possess poisonous
properties. They are readily parted with, and when the animal is touched
by the hand considerable irritation is set up.

=Fertility of Spiders.=—Spiders vary greatly in the average number of
eggs laid by different species, and within the limits of each species
there is a very considerable variation in fertility. As a rule it
appears that the large and vigorous spiders are more prolific than the
smaller and weaker members of the order. Were all the facts before us,
however, we should no doubt find that the number of eggs laid bore a
direct proportion, not to the size of the species, but to the dangers to
which the young of that species are exposed. Where the total numerical
strength of a species is fairly stationary, such a proportion must of
course exist. Some species, no doubt, are tending to become extinct,
while others are increasing in numerical importance. As a general rule,
however, it is safe to infer that, if a species is especially prolific,
special dangers attend the rearing of the young. The largest of North
American Epeirids, _Argiope cophinaria_,[287] constructs a cocoon
containing, on an average, 1150 eggs. As many as 2200 have been counted
in exceptional cases. Even this number is exceeded in the case of some
of the great Aviculariidae. _Theraphosa leblondi_ deposits as many as
3000 eggs. The large European Epeirids, _E. quadrata_ and _E.
diademata_, lay about 600 eggs, those of _Lycosa narbonensis_ reaching
about the same number. Those American spiders which have been described
as stringing up a series of cocoons in their webs usually attain about
the same aggregate, the eggs being less numerous in each cocoon.

These are examples of fairly large and fertile spiders. In the case of
other species the number of eggs laid is exceedingly small. _Ero
furcata_ makes a single cocoon containing six eggs. _Synageles picata_,
an ant-like Attid, lays only three. _Oonops pulcher_ constructs several
cocoons, but each contains only two eggs. The eggs of Cave-spiders, and
such as live in dark and damp places, are generally few in number.
_Anthrobia mammouthia_, for example, an inhabitant of the great American
caves, deposits only from two to five eggs.

Our knowledge of the special perils which beset particular species is so
incomplete that we are often at a loss for the reason of this great
inequality in fertility. For instance, how does _Synageles picata_
maintain its numerical strength by laying only three eggs, when, as
M‘Cook points out, its resemblance to the ant, though advantageous to
the adult spider, affords no protection to the egg? Our knowledge must
be greatly extended before we are able to account for particular cases.
Many influences hostile to spiders as a group are, however, well known,
and we may here enumerate them.

=Natural Enemies.=—The precautions taken by the mother in constructing
the cocoon render the inclemency of the weather very much less
destructive to the eggs than to the newly-hatched young. Nevertheless,
among spiders inhabiting swampy regions great havoc is wrought by the
occasional wholesale swamping of the cocoons by floods. Professor Wilder
considers the great fertility of _Nephila plumipes_ necessary to
counterbalance the immense destruction worked by the heavy rains upon
their cocoons, which are washed in great numbers from the trees, to the
leaves of which they are attached. But such exposed situations are
avoided by many species, and their eggs, enclosed in their silken
envelope, are well protected against the severities of the weather.

A more universal enemy to the egg is found in Ichneumon flies. On
examining the cocoons of almost any species of spider, a large
proportion are almost certain to be found to contain Ichneumon larvae.
Mr. F. Smith, in the _Transactions of the Entomological Society_ for
1860, describes two species, _Hemeteles fasciatus_ and _H. formosus_,
which are parasitic on the eggs of _Agelena brunnea_. They are figured
in Mr. Blackwall’s book on British Spiders. _Pezomachus gracilis_
attacks the cocoons of many kinds of American spiders, appearing to have
no special preference for any particular species, while _Acoloides
saitidis_ seems to pay special attention to the eggs of certain of the
Jumping-spiders, and particularly of _Saitis pulex_.

The Ichneumons which thus regard the Spider’s eggs as convenient food
for their own larvae are probably very numerous. Nor are they themselves
always free from parasites. Occasionally the larvae of minute
Hymenopterous insects are found to be parasitic upon the eggs of an
Ichneumon which have been laid in a Spider’s cocoon.

It sometimes happens that the development of the young spiders has so
far advanced at the time of the Ichneumon’s intrusion that the latter’s
intention is frustrated, and its offspring, instead of devouring, are
themselves devoured. Again, some few of the eggs in an infested cocoon
occasionally escape the general destruction and reach the adult
condition, but there can be no doubt that Ichneumons are largely
instrumental in keeping down the numbers of most species of spiders. The
perils which attend the Spider after leaving the cocoon are no less
formidable, and much more numerous. The whole newly-hatched brood may be
destroyed by a heavy rain-storm. If there is not a sufficient supply of
food suitable to their feeble digestive powers they perish of inanition,
or eat one another. This cannibalistic propensity is a considerable
factor in the mortality among young spiders, and the adult animals
frequently prey upon one another.

_Argyrodes piraticum_, in California, invades the webs of larger spiders
of the family Epeiridae, which it seizes and devours. _A. trigonum_,
common in the eastern states of North America, has the same habit.[288]
Hentz found in Alabama a spider, which he named _Mimetus interfector_,
of still more ferocious and piratical habits. Its special quarry is
_Theridion tepidariorum_. Sometimes the _Theridion_ overcomes the
invader, and one case was observed in which a second _Mimetus_ was
devouring a _Theridion_ beside the dead body of its predecessor, who had
come off the worse in the combat.

The eggs of _Theridion tepidariorum_ are also sometimes devoured by this
spider, and a similar propensity has been observed in some English
species, for Staveley[289] states that it is common to see certain
spiders of the genus _Clubiona_ feeding upon the eggs which have been
laid by their neighbours. The larvae of some Hymenopterous insects are
parasitic upon Spiders themselves, and not upon their eggs. Blackwall
found this to be the case with the larvae of _Polysphincta carbonaria_,
an Ichneumon which selects spiders of the genera _Epeira_ and _Linyphia_
on which to deposit its eggs.[290] The spider thus infested does not
moult, and is soon destroyed by the parasite which it is unable to
dislodge from its back. Menge, in his _Preussische Spinnen_, enumerates
several cases of parasitism in which the larva, as soon as it has
developed from the egg, enters the spider’s body, there to continue its
growth. Spiders are also subject to the attack of a parasitic worm,
_Gordius_ (cf. vol. ii. p. 173).

Some of the most deadly foes of Spiders are the Solitary Wasps. There
are many species of _Pompilus_ (vol. vi. p. 101), which, having
excavated holes in clay banks, store them with spiders or other
creatures which they have paralysed by their sting. They then deposit an
egg in the hole, and immediately seal up the orifice. This habit is
found to characterise the solitary wasps of all parts of the world.
Belt[291] relates the capture of a large Australian spider by a wasp.
While dragging its victim along, it was much annoyed by the persistent
presence of two minute flies, which it repeatedly left its prey to
attempt to drive away. When the burrow was reached and the spider
dragged into it, the two flies took up a position on either side of the
entrance, doubtless with the intention of descending and laying their
own eggs as soon as the wasp went away in search of a new victim.
Fabre[292] gives an interesting account of one of the largest European
Pompilidae, _Calicurgus annulatus_, which he observed dragging a
“Tarentula” to a hole in a wall. Having with great difficulty introduced
its burden into the cavity, the wasp deposited an egg, sealed up the
orifice, and flew away. Fabre opened the cell and removed the spider,
which, though completely paralysed, lived for seven weeks.

The same indefatigable observer describes the method adopted by the
comparatively small _Pompilus apicalis_ in attacking the formidable
Wall-spider, _Segestria perfida_. The combatants are well matched, and
the issue of the battle would be doubtful if the wasp did not have
recourse to stratagem. Its whole energies are directed towards forcing
the spider away from its web. At home, it is confident and dangerous;
when once dislodged, it appears bewildered and demoralised. The wasp
darts suddenly towards the spider and seizes it by a leg, with a rapid
effort to jerk it forth, releasing its hold before the enemy has had
time to retaliate. The spider, however, as well as being anchored by a
thread from its spinnerets, is clinging to its web with its hind legs,
and if the jerk is not sufficiently energetic, it hastily scrambles back
and resumes its defensive position. Before renewing the attack the wasp
gives the spider time to recover from the excitement of the first onset,
seeking, meanwhile, the retreats of other victims. Returning, it
succeeds, by a more skilful effort, in drawing the spider from its
retreat and hurling it to the ground, where, terrified and helpless, it
falls an easy prey. Should the insect bungle in its first attack and
become entangled in the web, it would itself become the victim. Certain
wasps thus appear to seek out particular species of spiders as food for
their larvae. Others are less discriminate in their tastes. Again, some,
as in the cases cited above, store their egg-nest with a single spider,
while others collect many for the purpose.

The American “blue digger wasp” (_Chlorion caeruleum_) excavates its
nest in the ground, and inserts a single large spider of any
species.[293] Another wasp, of the genus _Elis_, selects the
Wolf-spiders, and especially _Lycosa tigrina_, for the use of its
larvae, while _Priocnemus pomilius_ shows a preference for the
Crab-spiders, or Thomisidae.

One of the most remarkable instances is that of _Pepsis formosa_, which
preys upon the gigantic spider _Eurypelma hentzii_, wrongly styled in
America the “tarantula,” but really belonging to an entirely different
family, the Aviculariidae.

Fabre’s most interesting researches have established the fact that the
instinct of the wasp leads it to sting the spider in a particular spot,
so as to pierce the nerve-ganglion in the thorax. The precision with
which this is effected is absolutely necessary for the purpose of the
insect. If stung elsewhere, the spider is either incompletely paralysed,
or it is killed outright, and thus rendered useless as food for the
future larvae of the wasp. On the one hand, therefore, the Tarantula has
acquired the habit of striking the wasp in the only point where its blow
is instantaneously fatal, while on the other the wasp, with a different
object in view, has been led to select the precise spot where its sting
will disable without immediately destroying the spider. The latter case
is, if anything, the more extraordinary, as the insect can hardly have
any recollection of its larval tastes, and yet it stores up for progeny,
which it will never see, food which is entirely abhorrent to itself in
its imago state.

Spiders taken from the egg-nests of wasps by M‘Cook survived, on the
average, about a fortnight, during which period they remained entirely
motionless, and would retain any attitude in which they were placed.

There are many animals which either habitually or occasionally feed upon
spiders. They are the staple food of some hummingbirds, and many other
birds appear to find in them a pleasing variation on their customary
insect diet. These creatures, moreover, are destructive to spiders in
another way, by stealing the material of their webs, and especially the
more closely textured silk of their egg-cocoons, to aid in the
construction of their nests. M‘Cook has observed this habit in the case
of _Vireo noveborocensis_, and he states, on the authority of others,
that the “Plover” and the “Wren” are addicted to it. The smaller species
of monkeys are extremely fond of spiders, and devour large numbers of
them. They are said, moreover, to take a mischievous delight in pulling
them in pieces. Armadillos, ant-eaters, snakes, lizards, and indeed all
animals of insectivorous habit, draw no distinction between Insecta and
Arachnida, but feed upon both indiscriminately. The army ants, so
destructive to insect life in tropical countries, include spiders among
their victims. These formidable insects march along in vast hordes,
swarming over and tearing in pieces any small animal which lies in their
path. They climb over intervening obstacles, searching every cranny, and
stripping them bare of animal life. Insects which attempt to save
themselves by flight are preyed upon by the birds, which are always to
be seen hovering above the advancing army. The spider’s only resource is
to hang from its thread in mid-air beneath the branch over which the
ants are swarming, for the spider line is impracticable to the ant.
Belt[294] has observed a spider escape the general destruction by this
means.

=Protective Coloration.=—Examples are numerous in which the spider
relies upon the inconspicuousness not of its nest, but of itself, to
escape its natural foes. Its general hues and markings are either such
as to render it not readily distinguishable among its ordinary
surroundings, or the principle has been carried still further, and a
special object has been “mimicked” with more or less fidelity.

This country is not rich in the more striking mimetic forms, but the
observer cannot fail to notice a very general correspondence in hue
between the spiders of various habits of life and their environment.
Those which run on the ground are usually dull-coloured; tree-living
species affect grey and green tints, and those which hunt their food
amongst sand and stones are frequently so mottled with yellow, red, and
grey, that they can scarcely be recognised except when in motion.

A few of our indigenous species may be mentioned as especially protected
by their colour and conformation. _Tibellus oblongus_ is a
straw-coloured spider with an elongated body, which lives among dry
grass and rushes. When alarmed it clings closely to a dry stem, remains
motionless, and escapes observation by its peculiarity of colour and
shape. _Misumena vatia_, another of the Thomisidae or Crab-spiders,
approximates in colour to the flowers in which it is accustomed to lurk
on the watch for prey. It is of a variable hue, generally yellow or
pink, and some observers believe that they have seen it gently waving
its anterior legs in a way which made them easily mistaken for the
stamens of the flower stirred by the breeze. Its purpose appears to be
to deceive, not its enemies, but its victims. It seems to be partial to
the blooms of the great mullein (_Verbascum thapsus_), and
Pickard-Cambridge has more than once seen it seize and overcome a bee
which had visited the flower in search of honey. He has also observed it
in the blossoms of rose and furze bushes.[295]

An Epeirid (_Tetragnatha extensa_) resembles _Tibellus_ in its method of
concealing itself when alarmed. It also possesses an elongated abdomen,
of a grey-green tint, which it closely applies to one of the twigs among
which it has stretched its net, at the same time extending its four long
anterior legs straight before it, and in this position it lies _perdu_,
and is very easily overlooked. Another Orb-weaver, _Epeira cucurbitina_,
is of an apple-green colour, which is admirably calculated to conceal it
among the leaves which surround its snare.

Most of our English Attidae, or Jumping-spiders, imitate closely the
prevailing tone of the surfaces on which they are accustomed to hunt.
This will be recognised in the familiar striped Wall-spider, _Salticus
scenicus_, and we may also mention the grey _Attus pubescens_, which
affects stone walls, and the speckled _Attus saltator_, which is hardly
distinguishable from the sand which it searches for food.

Examples may also be found among the Lycosidae or Wolf-spiders. Of the
prettily variegated _Lycosa picta_, Pickard-Cambridge says: “Much
variation exists in the extent of the different portions of the pattern
and in their depth of colouring, these often taking their prevailing
tint from the colour of the soil in which the spider is found. The best
marked, richest coloured, and largest examples are found on sandy and
gravelly heaths, where there is considerable depth and variety of
colouring.... But on the uniformly tinted greyish-yellow sandhills
between Poole and Christchurch I have found a dwarf, pale yellow-brown
variety, with scarcely any dark markings on it at all, the legs being of
a uniform hue, and wholly destitute of dark annuli.”[296]

=Mimicry.=—In the island of Portland, a locality remarkable for the
number of species peculiar to itself, there is found a spider, _Micaria
scintillans_, very closely resembling a large blackish ant which
frequents the same neighbourhood. Its movements, moreover, are
exceedingly ant-like, as it hurries along in a zigzag course, frequently
running up and down grass stems after the manner of those insects. It is
a great lover of sunshine, and disappears as soon as the sun is obscured
by a passing cloud.

Such resemblances, obvious enough in nature, and heightened by the
behaviour of the mimetic form, are often by no means striking in the
cabinet. In some American species of spiders, however, imitation of the
ant has passed beyond the stage of a general resemblance as regards size
and colour and method of progression. The head of the ant is well marked
off from the body, and the thorax is frequently divided into distinct
regions. These peculiarities are imitated by constrictions in the
cephalothorax of mimetic spiders. The resemblance, moreover, is much
increased by their habit of using but six legs for locomotion, and
carrying the second pair as ants do their antennae. The best known
examples of these spiders are _Synageles picata_ and _Synemosyna
formica_ (see Fig. 215, C, p. 420), and even more striking resemblances
have been observed among some undescribed South American species.

The object of such mimicry seems to vary in different cases. Sometimes
the spider preys upon the ant which it resembles. Sometimes, again, by
its disguise, it escapes the notice of the ant which would otherwise
feed upon it. More often spider and ant are neutral as regards each
other, but, under cover of its resemblance, the Arachnid is enabled to
approach an unsuspecting victim to which the ant is not a terror. Again,
the unpleasantly acid taste of ants is unpalatable to most birds, though
not to all, and the increased danger from specially ant-eating birds may
be more than counterbalanced by the immunity they acquire from other
birds.

There is quite a large class of Spiders of nocturnal habits, whose only
precaution by day is to sit perfectly still and be mistaken for
something else. We have referred to the adaptation in colour of our
English species, _Misumena vatia_, to the flowers in which it lies in
wait for prey. Bates[297] mentions exotic examples of the same family
which mimic flower-buds in the axils of leaves. Herbert Smith says of a
spider which sits upon a leaf waiting for prey: “The pink three-lobed
body appears just like a withered flower that might have fallen from the
tree above; to the flies, no doubt, the deception is increased by the
strong sweet odour, like jasmine.”

Trimen[298] describes a Cape Town species which is of the exact rose-red
of the flower of the oleander. “To more effectually conceal it, the
palpi, top of the cephalothorax, and four lateral stripes on the abdomen
are white, according remarkably with the irregular white markings so
frequent on the petals of _Nerium_.”

The same observer, approaching a bush of the yellow-flowered _Senecio
pubigera_, noticed that two of the numerous butterflies settled upon it
did not fly away with their companions. Each of these he found to be in
the clutches of a spider, whose remarkable resemblance to the flower lay
not only in its colour, but in the attitude it assumed. “Holding on to
the flower-stalk by the two hinder pairs of legs, it extended the two
long front pairs upward and laterally. In this position it was scarcely
possible to believe that it was not a flower seen in profile, the
rounded abdomen representing the central mass of florets, and the
extended legs the ray florets; while, to complete the illusion, the
femora of the front pair of legs, adpressed to the thorax, have each a
longitudinal red stripe which represents the ferruginous stripe on the
sepals of the flower.”

Cambridge found in Palestine some species of Thomisidae which, when at
rest, were indistinguishable from bits of coarse fleecy wool, or the
rough seeds of some plant.

There is perhaps no more curious case of mimicry than that of a spider,
_Phrynarachne_ (= _Ornithoscatoides_) _decipiens_, which Forbes
discovered in Java while butterfly-hunting. It appears that butterflies
of the Family Hesperidae have a custom of settling, for reasons best
known to themselves, upon the excreta of birds, dropped upon a leaf.
Forbes noticed one in this position. Creeping up, he seized the
butterfly, but found it mysteriously glued by the feet. On further
investigation the “excreta” proved to be a spider. So accurate was the
mimicry that he was again completely deceived by the same species in
Sumatra. Its habit is to weave upon a leaf a small white patch of web,
of a shape which greatly assists the deception, and in the midst of this
it lies on its back, holding on by the spines with which its legs are
furnished. It then folds its legs over its thorax, and waits for some
insect to settle upon it.

In rare cases spiders have come to resemble their enemies the Ichneumon
flies. A frequent habit of these insects is to deposit their eggs in the
newly-formed cocoon of the spider. The Ichneumon eggs are the first to
hatch, and the larvae have a convenient food-supply at hand. Sometimes,
however, they adopt another method, and insert their eggs into the body
of the spider itself. It is probably in order to avoid this unpleasant
contingency that the spider has evinced towards the Ichneumon the
sincerest form of flattery.


                        =The Senses of Spiders.=

SIGHT.—Though, as has been shown, spiders are well provided with eyes,
their power of vision, in most cases, is by no means remarkable. As
might be expected, it is less developed in those of sedentary than in
those of nomadic habit.

It is noticeable that, in most spiders, some of the eyes are of a pearly
grey colour, and others of a much darker hue. Simon designates the
former _nocturnal_ and the latter _diurnal_ eyes, according to the
special use which he believes them to subserve.

This view of the matter cannot be regarded as at all established, and
has not found general acceptation. Moreover, Pillai[299] has shown that
certain Attid spiders can change the colour of their eyes by a movement
of the internal mechanism. The Epeiridae, spinners of the round web, are
certainly, as a rule, very dim-sighted creatures. A fly may be held
within an inch of them, but, unless it buzz, it will excite no notice
whatever. A careful observation of the performances of the large
Garden-spider in securing her prey will soon convince the onlooker that
she is guided almost entirely by appeals to her sense of touch
communicated along the tremulous lines of her snare. Interpreting these
too hastily, she will sometimes rush straight past the entangled fly,
and wait for it to renew its struggles before making sure of its
whereabouts. Keen sight would be of little utility to such spiders, as
they are concerned with nothing beyond the limits of their snare, and
within its range they are furnished with the equivalent of complete
telegraphic communication.

That most of the vagabond spiders can see well within the range of
several inches there is no doubt, though some observers have been misled
by the result of certain experiments on the Lycosidae, or
“Wolf-spiders.” It will be remembered that the female Lycosid carries
her egg-bag about with her, attached usually to her spinnerets. If it be
removed and placed close at hand, the spider experiences the greatest
difficulty in finding it again. Lubbock attributed this to defective
sight, whereas it merely arises from unfamiliarity with the _appearance_
of the egg-bag, which, since its construction, has been so situated as
to be out of the view of the spider. Peckham found that spiders of the
genus _Theridion_, accustomed to the sight of their cocoons, readily
recognised them by that sense when removed to a distance.

The most keen-sighted of the spider tribe are undoubtedly the Attidae,
or Leaping-spiders. The little black and white striped Wall-spider,
_Salticus scenicus_, is probably a familiar object to most of our
readers, and a very little observation of its movements, like those of a
cat stalking a bird, will convince the observer that its visual powers
are wonderfully keen and accurate. Its attitude of “attention” on
sighting its prey, its stealthy manœuvring to approach it unobserved,
and the unerring certainty of its final leap, are very interesting to
witness.

It is somewhat noticeable that both in the Epeiridae and in the Attidae
the two portions of the body, cephalothorax and abdomen, have more than
the usual freedom of independent motion. In the Orb-weavers this gives
play to the spinnerets in binding up a captured insect, but in the
Leaping-spiders it allows of the rapid directing of the large anterior
eyes towards the quarry, as it continually alters its position.

Professor and Mrs. Peckham of Wisconsin[300] performed some interesting
experiments to ascertain the sensitiveness of the spider’s eye to
colour. Freely communicating compartments of differently coloured glass
were constructed, and spiders were confined in them, when it was found
that red was the most and blue the least attractive hue. This agrees
well with what Lubbock found to be the case with ants, but those insects
displayed a greater antipathy for blue and not so marked a preference
for red.

HEARING.—Most of our knowledge about the auditory sense of spiders is
due to experiments performed by C. V. Boys,[301] and repeated by
Professor and Mrs. Peckham.

The spider usually responds to the stimulus in one of two ways; it
either raises its front legs, extending them in the direction of the
sound, or it allows itself to drop suddenly, as though in alarm. It was
only in the case of the Epeiridae that any results were obtained, and
these spiders were more sensitive to low than to high notes. Now, as
M‘Cook points out, it is exceedingly strange that the nomadic and
hunting spiders, to which the sense of hearing might be expected to be
extremely useful, should be deficient in this faculty, while the
sedentary spiders, to which it would appear comparatively unimportant,
should possess it in a tolerably developed form. That writer may
possibly be correct in supposing that the sense, as possessed by
spiders, is hardly differentiated from that of ordinary touch, and that
the web-making species are only aware of sounds by the vibrations
communicated to their feet by the medium of the web. However this may
be, we must reluctantly but sternly reject the numerous and seemingly
authentic stories, often connected with historic personages, which
credit the spider with a cultivated taste for music.

We have seen that among the spiders which possess a stridulating
apparatus it is confined, in certain groups, to the male, or if present
in the female it exists only in a rudimentary form. If in these cases
stridulation has been rightly interpreted as a sexual call, the power of
hearing, at least in the female, is of course connoted. The spiders in
question are members of the Theridiidae, a family closely allied to the
Epeiridae, and therefore more likely than most groups to possess the
power of hearing.

Theraphosid spiders show no response to the stimulus of sound, and among
them stridulation is not confined to one sex. If, as is generally
believed, the organ is used to warn off enemies, it is not necessary
that the sound produced should be audible to the spider itself. If there
be any true hearing organ in spiders its location is quite uncertain.
Some have supposed the so-called lyriform organs in the legs to have an
auditory function, while others have supposed the power of hearing to
reside in certain hairs, of which there are several different types
distributed over the body and limbs of the animal.

=Spider Intelligence.=—The experiments performed by the Peckhams clearly
proved that spiders have short memories—a sure indication of a low state
of intelligence. Members of the Lycosid or “Wolf-spider” group, when
deprived of their cocoons, recognised them again after a few hours, but
in most instances they refused to resume them after a lapse of
twenty-four hours, and in every case an absence of two days sufficed to
prevent any sign of recognition on their restoration. Moreover, when,
after a shorter interval, the cocoons of other spiders, even of
different genera, were offered to them, they appeared equally satisfied,
and attached them in the orthodox manner, beneath the abdomen. The same
treatment was even accorded to pith balls, which, if of the right size,
seemed to be a perfectly satisfactory substitute. The contents of one
cocoon were replaced by a shot three or four times their weight, but the
spider accepted it with alacrity, spending half an hour in refixing it,
when its weight caused it to fall from its attachment.

The habit of “feigning death,” which seems to be especially
characteristic of the Epeiridae or orb-weaving spiders, probably arises
from no desire to deceive its adversary as to its _condition_, but from
an instinct to remain motionless, and therefore inconspicuous. Where a
nomadic spider seeks safety in flight, a sedentary species finds a
greater chance of escape in dropping a certain distance, and, while
still attached by its silken line, giving as little evidence of its
whereabouts as possible—trusting, in many cases, to its protective
colouring. This method, moreover, has the advantage of facilitating its
return to the web when the danger is past—a feat of which it would be
quite incapable were it once to relinquish its clue.

All the remarkable and apparently intelligent actions of these creatures
seem to be done in obedience to a blind instinct, which is obeyed even
when there is no longer any object to be served. We have seen how the
Trap-door spiders decorate the lids of their nests with moss even when
the surrounding ground is bare, and _Agelena labyrinthica_ has been
observed to go through the whole lengthy and laborious operation of
constructing its egg-cocoon though all its eggs were removed immediately
on being laid.[302]

=Mating Habits.=—The sex of a mature spider can readily be recognised by
the palpus which, as we have seen, is furnished in the male with a
“palpal organ.” After the last moult but one the palp appears tumid, but
it is only at the last moult that the organ is fully formed, and that
the genital orifice is visible under the anterior part of the abdomen.

No alteration takes place in the female palp at maturity, but it is only
after the last moult that the “epigyne” is distinguishable.

[Illustration:

  FIG. 198.—_Argiope aurelia_, ♂ and ♀, natural size.
]

That the palpal organs are used in the fertilisation of the female has
long been established. How they came to contain the sperm matured in the
abdomen was a problem which has only been solved comparatively recently.
No direct connection could be found by way of the palpus with the
abdominal organs, which, indeed, were seen to have an orifice between
the lung-sacs. It is now known that some spiders at all events spin a
slight web upon which they deposit a drop of spermatic fluid, which they
afterwards absorb into their palpal organs for transference to the
female. Secondary sexual differences are often very marked, the male
being almost invariably the smaller in body, though its legs are
frequently longer and more powerful than those of the female.

Among some of the sedentary spiders the disparity in size is excessive.
The most striking examples are furnished by the Epeirid genera _Argiope_
and _Nephila_, the male in some instances not attaining more than the
thousandth part of the mass of the female. The coloration of the sexes
is frequently quite dissimilar, the male being usually the darker,
though in the Attidae he is in many cases the more strikingly
ornamented.

In the minute Theridiid spiders of the group Erigoninae (see p. 404),
the male cephalothorax often presents remarkable and characteristic
excrescences not observable in the female. Some curious examples of this
phenomenon may be seen in Fig. 209.

To the ordinary observer male spiders will appear to be comparatively
rare, and to be greatly outnumbered by the females. This is probably to
some degree true, but the unsettled habits of the males and the shorter
duration of their life are calculated to give an exaggerated impression
of their rarity. They only appear in considerable numbers at the mating
season, shortly after which the males, in the case of many species, may
be sought for in vain, as, after performing their functions, they
quickly die. The snares they spin are often rudimentary, their
capabilities in this direction appearing to deteriorate after the adult
form is attained. Young spiders of indistinguishable sex make perfect
snares on a small scale, while such as eventually develop male organs
will often thereafter be content with a few straggling lines made with
very slight regard to symmetry. They become nomadic in their habits,
wandering off in search of the females, and pitching a hasty tent by the
way.

The relations between the sexes in the Spider tribe present points of
extreme interest, but in this connexion the various groups must be
separately treated on account of their very different habits of life.

In no group are these relations more curious than in the Epeiridae, the
constructors of the familiar wheel-like web. Love-making is no trifling
matter here. If the female is not in the mood for the advances of the
male she will probably regard him as a desirable addition to her larder.
Even if his wooing is accepted, he has to beat a precipitate retreat
after effecting his purpose, or he may fall a victim to his partner’s
hunger.

This strange peril braved by the male in courting the female, which has,
as far as is known, no parallel in any other department of the animal
kingdom, is frequently mentioned as universal among spiders. It
unquestionably exists, and may be verified by any patient observer in
the case of the large Garden-spider _Epeira diademata_, but it has only
been observed among certain species of the Epeiridae and Attidae. It
will be remembered that in the Epeiridae the males are sometimes
absurdly small in comparison with the females, and this diminution of
size is thought to have a direct connection with the danger undergone at
the mating season. Small active males stand a better chance of escape
from ferocious females, so that natural selection has acted in the
direction of reducing their size as far as is compatible with the
performance of their functions.

Pickard-Cambridge[303] cites an extreme case. He says: “The female of
_Nephila chrysogaster_, Walck. (an almost universally distributed
tropical Epeirid), measures 2 inches in the length of its body, while
that of the male scarcely exceeds ⅒th of an inch, and is less than
¹⁄₁₃₀₀th part of her weight.”

During the mating season the males may be looked for on the borders of
the snares of the females. Their action is hesitating and irresolute, as
it well may be, and for hours they will linger on the confines of the
web, feeling it cautiously with their legs, and apparently trying to
ascertain the nature of the welcome likely to be extended to them. If
accepted, they accomplish their purpose by applying their palps
alternately to the epigyne of their mate. If repulsed, they do their
best to make their escape, and wait for a more auspicious moment.
Emerton[304] says: “In these encounters the males are often injured;
they frequently lose some of their legs; and I have seen one, that had
only four out of his eight left, still standing up to his work.”

Among the other groups of sedentary spiders the relations between the
sexes seem to be more pacific, and there is even some approach to
domesticity. Males and females of _Linyphia_ may be found during the
mating season living happily together in their irregular snares. The
same harmony seems to exist among the Tube-weavers, and _Agelena
labyrinthica_ lingers for days unmolested about the web of the female,
though it is perhaps hardly correct to say that they have their home in
common.

Among the wandering spiders the male usually seeks out the female and
leaps on her back, from which position his sperm-laden palps can reach
their destination. This is the habit of the Thomisidae or Crab-spiders,
and of the quick-running Wolf-spiders, or Lycosidae.

[Illustration:

  FIG. 199.—Male _Astia vittata_ dancing before the female. (After
    Peckham.)
]

The sexual relations of the Leaping-spiders, or Attidae, are so
remarkable as to deserve a longer notice. This Family includes the most
beautiful and highly ornamented examples of spider life. Their
headquarters are the tropics, and their brilliant colouring led Wallace
to speak of those he saw in the Malay Archipelago as “perfect gems of
beauty.”

Now among these spiders the male is almost always more highly decorated
than the female, and Peckham’s observations would lead to the conclusion
that the female is influenced by the display of these decorations in the
selection of her mate.

The so-called “love-dances” of certain tropical birds are known to all
readers of natural history, but it was hardly to be expected that their
counterpart would exist among spiders. Yet the antics by which male
Attidae endeavour to attract the attention of the females afford an
almost exact parallel.

The following extract from the account of Professor and Mrs.
Peckham[305] of their observations on _Saitis pulex_ will make this
abundantly clear: “When some four inches from her he stood still, and
then began the most remarkable performances that an amorous male could
offer to an admiring female. She eyed him eagerly, changing her position
from time to time, so that he might be always in view. He, raising his
whole body on one side by straightening out the legs, and lowering it on
the other by folding the first two pairs of legs up and under, leaned so
far over as to be in danger of losing his balance, which he only
maintained by sidling rapidly towards the lowered side.... Again and
again he circles from side to side, she gazing towards him in a softer
mood, evidently admiring the grace of his antics. This is repeated until
we have counted a hundred and eleven circles made by the ardent little
male. Now he approaches nearer and nearer, and when almost within reach
whirls madly around and around her, she joining with him in a giddy
maze. Again he falls back and resumes his semicircular motions, with his
body tilted over; she, all excitement, lowers her head and raises her
body so that it is almost vertical; both draw nearer; she moves slowly
under him, he crawling over her head, and the mating is accomplished.”

[Illustration:

  FIG. 200.—Dancing attitude of male _Icius mitratus_. (After Peckham.)
]

A similar but not exactly identical performance was gone through by the
male of several different species, but it was noteworthy that the
particular attitudes he adopted were always such as to display to the
best advantage his special beauties, whether they consisted in crested
head, fringed palpi and fore-legs, or iridescent abdomen. Sometimes even
such exertions failed to captivate the female, and she would savagely
attack the male, occasionally with fatal effect.

In the case of some species, when the male had won the consent of his
mate, he would weave a small nuptial tent or web, into which he would
partly lead and partly drive the female, who no longer offered serious
resistance.


                           =Fossil Spiders.=

About 250 species of fossil spiders have been discovered. Of these about
180 are embedded in amber, a fossil resinous substance which exuded from
ancient coniferous trees, and quantities of which are annually washed up
from the Baltic upon the shores of northern Prussia.

The most ancient fossil spider known was obtained from the argillaceous
slate of Kattowitz in Silesia, and belongs, therefore, to the
Carboniferous strata of the Palaeozoic epoch. It has been named
_Protolycosa anthrocophila_. There is some doubt as to the affinities of
this spider. Roemer, who described it, placed it among the Citigradae,
while others have thought it to belong rather to the Territelariae.
Thorell, on account of its agreement in certain important points with
the very curious recent Malay spider _Liphistius_, has placed them both
in a separate sub-family, Liphistioidae. To the same epoch belongs the
American fossil spider _Arthrolycosa antiqua_, which was found in the
Coal measures of Illinois.

The other localities from which fossil spiders have been obtained are
the Swiss Miocene at Oeningen, the Oligocene deposits at Aix, the
Oligocene of Florissant, Colorado, Green River, Wyoming, and Quesnel,
British Columbia.

Many of the spiders from the rocks are so fragmentary that it is
impossible to decide with certainty on their systematic position, but a
considerable number of them—more than half—have been assigned to recent
genera.

The amber spiders are mostly well preserved, and can be classified with
more certainty. Many of them are surprisingly like existing forms,
though others, like _Archaea paradoxa_, differ greatly from most spiders
now extant, though they show some affinities with one or two remarkable
and aberrant forms.




                               CHAPTER XV
    ARACHNIDA EMBOLOBRANCHIATA (_CONTINUED_)—ARANEAE (_CONTINUED_)—
                             CLASSIFICATION


The systematic study of Spiders has hitherto presented very great
difficulties. There is an extensive literature on the subject, but the
more important works are costly, not commonly to be found in libraries,
and written in diverse languages. Moreover, the nomenclature is only now
emerging from a condition of chaos. Able and diligent Arachnologists
have done admirable work in studying and describing the Spider fauna of
their various countries, and occasional tentative suggestions have been
put forth with a view to reducing to some sort of order the vast mass of
heterogeneous material thus collected. Most schemes of classification,
based chiefly upon a knowledge of European forms, have proved quite
inadequate for the reception of the vast numbers of strange exotic
species with which recent years have made us acquainted. The number of
described species is very large, and is rapidly increasing; but though
we are very far indeed from anything like an exhaustive knowledge of
existing forms, it may now be said that almost every considerable area
of the earth’s surface is at least partially represented in the cabinets
of collectors, and it is possible to take a comprehensive view of the
whole Spider fauna, and to suggest a scheme of classification very much
less likely than heretofore to be fundamentally deranged by new
discoveries.

The first to apply the Linnaean nomenclature to Spiders was Clerck, in
his _Araneae Suecicae_ (1757), which gives an account of seventy
spiders, some of which are varieties of the same species. A few new
species were added by Linnaeus, De Geer, Scopoli, Fabricius, etc., but
the next work of real importance was that of Westring (1861), who, under
the same title, described 308 species, divided among six families.
Blackwall’s beautiful work, the _Spiders of Great Britain and Ireland_,
was published by the Ray Society in 1864. He divides spiders into three
tribes, Octonoculina, Senoculina, and Binoculina, according to the
number of the eyes, and describes 304 British species, distributed among
eleven families.

His successor in this country has been Pickard-Cambridge, whose work,
under the modest title of _The Spiders of Dorset_ (1879–81), is
indispensable to British collectors.

Blackwall’s division of the order into tribes was evidently artificial,
and has not been followed by later Arachnologists. Dufour (1820) founded
two sub-orders, Dipneumones and Tetrapneumones, based on the presence of
two or four pulmonary sacs. Latreille (1825) established, and many
Arachnologists adopted, a division into tribes based upon habits,
Orbitelariae, Retitelariae, Citigradae, Latigradae, etc., and this
method of classification was followed in the important work of Menge,
entitled _Preussische Spinnen_, which was published between 1866 and
1874.

Since 1870 determined efforts have been made to grapple with the
difficult subject of Spider classification, notably by Thorell and
Simon. The latter, undoubtedly the foremost living Arachnologist, writes
with especial authority, and it is inevitable that he should be largely
followed by students of Arachnology, who cannot pretend to anything like
the same width of outlook.

It is indicative of the transition stage through which the subject is
passing that Simon in his two most important works,[306] propounds
somewhat different schemes of classification, while in the _Histoire
naturelle_, where his latest views are to be found, he introduces in the
course of the work quite considerable modifications of the scheme set
forth in the first volume.

In that work the order is divided into two sub-orders, ARANEAE
THERAPHOSAE and ARANEAE VERAE, the first sub-order containing
_Liphistius_ and the Mygalidae or Theraphosidae of other authors, while
all other spiders fall under the second sub-order. The Araneae verae are
subdivided into CRIBELLATAE and ECRIBELLATAE, according to the presence
or absence of “cribellum” and “calamistrum” (see p. 326) in the female.
Important as these organs doubtless are, the Cribellatae do not appear
to form a natural group, some of the families having apparently much
closer affinities with certain of the Ecribellatae than with one
another. This is especially evident in the case of the cribellate
Oecobiidae and the ecribellate Urocteidae (see p. 392), which most
authors unite in a single family.

After all, the larger divisions of the order are not of great
importance, and in the present chapter Simon’s linear arrangement of
families will in the main be followed, except for the distribution of
the eight families which constitute his Cribellatae[307] to the
positions which a more general view of their structure would seem to
indicate.

=Fam. 1. Liphistiidae.=—_Spiders with segmented abdomen, as shown by the
presence of a series of tergal plates. Eight spinnerets in the middle of
the ventral surface of the abdomen, far removed from the anal tubercle.
Sternum long and narrow. Eight compact eyes on a small eminence. Four
pulmonary stigmata._

[Illustration:

  FIG. 201.—Profile (nat. size) and ocular area (enlarged) of
    _Liphistius desultor_.
]

This Family includes a single genus and two species of large spiders
(about two inches in length), one from Penang and one from Sumatra. Very
few examples have been found, and these are more or less defective and
in bad condition. In some respects, especially the distinct segmentation
of the abdomen, this genus much more nearly approaches the Pedipalpi
than do any others of the order. No other spider possesses more than six
spinning mammillae, but it is possible that eight was the more primitive
number, and that the “cribellum” (see p. 326) of the so-called
Cribellate spiders is derived from the pair now possessed by
_Liphistius_ alone.

Some Arachnologists consider the genus _Liphistius_ so different from
all other spiders as to constitute in itself a sub-order, for which, on
account of the position of its spinnerets, the name MESOTHELAE has been
suggested, all other forms falling into the sub-order OPISTHOTHELAE.

=Fam. 2. Aviculariidae. (Mygalidae).=[308]—_Spiders with independent
chelicerae, the paturon directed forward and the unguis or fang
articulating in a vertical plane. The eyes are eight (except_ Masteria,
_six), usually compact, and situated on an eminence. Pedipalpi very
leg-like, and palpal organs of male simple. No maxillae. Four pulmonary
stigmata. Spinnerets normally four. No colulus._

The Aviculariidae inhabit the warmer portions of the world, and are
entirely unrepresented in this country. The monster spiders which excite
wonder in zoological collections belong to this group, as do the
moderate-sized “Trap-door” Spiders which are found abundantly in the
Mediterranean region.

The Family has been divided into about a hundred and fifty genera,
nearly half of which, however, contain only a single species.

They have been grouped by Simon[309] into seven sub-families,
PARATROPIDINAE, ACTINOPODINAE, MIGINAE, CTENIZINAE, BARYCHELINAE,
AVICULARIINAE, and DIPLURINAE, of which the first three may be dealt
with very briefly.

(i.) The PARATROPIDINAE include only two American species, _Paratropis
scrupea_ from the Amazon, and _Anisaspis bacillifera_ from St. Vincent.
They have thick, rugose integuments, and the internal angle of the coxa
of the pedipalp is produced. The labium is fused with the sternum, which
is very broad. Nothing is known of their habits, but as they do not
possess a “rastellus” (see p. 320) they are probably not burrowing
spiders.

(ii.) The ACTINOPODINAE comprise three genera, _Stasinopus_ represented
by a single South African species, _S. caffrus_; _Eriodon_, of which
about ten species inhabit Australia; and _Actinopus_, of which about ten
species are found in Central and South America. They have the coxae of
the pedipalps very short and broad, and somewhat produced at the
internal angle. The eyes are not in the usual compact group, but are
somewhat extended across the caput. _Actinopus_ burrows a deep
cylindrical hole lined with silk, and furnished with a round, bevelled
trap-door.

(iii.) The sub-family MIGINAE is established for the reception of three
genera, _Moggridgea_ (South Africa), _Migas_ (Australia and South-West
Africa), and _Myrtale_, whose single species, _M. perroti_, inhabits
Madagascar. They are chiefly characterised by their very short and
downwardly-directed chelicerae. They are not terricolous, but inhabit
trees, either boring holes in the bark, or constructing a sort of silken
retreat fortified by particles of wood.

(iv.) The CTENIZINAE form a large group, including some forty genera.
All the “Trap-door” Spiders of the Continent fall under this sub-family,
which, moreover, has representatives in all the tropical and
sub-tropical regions of the world. A rastellus is always present, and
the eyes form a compact group on an eminence. The coxae of the pedipalps
are longer than in the groups previously mentioned, and there is no
production of the internal angle. The labium is generally free.

The commonest European genus is _Nemesia_, of which about thirty species
inhabit the Mediterranean region. The cephalothorax is rather flat, and
the central fovea is recurved (◠). The burrow is sometimes simple and
sometimes branched, and the trap-door may be either thin, or thick with
bevelled edges.

Allied genera are _Hermacha_ and _Rachias_ in South America,
_Spiroctenus_ in South Africa, _Genysa_ in Madagascar, _Scalidognathus_
in Ceylon, and _Arbanitis_ in New Zealand. The genus Cteniza (fovea
procurved ◡) possesses only a single species (_C. sauvagei_), found in
South-East France and Italy.

_Pachylomerus_ is a widely-distributed genus, being represented in North
and South America, Japan, and North Africa. The tibiae of the third pair
of legs are marked above by a deep impression near the base. A closely
allied genus, _Conothele_. inhabits Southern Asia and New Guinea.

The widely-distributed genus _Acanthodon_, which has representatives in
all the sub-tropical countries of the world, together with the South
American genera _Idiops_ and _Pseudidiops_, and the Indian genus
_Heligmonerus_, present a peculiar arrangement of the eyes, one pair
being situated close together in the middle of the front of the caput,
while the remaining six form a more or less compact group some distance
behind them.

Among the many other genera of the Ctenizinae may be mentioned
_Cyrtauchenius_, of which many species inhabit North-West Africa, and
its close ally _Amblyocarenum_, represented on both shores of the
Mediterranean, and in North and South America. They differ from
_Cteniza_ chiefly in the possession of strong scopulae on the tarsi and
metatarsi of the first pair of legs, and in the double row of teeth with
which the tarsal claws are furnished. Their burrows are often surmounted
by a sort of turret raised above the level of the ground.

(v.) The BARYCHELINAE are burrowing forms which resemble _Nemesia_, but
have only two tarsal claws. _Leptopelma_ is the only European genus, and
has close affinities with certain South American genera (_Psalistops_,
_Euthycoelus_, etc.). _Pisenor_ inhabits tropical Africa, and
_Diplothele_, unique in possessing only two spinning mammillae, is an
inhabitant of India.

(vi.) The AVICULARIINAE include all the large hairy spiders which are
commonly called _Mygale_. The genus _Phlogius_, which inhabits Southern
Asia, forms a lidless burrow, though it has no rastellus, but
practically all the other members of the group are non-terricolous,
living under stones or in holes in trees, where they weave a slight web.
They are nocturnal in their habits. They all possess two tarsal claws,
and the labium is free and spined at the tip. Of the four spinnerets the
posterior pair are long and three-jointed, while the anterior are short
and not very close together.

The particular form of the tarsi and the nature of the scopulae,[310]
“claw-tufts,” and spines upon them are of great importance in
distinguishing the members of this group.

The Aviculariinae comprise about sixty genera from all the tropical and
sub-tropical regions of the world.

The genus _Ischnocolus_ extends into the Mediterranean region, having
representatives besides in Southern Asia and in Central and South
America. All the tarsi have their scopulae divided longitudinally by a
band of hairs. _Chaetopelma_ inhabits Egypt, Syria, and Arabia, and
_Cyclosternum_ is found in West Africa as well as in Central and South
America. In these genera the scopulae of the last two pairs of legs are
alone divided. The largest known spider is _Theraphosa leblondi_, which
is a native of Guiana. It measures 9 cm. (about three and a half inches)
in length.

_Eurypelma_ is a genus of large spiders entirely confined to the New
World, where it possesses many species. The genus _Avicularia_ is also
American, and includes a number of large long-haired spiders with short
and very strong legs, on which the scopulae and claw-tufts are well
developed. Its nearest allies in the Old World are the Indian genus
_Poecilotheria_, and the West African genus _Scodra_. The stridulating
spider figured on p. 328 belongs to this group, _Chilobrachys_ being a
genus from Ceylon.

[Illustration:

  FIG. 202.—_Ischnothele dumicola_, ♀ × 2. (After Pocock.)
]

(vii.) The DIPLURINAE are a very aberrant group, including some twenty
genera of Aviculariidae, usually of medium size, and possessed, as a
rule, of very long posterior spinnerets. They do not burrow or live in
holes or under stones, but weave webs of close texture, much resembling
those characteristic of the Agelenidae (see p. 415). The tarsal claws
are three in number, and there are never any claw-tufts. The rastellus,
of course, is absent.

Two genera have representatives in Europe, _Brachythele_ inhabiting the
East Mediterranean region (as well as many other parts of the world),
while _Macrothele_ is found in Spain as well as in the Malay Peninsula
and New Zealand. _Ischnothele dumicola_ is a native of Western India.
_Diplura_ is a South American genus. _Trechona venosa_, a large species
remarkable for the orange bands which decorate its abdomen, is also a
native of South America. The New Zealand genus _Hexathele_, and the
genus _Scotinoecus_ from Chili, possess six spinnerets. _Masteria_
(Ovalan Island) and _Accola_ (Philippines and South America) differ from
the rest of the family in having only six eyes.

=Fam. 3. Atypidae.=—_Spiders with anteriorly projecting and vertically
articulating chelicerae, but with no trough on the paturon for the
reception of the unguis, which is guarded when closed by a single row of
teeth. The spinnerets are normally six, and the anal tubercle is above,
and well removed from the posterior spinnerets._

[Illustration:

  FIG. 203.—_Atypus affinis_, ♀.
]

The Atypidae are a small family of six genera, rather closely related to
the Aviculariidae, and by some Arachnologists incorporated with them.
They may be regarded as the representatives of that family in
sub-tropical and temperate regions. In form they are strongly built,
with smooth integuments, and their legs are short and powerful. Of the
twenty-four species hitherto described almost all belong to the northern
hemisphere. Five are natives of Europe, and two are included in the
English fauna. The best known is _Atypus affinis_, which has been found
in several localities in the south of England, and which has occurred on
the Devil’s Dyke, near Cambridge. The female measures about half an inch
in length, the male being smaller. It burrows a deep cylindrical hole at
the edge of a grassy or heathery bank and lines it with a loose tube of
silk, which extends considerably beyond the orifice of the burrow,
either lying flat on the ground, or raised up and attached to the
neighbouring herbage. There is no lid, but the upper end of the tube is
always found closed, whether by its elasticity or by the deliberate
operation of the spider is not known. The animal is nocturnal in its
habits. Another species, _A. beckii_, occurs very rarely in the south of
England.

The genus _Atypus_ has representatives in Central and South Europe,
North Africa, Japan, Java, and North America. Of the other genera,
_Calommata_ inhabits Central and South-East Asia and Japan,
_Brachybothrium_, _Atypoides_, and _Hexura_ are peculiar to North
America, while _Mecicobothrium_ comprises a single species (_M.
thorelli_) native to the Argentine.[311]

=Fam. 4. Filistatidae.=—_Cribellate Spiders of moderate size, usually
brown or yellow in colour, with smooth integuments and somewhat long
tapering legs. The eight eyes are compactly arranged, and the palpal
organs of the male are of simple structure. The six spinnerets are
short, the anterior pair being thick and separated. Two pulmonary sacs,
with two minute tracheal stigmata close behind them and widely
separate._

There is but one genus, _Filistata_, in this family. About fifteen
species have been described, five of which inhabit the Mediterranean
region. Three are found in America, and others inhabit Central Asia, the
Philippines, and Australia. The genus is not represented in this
country, but one species, _F. testacea_, has an extremely wide
distribution in the Old World, while _F. capitata_ extends throughout
the American continent.

The calamistrum of the female is short, only occupying a portion of the
metatarsus of the fourth leg. The cribellum is divided. These spiders
weave a web of close texture, of an irregular tubular form.

=Fam. 5. Oecobiidae (Urocteidae).=—Two very remarkable genera constitute
this family, _Oecobius_ and _Uroctea_.

The species of _Oecobius_, about fifteen in number, are small spiders,
inhabiting sub-tropical countries—and especially desert regions—and
spinning a slight web under stones, or in holes in walls. The female
possesses a small transverse cribellum, the two halves of which are
widely separated. The calamistrum is but feebly developed. No example
has occurred in this country, but nine species have been described in
the Mediterranean region.

[Illustration:

  FIG. 204.—=A=, _Oecobius maculatus_, much enlarged; =B=, _Uroctea
    durandi_, slightly enlarged. (After Simon.)
]

The three species of _Uroctea_ are rather large spiders, two being
native to Africa, while the third inhabits China and Japan. They are
ecribellate. These two genera very closely resemble each other, not only
superficially, but in certain structural details—notably the remarkably
developed and two-jointed anal tubercle—and their close affinity
supplies the strongest argument against separating the spiders which
possess cribellum and calamistrum into a group by themselves. In both
genera the cephalothorax is very broad and rounded at the sides. The
eight eyes are compactly arranged. The sternum is broad and
heart-shaped. The legs are nearly of equal length, and the posterior
spinnerets have very long terminal joints.

=Fam. 6. Sicariidae (Scytodidae).=—The Sicariidae are a small group of
six-eyed spiders, usually with weak legs and slow halting movements;
they live under stones or in outhouses. The cephalothorax is generally
smooth and devoid of the median fovea, and the palpal organs of the male
are extremely simple. The best known genus is _Scytodes_, one species of
which (_S. thoracica_) has on rare occasions been found in outhouses in
the south of England, in Dorsetshire, and Kent. This is a remarkable
spider, about one-third of an inch long, with a pale yellow
ground-colour, marked with black spots and patches. The cephalothorax is
smooth and dome-shaped, and highest near the posterior end.

All the other members of the family are exotic. _Loxosceles_ is found in
the Mediterranean region and all over America, as well as in Japan. The
median fovea is present in this genus. _Sicarius_ is a native of America
and South Africa. It is of stouter build than _Scytodes_, and the legs
are stronger. _Drymusa_ belongs to South Africa. The peculiar New
Zealand species _Periegops hirsutus_ is placed by Simon in this family,
as is also the North American genus _Plectreurys_, notwithstanding its
possession of eight eyes.

=Fam. 7. Hypochilidae.=—Two species only are included in this family,
_Hypochilus thorelli_ of North America, and _Ectatosticta davidi_, a
native of China. They have four pulmonary sacs, though they possess
little else in common with the “Theraphosae.” The pedipalpus of the male
is very remarkable, the tarsus being almost unmodified, and the very
small palpal organ being inserted at its extremity. These spiders are
cribellate.

=Fam. 8. Leptonetidae.=—The Leptonetidae are small spiders with smooth
and usually dull-coloured integuments. Most of them are cave-living, but
some are found amidst vegetable débris in damp spots in forests. The
eyes are six in number, and the legs are generally long and thin. There
are five genera. _Leptoneta_ has about ten species living in caves in
the Pyrenees. The single species of _Telema_ (_T. tenella_) has the same
habitat. _Ochyrocera_ has representatives in tropical Asia and America,
and is somewhat more ornate than most members of the group. _Usofila_
has a single species, inhabiting North America, while _Theotina_ is
found in caves in the Philippines and in Venezuela.

=Fam. 9. Oonopidae.=—The Oonopidae are very small spiders, seldom
exceeding 2 mm. in length (the largest 4 mm.), living among vegetable
débris. _Oonops pulcher_, the only English representative of the family,
is not rare under stones or in the débris at the bottom of hedges. It is
a small brick-red spider, easily recognised by its six comparatively
large oval eyes, which are pale-coloured, and occupy the whole of the
caput.

The minute spiders of this family were until recently overlooked by
collectors in foreign countries, but now more than a hundred species
have been described, belonging to some eighteen genera. Thirteen species
inhabit the Mediterranean region, occurring especially on the African
side. In several genera there is a “scutum” or hard plate on the
abdomen. This is the case with _Dysderina_, which has a wide
distribution, as have also _Ischnyothyreus_ and _Opopaea_, and the
non-scutate genus _Orchestina_.

=Fam. 10. Hadrotarsidae.=—This family contains only two species,
_Hadrotarsus babirusa_ from New Guinea, and _Gmogala scarabeus_ from
Sydney. In general appearance they resemble the scutate Oonopidae, but
they have eight eyes, curiously arranged, two large, somewhat triangular
eyes being situated near the middle of the cephalothorax, and two groups
of three small eyes on either side of the front part of the caput. These
spiders are very minute.

=Fam. 11. Dysderidae.=—_Six-eyed spiders, with long free labium, and
long maxillae provided with a well-developed scopula. The cephalothorax
is rather flat, and the abdomen is oval or cylindrical, the integument
being smooth and usually rather soft. The palpal organ of the male is of
simple structure._

The Dysderidae are divided into two sub-families, DYSDERINAE and
SEGESTRIINAE, for the most part confined to temperate regions.

(i.) The DYSDERINAE are easily recognised by a peculiarity of the
sternum. Instead of being merely excavated along its border for the
reception of the legs, its edge is folded round the coxae to meet the
carapace, and thus forms a series of collars or sockets in which the
limbs are articulated in perfect isolation from each other. These
spiders vary considerably in size, and are generally of a somewhat
uniform coloration, never marked with vivid patterns. There are eight
genera of this sub-family, two of which are represented in England.

_Dysdera cambridgii_ is not a rare spider under stones in rocky
localities, such as the Isle of Portland, and occurs, though less
commonly, all over the country in similar situations, and under the
loose bark of trees. It is half an inch in length, with a
chestnut-coloured cephalothorax and legs, and dull yellow abdomen. A
closely allied species, _D. crocota_, also occurs more rarely.

_Harpactes hombergii_ is common in vegetable débris and under decaying
bark. It is about a quarter of an inch in length, of slender form, with
black-brown cephalothorax and clay-coloured abdomen. The legs are
yellowish and annulated. More than forty exotic species of _Dysdera_ and
twenty-four of _Harpactes_ have been described. Another genus of the
Dysderinae is _Stalita_, which comprises three species, inhabiting the
caves of Dalmatia and Carniola.

(ii.) The SEGESTRIINAE include two genera, _Segestria_ and _Ariadna_.

_Segestria senoculata_ occurs in England in similar localities to those
where _Dysdera cambridgii_ is found. It is not much smaller than that
spider, and has a dark brown cephalothorax and legs and a dull yellow
abdomen, with a series of adder-like diamond-shaped black markings along
the middle. Two other species have occurred on rare occasions in
England, and twelve more are recorded from the various temperate regions
of the world.

_Ariadna_ is the only Dysderid genus which invades the tropical regions.
It includes about twenty species.

=Fam. 12. Caponiidae.=—This is a small family of three genera and about
twelve species, remarkable in having no pulmonary sacs but five tracheal
stigmata,[312] and in the peculiar arrangement of their six spinnerets,
those which are ordinarily median being in the same transverse line with
the anterior ones.

The single species of _Caponia_ (_C. natalensis_) inhabits South Africa,
while _Caponina_ has two species in South America. These spiders are
eight-eyed, but the two median posterior eyes are much the largest, and
these alone are present in the remarkable genus _Nops_, of which several
species inhabit South America and adjacent islands.

=Fam. 13. Prodidomidae.=—This small family includes about twenty species
of minute spiders from sub-tropical regions. They are eight-eyed, with
short smooth legs, terminated by two claws not dentated. The spinnerets
are especially characteristic.

_Prodidomus_ (_Miltia_) includes fifteen species from the Mediterranean
region, Africa, and America. _Zimris_ is an Asiatic genus. The single
species of _Eleleis_ (_E. crinita_) is from the Cape.

[Illustration:

  FIG. 205.—Drassid Spiders. 1. _Drassus lapidosus._ 2. _Clubiona
    corticalis._ 3. _Zora spinimana._ 4. _Micaria pulicaria._
]

=Fam. 14. Drassidae.=—_Elongate spiders with low cephalothorax. Legs
usually rather long, strong, and tapering, terminated by two pectinate
claws, armed with spines, and scopulate. The body is smooth or short
haired and frequently unicolorous and sombre-coloured, seldom ornate.
The eyes, normally eight, are in two transverse rows. The mouth-parts
(labium and maxillae) are long. Spinnerets as a rule terminal, and
visible from above._

This important family includes a large number of species from all parts
of the world, fifty-six being natives of the British Isles. There are
familiar examples in the brown or mouse-coloured spiders which scurry
away when stones are raised, or when loose bark is pulled off a tree.

The family may be divided into seven sub-families, of which four,
DRASSINAE, CLUBIONINAE, LIOCRANINAE, and MICARIINAE, are represented in
this country.

(i.) The DRASSINAE include more than twenty genera, some of which
possess numerous species and have a wide distribution. The following may
be mentioned:—

_Drassus_ contains twelve British species. The commonest is _D.
lapidosus_, a large dull brown spider, more than half an inch in length,
which lives beneath stones in all parts of the country. At least a
hundred species of this genus have been described.

_Melanophora_ (= _Prosthesima_)[313] includes a large number of species.
They are dark-coloured active spiders, many of them jet black and
glossy. Seven are recorded from the British Isles, the average size
being about a quarter of an inch. They are found under stones. A closely
allied genus is _Phaeocedus_, whose single species (_P. braccatus_) has
occurred, though very rarely, in the south of England. _Gnaphosa_ has
fifty-five species, of which twenty-eight are European, and four are
British.

(ii.) The CLUBIONINAE have the anterior spinnerets closer together, and
the eyes more extended across the caput than in the foregoing
sub-family. Nearly thirty genera have been established, of which three
claim special attention. _Clubiona_ includes more than 100 species,
chiefly inhabiting temperate regions. Fifteen are included in the
British list. They are mostly unicolorous, and yellow or brown in
colour, but a few (_C. corticalis_, _C. compta_, etc.) have a distinct
pattern on the abdomen. _Cheiracanthium_ is a large and widely spread
genus, counting three English species. There are more than a hundred
species of the genus _Anyphaena_, of which one only (_A. accentuata_)
occurs in this country, where it is common upon bushes and trees in the
south.

(iii.) The LIOCRANINAE include about twenty-four genera, of which
_Zora_, _Liocranum_, _Agroeca_, and _Micariosoma_ are sparingly
represented in this country.

(iv.) The MICARIINAE are a remarkable group of Spiders containing
numerous ant-like mimetic forms. Two species of _Micaria_ alone are
English, but that genus is abundantly represented on the Continent,
where the species mount up to forty. They are mostly small, dark,
shining spiders, which, though not particularly ant-like in form, recall
those insects both by their appearance and movements. Some of the exotic
genera, and particularly the South American genus _Myrmecium_, possess
remarkable instances of mimetic resemblance to ants. _Micaria pulicaria_
is a very pretty little spider, about a sixth of an inch in length,
black, with iridescent hairs, and some white marks on the abdomen. It
runs about in a very active ant-like fashion and does not object to the
sunshine. It is fairly abundant in England.

=Fam. 15. Palpimanidae.=—This family includes a few genera of exotic
spiders. They are especially characterised by the great development of
their anterior legs, which are not much used for locomotion, but are
frequently raised as the spider moves along, generally somewhat slowly,
by means of the other three pairs. The best known genera are _Metronax_
and _Stenochilus_ from India, _Huttonia_ from New Zealand, and
_Palpimanus_ from the Mediterranean region, Africa, and South Asia.

=Fam. 16. Eresidae.=—The Eresidae are a small family of cribellate
spiders whose systematic position has been the subject of much
discussion. In general appearance they resemble the Attidae (_vide
infra_), but this resemblance is quite superficial. On the whole they
seem more nearly allied to the following family than to any other. They
are stoutly built, with thick, strong legs, and live either in the
ground or on bushes, where they weave a close-textured web. One species,
_Eresus cinnaberinus_, has occurred on rare occasions in the south of
England, and the male, which is a third of an inch in length, is perhaps
the most striking member of our Spider fauna, the abdomen being scarlet,
with four (or sometimes six) black spots edged with white hairs. The
cephalothorax is black, with red on the postero-lateral borders. The
abdomen of the female is black.

=Fam. 17. Dictynidae.=—_Cribellate spiders, with oval cephalothorax and
broad convex caput, with the eyes, normally eight, ranged across it in
two straight or slightly curved transverse rows. Basal joints of
chelicerae long and strong, often bowed. Legs rather strong. Tarsi
three-clawed and devoid of scopula._

The Dictynidae are sedentary spiders which weave a web of irregular
strands, covered by the close weft which is the product of the
cribellum. Some live under stones or in holes in walls, while others
spin their webs in bushes or herbage. There are about sixteen genera, of
which _Dictyna_ and _Amaurobius_ are the most important.

Nearly a hundred species of _Dictyna_ have been described. They are
small spiders, usually living in grass and herbage. Thirty species
inhabit Europe and the neighbouring coast of Africa, and eight of these
are natives of Britain. _D. arundinacea_ is very abundant, especially in
heather. It is about an eighth of an inch in length. _D. uncinata_ is
also often met with. _Amaurobius_, of which about eighty species are
known, includes some species of much larger size. Three species are
native to this country, _A. ferox_, _A. similis_, and _A. fenestralis_.
_A. ferox_ is a large and rather formidable-looking spider, more than
half an inch in length, with powerful chelicerae. It is found under
stones and bark, and in cellars and outhouses. _A. similis_ is the
commonest species in England, though _A. fenestralis_ somewhat replaces
it in the north. They are smaller than _A. ferox_, but are found in
similar situations.

=Fam. 18. Psechridae.=—This is a small family of cribellate spiders,
consisting only of two genera, _Psechrus_ and _Fecenia_, and some eight
species, all natives of Southern Asia and the adjacent islands. The two
species of _Psechrus_ are large spiders. They make large domed webs,
which they stretch between trees or rocks, and beneath which they hang
in an inverted position.

