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                        THE LIFE OF CRUSTACEA

                                  BY
                         W. T. CALMAN, D.Sc.

            WITH THIRTY-TWO PLATES AND EIGHTY-FIVE FIGURES

                          METHUEN & CO. LTD.
                         36 ESSEX STREET W.C.
                                LONDON




                       _First Published in 1911_




PREFACE


This sketch of the Natural History of the Crustacea deals chiefly with
their habits and modes of life, and attempts to provide, for readers
unfamiliar with the technicalities of Zoology, an account of some of the
more important scientific problems suggested by a study of the living
animals in relation to their environment.

I am indebted to the Trustees of the British Museum for leave to
reproduce certain figures prepared for the "Guide to the Crustacea,
Arachnida, Onychophora, and Myriopoda exhibited in the Department of
Zoology"; also to Sir Ray Lankester, K.C.B., F.R.S., and to Messrs. A.
and C. Black for the use of a number of figures from my volume on
Crustacea in the "Treatise on Zoology," edited by Sir Ray Lankester.

The source of these figures is indicated in the explanation attached to
each. Of the remaining illustrations, some are reproduced from
photographs of specimens in the collection of the British Museum; the
others have been drawn from Nature, or copied from the original figures
of various authors, by Miss Gertrude M. Woodward, to whom I am much
indebted for the care and skill which she has given to their
preparation.

                                                          W. T. C.




CONTENTS


CHAPTER                                                             PAGE

  I. INTRODUCTORY                                                      1

  II. THE LOBSTER AS A TYPE OF CRUSTACEA                               6

  III. THE CLASSIFICATION OF CRUSTACEA                                34

  IV. THE METAMORPHOSES OF CRUSTACEA                                  66

  V. CRUSTACEA OF THE SEASHORE                                        88

  VI. CRUSTACEA OF THE DEEP SEA                                      117

  VII. FLOATING CRUSTACEA OF THE OPEN SEA                            138

  VIII. CRUSTACEA OF FRESH WATERS                                    157

  IX. CRUSTACEA OF THE LAND                                          188

  X. CRUSTACEA AS PARASITES AND MESSMATES                            207

  XI. CRUSTACEA IN RELATION TO MAN                                   237

  XII. CRUSTACEA OF THE PAST                                         256


APPENDIX:

  I. METHODS OF COLLECTING AND PRESERVING CRUSTACEA                  271

  II. NOTES ON BOOKS                                                 277


INDEX                                                                280




LIST OF ILLUSTRATIONS IN THE TEXT


FIG.                                                                PAGE

  1. THE COMMON LOBSTER (_Homarus gammarus_), FEMALE,
        FROM THE SIDE                                                  7

  2. ONE OF THE ABDOMINAL SOMITES OF THE LOBSTER, WITH ITS
        APPENDAGES, SEPARATED AND VIEWED FROM IN FRONT                 9

  3. THIRD MAXILLIPED OF LOBSTER                                      11

  4. WALKING LEGS OF LOBSTER                                          12

  5. APPENDAGES OF LOBSTER IN FRONT OF THIRD MAXILLIPED               13

  6. DISSECTION OF MALE LOBSTER, FROM THE SIDE                        16

  7. GILLS OF THE LOBSTER, EXPOSED BY CUTTING AWAY THE
        SIDE-FLAP OF THE CARAPACE (BRANCHIOSTEGITE)                   18

  8. FIRST LARVAL STAGE OF THE COMMON LOBSTER. × 4                    28

  9. SIDE-VIEW OF ROSTRUM OF (A) COMMON LOBSTER
        (_Homarus gammarus_) AND (B) AMERICAN LOBSTER
        (_Homarus americanus_)                                        32

  10. THE "FAIRY SHRIMP" (_Chirocephalus diaphanus_), MALE. × 2       35

  11. _Estheria obliqua_, ONE OF THE CONCHOSTRACA                     36

  12. _Daphnia pulex_, A COMMON SPECIES OF "WATER-FLEA."
        MUCH ENLARGED                                                 37

  13. SHELLS OF OSTRACODA. MUCH ENLARGED                              38

  14. _Cyclops albidus_, A SPECIES OF COPEPOD FOUND IN FRESH
        WATER                                                         39

  15. _Nebalia bipes._ ENLARGED                                       44

  16. _Mysis relicta_, ONE OF THE MYSIDACEA. ENLARGED                 47

  17. _Gnathophausia willemoesii_, ONE OF THE DEEP-SEA MYSIDACEA.
        HALF NATURAL SIZE                                             48

  18. _Diastylis goodsiri_, ONE OF THE CUMACEA. ENLARGED              49

  19. _Apseudes spinosus_, ONE OF THE TANAIDACEA. ENLARGED            50

  20. A WOODLOUSE (_Porcellio scaber_), ONE OF THE ISOPODA.
        ENLARGED                                                      51

  21. AN AMPHIPOD (_Gammarus locusta_). ENLARGED                      53

  22. TWO SPECIES OF CAPRELLIDÆ                                       54

  23. _Paracyamus boopis_, THE WHALE-LOUSE OF THE HUMPBACK
        WHALE                                                         55

  24. _Meganyctiphanes norvegica_, ONE OF THE EUPHAUSIACEA.
        TWICE NATURAL SIZE                                            56

  25. LARVAL STAGES OF THE COMMON SHORE CRAB (_Carcinus
        mænas_--SEE PLATE IX.)                                        68

  26. LAST LARVAL STAGE OF THE COMMON PORCELAIN CRAB
        (_Porcellana longicornis_--SEE FIG. 41, P. 113). × 9          70

  27. FIRST LARVAL STAGE OF _Munida rugosa_ (SEE PLATE VI.) × 10      71

  28. THE PHYLLOSOMA LARVA OF THE COMMON SPINY LOBSTER
        (_Palinurus vulgaris_--SEE PLATE V.). MUCH ENLARGED           72

  29. LARVAL STAGES OF THE PRAWN _Penæus_ (SEE PLATE IV.). × 45       74

  30. NEWLY-HATCHED YOUNG OF A CRAYFISH (_Astacus fluviatilis_).
        ENLARGED                                                      76

  31. YOUNG SPECIMEN OF AN AFRICAN RIVER CRAB (_Potamon
        johnstoni_), TAKEN FROM THE ABDOMEN OF THE MOTHER.
        MUCH ENLARGED                                                 78

  32. EARLY LARVAL STAGE OF A SPECIES OF SQUILLA, PROBABLY
        _S. dubia_. × 10                                              80

  33. LARVAL STAGES OF THE BRINE SHRIMP (_Artemia salina_)            81

  34. EARLY NAUPLIUS LARVA OF A COPEPOD (_Cyclops_). MUCH
        ENLARGED                                                      82

  35. LARVAL STAGES OF THE COMMON ROCK BARNACLE
        (_Balanus balanoides_--SEE PLATE III.)                        83

  36. A COMMON HERMIT CRAB (_Eupagurus bernhardus_) REMOVED
        FROM THE SHELL                                                91

  37. _Pylocheles miersii_, A SYMMETRICAL HERMIT CRAB                 94

  38. _Callianassa stebbingi_ (FEMALE), A SAND-BURROWING
        THALASSINID FROM THE SOUTH COAST OF ENGLAND.
        NATURAL SIZE                                                 103

  39. THE COMMON SAND-HOPPER (_Talitrus saltator_), MALE,
        FROM THE SIDE. × 3                                           108

  40. A, A PIECE OF A TROPICAL SEA-WEED (_Halimeda_); B, A
        CRAB (_Huenia proteus_) WHICH LIVES AMONG THE FRONDS
        OF _Halimeda_, AND CLOSELY RESEMBLES THEM IN FORM
        AND COLOUR. REDUCED                                          110

  41. THE COMMON PORCELAIN CRAB (_Porcellana longicornis_),
        SLIGHTLY ENLARGED, AND ONE OF THE THIRD MAXILLIPEDS
        DETACHED AND FURTHER ENLARGED TO SHOW
        THE FRINGE OF LONG HAIRS                                     113

  42. A DEEP-SEA LOBSTER (_Nephropsis stewartii_), FROM THE
        BAY OF BENGAL. REDUCED                                       122

  43. _Munidopsis regia_, A DEEP-SEA GALATHEID FROM THE BAY
        OF BENGAL. REDUCED                                           123

  44. _Thaumastocheles zaleucus._ REDUCED                            129

  45. A DEEP-SEA CRAB (_Platymaia wyville-thomsoni._) REDUCED        131

  46. _Polycheles phosphorus_, ONE OF THE ERYONIDEA, FEMALE,
        FROM THE INDIAN SEAS                                         133

  47. _Eryon propinquus_, ONE OF THE FOSSIL ERYONIDEA, FROM
        THE JURASSIC ROCKS OF SOLENHOFEN                             135

  48. _Conchoecia curta_, AN OSTRACOD OF THE PLANKTON. × 40          144

  49. _Mimonectes loveni._ A FEMALE SPECIMEN SEEN FROM THE
        SIDE AND FROM BELOW, SHOWING THE DISTENDED-BALLOON-LIKE
        FORM OF THE ANTERIOR PART OF THE
        BODY. × 3                                                    145

  50. THE ZOËA LARVA OF A SPECIES OF _Sergestes_, TAKEN BY
        THE "CHALLENGER" EXPEDITION. × 25                            146

  51. THE NAUPLIUS LARVA OF A SPECIES OF BARNACLE OF
        THE FAMILY LEPADIDÆ, SHOWING GREATLY-DEVELOPED
        SPINES. FROM A SPECIMEN TAKEN IN THE ATLANTIC
        OCEAN, NEAR MADEIRA. × 11                                    147

  52. _Calocalanus pavo_, ONE OF THE FREE-SWIMMING COPEPODA
        OF THE PLANKTON. ENLARGED                                    148

  53. _Copilia quadrata_ (FEMALE), A COPEPOD OF THE FAMILY
        CORYCÆIDÆ, SHOWING THE PAIR OF LARGE "TELESCOPIC"
        EYES. × 20                                                   153

  54. _Phronima colletti_, MALE. FROM A SPECIMEN TAKEN IN
        DEEP WATER NEAR THE CANARY ISLANDS. × 12                     154

  55. THE BRINE SHRIMP (_Artemia salina_)                            164

  56. _Chydorus sphæricus_, A COMMON SPECIES OF WATER-FLEA.
        × 50                                                         166

  57. A WATER-FLEA (_Daphnia pulex_), FEMALE, WITH EPHIPPIUM
        CONTAINING TWO "RESTING EGGS." × 20                          167

  58. _Bythotrephes longimanus_, FEMALE, WITH EMBRYOS IN THE
        BROOD-SAC. × 12                                              169

  59. _Diaptomus coeruleus_, FEMALE. × 25                            171

  60. _Asellus aquaticus_, FEMALE. × 4                               173

  61. MAP SHOWING THE DISTRIBUTION OF CRAYFISHES                     175

  62. A WELL SHRIMP (_Niphargus aquilex_). × 7.                       185

  63. THE SEA-SLATER (_Ligia oceanica_). ABOUT TWICE NATURAL
        SIZE                                                         200

  64. STRUCTURE OF THE BREATHING ORGANS OF _Porcellio
        scaber_                                                      202

  65. _Armadillidium vulgare._ × 2-1/2                               203

  66. TWO BRANCHES OF A CORAL (_Seriatopora_) SHOWING
        "GALLS" INHABITED BY THE CRAB _Hapalocarcinus
        marsupialis_. ON THE RIGHT THE FEMALE CRAB,
        EXTRACTED FROM THE GALL AND FURTHER ENLARGED                 211

  67. _Hyperia galba_, FEMALE. ENLARGED                              213

  68. A, THE CRAB _Melia tessellata_ CLINGING TO A BRANCH OF
        CORAL, AND CARRYING IN EACH CLAW A LIVING SEA-ANEMONE;
        B, ONE OF THE CLAWS FURTHER ENLARGED
        TO SHOW THE WAY IN WHICH THE ANEMONE IS HELD                 216

  69. THE COMMON PEA CRAB (_Pinnotheres pisum_), FEMALE.
        NATURAL SIZE                                                 217

  70. _Cirolana borealis._ ABOUT TWICE NATURAL SIZE                  219

  71. A, FRONT PART OF BODY OF A PRAWN (_Spirontocaris
        polaris_), FROM ABOVE, SHOWING ON THE RIGHT SIDE A
        SWELLING OF THE CARAPACE CAUSED BY THE PRESENCE OF THE
        PARASITE _Bopyroides hippolytes_ IN THE GILL CHAMBER;
        B, THE FEMALE PARASITE EXTRACTED AND FURTHER ENLARGED;
        C, THE MALE PARASITE ON SAME SCALE AS THE FEMALE             222

  72. A FISH-LOUSE (_Caligus rapax_), FEMALE. × 5                    225

  73. STAGES OF DEVELOPMENT OF _Lernæa branchialis_. F IS
        SLIGHTLY, THE OTHER FIGURES GREATLY, ENLARGED                226

  74. STAGES OF THE LIFE-HISTORY OF _Hæmocera danæ_, ONE
        OF THE MONSTRILLIDÆ                                          229

  75. FREE-SWIMMING STAGES OF _Sacculina carcini_. MUCH
        ENLARGED                                                     232

  76. EARLY STAGE OF _Sacculina_ WITHIN THE BODY OF A CRAB           234

  77. ROSTRUM AND FORE PART OF CARAPACE, SEEN FROM ABOVE, OF
        (A) RED-CLAWED CRAYFISH (_Astacus fluviatilis_) AND
        (B) WHITE-CLAWED OR ENGLISH CRAYFISH
        (_Astacus pallipes_)                                         242

  78. THE COMMON SHRIMP (_Crangon vulgaris_). NATURAL SIZE           244

  79. THE NORWEGIAN DEEP-WATER PRAWN (_Pandalus borealis_),
        FEMALE                                                       246

  80. THE GRIBBLE (_Limnoria lignorum_). MUCH ENLARGED               254

  81. RESTORATION OF A TRILOBITE (_Triarthrus becki_), SHOWING
        THE APPENDAGES. UPPER SIDE ON RIGHT, UNDER SIDE ON
        LEFT. SLIGHTLY ENLARGED                                      258

  82. _Ceratiocaris papilio_, ONE OF THE FOSSIL PHYLLOCARIDA         262

  83. _Pygocephalus cooperi_, FROM THE COAL-MEASURES: UNDER
        SIDE OF A FEMALE SPECIMEN, SHOWING THE OVERLAPPING
        PLATES OF THE BROOD-POUCH                                    263

  84. THE TASMANIAN "MOUNTAIN SHRIMP" (_Anaspides tasmaniæ_),
        A LIVING REPRESENTATIVE OF THE SYNCARIDA.
        SLIGHTLY ENLARGED                                            264

  85. _Præanaspides præcursor_, ONE OF THE FOSSIL SYNCARIDA,
        FROM THE COAL-MEASURES OF DERBYSHIRE. SLIGHTLY
        ENLARGED                                                     265


FULL-PAGE PLATES

PLATE                                                        FACING PAGE

  I. MALE AND FEMALE LOBSTERS, SHOWING THE DIFFERENCE IN THE
        RELATIVE BREADTH OF THE ABDOMEN IN THE TWO SEXES. THIS
        FIGURE ALSO ILLUSTRATES THE DISSIMILARITY OF THE LARGE
        CLAWS, AND THE FACT THAT THE "CRUSHING CLAW" MAY BE ON
        EITHER THE RIGHT OR LEFT SIDE OF THE BODY.
        (_From Brit. Mus. Guide_)                                     26

  II. _Apus cancriformis_ FROM KIRKCUDBRIGHTSHIRE. SLIGHTLY
        ENLARGED                                                      36

        { GROUP OF SPECIMENS OF THE GOOSE BARNACLE             }
        {   (_Lepas anatifera_), ONE SHOWING THE CIRRI         }
        {   EXTENDED AS IN LIFE. NATURAL SIZE.                 }
  III.  {   (_From Brit. Mus. Guide_)                          }      42
        {                                                      }
        { GROUP OF A COMMON SPECIES OF ACORN-SHELL OR ROCK     }
        {   BARNACLE (_Balanus balanoides_). NATURAL SIZE      }

  IV. _Penæus caramote_, FROM THE MEDITERRANEAN. ABOUT
        HALF NATURAL SIZE. (_From Brit. Mus. Guide_)                  57

  V. THE COMMON SPINY LOBSTER (_Palinurus vulgaris_).
        MUCH REDUCED. (_From Brit. Mus. Guide_)                       59

  VI. _Munida rugosa._ BRITISH. REDUCED                               60

  VII. THE COMMON HERMIT CRAB, _Eupagurus bernhardus_, IN THE
        SHELL OF A WHELK. REDUCED. (_From Brit. Mus. Guide_)          62

  VIII. THE "NORTHERN STONE CRAB," _Lithodes maia_. MUCH REDUCED.
        THE LAST PAIR OF LEGS ARE FOLDED OUT OF SIGHT IN THE
        GILL CHAMBERS. (_From Brit. Mus. Guide_)                      63

        { THE COMMON SHORE CRAB (_Carcinus mænas_). REDUCED     }
  IX.   {                                                       }
        { _Dromia vulgaris_, CARRYING ON ITS BACK A MASS OF     }     68
        {   THE SPONGE, _Clione celata_. BRITISH. REDUCED       }

  X. _Calappa flammea._ BRAZIL. REDUCED                               72

  XI. THE GIANT JAPANESE CRAB, _Macrocheira kæmpferi_. MALE. THE
        SCALE OF THE FIGURE IS GIVEN BY A TWO-FOOT RULE PLACED
        BELOW THE SPECIMEN. (_From Brit. Mus. Guide_)                 76

  XII. _Squilla mantis_, FROM THE MEDITERRANEAN. ABOUT ONE-HALF
        NATURAL SIZE. (_From Brit. Mus. Guide_)                       82

        { A SWIMMING CRAB, _Portunus depurator_. BRITISH.       }
        {   REDUCED                                             }
  XIII. {                                                       }
        { A SPIDER CRAB, _Maia squinado_, DRESSED IN FRAGMENTS  }     96
        {   OF WEEDS. BRITISH. REDUCED                          }

        { _Corystes cassivelaunus._ MALE (ON LEFT) AND FEMALE   }
        {   (ON RIGHT). BRITISH. REDUCED                        }
  XIV.  {                                                       }    100
        { _Albunea symnista_, ONE OF THE HIPPIDEA. INDIAN       }
        {   SEAS. REDUCED                                       }

        { _Ocypode cursor._ WEST AFRICA. REDUCED                }
  XV.   {                                                       }
        { _Gelasimus tangeri._ MALE ABOVE, FEMALE BELOW.        }    104
        {   WEST AFRICA. REDUCED                                }

  XVI. A DEEP-SEA HERMIT CRAB, _Parapagurus pilosimanus_,
        SHELTERED BY A COLONY OF _Epizoanthus_. FROM DEEP WATER
        OFF THE WEST OF IRELAND. SLIGHTLY REDUCED                    124

  XVII. A DEEP-SEA PRAWN, _Nematocarcinus undulatipes_.
        SLIGHTLY REDUCED. (_From Brit. Mus. Guide_)                  128

  XVIII. _Bathynomus giganteus._ ABOUT ONE-HALF NATURAL SIZE.
        (_From Lankester's "Treatise on Zoology," after
        Milne-Edwards and Bouvier_)                                  131

        { _Latreillia elegans_, ONE OF THE DROMIACEA WHICH      }
        {   RESEMBLES A SPIDER CRAB. FROM THE MEDITERRANEAN.    }
  XIX.  {   NATURAL SIZE                                        }    155
        {                                                       }
        { THE GULF-WEED CRAB, _Planes minutus_. SLIGHTLY        }
        {   ENLARGED                                            }

        { THE MURRAY RIVER "LOBSTER," _Astacopsis spinifer_.    }
  XX.   {   NEW SOUTH WALES. MUCH REDUCED                       }    177
        { THE LAND CRAYFISH, _Engæus cunicularis_. TASMANIA.    }
        {  NATURAL SIZE                                         }

  XXI. _Palæmon jamaicensis._ A LARGE FRESHWATER
        PRAWN OF THE FAMILY PALÆMONIDÆ. WEST
        INDIES. MUCH REDUCED                                         179

  XXII. _Atya scabra._ A FRESHWATER PRAWN OF THE
        FAMILY ATYIDÆ, WEST INDIES. REDUCED                          180

        { THE RIVER CRAB OF SOUTHERN EUROPE, _Potamon edule_    }
        {   (OR _Telphusa fluviatilis_). REDUCED                }
  XXIII.{                                                       }    182
        { _Sesarma chiragra._ A FRESHWATER CRAB OF THE FAMILY  }
        {   GRAPSIDÆ. FROM BRAZIL. SLIGHTLY REDUCED             }

  XXIV. _Æglea lævis._ SOUTH AMERICA. NATURAL SIZE                   184

  XXV. THE BLIND CRAYFISH OF THE MAMMOTH CAVE OF KENTUCKY,
        _Cambarus pellucidus_. NATURAL SIZE                          186

        { A WEST INDIAN LAND CRAB, _Gecarcinus ruricola_.       }
  XXVI. {   REDUCED                                             }    190
        {                                                       }
        { A LAND HERMIT CRAB, _Coenobita rugosa_. REDUCED       }

  XXVII. THE COCONUT CRAB, _Birgus latro_. MUCH REDUCED              196

  XXVIII. GROUP OF BARNACLES, _Coronula diadema_, ON THE
           SKIN OF A WHALE. JAPAN. REDUCED                           209

        { _Cymothoa oestrum._ AN ISOPOD PARASITE OF FISH.       }
        {   SLIGHTLY ENLARGED                                   }
  XXIX. {                                                       }    220
        { _Sacculina carcini_ ATTACHED UNDER THE ABDOMEN        }
        {   OF A COMMON SHORE CRAB. REDUCED                     }

  XXX. THE "NORWAY LOBSTER," _Nephrops norvegicus_. ABOUT
        ONE-THIRD NATURAL SIZE. (_From Brit. Mus. Guide_)            240

  XXXI. THE COMMON EDIBLE CRAB, _Cancer pagurus_. BRITISH.
        MUCH REDUCED                                                 248

  XXXII. PIECE OF TIMBER FROM RYDE PIER, SHOWING DAMAGE CAUSED
        BY _Limnoria_ AND _Chelura_. (_From Brit. Mus. Guide_)       255




THE LIFE OF CRUSTACEA




CHAPTER I

INTRODUCTORY


Everyone has some acquaintance with the animals that are grouped by
naturalists under the name Crustacea. The edible Crabs, Lobsters,
Prawns, and Shrimps, are at least superficially familiar, either as
brought to the table or as displayed in the fishmonger's, and the most
unobservant of seaside visitors must have had his attention attracted by
living specimens of some of the more obtrusive species, such as the
common Shore Crab. Many, however, will be surprised to learn that the
Barnacles coating the rocks on the seashore, the Sand-hoppers of the
beach, and the Woodlice of our gardens, are members of the same class.
Still less is it suspected, by those who have not given special
attention to the subject, that the living species of the group number
many thousands, presenting strange diversities of structure and habits,
and playing important parts in the general economy of Nature.

In addition to those just mentioned, a few Crustacea are sufficiently
well known to be distinguished by popular names, such, for example, as
Crayfish and Hermit Crabs, but for the vast majority no names are
available except those of technical zoology. In the following pages,
therefore, while technical terms have been introduced as sparingly as
possible, the unfamiliarity of the animals themselves makes it needful
to use many unfamiliar names.

In the classification of the Animal Kingdom, the Crustacea form one of
the divisions of a comprehensive group, or Phylum, known as Arthropoda.
The typical members of this group have a more or less firm external
skeleton, the body is divided into segments, there are jointed limbs,
and some of these are modified to serve as jaws. The chief divisions or
classes of the Arthropoda are--(i.) _Insecta_, including Butterflies,
Moths, Bees, Wasps, Flies, Beetles, and the like; (ii.) _Chilopoda_, or
Centipedes; (iii.) _Diplopoda_, or Millipedes[1]; (iv.) _Onychophora_,
including the curious worm-like _Peripatus_; (v.) _Arachnida_, or
Scorpions, Spiders, Mites, and their allies; and (vi.) _Crustacea_.

  [1] The Chilopoda and Diplopoda are sometimes regarded as forming a
  single class--Myriopoda.

It is not easy to summarize in a few words the characters common to all
Crustacea, and distinguishing them from the other groups of Arthropoda.
As a rough guide to classification, it is useful to remember that an
Insect can generally be recognized by having three pairs of walking
legs, an Arachnid by having four pairs, and a Centipede or Millipede by
having a great many pairs, all nearly alike. The Crustacea, on the other
hand, show great diversity in the number and arrangement of their
walking or swimming legs, but they rarely show any special resemblance
to those of the other large groups of Arthropoda. Thus, for example, a
common species of Woodlouse, _Armadillidium vulgare_, is very similar at
first sight to the Millipede _Glomeris marginata_, but it has only seven
pairs of walking legs, while the Millipede has seventeen or nineteen
pairs.

More precisely, it may be said of the great majority of Crustacea that
they are aquatic animals, breathing by gills or by the general surface
of the body, having two pairs of "feelers," or antennæ, on the front
part of the head, and at least three pairs of jaws. Exceptions to each
of these statements will be mentioned in later chapters in dealing with
parasites and other highly modified types. In such cases, however, the
larval or young stages afford indications of affinity, and comparison
with less modified forms enables us to trace a connection with the
typical Crustacea.

The best way to form a conception of a group of animals, however, is not
to attempt in the first place to define its limits, but to begin by
studying the structure of some typical and central species, and
afterwards to note the divergences from this type presented by other
members of the group. Speaking very generally, it may be said that these
divergences are of two kinds. On the one hand there are characters that
have no apparent relation to the animal's habits and mode of life, and
on the other hand there are modifications of structure which are more or
less plainly of use to the animal. It is to characters of the former
class that we look for evidence of an animal's affinities, and it is
upon them that our systems of classification are chiefly based. The
characters of the second class--"adaptive" characters, as they are
called--become of importance when we study the animal "as a going
concern," so to speak, and endeavour to understand how its life is
carried on in relation to its surroundings.

In pursuance of this plan of study, the next chapter will be devoted to
a description of the Common Lobster as a type of the Crustacea. In the
third chapter a survey of the classification of the group will be given;
since, however, the characters on which the classification is based
cannot be explained fully without entering into technical details which
are beyond the scope of this work, this survey will be restricted to
what is necessary for comprehension of the succeeding chapters. In the
fourth chapter some account is given of the young or larval stages of
Crustacea, and of the changes they undergo in the course of
development.

In the next five chapters the Crustacea are classified according to
their habitats, and those living in the shallow waters, the depths, and
the surface of the ocean, in the fresh waters, and on land, are
discussed in turn; while a separate chapter is devoted to the curious
forms that live as parasites on, or as associates with, other animals.
The last two chapters deal respectively with the Crustacea as they
affect man, and with the past history of the group as revealed by fossil
remains.




CHAPTER II

THE LOBSTER AS A TYPE OF CRUSTACEA


The most noticeable feature distinguishing the Lobster[2] (Fig. 1) at
first sight from other familiar animals is the jointed shelly armour
that encases its body and limbs. Over the fore part of the body this
armour is continuous, forming a shield, or _carapace_, which projects in
front, between the eyes, as a toothed beak, or _rostrum_; on the hinder
part--the tail, or _abdomen_--it is divided into six segments, or
_somites_, connected with each other by movable joints. Each of these
somites carries on the under-side a pair of fin-like limbs, or
_swimmerets_, the last pair of which (uropods) are much larger than the
others, and are spread out at the sides of a middle tail-plate, or
_telson_, forming what is known as the _tail-fan_. Since the fore part
of the body also has a series of paired limbs, constructed, as will be
shown later, on the same plan as the swimmerets, it is concluded that
this part also is built up of somites, which have become soldered
together. That this conclusion is correct is shown by comparison with
some of the lower Crustacea in which this part of the body is divided up
into eight separate somites, like those of the abdomen, each carrying,
in place of the swimmerets, a pair of walking legs. In front of these
eight somites, forming what is called the _thorax_, is the head--a part
of the body which is never, in any Crustacean, broken up into distinct
somites, but which, since it carries five pairs of appendages, must
consist of at least five somites. The part of the body covered by the
Lobster's carapace includes both the head and the thorax, and is known,
therefore, as the _cephalothorax_. It is necessary to bear in mind that
the parts of the body to which the names head, thorax, and abdomen, are
applied in Crustacea are by no means exactly equivalent to those which
bear the same names in Insects, for example, and that, beyond a rough
similarity in position, they have no sort of relation to the parts so
named in the body of a vertebrate animal.

  [2] The account given here of the structure of the Lobster applies
  almost equally well to the River Crayfish or the Norway Lobster. The
  student is recommended to follow the description with a specimen of
  one of these animals before him.

[Illustration: FIG. 1--THE COMMON LOBSTER (_Homarus gammarus_,) FEMALE,
FROM THE SIDE. (From British Museum Guide.)]

There are altogether twenty pairs of appendages attached to the body of
the Lobster. In front of the head are the stalked _eyes_ (of which the
nature will be discussed later) and two pairs of feelers--the
_antennules_ and _antennæ_ (sometimes called the first and second
antennæ). Near the mouth on the under-side of the head are three pairs
of jaw-appendages--the strong _mandibles_ and the flattened, leaf-like
_maxillulæ_ and _maxillæ_. Following these are the appendages of the
thorax, of which the first three are intermediate in form between the
true jaws and the legs, and are therefore termed foot-jaws, or
_maxillipeds_. The remaining five pairs of thoracic limbs are the
_legs_, the first pair forming the large and powerful pincer-claws, or
_chelipeds_, while the others are the walking legs. The six pairs of
swimmerets on the abdomen have already been mentioned.

If one of the somites of the abdomen be separated from the others, it
will be seen (Fig. 2) to consist of a shelly ring, to which the two
swimmerets are attached, wide apart, on the under-side. The arched
upper part of the ring is known as the _tergum_, and the more flattened
under-part as the _sternum_. On each side the tergum overlaps the
sternum, and hangs down as a side-flap, or _pleuron_. On the upper side
of the abdomen the terga of the somites overlap, the front part of each
being pushed under the tergum in front when the abdomen is straightened,
and only exposed to view when the abdomen is bent. Below, the sternum of
each somite is seen to be only a narrow bar, connected with those in
front and behind by soft membrane, and there is no overlapping. At the
sides the somites are connected together by hinge-joints, which allow
them to move only in a vertical plane. Thus the abdomen can be
straightened out or bent downwards and forwards, but cannot be moved
from side to side. In life the Lobster can swim backwards through the
water by vigorously flapping the abdomen.

[Illustration: FIG. 2--ONE OF THE ABDOMINAL SOMITES OF THE LOBSTER, WITH
ITS APPENDAGES, SEPARATED AND VIEWED FROM IN FRONT. (From British Museum
Guide.)]

The carapace which covers the upper side of the head and thorax is not
formed, as might be thought, simply by the terga of the somites becoming
soldered together. This is shown by a comparison with certain
shrimp-like Crustacea (_Mysidacea_) in which the carapace arises, like a
fold of the skin, from the hinder edge of the head, and envelops, like a
loose jacket, the distinctly segmented thorax. In the Lobster this fold
has become adherent to the thoracic somites down the middle of the back,
but at the sides it hangs free, enclosing on each side a cavity within
which lie the gills.

It seems at first sight strange to include in the same category as
"limbs" or "appendages" organs which differ so much in form and function
as do the swimmerets, the walking legs, the jaws, and the antennæ.
Nevertheless it can easily be demonstrated that all of them are
constructed on the same general plan, and arise in the embryo from
rudiments which are, for the most part, exactly alike. This is expressed
in technical language by saying that the appendages of the whole series
are _homologous_ with one another. A full discussion of this interesting
fact would require more space than can be devoted to it here, but a few
examples may be given to illustrate what is meant by the "serial
homology" of the appendages in Crustacea.

If one of the swimmerets be detached from the third abdominal somite, it
will be seen (Fig. 2) to consist of a stalk, known as the _protopodite_,
bearing two branches, of which that on the outer side is called the
_exopodite_, and that on the inner side the _endopodite_. The
protopodite consists of two segments, the first very short, and the
second much longer. It can easily be seen that the side-plates of the
tail-fan (the middle plate, as already mentioned, is the telson) are
simply the swimmerets of the sixth abdominal somite. They are much
larger than the other swimmerets, and have the endopodite and exopodite
broadened out into large plates; while the protopodite is very short,
and not divided into segments.

[Illustration: FIG. 3--THIRD MAXILLIPED OF LOBSTER. (From British Museum
Guide.)]

If now the third maxilliped (Fig. 3) be examined, it will be found that,
like the swimmeret, it consists essentially of two branches springing
from a stalk of two segments. The exopodite, however, is much smaller
than the endopodite, and it ends in a flexible lash made up of many
small segments. The endopodite forms the main part of the limb, and has
five segments, so that, with the two segments of the protopodite, there
are _seven_ segments in the main axis of the limb; the second and third
segments are partly soldered together, but the line of union can be
plainly seen. Attached to the outer side of the first segment is a
membranous plate, known as the _epipodite_, on which is inserted, near
its base, a brush-like structure, which is one of the gills. In the
natural position the epipodite and its gill lie in the gill chamber,
hidden from view by the side-flap of the carapace.

[Illustration: FIG. 4--WALKING LEGS OF LOBSTER

A, Of first pair; B, of third pair]

The legs (Fig. 4) can, without difficulty, be seen to consist each of
seven segments like those of the maxillipeds, but there is no exopodite.
In the young Lobster, when just hatched from the egg, however, each of
the legs has a large exopodite like that of the third maxilliped. These
exopodites, which are used in swimming, are afterwards lost as the
animal grows; but their presence in the young is interesting as
confirming the conclusion that the legs, like the maxillipeds, are built
on the same plan as the swimmerets. The large claws, and also the first
and second pairs of walking legs, end in pincers, or _chelæ_, the
penultimate segment projecting in a thumb-like process against which the
last segment works. Each leg, except those of the last pair, has on its
first segment an exopodite with a gill like those of the maxilliped.

[Illustration: FIG. 5--APPENDAGES OF LOBSTER IN FRONT OF THIRD
MAXILLIPED

A, Eye-stalk; B, antennule; C, antenna (the flagellum is cut short); D,
mandible; E, maxillula; F, maxilla; G, first maxilliped; H, second
maxilliped. _en_, Endopodite; _ep_, epipodite; _ex_, exopodite; _gn_,
gnathobases, or jaw-plates; _p_, palp of mandible; _sc_, scaphognathite]

Following the series of appendages forwards from the third maxilliped
(Fig. 5), it is easy to trace the gradual reduction of the endopodite
and exopodite; while the two segments of the protopodite become
flattened and broadened inwards to form the jaw-plates. The mandibles
(Fig. 5, D), which are the chief organs of mastication, consist mainly
of the much enlarged basal segment of the protopodite, with a strongly
toothed inner edge, where it works against its fellow of the opposite
side; and the rest of the limb is reduced to a small sensory "palp,"
which represents the second segment of the protopodite and the
endopodite.

The antennæ (Fig. 5, C) can be shown, without difficulty, to conform to
the same plan of structure as the other appendages. The two segments of
the protopodite are short, but distinct; the endopodite forms the long
lash, or _flagellum_, composed of very numerous small segments; the
exopodite is reduced to a small movable scale or spine.

The antennules (Fig. 5, B) seem at first sight to present the
two-branched type of structure in its simplest form; but there is
considerable doubt as to whether the two lashes which each bears on a
three-segmented stalk are really equivalent to the endopodite and
exopodite.

The movable stalks which carry the eyes (Fig. 5, A) have been considered
by some to belong to the series of the appendages, and to be, in fact,
modified limbs. If this be the case, we have here the greatest
simplification which the limb undergoes in the Lobster, for each
eye-stalk consists only of two segments: the first small and
incompletely formed, the second in the form of a short cylinder, having
the eye at its end. There are, however, reasons for doubting whether the
eye-stalks are really appendages.

The hard outer covering of the Lobster not only protects and gives
support to the internal organs, but also affords points of attachment
for the muscles by means of which the animal moves. In other words, it
plays the part of a skeleton; but since, unlike the skeleton of
vertebrate animals, it is _outside_ instead of _inside_ the soft parts
of the body, it is known as an _exoskeleton_. Closer examination shows
that this outer covering is really continuous over the whole of the body
and limbs, but is thin and soft at the joints, allowing the parts to
move one upon another. It is composed of a horn-like substance known as
_chitin_, which, except at the joints, is hardened by the deposition in
it of carbonate and other salts of lime.

As this external covering does not increase in size after it has been
formed, and as it cannot stretch to any great extent, the Lobster
requires to cast its shell at intervals as it grows. In this process of
_moulting_ the integument of the back splits between the carapace and
the first abdominal somite. The body and limbs are gradually worked
loose and withdrawn through the opening, leaving the cast shell with all
its appendages almost entire. The new covering, which had been formed
underneath the old before moulting, is at first quite soft, and the
animal rapidly increases in size owing to the absorption of water. The
shell then gradually hardens by the deposition of lime salts.

[Illustration: FIG. 6--DISSECTION OF MALE LOBSTER, FROM THE SIDE. (From
British Museum Guide.)]

The internal anatomy (Fig. 6) presents many points of interest which can
only be briefly touched on here. The food-canal consists of a short
gullet leading into a capacious stomach, from which the straight
intestine runs to the vent on the under-side of the telson. The stomach
has a most remarkable and complicated structure. It consists of two
chambers, a larger in front and a smaller behind, which are lined by a
continuation of the chitinous outside covering of the body. This
chitinous lining is thickened in places to form a system of plates and
levers connected with three strong teeth set in the narrow opening
between the two chambers. By the action of muscles attached to certain
of these plates the teeth work together so as to divide up the food more
finely than had been done by the mandibles and other jaws. The whole
apparatus, in fact, serves as a kind of gizzard, and is known as the
_gastric mill_.

A small part of the intestine at the hinder end is lined, like the
stomach, by a continuation of the chitinous covering, which is turned in
at the vent. This lining and that of the stomach, with the plates and
teeth of the gastric mill, are cast and renewed when the shell is
moulted.

On each side of the food-canal in the thorax lies a large mass of soft
tissue, yellowish-green in colour. This is the _digestive gland_, or
"liver," which secretes the digestive juice, discharging it into the
food-canal by a short duct on each side just behind the stomach.

The _heart_ lies in the middle of the back, just under the hinder part
of the carapace, and gives off, in front and behind, a number of
arteries which carry the blood to the various organs of the body. From
the smaller branches of these arteries the blood passes, not, as in
vertebrate animals, into capillaries, but into the spaces lying between
the organs of the body, and it finds its way back to the heart, not in
definite veins, but by ill-defined venous channels which open into the
pericardium, or space surrounding the heart. From the pericardium the
blood enters the heart by six openings in its walls, each guarded by a
pair of valves which close when the heart contracts, and prevent the
blood from returning to the pericardium.

[Illustration: FIG. 7--GILLS OF THE LOBSTER, EXPOSED BY CUTTING AWAY THE
SIDE-FLAP OF THE CARAPACE (BRANCHIOSTEGITE)]

The venous channels which convey the blood back to the heart are so
arranged that most of the blood passes first through the _gills_, for
the purpose of respiration, before it reaches the heart and is again
distributed through the body. These gills, as already mentioned, lie in
the two branchial chambers under the side-flaps of the carapace (Fig.
7), and are attached, some to the epipodites of the thoracic limbs (as
described above), and some to the soft membrane of the joints between
the limbs and the body; while others are attached to the side-wall of
the thorax itself. Each gill is somewhat like a bottle-brush in shape,
consisting of a central stalk set round with rows of soft hair-like
processes. As the blood streams through the minute channels inside these
filaments, it is separated only by a thin membrane from the surrounding
water, and the absorption of oxygen and discharge of carbon dioxide can
go on easily. For this purpose, however, it is necessary that the water
within the gill chamber should be constantly renewed, and this is
effected in the following way: the front part of the gill chamber forms
a narrow channel running forward under the side-wall of the carapace.
Within this channel lies a large plate known as the _scaphognathite_,
attached to the outer side of the maxilla, which during life is
constantly in movement, causing a current of water to flow forwards
through the channel. The water enters the gill chamber by the narrow
slit-like space between the lower edge of the carapace and the bases of
the legs, and is discharged in front at the sides of the head, where its
movement is helped by the vibrating exopodites of the maxillipeds.

At the sides of the stomach, in the front part of the head, lie a pair
of glands which, from their colour, are known as the _green glands_.
These are the excretory organs, corresponding in function to the kidneys
of the higher animals. Each has connected with it a thin-walled bladder,
which opens to the outside through a small perforation on the
under-side of the first segment of the antenna.

The chief part of the _nervous system_ is the ventral nerve-chain, which
runs along the under-side of the body. This is a long cord having at
intervals a series of knots or swellings, the ganglia or nerve-centres,
from which nerves are given off to the appendages and to the organs of
the body. In the hinder part of the thorax and in the abdomen there is a
ganglion in each somite, but in front these ganglia become crowded
together and coalesced, so that we find only a single large ganglion,
corresponding to the somites from that of the mandibles to that of the
third maxillipeds. Between the ganglia the cord is really double,
although for the greater part of its length the two parts are more or
less completely fused into one. In front of the head and above the
gullet is a ganglion which sends nerves to the eyes, antennules and
antennæ, and is known as the _brain_, although it is, perhaps, hardly so
important as that name would suggest. It is connected with the ventral
chain by two cords that pass on either side of the gullet.

The _eyes_, as already mentioned, are set on movable stalks, so that
they can be turned in any direction at the will of the animal, and are
of the type known as "compound eyes." If the convex black area at the
end of the eye-stalk be examined with a strong lens, it will be seen
that the membrane which covers it is divided up into a beautifully
regular series of square facets. This membrane is a thin and transparent
continuation of the chitinous covering of the body, and if it be
stripped off and examined under a microscope, it will be found that each
facet is capable of acting as a lens and forming an image of external
objects. It is not to be supposed, however, that the Lobster sees a
separate image in each of the facets, some thirteen thousand in number,
which go to make up each eye. In the interior of the eye, at some
distance from the surface, are a large number of rod-like bodies,
connected with the fibres of the optic nerve, and believed to be the
actual organs for the perception of light. Each rod corresponds to one
of the facets, and as it lies at the bottom of a long conical tube, of
which the walls are covered with dark pigment, it can only receive light
from a single point in line with the axis of the tube. In this way the
image of any object will be built up, like a mosaic, out of the
impressions of light and darkness received through the separate facets,
and transmitted to the underlying rods. It has been shown in some
Crustacea that, when the animal is in a very dim light, the curtain of
pigment separating the tubes is partially withdrawn, so that the light
from each facet can reach, not one, but several rods. In this way the
images of objects received are much brighter, although they are less
sharply defined.

It might be thought that in animals like the Lobster, enclosed in a
hard shelly covering, the sense of _touch_ must be very dull, if not
altogether absent. This, however, is not the case. What is probably a
very delicate tactile sense is provided for by the numerous hairs which
are found, of many sorts and sizes, all over the body and limbs. Each of
these hairs is really a hollow outgrowth of the chitinous covering,
containing a delicate prolongation of the soft tissues underneath, and
also supplied, in many if not in all cases, with a nerve-fibre, so that
the slightest movement of the hair caused by contact with a solid body
is perceived by the animal. Many of these hairs are themselves beset
with delicate secondary hairs, arranged so that the whole looks like a
feather or like a bottle-brush. These hairs are adapted for detecting
slight movements or vibrations in the surrounding water.

