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I

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London : KEG AN PAUL, TRENCH, TRUBNER, & CO., Ltd.

The International Scientific Series.

vol. XXVIII.

THE COMMON CRAYFISH.
(Astacus Jluviatilis, Male.)

Frontispiece.]

THE CRAYFISH

AN INTRODUCTION TO

THE STUDY OF ZOOLOGY

BY

T. H. HUXLEY, F.E.S-

WITH EIGHTY-TWO ILLUSTRATIONS

SIXTH EDITION

LONDON
KEGAN PAUL, TRENCH, TRUBNER & CO., Lt?.
1896

"Aid 5tt fxrj Svax^po-ivet^ iratStKws rrjv irepl tu>v aTijtAOTeptot> £a)W iniaicexj/iv' e'v Trdtri
yap rot? <|)U(rt/cot5 eWoTt rt Oavfxao-Tov," — Aristotle, De Partibus, I. 5.

"Quienim Autorum verba legentes, rerum ipsarum imagines (eorum verbis com-
prehensa) sensibus propriis non abstrahunt, hi non veras Ideas, sed falsa Idola et
phantasinata inania mente concipiunt

" InsusuiTo itaque in aurem tibi (amice Lector !) ut qusecunque a nobis in hisce ....
exercitationibus tractabuntur, ad exactam experiential trutinam pensites : fidemque
iis non aliter adhibeas, nisi qnatenus eadem indubitato sensuum testimonio firmissime
stabiliri deprelienderis."— Harvey. Exercitationes de Generations Prcefatio.

"La seule et vraie Science est la connaissance des faits : l'esprit ne peut pas y
suppleer et les faits sont dansles sciences ce qu'est 1' experience dans la vie civile."

" Le seul et le vrai moyen d'avancer la science est de travailler a la description et
a l'histoire des differentes choses qui en font l'objet." — Buffon. Discours de la

maniere d'etudier et de trailer VHistoire Natxirelle.

" Ebenso hat mich audi die genauere Untersuchung unsers Krebses gelehret, dass,
so gemein und geringschatzig solcher auch den meisten zu seyn scheinet, sich an
selbigem doch so viel Wunderbares findet, dass es auch den grossten Naturforscher
schwer fallen sollte solehes alles deutlich zu beschreiben." — Roesel v. Rosenhof.
Insecten Belustigungen. — " Der Flusslcrebs hiesiges Landes mit seinen merkwurdigen
Eigenschaften."

PREFACE.

In writing this book about Crayfishes it has not
been my intention to compose a zoological mono-
graph on that group of animals. Such a work, to
be worthy of the name, would require the devotion
of years of patient study to a mass of materials
collected from many parts of the world. Nor has
it been my ambition to write a treatise upon
our English crayfish, which should in any way pro-
voke comparison with the memorable labours of
Lyonet, Bojanus, or Strauss Durckheim, upon the
willow caterpillar, the tortoise, and the cockchafer.
What I have had in view is a much humbler, though
perhaps, in the present state of science, not less use-
ful object. I have desired, in fact, to show how
the careful study of one of the commonest and most
insignificant of animals, leads us, step by step, from
every-day knowledge to the widest generalizations

Vi

P LIE FACE.

and the most difficult problems of zoology ; and,
indeed, of biological science in general.

It is for this reason that I have termed the book
an " Introduction to Zoology." For, whoever will
follow its pages, crayfish in hand, and will try to
verify for himself the statements which it contains,
will find himself brought face to face with all the
great zoological questions which excite so lively an
interest at the present day ; he will understand the
method by which alone we can hope to attain to
satisfactory answers of these questions; and, finally,
he will appreciate the justice of Diderot's remark,
" II faut etre profond dans Part ou dans la science
pour en bien posseder les elements."