The calamistrum of these spiders is short, about half the length of the
fourth metatarsus.

=Fam. 19. Zodariidae (Enyoidae).=—In this family are included a number
of remarkable exotic spiders, most of them somewhat Drassid-like in
appearance, but generally with three-clawed tarsi. The group appears to
be a somewhat heterogeneous one, the twenty genera of which it consists
presenting rather a wide range of characteristics.

_Cydrela_ is an African genus of moderate sized spiders, containing five
species of very curious habits. They scramble about and burrow in the
sand, in which, according to Simon,[314] they appear to swim, and their
chief burrowing implements are their pedipalpi, which are specially
modified, the tarsi in the female bristling with spines, and being armed
with one or more terminal claws.

_Laches_ (_Lachesis_) includes some larger pale-coloured spiders found
in Egypt and Syria, under stones in very hot and dry localities.

[Illustration:

  FIG. 206.—_Hermippus loricatus_, ♂ × 2½. (After Simon.)
]

_Storena_ has representatives in all the tropical and sub-tropical parts
of the world, and numbers about fifty species. They are of moderate
size, with integuments smooth and glossy or finely shagreened, usually
dark-coloured, with white or yellow spots on the abdomen. _Hermippus_
(Fig. 206) is also African. _Zodarion_ (_Enyo_) includes about
thirty-five species of rather small, generally unicolorous spiders, very
active and fond of the sunshine. They spin no web, but have a retreat
under a stone. Their chief prey appear to be ants. Most of the species
are native to the Mediterranean region, the others belonging to Central
and Southern Asia.

Simon includes in this family the remarkable genus _Cryptothele_, found
in Ceylon, Malacca, New Guinea, and various Oceanic islands. They are
moderate sized brownish spiders, with hard integuments rugged with
tubercles and projections. Their most curious characteristic is their
power of retracting their spinnerets within a sort of sheath, so that
they become entirely invisible.

[Illustration:

  FIG. 207.—_Hersilia caudata_, ♀. (After Pickard-Cambridge.)
]

=Fam. 20. Hersiliidae.=—This is a very distinct family of spiders, with
broad cephalothorax, with well-marked fovea and striae, and small, well
defined caput. The eyes, usually eight, are black except the median
anterior pair. The legs are long and thin, and the tarsi three-clawed.
The abdomen is oval or sub-globular, short haired, and generally of
greyish coloration. The spinnerets supply the chief characteristic, the
posterior pair being long—often excessively long—and two-jointed, the
terminal joint tapering and flexible. The colulus is large. They are
very active spiders, living on tree trunks or walls, or under stones,
but spreading no snare. Some of them are of considerable size.

_Hersilia_ includes nine species native to Africa and Asia. _Tama_ is
the only genus represented in the New World, two of its species being
found in South America, while others inhabit Africa, Asia, and
Australia. Another genus, _Hersiliola_, is principally African, but
extends into Spain.

=Fam. 21. Pholcidae.=—This is another very well-marked family. The most
striking peculiarity of its members is the possession of extremely long
and thin legs, the metatarsi being especially elongated, and the tarsi
furnished with several false articulations.

The eyes are also very characteristic. They are usually eight in number,
the two anterior median eyes being black, while the other six are white,
and arranged in lateral groups of three, sometimes on prominences or
stalks. The abdomen is sometimes nearly globular, but more often long
and cylindrical. Most of the genera, which, including several new genera
lately established by Simon, number more than twenty, are poor in
species, but enjoy a very wide distribution. This is explained by the
fact that many of them live in cellars and outhouses. This is the case
with the genus _Pholcus_, of which the sole English species _Ph.
phalangioides_ is a perfect nuisance in buildings in the most southern
parts of the country, “spinning large sheets of irregular webs in the
corners and angles, and adding to them year by year.”[315] Other genera
are _Artema_ (Africa, South Asia, Polynesia, America), which includes
the largest examples, and _Spermophora_, a six-eyed genus whose few
species are widely distributed.

=Fam. 22. Theridiidae.=—_Sedentary spiders, usually with feeble
chelicerae and relatively large abdomen. Snare irregular._

The Theridiidae, as here understood, are a very extensive family, and
more than half the British spiders (about 270 species) are included
within it. This family and the next present unusual difficulties of
treatment, and there is great divergence of opinion as to the most
satisfactory way of dealing with them. This is chiefly due to the fact
that, notwithstanding an infinite variation of facies, important points
of structure are wonderfully uniform throughout both the two groups,
while any differences that do occur are bridged over by intermediate
forms which merge into each other.

Simon[316] has become so impressed with the difficulty of drawing any
clear line between certain groups which he previously classed under the
Theridiidae and the spiders commonly known as Epeiridae, that he has
recently removed them from the Theridiidae and united them with the
orb-weaving spiders to form the Family Argiopidae, the family name
Epeiridae being discarded. The groups which, in his view, belong to the
Argiopidae will be indicated below. This view has not met with universal
acceptance, and notwithstanding the undoubted difficulty of clearly
distinguishing between the two families, it is more convenient in the
present work to maintain as a separate family a group of spiders nearly
all of whose members possess the easily recognised characteristic of
spinning a circular snare.

The Theridiidae and the Epeiridae form the great bulk of the sedentary
spiders. They do not wander in search of prey, but sit in snares of
various structure and wait for their victims to entangle themselves. The
spinnerets, organs whose peculiarities are often strongly marked in
other families, are here wonderfully constant in their arrangement and
general appearance, forming a compact rosette-like group beneath the
abdomen. Their eyes, normally eight in number, present an infinite
variety of arrangement. Their chelicerae and mouth-parts vary
considerably, but no abruptness of variation is distinguishable. This is
unsatisfactory from a systematic point of view, and the necessary result
is that certain groups might with equal propriety be classed with the
Theridiidae or the Epeiridae. The latter family will here be taken as
including all the orb-weaving spiders and a few groups which appear
inseparable from them.

We shall consider the Theridiidae as comprising the seven sub-families,
ARGYRODINAE, EPISININAE, THERIDIONINAE, PHORONCIDIINAE, ERIGONINAE,
FORMICINAE, and LINYPHIINAE, and shall briefly deal with them in this
order.

(i.) The ARGYRODINAE are very curious spiders with very long and often
flexible abdomen. They are commonly parasitic on the circular snares of
Epeirid spiders, between the rays of which they spin their own irregular
webs. There are three genera, _Argyrodes_, _Ariamnes_, and _Rhomphaea_,
which are distributed in the tropical and sub-tropical regions all over
the world.

(ii.) The EPISININAE hardly conform to the character of sedentary
spiders, being frequently found outside their webs. In most species the
abdomen is narrow in front and broader behind, where it is abruptly
truncated or bluntly pointed. The genus _Episinus_ is widely
distributed, and one species, _E. truncatus_, is one of our most
peculiar English spiders. It occurs occasionally under ledges of grassy
or heathery banks. The genus _Tomoxena_ is an inhabitant of tropical
Asia. _Janulus_ is found in the same regions, and in tropical America.

(iii.) The THERIDIONINAE are a large group of spiders, often very
ornate, and spinning snares of irregular threads running in all
directions. The abdomen is usually more or less globular. The chelicerae
are small and weak, and the paturon is transversely (not obliquely)
truncated for the reception of the small unguis or fang. The somewhat
long thin legs are almost or entirely destitute of spines.

We may consider certain genera as typical of the various groups into
which this sub-family naturally falls. _Theridion_ is the richest genus
of the entire order, numbering some 320 species, of which seventeen
inhabit the British Isles. During the summer months nearly every bush is
studded with the irregular webs of these little spiders, generally
prettily coloured, and with globular abdomen. The commonest is _T.
sisyphium_, which swarms on hollies and other bushes all over the
country. One of the handsomest is _T. formosum_, a rather local species,
about a sixth of an inch in length, with the abdomen beautifully marked
with oblique lines of white, yellow, red, and black. _T. tepidariorum_,
common in conservatories, is like a large and plainer edition of _T.
formosum_. _T. riparium_ is remarkable for the curious earth-encrusted
tube which it forms for the reception of its egg-cocoon. _T.
bimaculatum_ may often be seen among coarse herbage, holding on to its
ridiculously large egg-cocoon; it is a small spider, and the sexes are
more than usually unlike.

_Latrodectus_ and _Dipoena_ are associated exotic genera, including some
of the largest species of the group. _Latrodectus_ is peculiarly
interesting on account of the great reputation for especially poisonous
properties which some of its species have acquired. The New Zealand
“Katipo” is _L. scelio_, while _L. 13–guttatus_ enjoys an almost equally
evil reputation as the “malmignatte” in Corsica. The American species
_L. mactans_ (Fig. 197, p. 362) is also considered highly venomous.
These spiders form their irregular webs on low bushes, and it is curious
that they are usually marked with red or yellow spots on the abdomen.
They have been referred to in the section on the venom of spiders (see
p. 362).

The genus _Steatoda_ possesses one English species (_S. bipunctata_)
which is extremely common in buildings and in the angles of walls, and
is a rather striking spider, with dark cephalothorax, and livid brown
abdomen with a broken white stripe down the middle. Several closely
allied genera are also sparingly represented in this country, among
which may be mentioned _Crustulina_ (two species), _Asagena_ (one
species), _Teutana_ (two species), _Lithyphantes_ (one species),
_Laseola_ (five species), and _Euryopis_ (two species). In some of these
the male is provided with a stridulating organ between the thorax and
abdomen (Fig. 183, p. 327). The remarkable genus _Tetrablemma_ (see p.
318) is considered by Simon to have affinities with this group, though
Pickard-Cambridge, who first described it, is inclined to rank it among
the Dysderidae.

[Illustration:

  FIG. 208.—_Trithena tricuspidata_ ♀. × 3½. (After Simon.)
]

(iv.) The PHORONCIDIINAE are a remarkable group of spiny Theridiids
whose superficial resemblance to the Gasteracanthinae of the Epeiridae
(see p. 409) has often deceived Arachnologists as to their true
affinities. There are eight genera, all exotic, inhabiting hot
countries, and spinning a _Theridion_-like web on bushes. _Phoroncidia_
has twelve species in South Asia and Madagascar. _Trithena_ (Fig. 208)
is its American representative, five species being found in South
America. _Ulesanis_ has about twenty species, and extends from South
America to Australia.

(v.) The ERIGONINAE are an immense group of minute, sober-coloured
spiders, which include the “Money-spinners” of popular nomenclature, and
are largely responsible for the gossamer which fills the air and covers
every tuft of grass in the autumn. The number of species described is
very large and constantly increasing, and more than a hundred are
recognised as British.

Desperate efforts have been made of late years to grapple with this
almost unmanageable group, but the multitude of genera which have been
proposed can hardly as yet be considered to be finally established. The
small size of these spiders, which renders the aid of a microscope
necessary to make out their structural peculiarities, robs them of their
attractiveness to any but the ardent Arachnologist, but they number
among them some of our most remarkable English forms, and many of them
well repay examination. The smallest English species, _Panamomops
diceros_, measures about 1 mm. (about ¹⁄₂₅ inch) in length. Many of the
groups are jet black, some with dull and others with shiny integuments.
They are never greatly variegated in hue, but the glossy black of the
cephalothorax, combined with red-brown or yellow legs, gives to some
species a rather rich coloration.

[Illustration:

  FIG. 209.—Profile of cephalothorax of 1, _Lophocarenum insanum_; 2,
    _Dactylopisthes digiticeps_; 3, _Walckenaera acuminata_ (+ abdomen);
    4, _Diplocephalus bicephalus_; 5, _Metopobractus rayi_.
]

It is impossible here to deal with this sub-family in detail. Some of
its members must be familiar enough to everybody, and the reader is
recommended to spend an hour of a warm autumn day in watching them
depart on the ballooning excursions, of which a description has been
given (see p. 341), from the knobs which surmount iron railings in a
sunny spot. Among them he is pretty sure to find the genus _Erigone_—
containing some of the largest members of the group—strongly
represented.

In some species the male presents a remarkable difference from the
female in the structure of its cephalothorax, which has the head region
produced into eminences sometimes of the oddest conformation. An extreme
example is seen in _Walckenaera acuminata_, a fine species in which the
male caput is produced into a sort of spire, bearing the eyes, and
nearly as high as the cephalothorax is long (Fig. 209, 3).

(vi.) The FORMICINAE include only two genera, _Formicina_ (South Europe)
and _Solenysa_ (Japan). They are somewhat ant-like in appearance.

(vii.) The LINYPHIINAE are closely allied to the Erigoninae, but the
legs are usually armed with spines, and very commonly the female has a
dentated claw at the end of the pedipalp.

We include here about thirty genera of spiders of moderate or small
size, living for the most part on bushes or herbage. The characteristic
Linyphian web is a horizontal sheet of irregular strands, anchored to
neighbouring twigs or leaves by cross threads in all directions, and the
spider generally lurks beneath the web in an inverted position. Some of
the larger species are very familiar objects, _Linyphia triangularis_
being one of the most abundant English spiders, filling furze and other
bushes with its extensive spinning work.

The sub-family may be roughly divided into three groups, of which the
first is small, consisting of only three exotic genera of one species
each. _Donachochara_ may be taken as the type genus. They are
moderate-sized spiders with rather short legs, found in France and
Holland.

The second group consists of a number of genera of small spiders,
sober-coloured, and generally more or less unicolorous in brown, yellow,
or black, living in herbage. The sexes are much alike, the males never
exhibiting the excrescences on the caput so often met with in the
Erigoninae. The genus _Tmeticus_ may be considered the type. It includes
about forty species, of which about half are British. They are mostly
dull yellow or brown spiders, averaging perhaps the eighth of an inch in
length. Allied genera which are represented in England are _Porhomma_
(twelve species), _Microneta_ (twelve species), _Sintula_ (twelve
species). The American cave-genus _Anthrobia_ comes here.

The third and last group is that including _Linyphia_ and allied genera.
They are moderate-sized or small spiders with long spiny legs and
particularly long tarsi. The abdomen is generally decorated. The caput
is frequently rather prominent and crowned with hairs.

Of the large number of spiders which have been described under the
generic name of _Linyphia_, Simon[317] only admits about fifty species.
Ten are included in the British list. _L. triangularis_ has already been
mentioned, but there are other common species, as _L. montana_, _L.
marginata_, and _L. clathrata_. The members of most of the associated
genera are rather small in size. We may briefly mention _Bolyphantes_,
_Bathyphantes_, _Lephthyphantes_, and _Labulla_, all of which include
English species.[318]

=Fam. 23. Epeiridae.=—This family includes all the spiders which spin
circular or wheel-like snares, the highest form of spider industry,
together with a few forms so closely allied in structure to orb-weaving
species as to be systematically inseparable from them. It is practically
co-extensive with the Argiopinae, Tetragnathinae, and Nephilinae of
Simon’s Argiopidae in the _Histoire naturelle des araignées_.[319]

No one is unfamiliar with the orbicular snares, the structure of which
has already been described with some minuteness (see p. 344), and some
of the spiders which construct them are among the best known members of
the order.

It is impossible here to deal with the multitudinous forms embraced by
this family. We must mention those genera richest in species, and some
others of special interest. It will be convenient to indicate eight
sub-families or groups, which include most of the forms likely to be met
with. These are the THERIDIOSOMATINAE, TETRAGNATHINAE, ARGIOPINAE,
NEPHILINAE, EPEIRINAE, GASTERACANTHINAE, POLTYINAE, and ARCYINAE.

(i.) The THERIDIOSOMATINAE are a small group which might with equal
propriety be classed with the Theridiidae or the Epeiridae.
_Theridiosoma argenteolum_ is a rare spider in Dorsetshire. It is a
minute spider, one-twelfth of an inch in length, with silvery white
globular abdomen variegated with reddish brown, and yellow cephalothorax
with darker caput. Some allied spiders spin a roughly circular snare.

(ii.) The TETRAGNATHINAE consist chiefly of two genera, _Pachygnatha_
and _Tetragnatha_. The first consists of spiders which are not
orb-weavers, but live in herbage, especially in swampy places. Two
species, _Pachygnatha clerckii_ and _P. degeerii_, are common in
England, and a third, _P. listeri_, is sometimes met with. They are
rather striking, prettily marked spiders, with strongly developed
chelicerae.

The species of _Tetragnatha_ are true orb-weavers, and may easily be
recognised by their cylindrical bodies, elongated chelicerae, and long
legs, stretched fore and aft along the rays of their webs. Five species
have been recorded from England, and the genus contains at least a
hundred species in all; almost every country in the world, regardless of
its latitude, supplying examples.

Simon associates with these spiders the genus _Meta_, which includes
perhaps our commonest Epeirid, _Meta segmentata_, a smallish and not
very striking Orb-weaver, with a rather elongated or subcylindrical
abdomen. Every garden is pretty sure to abound in it.

(iii.) The ARGIOPINAE include many large and very striking members of
the Epeiridae. There are about a hundred species of _Argiope_ (Fig. 198,
p. 379) spread over the tropical and sub-tropical countries of the
world. They rarely invade the temperate regions, but _A. bruennichi_ is
found in South Europe, and _A. trifasciata_ in Canada. The large spiders
with transverse bars of yellow or orange on their abdomen, and often
with a silvery sheen, belong to this genus. The species of the allied
genus _Gea_ are generally much smaller, and their abdomen more
elongated. Both genera are found in tropical and sub-tropical regions
all over the world. _Argiope_ always sits in the middle of its circular
web. There are invariably some flossy zigzag bands of silk stretched
between two of the rays, and the web is generally accompanied by an
irregular net on its border, where the much smaller male may be found.

(iv.) Among the NEPHILINAE are to be found the largest Epeirids. Indeed,
the largest yield in size only to the Aviculariidae. _Nephila_ is a
tropical genus, numbering about sixty species. The abdomen is generally
elongated and somewhat cylindrical, and is strikingly variegated. It is
in this group that the disparity in size between the sexes is most
marked (see p. 379).

(v.) The EPEIRINAE[320] include the bulk of the Orb-weavers, and form a
very extensive group. Five genera and twenty-eight species are in the
British list.

No spider is more familiar than _Epeira diademata_ (Fig. 181, p. 325),
_the_ Garden-spider, _par excellence_, which attains its greatest size
and spreads its largest snares in the autumn. The smaller and much less
conspicuous _Zilla x-notata_ is sure to be found abundantly in the same
locality. Several other Epeirids are to be found in this country,
especially in the south, by sweeping heather or bushes with a net, or
shaking the boughs of trees over an umbrella or other receptacle. The
little apple-green species is _Epeira cucurbitina_. _E. cornuta_ is
extremely common in marshy places all over the country. In furze bushes,
and often among sedge in swampy places, will frequently be found _E.
quadrata_, one of the largest and handsomest species we possess. The
ground-colour may vary from orange-red to green, and there are four
conspicuous white spots on the abdomen. The tent-like retreat which this
spider makes near its snare often catches the eye.

[Illustration:

  FIG. 210.—_Epeira angulata_, ♀.
]

_E. umbratica_ is a dark flat, somewhat toad-like Epeirid of retiring
habits, which stretches its snare usually on wooden palings, between the
timbers of which it squeezes its flat body, and waits for insects to
entangle themselves.

Two of our finest Epeiras, _E. pyramidata_ and _E. angulata_ (Fig. 210),
are seldom met with, and only in the south.

Our only _Cyclosa_ (_C. conica_) is easily recognised by the peculiar
form of its abdomen, which is greatly prolonged beyond the spinnerets.
It is a small, rather dark species, which constructs a particularly
perfect snare.

Five British Epeirids belong to the genus _Singa_. They are small
creatures, not exceeding a sixth of an inch in length. They live in
heathery and marshy localities.

(vi.) The GASTERACANTHINAE are a remarkable group of Epeirids,
characterised by the hard and coriaceous integument covering the
abdomen, which is usually furnished with a number of more or less
formidable thorn-like spines, calculated to render these spiders by no
means pleasant eating for insectivorous birds. An even more constant
characteristic is the presence on the back of the abdomen of a number of
“sigilla,” or somewhat seal-like impressions arranged symmetrically,
four forming a trapezium in the middle, while the others are distributed
round the border.

[Illustration:

  FIG. 211.—_Gasteracantha minax_, ♀.
]

There are about 200 species of _Gasteracantha_, all natives of tropical
countries.

The spiders of the genus _Micrathena_ (_Acrosoma_) have a more elongate
cephalothorax, and sometimes the spines are exceedingly long, far
exceeding the length of the body proper. Among the less spiny members of
this group are some remarkable mimetic ant-like forms.

(vii.) The POLTYINAE include some remarkable spiders, found in Africa
and South Asia for the most part, though sparingly represented in
America and Oceania. They are generally largish spiders, often with a
very odd conformation of the abdomen, which is generally much raised.
The type genus is _Poltys_.

(viii.) The ARCYINAE, which are more characteristic of Australia and the
neighbouring islands, are a small group of spiders, usually yellow with
black markings, and with the somewhat square-shaped cephalothorax
usually prominent at the angles. The type genus is _Arcys_.

=Fam. 24. Uloboridae.=—The Uloboridae are cribellate spiders, with
rather elongate cephalothorax, devoid of median fovea. The cribellum is
transverse and generally undivided. The first pair of legs are usually
much the longest. The metatarsi of the fourth legs, in addition to the
calamistrum, bear a number of generally regularly arranged spines. The
eyes are often situated on tubercles. Three sub-families are recognised,
DINOPINAE, ULOBORINAE, and MIAGRAMMOPINAE.

(i.) The DINOPINAE are a small group comprising only two genera,
_Dinopis_ and _Menneus_. The calamistrum is short, occupying not more
than half of the metatarsus. Twenty species of _Dinopis_ and six of
_Menneus_ are scattered over the tropical regions of the world.

(ii.) The ULOBORINAE include a number of spiders which have been
described under several generic names, but are now considered to fall
into two genera, _Sybota_ and _Uloborus_. _Sybota_ has only two species,
one in the Mediterranean region and one in Chili. There are about sixty
species of _Uloborus_, some of which have a wide distribution, while
many (_e.g._ _U. republicanus_, of Venezuela) are social. The type
species, _U. walckenaerius_, is a very rare spider in England.

(iii.) The MIAGRAMMOPINAE include two genera containing some very
interesting forms. The genus _Miagrammopes_, of which twenty species
have been described, though the number is probably far greater, is
characterised by a very long cylindrical abdomen, and by the apparent
possession of only four eyes, in a transverse row. These are really the
posterior eyes; and the anterior eyes, or some of them, are present in a
very reduced condition. Little is known of the habits of these spiders.

[Illustration:

  FIG. 212.—_Hyptiotes paradoxus_, ♀.
]

The other genus, _Hyptiotes_, though only boasting three species,
possesses a special interest on account of the remarkable snare
constructed by the spiders which belong to it. This has already been
described in the section upon defective orb-webs (see p. 349).

The type species, _H. paradoxus_, is very rare in England, and though
small and inconspicuous, it is certainly one of the most curious members
of our Spider fauna.

=Fam. 25. Archeidae.=—This small family includes certain remarkable
fossil spiders from Baltic amber, and two rare recent forms, _Archea_
(_Eriauchenus_) _workmani_ from Madagascar, and _Mecysmauchenius
segmentatus_ from America. The chelicerae, which are extraordinarily
long, are articulated far away from the mouth-parts. The caput is
clearly marked off from the thorax, and is much raised. In several other
respects these spiders are very distinct from all other members of the
order.

=Fam. 26. Mimetidae.=—The Mimetidae form a small group in general
appearance recalling the Theridiidae, with which family they were for a
long time incorporated. The chief genera are _Ero_, _Mimetus_, and
_Gelanor_. _Ero furcata_ (= _thoracica_) is a pretty little spider, not
rare among grass in England. The upper side of its very convex abdomen
is marked with red, yellow, and black, and bears two little
protuberances or humps near the middle. It is only about an eighth of an
inch long. Its interesting egg-cocoon has already been alluded to (see
p. 358). _E. tuberculata_ has been found on rare occasions in this
country. There are about ten other species of _Ero_, all small spiders,
and living in temperate regions. The genus _Mimetus_ (in which is merged
Blackwall’s _Ctenophora_) includes a number of larger, more
strongly-built spiders, living for the most part in tropical countries.

The genus _Gelanor_ (_Galena_) is the American representative of the
group, its three species being rather large spiders, inhabiting Central
and South America. The males of this genus have remarkably long and
slender pedipalpi, much longer than the whole body.

=Fam. 27. Thomisidae.=—The Thomisidae are the Latigrade spiders of
Latreille, and the “Crab-spiders” of popular nomenclature. Their legs
are extended more or less laterally instead of in the normal fore and
aft directions, and their progression is frequently strikingly
crab-like. They form a very large group of more than 140 genera,
including spiders of every size, and they are to be found in every
quarter of the world. Forty-three species are British. Many strange
forms are included in this group, and several of the sub-families into
which it has been divided contain only one or two genera. The bulk of
its members fall into the sub-families THOMISINAE, PHILODROMINAE, and
SPARASSINAE.

(i.) The THOMISINAE (MISUMENINAE of Simon’s _Hist. Nat._) include what
may be called the more normal members of the family, distributed among
more than sixty genera. Six of these genera are represented in the
British Isles. Our commonest Crab-spider is probably _Xysticus
cristatus_, abundant everywhere in grass and herbage. Young specimens
may often be seen upon iron railings in the autumn. Twelve other species
of that genus are on the British list. They are of small or moderate
size, rarely exceeding a quarter of an inch in length. A closely allied
genus is _Oxyptila_, of which we have seven species. The more striking
members of this sub-family to be found in England are our single
representatives of the genera _Misumena_, _Diaea_, and _Thomisus_.
_Misumena vatia_ is a handsome species, the female measuring sometimes
more than a third of an inch, and having its large yellow or green
abdomen marked, in many specimens, with a pair of bright red bands,
which, however, are not always present. The males are much smaller and
darker. It is common in some parts of England, especially in the south,
where it is to be sought for in bushes and trees.

[Illustration:

  FIG. 213.—Thomisid spiders. =A=, _Micrommata virescens_, ♀; =B=,
    _Xysticus pini_, ♀; =C=, _Philodromus margaritatus_, ♂; =D=,
    _Tibellus oblongus_, ♀.
]

_Diaea dorsata_ is one of our prettiest British species, with light
green legs and cephalothorax, and a yellow abdomen with a red-brown
central marking. It is common in the New Forest and other southern
localities. The female attains a quarter of an inch in length.

_Thomisus onustus_, a rare spider among heather, is recognisable by the
shape of its abdomen, which is broadest behind and abruptly truncated.
When adult the abdomen is a pale yellow, but the young are suffused with
a pink hue closely corresponding with that of the heather blossom in
which they are frequently found sitting.

(ii.) The PHILODROMINAE have the cephalothorax more rounded in front,
and the legs, especially the second pair, usually longer than in the
Thomisinae. There are ten genera, of which the most important is
_Philodromus_, which numbers about a hundred species. They are active
spiders, living upon bushes and trees, and most of them are inhabitants
of temperate regions. We have about twelve species in the British Isles.
The commonest is _Ph. aureolus_, which is abundant on bushes in most
parts of the country. Some species are very prettily marked, and one,
_Ph. margaritatus_ (Fig. 213, C) presents a very good example of
protective coloration, being almost indistinguishable on the blue-grey
lichen on tree trunks, where it lies in wait for insects.

Another important genus, including some fifty species, is _Thanatus_,
extending from tropical to arctic regions, but very sparingly
represented in England. _Th. striatus_ (= _hirsutus_) occurs
occasionally, and one example of the fine species _Th. formicinus_ has
been taken in the New Forest. The members of this genus as a rule affect
dry and sandy habitats.

The genus _Tibellus_ includes few species, but has a wide distribution.
The type species _T. oblongus_ (Fig. 213, D) is found in the temperate
regions all over the world, and is common in England. It is a pale
straw-coloured spider with a much elongated abdomen. It closely
resembles the stems of dry grass in hue, and when alarmed it remains
perfectly still with its legs embracing the stem and its abdomen closely
applied to it.

(iii.) The SPARASSINAE[321] include most of the large Latigrade forms,
and number about forty genera.

_Heteropoda venatoria_ is a cosmopolitan species, and though proper to
warm countries, is often introduced here on hothouse plants, and has
been known to establish itself in the open air in botanical gardens. Our
only indigenous member of this sub-family is _Micrommata virescens_
(Fig. 213, A). This striking spider is found, though rarely, in the
south of England. The female is half an inch in length and of a vivid
green hue, while the more cylindrical abdomen of the male is yellow with
three longitudinal scarlet lines. Other genera are _Sparassus_,
_Torania_, and _Delena_.

(iv.) The APHANTOCHILINAE include two curious genera which are
exclusively American. The labium is much reduced and the sternum is
shortened, terminating between the third pair of legs. The species of
_Aphantochilus_ are largish, glossy-black spiders, sometimes spotted
with white. Some of them mimic ants of the genus _Cryptocerus_. The
other genus is _Bucranium_.

(v.) The STEPHANOPSINAE include about sixteen genera, of which the best
known are _Stephanopsis_ and _Regillus_. There are about fifty species
of _Stephanopsis_, most of them Australian, while the eight species of
_Regillus_ belong to Africa and South Asia.

The mimetic form _Phrynarachne decipiens_ has already been alluded to
(see p. 374).

(vi.) The SELENOPINAE consist of a single genus, _Selenops_, of which
ten or twelve species are known, some of which are very widely
distributed, though confined to hot regions. These spiders, which are
all large, are easily recognised by their extremely flat bodies and the
peculiar arrangement of their eyes, all eight of them being placed more
or less in a single transverse line.

=Fam. 28. Zoropsidae.=—The Zoropsidae are cribellate spiders of large
size, with well-developed scopulae on tarsi and metatarsi. The cribellum
is divided, and the calamistrum, which is very short, is not well
developed. Most are inhabitants of hot regions, where they live under
stones or bark. _Zoropsis_ has six species, chiefly inhabitants of North
Africa, though representatives occur on the European side of the
Mediterranean. _Acanthoctenus_ has two species in South and Central
America.

=Fam. 29. Platoridae.=—The Platoridae are Thomisid-like, medium-sized
spiders, generally with a uniform yellow or brown coloration. The
spinnerets are their most characteristic features. The median pair
present a large flat surface studded with two parallel rows of large
fusulae, while the anterior pair are situated outside them, and are thus
widely separated. There are only three genera, and very few species of
this family. _Plator insolens_ is a Chinese species. _Doliomalus_ and
_Vectius_ belong to South America.

=Fam. 30. Agelenidae.=—_Sedentary spiders with slight sexual dimorphism;
with three tarsal claws and devoid of scopulae._

The Agelenidae spin a more or less extensive web of fine texture,
usually accompanied by a tubular retreat. Our commonest cellar spiders
belong to this group, which may be divided into three sub-families,
CYBAEINAE, AGELENINAE, and HAHNIINAE.

(i.) The CYBAEINAE include some sixteen genera, of which two deserve
special mention on account of the peculiar habits of the spiders
belonging to them.

_Desis_ is a genus of marine spiders, said to live on coral reefs below
high-water mark, and to remain in holes in the rock during high tide,
enclosed in cocoons impermeable to the sea-water. At low tide it is
stated that they come forth and prey upon small crustaceans.
_Argyroneta_ has only one species, _A. aquatica_, spread throughout
Europe and North and Central Asia. It is the well-known “Water-spider,”
which is so often an object of interest in aquaria.

(ii.) The AGELENINAE also contain sixteen genera, but it is a much
larger group, some of the genera being rich in species. They are mostly
moderate or large-sized hairy spiders, living in temperate or cold
climates. There are about fifty species of _Tegenaria_, seven of which
have been recorded as British.

Our commonest Cellar-spider is _T. derhamii_, but the very large
long-legged species found in houses in the southern counties of England
is _T. parietina_ (= _guyonii_ = _domestica_). There are not many
species of _Agelena_, but one, _A. labyrinthica_, is a common object in
this country, with its large, close-textured web and accompanying tube
spread on grassy banks by the wayside. _Coelotes atropos_ is a
formidable-looking spider, found occasionally under stones in England
and Wales. Another genus, _Cryphoeca_, has three British
representatives.

(iii.) The HAHNIINAE are recognised at once by their spinnerets, which
are arranged in a single transverse line, the posterior pair being on
the outside, and generally much the longest. _Hahnia_ contains several
species of very small spiders, of which four or five are British,
usually occurring among moss or herbage. The aberrant form _Nicodamus_
(_Centropelma_), usually placed among the Theridiidae, is removed by
Simon to the Agelenidae, forming by itself the sub-family (iv.)
NICODAMINAE.

=Fam. 31. Pisauridae.=—The Pisauridae are hairy, long-legged spiders,
intermediate, both in structure and in habits, between the Agelenidae
and the Lycosidae. Many new genera have recently been added to the
group, but many of them only include one or two species.

_Pisaura_ is spread throughout the temperate regions of the Old World,
and _P._ (_Ocyale_) _mirabilis_ is common in England, being found
abundantly in woods and on commons. It is a striking spider, more than
half an inch in length, and its elongate abdomen is marked on either
side with a sinuous longitudinal white band.

There are some thirty species of _Dolomedes_ scattered over the
temperate regions of the world. _D. fimbriatus_ is a rare species in
marshy spots in the south of England, and is one of the largest British
spiders. The ground-colour is deep brown, with two longitudinal
yellowish stripes both on cephalothorax and abdomen.

The genus _Dolomedes_ is replaced by _Thaumasia_ in South America.

=Fam. 32. Lycosidae.=—These are what are popularly known as
“Wolf-spiders.” They are vagabond hunting spiders, spinning no snare,
but chasing their prey along the ground, and in the breeding season
carrying their egg-bags with them, attached beneath the abdomen. Some of
them burrow in the loose earth or sand, but others seem to have nothing
in the way of a habitation.

[Illustration:

  FIG. 214.—Lycosid Spiders. =1=, _Lycosa fabrilis_, ♀; =2=, _Lycosa
    picta_, ♀; =3=, _Pardosa amentata_, ♀.
]

The arrangement of the eyes is very characteristic. They are in three
rows. The front row consists of four small eyes above the insertion of
the chelicerae, and directed forwards. Two comparatively very large eyes
form the next row, and occupy the upper angles of the facies, being also
directed forwards. The third row consists of two medium-sized eyes
placed dorso-laterally on the caput, some distance behind the rest, and
looking upwards. The tarsi are three-clawed. The so-called “Tarantula”
spiders belong to this group, though the name has been so abused in
popular usage, and has passed through so many vicissitudes in scientific
nomenclature, that it is difficult to tell what creature is intended by
it. In America the Aviculariidae are commonly called Tarantulas.

The two chief genera of this extensive family are _Lycosa_ and
_Pardosa_.

The genus _Lycosa_ includes about 400 species. It has been broken up
from time to time into various genera (_Trochosa_, _Pirata_,
_Tarentula_, etc.), but these glide into each other by imperceptible
degrees, and are now discarded. They are large or moderate-sized
spiders, found in every part of the world. About twenty species are
British, some of them being fine and handsomely marked. One of the
prettiest is _Lycosa picta_, common on the sandhills in some localities.

Some exotic species are very large, _Lycosa ingens_, from Madeira,
measuring sometimes more than an inch and a half in length.

_Pardosa_ (Fig. 188, p. 341) is not so rich in species, but the
individuals of some species are wonderfully numerous. Hundreds of _P.
lugubris_, for example, may be seen scampering over the dead leaves of a
wood in the autumn. These spiders are generally sombrely coloured and
well covered with hair. Perhaps the commonest and most widely spread
species in this country is _P. amentata_.

=Fam. 33. Ctenidae.=—The Ctenidae are _Lycosa_-like spiders, having in
certain points of structure close affinities with the Pisauridae and the
Sparassinae of the Thomisidae. The limits of the family are not well
defined, and many arachnologists place in it some of the genera allotted
above to the Pisauridae, while others do not consider the group
sufficiently marked off to constitute a separate family at all. As here
understood they are equivalent to the Cteninae of the Clubionidae in
Simon’s _Histoire naturelle_. The eyes are arranged in the _Lycosa_
fashion, but the tarsi have only two terminal claws and well-developed
“claw-tufts,” frequently accompanied by a scopula. There are strong,
regularly-arranged spines under the tibiae and tarsi.

There are about fifteen genera. _Uliodon_ numbers six species of large
hairy spiders in Australia. _Ctenus_ is rich in species, having about
sixty, found in all hot countries, but especially in America and Africa.
They are also of large size and usually of yellowish coloration, often
diversified by a pattern on the abdomen. The fifteen species of
_Leptoctenus_ are proper to tropical Asia. Acantheis from South Asia and
_Enoplectenus_ from Brazil are more slender, elongate forms, recalling
_Tetragnatha_. _Caloctenus_ includes a number of _Pardosa_-like spiders
found at a high elevation in South America.

The Ctenidae have the habits of the Lycosidae, and are wandering
spiders, some forming a burrow in the ground.

=Fam. 34. Senoculidae.=—The South American genus _Senoculus_
(_Labdacus_) alone constitutes this family. The species are probably
numerous, but ten only have been described. They are moderate-sized
spiders, spinning no web, but running with astonishing speed over the
leaves and stems of plants. The generic name is really inapplicable, as
there are eight eyes, but the anterior laterals are much reduced. The
abdomen is long, and the legs are long and unequal, the first pair much
the longest and the third much the shortest.

=Fam. 35. Oxyopidae.=—The Oxyopidae form a well-marked group, with oval
cephalothorax somewhat narrowed in front, and lanceolate abdomen. The
eight black eyes have a characteristic arrangement, and the anterior
medians are always very small. The legs are long and tapering, and not
very unequal, and are furnished with particularly long spines, which
give these spiders a very characteristic appearance. There are eight
genera, of which the most important are _Pucetia_ and _Oxyopes_.

_Pucetia_ contains a number of rather large spiders, generally bright
green, often variegated with red. They affect particular plants. For
instance, _P. viridis_, which occurs in Spain, is always found on
_Ononis hispanica_. There are about thirty species of this genus
distributed over the tropical and sub-tropical regions of the world.
_Oxyopes_ numbers many species, certainly more than fifty, and has a
similar distribution, but some of its members invade colder regions.
They are of rather small size. _O. lineatus_ is a very rare spider in
the south of England.

The Oxyopidae are diurnal spiders, running over plants in search of
prey, and often leaping, after the fashion of members of the following
family.

=Fam. 36. Attidae (Salticidae).=—_Wandering spiders with cephalothorax
broad anteriorly, and bearing eight homogeneous eyes in three rows. Four
eyes, largely developed, are directed forward; the remaining four eyes
are placed dorsally in two rows, the first pair being much reduced in
size._

The Attidae or Jumping-spiders form the most extensive family of the
whole order, the known species amounting to something like four
thousand. It is only of late years that their vast numbers have begun to
be realised, for their vagabond habits and great activity enabled them
to a great extent to elude the earlier collectors, whose methods were
not as thorough as those now in vogue. Their real home is in the
tropical regions, temperate fauna being comparatively poor in Attid
species. France boasts nearly 150, but only 37 are recorded for the
British Isles, and 2 at least of these are recent introductions.

Some of the tropical forms are most brilliantly coloured, glowing with
vivid colours and metallic hues, and they have frequently excited the
admiration of travellers. The coloration is nearly always due to the
hairs and scales with which the spiders are clothed, and is,
unfortunately, almost incapable of preservation in the collector’s
cabinet.

These spiders are all wanderers, spinning no snares, though they form a
sort of silken cell or retreat, in which the female lays her eggs. Their
habits are diurnal, and they delight in sunshine. They stalk their prey
and leap upon it with wonderful accuracy. They invariably attach a
thread at intervals in their course, and on the rare occasions when they
miss their aim while hunting on a perpendicular surface, they are saved
from a fall by the silken line proceeding from the spot whence the leap
was made.

[Illustration:

  FIG. 215.—Attid Spiders. =A=, _Salticus scenicus_, ♂; =B=, _Marpissa
    muscosa_, ♀; =C=, _Synemosyna formica_, ♀; =D=, _Ballus variegatus_,
    ♀.
]

The movements of these spiders are sufficient to indicate their
systematic position without entering upon structural details, but their
eyes deserve a special mention. They are all dark-coloured and very
unequal in size, and they occupy the whole area of the caput, usually
forming a large quadrilateral figure. Four large eyes occupy the facies
or “forehead,” the medians being especially large. Next come two very
small eyes, behind the anterior laterals, and lastly two of medium size
at the posterior corners of the caput.

This vast family does not lend itself easily to division into
sub-families, and it will be impossible here to do more than indicate a
very few of the multitudinous forms.

The most familiar British example is _Salticus scenicus_ (_Epiblemum
scenicum_), the little black and white striped spider to be seen hunting
on walls and fences during the summer. _Marpissa muscosa_ is the largest
English species, measuring about half an inch. It has a brownish-yellow
coloration, and is found, though not commonly, in similar situations.
_Attus pubescens_ affects grey stone walls, on which it is nearly
invisible except when moving. The other British species are mostly to be
found on trees and shrubs or among herbage, or hunting over bare sandy
spots in the sunshine. A few (_Marpissa pomatia_, _Hyctia nivoyi_) are
fen species. _Hasarius falcatus_ is a handsome spider, common in woods
in some localities.

The species differ much in their jumping powers; the Marpissas, for
example, are not great leapers, but the little _Attus saltator_, found
on sandhills, jumps like a flea, and the North American species _Saitis
pulex_ has a suggestive specific name.

Again, in this family there are mimetic forms resembling ants.
_Myrmarachne formicaria_ (_Salticus formicarius_) is found very rarely
in England, but is not uncommon on the Continent.

_Synageles_ and _Synemosyna_ are allied genera. _Phidippus_ is a genus
well represented in America, and _Ph. morsitans_ has already been
mentioned (p. 365) in connexion with its poisonous reputation. _Astia_
and _Icius_ have American representatives (see pp. 381, 382), though the
type species belongs to the Old World.




                              CHAPTER XVI
    ARACHNIDA EMBOLOBRANCHIATA (_CONTINUED_)—PALPIGRADI—SOLIFUGAE =
                SOLPUGAE—CHERNETIDEA = PSEUDOSCORPIONES


                        =Order IV. Palpigradi.=

_Minute Arachnids with three-jointed chelate chelicerae, and with the
last two joints of the cephalothorax free. The abdomen consists of
eleven segments with a fifteen-jointed flagellum._

In 1885 Grassi discovered, at Catania, a minute Arachnid which did not
fall into any of the established orders of Arachnida. He named it
_Koenenia mirabilis_. In 1893 Hansen collected several specimens in
Calabria, near Palmi and Scilla, and carefully redescribed the species
in conjunction with Sörensen.[322] It has been studied still more
minutely by Börner.[323]

There is a “head” portion, covered by a carapace, and bearing the
chelicerae, pedipalpi, and two pairs of legs. The two free thoracic
segments bear the third and fourth pairs of legs, recalling the
Schizonotidae (see p. 312), where the portion of the thorax bearing
these legs is separate, though covered by a single dorsal plate. There
are no eyes, but two hair-structures, believed to be sensory, are
present on the cephalothorax, and Börner has observed openings in the
second joint of the first pair of legs which have all the appearance of
“lyriform” organs, as found in Spiders (see p. 325).

The last three abdominal segments narrow rapidly, the last bearing the
anus. A fifteen-jointed caudal flagellum is carried, Scorpion-like,
above the animal’s back. The body and tail are each about a millimetre
in length, and the animal is of a translucent white colour.

The mouth is extremely simple, being merely a slit upon a slight
eminence. There are two sternal plates beneath the “head,” and one
beneath each free thoracic segment. The genital operculum is
complicated, and is situated beneath the second abdominal segment.

[Illustration:

  FIG. 216.—_Koenenia mirabilis_, much enlarged. (After Hansen.)
]

Since 1885 several other species have been discovered in various parts
of the world. Two American forms possess three pairs of lung-sacs on
segments 4, 5, and 6 of the abdomen. Rucker[324] has suggested for them
the generic name of _Prokoenenia_, including _P. wheeleri_, Rucker, from
Texas, and _P. chilensis_, Hansen, from Chili. The others, styled by
that author _Eukoenenia_, have no lung-sacs. There are about ten
species, mostly from the Mediterranean region, but _E. augusta_, Hansen,
is found in Siam, _E. florenciae_, Rucker, in Texas, and _E. grassii_,
Hansen, in Paraguay.


                    =Order V. Solifugae (Solpugae).=

_Tracheate Arachnids, with the last three segments of the cephalothorax
free and the abdomen segmented. The chelicerae are largely developed and
chelate, and the pedipalpi are leg-like, possessing terminal
sense-organs._

The Solifugae are, in some respects, the most primitive of the tracheate
Arachnida. Their general appearance is very spider-like, and by the old
writers they are uniformly alluded to as spiders. The segmented body and
the absence of spinning organs, however, make them readily
distinguishable on careful inspection. They are for the most part
nocturnal creatures, though some seem to rove about by day, and are even
called “Sun-spiders” by the Spaniards. The night-loving species are
attracted by light. They are, as a rule, exceedingly hairy. Some are
extremely active, while the short-legged forms (e.g. _Rhagodes_, see p.
429) move slowly. They are capable of producing a hissing sound by the
rubbing together of their chelicerae. Only the last three pairs of legs
are true ambulatory organs, the first being carried aloft like the
pedipalps, and used for feeling and manipulating the prey.

There has been much controversy as to the poisonous properties with
which these creatures have been very widely credited by both ancient and
modern writers. The people of Baku on the Caspian consider them
especially poisonous after their winter sleep. The Russians of that
region much dread the “Falangas,” as they call them, and keep a Falanga
preserved in oil as an antidote to the bite. The Somalis, on the other
hand, have no fear of them, and, though familiar with these animals,
have not thought them worthy of the dignity of a name.

Several investigators have allowed themselves to be bitten without any
special result. Some zoologists have found and described what they have
taken to be poison-glands, but these appear to be the coxal glands,
which have an excretory function. Bernard[325] suggests that, if the
bite be poisonous, the virus may exude from the numerous setal pores
which are found on the extremities of the chelicerae. The cutting powers
of the immensely-developed chelicerae are usually sufficient to ensure
fatal results on small animals without the agency of poison.
Distant,[326] indeed, thinks they cannot be poisonous, for when birds
attack them they flee before their assailants.

The Solifugae require a tolerably warm climate. In Europe they are only
found in Spain, Greece, and Southern Russia. They abound throughout
Africa, and are found in South-Western Asia, the southern United States,
and the north of South America. They appear to be absent from Australia,
nor have any been found in Madagascar. Their usual food appears to be
insects, though they devour lizards with avidity. Some interesting
observations on their habits are recorded by Captain Hutton,[327] who
kept specimens in captivity in India. An imprisoned female made a burrow
in the earth with which her cage was provided, and laid fifty eggs,
which hatched in a fortnight, but the young remained motionless for
three weeks longer, when they underwent their first moult, and became
active.

A sparrow and musk rats were at different times placed in the cage, and
were speedily killed, but not eaten. Two specimens placed in the same
cage tried to avoid each other, but, on coming into contact, fought
desperately, the one ultimately devouring the other. It was noteworthy
that the one which was first fairly seized immediately resigned itself
to its fate without a struggle. As is the case with some spiders, the
female is said occasionally to kill and devour the male. A Mashonaland
species, _Solpuga sericea_, feeds on termites,[328] while a South
Californian _Galeodes_ kills bees,[329] entering the hives in search of
them. They are fairly good climbers. In Egypt _Galeodes arabs_ climbs on
to tables to catch flies, and some species have been observed to climb
trees.

[Illustration:

  FIG. 217.—_Rhagodes_ sp., ventral view. Nat. size. _a_, Anus; _ch_,
    chelicerae; _g.o_, genital operculum; _n_, racket organs; _p_,
    pedipalp; 1, 2, 3, 4, ambulatory legs. (After Bernard.)
]

That their pedipalps, in addition to their sensory function (see p.
426), possess a sucking apparatus, is clear from an observation of
Lönnberg,[330] who kept specimens of _Galeodes araneoides_ imprisoned in
rectangular glass boxes, up the perpendicular sides of which they were
able to climb for some distance by their palps, but, being able to
obtain no hold by their legs, they soon tired.

=External Anatomy.=—The body of _Galeodes_ consists of a cephalothorax
and an abdomen, both portions being distinctly segmented. The
cephalothorax consists of six segments, the first thoracic segment being
fused with the two cephalic segments to form a sort of head, while the
last three thoracic segments are free, and there is almost as much
freedom of movement between the last two thoracic segments as between
the thorax and the abdomen. The “cephalic lobes,” which give the
appearance of a head, have been shown by Bernard[331] to be due to the
enormous development of the chelicerae, by the muscles of which they are
entirely occupied.

The floor of the cephalothorax is for the most part formed by the coxae
of the appendages, and the sternum is hardly recognisable in many
species. In _Solpuga_, however (see p. 429), it exists in the form of a
long narrow plate of three segments, ending anteriorly in a
lancet-shaped labium.

A pair of large simple eyes are borne on a prominence in the middle of
the anterior portion of the cephalothorax, and there are often one or
two pairs of vestigial lateral eyes.

The first pair of tracheal stigmata are to be found behind the coxae of
the second legs.

The mouth-parts take the form of a characteristic beak, consisting of a
labrum and a labium entirely fused along their sides. The mouth is at
the extremity of the beak, and is furnished with a straining apparatus
of complicated hairs.

The abdomen possesses ten free segments, marked off by dorsal and
ventral plates, with a wide membranous lateral interval. The ventral
plates are paired, the first pair forming the genital opercula, while
behind the second and third are two pairs of stigmata. Some species have
a single median stigma on the fourth segment, but this is in some cases
permanently closed, and in the genus _Rhagodes_ entirely absent, so that
it would seem to be a disappearing structure.

The appendages are the six pairs common to all Arachnids—chelicerae,
pedipalpi, and four pairs of legs. The chelicerae, which are enormously
developed, are two-jointed and chelate, the distal joint being
articulated beneath the produced basal joint. In the male there is
nearly always present, on the basal joint, a remarkable structure of
modified hairs called the “flagellum,” and believed to be sensory. It
differs in the different genera, and is only absent in the Eremobatinae
(see p. 429). The pedipalpi are strong, six-jointed, leg-like
appendages, without terminal claw. They end in a knob-like joint,
sometimes movable, sometimes fixed, which contains a very remarkable
eversible sense-organ, which is probably olfactory. It is concealed by a
lid-like structure, and when protruded is seen to be furnished, on its
under surface, with a pile of velvet-like sensory hairs.

The legs differ in the number of their joints, as the third and fourth
pairs have the femora divided, and the tarsus jointed. The first pair
has only a very small terminal claw, but two well-developed claws are
borne by the tarsi of the other legs. Each of the last legs bears, on
its under surface, five “racket organs,” believed to be sensory.

=Internal Structure.=—The alimentary canal possesses a sucking chamber
_within_ the beak, after which it narrows to pass through the
nerve-mass, and after an S-shaped fold, joins the mid-gut. This gives
off four pairs of thin diverticula towards the legs, the last two
entering the coxae of the third and fourth pairs.

At the constriction between the cephalothorax and the abdomen there is
no true pedicle, but there is a transverse septum or “diaphragm,”
through which the blood-vessel, tracheal nerves, and alimentary canal
pass. The gut narrows here, and, on entering the abdomen, proceeds
straight to a stercoral pocket at the hind end of the animal, but gives
off, at the commencement, two long lateral diverticula, which run
backwards parallel with the main trunk. These are furnished with
innumerable secondary tube-like diverticula, which proceed in all
directions and fill every available portion of the abdomen. The caeca,
which are so characteristic of the Arachnidan mid-gut, here reach their
extreme development. A pair of Malpighian tubules enter the main trunk
in the fourth abdominal segment.

Other excretory organs are the coxal glands, which form many coils
behind the nerve-mass, and open between the coxae of the third and
fourth legs. They have been taken for poison-glands.

There is a small endosternite in the hinder portion of the cephalothorax
under the alimentary canal.

The vascular system is not completely understood. The heart is a very
long, narrow, dorsal tube, extending almost the entire length of the
animal, and possessing eight pairs of ostia, two in the cephalothorax
and six in the abdomen. It gives off an anterior and a posterior vessel,
the latter apparently a vein, as it is guarded at its entrance by a
valve. The blood seems to be delivered by the anterior artery on to the
nerve-mass, and, after percolating the muscles and viscera, to divide
into two streams—the one returning to the heart by the thoracic ostia,
the other passing through the diaphragm and bathing the abdominal
organs, finally to reach the heart either by the abdominal ostia or by
the posterior vein.

[Illustration:

  FIG. 218.—Nervous system of _Galeodes_. _abd.g_, Abdominal ganglion;
    _ch_, cheliceral nerve; _ch.f_, chitinous fold; _ch.r_, chitinous
    rod; _g.n_, generative nerve; _l_, labial nerve; _st_, position of
    stigma. (After Bernard.)
]

The nervous system, notwithstanding the fact that the three last
thoracic segments are free, is chiefly concentrated into a mass
surrounding the oesophagus. Nerves are given off in front to the eyes,
the labrum, and the chelicerae, while double nerves radiate to the
pedipalps and to the legs. From behind the nerve-mass three nerves
emerge, and pass through the diaphragm to enter the abdomen. The median
nerve swells into an “abdominal ganglion” just behind the diaphragm, and
is then distributed to the diverticula of the alimentary canal. The
lateral nerves innervate the generative organs.

The respiratory system consists of a connected network of tracheae
communicating with the exterior by the stigmata, whose position has
already been described. There are two main lateral trunks extending
nearly the whole length of the body, and giving off numerous
ramifications, the most important of which are in the cephalothorax, and
supply the muscles of the chelicerae and of the other appendages.

The generative glands do not essentially differ from the usual Arachnid
type, though the paired ovaries do not fuse to form a ring. There are no
external organs, and the sexes can only be distinguished by secondary
characteristics, such as the “flagellum” already mentioned.

=Classification.=—There are about a hundred and seventy species of
Solifugae inhabiting the warm regions of the earth. No member of the
group is found in England, or in any except the most southern portions
of Europe.

Kraepelin[332] has divided the group into three families—Galeodidae,
Solpugidae, and Hexisopodidae.

=Fam. 1. Galeodidae.=—The Galeodidae have a lancet-shaped flagellum,
directed backwards. There is a characteristic five-toothed plate or comb
covering the abdominal stigmata. The tarsus of the fourth leg is
three-jointed, and the terminal claws are hairy.

[Illustration:

  FIG. 219.—Chelicera and flagellum of _Galeodes_. (After Kraepelin.)
]

There are two genera, _Galeodes_, with about twelve species, and
_Paragaleodes_, with six species, scattered over the hot regions of the
Old World.

=Fam. 2. Solpugidae.=—The Solpugidae comprise twenty-four genera,
distributed under five sub-families. The toothed stigmatic plate is
absent, and the tarsal claws are smooth. The ocular eminence is
furnished with irregular hairs. The “flagellum” is very variable.

[Illustration:

  FIG. 220.—Chelicerae and flagella of =A=, _Rhagodes_; =B=, _Solpuga_;
    and =C=, _Daesia_. (After Kraepelin.)
]

(i.) The RHAGODINAE include the two genera, _Rhagodes_ (_Rhax_) and
_Dinorhax_. The first has twenty-two species, which inhabit Africa and
Asia. The single species of _Dinorhax_ belongs to East Asia. These
creatures are short-legged and sluggish.

(ii.) The SOLPUGINAE contain two genera—_Solpuga_ with about fifty
species, and _Zeriana_ with three. They are all inhabitants of Africa,
and some occur on the African shore of the Mediterranean.

(iii.) The DAESIINAE number about forty species, divided among several
genera, among which the principal are _Daesia_, _Gluvia_, and
_Gnosippus_. They are found in tropical regions of both the Old and the
New World.

(iv.) The EREMOBATINAE are North American forms, the single genus
_Eremobates_ numbering about twenty species. The flagellum is here
entirely absent.

[Illustration:

  FIG. 221.—Chelicera and flagellum of _Hexisopus_. (After Kraepelin.)
]

(v.) The KARSHIINAE include the five genera _Ceroma_, _Gylippus_,
_Barrus_, _Eusimonia_, and _Karshia_. They are universally distributed.

=Fam. 3. Hexisopodidae.=—This family is formed for the reception of a
single aberrant African genus, _Hexisopus_, of which five species have
been described.

There are no claws on the tarsus of the fourth leg, which is beset with
short spine-like hairs, and in other respects the genus is peculiar.


                        =Order VI. Chernetidea.=


                     (CHERNETES, PSEUDOSCORPIONES.)

_Tracheate Arachnids, with the abdomen united to the cephalothorax by
its whole breadth. Eyeless, or with two or four simple eyes placed
laterally. Abdomen segmented, with four stigmata. Chelicerae chelate,
bearing the openings of the spinning organs. Pedipalpi large,
six-jointed, and chelate. Sternum absent or rudimentary._

The Chernetidea or “False-scorpions” constitute the most compact and
natural order of the Arachnida. There are no extreme variations within
the group as at present known, while all its members differ so markedly
from those of other Arachnidan orders that their true affinities are by
no means easy to determine.

The superficial resemblance to Scorpions which has won these animals
their popular name is almost entirely due to the comparative size and
shape of their pedipalpi, but it is probable that they are structurally
much more closely allied to the Solifugae.

Chernetidea are not creatures which obtrude themselves on the general
notice, and it is highly probable that many readers have never seen a
living specimen. This is largely due to their minute size. _Garypus
littoralis_, a Corsican species, nearly a quarter of an inch in length
of body, is a veritable giant of the tribe, while no British species
boasts a length of more than one-sixth of an inch.

Moreover, their habits are retiring. They are to be sought for under
stones, under the bark of trees, and among moss and débris. One species,
probably cosmopolitan, certainly lives habitually in houses, and is
occasionally noticed and recognised as the “book-scorpion,” and one or
two other species sometimes make themselves conspicuous by the
remarkable habit of seizing hold of the legs of flies and being carried
about with them in their flight. With these exceptions, the Chernetidea
are not likely to be seen unless specially sought for, or unless
casually met with in the search for small beetles or other creatures of
similar habitat. Nevertheless they are very widely distributed, and
though more numerous in hot countries, are yet to be found in quite cold
regions.

Though comparatively little attention has been paid to them in this
country, twenty British species have been recorded, and the known
European species number about seventy.

As might be expected from their small size and retiring habits, little
is known of their mode of life. They are carnivorous, feeding apparently
upon any young insects which are too feeble to withstand their attacks.
The writer has on two or three occasions observed them preying upon
Homopterous larvae. As a rule they are sober-coloured, their livery
consisting of various shades of yellow and brown. Some species walk
slowly, with their relatively enormous pedipalps extended in front and
gently waving, but all can run swiftly backwards and sideways, and in
some forms the motion is almost exclusively retrograde and very rapid. A
certain power of leaping is said to be practised by some of the more
active species. The Chernetidea possess spinning organs, opening on the
movable digit of the chelicera. They do not, however, spin snares like
the Spiders, nor do they anchor themselves by lines, the sole use of the
spinning apparatus being, apparently, to form a silken retreat at the
time of egg-laying or of hibernation.

=External Structure.=—The Chernetid body consists of a cephalothorax,
and an abdomen composed of twelve segments. The segmentation of the
abdomen is emphasised by the presence of chitinous plates dorsally and
ventrally, but the last two dorsal plates and the last four ventral
plates are fused, so that ordinarily only eleven segments can be counted
above and nine below.

The cephalothorax presents no trace of segmentation in the Obisiinae
(see p. 437), but in the other groups it is marked dorsally with one or
two transverse striae. The eyes, when present, are either two or four in
number, and are placed near the lateral borders of the carapace towards
its anterior end. They are whitish and only very slightly convex, and
are never situated on prominences. Except in _Garypus_ there is no trace
of a sternum, the coxae of the legs and pedipalps forming the ventral
floor of the cephalothorax.

In the Obisiinae a little triangular projection in front of the
cephalothorax is regarded by Simon[333] as an _epistome_. It is absent
in the other sub-orders.

The abdomen is armed, dorsally and ventrally, with a series of chitinous
plates with membranous intervals. The dorsal plates are eleven in number
(except in _Chiridium_, which has only ten), and are frequently bisected
by a median dorsal membranous line. There are nine ventral plates. There
is a membranous interval down each side between the dorsal and ventral
series of plates.

[Illustration:

  FIG. 222.—=A=, _Chernes_ sp., diagrammatic ventral view, × about 12.
    _a_, Anus; _ch_, chelicera; _g_, generative opening; _p_, pedipalp;
    1, 2, 3, 4, legs. (The stigmata are at the postero-lateral margins
    of the 1st and 2nd abdominal segments.) =B=, Tarsus, with claws and
    sucking disc.
]

The chitinous membrane between the plates is very extensible, rendering
measurements of the body in these animals of little value. In a female
full of eggs the dorsal plates may be separated by a considerable
interval, while after egg-laying they may actually overlap. The four
stigmata are not situated on the plates, but ventro-laterally, at the
level of the hinder borders of the first and second abdominal plates.

The first ventral abdominal plate bears the genital orifice. In the same
plate there are two other orifices, an anterior and a posterior, which
belong to the “abdominal glands.” They were taken by some authors for
the spinning organs, but their function is probably to supply material
for the capsule by which the eggs are suspended from the body of the
mother (see p. 434).

The Chernetidea possess chelicerae, pedipalpi, and four pairs of
ambulatory legs, all articulated to the cephalothorax.

[Illustration:

  FIG. 223.—Chelicera of _Garypus_. _f_, Flagellum; _g_, galea; _s_,
    serrula. (After Simon.)
]

The chelicerae are two-jointed, the upper portion of the first joint
being produced forward into a claw, curving downward. The second joint
is articulated beneath the first, and curves upward to a point, the
appendage being thus chelate. This second joint, or movable digit,
bears, near its extremity, the opening of the spinning organ, and is
furnished, at all events in the Garypinae and Cheliferinae (see p. 437),
with a pectinate projection, the “galea,” arising at its base, and
extending beyond the joint in front. In the Obisiinae it is only
represented by a slight prominence.

Two other organs characterise the chelicerae of all the Chernetidea;
these are the “serrula” and the “flagellum.” Their minute size and
transparency make them very difficult of observation, and for a long
time they escaped notice. The serrula is a comb-like structure attached
to the inner side of the distal joint. The flagellum is attached to the
outer side of the basal joint, and recalls the antenna of a Lamellicorn
beetle, or the “pectines” of scorpions, a resemblance which gave rise to
the supposition that they are olfactory organs. It is more likely,
however, that they are of use in manipulating the silk.

The pedipalpi are six-jointed and are very large, giving these animals a
superficial resemblance to scorpions. According to Simon,[334] the
patella is absent, and the joints are _coxa_, _trochanter_, _femur_,
_tibia_, _tarsus_, with an apophysis forming the fixed digit of the
chela, while the sixth joint is the movable digit, and is articulated
behind the tarsus. These joints, especially the tarsus, are often much
thickened, but however strongly developed, they are always narrow and
pediculate at the base. The coxae of the pedipalps are closely
approximated, and are enlarged and flattened. They probably assist in
mastication, but there is no true maxillary plate articulated to the
coxa as in some Arachnid groups.

The legs are usually short and feeble, and the number of their
articulations varies from five to eight, so that it is not easy to be
certain of the homologies of the individual joints to those of other
Arachnids. The coxae are large, and form the floor of the cephalothorax.
They are succeeded by a short trochanter, which may be followed by
another short joint, the “trochantin.” Then come the femur and tibia,
elongated joints without any interposed patella, and finally the tarsus
of one or two joints, terminated by two smooth curved claws, beneath
which is situated a trumpet-shaped membranous sucker.