Whether Crustacea living in water can _hear_, in the sense in which the
word is used of animals living in air, is doubtful; but it is certain
that they are extremely sensitive to vibrations only a little coarser,
so to speak, than those we know as sound. The Lobster, and many other
Crustacea, do indeed possess a structure which was long supposed to be
an organ of hearing, and may possibly in part fulfil that function,
although it is now known that that is not its only or even its chief
use. It consists of a small cavity in the basal segment of the stalk of
the antennule, opening to the outside by a narrow slit on the upper
surface of the segment. The cavity is lined by a delicate continuation
of the chitinous covering of the body, and has on its inner surface a
series of feathered hairs of the kind described above, which are richly
supplied with nerve-fibres from a large nerve entering the base of the
antennule. Within the cavity, and for the most part entangled among
these hairs, are a number of grains of sand. When the Lobster moults,
the lining membrane of this cavity is thrown off like the rest of the
exoskeleton, and with it the contained sand-grains. While the shell is
still soft after moulting, and the lips of the slit are not rigid, as
they afterwards become, fresh sand-grains find their way into the cavity
to take the place of those which have been cast off. Perhaps, like some
other Crustacea, the Lobster buries its head in the sand to insure that
some grains may find their way in; for its pincers are too clumsy for it
to pick up sand-grains and to place them in the cavity, as some Prawns
have been seen to do. At all events, if a freshly moulted Prawn be
placed in a vessel of sea-water, and supplied, instead of sand, with
powdered glass or metal filings, particles of glass or metal will after
a short time be found in its antennular cavities. This habit has been
utilized in a very ingenious experiment by which the function of these
organs was demonstrated. A Prawn had been induced in this way to place
particles of iron filings in the cavities, and a strong electro-magnet
was brought near the side of the vessel in which it was kept. It was
observed that the Prawn, which had been swimming in the usual horizontal
position, at once turned the under-side of its body towards the magnet,
and swam about on its side as long as the magnet was in action. When the
current exciting the magnet was cut off, the animal resumed its ordinary
position. This experiment shows that these organs, to which we may now
give their proper name of _statocysts_, are organs for perceiving the
direction of the force of gravity. The magnetic force acted on the
particles of iron in the same way that the force of gravity acts on the
sand-grains in normal conditions, and the Prawn felt the weight of them,
so to speak, pulling towards the side instead of the bottom of the
vessel, and turned its body accordingly, to swim, as it supposed, right
side up. It is now known that those parts of the human ear called the
"semicircular canals" have a somewhat similar function as "organs of
orientation," although to animals walking on the solid ground this
function is not so important as it doubtless is to animals swimming in
water.

The sense of _smell_ is believed to have its seat chiefly in the
antennules. The outer branch of each antennule bears tufts of peculiar
hairs, in which the chitinous covering is extremely delicate, so that
substances dissolved in the water can easily pass through and affect
the nerve-endings within. These hairs are known as "olfactory
filaments."

The sense of _taste_ in aquatic animals is, perhaps, not sharply defined
from that of smell, but it is not very rash to assume that certain hairs
on the mouth parts and on the fleshy upper and lower lips which bound
the opening of the mouth have to do specially with this sense.

The relative importance of the various senses in the Lobster is well
illustrated in the following account of its habits given by Dr. H. C.
Williamson in the Report of the Scottish Fishery Board for 1904. After
noticing that, in daylight at least, the Lobster appears to be
"purblind," only distinguishing light from shadow, Dr. Williamson goes
on: "It tests a shadow with its antennæ, or sometimes, when a strong
shadow is thrown on it, it jumps at it with its chelæ outstretched and
snapping. It is dependent on its antennæ for guiding it in safe places.
It is especially careful in testing any hole before it is satisfied with
it. It discovers the cavity by means of its antenna, which is waved well
out to the side and in front as it walks. It searches the innermost
depths of the hole with the antenna, and then inserts its chela. If the
examination with the chela is also satisfactory, it immediately turns
and backs smartly into the hole. In feeding it is guided to the food by
the antennules. A piece of food which is dropped near a Lobster may fall
quite unnoticed unless it happens to touch the antenna or the [legs].
It is not seen at all. But sooner or later, according as the distance is
short or great, the scent of the food, carried by the currents set up by
the exopodites of the maxillipeds, reaches the Lobster. The Lobster is
immediately excited, although previously it was lying quite inert in its
hole. It whips the water with its antennules in a staccato fashion, and
feels about with the antennæ and chelæ, at first without leaving its
hole. At once both antennules are seen to be whipping in the direction
in which the food is lying, and an active search is made with the
antennæ. If they do not succeed in locating the bait, the Lobster rather
reluctantly leaves its hole, but cautiously, feeling all round about
with its antennæ. It goes off straight in the direction in which the
food is lying, and, if it misses it with its antennæ and chelæ, walks
over it and gets it with its chelate [walking legs]; it usually picks up
its food with the second [walking leg]. Meanwhile the expected feast has
by association stimulated the maxillipeds, which are actively working as
if they were already masticating the food. Once the food is seized it is
conveyed to the maxillipeds, and the Lobster retreats to its hole, there
to enjoy its meal."

Lobsters, like most other Crustacea, are of separate sexes. The females
(see Plate I.) may be distinguished from the males by the fact that the
abdomen is broader and has deeper side-plates, and by differences in
the form of the first two pairs of swimmerets. In the female the first
pair, which have only one branch, are short and slender filaments, while
in the male they are stout and peculiarly twisted rods. The second pair
in the female are similar in form to the succeeding pairs, but in the
male they have an additional lobe on the inner branch. The openings of
the generative organs will be found in the male on the basal segments of
the last pair of legs, while in the female they occupy the same position
on the legs of the last pair but two. The testis of the male lies in the
thorax, just below the heart. The ovary, which has the same position in
the female, is usually much more conspicuous, and from its red colour in
the cooked Lobster it is known as the "coral." On the under-side of the
thorax of the female, between the last two pairs of legs, is a
three-lobed structure enclosing a cavity known as the "sperm-receptacle."
Its function is to receive the fertilizing substance from the male, and
to retain it until the eggs are ready to be deposited.

[Illustration: _PLATE I_

     MALE AND FEMALE LOBSTERS, SHOWING THE DIFFERENCE IN THE RELATIVE
     BREADTH OF THE ABDOMEN IN THE TWO SEXES. THIS FIGURE ALSO
     ILLUSTRATES THE DISSIMILARITY OF THE LARGE CLAWS AND THE FACT THAT
     THE "CRUSHING-CLAW" MAY BE ON EITHER THE RIGHT OR LEFT SIDE OF THE
     BODY

                    (_From Brit. Mus. Guide_)]

In the Lobster, as in many other Crustacea, the eggs are carried by the
female until they hatch. After being extruded from the oviducts, they
are attached by a kind of cementing substance to the swimmerets, where
they hang in bunches. The swimmerets are kept constantly moving, so that
the eggs may obtain the oxygen necessary for the developing embryos
within. A female Lobster carrying eggs in this way is said by the
fishermen to be "in berry," and may carry, according to its size, from
about 3,000 to nearly 100,000 eggs. A period of about ten months elapses
between the deposition of the eggs and hatching.

[Illustration: FIG. 8--FIRST LARVAL STAGE OF THE COMMON LOBSTER. × 4.
(After Sars.)]

The young Lobster when first hatched (Fig. 8) differs considerably in
general appearance from the adult animal. The abdominal somites have a
row of spines down the middle of the back, and the telson has a forked
shape. There are no swimmerets, but, as already mentioned, the legs bear
large exopodites, which are used like oars, and by means of these the
larval Lobster swims about at the surface of the sea. The claws or chelæ
are at first hardly larger than the other legs, but later they increase
in size, the swimmerets are developed, the exopodites of the legs are
lost, and the young Lobster, sinking to the bottom of the sea, takes on
the creeping habits and gradually assumes the shape of the adult.

In many Crustacea the changes of shape or metamorphoses undergone after
hatching are much greater than in the Lobster. Some of these changes and
their probable significance will be considered at greater length in a
later chapter.

The two large claws of the Lobster (see Plate I.) are not quite alike in
size or in shape. The smaller of the two has the inner edges of the
fingers sharp and set with saw-like teeth; the larger has the fingers
armed with blunt rounded knobs. The larger claw is adapted for crushing
the shells of the animals on which the Lobster feeds, while the smaller
serves for holding and tearing the prey. In the Lobster, as in many of
the higher Crustacea in which this asymmetry occurs, the larger claw may
be indifferently on either side of the body. There are certain cases,
however, among Crabs where the large claw is constantly on the same side
of the body, or, in other words, all the individuals are either
right-handed or, more rarely, left-handed.

If a Lobster be caught by one of its claws or by a leg, it very readily
parts with the limb in its struggles to escape; and if one of the limbs
be crushed or otherwise injured, it is often cast off by the animal. The
separation always takes place at the same point, near the base of the
limb, and is not simply due to the limb breaking at its weakest part. It
is a reflex act, brought about by a spasmodic contraction of some of the
leg muscles. At the place of separation, corresponding to the junction
of the second and third segments of the limb, which, as already
mentioned, are soldered together, the internal cavity is crossed by a
transverse partition, having only a small aperture in the centre through
which the nerves and bloodvessels pass. When the limb is cast off, this
small opening quickly becomes closed by a clot of blood, and further
bleeding is stopped. If, as sometimes happens, a limb which has been
seriously injured is not cast off, the animal not infrequently bleeds to
death. This power of self-mutilation or _autotomy_, as it is called, is
frequently used by Crustacea as a means of escaping from their enemies,
and is closely connected with the power of _regeneration_ of lost
appendages. Beneath the scar which forms on the stump of a separated
limb a sort of bud grows, and gradually assumes the form of the lost
segments. At the next moult this straightens out, and, increasing in
size at succeeding moults, it ultimately provides, in normal cases, a
new member similar in every detail to that which had been lost.
Occasionally it happens, under circumstances not yet altogether
understood, that the process of regeneration may, so to speak, go wrong,
and in this way various malformations and abnormalities result. For
instance, it has been found that, if the larger, crushing claw of a very
young Lobster be removed by operation or by accident, the limb which
grows in its place may assume the form of the smaller, toothed claw.
Further, in some other Crustacea (but not in the Lobster, except in the
very youngest stages), it is found in such cases that, after removal of
the large claw, the claw of the other side assumes at the next moult the
form of a crushing claw, so that there is a "reversal of asymmetry."

A still more remarkable change sometimes occurs when one of the
eye-stalks is injured. If only the tip of the eye-stalk be cut off, so
that the nerve-ganglion which lies in the basal part of the stalk
remains uninjured, it will be found that a normal eye is in course of
time regenerated. If, however, the whole eye-stalk be amputated, and
with it the optic ganglion, there grows in its place, not a new
eye-stalk, but a segmented appendage similar to one of the flagella of
the antennules. This fact is considered by some zoologists to indicate
that the eye-stalks are, like the antennules, true appendages,
homologous with the mouth parts and limbs, but this is a much-disputed
question into which we cannot enter further here.

Lobsters vary a good deal in colour, but as a rule a living Lobster is
of a more or less mottled dark blue, becoming nearly black on the back,
and shaded off into orange yellow or red on the under-side. This
coloration resides in the shell, and does not change much after the
shell has hardened. In this respect the Lobster is unlike many of the
smaller Crustacea which have a thin and more or less transparent
exoskeleton, and in which the colour resides in certain living cells
(chromatophores) of the underlying skin. Many of these Crustacea possess
the power of changing their colours to a remarkable degree, by the
expansion and contraction of the branched chromatophores.

The question which is often asked, "Why does a Lobster turn red when it
is boiled?" is one to which it is not easy to give a simple answer. A
chemical change takes place under the influence of heat in the pigment
of the shell, which changes it from blue to red; how slight the change
is, is perhaps shown by the fact that occasionally living Lobsters are
found of a red colour almost as brilliant as that which is assumed on
boiling.

[Illustration: FIG. 9--SIDE-VIEW OF ROSTRUM OF (A) COMMON LOBSTER
(_Homarus gammarus_) AND (B) AMERICAN LOBSTER (_Homarus americanus_)]

The Common Lobster is found on the coasts of Western Europe, from Norway
to the Mediterranean, living in shallow water, generally a little way
below low-tide mark, wherever a rough, rocky bottom affords suitable
lurking-places. On the Atlantic coast of North America, Lobsters are
also found abundantly in similar situations. These American Lobsters, if
examined carefully, will be found to differ from the European kind in
certain small details of structure, of which the most conspicuous is the
presence, on the under-side of the rostrum, of two spines or teeth. In
the European Lobsters the under-side of the rostrum is smooth (Fig. 9).
In the nomenclature of technical zoology, these two kinds or _species_
of Lobster are said to constitute (along with a third species found at
the Cape of Good Hope) the genus _Homarus_, the European species being
known as _Homarus gammarus_, and the American as _Homarus americanus_.
The so-called "Norway Lobster" or "Dublin Prawn," which differs from the
Common Lobster in having large kidney-shaped eyes and long and slender
claws, and in many other details of structure, is placed in a distinct
genus, and is known as _Nephrops norvegicus_. The genera _Homarus_ and
_Nephrops_, together with some others, constitute the family Homaridæ,
which again is grouped with other families in a tribe, Nephropsidea,
forming a part of the order Decapoda. These groups are intended to
express the varying degrees of resemblance and difference in structure
between the species of animals which make up the class Crustacea. Since
we have good grounds for believing that all these species have arisen by
some mode of evolution, this classification also represents the varying
degrees of actual relationship between the different forms, so far as
this relationship can be discovered. In the next chapter a brief sketch
of the chief subdivisions of the Crustacea is given, with such details
as to the characteristics of each as are necessary to render
intelligible the succeeding chapters on their habits and modes of life.




CHAPTER III

THE CLASSIFICATION OF CRUSTACEA


  _Table of Classification of Crustacea_

  CLASS CRUSTACEA.

    Subclass BRANCHIOPODA         - {Order   Anostraca.
                                    {   "    Notostraca.
                                    {   "    Conchostraca.
                                    {   "    Cladocera.

         "   OSTRACODA            - {   "    Myodocopa.
                                    {   "    Podocopa.

         "   COPEPODA             - {   "    Eucopepoda.
                                    {   "    Branchiura.

         "   CIRRIPEDIA           - {   "    Thoracica.
                                    {   "    Rhizocephala.

         "   MALACOSTRACA.

       Series   _LEPTOSTRACA_     -     "    Nebaliacea.

         "      _EUMALACOSTRACA._

          Division _Syncarida_    -     "    Anaspidacea.

             "     _Peracarida_   - {   "    Mysidacea.
                                    {   "    Cumacea.
                                    {   "    Tanaidacea.
                                    {   "    Isopoda.
                                    {   "    Amphipoda.

             "     _Eucarida_    -  {   "    Euphausiacea.
                                    {   "    Decapoda.

             "     _Hoplocarida_ -      "    Stomatopoda.

[Illustration: FIG. 10--THE "FAIRY SHRIMP" (_Chirocephalus diaphanus_),
MALE. × 2. (After Baird.)]

Occasionally there may be found in rain-water puddles and the like, in
the South of England, a beautiful, transparent, shrimp-like animal, an
inch or more in length, to which the name of "Fairy Shrimp" has been
given (Fig. 10). It is known in technical zoology as _Chirocephalus
diaphanus_, and is a representative of the subclass BRANCHIOPODA. The
members of this group are distinguished from other Crustacea by their
flattened, leaf-like feet, each of which is divided into a number of
lobes, and has a gill plate on the outer side. In _Chirocephalus_ there
is no carapace, and the head is followed by eleven distinct body
segments, each bearing a pair of leaf-like, or rather fin-like, feet.
The hinder part of the body has no appendages, and ends in a forked
tail. In the female a large pouch hangs from the under-side of the body,
just behind the limb-bearing part, and is often found filled with eggs.
In the male, a pair of remarkable-looking appendages, each shaped
somewhat like a hand with webbed fingers, hang in front of the head.
These are connected with the antennæ, and are known as the "claspers,"
from their function in seizing and holding the female. The eyes are set
on movable stalks. Those Branchiopoda which, like _Chirocephalus_, have
no carapace, form the order ANOSTRACA.

[Illustration: _PLATE II_

     _Apus cancriformis_, FROM KIRKCUDBRIGHTSHIRE. (SLIGHTLY ENLARGED)]

A second order, the NOTOSTRACA, is represented by _Apus cancriformis_
(Plate II.), which occurs in many places in Europe in ponds and puddles,
and very rarely indeed in Britain. In _Apus_ there is a large dorsal
shield, or carapace, covering the greater part of the body, which
consists of a large number of segments (about twenty-eight), and ends
behind in a pair of long antenna-like filaments. The fin-like feet are
also very numerous (about sixty-three pairs). The eyes are not stalked,
but are set close together on the upper surface of the carapace.

[Illustration: FIG. 11--_Estheria obliqua_, ONE OF THE CONCHOSTRACA.
(After Sars, from Lankester's "Treatise on Zoology.")

A, Shell of female, from the side; B, male, from the side, after removal
of one valve of the shell. (Enlarged.) _a´_, Antennule; _a´´_, antenna;
_ad_, muscle which draws together the valves of the shell; _f_, tail
fork; _md_, mandible]

The third order of the Branchiopoda, the CONCHOSTRACA (Fig. 11), are not
represented in Britain, though several species occur on the Continent of
Europe. In these the carapace forms a bivalved shell, completely
enclosing the body and limbs, and closely resembling that of a small
Mollusc.

[Illustration: FIG. 12--_Daphnia pulex_, A COMMON SPECIES OF
"WATER-FLEA." MUCH ENLARGED. (From British Museum Guide.)

Female carrying eggs in the brood-chamber]

The fourth order, the CLADOCERA, comprises the so-called "Water-fleas,"
which are abundant everywhere in ponds and lakes (Fig. 12). They are
all of small size, almost or quite microscopic. The carapace, as in the
Conchostraca, forms a bivalved shell, but does not enclose the head.
There is a single large eye, which really corresponds to two eyes fused
together. A pair of large antennæ, each with two branches, carrying long
feathered hairs, project at the sides of the head, and are used in
swimming with a peculiar jumping motion, from which the popular name of
the animals is derived. There are not more than six pairs of feet. The
"Water-fleas," of which _Daphnia pulex_ is one of the commonest species,
are very beautiful and interesting objects for microscopic examination,
on account of their transparency, which allows many details of their
internal structure to be studied in the living animal.

[Illustration: FIG. 13--SHELLS OF OSTRACODA. MUCH ENLARGED. (From
Lankester's "Treatise on Zoology," after Brady and Norman, and Müller.)

A, _Philomedes brenda_ (Myodocopa); B, _Cypris fuscata_ (Podocopa); C,
_Cythereis ornata_ (Podocopa). _n_, Notch characteristic of the
Myodocopa; _e_, the median eye; _a_, mark of attachment of the muscle
connecting the two valves of the shell. A and C are marine species; B is
from fresh water]

The OSTRACODA (Fig. 13), which form the second subclass in the system
of classification here adopted, are nearly all microscopic animals, and
are found abundantly in fresh water as well as in the sea. The carapace
forms a bivalved shell, which completely encloses the body and limbs,
and is often sculptured in an elegant fashion. The Ostracoda are
remarkable for the very small number of their appendages. There are not
more than two pairs of limbs behind the maxilla. Most of the species are
included in two orders, the _Myodocopa_ and the _Podocopa_, of which the
former may generally be distinguished by a notch in the anterior part of
the margin of the shell (Fig. 13, A, _n_). In the _Podocopa_ the margin
is entire.

[Illustration: FIG. 14--_Cyclops albidus_, A SPECIES OF COPEPOD FOUND IN
FRESH WATER. (After Schmeil.)

Female specimen carrying a pair of egg-packets. The actual length is
about one tenth of an inch]

The subclass COPEPODA comprises animals, for the most part of
microscopic size, which are abundant in fresh water and in the sea. The
common fresh-water genus _Cyclops_ (Fig. 14) furnishes a good example of
the type of structure characteristic of the class. The body is somewhat
pear-shaped, with a narrow abdomen ending in a "caudal fork." The body
is divided into somites, and there is no overlapping carapace, although
the head and the first two thoracic somites are coalesced. There are
four pairs of two-branched, oar-like, swimming feet, and a fifth pair,
found in some other Copepoda, is represented in _Cyclops_ by minute
vestiges on the first segment of the narrow posterior part of the body.
The antennules are very large, unbranched and composed of numerous
segments; the antennæ are much smaller. In addition to the usual
mandibles, maxillulæ, and maxillæ, there is a pair of maxillipeds which
really represent the first pair of trunk limbs. There is a single red
eye in the middle of the front of the head. This eye is not formed, like
the single eye of the Cladocera, by fusion of a pair of eyes, but it
corresponds to a median eye of simple structure which is found in the
Branchiopoda, Ostracoda, and many other Crustacea, in addition to the
paired compound eyes. From the fact that this median eye is the only one
present in the earliest larval stage of Crustacea, the Nauplius (see
Chapter IV.), it is sometimes known as the "nauplius eye." The female
_Cyclops_ carries her eggs until they hatch, in two oval packets
attached to the sides of the body.

Forming a separate order (BRANCHIURA) apart from the more normal
Copepoda (order EUCOPEPODA) is the little group of the Carp-lice, one of
which, _Argulus foliaceus_, is common in England, living as a parasite
on different species of fresh-water fish, and often found swimming free
in ponds and rivers. It has a broad, flat, and very transparent body,
about three-sixteenths of an inch in length. It differs from _Cyclops_
in a great many points, of which, perhaps, the most conspicuous is the
possession of a pair of true compound eyes in addition to the median
eye. On the under-side of the head are a pair of large round suckers, by
means of which the animal fixes itself on to its prey. A study of their
development shows that these suckers are really the maxillæ, which in
the young animal are jointed limbs ending in a strong claw, but later
become changed into the suckers of the adult. A sharp spine, which can
be protruded in front of the mouth, is connected with what is believed
to be a poison-gland. The eggs are not carried in packets by the female
as in _Cyclops_, but are deposited on stones or water-weeds.

The fourth subclass, CIRRIPEDIA, comprises the Barnacles and
Acorn-shells. These are very unlike any of the other Crustacea, and, in
fact, they were long classed by naturalists with the Mollusca. It was
not until their larval development was made known that they were
recognized as Crustacea. The common Goose Barnacle (_Lepas
anatifera_--Plate III.) is found adhering to the bottoms of ships and to
floating timber. It has a fleshy stalk or peduncle which is fixed at one
end to the supporting object, and bears at the other end a shell, made
up of five separate plates, enclosing the body of the animal. The stalk
corresponds to the front part of the head, and careful examination may
discover at its end, among the hardened cement which fixes it to the
support, the remains of the antennules by which the attachment of the
young animal was first effected. The body of the animal within the
carapace or shell bears the usual mandibles, maxillulæ, and maxillæ,
close to the mouth, and six pairs of long, tendril-like feet. These feet
have each two branches, composed of numerous short segments and fringed
with long hairs. They can be protruded from the slit-like opening of the
shell, forming a sort of "casting-net" for the capture of minute
floating prey.

The Acorn-shells, of which one species (_Balanus balanoides_--Plate
III.) is abundant everywhere on our coasts, covering rocks and stones
just below high-water mark, differ from _Lepas_ and its allies in having
no peduncle. The shell is cemented directly to the rock, and is conical
in shape, like a small limpet, with a hole at the top which is closed
by four movable valves.

[Illustration: _PLATE III_

     GROUP OF SPECIMENS OF THE GOOSE-BARNACLE (_Lepas anatifera_), ONE
     SHOWING THE CIRRI EXTENDED AS IN LIFE. (NATURAL SIZE)

                    (_From Brit. Mus. Guide_)

     GROUP OF A COMMON SPECIES OF ACORN-SHELL OR ROCK BARNACLE (_Balanus
     balanoides_)(NATURAL SIZE)]

The Stalked Barnacles, like _Lepas_ (suborder _Pedunculata_), and the
Sessile Barnacles, or Acorn-shells, like _Balanus_ (suborder
_Operculata_), together form the order THORACICA. Of the other orders
which compose the subclass Cirripedia, the only one that need be
mentioned here is the RHIZOCEPHALA, which comprises strangely degenerate
parasites living on other Crustacea.

The Cirripedia are unlike nearly all other Crustacea in the fact that,
with few exceptions, they are hermaphrodite, having both sexes united in
each individual. In certain species of the Stalked Barnacles, however,
there are minute male individuals that are attached, like parasites, to
the large hermaphrodites. In a few species the large individuals only
possess female organs, so that the separation of the sexes is complete.

The remarkable larval metamorphoses of Cirripedes and the modifications
of structure presented by some parasitic forms will be described in
later chapters.

The fifth and last subclass, that of the MALACOSTRACA, is by far the
largest and most important, and will require to be considered in more
detail than any of the others. The animals composing the various orders
into which the subclass is divided differ very greatly in structure, but
they all agree in having typically the same number of appendages as the
Lobster--namely, nineteen pairs (or twenty, if the eye-stalks be
included). They also agree in the very important character that the
trunk limbs are divided into two sets, thoracic and abdominal, the
former of eight, and the latter of six pairs.

[Illustration: FIG. 15--_Nebalia bipes._ ENLARGED. (From Lankester's
"Treatise on Zoology," after Claus.)

_a´_, Antennule; _a´´_, antenna; _ab_1-_ab_6, the abdominal limbs; _ad_,
muscle joining the two valves of the shell; _f_, tail-fork; _p_, palp of
maxillula; _r_, rostral plate; _t_, telson; 1-7, the seven somites of
the abdomen]

The first order of the Malacostraca, the NEBALIACEA, comprises a few
Crustacea of small size, which differ in some very important characters
from all the other orders. _Nebalia bipes_ (Fig. 15), which occurs on
the southern coasts of the British Isles, has a large bivalved carapace
enclosing most of the limbs. In front, a small "rostral plate" is joined
to the carapace by a movable hinge, and partly covers the stalked eyes.
The eight pairs of thoracic feet are all alike, and are flattened and
leaf-like in form, resembling those of the Branchiopoda. The first four
pairs of abdominal limbs are large two-branched swimming feet, but the
last two pairs are reduced to small vestiges. Two of the most important
points in which the Nebaliacea differ from all the other Malacostraca
are that there are seven instead of six somites in the abdomen (the last
somite has no appendages), and that the telson has connected with it a
pair of movable rods forming a "caudal fork" like that of the
Branchiopoda. On account of the leaf-like thoracic feet and the
possession of a caudal fork and other features, the Nebaliacea were
formerly classified with the Branchiopoda, but a closer examination of
their structure has shown that they are true Malacostraca. In having an
additional somite in the abdomen and in other points, however, they may
be regarded as forming a link between the Malacostraca and the lower
forms of Crustacea, and for this reason they are set apart as a series
LEPTOSTRACA, while the other orders form a series EUMALACOSTRACA.

The orders of the Eumalacostraca, again, are grouped, as shown in the
table of classification, into four divisions. The first of these, the
SYNCARIDA, includes only one order, comprising a few small Crustacea
(see Fig. 84, p. 264) which have recently been discovered in fresh water
in Tasmania and Australia. They have no carapace, and all the thoracic
somites, or all but the first, are distinct. The antennules are
two-branched, the antennæ may have a scale-like exopodite, and the last
pair of abdominal appendages form, with the telson, a tail-fan. The eyes
are sometimes stalked, but in one species they are sessile. The thoracic
limbs, which are not clearly divided into maxillipeds and legs, carry a
double series of plate-like gills or epipodites. As will be shown later,
the living Syncarida are especially interesting on account of their
resemblance to certain very ancient fossil Crustacea.

The second division of the Eumalacostraca, the PERACARIDA, includes five
orders, the members of which differ very greatly in appearance. They all
agree, however, in certain important points of structure, of which the
most conspicuous is the possession, in the female sex, of a brood-pouch
for carrying the eggs and young. This brood-pouch is formed by a series
of overlapping plates attached to the bases of the thoracic limbs.

[Illustration: FIG. 16--_Mysis relicta_, ONE OF THE MYSIDACEA. ENLARGED.
(From Lankester's "Treatise on Zoology," after Sars.)

_cs_, Cervical groove of the carapace; _m_, brood-pouch]

The first order of the Peracarida, the MYSIDACEA, consists of small,
free-swimming, shrimp-like animals (Fig. 16). Many species are common in
the sea round the British coasts, and from their possession of a
brood-pouch, in which the young are carried, they are sometimes known as
"Opossum Shrimps." The eyes are stalked, and the carapace is well
developed, although it does not unite with all the thoracic somites. The
antennæ have a flattened, scale-like exopodite, probably of use for
keeping the animal balanced in swimming. Only one pair of the thoracic
limbs are modified to form maxillipeds, and all the legs (as in the
larval Lobster) have exopodites which form the chief swimming organs.
The uropods and telson form a "tail-fan." One of the most curious points
in the organization of some Mysidacea is the possession of a pair of
statocysts in the endopodites of the uropods. Each statocyst consists of
a small cavity containing a cake-shaped concretion known as a
"statolith," resting on a group of sensory hairs. There is reason to
believe that these organs have the same function as the statocysts of
the Lobster, although they are placed at the other end of the body. The
statolith serves the same purpose as the sand-grains found in the
Lobster's statocyst, although, unlike these, it is not introduced from
outside, but is formed in position by secretion from the walls of the
sac.

[Illustration: FIG. 17--_Gnathophausia willemoesii_, ONE OF THE DEEP-SEA
MYSIDACEA. HALF NATURAL SIZE. (From Lankester's "Treatise on Zoology,"
after Sars.)

_gr_, A groove dividing the last abdominal somite]

Most of the Mysidacea have no special organs of respiration, that
function being discharged (as in many of the smaller Crustacea) by the
general surface of the body, and especially by the thin carapace; but
certain deep-sea Mysidacea (Fig. 17) have tufted gills attached at the
base of the thoracic legs. In all cases the maxilliped has a plate-like
epipodite, which lies under the side-fold of the carapace, and no doubt
assists respiration, causing by its movements a current of water to flow
under the carapace.

[Illustration: FIG. 18--_Diastylis goodsiri_, ONE OF THE CUMACEA.
ENLARGED. (From Lankester's "Treatise on Zoology," after Sars.)

_a´_, Antennule; _l_1-_l_5, the five pairs of walking legs; _m_,
brood-pouch; _ps_, "pseudo-rostrum" formed by lateral plates of the
carapace; _t_, telson; _ur_, uropods]

The members of the second order of the Peracarida, the CUMACEA (Fig.
18), are small marine Crustacea in which the anterior part of the body
is generally stout, while the abdomen is slender and very mobile. The
short carapace does not cover more than the first three or four of the
thoracic somites. The eyes are not stalked, and are usually fused
together to form a single organ on the front part of the head. Swimming
branches (exopodites) are usually present on some of the thoracic legs,
at least in the males, which are more active swimmers than the females.
In the males, also, the swimmerets of the abdomen are often more or less
developed, but they are always absent in the females. The uropods do not
form a tail-fan, but are slender forked rods carrying comb-like rows of
spines, said to be used in cleaning the anterior appendages from the mud
among which these animals generally live. The telson is often absent,
or, rather, it is coalesced with the last somite of the abdomen. Under
the side-fold of the carapace on each side lies, as in the Mysidacea,
the epipodite of the maxilliped; but in this order it forms a gill, and
usually carries a row of flattened gill lobes.

[Illustration: FIG. 19--_Apseudes spinosus_, ONE OF THE TANAIDACEA.
ENLARGED. (From Lankester's "Treatise on Zoology," after Sars.)

_ex_, Vestiges of exopodites on second and third thoracic limbs; _oc_,
the small and immovable eye-stalks; _sc_, scale or exopodite of antenna;
_ur_, uropod]

The third Order, that of the TANAIDACEA (Fig. 19), is of special
interest, since in many respects it forms a transition to the next. It
comprises a number of minute Crustacea, generally found burrowing in mud
in the sea. They have a small carapace, which only involves the first
two thoracic somites, the rest of the somites being distinct. The
side-folds of the carapace enclose a pair of small cavities, within
which lie, as in the case of the last two orders, the epipodites of the
maxillipeds. The eyes are not movable, although they are set on little
side-lobes of the head, representing the vestiges of eye-stalks. The
first pair of thoracic limbs are maxillipeds, and the second pair are
very large, and form pincer-claws (chelæ). Minute vestiges of exopodites
are sometimes found on the second and third pairs, but they are not
used for swimming, and only help to keep a current of water flowing
through the gill cavities. The abdomen is very short, with small
swimmerets, and the telson is not separated from the last somite. The
uropods are generally very small, and do not form a tail-fan.

[Illustration: FIG. 20--A WOODLOUSE (_Porcellio scaber_), ONE OF THE
ISOPODA. ENLARGED. (From Lankester's "Treatise on Zoology," after Sars.)]

Unlike the Tanaidacea, the ISOPODA, which form the fourth order of the
Peracarida, are very numerous in species, and very varied in structure
and habits. The most familiar are the Woodlice, or Slaters, which are
commonly found in damp places, under stones and the like. Besides these,
however, the order includes a vast number of forms living in the sea and
a few that live in fresh water. The examination of a common Woodlouse,
such as _Oniscus_ or _Porcellio_ (Fig. 20), will give a general idea of
the form and structure of a typical Isopod, although many curious
modifications are found, some of which will be mentioned in later
chapters.

There is no distinct carapace, but the last vestige of one may be
indicated by the fact that the first thoracic somite is completely
fused with the head. All the other somites of the body are distinct (in
some Isopods, however, the abdominal somites are coalesced), but the
telson is not separate from the last somite. The eyes are not stalked,
but are sessile on the sides of the head. The antennules have only a
single branch, and in the Woodlice are very small. The antennæ have no
exopodite, although in a few other Isopods a minute vestige is present.
The thoracic limbs never have any trace of exopodites. The first pair
are maxillipeds, and if they carry an epipodite it is never enclosed in
a gill cavity, as in Tanaidacea. The swimmerets form one of the most
characteristic features of the Isopoda, for they are always flattened
into thin plates, which act as gills. In the Woodlice, which breathe
air, certain curious modifications of the swimmerets are found, which
will be described in a later chapter. In some Isopods that live as
parasites on fish or on other Crustacea, each individual is at first a
male, and later becomes a female. They are almost the only Crustacea,
except the Cirripedes already mentioned, which are normally
hermaphrodite.

[Illustration: FIG. 21--AN AMPHIPOD (_Gammarus locusta_). ENLARGED.
(From Lankester's "Treatise on Zoology," after Sars.)

_a´_, Antennule; _a´´_, antenna; _acc_, accessory (inner) flagellum of
antennule; _br_, gill plate; _cx_, coxal plate (the expanded first
segment of the leg); _gn_, the two pairs of gnathopods (prehensile
legs); _plp´´´_, abdominal appendage of third pair; _prp´_, _prp´´_,
first and second peræopods, or walking-legs; _t_, telson; _ur_, uropod;
II, VIII, second and eighth thoracic somites; 1, 6, first and sixth
abdominal somites]

The fifth order of the Peracarida, the AMPHIPODA, is also a very large
one. The "Sand-hoppers," which are very common on sandy coasts, belong
to this order, as do also a very large number of other forms found in
the sea and in fresh water, which have no popular names. A common
species is _Gammarus pulex_, sometimes called the "Fresh-water Shrimp,"
which is found everywhere in streams and ditches. Several closely allied
species, such as _G. locusta_ (Fig. 21), are found in the sea. The body
is flattened from side to side, and the abdomen is generally bent upon
itself. There is no carapace, but, as in the Isopods, the first thoracic
somite is fused with the head. The eyes are sessile on the sides of the
head. The antennules have a small inner branch, and the antenna have no
exopodites. The thoracic limbs, of which the first pair form
maxillipeds, have no exopodites, and are partly hidden by a row of
shield-like plates along the sides of the thorax. These plates are
formed by the enlarged and flattened basal segments of the limbs
themselves, and on the inner side they carry a series of oval plates,
which are the gills. The abdominal appendages are divided into two
sets: the first three pairs have each two slender, many-jointed
branches, and are used in swimming; the last three pairs are short,
stiff, and directed backwards, and are used in pushing the animal
through mud or among water-weeds. In many Amphipods, such as the
Sand-hoppers, the last three pairs of abdominal limbs are used in
jumping by sudden backward strokes of the abdomen.

[Illustration: FIG. 22--TWO SPECIES OF CAPRELLIDÆ. (From Lankester's
"Treatise on Zoology," after Sars.)

A, _Phtisica marina_, a species which retains the fourth and fifth pairs
of thoracic limbs (_prp´_, _prp´´_); B, _Caprella linearis_, in which
these limbs are represented only by the gills (_br_). (Enlarged.) _a´_,
Antennule; _a´´_, antenna; _abd_, vestigial abdomen; _gn_, gnathopods;
_m_, brood-pouch; IV, V, fourth and fifth thoracic somites]

[Illustration: FIG. 23--_Paracyamus boopis_, THE WHALE-LOUSE OF THE
HUMPBACK WHALE. (From Lankester's "Treatise on Zoology," after Sars.)

A, Male, dorsal view, enlarged; B, the maxillipeds detached and further
enlarged. _a´_, Antennule; _a´´_, antenna; _abd_, vestigial abdomen;
_br_, gills; _gn_, gnathopods; IV, V, fourth and fifth thoracic somites]

Two families of the Amphipoda differ so much in general appearance from
the others that they deserve mention. The Caprellidæ (Fig. 22) have the
body drawn out to a thread-like slenderness, and the abdomen reduced to
a mere vestige. The fourth and fifth pairs of thoracic limbs are
generally absent, though the corresponding gills remain. The animals
live in the sea, clambering among sea-weeds or zoophytes in a fashion
which recalls the movements of "looper" caterpillars. The _Cyamidæ_, or
"Whale-lice" (Fig. 23), are, as the name implies, parasites on the skin
of whales, and are closely related to the Caprellidæ. They have,
however, a broad, flattened body, more like that of an Isopod than an
ordinary Amphipod, and their legs have strong curved claws with which
they cling to the skin of their host.

[Illustration: FIG. 24--_Meganyctiphanes norvegica_, ONE OF THE
EUPHAUSIACEA. TWICE NATURAL SIZE. (From Lankester's "Treatise on
Zoology.")]

The third division of the Malacostraca, the EUCARIDA, consists of two
orders of very unequal interest and importance. The first of these, the
order EUPHAUSIACEA (Fig. 24), comprises only a single family of small,
shrimp-like Crustacea found swimming freely at the surface or in the
depths of the sea. In these the carapace fuses with all the thoracic
somites, the eyes are stalked, the antennules have two flagella, and the
antennæ have a broad scale. None of the thoracic limbs are modified into
maxillipeds, and all carry swimming exopodites. The uropods and telson
form a tail-fan. A single series of feathery gills are attached to the
bases of the thoracic limbs. Nearly all the Euphausiacea possess the
power of emitting light, and are furnished for the purpose with a number
of organs which were formerly supposed to be "accessory eyes."

The second order of the Eucarida, the DECAPODA, is by far the largest of
the orders of Crustacea, and it includes all the larger and more
familiar members of the class. It is necessary, therefore, to give a
considerably fuller account of its subdivisions than has been given in
the case of the other orders. The typical characters of the Decapoda are
well illustrated by the Lobster, which has been already described. As in
the Euphausiacea, the eyes are stalked, and the carapace fuses with all
the thoracic somites. From the Euphausiacea the Decapoda differ in the
fact that three pairs of the thoracic limbs are modified as maxillipeds,
the remaining five pairs forming the "ten legs" to which the name of the
order alludes. Further, the gills are arranged in more than one series,
not all attached to the bases of the legs, as in the Euphausiacea, and
covered over by the side-flaps of the carapace instead of being freely
exposed. While agreeing in these essential characters, however, the
members of the order Decapoda differ very widely among themselves in
structure and in general form, and they are classified (in the
arrangement adopted here) in two suborders, which are again subdivided
into sections and tribes.

  ORDER DECAPODA.

    Suborder NATANTIA   -      {  Tribe Penæidea.
                               {   "  Stenopidea.
                               {   "  Caridea.

       "     REPTANTIA.

     Section _Palinura_     -  {   "  Scyllaridea.
                               {   "  Eryonidea.

        "    _Astacura_     -      "  Nephropsidea.

        "    _Anomura_      -  {   "  Galatheidea.
                               {   "  Thalassinidea.
                               {   "  Paguridea.
                               {   "  Hippidea.

        "    _Brachyura_    -  {   "  Dromiacea.
                               {   "  Oxystomata.
                               {   "  Brachygnatha.
                                    Subtribe Brachyrhyncha.
                                        "     Oxyrhyncha.

The suborder NATANTIA includes the numerous species of what are commonly
known as Prawns and Shrimps. These are characteristically powerful
swimmers, with lightly armoured bodies, more or less flattened from side
to side, with a thin, saw-edged rostrum, and with large swimmerets which
are the chief organs of swimming; in addition, some of the more
primitive Natantia have swimming branches, or exopodites, like those of
the Euphausiacea, on the thoracic legs. This suborder is divided into
three tribes. The _Penæidea_ include the large Prawns of tropical
seas (_Penæus_--Plate IV.), which have the first three pairs of legs
provided with chelæ, and not differing greatly in size. The _Stenopidea_
are a small group of forms resembling the _Penæidea_ in having chelæ on
the first three pairs of legs, but the third pair are much larger than
the others. The _Caridea_ comprise our common Prawns (_Leander,
Pandalus_) and Shrimps (_Crangon_), besides a host of less generally
known forms; in these the third legs are never chelate, although the
first and second often are.

[Illustration: _PLATE IV_

     _Penæus caramote_ FROM THE MEDITERRANEAN. (ABOUT HALF NATURAL SIZE)

                    (_From Brit. Mus. Guide_)]

The second suborder, that of the REPTANTIA, is much more diversified,
but the animals composing it are united by certain characteristics, of
which the most obvious are their creeping habits (although some species
can swim well), their heavily armoured bodies, often more or less
flattened from above downwards, with the rostrum never thin and
saw-edged, and the swimmerets not used to any great extent for swimming.

The first section of the Reptantia, the _Palinura_, includes the Spiny
Lobsters, Rock Lobsters, or Sea-Crawfish, and their allies, forming the
tribe _Scyllaridea_. They are distinguished by having no large
pincer-claws, though the last pair of legs may have small pincers in the
female sex. One species, the Common Spiny Lobster (Plate V.), is found
on the southern and western coasts of the British Islands. The other
tribe belonging to this section is the _Eryonidea_, comprising a number
of small lobster-like forms living in the deep sea. They have
pincer-claws on the first four, or on all five, pairs of legs, and they
are of special interest on account of their geological antiquity.

[Illustration: _PLATE V_

     THE COMMON SPINY LOBSTER (_Palinurus vulgaris_). (MUCH REDUCED)

                    (_From Brit. Mus. Guide_)]

The section _Astacura_ contains only a single tribe, _Nephropsidea_,
formed by the true Lobsters and the fresh-water Crayfishes. They have
pincer-claws on the first three pairs of legs, and the first pair are
much larger than the others.

The third section of the Reptantia, the _Anomura_, comprises forms in
which the abdomen is variously modified, being either bent upon itself
or, if extended, more or less soft and feebly armoured. The last pair of
legs are commonly reduced in size, and not used in walking. The members
of the four tribes composing the section differ widely in their general
appearance.

The _Galatheidea_ (Plate VI.) are small, flattened, lobster-like animals
which have the abdomen bent under the body. In one family
(_Porcellanidæ_) the animals have quite the appearance of little Crabs
(see Fig. 41, p. 113), but they may be distinguished from the true Crabs
(Brachyura) by the fact that there are only three pairs of walking legs
behind the great chelæ, the last pair of legs being very small and
carried folded up at the sides of the body, or even within the gill
chambers.

[Illustration: _PLATE VI_

     _Munida rugosa._ BRITISH. (REDUCED)]

The _Thalassinidea_ are small lobster-like animals which burrow in sand
and mud, and have generally a more or less soft abdomen (see Fig. 38, p.
103).