And these benefits will accrue to the student
whatever shortcomings and errors in the work itself
may be made apparent by the process of verification.
" Common and lowly as most may think the cray-
fish," well says Eoesel von Eosenhof, "it is yet so
full of wonders that the greatest naturalist may be
puzzled to give a clear account of it." But only

PREFACE.

vii

the broad facts of the case are of fundamental im-
portance; and, so far as these are concerned, I ven-
ture to hope that no error has slipped into my
statement of them. As for the details^ it must be
remembered, not only that some omission or mis-
take is almost unavoidable, but that new lights
come with new methods of investigation ; and that
better modes of statement follow upon the improve-
ment of our general views introduced by the gradual
widening of our knowledge.

I sincerely hope that such amplifications and
rectifications may speedily abound; and that this
sketch may be the means of directing the attention of
observers in all parts of the world to the crayfishes.
Combined efforts will soon furnish the answers to
many questions which a single worker can merely
state; and, by completing the history of one group
of animals, secure the foundation of the whole
of biological science.

In the Appendix, I have added a few notes re-
specting points of detail with' which I thought it

viii

PREFACE.

unnecessary to burden the text ; and, under the
head of Bibliography, I have given some references
to the literature of the subject which may be useful
to those who wish to follow it out more fully.

I am indebted to Mr. T. J. Parker, demonstrator
of my biological class, for several anatomical draw-
ings ; and for valuable aid in supervising the
execution of the woodcuts, and in seeing the work
through the press.

Mr. Cooper has had charge of the illustrations,
and I am indebted to him and to Mr. Coombs,
the accurate and skilful draughtsman to whom
the more difficult subjects were entrusted, foi
such excellent specimens of xylographic art as
the figures of the Crab, Lobster, Eock Lobster,
and Norway Lobster.

T. H. H.

London,

November, 1879.

CONTENTS.

1 — -

PAGE

PREFACE v

LIST OF WOODCUTS . . xi

CHAPTER I.

The Natural History of the Common Crayfish ... 1

CHAPTER II.

The Physiology of the Common Crayfish. The Mechanism
by which the parts of the living engine are supplied
with the materials necessary for their maintenance
and growth 46

CHAPTER III.

The Physiology of the Common Crayfish. The Mechanism
by which the Living Organism adjusts itself to sur-
rounding conditions and reproduces itself 87

CHAPTER IV.

The Morphology of the Common Crayfish. The structure

and the development of the individual . . . . 137

CHAPTER V.

PAGE

The Comparative Morphology of the Crayfish. The struc-
ture AND THE DEVELOPMENT OF THE CRAYFISH COMPARED
WITH THOSE OF OTHER LIVING BEINGS 227

CHAPTER VI.

The Distribution and the Etiology of the Crayfishes . . 288

NOTES 347

BIBLIOGRAPHY . 357

INDEX. * . . 363

LIST OF WOODCUTS.

f —

PAGE

Frontispiece. The Common Crayfish, Astacus fluviatilis, (male)

Fig. 1. Astacus fluviafilis. Side view of the male. . . 6

,, 2. „ ,, Dorsal views of male and female 18

3. „ „ Ventral views OF MALE and female 21

4. ,, ,, The gills 26

,,5. ,, „ Dissection from the dorsal side

(male) 28

„ 6. ,, ,, Longitudinal vertical section of

the alimentary canal . . 29

„ 7. „ A GASTROLITH OR " CRAB'S EYE " . 30

,, 8. „ Attachment of young to swim-

meret of mother . . . 41
,, 9. ,, „ Structure of the stomach . . 53
„ 10. „ Longitudinal section of the sto-
mach 56

„ 11. ,, Roof of the stomach, from within 60

,, 12. „ Dissection from the side (male). 62

13. ,, Alimentary canal from above . 65

,,14. ,, Blood corpuscles . . 68

„ 15. ,, „ Transverse section of thorax .. 70

„ 16. ,, „ The heart 72

,, 17. „ „ Structure of the gills . . . 76

„ 18. „ „ The green gland .... 83

xii

LIST OF WOODCUTS.

. 10. Astacus fluviatilis. Muscular tissue . .

'20. ,, ,, Muscles of chela .

21. ,, „ Articulation of abdominal so

mites

22. ,, „ Muscular system .

23. ,, ,, Nerve fibres

24. ,, Nerve ganglia . . .
•2.*). Nervous system

26. ,, ,, Olfactory and auditory organs

27. „ ,, Auditory sac ....