=Internal Structure.=[335]—The internal structure of the Chernetidea, as
far as their small size has permitted it to be made out, bears a
considerable resemblance to that of the Phalangidea. The alimentary
canal dilates into a small sucking pharynx before passing through the
nerve-mass into the large many-lobed stomach, but the narrow intestine
which terminates the canal is convoluted or looped, and possesses a
feebly-developed stercoral pocket.

Above the stomach are situated the spinning glands, the products of
which pass, by seven or more tubules, to the orifice already mentioned
on the distal joint of the chelicerae. The abdominal or cement-glands
are in the anterior ventral portion of the abdomen. No Malpighian tubes
have been found.

The tracheae from the anterior stigmata are directed forward; those from
the posterior stigmata backward. Bernard[336] has found rudimentary
stigmata on the remaining abdominal segments.

The heart is the usual dorsal tube, situated rather far forward, and
probably possessing only one pair of ostia. The nerve-cord is a double
series of ventral masses, united by transverse commissures. These
undergo great concentration in the last stages of development, but in
the newly-hatched Chernetid a cerebral mass and five pairs of
post-oesophageal ganglia can be distinguished.

There are two peculiar eversible “ram’s-horn organs,” opening near the
genital opening. They are said to be present only in the male, and have
been taken for the male organs, though other writers consider them to be
tracheal in function.

=Development.=—Some points of peculiar interest are presented by the
embryology of these animals, the most striking facts being, first, that
the whole of the egg is, in some cases at all events, involved in the
segmentation; and, secondly, that there is a true metamorphosis, though
the larva is not free-living, but remains enclosed with others in a sac
attached to the mother.

At the beginning of winter the female immures herself in a silken
retreat, her body distended with eggs and accumulated nourishment. About
February the egg-laying commences, thirty eggs, perhaps, being extruded.
They are not, however, separated from the mother, but remain enclosed in
a sac attached to the genital aperture, and able, therefore, to receive
the nutritive fluids which she continues to supply throughout the whole
period of development.

The eggs, which line the periphery of the sac, develop into embryos
which presently become _larvae_, that is to say, instead of further
development at the expense of yolk-cells contained within themselves,
they develop a temporary stomach and a large sucking organ, and become
for a time independent sucking animals, imbibing the fluids in the
common sac, and arranged around its circumference with their mouths
directed towards the centre. Subsequently a second embryonic stage is
entered upon, the sucking organ being discarded, and the albuminous
matter which the larva has imbibed being treated anew like the original
yolk of the egg.

It is an interesting fact that in this second embryonic stage a
well-marked “tail” or post-abdomen is formed, and the ganglionic
nerve-masses increase in number, a cerebral mass being followed by eight
pairs of ganglia in the body and eight in the tail. Subsequently a great
concentration takes place till, besides the cerebral mass, only five
closely-applied pairs of ganglia remain, corresponding to the pedipalpi
and the four pairs of legs. Moreover, the first pair advances, so as to
lie on the sides of, and not behind, the oesophagus.

[Illustration:

  FIG. 224.—Three stages in the development of _Chelifer_. =A=,
    Segmenting ovum; =B=, embryo, with post-abdomen, maximum number of
    ganglia, and developing sucking apparatus; =C=, larva. (After
    Barrois.)
]

There are two ecdyses or moults during development, a partial moult,
concerning only the ventral surface of the “pro-embryo” as it assumes
the larval form, and a complete moult at the final stage, before
emergence from the incubating sac.

At the end of winter the mother cuts a hole in the silken web, and the
young brood issues forth.[337]

=Classification.=—The order Chernetidea consists of a single family,
=Cheliferidae=. Nine genera are recognised by most authors, but their
grouping has been the subject of a good deal of difference of opinion,
largely dependent on the different systemic value allowed by various
arachnologists to the absence or presence of eyes, and to their number
when present. Simon takes the extreme view that the eyes are only of
specific value, and he is thus led to suppress two ordinarily accepted
genera, _Chernes_ and _Roncus_, which are separated chiefly by
eye-characters from _Chelifer_ and _Obisium_ respectively. He relies
rather on such characters as the presence or absence of _galea_,
_epistome_, and _trochantin_, and establishes three sub-families as
follows:—

(i.) CHELIFERINAE.—Galea. No epistome. Trochantin on all legs. Eyes two
or none. Sole genus, _Chelifer_ (_Chelifer_ + _Chernes_).

(ii.) GARYPINAE.—Galea. No epistome. Trochantin on legs 3 and 4 only.
Eyes four or none. Genera _Chiridium_, _Olpium_, and _Garypus_.

(iii.) OBISIINAE.—No galea. An epistome. No trochantin. Genera
_Chthonius_ and _Obisium_ (which includes _Roncus_).

Whatever be the value of the eyes in the classification of this group—
and Simon adduces strong arguments for his view—there can be no doubt of
their convenience in practical identification. Moreover, as
Pickard-Cambridge[338] points out, a grouping of the genera according to
the eyes results, as regards British species, in pretty much the same
linear arrangement as Simon’s classification, and it may therefore be
convenient to mention that, of the six genera represented in this
country, _Chthonius_ and _Obisium_ are four-eyed, _Roncus_ and
_Chelifer_ two-eyed, while _Chernes_ and _Chiridium_ are eyeless.

=Sub-Fam. 1. Cheliferinae.=—These Chernetidea have the cephalothorax
slightly narrowed in front, and generally marked dorsally with two
transverse striae, while the abdominal plates are bisected by a dorsal
longitudinal line. With the exception of _Chelifer cancroides_, which is
always found in houses, all the species are to be sought under bark,
though occasionally they are discovered under stones.

The two genera of this sub-family are _Chelifer_ and _Chernes_, the
species of _Chelifer_ being two-eyed, and those of _Chernes_ blind.

As already stated, Simon does not consider the possession of the two—
often very feebly developed—eyes of generic importance, and admits only
the genus _Chelifer_.

Five species of _Chelifer_ (including _Ch. cancroides_) and five of
_Chernes_ have been recorded in England.

[Illustration:

  FIG. 225.—_Chelifer cyrneus_, enlarged. (After Simon.)
]

[Illustration:

  FIG. 226.—_Chiridium museorum_, enlarged. (After Simon.)
]

=Sub-Fam. 2. Garypinae.=—The Garypinae have the cephalothorax greatly
contracted in front and often projecting considerably.

[Illustration:

  FIG. 227.—_Olpium pallipes_, enlarged. (After Simon.)
]

There are three genera, _Chiridium_, _Olpium_, and _Garypus_.
_Chiridium_ is eyeless, and appears to have only ten segments in the
abdomen, the tergal plates of which are bisected. _C. museorum_ is found
in England, and is the only Chernetid except _Chelifer cancroides_ which
habitually lives in houses. _C. ferum_ is found under bark in the south
of France.

Neither _Olpium_ nor _Garypus_, which both possess four eyes and eleven
abdominal segments, have as yet been found in this country. _Garypus_,
like _Chiridium_, has the dorsal abdominal plates bisected. There is a
transverse stria on the cephalothorax, and the eyes are far from the
anterior border. In _Olpium_ the dorsal plates are undivided and the
eyes more anterior.

=Sub-Fam. 3. Obisiinae.=—The cephalothorax of the Obisiinae does not
narrow—and is, indeed, sometimes broadest—anteriorly. The chelicerae are
notably large, and the dorsal abdominal plates undivided. They are the
most active of the Chernetidea, ordinarily running backwards or sideways
with their pedipalpi closely folded up against the body. Four genera
usually admitted fall within this group:—_Obisium_, _Roncus_,
_Blothrus_, and _Chthonius_.

_Obisium_ has four eyes, parallel-sided cephalothorax, and curved chelae
on the palps. _Roncus_ is like _Obisium_ except in having only two eyes,
and is therefore disallowed by Simon, who also considers _Blothrus_ to
comprise merely eyeless species of _Obisium_. In _Chthonius_ the
cephalothorax is broadest in front, and the digits of the chelae are
straight.

The Obisiinae are found in moss and débris, and under stones. Three
species of _Obisium_, two of _Roncus_, and four of _Chthonius_ are
recorded in England.

The subjoined list of British species of Chernetidea is taken from the
monograph of the Rev. O. P. Cambridge, cited above:—

                     GROUP I.—Four eyes.

                 _Chthonius orthodactylus_, Leach.
                     „     _rayi_, L. Koch.
                     „     _tetrachelatus_, Preyssler.
                     „     _tenuis_, L. Koch.
                 _Obisium muscorum_, Leach.
                     „   _sylvaticum_, C. L. Koch.
                     „   _maritimum_, Leach.

                     GROUP II.—Two eyes.

                 _Roncus cambridgii_, L. Koch.
                     „    _lubricus_, L. Koch.
                 _Chelifer hermannii_, Leach.
                     „    _cancroides_, Linn.
                     „    _meridianus_, L. Koch.
                     „    _subruber_, Simon.
                     „    _latreillii_, Leach.

                     GROUP III.—No eyes.

                 _Chernes nodosus_, Schr.
                     „   _insuetus_, Camb.
                     „   _cimicoides_, Fabr.
                     „   _dubius_, Camb.
                     „   _phaleratus_, Simon.
                 _Chiridium museorum_, Leach.




                              CHAPTER XVII
    ARACHNIDA EMBOLOBRANCHIATA (_CONTINUED_)—PODOGONA—PHALANGIDEA =
               OPILIONES—HABITS—STRUCTURE—CLASSIFICATION


                   =Order VII. Podogona (Ricinulei).=

_Tracheate Arachnids with two-jointed chelate chelicerae and prehensile
pedipalpi. The tarsus of the third leg of the male bears a copulatory
organ._

[Illustration:

  FIG. 228.—_Cryptocellus simonis_, × 4. (After Hansen and Sörensen.)
]

In 1838 Guérin-Méneville[339] described an Arachnid from West Africa
which he named _Cryptostemma westermannii_. At rare intervals occasional
specimens of allied forms have been taken in the same region until six
species of _Cryptostemma_ have been established. In South America, also,
two unique examples of very similar creatures are the only known
representatives of the two species of the allied genus _Cryptocellus_.
All the examples hitherto found are of fair size (between ⅕ inch and ½
inch in length), and bear some general, though superficial, resemblance
to the Trogulidae, which has led to their being placed among the
Phalangidea by almost all the Arachnologists who have noticed them.
Their claim to this systematic position, however, is extremely doubtful,
and Hansen and Sörensen, who have had the opportunity of studying the
group much more minutely than previous writers, are of the opinion that
they ought to constitute a separate order of Arachnids, more nearly
allied to the Pedipalpi than to the Phalangidea. In this place it is
only possible to indicate some of their peculiar characteristics. Their
integuments are particularly hard and coriaceous. The cephalothorax is
united to the abdomen by a rather broad pedicle, but there is also a
remarkable coupling apparatus which makes the constriction between
cephalothorax and abdomen appear very slight. There is a movable
anterior projection of the cephalothorax, the “cucullus.” The
two-jointed chelicerae terminate in minute chelae, as also do the
five-jointed pedipalps. There are no spiracles on the abdomen, but two
are situated on the thorax above the coxae of the third pair of legs.
Perhaps the most remarkable fact is that, as in the Araneae, a modified
limb is used by the male for the fertilisation of the female; but in
this case it is not the tarsus of the pedipalp, but of the third leg of
the male, which is specially developed as an intromittent organ.

Ordinal rank is not universally accorded to the group, but whatever its
true position, the known forms fall under a single family
=Cryptostemmatidae=, including the two genera _Cryptostemma_ and
_Cryptocellus_.


                 =Order VIII. Phalangidea (Opiliones).=

_Tracheate Arachnids, with abdomen united to the cephalothorax by its
whole breadth. They are oviparous, and undergo no metamorphosis. Abdomen
always segmented. A pair of odoriferous glands opening on the thorax.
Two simple eyes; three-jointed chelate chelicerae; pedipalpi not
chelate. Spinning organs absent._

“Harvesters,” “Harvestmen,” or “Harvest-spiders,” as these animals are
popularly called, need never be confounded with true Spiders if the
absence of a constriction between the cephalothorax and abdomen be
noted. They are more difficult to distinguish from Mites, members of
which group have sometimes been described as Phalangids. The Phalangid
is, however, generally recognisable by its segmented abdomen, and as a
further point of distinction, it may be noted that, whereas the anal
orifice is always transverse or circular in Phalangids, it is uniformly
longitudinal in the Acarines.

[Illustration:

  FIG. 229.—_Oligolophus spinosus_. (After Pickard-Cambridge.)
]

Members of this group vary considerably in habit. The best known forms
are exceedingly active, and trust to their speed in endeavouring to
escape from danger, at the same time emitting an odorous fluid from two
apertures situated just above the coxae of the first pair of legs. These
active Harvestmen are only found in the mature state at certain seasons
of the year, and are believed, therefore, to live only for a single
season. Slow-moving forms, like the Nemastomatidae and the Trogulidae,
which live amidst grass and herbage, have a much longer duration of
life. In danger they remain perfectly still, and trust to their earthy
appearance to escape observation.

They are stated to be extremely thirsty animals, and have been observed
drinking from the dewdrops on herbage. It is probably on this account
that they are sometimes seen attacking juicy vegetable matter, for
without doubt they are essentially carnivorous. The larvae of insects,
young spiders, mites, and myriapods are their customary food. It is not
requisite that the prey should be alive, but they will not touch
anything mouldy.

Notwithstanding their apparently weak mouth-parts, they do not merely
suck the juices of their victims, but masticate and swallow solid
particles. Cannibalism is frequently observed among them.

The males fight fiercely with one another at the breeding time. The
females, with their long extrusible ovipositors, place groups of twenty
to forty eggs in small holes in the ground or under stones or bark,
unprotected by any form of cocoon. The eggs hatch into fully-formed
Phalangids, which are at first white, but attain their coloration after
the first moult. They subsequently moult from five to nine times.

The distribution of this group is world-wide, and some of the exotic
species are very remarkable in form. Only twenty-four species have as
yet been recorded in this country.

=External Structure.=—In the Phalangidea there is no constriction
between the cephalothorax and the abdomen, and in the Ischyropsalidae
alone is the distinction between them readily observable. This is due to
the partial or complete fusion of the first five segments of the abdomen
with the carapace or cephalothoracic shield in most species, these
segments being indicated, if at all, merely by faint striae or
successive transverse rows of spines or tubercles. In the forms
possessing hard integuments (Gonyleptidae, Nemastomatidae, Trogulidae)
this fusion results in a dorsal “scutum,” the component parts of which
cannot easily be distinguished.

The cephalothorax is often surmounted by a turret—usually grooved
dorsally, and beset on its edges with a spiny armature—on the sides of
which are the two simple eyes. The position and shape of this turret and
the arrangement of its spines are of importance in the classification of
the group.

[Illustration:

  FIG. 230.—Hood of _Metopoctea_. (After Simon.)
]

In the Trogulidae the base of the turret gives rise to a remarkable,
forwardly-directed, bifurcate structure, furnished with numerous strong
tubular bristles. This is called the “hood,” and its hollowed-out under
surface forms a chamber, the “camerostome,” in which lie the basal
joints of the pedipalpi.

In most European Phalangids the under surface of the cephalothorax is
almost entirely concealed by the forwardly-projecting portion of the
abdomen bearing the generative opening, and by the gnathobases, not only
of the pedipalpi, but of the first and sometimes of the second legs. As
in Spiders, however, there is always present a “sternum” and generally a
“labium.” The sternum is long and narrow in the Mecostethi, and
Cyphophthalmi, but in the Plagiostethi, which include most of the forms
found in temperate regions, it is very short and transverse, and is
hidden by the abdominal prolongation before mentioned.

[Illustration:

  FIG. 231.—Mouth-parts of _Phalangium_. =A=, =B=, =C=, Gnathobases of
    pedipalp and first and second legs; _ch_, chelicera; _ep_, epistome;
    _lab_, labium; _m_, mouth; _ped_, pedipalp; _pre.ep_, pre-epistome;
    _st_, sternum, shown by the removal of the anterior part of the
    genital process, which extends to the dotted line; 1, 2, 3, 4, legs.
]

The anterior wall of the mouth is formed by a beak-like plate, the
“epistome,” the basal portion of which is covered externally by a second
plate, for which Simon[340] proposes the name “pre-epistome.” In some
Phalangids there are three little chitinous plates, one median and two
lateral, on the clypeus, between the anterior border of the carapace and
the insertion of the chelicerae. They are best seen in _Nemastoma_.

The abdomen always presents evidences of segmentation, though there is a
difference of opinion as to the number of segments of which it is
composed. This is due to the already mentioned partial or complete
fusion of the anterior segments with the cephalothorax. From the
admirable researches of Hansen and Sörensen[341] it seems likely that
the normal number of abdominal segments is ten. Ventrally, the abdomen
is produced forward into a “sternal process” which is capped by a
genital plate, hardly distinguishable in the Phalangidae, but readily
visible in the other families, which surrounds and masks the unpaired
genital orifice. Two stigmata or breathing pores are situated on the
sides of the first ventral plate, which these authors consider to be
composed of two fused sternites.

As in other Arachnids there are six pairs of appendages articulated to
the cephalothorax. They are the chelicerae, the pedipalpi, and the four
pairs of ambulatory legs.

The chelicerae are three-jointed and chelate, the second joint having
its _inner_ portion produced into an apophysis to which the final joint
is apposed. In certain forms (Gonyleptidae, _Ischyropsalis_) the
chelicerae are remarkably long, and may considerably exceed the total
length of the trunk.

The pedipalpi are six-jointed, possessing coxa, trochanter, femur,
patella, tibia, and tarsus. They are leg-like and are never chelate, but
in some forms terminate in a single movable claw. The coxal joints are
provided with maxillary plates.

The legs are normally seven-jointed, as in Spiders, the penultimate
joint being the metatarsus. The tarsus is always multi-articulate, the
number of its joints being variable. It bears terminally one or two
simple claws. “False articulations” (where the parts are not inserted
one into the other, but are only marked off by a membranous ring) are of
frequent occurrence in the legs of these creatures. The first legs, like
the pedipalps, bear maxillary plates, as do also the second in most
Phalangids. The maxillae of the second legs are, however, entirely
absent in _Nemastoma_, and rudimentary in the Gonyleptidae and the
Ischyropsalidae. The coxae of the legs are all largely developed, but
are not capable of free motion, being soldered to, and practically
forming part of, the cephalothoracic floor. In some forms they are only
separated from one another by slight grooves. The extreme length of the
legs, and their hard and brittle nature, are characteristic features of
the Phalangids, though in some species (Trogulidae) they are
comparatively short. The first pair of legs are always the shortest, and
the second the longest.

The sexual organs of Phalangids are ordinarily concealed, and the sexes
can only be distinguished by certain very variable secondary characters,
the males being usually smaller of body and longer of leg than the
females, besides being more distinctly coloured and being armed with
more numerous and longer spines. Sometimes the male chelicerae are
highly characteristic.

Phalangids are usually destitute of spinning organs, but such have been
discovered, in a rudimentary state, in the Cyphophthalmi, which are said
to spin slight webs.

=Internal Structure.=—In _Phalangium_ the mouth leads upwards into a
membranous pharynx, wider than that of Spiders, but narrowing into an
oesophagus which passes between the cerebral and thoracic ganglionic
nerve-masses. It then turns backwards over the thoracic ganglion, being
slightly dilated at that point. Immediately afterwards it dilates into a
flask-like gastric sac which occupies almost the whole width of the
abdomen, and proceeds straight to the anus. Viewed from above, the shape
of this sac is entirely concealed by the large number of caeca (thirty)
to which it gives rise dorsally and laterally. The two largest of these
caeca extend, parallel to each other, over the whole of the abdominal
portion of the gastric sac, and are flanked by four lateral pairs of
smaller caeca, while there is a cluster of small caeca covering the
anterior and narrower portion of the flask-like stomach.

The large hepatic mass so conspicuous on opening dorsally the abdomen of
a Spider is here entirely absent, but its functions are believed to be
performed by certain wrinkled, tubular, longitudinally parallel bodies,
about seven in number, closely applied to the _under_ surface of the
flask.

The masticating portions of the maxillae of the pedipalpi and the first
pair of legs are hollow distensible sacs, often seen in a swollen
condition in specimens kept in spirits. They are furnished, on the inner
surface, with a horny ridge.

Owing to the fixity of the coxae of the legs, their maxillary plates are
incapable of much lateral motion, but are rubbed against each other
vertically.

Beyond the fact that the heart is a dorsal tube lying along the anterior
two-thirds of the alimentary canal, and divided by constrictions into
three well-marked and equal portions, little is known of the
blood-system of these animals. It is probably essentially like that of
Spiders, but the presence of a pericardial sac has not yet been
established, nor has the course of the blood-vessels been described in
detail.

As in other Arachnids, the principal ganglionic nerve-masses closely
embrace the oesophagus. Immediately anterior to it, forming a conical
mass with its base on the oesophagus, is the cerebral ganglion, while
just behind it is the transverse portion of the large thoracic
nerve-centre. In _Phalangium opilio_, according to Tulk,[342] a median
nerve is given off from the apex of the cerebral mass (the paired nature
of which is apparent) and bifurcates to the two eyes. Two lateral nerves
proceed to certain organs near the origin of the second pair of legs,
which were thought by the old writers to be lateral eyes, but which are
now known to be glands for the manufacture of the odorous fluid which
these animals can exude.

[Illustration:

  FIG. 232.—Nervous and respiratory systems of a Phalangid. Nerves
    black, tracheae white. _c.g_, Cerebral ganglion; _g′_, _g″_, _g‴_,
    ganglia supplying viscera; _m.n_, median abdominal nerve; _oe_,
    passage for oesophagus; _st_, stigma; _th.g_, thoracic ganglion;
    _tr_, main trunk of tracheae.
]

The thoracic ganglion expands, on either side of the oesophagus, into a
mass which extends nearly as far forward as the apex of the cerebral
ganglion. These lateral masses give off nerves to the appendages. From
the back of the transverse portion proceed three nerves. The median
nerve passes above the generative organs, and soon branches into two
nerves which presently swell out to form ganglia of considerable size,
beyond which they soon join again and give off an anastomosing network
of nerve-fibres. The lateral nerves immediately branch. The outer branch
dilates into a ganglion which supplies the external part of the
generative organ. The inner branch, which is longer, also forms a
ganglion the nerves from which are chiefly distributed to the under
surface of the alimentary canal.

The respiratory organs consist of two large tracheal tubes with numerous
branches, having their external openings or “stigmata” near the base of
the fourth pair of legs. The two main tubes are directed forwards, and
are mainly concerned with supplying the largely developed muscles of the
legs. The distribution of branches to the abdomen is comparatively
feeble. The particular arrangement of tubes in _P. opilio_, according to
Tulk, may be seen in the accompanying figure. There are a pair of coxal
glands, of excretory function, opening in the neighbourhood of the coxae
of the third pair of legs.

The Phalangidea are remarkable among Arachnids in the possession of
large protrusible external organs of generation. The ovipositor of the
female may be as long as the whole body of the animal, and the
intromittent organ of the male is of almost equal length. The pedipalpi
take no part in the fertilisation of the female, which is accomplished
directly.

The protrusible organs are concealed under the forwardly-projecting
anterior segment of the abdomen beneath, the genital orifice being thus
in many cases quite near the head region. The internal sexual organs are
not very complex. The ovary re-enters upon itself, forming a ring, and
from the point of re-entry a tube proceeds towards the centre of the
ring, dilating to form an ovisac. It then narrows, turns forward,
dilates once more into a second ovisac, from which the oviduct proceeds
to the base of the ovipositor. This is a flattened organ, grooved on its
upper surface and bifid at its extremity. The testis of the male is a
single sac-like gland, from either end of which proceeds a vas deferens,
which, after several convolutions, unite into a sperm-sac which opens at
the base of the penis.

Partial hermaphroditism is a very frequent phenomenon among the
Phalangids, the testis often producing ova as well as spermatozoa.

Though the males fight fiercely at the breeding time, the animals for
the most part live peacefully together. Henking[343] found that the eggs
of _Liobunum_, which were about half a millimetre in diameter, were laid
during October and hatched out in the following April.

=Classification.=—The Order Phalangidea is divided into three
Sub-orders: 1, CYPHOPHTHALMI; 2, MECOSTETHI; 3, PLAGIOSTETHI.


                      =Sub-Order 1. Cyphophthalmi=

_Phalangids with dorsal and ventral scutum, only the last abdominal
segment remaining free. Eyes two or absent. Maxillary lobe on coxae of
first pair of legs rudimentary. Sternum long and narrow. Anterior
segment of abdomen not projecting ventrally beyond the coxae of the
fourth pair. Odoriferous glands open on prominences._

In 1875 Stecker published a description of a remarkable creature which
he said he had found in Bohemia, and which he named _Gibocellum
sudeticum_. Among other points it possessed four eyes and four spinning
mammillae, and it differed so much from other Cyphophthalmi as to
necessitate the foundation of a family, Gibocellidae, for its reception.
No one else appears to have seen the animal, or any of Stecker’s
preparations of it, and Hansen and Sörensen[344] adduce grave reasons
for believing that it never existed at all. If this species is to be
disallowed, the Cyphophthalmi all fall into a single family.

[Illustration:

  FIG. 233.—_Parasiro corsicus_, enlarged. (After Simon.)
]

=Fam. Sironidae.=—These somewhat Mite-like Phalangids are rarely met
with, partly, no doubt, because of their retiring habits and small size,
the known forms ranging from 6 mm. to less than 2 mm. in length. Of the
seven genera which have been established, _Stylocellus_ numbers eight
species from Borneo and Sumatra, and _Pettalus_ two species from Ceylon.
_Ogovia_, _Miopsalis_, and _Purcellia_ have one species each, from South
Africa, Further India, and the Cape, respectively. The only European
forms are the two species of _Siro_ (France and Austria), and _Parasiro
corsicus_. No species has yet been found in England.


                    =Sub-Order 2. Mecostethi.=[345]

(LANIATORES).

_Sternum long and narrow. Dorsal scutum leaving at least the last three
segments free. Openings of odoriferous glands not on prominences. The
fourth pair of legs usually long and powerful. One terminal claw on each
of the first two pairs of legs; two on the last two pairs._

The Mecostethi are essentially tropical forms, though a few
representatives are found in the caves of Southern Europe. One family
(Phalangodidae) has its headquarters in the hot regions of the Old
World, while the other two (Cosmetidae, Gonyleptidae) are confined to
Central and South America.

=Fam. 1. Phalangodidae.=—_Body piriform or triangular, broadest behind.
Last ventral segment of abdomen much the largest. Very narrow sternum.
Eye-turret near anterior border of cephalothorax. Chelicerae narrow at
base. Pedipalpi long and strong. Maxillary plates on first pair of legs
rudimentary. No stigmata visible._

The only European forms of this family belong to the genus
_Phalangodes_. They all avoid the light, and are usually found in caves.
Simon[346] records six species found in France. A North American
species, _P. armata_, is entirely destitute of eyes.

[Illustration:

  FIG. 234.—_Phalangodes terricola_, enlarged. (After Simon.)
]

The family has representatives in Australia and in tropical Africa and
Asia. _Mermerus_, _Epidanus_, _Maracaudus_, and _Sitalces_ are some of
the exotic genera.

The other two families of this Sub-order—Fam. 2, =Cosmetidae=; Fam. 3,
=Gonyleptidae=—include a large number of species, some of considerable
size (up to an inch in length of body), found in Central and South
America.


                   =Sub-Order 3. Plagiostethi.=[347]
                             (PALPATORES.)

_First abdominal segment produced forward ventrally to the level of the
first pair of legs, bringing the mouth and the genital opening very near
together. Sternum consequently much reduced. Pedipalpi thin, with
terminal claw absent or rudimentary. Terminal claws of the legs single._

The Plagiostethi include most of the Harvestmen of temperate regions,
the most familiar examples of these creatures belonging to the large
family Phalangidae, and being much more in evidence than the slow-moving
and ground-living forms included in the other families.

=Fam. 1. Phalangiidae.=—_Eye-turret always far removed from anterior
border of cephalothorax. Second pair of legs with well-marked maxillary
lobes. Legs similar, without the false joint called “trochantin.”
Multiarticulate tarsi. Simple pedipalpi, with tarsus much longer than
tibia, and possessing terminal claw. Some have soft, some coriaceous
integuments._

The Phalangidae fall naturally into two groups or sub-families, named by
Simon SCLEROSOMATINAE and PHALANGIINAE. The first group consists of more
or less coriaceous forms living among moss and herbage. They are not
very numerous, there being only about twelve known European species
divided among the three genera, _Sclerosoma_, _Mastobunus_, and
_Astrobunus_.

[Illustration:

  FIG. 235.—_Sclerosoma quadridentatum._ (After Pickard-Cambridge.)
]

Two species of _Sclerosoma_ are found in England, _S. quadridentatum_
occurring not uncommonly among moss or under stones in various parts of
the country. Its back is studded with wart-like tubercles, which give it
a characteristic appearance.

The PHALANGIINAE are soft-bodied Harvestmen, always with long legs,
which in the genus _Liobunum_ attain an inordinate length. There are
nine European genera, _Liobunum_, _Prosalpia_, _Gyas_, _Oligolophus_,
_Acantholophus_, _Phalangium_, _Dasylobus_, _Platybunus_, and
_Megabunus_, comprising in all about fifty species. Five of these genera
are represented in England.

The familiar Phalangids, with small, almost spherical bodies and
ridiculously long legs, belong to the genus _Liobunum_, _L. rotundum_
being the common species. It is mature in autumn, when it may be seen
scampering at a great pace among the herbage. It very readily parts with
its limbs, and Pickard-Cambridge[348] relates that he once “saw one
running with very fair speed and facility, having lost all but two legs,
an anterior one on one side and a posterior one on the other.”

The Harvestmen so frequently seen on walls belong, as a rule, to the
genus _Phalangium_. The best known example is _Phalangium opilio_ (the
_P. cornutum_ of Linnaeus), the male of which possesses a remarkable
development of the chelicerae.

The genus _Oligolophus_ is well represented in this country, nine
species having been recorded. They do not differ greatly from
_Phalangium_, but have, as a rule, more massive bodies, and rather
stout, though tolerably long legs. The largest English Harvestman, not
rare under stones at Cambridge, is _O. spinosus_, whose body measures
half an inch in length. _O. agrestis_ is perhaps the commonest British
Phalangid, and is abundant in woods and among herbage, and on low trees.

[Illustration:

  FIG. 236.—_Oligolophus spinosus._ (After Pickard-Cambridge.)
]

_Platybunus_ has two, and _Megabunus_ one British representative. They
are of small size, and are to be sought for among heather or dead leaves
in spring or early summer.

=Fam. 2. Ischyropsalidae.=—_Coriaceous Phalangids, with eye-turret far
removed from anterior border of cephalothorax. Maxillary lobes of second
pair of legs rudimentary, in the form of tubercles. Legs similar,
without “trochantin.” Multiarticulate tarsi. Tarsus of pedipalp without
claw, and shorter than metatarsus. Pedipalps long and horizontal._

This family includes a small number of large or moderate-sized
Phalangids, which are found occasionally in thick moss, or in caves, in
mountainous regions of the south of Europe, and belong to the genera
_Ischyropsalis_ and _Sabacon_. There is a North American genus,
_Taracus_.

=Fam. 3. Nemastomatidae.=—_Coriaceous Phalangids, with cephalothorax
fused with the first five segments of the abdomen, forming a scutum.
Eye-turret near anterior border. No maxillary lobe on second coxae.
Similar legs, without “trochantin.” Multiarticulate tarsi. Tarsus of
pedipalp without claw, and shorter than metatarsus._

[Illustration:

  FIG. 237.—_Nemastoma lugubre._
]

There is but one genus, _Nemastoma_, in this family, and the members of
it are, as a rule, rather small and dark Phalangids, which live under
stones or in moss or débris, and are found in the mature state at all
seasons of the year. There are about twenty European species, but only
two of these, _N. lugubre_ and _N. chrysomelas_, have as yet been found
in Britain. _N. lugubre_ is a very common animal, and though it does not
obtrude itself upon public notice, its little black body with two pearly
white spots must be a familiar object to all insect collectors who have
occasion to search under stones or among moss in damp places. Its legs
are short and stout, but those of _N. chrysomelas_, which is a brighter
coloured Harvestman with spots of dull gold colour, are long and
slender.

=Fam. 4. Trogulidae.=—_Coriaceous and very hard integument. Anterior
part of cephalothorax produced into a bifurcate “hood.” Often a
“trochantin.”_

[Illustration:

  FIG. 238.—_Trogulus aquaticus._ _a_, Hood. (After Simon.)
]

The Trogulidae are very slow-moving Phalangids of moderate or large size
(a sixth to half an inch in body), found under stones or in damp moss
and débris. They are Mite-like in general appearance, and may readily be
distinguished from all other Harvestmen by the presence of the “hood”
(Fig. 230, p. 442), the hollowed-out under surface of which forms a
chamber, called by Simon the “camerostome,” in which lie the basal
portions of the pedipalps.

Only a single immature specimen has been found in England, belonging
probably to the species _Trogulus tricarinatus_. It was found in
Dorsetshire. Some members of the family are not uncommon in various
regions of the Continent. There are four genera, _Dicranolasma_,
_Anelasmocephalus_, _Calathocratus_, and _Trogulus_. Two other genera,
_Amopaum_ and _Metopoctea_, have been established, but the former is
probably the young of _Dicranolasma_ and the latter of _Trogulus_.

According to the monograph on the British Phalangidea by the Rev. O.
Pickard-Cambridge, cited above, the following species have been recorded
in this country. They all fall under the sub-order Plagiostethi:—

                    BRITISH PHALANGIDEA.

                        PHALANGIIDAE.

                _Sclerosoma quadridentatum_, Cuvier.
                    „      _romanum_, L. Koch.
                _Liobunum rotundum_, Latr.
                    „    _blackwallii_, Meade.
                _Phalangium opilio_, Linn.
                      „    _parietinum_, De Geer.
                      „    _saxatile_, C. L. Koch.
                      „    _minutum_, Meade.
                _Platybunus corniger_, Meade.
                      „    _triangularis_, Herbst.
                _Megabunus insignis_, Meade.
                _Oligolophus morio_, Fabr.
                      „     _alpinus_, Herbst.
                      „     _cinerascens_, C. L. Koch.
                      „     _agrestis_, Meade.
                      „     _tridens_, C. L. Koch.
                      „     _palpinalis_, Herbst.
                      „     _ephippiatus_, C. L. Koch.
                      „     _spinosus_, Bosc.

                NEMASTOMATIDAE.

                _Nemastoma lugubre_, Müller.
                    „     _chrysomelas_, Hermann.

                TROGULIDAE.

                _Anelasmocephalus cambridgii_, Westwood.
                _Trogulus tricarinatus_, Linn.




                             CHAPTER XVIII
ARACHNIDA EMBOLOBRANCHIATA (_CONTINUED_)—ACARINA—HARVEST-BUGS—PARASITIC
   MITES—TICKS—SPINNING MITES—STRUCTURE—METAMORPHOSIS—CLASSIFICATION


                 =Order IX. Acarina (Acari, Acaridea).=

_Arachnids with unsegmented,[349] non-pediculated abdomen. Respiration
by tracheae, or by the general surface of the body. Month parts
suctorial, but frequently capable of biting or piercing. Metamorphosis
always observable._

The Acarina or Mites are remarkable not so much for the number of their
species, which is very considerable, as for the vast multitude of
individuals of the Order, which is far in excess of that of any other
Arachnid group. This fact is correlated with their minute size. Very few
Mites exceed half an inch in length, while very many are microscopic
creatures, often measuring less than the hundredth of an inch. Taken all
round, a millimetre may be considered a large size for a Mite.

There is much variety of habit within the Order. All Mites live
principally on fluid nutriment, but it may be obtained from living
animals or plants or from decaying organic matter. Some are entirely
parasitic upon plants or animals; others attach themselves to animals in
their larval stage, but are free when adults; while others, again, live
an entirely independent and predaceous life.

The greater number of the Mites are too small to strike the eye. Some of
them have, however, contrived to attract attention, in no very agreeable
manner. Every one knows the Mite popularly called the “Harvest-bug,” but
to this day there is some uncertainty as to its identity. It was
described as a separate species under the name of _Leptus autumnalis_,
and Mégnin was the first to show that it was the larval form of one of
the Trombidiidae (see p. 472). Most authors have considered it the larva
of _Trombidium holosericeum_, but Murray referred it to the genus
_Tetranychus_. The difficulty is that the minute creature cannot be
removed from its victim without such injury as to prevent it from being
bred out and the mature form determined. Brucker[350] has recently
compared a large number of “Harvest-bugs” taken from human beings with
the figures and descriptions of the larvae of certain Trombidiidae given
by Henking and Berlese, and he determined them as the larvae of _T.
gynopterorum_. Quite possibly, however, more than one genus is concerned
in the production of this pest.

That certain skin-diseases are due to Mites (Demodicidae, Sarcoptidae)
is a fact which is widely known. The fruit-grower, too, has to take
cognisance of the Order, for his trees may suffer from “Red-spider”
(_Tetranychus telarius_), and his black-currant bushes fail under the
attack of the “Gall-mite” (_Eriophyes_ or _Phytoptus ribis_). The
curious swellings or galls which disfigure the leaves of many trees are
sometimes of insect origin, but they are often due to Mites.

Domestic pets suffer greatly from Acarine parasites. A large number of
species confine their attention exclusively to the feathers of birds
(_Analges_, etc., see p. 466). One curious genus, _Syringophilus_, is
parasitic _within_ the feathers, feeding upon the pith of the quill.
Heller of Kiel discovered them in 1879, but the researches of Trouessart
first showed their frequent presence and very wide distribution. He
found that they entered by the superior umbilicus of the feather, and
disappeared by the inferior umbilicus when the feathers moulted or the
infested bird died.

It is probable that the comparatively large Mites of the group
_IXODOIDEA_ (see p. 468), commonly called “Ticks,” are the most widely
known of the order. They attack wild and domestic animals and man, and
are nearly always acquired from vegetables, such as brush or herbage. It
would seem likely that many of these creatures can never have the chance
of attaching themselves to animals, and it has been suggested that
animal juices are a luxury but not a necessity to them, and that they
can live, if need be, on vegetable sap, but further investigations have
quite dispelled this view.

The suspected connection between the North American Tick, _Boophilus
annulatus_, and the cattle disease known as Texas fever or “red water,”
since clearly proved by the researches of Smith and Kilborne, led to the
careful investigation of the life-history of that creature, and this was
undertaken by Curtice.[351]

The female Ticks laid eggs a few days after dropping off the cattle,
egg-laying lasting a week or more. The eggs took from three to five
weeks to hatch, and the larvae attached themselves to cattle, on which
they remained a fortnight, becoming mature and fertilised before they
again sought the ground. The whole cycle occupied a time varying from
six to ten weeks, a period apparently much exceeded by some members of
the family.

Lounsbury[352] has recently made out the life-history of the South
African “Bont” tick, _Amblyomma hebraeum_.

The eggs are deposited in the soil, ten to twenty thousand eggs in all
being laid by one female. The larvae climb neighbouring plants and seize
passing animals. After the third day of attachment they begin to
distend, and they generally fall off, fully distended, on the sixth day,
immediately seeking a place of concealment, where they become torpid.
Under natural conditions the nymph does not emerge for at least eleven
weeks, and then it behaves in the same way as the larva, again attaching
itself to an animal for six days. A new time of torpidity and
concealment ensues, again of at least eleven weeks’ duration, when the
final moult takes place and the mature tick emerges. The males at once
attach themselves to animals, but the females hesitate to fix
themselves, except close by a male. For four days after fixation the
male appears to exercise no attraction for the female, but after that
period he shows great excitement at her approach. She, however, does all
the courting, the male remaining fixed in the skin of the host. After
pairing, the female distends greatly, attaining her maximum size (nearly
one inch in length) in about a week, when she lets go and descends to
the earth to lay eggs. If unmated, she detaches herself within a week,
and seeks another host. Oviposition lasts from three to nine weeks, and
the development of the egg from eleven weeks to six months. At least a
year is occupied in the whole cycle. These ticks, and many others,
communicate disease[353] by inoculation, conveying it from one animal to
another.

No poison-glands have been demonstrated in the Acari, the function of
the salivary glands of the Ticks being probably to prevent the
coagulation of the blood of their victims.

It is an important point in the mode of life of the Ticks that they can
live for a long time without food. Mégnin[354] states that he kept an
_Argas_ alive for four years, entirely without nutriment.

In the Tetranychinae (see p. 472), glands apparently homologous with the
salivary glands of the Ticks have taken on the function of spinning
organs. According to Donnadieu,[355] these glands, which resemble
bunches of grapes, and are possessed by both sexes, open into the buccal
cavity at the base of the chelicerae. The gummy fluid exudes from the
mouth, and is combed into threads by the pedipalps. The legs of these
mites are furnished terminally with curious hairs ending in a round
knob, which are supposed to have some relation to their spinning habits.

The males are the busiest spinners, the time of the females being
largely occupied in laying eggs among the excessively fine threads of
silk with which the Mites cover the under surface of leaves. In the
Eriophyidae (see p. 464) corresponding glands are thought to furnish an
irritant fluid which causes abnormal growths or galls upon vegetable
tissues.

=External Structure.=—It is often stated, but erroneously, that there is
no distinction between cephalothorax and abdomen in the Mites. Certainly
no such division can be made out in the Hydrachnidae (see p. 472) or in
some other forms, but in the majority of Acari the cephalothorax is
clearly marked off by a transverse groove or suture. In some cases the
anterior portion of the cephalothorax is movably articulated with the
rest, and forms a sort of false head called a “capitulum.” In most Mites
the chitinous integument is soft and non-resistant, but it is otherwise
with the Oribatidae or “Beetle-mites” (see p. 467), which are nearly all
covered by an extremely hard and coriaceous armature.

Eyes are sometimes absent, sometimes present in varying numbers. They
seem here to be of remarkably little systematic importance, as otherwise
closely allied species may be either eyed or eyeless.

Normally Mites possess the usual Arachnid appendages, chelicerae,
pedipalpi, and four pairs of ambulatory legs. The anterior appendages
are, however, subject to a very great degree of modification, while in
one Family, the Eriophyidae (Phytoptidae), the legs are apparently
reduced to two pairs.

The chelicerae are sometimes chelate, in which case they are
two-jointed, the distal joint or movable finger being always articulated
_below_ the immovable finger. Sometimes they terminate in a single claw
or blade, the movable joint being obsolete. In the Ticks they exist as
two long styles or piercing weapons, serrate on the outer edge.

The pedipalpi vary very much in structure, according to the habits of
the particular form to which they belong. In the Sarcoptidae (see p.
466) they are hardly recognisable owing to the extent to which they have
coalesced with the maxillary plate. In many of the free-living forms
they are leg-like feeling organs, but in others they are raptorial,
being not precisely chelate, but terminating in a “finger-and-thumb”
arrangement which is of use in holding prey. The extreme development of
the raptorial palp is found in _Cheyletus_ (see p. 473), in which the
whole appendage is remarkably thick and strong, and the “finger” is a
powerful chitinous claw, while the “thumb” is replaced by movable
pectinated spines of chitin. The Water-mites have a palpus adapted for
anchoring themselves to water-weeds, the last joint being articulated
terminally with the penultimate joint, and bending down upon it.
Finally, in the “Snouted mites” (Bdellidae, see p. 471) the palpi are
tactile or antenniform, often strongly recalling the antennae of
weevils.

The maxillary plates which arise from the basal joints of the pedipalps
are always more or less fused, in the Mites, to form a single median
transverse plate, constituting the lower lip or “labium” of some
authors. In some of the Oribatidae the fusion of the maxillae is only
complete at the base, and the free points are still of some use as
masticating organs. In those free living Mites which have undergone no
great modification of the mouth-parts two other portions can be
distinguished, the upper lip or “epipharynx,” and the “lingua,” which
forms the floor of the mouth, and is for the most part concealed by the
maxillary plate.

The legs are usually six- or seven-jointed, and are subject to great
variation, especially as regards the tarsus or terminal joint. This may
bear claws (1–3) or sucking disks, or a combination of the two, or may
simply take the form of a long bristle or hair.

The Cheese-mite has a claw surrounded by a sucker—like Captain Cuttle’s
hook within his sleeve. The claws of those species which are parasitic
on the hairs of animals are sometimes most remarkably modified.

[Illustration:

  FIG. 239.—Diagram of the viscera of an Oribatid Mite, greatly
    enlarged. C, C, Lateral caeca of stomach; _g_, cerebral ganglion;
    _od_, _od_, oviducts; _oe_, oesophagus; _pr.g_, pro-ventricular
    gland; _ps_, pseudo-stigmatic organ; _st_, stomach; _tr_, _tr_,
    tracheae. (Partly after Michael.)
]

=Internal Structure.=—The minute size of most Mites has rendered
research upon their internal structure a matter of great difficulty, and
there are still many obscurities to be removed. Those forms which have
been subjected to examination present a tolerable uniformity in the
structure of the principal organs, but the brief description here given
will not, of course, apply to aberrant groups like the Vermiformia. A
marked concentration is noticeable throughout the Order, and is best
exemplified by the nervous system.

The mouth leads into a sucking pharynx, which narrows to form the
oesophagus. This passes through the nerve-mass in the usual Arachnid
fashion, and widens to form the ventriculus or stomach. The oesophagus
varies considerably in width in the various groups, being very narrow in
those Mites which merely suck blood, but wider in vegetable-feeders like
the Oribatidae.

The stomach is always provided with caeca, but these are not nearly so
numerous as in some other Orders of Arachnida. There are always two
large caeca directed backwards, and there may be others. They are most
numerous in the Gamasidae (see p. 470), which sometimes possess eight,
some being prolonged into the coxae of the legs, as in Spiders. At the
sides of the anterior part of the stomach there are usually two
glandular bodies, the pro-ventricular glands. In those Mites in which
the alimentary canal is most differentiated (_e.g._ Oribatidae) three
parts are distinguishable behind the stomach, a small intestine, a
colon, and a rectum, but in most groups the small intestine is
practically absent. The Malpighian tubes, very variable in length, enter
at the constriction between colon and rectum.

In some of the Trombidiidae there appears to be a doubt as to the
existence of a hind-gut at all. A body having the appearance of the
hind-gut, and occupying its usual position, is found to contain, not
faecal matter, but a white excretory substance, and all efforts to
discover any passage into it from the stomach have been unsuccessful.
Both Croneberg[356] and Henking[357] came to the conclusion that the
stomach ended blindly, and that the apparent hind-gut was an excretory
organ. Michael,[358] in his research upon a Water-mite, _Thyas
petrophilus_, met with precisely the same difficulty, and was led to the
belief that what was originally hind-gut had become principally or
entirely an excretory organ.

The nervous system chiefly consists of a central ganglionic mass,
usually transversely oval, and presenting little or no indication of the
parts which have coalesced in its formation. Nerves proceed from it in a
radiate manner, but no double nerve-cord passes towards the posterior
end of the body. As above stated, it is perforated by the oesophagus.

The vascular system is little understood. In 1876 Kramer[359] wrote that
he was able to perceive an actively pulsating heart in the posterior
third of the abdomen in specimens of _Gamasus_ which had recently
moulted, and were therefore moderately transparent. No other
investigator has been equally fortunate, though many capable workers
have sought diligently for any trace of a dorsal vessel in various
Acarine groups.

It would appear that the blood-flow in most Mites is lacunar and
indefinite, aided incidentally by the movements of the muscles, and
perhaps by a certain rhythmic motion of the alimentary canal, which has
been observed to be most marked during the more quiescent stages of the
life-history.

The internal reproductive organs have the ringed arrangement generally
observed in the Arachnida. The two testes, which are sometimes bi-lobed,
are connected by a median structure which may serve as a vesicula
seminalis, and there are two vasa deferentia which proceed to the
intromittent organ, which is sometimes situated quite in the anterior
part of the ventral surface, but at others towards its centre. The male
Mite is often provided with a pair of suckers towards the posterior end
of the abdomen, and sometimes accessory clasping organs are present.

In some Mites there is no intromittent organ, and Michael[360] has
described some remarkable cases in which the chelicerae are used in the
fertilisation of the female, a spermatophore, or bag containing
spermatozoa, being removed by them from the male opening and deposited
in that of the female. The most remarkable instance is that of _Gamasus
terribilis_, the movable joint of whose chelicera is perforated by a
foramen through which the spermatophore is, so to speak, blown and
carried as a bi-lobed bag, united by the narrow stalk which passes
through the foramen, to the female aperture.

The ovaries are fused in the middle line, and connected by oviducts with
the tube (vagina or uterus) which passes to the exterior. There is often
an ovipositor.

Professor Gené of Turin[361] described, in 1844, some remarkable
phenomena in connection with the reproduction of Ticks. The male
_Ixodes_ introduced his rostrum into the female aperture, two small
white fusiform bodies emerging right and left from the labium at the
moment of introduction. On retraction they had disappeared. When the
female laid eggs, a bi-lobed vesicle was protruded from beneath the
anterior border of the scutum and grasped the egg delivered to it by the
ovipositor, appearing to manipulate it for some minutes. Then the
vesicle was withdrawn, and the egg was left on the rostrum, and
deposited by it in front of the animal. When the vesicle was punctured,
and so rendered useless, the unmanipulated eggs quickly shrivelled and
dried up.

Lounsbury[362] has recently confirmed Professor Gené’s observation as to
oviposition in the case of a South African Tick, _Amblyomma hebraeum_.

The respiratory organs, if present, are always in the form of tracheae.
These are usually long and convoluted, but not branching. The spiral
structure is difficult to make out in these animals, and in the
Oribatidae at least, instead of the external sheath being fortified with
a spiral filament of chitin, there is a very delicate enveloping
membrane with an apparently unbroken chitinous lining, which can,
however, by suitable treatment, be resolved into a ribbon-like spiral
band.[363] The position of the stigmata is very variable, and is
utilised to indicate the main groups into which the Mites have been
divided.

The Oribatidae possess two curious cephalothoracic organs which were for
a long time considered respiratory. These are in the form of two bodies,
like modified hairs, which protrude from sockets on the dorsal surface
of the cephalothoracic shield. Michael[364] has shown that these have no
connection with the tracheae, and he regards them as sensory organs—
possibly olfactory. They are generally referred to as the
“pseudo-stigmatic” organs.

In the Oribatidae, at all events, well-developed coxal glands are
present. In many Mites, especially the Ixodoidea or Ticks, the salivary
glands are large and conspicuous.

=Metamorphosis.=—All Mites undergo a metamorphosis, varying in
completeness in the different groups. Altogether six stages can be
recognised, though they are seldom or never all exhibited in the
development of a single species. These are ovum, deutovum, larva, nymph,
hypopial stage, and imago.

THE OVUM.—All Mites lay eggs. It is frequently stated that the
Oribatidae are viviparous exceptions, but though some of them are
perhaps ovoviviparous, most deposit eggs like the rest of the Order. A
phenomenon which has probably helped to foster this erroneous view is
the occasional emergence from the dead body of the mother of
fully-formed larvae. Towards winter it is not unusual for the mother to
die at a time when her abdomen contains a few ripe eggs, and these are
able to complete their development internally.

THE DEUTOVUM.[365]—In a few cases (_Atax_, _Damaeus_) a stage has been
observed in which the outer envelope of the egg becomes brown and hard,
and splits longitudinally, so as to allow the thin inner membrane to
become visible through the fissure. More room is thus obtained for the
developing larva, which is, moreover, protected, over most of its
surface, by a hard shell. The deutovum stage may occur either within the
body of the mother, or after the egg has been laid.

THE LARVA.—Omitting, for the moment, the very aberrant Vermiformia (see
p. 464), it is the almost universal rule for the egg to hatch out as a
hexapod larva. The larvae of the genus _Pteroptus_ are said to be
eight-legged. Winkler has stated that the early embryo of _Gamasus_
possesses eight legs, of which the last pair subsequently atrophy, but
this observation requires confirmation.

THE NYMPH.—The nymph-stage commences on the acquisition of eight legs,
and lasts until the final ecdysis which produces the imago. This is the
most important period of Acarine life, and is divided into a prolonged
active period, during which the animal feeds and grows, and an inert
period, sometimes prolonged, but at others very short, and differing
little from the quiescence observable at an ordinary moult, during which
the imago is elaborated. In many species the nymph is strikingly
different from the imago; in others there is a close resemblance between
them. It would appear, from the cases which have been most thoroughly
investigated, that the imago is not developed, part for part, from the
nymph, but that there is an “histolysis” and “histogenesis” similar to
that which occurs among certain insects (see vol. v. p. 165). There may
be more than one nymphal stage.

THE HYPOPIAL STAGE occurs in the Tyroglyphinae, the “Cheese-mite”
sub-family. Here some of the young nymphs assume an entirely different
form, so different that it was for a long time considered to constitute
a separate genus, and was named _Hypopus_. The animal acquires a hard
dorsal covering. The mouth-parts are in the form of a flat blade with
two terminal bristles, but with no discernible orifice. The legs are
single-clawed, and all more or less directed forward, and they are
articulated near the middle line of the ventral surface. Suckers are
always present under the hind part of the abdomen.

It appears that these remarkably modified nymphs are entrusted with the
wider distribution of the species, and that they are analogous to the
winged individuals which occur in the parthenogenetic generations of the
Aphidae. The ordinary _Tyroglyphus_ is soft-bodied, and requires a moist
environment, and exposure to the sun or prolonged passage through the
air would be fatal to it. The hypopial form is much more independent of
external conditions, and its habit is to attach itself by its suckers to
various insects, and by this means to seek a new locality, when it
resumes the ordinary nymph-form and proceeds with its development.

=Classification.=—There is no generally accepted classification of the
Acarina, though several eminent Arachnologists have attempted of late
years to reduce the group to order. Widely different views are held
concerning the affinities of certain groups, and there is no agreement
as to the value to be accorded to the characters which all recognise.
Thus Canestrini[366] allows thirty-four families, while according to
Trouessart[367] there are only ten.

Trouessart’s scheme of classification is in the main followed in the
present chapter.


                      =Sub-Order 1. Vermiformia.=

This Sub-order includes the lowest and most aberrant forms of the Mites.
They are entirely parasitic, and of very small size. The abdomen is much
elongated, and is transversely striated. There are two families,
Eriophyidae[368] (Phytoptidae) and Demodicidae.

=Fam. 1. Eriophyidae (Phytoptidae).=—These are the so-called Gall-mites.
The curious excrescences and abnormal growths which occur on the leaves
and buds of plants are familiar to every one. Various creatures are
responsible for these deformities, many being the work of insects,
especially the Cynipidae among the Hymenoptera, and the Cecidomyiidae
among the Diptera. Others, again, are due to Eriophyid Mites.

Though the galls originated by Mites are often outwardly extremely
similar to those of insect origin, they can be at once distinguished on
close examination. Mite-galls contain a single chamber, communicating
with the exterior by a pore, usually guarded with hairs, and the Mites
live gregariously within it, apparently feeding upon the hairs which
grow abundantly on its inner surface. In Insect-galls each insect larva
lives in a separate closed chamber.

[Illustration:

  FIG. 240.—Vermiform Mites, highly magnified. =A=, _Demodex
    folliculorum_; =B=, _Eriophyes_ (_Phyptoptus_) _ribis_.
]

The Eriophyidae are unique among Mites in possessing only two pairs of
legs, situated quite at the anterior part of the body. The mouth-parts
are very simple.

There are three genera, _Eriophyes_ (_Phyptoptus_) with about one
hundred and fifty known species, _Monochetus_ with a single species, and
_Phyllocoptes_ with about fifty species.

Among the best known examples are _Eriophyes tiliae_, which produces the
“nail-galls” on lime-leaves, and _E. ribis_, the “black-currant
Gall-mite,” which feeds between the folded leaves of the leaf-buds, and
gives rise to swelling and distortion.

=Fam. 2. Demodicidae.=—The single genus _Demodex_ which constitutes this
family consists of a few species of microscopic Mites which inhabit the
hair-follicles of mammals, and are the cause of what is known as
“follicular mange,” some other forms of mange being due to members of
the succeeding family. _Demodex_ possesses eight short, three-jointed
legs, each terminated by two claws. The abdomen is much produced, and is
transversely striated. About ten species have been described, but of
these five are probably varieties of _D. folliculorum_ (Fig. 240, A),
which infests Man.


                       =Sub-Order 2. Astigmata.=

The Astigmata are Mites of more or less globular form, with chelate
chelicerae and five-jointed legs. All members of the group are eyeless.
Their habits are very various, some feeding on vegetable matter and
others on carrion, while a large number are parasitic on animals.
Tracheae are absent. There is only one family.

[Illustration:

  FIG. 241.—=A=, Leg of a fowl infested with “leg-scab”; =B=, female of
    _Sarcoptes mutans_, greatly magnified. (After Neumann.)
]

=Fam. 1. Sarcoptidae.=—No tracheae or stigmata. Apical rostrum.
Oviparous or ovoviviparous. The seventy genera and 530 odd species of
this family are divided into a number of sub-families, of which the
principal are the Sarcoptinae, the Analgesinae, and the Tyroglyphinae.

(i.) The Sarcoptinae are the so-called “Itch-mites.” They are minute
animals, with bodies transversely wrinkled and legs terminating in
suckers or bristles. The genus _Sarcoptes_, which includes about fifteen
species, lives in tunnels which it burrows in the skin of mammals.

(ii.) The Analgesinae are the “Birds’-feather Mites.” The principal
genera are _Pterolichus_ (120 species), _Pteronyssus_ (33 species),
_Analges_ (23 species), _Megninia_ (42 species), and _Alloptes_ (33
species).

(iii.) The Tyroglyphinae[369] have received the popular name of
“Cheese-mites,” from the best known example of the group. They are
smooth-bodied, soft-skinned white Mites, with legs usually terminating
in a single claw, sometimes accompanied by a sucker. They are for the
most part carrion-feeders, living upon decaying animal or vegetable
matter, but a few are parasitic on mammals, insects, and worms.

There are sixteen genera, including about fifty species. _Tyroglyphus
siro_ and _T. longior_ are common Cheese-mites. Other species live in
decaying vegetables and food-stuffs. Some of the genus _Glycyphagus_
(_G. palmifer_, _G. plumiger_) are very remarkable for the palmate or
plumose hairs which decorate their bodies. The remarkable hypopial stage
in the development of _Tyroglyphus_ has been mentioned on p. 463. The
Tyroglyphinae are the lowest of the free-living Acarine forms.


                      =Sub-Order 3. Metastigmata.=

The four families which constitute this sub-order comprise a large
number of Mites in which the tracheae open near the articulation of the
legs, and consequently in a somewhat posterior situation. The families
are Oribatidae, Argasidae, Ixodidae, and Gamasidae.

=Fam. 1. Oribatidae.=—The Oribatidae or “Beetle-mites” are free-living
Acari, with tracheae of which the stigmata are concealed by the
articulation of the legs. The cephalothorax is distinctly marked off
from the abdomen, and bears dorsally two “pseudo-stigmatic” organs. The
rostrum is inserted below the cephalothorax. These Mites gain their
popular name from the beetle-like hardness of their integuments. They
are oviparous or ovoviviparous. Eyes are always absent.

[Illustration:

  FIG. 242.—Oribatid Mites. =A=, _Cepheus ocellatus_, × 24; =B=, ventral
    view of _Hoploderma magnum_, closed, × 20. (After Michael.)
]

These are small creatures, seldom attaining the twentieth of an inch in
length. They are vegetable-feeders (except, perhaps, _Pelops_), and are
to be found in dead wood or vegetable débris, under bark, or among moss
and lichen. In winter they often take refuge under stones. It is
impossible at present to estimate the number of existing species, for
only a few localities have been systematically worked for them, and
their small size has prevented their inclusion, in any numbers, in the
collections of scientific expeditions. Our knowledge of the group is
likely, however, to be largely extended, for it has been found that they
reach England alive and in good condition from the most remote regions
if moss or other material in which they live is collected when not too
dry, and hermetically sealed up in tin cases.

About twenty genera and more than 220 species are at present known.
_Pelops_ has much elongated chelicerae, with very small chelae at the
end. There are ten species, found in moss and on bushes. _Oribata_
numbers about fifty species, found in moss and on trees. _Notaspis_, in
which the last three legs are inserted at the margin of the body, has
about thirty species, found among moss and dead leaves. _Nothrus_ is a
short-legged genus with flat or concave dorsal plate, often produced
into very remarkable spiny processes. There are twenty-two species found
under bark and among moss and lichen. _Hoploderma_ (_Hoplophora_) is
remarkable for its power of shutting down its rostrum and withdrawing
its legs in a manner which leaves it as unassailable as a tortoise or an
armadillo.

Though the Oribatidae are all eyeless, they are distinctly sensitive to
light, not wandering aimlessly till they reach a shadow, but apparently
making straight for a dark spot when subjected to strong illumination.
Some species have a curious habit of collecting dirt and débris on their
backs, so as entirely to obscure the often very remarkable disposition
of the spines and processes with which they are furnished.


[Illustration:

  FIG. 243.—Capitulum of _Boophilus australis_; ventral view. _p^1_,
    _p^2_, _p^3_, _p^4_, The four articles of the palp; _m_, the
    mandible or chelicera; _d_, its digit; _n_, the hypostome.
]

The next two families include the animals commonly known as Ticks, the
largest and most familiar of the Mite tribe. Of recent years they have
attracted much attention as the conveyers, to man and domestic animals,
of certain diseases due to blood-parasites (see p. 457, _n._), and our
knowledge of their structure and habits has greatly increased in
consequence. Hitherto they have generally been considered to constitute
a single family, the Ixodidae, but a section of them so differ from the
rest as to require their removal to another family, the Argasidae, so
that it is necessary to employ a super-family name—IXODOIDEA—to embrace
the whole group.

Ticks are parasitic on mammals, birds, and reptiles, some shewing a
marked partiality for a particular host, others being much more catholic
in their tastes. Both sexes in the Argasidae, but the females only of
the Ixodidae, are capable of great distension, but when unfed they are
all somewhat flat animals with laterally extended legs and rather
crab-like movement.

All Ticks possess a small, movable “false head” or _capitulum_ bearing
mouth-parts which are exceedingly characteristic of the group. The
chelicerae are cutting instruments with their distal ends serrated
outwardly, and there is always present a hypostome beset with recurved
teeth which serve to maintain a firm hold on the tissues into which it
is thrust. On either side of the chelicerae are the four-jointed palps,
leg-like in the Argasidae, but more rigid and rod-like in the Ixodidae,
where their inner margin is often hollowed so as to enclose the
chelicerae and hypostome when the palps are apposed. There is a
conspicuous pair of spiracles near the coxae of the fourth pair of legs.

[Illustration:

  FIG. 244.—_Ornithodoros talaje_, under surface, × 5. (After
    Canestrini.)
]

=Fam. 2. Argasidae.=—The Argasidae are leathery Ticks without a shield
or _scutum_, and with free, leg-like palps. The capitulum is never more
than partially visible when the adult animal is viewed dorsally. Their
hosts are always warm-blooded animals. Two genera are usually
recognised, _Argas_ and _Ornithodoros_, though recent discoveries of new
forms have tended towards their fusion. _Argas reflexus_ and _A.
persicus_ have been proved to convey a Spirochaete disease to fowls, and
the latter, under the name of the “Mianeh Bug” has long possessed an
evil reputation for the “poisonous” effect of its bite on human beings.
In Mexico the “Turicata” (_Ornithodoros turicata_) and the “Garapata”
(_O. megnini_) are greatly dreaded, while human “tick fever” on the
Congo has been traced to the instrumentality of _O. moubata_.

[Illustration:

  FIG. 245.—Female Sheep-tick, _Ixodes ricinus_.
]

=Fam. 3. Ixodidae.=—These are the more familiar Ticks, possessing a
_scutum_ or shield, which covers the whole back of the male, which is
capable, therefore, of little distension, whereas it forms only a small
patch on the front part of the body of the distended female. There are
ten genera, _Ixodes_, _Haemaphysalis_, _Dermacentor_, _Rhipicentor_,
_Rhipicephalus_, _Boophilus_, _Margaropus_, _Hyalomma_, _Amblyomma_, and
_Aponomma_.