[Illustration: _PLATE VII_

     THE COMMON HERMIT-CRAB, _Eupagurus bernhardus_, IN THE SHELL OF A
     WHELK (REDUCED)

                    (_From Brit. Mus. Guide_)]

The tribe _Paguridea_ includes the Hermit Crabs (_Paguridæ_) and their
allies. The typical Hermit Crabs (Plate VII.), which are familiar
objects in seaside rock-pools, live in the empty shells of Whelks and
other Gasteropod Molluscs, which they carry about with them as portable
shelters. The structure of the animals is modified in adaptation to this
curious habit. The abdomen, which is protected during life by the
borrowed shell, is soft and unarmoured, and is spirally twisted. The
swimmerets, which have only the function of carrying the eggs in the
female, are much reduced, and are usually present only on one side of
the body. The uropods no longer form a tail-fan, but are adapted for
firmly wedging the hind part of the body into the coils of the shell.
One of the chelipeds is much larger than the other, and serves to block
up the opening when the animal withdraws into its shelter. In tropical
countries certain Hermit Crabs (_Coenobitidæ_) have become adapted to a
life on land, and one of these, the well-known Coconut Crab, or Robber
Crab (_Birgus latro_), which is the largest species of the tribe, has
given up the habit of protecting itself with a shell, and its abdomen
has again acquired a strong armour on the upper side. The marine
_Lithodidæ_--to which the British Stone Crab, _Lithodes maia_ (Plate
VIII.) belongs--seem at first sight to have little resemblance to the
Hermit Crabs, for they have the abdomen very small, and tucked up under
the body as in the true Crabs. Like the Porcellanidæ, mentioned above,
however, the Lithodidæ have only three pairs of walking legs behind the
chelipeds, the last pair being feeble and usually folded out of sight
within the gill chambers. The relationship of the Lithodidæ to the
Hermit Crabs is shown by the abdomen, which is more or less twisted to
one side, and has swimmerets only on one side in the female, and quite
wanting in the male.

[Illustration: _PLATE VIII_

     THE "NORTHERN STONE-CRAB," _Lithodes maia_, MUCH REDUCED. THE LAST
     PAIR OF LEGS ARE FOLDED OUT OF SIGHT IN THE GILL CHAMBERS

                    (_From Brit. Mus. Guide_)]

The _Hippidea_ are curious little Crabs found burrowing in sandy beaches
in the warmer seas. They have the abdomen tucked under the body, and the
legs flattened for shovelling the sand.

The BRACHYURA, or true Crabs, form the fourth section of the Reptantia,
and are distinguished by having the abdomen reduced to a tail-flap,
which is doubled up under the cephalothorax, and is usually without any
trace of the uropods which are present in all the groups already
mentioned, with the single exception of the Lithodidæ. At the sides of
the head the side-plates of the carapace become firmly soldered to the
"epistome," a plate which lies in front of the mouth, and in this way
there is formed the "mouth-frame," within which lie the jaws, covered in
by a pair of "folding-doors" formed by the flattened third
maxillipeds.

[Illustration: _PLATE IX_

     THE COMMON SHORE-CRAB (_Carcinus mænas_). (REDUCED)

     _Dromia vulgaris_, CARRYING ON ITS BACK A MASS OF THE SPONGE _Clione
     celata_. BRITISH. (REDUCED)]

The first tribe of the Brachyura, the _Dromiacea_, comprises a number of
Crabs that in many points of structure resemble the Lobsters, and are
regarded as the most primitive members of the section. _Dromia vulgaris_
(Plate IX.), a furry, clumsy-looking Crab, occasionally found on our
southern coasts, has the last two pairs of legs short and carried up
over the back, where they are used for holding a mass of living sponge
which the Crab uses as a cloak to protect and conceal itself. At the
sides of the abdomen, wedged in between the telson and the last somite,
a pair of small plates may be seen, which are the last vestiges of the
uropods. These are wanting in the other tribes of the Brachyura.

[Illustration: _PLATE X_

     _Calappa flammea_, BRAZIL. (REDUCED)]

The _Oxystomata_ (Plate X.), which form the second tribe of the
Brachyura, are distinguished by the form of the mouth-frame, which is
narrowed in front so as to be triangular instead of square in outline.
The passages through which the water passes out from the gills, which in
other Crabs open at the front corners of the mouth-frame, are carried
forwards to the front of the head. The Oxystomata are most abundant in
tropical seas, but are represented on the British coasts by species of
_Ebalia_, small and compact Crabs which are not unlike pebbles of the
gravel among which they live.

The remaining Crabs form the tribe _Brachygnatha_, in which the
mouth-frame and the maxillipeds that close it are more or less
quadrilateral in shape. The tribe is divided into two subtribes, which
may be recognized by the general shape of the carapace. In the
_Brachyrhyncha_ this is generally rounded or square-cut in front,
without a projecting rostrum. In this subtribe are included the great
majority of Crabs. The Edible Crab and the Shore Crab (Plate IX.) are
familiar examples. In the _Oxyrhyncha_, on the other hand, the carapace
is generally narrowed in front, with a projecting rostrum, either simple
or forked, and is often armed with spines. In this subtribe are included
the long-legged Spider Crabs, several species of which are common on our
coasts. The Giant Spider Crab of Japan (Plate XI.) is the largest of
living Crustacea.

[Illustration: _PLATE XI_

     THE GIANT JAPANESE CRAB, _Macrocheira kæmpferi_, MALE. THE SCALE OF
     THE FIGURE IS GIVEN BY A TWO-FOOT RULE PLACED BELOW THE SPECIMEN

                    (_From Brit. Mus. Guide_)]

The last division of the Eumalacostraca, the HOPLOCARIDA (Plate XII.),
is one of very small extent, comprising only a single order
(_Stomatopoda_) of very remarkable Crustacea which are common in
tropical seas, and of which at least one species, _Squilla desmarestii_,
is occasionally captured on the south coast of England. The Stomatopoda
are prawn-like Crustaceans, usually with a flattened body, and are
easily recognized by the form of the large claws (the second pair of
thoracic limbs), in which the last segment shuts down, like the blade of
a pocket-knife, on the preceding segment, and forms a very efficient
weapon, so that the larger species are not to be handled without
caution. The resemblance of these claws to those of the mantis-insect
of Southern Europe led to a common Mediterranean species receiving long
ago the name _Squilla mantis_ (Plate XII.).

[Illustration: _PLATE XII_

     _Squilla mantis_, FROM THE MEDITERRANEAN. ABOUT ONE-HALF NATURAL SIZE

                    (_From Brit. Mus. Guide_)]

The Stomatopoda have a small carapace, which does not cover the last
four thoracic somites, and has in front a small flattened rostrum,
attached by a movable hinge, like that of the Leptostraca. The eyes are
stalked, and, like the antennules, are attached to a separate movable
segment of the front part of the head--a peculiarity not found in any
other Crustacea. There are small plate-like gills attached to the bases
of some of the thoracic limbs, but the chief organs of respiration are
large feathery gills attached to the pleopods or swimmerets.

The Stomatopoda are all found in the sea, generally in shallow water,
burrowing in sand or hiding in crevices of rocks or corals. Some species
are more than a foot in length.




CHAPTER IV

THE METAMORPHOSES OF CRUSTACEA


The great majority of Crustacea are hatched from the egg in a form very
different from that which they finally assume, and reach the adult state
only after passing through a series of transformations quite as
remarkable as those which a caterpillar undergoes in becoming a
butterfly, or a tadpole in becoming a frog. Many of these young stages
were known for a long time before their larval nature was suspected, and
it is one of the curiosities of the history of zoology that, even after
the actual changes from one form to another had been observed and
described in several Crustacea, many eminent naturalists refused to
believe in the possibility of their occurrence. This scepticism was
largely due to the fact that the common fresh-water Crayfish, when
hatched from the egg, has practically the same structure as the adult,
and it was assumed that other Crustacea were developed in a similar
fashion. Although certain cases of metamorphosis had been actually seen
and described by naturalists in the eighteenth century, these
observations were forgotten or misunderstood till they were confirmed
by Mr. J. Vaughan Thompson, a naval surgeon stationed at Cork, the first
part of whose "Zoological Researches" was published in 1828. Thompson's
statements were much disputed at the time, but they have been confirmed
by subsequent research, and it is now known that the majority of
Crustacea undergo a more or less extensive metamorphosis after leaving
the egg, although, as will be seen later, there are many important
exceptions to this rule.

If a fine muslin net be towed at the surface of the sea on a calm day,
and the contents turned out into a jar of sea-water, it will usually be
found to have captured, among other things, clouds of animated specks,
which dance in the water or dart hither and thither with great rapidity.
Many of these specks, when examined with the microscope, will be found
to be Crustacea. Besides adult animals belonging to various groups, such
as the Copepoda, which pass the whole of their life swimming near the
surface of the sea, there will be numerous larval stages of species
which in their adult form live on the sea-bottom. The identification of
the species to which the various larvæ belong is a matter of
considerable difficulty, and, although the general course of development
is now well known for all the chief groups of Crustacea, there are very
many even of the common British species in which the larval
transformations have not yet been worked out in detail.

[Illustration: FIG. 25--LARVAL STAGES OF THE COMMON SHORE CRAB
(_Carcinus mænas_--SEE PLATE IX.). (Partly after Williamson.)

A, Young zoëa, shortly after hatching; B, megalopa stage; C, young Crab.
A × 20, B and C × 10]

As an example of the larval history of the higher Crustacea, we may take
the case of the Common Shore Crab, _Carcinus mænas_ (Fig. 25). The young
stages are common in tow-net gatherings round the British coasts in the
summer-time. The youngest larvæ (Fig. 25, A) are translucent little
creatures about one-twentieth of an inch long. They have the head and
front part of the body covered by a helmet-shaped carapace, with a long
spine standing out from the middle of the back, and another
projecting, like a beak, in front.

The narrow abdomen or tail is very flexible, and can be doubled up under
the body or stretched out behind; it ends in a forked telson. There are
two pairs of swimming limbs, each with endopodite and exopodite, and the
short antennules and antennæ are seen on either side of the rostrum.
There are a pair of very large compound eyes, which are not set on
movable stalks, but are under the front part of the carapace. The
two-branched swimming feet are really the first and second maxillipeds
(the mandibles, maxillulæ, and maxillæ, can be found in front of them),
but none of the other thoracic limbs are yet developed, and, although
the somites of the abdomen are distinct, there are no swimmerets. This
type of larva is known as a _zoëa_, a name which was given to it when it
was supposed to be an independent species of Crustacean. As a matter of
fact, the zoëa just described is not quite the earliest stage of the
Shore Crab, for when hatched from the egg it is without the spines on
the carapace, and is slightly different in other respects. A few hours
after hatching, however, it casts its skin for the first time, and
becomes a fully-formed zoëa. It swims rapidly about at the surface of
the sea, feeding on the minute floating animals and plants which are
found there, and growing in size with repeated castings of its skin. In
the later stages of the zoëa the rudiments of the hinder thoracic limbs
and of the swimmerets appear as little buds. In the next stage (Fig. 25,
B) all the appendages are present, the dorsal spine of the carapace has
disappeared, the eyes are stalked and movable, and the animal has all
the appearance of a little Crab, except that the abdomen is stretched
out instead of being tucked up under the body, and the swimmerets are
used as paddles in swimming. In this stage the larva, which is known as
a _megalopa_, swims at the surface of the sea, but later it sinks to the
bottom, and, moulting again, appears as a little Crab (Fig. 25, C), with
tucked-up abdomen and swimmerets no longer adapted for locomotion.

[Illustration: FIG. 26--LAST LARVAL STAGE OF THE COMMON PORCELAIN CRAB
(_Porcellana longicornis_--SEE FIG. 41, p. 113). × 9. (After Sars.)]

Most of the true Crabs (Brachyura) have a larval history similar to that
just described, and pass through zoëa and megalopa stages which differ
only in details from those of _Carcinus_. The Anomura are also hatched
as zoëæ, and one of the most remarkable forms common in tow-nettings in
British waters is the zoëa of the little Porcelain Crabs
(_Porcellana_--Fig. 26). In this larva the carapace has two long spines
behind, and a rostral spine which is several times as long as the body
of the animal. A great development of spines also characterizes the
larva of _Munida_ (Fig. 27).

[Illustration: FIG. 27--FIRST LARVAL STAGE OF _Munida rugosa_ (SEE PLATE
VI.). × 10. (After Sars.)]

The larval form of the Common Lobster has already been described, and it
will be noticed that the differences from the adult are much less than
in the case of the Crab. From the fact that this larva has swimming
exopodites on its legs, like the adult Mysidacea and Euphausiacea
(formerly grouped together as "Schizopoda"), it is said to be in the
"schizopod stage." The larva of the Norway Lobster (_Nephrops
norvegicus_) is essentially of the same type, but the great development
of the spines on the abdomen and of the forked telson gives it a
striking appearance.

A very remarkable type of larva is found among the Spiny Lobsters and
their allies (Scyllaridea). This larva, known by the name of
_phyllosoma_ (Fig. 28), is very broad, thin, and leaf-like, and quite
transparent, so that some of the larger kinds were formerly known as
"Glass Crabs." The thin oval carapace does not cover the whole of the
thoracic region, which is disc-shaped, with four pairs of long slender
legs, each with an exopodite. The abdomen is relatively small. The
intermediate stages between the phyllosoma and the adult are still very
imperfectly known. In tropical seas phyllosoma larvæ of large size are
found, sometimes reaching two or three inches in length. The larva of
the Common Spiny Lobster (_Palinurus vulgaris_), however, does not
exceed half an inch in length.

[Illustration: FIG. 28--THE PHYLLOSOMA LARVA OF THE COMMON SPINY LOBSTER
(_Palinurus vulgaris_--SEE PLATE V.). MUCH ENLARGED. (After J. T.
Cunningham.)]

The Shrimps and Prawns of the tribe Caridea are mostly hatched as zoëæ,
and pass through a "schizopod" stage comparable to that of the Lobster,
in which they swim by means of exopodites on the legs. Some of the
Prawns belonging to the tribe Penæidea, however, have a still more
remarkable metamorphosis, which is very important on account of the
resemblance of the earlier stages to those of the lower Crustacea. Fritz
Müller discovered in 1863 that _Penæus_ is hatched from the egg as a
_Nauplius_ (Fig. 29, A), a form of larva which was previously known
among the Copepoda, Branchiopoda, and Cirripedes. The nauplius, unlike
the larvæ which we have been considering, has an unsegmented body, and
has only three pairs of limbs. The body is pear-shaped in outline, and
near the front end is seen the median eye, sometimes called, from its
presence in this type of larva, the "nauplius-eye"; the paired eyes are
not yet developed. The three pairs of limbs are shown by their later
development to be the antennules, antennæ, and mandibles; the first pair
are unbranched, the second and third divided into exopodite and
endopodite. It is interesting to notice that the antennæ and mandibles,
which in the adult animal are so widely different that it is difficult
to trace any resemblance between them, are in the nauplius almost
identical in form. Further, the antennæ, instead of being placed in
front of the mouth as in the adult, lie on either side of it, and each
has at its base a hooked spine which projects inwards and serves for
seizing particles of food and passing them into the mouth; the antennæ
of the nauplius, in fact, serve as jaws, while it is only later that the
mandibles take on this function.

[Illustration: FIG. 29--LARVAL STAGES OF THE PRAWN--_Penæus_ (SEE PLATE
IV.). × 45. (After F. Müller.)

A, Nauplius; B, young zoëa; C, older zoëa; D, early "schizopod" stage]

In the further development of the larva, the body increases in length
and becomes divided into somites which increase in number by new somites
appearing behind those already marked off; the rudiments of the limbs
also appear in regular order from before backwards; the dorsal shield of
the nauplius grows out into a carapace, beneath which the paired eyes
begin to develop in front. Thus after passing through _metanauplius_ and
_protozoëa_ stages (Fig. 29, B) the larva becomes a _zoëa_ (Fig. 29, C),
resembling that of the Crab already described in that the swimming
organs are the maxillipeds, but differing in having the uropods well
developed and forming a tail-fan at the end of the abdomen, the hinder
thoracic somites marked off and their appendages present as rudiments,
and the stalked eyes free from the carapace. This is followed by a
_schizopod_ stage (Fig. 29, D), in which the prawn-like shape is assumed
and the thoracic legs have large exopodites used for swimming. Later
these exopodites diminish in size, though they do not quite disappear in
the adult _Penæus_, and the function of swimming organs is taken over by
the abdominal swimmerets.

In _Penæus_ the larvæ are of comparatively simple form, but in the
allied genus _Sergestes_ the zoëa has a very remarkable appearance. The
carapace is armed with long spines, each bearing two comb-like rows of
secondary spines. The development of spines and other outgrowths of the
surface of the body is a very common characteristic of organisms that,
like these larvæ, float or swim in the open sea; its probable
significance will be discussed in a later chapter.

The shrimp-like Euphausiacea have a larval development very like that of
_Penæus_. Most, if not all, of the species are hatched from the egg in
the nauplius stage, and pass through stages very similar to those
described above. The adult animals, however, may be said to remain in
the "schizopod" stage, since the exopodites of the thoracic legs remain
large and are used in swimming.

[Illustration: FIG. 30--NEWLY-HATCHED YOUNG OF A CRAYFISH (_Astacus
fluviatilis_). ENLARGED]

Even among the Decapoda, however, there are many species that are
hatched from the egg in a form that does not differ essentially from the
adult, and are therefore said to have a direct development. This is
often the case with species which live in fresh water or in the depths
of the sea. For example, the young of the fresh-water Crayfish (Fig.
30), when hatched, possess all the appendages of the adult except the
first pair of swimmerets and the uropods, or outer plates of the
tail-fan. The carapace is almost globular, owing to the presence inside
the body of a large amount of food-yolk, which supplies the nourishment
necessary for the young animal in the early stages of its development.
The chelæ have hooked tips, by means of which the young animal clings
securely to the swimmerets of the mother. After a time it moults, and
the uropods are set free, the chelæ lose their hooked tips, the carapace
assumes nearly its final shape (the food-yolk having been largely
absorbed), and the young Crayfish leaves the protection of its parent,
to shift for itself. The essential point of difference between the
development of the Crayfish and that of the closely related Lobster (see
Fig. 8, p. 28) is not so much that the changes in structure which occur
after hatching are less profound in the former case, but that there is
no free larval stage. In the Lobster the earlier stages are capable of
independent existence, and they differ from the full-grown animal not
only in structure, but also in habits, swimming at the surface instead
of creeping at the bottom of the sea.

A similar case to that of the Crayfishes is found in the River Crabs of
tropical countries, belonging to the family Potamonidæ. These Crabs are
as closely related to some marine Crabs as are the Crayfishes to the
Lobsters, yet the difference in their mode of development is even more
pronounced. Instead of beginning life as minute pelagic zoëæ, they leave
the shelter of the mother's abdomen as perfectly-formed little Crabs
(Fig. 31).

[Illustration: FIG. 31--YOUNG SPECIMEN OF AN AFRICAN RIVER CRAB
(_Potamon johnstoni_), TAKEN FROM THE ABDOMEN OF THE MOTHER. MUCH
ENLARGED

The adult of an allied species is figured on Plate XXIII]

Amongst the Decapoda, instances of direct development like those just
described are exceptional, but in some of the other orders of the
Malacostraca direct development is the rule. In the great division
Peracarida, as we have already seen, the females are provided with a
pouch, or marsupium (from which the name of the division is derived), in
which the eggs are carried. Within this pouch the young undergo the
whole of their development, and they only leave it, as a rule, when they
have attained the structure of the adults. Among the more familiar
representatives of this division, the Sand-hoppers (Amphipoda), the
Woodlice (Isopoda), and the Opossum Shrimps (Mysidacea), may be
mentioned as examples of this mode of development. The Woodlice and
their immediate allies differ a little from the other members of the
division in the fact that the young leave the brood-pouch with the last
pair of legs still undeveloped, though in other respects they are like
miniature adults.

In those Crustacea which have a direct development without free-swimming
larval stages, it is sometimes possible to find traces of such stages in
the early development of the embryo. This is shown most clearly,
perhaps, in the Opossum Shrimps (Mysidacea). In these the embryo becomes
free from the egg-membrane (or may, in a sense, be said to "hatch") at a
very early stage, and lies free within the brood-pouch as a
maggot-shaped body, on which three pairs of rudimentary limbs can be
made out. The later development shows that these three rudiments
correspond to the antennules, antennæ, and mandibles, so that the
maggot-shaped embryo is, in fact, a disguised nauplius without the power
of swimming or of leading an independent existence. In other cases--as,
for instance, in the Crayfish, where the earlier stages are confined
within the egg-membrane (or "egg-shell")--the nauplius stage, although
more difficult to examine, is quite as well marked.

Of the other groups of the Malacostraca, the Syncarida and Leptostraca
are hatched in nearly the adult form, but the Stomatopoda have a long
series of larval stages. These larvæ (Fig. 32) are all distinguished by
the large size of the carapace, which in some cases envelops the greater
part of the body. Some Stomatopod larvæ, in the warmer seas, attain to a
relatively great size, sometimes exceeding 2 inches in length, and their
glass-like transparency gives them a very striking appearance.

[Illustration: FIG. 32--EARLY LARVAL STAGE OF A SPECIES OF SQUILLA,
PROBABLY _S. dubia_. × 10. (After Brooks.)]

As we have seen, it is exceptional to find a free-swimming nauplius
larva among the Malacostraca, but it is the commonest larval stage in
the other subclasses of Crustacea. Most of the Branchiopoda are hatched
in this form (Fig. 33), and reach the adult state by a very gradual
series of changes in which new somites and appendages are added in
regular order from before backwards till the full number is reached. The
Water-fleas (Cladocera), however, differ from most of the other
Branchiopoda in having a direct development. The eggs are carried in a
brood-pouch under the back of the carapace, and in this the embryos
undergo their development. In the common _Daphnia_, for instance,
numerous eggs or young can generally be seen through the transparent
carapace (see Fig. 12, p. 37).

[Illustration: FIG. 33--LARVAL STAGES OF THE BRINE SHRIMP (_Artemia
salina_). (After Sars.)

A, Nauplius, just hatched; B-E, later stages, showing progressive
increase in number of somites and appendages. The adult form of this
species is shown in Fig. 55, p. 164]

Many of the Ostracoda have a direct development, but in some cases the
young animal, on hatching, has only the first three pairs of
appendages, and is therefore regarded as a nauplius, although it
possesses a bivalved shell like that of the adult, and is very unlike
the nauplius larvæ of other Crustacea.

[Illustration: FIG. 34--EARLY NAUPLIUS LARVA OF A COPEPOD (_Cyclops_).
MUCH ENLARGED. (From Lankester's "Treatise on Zoology.")

_a´_, Antennule; _a´´_, antenna; _gn_, jaw-spine of antenna; _lbr_,
upper lip; _md_, mandible]

Most of the Copepoda also leave the egg in the nauplius stage; and,
indeed, it was to the young of the common fresh-water _Cyclops_ (Fig.
34) that the name of _Nauplius_ was first given by the Danish
naturalist, O. F. Müller, in the eighteenth century, in the belief that
it was an adult and independent species of Crustacea. In the Copepoda,
the changes which transform the nauplius into the adult are gradual, and
consist chiefly in the successive addition of new somites and
appendages.

[Illustration: FIG. 35--LARVAL STAGES OF THE COMMON ROCK BARNACLE
(_Balanus balanoides_--SEE PLATE III.)

A, Nauplius stage (after Hoek); B, cypris stage (after Spence Bate)]

The development of the Cirripedia is of special interest, since it
was the discovery of the larval stages by J. Vaughan Thompson that first
demonstrated to naturalists that the Barnacles were Crustacea and not,
as had been supposed, Molluscs. The earliest stage is generally a
nauplius (Fig. 35, A) of very peculiar and characteristic form, with a
pair of horns projecting sideways from the front corners of the dorsal
shield, and a forked spine on the under-side behind. The later
development is very unlike those which have been described above, for
after a series of nauplius stages the larva passes suddenly, at a single
moult, into a stage in which the body and limbs are enclosed in a
bivalved shell (Fig. 35, B). From the superficial resemblance of the
shell to that of an Ostracod, this is known as the _cypris_ stage.
Through the valves of the shell a pair of large compound eyes can be
seen, as well as six pairs of two-branched swimming feet, while in front
a pair of antennules project between the valves. On each antennule is a
sucker-like disc by means of which the larva, after swimming freely for
some time, attaches itself to a stone or some other object, where it
remains fixed for the rest of its life. A cementing substance produced
by a gland at the base of the antennules attaches the front part of the
head firmly to the support; the valves of the shell are cast off, and
replaced by the rudimentary valves of the adult shell; the six pairs of
swimming feet grow out into tendril-like cirri; the compound eyes
disappear, and the animal assumes the structure of the adult.

The parasitic Rhizocephala have a very remarkable life-history, which
will be described in a later chapter; but it may be mentioned here that
their free-swimming larval stages resemble very closely those of the
ordinary Barnacles. It was the discovery of this fact which led to its
being recognized that the Rhizocephala are highly modified and
degenerate Cirripedes, although their structure in the adult state gives
little evidence of their affinities.

A number of interesting problems in speculative biology are suggested by
the larval stages of Crustacea. A full discussion of these problems
would involve matters too technical for these pages, but some indication
of the broader issues may be attempted.

The obvious question, Why do some Crustacea pass through a complicated
metamorphosis while others do not? is, like many obvious and simple
questions, one of the most difficult to answer. It will be pointed out
later, in dealing with the fresh-water Crustacea, that one of the most
general characters of fresh-water animals as compared with their marine
allies is the absence of free-swimming larval stages. This applies, for
instance, to the case of the Crayfishes and the marine Lobsters, and to
that of the River Crabs, as compared with those which live in the sea.
But it does not apply to all fresh-water Crustacea, and, on the other
hand, there are many cases of direct development in marine species.

Some of the advantages gained by the possession of free-swimming larval
stages are obvious enough. Many Crustacea which live on the sea-bottom,
and are not very powerful swimmers, have their progeny scattered far and
wide by winds and currents while in the surface-living larval stages. In
the extreme case of the Barnacles, which are fixed to one spot when
adult, a locomotive larval stage is clearly a necessity. But, here as
elsewhere, to demonstrate the usefulness of any character is to go only
a very little way towards explaining its origin. Moreover, the mere
necessity for a locomotive larva throws no light on the remarkable
resemblances between the larval stages of widely different species. In
the adult state, a Branchiopod, a Copepod, an Ostracod, a Barnacle, and
a Penæid Prawn, are separated by enormous differences of form and
structure; yet, as we have seen, all these are hatched from the egg as
six-limbed nauplius larvæ differing from each other only in trivial
details. It seems hardly possible to imagine any other interpretation of
this very striking fact than is afforded by the theory of Evolution. We
are forced to assume that all these diverse forms of Crustacea are
descended from very similar or identical ancestral types, and that the
modifications arising in the course of their evolution have affected the
adult but not the larval stages. Some naturalists would go farther than
this, and would apply the so-called "theory of recapitulation" to the
larval stages of the Crustacea. According to this theory, the stages in
the development of any animal tend to recapitulate, more or less
closely, the history of the race. Thus it is assumed, for instance, that
the nauplius reproduces the structure of a six-limbed ancestral form,
from which, in the distant past, all the diverse branches of the
Crustacean class took their origin. There are, however, considerable
difficulties in the way of this view. That some such ancestral type did
exist may be regarded as tolerably certain; that it resembled in its
adult state the nauplius larvæ of present-day Crustacea is, on the
whole, unlikely; but it is not at all improbable, whatever its adult
structure may have been, that it hatched from the egg as a nauplius
larva.

With regard to some of the other larval forms, it is possible to speak
with a little more confidence. There are good grounds for believing,
apart from the evidence of development, that the Lobster and its allies
have descended from Crustacea which, like the existing Euphausiacea,
possessed swimming branches (exopodites) on the thoracic legs; and there
seems no reason to doubt that the "schizopod" larva of the Lobster does
recapitulate this stage in the evolution of the race. On the other hand,
it is impossible to believe that any of the ancestors of the Shore Crab
resembled, even remotely, the zoëa stage with which the life-history of
the individual now begins.




CHAPTER V

CRUSTACEA OF THE SEASHORE


The tract of seashore which is laid bare by the retreat of the tide
offers on most coasts a rich collecting-ground to the student of
Crustacea. In places where shelving, weed-covered rocks run out to sea,
innumerable Crustacea have their home in the rock-pools, or lurk in
crannies awaiting the return of the tide. On sandy beaches, at first
sight apparently barren of life, a closer search will reveal a whole
fauna, amongst which burrowing Crustacea of various orders are
prominent. Further, the shore collector will find from time to time
stray specimens of forms that have their proper habitat beyond low-tide
mark, and occasionally their remains are thrown in quantities on the
beach by storms. It is convenient, therefore, to treat the Crustacea of
the shore as a sample of those inhabiting the shallower waters of the
ocean. In these shallower waters--down to the limit where light no
longer penetrates from above, where vegetable life ceases, and where the
strangely modified inhabitants of the deep sea begin to appear--the
sea-bottom is perhaps the most densely populated of all parts of the
earth's surface. Nowhere, at all events, do we find so wide a range of
animal forms, from the simplest organisms (Protozoa) up to
highly-organized Vertebrates. Nowhere, perhaps, is the struggle for
existence more keen, and it is not without justice that some naturalists
have regarded the shallow waters of the sea as "one of the great
battle-fields of life," where, in the long course of evolution, the main
branches of the animal kingdom have had their origin.

Conspicuous among the animals of this region are Crustacea of all sorts
and sizes. To identify all the species that may be obtained in a single
haul of the dredge in British seas would sometimes be a hard task even
for the most expert student of the group. Our present purpose, however,
is not to compile a faunistic catalogue, but merely to give some idea of
the endless diversity of form, and to note a few of the "shifts for a
living"--of the ways in which structure and habit are adapted to the
conditions of life in the Crustacea of the shore and of shallow water.

Though it might seem that the heavily armoured Lobsters and the larger
Crabs would be sufficiently protected against most enemies when once
they have attained their full size, yet they are preyed upon by the
Octopus, which seizes them with its suckers and pierces their armour
with its powerful beak, injecting a poison that paralyzes its victims.
Some years ago a "plague" of Octopus very seriously affected the Lobster
fishery in the English Channel. To escape from enemies such as these,
the Lobsters and many Crabs have the habit of lurking in crevices of the
rocks, while in case of sudden alarm the Lobster may escape from danger
by swimming, or rather darting, with great swiftness, tail foremost,
through the water by powerful strokes of the abdomen and tail-fan. In
the more lightly armed Prawns and other Crustacea of the tribe Natantia,
which are characteristically swimmers, the power of rapid motion is
probably the chief means of protection against enemies. There is reason
to believe that the Lobsters have been derived from prawn-like swimming
forms which have sacrificed some of their agility in developing their
heavy armour-plating, retaining, however, the power of sudden and rapid
motion in emergency. This power, again, has been lost by the typical
Crabs (Brachyura), in which the abdomen is reduced in size and without a
tail-fan, so as to be useless for swimming. While most of the Crabs,
however, are somewhat slow of movement, trusting to their armour and
their powerful pincers for defence, the Swimming Crabs (Portunidæ--Plate
XIII.) have reacquired the power of swimming by means of the
paddle-shaped legs of the last pair. Some of the tropical species of
Portunidæ are probably the most expert swimmers among the Crustacea,
and are described as shooting through the water like fish.

[Illustration: FIG. 36--A COMMON HERMIT CRAB (_Eupagurus bernhardus_)
REMOVED FROM THE SHELL]

The Lobster's habit of seeking shelter in rock-crevices or under stones
is one which is shared by a very large number of shore Crustacea. From
some primitive kind of Lobster which discovered the advantages of a
portable shelter have been derived the Hermit Crabs. In rock-pools one
may often see whelk or periwinkle shells tumbling about with an
activity quite foreign to the nature of their original molluscan
inhabitants, and closer examination will show that each contains a
Hermit Crab, which retreats into the shell when disturbed. If extracted
from the shell, the Crab (Fig. 36) can be seen to be most beautifully
adapted to its peculiar mode of life. The abdomen is soft and spirally
twisted to fit into the interior of the spiral shell, and the uropods,
instead of forming a tail-fan, are modified into holding organs, with
roughened, file-like surfaces which can be pressed outwards against the
walls of the shell, and wedge the body so firmly that an attempt to drag
the animal forcibly from its retreat often results in tearing it in
half. The front part of the body, which is exposed when the animal is
walking, retains its shelly armour. One of the pincer-claws, most
commonly the right, is much larger than the other, and serves to block
the opening of the shell when the body is withdrawn into it. The next
two pairs of legs are long and slender, and are used for walking; but
the last two pairs are short, with a roughened surface at the end, and
serve to steady the body in the mouth of the shell. The swimmerets on
the right side of the body, which is pressed against the central pillar
of the shell, have disappeared, but those of the left side remain.

As the Hermit grows, it is necessary for him to remove from time to time
into a larger dwelling. It has been stated that he will sometimes
dispossess the rightful owner of a whelk-shell for this purpose,
dragging him out piecemeal and eating him; but other observers deny that
this ever happens, and in most cases, at all events, the Hermit is
content to wait until he finds an empty shell of suitable size. After
turning this over and exploring the interior with his claws, to satisfy
himself that it is unoccupied, he deftly whips the unprotected hinder
part of his body into the new habitation, keeping hold of the old one
meanwhile, so that he can return to it if the other proves unsuitable.
The Hermits are very pugnacious, and fight with one another for the
possession of desirable shells, the victor dragging his opponent out and
establishing himself in his place. Besides appropriating the shell of a
dead Mollusc, many Hermits seem to go into partnership with living
animals of various kinds, and some of these associations will be noticed
in a later chapter. A number of species adopt other dwellings than
molluscan shells, and some tropical Hermits, for instance, are found
living in the cavities of water-logged stems of bamboo (Fig. 37); while
others, relinquishing the advantages of a portable shelter, live in
holes in corals or in the canals of living sponges. Although in some of
these cases the body is straight, it usually shows traces of its
original adaptation to a spiral shell in having no swimmerets on the
left side.

[Illustration: FIG. 37--_Pylocheles miersii_, A SYMMETRICAL HERMIT CRAB.
(After Alcock.)

The upper figure gives an end view of the animal lodged in a tube of
water-logged mangrove or bamboo, its large claws closing the opening.
The lower figure shows the animal removed from its shelter.]

The only Hermits which have a full series of swimmerets are the
primitive Pylochelidæ (Fig. 37), which come very near to what we imagine
the ancestral form of the group to have been like, and can hardly be
separated from the mud-burrowing, lobster-like Thalassinidea. A few
Hermits have given up altogether the use of any protective covering. One
of these is the Coconut Crab (_Birgus_), to be mentioned when we come to
deal with the Crustacea of the land. Another is the Stone Crab
(_Lithodes_--Plate VIII.) of our own seas, and its kindred, which have
redeveloped shelly plates on the back of the abdomen, but carry it
doubled up under the body like the true Crabs. These also preserve some
traces of the original twisting of the abdomen, and have swimmerets only
on one side.

Some Crustacea construct habitations for themselves. On turning over a
flat stone between tide-marks, one often finds a little mass of bits of
weed and rubbish attached to it, and if this be torn open a
greenish-brown, shrimp-like animal, about three-quarters of an inch
long, is seen slithering away on its side. This is an Amphipod
(_Amphithoë rubricata_) which builds the shelter for itself, sticking
the fragments together with threads of a cementing material produced by
glands on the surface of its body and legs. Other Amphipods construct
more neatly finished tubular dwellings of mud, or even of small stones,
which are attached to sea-weeds and the like; and some make portable
shelters of the same kind, which they carry about with them like the
caddis-worms of fresh-water streams.

Some of the true Crabs also employ portable shields for purposes of
defence or of concealment. The species of _Dorippe_ which are found in
tropical seas have the last two pairs of legs short, elevated on the
back so that they cannot be used for walking, and ending in a kind of
grasping claw. By means of these claws the Crab holds over its back some
object, generally one valve of a molluscan shell, sometimes even a
mangrove-leaf, to supplement the protection afforded by its carapace.
The "Sponge Crabs" (_Dromiidæ_), of which one species, _Dromia
vulgaris_ (Plate IX.), occurs on the southern coasts of Britain, have
also the last two pairs of legs elevated on the back and used in a
similar way; but in this case the covering is usually a mass of living
sponge, one of the Sea-squirts (Tunicata), or some similar organism.

[Illustration: _PLATE XIII_

     A SWIMMING CRAB, _Portunus depurator_. BRITISH. (REDUCED)

     A SPIDER-CRAB, _Maia squinado_, DRESSED IN FRAGMENTS OF WEED. BRITISH
     (REDUCED)]

Even more remarkable are the "masking" habits of the Spider Crabs
(Oxyrhyncha). In these the carapace is almost always covered with
sea-weeds, zoophytes, and other organisms which afford a very effective
disguise. For example, specimens of the British species of _Hyas_ (_H.
araneus_ and _H. coarctatus_) and _Maia_ (_M. squinado_--Plate XIII.),
which are very common on our coasts, readily escape the notice of the
collector, as they lurk in the rock-pools. They are slow-moving animals,
and the carapace and limbs are usually quite hidden by dense tufts of
growing sea-weed, sponges, and other organisms. By observing the Crabs
in an aquarium, it has been found that they actually dress themselves,
plucking pieces of weed and the like and placing them on the carapace,
where they are held in position by numerous hooked hairs. The
transplanted fragments continue to live and grow until the Crab appears
like a miniature moving forest. Still more strange is the fact that the
Crabs appear to be able in some degree to adapt the nature of their
covering to their surroundings. It has been found that specimens dressed
in sea-weeds, when placed in an aquarium among sponges, picked off
the weeds from their bodies and limbs, and planted fragments of sponge
in their place. Not only does this habit afford the Crabs protective
concealment, but it may also in some cases serve as a source of
food-supply. The late Dr. David Robertson, of Cumbrae, one of the most
observant of marine naturalists, saw the Crab _Stenorhynchus_ (or
_Macropodia_) _longirostris_ picking food-particles from among the
vegetation on its body, and conveying them to its mouth.

Many Crustacea of different orders seek concealment and protection by
burying themselves in sand. A pool left by the tide on a sandy beach may
at first sight appear empty of all life, but if it be watched for a
little while a greyish, shadowy form may often be seen to dart across
it, to settle on the bottom with a little puff of sand, and to
disappear. Even a close scrutiny of the spot will hardly discover
anything, but with a hand-net one may succeed in scooping up, before it
can dart away again, a specimen of the Common Shrimp (_Crangon
vulgaris_--see Fig. 78, p. 244), whose translucent body is finely
mottled with greyish-brown so as to match exactly the sand among which
it rests.

If a spadeful of sand from between tide-marks be stirred up in a bucket
of sea-water and allowed to settle for a few seconds, and the water then
poured off through a fine muslin net, a wonderful assemblage of minute
Crustacea may often be obtained. Numerous species of Ostracods,
Copepods, and Amphipods, and some Isopods, can be collected in this way,
and some of these, at least, show peculiarities of structure which
appear to be adapted to a sand-burrowing habit. Perhaps the most
remarkable Crustacea living in such situations, however, are the
Cumacea. In these, as already mentioned, the gills, which are attached
to the first pair of thoracic limbs, lie one on each side of the thorax
in a cavity enclosed by the carapace. These cavities are continued
forwards to the front of the head, where they unite in a single opening
from which a transparent tube (or a pair of tubes) can be protruded. It
appears probable that this very peculiar arrangement of the respiratory
system is adapted to enable the animals to breathe while buried in sand
or mud. The water is probably drawn in behind through the narrow slit
between the side-plates of the carapace and the bases of the legs, and
is expelled through the tube which is protruded from the front of the
head. In this way the delicate gills are protected from injury and kept
from becoming clogged with sand, while the effete water, loaded with the
products of respiration, is carried off to a safe distance, so that it
does not re-enter the gill chamber.

In the case of such minute forms, however, it is very difficult to
determine the precise details of their mode of life by observation of
the living animals. In the larger Decapods, which can be watched in
their natural haunts, or more closely in aquaria, many interesting
adaptations to burrowing in sand have been discovered. Many Crabs
belonging to the tribe Brachyrhyncha often take refuge in sand or
gravel, burying themselves till only the eyes remain exposed. The
Swimming Crabs (Portunidæ--Plate XIII.) of our own coasts have been
found to use the paddle-shaped last pair of legs for digging as well as
for swimming. In the sand, the Crab keeps its large claws, or chelipeds,
folded close up to the front edge of the carapace, which is cut into
sharp, saw-like teeth. Between these teeth the water passes, to reach
the entrance to the gill chamber which lies at the base of each
cheliped, and in this way an efficient strainer is provided, which in
coarse sand at least prevents the clogging of the respiratory passages.
The out-going current of water from the gills passes through channels
that open on either side of the mouth-frame.

[Illustration: _PLATE XIV_

     _Corystes cassivelaunus._ MALE (ON LEFT) AND FEMALE (ON RIGHT).
     BRITISH (REDUCED)

     _Albunea symnista_, ONE OF THE HIPPIDEA. INDIAN SEAS. (REDUCED)]

A more complex adaptation of structure to the habit of sand-burrowing is
found in the Masked Crab (_Corystes cassivelaunus_--Plate XIV.). This
Crab is common on the British coast, living in moderately deep water
wherever the bottom is sandy, and it has received its English name from
the fact that the furrows on the back of the carapace give it a
grotesque resemblance to a human face. It is noteworthy, among other
things, for the marked difference between the sexes, the male having
very long, slender chelipeds, while those of the female are quite
short. The most remarkable features of its organization, however, have
to do with its habit of burrowing in sand. The antennæ, which in most
Crabs are extremely short, are in this species as long as the body, and
each bears a double fringe of stiff hairs disposed along the upper and
under sides of the antenna, but curved inwards, so that when the two
antennæ are brought together parallel with each other, the hairs
interlock and form a long tube. At its base this tube communicates with
a space in front of the mouth, into which open the channels from the
gill chamber at the front corners of the mouth-frame. The Crab burrows
in fine sand, and the process is thus described by Professor Garstang:
"The Crab sits upright on the surface of the sand; the elongated,
talon-like claws of the four hindmost pairs of legs dig deeply into the
sand; the body of the Crab is thus forcibly pulled downwards by the grip
of the legs, and the displaced sand is forced upwards on the ventral
side of the body by the successive diggings and scoopings of the legs;
the slender chelate arms of the first thoracic pair assist in the
process of excavation by thrusting outwards the sand which accumulates
round the buccal region of the descending Crab." In this way the Crab
descends deeper and deeper, until nothing is visible above the surface
of the sand but the tips of the antennæ. The antennal tube keeps open a
channel leading from the buried Crab to the water above. Since this
tube communicates at its base with the passages through which the water
passes _out_ from the gill chamber in most Crabs, it was assumed by the
older observers that the antennal tube served to carry the outflowing
water to the surface of the sand. It has recently been shown, however,
by Professor Garstang that when the Masked Crab is buried in sand the
normal respiratory current is reversed, water being drawn _down_ the
antennal tube, into the gill chambers, and passing out through the
openings at the base of the chelipeds which, when the Crab is not
buried, serve for its entrance.

Most, if not all, of the Crabs belonging to the tribe Oxystomata are
sand-burrowers, and the structure of the mouth parts characteristic of
the tribe appears to have been acquired as an adaptation to this habit.
As already mentioned, the mouth-frame in these Crabs is triangular
instead of square, being produced forwards between the eyes, and the
third maxillipeds, which cover it, are also elongated. In this way the
exhalent channels carrying the water from the gill chambers open on the
front margin of the head, and are exposed even when the Crab is buried.
In the different families of this tribe the inhalent openings by which
the water enters the gill cavities are protected in various ways, and so
arranged that respiration can go on without danger of the gills becoming
clogged by sediment.

The members of the tribe Hippidea (sometimes called "Mole Crabs"), among
the Anomura, have habits somewhat similar to those of the Crabs just
described. They are common on sandy beaches in the warmer parts of the
globe, and they burrow with great rapidity by means of the curved,
flattened end-segments of the legs. The carapace is generally smooth and
oval, and the body is compact, the short abdomen being folded up as in
the Crabs.

In _Albunea_ (Plate XIV.), which belongs to this tribe, a long "antennal
tube," which looks very like that of _Corystes_, is believed to have a
similar function in connection with respiration when the animal is
buried. In this case, however, the tube is formed, not by the antennæ,
as in _Corystes_, but by the antennules, so that it affords a striking
example of the independent evolution of similar structures from quite
different origins.