28. Structure of eye

29. ,, Diagram of eye

30. „ Female reproductive organs

31. Male reproductive organs .

32. „ Structure of uvary .

33. Structure of testis

34. ,, Spermatozoa

35. ,, ,, the last thoracic sternum in the

MALE and female

36. ,, Transverse section of abdomen

37. Abdominal appendages
33. ,, ,, Connection between thorax and

ABDOMEN ....

39. „ „ Cephalothoracic STERNA AND EN

DOPHRAGMAL SYSTEM

40. „ Ophthalmic and antennulary so

mites

41. The rostrum ....

42. ,, ,, A Segment of the endophragmal

SYSTEM ....

43. 99 „ Longitudinal section of cephalo

THORAX

44. ,, „ The third maxillipede .

LIST OF WOODCUTS.

xiii

45. Astacus fluviatilis.

46.
47.
48.

49.
50.
51.
52.
53.
54.
55.
56.
57.
58.

60.

61.

62.

63.
64.
65.
66.

67.

torrentium.

nobilis,

nigrescens.

torrentium.

nobilis.

The first and second maxillt-

PEDES . . . . . .

The second ambulatory leg .
The mandible and maxilla . .
The eye-stalk, antennule, and

ANTENNA

Blood corpuscles . • . . .

Epithelium

Connective tissue . . . .
Muscular tissue .
Muscular tissue . . . .
Nerve ganglia ....

Nerve fibres

cut1cular tissue ....
Sections of embryos . . . .
Earlier stages of development
Later stages of development .
Newly hatched young . . .
Comparative views of the cara-
pace, THIRD ABDOMINAL SOMITE,

and telson ....
Comparative views of the first
and second abdominal appen-
dages of the male . . .

^ nigrescens,

Cambarus Clarkii

Parastacus brasiliensis ........

Astacoides madagascarensis

Diagram of the morphological relations of the
Astacidce .........

Homarus vulgaris ........

( Parastacus. )

J Nephrops. L Podobrancht;e
[Palcemon. J

166
169
171

172
176
178
179
181
182
188
189
191
208
210
216
220

233

245
248
250
251

253
258

259

xiv LIST OF WOODCUTS.

PAGE

Fig. 69. Nephrops norrcgicus . ....... 260

70. Palinurus vulgaris 262

71. Palcemon jamaicnisis » . . . . . . 269

,, 72. Cancer pagur us ..273

73. Penceus 281

,, 74. Cancer pagurus. Development 282

75. Astacus leptodactylis 301

,, 76. Australian Crayfish . . 307

77. Map of the distribution of Crayfishes . . . 309

78. Cambarus. Walking leg. 312

79. Palcemon jamaicensis 329

( Pseudastacus pustulosus )

lf 80. \ [ 340

( Eryma modestiformis )

„ 81. Eoploparia longimana 0 « 9 , . . 34*2

THE CEAYFISH:

AN INTRODUCTION TO THE STUDY OF ZOOLOGY.

CHAPTER I.

THE NATURAL HISTORY OF THE COMMON CRAYFISH
(Astacus fluviatilis.)

Many persons seem to believe that what is termed
Science is of a widely different nature from ordinary
knowledge, and that the methods by which scientific
truths are ascertained involve mental operations of a
recondite and mysterious nature, comprehensible only by
the initiated, and as distinct in their character as in
their subject matter, from the processes by which we
discriminate between fact and fancy in ordinary life.

But any one who looks into the matter attentively will
soon perceive that there is no solid foundation for the
belief that the realm of science is thus shut off from that
of common sense ; or that the mode of investigation which
yields such wonderful results to the scientific inves-
tigator, is different in kind from that which is employed

B

2 THE NATURAL HISTORY OF THE COMMON CRAYFISH.

for the commonest purposes of everyday existence.
Common sense is science exactly in so far as it fulfils
the ideal of common sense ; that is, sees facts as they
are, or, at any rate, without the distortion of prejudice,
and reasons from them in accordance with the dictates
of sound judgment. And science is simply common sense
at its best ; that is, rigidly accurate in observation, and
merciless to fallacy in logic.