_Ixodes ricinus_ is the common English sheep-tick. Species of
_Boophilus_ are parasitic on cattle the world over, and _B. annulatus_
is the transmitter of Texas fever. _Rhipicephalus_ and _Amblyomma_ are
large genera which include several species of economic importance. For
example, _R. sanguineus_ conveys canine piroplasmosis, and _A. hebraeum_
causes “heart-water” in South African cattle. The genus _Aponomma_
confines its attention to reptiles, and some of its species are
exceedingly ornate.

Neglecting _Margaropus_ and _Rhipicentor_, which include only a very few
aberrant forms, the following entirely artificial key will serve to
differentiate the genera of the Ixodidae:—

 1. A pair of eyes on the lateral borders of the scutum                2

    No eyes                                                            6

 2. Capitulum long, much longer than broad                             3

    Capitulum short                                                    4

 3. Unicolorous, ♂ with chitinous plates near anus            _Hyalomma_

    Generally ornate, ♂ without anal plates                  _Amblyomma_

 4. Generally ornate, ♂ without anal plates, but with
      enlarged 4th coxae                                   _Dermacentor_

    Unicolorous, ♂ with anal plates and normal coxae                   5

 5. Palpi very short, spiracle circular                      _Boophilus_

    Palpi medium, spiracle comma-shaped                  _Rhipicephalus_

 6. Capitulum short; 2nd article of palp projecting
      laterally                                          _Haemaphysalis_

    Capitulum long                                                     7

 7. Unicolorous, elongate, on birds or mammals                  _Ixodes_

    Generally ornate, broad-oval, on reptiles                 _Aponomma_

Neumann has recently revised the Ixodoidea in a series of papers
published in the _Mémoires de la Société zoologique de France_,[370] but
the work is not obtainable as a whole. A monograph, by Nuttall,
Warburton, Cooper, and Robinson, is now in course of publication at the
Cambridge University Press.[371]

=Fam. 4. Gamasidae.=—The Gamasidae are carnivorous Mites, either
free-living or parasitic on animals. The chelicerae are chelate, and the
palps are free. The tarsi have two claws, accompanied by a “caruncle” or
sucking disc. They are mostly pale-coloured Mites, with a smooth, more
or less scutate covering. The three principal sub-families are
Gamasinae, Uropodinae, and Dermanyssinae.

Of the GAMASINAE, _Gamasus coleoptratorum_ is the well-known
Beetle-parasite so frequently seen on _Geotrupes_. It is often
confounded with another species of similar habits, _G. crassipes_.

The curious Beetle-parasites attached to their victim by a thread belong
to the genus _Uropoda_ of the UROPODINAE. The connecting filament, which
the Mite can sever at will, for a long time puzzled observers. It was
variously construed as a silken cord of attachment, and as a sort of
umbilical cord, through which the Mite drew nourishment from the Beetle.
On more careful investigation it proved to be connected with the anus of
the Mite, and to consist of its consolidated excrement.

The DERMANYSSINAE are all parasitic on warm-blooded animals, principally
birds and bats. _Dermanyssus avium_ is the common parasite infesting
fowls and cage-birds.


                     =Sub-Order 4. Heterostigmata.=

=Fam. Tarsonemidae.=—This is the sole family of the sub-order. It
comprises a number of minute vegetable-feeding Mites which have been
little studied, though they are probably the cause of considerable
injury to the leaves and buds of plants.


                      =Sub-Order 5. Prostigmata.=

In these Mites the stigmata are situated anteriorly, in the rostrum or
the thorax. In the Water-mites the tracheae have atrophied, but these
creatures are clearly Trombidiidae which have taken to an aquatic life.

[Illustration:

  FIG. 246.—_Bdella lignicola_, x about 50. (After Canestrini.)
]

=Fam. 1. Bdellidae.=—The Bdellidae are sometimes known as the “Snouted
Mites” on account of the very prominent forwardly-directed “capitulum”
or false head. They have chelate chelicerae and tactile palps, which are
often “elbowed,” like the antennae of weevils. Eyes may be present or
absent. They are usually of a bright red colour, and are free-living and
predaceous, though in their larval stages they may often be found
attached to the limbs of insects and spiders.

The minute active scarlet Mites of the genus _Eupodes_ and its allies
perhaps come within this family. Their legs are six-jointed.

The remaining families of the Prostigmata (Halacaridae, Hydrachnidae,
and Trombidiidae) all have raptorial palps, and clawed or piercing
chelicerae.

=Fam. 2. Halacaridae.=—This is a small group of marine Mites. In their
usually prominent capitulum they resemble the Bdellidae. In some
respects they recall the Oribatidae, having hard integuments, and their
legs being articulated near the margin of the body. They do not swim,
but crawl upon weeds and zoophytes, or burrow in the mud.

[Illustration:

  FIG. 247.—_Atax alticola_, x 16. (After Canestrini.)
]

=Fam. 3. Hydrachnidae.=—The Hydrachnidae are the Fresh-water Mites.
Their legs are provided with long close-set hairs, and thus adapted for
swimming. They are predaceous, and in their young stages are often
parasitic on water insects. A familiar example is _Atax bonzi_, which
lives within the shell of the fresh-water mussel.

=Fam. 4. Trombidiidae.=—The predaceous palps of the Trombidiidae are
generally of the “finger-and-thumb” type. The tarsi are two-clawed,
without caruncle. This group may be divided into six sub-families.

[Illustration:

  FIG. 248.—_Tetranychus gibbos_us, x 50. (After Canestrini.)
]

(i.) The LIMNOCHARINAE or “Mud-mites” connect the Hydrachnidae with the
typical Trombidiidae. They are usually velvety and of a red colour. They
do not swim, but creep. The larva of _Limnocharis aquaticus_ is
parasitic on _Gerris lacustris_.

(ii.) The CAECULINAE bear a strong general resemblance to the Harvestmen
or Phalangidae. _Caeculus_ is so similar to the Phalangid genus
_Trogulus_ that it was considered by Dufour to belong to the same order.

(iii.) The TETRANYCHINAE or “Spinning mites” are phytophagous, and do
much injury to plants, sucking the sap from the leaves and giving them a
blistered appearance. _Tetranychus telarius_ is the “Red-spider” of
popular nomenclature.

(iv.) The CHEYLETINAE are remarkable Mites with fleshy, semi-transparent
body, and enormously developed raptorial pedipalpi, which are extremely
formidable weapons of attack. They do not creep or run like most Mites
but proceed by a series of short leaps. _Cheyletus_ is the principal
genus.

The curious genus _Syringophilus_, which is parasitic in the interior of
birds’ feathers, appears to be a degenerate Cheyletine.

(v.) The ERYTHRAEINAE are minute, active Mites, usually red in colour,
free-living and predaceous.

(vi.) The TROMBIDIINAE include most of the moderate-sized, velvety red
Mites which are commonly known as “Harvest-mites,” and their larvae, the
so-called Harvest-bugs, frequently attack man. _Trombidium holosericeum_
is a well-known example.


                   =Sub-Order 6. Notostigmata.=[372]

This sub-order has been established for the reception of the curious
genus _Opilioacarus_.

=Fam. Opilioacaridae.=—Mites with segmented abdomen, leg-like palps,
chelate chelicerae, and two pairs of eyes. There are four dorsal
abdominal stigmata. Four species of the sole genus _Opilioacarus_ have
been recorded, _O. segmentatus_ from Algeria, _O. italicus_ from Italy,
_O. arabicus_ from Arabia, and _O. platensis_[373] from South America.




                        APPENDICES TO ARACHNIDA
                               I. AND II.


                      TARDIGRADA AND PENTASTOMIDA


                                   BY

                    ARTHUR E. SHIPLEY, M.A., F.R.S.

 Fellow and Tutor of Christ’s College, Cambridge, and Reader in Zoology
                           in the University




                              CHAPTER XIX
                               TARDIGRADA


OCCURRENCE—ECDYSIS—STRUCTURE—DEVELOPMENT—AFFINITIES—BIOLOGY—DESICCATION—
                          PARASITES—SYSTEMATIC

The animals dealt with in this chapter lead obscure lives, remote from
the world, and few but the specialist have any first-hand acquaintance
with them. Structurally they are thought to show affinities with the
Arachnida, but their connexion with this Phylum is at best a remote one.

Tardigrades are amongst the most minute multicellular animals which
exist, and their small size—averaging from ⅓ to 1 mm. in length—and
retiring habits render them very inconspicuous, so that as a rule they
are overlooked; yet Max Schultze[374] asserts that without any doubt
they are the most widely distributed of all segmented animals. They are
found amongst moss, etc., growing in gutters, on roofs, trees or in
ditches, and in such numbers that Schultze states that almost any piece
of moss the size of a pea will, if closely examined, yield some members
of this group, but they are very difficult to see. The genus
_Macrobiotus_ especially affects the roots of moss growing on stones and
old walls. _M. macronyx_ lives entirely in fresh water, and _Lydella
dujardini_ and _Echiniscoides sigismundi_ are marine; all other species
are practically terrestrial, though inhabiting very damp places.

In searching amongst the heather of the Scotch moors for the ova and
embryos of the Nematodes which infest the alimentary canal of the
grouse, I have recently adopted a method not, as far as I am aware, in
use before, and one which in every case has yielded a good supply of
Tardigrades otherwise so difficult to find. The method is to soak the
heather in water for some hours and then thoroughly shake it, or to
shake it gently in a rocking machine for some hours. The sediment is
allowed to settle, and is then removed with a pipette and placed in a
centrifugaliser. A few turns of the handle are sufficient to concentrate
at the bottom of the test-tubes a perfectly amazing amount of cryptozoic
animal life, and amongst other forms I have never failed to find
Tardigrades.

[Illustration:

  FIG. 249.—Dorsal view of _Echiniscus testudo_, C. Sch., × 200, showing
    the four segments 1, 2, 3, 4. (From Doyère.)
]

[Illustration:

  FIG. 250.—Cast-off cuticle of _Macrobiotus tetradactylus_, Gr., ×
    about 150, containing four eggs in which the boring apparatus of the
    embryo can be distinguished. (From R. Greeff.)
]

Many Tardigrades are very transparent; their cells are large, and
arranged in a beautifully symmetrical manner; and since those of them
that live in moss, and at times undergo desiccation, are readily thrown
into a perfectly motionless state, during which they may be examined at
leisure, it is not surprising that these little creatures have been a
favourite object for histological research. One way to produce the
above-mentioned stillness is partly to asphyxiate the animals by placing
them in water which has been boiled, and covering the surface of the
water with a film of oil.

[Illustration:

  FIG. 251.—_Echiniscus spinulosus_, C. Sch., × about 200, seen from the
    side. (From Doyère.)
]

The whole body is enclosed in a thin transparent cuticle, which must be
pierced by a needle if it be desired to stain the tissues of the
interior. As a rule the cuticle is of the same thickness all over the
body, but in the genus _Echiniscus_ the cuticle of the dorsal surface is
arranged in thickened plates, and these plates are finely granulated.
From time to time the cuticle is cast, and this is a lengthy process, so
that it is not unusual to find a Tardigrade ensheathed in two cuticles,
the outer of which is being rubbed off. The Macrobioti lay their eggs in
their cast cuticle (Fig. 250). The end of each of the eight legs bears
forked claws of cuticular origin. The legs are not jointed except in the
genus _Lydella_, where two divisions are apparent.

Within the cuticle is the epidermis, a single layer of cells arranged in
regular longitudinal and transverse rows along the upper and under
surface, where the cells are as uniformly arranged and as rectangular as
bricks. The cells on the sides of the body are polygonal, and not in
such definite rows. The nuclei show the same diagrammatic symmetry as
the cells which contain them, and lie in the same relative position in
neighbouring cells. In a few places, such as the end of each limb and
around the mouth and arms, the cells of the epidermis are heaped up and
form a clump or ridge. In some genera a deposit of pigment in the
epidermis, which increases as the animal grows old, obscures the
internal structures. It is generally brown, black, or red in colour.

[Illustration:

  FIG. 252.—_Macrobiotus schultzei_, Gr., × 150. (Modified from Greeff.)
    _a_, The six inner papillae of the mouth; _b_, the chitin-lined
    oesophagus; _c_, calcareous spicule; _d_, muscle which moves the
    spicule; _e_, muscular pharynx with masticating plates; _f_,
    salivary glands; _g_, stomach; _h_, ovary; _i_, median dorsal
    accessory gland; _k_, diverticula of rectum.
]

The cuticle and epidermis enclose a space in which the various internal
organs lie. This space is traversed by numerous symmetrically disposed
muscle-fibres, and contains a clear fluid—the blood—which everywhere
bathes these organs. This fluid evaporates when desiccation takes place,
and is soon replaced after rain; it forms no coagulum when reagents are
added to it, and it probably differs but little from water. Floating in
it are numerous corpuscles, whose number increases with age. In well-fed
Tardigrades the corpuscles are packed with food-reserves, often of the
same colour—green or brown—as the contents of the stomach, which soon
disappear when the little creatures are starved.

The alimentary canal begins with an oral cavity, which is in many
species surrounded by chitinous rings. The number of these rings and
their general arrangement are of systematic importance. The oral cavity
opens behind into a fine tube lined with chitin, very characteristic of
the Tardigrada, which has been termed the mouth-tube. By its side,
converging anteriorly, lie the two chitinous teeth, which may open
ventrally into the mouth-tube, as in _Macrobiotus hufelandi_ and
_Doyeria simplex_, or may open directly into the oral cavity, as in
_Echiniscus_, _Milnesium_, and some species of _Macrobiotus_. In some of
the last named the tips of the teeth are hardened by a calcareous
deposit. The hinder end of each stylet or tooth is supported by a second
chitinous tooth-bearer,[375] and the movement of each is controlled by
three muscles, one of which, running forwards to the mouth, helps to
protrude the tooth, whilst the other two running upwards and downwards
to the sheath of the pharynx, direct in what plane the tooth shall be
moved.

The mouth-tube passes suddenly into the muscular sucking pharynx, which
is pierced by a continuation of its chitinous tube. Roughly speaking,
the pharynx is spherical; the great thickness of its walls is due to
radially arranged muscles which run from the chitinous tube to a
surrounding membrane. When the muscles contract, the lumen of the tube
is enlarged, and food, for the most part liquid, is sucked in. Two large
glands, composed of cells with conspicuous nuclei, but with ill-defined
cell outlines, pour their contents into the mouth in close proximity to
the exit of the teeth. The secretion of the glands—often termed salivary
glands—is said in many cases to be poisonous.

The pharynx may be followed by a distinct oesophagus, or it may pass
almost immediately into the stomach, which consists of a layer of
six-sided cells arranged in very definite rows. In fully-fed specimens
these cells project into the lumen with a well-rounded contour.
Posteriorly the stomach contracts and passes into the narrow rectum,
which receives anteriorly the products of the excretory canals and the
reproductive organs, and thus forms a cloaca. Its transversely placed
orifice lies between the last pair of legs. The food of Tardigrades is
mainly the sap of mosses and other humble plants, the cell-walls of
which are pierced by the teeth of the little creatures.

The organs to which an excretory function has been attributed are a pair
of lateral caeca, which vary much in size according as the possessor is
well or ill nourished. They recall the Malpighian tubules of such Mites
as _Tyroglyphus_. Nothing comparable in structure to nephridia or to
coxal glands has been found.

The muscles show a beautiful symmetry. There are ventral, dorsal, and
lateral bundles, and others that move the limbs and teeth, but the
reader must be referred to the works of Basse, Doyère,[376] and
Plate[377] for the details of their arrangement. The muscle-fibres are
smooth.

[Illustration:

  FIG. 253.—Brain of _Macrobiotus hufelandi_, C. Sch., × about 350.
    (From Plate.) Seen from the side. _ap_, Lobe of brain bearing the
    eye; _ce_, supra-oesophageal ganglion; _d_, tooth; _Ga_, first
    ventral ganglion; _ga’_, sub-oesophageal ganglion; _k_, thickening
    of the epidermis round the mouth; _oc_, eye-spot; _oe_, oesophagus;
    _op_, nerve running from the ocular lobe of the brain to the first
    ventral ganglion; _ph_, pharynx.
]

The nervous system consists of a brain or supra-oesophageal ganglion,
whose structure was first elucidated by Plate, and a ventral chain of
four ganglia. Anteriorly the brain is rounded, and gives off a nerve to
the skin; posteriorly each half divides into two lobes, an inner and an
outer. The latter bears the eye-spot when this is present. Just below
this eye a slender nerve passes straight to the first ventral ganglion.
The brain is continued round the oral cavity as a thick nerve-ring, the
ventral part of which forms the sub-oesophageal ganglion, united by two
longitudinal commissures to the first ventral ganglion. Thus the brain
has two channels of communication between it and the ventral nerve-cord
on each side, one by means of the slender nerve above mentioned, and one
through the sub-oesophageal ganglion. The ventral chain is composed of
four ganglia connected together by widely divaricated commissures. Each
ganglion gives off three pairs of nerves, two to the ventral
musculature, and one to the dorsal. The terminations of these nerves in
the muscles are very clearly seen in these transparent little creatures,
though there is still much dispute as to their exact nature.

The older writers considered the Tardigrada as hermaphrodites, but Plate
and others have conclusively shown that they are bisexual, at any rate
in the genus _Macrobiotus_. The males are, however, much rarer than the
females. The reproductive organs of both sexes are alike. Both ovary and
testis are unpaired structures opening into the intestine, and each is
provided with a dorsal accessory gland placed near its orifice. In the
ovary many of the eggs are not destined to be fertilised, but serve as
nourishment for the more successful ova which survive.

No special circulatory or respiratory organs exist, and, as in many
other simple organisms, there is no connective tissue.

[Illustration:

  FIG. 254.—Male reproductive organs of _Macrobiotus hufelandi_, C.
    Sch., × about 350. (From Plate.) _a.ep_, Epidermal thickening round
    anus; _cl_, cloaca; _gl.d_, accessory gland; _gl.l_, Malpighian
    gland; _st_, stomach; _te_, testis; _x_, mother-cells of
    spermatozoa.
]

The segmentation of the egg in _M. macronyx_ is total and equal,
according to the observations of von Erlanger.[378] A blastula, followed
by a gastrula, is formed. The blastopore closes, but later the anus
appears at the same spot. There are four pairs of mesodermic diverticula
which give rise to the coelom and the chief muscles. The reproductive
organs arise as an unpaired diverticulum of the alimentary canal, which
also gives origin to the Malpighian tubules. The development is thus
very primitive and simple, and affords no evidence of degeneration.

With regard to their position in the animal kingdom, writers on the
Tardigrada are by no means agreed. O. F. Müller placed them with the
Mites; Schultze and Ehrenberg near the Crustacea; Dujardin and Doyère
with the Rotifers near the Annelids; and von Graff with the Myzostomidae
and the Pentastomida. Plate regards them as the lowest of all
air-breathing Arthropods, but he carefully guards himself against the
view that they retain the structure of the original Tracheates from
which later forms have been derived. He looks upon Tardigrades as a side
twig of the great Tracheate branch, but a twig which arises nearer the
base of the branch than any other existing forms. These animals seem
certainly to belong to the Arthropod phylum, inasmuch as they are
segmented, have feet ending in claws, Malpighian tubules, and an entire
absence of cilia. The second and third of these features indicate a
relationship with the Tracheate groups; on the other hand there is an
absence of paired sensory appendages, and of mouth-parts. Von Erlanger
has pointed out that the Malpighian tubules, arising as they do from the
mid-gut, are not homologous with the Malpighian tubules of most
Tracheates, and he is inclined to place this group at the base or near
the base of the whole Arthropod phylum. They, however, show little
resemblance to any of the more primitive Crustacea. The matter must
remain to a large extent a matter of opinion, but there can be no doubt
that the Tardigrades show more marked affinities to the Arthropods than
to any other group of the animal kingdom.

=Biology.=—Spallanzani, who published in the year 1776 his _Opuscules de
physique animale et végétale_, was the first satisfactorily to describe
the phenomena of the desiccation of Tardigrades, though the subject of
the desiccation of Rotifers, Nematodes, and Infusoria had attracted much
notice, since Leeuwenhoek had first drawn attention to it at the very
beginning of the century. In its natural state and in a damp atmosphere
Tardigrades live and move and have their being like other animals, but
if the surroundings dry up, or if one be isolated on a microscopic slide
and slowly allowed to dry, its movements cease, its body shrinks, its
skin becomes wrinkled, and at length it takes on the appearance of a
much weathered grain of sand in which no parts are distinguishable. In
this state, in which it may remain for years, its only vital action must
be respiration, and this must be reduced to a minimum. When water is
added it slowly revives, the body swells, fills out, the legs project,
and gradually it assumes its former plump appearance. For a time it
remains still, and is then in a very favourable condition for
observation, but soon it begins to move and resumes its ordinary life
which has been so curiously interrupted.

All Tardigrades have not this peculiar power of revivification—
anabiosis, Preyer calls it—it is confined to those species which live
amongst moss, and the process of desiccation must be slow and, according
to Lance,[379] the animal must be protected as much as possible from
direct contact with the air.

According to Plate, the Tardigrada are free from parasitic Metazoa,
which indeed could hardly find room in their minute bodies. They are,
however, freely attacked by Bacteria and other lowly vegetable
organisms, and these seem to flourish in the blood without apparently
producing any deleterious effects on the host. Plate also records the
occurrence of certain enigmatical spherical bodies which were found in
the blood or more usually in the cells of the stomach. These bodies
generally appeared when the Tardigrades were kept in the same unchanged
water for some weeks. Nothing certain is known as to their nature or
origin.

=Systematic.=—A good deal of work has recently been done by Mr. James
Murray on the Polar Tardigrades and on the Tardigrades of Scotland, many
of which have been collected by the staff of the Lake Survey.[380] Over
forty species have been described from North Britain.

The following table of Classification is based on that drawn up by
Plate:—


                           =Table of Genera.=

  I. The claws of the legs are simple, without a second hook. If there
    are several on the same foot they are alike in structure and size.

      A. The legs are short and broad, each with at least two claws.

                2–4 claws      Gen. 1. _ECHINISCUS_, C. SCH. (Fig. 249).

                7–9 claws      Sub-gen. 1_a_. _ECHINISCOIDES_, PLATE.

      B. The legs are long and slender; each bears only one small claw.

                                                 Gen. 2. _LYDELLA_, DOY.

    II. The claws of the legs are all or partly two- or three-hooked.
    Frequently they are of different lengths.

      A. There are no processes or palps around the mouth.

          I. The muscular sucking pharynx follows closely on the
            mouth-tube.

              α. The oral armature consists on each side of a stout
                tooth and a transversely placed support.

                              Gen. 3. _MACROBIOTUS_, C. SCH. (Fig. 252).

              β. The oral armature consists on each side of a
                stylet-like tooth without support.

                                               Gen. 4. _DOYERIA_, PLATE.

            II. The mouth-tube is separated from the muscular sucking
            pharynx by a short oesophagus.

                                  Gen. 5. _DIPHASCON_, PLATE (Fig. 255).

        B. Six short processes or palps surround the mouth, and two
        others are placed a little farther back.

                                               Gen. 6. _MILNESIUM_, DOY.

1. Genus _ECHINISCUS_ (= _EURYDIUM_, DOY.).—The dorsal cuticle is thick,
and divided into a varying number of shields, which bear thread- or
spike-like projections. The anterior end forms a proboscis-like
extension of the body. Two red eye-spots. There are many species, and
the number has increased so rapidly in the last few years that
specialists are talking of splitting up the genus. _E. arctomys_, Ehrb.;
_E. mutabilis_, Murray; _E. islandicus_, Richters; _E. gladiator_,
Murray; _E. wendti_, Richters; _E. reticulatus_, Murray; _E. oihonnae_,
Richters; _E. granulatus_, Doy.; _E. spitzbergensis_, Scourfield;[381]
_E. quadrispinosus_, Richters; and _E. muscicola_, Plate, are all
British. More than one-half of these species are also Arctic, and _E.
arctomys_ is in addition Antarctic. In fact, the group is a very
cosmopolitan one. The genus is also widely distributed vertically,
specimens being found in cities on the sea level and on mountains up to
a height of over 11,000 feet.

[Illustration:

  FIG. 255.—_Diphascon chilenense_, Plate, × about 100. (From Plate.)
    _ce_, Brain; _k_, thickening of the epidermis above the mouth; _o_,
    egg; _oe_, oesophagus; _p_,?salivary glands; _ph_, pharynx; _sa_,
    blood corpuscles; _st_, stomach.
]

1a. Sub-genus _ECHINISCOIDES_ differs from the preceding in the number
of the claws, the want of definition in the dorsal plates, and in being
marine. The single species _E. sigismundi_, M. Sch., is found amongst
algae in the North Sea (Ostend and Heligoland).

2. Genus _LYDELLA_.[382]—The long, thin legs of this genus have two
segments, and in other respects approach the Arthropod limb. Marine.
Plate suggests the name _L. dujardini_ for the single species known.

3. Genus _MACROBIOTUS_ has a pigmented epidermis, but eye-spots may be
present or absent. The eggs are laid one at a time, or many leave the
body at once. They are either quite free or enclosed in a cast-off
cuticle. The genus is divided into many species and shows signs of
disruption. They mostly live amongst moss; but _M. macronyx_, Doy., is
said to live in fresh water. The following species are recorded from
North Britain: _M. oberhäuseri_, Doy.; _M. hufelandi_, Schultze; _M.
zetlandicus_, Murray; _M. intermedius_, Plate; _M. angusti_, Murray; _M.
annulatus_, Murray; _M. tuberculatus_, Plate; _M. sattleri_, Richters;
_M. papillifer_, Murray; _M. coronifer_, Richters; _M. crenulatus_,
Richters; _M. harmsworthi_, Murray; _M. orcadensis_, Murray; _M.
islandicus_, Richters; _M. dispar_, Murray; _M. ambiguus_, Murray; _M.
pullari_, Murray; _M. hastatus_, Murray; _M. dubius_, Murray; _M.
echinogenitus_, Richters; _M. ornatus_, Richters; _M. macronyx_?, Doy.

4. Genus _DOYERIA_.—The teeth of this genus have no support, and the
large salivary glands of the foregoing genus are absent; in other
respects _Doyeria_, with the single species _Doyeria simplex_, Plate,
resembles _Macrobiotus_, and is usually to be found in consort with _M.
hufelandi_, C. Sch.

5. Genus _DIPHASCON_ resembles _M. oberhäuseri_, Doy., but an oesophagus
separates the mouth-tube from the sucking pharynx, and the oral armature
is weak. The following species are British, the first named being very
cosmopolitan, being found at both Poles, in Chili, Europe, and Asia: _D.
chilenense_, Plate; _D. scoticum_, Murray; _D. bullatum_, Murray; _D.
angustatum_, Murray; _D. oculatum_, Murray; _D. alpinum_, Murray; _D.
spitzbergense_, Murray.

6. Genus _MILNESIUM_ has a soft oral armature, and the teeth open
straight into the mouth. A lens can usually be distinguished in the
eyes. Two species have been described, _M. tardigradum_, Doy., British,
and _M. alpigenum_, Ehrb. Bruce and Richters consider that these two
species are identical.




                               CHAPTER XX
                           PENTASTOMIDA[383]


 OCCURRENCE—ECONOMIC IMPORTANCE—STRUCTURE—DEVELOPMENT AND LIFE-HISTORY—
                               SYSTEMATIC

Pentastomids are unpleasant-looking, fluke-like or worm-like animals,
which pass their adult lives in the nasal cavities, frontal sinuses, and
lungs of flesh-eating animals, such as the Carnivora, Crocodiles, and
Snakes; more rarely in Lizards, Birds, or Fishes. From these retreats
their eggs or larvae are sneezed out or coughed up, or in some other way
expelled from the body of their primary host, and then if they are
eaten, as they may well be if they fall on grass, by some
vegetable-feeding or omnivorous animal, they undergo a further
development. If uneaten the eggs die. When once in the stomach of the
second host, the egg-shell is dissolved and a larva emerges (Fig. 260,
p. 494), which bores through the stomach-wall and comes to rest in a
cyst in some of the neighbouring viscera. Here, with occasional
wanderings which may prove fatal to the host, it matures, and should the
second host be eaten by one of the first, the encysted form escapes,
makes its way to the nasal chambers or lungs, and attaching itself by
means of its two pairs of hooks, comes to rest on some surface capable
of affording nutriment. Having once taken up its position the female
seldom moves, but the males, which are smaller than the females, are
more active. They move about in search of a mate. Further, should the
host die, both sexes, after the manner of parasites, attempt to leave
the body. Like most animals who live entirely in the dark they develop
no pigment, and have a whitish, blanched appearance.

The only species of Pentastomid which has any economic importance is
_Linguatula taenioides_ of Lamarck, which is found in the nose of the
dog, and much more rarely in the same position in the horse, mule, goat,
sheep, and man. It is a comparatively rare parasite, but occurred in
about 10 per cent of the 630 dogs in which it was sought at the
laboratory of Alfort, near Paris, and in 5 out of 60 dogs examined at
Toulouse. The symptoms caused by the presence of these parasites are not
usually very severe, though cases have been recorded where they have
caused asphyxia. The larval stages occur in the rabbit, sheep, ox, deer,
guinea-pig, hare, rat, horse, camel, and man, and by their wandering
through the tissues may set up peritonitis and other troubles.

As in the Cestoda, which they so closely resemble in their life-history,
the nomenclature of the Pentastomids has been complicated by their
double life. For long the larval form of _L. taenioides_ was known by
different names in different hosts, _e.g._ _Pentastoma denticulatum_,
Rud., when found in the goat, _P. serratum_, Fröhlich, when found in the
hare, _P. emarginatum_ when found in the guinea-pig, and so on. In the
systematic section of this article some of the species mentioned are
known in the adult state, some in the larval, and in only a few has the
life-history been fully worked out.

=Structure.=[384]—The body of a Pentastomid is usually white, though in
the living condition it may be tinged red by the colour of the blood
upon which it lives. The anterior end, which bears the mouth and the
hooks (Fig. 256), has no rings; this has been termed the cephalothorax.
The rest of the body, sometimes called the abdomen, is ringed, and each
annulus is divided into an anterior half dotted with the pores of
certain epidermal glands and a hinder part of the ring in which these
are absent.

On the ventral surface of the cephalothorax, in the middle line, lies
the mouth, elevated on an oral papilla, and on each side of the mouth
are a pair of hooks whose bases are sunk in pits. The hooks can be
protruded from the pits, and serve as organs of attachment. Their shape
has some systematic value.

[Illustration:

  FIG. 256.—_Porocephalus annulatus_, Baird. =A=, Ventral view of head,
    × 6; =B=, ventral view of animal, × 2.
]

There are a pair of peculiar papillae which bear the openings of the
“hook-glands,” lying just in front of the pairs of hooks, and other
smaller papillae are arranged in pairs on the cephalothorax and anterior
annuli. The entire body is covered by a cuticle which is tucked in at
the several orifices. This is secreted by a continuous layer of ectoderm
cells. Some of these subcuticular cells are aggregated together to form
very definite glands opening through the cuticle by pores which have
somewhat unfortunately received the name of stigmata. Spencer attributes
to these glands a general excretory function. There is, however, a very
special pair of glands, the hook-glands, which extend almost from one
end to the other of the body; anteriorly these two lateral glands unite
and form the head-gland (Fig. 257). From this on each side three ducts
pass, one of which opens to the surface on the primary papilla; the
other two ducts open at the base of the two hooks which lie on each side
of the mouth. Leuckart has suggested that these important glands secrete
some fluid like the irritating saliva of a Mosquito which induces an
increased flow of blood to the place where it is of use to the parasite.
Spencer, however, regards the secretion as having, like the secretion of
the so-called salivary cells of the Leech, a retarding action on the
coagulation of the blood of the host.

The muscles of Pentastomids are striated. There is a circular layer
within the subcuticular cells, and within this a longitudinal layer and
an oblique layer which runs across the body-cavity from the
dorso-lateral surface to the mid-ventral line, a primitive arrangement
which recalls the similar division of the body-cavity into three
chambers in _Peripatus_ and in many Chaetopods. Besides these there are
certain muscles which move the hooks and other structures.

The mouth opens into a pharynx which runs upwards and then backwards to
open into the oesophagus (Fig. 257). Certain muscles attached to these
parts enlarge their cavities, and thus give rise to a sucking action by
whose force the blood of the host is taken into the alimentary canal.
The oesophagus opens by a funnel-shaped valve into the capacious stomach
or mid-gut, which stretches through the body to end in a short rectum or
hind-gut. The anus is terminal.

[Illustration:

  FIG. 257.—Diagrammatic representation of the alimentary, secretory,
    nervous, and reproductive systems of a male _Porocephalus
    teretiusculus_, seen from the side. The nerves are represented by
    solid black lines. (From W. Baldwin Spencer.)

  1, Head-gland; 2, testis; 3, hook-gland; 4, hind-gut; 5, mid-gut; 6,
    ejaculatory
  duct; 7, vesicula seminalis; 8, vas deferens; 9, dilator-rod sac; 10,
    cirrus-bulb;
  11, cirrus-sac; 12, fore-gut; 13, oral papillae.
]

There appears to be no trace of circulatory or respiratory organs,
whilst the function usually exercised by the nephridia or Malpighian
tubules or by coxal glands, of removing waste nitrogenous matter, seems,
according to Spencer, to be transferred to the skin-glands.

The nervous system is aggregated into a large ventral ganglion which
lies behind the oesophagus. It gives off a narrow band devoid of
ganglion-cells, which encircles that tube. It also gives off eight
nerves supplying various parts, and is continued backward as a ninth
pair of prolongations which, running along the ventral surface, reach
almost to the end of the body (Fig. 257). The only sense-organs known
are certain paired papillae on the head, which is the portion that most
closely comes in contact with the tissues of the host.

Pentastomids are bisexual. The males are as a rule much less numerous
and considerably smaller than the females, although the number of annuli
may be greater.

The ovary consists of a single tube closed behind. This is supported by
a median mesentery. Anteriorly the ovary passes into a right and left
oviduct, which, traversing the large hook-gland, encircle the alimentary
canal and the two posterior nerves (Fig. 258). They then unite, and at
their point of union they receive the ducts of the two spermathecae,
usually found packed with spermatozoa. Having received the orifices of
the spermatheca, the united oviducts are continued backward as the
uterus, a highly-coiled tube in which the fertilised eggs are stored.
These are very numerous; Leuckart estimated that a single female may
contain half a million eggs. The uterus opens to the exterior in the
mid-ventral line a short distance—in _P. teretiusculus_ on the last ring
but seven—in front of the terminal anus. In _L. taenioides_ the eggs
begin to be laid in the mucus of the nose some six months after the
parasite has taken up its position.

[Illustration:

  FIG. 258.—Diagrammatic representation of the alimentary, secretory,
    nervous, and reproductive systems of a female _Porocephalus
    teretiusculus_, seen from the side. The nerves are represented by
    solid black lines. (From W. Baldwin Spencer.)

  1, Head-gland; 2, oviduct; 3, hook-gland; 4, mid-gut; 5, ovary; 6,
    hind-gut; 7,
  vagina; 8, uterus; 9, accessory gland; 10, spermatheca.
]

The testis is a single tube occupying in the male a position similar to
that of the ovary in the female. Anteriorly it opens into two vesiculae
seminales, which, like the oviducts, pierce the hook-glands and encircle
the alimentary canal (Fig. 257). Each vesicula passes into a vas
deferens with a cuticular lining. Each vas deferens also receives the
orifice of a muscular caecal ejaculatory duct, which, crowded with
mature spermatozoa, stretches back through the body. Anteriorly the vas
deferens passes into a cirrus-bulb, which is joined by a cirrus-sac on
one side and a dilator-rod sac on the other, structures containing
organs that assist in introducing the spermatozoa into the female. The
two tubes then unite, and having received a dorsally-placed accessory
gland, open to the exterior by a median aperture placed ventrally a
little way behind the mouth.

=Life-history.=—The egg undergoes a large portion of its development
within the body of the mother. In _Linguatula taenioides_, which lives
in the nasal cavities of the dog, the eggs pass away with the nasal
excretions. If these, scattered about in the grass, etc., be eaten by a
rabbit, the egg-shell is dissolved in the stomach of the second host and
a small larva is set free. In _Porocephalus proboscideus_ and others,
which inhabit the lungs of snakes, the eggs pass along the alimentary
canal and leave the body with the faeces. They also must be eaten by a
second host if development is to proceed.

[Illustration:

  FIG. 259.—A late larval stage of _Porocephalus proboscideus_, seen
    from the side. Highly magnified. (From Stiles.) 1, primordium of
    first pair of chitinous processes; 2, primordium of second pair of
    chitinous processes; 3, mouth; 4, ventral ganglion; 5, receptaculum
    seminis; 6, oviduct; 7, ovary; 8, anus; 9, vagina.
]

The larva which emerges when the egg-shell is dissolved has a rounded
body provided with two pairs of hooked appendages, and a tail which is
more or less prominent in different species (Figs. 259, 260). Each
appendage bears a claw, and is strengthened by a supporting rod or
skeleton. Anteriorly the head bears a boring apparatus of several
chitinous stylets. The various internal organs are in this stage already
formed, though in a somewhat rudimentary state, and it is doubtful if
the anus has yet appeared.

[Illustration:

  FIG. 260.—Larva of _Porocephalus proboscideus_, seen from below.
    Highly magnified. (From Stiles.) 1, Boring, anterior end; 2, first
    pair of chitinous processes seen between the forks of the second
    pair; 3, ventral nerve-ganglion; 4, alimentary canal; 5, mouth; 6
    and 7, gland-cells.
]

By means of its boring apparatus, and aided by its hooked limbs, the
larva now works its way through the stomach-walls of its second host,
and comes to rest in the liver or in some other viscus. Its presence in
the tissues of its second host causes the formation of a cyst, and
within this the larva rests and develops. In man, at least, the cysts
often undergo a calcareous degeneration, and Virchow states “dass beim
Menschen das _Pentastomum_ am häufigsten von allen Entozoen zu
Verwechselungen mit echten Tuberkeln Veranlassungen giebt.” The larva
moults several times, and loses its limbs, which seem to have no
connexion with the paired hooks in the adult (Fig. 256). The internal
organs slowly assume the form they possess in the adult. The larva is at
first quite smooth, but as it grows the annulations make their
appearance, arising in the middle and spreading forward and backward
(Fig. 259). In this encysted condition the larva remains coiled up for
some months, according to Leuckart; six in the case of _L. taenioides_,
and a somewhat shorter period, according to Stiles,[385] in the case of
_P. proboscideus_.

[Illustration:

  FIG. 261.—Encysted form of _Porocephalus protelis_, × 1, lying in the
    mesentery of its host. (From Hoyle.)
]

The frequency of what used to be called _Pentastoma denticulatum_ (= the
larval form of _L. taenioides_) in the body of man depends on the
familiarity of man with dogs. Klebs and Zaeslin found one larva in 900
and two in 1914 autopsies. Laenger[386] found the larva fifteen times in
about 400 dissections, once in the mesentery, seven times in the liver,
and seven times in the wall of the intestine. After remaining encysted
for some time it may escape, and begins wandering through the tissues,
aided by its hooks and annulations, a proceeding not unaccompanied by
danger to its host. Should the latter be eaten by some carnivorous
animal, the larva makes its way into the nasal cavities or sinuses, or
into the lungs of the flesh-eating creature, and there after another
ecdysis it becomes adult. If, however, the second host escapes this
fate, the larvae re-encyst themselves, and then if swallowed they are
said to bore through the intestine of the flesh-eater, and so make their
way to their adult abode.

=Systematic.=[387]—The Pentastomida are a group much modified by
parasitism, which has so deeply moulded their structure as to obscure to
a great extent their origin and affinities. The larva, with its clawed
limbs, recalls the Tardigrades and certain Mites, e.g. _Phytoptus_,
where only two pairs of limbs persist, and where the abdomen is
elongated and forms a large proportion of the body. The resemblances to
a single and somewhat aberrant genus must not, however, be pressed too
far. The striated muscles, the ring-like nature of the reproductive
organs and their ducts, perhaps even the disproportion both in size and
number of the females to the males, are also characters common to many
Arachnids.

The Pentastomida include three genera, _Linguatula_, Fröhlich,
_Porocephalus_, Humboldt, and _Reighardia_, Ward.[388] The first two
were regarded by Leuckart as but sub-genera, but Railliet[389] and
Hoyle[390] have raised them to the rank of genera. They are
characterised as follows:—

_Linguatula_, body flattened, but dorsal surface arched; the edges of
the fluke-like body crenelated; the body-cavity extends as diverticula
into the edges of the body.

_Porocephalus_, body cylindrical, with no diverticula of the
body-cavity.

_Reighardia_, devoid of annulations, transparent, with poorly developed
hooks and a mouth-armature.

The following is a list of the species with their primary and secondary
or larval hosts:—

  i. _Linguatula pusilla_, Diesing, found in the intestine of the
    fresh-water fish _Acara_, a South American genus of the Cichlidae.
    This is possibly the immature form of _L. subtriquetra_.

  ii. _L. recurvata_, Diesing, found in the frontal sinuses and the
    trachea of _Felis onca_.

  iii. _L. subtriquetra_, Diesing, found in the throat of _Caiman
    latirostris_ and _C. sclerops_, perhaps the mature form of _L.
    pusilla_.

  iv. _L. taenioides_, Lamarck, found in the frontal sinuses and nasal
    chambers of the dog and ounce, and in the nasal cavities of the
    wolf, fox, goat, horse, mule, sheep, and man, and in the trachea of
    the ounce. The immature form has been found in or on the liver of
    the cat, guinea-pig, and horse; in the lungs of the ox, cat,
    guinea-pig, porcupine, hare, and rabbit; in the liver and connective
    tissue of the small intestine of man; and in the mesenteric glands
    of the ox, camel, goat, sheep, antelope, fallow-deer, and mouse.

  v. _Porocephalus annulatus_, Baird, found in the lungs of the Egyptian
    cobra, _Naja haje_; the immature form is thought to live encapsuled
    in a species of _Porphyrio_[391] and in the Numidian Crane.

  vi. _P. aonycis_, Macalister, from the lungs of an Indian otter taken
    in the Indus.

  vii. _P. armillatus_, Wyman, found in the adult state in the lungs of
    certain African pythons, and in the lion; in the larval form it
    occurs encysted in the abdomen of the Aard-wolf, the mandril, and
    man—usually in negroes. Its migrations in the body of its second
    host sometimes cause fatal results.

  viii. _P. bifurcatus_, Diesing, found in the body-cavity of certain
    snakes, and in the lungs of boa-constrictors and the legless lizard,
    _Amphisbaena alba_. Possibly an immature form.

  ix. _P. clavatus_, Lohrmann, found in the lungs of the Monitor lizard.

  x. _P. crocidura_, Parona, found in the peritoneum of the “musk-rat”
    _Crocidura_ in Burmah. Probably a larval form.

  xi. _P. crotali_, Humboldt, found in the lungs, body-cavity, kidneys,
    spleen, and mesentery of many snakes and lizards, and of the lion
    and leopard. The immature forms occur in the liver and abdominal
    cavity of species of opossum, armadillo, mouse, raccoon, bat, and
    marmoset.

  xii. _P. geckonis_, Dujardin, found in the lungs of a Siamese gecko.

  xiii. _P. gracilis_, Diesing, found free in the body-cavity or
    encapsuled on the viscera and mesenteries of South American fishes,
    snakes, and lizards,

  xiv. _P. heterodontis_; Leuckart, found encapsuled in the abdominal
    muscles and mesentery of a species of _Heterodon_.

  xv. _P. indicus_,[392] v. Linst., found in the trachea and lungs of
    _Gavialis gangeticus_.

  xvi. _P. lari_, Mégnin, found in the air-sacs of the Burgomaster or
    Glaucous gull, _Larus glaucus_ of the Polar seas.

  xvii. _P. megacephalus_, Baird, found embedded in the flesh of the
    head of an Indian crocodile, _C. palustris_, the “Mugger.” Probably
    a larval form.

  xviii. _P. megastomus_, Diesing, found in the lungs of a fresh-water
    tortoise, _Hydraspis geoffroyana_.

  xix. _P. moniliformis_, Diesing, found in the lungs of pythons.

  xx. _P. najae sputatricis_, Leuckart, found encapsuled in the
    abdominal muscles and peritoneum of the cobra, _Naja tripudians_.
    Probably a larval form.

  xxi. _P. oxycephalus_, Diesing, found in the lungs of crocodiles and
    alligators.

  xxii. _P. platycephalus_, Lohrmann, habitat unknown.

  xxiii. _P. subuliferus_, Leuckart, in the lungs of the cobra _Naja
    haje_.

  xxiv. _P. teretiusculus_, Baird, found in the lungs and mouth of
    certain Australian snakes.

  xxv. _P. tortus_, Shipley, found in the body-cavity of a snake,
    _Dipsadomorphus irregularis_, taken in New Britain.

  xxvi. _Reighardia_, sp., Ward, found in the air-sacs of Bonaparte’s
    gull and the common North American tern.




                              PYCNOGONIDA


                                   BY

             D’ARCY W. THOMPSON, C.B., M.A. TRINITY COLLEGE

       Professor of Natural History in University College, Dundee




                              CHAPTER XXI
                            PYCNOGONIDA[393]


Remote, so far as we at present see, from all other Arthropods, while
yet manifesting the most patent features of the Arthropod type, the
Pycnogons constitute a little group, easily recognised and
characterised, abundant and omnipresent in the sea. The student of the
foreshore finds few species and seldom many individuals, but the dredger
in deep waters meets at times with prodigious numbers, lending a
character to the fauna over great areas.

[Illustration:

  FIG. 262.—_Pycnogonum littorale_, Ström, × 2.
]

The commonest of our native species, or that at least which we find the
oftenest, is _Pycnogonum littorale_ (_Phalangium littorale_, Ström,
1762). We find it under stones near low water, or often clinging
louse-like to a large Anemone. The squat segmented trunk carries, on
four pairs of strong lateral processes, as many legs, long, robust,
eight-jointed, furnished each with a sharp terminal claw. In front the
trunk bears a long, stout, tubular proboscis, at the apex of which is
the mouth, suctorial, devoid of jaws; the body terminates in a narrow,
limbless, unsegmented process, the so-called “abdomen,” at the end of
which is the anal orifice. The body-ring to which is attached the first
pair of legs, bears a tubercle carrying four eye-spots; and below, it
carries, in the male sex, a pair of small limbs, whose function is to
grasp and hold the eggs, of which the male animal assumes the burden,
carrying them beneath his body in a flattened coherent mass. In either
sex a pair of sexual apertures open on the second joints of the last
pair of legs. The integument of body and limbs is very strongly
chitinised, brown in colour, and raised into strong bosses or tubercles
along the middle line of the back, over the lateral processes, and from
joint to joint of the limbs. The whole animal has a singular likeness to
the Whale-louse, _Cyamus mysticeti_ (well described by Fr. Martins in
1675), that clings to the skin of the Greenland Whale as does
_Pycnogonum_ to the Anemone, a resemblance close enough to mislead some
of the older naturalists, and so close that Linnaeus, though in no way
misled thereby, named it _Phalangium balaenarum_. The substance of the
above account, and the perplexity attending the classification of the
animal, are all included in Linnaeus’s short description:[394]
“Simillimus Onisco Ceti, sed pedes omnes pluribus articulis, omnes
perfecti, nec plures quam octo. Dorsum rubrum, pluribus segmentis;
singulis tribus mucronibus. Cauda cylindrica, brevissima, truncata.
Rostrum membranaceum, subsubulatum, longitudine pedum. Genus dubium,
facie Onisci ceti; rostro a reliquis diversum. Cum solo rostro absque
maxillis sit forte aptius Acaris aut proprio generi subjiciendum....
Habitat in mari norvegico sub lapidibus.”[395]

[Illustration:

  FIG. 263.—Dorsal view of _Nymphon brevirostre_, Hodge, × 6. Britain.
]

The common _Pycnogonum_ is, by reason of the suppression of certain
limbs, rather an outlying member than a typical representative of the
Order, whose common characters are more strikingly and more perfectly
shown in species, for instance, of _Nymphon_. Of this multiform genus we
have many British species, some of the smaller being common below
tide-marks, creeping among weeds or clinging like Caprellae with
skeleton limbs to the branches of Zoophytes, where their slender forms
are not easily seen. In contrast to the stouter body and limbs of
_Pycnogonum_, the whole fabric of _Nymphon_ tends to elongation; the
body is drawn out so that the successive lateral processes stand far
apart, and a slender neck intervenes between the oculiferous tubercle
and the proboscis; the legs are produced to an amazing length and an
extreme degree of attenuation: “mirum tam parvum corpus regere tam
magnos pedes,” says Linnaeus. Above the base of the proboscis are a pair
of three-jointed appendages, the two terminal joints of which compose a
forcipate claw; below and behind these come a pair of delicate,
palp-like limbs of five joints; and lastly, on the ventral side, some
little way behind these, we find the ovigerous legs that we have already
seen in the male _Pycnogonum_, but which are present in both sexes in
the case of _Nymphon_. At the base of the claw which terminates each of
the eight long ambulatory legs stands a pair of smaller accessory or
“auxiliary” claws. The generative orifices are on the second joint of
the legs as in _Pycnogonum_, but as a rule they are present on all the
eight legs in the female sex, and on the two hindmost pairs in the male.
One of the Antarctic Nymphonidae (_Pentanymphon_) and one other
Antarctic genus less closely related (_Decolopoda_) have an extra pair
of legs. No other Pycnogon, save these, exhibits a greater number of
appendages than _Nymphon_ nor a less number than _Pycnogonum_, nor are
any other conspicuous organs to be discovered in other genera that are
not represented in these two: within so narrow limits lie the varying
characters of the group.

[Illustration:

  FIG. 264.—_Nymphon brevirostre_, Hodge. Head, from below, showing
    chelophores, palps, and ovigerous leg.
]

In framing a terminology for the parts and members of the body, we
encounter an initial difficulty due to the ease with which terms seem
applicable, that are used of more or less analogous parts in the Insect
or the Crustacean, without warrant of homology. Thus the first two pairs
of appendages in _Nymphon_ have been commonly called, since Latreille’s
time, the mandibles and the palps (Linnaeus had called them the palps
and the antennae), though the comparison that Latreille intended to
denote is long abandoned; or, by those who leaned, with Kröyer and
Milne-Edwards, to the Crustacean analogy, mandibles and maxillae. Dohrn
eludes the difficulty by denominating the appendages by simple numbers,
I., II., III., ... VII., and this method has its own advantages; but it
is better to frame, as Sars has done, a new nomenclature. With him we
shall speak of the Pycnogon’s body as constituted of a trunk, whose
first (composite) segment is the cephalic segment or head, better
perhaps the cephalothorax, and which terminates in a caudal segment or
abdomen; the “head” bears the proboscis, the first appendages or
“chelophores,” the second or “palps,” the third, the false or
“ovigerous” legs, and the first of the four pairs of “ambulatory” legs.
The chelophores bear their chela, or “hand,” on a stalk or scape; the
ambulatory legs are constituted of three coxal joints, a femur, two
tibial joints, a tarsus, and a propodus, with its claws, and with or
without auxiliary claws.

=The Body.=—The trunk with its lateral processes may be still more
compact than in _Pycnogonum_, still more attenuated than in _Nymphon_.

In a few forms (e.g. _Pallene_, _Ammothea_, _Tanystylum_,
_Colossendeis_) the last two, or even more, segments of the trunk are
more or less coalescent. In _Rhynchothorax_ the cephalic segment is
produced into a sharp-pointed rostrum that juts forward over the base of
the proboscis. The whole body and limbs may be smooth, tuberculated,
furnished with scattered hairs, or sometimes densely hispid.

[Illustration:

  FIG. 265.—=A=, _Colossendeis proboscidea_, Sabine, Britain; =B=,
    _Ammothea echinata_, Hodge, Britain; =C=, _Phoxichilus spinosus_,
    Mont., Arctic Ocean. (The legs omitted.)
]

The proboscis varies much in shape and size. It may be much longer or
much shorter than the body, cylindrical or tumid, blunt or pointed,
straight or (e.g. _Decolopoda_) decurved; usually firmly affixed to the
head and pointing straight forwards; sometimes (_Eurycide_,
_Ascorhynchus_) articulated on a mobile stalk and borne deflexed beneath
the body.

=Chelophores.=—The first pair of appendages or chelophores are wanting
in the adult _Pycnogonum_, _Phoxichilus_, _Rhynchothorax_, and
_Colossendeis_.[396]

In _Ammothea_ and its allies they are extremely rudimentary in the
adult, being reduced to tiny knobs in _Tanystylum_ and _Trygaeus_, and
present as small two-jointed appendages in _Ammothea_; in this last, if
not in the others also, they are present in complete chelate form in the
later larval stages.

[Illustration:

  FIG. 266.—=A=, =B=, Chelophores of _Ascorhynchus abyssi_, G.O.S. A,
    Young; B, adult. (After Sars.) =C=, Anterior portion of _Ammothea
    hispida_, Hodge, Jersey: late larval stage (= _Achelia longipes_,
    Hodge), showing complete chelae. =D=, Chela of _Eurycide hispida_,
    Kr.
]

In _Eurycide_, _Ascorhynchus_, and _Barana_ they are usually less
atrophied, but yet comparatively small and with imperfect chelae, while
in some Ascorhynchi (_A. minutus_, Hoek) they are reduced to stumps.

[Illustration:

  FIG. 267.—Chelae of species of Nymphonidae: =A=, _Nymphon
    brevirostre_, Hodge; =B=, _Boreonymphon robustum_, Bell; =C=,
    _Chaetonymphon macronyx_, G.O.S.; =D=, _Nymphon elegans_, Hansen.
]

[Illustration:

  FIG. 268.—Proboscis and chelophores of _Cordylochele longicollis_,
    G.O.S. (After Sars.)
]

In _Pallenopsis_ the scape of the chelophore consists of two joints, as
also in _Decolopoda_ and some _Ascorhynchus_: in _Nymphon_,
_Phoxichilidium_, _Pallene_, and _Cordylochele_ of one only; in all
these the terminal portion or “hand” forms a forcipate “chela,” of which
the ultimate joint forms the “movable finger.” In some species of
_Nymphon_ the chela is greatly produced and attenuated, and armed with
formidable serrate teeth on its opposing edges; in others it is
shortened, with blunter teeth; in _Boreonymphon robustum_ the claws are
greatly curved, with a wide gap between. In this last, and in
_Phoxichilidium_, the opposing edges are smooth and toothless. In
_Cordylochele_ the hand is almost globular, the movable finger being
shortened down, and half enclosed by the other.

[Illustration:

  FIG. 269.—_Eurycide hispida_, Kr., showing stalked proboscis and
    zigzag palps.
]

=Palpi.=—The second pair of appendages, or palps, are absent, or all but
absent, in the adult _Pycnogonum_, _Phoxichilus_, _Phoxichilidium_,
_Pallene_, and their allies. In certain of these cases, _e.g._
_Phoxichilidium_, a knob remains to mark their place; in others, _e.g._
_Pallenopsis_, a single joint remains; in a few Pallenidae a sexual
difference is manifested, reduction of the appendage being carried
further in the female than in the male. The composition of the palps
varies in the genera that possess them. In _Nymphon_ there are five
joints, and their relative lengths (especially of the terminal ones) are
much used by Sars in defining the many species of the genus. The
recently described _Paranymphon_, Caullery, has palps of six or seven
joints. In the Ammotheidae the number of joints ranges from five or six
in _Tanystylum_ to nine (as a rule) in _Ammothea_ and _Oorhynchus_, or
ten, according to Dohrn, in certain species of _Ammothea_.
_Colossendeis_ and the Eurycididae have a ten-jointed palp, which in
this last family is very long and bent in zigzag fashion, as it is, by
the way, also in _Ammothea_. The terminal joints of the palp are in all
cases more or less setose, and their function is conjecturally tactile.

=Ovigerous Legs.=—Custom sanctions for these organs an inappropriate
name, inasmuch as it is only in the males that they perform the function
which the name connotes.[397] They probably also take some part, as
Hodgson suggests, in the act of feeding.

[Illustration:

  FIG. 270.—Ovigerous legs of =A=, _Phoxichilus spinosus_, Mont.; =B=,
    _Phoxichilidium femoratum_, Rathke; =C=, _Anoplodactylus
    petiolatus_, Kr.; =D=, _Colossendeis proboscideus_, Sab.
]

[Illustration:

  FIG. 271.—Terminal joints of ovigerous leg of _Rhynchothorax
    mediterraneus_, Costa.
]

[Illustration:

  FIG. 272.—_Nymphon brevirostre_, Hodge. Terminal joints of ovigerous
    leg, with magnified “tooth.”
]

In _Pycnogonum_, _Phoxichilus_, _Phoxichilidium_, and their immediate
allies they are absent in the female; in all the rest they are alike
present in both sexes, though often somewhat smaller in the female than
in the male. They are always turned towards the lower side of the body,
and in many cases even their point of origin is wholly ventral. The
number of joints varies: in _Phoxichilidium_ five, _Anoplodactylus_ six,
_Phoxichilus_ seven; in _Paranymphon_ eight; in _Pycnogonum_ nine, with,
in addition, a terminal claw; in the Ammotheidae from seven (_Trygaeus_)
to ten, without a claw; in Pallenidae ten, with or without a claw; in
_Rhynchothorax_, _Colossendeis_, _Eurycide_, _Ascorhynchus_, _Nymphon_,
ten and a claw. The appendage, especially when long, is apt to be wound
towards its extremity into a spiral, and its last four joints usually
possess a peculiar armature. In _Rhynchothorax_ this takes the form of a
stout toothed tubercle on each joint; in _Colossendeis_ of several rows
of small imbricated denticles; in _Nymphon_ and _Pallene_ of a single
row of curious serrate and pointed spines, each set in a little
membranous socket.

[Illustration:

  FIG. 273.—_Nymphon strömii_, Kr. Male carrying egg-masses on his
    ovigerous legs.
]

[Illustration:

  FIG. 274.—Terminal joints (tarsus and propodus) of legs. =1=,
    _Chaetonymphon hirtum_, Fabr.; =2=, _N. strömii_, Kr.; =3=, _Nymphon
    brevirostre_, Hodge; =4=, _Ammothea echinata_, Hodge; =5=,
    _Ascorhynchus abyssi_, G.O.S. (All after Sars.)
]

=Legs.=—The four pairs of ambulatory legs are composed, in all cases
without exception, of eight joints if we exclude, or nine if we include,
the terminal claw. They vary from a length about equal to that of the
body (_Pycnogonum_, _Rhynchothorax_, _Ammothea_) to six or seven times
as much, perhaps more, in _Nymphon_ and _Colossendeis_, the fourth,
fifth, and sixth joints being those that suffer the greatest elongation.
The seventh joint, or tarsus, is usually short, but in some Nymphonidae
is much elongated; the eighth, or propodus, is usually somewhat curved,
and usually possesses a special armature of simple or serrate spines.
The auxiliary claws, sometimes large, sometimes small, lie at the base
of the terminal claw in Ammotheidae, Phoxichilidae, in _Phoxichilidium_,
in most Pallenidae, in nearly all Nymphonidae. Their presence or absence
is often used as a generic character, helping to separate, e.g.,
_Pallene_ from _Pseudopallene_ and _Pallenopsis_, and _Phoxichilidium_
from _Anoplodactylus_; nevertheless they may often be detected in a
rudimentary state when apparently absent. The legs are smooth or hirsute
as the body may happen to be.

[Illustration:

  FIG. 275.—Legs of =A=, _Pallene brevirostris_, Johnston; =B=,
    _Anoplodactylus petiolatus_, Kr.; =C=, _Phoxichilus spinosus_,
    Mont.; =D=, _Colossendeis proboscidea_, Sabine; =E=, _Ammothea
    echinata_, Hodge, ♂.
]

[Illustration:

  FIG. 276.—_Boreonymphon robustum_, Bell. Male with young, slightly
    enlarged. Faeroe Channel.
]

=Glands.=—In some or all of the appendages of the Pycnogonida may be
found special glands with varying and sometimes obscure functions. The
glands of the chelophores (Fig. 280, p. 522) are present in the larval
stages only. They consist of a number of flask-shaped cells[398] lying
within the basal joint of the appendage, and generally opening at the
extremity of a long, conspicuous, often mobile, spine (e.g. _Ammothea_
(Dohrn), _Pallene_, _Tanystylum_ (Morgan), _Nymphon brevicollum_ and _N.
gracile_ (Hoek)). They secrete a sticky thread, by means of which the
larvae attach themselves to one another and to the ovigerous legs of the
male parent. In _Nymphon hamatum_, Hoek, the several filaments secreted
by the separate sacculi of the gland issue separately. In _Pycnogonum_
the spine on which the gland opens is itself prolonged into a long fine
filament, and here, according to Hoek, the gland is in all probability
functionless and rudimentary. Hoek has failed to find the gland in
_Ascorhynchus_, and also in certain Nymphonidae (e.g. _Boreonymphon
robustum_, Bell), in which the young are more than usually advanced at
the time of hatching. The gland has also been described by Lendenfeld
and others in _Phoxichilidium_, whose larvae do not cling together but
live a parasitic life; in this genus the long spine or tubercle is
absent on which the orifice is usually situated, and, according to
Lendenfeld, the secretion issues from many small orifices set along the
opposing edges of the chela. Of the two species described by Dohrn as
_Barana castelli_ and _B. arenicola_, the former has the spine of
inordinate length, more than twice as long as the whole body, chelophore
and all; while in the latter (which species rather resembles
_Ascorhynchus_) the spine is altogether absent.

In the palps and ovigerous legs of the adult are found glandular bodies
of a hollow vesicular form with a simple lining of cells, the vesicle
being divided within by a septum with a central orifice, the outer and
smaller half opening to the exterior. These glands are probably of
general occurrence, but they have been but little investigated. They lie
usually in the fourth and fifth joints of the palp, and the third and
fourth joints of the ovigerous leg. Hoek describes them in
_Discoarachne_ (_Tanystylum_) as lying within the elongated third joint
of the palp, and opening by a sieve-plate at the end of the second
joint. In _Ammothea_ (Dohrn) and _Ascorhynchus_ (Hoek) they open on a
small tubercle situated on the fifth joint of the palp. In _Nymphon_,
Hoek describes them as opening by a small pore on the fourth joint of
the ovigerous leg. Dohrn failed to find them in _Pycnogonum_, but in
_Phoxichilus_, _Phoxichilidium_ and _Pallene_ he discovered the glands
appertaining to the palps, though the palps themselves have disappeared
in those genera; he has found the glands also in _Ammothea_, in larvae
that have not yet attained their full complement of legs.

The males in nearly all cases are known to possess glands in the fourth
joints or thighs of all the ambulatory legs, and these glands without
doubt act as cement-glands, emitting, like the chelophoral glands of the
larvae, a sticky thread or threads by which the eggs and young are
anchored to the ovigerous legs. In some species of _Nymphon_ and of
_Colossendeis_ Hoek could not find these, and he conjectures them to be
conspicuous only in the breeding season. While in most cases these
glands open by a single orifice or by a few pores grouped closely
together, in _Barana_, according to Dohrn, and especially in _B.
arenicola_, the pores are distributed over a wide area of the femoral
joint.[399] In _Discoarachne_ (Loman) and _Trygaeus_ they open into a
wide chitinised sac with tubular orifice. While the function of these
last glands and of the larval glands seems plain enough, that of those
which occur in the palps and ovigerous legs of both sexes remains
doubtful.

In their morphological nature the two groups of glands are likewise in
contrast, the former being unicellular glands, such as occur in various
parts of the integument of the body and limbs of many Crustacea; while
the latter are segmentally arranged and doubtless mesoblastic in origin,
like the many other segmental excretory organs (or coelomoducts) of
various Arthropods.