_Hippa emerita_, which is found on the coasts of North and South
America, has the mouth parts imperfectly formed, and not adapted for
biting; and it is stated by Professor S. I. Smith that the animal feeds
in the way that an earthworm does, swallowing the sand through which it
burrows, and extracting the nutriment which it may contain. This habit,
however, is not followed by other members of the tribe, for Mr.
Borradaile found that a species of _Remipes_ in the Maldive Islands
could "easily be caught by a bait of Crab at the end of a line,
pouncing on it with its sharp maxillipeds, and allowing itself to be
flicked out of the sand if the rod be sharply lifted."

In the cases mentioned above, the Crustacea do not bury themselves much
below the surface of the sand, and do not form definite burrows; but
there are many Crustacea which live in open tunnels dug deep into the
sand. Some of these belong to the category of amphibious forms, to be
mentioned presently; but there are others which live in deeper water,
and of which the habits are less open to observation.

[Illustration: FIG. 38--_Callianassa stebbingi_ (FEMALE), A SAND-BURROWING
THALASSINID FROM THE SOUTH COAST OF ENGLAND. NATURAL SIZE]

Nearly all the Thalassinidea (Fig. 38) live in burrows, often of
considerable depth, in sand or mud. Although now classed with the
Anomura, these animals are lobster-like in form, loosely built,
generally with short carapace and long, soft abdomen. They have usually
very small eyes, which appear as if they were not of much use for
vision; and some of the hinder pairs of legs are short, and carried
folded against the sides of the body, probably for use when the animal
is moving up or down in its burrow.

Most of the Stomatopoda resemble the Thalassinidea in their mode of
life, and show some curious similarities to them in structure, although
by no means closely related. They are described as lying in wait for
prey at the mouth of their burrows, darting out on passing fish or other
animals, which they seize with their great saw-toothed claws, and
retreating with great rapidity to the bottom of the burrow.

Most of the Crustacea mentioned live below tide-marks, and at all events
are rarely seen when the sand in which they burrow is left bare by the
tide; but there are others, especially on tropical shores, which seem to
have their chief period of activity when the sand or mud banks on which
they live are exposed to the air. Chief among these amphibious forms in
the warmer seas are the Crabs of the genera _Ocypode_ and _Gelasimus_
and some of their allies.

[Illustration: _PLATE XV_

     _Ocypode cursor._ WEST AFRICA. (REDUCED)

     _Gelasimus tangeri._ MALE ABOVE, FEMALE BELOW. WEST AFRICA. (REDUCED)]

Some of the species of _Ocypode_ (Plate XV.) dig their burrows between
tide-marks, where they are swamped by the advancing tide, and must be
excavated afresh when the water retreats. Other species, however, live
above high-water mark, and are practically terrestrial animals, only
entering the water occasionally, and, indeed, unable to survive
prolonged immersion. The work of excavating the burrows has been watched
in several species. The Crab comes out of the burrow sideways, carrying
a load of sand between two of the walking legs on the rear side. By a
sudden movement the sand is jerked away to some distance, where it
accumulates in a little heap, and the Crab dives into the burrow for
another load. Most of the Crabs belonging to this genus possess a
curious "stridulating organ" on one of the large claws, by means of
which they can produce a buzzing or hissing sound. On the inner surface
of the "hand" there is a raised patch, which, when examined with a lens,
is seen to be made up of a series of fine ridges, like the teeth of a
file. When the limb is bent in towards the body, this patch can be
rubbed up and down against a sharp-edged ridge or scraper on the third
segment of the limb, and in this way the sound is produced. What the use
of the sound may be is not quite clear, but there is probability in Dr.
Alcock's suggestion that it serves to warn intruders that the burrow is
already occupied. These Crabs run very swiftly, and one species was seen
by Professor S. I. Smith to catch Sand-hoppers (Amphipods of the family
Talitridæ) by springing on them suddenly, "very much as a cat catches
mice," but it also fed on dead fish and the like.

Of somewhat similar habits are the numerous species of the genus
_Gelasimus_ ("Fiddler Crabs"--Plate XV.), which abound on sand and mud
flats of tropical shores. These little Crabs are remarkable for the
great dissimilarity between the sexes in the form of the chelipeds. In
the female both chelipeds are small and feeble, but in the males one of
them, either the right or the left, is enormously enlarged, sometimes
exceeding in length and breadth the body of the Crab which carries it.
What the precise use of this enormous claw may be does not seem to be
quite certainly known. It is said to be used as a weapon by the males in
fighting with one another, but it seems too clumsy to be very efficient
for this purpose. It is often brilliantly coloured, and has been
supposed to be a sexual adornment.

In _Ocypode_ and _Gelasimus_ the respiratory apparatus is modified for
the purpose of breathing air. The gills are similar to those of purely
aquatic Crabs, and no doubt serve for respiration when the animal is in
the water; but the gill chambers are much more spacious than usual, and
the lining membrane is richly supplied with bloodvessels. Air is
admitted to the gill chambers by an opening, protected by a brush of
hairs, between the second and third pairs of walking legs on each side.
It is believed that in this way the gill chamber is fitted to be used as
a lung when the animals are out of the water. Similar arrangements in
some of the more exclusively terrestrial Crustacea will be mentioned in
a later chapter.

There are many Shore Crabs, however, which lead a more or less
amphibious existence without showing any marked modifications of
structure as compared with their more purely aquatic relatives. On our
own coasts, the Common Shore Crab (_Carcinus mænas_--Plate IX.) commonly
spends several hours each day exposed to the air, and in an aquarium it
will voluntarily leave the water if the opportunity be afforded it. On
tropical coasts the species of _Grapsus_ and allied genera are often
seen clambering with great agility about exposed rocks.

Analogous habits to those of the sand-burrowing, amphibious Crabs
described above are shown on a small scale by the Amphipods of the
family Talitridæ, known as "Sand-hoppers" or "Beach-fleas." Everyone who
has walked over the firm sand near high-water mark on our own shores
must have noticed the myriads of actively hopping little creatures
disturbed at every step. The commonest species of Sand-hopper on the
British coasts is _Talitrus saltator_ (Fig. 39), but _Orchestia
gammarellus_ is also common. Both species occur together on sandy
beaches or among decaying sea-weeds, and are among the most important
scavengers of the seashore, picking clean the bones of fish or other
animals cast up by the tide. In this country the Sand-hoppers do not, as
a rule, venture far above high-water mark; but in warmer climates
species of Talitridæ live in the damp forests at great distances from
the sea, and deserve to be ranked among the terrestrial Crustacea.

[Illustration: FIG. 39--THE COMMON SAND-HOPPER (_Talitrus saltator_),
MALE, FROM THE SIDE. × 3. (After Sars.)]

It has been mentioned above that the Common Shrimp is protected, not
only by its habit of lying half buried in sand, but also by its close
resemblance in colour to the sand among which it lives. There are many
others among the shore Crustacea which show what seems to be a
"protective resemblance" in colour and form to their surroundings. It is
necessary to be cautious in interpreting these resemblances as
necessarily protective, since the fish and other enemies which prey on
these Crustacea see them with eyes very different from ours, and
probably, in many cases, are guided to their prey by the sense of smell
rather than by sight. The "masking" habit of the Spider Crabs, already
described, strongly suggests, however, that concealment from sight is an
important protection to some shore Crustacea, and helps to make it
probable that the same end is reached in other cases by modifications of
form and colour.

There can be no doubt, at all events, that many Crustacea are very
inconspicuous to human eyes when they remain motionless in their natural
surroundings. Thus, for example, the Caprellidæ, or "Skeleton Shrimps"
(see Fig. 22, p. 54), are hard to detect without very close search, as
they cling to the feathery branches of the hydroid zoophytes among which
they are usually found. They are strangely modified Amphipods, in which
the body is slender and thread-like, and generally of a
semi-transparent, whitish or yellowish colour, like the zoophytes on
which they live. They clamber about among the branches with a movement
like that of a "looper" caterpillar, and often remain clinging by means
of the hooked claws of the hinder pairs of legs, with the fore part of
the body gently waving about.

The little Crabs of the family Leucosiidæ (Oxystomata), of which the
British representatives are several species of the genus _Ebalia_, are
often extremely like pebbles of the gravel among which they live. In
many tropical species the carapace is pitted and eroded, so as to
resemble a worn fragment of coral shingle. One of the most striking
cases among the Crabs, however, is that of _Huenia proteus_ (Fig. 40),
one of the Spider Crabs (Oxyrhyncha), which is found in the Indian and
Pacific Oceans. In this little Crab the carapace is flat, and is
extraordinarily variable in form. In most of the males it is triangular
in outline, but in most of the females and in some males it is broadened
by leaf-like expansions of the side edges. Borradaile has pointed out
that these broad individuals are usually found among the sea-weed
_Halimeda_, and that they closely resemble the fronds of this weed in
form and in their greenish colour.

[Illustration: FIG. 40--A, A PIECE OF A TROPICAL SEA-WEED (_Halimeda_);
B, A CRAB (_Huenia proteus_) WHICH LIVES AMONG THE FRONDS OF _Halimeda_,
AND CLOSELY RESEMBLES THEM IN FORM AND COLOUR. REDUCED. (After
Borradaile.)]

A number of Crustacea are known to possess a chameleon-like power of
changing their colour. The mechanism by which this change is effected is
similar to that found in other animals, such as fish and frogs, which
have the same power. The pigment which gives its colour to the animal is
lodged in microscopic star-shaped bodies known as _chromatophores_,
lying for the most part just below the skin. Each chromatophore consists
of a central body from which a number of branching filaments radiate.
The pigment may contract into the centre of the chromatophore, forming
a minute and hardly visible speck, or it may spread out into the
branching filaments, forming a distinct spot of colour. Each
chromatophore may in some cases contain several colours of pigment, and
these may expand or contract independently of each other, so that a
whole series of changes may be produced by a single chromatophore. In
the larger Crabs and Lobsters the visible colour of the animals depends
on pigment in the shelly exoskeleton, which is thick enough to hide the
chromatophores in the living tissues underneath, and no very rapid or
considerable changes are apparent; but in the smaller forms, in which
the exoskeleton is thin and translucent enough to allow the underlying
colours to appear through it, the changes in the chromatophores may
produce striking effects. Thus, Fritz Müller describes a species of
Fiddler Crab of the genus _Gelasimus_, in which the hinder part of the
carapace was brilliantly white, but five minutes after the Crab was
captured it had changed to a dull grey. Many other cases of colour
change have been described, but most remarkable and the most fully
studied is that of the Prawn, _Hippolyte varians_, which is very common
on our own coasts, and has recently been the subject of a very elaborate
series of researches by Professors Keeble and Gamble. The specimens of
this Prawn show "a bewildering variety of colour and of colour-pattern";
they may be uniformly coloured in various shades of brown, green, or
red, or they may be "blotched," "barred," or "lined," with colour. These
different varieties are generally found among sea-weeds, which they
resemble in colour and pattern, the "lined" forms, for instance,
frequenting finely branched and feathery weed. Like many other
protectively coloured animals, they are of sedentary habits, clinging to
the weed, and seldom moving by day. If a specimen be removed from its
habitat and placed in an aquarium with different kinds of sea-weed, it
will take refuge among that which it most closely resembles. It appears
that this resemblance in colour-pattern is acquired during the growth of
the Prawn, and that a young specimen kept among finely branched sea-weed
will acquire the "lined" pattern, while others, living among coarser
weed, become "barred," "blotched," or "monochrome." Even in the adult
Prawns the colour (though not the pattern) becomes changed in a day or
two if they are placed among weed of a different colour--from green to
brown, or the like. Within certain limits still more rapid changes of
colour take place. If kept in the dark, or if placed on a white
background (for example, in a porcelain dish) in the light, the Prawn
quickly becomes nearly colourless, by contraction of the chromatophores,
a transparent bluish tint alone remaining, due to a substance which
diffuses from the chromatophores into the fluids of the body. In natural
conditions this phase is assumed at night; and the interesting
observation has been made that Prawns kept in the dark continue for
three or four days to show a periodic expansion and contraction of the
chromatophores, corresponding to the alternation of day and night. It
seems that the rhythm of light and darkness has become impressed on the
chromatophore system of the animal, and the movement of the pigments is
regulated by something analogous to memory.

[Illustration: FIG. 41--THE COMMON PORCELAIN CRAB (_Porcellana
longicornis_), SLIGHTLY ENLARGED, AND ONE OF THE THIRD MAXILLIPEDS
DETACHED AND FURTHER ENLARGED TO SHOW THE FRINGE OF LONG HAIRS]

It has already been mentioned, in dealing with the Lobster, that certain
Crustacea have the power of voluntarily throwing off some of their limbs
(autotomy). In many cases, as in the Lobster, this power is mainly of
use in enabling the animal to discard an injured limb; but there are
some Crustacea which seem to adopt it as a means of escaping from the
attack of an enemy. On our own coasts the shore collector will often
find, on turning over a large stone, one or more specimens of the little
Porcelain Crabs (_Porcellana platycheles_, or _P. longicornis_--Fig. 41)
clinging to its under-side. If these Crabs be seized by one of the large
claws, they frequently leave the claw in the captor's hand and scuttle
off without it; and it cannot be doubted that, as in the case of lizards
and other animals which have a similar power of self-mutilation, this
habit often enables them to escape from their natural enemies.

Although the Crustacea as a whole are predominantly active animals, many
examples have already been mentioned of species which are more or less
sluggish and sedentary in their habits. The extreme degree of passivity
is reached by the Barnacles (Cirripedia), which differ from all other
Crustacea (except some parasites) in being fixed to one spot, and quite
without the power of locomotion in the adult state. Most of the
Barnacles met with on the shore or in shallow water belong to the
division of the Sessile Barnacles or Acorn-shells (Operculata). Every
visitor to the seashore has noticed the little conical shells which
cover exposed rocks as if with a coat of rough-cast. On the British
coasts the commonest species is _Balanus balanoides_ (Plate III.),
though other species closely resembling it are also common. They are to
be found almost up to high-water mark in situations where they are left
uncovered for many hours every day; but the valves which close the
opening of the shell fit so tightly that a little sea-water is enclosed,
and the animal is protected from drying up even when exposed to the heat
of the sun. If a stone or a chip of rock, with a few of these animals on
it, be placed in a jar of sea-water, their peculiar mode of obtaining
food can easily be watched. The valves will presently be seen to open a
little, and the curled cirri will be protruded, opened out like the
fingers of a hand, and withdrawn again with a sort of grasping motion.
These movements are continued without stopping while the animal is under
water. If the cirri be examined with a pocket-lens or under a
microscope, it will be seen that they are fringed with stiff bristles,
so that, when they are opened out, the whole forms a kind of
"casting-net." As it is swept through the water, this net entangles
minute floating particles of animal or vegetable matter, and carries
them into the shell, so that they can be seized by the jaws and
swallowed. The cirri, as we have already seen, are really the modified
thoracic limbs, so that, in Huxley's words, "A Barnacle may be said to
be a Crustacean fixed by its head, and kicking the food into its mouth
with its legs."

A mode of obtaining food by "net-fishing," not unlike that employed by
the Barnacles, is found in certain Crustacea belonging to a widely
different group--the little "Porcelain Crabs" (Fig. 41) mentioned above.
Mr. Gosse observed that the Broad-clawed Porcelain Crab (_Porcellana
platycheles_) employed its third pair of maxillipeds, which are thickly
fringed with long feathered hairs, in making alternate casting movements
"exactly in the manner of the fringed hand of a Barnacle, of which both
the organ and the action strongly reminded me."




CHAPTER VI

CRUSTACEA OF THE DEEP SEA


It has already been mentioned that the animals living on the sea-bottom
in shallow water do not differ greatly in character from those that may
be found between tide-marks. As we go farther out from land, however,
into the deeper water, the character of the fauna gradually changes. One
by one the species found near the shore become rare and disappear, and
their places are taken by others characteristic of the intermediate
depths. These in their turn give way to others, till in the abysses of
the great oceans we find an assemblage of strange animals adapted to the
conditions of life in the great depths, and differing widely in many
respects from the more familiar inhabitants of the coastal waters. In
this "fauna of the deep sea," which extends to the greatest depths
reached by the dredge or trawl, the Crustacea occupy a prominent place.
Before proceeding to discuss some of these peculiar forms, however, it
is necessary to attempt to form some idea of the conditions under which
they live.

In the first place, the character of the sea-bottom changes very greatly
as we pass away from the coast. Near the shore it is extremely
diversified, consisting in one place of rocks swept bare by the tides or
overgrown with jungles of sea-weed, in another of banks of gravel or
shingle, of sand or of mud, but in all cases derived from the "waste" of
the land, as it is eaten away by the waves or washed down by the rivers.
As the distance from land increases, the deposits become finer and
finer, till they shade off into a soft oozy mud, composed of the finest
particles brought down by the rivers. In the neighbourhood of large
rivers this mud may sometimes extend for hundreds of miles from the
land, but there is a limit to the distance to which even the finest
particles can drift before they settle to the bottom, and beyond this
limit the floor of the ocean is covered by sediments which owe their
origin, not to the land, but to the ocean itself. The surface waters of
the ocean everywhere teem with a vast variety of floating animals and
plants, and, as these die, their remains sink to the bottom "like a
perpetual shower of rain."

Among the most abundant floating organisms--in the warmer seas, at any
rate--are certain minute animals known as _Foraminifera_, which belong
to the lowest class of the animal kingdom, and have shells composed, in
most cases, of carbonate of lime. Over vast areas the bottom of the
ocean is covered with a soft grey ooze, made up almost entirely of the
dead shells of Foraminifera rained down from above. Since the commonest
species of Foraminifera found under these circumstances belong to the
genus _Globigerina_, the deposit is known as "Globigerina ooze."

In certain regions of the ocean the shells of other floating organisms
largely replace those of the Foraminifera in covering the ocean floor,
and in the deepest abysses--so deep that the shells of surface animals
are dissolved before they can sink to the bottom--there is found a
deposit known as the "red clay," which appears to be derived largely
from the impalpable volcanic and cosmic dust that floats in the
atmosphere. It is not necessary for our present purpose to enter more
fully into the interesting questions connected with these deep-sea
deposits, but it is important to remember that, generally speaking, the
floor of the deep sea is everywhere soft ooze, without rocks or stones,
except for an occasional water-logged lump of pumice or a stone dropped
by a melting iceberg. This fact is probably of great importance in the
life of deep-sea animals.

One of the most peculiar and characteristic of the physical conditions
in the deep sea is the enormous pressure under which life has to be
carried on. At the surface of the sea the pressure of the atmosphere is,
roughly speaking, 14-1/2 pounds per square inch. At a depth of only 33
feet of water this pressure is doubled, and at greater depths the
pressure increases in proportion, till at 2,000 fathoms it is more than
2-1/2 tons on the square inch. As a matter of fact, however, the animals
at the bottom of the sea are probably but little affected by this
enormous pressure. Only, when they are brought up by the dredge the
sudden release of pressure causes the fluids of the body to expand and
destroys the tissues, so that the animals are generally dead or dying
when they reach the surface.

More important than the pressure in its influence on life is the
darkness of the depths. The light of the sun only penetrates the water
of the sea to a comparatively small depth. At 200 fathoms there is not
enough light to produce any effect on a photographic plate. Even at a
considerably less depth the absence of light puts an end to all
plant-life, except for the ubiquitous bacteria, and it follows that all
the animals of the deep sea ultimately depend for their food-supply on
the rain of dead bodies of surface animals which, as already mentioned,
is constantly falling on the sea-bottom.

The temperature at the bottom of the deep sea is always very low. Dr.
Alcock states that "in the open part of the Bay of Bengal, where the
mean surface temperature is about 80° F., the temperature at a depth of
100 fathoms is only about 60° F., at a depth of 300 fathoms not quite
50° F.; while at a depth of 2,000 fathoms the temperature all the year
round is only 3° above freezing-point."

Finally, it is important to notice the _uniformity_ of the conditions at
the bottom of the sea; not only are the alternation of night and day and
the progress of the seasons unfelt in the abysses, but the conditions
are practically the same over vast areas in all the oceans.

In the case of deep-sea Crustacea, we are frequently confronted with a
difficulty which does not occur in the case of some other groups of
animals--Corals or Echinoderms, for example--the difficulty, namely, of
deciding whether the animals really lived on or near the bottom, or were
captured by the open mouth of the trawl on its way to the surface. When
the animals are plainly not well adapted for swimming--as, for instance,
most of the Crabs--it may be assumed that they did actually live on the
bottom; but, with the prawn-like forms, the possibility that they may
really be inhabitants of the intermediate depths must always be taken
into consideration.

[Illustration: FIG. 42--A DEEP-SEA LOBSTER (_Nephropsis stewartii_),
FROM THE BAY OF BENGAL. REDUCED. (After Alcock and Anderson.)]

[Illustration: FIG. 43--_Munidopsis regia_, A DEEP-SEA GALATHEID FROM
THE BAY OF BENGAL. REDUCED. (After Alcock and Anderson.)]

In animals that live in perpetual darkness we should expect to find, in
accordance with the principle of adaptation which runs through the whole
of organic nature, that the eyes are wanting or imperfectly developed.
In a great many deep-sea animals this is indeed the case. The deep-sea
Lobsters of the genus _Nephropsis_ (Fig. 42), which are very closely
allied to the Norway Lobster (_Nephrops_) of shallow water, have very
short and slender eye-stalks hidden under the rostrum, and showing at
the tip only the merest traces of what was once an eye. In the
lobster-like Eryonidea (see Fig. 46, p. 133), the reduced eye-stalks are
firmly fixed in notches in the front edge of the carapace. Some of the
deep-sea Crabs and Prawns seem also to be totally blind. In a great many
cases degeneration has not quite gone so far, and the eyes are present,
although much reduced and modified. Thus, the very numerous deep-sea
species of Galatheidæ, belonging to the genus _Munidopsis_ (Fig. 43) and
its allies have, as Alcock says, "pallid, milky-yellow, lack-lustre
eyes which, though they may perhaps serve to distinguish between light
and darkness, can never form a definite visual image." It is probable,
indeed, that these pale-coloured eyes are specially adapted for vision
in a dim light, for it has been shown that in certain deep-sea
Euphausiacea the pigment-sheaths between the separate elements of the
compound eyes are greatly reduced, and are fixed in the position
temporarily assumed by those in the eyes of normal Crustacea when kept
in the dark. Be this as it may, there are many deep-sea Crustacea which
have well-developed and darkly-pigmented eyes. Some of these are
swimming forms, which may at times migrate into the upper strata of
water to which some rays of light penetrate; but there are some cases of
Crabs and other bottom-living species that have well-developed eyes,
although they live at great depths. This would seem to suggest that,
although shut off from the light of day, they are not condemned to grope
in perpetual darkness. Many deep-sea animals are known to be
phosphorescent, and it seems probable that the large-eyed species may
profit by the light emitted by the glow-worms and fireflies of the
abyss. Thus, Alcock points out that the deep-sea Hermit Crab
_Parapagurus pilosimanus_ (Plate XVI.), which lives in partnership with
a colony of Sea-anemones which it carries about with it, has large eyes,
although it descends to depths of at least 2,000 fathoms; and he
suggests that the Crab may be able to see its way by the light emitted
by the zoophytes.

[Illustration: _PLATE XVI_

     DEEP-SEA HERMIT-CRAB, _Parapagurus pilosimanus_, SHELTERED BY A
     COLONY OF _Epizoanthus_. FROM DEEP WATER OFF THE WEST OF IRELAND
     (SLIGHTLY REDUCED)]

Some of the Crustacea, however, are themselves luminous. Thus, Alcock
records how specimens of a deep-sea Prawn, _Heterocarpus alphonsi_,
"poured out, apparently from the orifices of the 'green glands' at the
base of the antennæ, copious clouds of a ghostly blue light of
sufficient intensity to illuminate a bucket of sea-water so that all its
contents were visible in the clearest detail." Certain other Prawns are
known to possess special light-producing organs on various parts of the
body and limbs. It is in the Euphausiacea, however, that these organs
have been most fully examined, and although the members of this group
(see Fig. 24, p. 56) are by no means all deep-sea animals, some of them
occurring at the surface of the sea, the structure of their luminous
organs, or "photophores," may appropriately be described here. They are
situated on the under-surface of the abdomen, in the basal segments of
some of the thoracic legs, and on the upper surface of the eye-stalks.
Each consists of a globular capsule covered by a layer of pigment,
except on the outer side, where there is a transparent biconvex lens. In
the centre of the capsule is a peculiar "striated body" which seems to
be the actual seat of luminescence, and behind it is a concave reflector
composed of concentric lamellæ, and having a silvery lustre. Before
their luminosity was observed, these organs were described as "accessory
eyes," but there can be little doubt that they serve rather as
searchlights, although, from the positions that some of them occupy on
the body, it is not easy to see how they can illuminate objects within
range of the eyes. That the function of phosphorescent organs is not
always that of giving light for their possessor to see by is shown by
the fact that many luminous animals are blind. It is important to
notice, however, that these blind animals never have complex
"photophores" like those just described, but only exhibit a diffuse
luminosity or give off luminous secretions; as an example among
Crustacea, the blind Eryonidea (see Fig. 46, p. 133) may be mentioned,
one species of which was observed by Alcock to be "luminous at two
points between the last pair of thoracic legs, where there is a
triangular glandular patch." In a recent discussion of the whole
question of phosphorescence in marine organisms, Dr. Doflein concludes
that the part it plays in the life of the animal probably differs in the
different cases. In some it may serve to attract prey, as moths are
attracted to a candle; in others it may help individuals of the same
species to keep together in a swarm or to find their mates, the varying
arrangement of the photophores producing characteristic light-patterns
that serve as "recognition marks" like the colour-patterns of animals
that live in the light of day. The clouds of luminous secretion thrown
out by _Heterocarpus_ and other Prawns, and by certain Mysidacea and
Ostracods, may serve to baffle pursuers, like the cloud of ink thrown
out by a Cuttlefish, and in some cases the more complex organs may
illuminate objects within the range of vision. That this does not
exhaust the possibilities of speculation on the subject, however, is
shown by the case of certain deep-sea Prawns which have been recently
discovered to possess photophores placed so as to illuminate the
interior of the gill cavities. What function they can discharge in this
position seems beyond conjecture.

The colours of deep-sea Crustacea are very curious. Few of them have the
blanched appearance common, for instance, in animals that live in the
darkness of caves; on the contrary, their colours are often very vivid,
but they are nearly always uniform, without spots or markings, and in a
large proportion of cases are in some shade of red or orange. This red
colour seems to be associated, in some way that we do not understand,
with the darkness of their habitat. The general absence of markings is
very striking. Dr. Alcock remarks that in deep-sea Crustacea we never
see "those freaks of colour, or those labyrinthine mottlings and
dapplings, that excite our curiosity when handling the Crabs and Shrimps
of the reefs." Possibly the explanation of this may be that in these
dwellers in darkness colour is merely, as it were, an accident, a
by-product of physiological processes directed to other ends, not a
character of protective or warning value, as in animals that hunt and
are hunted in the light of day. It is a curious fact, which may have
some bearing on this problem, that in many cases, while the adults are
coloured in some shade of red, the eggs carried by the female are bright
blue or green.

Some of the peculiarities of structure observed in deep-sea Crustacea
seem to be correlated with the difficulties of resting or moving about
with security on the soft ooze of the sea-floor. Among the Crabs we find
a preponderance of long-legged species, not only among the true Spider
Crabs (Oxyrhyncha), but also in other groups (Dromiacea like
_Latreillia_, figured on Plate XIX., and Oxystomata), the members of
which assume the same spider-like form. In some cases the legs are
fringed with long stiff hairs, which may help to prevent the animal from
sinking in the ooze, and the spines on the body and legs of many species
may have the same effect. Among the deep-sea Prawns, the species of the
family Nematocarcinidæ (Plate XVII.) have extremely long and slender
legs, which we may assume to be used like stilts for walking over the
soft ooze.

[Illustration: _PLATE XVII_

     A DEEP-SEA PRAWN, _Nematocarcinus undulatipes_. (SLIGHTLY REDUCED)

                    (_From Brit. Mus. Guide_)]

Not much is known regarding the food of deep-sea animals. In the absence
of plant-life they must of necessity be all carnivorous, and all
ultimately dependent on the food-supply falling from above. Some
species have been found to have the food-canal filled with Globigerina
ooze, which they no doubt swallow, as earth-worms do the soil in which
they burrow, for the purpose of extracting the nutriment that it
contains. In one species of deep-sea Cumacea (_Platycuma holti_), which
appears to feed in this manner, the food-canal is coiled, a condition
very rare in Crustacea; in all probability this is due to the necessity
for an increase of the absorptive surface, since it is common to find
such an increase, either by lengthening and consequent coiling of the
gut, or by infolding of its walls, in animals that have to swallow large
quantities of relatively innutritious food material. Many species,
however, no doubt have more selective habits of feeding. The
lobster-like _Thaumastocheles_ (Fig. 44), which was dredged by the
_Challenger_ expedition in the West Indies at a depth of 450 fathoms,
and has since been got from deep water off the Japanese coast, has one
of the chelæ enormously enlarged, with long and slender fingers set with
spines like the teeth of a rake. It has been suggested that this
remarkable claw may be used for raking or sifting the ooze for small
animals on which the _Thaumastocheles_ feeds. A similar function may be
suggested for the long and spiny first pair of walking legs in the
Spider Crab _Platymaia_ (Fig. 45).

[Illustration: FIG. 44--_Thaumastocheles zaleucus._ REDUCED. (After
Spence Bate.)]

[Illustration: FIG. 45--A DEEP-SEA CRAB (_Platymaia wyville-thomsoni_).
REDUCED. (After Miers.)]

In many deep-sea Crustacea the eggs are of very large size, indicating
that the young are hatched in an advanced stage of development. For
example, in the numerous species of the genus _Munidopsis_ the eggs are
always large and correspondingly few in number, in striking contrast to
the closely allied genus _Galathea_, from shallow water, in which the
eggs are small and very numerous. Alcock mentions that a deep-sea Prawn
of the genus _Psathyrocaris_, although only about 3-1/2 inches long, has
eggs nearly a quarter of an inch in length. It would seem that, in some
way or other, the conditions are unfavourable for a free-swimming larval
life; but they cannot be altogether prohibitive, for there are a good
many characteristically deep-sea Crustacea, such as the Eryonidea, that
have small eggs and presumably a larval metamorphosis.

[Illustration: _PLATE XVIII_

     _Bathynomus giganteus_, ABOUT ONE-HALF NATURAL SIZE.

                    (_From Lankester's "Treatise on Zoology," after
                    Milne-Edwards and Bouvier_)]

The uniformity of the physical conditions over vast areas in the deep
sea is no doubt the cause of the enormously wide geographical range of
many species of deep-sea animals. There are many examples of this among
Crustacea, and they are added to by every deep-sea dredging expedition.
For example, the giant Isopod _Bathynomus_ (Plate XVIII.) was first
discovered in West Indian seas, and the same species has since been
dredged near Ceylon, while a second species has been found off the
Japanese coast. Of the strange lobster-like _Thaumastocheles_ (Fig.
44), mentioned above, only four specimens are known--one dredged by the
_Challenger_ in the West Indies, and three others more recently brought
from Japan.

The low temperatures prevailing in deep water, even in tropical seas,
render it possible for many Crustacea to live there which are closely
allied to, or identical with, species occurring in shallow water in the
colder seas of the North and South. Many examples of this are mentioned
by Dr. Alcock in his discussion of the deep-sea fauna of Indian seas;
for example, the Lobster _Nephrops andamanicus_, found at depths of 150
to 400 fathoms in the Indian seas, is very closely allied to the Norway
Lobster (_Nephrops norvegicus_) of our own coasts. To some extent this
fact affords an explanation of the phenomenon that has been called
"bipolarity" in the distribution of marine animals. It has been observed
that certain families, genera, and even species, are found in the Arctic
and Antarctic seas, although they seem to be entirely absent from the
intervening tropical zones. In some cases, however, it has been found
that these forms occur in the deep sea in the warmer regions where the
cold water offers them a connection between North and South without any
great difference of temperature.

In the early days of deep-sea exploration, when naturalists were
becoming aware of the rich fauna inhabiting the abysses of the ocean,
which till then had been supposed to be barren of all life, it was
confidently expected that representatives would be discovered of some of
the animals known as fossils from the earlier geological periods. It was
believed that the great ocean basins had remained unchanged for vast
periods of geological time, and that numerous "living fossils" would be
found surviving in the depths. These hopes have not been fully realized,
for the deep-sea fauna as a whole has proved to be of a comparatively
modern type; nevertheless, it does include a considerable number of
primitive and old-fashioned forms of life, some of which belong to
groups elsewhere extinct. This is conspicuously the case among the
Crustacea. The lobster-like Eryonidea, which at the present day are only
found in the deep sea, were long known as fossils before they were
discovered to survive as living animals. The existing species (Fig. 46)
are all blind, with only vestiges of eye-stalks, and they may be readily
distinguished by the fact that the first four, and sometimes all five,
pairs of legs end in chelæ, no other Decapods having more than three
pairs of chelate legs. The fossils occur in rocks of the Secondary
Period, from the Trias to the early Cretaceous. Some of them, at least,
had well-developed eyes, and probably lived in shallow water. This was
almost certainly the habitat of those (Fig. 47) that are found preserved
in a marvellously perfect state in the lithographic limestone of
Solenhofen (famous for the discovery of _Archæopteryx_ and many other
remarkable fossils), which is believed to have been deposited in a
lagoon. After the early part of the Cretaceous epoch, the Eryonidea are
no longer found as fossils, and it is, at all events, a probable
conjecture that about that period they forsook the shallow waters for
the deeper recesses of the ocean, where their descendants have held
their own till the present day.

[Illustration: FIG. 46--_Polycheles phosphorus_, ONE OF THE ERYONIDEA,
FEMALE, FROM THE INDIAN SEAS. (From British Museum Guide, after Alcock.)]

[Illustration: FIG. 47--_Eryon propinquus_, ONE OF THE FOSSIL ERYONIDEA,
FROM THE JURASSIC ROCKS OF SOLENHOFEN. (From Lankester's "Treatise on
Zoology," after Oppel.)]

Another group of deep-sea Crustacea which has affinities with certain
fossil forms is the little family Homolodromiidæ among the Crabs. It has
already been mentioned that the Dromiacea are the most primitive tribe
of the Brachyura, and Professor Bouvier has shown that among these the
Homolodromiidæ approach most nearly to the lobster-like forms from which
the Crabs have been derived. He has further shown that the members of
this family closely resemble in the arrangement of the grooves upon the
carapace the extinct Prosoponidæ, which are known as fossils from
Jurassic and Cretaceous rocks.

It is in the deep sea also that we find the curious Hermit Crabs of the
family _Pylochelidæ_ (Fig. 37, p. 94), which are perfectly symmetrical
and show no trace of having ever adopted the habit of living in
Gastropod shells; so primitive, indeed, are these forms that it is not
easy to find characters by which to define them from the lobster-like
Thalassinidea or from the true Lobsters themselves, and, although no
fossil representatives are yet known, there seems no reason to doubt
that the Pylochelidæ are nearly related to the primitive stock from
which the other Hermit Crabs have been evolved. Among the deep-sea
Prawns there are many forms, both of Penæidea and of Caridea, which are
more primitive than most of their relatives from shallow water; and
although in these cases also the geological records are faulty, we may
assume, if we cannot prove in detail, a general similarity to the fossil
Prawns of Mesozoic rocks.

When all has been said, however, perhaps the most surprising thing about
the deep-sea fauna is, not that the animals are unlike those living in
shallow water, but that they differ from them so little. When we
consider the physical conditions of the oceanic abysses--the absolute
darkness, the freezing cold, the pressure measured in tons on the square
inch--it would seem inevitable that the physiological processes of
deep-sea animals must differ greatly from those of animals living in
shallow water; yet in very many cases these differences of function are
accompanied only by the most trivial differences in structure. To take
one example, the "Pink Shrimp" (_Pandalus montagui_), which we may find
commonly between tide-marks on our own coasts, differs only in
inconspicuous details from species of the same genus living at a depth
of 600 fathoms; while other genera of the family Pandalidæ range
downwards to 2,000 fathoms or more, without any important divergences in
structure.




CHAPTER VII

FLOATING CRUSTACEA OF THE OPEN SEA


It is only rarely that the floating organisms of the surface of the sea
are so large or so abundant as to catch the attention of the casual
observer. Except for an occasional shoal of porpoises or of flying-fish,
the waste of waters seen from the deck of a ship in mid-ocean usually
seems to be barren of life. Nevertheless, there is probably no region of
the ocean where the tow-net will not reveal the existence of a more or
less varied fauna and flora. Sometimes, indeed, these organisms, though
minute, are so numerous as to discolour the water over large areas;
whalers in the Arctic seas know by the appearance of "whale-food" where
whales are likely to be found, and Herring or Mackerel fishermen
recognize the changes in colour of the water among the "signs" which
guide them when and where to shoot their nets.

The organisms which make up this "pelagic" fauna and flora may be
grouped into two classes, which may be termed the "swimmers," or
Necton, and the "drifters," or Plankton. The former include the larger
and more active animals, such as fish, whales, and the like, whose
movements are more or less independent of the movements of the water;
the latter comprise the plant-life and the floating or feebly swimming
animals that drift at the mercy of waves and currents. A great deal of
attention has been given in recent years to the study of the plankton,
and it has come to be recognized as filling a very important place in
the balance of life in the sea. In the sea, as on land, all the animals
are ultimately dependent on plants for their food. The larger and more
conspicuous sea-weeds which grow on the sea-bottom, however, can only
flourish in comparatively shallow water, and the region which they
occupy forms only a narrow fringe round the land-masses of the globe. It
is only necessary to look at a map of the world, showing the depth of
the sea, to realize what an insignificant part of the area of the oceans
contributes in this way to the food-supply of marine animals. The
microscopic plant-life of the plankton, however, makes up for the
individual minuteness of its constituents by their incalculable numbers.
The lowly organisms known as "diatoms," familiar to the microscopist
from the beauty of their flinty skeletons, are among the most numerous
and important of these, and they are associated with a great variety of
other single-celled algæ and allied organisms, some of them so minute
that they pass through the finest silk plankton-nets, and have to be
sought for by special methods of collection recently devised for the
purpose. All these organisms possess the green colouring matter
(chlorophyll) that enables them to live, as the higher plants do, on the
carbon dioxide and other substances dissolved in the water. The smaller
animals of the plankton feed on these vegetable organisms, and in their
turn serve as food for larger animals. The Herring, the Mackerel, the
gigantic Basking Shark, and the still more gigantic Greenland Whale, all
feed directly on the animal plankton, and we have already seen that the
animals of the deep sea depend entirely on the same source of
food-supply. Further, very many of the bottom-living animals of shallow
water swim at the surface in the early stages of their life, and feed on
the other plankton animals and plants. Indeed, it is no exaggeration to
say that "all fish is diatom" in the same physiological sense as "all
flesh is grass," and the study of the plankton is thus of practical
importance as well as of scientific interest.

Of all the minute animals that form the intermediate links in the chain
between diatom and fish or whale, the Crustacea are the most important
and the most numerous both in species and in individuals. The Copepoda
are more richly represented than any of the other groups, and it would
be difficult to find a sample of marine plankton from which they were
altogether absent. Associated with them we find one or two species of
Cladocera, a larger number of Ostracoda (chiefly of the family
Halocypridæ), a few Mysidacea, the Amphipoda of the suborder Hyperiidea,
the Euphausiacea, and some of the shrimp-like Decapods; while the larval
stages of these and other groups also form an important part of the
plankton.

It is necessary to make a distinction between the "neritic" plankton of
shallow water near the coast and the "oceanic" plankton of the open sea.
In the inshore waters the plankton consists not only of organisms that
pass the whole of their life at or near the surface, but also, and very
largely, of the free-swimming larvæ of bottom-living species, and of
others that make occasional and temporary excursions to the surface. For
example, if the tow-net be used a short distance from land--say in some
sheltered bay on our own coasts--the catch will often be found to
consist largely of larval Crustacea. The zoëa and megalopa stages of
Crabs, the zoëa and schizopod stages of Prawns and Shrimps, are often
conspicuous by their numbers, or we may find swarms of the nauplius and
cypris larvæ of Barnacles. Sometimes, and especially at night, numbers
of Cumacea may be found in the tow-net; and it is noteworthy that these
are usually males, which leave the females burrowing in the mud at the
bottom, and swarm to the surface for a brief period of activity.
Besides all these more or less temporary visitors, however, there are
numerous species, even in the inshore waters, which are adapted to a
floating life, and pass their whole existence as members of the
plankton. Copepoda of many kinds, some Mysidæ, Amphipods like
_Hyperia_--which is commonly found sheltering under large
jellyfish--some species of actively swimming Isopods, and many other
forms, are only to be captured by the tow-net; and now and then, in
certain localities, winds and currents may drive into coastal waters
shoals of species whose proper home is the open ocean.

In a similar way the strictly neritic forms may sometimes be carried far
out to sea, so that it is nowhere possible to draw a hard-and-fast line
between the regions occupied by the neritic and the oceanic plankton.
With increasing distance from land, however, the larval stages of
bottom-living species become fewer, and finally disappear altogether,
and there is left an assemblage of animals whose whole existence is
passed floating at the surface or at the intermediate depths. How far
down from the surface this floating fauna actually descends is a
question which has been much debated. It appears now to be certain that
there is no stratum of water between the surface and the bottom of the
ocean which is devoid of life, although the upper layers (not at, but
some distance below, the surface) are probably much more densely
populated than those of the abyss. Many of the species appear to
undertake more or less extensive migrations in a vertical direction,
coming nearer the surface at certain stages of their life-history, and
sinking into deeper water at others. Further, some species at least seem
to rise to the surface at night, and to sink again during the day. Apart
from these vertical movements, which are as yet only imperfectly
understood, it is desirable to distinguish between the "epiplankton,"
comprising the organisms which inhabit the superficial strata of the
ocean down to about 100 fathoms, and the "mesoplankton," found at
greater depths. The plant-life which is dependent on sunlight belongs to
the epiplankton, while the animals of the mesoplankton are dependent,
like the bottom animals of the deep sea, on the supply of dead food
material falling from above. A third division, the "hypoplankton," has
been established for those animals which live immediately above the
bottom, but its distinctness from the mesoplankton has not yet been
satisfactorily established. Indeed, many of the swimming forms which
have already been mentioned in dealing with the Crustacea of the deep
sea are probably rather to be considered as belonging to the deep
mesoplankton--at least, where their size and swimming powers do not
entitle them to be ranked with the "necton."

[Illustration: FIG. 48--_Conchoecia curta_, AN OSTRACOD OF THE PLANKTON.
× 40. (Partly after G. W. Müller.)]

Many of the modifications in structure characteristic of pelagic
animals may be traced to the necessity for keeping continuously afloat
with a minimum of exertion. The Crustacea of the plankton never carry
the heavy armour found in bottom-living species. Thus, the thick-shelled
Ostracoda of the bottom are represented in the plankton chiefly by the
family Halocypridæ (Fig. 48), in which the shell is thin, uncalcified,
and almost membranous. Many species, particularly of the Copepoda, are
seen, under the microscope, to have large globules of oil distributed
through the tissues of the body, and these no doubt serve as floats,
increasing the buoyancy of the animal. The same purpose is probably
served, in many cases, by having large spaces, filled with fluid, within
the body. This is characteristic of pelagic animals, and is well seen in
many of the Crustacea in which the viscera and muscles occupy a
relatively small part of the interior of the animals, the intervening
spaces being filled with colourless transparent fluid. Many of the
Hyperid Amphipoda show this peculiarity--for example, the relatively
gigantic _Cystisoma_, which is mesoplanktonic in deep water; and it
reaches its extreme in _Mimonectes_ (Fig. 49), in which the anterior
part of the body is, as it were, blown out into a balloon, giving the
animal the aspect of a small jellyfish rather than an Amphipod.

[Illustration: FIG. 49--_Mimonectes loveni._ A FEMALE SPECIMEN SEEN FROM
THE SIDE AND FROM BELOW, SHOWING THE DISTENDED-BALLOON-LIKE FORM OF THE
ANTERIOR PART OF THE BODY. × 3. (After Bovallius.)]