By adding colouring matters (acid-fuchsin, etc.) to the water in which
the animals were living, Kowalevsky demonstrated the presence of what he
believed to be excretory organs in _Phoxichilus_, _Ammothea_, and
_Pallene_. These are small groups of cells, lying symmetrically near the
posterior borders of the first three body-segments, and also near the
bases of the first joints of the legs, dorsal to the alimentary
canal.[400]

[Illustration:

  FIG. 277.—Longitudinal section through one “antimere” of the proboscis
    in _Phoxichilus charybdaeus_. _G_, _g′_, Principal and secondary
    ganglia; _h_, sieve-hairs; _L_, lip; _mt_, oral tooth; _N_, _N′_,
    inner and outer nerve-cords; _t_, proboscis-teeth. (After Dohrn.)
]

=Alimentary System.=—The proboscis is a very complicated organ, and has
been elaborately described by Dohrn.[401] It is a prolongation of the
oral cavity, containing a highly developed stomodaeum, but showing no
sign of being built up of limbs or gnathites. The mouth, situated at its
apex, is a three-sided orifice, formed by a dorsal[402] and two lateral
lobes; and hence the proboscis has been assumed by some, on no competent
evidence, to be constituted of a degenerate pair of appendages and a
labrum or upper lip. Each of the three lobes which bounds the mouth
shows the following structures: firstly, a lappet of external chitinised
integument, overlapping, as the finger-nail overlaps the finger, a
cushion-like lip, ridged after the fashion of a fine-cut file in some
species, hairy in others, on the inner surface where the three lips meet
to close the orifice of the mouth. Below this again is a prominent tooth
(Fig. 277, _mt_), supported, as are the lips, by a system of chitinous
rods, which are but little developed in the genus here figured, though
conspicuous and complicated in others. Transverse ridges run across the
angles where adjacent lips meet, and the whole mechanism constitutes an
efficient valve, preventing the escape of swallowed food. The greater
portion of the proboscis is occupied by a masticating or triturating
apparatus, the oesophageal cavity expanding somewhat and having its
walls densely covered, in three bands corresponding to the antimeres,
with innumerable minute spines (_h_) or needles, sometimes supplemented
by large teeth (_t_) that point forwards somewhat obliquely to the axis
of the proboscis.[403]

In the curious East Indian genus _Pipetta_ (Loman) the sucking and
sifting mechanism is low down in the proboscis, and the organ is
prolonged into a very fine tube, the lips growing together till they
leave an aperture of only ·007 mm. for the absorption of liquids.

In some cases, where the proboscis itself is short, as in _Pallene_,
this mechanism is carried backwards into the fore-part of the body; and,
in the latter genus, the narrow oesophagus which succeeds the
masticatory apparatus is likewise provided with extrinsic muscles.

[Illustration:

  FIG. 278.—Transverse sections through the proboscis of _Ph.
    charybdaeus_. =A=, Anterior, through the principal ganglionic mass
    (_G_); =B=, posterior, at the level of the sieve-hairs (_h_).
    _Coec_, Intestinal caeca; _Dil. M_, dilator muscles; _N_, inner
    nerve-ganglion, with circular commissure; _N′_, outer nerve; _or_,
    chitinous lining of oral cavity; _R M_, _Ret.M_, retractor muscles.
    (After Dohrn.)
]

[Illustration:

  FIG. 279.—Transverse section through the basal joint of the third leg
    in _Phoxichilus charybdaeus_, ♀. _Cut_, Cuticle; _Hyp_, hypodermis;
    _Int_, intestinal caecum; _N_, nerve-cord; _Ov_, ovary; _Sept_,
    septum. (After Dohrn.)
]

The oesophagus is followed by a long gastric cavity, which sends forth
caecal diverticula into the chelophores (when these are present), and
four immensely long ones into the ambulatory legs. The caeca are
attached to the walls of the limb cavities, especially at their
extremities in the tarsi, by suspensory threads of connective tissue,
and the whole gut, central and diverticular, is further supported by a
horizontal septal membrane, running through body and legs, which
separates the dorsal blood-vessel and sinus from the gut, the nervous
system and the ventral sinus, giving support also to the reproductive
glands. A short and simple rectum follows the gastric cavity.

In _Phoxichilus_, which lacks the three anterior appendages in the
female and the two anterior in the male, two pairs of caeca run from the
gut into the cavity of the proboscis (Fig. 278, B, _coec_.).[404]

=Circulatory System.=—The heart has been especially studied by Dohrn in
_Phoxichilus_. It consists of a median vessel running from the level of
the eyes to the abdomen, furnished with two pairs of lateral valvular
openings, and sometimes, though not always, with an unpaired one at the
posterior end. The walls are muscular, but with this peculiarity that
the muscular walls do not extend around the heart dorsally, in which
region its lumen is only covered by the hypodermis and cuticle of the
back. The blood-spaces of the body are separated into dorsal and ventral
halves by the septal membrane already referred to, which is perforated
in the region of the lateral processes by slits placing the two cavities
in communication; this septal membrane runs through the limbs to their
tips, and far into the proboscis, where it is attached to the edge of
the superior antimere. The blood is a colourless plasma with several
kinds of corpuscles, of which the most remarkable are amoeboid, actively
mobile, often coalescing into plasmodia. The course of the circulation
is on the whole outwards in the inferior or ventral sinus, inwards
towards the heart in the superior, save in the proboscis, where the
systole of the heart drives the blood forwards in the dorsal channel.
The beat is rapid, two or three times in a second, according to Loman,
in _Phoxichilidium_. Especially in the species with small body and
exaggerated legs, the movement of the circulatory fluid is actuated more
by the movements of the limbs and the contractions of the intestinal
caeca than by the direct impulse of the heart.

=Nervous System.=—The nerve-chain consists of a fused pair of
supra-oesophageal ganglia, which innervate (at least in the adult) the
chelophores, and of ventral ganglia, whence proceed the nerves to the
other limbs. The ganglia of the second and third appendages are fused
with one another, sometimes also with the ganglia of the first
ambulatory legs; the ganglia of the three posterior pairs of legs are
always independent (though the development of their longitudinal
commissures varies with the body-form), and they are succeeded by one or
two pairs of ganglia, much reduced in size, situated in the abdomen, of
which the posterior one innervates the muscles of the abdomen and of the
anal orifice. Each lateral nerve divides into two main branches, which
supply the parts above and below the septal membrane. The nerve-supply
of the proboscis is very complicated. Its upper antimere is supplied
from the pre-oral, its two lateral antimeres from the first post-oral,
ganglion, and each of these three nerves divides into two branches, of
which the inner bears six to eight or more small ganglia, which annular
commissures passing round the pharynx connect one to another. Of these
ganglia and commissures the anterior are the largest, and with these the
outer lateral nerve-branches of the proboscis merge. The immediate
origin of the nerves to the chelophores is from the median nerve that
springs from the under side of the supra-oesophageal ganglion to run
forward into the proboscis, but it is noteworthy that the chelophores
receive twigs also from the lateral nerves of the proboscis which arise
from the post-oral ganglia.

=Eyes.=—Eyes are the only organs of special sense known in the
Pycnogons. The deep-water Pycnogons, in general those inhabiting depths
below four or five hundred fathoms, have in most cases imperfect organs,
destitute of lens and of pigment, so imperfect in many cases as to be
described as wanting. It is rare for the eyes to be lacking in
shallow-water species, as they are, for instance, in _Ascorhynchus
minutus_, Hoek, dredged by the _Challenger_ in 38 fathoms, but, on the
other hand, it is no small minority of deep-water species that possess
them of normal character and size, even to depths of about 2000 fathoms.

In all cases where eyes are present, they are simple or “monomeniscous”
eyes, four in number, and are situated in two pairs on an “oculiferous
tubercle,” sometimes blunt and low, sometimes high and pointed, placed
on the so-called cephalothorax, or first, compound, segment of the body.
The anterior pair are frequently a little larger, sometimes, as in
_Phoxichilidium mollissimum_, Hoek, very much larger, than the
posterior. The minute structure of the eye has been investigated by
Dohrn, Grenacher, Hoek, and Morgan. The following account is drawn in
the first instance from Morgan’s descriptions.[405]

The eye of a Pycnogon (_Phoxichilidium_) is composed of three layers, an
outer layer of specialised ectoderm cells (hypodermis) that secrete the
cuticular lens, a middle layer of visual or retinal elements, and an
inner layer of pigment-cells. The elements of the middle layer consist
of much elongated cells, whose branching outer ends are connected with
nerve-fibrils and interwoven in a protoplasmic syncytium, whose middle
parts are occupied by the nuclei and whose inwardly directed ends form
the retinal rods or bacilli. The pigment-cells of the inner layer are of
various forms, those towards the middle of the eye being small and
flattened, those at the sides being, for the most part, long and
attenuated, so seeming, as Morgan remarks, to approximate in character
to the retinal elements. The pigment-layer is easily dispersed and
reveals beneath it a median vertical raphe, caused by the convergence of
the cells of the middle layer from either side, and along the line of
this raphe the optic nerve joins the eye, though its subsequent course
to its connection with the retinal elements is obscure. It is at least
clear that the retina is an “inverted” retina, with the nerve-connected
bases of its cells lying outwards and their bacillar extremities
directed inwards.

In a longitudinal vertical section of the eye of a larva (_Tanystylum_),
at a stage when three pairs of walking legs are present, Morgan shows us
the pigment-layer apparently continuous with the hypodermis just below
the eye, and in close connection with the middle layer at the upper part
of the eye. From this we are permitted to infer a development by
invagination, in which the long invaginated sac is bent and pushed
upwards till it comes into secondary contact with the hypoderm, so
giving us the three layers of the developed eye. This manner of
formation is precisely akin to that described by Parker, Patten, Locy,
and others for the median eyes of Scorpions and of Spiders, and the
organ is structurally comparable to the Nauplius- or median eye of
Crustacea. But neither in these cases nor in that of the Pycnogon is the
whole process clear, in consequence chiefly of the obscurity that
attends the course of the optic nerve in both embryo and adult. For
various discussions and accounts, frequently contradictory, of these
phenomena, the reader is referred to the authors quoted, or to Korschelt
and Heider’s judicious summary.[406]

There seems to be a small structure, of some sort or other, between the
ocelli on either side. Dohrn thought it might be auditory, Loman that it
might be secretory, but its use is unknown.

=Integument.=—The chitinised integument is perforated by many little
cavities, some of them conical and tapering to a minute external pore,
the others more regularly tubular. Sometimes, but according to Hoek
rarely, the tubular pore-canals communicate with, or arise from, the
conical cavities. The pore-canals transmit a nerve for the supply of
sensory hairs, often forked, which arise from the orifice of the canal
in little groups of two or more, sometimes in rosettes of eight or nine.
These setae are small or rudimentary in _Ascorhynchus_ and totally
wanting in _Colossendeis_; they appear to be extremely large and
stellate in _Paranymphon_. The conical cavities contain proliferated
epithelial cells, blood corpuscles, and cells of more doubtful nature
that are perhaps glandular. According to Dohrn, glands exist in
connection with both kinds of integumentary perforations, and he
suspects that they secrete a poisonous fluid in response to stimuli
affecting the sensory hairs; Hoek, on the other hand, is inclined to
ascribe a respiratory function to the cavities; but indeed, as yet, we
must confess that their use is undetermined.

=Reproductive Organs.=—In each sex the generative organs consist of a
pair of ovaries or testes lying above the gut on either side of the
heart; in the adult they are fused together posteriorly at the base of
the abdomen, and send long diverticula into the ambulatory legs. In the
female _Phoxichilidium_, at least, as Loman has lately shown, the fusion
is complete, and the ovary forms a thin broad plate, spreading through
the body and giving off its lateral diverticula. The diverticula of the
testes reach to the third joint of the legs, those of the ovaries to the
fourth, or sometimes farther. The ova ripen within the lateral
diverticula, chiefly, and sometimes (_Pallene_) exclusively, in the
femora or fourth joints of the legs,[407] which, in many forms, are
greatly swollen to accommodate them; the spermatozoa, on the other hand,
are said to develop both within the legs and within the thoracic
portions of the testis. The genital diverticula may end blindly within
the leg, or communicate through a duct with the exterior by a valvular
aperture placed on the second coxal joint. Such apertures occur, as a
rule, on all the legs in the females, in _Rhynchothorax_ and
_Pycnogonum_ on the last only. In the males an aperture is present on
all the legs in _Decolopoda_ and _Phoxichilidium_; on the last three in
_Nymphon_ and _Phoxichilus_; in most genera on the last two; in
_Pycnogonum_ and _Rhynchothorax_ on the last only.

Very commonly the female individuals are somewhat larger than the males,
and in some species (_Ammothea_, _Trygaeus_) the latter are
distinguished by a greater development of spines or tubercles on the
body and basal joints of the legs (Dohrn).

The act of fecundation has been observed by Cole[408] in
_Anoplodactylus_. The animal reproduces towards the end of August.
Consorting on their _Eudendrium_ (Hydroid) colony, the male climbs upon
the female and crawls over her head to lie beneath her, head to tail;
and then, fertilisation taking place the while, the hooked ovigerous
legs of the male fasten into the extruding egg-masses and tear them
away. The whole process is over in five minutes. The fresh egg-masses
are more or less irregular in shape, and white in colour like little
tufts of cotton.

Each ball of eggs that the male carries represents the entire brood of
one female, and in _Phoxichilidium_ Loman has seen a male carrying as
many as fourteen balls. Fertilisation is external, taking place while
the eggs are being laid. The spermatozoa have small rounded heads and
long tails, and are thus unlike the spermatozoa of most Crustacea.

=Development.=—Until the hatching of the embryo, the eggs of the
Pycnogons are carried about, agglutinated by cement-substance into
coherent packets, on the ovigerous legs of the males. They are larger or
smaller according to the amount of yolk-substance present, very small in
_Phoxichilidium_ and _Tanystylum_ (Morgan), where they measure only ·05
mm. in diameter; larger in _Pallene_ (·25 mm.); larger still (·5–·7 mm.)
in _Nymphon_. In _Pallene_ each egg-mass commonly contains only two
eggs; in the other genera they are much more numerous, rising to a
hundred or more in _Ammothea_ (Dohrn). The egg-masses may be one or more
on each ovigerous leg, sometimes (_Phoxichilidium angulatum_, Dohrn) a
single egg-mass is held by both legs; they are extremely numerous in
_Phoxichilus_, and in _Pycnogonum_ they coalesce to form a broad pad
beneath the body. The fact that it is the male and not the female that
carries the eggs was only announced in 1877 by Cavanna;[409] before, and
by some even after his time, the two sexes were constantly
confused.[410]

[Illustration:

  FIG. 280.—Young larva (nat. size ·1 mm.) of _Ammothea fibulifera_,
    Dohrn. _C.G_, Brain; _gl_, _gld_, gland and duct of chelophore;
    _pr_, proboscis; I, II, III, IV, appendages. (After Dohrn.)
]

Segmentation is complete, symmetrical in the forms with smaller eggs,
unequal in those burdened with a preponderance of yolk (Morgan). In
_Pallene_, as in the Spider’s egg, what is described as at first a total
segmentation passes into a superficial or centrolecithal one by the
migration outwards of the nuclei and the breaking down of the inner ends
of the wedge-shaped segmentation-cells. The blastoderm so formed becomes
concentrated at the germinal pole of the egg. A thickened portion of the
blastoderm (which Morgan compares to the “cumulus primitivus” of the
Spider’s egg) forms an apparently blastoporal invagination (though
Morgan calls it the stomodaeum), and from its sides are budded off the
mesodermal bands. Meisenheimer has recently given a minute account of
the early development of _Ammothea_, a form with small yolkless eggs.
Here certain cells of the uniform and almost solid blastosphere grow
inwards till their nuclei arrange themselves in an inner layer of what
(so far as they are concerned) is a typical gastrula, but without any
central cavity. The inner layer subsequently, but slowly, differentiates
into the mid-gut, and into dorsal and lateral offshoots, the sources of
the heart and of the muscles and connective tissues respectively. The
further development of the egg takes place, as is usual in Arthropods,
by the appearance, in a longitudinal strip or germ-band which enwraps
the yolk, of paired thickenings which represent the cerebral and
post-oral ganglia, and of others from which arise the limbs. Of these
latter, the chelophores are the first to appear, on either side of the
mouth; in _Pallene_ the fourth pair appears next in order, followed by
the fifth and sixth, and by the third and seventh just before the
hatching out of the embryo; the second is lacking in this particular
genus. Thus in _Pallene_ (Dohrn, Morgan), and in some others, e.g.
_Nymphon brevicollum_ (Hoek), the free larva is from the first provided
with its full complement of limbs. Certain other species of _Nymphon_
hatch out in possession of four or five pairs of limbs, but in the great
majority of cases studied the larval Pycnogon is at first provided with
three pairs only, the three anterior pairs of the typical adult.[411]
Numerical coincidence, and that alone, has often led this “Protonymphon”
larva to be compared with the Crustacean Nauplius. In the annexed figure
of a young larval _Ammothea_ (_Achelia_), we see the unsegmented body,
the already chelate chelophores (furnished with the provisional
cement-glands already described), the other two pairs of appendages each
with a curious spine at its base, the gut beginning to send out
diverticula (of which the first pair approach the chelophores) but still
destitute of the anus (which is only to be formed after the development
of the abdomen), the proboscis, and one pair of eyes situated close over
the pre-oral ganglia. The subsequent changes are in this genus extremely
protracted, and terminate with the loss of the chelae, a process which
occurs so late in life that the chelate individuals were long looked
upon as belonging to a separate genus, the original _Ammothea_ of Hodge,
until Hoek proved their identity with the clawless _Achelia_.

The developmental history of _Phoxichilidium_ and _Anoplodactylus_ is
peculiar. The young larvae have the claws of the second and third
appendages hypertrophied to form enormous stiff tendril-like organs,
with which they affix themselves to the bodies of Hydroid Zoophytes
(_Coryne_, _Eudendrium_, _Tubularia_, _Hydractinia_, etc.), feeding as
the adults do: afterwards losing these elongated tendrils in a moult,
they pass into the gastral cavity of the Hydroid; in our native species
the larva issues from the Hydroid and begins its independent life at a
stage when three pairs of ambulatory legs are present and the fourth is
in bud.[412] The _Phoxichilidium_ larvae were first noticed by Gegenbaur
in _Eudendrium_,[413] again by Allman in _Coryne eximia_.[414] George
Hodge made detailed and important observations,[415] and showed, in
opposition to Gegenbaur, that it was the larva which entered the Hydroid
and not the egg that was laid therein.[416]

[Illustration:

  FIG. 281.—Larva of _Phoxichilidium_ sp., showing tendril-like
    appendages of the larval palps and ovigerous legs. (After Dohrn.)
]

Moseley has the following interesting note in his _Challenger
Report_:[417] “The most interesting parasite observed was a form found
in the gastric cavities of the gastrozoids of _Pliobothrus symmetricus_
(West Indies, 450 f.), contained in small capsules. These capsules were
badly preserved, but there seemed little doubt that they contained the
remains of larvae of a Pycnogonid, so that the deep-sea Pycnogonids,
which are so abundant, very possibly pass through their early stages in
deep-sea Stylasteridae.... The gastrozoids containing the larvae were
partly aborted.”

A Pycnogon larva, doubtfully ascribed to _Nymphon_, has been found
living in abundance ectoparasitically on _Tethys_ in the Bay of
Naples.[418]

=Habits.=—Of the intimate habits of the Pycnogons we can say little.
_Pycnogonum_ we often find clinging, as has been said, close appressed
to some large Anemone (_Tealia_, _Bolocera_, etc.), whose living juices
it very probably imbibes. The more slender species we find climbing over
sea-weeds and Zoophytes, where sometimes similarity of colour as well as
delicacy of form helps to conceal them; thus _Phoxichilidium femoratum_
(_Orithyia coccinea_, Johnston) is red like the Corallines among which
we often find it, _P. virescens_ green like the filamentous Ulvae, the
Nymphons yellowish like the _Hydrallmania_ and other Zoophytes which
they affect. On the New England coast, according to Cole, the dark
purple _Anoplodactylus lentus_, Wilson (_Phoxichilidium maxillare_,
Stimpson), is especially abundant on colonies of _Eudendrium_, whose
colour matches its own, the yellowish _Tanystylum orbiculare_ frequents
a certain yellowish Hydroid, and of these two species neither is ever
found on the Hydroid affected by the other; while, on the other hand,
_Pallene brevirostris_, whose whitish, almost transparent body is
difficult to see, is more generally distributed.[419] The deep-sea
Pycnogons (_Colossendeis, Nymphon_) are generally (if not universally)
of a deep orange-scarlet colour, a common dress of many deep-sea
Crustacea.

The movements of the Pycnogons are singularly slow and deliberate; they
are manifestly not adapted to capture or to kill a living prey. Linnaeus
accepted from J. C. König the singular statement that they enter and
feed upon bivalve shells, “Mytilorum testes penetrat et exhaurit”; but
the statement has never been reaffirmed.[420]

Loman describes _Phoxichilidium_ as feeding greedily on _Tubularia
larynx_, and especially on the gonophores. It grasps them with its
claws, sucks them in bit by bit till the proboscis is filled as far as
the sieve, whereupon that part of the proboscis squeezes and kneads the
mass, letting only juices and fine particles pass through into the
alimentary canal. The lateral caeca and the rectum are separated by
sphincter muscles from the stomach; the former are in turn filled with
food and again emptied; the contents of the alimentary canal are in
constant rolling movement, and the faeces are eliminated by the action
of a pair of levatores ani, in round pellets.

The Pycnogons, or some of them, can swim by “treading water,” and
_Pallene_ is said by Cole to swim especially well; they more often
progress half by swimming, half by kicking on the bottom. They move
promptly towards the light, unless they have Hydroids to cling to, and
Cole points out that when they crawl with all their legs on the bottom
they move forwards towards the light,[421] but backwards when they swim
in part or whole. The legs move mostly in a vertical plane, horizontal
movements taking place chiefly between the first and second joints.
_Tanystylum_ is uncommonly sluggish and inert; it sinks to the bottom,
draws its legs over its back and remains quiet, while _Pallene_, by
vigorous kicks, remains suspended.

The long legs of the Pycnogons are easily injured or lost, and easily
repaired or regenerated. This observation, often repeated, is as old as
Fabricius: “Mutilatur etiam in libertate sua, redintegrandum tamen; vidi
enim in quo pedes brevissimi juxta longiores enascentes, velut in
asteriis cancris aliisque redintegratis.” In such cases of
redintegration of a leg, the reproductive organ, the genital orifice,
and the cement-gland are not restored until the next moult.[422]

=Systematic Position.=—To bring this little group into closer accord
with one or other of the greater groups of Arthropods is a problem
seemingly simple but really full of difficulty.

The larval Pycnogon, with its three pairs of appendages, resembles the
Crustacean Nauplius in no single feature save this unimportant numerical
coincidence; nor is there any significance in the apparent outward
resemblance to isolated forms (e.g. _Cyamus_) that induced some of the
older writers, from Fabricius downwards and including Kröyer and the
elder Milne-Edwards, to connect the Pycnogons with the Crustacea. To
refer them, or to approximate them to the Arachnids, has been a stronger
and a more lasting tendency.[423] Linnaeus (1767) included the two
species of which he was cognisant in the genus _Phalangium_, together
with _P. opilio_. Lamarck, who first formulated the group Arachnida
(1802), let it embrace the Pycnogons; and Latreille (1804, 1810), who
immediately followed him, defined more clearly the Pycnogonida as a
subdivision of the greater group, side by side with the subdivision that
corresponds to our modern Arachnida (“Arachnides acères”), and together
with a medley of lower Crustacea, Myriapoda, Thysanura, and Parasitic
Insects; he was so cautious as to add “j’observerai seulement, que je ne
connais pas encore bien la place naturelle des Pycnogonides et des
Parasites,” and Cuvier, setting them in a similar position, adds a
similar qualification.[424]

Leach (1814), whose great service it was to dissociate the
Edriophthalmata and the Myriapoda from the Latreillian medley, left the
group Arachnida as we still have it (save for the inclusion of the
Dipterous Insect _Nycteribia_), and divided the group (with the same
exception) into four Orders of which the Podosomata, i.e. the
Pycnogonida, are one. Savigny (1816), less philosophical in this case
than was his wont, assumed the Crustacean type to pass to the Arachnidan
by a loss of several anterior pairs of appendages, and appears to set
the Pycnogons in an intermediate grade, marking the pathway of the
change. He considered the seven pairs of limbs of the Pycnogons to
represent thoracic limbs of a Malacostracan, and, like so many of his
contemporaries, was much biased by the apparent resemblance of _Cyamus_
to _Pycnogonum_. The reader may find in Dohrn’s Monograph a guide to
many other opinions and judgments, some of them of no small
morphological interest and historical value[425]; but it behoves us to
pass them by, and to inspect, in brief, the case as it stands at
present. The obvious features in which a Pycnogon resembles a Spider or
other typical Arachnid, are the possession of four pairs of walking
legs, and the pre-oral position and chelate form of the first pair of
appendages; we may perhaps also add, as a more general feature of
resemblance, the imperfect subservience of limbs to the mouth as
compared with any of the Crustacea. The resemblance would still be
striking, in spite of the presence of an additional pair of legs in a
few Pycnogons, were it not for the presence of the third pair of
appendages or ovigerous legs of the Pycnogon, whose intercalation spoils
the apparent harmony. We are neither at liberty to suppose, with Claus,
that these members, so important in the larva, have been interpolated,
as it were, anew in the Pycnogon; nor that they have arisen by
subdivision of the second pair, as Schimkewitsch is inclined to suppose;
nor that they have dropped out of the series in the Arachnid, whose body
presents no trace of them in embryo or adult. In a word, their presence
precludes us from assuming a direct homology between the apparently
similar limbs of the two groups,[426] and at best leaves it only open to
us to compare the last legs of the Pycnogon with the first abdominal, or
genital, appendages of the Scorpion and the Spider. On the other hand,
if we admit the seventh (as we must admit the occasional eighth) pair of
appendages of Pycnogons to be unrepresented in the prosoma of the
Arachnids, then, in the cephalothorax of the former, with its four pairs
of appendages, we may find the homologue of the more or less free and
separate part of the cephalothorax in _Koenenia_, _Galeodes_, and the
Tartaridae. There is a resemblance between the two groups in the
presence of intestinal diverticula that run towards or into the limbs,
as in Spiders and some Mites, and there are certain histological and
embryological resemblances that have been in part referred to above; but
these, such as they are, are not adequate guides to morphological
classification. We must bear in mind that such resemblances as the
Pycnogons seem to show are not with the lower Arachnids but with the
higher; they are either degenerates from very advanced and specialised
Arachnida, or they are lower than the lowest. Confronted with such an
issue, we cannot but conclude to let the Pycnogons stand apart, an
independent group of Arthropods[427]; and I am inclined to think that
they conserve primitive features in the usual presence of generative
apertures on several pairs of limbs, and probably also in the
non-development of any special respiratory organs. But inasmuch as the
weight of evidence goes to show that subservience of limbs to mouth is a
primitive Arthropodan character, the fact that the basal elements of the
anterior appendages have here (as in _Koenenia_) no such relation to the
mouth must be taken as evidence, not of antiquity, but of
specialisation. In like manner the suctorial proboscis cannot be deemed
a primitive character, and the much reduced abdomen also is obviously
secondary and not primitive.

=Classification.=—No single genus more than another shows signs of
affinity with other groups, and no single organ gives us, within the
group, a clear picture of advancing stages of complexity. On the
contrary, the differences between one genus and another depend very much
on degrees of degeneration of the anterior appendages, and we have no
reason to suppose that these stages of degeneration form a single
continuous series, but have rather reason to believe that degeneration
has set in independently in various ways and at various points in the
series. But while we are unable at present to form a natural
classification[428] of the Pycnogons, yet at the same time a purely
arbitrary or artificial classification, conveniently based on the
presence or absence of certain limbs, would run counter to such natural
relationships as we can already discern.

The classification here adopted is a compromise between a natural
system, so far as we can detect it, and an artificial one.

Two forms, separated from one another by many differences, show a
minimum of degeneration, namely _Decolopoda_ on the one hand, and the
Nymphonidae on the other. The former genus has five pairs of legs, and
this peculiarity is shared by _Pentanymphon_. In both groups the three
anterior limbs are all present and well formed, save only that the
ovigerous legs, which have ten joints in _Decolopoda_, are reduced to
five joints in the Nymphons, and their denticulate spines, of which
several rows are present in the former, are reduced to one row in the
latter; on the other hand, a greater or a less degeneration of these
limbs marks each and all of the other families.

_Decolopoda_ is very probably the most primitive form known, though it
has characters which seem to be the reverse of primitive in the dwarfish
size of its chelophores and the crowded coalescent segmentation of the
trunk. _Colossendeis_, in spite of its vanished chelophores, is probably
closely allied: the shape and segmentation of the body and the several
rows of smooth denticles on the ovigerous legs are points in common. The
Eurycydidae are closely allied to Colossendeidae; they agree with
_Decolopoda_ in the two-jointed scape of the chelophore, and with
Ammotheidae in the deflexed mobile proboscis. The true position of
_Rhynchothorax_ is very doubtful.

The Nymphonidae and Pallenidae are closely allied, and the
Phoxichilidiidae have points of resemblance, especially with the latter.
_Nymphon_ compares with _Decolopoda_ in the completeness of its parts,
and is more typical in its long well-segmented body, and in its
highly-developed chelae; but it already shows reduction in the scape of
the chelophore, in the palps, and in the armature of the ovigerous legs.

The Phoxichilidae and Pycnogonidae (Agnathonia, Leach; Achelata, Sars),
though differing greatly in aspect, are not improbably allied to one
another; and whether this be so or not, the complete absence of
chelophores and of palps affords an arbitrary character by which they
are conveniently separated from all the rest.

The following table epitomises the chief characters of the several
families:—

 ┌────────────────────┬──────────┬────────────┬───────────┬─────────┐
 │    PYCNOGONIDA.    │Proboscis.│Chelophores.│  Palps.   │Ovigerous│
 │                    │          │            │           │  legs.  │
 ├────────────────────┼──────────┼────────────┼───────────┼─────────┤
 │                    │          │            │           │         │
 ├────────────────────┼──────────┼────────────┼───────────┼─────────┤
 │(=Cryptochelata=,   │          │            │           │         │
 │Sars)—              │          │            │           │         │
 │ DECOLOPODIDAE      │  Fixed,  │ Complete,  │ 10 joints │10 joints│
 │                    │ decurved │small, scape│           │  ♂, ♀   │
 │                    │          │ 2–jointed  │           │         │
 │                    │          │            │           │         │
 │                    │          │            │           │         │
 │ COLOSSENDEIDAE     │ Somewhat │     0      │    10     │ 10 ♂, ♀ │
 │                    │ mobile,  │            │           │         │
 │                    │sometimes │            │           │         │
 │                    │ decurved │            │           │         │
 │ EURYCIDIDAE        │ Mobile,  │   Scape    │    10     │ 10 ♂, ♀ │
 │                    │ stalked, │ 2–jointed, │           │         │
 │                    │ deflexed │   chelae   │           │         │
 │                    │          │rudimentary │           │         │
 │ _Hannonia_         │    „     │Rudimentary │     0     │ 10 ♂, ♀ │
 │                    │          │            │           │         │
 │ AMMOTHEIDAE        │ Mobile,  │     „      │    4–9    │ 10 (or  │
 │                    │ deflexed │            │           │less) ♂, │
 │                    │          │            │           │    ♀    │
 │                    │          │            │           │         │
 │ ? RHYNCHOTHORACIDAE│  Large,  │     0      │   8 (5)   │ 10 ♂, ♀ │
 │                    │  fixed,  │            │           │         │
 │                    │ aberrant │            │           │         │
 ├────────────────────┼──────────┼────────────┼───────────┼─────────┤
 │(=Euchelata=, Sars)—│          │            │           │         │
 │ NYMPHONIDAE        │  Large,  │Large, scape│   5 (7)   │8–10 ♂, ♀│
 │                    │  fixed   │ 1–jointed  │           │         │
 │                    │          │            │           │         │
 │                    │          │            │           │         │
 │ PALLENIDAE         │    „     │     „      │   0 or    │ 10 ♂, ♀ │
 │                    │          │            │rudimentary│         │
 │                    │          │            │           │         │
 │ PHOXICHILIDIIDAE   │    „     │     „      │     0     │  5–6 ♂  │
 │                    │          │            │           │         │
 │                    │          │            │           │         │
 ├────────────────────┼──────────┼────────────┼───────────┼─────────┤
 │(=Achelata=, Sars)  │          │            │           │         │
 │ PHOXICHILIDAE      │  Large,  │     0      │     0     │   7 ♂   │
 │                    │  fixed   │            │           │         │
 │                    │          │            │           │         │
 │ PYCNOGONIDAE       │    „     │     0      │     0     │   9 ♂   │
 │                    │          │            │           │         │
 └────────────────────┴──────────┴────────────┴───────────┴─────────┘

 ┌────────────────────┬──────────┬─────┬───────────────┬─────────┐
 │    PYCNOGONIDA.    │ Teeth on │Legs.│Trunk-segments.│ Genital │
 │                    │   do.    │     │               │Openings.│
 ├────────────────────┼──────────┼─────┼───────────────┼────┬────┤
 │                    │          │     │               │ ♂  │ ♀  │
 ├────────────────────┼──────────┼─────┼───────────────┼────┼────┤
 │(=Cryptochelata=,   │          │     │               │    │    │
 │Sars)—              │          │     │               │    │    │
 │ DECOLOPODIDAE      │Four rows,│  5  │  Condensed,   │ 1, │ 1, │
 │                    │  simple  │     │  coalescent   │ 2, │ 2, │
 │                    │          │     │               │ 3, │ 3, │
 │                    │          │     │               │ 4, │4, 5│
 │                    │          │     │               │ 5, │    │
 │ COLOSSENDEIDAE     │Many rows,│  4  │  Coalescent   │ 1, │ 1, │
 │                    │  simple  │     │               │ 2, │ 2, │
 │                    │          │     │               │3, 4│3, 4│
 │                    │          │     │               │    │    │
 │ EURYCIDIDAE        │More than │  4  │Well segmented │3, 4│ 1, │
 │                    │ one row, │     │               │    │ 2, │
 │                    │ serrate  │     │               │    │3, 4│
 │                    │          │     │               │    │    │
 │ _Hannonia_         │Scattered │  4  │       „       │ „  │ „  │
 │                    │  spines  │     │               │    │    │
 │ AMMOTHEIDAE        │   Few,   │  4  │  Condensed,   │ „  │ „  │
 │                    │scattered,│     │   segmented   │    │    │
 │                    │serrate or│     │               │    │    │
 │                    │  smooth  │     │               │    │    │
 │ ? RHYNCHOTHORACIDAE│ Toothed  │  4  │       „       │ 4  │ 4  │
 │                    │tubercles │     │               │    │    │
 │                    │          │     │               │    │    │
 ├────────────────────┼──────────┼─────┼───────────────┼────┼────┤
 │(=Euchelata=, Sars)—│          │     │               │    │    │
 │ NYMPHONIDAE        │ One row, │ 4–5 │Well segmented │ 2, │ 1, │
 │                    │ serrate  │     │               │3, 4│ 2, │
 │                    │          │     │               │(5) │3, 4│
 │                    │          │     │               │    │(5) │
 │ PALLENIDAE         │    „     │  4  │       „       │(1, │ „  │
 │                    │          │     │               │2), │    │
 │                    │          │     │               │3, 4│    │
 │ PHOXICHILIDIIDAE   │ One row, │  4  │       „       │ 1, │ „  │
 │                    │  simple  │     │               │ 2, │    │
 │                    │          │     │               │3, 4│    │
 ├────────────────────┼──────────┼─────┼───────────────┼────┼────┤
 │(=Achelata=, Sars)  │          │     │               │    │    │
 │ PHOXICHILIDAE      │Scattered,│  4  │Well segmented │ 2, │ 1, │
 │                    │  simple  │     │               │3, 4│ 2, │
 │                    │          │     │               │    │3, 4│
 │ PYCNOGONIDAE       │  Small,  │  4  │  Segmented,   │ 4  │ 4  │
 │                    │irregular │     │   condensed   │    │    │
 └────────────────────┴──────────┴─────┴───────────────┴────┴────┘


                        CLASS PYCNOGONIDA.[429]

Marine Arthropoda, with typically seven (and very exceptionally eight)
pairs of appendages, of which none have their basal joints subservient
to mastication, the first three are subject to suppression, the first
(when present) are chelate, the second palpiform, the third ovigerous,
and the rest form ambulatory limbs, usually very slender and long; with
a suctorial proboscis, a limbless, unsegmented abdomen, and no manifest
respiratory organs.

[Illustration:

  FIG. 282.—_Decolopoda australis_, Eights. =A=, × 1: from a specimen
    obtained at the South Shetlands by the _Scotia_ Expedition. =B=,
    First appendage, or chelophore. (=A=, original; =B=, after Hodgson.)
]

=Fam. 1. Decolopodidae.=—Appendage I. dwarfed, but complete and chelate,
scape with two joints; II. 9–10–jointed; III. well developed in both
sexes, 10–jointed, the terminal joints with about four rows of teeth;
five pairs of legs, destitute of accessory claws; genital apertures on
all the legs (Bouvier).

_Decolopoda australis_, Eights[430] (1834), a remarkable form from the
South Shetlands, recently re-discovered by the _Scotia_ expedition. The
animal is large, seven inches or more in total span, in colour scarlet;
it was found in abundance in shallow water and cast upon the shore. The
body is greatly condensed, the proboscis is “clavate, arcuated
downwards,” and beset with small spines. A second Antarctic species, _D.
antarctica_, has been described by Bouvier. The presence of a fifth pair
of legs distinguishes _Decolopoda_ from all known Pycnogons, except
_Pentanymphon_. Stebbing would ally _Decolopoda_ with, or even include
it in, the Nymphonidae; but the presence of a second joint in the
chelophoral scape, the number of joints in, and the armature on, the
ovigerous legs, and the deflexed proboscis, are all characters either
agreeing with or tending towards those of the Eurycididae; while the
Colossendeidae would be very like _Decolopoda_ were it not for the
complete suppression of the chelophores. It seems convenient to
constitute a new family for this remarkable form.

=Fam. 2. Colossendeidae= (=Pasithoidae=, Sars).—Appendage I. absent in
adult; appendage II. very long, 10–jointed; appendage III. 10–jointed,
clawed, with many rows of teeth; auxiliary claws absent; segments of
trunk fused; proboscis very large, somewhat mobile; genital apertures,
in at least some cases, on all the legs.

_Pasithoe_, Goodsir (1842), which Sars assumes as the type of the
family, is here relegated to _Ammothea_.[431] _Colossendeis_, Jarszynsky
(1870) (_Anomorhynchus_, Miers (1881), _Rhopalorhynchus_, Wood-Mason
(1873)), remains as the only genus commonly accepted: large, more or
less slender short-necked forms; world-wide, principally Arctic,
Antarctic, and deep-sea; about twenty-five species.[432] The largest
species, _C. gigas_, Hoek, from great depths in the Southern Ocean, has
a span of about two feet. The North Atlantic _C. proboscidea_ and
Antarctic _C. australis_ are very closely related to one another.
Carpenter would retain the genus _Rhopalorhynchus_ for _R. kröyeri_,
W.-M. (Andamans), _R. clavipes_, Carp. (Torres Straits), and _R.
tenuissimus_, Haswell (Australia), all more or less shallow-water
species, excessively attenuated, with the second and third body-segments
elongated, the caudal segment excessively reduced, the club-shaped
proboscis on a slender stalk, and other common characters. _Pipetta
weberi_, Loman (1904), is a large and remarkable form from the Banda
Sea, apparently referable, in spite of certain abnormal features, to
this family; the proboscis is extraordinarily long and slender; the
palps have eight joints, the ovigerous legs eleven.

=Fam. 3. Eurycididae= (=Ascorhynchidae=, Meinert).—Appendage I. more or
less reduced; appendage II. 10–jointed (absent in _Hannonia_); appendage
III. 10–jointed, clawed, with more than one tow of serrated teeth;
proboscis movably articulated and more or less bent under the body;
auxiliary claws absent.

[Illustration:

  FIG. 283.—_Eurycide hispida_, Kr.; side view.
]

_Eurycide_, Schiödte (1857) (_Zetes_, Kröyer, 1845): Appendage I. with
two-jointed scape, without chelae in adult; one species (_E. hispida_,
(Kr.)), from the North Atlantic and Arctic, and two others from the East
Indies, recently described by Loman. _Barana arenicola_, Dohrn (1881),
is nearly allied. _Ascorhynchus_, G. O. Sars (1876) (_Gnamptorhynchus_,
Böhm, 1879; _Scaeorhynchus_, Wilson, 1881), very similar to _Eurycide_,
with which, according to Schimkewitsch, it should be merged, includes
large, smooth, elongated forms, with long neck and expanded frontal
region, and a long proboscis lacking the long scape that supports the
proboscis in _Eurycide_; about twelve species, world-wide, mostly
deep-water. _Barana castelli_, Dohrn, from Naples is akin to the
foregoing genera, but seems to deserve generic separation from _B.
arenicola_. _Ammothea longicollis_, Haswell, from Australia, is, as
Schimkewitsch has already remarked, almost certainly a _Eurycide_, as is
also, probably, _Parazetes auchenicus_, Slater, from Japan.

_Hannonia typica_, Hoek (1880), from Cape Town, is a remarkable form,
lately redescribed by Loman. The chelophores are much reduced, the palps
are absent; the ovigerous legs are 10–jointed, and clawed; the terminal
joints of the latter bear long straight spines, scattered over their
whole surface; the proboscis is borne on a narrow stalk, and sharply
deflexed. The eggs form a single flattened mass, as in _Pycnogonum_.
While the lack of palps would set this genus among the Pallenidae, the
remarkable proboscis seems to be better evidence of affinity with
_Ascorhynchus_ and _Eurycide_.[433]

_Nymphopsis_, Haswell (1881), is a genus of doubtful affinities, placed
here by Schimkewitsch. The first appendage is well-developed and
chelate; the palps are 9–jointed, the ovigerous legs are 7–jointed, none
of the joints being provided with the compound spines seen in _Nymphon_
and _Pallene_. It is perhaps an immature form. Schimkewitsch has
described another species, _N. korotnevi_, and Loman a third, _N.
muscosus_, both from the East Indies.

=Fam. 4. Ammotheidae.=—Akin to Eurycididae in having the proboscis more
or less movably jointed to the cephalic segment, and appendage I.
reduced, non-chelate in the adult; the body is compact and more or less
imperfectly segmented; appendage II. 4–9–jointed; appendage III.
clawless, and the number of joints sometimes diminished, with a sparse
row of serrated spines; auxiliary claws usually present.

_Ammothea_, Leach (1815) (including _Achelia_, Hodge (1864) = the old
non-chelate individuals): appendage I. very small, 2–jointed; appendage
II. 8–9–jointed; caudal segment fused with last body-segment; about
eighteen species, four from the South Seas, two or three from the East
Indies, the rest mostly Mediterranean and North Atlantic, in need of
revision. _Ammothea longipes_, Hodge, is the young of _Achelia hispida_,
Hodge; and _Ammothea magnirostris_, Dohrn, is apparently the same
species. _A. fibulifera_, Dohrn, seems identical with _Achelia
echinata_, Hodge (of which _A. brevipes_, Hodge, is the young), and so
probably is _A. achelioides_, Wilson; _Endeis didactyla_, Philippi
(1843), is very probably the same species. _A. uniunguiculata_, Dohrn (?
_Pariboea spinipalpis_, Philippi (1843)), has no auxiliary claws.
_Leionymphon_, Möbius (1902), contains nine Antarctic forms, allied to
_Ammothea_ (including _A. grandis_, Pfeffer, and _Colossendeis gibbosa_,
Möb., which two are probably identical), with characteristic transverse
ridges on the body, a large proboscis, a 9–jointed palp, and somewhat
peculiar ovigerous legs. _Cilunculus_, _Fragilia_, and _Scipiolus_ are
new genera more or less allied to _Leionymphon_, described by Loman
(1908) from the Siboga Expedition.[434] _Tanystylum_, Miers (1879)
(including _Clotenia_, Dohrn (1881), and _Discoarachne_, Hoek (1880)),
has appendage I. reduced to a single joint or a small tubercle, and
appendage II. 4–6–jointed; world-wide; about eight species. _Austrodecus
glacialis_ and _Austroraptus polaris_ are two allied Antarctic species,
described by Hodgson (1907), the former a curious little form with a
pointed, weevil-like proboscis, no chelophores, and 6–jointed palp.
_Trygaeus communis_, Dohrn (1881), from Naples, has a 7–jointed, and
_Oorhynchus aucklandiae_, Hoek (1881), a 9–jointed palp; the former has
only seven joints in the ovigerous leg. _Lecythorhynchus armatus_, Böhm
(1879), with rudimentary 2–jointed chelophores, and _L._ (_Corniger_)
_hilgendorfi_, Böhm, with small tubercles in their place, both from
Japan, have also 9–jointed palps: the former, at least, is apparently an
_Ammothea_. Several insufficiently described genera, _Phanodemus_, Costa
(1836), _Platychelus_, Costa (1861), _Oiceobathes_, Hesse (1867), and
_Böhmia_, Hoek (1880), seem to be referable to this group; all have
chelate mandibles, and may possibly be based on immature forms.

Goodsir’s _Pasithoe vesiculosa_[435] is, in my opinion, undoubtedly
_Ammothea hispida_, Hodge, and so also, I believe, is his _Pephredo
hirsuta_; _P. umbonata_, Gould[436] (Long Island Sound), is, with as
little doubt, _Tanystylum orbiculare_, Wilson.

=Fam. 5. Rhynchothoracidae.=—The animal identified by Dohrn as
_Rhynchothorax mediterraneus_, Costa (1861), is a minute and very
remarkable form, without chelophores, with large 8–jointed palps,
reduced by fusion to five joints, and 10–jointed, clawed ovigerous legs,
which last are provided on the last five joints with peculiar toothed
tubercles. The general aspect of the body is somewhat like that of an
_Ammothea_, which genus it resembles in the ventral insertion of the
ovigerous legs and the somewhat imperfect segmentation of the body. It
differs from Ammotheidae in the possession of a claw on appendage III.
It is highly peculiar in the structure of the mouth, in having a long
forward extension of the oculiferous tubercle jutting out over the
proboscis, in the extreme shortness of the intestinal caeca and ovaries
which scarcely extend into the legs, and in the absence of cement-glands
from the fourth joint of the legs; these last are present only in the
third joint of the penultimate legs. A single pair of generative
orifices are found on the last legs. A second species, _R. australis_,
Hodgson, comes from the Antarctic.

[Illustration:

  FIG. 284.—_Rhynchothorax mediterraneus_, Costa. =A=, Body and bases of
    legs; =B=, terminal joints of palp. (After Dohrn.)
]

=Fam. 6. Nymphonidae.=—Appendage I. well-developed, chelate; II.
well-developed, usually 5–jointed; III. well-developed in both sexes,
usually 10–jointed, the terminal joints with one row of denticulated
spines.

_Nymphon_, Fabr. (1794), about forty-five recognised species, of which
some are but narrowly defined. Closely allied are _Chaetonymphon_, G. O.
Sars (1888), including thick-set, hairy species, about eight in number,
from the North Atlantic, Arctic, and Antarctic; and _Boreonymphon_, G.
O. Sars (1888), with one species (_B. robustum_, Bell, Fig. 276), also
northern, in which the auxiliary claws are almost absent. _Nymphon
brevicaudatum_, Miers (= _N. horridum_, Böhm), an extraordinary hispid
form from Kerguelen,[437] is also peculiar. _Pentanymphon_, Hodgson
(1904), from the Antarctic (circumpolar), differs in no respect save in
the presence of a fifth pair of legs; one species.

The only other genus is _Paranymphon_, Caullery (1896) (one species,
Gulf of Gascony, West of Ireland, Greenland), in which the palp is
(6–)7–jointed, the ovigerous leg 8–jointed, and the auxiliary claws are
absent.

=Fam. 7. Pallenidae.=—As in _Nymphon_, but appendage II. absent or
rudimentary.

[Illustration:

  FIG. 285.—_Pallene brevirostris_, Johnston, ♀, Plymouth.
]

_Pallene_, Johnston (1837): about ten species (Mediterranean, North
Atlantic, Arctic, Australia). _P. languida_, Hoek, Australia, lacks
auxiliary claws, and is otherwise distinct; but _P. novaezealandiae_, G.
M. Thomson, is typical. _Pseudopallene_, Wilson (1878):[438] appendage
III. clawed; auxiliary claws absent; four (or more) species (North
Atlantic, Arctic, Antarctic). _P._ (_Phoxichilus_) _pygmaea_, Costa
(1836), and _P. spinosa_, Quatref., seem to belong to this genus or to
_Pallene_. _Cordylochele_, G. O. Sars (1888): closely allied, but with
front of cephalic segment much expanded and chelae remarkably swollen,
includes three very smooth, elongated, northern species, to which
Bouvier has added one from the Antarctic; _Pallene laevis_, Hoek, from
Bass’s Straits, is somewhat similar. _Neopallene_, Dohrn (1881): as in
_Pallene_, but with a rudimentary second appendage in the female, and no
generative aperture on the last leg in the male (one species,
Mediterranean). _Parapallene_, Carpenter (1892): as in _Pallene_, but
without auxiliary claws, and with the two last segments of the trunk
(which in _Pallene_ are coalesced) independent (about ten species, East
Indies and Australia); _Pallene grubii_, Hoek (_Phoxichilidium_ sp.,
Grube, 1869), is probably congeneric. _Pallenopsis_, Wilson (1881):
appendage I. 2–jointed; appendage II. rudimentary, 1–jointed; appendage
III. clawless; auxiliary claws present; slender forms, including some
formerly referred to _Phoxichilidium_; about fifteen species,
world-wide. _Pallene dimorpha_, Hoek, from Kerguelen, with 4–jointed
palps, deserves a new generic appellation. _P. longiceps_, Böhm, from
Japan, with rudimentary 2–jointed palps in the male, is also peculiar.

[Illustration:

  FIG. 286.—_Phoxichilidium femoratum_, Rathke, Britain. =A=, The animal
    with its legs removed; =B=, leg and chela.
]

=Fam. 8. Phoxichilidiidae.=—Appendage I. well-developed; II. absent;
III. present only in the male, having a few simple spines in a single
row. The last character is conveniently diagnostic, but nevertheless the
Phoxichilidiidae come very near to the Pallenidae, with which, according
to Schimkewitsch and others, they should be merged; the two families
resemble one another in the single row of spines on the ovigerous legs
and in the extension of the cephalic segment over the base of the
proboscis.

_Phoxichilidium_, M.-E. (1840): appendage III. 5–jointed; five or six
species (Mediterranean, North Atlantic, Arctic, Australia, Japan).
_Anoplodactylus_, Wilson (1878): appendage III. 6–jointed; auxiliary
claws absent or very rudimentary; about twelve species, cosmopolitan, of
which many were first referred to _Phoxichilidium_. _A. neglectus_,
Hoek, comes from 1600 fathoms off the Crozets. _Oomerus stigmatophorus_,
Hesse (1874), from Brest, seems to belong to one or other genus, but is
unrecognisable. _Anaphia_, Say (1821), is in all probability identical
with _Anoplodactylus_, and if so the name should have priority.
_Halosoma_, Cole (1904), is an allied genus from California.

[Illustration:

  FIG. 287.—_Anoplodactylus petiolatus_, Kr., Britain. =A=, Dorsal view;
    =B=, side view.
]

=Fam. 9. Phoxichilidae.=[439]—Appendage I. and II. absent; appendage
III. present only in the males, 7–jointed, with minute scattered spines;
auxiliary claws well-developed; body and legs slender. The only genus is
_Phoxichilus_ (auctt., non Latreille, _Chilophoxus_, Stebbing, 1902);
the type is _P. spinosus_, Mont. (non Quatrefages), from the N.
Atlantic, and _P. vulgaris_, Dohrn, _P. charybdaeus_, Dohrn, and _P.
laevis_, Grube, are all very similar. _Endeis gracilis_, Philippi
(1843), is probably identical with _P. spinosus_, or one of its close
allies. There are also known _P. meridionalis_, Böhm, _P. mollis_,
Carp., and _P. procerus_, Loman, from the East Indies; _P. australis_,
Hodgson, from the Antarctic; _P. böhmii_, Schimk., of unknown locality;
and forms ascribed to _P. charybdaeus_ by Haswell and by Schimkewitsch
from Australia and Brazil.

=Fam. 10. Pycnogonidae.=—Appendages I. and II. absent; appendage III.
present only in the male, 9–jointed, with small, simple spines;
auxiliary claws absent or rudimentary; body and legs short, thick-set.

The only genus is _Pycnogonum_, Brünnich (1764) (_Polygonopus_, Pallas,
1766); the type is _P. littorale_, Ström, of the N. Atlantic (0–430
fathoms), to which species have also been ascribed forms from various
remote localities, _e.g._ Japan, Chile, and Kerguelen. _P.
crassirostre_, G. O. Sars, a northern and more or less deep-sea form, is
distinct, and so also are _P. nodulosum_ and _P. pusillum_, Dohrn, from
Naples. _P. stearnsi_, Ives, from California, is like _P. littorale_,
except for the rostrum, which resembles that of _P. crassirostre_. _P.
magellanicum_, Hoek, _P. magnirostre_, Möbius, both from the Southern
Ocean; _P. microps_, Loman, from Natal, and four others described by
Loman from the East Indies, are the other authenticated species. Of _P.
philippinense_, Semper, I know only the bare record; and _P. australe_,
Grube, is described only from a larval form with three pairs of legs.
_P. orientale_, Dana (first described as _Astridium_, n.g.), is also
described from an immature specimen, and more resembles a _Phoxichilus_.


                        =The British Pycnogons.=

Dr. George Johnston,[440] the naturalist-physician of Berwick-on-Tweed,
Harry Goodsir,[441] brother of the great anatomist, who perished with
Sir John Franklin, and George Hodge[442] of Seaham Harbour, a young
naturalist of singular promise, dead ere his prime, were in former days
the chief students of the British Pycnogons. Of late, Carpenter[443] has
studied the Irish species; and the cruises of the _Porcupine_, _Triton_,
and _Knight Errant_ have given us a number of deep-water species from
the verge of the British area.

In compiling the following list, I have had the indispensable advantage
of access to Canon Norman’s collection, and the still greater benefit of
his own stores of endless information.[444]

  _Pseudopallene circularis_, Goodsir: Firth of Forth.

  _Phoxichilidium femoratum_, Rathke (_P. globosum_, Goodsir; _Orithyia
    coccinea_, Johnston) (Figs. 270, B; 286): East and West coasts,
    Shetland, Ireland.

  _Anoplodactylus virescens_, Hodge (? _Phoxichilidium olivaceum_,
    Gosse): South coast.

  _A. petiolatus_, Kr. (Figs. 270, C; 275, B; 287) (_Pallene attenuata_
    and _pygmaea_, Hodge; _Phoxichilidium exiguum_ and _longicolle_,
    Dohrn): Plymouth, Firth of Forth, Cumbrae, Irish coasts.

  _Ammothea_ (_Achelia_) _echinata_, Hodge (Fig. 265, B; 274, 4; 275,
    E): Plymouth, Channel Islands, Isle of Man, Cumbrae, Durham (Hodge),
    West of Ireland. We have not found it on the East of Scotland. _A.
    brevipes_, Hodge, is presumed to be the young. Two of Dohrn’s
    Neapolitan species, _A. fibulifera_ and _A. franciscana_, are in my
    opinion not to be distinguished from one another, nor from the
    present species.

  _A. hispida_, Hodge (Fig. 266, C) (_A. longipes_, Hodge (_juv_); _A.
    magnirostris_, Dohrn;? _Pasithoe vesiculosa_, Goodsir;? _Pephredo
    hirsuta_, Goodsir): Cornwall and Devon (Hodge and Norman), Jersey.
    The form common on the East of Scotland would seem to be this
    species. The Mediterranean _A. magnirostris_, Dohrn, appears to be
    identical.

  _A. laevis_, Hodge: Cornwall (Hodge), Devon (Norman), Jersey (Sinel).

  _Tanystylum orbiculare_, Wilson (_Clotenia conirostre_, Dohrn):
    Donegal (Carpenter).

  _Phoxichilus spinosus_, Mont. (Fig. 265, C; 270, A; 275, C): South
    Coast, Moray Firth, Firth of Clyde, Ireland. A smaller and less
    spiny form occurs, which Carpenter records as _P. laevis_, Grube,
    but Norman unites the two under the name of _Endeis spinosus_
    (Mont.).

  _Pycnogonum littorale_, Ström (Fig. 262): on all coasts, and to
    considerable depths (150 fathoms, West of Ireland).

  _Nymphon brevirostre_, Hodge (_N. gracile_, Sars) (Figs. 263, 264,
    267, A; 272, 274, 3): common on the East Coast; Herm (Hodge),
    Dublin, Queenstown (Carpenter). Our smallest species of _Nymphon_.

  _N. rubrum_, Hodge (_N. gracile_, Johnston; _N. rubrum_, G. O. Sars):
    common on the East Coast; Oban (Norman), Ireland (Carpenter).

  _N. grossipes_, O. Fabr., Johnston (_N. johnstoni_, Goodsir):
    Northumberland, East of Scotland, Orkney, etc., not uncommon.

  _N. gracile_, Leach (_N. gallicum_, Hoek; ♂ _N. femoratum_, Leach):
    South of England, West of Scotland, and Ireland.

  _N. strömii_, Kr. (_N. giganteum_, Goodsir) (Figs. 273, 274, 2): East
    Coast, from Holy Island to Shetland.

  _Chaetonymphon hirtum_, Fabr. (Fig. 274, 1): Northumberland (Hodge),
    Margate (Hoek), East of Scotland, and Ireland, not uncommon. There
    seems to be no doubt that British specimens agree with this species
    as figured and identified by Sars. _N. spinosum_, Goodsir (East of
    Scotland, Goodsir; Belfast, W. Thompson), is, according to Norman,
    the same species. Sars’ Norwegian specimens figured under the latter
    name are not identical, and have been renamed by Norman _C.
    spinosissimum_, but are said by Meinert and Möbius to be identical
    with _C. hirtipes_, Bell.

  Hodge (1864) records _Nymphon mixtum_, Kr., and _N. longitarse_, Kr.,
    from the Durham coast. His full list of the recorded species of
    other authors also includes the following doubtful or unrecognised
    species: _N. pellucidum_, _N. simile_, and _N. minutum_, all of
    Goodsir.

  _Pallene brevirostris_, Johnston (_P. empusa_, Wilson;? _P. emaciata_,
    Dohrn) (Figs. 275, A; 285): all coasts. Examples differ considerably
    in size and proportions, as do Dohrn’s Neapolitan species one from
    another. We have specimens from the Sound of Mull that come very
    near, and perhaps agree with, Sars’ _P. producta_, a species that
    scarcely differs from _P. brevirostris_, save in its greater
    attenuation; the same species has also been recorded from Millport
    and from Port Erin.

  _P. spectrum_, Dohrn: Plymouth (A. H. Norman).


Besides the above, all of which are littoral or more or less
shallow-water species, we have another series of forms, or, to speak
more correctly, we have two other series of forms, from the deep
Atlantic waters within the British area. In the cold area of the Faeroe
Channel we have _Boreonymphon robustum_, Bell; _Nymphon elegans_,
Hansen; _N. sluiteri_, Hoek; _N. stenocheir_, Norman; _Colossendeis
proboscidea_, Sabine; _C. angusta_, Sars. In the warm waters south and
west of the Wyville-Thomson ridge we have _Chaetonymphon spinosissimum_,
Norman; _Nymphon gracilipes_, Heller (non Fabr.); _N. hirtipes_, Bell;
_N. longitarse_, Kr.; _N. macrum_, Wilson; _Pallenopsis tritonis_, Hoek
(= _P. holti_, Carpenter); _Anoplodactylus oculatus_, Carpenter, and _A.
typhlops_, G. O. Sars; and to the list under this section Canon Norman
has lately made the very interesting addition of _Paranymphon spinosum_,
Caullery, from the _Porcupine_ Station XVII., S.S.E. of Rockall, in 1230
fathoms. Lastly, and less clearly related to temperature, we have
_Chaetonymphon tenellum_, Sars; _N. gracilipes_, Fabr.; _N.
leptocheles_, Sars; _N. macronyx_, Sars; _N. serratum_, Sars; and
_Cordylochele malleolata_, Sars.

Of the species recorded in the above list as a whole, _Anoplodactylus
virescens_, _Nymphon gracile_, and _Pallene spectrum_ reach their
northern limit in the southern parts of our own area; _Ammothea
echinata_, _Anoplodactylus petiolatus_, _Pallene brevirostris_, and
_Phoxichilus spinosus_ (or very closely related forms) range from the
Mediterranean to Norway, the last three also to the other side of the
Atlantic; _Nymphon brevirostre_ and _N. rubrum_ range from Britain,
where they are in the main East Coast species, to Norway. Of the
Atlantic species, other than the Arctic ones, the majority are known to
extend to the New England coast.




                                 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.