If, as seems probable, the body-fluid of these animals is of a lower
specific gravity than the sea-water, it will act like the oil-globules
of the Copepoda in keeping the animals afloat. Even if the specific
gravity be the same, however, the distension of the body with fluid
acts in another way, by increasing the surface exposed to friction with
the surrounding water, and so retarding sinking. The principle involved
is illustrated by the fact that a soap-bubble sinks much more slowly
through the air than the drop of water into which it collapses. The
same result is produced if the surface is increased by outstanding
spines or hairs, just as, for instance, a downy feather sinks slowly
through the air, but drops rapidly if it is rolled into a ball between
the fingers. This is, no doubt, one function of the spines with which
plankton Crustacea, and particularly larvæ, are frequently provided,
though they may also serve in some cases as a protection against
enemies. The spines have been already alluded to in describing the
various larvæ, but it may be noted here that they are most strongly
developed in larvæ which live in the open ocean; for example, the most
elaborately armed of all Decapod larvæ are the zoëa stages of
_Sergestes_ (Fig. 50), which, like the adults, belong to the oceanic
plankton. The nauplius larvæ of Cirripedes are all more or less spiny,
and the spines reach an exaggerated development in the larvæ of the
genus _Lepas_ (Fig. 51), of which the adults are attached to floating
drift-wood or the like, and belong to the oceanic fauna, although hardly
to be classed with the plankton.

[Illustration: FIG. 50--THE ZOËA LARVA OF A SPECIES OF _Sergestes_,
TAKEN BY THE "CHALLENGER" EXPEDITION. × 25. (After Spence Bate.)]

[Illustration: FIG. 51--THE NAUPLIUS LARVA OF A SPECIES OF BARNACLE OF
THE FAMILY LEPADIDÆ, SHOWING GREATLY-DEVELOPED SPINES. FROM A SPECIMEN
TAKEN IN THE ATLANTIC OCEAN, NEAR MADEIRA. × 11. (After Chun.)]

The large feathered bristles that decorate the limbs or tail of many
plankton Copepoda have no doubt the same function in assisting
flotation. In the genus _Calocalanus_ (Fig. 52), for example, the tail
setæ are large and brilliantly coloured feathery plumes, and in one
species, _C. plumulosus_, one of these setæ is of relatively enormous
size, five or six times as long as the body of the animal itself.

[Illustration: FIG. 52--_Calocalanus pavo_, ONE OF THE FREE-SWIMMING
COPEPODA OF THE PLANKTON. ENLARGED. (From Lankester's "Treatise on
Zoology," after Giesbrecht.)]

Among the most singular of plankton Crustacea are the _Phyllosoma_ larvæ
(see Fig. 28, p. 72) of the Spiny Lobsters and their allies
(Scyllaridea), which have been already described. These larvæ are
sometimes found far out at sea, and it seems likely that their larval
life is unusually prolonged, and that they may be drifted to great
distances by ocean currents. At all events, they are well adapted for
pelagic life, since the broad flat body, hardly thicker than a sheet of
paper, can be sustained in the water like a "hydroplane" by
comparatively slight efforts of the swimming legs.

The watery character of the body, together with the thinness of the
exoskeleton, helps to explain the glassy transparency which is a feature
of most plankton Crustacea. This transparency has been regarded as a
protective adaptation rendering the animals inconspicuous in the water,
and it has indeed that effect to human eyes, but it is very doubtful
whether the animals derive much benefit from this. Many of the
animals--such as Herring and other pelagic fishes--that prey upon
plankton Crustacea appear to swallow them in bulk, without much
selection; and the Greenland Whale, as it swims open-mouthed through the
sea, is not likely to be guided by the greater or less visibility of the
Copepods that it sifts out on its baleen plates. Further, this
glass-like transparency is by no means universal, for many plankton
Copepoda are brightly coloured. In some, as in the beautiful blue
_Anomalocera_, common in British waters, the colour is due to pigment in
the fluids and tissues of the body; in others the feathery hairs on the
body and limbs show brilliant metallic colours, produced, like the
colours of a peacock's feather, not by pigments, but by the diffraction
of light in the texture of the organ. The most beautiful of all Copepoda
is _Sapphirina_, in which the surface of the body absolutely sparkles
with iridescent colours.

The striking phenomenon known as the "phosphorescence of the sea" is
familiar to every ocean voyager, and is seen from time to time on our
own coast. On a dark night the crest of every wave often seems to break
in a pale glow, the wake of the vessel is a trail of light, and an oar
dipped in the water seems on fire. This luminosity is due to the animals
of the plankton, largely to the lowly Protozoa and the jellyfishes, but
in part also to certain Crustacea. A number of pelagic Copepoda have
been shown by Giesbrecht to secrete, from special glands on the surface
of the body, a substance which becomes luminous on coming in contact
with the water. Even specimens which had been dried were found to give
out light on being wetted. Some pelagic Ostracods of the family
Halocypridæ have been observed to emit clouds of a luminous secretion
from a gland in the neighbourhood of the mouth. A similar habit has been
seen, as already mentioned, in certain deep-sea Prawns and Mysidacea,
which may perhaps belong to the deeper part of the mesoplankton rather
than to the bottom fauna. The complex light-producing organs of the
Euphausiacea have already been described in dealing with deep-sea
Crustacea. A great many species of this group, however, are members of
the epiplankton, and in these the phosphorescent apparatus is quite as
fully developed as in species coming from greater depths.
_Meganyctiphanes norvegica_ (Fig. 24, p. 56), which is one of the
largest of the Euphausiacea, is common at no great depths in many places
in British seas. If a jar of sea-water in which specimens of this
species are swimming be brought into a dark room, a tap on the glass
will cause the photophores to flash out like a row of tiny lamps along
the side of the body. After shining for a few seconds the light dies
out, to appear again if the tapping be repeated.

There are certain peculiarities in the structure of the eyes in some
plankton Crustacea which suggest that the sense of sight is of special
importance to their possessors, although we can hardly do more than
guess at their special significance. Most Copepoda have only a single
eye in the middle of the head, corresponding to the single eye of the
nauplius larva, and of far simpler structure than the paired compound
eyes of most other Crustacea. In many plankton species, however, this
simple eye becomes much enlarged and complicated in various ways. The
three parts of which it is normally made up may become separated from
each other, and are sometimes increased in number to five, while lenses
serving to concentrate the light are often developed by thickening of
the overlying cuticle. The most elaborately constructed eyes are found
in the family Corycæidæ. In _Copilia_ (Fig. 53) a pair of eyes of
relatively enormous size are present. Each has in front a large biconvex
lens set at the end of a conical tube which extends backwards to a
smaller lens (like a telescope with object-glass and eyepiece), behind
which, again, are the sensory cells, corresponding to the retina,
enclosed in a tube of dark pigment, the whole apparatus being more than
half the length of the body. These eyes, although paired, do not
correspond to the paired compound eyes of other Crustacea, but have
arisen by the separation and enlargement of two of the three divisions
of the typical median Copepod eye.

[Illustration: FIG. 53--_Copilia quadrata_ (FEMALE), A COPEPOD OF THE
FAMILY CORYCÆIDÆ, SHOWING THE PAIR OF LARGE "TELESCOPIC" EYES. x 20.
(After Giesbrecht.)]

A peculiarity of the paired compound eyes found in plankton Crustacea of
several different orders consists in the division of each eye into two
parts, which differ in structure. In many Euphausiacea and Mysidacea,
especially in those haunting the deeper strata (mesoplankton), this
division of the eyes is well marked, a frontal or dorsal part having the
separate elements of the eye (ommatidia) greatly lengthened and with
reduced pigment, while the lateral part is of more normal structure. It
seems probable, from the researches of Professor Chun, that the
fronto-dorsal division is adapted for the perception of very faint
light, while the lateral division will give a more accurate image of
brightly illuminated objects.

[Illustration: FIG. 54--_Phronima colletti_, MALE. FROM A SPECIMEN TAKEN
IN DEEP WATER NEAR THE CANARY ISLANDS. × 12. (After Chun.)]

In the pelagic Amphipoda, forming the suborder Hyperiidea, the eyes are
of very large size, generally occupying almost the whole surface of the
head, and giving the animals a very characteristic appearance, in
contrast to the small-eyed, bottom-living Gammaridea. In the family
Phronimidæ (Fig. 54) the eyes are each divided into two parts, differing
in structure in the way just described.

There are a few Crustacea living habitually on the high seas which
cannot be reckoned as belonging either to the true plankton or to the
necton, since they depend on outside help for keeping themselves afloat.
Among these are the Barnacles which cluster on logs of drift-wood, and
are among the most important causes of the "fouling" of ships' hulls on
long voyages. The stalked Barnacles of the genus _Lepas_ are especially
common in such situations, and the characters of their larvæ have been
already alluded to. Certain species of sessile Barnacles are constantly
found attached to large marine animals. For example, _Chelonobia_
adheres to the shell of Turtles, while _Coronula_ and some allied genera
are found on Whales.

[Illustration: _PLATE XIX_

     _Latreillia elegans_, ONE OF THE DROMIACEA WHICH RESEMBLES A
     SPIDER-CRAB. FROM THE MEDITERRANEAN. (NATURAL SIZE)

     THE GULF-WEED CRAB, _Planes minutus_. (SLIGHTLY ENLARGED)]

The little "Gulf-weed Crab" (_Planes minutus_--Plate XIX.) is found
clinging to floating drift-weed nearly everywhere throughout the
temperate and tropical seas of the globe, and is especially common in
the area known as the Sargasso Sea, in mid-Atlantic. It is occasionally
drifted to the south coasts of the British Islands. In Sloane's "Natural
History of Jamaica," published in 1707-1725, it is stated of the
Gulf-weed Crab that "Columbus, finding this alive on the Sargasso
floating in the sea, conceived himself not far from some land, on the
first voyage he made on the discovery of the West Indies."

A few other Crustacea also form part of the peculiar fauna which is
associated with the Sargasso weed, notably a swimming Crab, _Neptunus
sayi_, and two or three species of Prawns. All of these are coloured
olive-green, like the weed among which they live.




CHAPTER VIII

CRUSTACEA OF FRESH WATERS


The Crustacean fauna of fresh water is much less rich and varied than
that of the sea. Although the number of individuals in a pond or lake
may be enormous, they will be found to belong to a comparatively small
number of species. All the subclasses of Crustacea with the exception of
the Cirripedia have representatives in fresh water, but in most of them
only a very few of the families and genera comprise truly fresh-water
species. In spite of the comparative poverty of the fauna, however, it
is of very great interest, more especially with regard to the problems
of geographical distribution; and the ease with which specimens may be
collected everywhere, and kept in small aquaria, renders it a
particularly attractive subject of study for the amateur naturalist.

The general uniformity of the fresh-water fauna throughout the world has
often been remarked. Darwin says: "When first collecting in the fresh
waters of Brazil, I well remember feeling much surprise at the
similarity of the fresh-water insects, shells, etc., and at the
dissimilarity of the surrounding terrestrial beings, compared with those
of Britain." This uniformity is well illustrated by many of the smaller
Crustacea. In a gathering of Cladocera, Copepoda, and Ostracoda, from
Central Africa or from Australia, we find that most of the genera, and
even some of the species, are identical with those found in similar
situations in this country. It is by no means the case that all the
species and genera are thus universally distributed, for there are many,
especially among the larger forms, which have a very restricted range;
but this does not render less striking the general uniformity of the
fauna over very wide areas.

When we consider the physical environment of fresh-water animals, it
seems at first sight as if this wide distribution were the reverse of
what might have been expected, for the area occupied by them is far more
discontinuous than in the case of terrestrial or marine animals. The
inhabitants of a pond or lake are to a great extent isolated; and
although they may spread to other ponds and lakes by way of
communicating streams or rivers, where these are not too swiftly flowing
and are not interrupted by falls, yet direct passage from one river
system to another is rarely possible. Further, since practically the
whole of the fresh water on the surface of the globe is constantly
flowing, more or less rapidly, towards the sea, the smaller feebly
swimming forms tend to be swept down with the current, and ultimately
carried to perish in the sea. It follows that only those forms which
possess special adaptations for dispersal are able to flourish in fresh
water. In many cases, as will be described below, the eggs of the
smaller Crustacea can survive being dried up, and in this state they may
be blown about by wind or carried to great distances in mud, adhering to
the feet of migratory wading birds. Darwin says: "The wide-ranging power
of fresh-water productions can, I think, in most cases be explained by
their having become fitted, in a manner highly useful to them, for short
and frequent migrations from pond to pond, or from stream to stream,
within their own countries; and liability to wide dispersal would follow
from this capacity as an almost necessary consequence" ("Origin of
Species," sixth edition, chapter xiii.). In accordance with this, we
find that it is just those groups of Crustacea which show these
adaptations for dispersal that are most universally distributed in fresh
water. On the other hand, the larger Crustacea, like the Crayfishes and
River Crabs, which cannot so easily be transported from one locality to
another, have as a rule a more restricted range. These larger forms,
from their size and powers of swimming or creeping, can make their way
upstream and spread throughout a river system, and in some cases they
can leave the water and journey for short distances overland. On the
other hand, since free-swimming larvæ would be liable to be swept out to
sea, most of them have a direct development, the young only leaving the
protection of the mother when they have attained the form and habits of
the adult. When all these factors have been taken into account, however,
there still remain many cases where the distribution of individual
species or of groups is hard to explain, and shows indications of dating
from a time when the outlines of continents and the connections of river
systems were different from what they are now.

Before proceeding to mention some of the more characteristic forms of
fresh-water Crustacea, it should be mentioned that in large lakes, as in
the sea, we can distinguish a littoral fauna in the shallow waters close
to the shore, a plankton fauna of the surface waters, and a deep-water
fauna. The littoral fauna does not differ in general characters from
that found in smaller ponds and gently-flowing rivers; the plankton
comprises many peculiar species showing adaptations for flotation, as in
the case of the marine plankton; and the deep-water fauna is very poor
in species and in individuals, and shows some relations with the
subterranean fauna to be mentioned later.

Of all the subclasses of Crustacea, the Branchiopoda are the most
characteristically fresh-water animals, only a few Cladocera being found
in the sea, and some Anostraca in salt lakes and brine pools.

The larger Branchiopoda (Anostraca, Notostraca, and Conchostraca) are
generally found in small, shallow ponds which are liable to be dried up
in summer. The "Fairy Shrimp" (_Chirocephalus diaphanus_; see Fig. 10,
p. 35) has been found in swarms in the water standing in deep cart-ruts
in a country lane in England, and _Apus_ sometimes appears suddenly in
rain-water puddles of a few square yards in area, which dry up after a
few weeks of hot weather. The eggs of these animals, when dried in the
mud, may remain dormant for long periods, and many species have been
hatched out from samples of dried mud brought by travellers from distant
countries. In such a sample from the Pool of Gihon at Jerusalem, it is
recorded that the eggs of _Estheria_ (see Fig. 11, p. 36) were found to
be capable of hatching after being kept dry for nine years. In some
species it is said that the eggs will not develop unless they have been
first dried, but this is not the case with _Chirocephalus_. In
favourable conditions development takes place very rapidly. Messrs.
Spencer and Hall, in describing the Branchiopoda of Central Australia,
say: "Certainly not more than two weeks after a fall of rain, and
probably only a few days, numberless specimens of _Apus_, measuring in
all about 2-1/2 to 3 inches in length, were swimming about; and, as not
a single one was to be found in the water-pools prior to the rain,
these must have been developed from the egg."

From what has been said, it is apparent that the larger Branchiopoda are
particularly well fitted to be distributed by the agency of birds, and
this is no doubt the explanation of the way in which many of the species
suddenly appear in localities where they were previously unknown, and,
after swarming for a longer or shorter time, sometimes for several
successive seasons, as suddenly vanish. A striking example of this is
afforded by _Apus cancriformis_ (see Plate II.), which formerly occurred
in several localities in the South of England, and appears more or less
irregularly in many parts of the Continent of Europe. No British
specimens had been recorded for over forty years, and the species was
believed to be extinct in this country, when it was found in 1907 by Mr.
F. Balfour Browne in a brackish marsh near Southwick, in
Kirkcudbrightshire. It can hardly be supposed that so large an animal as
_Apus_, and one so easily recognized, would have escaped notice
altogether had it occurred regularly in any part of the British Islands.
It is much more probable that the Scottish specimens found in 1907 had
developed from eggs accidentally transported by some bird from the
Continent. In 1908 a careful search in the same locality failed to
reveal a solitary specimen.

The Anostraca and Notostraca usually swim with the back downwards.
Particles of mud and of animal and vegetable matter are drawn by the
currents produced in swimming, into the ventral groove between the pairs
of feet, and are passed forwards to the mouth to serve as food. Some
species of Conchostraca are said to swim in the same inverted position;
but Messrs. Spencer and Hall, in the memoir already quoted, state that
the Australian Conchostraca swim back uppermost. They attribute the
difference in habit between the Conchostraca and Notostraca to the fact
that in the former group the valves of the shell can be rapidly closed
to protect the soft and vulnerable appendages, while no such protection
is possible in the Notostraca. They found on one occasion a specimen of
_Apus_ (Notostraca) attacked by three Water-beetles, which were tearing
its soft appendages, and they suppose that _Apus_ generally escapes such
attacks by swimming upside down.

The breeding habits of the Branchiopoda are also of interest, from the
prevalence in many species of reproduction by unfertilized eggs, or
"parthenogenesis." This may go on for many generations, and in _Apus_,
for instance, it is possible to examine thousands of specimens before
finding a single male, although, for some unexplained reason, males are
sometimes comparatively common. It is probable that males must appear
sooner or later, otherwise the series of parthenogenetic generations
will come to an end; but it is not certain that this is the case, and
there are some species of Conchostraca of which the males have never
been seen.

[Illustration: FIG. 55--THE BRINE SHRIMP (_Artemia salina_). (After
Sars.)

A, Female, under-side, × 6; B, head of male, upper side, further
enlarged, showing the large clasping antennæ. The larval stages of this
species are shown in Fig. 33, p. 81]

The genus _Artemia_ (Fig. 55), among the Anostraca, is peculiar in its
habitat; for, while most of the Branchiopoda inhabit fresh or brackish
water, it flourishes in concentrated brine. In the South of Europe it is
found, as it was formerly in England, in the shallow ponds in which
sea-water is exposed to evaporation for the manufacture of salt, and in
these it occurs in such numbers as to give the water a reddish colour.
It is also found in salt lakes, like the Great Salt Lake of Utah, in the
United States, and in many other parts of the world. The specimens from
different localities often differ considerably, especially in the form
of the tail-lobes; but it has been shown that these differences are more
or less directly correlated with the degree of salinity of the water in
which the animals live, and it is probable that the forms which have
been described are all variations of a single cosmopolitan species
ranging from Greenland to Australia, and from the West Indies to Central
Asia. _Artemia_ is the only one of the Anostraca that is known to be
parthenogenetic, some colonies consisting entirely of females, while in
others males are abundant. The reddish colour above alluded to is found
also in _Branchipus_, _Apus_, and other Branchiopoda, and is due, as Sir
Ray Lankester first showed, to the presence in the body-fluids of
hæmoglobin, the red colouring matter of the blood of Vertebrates, which
is important in the process of respiration.

[Illustration: FIG. 56--_Chydorus sphæricus_, A COMMON SPECIES OF
WATER-FLEA. × 50. (After Lilljeborg.)]

The smaller Branchiopoda known as "Water-fleas," forming the order
Cladocera, are abundant everywhere in fresh water. _Daphnia pulex_ and
other species of the genus, and the little Lynceidæ, of which _Chydorus
sphæricus_ (Fig. 56) is the commonest species, are to be found in ponds
and ditches, and often swarm in farmyard ponds where the water is foul
with decaying matter. In most gatherings from such localities only
female specimens will be found, and nearly all of these will be seen to
carry a cluster of eggs or of developing embryos in the "brood-chamber"
between the back part of the body and the shell. In _Daphnia pulex_ (see
Fig. 12, p. 37) a single brood may consist of thirty young, and
occasionally of more than twice that number. As the broods may succeed
each other at intervals of two or three days, it will be seen that the
multiplication of the species in favourable circumstances may be
exceedingly rapid. It has been calculated that in sixty days the progeny
of a single female might amount to about 13,000,000,000. In addition to
these parthenogenetic eggs, which hatch at once while still within the
brood-chamber, the Cladocera produce, at certain seasons, another kind
of egg which requires to be fertilized by the male before it will
develop. These eggs are dark in colour and are enclosed in a thick
shell, and they do not hatch at once, but are cast off when the shell of
the female is moulted. Very commonly these "resting eggs," as they are
called, are produced in the autumn and lie dormant until the following
spring, and they can survive drying or freezing without injury, while
the thin-shelled parthenogenetic eggs within the brood-chamber of the
mother are easily killed. In addition to having thick shells, the
resting eggs are further protected in most, but not in all, cases by the
moulted carapace of the parent, which is specially thickened for the
purpose. This modification of the carapace is most highly developed in
the family Daphniidæ (Fig. 57), where a saddle-shaped area on the dorsal
side, known as the "ephippium," becomes thickened, and on moulting
separates from the rest of the carapace to form a compact case enclosing
the two resting eggs. The outer wall of the ephippium is divided up into
small hexagonal cells, which become filled with air, causing the
ephippium to float at the surface of the water. In this position the
ephippia readily become entangled in the feathers of birds, and in some
cases the shell is provided with spines or hooks, which facilitate
transport to other localities by such means.

[Illustration: FIG. 57--A WATER-FLEA, (_Daphnia pulex_), FEMALE, WITH
EPHIPPIUM CONTAINING TWO "RESTING EGGS." × 20. (Partly after Lilljeborg.)

The Antenna is cut short. Compare Fig. 12, p. 37.]

The appearance of males and the production of ephippial eggs--in other
words, the "sexual period"--is generally more or less restricted to one
season of the year. In most species, particularly in those which live
in lakes, the sexual period occurs in the late autumn, and the ephippial
eggs lie dormant during the winter, and hatch in the spring. In species
living in small ponds exposed to the risk of overheating or of drying up
during summer, there is often a distinct sexual period in the spring,
when ephippial eggs are produced to tide over the unfavourable
conditions of the warmer months of the year. Although no species is
known to be exclusively parthenogenetic, yet it appears that purely
parthenogenetic colonies of certain species may be found in favourable
localities, where they may reproduce from year to year without males
ever being found.

Certain species of Cladocera belong to the plankton of lakes and large
ponds, and show modifications which adapt them for a floating life. Some
of these belong to the genus _Daphnia_, and differ from the species
found in other situations by their glassy transparency. As in the case
of many marine plankton Crustacea, this transparency is probably due to
the thinness of the shell and to the general watery condition of the
body, giving the necessary buoyancy to enable the animal to remain
constantly afloat. The same effect is no doubt produced by the long
terminal spine of the carapace and by the great helmet-shaped crest into
which the upper part of the head is often produced. A form very
characteristic of the plankton of large lakes is _Bythotrephes_ (Fig.
58), which is found in the lakes of Scotland, Ireland, Wales, and the
Lake District of England. In _Bythotrephes_ the carapace does not
enclose the body, but is reduced to a small brood-sac; the abdomen,
however, is drawn out into a long spine, which may be two or three times
as long as the body. A further point of interest is the division of the
eye into a dorsal and a ventral portion, differing in structure in much
the same way as do the two divisions of the eyes in certain marine
plankton Crustacea (see p. 152). Another very remarkable lacustrine form
is _Leptodora_, the largest of all the Cladocera, being sometimes more
than half an inch in length. In this case also the carapace is very
small, and does not enclose the body. The swimming antennæ are very
large, and the abdomen is long and divided into several segments.

[Illustration: FIG. 58--_Bythotrephes longimanus_, FEMALE, WITH EMBRYOS
IN THE BROOD-SAC. × 12. (After Lilljeborg.)]

_Leptodora_ is further remarkable on account of its mode of development.
The parthenogenetic eggs, as in other Cladocera, develop directly, but
the resting eggs give rise to larvæ of the nauplius type.

_Holopedium_, which is found in similar situations, surrounds itself
with a mass of a jelly-like substance which it secretes. A similar
envelope of jelly is found in some marine plankton animals, though not,
so far as is known, in any Crustacea, and it no doubt serves to give
buoyancy to the animal.

The Copepoda of fresh water are as abundant and universally distributed
as the Cladocera. Species of the genus _Cyclops_ (see Fig. 14, p. 39),
easily recognized by the pear-shaped body and the two egg-packets
carried by the female, are to be found in almost every pond and ditch.
The genus _Canthocamptus_ comprises species of smaller size, with
slender, flexible body, and carrying only a single egg-packet. The
plankton of lakes and ponds includes species of _Diaptomus_ (Fig. 59),
which have a narrow body and very long antennules. The latter are held
out stiffly while the animal swims by rapid movements of the antennæ and
mouth parts, making occasional sudden leaps by means of its oar-like
feet. In this genus also the egg-packet is single. The development can
easily be studied by keeping egg-carrying females of _Cyclops_ in a jar
of water, when the nauplius larvæ will soon hatch out.

[Illustration: FIG. 59--_Diaptomus coeruleus_, FEMALE. × 25. (After
Schmeil.)]

Although the Copepoda, unlike the Cladocera, are not parthenogenetic, it
has been found that certain species of _Diaptomus_ produce resting eggs
capable of surviving freezing or drying. In the early part of the
breeding season the eggs have thin shells, and they hatch after a short
time. In the autumn, however, thick-shelled eggs are produced, which lie
dormant in the mud until the following spring. It has recently been
discovered that species of _Cyclops_ and _Canthocamptus_ pass through a
resting stage, in which the animal surrounds itself with a cocoon-like
capsule of mud held together by a glutinous secretion produced by glands
on the surface of the body and limbs. The encapsuled animals, in the
cases observed, lie dormant in the mud during the summer, to resume
active life in the colder months of the year. It is very probable that
they can also be dried without injury, and that the "cocoons" serve the
same purpose as the resting eggs of other species.

Numerous species of Ostracods, belonging to the genus _Cypris_ (see Fig.
13, B, p. 38), and other closely related genera, occur in fresh water.
Like the Cladocera, they reproduce largely by parthenogenesis, and the
males of many species are rarely found, while in some species they have
not yet been discovered. In Professor Weismann's laboratory at Freiburg
a colony of _Cypris_ was kept in an aquarium for eight years, and during
the whole of that time no males made their appearance, the colony
reproducing exclusively by parthenogenesis. Probably in all species the
eggs survive drying.

The common "Freshwater Shrimp" (_Gammarus pulex_), which has already
been described, may be taken as a type of a large number of Amphipoda,
for the most part closely allied, which are widely distributed in most
regions of the world, with the exception of the tropics. _G. pulex_
itself ranges from the British Islands to Mongolia. As the eggs are
carried, till they hatch, in the brood-pouch of the parent, and are not
known to survive drying, it is difficult to understand in what way
_Gammarus_ and its allies contrive to spread from one locality to
another.

The little fresh-water Isopod _Asellus aquaticus_ (Fig. 60) is common in
ponds and canals in this country. It may be recognized by its general
resemblance to a Woodlouse, with very long antennæ, and with a pair of
long, slender, forked uropods projecting behind. The species is widely
distributed in Europe, and other species of the same and closely related
genera are found in North America.

[Illustration: FIG. 60--_Asellus aquaticus_, FEMALE. × 4. (After Sars.)]

In Australia and New Zealand the Isopoda are represented in fresh waters
by a very peculiar group of species, forming the suborder Phreatoicidea,
which have more the aspect of Amphipods than of Isopods, since the body
is more or less flattened from side to side, instead of from above
downwards.

With regard to the mode of distribution of the fresh-water Isopoda,
there is the same difficulty as in the case of the Amphipoda, for the
eggs are carried in a brood-pouch, and do not seem to be in any way
protected against drought. It is no doubt in consequence of this that
the fresh-water species and genera of both Amphipoda and Isopoda, though
widely distributed, do not have the world-wide range of many of the more
minute Crustacea described above.

The common Crayfish, _Astacus_ (or _Potamobius_) _pallipes_, is the only
truly fresh-water Decapod found in England, although a small Prawn,
_Palæmonetes varians_, which usually inhabits brackish water, may
occasionally be found in places where the water is practically fresh.
The structure of the Crayfish is very similar to that of the Lobster,
but, as already mentioned, it differs in its mode of development, having
no free-swimming larval stage. From its size, and from the fact that the
eggs are carried by the female, the Crayfish cannot be transported from
one locality to another by the agencies which distribute the smaller
fresh-water Crustacea. On the other hand, the adult animals can live out
of the water for days, or even weeks, if they are kept moist, and the
English species is stated to leave the water occasionally, and to make
short excursions on land. Many species found in foreign countries are
still more truly amphibious in their habits. It is clear, however, that
the means of dispersal of the Crayfishes are very limited, and on this
account the problems connected with their geographical distribution
are of great interest. An admirable discussion of the subject will be
found in Professor Huxley's book on the Crayfish, and the conclusions
reached by him have hardly been modified by thirty years of subsequent
research. Only a very brief outline can be attempted here.

[Illustration: FIG. 61--MAP SHOWING THE DISTRIBUTION OF CRAYFISHES.
(Partly after Ortmann.)

The dotted areas are those occupied by the Northern Crayfishes (family
Astacidæ). The black patches mark the areas inhabited by the Southern
Crayfishes (family Parastacidæ)]

Crayfishes are found in the fresh waters of the Northern and Southern
Hemispheres (Fig. 61), but in each case they are practically confined to
the temperate regions, and are absent from a broad intervening tropical
zone. The Northern Crayfishes, forming the family Astacidæ (or
Potamobiidæ) are distinguished, among other characters, by having a pair
of appendages on the first abdominal somite, at least in the male sex;
the Southern Crayfishes have no appendages on that somite, and for this
and other reasons are regarded as constituting a distinct
family--Parastacidæ. There is thus a general correspondence between the
geographical distribution of the Crayfishes and the more important
structural differences expressed in their classification. There can be
no doubt that the two families have been derived from a common stock of
marine lobster-like animals, and it is reasonable to suppose that two
branches of this stock became independently adapted to a fresh-water
habitat in the North and in the South, giving rise to the Astacidæ and
the Parastacidæ respectively.

The distribution of the individual genera is, however, not so easy to
understand. The species found in Europe all belong to the genus
_Astacus_, which also penetrates into Asia as far as Turkestan and the
basin of the River Obi.

Throughout the greater part of Asia no Crayfishes are found until we
come to the Far East, where we find an isolated colony in the
river-system of the Amur, in Korea, and in the north of Japan. These far
eastern Crayfishes, however, differ so much from the typical species of
_Astacus_ that they are now placed in a subgenus (sometimes regarded as
a distinct genus), _Cambaroides_. Curiously enough, the typical genus
_Astacus_ reappears again on the other side of the Pacific, where
several species occur in that part of North America which lies west of
the Rocky Mountains. East of the Rockies, again, numerous species are
found belonging to a distinct genus, _Cambarus_, which ranges from
Canada to Central America and Cuba, and this genus is allied in certain
respects to the _Cambaroides_ of Eastern Asia. If the systematic
relations of these genera have been properly interpreted, it is by no
means easy to understand in what way their present distribution has been
brought about.

[Illustration: _PLATE XX_

     THE MURRAY RIVER "LOBSTER," _Astacopsis spinifer_. NEW SOUTH WALES.
     (MUCH REDUCED)

     THE LAND CRAYFISH, _Engæus cunicularis_. TASMANIA (NATURAL SIZE)]

The Southern Crayfishes have an even more scattered and discontinuous
range. In New Zealand the genus _Paranephrops_ occurs, in Australia and
Tasmania the genera _Astacopsis_ (Plate XX.), _Cheraps_ and _Engæus_
(Plate XX.). A single species of _Cheraps_ has been recorded from New
Guinea, but no Crayfishes are found in any part of the Malay
Archipelago, in Southern Asia, or on the continent of Africa, although,
curiously enough, a single species of a peculiar genus (_Astacoides_) is
found in Madagascar. In South America species of _Parastacus_ are found
in Southern Brazil, Argentina, and Chili. It is evident that these
various genera of Parastacidæ, which are now so widely isolated from
each other, must have reached their present habitats when the relative
distribution of land and sea in the Southern Hemisphere was very
different from what it is now. What exactly the nature of the land
connection between the various islands and continents was, whether by
way of an Antarctic continent or otherwise, is a question that can only
be suggested here. To attempt to answer it would involve the
consideration of the distribution of many other groups of animals
besides Crayfishes.

Before leaving the Crayfishes, it may be mentioned that certain species
have become adapted to almost terrestrial habits. A number of species of
_Cambarus_ in North America are often found at considerable distances
from open water, burrowing in damp earth, their burrows reaching down to
the ground-water. In many cases they throw up chimney-like piles of mud
at the mouths of their burrows, and in places their chimneys are so
numerous as to "hamper farming operations by interfering with the
harvesting machines, clogging and ruining them." The species of
_Engæus_ (Plate XX.), found in Tasmania, are there known as "Land
Crabs," and burrow in marshy places and in the forests up to an
elevation of 4,000 feet.

[Illustration: _PLATE XXI_

     _Palæmon jamaicensis_, A LARGE FRESHWATER PRAWN OF THE FAMILY
     PALÆMONIDÆ. WEST INDIES. (MUCH REDUCED)]

The broad equatorial belt which separates the regions inhabited by the
Northern and the Southern Crayfishes is characterized by the presence of
several other groups of fresh-water Decapoda. The large River Prawns,
which are found nearly everywhere within the tropics, belong to the
genus _Palæmon_ (Plate XXI.), which is very closely related to the
common marine Prawns (_Leander_) of our own coasts. Some of these Prawns
grow to a foot or more in length of body, and the large claws may
measure as much again. From the Crayfishes, for which they are sometimes
mistaken, they may be easily distinguished by the fact that the large
pincer-claws are not the first, but the second pair of legs. Another
widely-spread group of River Prawns, for the most part of small size, is
the family Atyidæ (Plate XXII.), in which the two pairs of pincer-claws
are feeble, and have the fingers tipped with brushes of long hairs, used
in sweeping up minute particles of food from the mud. The distribution
of these Prawns presents many difficult problems, as an example of which
we may mention the presence of identical or closely related species in
the fresh waters of West Africa and of the West Indies.

[Illustration: _PLATE XXII_

     _Atya scabra_, A FRESH-WATER PRAWN OF THE FAMILY ATYIDÆ, WEST INDIES
     (REDUCED)]

The Brachyura (or Crabs) include many species that live in fresh water.
Some of these, like the species of _Sesarma_ (see Plate XXIII.) and some
other genera of the family Grapsidæ, are common throughout the tropics,
passing up the rivers from the brackish water of estuaries, and being
often found long distances inland in quite fresh water. The true River
Crabs, however, belong to the family Potamonidæ, and are very common
throughout the warmer regions of the globe. One species, _Potamon edule_
(Plate XXIII.), formerly called _Telphusa fluviatilis_, is found in the
South of Europe (Italy, Greece, etc.). Very numerous species, as yet
only imperfectly known, occur throughout the whole of Africa, in
Southern Asia, and in the Malay Islands, extending to Australia in the
south and Japan on the north. In the New World the River Crabs are found
in South America, and extend north to Mexico and the West Indian
Islands. Many of the River Crabs are amphibious in habits, and may be
found burrowing in marshy ground or in damp forests. The young are
hatched from the egg with all the appendages developed, and they remain
clinging to the abdomen of the mother until after the first moult, when
they are perfectly-formed little Crabs (see Fig. 31, p. 78).

[Illustration: _PLATE XXIII_

     THE RIVER-CRAB OF SOUTHERN EUROPE, _Potamon edule_ (OR _Telphusa
     fluviatilis_) (REDUCED)

     _Sesarma chiragra_, A FRESHWATER CRAB OF FAMILY GRAPSIDÆ. FROM
     BRAZIL. (SLIGHTLY REDUCED)]

The groups which have been mentioned are all characteristic inhabitants
of the fresh waters over considerable areas of the surface of the globe.
There are, however, in addition to these, certain Crustacea which occur
in isolated localities, and have no close allies in fresh waters
elsewhere. In the streams of Southern Brazil and Chili there is found a
small Crustacean (_Æglea lævis_--Plate XXIV.), not unlike the Galatheas
of our own coasts, which is interesting as being the only species of the
Anomura found in fresh water. Still more remarkable are the Syncarida,
which are represented by two species of "Mountain Shrimps" (see Fig. 84,
p. 264) in Tasmania, and by a third species found near Melbourne. These
forms have no near allies among living Crustacea, but appear to be
related, as will be shown in a later chapter, to certain fossil
Crustacea found in Palæozoic rocks.

[Illustration: _PLATE XXIV_

     _Æglea lævis._ SOUTH AMERICA. (NATURAL SIZE)]

Belonging to a different category from any of those mentioned are
certain Crustacea closely allied to, or identical with, species living
in the sea, which inhabit inland lakes where no direct passage from the
sea is now possible. Attention was first called to these in the case of
some of the large lakes of Sweden, in which Professor Lovén found some
Crustacea--_Mysis relicta_ (see Fig. 16, p. 47), _Mesidotea entomon,
Pontoporeia affinis_--almost or quite identical with species inhabiting
the Baltic, the Arctic Ocean, and the North Atlantic. There is
geological evidence to show that these lakes were once fjords, or arms
of the sea, and have become cut off from communication with the Baltic
by gradual elevation of the land. The marine animals which they
contained would thus be imprisoned, and as the water became less and
less salt, by the inflow of rivers, certain species which were able to
accommodate themselves to the altered conditions would survive. Some of
the species living in the Swedish lakes have since been found to have a
wider distribution. Thus, _Mysis relicta_, which should perhaps be
reckoned as only a variety of the _Mysis oculata_ of Arctic seas, has
been found in lakes in Russia, North Germany, and North America (Lake
Superior and others), and has lately been discovered in Lough Neagh and
some other lakes in Ireland.

The brackish waters of the Caspian Sea contain a very remarkable
assemblage of animals, including many Crustacea, which, although now
quite isolated from the oceans, are certainly of marine, and in part of
Arctic, origin. Among these are some species closely allied to or
identical with those of the Swedish lakes already mentioned, together
with a great variety of species of Mysidacea, Cumacea, and Amphipoda,
which appear to have been evolved from marine forms since the Caspian
was cut off from communication with the Arctic Ocean.

To such assemblages of animals derived from marine species and isolated
in inland lakes the name of "relict" faunas has been given. It is
necessary to use caution, however, in extending this explanation of
their origin to every case of peculiar lake faunas. For example, there
are difficulties in the way of supposing that Lake Baikal was ever in
open and direct communication with the sea, although it contains many
animals, such as seals, which are certainly of marine origin. The chief
Crustacea of the lake are numerous species of Amphipods belonging to the
genus _Gammarus_, and other genera closely related thereto, and for
these, at all events, there is no need to assume a "relict" origin.

One of the most remarkable lakes in the world from a zoological point of
view is Lake Tanganyika in Africa. When it was found that this lake
contained a fauna very different from that of the other great lakes of
Africa, it was rashly assumed that it must be of relict origin, and some
remarkable speculations were indulged in as to the former connection
between the lake and the sea. Further research, while it has greatly
emphasized the peculiar nature of the fauna, has entirely disposed of
the view that it originated in this way. The Crabs and Prawns, for
example, are not nearly related to marine forms, but belong to groups
that are characteristic of fresh waters in the tropics. While Nyassa and
the Victoria Nyanza have as yet only yielded a single species of Prawn,
and that one of enormously wide distribution (from the Nile to
Queensland), Tanganyika contains no fewer than twelve species, all of
which are peculiar to the lake, while all except one belong to genera
unrepresented elsewhere. Similarly, the Crabs found in the other great
lakes of Africa belong to commonplace types of River Crabs of the genus
_Potamon_; in Tanganyika, in addition to some of these, there are three
species of a remarkable genus, _Platytelphusa_, not known from any other
locality. The Copepoda and Ostracoda of Tanganyika comprise a remarkably
large number of species, many of them peculiar to the lake. A most
unusual feature is the entire absence of Cladocera. It is not easy to
explain the occurrence of this remarkable fauna in Tanganyika, but the
evidence from other groups of animals, such as Mollusca and fishes,
tends to suggest that the lake must have been, until recently,
completely isolated from the other lakes and river-systems of Africa,
that it had no outlet, and that the water was consequently more or less
brackish. Under these conditions the fauna of the lake, originally
similar to that of the other African lakes, has evolved along lines of
its own.

[Illustration: FIG. 62--A WELL SHRIMP (_Niphargus aquilex_). × 7. (After
Wrzesniowski.)]

A very interesting division of the fresh-water fauna is constituted by
those animals which inhabit underground waters. In the South of England
there is found not unfrequently in the water of wells a small colourless
transparent Amphipod known as the "Well Shrimp" (_Niphargus
aquilex_--Fig. 62), distinguished from the common fresh-water _Gammarus_
by the slenderness of its body, by the elongation of the last pair of
tail appendages (uropods), and by the absence of eyes. The proper
habitat of _Niphargus_ is not actually in the wells, but in the
subterranean reservoirs and streams by which the wells are fed. These
subterranean channels intercommunicate over wide areas, and are now
known in many parts of the world to contain a peculiar assemblage of
animals which become accessible to the naturalist in wells and in the
streams and lakes of large caves. Further, the scanty "abyssal" fauna of
deep lakes is partly made up of species which enter the lakes by
subterranean channels, and find a suitable habitat in the deep water.
Species of _Niphargus_, for example, have been dredged in Lough Mask in
Ireland and in some of the Swiss lakes.

Several species of blind Crayfishes have been found in caves in North
America, the best known being one (_Cambarus pellucidus_--Plate XXV.)
found in the Mammoth Cave in Kentucky; and blind Prawns belonging to
various genera have been discovered in caves in America and Europe.

[Illustration: _PLATE XXV_

     THE BLIND CRAYFISH OF THE MAMMOTH CAVE OF KENTUCKY, _Cambarus
     pellucidus_. (NATURAL SIZE)]

A very remarkable feature of the subterranean fauna is that a number of
the animals appear to be more closely allied to marine species than to
any known from fresh waters above-ground. This is especially the case
with some of the Isopoda belonging to typically marine families like the
Cirolanidæ and Anthuridæ, and it has been suggested that these have been
derived from marine species which have entered the underground waters
directly from the sea by way of submarine fissures in the crust of the
earth.

The environment in which these subterranean animals live resembles that
of the deep-sea animals in the absence of light, and the consequent
absence of plant-life. They must ultimately depend for food on animal
and vegetable débris washed down from the surface, but the food-supply
must be scanty, for the water in which they live is usually very clear
and free from organic matter. It is not surprising to find that nearly
all of them are blind, and the few species provided with visual organs
which have been described, from caves, are probably only temporary or
accidental immigrants. Whether the degeneration of the eyes is the
direct effect of disuse, or is due to natural selection ceasing to keep
the eyes up to the standard of usefulness, is a question which has been
much debated, and its answer, were we sure of it, would settle some
of the most fundamental problems of the evolution theory.

At all events, we do not find in any truly subterranean species large
and peculiarly modified eyes like those of many deep-sea animals, and
this may be associated with the complete darkness of their habitat, not
lighted by phosphorescent organisms as the deep sea is. In another
respect these animals differ from those of the deep sea, for they are
all colourless or nearly so; while many of the inhabitants of the deep
sea, as we have already seen, are brilliantly coloured.