 _Abalius_, _312_

 Abdomen, of Malacostraca, 110;
   of _Acantholithus_, =178=;
   of _Birgus_, =176=;
   of _Cenobita_, =176=;
   of _Dermaturus_, =178=;
   of _Hapalogaster_, =178=;
   of _Lithodes_, =178=;
   of _Pylopagurus_, =178=;
   of Trilobites, 235;
   of Scorpions, 297;
   of Pedipalpi, 309;
   of Spiders, 317;
   of Palpigradi, 422;
   of Solifugae, 426;
   of Pseudoscorpions, 431;
   of Podogona, 440;
   of Phalangidea, 440, 443;
   of Acarina, 457;
   of Pentastomida, 489;
   of Pycnogonida, 502

 Abdominal glands, of Chernetidea, 432

 Abyssal region (marine), 204;
   (lacustrine), 209

 _Acantheis_, _418_

 _Acanthephyra_, _163_

 Acanthephyridae, _163_

 _Acanthoctenus_, _415_

 _Acanthodon_, _388_

 _Acanthogammarus_, _138_

 _Acantholeberis_, _53_

 _Acantholithus_, _181_;
   _A. hystrix_, =178=

 _Acanthophrynus_, _313_

 Acari, 454 (= Acarina, _q.v._)

 Acaridea, 454 (= Acarina, _q.v._)

 Acarina, _258_, _454_ f.;
   parasitic, 455;
   external structure, 457;
   spinning organs, 457;
   internal structure, =459=;
   metamorphosis, 462;
   classification, 464

 _Acaste_, _249_

 _Accola_, _390_

 _Acerocare_, _247_

 Achelata, _529_

 _Achelia_, _534_;
   _A. longipes_, =506=

 _Achtheres_, _75_;
   _A. percarum_, =75=

 Acidaspidae, _251_

 _Acidaspis_, 226, 227, 230, 231, 235, 241, _251_;
   _A. dufrenoyi_, =250=;
   _A. tuberculata_, larva, =240=;
   _A. verneuili_, 231;
   _A. vesiculosa_, 231

 Aciniform glands, =335=, 349

 _Acoloides saitidis_, 367

 _Acroperus_, _53_;
   _A. leucocephalus_, =52=

 _Acrosoma_, _410_

 Acrothoracica, _92_

 _Actaea_, _191_;
   habitat, 198

 Actinopodinae, _387_

 _Actinopus_, _387_

 Aculeus, of scorpion, 303

 _Admetus_, _313_

 Aegidae, _126_

 _Aegisthus_, _61_

 _Aeglea laevis_, _169_;
   distribution, 212

 Aegleidae, _169_

 _Aeglina_, 227, _249_;
   _Ae. prisca_, =248=

 _Agelena_, _416_;
   _A. brunnea_, 367;
   _A. labyrinthica_, 352, 353, 378, 380, 381, 416;
   _A. naevia_, 339

 Agelenidae, 325, 352, 353, _415_

 Ageleninae, _416_

 Aggregate glands, 335, 349

 _Aglaspis_, _279_

 _Agnathaner_, _66_

 Agnathonia, _529_

 Agnostidae, _244_

 Agnostini, _243_

 _Agnostus_, 222, 223, 225, 231, 234, _245_;
   _A. integer_, =245=

 _Agraulos_, _247_

 _Agroeca_, _397_;
   _A. brunnea_, cocoon, =358=

 _Albunea_, _171_;
   respiration, 170;
   distribution, 201

 Albuneidae, _171_

 _Alcippe_, _92_;
   _A. lampas_, =92=, 93

 Alcock, on Oxyrhyncha, 192;
   on phosphorescence, 151

 _Alepas_, _89_

 Alima, larva of _Squilla_, 143

 Alimentary canal, of Crustacea, 14;
   of Phyllopoda, 28;
   of Cladocera, 42;
   of _Squilla_, 142;
   of Malacostraca, 110;
   of
 Trilobites, 222;
   of Arachnida, 256;
   of _Limulus_, 268;
   of Scorpions, 304;
   of Pedipalpi, 310;
   of Spiders, 329;
   of Solifugae, 427;
   of Pseudoscorpions, 134;
   of Phalangidea, 444;
   of Acarina, 459;
   of Tardigrada, 480;
   of Pentastomida, 491;
   of Pycnogous, 513

 _Alitropus_ (Aegidae), habitat, 211

 Allman, on larvae of Pycnogons, 523

 _Alloptes_, _466_

 _Alona_ (including _Leydigia_, _Alona_, _Harporhynchus_,
    _Graptoleberis_), _53_

 _Alonopsis_, _53_

 Alpheidae, _163_;
   habitat, 198

 _Alpheus_, _163_;
   reversal of regeneration, 156

 Alveolus, of palpal organ of Spiders, 322

 _Amaurobius_, _399_;
   _A. fenestralis_, 399;
   _A. ferox_, 399;
   _A. similis_, 399;
   spinnerets, =326=

 _Amblyocarenum_, _388_

 _Amblyomma_, _470_;
   _A. hebraeum_, 456, 470

 Amblypygi, _312_

 _Ammothea_, 505, _534_;
   _A. achelioides_, 534;
   _A. brevipes_, 541;
   _A. echinata_, =505=, =509=, =510=, 534, 541, 542;
   _A. fibulifera_, =522=, 534, 541;
   _A. franciscana_, 541;
   _A. grandis_, 534;
   _A. hispida_, 534, 535, 541;
   _A. laevis_, 541;
   _A. longicollis_, 533;
   _A. longipes_, =506=, 534, 541;
   _A. magnirostris_, 534, 541;
   _A. typhlops_, 542;
   _A. uniunguiculata_, 534

 Ammotheidae, _534_

 _Amopaum_, _452_

 Ampharthrandria, _61_

 Amphascandria, _57_

 _Amphion_, _251_

 Amphipoda, _136_ f.;
   pelagic, 202;
   fresh water, 211

 Ampullaceal glands, 335, 349

 Ampycini, _243_

 _Ampyx_, 231, _245_;
   _A. roualti_, =230=

 Anabiosis, in Tardigrada, 484

 _Analges_, 455, _466_

 Analgesinae, _466_

 _Ananteris_, _306_

 _Anaphia_, _539_

 Anaspidacea, _115_;
   distribution, 211, 217

 Anaspidae, _89_

 _Anaspides_, _115_, 117;
   relation to Schizopoda, 112;
   distribution, 211;
   _A. tasmaniae_, 115, =116=;
   habitat, 211

 Anaspididae, _115_

 _Anelasma squalicola_, _89_

 _Anelasmocephalus_, _452_

 _Angelina_, _247_

 _Anisaspis bacillifera_, _387_

 Anisopoda, _122_

 _Anomalocera pattersoni_, =60=;
   distribution, 202, 203

 Anomopoda, _51_

 _Anomorhynchus_, _532_

 Anomura, _167_;
   relation to Thalassinidea, 167

 _Anoplodactylus_, 511, _538_;
   _A. lentus_, 524;
   _A. neglectus_, 539;
   _A. oculatus_, 542;
   _A. petiolatus_, =508=, =510=, =539=, 541, 542;
   _A. virescens_, 540, 542

 _Anopolenus_, _247_

 Antarctic zone (marine), 200

 Antarctica, evidence on, 200, 217

 Antennae, of Crustacea, 5, 8;
   of Phyllopoda, 24;
   of Cladocera, 37;
   of Copepoda, 55;
   of Cirripedia, 81 f.;
   of Ostracoda, 107;
   of Malacostraca, 110;
   of Anomura, 168;
   of _Corystes cassivelaunus_, 170, 183, 189;
   used in respiration, 170;
   of Trilobites, 237

 Antennary gland, 13 (= green gland, _q.v._)

 _Anthrobia_, _406_;
   _A. mammouthia_, 334, 366

 _Anthura_, _124_

 Anthuridae, _124_

 Ants and spiders, 370

 _Anyphaena accentuata_, _397_

 Aphantochilinae, _414_

 _Aphantochilus_, _414_

 Apoda, _94_

 Apodidae, 19, 21, 22, 23, 27, 28, 29, 31, _36_, 241

 _Aponomma_, _470_

 Appendages (_incl._ legs, limbs), of Crustacea, 7;
   of Entomostraca, 18;
   of Phyllopoda, 24;
   of Cladocera, 40;
   of Copepoda, 55;
   of Cirripedia, 80 f.;
   of Ostracoda, 107;
   of Malacostraca, 110;
   of _Nebalia_, 111;
   of Eumalacostraca, 113;
   of _Anaspides_, 115;
   of Mysidacea, 118 f.;
   of Cumacea, 120;
   of Isopoda, 121 f.;
   of Amphipoda, 136 f.;
   of Stomatopoda, 142;
   of Euphausiacea, 144 f.;
   of Decapoda, 152;
   of Macrura, 153;
   of their larvae, 159;
   of Anomura, 167 f.;
   of _Birgus_, 175;
   of Brachyura, 181 f.;
   alterations caused by parasites, 100 f.;
   by hermaphroditism, 102 f.;
   of Trilobita, 236, =237=;
   of Arachnida, 255 f.;
   of _Limulus_, =262=, 263;
   of _Eurypterus_, 285 f.;
   of Scorpions, 301, 303;
   of Pedipalpi, 309;
   of Spiders, 319;
   of Palpigradi, 422;
   of Solifugae, 426;
   of Pseudoscorpions, 432;
   of Podogona, 440;
   of Phalangidea, 443;
   of Acarina, 458;
   of Tardigrada, 479;
   of Pentastomida, 493;
   of Pycnogons, 503 f.

 _Apseudes spinosus_, =123=

 Apseudidae, _122_

 Apstein, 335

 _Apus_, 21, 23, 25, 28, 30, 32, 34, _36_, 221, 242, 243;
   segmentation, 6;
   _A. australiensis_, 36;
   _A. cancriformis_, 36;
   habitat, 34

 Arachnida, introduction to, 255;
   segmentation of body, 255–6;
   primitive, 256–7;
   coxal glands, 257;
   endosternite, 257;
   sense-organs, 257;
   classification, 258

 Araneae, _258_, _314_ f.

 Araneida, _314_

 Araneina, _314_

 _Araneus_, 408 n.

 _Aratus pisonii_, 195

 _Arbanitis_, _388_

 _Archaeolepas_, _84_;
   _A. redtenbacheri_, =84=

 _Archea_, _411_;
   _A. paradoxa_, 383;
   _A. workmani_, 411

 Archeidae, 321, _411_

 _Archisometrus_, _306_

 Arctic zone, 199

 Arcturidae, _127_

 _Arcturus_, _127_

 Arcyinae, _410_

 _Arcys_, _410_

 _Arethusina_, 223, 230, _251_;
   _A. konincki_, =250=

 _Argas_, 457, _469_;
   _A. persicus_, 469;
   _A. reflexus_, 469

 Argasidae, _469_

 _Arges_, _252_

 _Argiope_, _408_;
   _A. aurelia_, 340, =379=;
   _A. bruennichi_, 408;
   _A. cophinaria_, 349, 365;
   _A. trifasciata_, 408

 Argiopidae, 406 n.

 Argiopinae, _408_

 Argulidae, _76_

 _Argulus foliaceus_, =77=

 _Argyrodes_, _402_;
   _A. piraticum_, 367;
   _A. trigonum_, 367

 Argyrodinae, _402_

 _Argyroneta_, 336, _415_;
   _A. aquatica_, 357, 415

 _Ariadna_, _395_

 _Ariamnes_, _402_;
   _A. flagellum_, =318=

 _Arionellus_, _247_

 _Aristaeus_, _162_;
   _A. crassipes_, 159;
   _A. coruscans_, phosphorescence, 151

 _Armadillidium_, _129_

 _Artema_, _401_

 _Artemia_, 23, 24, _35_;
   _A. fertilis_, anal region, =23=;
   head, =26=;
   limb, 27;
   _A. salina_, 23, 33, 36;
   _A. urmiana_, 23

 _Arthrolycosa antiqua_, 383

 Arthropoda, 4;
   segmentation, 7;
   a natural group, 17

 Arthrostraca, _121_

 _Asagena_, _404_

 _Asaphellus_, _249_

 Asaphidae, _249_

 Asaphini, _243_

 _Asaphus_, 222, 225, 227, 229, 235, 236, _249_;
   _A. cornigerus_, 227;
   _A. fallax_, eye, =228=;
   _A. kowalewskii_, 227;
   _A. megistos_, 236;
   _A. platycephalus_, 236

 _Ascidicola rosea_, _66_

 Ascidicolidae, _66_

 Asconiscidae, _130_

 _Ascorhynchus_, 505, _533_;
   _A. abyssi_, =506=, =509=, 519;
   _A. cryptopygius_, 513 n.;
   _A. minutus_, 517;
   _A. ramipes_, 513 n.

 Ascothoracica, _93_

 Asellidae, _128_

 Asellota, _127_

 _Asellus_, _127_;
   habitat, 209, 211;
   _A. aquaticus_, 127, 209;
   _A. cavaticus_, 209, 210;
   _A. forelii_, 209

 _Aspidoecia_, _76_

 Astacidae, _157_;
   distribution, 213, 216

 _Astacoides_, _157_;
   distribution, 213

 _Astacopsis_, _157_;
   distribution, 213;
   _A. franklinii_, 214

 _Astacus_, 104, _157_;
   appendages, =10=;
   distribution, 213;
   hermaphroditism, 104

 _Astacus gammarus_ (= _Homarus vulgaris_), _154_

 _Asterocheres violaceus_, =67=

 Asterocheridae, _67_

 _Asterope oblonga_, =108=

 _Astia_, _421_;
   _A. vittata_, =381=

 Astigmata, _465_

 _Astridium_, _540_

 _Atax_, 462, _472_;
   _A. alticola_, =472=;
   _A. bonzi_, 472

 Atelecyclidae, _190_

 _Atelecyclus_, _191_;
   respiration, 189

 _Atops_, _247_

 Attidae, 376, 381, _419_

 _Attus_, _421_;
   _A. pubescens_, 372, 421;
   _A. saltator_, 372, 421

 _Atya_, _163_

 _Atyephyra_, _163_;
   habitat, 210

 _Atyidae_, 159, _163_;
   distribution, 212

 Atypidae, _390_

 _Atypoides_, _391_

 _Atypus_, _391_;
   _A. abboti_, 356;
   _A. affinis_, 356, =391=;
   _A. beckii_, 391

 Auditory organ, of _Anaspides_, 116;
   of Decapoda, 153;
   of Mysidae, 119

 _Augaptilus filigerus_, _59_

 _Austrodecus glacialis_, _535_

 _Austroraptus polaris_, _535_

 Autotomy, 155

 _Avicularia_, _389_

 Aviculariidae, 316, 327, _386_;
   bite of, 365;
   poisonous hairs of, 365

 Aviculariinae, _389_

 Axial furrows, 223


 Baglivi, 361

 Baikal, Lake, Crustacea of, 212

 _Balanus_, _91_;
   _B. porcatus_, shell, =90=;
   _B. tintinnabulum_, 91;
   anatomy, =90=

 _Ballus variegatus_, =420=

 _Barana_, 506, 513, _533_;
   _B. arenicola_, 512, 513, 533;
   _B. castelli_, 512, 513 n., 533

 Barnacles, origin of term, 79

 Barrande, J., on development of Trilobites, 238;
   on their classification, 243

 _Barrandia_, _249_

 Barrois, 435 n.

 _Barrus_, _429_

 Barychelinae, _389_

 Basse, on Tardigrada, 481

 Baster, Job, 503

 Bates, 373

 _Bathynomus giganteus_, _126_;
   habitat, 205

 _Bathynotus_, _247_

 _Bathyphantes_, _406_

 _Bdella lignicola_, =471=

 Bdellidae, 458, _471_

 Beecher, C. E., on facial sutures of _Agnostus_ and _Olenellus_, 225;
   on development of Trilobites, 238;
   on their classification, 243

 Beetle-mites, 467

 Beetle-parasites, 470

 _Belinurus_, 275, 279;
   _B. reginae_, =278=

 _Belisarius_, _308_

 Belt, 368, 371

 _Beltina_, 283 n.

 Bernard, 311, 424, 426, 433 n., 434 n.

 Bertkau, 323, 365, 395 n.

 Beyrich, E., on facial suture of _Trinucleus_, 226

 Billings, E., on appendages of Trilobites, 236

 Bipolarity, 200

 Birds and Spiders, 370

 Birds’ feather Mites, 466

 _Birgus_, _181_;
   _B. latro_, habits, 174;
   structure, =175=, =176=

 Black Corals, Cirripedia parasitic on, 93, 94

 Blackwall, 348, 359 n., 365, 368, 385

 Blindness, in Crustacea, 149, 209, 210;
   in Spiders, 334

 Blood, haemoglobin supposed in, 30, 68

 Boas, on classification of Malacostraca, 113

 _Boeckella_, distribution, 216

 _Boeckia_, _138_

 _Böhmia_, _535_

 _Bolocera_, _Pycnogonum_ with, 524

 _Bolyphantes_, _406_

 Bomolochidae, _71_

 _Bomolochus_, _71_, =72=

 Bon, 360

 Bont-tick, 456

 _Boophilus_, 456, _469_;
   _B. australis_, capitulum of, =468=

 Bopyridae, 130, _133_

 Bopyrina, 129, 130, _132_

 _Bopyrus fougerouxi_, _133_;
   male, =133=;
   adult female, =134=

 Bopyrus larva, of Bopyrina, 129, 133

 _Boreomysis_, _120_;
   _B. scyphops_, distribution, 201

 _Boreonymphon_, _536_;
   _B. robustum_, =506=, 507, =511=, 512, 542

 _Bosmina_, =52=, _53_;
   occurrence in Southern hemisphere, 216;
   _B. longirostris_, habitat, 206

 Bosminidae, _53_;
   appendages, 41;
   alimentary canal, 42

 Bothriuridae, 306, _308_

 _Bothriurus_, _308_

 Bouvier, 528 n.

 Boys, 348, 360, 376

 _Brachybothrium_, _391_

 _Brachymetopus_, _251_

 _Brachythele_, _390_

 Brachyura, _181_;
   eyes, 150

 Branchiae (= gills) of Crustacea, 16;
   of Decapoda, 152;
   of _Limulus_, 269;
   of Eurypterids, 288

 _Branchinecta_, 25, _35_;
   _B. paludosa_, 35;
   range, 34

 Branchiopoda, _18_ f.

 _Branchiopodopsis_, _35_;
   _B. hodgsoni_, 35

 Branchiostegite, 152

 Branchipodidae, 19, 22, _35_, 241

 _Branchipus_, 25, _35_, 233, 242, 511 n.;
   thoracic limb, =10=;
   nervous system, 30;
   _B. spinosus_, habitat, 33;
   _B. stagnalis_, 35;
   eggs, 32

 Branchiura, _76_

 Brauer, on development of Scorpions, 263, 301 n., 305

 Breeding (see Reproduction)

 British forms, of Cladocera, 51;
   of Pycnogons, 540

 Bronteidae, _249_

 _Bronteus_, 228, 235, _249_;
   _B. brongniarti_, eye, 229;
   _B. palifer_, eye, 229;
   _B. polyactin_, hypostome, =233=;
   _B. irradians_, macula, =233=

 Brood-pouch, of Cladocera, 46, 47;
   of Peracarida, 118

 _Broteas_, _308_

 _Broteochactas_, _308_

 Brünnich, 502

 Buckler, 330

 _Bucranium_, _414_

 Bulb, of palpal organ of Spiders, 322

 _Bumastus_, 235, 236, _249_

 _Bunodella_, _279_

 _Bunodes_, _279_

 Buthidae, _306_

 Buthinae, _306_

 _Buthus_, _306_;
   _B. occitanus_, =299=, =300=, =302=

 _Bythotrephes_, 38, _54_;
   reproduction, 47;
   _B. cederströmii_, =42=


 Cabiropsidae, _130_

 _Caecidotea nickajackensis_, habitat, 210;
   _C. stygia_, habitat, 210

 Caeculinae, _472_

 _Caeculus_, _472_

 Calamistrum, =326=, 354, 385, 392, 399, 410

 Calanidae, _57_

 _Calanus_, _57_;
   _C. finmarchicus_, distribution, 203, 204;
   _C. hyperboreus_, 55, =56=, 58

 _Calappa_, _187_;
   respiration, 186;
   habitat, 198;
   distribution, 201;
   _C. granulata_, =186=

 Calappidae, _187_

 _Calathocratus_, _452_

 _Calathura brachiata_ (Anthuridae), _Duplorbis_ parasitic on, 95

 _Calicurgus annulatus_, 369

 Caligidae, _73_

 _Caligus nanus_, =74=;
   _C. rapax_, 74;
   _C. lacustris_, 74

 _Callianassa_, _167_;
   habitat, 198;
   _C. subterranea_, 167;
   gut, 14

 Callianassidae, _167_

 _Callinectes_, _191_;
   _C. sapidus_, 191

 Calman, on classification of Crustacea, 112, 113

 _Calocalanus plumulosus_, =58=

 _Caloctenus_, _418_

 _Calommata_, _391_

 _Calymene_, 225, 230, 235, _249_;
   _C. senaria_, 236;
   _C. tuberculata_, =224=

 Calymenidae, _247_

 Calyptomera, 38, _51_

 Calyptopis, larva of _Euphausia pellucida_, =144=

 _Cambaroides_, distribution, 213

 _Cambarus_, _157_;
   hermaphroditism, 103;
   distribution, 213;
   _C. stygius_, distribution, 213

 Camerostome, 452

 Campbell, 327

 _Camptocercus_, _53_;
   _C. macrurus_, 48

 _Cancer_, _191_;
   _C. pagurus_, 191

 _Cancerilla_, _68_;
   _C. tubulata_, 68

 Cancridae, _191_

 _Candace_, _60_;
   _C. pectinata_, 60

 Candacidae, _60_

 _Candona_, _107_;
   _C. reptans_, =107=

 Canestrini, 464

 _Canthocamptus_, _62_;
   habitat, 206

 Capitulum, of Cirripedia, 81;
   of Acarina, 457, 468, 471

 _Caponia natalensis_, _395_

 Caponiidae, _395_

 _Caponina_, _395_

 _Caprella acutifrons_, _140_;
   _C. grandimana_, =139=

 Caprellidae, _139_

 Carapace, of Phyllopoda, 19 f.;
   of Cladocera, 38;
   absence of, in Copepoda, 55;
   of Malacostraca, 114

 Carcinoplacidae, _195_

 _Carrinoscorpius_, _277_;
   _C. rotundicauda_, 277

 _Carcinus_, _191_;
   _C. maenas_, 188, 191;
   gut, 14;
   respiration, 189, 190;
   distribution, 198;
   _Portunion_ parasitic in, =135=;
   _Sacculina_ parasitic on, 96

 _Cardisoma_, _196_;
   distribution, 201

 Caridea, _158_, _163_;
   metamorphosis, 161

 _Caridina_, _163_;
   _C. nilotica_, distribution, 212

 Carniola, caves of, 34

 Carpenter, on segmentation of Arthropods, 6, 263;
   on affinities of Trilobites, 242;
   on Irish Pycnogons, 540

 Caruncle, 470

 Caspian Sea, Crustacea of, 215

 _Caspiocuma_, _121_

 Catometopa, _193_ f.;
   habits, 194, 195

 _Catophragmus_, =91=

 Caudal organs, 311

 Caullery, on Liriopsidae, 132 n.

 Causard, 332

 Cavanna, 520

 _Cecrops_, _74_

 _Cenobita_, _181_;
   relation to _Birgus_, =176=

 Cenobitidae, _181_

 _Centropages hamatus_, 203;
   _C. typicus_, distribution, 203

 Centropagidae, _58_

 _Centropelma_, _416_

 _Centropleura_, _247_

 Centrurinae, _306_

 _Centrurus_, _306_

 Cephalic shield, 223

 _Cepheus ocellatus_, =467=

 _Cerataspis_, _162_

 _Ceratolichas_, _252_

 _Ceratopyge_, _247_

 _Cercophonius_, _308_

 _Ceriodaphnia_, 37, 39, _51_

 _Ceroma_, _429_

 _Chactas_, _308_

 Chactidae, 306, _307_

 Chaerilidae, 306, _307_

 _Chaerilus_, _307_

 _Chaetolepas_, _89_

 _Chaetonymphon_, _536_;
   _C. hirtipes_, 541;
   _C. hirtum_, =509=, 541;
   _C. macronyx_, =506=;
   _C. spinosissimum_, 541, 542;
   _C. tenellum_, 542

 _Chaetopelma_, _389_

 Charontinae, _313_

 _Chasmops_, _249_

 Cheeks, of Trilobites, 223, 225

 Cheese-mites, 466

 _Cheiracanthium_, _397_

 Cheiruridae, _250_

 _Cheirurus_, 235, _251_;
   _C. insignia_, =250=;
   _C. pleurexacanthus_, 236

 Chelicerae, of Xiphosura, 263 f.;
   of Eurypterida, 285;
   of Scorpions, 303;
   of Pedipalpi, 309;
   of Spiders, 319;
   of Palpigradi, 422;
   of Solifugae, 426;
   of Pseudoscorpions, 432;
   of Podogona, 439;
   of Phalangids, 443;
   of Acarina, 458

 _Chelifer_, 436, _437_;
   development, =435=;
   _C. cancroides_, 437;
   _C. cyrneus_, =437=;
   _C. ferum_ 437

 Chelifera, _122_

 Cheliferidae, _436_

 Chelophores, of Pycnogons, 505

 _Chernes_, =432=, 436, 437, _438_

 Chernetes, 430

 Chernetidea, _258_, _430_ f.

 Cheyletinae, _473_

 _Cheyletus_, 458, _473_

 Chilaria, 260, 271, 287, 292

 _Chilobrachys_, _390_;
   _C. stridulans_, =328=, 329

 _Chilophoxus_, _539_

 _Chiltonia_, _139_;
   distribution, 217

 _Chiridium_, 432, 436, _437_;
   _C. museorum_, =437=

 _Chirocephalus_, _35_;
   _C. diaphanus_, =20=, =24=, =25=, =27=, 29, 32, 33, 35

 _Chlorodinus_, habitat, 198

 _Chlorodius_, _191_

 Chondracanthidae, _72_

 _Chondracanthus zei_, =72=

 _Choniostoma_, _76_

 Choniostomatidae, _76_

 _Chthonius_, 436, _438_

 Chun, on phosphorescence and eyes, 150

 _Chydorus_, _54_

 _Cilunculus_, _535_

 Circulatory (= vascular) system, of Crustacea, 11;
   of Arachnids, 256;
   of _Limulus_, 268 f.;
   of Tardigrada, 482;
   of Pentastomida, 491;
   of Pycnogons, 516

 _Cirolana_, _126_

 Cirripedia, _79_ f.;
   metamorphosis, 80;
   anatomy, 83;
   sex, 87, 105

 CLADOCERA, _19_, _37_ f.;
   carapace, 38;
   dorsal organ, 39;
   appendages, 40 f.;
   alimentary canal, 42;
   heart, 43;
   reproduction, 43–50;
   British genera, 51–54;
   extra-European, 54;
   pelagic, 207, 208

 Claparède, 331, 462 n.

 Clarke, J. M., on the eye of _Calymene senaria_, 229;
   of _Harpes_, 231

 Claus, on Copepoda, 55;
   on _Nebalia_, 111;
   on discovery of metamorphosis of Decapods, 153 n.;
   on Pycnogonida, 527

 Claw-tufts, 389

 Clerck, 384, 408 n.

 _Clibanarius_, _181_

 _Clotenia conirostre_, 541

 _Clubiona_, 337, 368, _397_;
   _C. compta_, 397;
   _C. corticalis_, =396=, 397

 Clubioninae, _397_

 Clypeus, 316

 _Clytemnestra_, _61_

 _Coelotes atropos_, _416_

 Cole, 520, 524, 525, 528 n.

 _Colossendeis_, 505, _532_;
   _C. angusta_, 542;
   _C. australis_, 533;
   _C. gibbosa_, 534;
   _C. gigas_, 532;
   _C. gracilis_, 505 n.;
   _C. proboscidea_, =505=, =508=, =510=, 533, 542

 Colour, adaptation in, of Crustacea, 159

 Colulus, =317=, 319

 Commensalism, of Hermit-crabs, 172;
   of _Pinnotheres_, 195

 Complemental males, of Cirripedes, 83, 86, 99, 106

 _Conchoderma_, _88_;
   _C. virgata_, =88=

 Conocephalidae, _247_

 _Conocoryphe_, 231, _247_;
   _C. sulzeri_, =248=

 Conocoryphidae, _247_

 _Conolichas_, _252_

 _Conothele_, _388_

 _Constantia_ (_Macrohectopus_), _138_;
   occurrence, 212

 Cook, 425 n.

 Copepoda, _55_ f.;
   fresh-water, 59, 62;
   pelagic, 202;
   life-cycle of fresh-water, 209

 _Copilia vitrea_, _69_, =70=

 _Cordylochele_, 506, _537_;
   _C. longicollis_, =507=;
   _C. malleolata_, 542

 _Corniger hilgendorfi_, _535_

 _Coronula diadema_, _91_

 Corophiidae, _139_

 _Corophium_, _139_

 Corycaeidae, _69_

 _Corystes_, _188_, _190_;
   habitat, 198;
   _C. cassivelaunus_, respiration, 170, 189;
   metamorphosis, =182=, =183=

 Corystidae, _190_

 Cosmetidae, _449_

 Costa, da, 221

 Coxal glands, 257;
   of _Limulus_, 270;
   of Scorpions, 306;
   of Pedipalpi, 311;
   of Spiders, 337

 Coxopodite, of Trilobites, 237

 Crab, Hermit-, 171–173;
   River-, 214;
   Robber-, 174;
   Shore-, 188, 189, 198;
   Edible, 188;
   Spider-, 191;
   Land-, 195;
   enemies of, 192

 Crab-spiders, 412 (= Thomisidae, _q.v._)

 _Crangon_, _164_;
   _C. antarcticus_, distribution, 200;
   _C. franciscorum_, distribution, 200;
   _C. vulgaris_, 158, 164;
   distribution, 199

 Crangonidae, _164_;
   distribution, 199

 _Crangonyx_, _138_

 Crayfish, 154, _157_;
   distribution, 213, 215

 Crevettina, _137_

 Cribellatae, 324, 385, 386 n.

 Cribellum, =326=, 354, 385, 386, 392, 398, 410

 Croneberg, 460

 _Cruregens_, _124_;
   _C. fontanus_, habitat, 210

 Crustacea, organisation, 1 f.;
   segmentation, 5;
   appendages, 8 f.;
   body-cavity and coelom, 11;
   kidneys, 13;
   alimentary canal, 14;
   reproductive organs, 15;
   respiratory organs, 16;
   compound eyes, 146;
   growth and sex in, 100;
   metabolism, 104;
   distribution, 197;
   pelagic, 202, 207;
   littoral, 197, 206;
   abyssal, 204, 209;
   fresh-water, 205;
   subterranean and cave, 209

 Crustacés aranéiformes, 501 n.

 _Cryphaeus_, _249_

 _Cryphoeca_, _416_

 _Cryptocellus_, _439_;
   _C. simonis_, =439=

 _Cryptocerus_, _414_

 Cryptoniscidae, _130_

 Cryptoniscina, _129_, 130

 Cryptoniscus, larva of Epicarida, 129, =131=, 132

 _Cryptophialus_, _92_;
   _C. minutus_, 92, 93;
   _C. striatus_, 93

 _Cryptostemma westermannii_, _439_

 Cryptostemmatidae, _440_

 _Cryptothele_, _400_

 Ctenidae, _418_

 Cteninae, _418_

 _Cteniza_, _388_;
   _C. ariana_, 355

 Ctenizinae, _388_

 _Ctenocephalus_, _247_

 _Ctenophora_, _412_

 Ctenopoda, _51_

 _Ctenopyge_, 232, _247_

 _Ctenus_, _418_

 Cucullus, 440

 _Cuma_, _121_

 Cumacea, 114, _120_;
   of the Caspian, 215

 Cumidae, _121_

 Cyamidae, _140_

 _Cyamus ceti_, _140_

 Cybaeinae, _415_

 _Cybele_, _251_

 _Cyclaspis_, _121_

 _Cyclestheria_, _37_;
   _C. hislopi_, 37

 _Cyclodorippe dromioides_, eyes, 149

 _Cyclograpsus_, _196_;
   distribution, 200

 Cyclometopa, _188_ f.;
   respiration, 189, 190

 Cyclopidae, _61_, 62;
   subterranean, 209

 _Cyclops_, _62_;
   _C. fuscus_, habitat, 207;
   _C. strenuus_, habitat, 207, 208;
   _C. stygius_, habitat, 210

 _Cyclosa conica_, _409_

 _Cyclosternum_, _389_

 _Cydrela_, _399_

 _Cymodoce_, _126_

 _Cymonomus_, _188_;
   _C. granulatus_, =185=;
   eyes, 149, 186;
   _C. normani_, 186;
   _C. quadratus_, _186_

 _Cymothoa_, _126_;
   habitat, 211

 Cymothoidae, _126_

 _Cyphaspis_, _251_

 Cyphophthalmi, 443, 444, _447_

 Cypridae, _107_;
   subterranean, 209

 Cypridinidae, _108_

 _Cypris_, _107_;
   _C. reptans_, parthenogenesis, 108

 Cypris larva, of Cirripedia, 80, =82=;
   of _Sacculina_, =97=, =99=

 _Cyrtauchenius_, _388_;
   _C. elongatus_, funnel of, =356=

 _Cythere dictyon_, _108_

 Cytherellidae, _109_

 Cytheridae, _107_


 _Dactylopisthes digiticeps_, =405=

 _Dactylopus tisboides_, _62_

 _Daesia_, _429_

 Daesiinae, _429_

 Dajidae, _130_

 _Dalmanites_, _249_;
   _D. imbricatulus_, eye, =228=;
   _D. limulurus_, =250=;
   _D. socialis_, larvae, =240=

 _Danalia curvata_, _130_, =131=, =132=

 _Daphnella_, _51_;
   testes, 44

 _Daphnia_, 37, 38, 39, _51_;
   ovary, =45=, 48;
   _D. magna_, 50;
   _D. obtusa_, =51=

 Daphniidae, _51_;
   appendages, 40;
   alimentary canal, 42;
   reproduction, 48;
   reactions, 50

 Darwin, on Cirripedia, 80, 85, 86, 92, 94

 _Dasylobus_, _450_

 Decapoda, _152_ f.;
   systematic position, 114;
   alimentary canal, 14;
   pelagic, 202;
   subterranean, 210;
   Rhizocephala parasitic on, 95, 101;
   Bopyridae parasitic on, 133

 _Dechenella_, _251_

 _Decolopoda_, 504, _529_, _532_;
   _D. antarctica_, 532;
   _D. australis_, =531=, 532

 Decolopodidae, _531_

 Defective orb-webs, 349

 _Deiphon_, 235, _251_;
   _D. forbesi_, =250=

 _Delena_, _414_

 Delobranchiata, _258_, 259 f.

 _Demodex_, _465_;
   _D. folliculorum_, =465=

 Demodicidae, 455, _465_

 _Dendrogaster astericola_, _94_

 _Dermacentor_, _469_

 Dermanyssinae, _471_

 _Dermanyssus avium_, _471_

 _Dermaturus_, _181_;
   _D. hispidus_, =178=

 _Desis_, _415_

 Deutovum, 462

 Development, of Monstrillidae, 64;
   of Cirripedia, 80;
   of Rhizocephala, 96;
   of Epicarida, 130;
   of Stomatopoda, 142;
   of Shrimps and Prawns, 159;
   of Loricata, 165;
   of Hermit-Crabs, 179;
   of Brachyura, 181;
   of Trilobites, 238 f.;
   of _Limulus_, 275;
   of _Scorpio_, 305;
   of Pseudoscorpions, 434;
   of Mites, 462;
   of Tardigrada, 483;
   of Pentastomida, 493;
   of Pycnogons, 520

 _Diaea_, _412_;
   _D. dorsata_, 413

 Diaphragm, of Solifugae, 427

 _Diaptomus_, _59_;
   distribution, 208, 216;
   _D. caeruleus_, habitat, 208;
   _D. castor_, habitat, 206;
   _D. gracilis_, habitat, 206

 Diastylidae, _121_

 _Diastylis_, _121_;
   _D. goodsiri_, 121;
   _D. stygia_, =120=

 _Dichelaspis_, _88_

 Dichelestiidae, _68_;
   classification, 63

 _Dichelestium_, _68_

 Dick, 363

 _Dicranogmus_, _252_

 _Dicranolasma_, _452_

 _Dictyna_, _398_;
   _D. arundinacea_, 399;
   _D. uncinata_, 399

 Dictynidae, 352, 353, _398_

 Digestive system, = alimentary canal, _q.v._

 _Dikelocephalus_, _247_

 Dimorphism, high and low;
   in Decapoda, 103;
   in Tanaids, 123

 _Dindymene_, _251_

 Dinopinae, _410_

 _Dinopis_, _410_

 _Dinorhax_, _429_

 _Diogenes_, _181_

 _Dionide_, _245_

 _Diphascon_, 485;
   _D. alpinum_, 487;
   _D. angustatum_, 487;
   _D. bullatum_, 487;
   _D. chilenense_, =486=, 487;
   _D. oculatum_, 487;
   _D. scoticum_, 487;
   _D. spitzbergense_, 487

 Diplocentrinae, 306, _307_

 _Diplocentrus_, _307_

 _Diplocephalus bicephalus_, =405=

 Diplostichous eyes, 301

 _Diplura_, _390_

 Diplurinae, _390_

 _Dipoena_, _403_

 _Discoarachne_, 512, _535_

 Distribution, of Crustacea, 197 f.;
   (stratigraphical) of Trilobites, 222

 Doflein, on eyes of deep-sea Crustacea, 148, 150

 Dohrn, 504, 513, 519

 Doleschall, 365

 _Dolichopterus_, 283, _291_

 _Doliomelus_, _415_

 _Dolomedes fimbriatus_, _416_

 _Dolops_, _78_

 Domed webs, 350

 _Donachochara_, _406_

 Donnadieu, 457

 _Dorippe_, 185, _188_

 Dorippidae, _188_

 _Doropygus_, _66_;
   _D. pulex_, =66=

 Dorsal organ, of Phyllopoda, 22;
   of Cladocera, 39

 Doublure, 232

 Doyère, on Tardigrada, 481;
   on their systematic position, 483

 _Doyeria_, 485;
   _D. simplex_, 480, 487

 Drassidae, 324, _396_

 Drassinae, _396_

 _Drassus_, _397_;
   _D. lapidosus_, =396=, 397

 _Drepanothrix_, _53_

 _Dromia_, _184_;
   _D. vulgaris_, =184=

 Dromiacea, _183_;
   metamorphosis, 182;
   relation to Macrura, 184;
   habitat, 198

 _Dromidia_, distribution, 200

 Dromiidae, _184_

 _Drymusa_, _393_

 Dufour, 385

 Dujardin, 464 n.;
   on systematic position of Tardigrada, 483

 _Duplorbis_, _95_;
   _D. calathurae_, 99

 _Dynomene_, _184_

 Dynomenidae, _184_

 _Dysdera_, _394_;
   _D. cambridgii_, 394;
   _D. crocota_, 395

 Dysderidae, 317, 319, 336, _394_

 _Dysderina_, _394_

 Dysderinae, _394_


 _Ebalia_, _188_

 _Echiniscoides_, 485;
   _E. sigismundi_, 477, 486

 _Echiniscus_, 480, 485;
   _E. arctomys_, 486;
   _E. gladiator_, 486;
   _E. granulatus_, 486;
   _E. islandicus_, 486;
   _E. muscicola_, 486;
   _E. mutabilis_, 486;
   _E. oihonnae_, 486;
   _E. quadrispinosus_, 486;
   _E. reticulatus_, 486;
   _E. spinulosus_, =479=;
   _E. spitzbergensis_, 486;
   _E. testudo_, =478=;
   _E. wendti_, 486

 Echinoderms, _Dendrogaster_ parasitic on, 94

 _Echinognathus_, 283

 Ecribellatae, _385_

 _Ectatosticta davidi_, _393_

 _Ectinosoma_, _62_

 Edriophthalmata, 112, 121

 Eggs, of Phyllopoda, 32;
   of Cladocera, 44;
   of Copepoda, 59, 62, =66=, =67=, =71=, =74=;
   of Branchiura, 77;
   of Syncarida, 114;
   of Peracarida, 123;
   of Hoplocarida, 141;
   of Eucarida, 144;
   of Trilobites, 238;
   of _Limulus_, 275;
   of Pedipalpi, 309;
   of Spiders, 358;
   of Solifugae, 424;
   of Pseudoscorpions, 434;
   of Phalangidea, 442;
   of Acarina, 456;
   of Tardigrada, 478;
   of Pentastomida, 493;
   of Pycnogons, 520

 Ehrenberg, on systematic position of Tardigrada, 483

 _Eleleis crinita_, _396_

 _Ellipsocephalus_, 224, 235, _247_;
   _E. hoffi_, =248=

 Embolobranchiata, _258_, 259, 297 f.

 Emmerich, on facial suture of _Trinucleus_, 226

 _Encephaloides_, _193_;
   _E. armstrongi_, 192, =193=;
   habitat, 205

 Encrinuridae, _251_

 _Encrinurus_, 227, 235, _251_

 _Endeis didactyla_, _534_;
   _E. gracilis_, _539_;
   _E. spinosus_, 541

 Endite, 9, =10=

 Endopodite, 9, =10=;
   of Trilobites, =237=

 Endosternite, 257, 305, 330

 Endostoma, of _Eurypterus_, 287

 _Engaeus_, _157_;
   _E. fossor_, distribution, 213

 _Enoplectenus_, _418_

 _Enterocola_, _67_;
   _E. fulgens_, =67=

 Entomostraca, defined, 6;
   diagnosis, 18;
   of littoral zone, 197;
   fresh-water, of southern hemisphere, 216

 Entoniscidae, 130, _134_

 _Enyo_, _400_

 Enyoidae, _399_

 _Eoscorpius_, 298

 _Epeira_, _409_;
   _E. angulata_, =315=, =409=;
   _E. basilica_, 350, 351;
   web of, =351=;
   _E. bifurcata_, 359;
   _E. caudata_, 359;
   _E. cornuta_, 409;
   _E. cucurbitina_, 372, 409;
   _E. diademata_, 335, 340, 343, 345, 359, 366, 380, 409;
   anatomy, =332=;
   cocoon, =358=;
   silk, 360;
   spinnerets, =325=;
   _E. labyrinthea_, 350;
   _E. madagascarensis_, 360;
   _E. mauritia_, 349;
   _E. pyramidata_, 409;
   _E. quadrata_, 366, 409;
   _E. triaranea_, 350;
   _E. umbratica_, 409

 Epeiridae, 376, 377, _406_

 Epeirinae, _408_

 Ephippium, =48=

 _Epiblemum_, _420_

 Epicarida, _129_;
   sex in, 105

 Epicaridian, larva of Epicarida, =130=

 Epicoxite, of _Eurypterus_, 287

 _Epidanus_, _449_

 Epigyne, 319, 333, 378

 Epipharynx, 459

 Epipodite, 9, =10=

 Episininae, _402_

 _Episinus truncatus_, _403_

 Epistome, of Eurypterida, 291;
   of Pseudoscorpions, 431, 436;
   of Phalangidea, 443

 Erber, 355, 356

 _Eremobates_, _429_

 Eremobatinae, _429_

 Eresidae, _398_

 _Eresus cinnaberinus_, _398_

 _Eriauchenus_, _411_

 Erichthoidina, larva of Stomatopod, =143=

 Ericthus, larva of Stomatopod, =143=

 _Erigone_, _405_

 Erigoninae, _404_

 _Eriophyes_, _465_;
   _E. ribis_, 455, =465=;
   _E. tiliae_, 465

 Eriophyidae, _464_

 _Eriphia_, _191_;
   _E. spinifrons_, 191

 Erlanger, von, on development and position of Tardigrada, 483

 _Ero_, _411_;
   _E. furcata_, 366, 411;
   cocoon, =358=;
   _E. tuberculata_, 412

 Eryonidae, _158_;
   habitat, 204

 Eryonidea, _157_

 Erythraeinae, _473_

 _Estheria_, 21, 22, =23=, _36_;
   _E. gubernator_ and _E. macgillivrayi_, habitat, 33;
   _E. tetraceros_, 36

 Eucarida, 114, _144_ f.

 _Euchaeta norwegica_, _58_

 Eucopepoda, _57_ f.

 _Eucopia australis_, =119=

 Eucopiidae, 113, 114, _118_

 _Eudendrium_, Pycnogons on, 520

 _Eudorella_, _121_

 _Eukoenenia_, _423_;
   _E. augusta_, 423;
   _E. florenciae_, 423;
   _E. grassii_, 423

 _Eulimnadia_, _36_;
   _E. mauritani_, 36;
   _E. texana_, 36

 _Euloma_, 230

 Eumalacostraca, _112_ f.

 Eupagurinae, _180_

 _Eupagurus_, _180_;
   _E. bernhardus_, commensalism, 172;
   distribution, 199;
   _E. excavatus_, parasitic castration of, 101;
   _E. longicarpus_, metamorphosis, =179=;
   _E. prideauxii_, commensalism, 172;
   _E. pubescens_, distribution, 199

 _Euphausia pellucida_, _145_, =146=

 Euphausiacea, _144_

 Euphausiidae, 113, 114, _144_;
   larval history, 145;
   eyes, 150

 _Eupodes_, _471_

 _Euproöps_, 278

 _Eurycare_, 232, _247_

 _Eurycercus_, _53_;
   alimentary canal, 42;
   _E. lamellatus_, habitat, 207

 _Eurycide_, 505, _533_;
   _E. hispida_, =506=, =507=, =533=

 Eurycididae, _533_

 _Eurydium_, 485

 _Euryopis_, _404_

 _Eurypelma_, _389_;
   _E. hentzii_, 361, 370

 _Euryplax_, _195_

 Eurypterida, _258_, 278, 283 f.

 Eurypteridae, _290_ f.

 _Eurypterus_, 283 f., 290, 291, 292;
   _E. fischeri_, =284=, =286=, =289=

 _Eurytemora_, _59_;
   _E. affinis_, habitat, 206

 _Eusarcus_, 283, _291_

 Euscorpiinae, _308_

 _Euscorpius_, 298, _308_;
   _E. carpathicus_, 299

 _Eusimonia_, _429_

 _Euterpe acutifrons_, _61_, =61=;
   distribution, 203

 _Euthycoelus_, _389_

 _Evadne_, _54_;
   young, 47

 Excretory system (including Renal organs), in Crustacea, 12;
   in Arachnids, 257;
   in _Limulus_, 270;
   in Tardigrada, 481;
   in Pentastomida, 491

 Exner, on mosaic vision, 148

 Exopodite, 9, =10=;
   of Trilobites, =237=

 Eyes, compound, of Crustacea, 146, =147=;
   physiology of, 148;
   of deep-sea Crustacea, 149;
   connexion with phosphorescent organs, 151;
   regeneration of, 6;
   of Trilobites, 227 f., =228=;
   of _Limulus_, 271;
   of Eurypterida, 285;
   of Scorpions, 301;
   of Pedipalpi, 309;
   of Spiders, 315, 334; of Solifugae, 426;
   of Pseudoscorpions, 431;
   of Phalangidea, 442;
   of Acarina, 458;
   of Pycnogons, 517


 Fabre, on habits of Spiders, 298 f.;
   of Tarantula, 361 f.;
   on Wasp _v._ Spider, 368 f.

 Facet, of Trilobites, 235

 Facial suture, 225 f., 232

 Falanga, 424

 False articulations, 444

 False-scorpions, 430

 _Fecenia_, _399_

 _Filistata_, _391_;
   _F. capitata_, 392;
   _F. testacea_, 392

 Filistatidae, 319, 336, _391_

 Finger-keel, 303

 Fixed cheek, 225, 226, 227

 Flabellifera, _124_ f.

 Flabellum, 270

 Flacourt, 363

 Flagellum, in Solifugae, 426, 428;
   in Pseudoscorpions, 433

 Forbes, 374

 Ford, S. W., on development of Trilobites, 238

 Forel, on Lake of Geneva, 206

 _Formicina_, _405_

 Formicinae, _405_

 _Formicinoides brasiliana_, =318=

 _Fragilia_, _535_

 Free cheek, 225, 226, 227

 Fresh-water, Crustacea, 205 f.;
   Spiders, 357

 Furcilia (Metazoaea), larva of _Euphausia_, 145

 Fusulae, 325, 335


 _Galathea_, 169, _170_;
   _G. intermedia_, _Pleurocrypta_ parasitic on, =133=;
   _G. strigosa_, =170=;
   gut of, 15

 Galatheidae, _169_

 Galatheidea, _169_

 Galea, 433, 436

 _Galena_, _412_

 _Galeodes_, _429_, 527;
   nervous system, =428=;
   chelicera, =429=;
   _G. arabs_, 425;
   _G. araneoides_, 425

 Galeodidae, _428_

 Gall-mites, 455, 464

 Gamasidae, _470_

 Gamasinae, _470_

 _Gamasus_, 460, 461, 463, _470_;
   _G. coleoptratorum_, 470;
   _G. crassipes_, 470;
   _G. terribilis_, 461

 Gammaridae, _138_

 _Gammarus_, 137, _138_;
   of Lake Baikal, 212;
   of Australia, 216;
   _G. locusta_, 138, =138=;
   _G. pulex_, 138

 _Gampsonyx_, _115_, 118

 Garstang, on respiration of crabs, 186 n.

 Garypinae, 436, _437_

 _Garypus_, 431, 436, _437_, 438;
   chelicera, =432=;
   _G. littoralis_, 430

 Gaskell, 270, 277, 334

 _Gasteracantha_, _410_;
   _G. minax_, =410=

 Gasteracanthinae, 317, _409_

 _Gastrodelphys_, _73_

 Gastrolith, of Lobster, 155

 Gaubert, 525 n.

 _Gebia littoralis_, _167_

 Gecarcinidae, _196_

 _Gecarcinus_, 194, 195, _196_

 Gegenbaur, 523

 _Gelanor_, _411_, 412

 _Gelasimus_, 194, _196_;
   habitat, 198;
   distribution, 210;
   _G. annulipes_, =194=

 Genal angle, 225

 Gené, 461

 Genital operculum, of Eurypterida, 288, =289=, 291

 _Genysa_, _388_

 _Gerardia_, _Laura_ parasitic on, 93

 _Geryon_, _195_

 _Giardella callianassae_, _73_

 Gibocellidae, 448

 _Gibocellum sudeticum_, _447_

 Giesbrecht, on Copepoda, 57;
   on phosphorescence, 59

 Gigantostraca, _258_, 283 f.

 Gill-book, =270=

 Glabella, 223

 Glabella-furrows, 223

 Glands, of Tardigrada, 481;
   of Pentastomida, 490, 491;
   of Pycnogons, 511;
   coxal, of Arachnids, 257, 270, 337;
   green, of Malacostraca, 110;
   poison-, of Arachnids, 337, 360;
   spinning, of Spiders, 335;
   of Pseudoscorpions, 434

 Glaucothoe, larva of _Eupagurus_, =179=, 180

 _Gluvia_, _429_

 _Glycyphagus_, _466_;
   _G. palmifer_, 466;
   _G. plumiger_, 466

 _Glyphocrangon_, _164_;
   _G. spinulosa_, 158, =164=

 Glyphocrangonidae, _164_

 _Glyptoscorpius_, 283, 291, 294

 _Gmelina_, _138_

 _Gmogala scarabaeus_, _394_

 _Gnamptorhynchus_, _533_

 _Gnaphosa_, _397_

 _Gnathia maxillaris_, _124_;
   life-history of =125=

 Gnathiidae, _124_

 Gnathobase, 10, 264

 _Gnathophausia_, _119_, 256 n.;
   maxillipede of, =10=

 Gnathostomata, 56

 _Gnosippus_, _429_

 Goldsmith, 362

 Gonads, = reproductive organs, _q.v._

 _Gonodactylus_, _143_;
   _G. chiragra_, 143

 Gonoplacidae, _195_

 _Gonoplax_, _195_;
   _G. rhomboides_, 195

 Gonyleptidae, 442, 448, _449_

 Goodsir, Harry, 535, 540

 _Gordius_, parasitic in Spiders, 368

 Gossamer, 342

 Graells, 364

 _Graeophonus_, _309_

 Graff, von, on position of Tardigrada, 483

 Grapsidae, 193, _195_;
   habitat, 198, 201

 _Graptoleberis_, _53_

 Grassi, 422

 Green gland, 110 (= antennary gland, _q.v._)

 Gregarious Spiders, 340

 Grenacher, 517

 _Griffithides_, _251_

 Gruvel, on Cirripedia, 80, 86

 Guérin-Méneville, 439

 Gurney, on Copepoda, 62;
   on Brachyuran metamorphosis, 181 n.

 _Gyas_, _450_

 _Gylippus_, _429_

 _Gymnolepas_, _89_

 Gymnomera, 38, _54_

 Gymnoplea, _57_


 Hadrotarsidae, _394_

 _Hadrotarsus babirusa_, _394_

 Haeckel, on plankton, 203

 _Haemaphysalis_, _469_

 Haematodocha, 322

 _Haemocera_, _64_;
   _H. danae_, life-history, =64=, =65=

 Haemocoel, 5, 11

 _Hahnia_, 325, _416_

 Hahniinae, _416_

 Halacaridae, _472_

 Halocypridae, _108_

 _Halosoma_, _539_

 _Hannonia typica_, _533_

 Hansen, on Choniostomatidae, 76;
   on Cirripede Nauplii, 94;
   on classification of Malacostraca, 113

 Hansen and Sörensen, 422, 439, 443, 448

 _Hapalogaster_, _181_;
   _H. cavicauda_, =178=

 Hapalogasterinae, _181_

 _Harpactes hombergii_, _395_

 Harpacticidae, _61_, 62;
   habitat, 206

 Harpedidae, _245_

 _Harpes_, 225, 226, 230, 231, 234, _246_;
   _H. ungula_, =248=;
   _H. vittatus_, eyes, =228=

 _Harporhynchus_, _53_

 Harvest-bugs, 454, 473

 Harvestmen, 440, = Phalangidea, _q.v._

 Harvest-spiders, 440, = Phalangidea, _q.v._

 Harvesters, 440, = Phalangidea, _q.v._

 _Hasarius falcatus_, _421_

 Haustellata, 501 n.

 Haustoriidae, _137_

 _Haustorius arenarius_, _137_

 Hay, on name _Lydella_, 486 n.

 Heart, of Phyllopoda, 29;
   of Cladocera, 43;
   of _Nebalia_, 112;
   of Syncarida, 115;
   of Peracarida, 118;
   of Isopoda, 122;
   of _Danalia_, 132;
   of Amphipoda, 136;
   of _Squilla_, 142;
   of Eucarida, 144;
   of _Limulus_, 268;
   of Scorpions, 305;
   of Pedipalpi, 311;
   of Spiders, 331;
   of Solifugae, 427;
   of Pseudoscorpions, 434;
   of Phalangidea, 445;
   of Acarina, 460;
   of Pycnogons, 516

 Heart-water, 470

 Hedley, on home of cocoa-nut, 174

 _Heligmonerus_, _388_

 Heller, 455

 _Hemeteles fasciatus_, 367;
   _H. formosus_, 367

 _Hemiaspis_, 278;
   _H. limuloides_, =278=

 Hemioniscidae, _130_

 _Hemiscorpion lepturus_, _307_

 Hemiscorpioninae, 306, _307_

 Henking, 447, 460

 Hentz, 367

 Herbst, on regeneration of eye, 6 n.

 _Hermacha_, _388_

 Hermaphroditism, 15;
   caused by parasite, 101, 102;
   partial and temporary, 102;
   normal, 105;
   in Cymothoidae, 126;
   in Isopoda Epicarida, 129;
   in Entoniscidae, 135;
   in _Caprella_, 140

 _Hermippus_, 317, _399_;
   _H. loricatus_, =400=

 Hermit-crab, 167, 171;
   commensalism, 172;
   reacquisition of symmetry, 173;
   regeneration of limbs, 156

 Hermit-lobster, 167

 Herrick, on the Lobster, 154

 _Hersilia_ (Araneae), _401_;
   _H. caudata_, =400=

 Hersiliidae (Araneae), 326, _400_

 Hersiliidae (Copepoda), _73_

 _Hersiliola_, _401_

 Heterarthrandria, _58_

 _Heterocarpus alphonsi_ (Pandalidae), phosphorescence, 151

 _Heterochaeta papilligera_, _60_

 _Heterocope_, _59_

 _Heterogammarus_, _138_

 _Heterometrus_, _307_

 _Heterophrynus_, _313_

 _Heteropoda venatoria_, _414_

 Heterostigmata, _471_

 _Heterotanais_, _123_

 Hexameridae, _91_

 _Hexathele_, _390_

 Hexisopodidae, _429_

 _Hexisopus_, _429_, =429=

 _Hexura_, _391_

 _Hippa_, _171_;
   _H. emerita_, distribution, 202

 Hippidae, _171_

 Hippidea, _170_;
   habitat, 198

 _Hippolyte_, _164_;
   distribution, 200;
   _H. varians_, 164

 Hippolytidae, _164_;
   distribution, 199

 Hodge, George, 523, 540

 Hodgson, 508

 Hoek, on Cirripedia, 80;
   on Pycnogons, 505, 512, 513

 Holm, G., on _Agnostus_, 225;
   on _Eurypterus_, 285 n.

 _Holmia_, 236, 242, _247_;
   _H. kjerulfi_, 242, =246=

 Holochroal eye, 228

 Holopediidae, _51_

 _Holopedium_, 38, _51_

 _Homalonotus_, 222, _249_;
   _H. delphinocephalus_, =223=

 _Homarus_, _154_;
   habitat, 200;
   excretory
 glands, 13;
   _H. americanus_, 154;
   _H. vulgaris_, 154

 _Homoeoscelis_, _76_

 _Homola_, _184_;
   distribution, 205

 Homolidae, _184_

 _Homolodromia_, _184_;
   _H. paradoxa_, resemblance to Nephropsidae, 184

 Hood, of Phalangidea, =442=, 452

 Hoplocarida, 114, _141_

 _Hoploderma_, _468_;
   _H. magnum_, =467=

 _Hoplophora_, _468_

 Horse-foot crab, = _Limulus_, _q.v._

 Hoyle, on classification of Pentastomids, 495

 _Hughmilleria_, 283, 290, 292

 Humboldt, on _Porocephalus_, 488 n.

 Hutton, 424

 _Huttonia_, _398_

 _Hyale_, _139_

 _Hyalella_, 137, _139_;
   distribution, 211, 217

 _Hyalomma_, _469_

 _Hyas_, 192, _193_;
   distribution, 200

 _Hyctia nivoyi_, _421_

 Hydrachnidae, _472_

 _Hydractinia_, Pycnogons on, 523

 _Hydrallmania_, Pycnogons on, 524

 _Hymenocaris_, _112_

 _Hymenodora_, _163_

 _Hymenosoma_, _193_;
   distribution, 200

 Hymenosomatidae, _193_

 Hyperina, _140_

 Hypochilidae, _393_

 _Hypochilus_, 336, _393_;
   _H. thorelli_, 393

 _Hypoctonus_, _312_

 Hypoparia, 243

 Hypopus, 463

 Hypostome, of Trilobites, 233, 237;
   of _Bronteus_, =233=;
   of Acarina, 469

 _Hyptiotes_, 349, _411_;
   _H. cavatus_, snare, =350=;
   _H. paradoxus_, 350, =411=


 _Iasus_, 165, _167_;
   distribution, 200

 _Ibacus_, _167_

 _Ibla_, _88_;
   _I. cumingii_, =88=;
   _I. quadrivalvis_, 88, 89

 Ichneumon flies, and Spiders, 367

 _Icius_, _421_;
   _I. mitratus_, =382=

 _Idiops_, _388_

 _Idothea_, habitat, 211

 Idotheidae, _127_

 Ihle, J. E. W., 526 n.

 _Ilia_, _188_;
   _I. nucleus_, =188=;
   respiration, 187

 _Illaenus_, 229, 231, 235, _249_;
   _I. dalmanni_, =248=

 _Ilyocryptus_, 40, _53_

 _Inachus_, 192, _193_;
   _I. mauritanicus_, _Sacculina_ parasitic on, 97 f.;
   parasitic castration in, =101=;
   temporary hermaphroditism of, =103=;
   _Danalia_ and _Sacculina_ parasitic on, =131=

 Integument, of Pycnogons, 518

 Irregular Spider-snares, 351

 _Ischnocolus_, _389_

 _Ischnothele dumicola_, =390=

 Ischnurinae, 306, _307_

 _Ischnurus ochropus_, _307_

 _Ischnyothyreus_, _394_

 Ischyropsalidae, _451_

 _Ischyropsalis_, 444, _451_

 Isokerandria, _69_ f.

 _Isometrus europaeus_, _306_

 Isopoda, _121_ f., 242

 _Ixodes_, _469_;
   _I. ricinus_, =469=

 Ixodidae, _469_

 Ixodoidea, 455, 462, _468_


 _Janulus_, _403_

 Jaworowski, on vestigial antennae in a Spider, 263

 Johnston, George, 540

 Jumping-Spiders, 419


 _Karshia_, _429_

 Karshiinae, _429_

 Katipo, 363, 403

 King-crab, =_Limulus_, _q.v._

 Kingsley, on Trilobites, 239, 243 n.;
   on breeding habits of _Limulus_, 271

 Kishinouye, on _Limulus_, 274, 275

 Klebs, on the frequency of human Pentastomids, 494

 Knight Errant, 540

 Koch, C., 397 n.

 Koch, L., 397 n.

 _Kochlorine_, _92_;
   _K. hamata_, 93

 _Koenenia_, _422_, 527, 528;
   _K. mirabilis_, =423=

 Koltzoff, 15

 König, 524

 _Koonunga cursor_, _117_;
   distribution, 211

 Koonungidae, _117_

 Korschelt and Heider, on neuromeres in Arachnids, 263

 Kowalevsky, 513

 Kraepelin, 303, 306, 312 n., 428

 Kramer, 460

 Kröyer, 504, 526


 _Labdacus_, _418_

 _Labochirus_, _312_

 Labrum, of Trilobites, 233

 _Labulla_, _406_

 _Laches_, _399_

 _Lachesis_, _399_

 Lacinia mobilis, 114

 Laemodipoda, _139_

 Laenger, on the frequency of human Pentastomids, 494

 Lakes, characters of fauna of, 206;
   English, 207, 208;
   Baikal, 212;
   Great Tasmanian, 216

 _Lambrus_, 192, _193_;
   _L. miersi_, =193=

 _Lamproglena_, _68_

 Lampropidae, _121_

 _Lamprops_, _121_

 Langouste, 165

 Laniatores, 448

 Lankester, on Crustacean limb, 9;
   on classification of Arachnids, 258, 277;
   on _Limulus_, 274, 305

 _Laophonte littorale_, 62;
   _L. mohammed_, 62

 _Laseola_, _404_

 _Lathonura_, _53_

 _Latona_, _51_

 Latreille, 385, 408 n., 412, 504, 526

 _Latreillia_, _185_;
   distribution, 205

 _Latreillopsis_, _185_;
   _L. petterdi_, 185

 _Latreutes ensiferus_, habitat, 202

 _Latrodectus_, 362, _403_;
   _L. 13–guttatus_, 364, 403;
   _L. mactans_, =362=, 363, 403;
   _L. scelio_, 403

 _Laura_, _93_;
   _L. gerardiae_, 93

 Laurie, 309 n., 310, 311

 Leach, 526

 _Lecythorhynchus armatus_, _535_

 Leeuwenhoek, on desiccation in Tardigrada, 484

 _Leionymphon_, _534_

 Lendenfeld, von, 512, 523

 _Lepas_, _87_;
   metamorphosis, 80;
   anatomy, 82;
   _L. australis_, Cypris, =82=;
   _L. fascicularis_, Nauplius, =81=;
   _L. pectinata_, pupa, =82=

 _Lephthyphantes_, 327, _406_

 _Lepidurus_, 23, 24, _36_;
   heart, 29;
   _L. glacialis_, range, 34;
   _L. patagonicus_, 36;
   _L. productus_, 36;
   carapace, 20;
   telson, =23=;
   _L. viridis_, 36

 _Leptestheria_, _36_;
   _L. siliqua_, 37

 _Leptochela_, _163_

 _Leptochelia_, _122_;
   _L. dubia_, dimorphism, 123

 _Leptoctenus_, _418_

 _Leptodora_, _54_;
   appendages, 42;
   alimentary canal, 43;
   ovary, 44, =45=;
   _L. hyalina_, =54=

 Leptodoridae, _54_

 Leptoneta, _393_

 Leptonetidae, _393_

 _Leptopelma_, _389_

 _Leptoplastus_, _247_

 Leptostraca, _111_, 242;
   defined, 6;
   segmentation, 7

 _Lernaea_, _74_;
   _L. branchialis_, =74=, =75=

 _Lernaeascus_, _73_

 Lernaeidae, _74_

 _Lernaeodiscus_, _95_

 _Lernaeopoda salmonea_, _76_

 Lernaeopodidae, _75_

 _Lernanthropus_, _68_;
   blood, 30, 68

 _Lernentoma cornuta_, =72=

 Leuckart, on Pentastomida, 490, 492;
   on development of, 494;
   on sub-genera of, 495

 _Leuckartia flavicornis_, _59_

 _Leucon_, _121_

 Leuconidae, _121_

 _Leucosia_, _188_

 Leucosiidae, _188_;
   respiration, 187;
   habitat, 199

 _Leydigia_, _53_

 Lhwyd, Edward, on Trilobites, 221

 Lichadidae, _252_

 _Lichas_, 222, _252_

 Lichomolgidae, _70_

 _Lichomolgus_, _71_;
   _L. agilis_, =71=;
   _L. albeus_, 71

 _Ligia oceanica_, =128=

 _Ligidium_, _129_

 Lilljeborg, on Cladocera, 51 n.