CHAPTER IX

CRUSTACEA OF THE LAND


There is every reason to believe that the Arthropoda, like the other
great groups of the animal kingdom, had their origin in the sea; but
they must have invaded the dry land at a very early period, and most of
the classes into which the group is divided--the Arachnids, Myriopods,
and Insects--are now predominantly terrestrial in their habits. The
Crustacea alone have remained for the most part aquatic animals, and
only in a comparatively few cases have they succeeded in adapting
themselves completely to an air-breathing existence. As already
mentioned, a considerable number, both of marine and of fresh-water
species, are more or less amphibious in their habits. Thus, the common
Shore Crab of our own coasts and the Grapsoid Shore Crabs of warmer seas
voluntarily leave the water and scramble about among the rocks between,
and even above, tide-marks. Some Crabs, like _Ocypode_ and _Gelasimus_
(see Plate XV.), have gone farther towards becoming land-dwellers, since
their gill chambers are adapted to serve as lungs for breathing air,
and some species may even be drowned by keeping them in water. The
marsh-dwelling or fresh-water Crabs of the genus _Sesarma_ (see Plate
XXIII.) and allied genera are also apparently to some extent
air-breathers, and one species, _Aratus pisonii_, is stated by Fritz
Müller to climb mangrove bushes and to feed on their leaves. Some
Crayfishes, like the _Engæus_ of Tasmania (see Plate XX.), already
mentioned, are practically land animals. Finally, some Amphipoda,
closely allied to the Sand-hoppers of British coasts, live in damp
places on land, although they do not show any conspicuous modifications
of structure to adapt them to this mode of life. Of one of these
Amphipoda, _Talitrus sylvaticus_, Mr. G. Smith writes: "This species of
land-hopper is widely distributed in the highlands of Tasmania, being
found under logs and leaves in the forests on Mount Wellington, and in
very great abundance in the beech-forests on the mountains of the west
coast."

It will thus be seen that it is impossible to draw any sharp distinction
between aquatic and terrestrial Crustacea, and it is chiefly from
motives of convenience that we have left to be dealt with in this
chapter three groups of land-dwelling Crustacea--the Land Crabs of the
family Gecarcinidæ, the Land Hermits (Coenobitidæ), and the Land
Isopods, or Woodlice (Oniscoidea).

[Illustration: _PLATE XXVI_

     A WEST INDIAN LAND-CRAB, _Gecarcinus ruricola_. (REDUCED)

     A LAND HERMIT CRAB, _Coenobita rugosa_. (REDUCED)]

The Gecarcinidæ are abundant in the tropics of the Old and New Worlds.
Some of the species at least, probably all, visit the sea at intervals
for the purpose of hatching off the eggs carried by the females, and the
larval stages are passed in the sea. In the case of _Gecarcinus
ruricola_ (Plate XXVI.), a species very common in the West Indies, the
migration to the sea takes place annually during the rainy season in
May. The Crabs are described as coming down from the hills in vast
multitudes, clambering over any obstacles in their way, and even
invading houses, in their march towards the sea. Stebbing states that
"The noise of their march is compared to the rattling of the armour of a
regiment of cuirassiers." The females enter the sea to wash off the eggs
which they carry attached to their abdominal appendages, or rather,
probably, to allow the young to hatch out. The Crabs then return whence
they came, and are followed later by the young, which, having passed
through their larval stages in the sea, leave the water, and are found
in thousands clinging to the rocks on the shore.

On Christmas Island, in the Indian Ocean, Dr. C. W. Andrews studied the
habits of another Land Crab, of which the proper name seems to be
_Gecarcoidea lalandii_. He says: "This is the commonest of the Land
Crabs inhabiting the island, and is found in great numbers everywhere,
even on the higher hills and the more central portion of the plateau.
In many places the soil is honeycombed by its burrows, into which it
rapidly retreats when alarmed. These Crabs seem to feed mainly on dead
leaves, which they carry in one claw held high over the back and drag
down into the burrows. From their enormous numbers, they must play a
great part in the destruction of decaying vegetable matter and its
incorporation into the soil."

"Once a year, during the rainy season, they descend to the sea to
deposit [or, rather, to hatch out] their eggs, and during this migration
hundreds may be seen on every path down steep slopes, and many descend
the cliff-face itself. They remain on the beach for a week or two, and
... afterwards gradually make their way back to their accustomed homes."

In the year of Dr. Andrews' first visit to the island (1898) this
migration occurred in January. On a subsequent visit to the island in
1908 he obtained specimens of a large Megalopa larva (see p. 70) which
occurred in enormous quantities in the sea shortly after the migration,
and also of a small Crab which appeared in similar numbers at a slightly
later date. It seems practically certain that these larvæ and young are
those of _Gecarcoidea lalandii_. A second species of Land Crab,
_Cardisoma hirtipes_, found on Christmas Island, has very different
habits from the foregoing. Dr. Andrews says of it: "In this island, at
any rate, this species must be regarded as a fresh-water form, and, in
fact, when a specimen was seen it might be taken as an indication that
fresh water was not far off. It lives in deep holes in the mud at the
sides and bottom of the brooks." Dr. Andrews tells me that he never saw
this species at or near the sea (in marked contrast to _Gecarcoidea_),
and this agrees with the observations of other travellers on species of
the genus _Cardisoma_, so that the breeding habits remain unknown. There
is every probability, however, that in this case, also, the young stages
are passed in the sea.

The student will find, in many textbooks on zoology, the statement that
some Land Crabs of the genus _Gecarcinus_ develop without metamorphosis.
Although it is impossible, with our present knowledge, to state
definitely that this is not the case, there is absolutely no evidence to
support it, and it is an interesting example of the way in which
erroneous statements sometimes gain currency in science.[3] It is based
upon the fact that in 1835 Professor J. O. Westwood described the early
stages of "a West Indian Land Crab," in a paper "On the Supposed
Existence of Metamorphosis in the Crustacea," published in the
Transactions of the Royal Society. Professor Westwood found that the
embryos extracted from the egg possessed all the appendages of the adult
except the swimmerets, and that young specimens clinging to the abdomen
of the parent were perfectly-formed little Crabs. The specimens which
he described were sent to him by the Rev. Lansdown Guilding, of St.
Vincent, who also deals with the subject in a note published in the
_Magazine of Natural History_ in the same year. Neither Westwood nor
Guilding refers to the Crab as a _Gecarcinus_, although Guilding calls
it the "Mountain Crab," a name which Patrick Browne in 1756 gives to the
_Gecarcinus ruricola_ of Jamaica. So far as I am aware, the first writer
to refer to Westwood's Crab as a _Gecarcinus_, was Professor T. Bell,
who in his "British Stalk-eyed Crustacea," published in 1853, states
that some of the original specimens had come into his possession. They
consisted of the detached abdomens of female Crabs, with eggs and young
adhering to them. It would be by no means easy to identify the species
of Crab to which a detached abdomen belonged, and there is nothing in
the whole history inconsistent with the supposition that these
observations really relate to a River Crab of the family Potamonidæ, of
which at least one species, _Pseudothelphusa dentata_, is known to occur
on the island of St. Vincent. As we have already seen, some of these
River Crabs are quite as much land animals as the Gecarcinidæ, and they
are known to have a direct development.

  [3] I am indebted to Mr. J. T. Cunningham for calling my attention to
  some of the facts here recorded.

The Gecarcinidæ possess well-developed gills, but in addition the gill
chambers are modified for air-breathing, as in some other amphibious
Crabs (_Ocypode_, _Gelasimus_, etc.). Each chamber is capacious and
vaulted, and the lining membrane is thick and richly supplied with
bloodvessels, and is folded so as to divide off the upper part of the
chamber as a sort of pocket.

The Land Hermit Crabs of the family Coenobitidæ are found on the coasts
of all tropical seas. Like the Gecarcinidæ, they visit the sea
periodically for the purpose of hatching off the eggs, and the larval
stages are marine. The species of the genus _Coenobita_ (Plate XXVI.)
resemble the marine Hermit Crabs in general shape, and like them use the
shells of Gasteropod Molluscs as portable shelters. Where shells are
scarce, other hollow objects are occasionally utilized; for example,
large individuals will sometimes carry about the shell of a broken
coconut, and a specimen has been seen to walk off in a cracked test-tube
discarded by a naturalist who was investigating their habits. In one
instance Professor Alcock saw an individual "so big that it seemed to
have given up hope of finding a house, and was wandering about
recklessly, with its tail behind it all unprotected."

The Coenobites often climb into bushes in search of food, and Dr. Alcock
"once found one of them busy, like a large bee, among the florets of a
coconut, which made me wonder whether they may not sometimes play a part
in fertilizing flowers." They are, however, by no means exclusively
vegetarians. The author just quoted describes a visit to Pitti Bank in
the Laccadive Archipelago, the breeding-ground of two species of terns.
The ground was everywhere strewn with the dead bodies and clean-picked
skeletons of the young birds. "We soon discovered that one great cause
of the wholesale destruction of young birds was the voracity of swarms
of large Hermit Crabs (_Coenobita_), for again and again we found
recently killed birds, in all the beauty of their first speckled
plumage, being torn to pieces by a writhing pack of these ghastly
Crustaceans. There were plenty of large Ocypode Crabs, too (_O.
ceratophthalmus_), aiding in the carnage."

On Christmas Island Dr. Andrews found a species of _Coenobita_ not
unfrequently in the higher parts of the island far from the sea, and he
remarks that the occurrence of large marine shells high up on the hills
seemed very puzzling until it was noticed that they were brought by the
Hermit Crabs.

The species of _Coenobita_ possess a very curious adaptation for aerial
respiration. The soft skin of the abdomen is traversed by a network of
bloodvessels and acts as a kind of lung, and a pair of contractile
vesicles at the base of the abdomen serve as accessory hearts in
promoting a specially active circulation in that part of the body. The
lining membrane of the gill chambers also appears to aid in respiration
as in other terrestrial Decapods.

[Illustration: _PLATE XXVII_

     THE COCONUT CRAB, _Birgus latro_. (MUCH REDUCED)]

The "Robber Crab" or "Coconut Crab" (_Birgus latro_--Plate XXVII.)
also belongs to the family Coenobitidæ, and has attracted much notice
from its relatively gigantic size and its singular habits. Although
resembling _Coenobita_ closely in essential structure, _Birgus_ differs
from it and from most other Hermit Crabs in not making use of a portable
shelter, perhaps owing to the difficulty of obtaining one of suitable
size. The necessary protection for the abdomen is obtained by a
redevelopment of the shelly plates (terga) on the upper surface of the
abdominal somites. The abdomen is carried doubled underneath the body to
protect the soft under-surface, and the animal, when threatened, seeks a
shelter for its vulnerable hinder part in the nearest hole or cranny.
The swimmerets are absent in the male sex, and are present only on one
side of the abdomen in the female. This unsymmetrical development of the
appendages is interesting as indicating the derivation of the Robber
Crab from ancestors adapted to living in the unsymmetrical shells of
Gasteropod Molluscs. The last pair of abdominal appendages, which in
other Hermit Crabs serve to hold the body in the shell, are here much
reduced in size, and quite useless for that purpose. The carapace is
very broad posteriorly, owing to the great development of the branchial
cavities, which are much too capacious for the very small gills. As in
the true Land Crabs, the lining membrane of the gill cavity is thick and
spongy, and traversed by numerous bloodvessels; but in this case its
efficiency as a lung is added to by numerous tufted papillæ, which
increase the surface exposed to the air.

As in other Hermit Crabs, the last two pairs of legs are shorter than
the others, and they end in small chelæ. The last pair are very slender,
and are usually carried folded up within the gill chambers, which they
possibly serve to keep clear from foreign bodies. The penultimate pair
of legs are stouter, and the two pairs in front of these are long
walking legs. The chelipeds are very strong and are of unequal size.
When attacked, the animal defends itself, not, as might have been
expected, with its chelipeds, but with the first pair of walking legs,
the sharp points of which form very efficient weapons.

The statement that the Robber Crab climbs lofty trees was first made by
the Dutch naturalist Rumphius, in the beginning of the eighteenth
century. Its accuracy has been often doubted or denied since then, and
only finally put beyond dispute by a photograph taken on Christmas
Island by Dr. Andrews, which shows one of these Crabs in the act of
descending the trunk of a sago-palm. It seems not impossible that the
habits of the animal may vary to some extent in different localities,
and that where food is abundant on the ground the tree-climbing habit
may be in abeyance. If this were so, it would explain the very definite
statements made by some observers, that _Birgus_ does _not_ climb
trees.

In localities where coconut palms abound, _Birgus_ feeds largely on the
nuts, tearing off the fibrous outer husk and breaking open the shell by
hammering with its powerful claws at one of the "eye-holes." According
to Darwin in his "Naturalist's Voyage," the pincers of the penultimate
pair of legs are used for extracting the contents of the nut, but this
observation does not seem to have been confirmed. In spite of its name
of "Coconut Crab," however, _Birgus_ by no means feeds exclusively on
coconuts. On Christmas Island, where until recently there were no
coconut palms, the Crabs are exceedingly abundant, and, according to Dr.
Andrews, they "eat fruits, the pith of the sago-palm and the
screw-pines, dead rats and other carrion, and any of their fellows that
may have been injured.... They are excellent scavengers, and have a
curious habit of often dragging their food long distances before
attempting to eat it. I have seen a Crab laboriously pulling a bird's
wing up the first inland cliff, half a mile or more from the camp whence
it had stolen it."

Large specimens of the Robber Crab may be at least a foot in length of
body when the abdomen is straightened out. Their great strength is
illustrated by the fact, related by Darwin, that specimens placed in a
strong biscuit-tin, of which the lid was secured by wire, escaped by
turning down the edges with their claws, and in doing so actually
punched holes quite through the tin.

The breeding habits and mode of development of the Robber Crab have
often formed the subject of inquiry by naturalists, but it is only
recently that Dr. Willey has been able to prove definitely that the
female visits the sea for the purpose of hatching off the eggs, and that
the young are hatched in the zoëa stage. The larvæ obtained by Dr.
Willey have been described by Mr. Borradaile, who finds that, as was to
be expected, they closely resemble those of _Coenobita_. There appears,
however, to be no such simultaneous migration of the Crabs towards the
sea as has been described in the case of the Gecarcinidæ. The statement,
quoted by Darwin, that _Birgus_ visits the sea every night for the
purpose of moistening its branchiæ, cannot be universally applicable,
since the Crabs are often found, as on Christmas Island, at distances
from the sea which put a nightly journey to it out of the question.

Of all Crustacea, the most completely adapted to terrestrial life are
the Land Isopods, or Woodlice, which may be found in every garden. It is
true that most species are found in damp places, although some that
inhabit the sandy deserts of Asia and Africa must be content with a very
slight degree of humidity; and in no case is their dependence on
moisture greater than, for instance, that of many Insects and Arachnids
which are regarded as typically terrestrial animals. Since there is
reason to believe that the Woodlice have been derived from marine
ancestors--they show no special affinities to the fresh-water Isopoda,
like _Asellus_--it is interesting to find that the most primitive forms,
which have departed least from the general Isopod type, are commonly
found on or near the seashore. The "Sea-slater," _Ligia oceanica_ (Fig.
63), which is abundant in rocky places on our own coast, is one of the
most primitive forms. It has a broad, flattened, greenish-brown body,
about an inch long, and it runs quickly, creeping into narrow crevices
of the rocks, so that it is not easy to catch. The antennules, as in the
other land Isopods, are very minute, but the antennæ are long, and have,
besides the five segments which form the "peduncle," a "flagellum" of
about twelve short segments. The uropods or tail appendages are long,
each with two slender, pointed branches. On the under-side of the
abdomen can be seen the five pairs of pleopods, each with two plate-like
branches attached to a very short peduncle. As in most aquatic Isopods,
the plates of the pleopods are soft and thin, and appear adapted to act
as gills, although the outer plate of each pair is somewhat stiffer than
the inner. The Sea-slater is generally found just above high-water mark,
probably always within reach of the salt spray, and it is said sometimes
to enter the water of rock-pools.

[Illustration: FIG. 63--THE SEA-SLATER (_Ligia oceanica_). ABOUT TWICE
NATURAL SIZE. (After Sars.)]

In almost every garden there may be found, under flower-pots and the
like, a Woodlouse, about two-thirds of an inch long, of a brown colour,
with yellowish blotches arranged in a row on each side of the back. This
is _Oniscus asellus_, a species widely distributed in Europe and North
America. It has the antennæ shorter than in _Ligia_, and the flagellum
is composed of only three segments. The uropods are quite short. The
endopodites of the pleopods are membranous gill-plates, which serve for
respiration in the moist air in which these animals generally live. The
exopodites are stiff plates which cover and protect the delicate
endopodites; it is probable that they also aid in respiration, for they
contain a system of minute channels, filled with air, where the cuticle
is separated from the underlying cells. As these channels are nowhere
open to the outside, the air must find its way in by diffusion through
the cuticle.

[Illustration: FIG. 64--STRUCTURE OF THE BREATHING ORGANS OF _Porcellio
scaber_. (From Lankester's "Treatise on Zoology," after Stoller.)

A, Exopodite of first pleopod, showing the tuft of air-tubes
("pseudo-tracheæ"), seen through the transparent cuticle; B, vertical
section through same; C, part of section more highly magnified. _art_,
Point of attachment of exopodite to peduncle; _c_, cuticle; _gr_,
grooved area of cuticle; _hy_, hypodermis, or layer of cells under the
cuticle; _n_, nucleus of hypodermis cell of air-tube; _o_, external
opening; _tr_, air-tubes]

Even more abundant than _Oniscus asellus_, and often found together with
it, is _Porcellio scaber_ (see Fig. 20, p. 51). It is usually of a dark
bluish-grey, but occasionally it is irregularly mottled with a lighter
colour. The flagellum of the antenna has only two segments. The most
interesting difference from _Oniscus_, however, is found in the
pleopods. If the under-side of the living animal be examined with a
pocket lens, a white spot will be seen on each exopodite of the first
two pairs of pleopods. When the structure of the pleopods is
investigated by means of microscopic sections (Fig. 64), it is found
that the white spots are tufts of fine branching tubes radiating into
the interior of the exopodite from a slit-like opening on the outer
edge. These tubes arise by an in-pushing of the integument, and they are
lined throughout by a delicate continuation of the external cuticle.
During life they are filled with air, and they serve to aerate the blood
circulating in the interior of the appendage.

[Illustration: FIG. 65--_Armadillidium vulgare._ × 2-1/2. (After Sars.)]

Another Woodlouse common in England is _Armadillidium vulgare_ (Fig.
65), a little slaty-grey species with a very convex body, which rolls
itself into a ball when touched. Like the last-mentioned species, it has
two segments in the flagellum of its short antennæ, and it has tufted
air-tubes in the exopodites of the first two pairs of pleopods. It is
often mistaken for an animal of widely different structure, which it
superficially resembles--the Pill Millipede (_Glomeris marginata_). The
latter, however, may easily be recognized by having either seventeen or
nineteen pairs of walking legs (instead of seven pairs), set close
together in the middle line of the body, and by lacking the plate-like
pleopods. The resemblance between the two animals can hardly be regarded
as a case of "mimicry," since there is no reason to believe that either
benefits by its likeness to the other. As in so many other cases of
"convergent resemblance" between animals of different structure, it does
not seem possible to get beyond the vague suggestion that a similarity
in habits may have led, in some way that we do not understand, to a
similarity in appearance.

The presence of air-tubes in the pleopods of many Woodlice raises some
questions which are of importance with reference to the classification
of the Arthropoda as a whole. The Six-legged Insects, most Spiders and
many of their allies, the Centipedes and Millipedes, and the worm-like
_Peripatus_, all breathe air by means of fine tubes which penetrate
throughout the body, and bring the air into close contact with the
tissues. These tubes, which are known as "tracheæ," arise as ingrowths
of the outer layer of the embryo, and are lined by a delicate
continuation of the external cuticle. It has been held by some
zoologists that so peculiar a system of breathing organs must indicate a
common descent of the animals that possess them, and accordingly it has
been proposed to separate the Insects, Arachnids, Myriopods, and
_Peripatus_, as a group, Tracheata, from the Crustacea and some other
Arthropods which have no tracheæ. The air-tubes of the Woodlice,
however, are precisely like tracheæ in structure and function, and only
differ from the tracheæ of the other groups in the fact that they are
confined to the appendages, and do not penetrate into the body. Since
the Woodlice are a small and highly specialized branch of the Crustacea,
we can hardly suppose that they derive their tracheæ from any ancestral
type which they had in common with the widely different Arachnids, for
example; and if tracheæ have been evolved independently in these two
groups, there seems no reason why those of the Insects may not have
arisen independently of either. This is only one example out of many
which go to show that, in attempting to reconstruct the genealogy, or
phylogeny, as it is called, of the animal kingdom, we must constantly
admit the possibility of "convergent evolution."

Although Woodlice are very common animals, comparatively little is known
of their habits. They seem to live chiefly on vegetable food, and
sometimes damage seedlings and tender plants in gardens and greenhouses,
but occasionally they are carnivorous, and even cannibalistic, in their
habits. A few species live as "guests" in ants' nests, and one of these,
the little blind white _Platyarthrus hoffmannseggii_, is common in many
localities in this country. Why the ants tolerate their presence we do
not know, for they do not seem to render any service to their hosts, as
do the plant-lice and some other insects that are kept by the ants for
the sake of the secretions which they yield.

The Woodlice, like some other Isopoda, have a peculiar method of
moulting. Instead of the whole exoskeleton being cast off at one time,
as in other Crustacea, that of the hinder half of the body is moulted
first, and it is only after two or three days, when the new cuticle has
hardened, that the exoskeleton of the anterior half follows. As a result
of this arrangement, it occasionally happens that specimens are found
with the fore part of the body differing in colour from the hind part,
owing to the one having been moulted more recently than the other.

Woodlice occur in most regions of the globe, and one of the most
remarkable features of their geographical distribution is the extremely
wide range of certain species. This is probably due, at least in many
cases, to their accidental transport by human agency. Thus, _Porcellio
scaber_, so common in this country, is also found in great abundance in
New Zealand; but Professor Chilton notes that it is usually found near
buildings, and only rarely in the native bush, so that there can be no
doubt that it has been introduced by artificial means.




CHAPTER X

CRUSTACEA AS PARASITES AND MESSMATES


The life of every animal is in more or less intimate relation with that
of all the living creatures which surround it. Some serve for its food,
or supply it with shelter or foothold; others prey upon it, or compete
with it for the necessaries of life; and others, again, influence it for
good or evil in countless ways more subtle than these, but equally
important. There are some associations of a closer and more enduring
nature, to which the names of Symbiosis, Commensalism, and Parasitism,
are applied, and it is with examples of these that the present chapter
is concerned.

The term Symbiosis is strictly applied to an intimate physiological
partnership, such as we find in some of the lower animals and plants,
and in this sense there are no truly symbiotic Crustacea. The word,
however, is sometimes used, in its literal sense of a "living together,"
to embrace all cases of animals living together for mutual advantage.
Commensalism means, literally, "sitting at the same table," and ought
to be applied only to cases where two or more animals, living together
as "messmates," partake of the same food; but it is sometimes used more
loosely to include instances where one of the animals does not actually
share in the food-supply of the other. Parasitism, again, implies that
the parasite lives permanently at the expense of its host, by sucking
its juices or otherwise, and in this case also there are innumerable
degrees and varieties of dependence, which defy inclusion in a strictly
logical scheme of classification. Even such typical parasites as
Tape-worms, for example, might strictly be regarded as commensals,
sharing in the host's food only after it has entered the alimentary
canal. Finally, in all these kinds of interrelation, we find cases where
the association is temporary, intermittent, or almost accidental, and
where there are no perceptible adaptations of structure directed to its
maintenance in either of the partners. From these we may trace a series
of gradations leading to cases where the associated organisms are never
found apart, and where the structure of both is profoundly modified in
adaptation to the particular form of association.

Perhaps the simplest form of association between two animals is found
where one utilizes the other as a means of transport. The little
Gulf-weed Crab, previously mentioned, is very often found clinging to
the carapace or skin of large marine Turtles. It is not a parasite,
since it can hardly derive any food from the Turtle itself; neither
is it a commensal, for there is no evidence that it shares in the
Turtle's meals. It probably takes to a Turtle, when it can find one, as
giving it a wider range of operations than is afforded by its usual
drift-log or tuft of sargasso-weed. A somewhat similar case is afforded
by some of the Barnacles that are found on the skin of Whales. The
species of _Conchoderma_, for instance, are often found on certain
Whales, but they may also occur on inanimate floating objects. Other
Whale-infesting Cirripedes, however, are specially adapted to their
habitat, and never occur elsewhere. For example, _Coronula_ (Plate
XXVIII.) is a genus of sessile Barnacles in which the shell is
elaborately folded, forming a series of chambers into which
prolongations of the Whale's epidermis grow, securely fixing the shell.
_Tubicinella_ is even more effectively protected against dislodgment,
for its shell is sunk in the thickness of the Whale's skin, with only
the opening exposed. Other genera of sessile Barnacles (_Chelonobia_,
etc.) are found adhering to the shell of Turtles. The increased
food-supply made available by the host's movements through the water is
probably the chief advantage that the Barnacles gain in such cases. This
is indicated by the fact that certain small stalked Barnacles
(_Dichelaspis_, etc.), found on large Crabs and Lobsters in tropical
seas, generally cluster on the mouth parts of their hosts, near the
entrances to, or even within, the gill chambers, profiting no doubt by
the respiratory currents and the food particles they carry.

[Illustration: _PLATE XXVIII_

     GROUP OF BARNACLES, _Coronula diadema_, ON THE SKIN OF A WHALE.
     JAPAN. (REDUCED)]

A great variety of Crustacea find shelter and defence in association
with Sponges, Corals, and other more or less sedentary animals. Sponges
are not eaten by many marine animals, the needle-like spicules which
often form their skeleton no doubt helping to render them distasteful,
and many small Crustacea, Amphipods, Isopods, Prawns, etc., profit by
their immunity from attack, and take up their abode in the internal
channels and cavities of the Sponge. The beautiful siliceous Sponge
known as "Venus's Flower-basket" (_Euplectella_) very often contains
imprisoned within it specimens of a delicate little Prawn (_Spongicola
venusta_) or of an Isopod (_Æga spongiophila_). As these Crustacea share
with the Sponge the food particles drawn in by the currents of water
passing through the pores in its walls, they are in the strict sense
commensals.

The Corals and various other animal organisms commonly known as
"Zoophytes," forming together with the Jellyfishes the group Coelentera,
are very effectively protected against the attacks of most predatory
animals by the possession of "stinging cells," and this protection is
shared by many other animals which shelter among them. Thus, the
branching Coral stocks which grow in great luxuriance on tropical coasts
support a rich and varied assemblage of animals, some of which may
actually prey upon the Coral polypes, but all of which profit by the
fact that few enemies venture to pursue them in their retreats.
Innumerable prawn-like animals of the Alpheidæ and other families, and
many kinds of Crabs, are found among living Corals. The Crabs of the
family Trapeziidæ are especially characteristic of such habitats, and
their thin, flat bodies seem to be adapted to slip into slits and
crannies of the Coral blocks. The most highly specialized of all Coral
Crabs, however, are the species of the family Hapalocarcinidæ, which
modify in various ways the growth of the corals on which they live. In
some of the more delicately branched kinds of Coral there may sometimes
be found hollow bulbous growths, each of which contains imprisoned
within it a little Crab--_Hapalocarcinus marsupialis_ (Fig. 66). It
seems that the female Crab (the habits of the male are not definitely
known) settles down among the branches of the Coral, and that the
irritation of its presence causes the branches to grow up and surround
it, coalescing with each other to form a kind of cage, and ultimately
leaving only one or two small openings. Through these openings water can
enter to enable the Crab to breathe, and no doubt food particles find
their way in, but it is not possible for the Crab to leave its prison.
The production of these abnormal growths of the Coral is closely
analogous to the formation of "galls" on plants as a result of the
irritation set up by the presence of insect larvæ or other parasites,
and it is not inappropriate, therefore, to speak of them as "Coral
galls."

[Illustration: FIG. 66--TWO BRANCHES OF A CORAL (_Seriatopora_) SHOWING
"GALLS" INHABITED BY THE CRAB _Hapalocarcinus marsupialis_. ON THE RIGHT
THE FEMALE CRAB, EXTRACTED FROM THE GALL AND FURTHER ENLARGED]

The Medusæ, or Jellyfishes, like other Coelentera, are provided with
poisonous stinging cells, which, in the larger species of our own seas,
are powerful enough to cause discomfort to bathers who come in contact
with them. The protection thus afforded is no doubt of advantage to the
little globular Amphipods of the genus _Hyperia_ (Fig. 67), which are
almost always to be found sheltering under the bells of the larger
Medusæ. In what way the Amphipods escape injury from the stinging cells
of their host is not known.

[Illustration: FIG. 67--_Hyperia galba_, FEMALE. ENLARGED. (After
Sars.)]

In all the cases mentioned, the advantages of the partnership seem to be
all on one side, but there are numerous instances in which both partners
seem to reap some benefit. A species of Hermit Crab very common in
moderately deep water on many parts of the British coasts, _Eupagurus
prideauxi_, is always found to have a Sea-anemone (_Adamsia palliata_)
attached to the shell which it carries. The Anemone has a broad base
which is wrapped round the shell, the mouth, surrounded by the
tentacles, being on the under-side next the opening of the shell. There
seems no reason to doubt that the presence of the Anemone does afford
some degree of protection to the Hermit, and that, on the other hand,
the Anemone benefits by being carried about, and shares in the crumbs
from the Hermit's meals. It is stated that, when the Hermit removes to a
new shell, it detaches the Anemone from the old shell with its pincers
and places it in position on the new one. It appears, however, that it
is not always necessary for the Hermit to remove to a larger shell as it
grows, for the enveloping Anemone, as it increases in size, extends
beyond the mouth of the shell, and so enlarges the shelter. Further, the
Anemone in course of time dissolves the shell almost entirely away, and
the Hermit is enveloped only by the soft fleshy mantle which it forms.

In a similar way the deep-sea Hermit Crab _Parapagurus pilosimanus_ (see
Plate XVI.) is always found lodged in a fleshy mass formed by a colony
of Sea-anemones (_Epizoanthus_), within which, when it is cut open, may
be found the remains of the shell which the Hermit first inhabited. A
further development of the same habit is given by _Paguropsis typica_,
found in deep water in Indian seas, which does not inhabit a shell at
any time, but carries a fleshy blanket formed by a colony of Anemones.

In dredging off the British coasts, we often find smooth rounded lumps
of a Sponge (_Suberites ficus_), generally yellowish-grey in colour,
having a round opening in which the claws of a small Hermit Crab
(_Eupagurus cuanensis_) may be seen. On cutting open the Sponge, the
body of the Hermit is seen to be lodged in a spiral cavity, and at the
apex may be found the remains of a shell that has been corroded away by
the Sponge which settled on and replaced it. Other species of Hermit
Crabs constantly have their shells covered with a horny crust formed by
Hydroid zoophytes (_Hydractinia_, etc.), and in this case also the
extension of the Hydroid colony beyond the lip of the shell relieves the
Hermit from the necessity of so frequently changing to a larger shell as
it grows.

A number of other animals are found associated with Hermit Crabs,
without, as far as we can see, rendering any service in return for the
house-room. The Whelk-shells inhabited by _Eupagurus bernhardus_ (see
Plate VII.) often contain one of the bristle-footed worms (_Nereilepas
fucata_), which may sometimes be observed to protrude its head from the
shell when the Crab is feeding, and to snatch away fragments of the prey
from the very jaws of its host. It is thus, in the strict sense of the
word, a commensal. Species of Copepods, Amphipods, Porcelain Crabs, and
even a Mysid, have been found sharing the lodging of Hermit Crabs in a
similar way, and in addition there are various parasites, presently to
be mentioned, found on the Crabs themselves, so that each Crab forms the
centre of a whole community of widely diverse organisms all more or less
directly dependent on it.

A habit similar to those of some Hermit Crabs is that of the Crab
_Dromia_ (see Plate IX.), mentioned in a previous chapter, which
carries, as a cloak, a mass of living sponge, holding it in position by
means of the last two pairs of legs. Even the "masking" habit of the
Spider Crabs, already described (p. 96), may be regarded as a kind of
symbiosis, since the sponges, zoophytes, etc., which grow on the Crabs
no doubt benefit by being carried about in return for the protection
they give.

[Illustration: FIG. 68--A, THE CRAB _Melia tessellata_ CLINGING TO A
BRANCH OF CORAL, AND CARRYING IN EACH CLAW A LIVING SEA-ANEMONE; B, ONE
OF THE CLAWS FURTHER ENLARGED TO SHOW THE WAY IN WHICH THE ANEMONE IS
HELD. (After Borradaile.)]

One of the strangest habits is that of certain little tropical Crabs, of
which _Melia tessellata_ (Fig. 68) is the best known, which carry in
each claw a living Sea-anemone and use it as a weapon. The claws or
chelipeds are in this case of small size, so that they would be of
little use by themselves for attack or defence; but the fingers are
provided with recurved teeth, enabling them to take a firm hold of the
slippery body of the Anemone. Particles of food caught by the tentacles
of the Anemone are removed and eaten by the Crab, which uses for the
purpose the long walking legs of the first pair. The same limbs are also
used in the process of detaching the Anemones from the stone on which
they may be growing. The Anemones do not appear to suffer from the rough
treatment to which they are subjected, but whether they can reap any
benefit from the partnership is very doubtful.

[Illustration: FIG. 69--THE COMMON PEA CRAB (_Pinnotheres pisum_),
FEMALE. NATURAL SIZE.]

From remote antiquity it has been known that a little Crab (Fig. 69) is
frequently found living within the shells of bivalve Molluscs, such as
Oysters, Mussels, and especially the large mussel-like _Pinna_, which is
common in the Mediterranean. Ancient writers regarded this as a case of
association for mutual advantage, believing that the _Pinnotheres_
warned the _Pinna_ of the approach of enemies or of the entrance of prey
between its gaping valves. It is even stated that the Pinna and Crab
were depicted in Egyptian hieroglyphics to symbolize the dependence of a
man on his friends.

As a matter of fact, however, there is no reason to believe that the
Molluscs which harbour species of _Pinnotheres_ and allied genera
benefit in any way by the presence of the Crabs. The latter probably
feed, as their hosts do, on particles brought in by the current of water
entering the mantle cavity. They are therefore strictly "commensals,"
though it is usual, and perhaps equally correct, to speak of them as
"parasites." The case is, indeed, an example of the difficulty of
defining these two terms. At all events, the Pinnotherid Crabs show one
of the characteristics of parasites in being to some extent degenerate
in their structure. The carapace and the rest of the exoskeleton, no
longer needed for protection, have become soft and membranous, and the
eyes and antennules, the chief organs of sense, are very minute. As in
many parasites, also, the eggs produced by the female are very numerous,
and the abdomen is very broad and deeply hollowed for their reception.

While most of the Pinnotheridæ live in bivalve Molluscs, some species
are associated with other invertebrate animals. _Pinnaxodes chilensis_
is found in a species of Sea-urchin (_Strongylocentrotus gibbosus_) on
the coast of Chili. On opening the shell of the Urchin, the Crab is
found enclosed in a thin-walled bag formed by enlargement of the
terminal part of the host's intestine.

It did not escape the notice of Aristotle that a little Shrimp sometimes
occurred in the _Pinna_ in place of the Crab. This is _Pontonia custos_,
and other species of the same and allied genera have similar habits.

The order Isopoda includes a very large number of parasitic species. The
extensive family Cymothoidæ presents a whole series of gradations in
habits and structure between actively swimming predatory species and
others which in the adult state are permanently fixed to their host,
usually a fish, and are incapable of movement. At one end of the series
are the species of _Cirolana_, which have powerful biting jaws. Of _C.
borealis_ (Fig. 70), Mr. Stebbing remarks that "it is a good swimmer,
tenacious of life, a savage devourer of fish, and not to be held in the
human hand with impunity." The species is not uncommon in British seas,
and numerous individuals will sometimes attack a Cod or other large
fish, perhaps after it has been caught on a hook, and gnaw their way
into its body, so that when brought to the surface the fish consists of
little more than skin and bone.

[Illustration: FIG. 70--_Cirolana borealis._ ABOUT TWICE NATURAL SIZE.
(After Sars.)]

The little _Eurydice achatus_, belonging to the same subfamily,
Cirolaninæ, is commonly taken in the tow-net in sandy bays on our own
coasts. It is said sometimes to attack bathers, and to "nip most
unpleasantly."

More definitely parasitic are the species of _Æga_ and allied genera,
which have piercing and suctorial mouth parts, and suck the blood of
fish. They are usually found adhering closely to the skin of their
victim by means of the strong hooked claws of the anterior pairs of
legs; but they have not lost the power of locomotion, and, as females
bearing eggs are never taken on fish, it would appear that they drop off
after gorging themselves with blood, and probably seek a retreat at the
bottom of the sea, where they may hatch their young in safety. The
digestive canal of _Æga_ dilates into a large bag, which becomes
distended with a semi-solid mass of blood. This mass, when extracted and
dried, is the "Peter's stone" of old Icelandic folklore, to which
magical and medicinal virtues were attributed. The species _Æga
spongiophila_, already mentioned, differs in its habits from all the
other species of the genus, since it lives, not on fish, but in the
interior of a sponge.

[Illustration: _PLATE XXIX_

     _Cymothoa oestrum_, AN ISOPOD PARASITE OF FISH (SLIGHTLY ENLARGED)

     _Sacculina carcini_ ATTACHED UNDER THE ABDOMEN OF A COMMON SHORE-CRAB
     (REDUCED)]

The most completely parasitic members of the Cymothoidæ are found in the
subfamily Cymothoinæ, including the typical genus _Cymothoa_ (Plate
XXIX.) and many others. The adult animals are found clinging to the skin
of fishes, the legs being provided with strong hook-like claws that give
them a very firm hold. Some species, especially common on Flying-fishes,
cling to the tongue of the fish, and almost prevent it from closing its
mouth. When young, the Cymothoinæ swim freely, and the shape of the body
is not unlike that of the Cirolaninæ; but after they have settled on a
host the body often becomes distorted and unsymmetrical. A still more
remarkable change occurs in the reproductive organs in some, if not in
all members of this subfamily. Each individual, when it first
attaches itself to a host, presents the characters of the male sex.
Later it becomes a female, develops a brood-pouch, and produces eggs.
The animals are, in fact, hermaphrodite; but it is to be noted that the
hermaphroditism is of a different kind from that presented by the
Cirripedia, since the organs of the two sexes are successively, not
simultaneously, developed. Where, as in this case, the male phase comes
first in the life-history of the individual, the condition is known as
"protandrous" hermaphroditism.

Another large group of parasitic Isopods is the suborder Epicaridea, all
the species of which are parasitic on other Crustacea. It is not
uncommon to find specimens of the common Prawn (_Leander serratus_)
which have a large swelling on one side of the carapace. If the lower
edge of the carapace be raised, it will be seen that this swelling is
due to the presence in the gill cavity of an Isopod parasite (_Bopyrus
squillarum_). A closely similar form, found on Prawns of the genus
_Spirontocaris_, is _Bopyroides hippolytes_, represented in Fig. 71.
Other allied species are found on Hermit Crabs and other Decapods. When
extracted, the parasite is seen to have a flat and curiously distorted
body, with extremely short legs ending in hooked claws. The under-side
is generally occupied by a relatively enormous mass of eggs, which is
only partly covered in by the small brood-plates. The mouth parts form
a short piercing beak with which the parasite sucks the blood of its
host. On the under-side of the abdomen may usually be found the minute
male, attached, like a secondary parasite, to the body of the female.

[Illustration: FIG. 71--A, FRONT PART OF BODY OF A PRAWN (_Spirontocaris
polaris_), FROM ABOVE, SHOWING ON THE RIGHT SIDE A SWELLING OF THE
CARAPACE CAUSED BY THE PRESENCE OF THE PARASITE _Bopyroides hippolytes_
IN THE GILL CHAMBER; B, THE FEMALE PARASITE EXTRACTED AND FURTHER
ENLARGED; C, THE MALE PARASITE ON SAME SCALE AS THE FEMALE. (After
Sars.)]

The species of Epicaridea are very numerous, and they infest Crustacea
belonging to nearly all the chief groups of the class, a few even being
parasitic on other Epicaridea. Many of them differ greatly from the
_Bopyrus_ just described, and in some cases it would be impossible to
guess from the structure of the adult animals that they were Isopoda, or
even Crustacea at all. The life-history is not yet completely known.
When hatched from the egg, the free-swimming larvæ have a short and
broad body, and, as in other Isopod larvæ, have only six instead of
seven pairs of legs. A later larval stage, just before attachment to the
final host, has a long narrow body and the full number of legs. It has
lately been shown, however, that, in all probability, between these two
free-swimming stages there intervenes a stage in which the larvæ is
temporarily parasitic on certain Copepoda. Further, some of the
Epicaridea, like the Cymothoinæ described above, are protandrous
hermaphrodites, developing the male organs when in the last larval
stage, and passing into the female phase after they have become attached
to the host. In _Bopyrus_ and many other genera, however, there is no
evidence that the males ever develop into females.

Some of the most remarkable Epicaridea are those belonging to the family
Entoniscidæ, which are parasitic on Crabs. In these the parasite
penetrates from the gill chamber into the interior of the body of the
host, remaining enveloped, however, by a delicate membrane which grows
in with it from the wall of the gill chamber. The body is distorted in
an extraordinary fashion, so that at first sight it seems impossible to
trace any resemblance to the form of a typical Isopod.

Among the Amphipoda there are a few species belonging to various
families of the Gammaridea which have suctorial mouth parts, and lead a
semi-parasitic existence; but the only completely parasitic forms are
the Whale-lice, forming the family Cyamidæ (see Fig. 23, p. 55) in the
suborder Caprellidea. Although differing greatly in the broad, flattened
shape of the body from the slender, thread-like Caprellidæ, they closely
resemble them in structure, particularly in having the abdomen reduced
to a mere knob. The fourth and fifth pairs of thoracic limbs have
disappeared, although the gills corresponding to them are very large;
and the last three pairs of legs have long curved claws which enable the
Whale-louse to cling firmly to the skin of its host. The mouth parts are
adapted for biting, not for sucking blood, and the animals seem to live
by gnawing the skin of the Whales. In one respect the Whale-lice are
unique among Crustacean parasites: they have not the power of swimming
at any period of their life-history. The young settle down near their
parents, and masses of many hundred individuals of all sizes are found
clinging close together on the skin of the host.

No group of Crustacea exhibits more numerous or more varied examples of
parasitism than the Copepoda. Every grade of transition between a free
predatory habit of life and the most complete dependence upon a host may
be traced in various families of the subclass. Only a few examples can
be mentioned here.

[Illustration: FIG. 72--A FISH-LOUSE (_Caligus rapax_), FEMALE. × 5.
(After Wilson.)]

The commonest "fish-lice" are the numerous species of the family
Caligidæ, many of which, belonging to the genera _Caligus_ (Fig. 72),
_Lepeophthirus_, etc., are found on marine fishes on our own coasts. In
these the body is broad and flat, but in many of them the resemblance,
even in general form, to the free-living Copepoda is easily traceable.
The maxillipeds form powerful hooked claws, by means of which the
animals cling to the skin of the fish they infest, and in _Caligus_ the
basal segments of the antennules have a pair of suckers which aid in
adhesion. The mouth parts are adapted for piercing, and are enclosed in
a suctorial proboscis.

When the young Caligid, after passing through the free-swimming larval
stages, first becomes attached to a fish, it adheres by means of a
thread-like process issuing from the front of the head, and formed by
the secretion of a gland. At this stage, formerly described as an
independent species under the generic name of _Chalimus_, the parasite
is unable to detach itself from its host; but later, in many species,
it re-acquires the power of swimming, and specimens of _Caligus_, for
instance, are commonly found free in tow-net gatherings.

[Illustration: FIG. 73--STAGES OF DEVELOPMENT OF _Lernæa branchialis_. F
IS SLIGHTLY, THE OTHER FIGURES GREATLY, ENLARGED. (After A. Scott.)

A, Nauplius, just hatched; B, young female taken from gills of Flounder;
C, free-swimming stage of female, after leaving Flounder; D,
free-swimming male; E, female just after settling on gills of Whiting;
F, fully-developed female.]

On the gills of Cod, Haddock, and other common fish, we often find a red
worm-like parasite, _Lernæa branchialis_ (Fig. 73, F), which at first
sight seems to bear no sort of resemblance to a Crustacean. The soft
body is curiously doubled up, and is attached to the host by a narrow
neck; while dissection will reveal a small head buried in the flesh of
the fish's gills, and having three branched outgrowths, which penetrate
into the surrounding tissues and make the attachment of the parasite
more secure. Near the hinder end of the body are two coiled threads,
which are the egg-masses. The reduced mouth parts and the microscopic
vestiges of the swimming feet may be detected on and near the head, but
apart from these it would be hard to find any characters to show that
the animal is a Crustacean.

The life-history of _Lernæa_ is very remarkable. The young are hatched
in the nauplius stage (Fig. 73, A), and after passing through some
further free-swimming stages they become parasitic on a fish. Curiously
enough, however, they choose a very different host from that on which
the adults are found, for at this stage (Fig. 73, B) they attach
themselves to the gills of one of the Flat-fishes (Pleuronectidæ), such
as the Flounder, Plaice, etc., attachment being effected by a frontal
cement gland similar to that of the larval Caligidæ, already mentioned.
The animal is now without the power of swimming, its appendages becoming
reduced to stumps and losing their setæ. After passing some time in
this condition, the larva again acquires the power of swimming, and
leaves its host. Both sexes become mature in this free-swimming stage
(Fig. 73, C, D), and impregnation is effected. The males die without
developing further, but the females seek a second host, a fish of the
family Gadidæ, such as the Cod, Haddock, etc., and, settling on the
gills, become metamorphosed (Fig. 73, E) into the adult form described
above.

Within the gill cavities of the strange-looking fish known as the Angler
or Fishing-frog (_Lophius piscatorius_) there may often be found
specimens of another parasitic Copepod, _Chondracanthus gibbosus_. It
has a soft, unsegmented body about half an inch long, provided with
numerous blunt lobes which give it a very irregular shape. On the
under-side, near the front, are forked lobes representing two pairs of
the swimming feet. At the hinder end are usually attached a pair of long
thread-like egg-masses. Just at the point where the egg-masses are
attached, close inspection of the under-side of the body will reveal a
very minute maggot-like object. This is a male individual, which is
attached, like a secondary parasite, to the body of the enormously
larger female.

[Illustration: FIG. 74--STAGES IN THE LIFE-HISTORY OF _Hæmocera danæ_,
ONE OF THE MONSTRILLIDÆ. (From Lankester's "Treatise on Zoology," after
Malaquin.)

A, Free-swimming nauplius larva; B, embryo after penetrating into the
body of the worm _Salmacina_; C, D, E, successive stages in the body of
the host; F, free-swimming adult female. (All greatly enlarged, not to
same scale.) _a´_, Antennule; _br_, brain; _e_, nauplius eye; _f_,
swimming feet; _g.s._, hairs on which the eggs are carried; _m_,
position of mouth; _md_, hooked mandible of nauplius; _n_, nerve cord;
_ov_, mass of eggs carried by female; _ovy_, ovary; _pr_, absorptive
processes.]

In all the cases mentioned, the animal is parasitic in the final state
of its existence--at least in the female sex--but there are a few
Copepoda which are free-swimming, both when young and when adult, but
parasitic in the intermediate stages. Among the Copepoda taken by the
tow-net in British seas, there may sometimes be found species of the
family Monstrillidæ (Fig. 74, F), which are remarkable for having no
appendages between the antennules and the first pair of swimming feet.
They have no trace of jaws, and only a minute vestige of a
mouth-opening; while internally there is no food-canal, so that the
animals are incapable of taking nourishment. Their development was for
long a mystery, but it is now known that the greater part of their life
is passed as internal parasites in certain bristle-footed worms
(Polychæta). The young are hatched as nauplius larvæ (Fig. 74, A)
without mouth or food-canal, but capable of swimming, and having the
third pair of appendages (mandibles) furnished with strong hooks, by
means of which they fasten on to the worm which is to serve as their
host. The nauplius bores through the skin of the worm, casting its
cuticle and losing all its appendages in the process, and making its way
into one of the bloodvessels in the form of a little oval mass of cells
(Fig. 74, B), within which no organs except the degenerating nauplius
eye can be detected. It later becomes enclosed in a delicate cuticle,
and from one end two long finger-like processes grow out, which are
believed to have the function of absorbing nourishment from the blood of
the host (Fig. 74, C, D). Within the cuticle the organs of the adult
animal are gradually differentiated (Fig. 74, E), and when fully formed
it bores its way through the tissues of its host by means of rows of
hook-like spines surrounding the pointed posterior end of the sac. On
reaching the surface the enclosing membrane bursts, and the adult animal
is set free.

Of all Crustacean parasites, however, perhaps the most remarkable in
their structure and life-history are the Cirripedes of the order
Rhizocephala. It is not uncommon on the British coasts to find specimens
of the common Shore Crab or other Crabs which carry under the abdomen an
oval fleshy body. This is the Rhizocephalan _Sacculina carcini_ (Plate
XXIX.), and it would hardly be possible to guess, from its appearance or
structure, that it was a Cirripede or a Crustacean at all. It is
attached to the under-side of the Crab's abdomen by a short stalk, and
in the middle of its opposite surface is a small opening which leads
into a cavity separating the outer "mantle" from the body of the animal.
Very often this mantle cavity will be found to be full of eggs enclosed
in sausage-shaped packets. At the point where the short stalk enters the
abdomen of the Crab, it gives off an immense system of fine branching
roots, which penetrate throughout the body of the Crab, and even into
its legs and other appendages. By means of these roots the _Sacculina_
absorbs nourishment from the body-fluids of its host. Like most
Cirripedes, _Sacculina_ is hermaphrodite, and the body within the mantle
cavity contains only the reproductive organs of the two sexes and a
small nerve ganglion representing the whole of the nervous system. There
is no mouth, no food-canal, and no trace of appendages. Another
Rhizocephalan, _Peltogaster_, is not uncommonly found attached to the
abdomen of Hermit Crabs. Although the nauplius larva of _Sacculina_ was
described, and its resemblance to that of the Cirripedia pointed out, as
long ago as 1836, by that acute observer, J. Vaughan Thompson, it is
only recently that the full life-history has been made known by the
researches of Professor Delage and Mr. Geoffrey Smith. The nauplius
larva (Fig. 75, A) resembles that of the normal Cirripedes, especially
in the shape of the dorsal shield, which is drawn out on either side in
front into a pair of fronto-lateral horns. It has, however, no mouth,
and the food-canal is quite absent. As in the normal Cirripedes, the
nauplius is followed by a _cypris_ stage (Fig. 75, B), also mouthless,
and it is in this form that the _Sacculina_ seeks the Crab on which it
is to become parasitic. It would be almost impossible for the _cypris_
larva to settle on that part of the Crab where the adult _Sacculina_ is
afterwards to appear, since the Crab usually has its abdomen closely
pressed against the under-side of its thorax. The larva therefore
attaches itself on some exposed part of the Crab, often on one of the
legs, clinging to a hair by means of its antennules. It bores through
the cuticle at the base of the hair, and the contents of its body pass
into the interior of the Crab as a little mass of cells, the empty
_cypris_ shell being cast off. This mass of cells, which constitutes the
embryo _Sacculina_, is carried about by the blood-currents of the Crab
till it reaches the under-side of the intestine, where it becomes
attached. It now begins to send out roots (Fig. 76), and as it grows the
central mass travels backwards along the intestine of the Crab till it
reaches the place where the adult parasite is to emerge. As the mass
increases in size, and the organs of the _Sacculina_ become
differentiated within it, its presence causes the living tissues
between it and the external cuticle to degenerate, so that when the Crab
moults an opening is left through which the body of the parasite
protrudes. Owing, no doubt, to the drain on its system due to the
presence of the _Sacculina_, the Crab ceases to grow, and it does not
moult again as long as the parasite remains alive.

[Illustration: FIG. 75--FREE-SWIMMING STAGES OF _Sacculina carcini_.
MUCH ENLARGED. (After Delage.)

A, Nauplius; B, cypris stage.]

[Illustration: FIG. 76--EARLY STAGE OF _Sacculina_ WITHIN THE BODY OF A
CRAB. (After G. Smith.)

_i_, Intestine of the Crab; _s_, body of the _Sacculina_, which
afterwards emerges on the under-surface of the Crab's abdomen; _r_,
roots of the _Sacculina_.]

In addition to this arrest of growth, _Sacculina_ produces in its hosts
other changes, which affect chiefly the reproductive organs and the
structures associated therewith. Crabs of either sex infected with
_Sacculina_ are incapable of breeding; the genital gland (ovary or
testis) is found on dissection to be shrivelled up, and the external
characters indicative of sex become strangely modified. The changes
have been most fully studied in the case of a kind of Spider Crab
common at Naples--_Inachus mauritanicus_. In this species it is found
that females infected with _Sacculina_ show no conspicuous external
modification, except that the abdominal appendages, which in the normal
females serve for the attachment of the eggs, are greatly reduced in
size. Infected males, however, may assume to a greater or less degree
the characters proper to the female sex. Some males show little change,
except that the chelipeds remain small and flattened, as in the females
and non-breeding males. Other specimens have, in addition, the abdomen
much broader than in normal males, and sometimes as broad as in the
females. Finally, some males develop on the abdomen, in addition to the
rod-like appendages on the first and second somites, characteristic of
the male sex, two-branched appendages on the next three somites, as in
the females; these individuals are, in fact, so completely intermediate
in character between the two sexes that it is only by dissection that it
is possible to recognize them as modified males.

An indication of the way in which the degenerate Rhizocephala have been
derived from normal Cirripedes is given by a peculiar species of
pedunculate Barnacle, _Anelasma squalicola_, which lives attached to
Sharks and Dogfish in the North Sea. In _Anelasma_ the peduncle becomes
deeply buried in the flesh of the Shark, and its surface is covered with
short branching, root-like filaments. As in the case of the
Rhizocephala, these roots appear to absorb nutriment from the host, and,
although _Anelasma_ possesses a food-canal and mouth, the cirri are
reduced in size and devoid of hairs, so that they cannot be used for
obtaining food as in ordinary Barnacles.




CHAPTER XI

CRUSTACEA IN RELATION TO MAN


The Crustacea come into relation with human life in the most obvious and
direct way in the case of those species that are used for food. The
number of species so used in various parts of the world is very large,
almost the only necessary condition being that the species shall be
sufficiently large and abundant to make it worth while to fish for it.

As most of the larger Crustacea belong to the Decapoda, it is this order
that supplies practically all the edible species, almost the only
exceptions being a few Barnacles which are eaten in various parts of the
world. Thus the sessile Barnacle _Balanus psittacus_, found on the
coasts of Chili, and growing to a length of 9 inches by 2 or 3 inches
diameter, is, according to statements quoted by Darwin, "universally
esteemed as a delicious article of food," and the pedunculate
_Pollicipes cornucopia_ is used for food on the coasts of Brittany and
Spain.

By far the most valuable of all the edible Crustacea are the European
and American Lobsters (_Homarus gammarus_ and _H. americanus_). The
former is found on the coasts of Europe from Norway to the
Mediterranean, living mostly a short distance below low-water mark
wherever the bottom is rocky. At some places, as for instance at
Worthing, Lobsters are common on a sandy bottom, but as a rule they seem
to prefer localities where the crevices of a rough hard bottom afford
abundance of shelter. They are usually caught in traps known as "Lobster
pots" or "creels," which vary in construction in different localities.
In some cases they are made of wicker-work, hemispherical in shape, with
a funnel-shaped opening on top, so devised as to permit the Lobsters to
enter easily, while preventing their escape. Another form is
semi-cylindrical, with a framework of wood covered with netting or with
wooden spars, and having two funnel-shaped entrances at the sides. These
traps are baited with pieces of fish, preferably stale, and are sunk in
suitable places, each attached by a line to a buoy or float.

Important Lobster fisheries are carried on in Norway, Scotland, England,
Ireland, Heligoland, and other parts of the coasts of Northern Europe.
In the South the Lobster fishery is of less importance, other large
Crustacea, especially the Spiny Lobster, being more abundant and more
highly esteemed.

The American Lobster, as already mentioned, closely resembles the
European species, the chief difference being in the form of the rostrum
(see Fig. 9, p. 32). It is found on the Atlantic coast from Labrador to
Cape Hatteras, but it is not abundant south of New Jersey. The canning
of Lobsters is a very important industry in Newfoundland, the Maritime
Provinces of Canada, and the Northern New England States.

The only other species of the genus _Homarus_ (_H. capensis_) is found
at the Cape of Good Hope, but it is of small size and is of no economic
importance.

The European Lobster rarely reaches a weight of 10 pounds, although
individuals of 14 pounds weight have been caught. In America, there are
authentic records of Lobsters weighing 20 and even 23 pounds.

The bad effects of over-fishing have become apparent of late years,
especially on the American coast, in the reduced average size of the
Lobsters caught rather than in a diminution of the total yield of the
fishery. Numerous experiments in legislation have been made with a view
to checking the depletion of the fishing-grounds, but in no case with
conspicuous success. A "close time" for the spawning Lobsters has often
been tried, but the fact that the female carries the eggs attached to
her body for nearly a year after spawning makes it quite impossible to
give effective protection by this means. In most Lobster-fishing
districts a minimum size is fixed by law, below which it is illegal to
take or sell Lobsters, and in many cases also the capture of females
carrying spawn, or, as it is termed, "in berry," is prohibited.

[Illustration: _PLATE XXX_

     THE "NORWAY LOBSTER," _Nephrops norvegicus_, ABOUT ONE-THIRD NATURAL
     SIZE

                    (_From Brit. Mus. Guide_)]

The so-called "Norway Lobster" (_Nephrops norvegicus_--Plate XXX.), the
"Dublin Prawn" of the London fishmongers, is a smaller and much less
valuable species than the common Lobster. It may be recognized at once
by its long and slender claws, furnished with rows of tubercles or blunt
spines, and by the sculptured markings on the somites of the abdomen.
When alive it is of an orange colour, beautifully marked with red and
white. It differs considerably in its habits from the common Lobster,
living at a considerably greater depth (30 to 60 fathoms in Norway), and
on a muddy bottom. It is generally taken by trawling, and is captured in
large quantities by trawlers fishing in various parts of the North Sea.
Since it must be cooked soon after it is caught, and cannot easily be
brought to market alive like the common Lobster, only a small number of
those actually caught are made use of. Formerly most of those sold in
London were caught in the Irish Sea (whence the name of "Dublin Prawn"),
but the North Sea is now the chief source of supply. The species is
found in suitable localities from Norway to the Mediterranean, and is
especially abundant in the Adriatic, where it is caught and sold in
Venice and elsewhere under the name of "Scampo."

The Spiny Lobster, Rock Lobster, or Sea-crawfish (_Palinurus
vulgaris_--Plate V.), is common on the south and south-west coasts of
the British Islands, becoming rare in the north, although specimens have
been found as far north as Orkney, and there is a single record of the
species from the West of Norway. It is far less commonly used for the
table in this country than in France, where it is known as "Langouste"
and is very highly esteemed.

Various species of Spiny Lobsters belonging to the same family
(Palinuridæ) as the European species are found in different parts of the
world. In tropical countries the species of _Panulirus_ are commonly
used for food (for example, _P. interruptus_ in California and _P.
fasciatus_ in India), as are species of _Jasus_ in South Africa,
Australia, and New Zealand. Recently a consignment of Spiny Lobsters
(_Jasus lalandii_) was sent to the London market from the Cape, but it
appears that the experiment was not altogether successful.

Belonging to the same tribe (Nephropsidea) as the Lobsters are the
fresh-water Crayfishes. The English Crayfish (_Astacus pallipes_) is
common in many rivers as far north as Lancashire, and in some parts of
Ireland, but is not found in Scotland. It is not much esteemed for the
table, and although small numbers are sent to Billingsgate, chiefly from
Leicestershire, they are said to be used only for garnishing dishes. The
same species occurs on the Continent of Europe, chiefly in the west and
south (France, Germany, Switzerland, Spain, Italy, and the Balkan
Peninsula). It is known in France as "Écrevisse à pattes blanches" (from
the whitish colour of the under-side of the large claws), and in Germany
as "Steinkrebs," and is distinguished, among other characters, by the
shape of the rostrum (Fig. 77, B), which has a tooth on each side close
to the point. Far more important as an article of food is the larger
_Astacus fluviatilis_, the "Écrevisse à pattes rouges" or "Edelkrebs,"
which is found in France, Germany, Austria, Southern Sweden, Russia,
etc. In this species the under-side of the large claws is generally of a
fine red colour, and the rostrum (Fig. 77, A) has a pair of side-teeth
about the middle of its length, and a long slender point. The red-clawed
Crayfish is an important article of commerce on the Continent, and is
sent to the London market in considerable numbers, chiefly from Germany
and South-West Russia. In France it is cultivated for the market in
"Crayfish farms" on a large scale.

[Illustration: FIG. 77--ROSTRUM AND FORE PART OF CARAPACE, SEEN FROM
ABOVE, OF (A) RED-CLAWED CRAYFISH (_Astacus fluviatilis_) AND (B)
WHITE-CLAWED OR ENGLISH CRAYFISH (_Astacus pallipes_)]

A species of Crayfish (_A. leptodactylus_) occurring in the Lower
Danube and in other rivers flowing into the Black Sea sometimes finds
its way to the London market, although it is less valued than the
red-clawed species. It is distinguished by its long and slender claws,
by the spiny edges of the rostrum, and by other characters. A fourth
species (_A. torrentium_), occurring chiefly in Central Europe, is very
closely allied to _A. pallipes_, and, like it, is of little value for
the table.

Within the last thirty years the Crayfish fisheries of Western Europe
have suffered heavily from outbreaks of an epidemic disease which has
all but exterminated these animals in certain districts. In this country
it is said to be responsible for the almost complete disappearance of
Crayfish from localities where they were formerly plentiful, as, for
instance, in the neighbourhood of Oxford. The cause of the disease is
believed to be a protozoan parasite belonging to the group Myxosporidia.

In other parts of the world it does not seem that the fresh-water
Crayfishes are of much importance as an article of food. Some species of
_Cambarus_ are so used to a limited extent in the United States, and the
gigantic _Astacopsis serratus_ (Plate XX.) is known as the "Murray River
Lobster" in the markets of Sydney and Melbourne.

[Illustration: FIG. 78--THE COMMON SHRIMP (_Crangon vulgaris_). NATURAL
SIZE]

The Decapods of the suborder Natantia comprise a large number of edible
species, generally known as Shrimps and Prawns. The Common Shrimp,
_Crangon vulgaris_ (Fig. 78), which is plentiful on the British coasts
wherever the bottom is sandy, is about two or three inches long, and
when alive is of a translucent greyish colour speckled with brown. It
differs from most of the Natantia in having the body somewhat flattened
from above downwards, and the rostrum very short. When boiled, it is of
a reddish-brown colour, and from this it is sometimes known as the
"Brown Shrimp." On many parts of the coast the Shrimp fishery is of
considerable importance. Most often the Shrimps are caught by means of a
large bag-net attached to a semicircular hoop with a long handle, and
pushed over the surface of the sand by a fisherman wading in the water
at ebb-tide.

A variety of species are sold in England under the name of Prawns. The
largest of the native species, to which the name of Common Prawn is
perhaps most properly restricted, is _Leander serratus_. It grows to a
length of over 4 inches, and has a long serrated rostrum extending
beyond the antennal scales and curving upwards at the point. The first
and second pairs of legs end in small pincer-claws. When alive the
animal is very transparent, and beautifully marked with bands of brown
and red on the body and limbs. A smaller species of the same genus (_L.
squilla_), distinguished by the much shorter and straighter rostrum, and
another very similar species of which the proper name appears to be _L.
adspersus_ (often known as _L. fabricii_), are said to be sold on some
parts of the English coast as "Cup Shrimps."

Much commoner, at least in the London market, than the species of
_Leander_ is _Pandalus montagui_, often sold under the general name of
Prawn, but sometimes called the "Pink Shrimp." This resembles _Leander
serratus_ in having a long, serrated, up-curved rostrum, but differs
from it strikingly in the form of the anterior pairs of feet. The first
pair appear to the naked eye to have no pincer-claws, but to end in a
sharp point, resembling the third maxillipeds, which are just in front
of them. As a matter of fact, they do have pincers, but so minute that
they can only be detected by microscopic examination. The feet of the
second pair are unequal in length on the two sides, that on the left
side being the longer, and are very slender. They end in small pincers,
and examination with a pocket-lens will show that the carpus, or
"wrist," and the segment below it (merus) are broken up into a large
number of short segments, so that the limb is extremely flexible. When
alive, the animal is even more handsomely marked than the Common Prawn.

[Illustration: FIG. 79--THE NORWEGIAN DEEP-WATER PRAWN (_Pandalus
borealis_), FEMALE. (After Sars.)

The second leg of the right side is indicated by dotted lines.]

A large species of Prawn is now imported to this country in considerable
quantities from Norway. This is _Pandalus borealis_ (Fig. 79), a species
closely allied to the last-named, but differing in the longer and more
slender rostrum and in many other characters, as well as in its larger
size (specimens have been recorded of 6 inches in total length). It
also differs in its habitat, for while _P. montagui_ lives in shallow
water, or even between tide-marks, _P. borealis_ occurs at depths of 30
to 60 fathoms in the Norwegian fjords. The recent development of the
fishery for _P. borealis_ in Norway is a striking example of the
practical value of zoological research. Until 1898 the species was
hardly known except to zoologists, although a small fishery was carried
on in the Drammen Fjord, near Christiania. The investigations of the
naturalists employed by the Norwegian Department of Fisheries showed
that the species existed in vast numbers in the deeper water of many of
the fjords, and that it could be captured in abundance by means of a
suitably-devised trawl-net. As a result, a very profitable fishery was
established, and the "deep-water Prawns" are now not only largely
consumed in Norway, but are exported in increasing quantities to the
English and other markets.

In the warmer seas the large Prawns of the genus _Penæus_ are of
considerable importance. Thus, in the Mediterranean countries, _Penæus
caramote_ (Plate IV.) is highly esteemed for food, and _P. setifer_ and
_P. brasiliensis_ are largely consumed in the Southern United States.
_P. monodon_ and other species are eaten in India. An attempt has been
made to send a species of the same genus (apparently _P. indicus_) in a
frozen state from Queensland to the London market.

Numerous other species of Natantia are used for food in various parts of
the world, but the only ones that need be further mentioned here are the
River Prawns of the genus _Palæmon_, which are abundant in the fresh
waters of most tropical countries, and sometimes grow to a very large
size. They are generally distinguished by the fact that the legs of the
second pair are very long, forming powerful pincer-claws. In the West
Indies and Central America, _P. jamaicensis_ (Plate XXI.), which reaches
a length of 10 inches exclusive of the great claws, is sold in the
markets, while in India and elsewhere in the East _P. carcinus_, which
grows to an even greater size, and other smaller species, are used for
food. The fresh-water Prawns of the family Atyidæ, on account of their
small size, are not of much importance from this point of view, but
Professor Hickson states that the little _Caridina nilotica_, a very
widely-distributed species, is eaten in Celebes.

[Illustration: _PLATE XXXI_

     THE COMMON EDIBLE CRAB, _Cancer pagurus_. BRITISH. (MUCH REDUCED)]

Among British Crustacea, the next in importance to the Lobster as an
article of food is the Edible Crab, _Cancer pagurus_ (Plate XXXI.),
known in Scotland as the "Partan." Like the Lobster, it is found on
rocky coasts in shallow water, and young specimens are often taken
between tide-marks. It grows to a size of more than 10 inches across the
shell, and may reach a weight of 12 pounds. The means used for its
capture are the same as in the case of the Lobster, and the fishery is
of considerable importance on many parts of the British coasts. On
the other hand, a Connemara fisherman, who was using these Crabs for
bait, received with incredulity the statement that they were good for
human food!

The Shore Crab, _Carcinus mænas_ (Plate IX.), is not of much importance
as food in this country, although it is recorded that fifty years ago
great numbers were brought to the London market. On the shores of the
Mediterranean and Adriatic, however, and especially in Venice, this
species is regarded as a delicacy, particularly in the soft-shelled
state after moulting.

On the Atlantic coast of North America, the most important edible
Crustacean after the Lobster is the "Blue Crab" (_Callinectes sapidus_),
one of the Swimming Crabs (Portunidæ). This is consumed in large
quantities, especially in the soft-shelled state. Several other species
of Crabs are eaten in America, including the little "Oyster Crab," a
species of _Pinnotheres_ living in the American Oyster. From its small
size, and the difficulty of obtaining it in numbers, it is a very costly
delicacy.

In the East Indies the most important edible Crabs are various species
of Portunidæ, especially the large _Scylla serrata_ and _Neptunus
pelagicus_.

Except as food, the Crustacea are of very little direct use to man.
Almost the only instance in which they are otherwise utilized is in the
case of a species of sessile Barnacle (_Balanus_) which is cultivated
in Japan for use as manure. The method of culture has been described by
Professor Mitsukuri. Bunches of bamboo "collectors," like those used for
the collection of oyster-spat, are fixed into the ground on tidal flats.
After two or three months they are taken up, and the Barnacles with
which they have become covered are beaten off and sold for use as
manure.

Apart from their direct utility, however, the Crustacea are indirectly
of great importance as providing a large part of the food-supply of
marketable fishes. From this point of view, a study of the habits and
distribution of the commoner species may be of practical value in
throwing light on the migrations and other obscure points in the
life-history of the fishes that prey upon them. As an example of this,
we may refer to some investigations on the Mackerel fishery recently
carried out by the naturalists of the Marine Biological Association at
Plymouth. In the spring and early summer months the Mackerel migrate
into inshore waters for the purpose of spawning. During this period the
fish congregate in shoals at the surface of the sea, and are captured in
drift-nets. The extent of this "shoaling" varies greatly from year to
year, and determines whether the season shall be a profitable one for
the fishermen or not. When shoaling, the fish feed exclusively on
plankton, consisting largely of Copepoda, and it has been shown by Mr.
G. E. Bullen that the fluctuations in the yield of the Mackerel fishery
from year to year follow very closely the fluctuations in the abundance
of the Copepod plankton on the fishing-grounds. The investigation has
been carried a step farther by Dr. E. J. Allen, who points out that the
abundance of Copepods is determined by the abundance of the Diatoms and
other minute vegetable organisms of the plankton. These organisms are
very largely influenced by the amount of sunshine during the period of
their development in the earlier months of the year. Dr. Allen gives a
diagram showing for each of seven years (1902-1908) the average number
of hours of bright sunshine during the months of February and March in
the South-West of England. With this he compares the number of fish
caught in the month of May in each of these years by certain vessels
engaged in the western Mackerel fishery. The correspondence between the
two is very striking indeed, and justifies his conclusion that the
amount of sunshine in the early months of the year determines the
abundance of the vegetable life of the plankton, and through it of the
Copepods and other animals which form the bulk of the plankton a little
later in the year; and although there are doubtless other influences at
work determining the success or failure of the fishery, it is largely a
matter of the richness or poverty of the plankton harvest.

None of the Crustacea can be regarded as directly harmful to man. They
have not the power of inflicting envenomed wounds which renders some
other Arthropods, such as Scorpions, some Spiders, Centipedes, and
Insects, formidable in spite of their small size; and although
blood-curdling tales of the ferocity of the Land Crabs are to be found
in the accounts of old voyages, even the largest of these is hardly an
antagonist to be dreaded.

A considerable number of invertebrate animals, not of themselves
noxious, are now known to be the indirect cause of much serious injury
to human life by harbouring and disseminating organisms which produce
disease. The progress of research is adding, almost every day, to the
number of species known to be disease-carriers, and it is possible that
in the future some Crustacea as yet unsuspected may be added to the
list.

At present, however, there is only one case in which a Crustacean has
been shown to be concerned in the transmission of a parasite of man. The
"Guinea-worm," _Filaria_ (or _Dracunculus_) _medinensis_, is a parasite
belonging to the group of "Thread-worms" (Nematoda) which causes
dangerous abscesses under the skin of the legs in many parts of tropical
Africa. It has been shown that the embryos of the worm, which are
discharged in vast numbers on the bursting of the abscess, do not
develop unless they fall into water containing certain species of the
Copepod _Cyclops_ (see Fig. 14, p. 39). In some way not yet understood,
the embryos penetrate into the body cavity of the _Cyclops_, where they
undergo a metamorphosis. For their further development it is necessary
that the _Cyclops_ should be swallowed by man, as may easily happen in
drinking water from a pond. When the _Cyclops_ is digested the larval
worms are set free, and they bore their way through the tissues of their
human host till they reach the place (generally under the skin of the
leg) where they complete their development and produce the innumerable
embryos that are set free in the way just described.

A few Crustacea inflict a certain amount of injury on man in more
indirect ways. In tropical countries, Land Crabs are often troublesome
in gardens, and may cause serious damage to young plants in sugar-cane
plantations and rice-fields. In gardens in this country, the Woodlice,
as already mentioned, are sometimes destructive to seedlings and
delicate plants. The little fresh-water Isopod, _Asellus aquaticus_, is
accused of destroying the nets used in fishing for Pollan in Lough Neagh
in Ireland.

[Illustration: FIG. 80--THE GRIBBLE (_Limnoria lignorum_). MUCH ENLARGED
(From British Museum Guide, after Sars.)]

Probably the most important of all Crustacea, however, as regards their
destructive activity, are the species which bore into wood, and
sometimes do extensive damage to the submerged timber of piers, jetties,
and similar structures. On our own coast the most destructive is a
little Isopod known as the "Gribble" (_Limnoria lignorum_--Fig. 80),
which is distributed from Norway to the Black Sea, and occurs also on
the Atlantic coast of North America. Several species of the same genus
having similar habits are found in other parts of the world. The Gribble
was first discovered as a British species by Robert Stevenson, the
celebrated lighthouse engineer, who found it in 1811 destroying the
woodwork employed in the erection of the Bell Rock Lighthouse, and sent
specimens to Dr. Leach of the British Museum. The animal is only about
one-eighth of an inch in length, and its cylindrical burrow is about
one-fifteenth of an inch in diameter, and penetrates for a depth of one
or two inches. The excavation of the wood is effected by means of the
mandibles, which are unusually strong; and when the animals are
numerous the burrows are driven so close together that the surface of
the wood is reduced to a spongy mass which is rapidly washed away by the
waves (Plate XXXII.). The Gribble is often accompanied by another
Crustacean of similar habits, the Amphipod _Chelura terebrans_. The
latter is about one-fifth of an inch in length, and differs from most
Amphipods in having the body somewhat flattened from above downwards
instead of from side to side. The burrows made by _Chelura_ are
shallower than those of the Gribble, and generally run more or less
parallel to the surface of the wood.

[Illustration: _PLATE XXXII_

     PIECE OF TIMBER FROM RYDE PIER SHOWING DAMAGE CAUSED BY _Limnoria_
     AND _Chelura_

     (_From Brit. Mus. Guide_)]




CHAPTER XII

CRUSTACEA OF THE PAST


Since the acceptance by naturalists of the theory of Evolution as
indicating the mode of origin of the various forms of life now existing,
one of the chief lines of biological investigation has had for its
object the reconstruction of the pedigree (or, as it is called, the
"phylogeny") of the larger groups of the animal and vegetable kingdoms.
In attempting to do this, there are three main sources from which
evidence may be drawn. The results of Comparative Anatomy enable us to
decide with more or less confidence as to the degrees of relationship
between the groups of organisms, and to distinguish between the more
primitive and the more specialized; the study of Embryology is, at
least, an indispensable adjunct to Comparative Anatomy, even if it does
not, as was once supposed, give us an actual recapitulation of ancestral
history; and, finally, the study of Fossil Remains holds out the hope
that we may be able to find the ancestral types themselves.

It is clear that evidence from the last-named source, when it is
available, is the most important of all, since the order of succession
of the various types is given by that of the rock strata in which they
occur, and we can be quite certain that we are dealing, if not with the
actual ancestors, at least with the forerunners of existing species. The
"imperfection of the geological record," however, is so great that the
organisms preserved in the fossil state represent only an insignificant
part of the whole number of organisms that have lived on the globe since
life began; and it is not surprising, therefore, that in many groups the
study of fossils has hitherto afforded little help towards the working
out of their genealogical history. Thus, among Crustacea there are many
important groups such as the Copepoda, which are entirely unknown as
fossils, their small and delicate bodies being ill adapted for
preservation, although there is every reason to suppose that they are a
very primitive and very ancient group. In many fossil Crustacea only the
hard shell or carapace has been preserved, the appendages being lost or
represented only by indecipherable fragments, and in some cases it is
hardly possible to guess at the affinities of the animals. Further,
several important groups are already represented in some of the oldest
of the fossil-bearing rocks at present known, and the differentiation of
these groups must have taken place in the dark ages before the record of
the fossils begins. In spite of these disadvantages, however, the study
of fossil Crustacea does throw considerable light on the evolution of
the group, and it is likely that interesting results in this direction
await future investigations.

[Illustration: FIG. 81--RESTORATION OF A TRILOBITE (_Triarthrus becki_),
SHOWING THE APPENDAGES. UPPER SIDE ON RIGHT, UNDER-SIDE ON LEFT.
SLIGHTLY ENLARGED. (After Beecher.)]

In the earliest fossiliferous rocks the most abundant and important
Arthropods are the Trilobites (Fig. 81), an extinct group which appears
to have been related to the primitive Crustacea. The name Trilobite
refers to the three-lobed form of the body when seen from the dorsal
side, most species having a pair of grooves running lengthwise which
divide off a middle lobe containing the principal organs of the body
from two lateral "pleural" expansions covering the limbs. The
head-shield shows indications of being composed of five segments, and
bears a pair of sessile compound eyes. It is followed by a number (up to
twenty-six) of free somites, and the body ends in a tail-shield, or
"pygidium," which is often plainly composed of several somites fused
together. Although Trilobites are among the commonest and most familiar
of fossils in the older rocks, the nature of their appendages remained
quite unknown until within recent years, when specimens of several
species showing the structure of the limbs and under-side of the body
were discovered in America. From these it appears that the head bore in
front a pair of long thread-like antennæ and four pairs of two-branched
appendages, each with a jaw process, or "gnathobase," turned towards the
mouth, which is covered below by a large anterior lip, or "hypostome."
It seems probable that the five pairs of head-appendages correspond
respectively to the antennules, antennæ, mandibles, maxillulæ, and
maxillæ, of Crustacea; but the second pair appear to have acted as jaws,
retaining the gnathobase which, among Crustacea, is only hinted at by
the hooked spine on the antenna of the nauplius larva.

Each of the free somites and of those forming the tail-shield bears a
pair of two-branched appendages, not differing greatly from the
posterior appendages of the head, but becoming smaller and more
flattened towards the hinder end of the body. The numerous genera and
species of Trilobites present great differences in the form and
ornamentation of the dorsal surface of the body, and it is probable that
considerable differences may also have existed in the structure of the
limbs, which are only known in two or three species. Some Trilobites are
among the most ancient of known fossils, being found in rocks of the
Lower Cambrian epoch. The group reaches its maximum development in the
Ordovician, and the number of the species and size of the individuals
gradually diminish through the Silurian and Devonian till they become
extinct at the close of the Carboniferous epoch, except for a single
species found in rocks of Permian age in America.

Although zoologists are not all agreed as to the precise systematic
place to be assigned to the Trilobites, there can be little doubt that
they were related more or less closely to the most primitive Crustacea,
and they are of special interest as preserving for us the stage in which
the second pair of appendages were still used as biting jaws, and had
not moved forwards in front of the mouth to become antennæ, as in all
living Crustacea.

Contemporary with some of the earliest Trilobites, however, are
undoubted Crustacea, which, so far as we know their structure, are not
very different from types now living. In the Cambrian epoch the
Branchiopoda appear to be represented by _Protocaris_, which in its
general form resembles _Apus_; and there are a variety of genera and
species of Ostracoda, although, since their shells alone are preserved,
it is not possible to determine their exact relations to existing forms.
In the succeeding Ordovician and Silurian epochs we first meet with the
remains of Barnacles, and it is interesting to note that some of them
are referred to the genera _Pollicipes_ and _Scalpellum_, which are
represented by numerous species in the seas of the present day. Along
with these, however, are some strange-looking forms (_Turrilepas_,
etc.), having the body covered with rows of overlapping plates. If these
are really Cirripedes, they must have differed considerably in structure
from the more modern types.

The Malacostraca are more interesting from the point of view of
palæontology than the other subclasses of Crustacea, since the evolution
of the group appears to have taken place within the period covered by
the fossil records, and it is possible to trace the course of that
evolution--at least, in its broad outlines. It has already been pointed
out that the most primitive of existing Malacostraca are the
Phyllocarida (_Nebalia_ and its allies), which are in several respects
intermediate between the higher Malacostraca and the Branchiopoda; and
it is interesting to find that fossils apparently belonging to the
Phyllocarida are found far earlier than any of the other Malacostraca.
In the Cambrian, and more abundantly in the Ordovician and Silurian,
there are found Crustacea (Fig. 82) that resemble _Nebalia_ in having a
large bivalved carapace, with a movable beak-like plate in front, a
projecting abdomen without conspicuous limbs, and a pair of large spines
at the sides of the telson. Unfortunately, we have almost no knowledge
of the structure of the limbs; but it can hardly be doubted that these
very ancient Crustacea were allied to the existing Phyllocarida, and
that they included the forerunners of the higher Malacostraca.

[Illustration: FIG. 82--_Ceratiocaris papilio_, ONE OF THE FOSSIL
PHYLLOCARIDA. (From Lankester's "Treatise on Zoology," after H.
Woodward.)

_a_, Traces of antennules; _m_, possibly mandibles; _r_, rostral plate]

It is in the Carboniferous epoch, in all probability, that we must look
for the origin of most of the existing orders of Malacostraca. In the
rocks of this age in different parts of the world there have been found
a number of undoubted Malacostraca, nearly all of the shrimp-like form
which there is good reason to believe to be a primitive characteristic.
Some of these (_Pygocephalus_--Fig. 83) have recently been shown to
possess a brood-pouch formed of overlapping plates on the under-side of
the thorax, and thus resemble the existing Mysidacea, which stand at the
base of the Peracaridan series of orders. Others have a pair of strong
side-spines near the tip of the telson, and in other ways resemble the
recent Euphausiacea, so that they may have been primitive members of the
Eucaridan series.

[Illustration: FIG. 83--_Pygocephalus cooperi_, FROM THE COAL-MEASURES:
UNDER-SIDE OF A FEMALE SPECIMEN, SHOWING THE OVERLAPPING PLATES OF THE
BROOD-POUCH. (From Lankester's "Treatise on Zoology," after H.
Woodward.)]

[Illustration: FIG. 84--THE TASMANIAN "MOUNTAIN SHRIMP" (_Anaspides
tasmaniæ_), A LIVING REPRESENTATIVE OF THE SYNCARIDA. SLIGHTLY ENLARGED

_c.gr._, "Cervical groove," marking off the first thoracic somite;
ii-viii, the remaining thoracic somites; 1-6, the abdominal somites]

Among the Crustacea of the Carboniferous and Permian epochs, there are a
number of forms of which the affinities were until recently quite
obscure. They have two-branched antennules, a scale-like exopodite on
the antenna, and the last pair of appendages (uropods) form, with the
telson, a tail-fan. In these points they resemble the shrimp-like
forms, but there is no carapace, and all the somites of the thorax are
distinct, so that the form of the body is rather that of an Amphipod or
Isopod. On the discovery of the remarkable Crustacean _Anaspides_ (Fig.
84), which lives in fresh-water pools in the mountains of Tasmania, it
was pointed out that it agreed with the fossil genera _Uronectes_,
_Palæocaris_, and their allies, in those very characters in which they
differed from all other Crustacea, and that it must be regarded as a
surviving representative of the ancient group to which the name of
Syncarida had been given. The more recent discoveries of living forms,
_Paranaspides_ from the Great Lake of Tasmania and _Koonunga_ from
fresh-water pools near Melbourne, and of the fossil _Præanaspides_
(Fig. 85) from the Coal-measures of Derbyshire, have tended to support
this conclusion. There can be little doubt that the Syncarida arose
during the Carboniferous epoch (or earlier) from primitive shrimp-like
forms which lost the carapace; but, after flourishing for a relatively
brief period, the group dwindled away, although a few survivors have
lingered on, like so many other "living fossils," in the isolated
Australian region.

[Illustration: FIG. 85--_Præanaspides præcursor_, ONE OF THE FOSSIL
SYNCARIDA, FROM THE COAL-MEASURES OF DERBYSHIRE. SLIGHTLY ENLARGED.
(After H. Woodward.)]

It must be pointed out that, in spite of the resemblance of the body of
_Anaspides_ to that of an Amphipod, the Syncarida can have had no close
relation to the origin of the Isopoda and Amphipoda. These have also
been derived from a shrimp-like type, but their possession of a
brood-pouch, among other characters, shows that they are linked on to
the Mysidacea, and must have arisen from some primitive member of that
group, like _Pygocephalus_. Although palæontology as yet gives little
help in tracing the course of their evolution, we can imagine what the
intermediate links must have been like by comparison with the living
Cumacea and Tanaidacea.

It is possible, indeed, that the divergence of the Isopod line of
descent from that of the Mysidacea took place earlier than the
Carboniferous epoch, for there has recently been discovered in rocks of
Devonian age in Ireland a single specimen of a fossil, to which the name
of _Oxyuropoda_ has been given, which has every appearance of being an
Isopod. At all events, undoubted Isopods make their appearance in rocks
of the Secondary Period, and some of those from the Jurassic epoch are
not very different in general form from types still existing.

Some of the Carboniferous shrimp-like Crustacea present characters which
seem to point in the direction of the Stomatopoda, and fossils which
clearly belong to that group are found in Jurassic and later deposits.
In the Cretaceous epoch there were Stomatopoda resembling modern types
so closely that they have been referred to the existing genus _Squilla_.
We are even able to say that they resembled the living Stomatopoda in
their mode of development, for larvæ of the type known as _Erichthus_
have been recognized in rocks of Cretaceous age from Lebanon. This is a
striking example of the way in which, by a fortunate accident as it
were, organisms apparently ill-adapted for fossilization may
occasionally be preserved.

Of the Decapoda the geological history is tolerably full, and it is
possible to trace in its broad outlines the course of evolution of the
various suborders. Here again it is likely that the beginnings of the
group are to be sought for in the Carboniferous epoch, and some of the
obscure shrimp-like forms of that age show hints of an affinity with the
Decapods. In the Triassic epoch, however, and more abundantly in the
succeeding Jurassic, a number of types are found which seem to include
primitive representatives of several of the existing groups of Natantia
and Reptantia. It is noteworthy that among them are some forms (_Æger_,
etc.) resembling the existing Stenopidea, a tribe which in some respects
is intermediate between these two suborders. In the Stenopidea the first
three pairs of legs bear pincer-claws, as in the Lobster, but the third
pair is much the largest; and _Æger_ agrees with them in this unusual
character, though there is little else, in what is known of its
structure, to help to determine its affinities.

The tribe Penæidea, which occupies in many respects a primitive place
among the Natantia, is abundantly represented in the Jurassic epoch,
especially in the lithographic stone (Upper Jurassic) of Solenhofen, and
by somewhat doubtful specimens from the earlier Trias. All these agree
in having the first three pairs of legs with pincer-claws, and not
differing greatly in size. Some of the Jurassic and later fossils are of
so modern a type that they have been referred to the existing genus
_Penæus_.

The Upper Jurassic rocks also preserve the earliest undoubted specimens
of true Prawns of the tribe Caridea, and some of these show swimming
branches (exopodites) on the thoracic legs, so that they were probably
related to the primitive family Acanthephyridæ, of which the existing
members are found in the deep sea. It is possible, however, that Caridea
were already in existence far earlier, for some of the obscure
Carboniferous forms seem to have the broadened side-plates of the second
abdominal somite, which, so far as we know, are exclusively
characteristic of that tribe.

The Reptantia, forming the other large division of the Decapoda, also
had their origin at least as early as the Triassic epoch, where
representatives of the tribes Eryonidea and Scyllaridea are found. The
history of the Eryonidea has already been discussed (p. 133) in dealing
with the deep-sea Crustacea. The oldest representatives of the
Scyllaridea belong to a family (Glyphæidæ) now wholly extinct, and are
in many respects more primitive and lobster-like than any of the living
Spiny Lobsters and their allies (Palinuridæ and Scyllaridæ). Forms with
greatly thickened antennæ, indicating a transition to the Palinuridæ,
begin to appear in the Jurassic; and in the later Cretaceous a genus,
_Podocrates_, occurs which is hardly to be distinguished from
_Linuparus_, now living in Japanese seas. The Scyllaridæ have the
antennæ modified into broad shovel-like plates, and perhaps take their
origin from _Cancrinus_, in the Solenhofen lithographic stone
(Jurassic), which has broad and apparently flattened antennæ. True
Scyllaridæ are certainly found in Cretaceous deposits, and some, from
the Upper Chalk, are even referred to the existing genus _Scyllarus_.