 _Limnadia_, 21, 22, _36_;
   _L. lenticularis_, 22, 36

 Limnadiidae, 20, 23, 28, 29, _36_

 _Limnetis_, 20, 21, 22, _36_;
   _L. brachyura_, =21=, 24, _36_

 Limnocharinae, _472_

 _Limnocharis aquaticus_, _472_

 _Limulus_, 256, 292;
   nervous system, 257;
   classification, 260, 276;
   segmentation, 260, =261=, =262=, =266=, =270=, 272;
   appendages, 263;
   habits, 265, 271;
   food, 267;
   digestive system, 268;
   circulatory system, 268;
   respiratory system, 269;
   excretory system, 270;
   nervous system, 270, =272=;
   eggs and larvae, 274, =275=;
   ecdysis, 274;
   used as food, 275–6;
   affinities, 277;
   fossil, 277;
   _L. gigas_, 276;
   _L. hoeveni_, 277;
   _L. longispina_, 264, 274;
   _L. moluccanus_, 264, 274, 276, 277;
   _L. polyphemus_, =261=, =262=, 264, 271;
   _L. rotundicauda_, 275, 277;
   _L. tridentatus_, 276

 Lindström, on facial suture of _Agnostus_ and _Olenellus_, 225;
   on eyes of Trilobites, 228 f.;
   on blind Trilobites, 231 f.;
   on maculae of Trilobites, 233

 Lingua, 459

 _Linguatula_, 488 n., 495;
   _L. pusilla_, 496;
   _L. recurvata_, 496;
   _L. subtriquetra_, 496;
   _L. taenioides_, 489, 492, 493, 494, 496;
   frequency of, 489;
   larvae of, 489, 494;
   hosts of, 496

 Linnaeus, 408 n., 502

 _Linyphia_, _406_;
   _L. clathrata_, 406;
   _L. marginata_, 406;
   _L. montana_, 406;
   _L. triangularis_, 406

 Linyphiinae, _405_

 _Liobunum_, 447, _450_

 Liocraninae, _397_

 _Liocranum_, _397_

 Liphistiidae, _386_

 Liphistioidae, 383

 _Liphistius_, 317, 383, 385, _386_;
   _L. desultor_, =386=

 Liriopsidae, _130_

 _Lispognathus thompsoni_, eyes, 149

 Lister, M., 341, 342

 _Lithodes_, _181_;
   _L. maia_, 176, =177=, =178=

 Lithodidae, _181_;
   evolution of, 176 f.

 Lithodinae, _181_;
   distribution, 199, 201

 _Lithoglyptes_, _92_;
   _L. varians_, 93

 _Lithotrya_, _87_;
   _L. dorsalis_, =87=

 _Lithyphantes_, _404_

 Littoral region, of sea, 197;
   of lakes, 206

 Liver (gastric glands), of Crustacea, 14;
   of Branchiopods, 29;
   of _Limulus_, 268;
   of Arachnids, 304 f., 331

 Lobster, distribution, 199;
   Mysis stage, 153;
   natural history, 154 f.

 Lockwood, on habits of _Limulus_, 265, 271

 Loeb, 525 n.

 Loman, 331, 514, 525

 Lönnberg, 425

 _Lophocarenum insanum_, =405=

 _Lophogaster_, _119_

 Lophogastridae, 113, 114, _119_

 Loricata, _165_

 Lounsbury, 456, 461

 Love-dances, among spiders, 381

 Lovén, on Trilobites, 226

 _Loxosceles_, _393_

 Lubbock, 375

 Lucas, 364

 _Lucifer_, _162_

 Lung-books, 297, 308, 336;
   origin of, 305

 _Lupa_, _191_;
   _L. hastata_, =191=;
   resemblance to _Matuta_, 187, 189

 _Lycosa_, _417_;
   _L. arenicola_, 357;
   _L. carolinensis_, turret of, =357=;
   _L. fabrilis_, =417=;
   _L. ingens_, 418;
   _L. narbonensis_, 361, 366;
   _L. picta_, 357, 372, =417=;
   _L. tigrina_, 357, 369

 Lycosidae, 359, 375, 381, _417_

 _Lydella_, 479, 485;
   _L. dujardini_, 477, 486

 Lynceidae, _53_;
   alimentary canal, 43;
   winter-eggs, 48;
   reproduction, 49

 Lyncodaphniidae, _53_

 Lyonnet, 319, 320

 Lyra, 328

 Lyriform organs, 325, 422

 _Lysianassa_, _137_

 Lysianassidae, _137_

 _Lysianax punctatus_, commensal with hermit-crab, 172


 M‘Cook, 334, 339, 340, 346, 350, 352 n., 365 n., 366, 367 n., 369 n.

 M‘Coy, F., on facial suture of _Trinucleus_, 226;
   on free cheek of Trilobites, 227

 M‘Leod, 336 n.

 _Macrobiotus_, 480, 485;
   _M. ambiguus_, 487;
   _M. angusti_, 486;
   _M. annulatus_, 486;
   _M. coronifer_, 487;
   _M. crenulatus_, 487;
   _M. dispar_, 487;
   _M. dubius_, 487;
   _M. echinogenitus_, 487;
   _M. harmsworthi_, 487;
   _M. hastatus_, 487;
   _M. hufelandi_, 480, =482=, =483=, 486;
   _M. intermedius_, 486;
   _M. islandicus_, 487;
   _M. macronyx_, 477, 483, 487;
   _M. oberhäuseri_, 486;
   _M. orcadensis_, 487;
   _M. ornatus_, 487;
   _M. papillifer_, 487;
   _M. pullari_, 487;
   _M. sattleri_, 487;
   _M. schultzei_, =480=;
   _M. tetradactylus_, =478=;
   _M. tuberculatus_, 487;
   _M. zetlandicus_, 486

 _Macrocheira kämpferi_, 192

 _Macrohectopus_ (= _Constantia_), _138_, 212

 _Macrophthalmus_, _196_

 _Macrothele_, _390_

 _Macrothrix_, 37, _53_

 Macrura, _153_ f.

 Macula, 233

 _Maia_, _193_;
   distribution, 205;
   _M. squinado_, 192;
   alimentary canal, 15

 Maiidae, _193_

 Malacostraca, _110_ f.;
   defined, 6;
   classification, 113, 114;
   fresh-water, 210 f.

 Malaquin, on _Monstrilla_, 63 n.

 Male Spider, devoured by female, 380

 Malmignatte, 364, 403

 Malpighian tubes or tubules, 12, 257, 311, 331, 427, 434, 460

 Mandibles, of Crustacea, 8;
   of Arachnida, 319

 Mange, 465

 _Maracaudus_, _449_

 _Margaropus_, _469_

 Marine Spiders, 415

 _Marpissa_, _421_;
   _M. muscosa_, =420=;
   _M. pomatia_, 421

 Martins, Fr., 502

 Marx, 350

 _Masteria_, _390_

 _Mastigoproctus_, _312_

 _Mastobunus_, _449_

 Matthew, G. F., on development of Trilobites, 238

 _Matuta_, _188_;
   habitat, 198;
   _M. banksii_, =187=

 Maxilla, 8;
   of Decapoda, 152;
   of Spiders, 321

 Maxillary gland, 13

 Maxillipede, 8;
   of Copepoda, 55, 78;
   of Malacostraca, 113;
   of Zoaea, 180, 181, 182

 _Mecicobothrium_, _391_

 Mecostethi, 443, 447, _448_

 _Mecysmauchenius segmentatus_, _411_

 Meek, 363

 _Megabunus_, _450_, 451

 Megacorminae, _308_

 _Megacormus granosus_, _308_

 _Megalaspis_, 222, _249_

 Megalopa, compared to Glaucothoe, 180;
   of _Corystes cassivelaunus_, =183=

 Mégnin, 455, 457

 _Megninia_, _466_

 Meinert, 522 n.

 Meisenheimer, 511 n.

 _Melanophora_, _397_

 Mena-vodi, 362

 Menge, 319, 368, 385

 _Menneus_, _410_

 _Mermerus_, _449_

 Merostomata, _258_, 259 f.

 Mertens, Hugo, 524 n.

 _Mesochra lilljeborgi_, 62

 _Mesonacis_, _247_;
   _M. asaphoides_, larva, =240=

 Mesosoma, of Arachnida, 256;
   of _Limulus_, 260, 263;
   of _Eurypterus_, 288;
   of Scorpion, 302

 Mesothelae, _386_

 _Meta segmentata_, _408_

 Metamorphosis, of Cirripedia, 80;
   of _Sacculina_, 97;
   of Epicarida, 130, 133, =135=;
   of _Squilla_, 142, 143;
   of _Euphausia_, 144;
   discovery of, in Decapoda, 153;
   of Lobster, 156;
   of Crayfish, 157;
   of _Peneus_, 159;
   primitive nature of, in Macrura, 161;
   of Loricata, 165, 166;
   of Hermit-crab, =179=;
   of Brachyura, 181, =182=;
   of Dromiacea, 182;
   of Trilobites, =239=;
   of _Limulus_, 275;
   of Pseudoscorpions, 435;
   of Acarina, 462;
   of Pentastomida, 493 f.;
   of Pycnogons, 521 f.

 Metasoma, of Arachnida, 256;
   of _Limulus_, 260, 263;
   of _Eurypterus_, 289;
   of Scorpion, 303

 Metastigmata, _467_

 Metastoma, of Trilobites, 234;
   of Eurypterida, 287, 292

 Metazoaea, 182

 _Metopobractus rayi_, =405=

 _Metopoctea_, _452_

 _Metridia_, _59_;
   _M. lucens_, distribution, 203

 _Metronax_, _398_

 Metschnikoff, 435 n.

 _Miagrammopes_, _411_

 Miagrammopinae, _411_

 _Micaria_, _397_;
   _M. pulicaria_, =396=, 397;
   _M. scintillans_, 372

 Micariinae, _397_

 _Micariosoma_, _397_

 Michael, 460, 461, 462, 466 n.

 _Micrathena_, _410_

 _Microdiscus_, 225, 231, _245_

 _Microlyda_, _486_ n.

 _Micrommata_, _414_;
   _M. virescens_, =413=, 414

 _Microneta_, _406_

 Microniscidae, _130_

 _Migas_, _387_

 Miginae, _387_

 Milne-Edwards, 504

 _Milnesium_, 480, 485;
   _M. alpigenum_, _487_;
   _M. tardigradum_, 487

 _Miltia_, _396_

 Mimetidae, _411_

 _Mimetus_, _411_;
   _M. interfector_, 368

 Mimicry, in Spiders, 372

 _Mimoscorpius_, _312_

 _Miopsalis_, _448_

 _Misumena_, _412_;
   _M. vatia_, 371, 373, 412

 Mites, = Acarina, _q.v._

 _Moggridge_, 354, 355 n.

 _Moggridgea_, _387_

 _Moina_, 37, _52_;
   reproduction, =46=, =47=, 48, 49;
   _M. rectirostris_, =46=, =47=, =52=

 Mole-crab, 170

 _Monochetus_, _465_

 _Monolistra_ (Sphaeromidae), habitat, 211

 _Monopsilus_, _54_

 Monostichous eyes, 301

 _Monstrilla_, _64_

 Moustrillidae, _63_

 Morgan, 517, 518, 521

 Mortimer, Cromwell, on Trilobites, 221

 Mosaic vision, 147

 Moseley, 523

 Moulting (Ecdysis), 154, 155, 225, 338

 Mouth, of Trilobites, 234

 Mud-mites, 472

 Müller, F., on Tanaids, 123

 Müller, O. F., on position of Tardigrada, 483

 _Munidopsis_, _170_;
   eyes, 149;
   _M. hamata_, =168=

 Munnopsidae, _128_

 _Munnopsis typica_, =127=

 Murray, 455

 Murray, J., on British Tardigrada, 485

 Muscular system, in Tardigrada, 481;
   in Pentastomida, 490

 _Mygale_, 337, _386_ n., 389

 Mygalidae, = Aviculariidae, _q.v._

 _Myrmarachne formicaria_, _421_

 _Myrmecium_, _397_

 _Myrtale perroti_, _387_

 Mysidacea, _118_

 Mysidae, 113, 114, _119_;
   habitat, 201;
   relation to _Nebalia_, 112

 _Mysis_, _120_;
   maxillipede, =10=, 11;
   resemblance to _Paranaspides_, 117;
   _M. oculata_, var. _relicta_, 120, 210;
   _M. vulgaris_, 118

 Mysis-larva, of Lobster, 156;
   of _Peneus_, =161=

 _Mytilicola_, _68_


 _Nanodamon_, _313_

 Nauplius, of _Haemocera danae_, =64=;
   of _Lepas fascicularis_, =81=;
   of _Sacculina_, =97=;
   of _Euphausia_, 144;
   an ancestral larval form, 145;
   of _Peneus_, =159=;
   compared with Protaspis, 239

 _Nebalia_, _111_, 112, 114;
   segmentation, 6, 7;
   limbs, =10=, 11;
   relation to Cumacea, 120;
   compared with Trilobita, 242;
   _N. geoffroyi_, =111=

 _Nebo_, _307_

 Neck-furrow, 224

 _Nemastoma_, 443, _451_;
   _N. chrysomelas_, 452;
   _N. lugubre_, =452=

 Nemastomatidae, _451_

 _Nematocarcinus_, _163_

 _Nemesia_, _388_;
   _N. congena_, 355, 357

 _Neolimulus_, 278, 279

 _Neoniphargus_, distribution, 216

 _Neopallene_, _537_

 _Nephila_, _408_;
   _N. chrysogaster_, 380;
   _N. plumipes_, 366

 Nephilinae, _408_

 _Nephrops_, _154_;
   _N. andamanica_, distribution, 205;
   _N. norwegica_, 205

 Nephropsidae, _154_;
   resemblance to Dromiacea, 184

 _Neptunus_, _191_;
   _N. sayi_, habitat, 202

 Nereicolidae, _73_

 Nervous system, of Crustacea, 5;
   of Branchiopoda, 30;
   of _Squilla_, 142;
   of Arachnida, 257;
   of _Limulus_, 270;
   of Scorpions, 305;
   of Pedipalpi, 311;
   of Spiders, =332=, 333;
   of Solifugae, =428=;
   of Pseudoscorpions, 434;
   of Phalangidea, 445, =446=;
   of Acarina, 460;
   of Tardigrada, 482;
   of Pentastomida, 491;
   of Pycnogons, 516

 Neumann, 470

 Nicodaminae, _416_

 _Nicodamus_, _416_

 _Nicothoe astaci_, _68_

 _Nileus_, 229, _249_;
   _N. armadillo_, eye, =228=

 _Niobe_, _249_

 _Niphargoides_, _138_

 _Niphargus_, 137, _138_;
   distribution, 216;
   _N. forelii_, 138;
   _N. puteanus_, habitat, 209, 210

 _Nogagus_, _73_

 _Nops_, 315, 336, _395_

 Norman, A. M., 540

 _Notaspis_, _467_

 _Nothrus_, _468_

 _Notodelphys_, _66_

 Notostigmata, _473_

 _Nyctalops_, _312_

 _Nycteribia_ (Diptera), 526

 Nymph, 463

 _Nymphon_, 503, _536_;
   _N. brevicaudatum_, 507, 536;
   _N. brevicollum_, 511, 521;
   _N. brevirostre_, =503=, =504=, =506=, =508=, =509=, 541, 542;
   _N. elegans_, =506=, 542;
   _N. femoratum_, 541;
   _N. gallicum_, 541;
   _N. gracile_, 511, 541, 542;
   _N. gracilipes_, 542;
   _N. grossipes_, 541;
   _N. hamatum_, 512;
   _N. hirtipes_, 542;
   _N. horridum_, 537;
   _N. johnstoni_, 541;
   _N. leptocheles_, 542;
   _N. longitarse_, 541, 542;
   _N. macronyx_, 542;
   _N. macrum_, 542;
   _N. minutum_, 541;
   _N. mixtum_, 541;
   _N. pellucidum_, 541;
   _N. rubrum_, 541, 542;
   _N. serratum_, 542;
   _N. simile_, 541;
   _N. sluiteri_, 542;
   _N. spinosum_, 541;
   _N. stenocheir_, 542;
   _N. strömii_, =509=, 541

 Nymphonidae, _536_

 Nymphopsinae, _535_ n.

 _Nymphopsis_, _534_, 535 n.;
   _N. korotnevi_, 534;
   _N. muscosus_, 534


 Obisiinae, 436, _437_

 _Obisium_, 436, _438_

 _Ochyrocera_, _393_

 Octomeridae, _91_

 _Octomeris_, _91_

 _Ocyale mirabilis_, _416_

 _Ocypoda_, 194, _196_;
   habitat, 198;
   distribution, 201

 Ocypodidae, _196_

 Oecobiidae, 386 n., _392_

 _Oecobius_, _392_;
   _Oe. maculatus_, =392=

 Oehlert, on facial suture of _Trinucleus_, 226

 _Ogovia_, _448_

 _Ogygia_, _249_

 _Oiceobathes_, _535_

 _Oithona_, _61_;
   _O. nana_, 203;
   _O. plumifera_, 203

 _Olenelloides_, _247_;
   _O. armatus_, =247=

 _Olenellus_, 225, 227, 232, 236, _247_

 Olenidae, _247_

 _Olenus_, 232, _247_;
   _O. truncatus_, =248=

 _Oligolophus_, _450_;
   _O. agrestis_, 450;
   _O. spinosus_, =441=, 450, =451=

 _Olpium_, 436, _437_;
   _O. pallipes_, =437=

 Ommatoids, 310, 311, 312

 _Oncaea_, _69_;
   _O. conifera_, phosphorescence, 60

 Oncaeidae, _69_

 Oniscoida, _128_

 _Oniscus_, _129_

 _Ononis hispanica_, Spiders on, 419

 Onychium, 324

 _Oomerus stigmatophorus_, _539_

 Oonopidae, 336, _393_

 _Oonops_, _394_;
   _O. pulcher_, 366, 394

 _Oorhynchus_, 507, _535_;
   _O. aucklandiae_, 535

 Oostegites, of Malacostraca, 114

 Operculata, _89_, =91=

 _Ophiocamptus_ (_Moraria_), _62_;
   _O. brevipes_, 62

 _Opilioacarus_, 454, _473_;
   _O. arabicus_, 473;
   _O. italicus_, 473;
   _O. platensis_, 473;
   _O. segmentatus_, 473

 Opiliones (= Phalangidea, _q.v._), _440_

 _Opisthacanthus_, _307_

 Opisthoparia, 244

 _Opisthophthalmus_, _307_

 Opisthothelae, _386_

 _Opopaea_, _394_

 _Orchestia_, _139_;
   hermaphroditism, 104;
   _O. gammarellus_, 137, 139;
   habitat, 211

 _Orchestina_, _394_

 _Oribata_, _467_

 Oribatidae, 457, 458, 459, 460, 462, _467_;
   anatomy, =459=

 _Orithyia coccinea_, 524, 540

 _Ornithodoros_, _469_;
   _O. megnini_, 469;
   _O. moubata_, 469;
   _O. talaje_, =469=;
   _O. turicata_, 469

 _Ornithoscatoides_, 374

 _Orometopus_, 226, _245_;
   _O. elatifrons_, =230=

 Ortmann, on Brachyura, 181 n.;
   on bipolarity, 200;
   on crayfishes, 213;
   on Pycnogons, 513 n.

 Ostracoda, _107_;
   pelagic, 202

 Oudemans, 528 n.

 Ovary, of Cladocera, 44, =45=;
   of _Danalia_, =132=;
   of Spiders, =332=

 _Oxynaspis_, _88_

 _Oxyopes_, _419_;
   _O. lineata_, 419

 Oxyopidae, _419_

 _Oxyptila_, _412_

 Oxyrhyncha, _191_ f.;
   habits, 192;
   enemies, 192;
   habitat, 198

 Oxystomata, _185_ f.;
   respiration, 186, 187


 _Pachycheles_, _170_;
   _P. panamensis_, distribution, 202

 _Pachygnatha_, _407_;
   _P. clerckii_, 407;
   _P. degeerii_, 407;
   _P. listeri_, 407

 _Pachygrapsus_, _196_;
   _P. marmoratus_, 193, =194=, 196

 _Pachylasma giganteum_, _91_

 _Pachylomerus_, _388_

 _Pachysoma_, _69_

 Pagurian, _180_;
   eyes of deep-sea, 149, 150

 Paguridea, _171_

 Pagurinae, _180_

 _Palaemon_, _164_;
   excretory glands, 13;
   fresh-water, 212;
   _P. serratus_, 158, 164;
   _Bopyrus_ parasitic on, 133

 _Palaemonetes_, _164_;
   _P. antrorum_, habitat, 210;
   _P. varians_, 161;
   distribution, 212

 Palaemonidae, 159, _164_

 _Palaeocaris_, _115_, 118

 _Palaeophonus_, 294, 298

 _Palamnaeus_, _307_;
   _P. swammerdami_, tarsus, =304=

 Palinuridae, _167_

 _Palinurus_, 165, _167_;
   habitat, 198, 202;
   _P. elephas_, 167;
   _P. quadricornis_, embryo, =165=

 _Pallene_, 505, _537_;
   _P. attenuata_, 541;
   _P. brevirostris_, =510=, 524, =537=, 541, 542;
   _P. dimorpha_, 538;
   _P. emaciata_, 541;
   _P. empusa_, 541;
   _P. grubii_, 538;
   _P. languida_, 537;
   _P. longiceps_, 538;
   _P. novaezealandiae_, 537;
   _P. producta_, 542;
   _P. pygmaea_, 537, 541;
   _P. spectrum_, 542;
   _P. spinosa_, 537

 Pallenidae, _537_

 _Pallenopsis_, _506_, 511;
   _P. holti_, 542;
   _P. tritonis_, 542

 Palp, of Pycnogons, 507

 Palpal organ, =322=, 378

 Palpebral lobe, 227

 Palpigradi, _258_, _422_

 Palpimanidae, 323, 325, _398_

 _Palpimanus_, _398_

 _Panamomops diceros_, _405_

 Pandalidae, _164_

 _Pandalus_, _164_;
   _P. annulicornis_, 164

 _Pandinus_, _307_

 _Panoplax_, _195_

 Pantopoda, 501 n. (= Pycnogonida, _q.v._)

 _Panulirus_, 165, _167_

 _Parabolina_, 232, _247_

 _Parabolinella_, _247_

 _Parabuthus_, 298;
   _P. capensis_, 298, 299

 _Paradoxides_, 222, 232, 236, _247_;
   _P. bohemicus_, =246=

 _Paragaleodes_, _429_

 _Paralomis_, 179, _181_

 _Paranaspides_, _117_;
   _P. lacustris_, 117;
   distribution, 210;
   habitat, 210

 _Paranebalia_, 242

 _Paranephrops_, _157_;
   distribution, 213

 _Paranthura_, _124_

 _Parantipathes_, _Synagoga_ parasitic on, 94

 _Paranymphon_, 507;
   _P. spinosum_, 542

 _Parapagurus_, _180_

 _Parapallene_, _537_

 _Parapeneus_, _162_;
   _P. rectacutus_, 159

 _Parapylocheles scorpio_, eyes, 149

 _Parasiro_, _448_;
   _P. corsicus_, =448=

 Parasites, in Tardigrada, 484

 Parasitic castration, 100, 136

 Parastacidae, _157_;
   distribution, 213

 _Parastacus_, _157_;
   distribution, 213

 Paratropidinae, _387_

 _Paratropis scrupea_, _387_

 _Parazetes auchenicus_, _533_

 _Pardosa_, _417_;
   female carrying young, =341=;
   _P. amentata_, =417=, 418;
   _P. lugubris_, 418

 _Pariboea spinipalpis_, _534_

 Parthenogenesis, in Phyllopoda, 32;
   in Cladocera, 44, 46, 49;
   in Ostracoda, 108

 _Parthenope_, _193_;
   _P. investigatoris_, 192

 Parthenopidae, _193_

 _Pasiphaea_, _163_

 Pasiphaeidae, _163_

 _Pasithoe_, _532_;
   _P. umbonata_, 535;
   _P. vesiculosa_, 535, 541

 Pasithoidae, _532_

 Patten, 270, 271, 277

 Patten and Redenbaugh, on _Limulus_, =266=, 270, =272=

 Paturon, 319, =320=

 Peckham, 376, 377, 378, 381, 382

 Pecten, 328

 Pectines, of Scorpions, 302, =302=;
   function of, 299;
   of _Glytoscorpius_, 294

 Pedicle, 317

 Pedipalpi, _258_, _308_;
   habits, 309;
   external structure, 309;
   legs, 309;
   internal structure, 310;
   alimentary canal, 310;
   nervous system, 311;
   classification, 312

 Pedipalpi (appendages), 263, 303, 309, 321, 422, 426, 433, 440, 458

 Pedunculata, _84_

 Pelagic Crustacea, marine, 202;
   lacustrine, 207

 _Pelops_, _467_

 Peltiidae, _63_

 _Peltogaster_, _95_;
   structure, =95=;
   males, 99;
   castration caused by, 100;
   _P. curvatus_, castration caused by, 100;
   _P. sulcatus_, 95

 _Peltura_, _247_

 Peneidae, _162_

 Peneidea, _158_, _162_;
   metamorphosis, 159

 _Penella sagitta_, _74_

 _Peneus_, 158, _162_;
   metamorphosis, 159, =159=, =160=, =161=

 _Pentanymphon_, 504, _537_

 Pentaspidae, _87_

 _Pentastoma_, 488 n.;
   _P. denticulatum_, 489, 494;
   _P. emarginatum_, 489;
   _P. serratum_, 489

 Pentastomida, _258_, _488_ f.;
   structure, 489;
   habitat, 488;
   life-history, 488, 493;
   hosts of, 496, 497

 _Pephredo hirsuta_, _535_, 541

 _Peracantha_, =43=, _53_;
   alimentary canal, 43

 Peracarida, 114, _118_

 Pereiopod, defined, 110;
   reduced hind, in Galatheidea, 168;
   in Hippidea, 170;
   in Paguridea, 172;
   in Dromiacea, 184;
   in Oxystomata, 185

 _Periegops hirsutus_, _393_

 _Peroderma cylindricum_, _75_

 _Petrarca bathyactidis_, _93_

 _Pettalus_, _448_

 _Pezomachus gracilis_, parasitic in cocoons of Spiders, 367

 Phacopidae, _249_

 Phacopini, 243

 _Phacops_, 223, 232, 235, _249_;
   _P. latifrons_, =227=;
   _P. sternbergi_, =248=

 _Phaeocedus braccatus_, _397_

 Phagocytes, in _Danalia_, 132

 Phalangidea, _258_, _440_ f.;
   habits, 441;
   external structure, 442;
   internal structure, 444;
   nervous system, =446=;
   classification, 447;
   British species, 453

 Phalangiidae, _449_

 Phalangiinae, _450_

 _Phalangium_, 444, _450_, 526;
   mouth-parts, =443=;
   _P. balaenarum_, 502;
   _P. cornutum_, 450;
   _P. littorale_, 501;
   _P. opilio_, 445, 446, 450, 526

 _Phalangodes_, _449_;
   _P. armata_, 449;
   _P. terricola_, =449=

 Phalangodidae, _448_

 _Phanodemus_, _535_

 _Phidippus_, _421_;
   _P. morsitans_, 365, 421

 Philichthyidae, _73_

 _Philichthys_, _73_;
   _P. xiphiae_, 73 n.

 _Phillipsia_, _251_;
   _P. gemmulifera_, =250=

 Philodrominae, _413_

 _Philodromus_, _413_;
   _P. aureolus_, 413;
   _P. margaritatus_, =413=

 _Philoscia muscorum_, _129_

 Pholcidae, 336, _401_

 _Pholcus_, 320, _401_;
   _P. phalangioides_, 401

 _Phoroncidia_, _404_;
   _P. 7–aculeata_, =318=

 Phoroncidiinae, 317, _404_

 Phosphorescence, of Copepoda, 59;
   relation to eyes in deep-sea Crustacea, 150, 151

 Phosphorescent organs, of Euphausiidae, 145;
   of _Stylocheiron mastigophorum_, =151=

 Phoxichilidae, _539_

 Phoxichilidiidae, _538_

 _Phoxichilidium_, 506, 512, 520, 521 n., =523=, 525, _538_;
   _P. angulatum_, 520;
   _P. exiguum_, 541;
   _P. femoratum_, =508=, 524, =538=, 540;
   _P. globosum_, 540;
   _P. mollissimum_, 517;
   _P. olivaceum_, 540

 _Phoxichilus_, 505, 512, _539_;
   _P. australis_, 539, 540;
   _P. böhmii_, 539;
   _P. charybdaeus_, 514, 515, 539;
   _P. laevis_, 537, 539, 541;
   _P. meridionalis_, 539;
   _P. mollis_, 539;
   _P. proboscideus_, 532;
   _P. procerus_, 539;
   _P. spinosus_, =505=, =508=, =510=, 537, 539, 541, 542;
   _P. vulgaris_, 539

 Phreatoicidae, _136_;
   distribution, 211, 217

 Phreatoicidea, _136_

 _Phreatoicopsis_, _136_;
   distribution, 211

 _Phreatoicus_, _136_;
   distribution, 210, 211, 217;
   _P. assimilis_, habitat, 210;
   _P. typicus_, habitat, 210

 _Phronima_, _140_;
   _P. sedentaria_, =140=

 _Phrynarachne_, _414_;
   _P. decipiens_, 374, 414

 Phrynichinae, _313_

 _Phrynichus_, _313_

 Phrynidae, 309, 310, _312_

 _Phrynopsis_, _313_

 _Phrynus_, _312_

 Phryxidae, _130_

 Phyllocarida, _111_, 242

 _Phyllocoptes_, _465_

 Phyllopoda, _19_ f.;
   appendages, 24 f.;
   alimentary canal, 29;
   vascular system, 29;
   nervous system, 30;
   reproductive organs, 31;
   habitat, 32;
   genera, 35

 Phyllosoma, larva of _Palinurus_, =166=

 Phytoptidae, _464_

 _Phytoptus_, 464 n., 495 (= _Eriophyes_, _q.v._)

 Pickard-Cambridge, F., 352

 Pickard-Cambridge, O., 318, 321 n., 323 n., 359 n., 372, 374, 380, 385,
    401 n., 436, 438, 450, 451, 452

 Pillai, 375

 _Pilumnus_, _191_

 _Pinnotheres pisum_, _195_

 Pinnotheridae, _195_

 _Pipetta_, 514, _533_;
   _P. weberi_, 533

 _Pirata_, _417_

 Piriform glands, =335=, 349

 _Pisa_, _193_

 _Pisaura mirabilis_, _416_

 Pisauridae, _416_

 _Placoparia_, _251_

 Plagiostethi, 443, 447, _449_, 452

 Plagula, 317

 _Planes minutus_, habitat, 202

 Plankton, characters of, 203;
   fresh-water, 207, 216;
   Cladocera in, 50

 Plastron, 316

 Plate, on Tardigrada, 481, 482, 484

 _Plator insolens_, _415_

 Platoridae, _415_

 _Platyarthrus hoffmannseggii_, _129_

 _Platyaspis_, _121_

 _Platybunus_, 450, _451_

 _Platycheles_, _535_

 _Plectreurys_, _393_

 Pleopod, defined, 110

 Pleura, 234 f.

 _Pleurocrypta microbranchiata_, =133=

 _Pleuromma_, _59_;
   _P. abdominale_, 59;
   _P. gracile_, 59

 _Pliobothrus symmetricus_, Pycnogon larvae in, 523

 Pocock, 298, 308 n., 312, 328, 329, 425 n., 534 n.

 Podasconidae, _130_

 Podogona, _258_, _439_

 _Podon_, _54_

 Podophthalmata, 112

 Podoplea, _61_

 Podosomata, 501 n. (= Pycnogonida, _q.v._)

 _Poecilotheria_, _390_

 Poisonous hairs, of Spiders, 365

 _Pollicipes_, _84_;
   fertilisation, 86;
   _P. cornucopia_, 85;
   _P. mitella_, =85=

 Pollock, 340

 Poltyinae, _410_

 _Poltys_, _410_;
   _P. ideae_, =318=

 _Polyartemia_, _36_;
   antennae, 26, 28;
   range of, 34;
   _P. forcipata_, 36

 Polyaspidae, _84_

 Polycopidae, _109_

 _Polygonopus_, _539_

 Polyphemidae, _54_;
   appendages, 42;
   ovary, 47;
   reproduction, 49

 _Polyphemus_, 47, _54_;
   _P. pediculus_, habitat, 206, 208

 _Polysphincta carbonaria_, parasitic on Spiders, 368

 Pompeckj, on Calymenidae, 244

 _Pompilus_, 368

 Pontellidae, _60_

 _Pontoporeia_, _137_;
   distribution, 212;
   _P. affinis_, 138;
   _P. femorata_, 138;
   _P. loyi_, 138;
   _P. microphthalma_, 138

 _Porcellana_, 168, _170_;
   Zoaea, 168;
   _P. platycheles_, 170

 Porcellanidae, _170_;
   habitat, 198

 _Porcellio_, _129_

 Porcupine, 540

 _Porhomma_, _406_

 _Porocephalus_, 488 n., 495;
   _P. annulatus_, =490=, 496;
   _P. aonycis_, 496;
   _P. armillatus_, 496;
   _P. bifurcatus_, 496;
   _P. clavatus_, 496;
   _P. crocidura_, 496;
   _P. crotali_, 496;
   _P. geckonis_, 496;
   _P. gracilis_, 496;
   _P. heterodontis_, 496;
   _P. indicus_, 496;
   _P. lari_, 496;
   _P. megacephalus_, 497;
   _P. megastomus_, 497;
   _P. moniliformis_, 497;
   _P. najae sputatricis_, 497;
   _P. oxycephalus_, 497;
   _P. platycephalus_, 497;
   _P. proboscideus_, =493=, 494;
     larvae of, 493, =494=;
     hosts of, 496;
   _P. protelis_, larva, =495=;
   _P. subuliferus_, 497;
   _P. teretiusculus_, 489, =491=, 492, =492=, 497;
   _P. tortus_, 497

 Portunidae, _191_

 _Portunion_, _134_;
   _P. maenadis_, 134;
   life-history, =135=, 136

 _Portunus_, _191_

 _Potamobius_ (= _Astacus_), _157_;
   distribution, 213

 _Potamocarcinus_, _191_;
   distribution, 213

 _Potamon_, _191_

 Potamonidae, _191_

 Praniza, larva of _Gnathia_, 125

 Prawn, 151, 153, 158, 164, 198;
   fresh-water, 212, 214

 Pre-epistome, 443

 _Prestwichia_ (_Euproöps_), 275, =278=, 279

 Preyer, on anabiosis in Tardigrades, 484

 _Prionurus_, 298, 299

 Prismatic eye, of Trilobites, 229

 Procurved eyes, 316

 Prodidomidae, _395_

 _Prodidomus_, _396_

 Proëtidae, _251_

 _Proëtus_, _251_;
   _P. bohemicus_, =248=

 _Prokoenenia_, _423_;
   _P. chilensis_, 423;
   _P. wheeleri_, 423

 _Prolimulus_, 279

 Promesosternite, in _Limulus_, 264

 Proparia, 244

 _Prosalpia_, _450_

 Prosoma, of Arachnida, 260;
   of _Limulus_, 260, 263;
   of Eurypterida, 285;
   of Scorpion, _301_

 _Prosthesima_, _397_

 Prostigmata, _471_

 Protaspis, 239, =239=, =240=

 _Proteolepas_, _94_;
   _P. bivincta_, =94=

 _Protocaris_, 243

 _Protolenus_, _247_

 _Protolimulus_, 279

 _Protolycosa anthrocophila_, 383

 Przibram, on regeneration in Crustacea, 156

 Psalidopodidae, _164_;
   habitat, 204

 _Psalidopus_, _164_

 _Psalistops_, _389_

 Psechridae, _399_

 _Psechrus_, _399_

 _Pseudalibrotus_, _137_

 _Pseudidiops_, _388_

 _Pseudocuma_, _121_;
   distribution, 215

 Pseudocumidae, _121_

 _Pseudoniscus_, 279

 _Pseudopallene_, 511, _537_;
   _P. circularis_, 540;
   _P. spinipes_, 537 n.

 Pseudoscorpiones, _258_, _430_ f.;
   habits, 430;
   external structure, 431, =432=;
   internal structure, 433;
   development, 434, =435=;
   classification, 436;
   British species, 438

 Pseudo-stigmatic organs, 467

 Pseudozoaea, larva of Stomatopod, 143

 _Pterocuma_, _121_

 _Pterolichus_, _466_

 _Pteronyssus_, _466_

 _Pterygometopus_, _249_

 _Pterygotus_, 283, 291, 292;
   _P. osiliensis_, =290=

 _Ptychoparia_, _247_

 _Pucetia viridis_, _419_

 Pupa, of Cirripedia, 81, =82=

 _Purcellia_, _448_

 Pychnogonides, 501 n.

 Pycnogonida, _501_ f.;
   body, 505;
   chelophores, 505;
   palpi, 507;
   ovigerous legs, 507;
   glands, 511;
   alimentary system, 513;
   circulatory system, 516;
   nervous system, 516;
   eyes, 517;
   integument, 518;
   reproductive organs, 519;
   eggs, 520;
   development, 520;
   habits, 524;
   systematic position, 525;
   classification, 528 f.;
   British species, 540 f.

 Pycnogonidae, _539_

 _Pycnogonum_, 503, _539_;
   _P. australe_, 540;
   _P. crassirostre_, 540;
   _P. littorale_, =501=, 540, 541;
   _P. magellanicum_, 540;
   _P. magnirostre_, 540;
   _P. microps_, 540;
   _P. nodulosum_, 540;
   _P. orientale_, 540;
   _P. philippinense_, 540;
   _P. pusillum_, 540;
   _P. stearnsi_, 540

 Pygidium, 235

 _Pylocheles_, _180_;
   _P. miersii_, =173=

 Pylochelidae, _180_;
   habitat, 204

 _Pylopagurus_, _180_;
   relation to Lithodidae, 177, =178=

 _Pyrgoma_, _92_


 _Rachias_, _388_

 Railliet, on classification of Pentastomids, 495

 _Ranina dentata_, _188_

 Raninidae, _188_

 Rastellus, 320, 387

 _Ratania_, _68_;
   mouth, 63

 Réaumur, 360

 Recurved eyes, 316

 Red spider, 455, 472

 Red-water, 456

 Regeneration, of Crustacean limbs, 155, 156

 _Regillus_, _414_

 Reichenbach, on embryology of _Astacus_, 12

 _Reighardia_, 495, 497;
   hosts of, 497

 _Remipes_, _171_;
   _R. scutellatus_, =171=

 _Remopleurides_, 232, _247_;
   _R. radians_, 229, =248=

 Reproduction (incl. Breeding), of Cladocera, 43 f.;
   of _Anaspides_, 116;
   of Lobster, 156;
   of _Limulus_, 274;
   of Spiders, 365; of Ticks, 461;
   of Pycnogons, 520

 Reproductive (generative) organs, of Crustacea, 15;
   of Phyllopods, 31;
   of Cladocera, 43;
   of Arachnids, 257;
   of _Limulus_, 271;
   of Scorpions, 305;
   of Spiders, 333;
   of Solifugae, 428;
   of Phalangidea, 446;
   of Acarina, 461;
   of Tardigrada, 482;
   of Pentastomida, 492;
   of Pycnogons, 519

 Respiration, of Crustacea, 16;
   of _Anaspides_, 115;
   of _Albunea_, 170;
   of _Corystes_, 170, 189
   of _Birgus_, 174;
   of Oxystomata, 186, 187;
   of Catometopa, 194, 195;
   of Arachnids, 256.
   (See also Respiratory organs.)

 Respiratory organs, of Arachnids, 256;
   of _Limulus_, 269, =270=;
   of Eurypterids, 288;
   of Scorpions, 305;
   of Spiders, 336;
   of Tardigrada, 482;
   of Pentastomida, 491.
   (See also Respiration.)

 _Rhagodes_, =425=, _429_

 Rhagodinae, _429_

 _Rhax_, _429_

 _Rhipicentor_, _469_

 _Rhipicephalus_, _469_;
   _R. sanguineus_, 470

 Rhizocephala, _95_ f.;
   compared with _Monstrilla_, 66;
   with _Anelasma_, 89;
   castration caused by, 100;
   males, 106;
   association with Entoniscidae, 136

 _Rhomphaea_, _402_

 _Rhopalorhynchus_, _532_;
   _R. clavipes_, 533;
   _R. kröyeri_, 533;
   _R. tenuissimus_, 533

 Rhynchothoracidae, _535_

 _Rhynchothorax_, 505, _535_;
   _R. australis_, 536;
   _R. mediterraneus_, =508=, 535, =536=

 Ricinulei, _439_

 Robber-crab, 173

 _Roncus_, 436, _438_

 Rucker, 423

 Rudolphi, on _Pentastoma_, 488 n.


 _Sabacon_, _451_

 _Sabelliphilus_, _71_

 _Sacculina_, _95_;
   life-history, 96 f.;
   males, =99=;
   castration caused by, 100 f.;
   _S. carcini_, _96_;
   _S. neglecta_, Nauplius, =97=;
   Cypris, =97=;
   internal stages, =98=;
   with parasitic _Danalia_, 130, =131=

 _Saitis_, _421_;
   _S. pulex_, 382, 421

 Salter, on facial suture of _Trinucleus_, 226;
   on classification of Trilobites, 243

 Salticidae, _419_

 _Salticus_, _420_;
   _S. scenicus_, 372, 376, =420=

 _Sao_, 235, _247_;
   _S. hirsuta_, development, =239=

 _Sapphirina_, _69_;
   colour, 60;
   _S. opalina_, 69

 _Sarcoptes_, _466_;
   _S. mutans_, =466=

 Sarcoptidae, 455, _466_

 Sarcoptinae, _466_

 Sars, G. O., on Calanidae, 58;
   on Isopoda, 122;
   on Crustacea of the Caspian, 215;
   on Pycnogons, 504

 Savigny, 526

 _Scaeorhynchus_, _533_

 _Scalidognathus_, _388_

 _Scalpellum_, _84_, 85;
   complemental male, =86=;
   sex, 86, 105 f.;
   _S. balanoides_, sex, 86;
   _S. ornatum_, sex, 87;
   _S. peronii_, male, =86=;
   sex, 87, 105;
   _S. velutinum_, sex, 87;
   _S. vulgare_, 85, =86=;
   male, =83=;
   sex, 86, 87

 Scaphognathite, 152

 _Scapholeberis_, 39, _52_;
   _S. mucronata_, =52=

 Schimkewitsch, 527, 534

 Schizochroal eye, =228=, 229

 Schizonotidae, 310, _312_

 _Schizonotus_, _312_

 Schizopoda, 112;
   re-classification, 113;
   relation to Macrura, 162

 _Schizorhynchus_, _121_

 Schmeil, on fresh-water Copepoda, 59, 62

 Schultze, on position of Tardigrada, 483

 _Scipiolus_, _535_

 _Sclerocrangon_, distribution, 200

 _Sclerosoma_, _450_;
   _S. quadridentatum_, =450=

 Sclerosomatinae, _449_

 _Scodra_, _390_

 Scopula, 324, =324=, 389 n.

 _Scorpio_, 305, _307_;
   _S. boehmi_, 307;
   _S. maurus_, 307

 Scorpion, 297 f.

 Scorpionidae, _306_

 Scorpionidea, _258_, _297_ f.;
   habits, 298;
   senses, 299;
   poison, 299, 301;
   mating habits, 300;
   external structure, 301;
   prosoma, 301;
   pre-cheliceral segment, 301;
   development of eyes, 301;
   mesosoma, 302;
   metasoma, 303;
   appendages, 303;
   pedal spurs, =304=, 306, 307, 308;
   tibial spurs, =304=, 306, 307, 308;
   internal anatomy, 304;
   alimentary canal, 304;
   vascular system, 305;
   nervous system, 305;
   endosternite, 305;
   generative organ, 305;
   development of, 305;
   classification, 306;
   fossil, 298;
   resemblance to Eurypterids, 292

 Scorpioninae, 306, _307_

 _Scorpiops_, _308_

 _Scotinoecus_, _390_

 Scott, on fish-parasites, 69 n.

 Scourfield, on Cladocera, 51 n.

 Scutum, of Spiders, 317, 394;
   of Ticks, 469

 _Scyllarus_, _167_;
   _S. arctus_, =165=, 167

 _Scytodes thoracica_, _393_

 Scytodidae, _393_

 _Segestria_, _395_;
   _S. perfida_, 369;
   _S. senoculata_, 395

 Segestriinae, _395_

 Segmentation, of Crustacea, 5 f.;
   of Trilobites, 223 f.;
   of Arachnida, 256;
   of _Limulus_, 263;
   of Pycnogons, 501 f.

 Selenopinae, _414_

 _Selenops_, _414_

 Semper, 521 n.

 Senoculidae, _418_

 _Senoculus_, _418_

 Sense-organs, of Arachnids, 257;
   of _Limulus_, 271, =272=;
   of Tardigrada, 482;
   of Pentastomida, 491 (see also Auditory organ, Eyes)

 _Sergestes_, _162_

 Sergestidae, _162_;
   Zoaea, 161;
   distribution, 204

 Serolidae, _126_

 _Serolis_, _126_;
   distribution, 200;
   _S. antarctica_, _S. bronleyana_, _S. schytei_—eyes, 149

 Serrula, 322, 433

 _Sesarma_, _196_;
   distribution, 213

 _Setella_, _61_

 Sex, in Crustacea, 100;
   in Trilobites, 235

 Sexual dimorphism, of Copepoda, 57, 67, 75;
   of _Inachus_, 103;
   of Tanaidae, 123;
   of _Gnathia_, 125;
   of Prawns, 159;
   of _Gelasimus_, 194;
   of Spiders, 379

 Sheet-webs, 352

 Shell-gland, 13

 Shipley, A. E., introduction to Arachnida, 253 f.;
   on Xiphosura, 259 f.;
   on Tardigrada, 475 f.;
   on Pentastomida, 488 f.

 Shrimp, 153, 158, 164, 198, 199

 _Shumardia_, _245_

 Shumardiidae, _245_

 Sicariidae, 327, _393_

 _Sicarius_, _393_

 _Sida_, _51_;
   reproduction, 49;
   _S. crystallina_, 22, 39, =40=

 Sididae, _51_;
   appendages, 40;
   heart, 43

 Siebold, von, 464 n.

 Sigilla, 410

 Silvestri, 473 n.

 _Simocephalus_, _52_;
   _S. vetulus_, =38=, =39=;
   appendages, =41=

 Simon, 303, 314 n., 326, 385, 386 n., 387, 391 n., 397 n., 400, 401 n.,
    406, 408 n., 414 n., 418, 431, 433, 449, 452

 _Singa_, _409_

 _Sintula_, _406_

 Siphonostomata, 56

 _Siriella_, _120_

 _Siro_, _448_

 Sironidae, _448_

 _Sitalces_, _449_

 _Slimonia_, 283, 290, 292;
   _S. acuminata_, =291=

 Smith, F., 367

 Smith, G. on Crustacea, 1 f.

 Smith, H., 373

 Smith and Kilborne, 456

 Snouted Mites, 458, 471

 _Solenopleura_, _247_

 _Solenysa_, _405_

 Solifugae, _258_, _423_ f.;
   habits, 423;
   climbing habits, 425;
   doubtfully poisonous, 424;
   external structure, 425;
   internal structure, 427;
   classification, 428

 _Solpuga_, _429_;
   _S. sericea_, 425

 Solpugae, 423

 Solpugidae, _429_

 Solpuginae, _429_

 Spallanzani, on desiccation of Tardigrada, 484

 Sparassinae, 323, _414_

 _Sparassus_, _414_

 Spencer, on Pentastomida, 489 n., 490

 Spermatheca, 15

 Spermatophore, 15

 Spermatozoa, of Crustacea, 15;
   of Malacostraca, 114

 _Spermophora_, _401_

 _Sphaerexochus_, _251_

 _Sphaeroma_, habitat, 211

 Sphaeromidae, _126_

 _Sphaeronella_, _76_

 _Sphaerophthalmus_, 232, _247_;
   _S. alatus_, eye, =228=

 Spiders, 314 f.;
   external structure, 314, =316=, =317=;
   appendages, 319 f.;
   rostrum, 320;
   maxilla, 321;
   palpal organs, 321;
   tarsi, =324=;
   spinnerets, =325=;
   stridulating organs, =327=, 404;
   internal anatomy, 329, =330=;
   alimentary system, 329;
   vascular system, 331;
   generative system, 333;
   nervous system, 333;
   sense-organs, 333;
   eyes, 315, 334, 375;
   spinning glands, 335;
   respiratory organs, 318, 336;
   coxal glands, 337;
   poison-glands, 337;
   ecdysis, 338;
   early life, 338;
   ballooning habit, 341, =342=;
   webs, 343 f.;
   nests, 354;
   cocoons, 358, =358=;
   commercial use of silk, 359;
   poison, 360;
   fertility, 365;
   cannibalism, 367;
   enemies, 368;
   protective coloration, 371;
   senses, 375 f.;
   sight, 375;
   hearing, 376;
   touch, 334;
   intelligence, 377;
   mating habits, 378;
   fossil, 383;
   classification, 384 f.

 Spinning glands, =335=

 Spinning Mites, 472

 _Spiroctenus_, _388_

 _Spongicola_, _162_

 _Squilla_, _141_, =141=, 142, 143;
   _S. desmaresti_, 141;
   _S. mantis_, 141

 Squillidae, 114, _143_;
   compared with Loricata, 166

 _Stalita_, _395_

 _Stasinopus caffrus_, _387_

 _Staurocephalus_, _251_

 _Steatoda_, _404_;
   _S. bipunctata_, =327=, 404

 Stebbing, on Amphipods, 137;
   on Pycnogons, 503 n., 527 n.

 Stecker, 447

 _Stegosoma testudo_, =318=

 _Stenochilus_, _398_

 _Stenochotheres_, _76_;
   _S. egregius_, 76, =76=

 _Stenocuma_, _121_

 Stenopodidae, _162_

 _Stenopus_, _162_

 _Stenorhynchus_, 192, _193_

 Stephanopsinae, _414_

 _Stephanopsis_, _414_

 Stiles, on larval Pentastomids, 493, 494

 Stomatopoda, 114, _141_ f.

 _Storena_, _399_

 _Strabops_, 283;
   eyes, 290

 Strauss-Durckheim, on _Limulus_, 277

 _Streblocerus_, _53_

 _Streptocephalus_, 25, _35_;
   range of, 34;
   _S. torvicornis_, 35

 Stridulating organs, in Arachnids, 257, 327, =327=, 404

 _Stygina_, _249_

 Style, of palpal organ of Spiders, 322

 _Stylocellus_, _448_

 _Stylonurus_, 283, 291;
   _S. lacoanus_, =293=

 _Sunaristes paguri_, _63_

 Sun-spiders, 423

 _Sybota_, _410_

 _Sylon_, _95_;
   sex, 99

 _Symphysurus_, _249_

 _Synageles_, _421_;
   _S. picata_, 366, 373

 _Synagoga mira_, _94_

 Syncarida, _114_

 _Synemosyna_, =420=, _421_;
   _S. formica_, 373

 _Synhomalonotus_, _249_

 _Syringophilus_, 455, _473_


 _Tachidius brevicornis_, 62;
   _T. littoralis_, 62

 Tachypleinae, _276_

 _Tachypleus_, _276_;
   _T. gigas_, 276;
   _T. hoeveni_, 277;
   _T. tridentatus_, 276

 Talitridae, _139_

 _Talitrus_, _139_;
   _T. sylvaticus_, 139;
   habitat, 211

 _Talorchestia_, _139_

 Tanaidae, _122_

 _Tanais_, _122_

 Tanganyika, Lake, prawns of, 212

 _Tanystylum_, 505, _535_;
   _T. orbiculare_, 524, 535, 541

 _Taracus_, _451_

 Tarantella, 361

 Tarantism, 361

 Tarantula (Spider), 361

 _Tarantula_, _313_;
   _T. reniformis_, 312

 Tarantulidae, 310, _312_

 Tarantulinae, _313_

 Tardigrada, _258_, _477_ f.;
   occurrence, 477;
   how to capture, 477;
   powers of resisting drying up, 484;
   classification, 485;
   British species, 486, 487

 _Tarentula_, _417_

 Tarsonemidae, _471_

 Tartaridae, _312_, 527

 _Tealia_, _Pycnogonum_ on, 524

 _Tegenaria_, _416_;
   _T. civilis_, 352;
   _T. domestica_, 416;
   palp, =321=;
   _T. parietina_, 352, 416

 _Telema tenella_, _393_

 Telson, 6, 7;
   of Phyllopoda, 22

 _Temora longicornis_, distribution, 203

 _Tethys_ (Mollusca), Pycnogon larva on, 524

 _Tetrabalius_, _312_

 _Tetrablemma_, 315, _404_;
   _T. medioculatum_, =318=

 _Tetraclita_, =91=, _92_

 _Tetragnatha_, _407_;
   _T. extensa_, 372

 Tetragnathinae, _407_

 Tetrameridae, _92_

 Tetranychinae, _472_

 _Tetranychus_, _472_;
   _T. gibbosus_, =472=;
   _T. telarius_, 455, 472

 Tetraspidae, _88_

 _Teutana_, _404_

 _Teuthraustes_, _308_

 Texas fever, 456, 470

 Thalassinidea, _167_

 _Thamnocephalus_, _36_;
   range of, 34;
   _T. platyurus_, 36

 _Thanatus_, _414_;
   _T. formicinus_, 414;
   _T. hirsutus_, 414;
   _T. striatus_, 414

 _Thaumasia_, _416_

 _Thelphusa_, _191_;
   _T. fluviatilis_, development, 190;
   distribution, 213

 Thelphusidae, _191_

 _Thelyphonellus_, _312_

 Thelyphonidae, 309, _312_

 _Thelyphonus_, =309=, 310, _312_;
   resemblance to Eurypterids, 294

 _Theotina_, _393_

 _Theraphosa_, _389_;
   _T. leblondi_, 366, 389

 Theraphosae, 319

 Theraphosidae, 391 n.

 Theridiidae, 327, 351, _401_

 _Theridion_, 376, _403_;
   _T. bimaculatum_, 403;
   _T. formosum_, 403;
   _T. pallens_, cocoon, =358=;
   _T. riparium_, 403;
   _T. sisyphium_, 340, 351, 359, 403;
   _T. tepidariorum_, 352, 368, 403

 Theridioninae, _403_

 _Theridiosoma argenteolum_, _407_

 Theridiosomatinae, _407_

 _Thersites gasterostei_, =71=

 Thomisidae, 323, 324, 369, 371, 381, _412_

 Thomisinae, _412_

 _Thomisus_, _412_;
   _T. onustus_, 413

 Thompson, D’A. W., on Pycnogonida, 499 f.

 Thoracica, _84_

 Thorax, of Trilobites, 234

 Thorell, 383

 _Thyas petrophilus_, 460

 _Tibellus_, _414_;
   _T. oblongus_, 371, =413=, 414

 Tick fever, 469

 Ticks, 468 f.;
   habits, 455, 461;
   synopsis of genera, 470

 _Titanodamon_, _313_

 _Tityus_, 298, _306_

 _Tmeticus_, _406_

 _Tomoxena_, _403_

 _Torania_, _414_

 Tracheae, in Arachnida, 256;
   in _Peripatus_, 256;
   in Spiders, 336;
   in Phalangids, 446;
   in Acarina, 462

 Trap-door Spiders, 354, 387, 388

 _Trechona venosa_, _390_

 _Triarthrus_, 230, 234, 236, _247_;
   thoracic limb, =10=;
   _T. becki_, =237=;
   Protaspis, =240=

 _Trichoniscus_, _129_

 _Trigonoplax_, _193_

 Trilobita, 219 f.

 Trilobite larva, of _Limulus_, 275, =276=

 _Trimerocephalus_, _249_;
   _T. volborthi_, 229

 Trinucleidae, =230=, _245_

 _Trinucleus_, 225, 226, 230, 231, 236, 238, _245_;
   _T. bucklandi_, =230=, 231;
   _T. seticornis_, 231

 _Tripeltis_, _312_

 _Trithena tricuspidata_, =404=

 _Trithyreus_, _312_

 Triton, cruise of the, 540

 Trochantin, 433, 436, 449, 451, 452

 _Trochosa_, _417_;
   vestigial antennae in, 256, 263

 _Troglocaris_, _163_;
   _T. schmidtii_, habitat, 210

 Trogulidae, 439, 442, 444, _452_

 _Trogulus_, _452_;
   _T. aquaticus_, =452=;
   _T. tricarinatus_, 452, 453

 Trombidiidae, _472_

 Trombidiinae, _473_

 _Trombidium_, _473_;
   _T. gymnopterorum_, 455;
   _T. holosericeum_, 455, 473

 Tropical zone (marine), 201

 Trouessart, 455

 _Trygaeus_, 506, _535_;
   _T. communis_, 535

 _Tubicinella trachealis_, _91_

 _Tubularia_, Pycnogons on, 522, 525

 Tubuliform glands, =335=, 349

 Tulk, 445, 446, 461 n.

 Turret-spider, 357

 _Turrilepas_, _84_;
   _T. wrightianus_, =84=

 _Tylaspis_, 179

 _Typhlocarcinus_, _195_

 _Typopeltis_, _312_

 Tyroglyphidae, _466_ n.

 Tyroglyphinae, _466_

 _Tyroglyphus_, 464, _466_, 481;
   _T. longior_, 466;
   _T. siro_, 466


 _Uliodon_, _418_

 Uloboridae, 350, _410_

 Uloborinae, _410_

 _Uloborus_, 352, _410_;
   snare of, =352=;
   _U. republicanus_, 411;
   _U. walckenaerius_, 411

 Unguis, 319, =320=

 _Uroctea_, _392_;
   _U. durandi_, =392=

 Urocteidae, 386, _392_

 Urodacinae, 306, _307_

 _Urodacus_, _307_

 _Uroplectes_, _306_

 _Uropoda_, _471_

 Uropodinae, _471_

 _Uroproctus_, _312_

 Uropygi, _312_

 _Usofila_, _393_


 Valvifera, _127_

 Vancoho, 362

 _Vectius_, _415_

 Vejdovský, 435 n.

 Vejovidae, 306, _308_

 _Vejovis_, _308_

 Vermiformia, _464_

 _Verruca_, 89, _91_

 Verrucidae, _91_

 Vesicle, of Scorpion, 303

 Vinson, 349, 360, 362

 _Virbius_, _164_;
   _V. acuminatus_, 164;
   habitat, 202

 Virchow, on human Pentastomids, 494

 Viscid globules, on Spider web, =347=


 Waite, 13

 Walckenaer, 365, 386 n., 408 n.

 _Walckenaera_, _405_;
   _W. acuminata_, =405=

 Walcott, on appendages of Trilobites, 236;
   on their development, 238;
   on early forms of Eurypterids, 283 n.

 Wallace, 381

 Wall-spider, 369

 Warburton, C., on Arachnida, 295 f., 344 n., 349 n., 378 n.

 Ward, on _Reighardia_, 495

 Wasps and Spiders, 368

 Water-mites, 460, 471, 472

 Water-spider, 357, 415

 Weismann, on Cladocera, 44, 49

 Weldon, W. F. R., on excretory glands, 13;
   on Branchiopoda, 18 f.;
   on respiration in _Carcinus_, 189

 Westring, 327, 384

 Whale-louse, 502

 Whip scorpions, 309

 White, Gilbert, 342

 Wilder, 366

 _Willemoesia_, _157_;
   _W. inornata_, =158=

 Winkler, 463

 With, 473 n.

 Wolf-spiders, 341, 356, 359, 369, 375, 377, 381, 417

 Wood-Mason, 328

 Woods, H., on Trilobita, 219 f.;
   on fossil Xiphosura, 277 f.;
   on Eurypterida, 281 f.


 Xanthidae, _191_

 Xantho, _191_;
   habitat, 198

 _Xenobalanus globicipitis_, _92_

 _Xiphocaris_, _163_;
   distribution, 210

 Xiphosura, _258_, _259_ f.;
   classification, 260, 276;
   fossil, 277 f.;
   affinities with Eurypterida, 292

 _Xiphosura_, _276_;
   _X. polyphemus_, 276

 Xiphosuridae, _276_

 Xiphosurinae, _276_

 _Xysticus_, _412_;
   _X. cristatus_, 412;
   _X. pini_, =413=


 _Zacanthoides_, _247_

 Zaeslin, on the frequency of human Pentastomids, 494

 _Zeriana_, _429_

 _Zilla_, _409_;
   _Z. x-notata_, 359, 409

 _Zimris_, _396_

 Zoaea, compared with Cumacea, 120;
   with Erichthus, 143;
   Calyptopis of _Euphausia_, =144=;
   of _Peneus_, =160=;
   of Sergestidae, 161;
   of _Porcellana_, =169=;
   of _Birgus_, 174;
   of _Eupagurus_, =179=;
   of _Corystes cassivelaunus_, =182=

 Zodariidae, 317, _399_

 _Zodarion_, _399_

 _Zora_, _397_;
   _Z. spinimana_, =396=

 _Zoropsis_, _415_

 Zoropsidae, _415_


                             END OF VOL. IV

           _Printed by_ R. & R. CLARK, LIMITED, _Edinburgh_.

-----

Footnote 1:

  The muscles are to a certain extent segmented in correspondence with
  the limbs; and the heart, in Phyllopoda and Stomatopoda, may have
  segmentally arranged ostia.

Footnote 2:

  Herbst, _Arch. Entwick. Mech._ ii., 1905, p. 544.

Footnote 3:

  _Quart. J. Micr. Sci._ xlix., 1906, p. 469.

Footnote 4:

  Present in Nebalia.

Footnote 5:

  As many as 37 ambulatory appendages may be present.

Footnote 6:

  _Quart. J. Micr. Sci._ xxi., 1881, p. 343.

Footnote 7:

  _Abhandl. Senckenberg. Nat. Gesellsch._ xiv., 1886.

Footnote 8:

  The Cumacea, Anaspidacea, and certain Isopods possess a maxillary
  gland only.

Footnote 9:

  _Quart. J. Micr. Sci._ xxxii., 1891, p. 279.

Footnote 10:

  _Arch. Zool. Exp._ (2) x., 1892, p. 57.

Footnote 11:

  _Bull. Mus. Comp. Zool. Harvard_, xxxv., 1899, p. 152.

Footnote 12:

  _Arch. f. mikr. Anat._ lxvii., 1906, p. 364.

Footnote 13:

  Vol. x., 1897, pp. 97, 264.

Footnote 14:

  For this use of the term Branchiopoda, cf. Boas, _Morph. Jahrb._
  viii., 1883, p. 519.

Footnote 15:

  Bernard, “The Apodidae,” _Nature Series_, 1892.

Footnote 16:

  _Arb. Zool. Inst. Wien_, vi., 1886, p. 267.

Footnote 17:

  I do not understand Packard’s account of the telson in
  _Thamnocephalus_.

Footnote 18:

  The nomenclature here adopted is not that of Lankester.

Footnote 19:

  [The red pigment in _Lernanthropus_, see p. 68, has been shown to be
  not haemoglobin, so that the presence of this substance in Phyllopod
  blood becomes doubtful.—G.S.]

Footnote 20:

  _Zeitschr. wiss. Zool._ lxxi., 1902, p. 508.

Footnote 21:

  Cf. Gaskell, _Journ. Anat. Physiol._ x., 1876, p. 153.

Footnote 22:

  Bernard’s statement that _Apus_ is hermaphrodite seems based on
  insufficient evidence.

Footnote 23:

  Sayce has since described it, _Proc. Roy. Soc. Victoria_, xv., 1903,
  p. 229.

Footnote 24:

  _A. cancriformis_ had been supposed to have disappeared from the
  British fauna for many years, but it was found in Scotland in 1907.
  See R. Gurney, _Nature_, lxxvi., 1907, p. 589.

Footnote 25:

  _Branchipodides_ has been described by H. Woodward, from Tertiary
  strata.

Footnote 26:

  Consult Baird, “Monograph of the Branchiopodidae,” _Proc. Zool. Soc._
  1852, p. 18. Packard, _12th Ann. Rep. U.S. Geol. Survey_, part i.,
  1879.

Footnote 27:

  _Arch. f. Math. og Naturvidensk._ xx., 1898, Nos. 4 and 6. Thiele,
  _Zool. Jahrb. System._ xiii., 1900, p. 563.

Footnote 28:

  Bernard, _loc. cit._ p. 19; Baird, _Proc. Zool. Soc._ 1852, p. 1;
  Sayce, _Proc. Roy. Soc. Victoria_, xv., 1903, p. 224.

Footnote 29:

  Sars, _Arch. f. Math. og Naturvidensk._ xx., 1898, Nos. 4 and 6.

Footnote 30:

  Sars, _Christiania Vidensk. Forhand._ 1887. For Australian Phyllopods,
  see Sars, _Arch. f. Math. og Naturvid._ xvii., 1895, No. 7, and Sayce,
  _loc. cit._ p. 36.

Footnote 31:

  _Simocephalus vetulus_ anchors itself to weeds, etc., by a modified
  seta on the exopodite of the second antenna. It does not employ a
  dorsal organ for purposes of fixation. [G. S.]

Footnote 32:

  _Zeitschr. wiss. Zool._ xxiv., 1874, p. 1.

Footnote 33:

  _Zeitschr. wiss. Zool._ xxvii., xxxiii., 1876, 1879.

Footnote 34:

  Consult Lilljeborg, _Nov. Acta Reg. Soc. Upsalensis_, 1901;
  Scourfield, _J. Quekett Micr. Club_, 1903–4.

Footnote 35:

  More properly =Chydoridae=, but the universally known name Lynceidae
  is convenient.

Footnote 36:

  _Grundzüge der Zoologie_, 4. Aufl. 1880, p. 543.

Footnote 37:

  _Fauna and Flora G. v. Neapel_, Monograph 19, 1892.

Footnote 38:

  _Ibid._ Monograph 25, 1899.

Footnote 39:

  _Norwegian North Polar Exp. Sci. Results_, vol. i. part v., 1900.

Footnote 40:

  They may assist the animal by retarding its sinking. Cf. Chun, “Aus
  den Tiefen des Weltmeeres,” 1905.

Footnote 41:

  Schmeil, _Bibliotheca Zoologica_, Hefte 11, 15, and 21.

Footnote 42:

  Giesbrecht, _Mitth. Zool. Stat. Neap._ xi., 1895, p. 648.

Footnote 43:

  _Loc. cit._ p. 59.

Footnote 44:

  Claus, _Copepodenstudien_, 1. Heft, Vienna, 1889.

Footnote 45:

  Malaquin, _Arch. Zool. Exp._ (3), ix., 1901, p. 81.

Footnote 46:

  Canu, _Trav. Inst. Zool. Litte._ vi., 1892.

Footnote 47:

  Giesbrecht, _Fauna and Flora G. v. Neapel_, Monogr. 25, 1899.

Footnote 48:

  _Arb. Zool. Inst. Wien_, ii. 1879, p. 268.

Footnote 49:

  _Ibid._ xv., 1905, p. 1.

Footnote 50:

  Heller, _Reise der Novara_, vol. iii., 1868.

Footnote 51:

  For fish-parasites in British waters consult Scott, _Fishery Board for
  Scotland, Scientific Investigations_, xix., 1900 _et seq._

Footnote 52:

  Canu, _loc. cit._ p. 66.

Footnote 53:

  The Cambridge Museum possesses two specimens of _Philichthys xiphiae_,
  from the frontal bones of a Swordfish (_Xiphias gladius_) taken off
  Lowestoft in 1892.

Footnote 54:

  Claus, _Arb. Zool. Inst. Wien_, vii., 1888, p. 281.

Footnote 55:

  _Proc. Biol. Soc. Liverpool_, i., 1887.

Footnote 56:

  _Zeitschr. wiss. Zool._ xlix., 1890, p. 71.

Footnote 57:

  The genus _Pennella_ also includes parasites on the whales
  _Hyperoodon_ and _Balaenoptera_.

Footnote 58:

  Claus, _Schriften d. Gesellsch. Marburg._ Suppl. 1868.

Footnote 59:

  Claus, _Zeitschr. wiss. Zool._ xi., 1861, p. 287.

Footnote 60:

  Hansen, “The Choniostomatidae,” Copenhagen.

Footnote 61:

  Claus, _Zeitschr. wiss. Zool._ xxv., 1875, p. 217.

Footnote 62:

  C. B. Wilson, _Proc. U.S. Nat. Museum_, xxv., 1902, p. 635.

Footnote 63:

  Max Müller (_Science of Language_, 2nd series, p. 534) gives
  references to a number of old authors who vouch for the truth of this
  legend, going back as far as Giraldus Cambrensis in the twelfth
  century. The legend appears to be of Scotch or Irish origin. Giraldus
  complains of the clergy in Ireland eating Barnacle geese at the time
  of fasting under the pretext that they are not flesh, but born of fish
  living in the sea. The form of the legend varies, certain authors
  alleging that the geese are produced from the fruits of a tree which
  drop into the water, others that they grow in shells (Barnacles)
  attached to floating logs. Aldrovandus (_De Avibus_, T. iii., 1603, p.
  174) ingeniously combines both versions in a woodcut representing
  undoubted Barnacles growing on a tree with luxuriant foliage at the
  water’s edge, below which a number of liberated geese are swimming.
  Müller ascribes an etymological origin to the legend, the Barnacle
  goose (deriv. _Hibernicula, bernicula_ = Irish goose) being confounded
  with pernacula, bernacula, a little shell.

Footnote 64:

  “A Monograph of the Cirripedia,” vols. i. and ii., _Ray Society_,
  1851, 1853.

Footnote 65:

  “Rep. on the Cirripedia, H.M.S. ‘Challenger,’” vols. viii. and x.,
  1883.

Footnote 66:

  “Monographie des Cirrhipèdes,” Paris, 1905, in which will be found
  full references to literature.

Footnote 67:

  _Arch. Biol._ xvi., 1899, p 27.

Footnote 68:

  Berndt, _Sitzb. Ges. Naturfr. Berlin_, 1903, p. 436.

Footnote 69:

  _Arch. Zool. Exp._ viii., 1880, p. 537.

Footnote 70:

  _Quart. J. Micr. Sci._ xxx., 1890, p. 107.

Footnote 71:

  _Plankton Expedition_, ii. G. d. 1899.

Footnote 72:

  Y. Delage, _Arch. Zool. Exp._ (2), ii., 1884, p. 417; G. Smith, _Fauna
  u. Flora G. von Neapel_, Monogr. 29, 1906.

Footnote 73:

  G. Smith, _Fauna u. Flora d. Golfes v. Neapel_, Monogr. xxix., 1906,
  pp. 60–64, 119–121.

Footnote 74:

  _Bull. Sc. Dép. Nord_ (2), 10 Ann. xviii., 1887, p. 1. _Ibid._ (3),
  i., 1888, p. 12; and other papers.

Footnote 75:

  G. Smith, _loc. cit._ chap. v. _I. scorpio_ should be _I.
  mauritanicus_ throughout this Monograph.

Footnote 76:

  F. A. Potts, _Quart. J. Micr. Sci._ l., 1906, p. 599.

Footnote 77:

  Faxon, _Ann. Mag. Nat. Hist._ (5), xiii., 1884, p. 147.

Footnote 78:

  G. Smith, _Mitth. Zool. Stat. Neapel_, xvii., 1905, p. 312.

Footnote 79:

  C. L. Boulenger, _Proc. Zool. Soc._ 1908, p. 42.

Footnote 80:

  Garnier, _C. R. Soc. Biol._ liii., 1901, p. 38.

Footnote 81:

  Gruvel, _Monographie des Cirrhipèdes_, 1905, p. 152.

Footnote 82:

  Claus, _Untersuchungen zur Erforschung des Crustaceensystems_, Wien,
  1876. Brady and Norman, “Monograph of the Marine and Fresh-Water
  Ostracoda of the N. Atlantic,” _Trans. R. Dublin Soc._ (2) iv., 1889,
  p. 63. Müller, _Fauna und Flora G. von Neapel_, Monogr. xxi., 1894;
  “Deutschlands Süsswasser-Ostracoden,” Chun’s _Zoologica_, xii., 1900.

Footnote 83:

  “The Germ Plasm,” _Contemp. Science Series_, 1893, p. 345.

Footnote 84:

  The term pereiopod is applied to those thoracic limbs which are used
  in locomotion, and are not specially differentiated for any other
  purpose.

Footnote 85:

  Claus, _Arb. Inst. Wien_, viii., 1889, p. 1.

Footnote 86:

  Robinson, _Quart. J. Micr. Sci._ 1., 1906, p. 383.

Footnote 87:

  _Morphol. Jahrb._ viii., 1883, p. 485.

Footnote 88:

  _Ann. Mag. Nat. Hist._ (7), xiii., 1904, p. 144.

Footnote 89:

  The lacinia mobilis is a movable tooth-like structure jointed on to
  the biting face of the mandible.

Footnote 90:

  _Trans. Linn. Soc._ (2), vi., 1894–1897, p. 285.

Footnote 91:

  _Trans. Roy. Soc. Edinburgh_, xxxviii., 1897, p. 787.

Footnote 92:

  G. Smith, _Proc. Roy. Soc._ 1908.

Footnote 93:

  This characteristic is found in the Crustacea elsewhere only in the
  Argulidae and certain Euphausiidae.

Footnote 94:

  _The Victorian Naturalist_, xxiv., 1907, p. 117.

Footnote 95:

  _Challenger Reports_, vol. xiii., 1885, p. 55.

Footnote 96:

  Sars, “Crustacea of Norway,” iii., 1900.

Footnote 97:

  Sars, “Crustacea Caspia,” _Bull. Acad. Imp. Sci. St. Pétersbourg_,
  series 4, xxxvi., 1894, and “Crustacea of Norway,” iii., 1900, p. 120.

Footnote 98:

  “Crustacea of Norway,” vol. ii., Isopoda, 1899, in which many
  references to literature will be found.

Footnote 99:

  Smith, _Mitth. Zool. Stat. Neapel_, xvii., 1905, p. 312.

Footnote 100:

  G. Smith, _Mitth. Zool. Stat. Neapel_, xvi., 1903, p. 469.

Footnote 101:

  Mayer, _Mitth. Zool. Stat. Neapel_, i., 1879, p. 165.

Footnote 102:

  Beddard, _Challenger Reports_, vol. xi., 1884.

Footnote 103:

  Hansen, _Quart. J. Micr. Sci._ xlix., 1906, p. 69.

Footnote 104:

  A useful little book on British Woodlice by Webb and Sillem (1906) may
  be profitably consulted. Budde Lund’s _Isopoda Terrestria_, 1900, is
  useful to the specialist.

Footnote 105:

  The pleopods are traversed by a system of minute tubes called
  pseudotracheae, somewhat resembling the tracheae of Insects.

Footnote 106:

  Bonnier, _Trans. Inst. Zool. Lille_, viii., 1900.

Footnote 107:

  G. Smith, _Fauna and Flora Neapel_, Monograph 29, chap. vi.; M.
  Caullery, _Mitth. Zool. Stat. Neapel_, xviii., 1908, p. 583.

Footnote 108:

  M. Caullery (_loc. cit._ p. 130) questions the truth of this
  observation, but I am convinced of its accuracy.

Footnote 109:

  _Trav. Inst. Lille_, v., 1887.

Footnote 110:

  Chilton, _Trans. Linn. Soc._ vi., 1894, p. 185.

Footnote 111:

  Spenser and Hall, _Proc. Roy. Soc. Victoria_, ix. p. 12.

Footnote 112:

  “Das Tierreich,” 21, _Amphipoda Gammaridea_, 1906.

Footnote 113:

  Cf. P. Mayer, _Fauna u. Flora G. von Neapel_, Monogr. vi., 1882;
  xvii., 1890.

Footnote 114:

  _Abhandl. königl. Gesellsch. Göttingen_, xvi., 1871.

Footnote 115:

  _Mem. Nat. Acad. Sci._ v., 1891.

Footnote 116:

  Sars, _Challenger Reports_, xiii., 1885; Chun, _Bibliotheca
  Zoologica_, xix., 1896, p. 139.

Footnote 117:

  _Die Physiologie der facettierten Augen von Krebsen und Insecten._
  Leipzig, Wien, 1891.

Footnote 118:

  _Valdivia Expedition_, vol. vi., 1904.

Footnote 119:

  _Ann. Sci. Nat._ (Zool.) (7), xiii., 1892, p. 185.

Footnote 120:

  _A Naturalist in Indian Seas_, 1902.

Footnote 121:

  “Atlantis,” _Bibliotheca Zoologica_, Heft 19, 1896, p. 193.

Footnote 122:

  _Loc. cit._ p. 150.

Footnote 123:

  Bell, _A History of the British Stalk-eyed Crustacea_, 1853; Heller,
  _Die Crustaceen des Südlichen Europa_, 1863.

Footnote 124:

  Cf. Claus, _Würzburger Naturwiss. Zeitschr._ ii., 1861, p. 23.

Footnote 125:

  _Arch. f. Naturg._ vi., 1840, p. 241.

Footnote 126:

  Spence Bate’s _Challenger Reports_.

Footnote 127:

  Some of the pereiopods remain biramous in certain Peneidea and Caridea
  (see p. 163).

Footnote 128:

  _Bull. U.S. Fish Commission_, xv., 1895.

Footnote 129:

  _Zool. Bulletin_, i., 1898, p. 287.

Footnote 130:

  _Archiv für Entw. Mech._ xi., 1901, p. 321.

Footnote 131:

  _Challenger Reports_, xxiv., 1888.

Footnote 132:

  _Loc. cit._ p. 150.

Footnote 133:

  Keeble and Gamble, _Phil. Trans._, Ser. B, cxcvi., 1904, p. 295. The
  chromatophores are also directly responsive to light, but the lasting
  adaptations to colour-backgrounds are brought about indirectly, the
  stimulus being transmitted through the eyes and nervous system. The
  influence of light may also affect the metabolism of the animal, the
  chromatophores being accompanied by a ramifying fatty tissue, which
  disappears if the animal is kept in the dark.

Footnote 134:

  _Challenger Reports_, xxiv., 1881.

Footnote 135:

  Borradaile’s useful paper on the classification of the Decapoda (_Ann.
  Mag. Nat. Hist._ (7), xix., 1907, p. 457) should be consulted for this
  and other Decapod groups. Also Alcock’s _Cat. of the Indian Mus._,
  “Decapod Crustacea.”

Footnote 136:

  Giard and Bonnier, _Compt. Rend. Soc. Biol._ 1892.

Footnote 137:

  Coutière, _Fauna and Geogr. Maldive and Laccadive Archipelagos_, ii.,
  1905, p. 852.

Footnote 138:

  Keeble and Gamble, _Phil. Trans._ Ser. B., cxcvi., 1904, p. 295. In
  the young a constant and very simple chromatophore-system is present,
  but in the adult a barred, lined, or monochrome colour-pattern may be
  present, which is ultimately induced by the nature of the environment,
  and does not subsequently change. In other species of _Hippolyte_, and
  in _Palaemon_ and _Crangon_, only one adult colour-pattern occurs.
  Thus _H. varians_, besides reacting to light by its chromatophores,
  possesses a permanent colour-pattern, which is also determined by
  environment.

Footnote 139:

  Claus, _Unt. z. Erforschung d. genealog. Grundlage d.
  Crustaceensystems_. Vienna, 1876.

Footnote 140:

  Milne Edwards and Bouvier, _Ann. Sci. Nat._ (7), xvi., 1894, p. 91.

Footnote 141:

  Garstang, _Quart. J. Micr. Sci._ xl., 1897, p. 211.

Footnote 142:

  Milne Edwards and Bouvier, _Bull. Soc. Philomath. Paris_ (8), ii.,
  1889; and _Expédition du Talisman_, “Crustacés Décapodes,” 1900.

Footnote 143:

  Alcock, _loc. cit._; Borradaile, _op. cit._ p. 162; i. p. 64.

Footnote 144:

  Brandt, _Bull. Phys. Math. Acad. St. Pétersbourg_, i. p. 171, and
  viii. p. 54; Boas, _K. Dansk. Vidensk. Selskab. Skrift. Naturvid. og
  Math. Afd._ 6, Bd. 2, 1880; Bouvier, _Ann. Sci. Nat. (Zool.)_ (7)
  xviii. p. 157.

Footnote 145:

  Vol. xxvii. p. 81.

Footnote 146:

  _Proc. Boston Soc. Nat. Hist._, xxxi., 1904, p. 147.

Footnote 147:

  For general literature consult Ortmann in _Bronn’s Tier-Reich_, v. 2,
  1901, p. 778. See also _Reports_ of _Challenger_, _Valdivia_, and
  _Talisman Expeditions_, etc.

Footnote 148:

  Gurney, _Quart. J. Micr. Sci._ xlvi., 1902, p. 461.

Footnote 149:

  Bouvier, _Bull. Soc. Philomath. Paris_, (8) viii., 1896.

Footnote 150:

  _Loc. cit._ p. 183.

Footnote 151:

  M‘Culloch, _Rec. Australian Mus._ vi. part 5, 1907, p. 353.

Footnote 152:

  Lankester, _Quart. J. Micr. Sci._ xlvii., 1903, p. 439.

Footnote 153:

  Garstang, _Quart. J. Micr. Sci._ xl., 1897, p. 211, and _Journ. Mar.
  Biol. Ass._ iv., 1895–97, p. 396.

Footnote 154:

  _Loc. cit._ p. 181.

Footnote 155:

  _Rep. Brit. Ass._ for 1898, p. 887.

Footnote 156:

  _Naturalist in Indian Seas_, 1902.

Footnote 157:

  There appears to be some doubt on this point, as Westwood (see p. 153)
  described direct development in a _Gecarcinus_. Possibly different
  species behave variously.

Footnote 158:

  Kingsley, _Proc. Acad. Nat. Sci. Philadelphia_, 1880, p. 187.

Footnote 159:

  _American Naturalist_, xxxiii., 1899, p. 583.

Footnote 160:

  _Planktonstudien_, Jena, 1890.

Footnote 161:

  “Report on the Plankton,” _Internat. Inst. Marine Biol._ 1903.

Footnote 162:

  _Internat. Inst. Mar. Biol._ 1903.

Footnote 163:

  _A Naturalist in Indian Seas._

Footnote 164:

  Scourfield, _J. Quekett Micr. Club_, 1903–4, gives a useful list of
  British Fresh-water Entomostraca. For the identification of
  fresh-water Cladocera, Lilljeborg’s “Cladocera Sueciae,” _Nov. Act.
  Reg. Soc. Upsalensis_, 1901; for Copepoda, Schmeil’s “Süsswasser
  Copepoden,” in _Bibliotheca Zoologica_, iv., v., and viii., 1892,
  1893, and 1895 are recommended.

Footnote 165:

  _Trans. Norfolk and Norwich Nat. Soc._ vii.

Footnote 166:

  _Le Lac Leman_, 3 vols., Lausanne, 1892.

Footnote 167:

  Consult Apstein, “Das Süsswasserplankton,” Kiel and Leipzig, 1896; and
  _Arch. f. Hydrobiologie u. Planktonkunde_, numerous papers.

Footnote 168:

  Mr. C. H. Martin points out to me that in the Scottish lochs, which
  from their geological nature are evidently not connected with
  subterranean waters, none of them nor similar forms occur; nor do they
  in the Tasmanian lakes which are on igneous diabase, so that Forel’s
  conclusion would seem to be of wide application.

Footnote 169:

  _See_ Chilton, _Trans. Linn. Soc._ (2) vi., 1894, p. 163, with review
  of literature.

Footnote 170:

  S. F. Harmer, _Trans. Norfolk and Norwich Nat. Soc._ ii., 1899, p.
  489.

Footnote 171:

  _Mem. Nat. Acad. Washington_, iii., 1886, p. 1.

Footnote 172:

  _Arch. Zool. Exp._ (4), ii., 1904, p. 1.

Footnote 173:

  See Calman, _Proc. Zool. Soc._ 1906, p. 187.

Footnote 174:

  _The Crayfish_, Internat. Scient. Series.

Footnote 175:

  _Mem. Harvard. Mus._ x., 1885.

Footnote 176:

  _Proc. Amer. Phil. Soc._ xli., 1902, p. 267, and xliv., 1905, p. 91.

Footnote 177:

  G. O. Sars, “Crustacea Caspia,” _Bull. Acad. Imp. Sc. St. Pétersbourg_
  (4), xxxvi., 1893–4, pp. 51 and 297; (5) i., 1894, pp. 179 and 243;
  also _Crustacea of Norway_, vol. ii. Isopoda, 1900, p. 73.

Footnote 178:

  Daday, _Termés Füzetek_, xxv., 1902, pp. 101 and 436.

Footnote 179:

  Daday, _Bibliotheca Zoologica_, Heft 44, 1905.

Footnote 180:

  On the cheek the furrow represents a pleural groove, and does not form
  the limit of the posterior cephalic segment.

Footnote 181:

  M‘Coy, _Synop. Sil. Foss. Ireland_, 1846, p. 56, and _Brit. Pal.
  Foss._, 1851, p. 146, pl. 1 E, fig. 16; Salter, _Quart. Journ. Geol.
  Soc._ iii., 1847, p. 251.

Footnote 182:

  Figures showing this suture are given by Oehlert, _Bull. Soc. géol. de
  France_ (3), xxiii., 1895, pl. 1, figs. 9, 12, 15.

Footnote 183:

  _Ann. Mag. Nat. Hist._ (2) iv., 1849, p. 396.

Footnote 184:

  Lindström, “Visual Organs of Trilobites,” _Svenska Vet. Akad. Handl._
  xxxiv., 1903. Exner, _Physiol. d. facett. Augen v. Krebsen u.
  Insecten_, 1891, p. 34, pl. ii. figs. 18, 19.

Footnote 185:

  _Journ. Morphol._ ii., 1889, p. 253, pl. 21.

Footnote 186:

  Watase, _Johns Hopkins Univ. Studies, Biol. Lab._ iv., 1890, p. 290.
  Lindström, _op. cit._ p. 27.

Footnote 187:

  A suture is said to be present at the external margin of the flattened
  cephalic border.

Footnote 188:

  Goldfuss, “Beitr. zur Petrefaktenkunde,” 1839, p. 359, pl. 33, fig.
  2_d._

Footnote 189:

  Spencer, _Geol. Mag._ 1903, p. 489.

Footnote 190:

  For an example of this see Salter, _Mon. Brit. Trilobites_, 1864–83,
  pls. 15, 16.

Footnote 191:

  _Bull. Mus. Comp. Zool. Harvard_, viii., 1881, p. 191.

Footnote 192:

  _Studies in Evolution_, 1901, pp. 197–225; _Geol. Mag._ 1902, p. 152.
  Walcott, _Proc. Biol. Soc. Washington_, ix., 1894, p. 89.

Footnote 193:

  _Syst. Sil. Bohême_, i., 1852, pp. 257–276.

Footnote 194:

  _American Geologist_, xx., 1897, p. 34.

Footnote 195:

  _Proc. R. Irish Acad._ xxiv., 1903, p. 332, and _Quart. Journ. Micr.
  Sci._ xlix., 1906, p. 469.

Footnote 196:

  This has received some support from H. Milne Edwards, _Ann. Sci. Nat.
  Zool._ (6), xii., 1881, p. 33; H. Woodward, _Quart. Journ. Geol._ Soc.
  xxvi., 1870, p. 487, and vol. 1., 1894, p. 433; Bernard, _ibid._ vol.
  1. p. 432.

Footnote 197:

  Kingsley does not admit this relationship, and regards the Trilobita
  as a group quite distinct from all other Crustacea. See _American
  Naturalist_, xxviii., 1894, p. 118, and _American Geologist_, xx.,
  1897, p. 33.

Footnote 198:

  Zittel states that _Apus_ appears first in the Trias.

Footnote 199:

  _Monogr. Brit. Trilobites_, 1864, p. 2.

Footnote 200:

  “A Natural Classification of Trilobites,” _Amer. Jour. Sci._ (4),
  iii., 1897, pp. 89–106, 181–207. Reprinted in Beecher’s _Studies in
  Evolution_, 1901, p. 109. A classification based on the character of
  the pygidium has been proposed by Gürich, _Centralbl. für Min. Geol.
  u. Pal._ 1907, p. 129. A classification based on the minute structure
  of the test has been given by Lorenz, _Zeitschr. d. deutsch. geol.
  Gesellsch._ lviii., 1906, p. 56.

Footnote 201:

  _Neues Jahrb. für Min. Geol. u. Pal._ 1898, i. p. 187.

Footnote 202:

  Lake, _Brit. Cambrian Tril._ 1907, p. 45.

Footnote 203:

  The British Carboniferous Proëtidae are described by H. Woodward,
  _Monogr. Brit. Carb. Trilobites_, Palaeont. Soc. 1883–84.

Footnote 204:

  This can be maintained in the Crustacea by counting the seventh
  abdominal segment, which appears in _Gnathophausia_; but this is not
  universally regarded as a true segment. See also _Nebalia_ (p. 111).

Footnote 205:

  This and the following Sub-class correspond with Lankester’s Sub-class
  Euarachnida. The Delobranchiata have gills patent and exposed, and
  adapted for breathing oxygen dissolved in water. The Embolobranchiata
  have either the gill-books (now termed lung-books) sunk into their
  body, or the gill-books are wholly or partially replaced by tracheae.
  In either case the members of this Sub-class breathe atmospheric
  oxygen.

Footnote 206:

  Woodward, “On some Points in the Structure of the Xiphosura, having
  reference to their relationship with the Eurypteridae,” _Quart. J.
  Geol. Soc._ xxiii., 1867, p. 28, and xxviii., 1871, p. 46. Milne
  Edwards, A., “Recherches sur l’anat. des Limules,” _Ann. Sci. Nat._
  (5), xvii., 1873, Art. 4. Lankester, E. R., “Limulus an Arachnid,”
  _Quart. J. Micr. Sci._ xxi., 1881, p. 504. Kingsley, J. S., “The
  Embryology of _Limulus_,” _Journ. Morph._ vii. p. 35, and viii. p.
  195, 1892–3. Kishinouye, “On the Development of _Limulus longispina_,”
  _Journ. Coll. Sci. Japan_, v., 1892, p. 53. Patten, W., and
  Redenbaugh, W. A., “Studies on _Limulus_,” _Journ. Morph._ xvi., 1900,
  pp. 1, 91.

Footnote 207:

  _Quart. J. Micr. Sci._ xlviii., 1905, p. 165.

Footnote 208:

  μηρός = a thigh.

Footnote 209:

  This segment, though present in embryo Scorpions, has disappeared in
  the adults of those animals.

Footnote 210:

  _Quart. J. Micr. Sci._ xlix., 1906, p. 469.

Footnote 211:

  _Zool. Anz._ xiv., 1891, pp. 164, 173.

Footnote 212:

  _Zeitschr. wiss. Zool._ lix., 1895, p. 351.

Footnote 213:

  They are described in great detail in Lankester’s article, “_Limulus_
  an _Arachnid_,” _Quart. J. Micr. Sci._ xxi., 1881, p. 504.

Footnote 214:

  _Tr. Linn. Soc._ xxviii., 1872, p. 471.

Footnote 215:

  _Tr. Linn. Soc._ xxviii., 1872, p. 472.

Footnote 216:

  A rudimentary ninth pair of ostia are described anteriorly.

Footnote 217:

  _J. Morph._ vii., 1892, p. 35.

Footnote 218:

  Kingsley, _loc. cit._

Footnote 219:

  _J. Coll. Tokyo_, v., 1893, p. 53.

Footnote 220:

  Lockwood, _Amer. Nat._ iv., 1870–71, p. 261.

Footnote 221:

  For a diagnosis of the species and a list of synonyms, see Pocock,
  _Ann. Mag. Nat. Hist._ (7), ix., 1902, p. 256.

Footnote 222:

  _Quart. J. Micr. Sci._ xxxi., 1890, p. 379; _Proc. Cambr. Phil. Soc._
  ix., 1895–1898, p. 19; _J. Anat. Physiol._ xxxiii., 1899, p. 154.

Footnote 223:

  _Quart. J. Micr. Sci._ xxxi., 1890, p. 317.

Footnote 224:

  I am indebted to Mr. Henry Woods for these paragraphs on fossil
  Xiphosura.

Footnote 225:

  The British fossil forms of this group are described and figured by H.
  Woodward, “Monograph of the Merostomata,” _Palaeontogr. Soc._ 1866–78,
  and _Geol. Mag._ 1907, p. 539.

Footnote 226:

  Packard, “Carb. Xiphos. N. America,” _Mem. Nat. Acad. Sci.
  Washington_, iii., 1885, p. 146, pl. vi. fig. 1_a_, pl. v. fig. 3_a_
  (restoration). Williams, _Amer. Journ. Sci._ (3), xxx., 1885, p. 45.
  Fritsch, _Fauna d. Gaskohle_, iv., 1901, p. 64, pl. 155, figs. 1–3,
  and text-figures, 369, 370.

Footnote 227:

  Walcott has described, under the generic name _Beltina_, imperfect
  specimens from the Algonkian (pre-Cambrian) of Montana, which he
  thinks may be the remains of Eurypterids (_Bull. Geol. Soc. America_,
  x., 1899, p. 238).

Footnote 228:

  Walcott, _Amer. Jour. Sci._ (3), xxiii., 1882, p. 213.

Footnote 229:

  Descriptions and figures of British Eurypterids are given in the
  following works:—Huxley and Salter, “Pterygotus,” _Mem. Geol. Survey,
  Brit. Org. Remains_, i., 1859; H. Woodward, “Monograph of the
  Merostomata,” _Palaeont. Soc._ 1866–78, and _Geol. Mag._ 1879, p. 196;
  1887, p. 481; 1888, p. 419; 1907, p. 277; Peach, _Trans. Roy. Soc.
  Edinb._ xxx., 1882, p. 511; Laurie, _ibid._ xxxvii., 1892, p. 151;
  xxxvii., 1893, p. 509; and xxxix., 1899, p. 575.

Footnote 230:

  A detailed account of _Eurypterus fischeri_ has been given by G. Holm,
  _Mém. Acad. Impér. Sci. St. Pétersbourg_ (8), viii. 2, 1898. See also
  F. Schmidt, _ibid._ (7), xxxi. 5, 1883. Descriptions of American forms
  of _Eurypterus_ are given by Hall, “Nat. Hist. New York,” _Palaeont._
  iii., 1859, p. 395; _ibid._ vii., 1888, p, 156; and _Second Geol.
  Survey Pennsylvania_, “Report of Progress,” PPP., 1884; Whiteaves,
  _Geol. and Nat. Hist. Surv. Canada_, “Palaeozoic Foss.,” iii., 1884,
  p. 42.

Footnote 231:

  It was this ornamentation found on fragments of _Pterygotus anglicus_
  which led the Scotch quarrymen to apply the name “Seraphim” to that
  Eurypterid. On this subject Hugh Miller writes: “The workmen in the
  quarries in which they occur, finding form without body, and struck by
  the resemblance which the delicately waved scales bear to the
  sculptured markings on the wings of cherubs—of all subjects of the
  chisel the most common—fancifully termed them ‘Seraphim’” (_The Old
  Red Sandstone_, ed. 6, 1855, p. 180).

Footnote 232:

  The third leg in the male possesses on the fifth joint a curved
  appendage which extends backwards to the proximal end of the second
  joint. This structure may have been a clasping organ.

Footnote 233:

  It has been suggested that the metastoma really belongs to a
  pregenital segment of the mesosoma which is absent in the adult, but
  has been found in the embryo of Scorpions.

Footnote 234:

  Sarle, _New York State Museum, Bulletin_ 69, Palaeont. 9, 1903, p.
  1087.

Footnote 235:

  Beecher, _Geol. Mag._ 1901, p. 561.

Footnote 236:

  Peach, _Nature_, xxxi., 1885, p. 295; Pocock, _Quart. Journ. Micr.
  Sci._ xliv., 1901, p. 291; Laurie, _Trans. Roy. Soc. Edinb._ xxxix.,
  1899, p. 575.

Footnote 237:

  Peach, _Trans. Roy. Soc. Edinb._ xxx., 1882, p. 516.

Footnote 238:

  Cf. p. 258.

Footnote 239:

  _Nature_, xlviii., 1893, p. 104.

Footnote 240:

  _Souvenirs entomologiques_, Sér. 9, 1907, p. 229.

Footnote 241:

  Brauer, _Zeitschr. wiss. Zool._ lix., 1895, p. 355.

Footnote 242:

  _Das Tierreich_, 8. Lief., 1899, p. 4.

Footnote 243:

  _Arachnides de France_, vii., 1879, p. 84.

Footnote 244:

  _Fauna of British India_, “Arachnida,” 1900, p. 8.

Footnote 245:

  _Tr. Zool. Soc._ xi. part x., 1885, p. 373.

Footnote 246:

  _Zeitschr. wiss. Zool._ lix., 1895, p. 351.

Footnote 247:

  _Das Tierreich_, 8. Lief., 1899.

Footnote 248:

  Pocock, _Fauna of British India_, “Arachnida.” London, 1900.

Footnote 249:

  Laurie, _J. Linn. Soc. Zool._ xxv., 1894, p. 30.

Footnote 250:

  _See_ M. Laurie in _J. Linn. Soc. Zool._ xxv., 1894, p. 20.

Footnote 251:

  _Tr. Linn. Soc._ (2) vi., 1896, p. 344.

Footnote 252:

  Bernard, _loc. cit._ p. 366.

Footnote 253:

  _J. Linn. Soc._ xxv., 1894, p. 29.

Footnote 254:

  See Pocock, _Ann. Nat. Hist._ (6), xiv., 1894, p. 120.

Footnote 255:

  Kraepelin, _Das Tierreich_, Berlin, 8. Lief., 1899, p. 234.

Footnote 256:

  The term mostly in use is Araneida, which should mean Araneus-like
  animals. This is clearly not allowable, unless there is a genus
  _Araneus_ or _Aranea_. For many years there has been no such genus
  recognised, but Simon now attempts to re-establish it, inadmissibly,
  as it appears to us. (See note, p. 408).

Footnote 257:

  _Mém. Mus. d’Hist. Nat._ xviii., 1829, p. 377.

Footnote 258:

  Pickard-Cambridge (_Spiders of Dorset_, 1879–1881) omits the coxal
  joint, which, with its lobe, he calls the maxilla, and therefore gives
  only five joints, which he names _axillary_, _humeral_, _cubital_,
  _radial_, and _digital_.

Footnote 259:

  Pickard-Cambridge, in his _Spiders of Dorset_, names them
  _exinguinal_, _coxal_, _femoral_, _genual_, _tibial_, _metatarsal_,
  and _tarsal_.

Footnote 260:

  _Nat. Hist. Tidsskr._ iv., 1843, p. 349.

Footnote 261:

  _J. Linn. Soc._ xv., 1881, p. 155.

Footnote 262:

  _Proc. Asiat. Soc. Beng._ 1875, p. 197.

Footnote 263:

  _Tijdschr. v. d. Nederl. Dierkundige Ver._ (2), i., 1885–1887, p. 109.

Footnote 264:

  _Études sur la circulation du sang chez les Aranées du genre Lycose._
  Utrecht, 1862.

Footnote 265:

  _Recherches sur l’appareil circulatoire des Aranéides._ Lille, 1896.

Footnote 266:

  _Arch. f. Naturg._ 55 Jahrg., i., 1889, p. 29.

Footnote 267:

  M‘Leod, _Bull. Ac. Belg._ (3), iii., 1882, p. 779.

Footnote 268:

  _American Spiders and their Spinning Work_, ii., 1890, p. 208.

Footnote 269:

  _Ann. Nat. Hist._ (3), xv., 1865, p. 459.

Footnote 270:

  _Voyage of the Beagle._

Footnote 271:

  _Correspondence of John Ray_, p. 77.

Footnote 272:

  Warburton, _Q. J. Micr. Sci._ xxi., 1890, p. 29.

Footnote 273:

  _Rep. Brit. Ass._ 1844, p. 77.

Footnote 274:

  _Nature_, xl., 1889, p. 250.

Footnote 275:

  See Warburton, _Quart. J. Micr. Sci._ xxi., 1890, p. 29.

Footnote 276:

  _Aranéides de la Réunion, Maurice et Madagascar_, Paris, 1863, p. 238.

Footnote 277:

  M‘Cook, _American Spiders and their Spinning Work_, i., 1889, p. 351;
  F. O. Pickard-Cambridge, _J. Micr. and Nat. Sci._ July 1890.

Footnote 278:

  Moggridge, _Harvesting Ants and Trap-door Spiders_. London, 1873, p.
  120.

Footnote 279:

  _Verh. Ges. Wien_, xviii., 1868, p. 905 (Abstract in _Zool. Rec._ v.,
  1868, p. 175).

Footnote 280:

  The figure of this cocoon has been accidentally inverted in the works
  of both Blackwall and Pickard-Cambridge.

Footnote 281:

  Fabre, _Nouveaux souvenirs entomologiques_, ch. xi.

Footnote 282:

  _Aranéides de la Réunior, Maurice et Madagascar_, Paris, 1863, p.
  xlvi.

Footnote 283:

  _Hist. de la grande île de Madagascar_, 1658, p. 156.

Footnote 284:

  _Science Gossip_, 1877, p. 46.

Footnote 285:

  _Insect Life_, i., 1889, p. 205.

Footnote 286:

  _Ann. Soc. ent. France_, xi., 1842, p. 205. Translated from the
  Spanish by L. Fairmaire.

Footnote 287:

  M‘Cook, _American Spiders and their Spinning Work_, ii., 1890, p. 188.

Footnote 288:

  M‘Cook, _t.c._ p. 389.

Footnote 289:

  _British Spiders_, 1861, p. 102.

Footnote 290:

  _Ann. Nat. Hist._ (1), xi., 1843, p. 1.

Footnote 291:

  _Naturalist in Nicaragua_, 2nd ed., 1888, p. 134.

Footnote 292:

  _Nouveaux souvenirs entomologiques_, ch. xii.

Footnote 293:

  M‘Cook, _t.c._ p. 384.

Footnote 294:

  _The Naturalist in Nicaragua_, p. 19.

Footnote 295:

  _Spiders of Dorset_, 1879–1881, p. 292.

Footnote 296:

  _Ibid._ p. 360.

Footnote 297:

  _Naturalist on the Amazon_, 1873, p. 54.

Footnote 298:

  _Protective Resemblances and Mimicry in Animals_, 1873, p. 4.

Footnote 299:

  _Nature_, lxviii., 1908, p. 631.

Footnote 300:

  _J. Morph._ (Boston, U.S.A.) i., 1887, p. 403.

Footnote 301:

  _Nature_, xxiii., 1880, p. 149.

Footnote 302:

  Warburton, _Ann. Nat. Hist._ (6), viii., 1891, p. 113.

Footnote 303:

  _Spiders of Dorset_, 1879–1881, p. xxvii.

Footnote 304:

  _Spiders, their Structure and Habits_, 1883, p. 98.

Footnote 305:

  _Sexual Selection in Spiders_, p. 37. (Occasional Papers of the Nat.
  Hist. Soc. of Wisconsin, I., 1889.)

Footnote 306:

  _Arachnides de France_ (vol. i., published 1874). _Histoire naturelle
  des araignées_ (2nd ed. vol i., published 1892).

Footnote 307:

  Simon’s Cribellatae comprise Hypochilidae, Uloboridae, Psechridae,
  Zoropsidae, Dictynidae, Oecobiidae, Eresidae, Filistatidae.

Footnote 308:

  The Spider genus _Mygale_ was established by Walckenaer in 1802, but
  the name was preoccupied, having been used by Cuvier (Mammalia) in
  1800.

Footnote 309:

  _Hist. Nat. des Araignées_ (2nd ed.), i., 1892, p. 76.

Footnote 310:

  The “scopula” is the pad of close-set thick hairs which covers the
  under surface of the tarsus and often of the metatarsus. The
  “claw-tufts” are groups of longer hairs, often extending beyond the
  claws, and giving the foot a bifid appearance.

Footnote 311:

  The three families mentioned above constitute the “Araneae
  Theraphosae” of Simon, the remaining families being distinguished as
  “Araneae Verae.” The Aviculariidae and the Atypidae are united by some
  authors to form the Theraphosidae.

Footnote 312:

  According to Bertkau (in a letter to Simon, cited in _Hist. Nat. des
  Ar._ i. p. 327), two pairs of linear stigmata under the anterior part
  of the abdomen lead, to pulmonary sacs, but to tracheae.

Footnote 313:

  L. Koch replaced _Melanophora_ by _Prosthesima_, believing the former
  to be preoccupied, but according to Simon (_Hist. Nat. des Ar._ i. p.
  341) C. Koch’s use of _Melanophora_ for an Arachnid was antecedent
  (1833) to Meigen’s employment of it for Diptera, 1838.

Footnote 314:

  _Hist. Nat. des Ar._ i. p. 416.

Footnote 315:

  Pickard-Cambridge, _Spiders of Dorset_, p. 77.

Footnote 316:

  _Hist. Nat. des Ar._ i. p. 594.

Footnote 317:

  _Hist. Nat. des Ar._ i. p. 692.

Footnote 318:

  The Erigoninae, Formicinae, and Linyphiinae, together with the
  Epeiridae, form Simon’s family of Argiopidae.

Footnote 319:

  _I.e._ as developed in the course of the work, not as set forth on p.
  594 of vol. i., where five sub-families are established
  (Theridiosomatinae, Arciinae, Eurycorminae, Amazulinae, Poltyinae),
  which are afterwards merged in the Argiopinae.

Footnote 320:

  Simon’s treatment of this group in his _Hist. Nat. Ar._ does not
  appear to us satisfactory. He revives the name _Araneus_ as a generic
  term, a proceeding to which there are very valid objections, and
  merges in it, in whole or in part, about twenty-five generally
  received genera, including 800 species. He then proceeds to break up
  the genus _Araneus_ into six entirely artificial “series,” according
  to the eyes. However unsatisfactory the merged genera may be, nothing
  seems to be gained by this proceeding. The facts about “Araneus” are
  these. Clerck and Linnaeus used the name “_Araneus_” for every member
  of the order. Latreille, in subdividing the order, retained the name
  for _A._ (_Epeira_) _diademata_ (1804), but later (1827) transferred
  it to _A._ (_Tegenaria_) _domestica_. Walckenaer, seeing the
  impropriety of using _Araneus_ as a generic term, discarded it,
  establishing _Epeira_, which has since obtained universal recognition.

Footnote 321:

  Simon, in his _Histoire naturelle des araignées_, removes the
  Sparassinae and the Selenopinae to the Clubionidae, considering that,
  notwithstanding the direction of their legs, they have a greater
  affinity with that group than with the other Thomisidae.

Footnote 322:

  _Ent. Tidsskr._ xviii., 1897, p. 223, pl. iv.

Footnote 323:

  _Zool. Anz._ xxiv., 1901, p. 537.

Footnote 324:

  _Quart. J. Micr. Sci._ xlvii., 1904, p. 215.

Footnote 325:

  _Trans. Linn. Soc._ (2), vi., 1896, p. 323.

Footnote 326:

  _Nature_, xlvi., 1892, p. 247.

Footnote 327:

  _Ann. Nat. Hist._ (1), xii., 1843, p. 81.

Footnote 328:

  Pocock, _Nature_, lvii., 1897, p. 618.

Footnote 329:

  Cook, _Nature_, lviii., 1898, p. 247.

Footnote 330:

  _Öfv. Ak. Förh._ lvi., 1899, p. 977.

Footnote 331:

  _Trans. Linn. Soc._ (2), vi., 1896, p. 310.

Footnote 332:

  _Das Tierreich_, Berlin, 12. Lief., Arachnoidea, 1901, p. 4.

Footnote 333:

  _Arachnides de France_, vii., 1879, p. 2.

Footnote 334:

  _Arachnides de France_, vii., 1879, p. 5.

Footnote 335:

  See Bernard, _J. Linn. Soc._ xxiv. (Zool.), 1893, p. 410.

Footnote 336:

  See Bernard, _J. Linn. Soc._ xxiv. (Zool.), 1893, p. 422.

Footnote 337:

  For the embryology of Chernetidea, see J. Barrois, “Mém. sur le
  développement des Chélifers,” _Rev. Suisse de Zool._ iii., 1896.
  Metschnikoff, _Zeitschr. wiss. Zool._ xxi., 1876, p. 514; and
  Vejdovský, _Congrès zool. international de Moscou_, 1892, p. 120, may
  also be consulted.

Footnote 338:

  _Monograph of the British Species of Chernetidea_, Dorchester, 1892.

Footnote 339:

  _Revue Zoologique par la Société Cuvierienne_, p. 10.

Footnote 340:

  _Arachnides de France_, vii., 1879, p. 122.

Footnote 341:

  _On two Orders of Arachnida_, Cambridge University Press, 1904.

Footnote 342:

  _Mag. Nat. Hist._ (i.), xii., 1843, p. 325.

Footnote 343:

  _Zool. Jahrb._ iii., 1888, p. 319.

Footnote 344:

  _T. C._ pp. 67–75.

Footnote 345:

  Long sternum (μῆκος = length; στῆθος = breast).

Footnote 346:

  _Arachnides de France_, vii., 1879.

Footnote 347:

  Transverse sternum (πλάγιος = transverse).

Footnote 348:

  _Monograph of the British Phalangidea_, Dorchester, 1890.

Footnote 349:

  The single exception is _Opilioacarus_, see p. 473.

Footnote 350:

  _C. R. Ac. Sci._ cxxv., 1897, p. 879.

Footnote 351:

  “The Biology of the Cattle Tick,” _Journ. Compar. Med. and Vet.
  Archives_, 1891, p. 313.

Footnote 352:

  _Entomological News_ (Philadelphia), vol. xi., Jan. 1900.

Footnote 353:

  For the Protozoa to which these and similar diseases are due, cf. vol.
  i. pp. 120 f.

Footnote 354:

  _C. R. Soc. Biol. Paris_ (7), iv., 1882, p. 305.

Footnote 355:

  _Ann. Soc. Linn. Lyon_, xxii., 1876, p. 29.

Footnote 356:

  _Bull. Soc. Nat. de Moscou_, liv. 1879, pt. i. p. 234.

Footnote 357:

  _Z. wiss. Zool._ xxxvii., 1882, p. 553.

Footnote 358:

  _P. Z. S._, 1895, p. 174.

Footnote 359:

  _Arch. f. Naturg._ i., 1876, p. 65.

Footnote 360:

  _Tr. Linn. Soc._ (2), v. Zool., 1890, p. 281.

Footnote 361:

  See account given by Tulk in _Mag. Nat. Hist._ xviii., 1846, p. 160.

Footnote 362:

  _Entomological News_ (Philadelphia), vol. xi., Jan. 1900.

Footnote 363:

  Michael, _British Oribatidae_ (Ray Soc.), i., 1883, p. 176.

Footnote 364:

  _Loc. cit._ p. 168.

Footnote 365:

  Claparède, _Z. wiss. Zool._ xviii., 1868, p. 455. Michael, _British
  Oribatidae_, i., 1883, p. 73, writes it “Deutovium.”

Footnote 366:

  _Atti Ist. Veneto_, ii., 1891, p. 699.

Footnote 367:

  _Rev. Sci. Nat. Ouest_, ii., 1892, p. 20.

Footnote 368:

  _Eriophyes_, v. Siebold, _Jahresber. Schles. Ges._ xxviii., 1850, p.
  89; _Phytoptus_, Dujardin, _Ann. Sci. Nat._ (3), xv., 1851, p. 166.

Footnote 369:

  See Michael, _British Tyroglyphidae_, published by the _Ray Society_,
  1901–2.

Footnote 370:

  The first paper appeared in _Mém. Soc. Zool._ ix., 1896, pp. 1–44.

Footnote 371:

  “Ticks, a Monograph of the Ixodoidea.” Part I. Argasidae, 1908.

Footnote 372:

  With, _Vid. Medd._ 1904, p. 137.

Footnote 373:

  Silvestri, _Redia_, ii., 1904, fasc. 2, p. 257.

Footnote 374:

  _Arch. mikr. Anat._ Bd. i., 1865, p. 428.

Footnote 375:

  A. Basse, _Zeitschr. wiss. Zool._ lxxx., 1906, p. 259.

Footnote 376:

  _Ann. Sci. nat._ (2), xiv., 1840, p. 269, and xvii., 1842, p. 193.

Footnote 377:

  _Zool. Jahrb. Anat._ iii., 1889. This paper contains a bibliography.

Footnote 378:

  _Morph. Jahrb._ xxii., 1895, p. 491.

Footnote 379:

  _C. R. Ac. Sci._ cxviii., 1894, p. 817.

Footnote 380:

  _Tr. R. Soc. Edinb._ xlv., 1908, p. 641. This contains a Bibliography
  of recent literature. See also Richters, _Zool. Anz._ xxx., 1906, p.
  125, and Heinis, _Zool. Anz._ xxxiii., 1908, p. 69.

Footnote 381:

  _P. Zool. Soc._ 1897, p. 790.

Footnote 382:

  Hay, in _P. Biol. Soc. Washington_, xix., 1906, p. 46, states that the
  name _Lydella_, Dujardin, is preoccupied, and suggests as a substitute
  _Microlyda_.

Footnote 383:

  The animals included in this group are usually called Linguatulidae or
  Pentastomidae after the two genera or sub-genera _Linguatula_ and
  _Pentastoma_. But the animal which Rudolphi in 1819 (_Synopsis
  Entozoorum_) named _Pentastoma_ had been described, figured, and named
  _Porocephalus_ by Humboldt (_Recueil d’observations de zoologie et
  anatomie comparee_, i. p. 298, pl. xxvi.) in 1811. The familiar name
  _Pentastoma_ may, however, be preserved by incorporating it in the
  designation of the group.

Footnote 384:

  This description is mainly based on the account of _P. teretiusculus_
  given by Spencer, _Quart. J. Micr. Sci._ xxxiv., 1893, p. 1.

Footnote 385:

  _Zeitschr. wiss. Zool._ lii., 1891, p. 85. This contains a very full
  bibliography, of 143 entries.

Footnote 386:

  _Centrbl. Bakter._ xl., 1906, p. 368; v. also Thiroux, _C. R. Soc.
  Biol._ lix., 1905, p. 78.

Footnote 387:

  Shipley, _Arch. parasit._ i., 1898, p. 52. This contains lists of
  synonyms and of memoirs published since Stiles’ paper, etc.

Footnote 388:

  H. B. Ward, _P. Amer. Ass._ 1899, p. 254.

Footnote 389:

  _Nouv. Dict. de méd., de chir. et d’hyg. vétérinaires_, xii. 1883.

Footnote 390:

  _Tr. R. Soc. Edinb._ xxxii., 1884, p. 165.

Footnote 391:

  Lohrmann, _Arch. Naturg. Jahrg._ 55, i., 1889, p. 303.

Footnote 392:

  Von Linstow, _J. R. Asiat. Soc. Bengal_, ii., 1906, p. 270.

Footnote 393:

  _Pycnogonides_, Latreille, 1804; _Podosomata_, Leach, 1815;
  _Pychnogonides ou Crustacés aranéiformes_, Milne-Edwards, 1834;
  _Crustacea Haustellata_, Johnston, 1837; _Pantopoda_, Gerstaecker,
  1863.

Footnote 394:

  _Syst. Nat._ ed. xii. 1767, vol. ii. p. 1027.

Footnote 395:

[Illustration: Fig. 4]

  Brünnich’s description (“Entomologia,” 1764), is still more accurate,
  and is worthy of transcription as an excellent example of early work.
  “Fig. iv. Novum genus, a R[ev.] D[on.] Ström inter _phalangiis_
  relatum, _Söndm._ Tom. i. p. 209, t. 1, f. 17. Exemplar hujus insecti,
  quod munificentia R. Autoris possideo, ita describo; Caput cum thorace
  unitum, tubo _b_ excavato cylindrico, antice angustiore, postice in
  thoracem recepto, prominens; Oculi iv. dorsales, _a_, in gibbositate
  thoracis positi; _c_, Antennae 2 tubo breviores moniliformes, subtus
  in segmento thoracis, cui oculi insident, radicatae; segmenta
  corporis, excepto tubo, iv., cum tuberculo e medio singuli segmenti
  prominulo. Pedes viii., singuli ex articulis vii. brevissimis
  compositi, ungue valido terminati. Ex descriptione patet insectum hoc
  a generibus antea notis omnino differre, ideoque novum genus, quod e
  crebris articulationibus Pycnogonum dico, constituit.” The confusion
  between _Cyamus_ and _Pycnogonum_ seems to have arisen with Job
  Baster, 1765; cf. Stebbing, _Knowledge_, February 1902, and
  _Challenger Reports_, “Amphipods,” 1888, pp. 28, 30, etc.

Footnote 396:

  Hoek, _Chall. Rep._ p. 15, mentions a specimen of _Colossendeis
  gracilis_, Hoek, “furnished with a pair of distinctly three-jointed
  mandibles; and the specimen was the largest of the three obtained.”

Footnote 397:

  As a rare exception, Hoek has found the eggs carried on the ovigerous
  legs in a single female of _Nymphon brevicaudatum_, Miers.

Footnote 398:

  Meisenheimer (_Zeitsch. wiss. Zool._ lxxii., 1902, p. 235) compares
  these with certain glands described in _Branchipus_ by Spangenberg and
  by Claus.

Footnote 399:

  Ortmann, who would unite _Barana_ with _Ascorhynchus_, observes: “Bei
  dieser Gattung [_Ascorhynchus_] konnte ich die Kittdrüsen beobachten,
  die bei _A. ramipes_ mit dem von _Barana castelnaudi_ [_castelli_]
  Dohrn, bei _A. cryptopygius_ mit _Barana arenicola_ übereinstimmen und
  also die primitivsten Formen der Ausbildung zeigen.”—_Zool. Jahrb.
  Syst._ v., 1891, p. 159.

Footnote 400:

  _Mém. Acad. Sci. St-Pétersb._ (vii.), xxxviii., 1892.

Footnote 401:

  _Fauna und Flora G. von Neapel_, iii. Monogr. 1881, p. 46; see also
  Loman, J. C. C., _Tijdschr. D. Ned. Dierk. Ver._ (2), viii., 1907, p.
  259.

Footnote 402:

  The dorsal lobe is absent in _Rhynchothorax_.

Footnote 403:

  For a very detailed account of this mechanism, here epitomised in the
  merest outline, and for an account of its modifications in diverse
  forms, the student must consult Dohrn’s Monograph (_t. cit._ pp.
  46–53).

Footnote 404:

  Dohrn, _t. cit._ p. 55.

Footnote 405:

  _Biol. Stud. Johns Hopkins Univ._ v., 1891, p. 49.

Footnote 406:

  _Vergl. Entwickl. d. wirbellosen Tiere_, Jena, 1893, p. 664.

Footnote 407:

  In the second joint in _Ascorhynchus abyssi_, Sars, and _A. tridens_,
  Meinert.

Footnote 408:

  _Biol. Bulletin Woods Holl_, vol. ii., Feb. 1901, p. 196.

Footnote 409:

  _Studi e ricerche sui Picnogonidi_, Firenze, 1876.

Footnote 410:

  Semper came near to discovering the fact when he saw, at Heligoland,
  ripe eggs in a _Phoxichilidium_ that was, nevertheless, totally
  destitute of ovigerous legs. The animal, he says, was adult and
  sexually mature: “Trotzdem fehlen dem Tiere die Eierträger
  vollständig; es muss sich also das Tier noch mindestens ein Mal häuten
  vor der Eierablag, und dabei müssen die Eierträger gebildet werden.”
  (_Arb. Inst. Würzburg_, 1874, p. 273).

Footnote 411:

  The correspondence is not universally admitted. Meinert (Ingolf
  Expedition, 1899) believes that the second and third appendages of the
  larva disappear, and that the palps and ovigerous legs are new
  developments; so giving to the normal Pycnogon nine instead of seven
  appendages. See also Carpenter “On the Relationship between the
  Classes of the Arthropoda,” _Proc. R. Irish Acad._ xxiv., 1903, pp.
  320–360. The latest observer (Loman) inclines to the older view.

Footnote 412:

  A slightly different account is given of the Australian _P.
  plumulariae_ by v. Lendenfeld (_Zeitschr. wiss. Zool._ xxxviii., 1883,
  pp. 323–329).

Footnote 413:

  _Zur Lehre vom Generationswechsel und Fortpflanzung bei Medusen und
  Polypen_, 1854.

Footnote 414:

  _Rep. Brit. Ass._ 1859; cf. “Gymnoblastic Hydroids,” _Ray Soc._ pl.
  vi. fig. 6.

Footnote 415:

  _Trans. Tyneside Field Club_, v. (1862–3), 1864, pp. 124–136, pls.
  vi., vii.; _Ann. Mag. Nat. Hist._ (3), ix., 1862, p. 33.

Footnote 416:

  See also Hallez, _Arch. Zool. Exp._ (4), v., 1905, p. 3; Loman,
  _Tijdschr. Ned. Dierk. Ver._ (2), x., 1906, p. 271, etc.

Footnote 417:

  “On Hydroid and other Corals,” 1881, p. 78.

Footnote 418:

  Hugo Mertens, _Mitth. Zool. Stat. Neapel_, xviii., 1906, pp. 136–141.

Footnote 419:

  One is tempted to explain such cases as the above of harmonious or
  identical coloration by the simple passage of pigments unchanged from
  the food.

Footnote 420:

  Fabricius says of his _Pycnogonum_ (_Nymphon_) _grossipes_, “Vescitur
  insectis et vermibus marinis minutis; quod autem testas mytilorum
  exhauriat mihi ignotum est, dum nunquam intra testam mytili illud
  inveni, licet sit verisimile satis,” _Fauna Groenlandica_, p. 231.

Footnote 421:

  Loeb (_Arch. Entw. Mech._ v. 2, 1897, p. 250) also says that the
  Pycnogons are positively heliotropic.

Footnote 422:

  See also P. Gaubert, “Autotomie chez les Pycnogonides,” _Bull. Soc.
  Zool. Fr._ xvii., 1892, p. 224.

Footnote 423:

  Cf. Carpenter, _Proc. R. Irish Acad._ xxiv., 1903, p. 320; Lankester,
  _Quart. J. Micr. Sci._ xlviii., 1904, p. 223; Bouvier, _Exp. Antarct.
  Fr._, “Pycnogonides,” 1907, p. 7, etc.

Footnote 424:

  “Nous ne les plaçons ici qu’avec doute,” _Règne Anim._ éd. 3, tom. vi.
  p. 298.

Footnote 425:

  Cf. also J. E. W. Ihle, “Phylogenie und systematische Stellung der
  Pantopoden,” _Biol. Centralbl._, Bd. xviii., 1898, pp. 603–609;
  Meisenheimer, _Verh. zool.-bot. Ges. Wien_, xii., 1902, pp. 57–64;
  also Stebbing, in _Knowledge_, 1902.

Footnote 426:

  The chelate form of the foremost appendages is of little moment. A
  chela consists merely of a more or less mobile terminal joint flexing
  on a more or less protuberant penultimate one, and in the Scorpions,
  in _Limulus_, throughout the Crustacea, and even in Insects (cf. vol.
  vi. p. 554), we see such a structure arising independently on very
  diverse appendages.

Footnote 427:

  Cf. Oudemans, _Tijdschr. d. Ned. Dierk. Ver._ (2), i., 1886, p. 41:
  “Jedermann weiss nun, dass diese Tiere eine ganz besondere Urgruppe
  bilden, ohne alle Verwandschaft mit irgend einer anderen
  Arthropodengruppe.”

Footnote 428:

  Cole (_Ann. Mag. Nat. Hist._ (7), xv., 1905, pp. 405–415) has
  attempted such a phylogenetic classification, starting with
  _Decolopoda_, and leading in two divergent lines, through _Nymphon_
  and _Pallene_ to the Pycnogonidae, and through _Eurycide_ and
  _Ammothea_ to _Colossendeis_. This hint is in part adopted in the
  subjoined classification. Bouvier, in his recent Report on the
  Pycnogons of the French Antarctic Expedition (_t. cit._), gives
  reasons for separating the Decolopodidae and Colossendeidae from all
  the rest. Loman, in _Die Pantopoden der Siboga Expedition_, 1908, has
  recently suggested another, and in many respects novel, classification
  of the whole group.

Footnote 429:

  See (_inter alia_) Dohrn, _l.c._; E. B. Wilson, _Rep. U.S. Fish.
  Comm._ (1878), 1880; Hoek, _Chall. Report_, 1881; G. O. Sars, _Norw.
  N. Atl. Exp._ 1891; Meinert, _Ingolf Exped._ 1899; Möbius, _Fauna
  Arctica_, 1901, _Valdivia Exped._ 1902; Cole, _Harriman Alaska Exped._
  1904; Hodgson, _Discovery Exped._ 1907; Bouvier, _Exp. Antarct. Fr._
  1907.

Footnote 430:

  _Boston Journ. Nat. Hist._ i., 1834, p. 203; Cf. Hodgson, _Pr. R.
  Phys. Soc. Edinburgh_, xvi., 1905, p. 35; _Zool. Anz._ xxv., 1905, p.
  254; _Discovery Exp._, “Pycnogonida,” 1907; Bouvier, _Exp. Antarct.
  Fr._ 1907.

Footnote 431:

  See pp. 535, 541. Cf. Dohrn (_t. cit._), p. 228.

Footnote 432:

  The first known species was described as _Phoxichilus proboscideus_,
  Sabine, from the shores of the North Georgian Islands (1821).

Footnote 433:

  Pocock (_Encycl. Brit._, 10th ed., Art. “Arachnida”) makes _Hannonia_
  the solitary type of a family. Cf. Loman, _Zool. Jahrb._, Syst., xx.,
  1904, p. 385.

Footnote 434:

  Loman conjoins all these genera, and also _Lecythorhynchus_, with
  _Nymphopsis_, as a sub-family Nymphopsinae of Ammotheidae.

Footnote 435:

  _Edinb. New Phil. Journal_, Oct. 1842, p. 367 (_P. capillata_ on
  Plate).

Footnote 436:

  _Proc. Boston Nat. Hist. Society_, vol. i., 1841–44, p. 92.

Footnote 437:

  Found by Sir John Ross’s expedition in 1840, and subsequently by the
  _Challenger_ expedition and other visitors.

Footnote 438:

  Stebbing has recently shown (_Knowledge_, Aug. 1902, p. 157) that the
  genus _Phoxichilus_ was instituted by Latreille (_Nouv. Dict. d’hist.
  nat._ 1804) for the _Pycnogonum spinipes_ of Fabricius, now
  _Pseudopallene spinipes_, auctt. Hence he changes _Pseudopallene_ to
  _Phoxichilus_, Latr., and Phoxichilidae and _Phoxichilus_, auctt., to
  Chilophoxidae, etc.; it also follows that the family known to all
  naturalists as Pallenidae should, according to the letter of the law
  of priority, be henceforth known as the Phoxichilidae. In my opinion
  this is a case where strict adherence to priority would serve no good
  end, but would only lead to great and lasting confusion (cf. Norman,
  _J. Linn. Soc._ xxx., 1908, p. 231).

Footnote 439:

  _Vide_ note 2, p. 537.

Footnote 440:

  _Mag. Nat. Hist._ vi., 1838, p. 42; _Mag. Zool. and Bot._ i., 1837, p.
  368.

Footnote 441:

  _Edinb. New Phil. Journ._ xxxii., 1842, p. 136; xxxiii., 1842, p. 367;
  _Ann. Mag. Nat. Hist._ (1), xiv., 1844, p. 4.

Footnote 442:

  _Ann. Mag. Nat. Hist._ (3), xiii., 1864, p. 113.

Footnote 443:

  _Proc. R. Dublin Soc._ (N.S.), viii., 1893, p. 195; _Fisheries,
  Ireland, Sci. Invest._ 1904, No. iv. (1905).

Footnote 444:

  Cf. A. M. Norman, _J. Linn. Soc._ xxx., 1908, pp. 198–238.

------------------------------------------------------------------------




                     THE CAMBRIDGE NATURAL HISTORY

 Edited by S. F. HARMER, Sc.D., F.R.S., and A. E. SHIPLEY, M.A., F.R.S.

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                               VOLUME IV.

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