The Anomura are almost unknown as fossils, but the true Crabs, or
Brachyura, are abundantly represented. They first appear about the
middle of the Jurassic epoch, and, as already pointed out, the earliest
forms (Prosoponidæ) are referred to the Dromiacea, and appear to be
closely related to the primitive Homolodromiidæ now living in the deep
sea (p. 134). One of the oldest, and at the same time one of the most
completely known, is _Protocarcinus_, from the Great Oolite of
Wiltshire, which is preserved (in the only known specimen) with the
abdomen partly extended, possibly indicating that the abdomen was less
closely doubled under the body than in modern Crabs.

The next group of Crabs to appear are the Oxystomata, which are found
from the middle of the Cretaceous epoch onwards. The Brachyrhyncha
perhaps begin to appear about the same time, but the affinities of the
earlier types are doubtful, and it is only in the Tertiary that they
become abundant and unmistakable. Several living genera, such as
_Cancer_, date back to the Eocene. The Spider Crabs (Oxyrhyncha) are
rare as fossils, and the earliest specimens are found near the beginning
of the Tertiary.




APPENDIX


I. METHODS OF COLLECTING AND PRESERVING CRUSTACEA

It may be useful to give here a few hints as to the methods of
collecting Crustacea. Of the species that live in the sea, many may be
found between tide-marks by turning over stones and searching among
sea-weeds and in rock crevices. A small hand-net, made by fastening a
bag of coarse muslin to a stout wire ring of a few inches diameter, is
useful for fishing in rock pools. Shore-collecting in this manner is
most productive at spring-tides, when the deeper levels of the shore are
open to exploration.

Many burrowing species are to be found by digging in the sand near
low-water mark. In addition to Crabs and other large species, many
minute forms, Amphipods, Cumacea, and the like, inhabit such localities.
The best way of collecting these is to stir up a spadeful of the sand in
a bucket of water, and, after allowing the sand to settle for a few
seconds, to pour off the water through a muslin bag. After repeating the
operation two or three times, the contents of the bag are washed out
into a jar or dish of sea-water for examination with a lens or under
the microscope.

Dredging is the most effective method of obtaining Crustacea that live
in deeper water. The dredge usually employed by naturalists consists of
a heavy rectangular iron frame to which is attached a strong bag-shaped
net. The two longer sides of the frame are sharp-edged and bevelled
outwards, so as to "bite" when the dredge is dragged along the
sea-bottom. To the shorter sides are hinged a pair of arms ending in
rings. The dredge-rope is made fast to one of these rings, while the
other is held only by a "stopping" of yarn, which gives way if the
dredge should catch on a rock, and permits it to be dragged sideways off
the obstruction. The size and weight of the dredge may vary according to
the depth at which it is to be used and the power available for working
it. A convenient size for use with a small sailing boat at moderate
depths has a frame 20 by 5 inches.

Apart from dredging, many specimens from moderately deep water may be
picked out from among the "rubbish" brought up on fishermen's lines or
by the trawl, and various Crustacea besides the edible species find
their way into Lobster and Crab pots. The true deep-sea fauna is, for
the most part, only to be reached by specially-equipped expeditions,
although specimens from great depths are occasionally obtained during
the operations for the repair of submarine telegraph cables.

The floating animals of the surface of the sea are to be captured by
means of the tow-net. This consists of a conical bag of muslin,
cheese-cloth, or, best of all, silk "bolting-cloth," attached to a
galvanized iron-ring of one or two feet in diameter, and having a zinc
can or a strong glass jar fixed to the narrow end. The ring is attached
by three equidistant cords to the towing line, and the net is towed
slowly at or near the surface of the sea. When taken on board, the
contents of the can are emptied into a jar of sea-water for examination.
The tow-net is best used when there is only enough way on the boat to
keep the net from sinking; if towed more rapidly, delicate organisms are
apt to be crushed by the pressure of the water, or the net itself may be
burst. The use of unnecessarily fine nets should be avoided. A
fine-meshed net may not capture a single specimen of the larger
Crustacea, even though these may be swarming in the water through which
it is drawn.

By weighting the tow-net it may be used at various depths to capture the
floating animals of mid-water. When it is so used, however, it is
impossible to tell from what depth any particular specimen may have
come, since it may have been captured during the hauling in of the net.
For more precise investigations in deep water, "closing tow-nets" of
various types have been devised, which can be opened by a "messenger"
sent down the line when the net has reached the desired depth, and
closed again by another "messenger" before it is hauled in.

A simple method that has proved very successful for collecting small
Crustacea living on a sandy bottom in shallow water is to employ a light
tow-net with a cane ring, and with a heavy sinker attached to the towing
line at a distance of a few feet in front of the net. As the sinker is
dragged along the bottom, the net floats up behind it, and catches small
animals stirred up by its passage.

For collecting the smaller fresh-water Crustacea--Water-fleas and the
like--a small muslin ring-net may be used in ponds and ditches. The
plankton of the open water of lakes is best obtained by means of a
tow-net like that described above for use in the sea.

The interesting blind species known as "Well Shrimps" are to be looked
for in the water of springs and wells. In wells fitted with a pump,
Professor Chilton found that "the Crustacea are often brought up most
abundantly when pumping is first commenced, and that jerking the handle
of the pump somewhat violently is often more successful than pumping at
the ordinary rate." In disused open wells, they may be trapped by
baiting a muslin ring-net with a piece of stale meat or fish, and
pulling it up rapidly after it has remained in the well for a few hours.
The subterranean waters of caves have yielded many curious species in
various parts of the world. For the capture of species living in the
deep water of large lakes, a special form of dredge has been devised
with runners to prevent it from sinking into the soft mud, while the
mouth of the net is raised a few inches above the bottom.

For preserving Crustacea the best medium to use is 70 per cent. alcohol.
Strong spirit diluted with a little less than one-third its bulk of
water gives about the required strength. If too strong spirit is used,
the specimens tend to be hard and brittle, and delicate organisms become
shrivelled. Methylated spirit as sold in the shops in this country
contains mineral naphtha, and turns milky when water is added, so that
it is unsuitable for preserving specimens. Methylated alcohol without
naphtha can be bought, by permission of the Inland Revenue authorities,
but only in considerable quantities at a time.

Formalin is very cheap and readily obtained, but it is much less
suitable than spirit for most Crustacea, as it tends to make them stiff
and fragile, and small forms containing much lime, such as Cumacea, may
become decalcified. For Crustacea collected by the tow-net, however,
formalin gives good results. A few drops of strong formalin, added to
the water into which the tow-net has been washed, kills the animals in a
few minutes. After they have sunk to the bottom, the liquid may be
poured off and replaced by formalin diluted with sea-water (for marine
plankton), or by a mixture of formalin and spirit. The most suitable
strength of formalin varies with different organisms, but 5 per cent.
(_i.e._, 1 part of commercial formalin to 19 parts of water) is perhaps
most generally useful.

Crabs, Prawns, and the like, if put alive into strong spirit, may throw
off some of their limbs, or else become so rigid that these break on
the slightest manipulation. This may often be avoided by killing the
animals in weak spirit (30 per cent. or less) before preserving in
strong spirit. Marine species may also be killed by placing them in
fresh water, care being taken not to allow them to remain in it longer
than is necessary, as it causes distortion of the membranous appendages.

The larger Crabs, Lobsters, and the like, may be preserved dry, although
in this state they are unsuitable for examination of the more delicate
appendages. The carapace should be detached, and the soft parts cleaned
away as far as possible, a bent wire being used, if necessary, to remove
the flesh from the legs. The specimens should be dried in the shade, to
preserve as much as possible of the natural colour.

With specimens intended for permanent preservation in spirit, the use of
corks should be avoided, as they discolour the spirit, and ultimately
the specimens. Small specimens are most conveniently kept in glass tubes
closed with a piece of clean elder-pith (not cotton-wool), and placed,
upside down, in a bottle of spirit. Labels to be placed inside the tubes
are best written with Indian ink, and allowed to dry before immersion in
the spirit.


II. NOTES ON BOOKS

The literature of Carcinology is bewildering in its extent, and is for
the most part scattered through the volumes of scientific periodicals
and the publications of learned societies in most of the languages of
Europe. A guide to the current literature is provided by the _Zoological
Record_, the latest volume of which, relating to the year 1909,
enumerates no fewer than 337 papers dealing wholly or in part with this
group of animals.

The following short list of books in the English language may be of some
help to the beginner. Most of them give references to the literature
which will provide the necessary guidance towards a further study of the
subject.


GENERAL WORK

     _Huxley, T. H._ The Crayfish: an Introduction to the Study of
     Zoology. International Science Series, vol. xxviii. London, 1880.

     _Stebbing, T. R. R._ A History of Crustacea: Recent Malacostraca.
     International Science Series, vol. lxxiv. London, 1893.

     _Calman, W. T._ Crustacea. A Treatise on Zoology, edited by Sir Ray
     Lankester. Part vii., fascicle iii. London, 1909.

     _Smith, G._, and _Weldon, W. F. R._ Crustacea. The Cambridge
     Natural History, vol. iv. London, 1909.

     _Lister, J. J._ Crustacea, in "A Student's Textbook of Zoology," by
     Adam Sedgwick. Vol. iii. London, 1909.


BRITISH CRUSTACEA

     _Baird, W._ The Natural History of the British Entomostraca. (Ray
     Society.) London, 1850.

     _Bell, T._ A History of the British Stalk-eyed Crustacea. London,
     1853.

     _Spence Bate, C._, and _Westwood, J. O._ A History of the British
     Sessile-eyed Crustacea. 2 vols. London, 1863 and 1868.

     _Brady, G. S._ A Monograph of the Free and Semiparasitic Copepoda
     of the British Islands. (Ray Society.) 3 vols. London, 1878-1880.

These works, although still valuable, and indeed indispensable, are now
more or less out of date. A list of British Malacostraca (except
Amphipoda) will be found in Mr. Stebbing's volume mentioned above.

     _Sars, G. O._ An Account of the Crustacea of Norway. Vol. i.,
     Amphipoda, 1890-1895. Vol. ii., Isopoda, 1896-1899. Vol. iii.,
     Cumacea, 1899-1900. Vols. iv. and v., Copepoda, 1903 (in progress).
     Christiana and Bergen.

A very large proportion of the British species in the groups mentioned
are described and figured in this splendid series of volumes. The text
is in English.

     _Norman, A. M._, and _Scott, T._ The Crustacea of Devon and
     Cornwall. London, 1906.

     _Webb, W. M._, and _Sillem, C._ The British Woodlice. London, 1906.

     Memoirs of the Liverpool Marine Biology Committee, edited by
     Professor _W. A. Herdman_. A useful series of monographs on the
     structure and life-history of common British marine animals and
     plants. The following relate to Crustacea:

     Memoir VI. Lepeophtheirus and Lernæa (Parasitic Copepoda). By _A.
     Scott_. 1901.

     Memoir XII. Gammarus (Amphipod). By _M. Cussans_. 1904.

     Memoir XIV. Ligia (Isopod). By _C. G. Hewitt_. 1907.

     Memoir XVI. Cancer (Edible Crab). By _J. Pearson_. 1908.

     Descriptions of all the British species of Barnacles will be found
     in _Darwin's_ "Monograph of the Sub-class Cirripedia." (Ray
     Society.) 2 vols. London, 1851-1854.




INDEX


  Abdomen of Lobster, 6

  Abyssal fauna of lakes, 185

  Acanthephyridæ, 268

  Acorn-shells, 41

  _Æga_: in Sponges, 210;
    parasitic on fish, 219

  _Æger_, 267

  _Æglea_, 181

  Air-breathing Crabs, 188

  _Albunea_, 102

  Alcock, A.: on habits of _Ocypode_, 105;
    on temperature of sea, 120;
    on eyes of deep-sea Crustacea, 124;
    on colour in deep-sea Crustacea, 127;
    on luminosity of deep-sea Crustacea, 125, 126;
    on eggs of deep-sea Prawn, 130;
    on _Nephrops_, 132;
    on habits of _Coenobita_, 194, 195

  Allen, E. J., on Mackerel fishery, 251

  Alpheidæ, 211

  American Lobster, 32

  Amphibious Crustacea, 104, 188

  Amphipoda, 52;
    seashore, 95, 107;
    plankton, 145;
    terrestrial, 189;
    fresh-water, 172;
    in Sponges, 210;
    on Jellyfishes, 212;
    and Hermit Crabs, 215;
    parasitic, 223;
    wood-boring, 255

  _Amphithoë_, 95

  _Anaspides_, structure and fossil allies, 264

  Andrews, C. W.:
    on Land Crabs, 190;
    on habits of _Coenobita_, 195;
    on habits of _Birgus_, 197, 198

  _Anomalocera_, 150

  Anomura, 60;
    fresh-water, 181

  Anostraca, 36;
    habits, 162, 164

  Antennæ of Lobster, 8, 14;
    use in respiration in _Corystes_, 100;
    in _Albunea_, 102

  Antennule of Lobster, 8, 14

  Ants, Woodlice living with, 205

  _Apseudes_, 50

  _Apus_, 36;
    habitats, 161;
    occurrence in Britain, 162;
    habits, 163;
    hæmoglobin in, 165;
    fossil allies of, 260

  Appendages of Lobster, 8;
    of Trilobites, 259

  _Aratus_, 189

  _Argulus_, 41

  Aristotle on Shrimp living with Mollusc, 218

  _Armadillidium_, 203

  _Artemia_:
    larvæ of, 81;
    habitat, 164

  Arthropoda, 2;
    classification, 204

  _Asellus_, 172;
    destroying fishing nets, 253

  Astacidæ, 176

  _Astacoides_, 178

  _Astacopsis_, 177;
    used for food, 243

  Astacura, 60

  _Astacus_:
    habits, 174;
    young, 76;
    distribution, 177;
    used for food, 241

  Asymmetry of Lobster, 29

  Atyidæ, 179;
    used for food, 248

  Autotomy, 113;
    in Lobster, 30


  Baikal, Lake, 182

  _Balanus_, 42;
    larvæ, 83;
    habits, 114;
    used for food, 237;
    cultivated for manure, 250

  Barnacles, 41;
    development, 83;
    seashore, 114;
    of high seas, 155;
    on Whales, Turtles, and Crabs, 209;
    parasitic, on fish, 235;
    used for food, 237;
    cultivated for manure, 250;
    fossil, 261

  _Bathynomus_, 131

  Beach-fleas, 107

  Bell, T., on development of Land Crabs, 193

  "Berry," Lobster in, 28

  Bipolarity, 132

  _Birgus_, 94;
    habits and structure, 195;
    breeding habits, 199.
    See also Robber Crab

  Bloodvessels of Lobster, 17

  Blue Crab, 249

  _Bopyroides_, 221

  _Bopyrus_, 221

  Borradaile, L. A.:
    on habits of _Remipes_, 102;
    on _Huenia_, 110;
    on larvæ of Robber Crab, 199

  Bouvier, E. L., on Dromiacea, 134

  Brachygnatha, 63

  Brachyrhyncha, 64;
    fossil, 270

  Brachyura, 62;
    fossil, 269

  Brain of Lobster, 20

  Branchiopoda, 35;
    larvæ, 80;
    habitats, 161;
    fossil, 260

  Branchiostegite, 18

  _Branchipus_, 165

  Branchiura, 41

  Brine Shrimp, 164;
    larvæ, 81

  Brood-pouch of Peracarida, 46

  Brown Shrimp, 244

  Browne, F. Balfour, on _Apus_ in Scotland, 162

  Browne, Patrick, on Mountain Crab, 193

  Bullen, G. E., on food of Mackerel, 251

  _Bythotrephes_, 168


  Caligidæ, 225

  _Caligus_, 225

  _Callianassa_, 103

  _Callinectes_, 249

  _Calocalanus_, 149

  _Cambaroides_, 177

  _Cambarus_:
    distribution, 177;
    habits, 178;
    blind species, 185;
    used for food, 243

  _Cancer_:
    used for food, 248;
    fossil, 270.
    See also Edible Crab

  _Cancrinus_, 269

  _Canthocamptus_, 170;
    resting stage, 171

  _Caprella_, 54

  Caprellidæ, 55;
    habits, 109

  Carapace of Lobster, 6, 9

  _Carcinus_:
    larval stages, 68;
    habits, 107.
    See also Shore Crab

  _Cardisoma_, 191

  Caridea, 59;
    zoëa of, 73;
    fossil, 268

  _Caridina_, 248

  Carp-lice, 41

  Caspian Sea, 182

  Caudal fork, 40

  Cephalothorax of Lobster, 8

  _Ceratiocaris_, 262

  _Chalimus_, 225

  Chela of Lobster, 12

  Cheliped of Lobster, 8

  _Chelonobia_, 155, 209

  _Chelura_, 255

  _Cheraps_, 177

  Chilton, C., on Woodlice in New Zealand, 206

  _Chirocephalus_, 35, 161

  Chitin, 15

  _Chondracanthus_, 228

  Chromatophores, 31, 110

  Chun, C., on eyes of plankton Crustacea, 154

  _Chydorus_, 165

  _Cirolana_, 218

  Cirripedia, 41;
    development, 82;
    on Whales, 209;
    parasitic, 231;
    fossil, 261.
    See also Barnacles

  Cladocera, 37;
    development, 80;
    habits, 165;
    in plankton of lakes, 168;
    absence from Tanganyika, 184

  Claspers of _Chirocephalus_, 35

  Classification of Crustacea, table, 34;
    of Decapoda, table, 58

  Close time for Lobsters, 239

  Coconut Crab. See Robber Crab

  Cocoons of Copepoda, 171

  _Coenobita_:
    habits, 194;
    respiration, 195

  Coenobitidæ, 61;
    habits, 194

  Colour of Lobster, 31;
    deep-sea Crustacea, 127;
    plankton Crustacea, 150;
    Branchiopoda, 165;
    subterranean Crustacea, 187

  Colour-change, 110

  Columbus and Gulf-weed Crab, 155

  Commensalism, 207

  _Conchoderma_, 209

  _Conchoecia_, 144

  Conchostraca, 37;
    habits, 163

  Convergent evolution, 205

  Copepoda, 40;
    development, 82;
    of open sea, 140;
    plankton, 149;
    luminosity, 150;
    eyes, 152;
    of fresh water, 170;
    of Tanganyika, 184;
    and Hermit Crabs, 215;
    parasitic, 224;
    as food of Mackerel, 250

  _Copilia_, 152

  Coral of Lobster, 27

  Corals, Crustacea on, 210

  _Coronula_, 155, 209

  Corycæidæ, 152

  _Corystes_, 99

  Crabs:
    true, 62;
    sand-burrowing, 99;
    of fresh water, 179;
    of Tanganyika, 183;
    on Corals, 211;
    carrying Sea-anemones, 215;
    living with Molluscs, 217;
    living in Sea-urchin, 218;
    Isopods parasitic on, 223;
    Rhizocephala parasitic on, 231;
    used for food, 248;
    fossil, 269.
    See also Shore Crab, Edible Crab, etc.

  _Crangon_, 59;
    habits, 97;
    fishery, 244

  Crawfish, Sea-, 59

  Crayfish:
    young of, 76;
    habits, 174;
    distribution, 176;
    terrestrial, 178, 189;
    blind, in caves, 185;
    in British Isles, 241;
    used for food, 241;
    disease of, 243

  Cumacea, 48;
    habits, 98;
    of deep sea, 129;
    of plankton, 141

  Cunningham, J. T., on development of Land Crabs, 192

  Cup Shrimps, 245

  Cyamidæ, 56;
    habits, 224

  _Cyclops_, 40;
    nauplius, 82;
    habits, 170;
    resting stage, 171;
    as host of Guinea-worm, 252

  _Cymothoa_, 220

  Cymothoidæ, habits, 218

  Cymothoinæ, habits, 220

  _Cypris_, 38;
    reproduction, 172

  Cypris stage of Barnacles, 84;
    of _Sacculina_, 233

  _Cystisoma_, 145

  _Cythereis_, 38


  _Daphnia_, 38;
    development, 81;
    habits, 165;
    in plankton of lakes, 168

  Darwin, C.:
    on fresh-water fauna, 157, 159;
    on habits of _Birgus_, 198, 199;
    on Barnacles used for food, 237

  Decapoda, 57;
    fossil, 267

  Deep-water Prawn, 246

  Delage, Y., on development of _Sacculina_, 232

  Development of Lobster, 28;
    of Crayfish, 77;
    of River Crab, 78;
    of Peracarida, 78;
    of Woodlice, 79;
    of Opossum Shrimp, 79;
    of Cladocera, 80;
    of Ostracoda, 81;
    of Copepoda, 82;
    of fresh-water Crustacea, 160;
    of Epicaridea, 223;
    of parasitic Copepoda, 225, 227, 230;
    of Rhizocephala, 232

  _Diaptomus_, 170

  _Diastylis_, 49

  Diatoms, 139;
    relation to Mackerel fishery, 251

  _Dichelaspis_, 209

  Digestive gland, 17

  Dispersal of fresh-water Crustacea, 159;
    of Crayfishes, 174

  Distribution of Woodlice, 206

  Doflein, F., on luminosity in marine animals, 126

  Dogfish, Barnacle parasitic on, 235

  _Dorippe_, 95

  _Dracunculus_, 252

  _Dromia_, 63;
    habits, 96;
    and Sponge, 215

  Dromiacea, 63;
    of deep Sea, 128, 134

  Dublin Prawn, 33;
    fishery, 240


  _Ebalia_, 63;
    protective resemblance, 109

  Écrevisse, 242

  Edelkrebs, 242

  Edible Crab, 64, 248

  Eggs of Lobster, 27;
    of deep-sea Crustacea, 130;
    of fresh water Branchiopoda, 161

  Endopodite, 11

  _Engæus_:
    distribution, 177;
    habits, 179, 189

  Entoniscidæ, 223

  Ephippium of Cladocera, 167

  Epicaridea, habits, 221

  Epiplankton, 143

  Epipodite, 11

  Epistome, 62

  _Erichthus_ larva, fossil, 266

  _Eryon_, 135

  Eryonidea, 60;
    eye-stalks of, 122;
    luminosity, 126;
    eggs of, 131;
    fossil and living species, 133, 268

  _Estheria_, 36;
    habitats, 161

  Eucarida, 56

  Eucopepoda, 41

  Eumalacostraca, 45

  _Eupagurus_, 91;
    commensals, 213

  Euphausiacea, 56;
    larvæ, 76;
    of deep sea, 124, 125;
    luminous organs, 125, 151;
    eyes, 153;
    fossil, 263

  European Lobster, 32

  _Eurydice_, 219

  Evolution, 256

  Excretory organs, 19

  Exopodite, 10

  Exoskeleton, 15

  Eyes of Lobster, 20;
    of _Cyclops_, 40;
    of deep-sea Crustacea, 121;
    of plankton Crustacea, 151;
    of _Bythotrephes_, 169;
    degeneration in subterranean Crustacea, 186

  Eye-stalk of Lobster, 14


  Fairy Shrimp, 35;
    habitats, 161

  _Filaria_, 252

  Fiddler Crab:
    habits, 106;
    colour-change, 111

  Fish:
    attacked by Isopods, 219;
    Crustacea as food of, 250

  Fish-lice, 224

  Flagellum, 14

  Foraminifera, 118

  Fossil Crustacea, 256


  _Galathea_, 130

  Galatheidea, 60

  Galls on Corals, 211

  Gamble, F. W., on colour-changes, 111

  Gammaridea, eyes of, 154

  _Gammarus_, 53;
    distribution, 172;
    in Lake Baikal, 183

  Ganglia of Lobster, 20

  Garstang, W., on habits of Masked Crab, 100

  Gastric Mill of Lobster, 17

  Geographical distribution of Lobsters, 32;
    of Crayfishes, 174

  Gecarcinidæ, 190

  _Gecarcinus_, 190;
    supposed metamorphosis, 192

  _Gecarcoidea_, 190

  _Gelasimus_, 188;
    habits, 106;
    colour-changes, 111

  Generative organs of Lobster, 27

  Giant Crab, 64

  Giesbrecht on luminosity of Copepoda, 150

  Gill chamber of Lobster, 18

  Gills of Lobster, 12, 18;
    of Mysidacea, 48

  Globigerina ooze, 119

  _Glomeris_, a Millipede, 3, 203

  Glyphæidæ, 268

  Gnathobases of Trilobites, 259

  _Gnathophausia_, 48

  Goose Barnacle, 42

  Gosse, P. H., on Porcelain Crabs, 115

  Grapsidæ, 180

  _Grapsus_, 107

  Green gland, 19

  Gribble, 253

  Guilding, L., on development of Land Crabs, 193

  Guinea-worm, 252

  Gulf-weed Crab, 155;
    on Turtles, 208


  Habitations of shore Crustacea, 95

  _Hæmocera_, 229

  Hæmoglobin in Branchiopoda, 165

  Hairs of Lobster, 22

  Hall. See Spencer and Hall

  Halocypridæ, 141, 144;
    luminosity, 151

  _Hapalocarcinus_, 211

  Head of Lobster, 7

  Hearing, sense of, in Lobster, 22

  Heart of Lobster, 17

  Hermaphroditism of Cirripedia, 43;
    in Isopoda, 52;
    of Cymothoinæ, 221;
    of Epicaridea, 223;
    of Rhizocephala, 231

  Hermit Crabs, 61;
    of seashore, 91;
    of deep sea, 124, 136;
    terrestrial, 194;
    commensals, 213;
    Isopods parasitic on, 221

  _Heterocarpus_, 125

  Hickson, S. J., on _Caridina_, 248

  _Hippa_, 102

  Hippidea, 62;
    habits, 102

  _Hippolyte_, colour-changes, 111

  _Holopedium_, 170

  Homaridæ, 33

  _Homarus_, 32;
    fishery, 238

  Homolodromiidæ, 134;
    fossil allies, 269

  Homologous organs, 10

  Hoplocarida, 64

  _Huenia_, 109

  Huxley, T. H.: on Barnacles, 115;
    on distribution of Crayfishes, 176

  _Hyas_, masking habits, 96

  _Hyperia_, 142;
    on Jellyfishes, 212

  Hyperiidea, 141;
    eyes, 154

  Hypoplankton, 143

  Hypostome, 259


  _Inachus_ infected with _Sacculina_, 235

  Intestine of Lobster, 16

  Isopoda, 51;
    deep-sea, 131;
    fresh-water, 172;
    subterranean, 186;
    land, 199;
    in Sponges, 210;
    parasitic, 218;
    wood-boring, 253;
    fossil, 266


  _Jasus_, 241

  Jellyfishes, Amphipods on, 212


  Keeble, F., on colour-changes, 111

  Kidney, 19

  _Koonunga_, 264


  Land Crabs, 189;
    injuring crops, 253

  Land Hermit Crabs, 194

  Land-hoppers, 189

  Langouste, 241

  Lankester, E. Ray, on hæmoglobin in Branchiopoda, 165

  Larvæ of Lobster, 28;
    of Norway Lobster, 71;
    of Stomatopoda, 80;
    of Brine Shrimp, 81;
    of Land Crabs, 191;
    of Robber Crab, 199;
    fossil, of Stomatopoda, 266

  Larval stages, 66;
    significance of, 85

  _Latreillia_, 128

  Leach, W. E., on the Gribble, 254

  _Leander_, 59, 179;
    Isopod parasitic on, 221;
    used for food, 245

  Legs of Lobster, 8, 12

  _Lepas_, 42;
    habitat, 155;
    nauplius, 148

  _Lepeophthirus_, 225

  _Leptodora_, 169

  Leptostraca, 45

  _Lernæa_, 226

  Leucosiidæ, 109

  _Ligia_, structure and habits, 200

  _Limnoria_, 254

  _Linuparus_, 269

  _Lithodes_, 62, 94

  Lithodidæ, 61

  Liver of Lobster, 17

  Lobsters:
    deep-sea, 121;
    fishery, 238

  Lovén, S., on relict Crustacea, 181

  Luminosity of deep-sea Crustacea, 125;
    of plankton Crustacea, 150

  Lynceidæ, 165


  Mackerel feeding on plankton, 250

  _Macropodia_, masking habits, 97

  _Maia_, masking habits, 96

  Malacostraca, 43;
    fossil, 261

  Mammoth Cave, Crayfish of, 185

  Mandible of Lobster, 8, 14

  Masked Crab, 99

  Masking of Crabs, 96, 215

  Maxilla of Lobster, 8;
    of _Argulus_, 41

  Maxillipeds of Lobster, 8, 11

  Maxillula of Lobster, 8

  Medusæ, Amphipods on, 212

  Megalopa of Shore Crab, 70

  _Meganyctiphanes_, 56, 151

  _Melia_, 215

  _Mesidotea_, 181

  Mesoplankton, 143

  Messmates, 208

  Metamorphosis of Lobster, 28;
    of Land Crabs, 191

  Metanauplius of _Penæus_, 75

  Mimicry, 204

  _Mimonectes_, 145

  Mitsukuri, K., on cultivation of Barnacles, 250

  Mole Crabs, 102

  Monstrillidæ, 230

  Moulting of Lobster, 15;
    of Woodlice, 206

  Mountain Shrimps, 181;
    structure and fossil allies, 264

  Mouth-frame of Crabs, 62

  Müller, Fritz:
    on larval stages of _Penæus_, 73;
    on habits of _Aratus_, 189

  Müller, O. F., on Nauplius, 82

  _Munida_, larva of, 71

  _Munidopsis_:
    eyes, 123;
    eggs, 130

  Murray River Lobster, 243

  Mussels and Pea Crab, 217

  Myodocopa, 39

  Mysidacea, 46;
    development, 79;
    luminosity, 127;
    eyes, 153;
    and Hermit Crabs, 215;
    fossil, 263

  _Mysis_, 47;
    in lakes, 181, 182


  Natantia, 58; used for food,
  243

  Nauplius
    of _Penæus_, 73;
    of Crayfish, 79;
    of Opossum Shrimp, 79;
    of Branchiopoda, 80;
    of _Cyclops_, 82;
    of Ostracoda, 82;
    of Barnacles, 83;
    of _Lepas_, 148;
    of _Leptodora_, 170;
    of _Sacculina_, 233;
    eye, 40

  _Nebalia_, 44;
    fossil allies, 261

  Nebaliacea, 44

  Necton, 138

  Nematocarcinidæ, 128

  _Nephrops_, 33;
    larva, 71;
    distribution, 132;
    fishery, 240

  Nephropsidea, 33, 60

  _Nephropsis_, 121

  _Neptunus_, 156;
    used for food, 249

  Neritic plankton, 141

  Nervous system of Lobster, 20

  _Niphargus_, 184

  Northern Crayfishes, 176

  Norway Lobster, 33;
    larva, 71;
    fishery, 240

  Norwegian Prawn, 246

  Notostraca, 36;
    habits, 162


  Oceanic plankton, 141

  Octopus preying on Crustacea, 89

  _Ocypode_, 188;
    habits, 104;
    carnivorous, 195

  Olfactory filaments of Lobster, 25

  _Oniscus_, 51;
    Structure and habits, 201

  Operculata, 43

  Opossum Shrimps, 46;
    development of, 79

  _Orchestia_, 107

  Orientation, organs of, 24

  Ostracoda, 38;
    development, 81;
    luminosity, 127, 151;
    plankton, 144;
    fresh-water, 172;
    of Tanganyika, 184;
    fossil, 261

  Ovary of Lobster, 27

  Oxyrhyncha, 64;
    masking habits, 96;
    fossil, 270

  Oxystomata, 63;
    habits, 101;
    protective resemblance, 109;
    deep-sea, 128;
    fossil, 269

  _Oxyuropoda_, 266

  Oyster Crab, 249

  Oysters and Pea Crab, 217


  Paguridea, 61

  _Paguropsis_ and Sea-anemones, 214

  _Palæmon_, 179;
    used for food, 248

  _Palæmonetes_, 174

  Palinura, 59

  Palinuridæ, 241;
    fossil, 269

  _Palinurus_, larva of, 72;
    fishery, 240

  Palp, 14

  _Pandalus_, 59, 137;
    used for food, 245

  _Panulirus_, 241

  _Paracyamus_, 55

  _Paranaspides_, 264

  _Paranephrops_, 177

  _Parapagurus_, 124;
    and Sea-anemones, 213

  Parasitism, 208

  Parastacidæ, 176

  _Parastacus_, 178

  Partan, 248

  Parthenogenesis, 163;
    of Cladocera, 166;
    of Ostracoda, 172

  Pea Crab, 217

  Pedunculata, 43

  _Peltogaster_, 232

  Penæidea, 59;
    fossil, 267

  _Penæus_, 59;
    larvæ, 73;
    used for food, 247;
    fossil, 268

  Peracarida, 46;
    development, 78

  Pericardium, 17

  _Peripatus_, tracheæ of, 204

  Peter's stone, 220

  _Philomedes_, 38

  Phosphorescence. See Luminosity

  Photophores of Euphausiacea, 125

  Phreatoicidea, 173

  _Phronima_, 154

  Phronimidæ, eyes of, 154

  _Phtisica_, 54

  Phyllocarida, 261

  _Phyllosoma_, 149;
    of Spiny Lobster, 72

  Phylogeny, 205, 256

  Pill Millipede, 203

  Pink Shrimp, 137, 245

  _Pinnaxodes_, 218

  _Pinnotheres_, 217;
    used for food, 249

  _Planes_, 155

  Plankton, 139;
    of fresh water, 160;
    of lakes, 168, 170;
    relation to fisheries, 250

  _Platyarthrus_, 205

  _Platycuma_, 129

  _Platymaia_, 130

  _Platytelphusa_, 184

  Pleuron, 9

  Podocopa, 39

  _Podocrates_, 269

  _Pollicipes_ used for food, 237;
    fossil, 261

  _Polycheles_, 133

  _Pontonia_, 218

  _Pontoporeia_, 181

  Porcelain Crab: zoëa of, 70;
    autotomy, 114;
    feeding, 115;
    and Hermit Crabs, 215

  _Porcellana_: zoëa of, 70;
    autotomy, 114;
    feeding, 116

  Porcellanidæ, 60

  _Porcellio_, 51;
    structure and habits, 202;
    distribution, 206

  Portunidæ: swimming, 90;
    sand-burrowing, 99;
    used for food, 249

  Potamobiidæ, 176

  Potamobius, 174

  _Potamon_, 180;
    young of, 78

  _Præanaspides_, 265

  Prawns, 58;
    luminosity of, 127;
    of deep sea, 136;
    of fresh water, 174, 179;
    of Tanganyika, 183;
    blind, in caves, 185;
    in Sponges, 210;
    Isopods parasitic on, 221;
    Common, 245;
    used for food, 245;
    deep-water, 247;
    fossil, 268

  Prosoponidæ, 135, 269

  Protandrous hermaphroditism, 221

  Protective resemblance, 108

  _Protocarcinus_, 269

  _Protocaris_, 260

  Protopodite, 10

  Protozoëa of _Penæus_, 75

  _Psathyrocaris_, 130

  _Pseudothelphusa_, 193

  Pseudo-tracheæ, 202

  Pygidium, 259

  _Pygocephalus_, 263

  _Pylocheles_, 94

  Pylochelidæ of deep sea, 136


  Recapitulation, theory of, 86

  Red-clawed Crayfish, 242

  Regeneration in Lobster, 30

  Relict faunas, 182

  _Remipes_, 102

  Reproduction of Cladocera, 166;
    of _Leptodora_, 169;
    of Cymothoinæ, 220

  Reptantia, 59

  Respiration in sand-burrowing Crabs, 99;
    in amphibious Crabs, 106;
    in Land Crabs, 193;
    in _Coenobita_, 195;
    in _Birgus_, 196;
    in Land Isopods, 201, 202, 203

  Resting eggs of Cladocera, 166;
    of Copepoda, 170

  Resting stage of Copepoda, 171

  Reversal of asymmetry in Lobster, 30

  Rhizocephala, 43;
    larvæ, 84;
    structure and development, 231

  River Crabs: young of, 78;
    habits and distribution, 180;
    development of, 193

  River Prawns, 179;
    used for food, 248

  Robber Crab, 61, 94;
    habits and structure, 195;
    breeding, 199

  Robertson, David, on habits of Crabs, 97

  Rock Lobster. See Spiny Lobster

  Rostrum of Lobster, 6


  _Sacculina_, 231

  Salt lakes, Branchiopoda of, 165

  Sand-burrowing Crustacea, 97

  Sand-hoppers, 52;
    habits, 107, 189

  _Sapphirina_, 150

  Sargasso Sea, 155

  Scampo, 240

  _Scapellum_, fossil, 261

  Scaphognathite, 19

  Schizopod Stage of Lobster, 71;
    of _Penæus_, 75

  _Scylla_, 249

  Scyllaridæ, fossil, 269

  Scyllaridea, 59;
    Phyllosoma larvæ, 149;
    fossil, 268

  _Scyllarus_, fossil, 269

  Sea-slater, 200

  Sea-anemones: and Hermit Crabs, 213;
    carried by Crab, 215

  Sea-crawfish. See Spiny Lobster

  Sea-urchin, Crab living in, 218

  Sedentary Crustacea, 114

  Self-mutilation in Lobster, 30

  Senses of Lobster, 25

  _Sergestes_, zoëa of, 75, 148

  Serial homology, 10

  _Sesarma_, habits, 180, 189

  Sessile Barnacles, 43

  Sexes of Lobster, 26

  Sexual characters of Crabs infected with _Sacculina_, 235

  Sharks, Barnacle parasitic on, 235

  Shore Crab, 64;
    larval stages, 68;
    habits, 107, 188;
    infested by _Sacculina_, 231;
    used for food, 249

  Shrimps, 58;
    fresh-water, 53, 172;
    Common, habits of, 97;
    protective resemblance, 108;
    living with Mollusc, 218;
    fishery, 244.
    See also Brown Shrimp, Pink Shrimp, etc.

  Sight, sense of, in Lobster, 21

  Skeleton Shrimps, 109

  Slaters, 51;
    Sea-, 200

  Sloane, H., on Gulf-weed Crab, 155

  Smell, sense of, in Lobster, 24

  Smith, G.: on terrestrial Amphipoda, 189;
    on development of _Sacculina_, 233

  Smith, S. I., on habits of _Ocypode_, 105

  Somites of Lobster, 6, 8

  Southern Crayfishes, 176

  Spencer and Hall on Australian Branchiopoda, 161, 163

  Sperm-receptacle of Lobster, 27

  Spider Crabs, 64;
    masking habits, 96, 215;
    deep-sea, 128

  Spiny Lobster, 59;
    larva of, 72;
    fishery, 240

  _Spirontocaris_, Isopod parasitic on, 221

  Sponge Crab, 95

  Sponges, Crustacea in, 210

  _Spongicola_, 210

  _Squilla_, 64;
    larva of, 80;
    fossil, 266

  Stalked Barnacles, 43

  Statocyst of Lobster, 24;
    of Mysidacea, 47

  Statolith, 47

  Stebbing, T. R. R.: on Land Crabs, 190;
    on habits of _Cirolana_, 219

  Steinkrebs, 242

  Stenopidea, 59;
    fossil, 267

  _Stenorhynchus_, 97

  Sternum, 9

  Stevenson, R., on the Gribble, 254

  Stomach of Lobster, 16

  Stone Crab, 61, 94

  Stomatopoda, 64;
    larvæ, 80;
    habits, 104;
    fossil, 266

  Stridulating organ of _Ocypode_, 105

  Subterranean Crustacea, 184

  Swimmerets of Lobster, 6, 10

  Swimming Crabs, 90;
    sand-burrowing, 99;
    used for food, 249

  Symbiosis, 207

  Syncarida, 45, 181;
    fossil, 264


  Tail-fan of Lobster, 6

  Talitridæ, 107

  _Talitrus_: habits, 107;
    terrestrial Species, 189

  Tanaidacea, 50

  Tanganyika, Lake, Crustacea of, 183

  Taste, sense of, in Lobster, 25

  _Telphusa_, 180

  Telson of Lobster, 6

  Tergum, 9

  Testis of Lobster, 27

  Thalassinidea, 61;
    habits, 103

  _Thaumastocheles_, 130, 132

  Thompson, J. Vaughan: discovery of metamorphoses of Crustacea, 67;
    on larvæ of Cirripedia, 83;
    on development of _Sacculina_, 232

  Thoracica, 43

  Thorax of Lobster, 7

  Thread-worms, 252

  Touch, sense of, in Lobster, 22

  Tracheæ, 204

  Tracheæ, pseudo-, 202

  Trapeziidæ, 211

  _Triarthrus_, 258

  Trilobites, structure and history, 258

  _Tubicinella_, 209

  _Turrilepas_, 261

  Turtles: Barnacles on, 209;
    Gulf-weed Crab on, 209


  Underground Crustacea, 184

  Uropods of Lobster, 6


  Venus's Flower-basket, Crustacea in, 210


  Water-fleas, 37, 165

  Weismann on parthenogenesis of _Cypris_, 172

  Well Shrimp, 184

  Westwood, J. O., on development of Land Crabs, 192

  Whales, Barnacles on, 209

  Whale-food, 138

  Whale-lice, 56;
    habits, 224

  White-clawed Crayfish, 242

  Willey, A., on breeding of Robber Crab, 199

  Williamson, H. C., on habits of Lobster, 25

  Wood-boring Crustacea, 253

  Woodlice, 51;
    development of, 79;
    habits and structure, 199;
    distribution of, 206;
    destructive in gardens, 253

  Worms: and Hermit Crabs, 215;
    Copepoda parasitic in, 230


  Zoëa of Shore Crab, 69;
    of Caridea, 73;
    of Porcelain Crab, 70;
    of _Munida_, 71;
    of _Penæus_, 75;
    of _Sergestes_, 75, 148;
    of Robber Crab, 199




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Transcriber's note:

In general every effort has been made to replicate the original text as
faithfully as possible, including some instances of non-standard
spelling and punctuation. Certain animal names (notably, "Lobster",
"Crab") have occasionally been changed from lowercase to capitalized
when the pattern of capitalization used by the author has permitted the
transcriber to judge that the omission of such capitalization was likely
to be typographical in nature and not intentional. The hyphenation is
occasionally variable; this has not been altered. A few obvious
punctuation errors have been corrected, as well as the following
apparently typographical errors:

  p. ix   (List of Illus.) "8. _Diastylis goodsiri_...," 8 changed to 18
  p. xiv  (List of Illus.) Plate II page ref., changed from 63 to 36
  p. xv   (List of Illus.) Plate XXI caption, Palamon changed to Palæmon
  p. xvi  (List of Illus.) Plate XXVII caption, "THE COCO NUT CRAB,
            Birgus latro," Coco nut changed to Coconut
  p. 78   Fig 31 caption, "The adult of an allied species is figured on
            Plate XXV," XXV changed to XXIII
  p. 165  "Daphnia pulex and other speces," speces changed to species
  p. 169  "the addomen, however, is drawn" addomen changed to abdomen
  p. 196  Plate XXVII caption, "THE COCO-NUT CRAB, Birgus latro,"
            COCO-NUT changed to COCONUT
  p. 208  Plate labeled XXVII (Group of Barnacles...) changed to XXVIII
  p. 214  "a fleshy mass formed by a colony of Sea-amenones," amenones
            changed to anemones
  p. 215  "woud be of little use by themselves," woud changed to would
  p. 288  (index) missing italics placed on "Sapphirina"
  p. 289  (index) tergum page reference 8 changed to 9

Some illustrations have been moved from their original locations to
paragraph breaks, so as to be nearer to their corresponding text, and
for ease of document navigation. Missing page numbers correspond to
pages not numbered  in the original document. References to scale in
illustration captions are those of the original publication, and
therefore do not correspond to the scale of the images in the HTML
version of this ebook.





End of Project Gutenberg's The Life of Crustacea, by William Thomas Calman