Whoso will question the validity of the conclusions of
sound science, must be prepared to carry his scepticism
a long way ; for it may be safely affirmed, that there is
hardly any of those decisions of common sense on
which men stake their all in practical life, which can
justify itself so thoroughly on common sense principles,
as the broad truths of science can be justified.

The conclusion drawn from due consideration of the
nature of the case is verified by historical inquiry ; and
the historian of every science traces back its roots' to the
primary stock of common information possessed by all
mankind.

In its earliest development knowledge is self-sown.
Impressions force themselves upon men's senses whether
they will or not, and often against their will. The
amount of interest which these impressions awaken is
determined by the coarser pains and pleasures which
they carry in their train, or by mere curiosity; ^and
reason deals with the materials supplied to it as far as
that interest carries it, and no farther. Such common

COMMON KNOWLEDGE AND SCIENCE.

3

knowledge is rather brought than sought ; and such
ratiocination is little more than the working of a blind
intellectual instinct.

It is only when the mind passes beyond this condition
that it begins to evolve science. When simple curiosity
passes into the love of knowledge as such, and the
gratification of the aesthetic sense of the beauty of com-
pleteness and accuracy seems more desirable than the
easy indolence of ignorance; when the finding out of
the causes of things becomes a source of joy, and he
is counted happy who is successful in the search ; common
knowledge of nature passes into what our forefathers
called Natural History, from whence there is but a step
to that which used to be termed Natural Philosophy, and
now passes by the name of Physical Science.

In this final stage of knowledge, the phenomena of
nature are regarded as one continuous series of causes
and effects ; and the ultimate object of science is to trace
out that series, from the term which is nearest to us, to
that which is at the furthest limit accessible to our means
of investigation.

The course of nature as it is, as it has been, and as it
will be, is the object of scientific inquiry; whatever lies
beyond, above, or below this, is outside science. But.
the philosopher need not despair at the limitation of his
field of labour : in relation to the human mind Nature is
boundless; and, though nowhere inaccessible, she is
everywhere unfathomable. B 2

4 THE NATURAL HISTORY OF THE COMMON CRAYFISH.

The Biological Sciences embody the great multitude
of truths which have been ascertained respecting living
beings ; and as there are two chief kinds of living things,
animals and plants, so Biology is, for convenience sake,
divided into two main branches, Zoology and Botany.

Each of these branches of Biology has passed through
the three stages of development, which are common to
all the sciences ; and, at the present time, each is in these
different stages in different minds. Every country boy
possesses more or less information respecting the plants
and animals which come under his notice, in the stage
of common knowledge; a good many persons have
acquired more or less of that accurate, but necessarily
incomplete and unmethodised knowledge, which is under-
stood by Natural History ; while a few have reached the
purely scientific stage, and, as Zoologists and Botanists,
strive towards the perfection of Biology as a branch of
Physical Science.

Historically, common knowledge is represented by the
allusions to animals and plants in ancient literature ;
while Natural History, more or less grading into Biology,
meets us in the works of Aristotle, and his continuators
in the Middle Ages, Bondoletius, Aldrovandus, and their
contemporaries and successors. But the conscious at-
tempt to construct a complete science of Biology hardly
dates further back than Treviranus and Lamarck, at
the beginning of this century, while it has received its
strongest impulse, in our own day, from Darwin.

MALE AND FEMALE CRAYFISHES.

7

backwards with rapid jerks, propelled by the strokes of a
broad, fan-shaped flipper, which terminates the hinder
end of the body (fig. 1, t, 20). In front of the foui? pairs
of legs, which are used in walking, there is a pair of
limbs of a much more massive character, each o$ wMtfh
ends in two claws disposed in such a manner as to
constitute a powerful pincer (fig. 1; 10). These
claws are the chief weapons of offence and defence
)f the crayfish, and those who handle them incautiously
will discover that their grip is by no means to be des-
pised, and indicates a good deal of disposable energy.
A sort of shield covers the front part of the body,
and ends in a sharp projecting spine in the middle
line (r). On each side of this is an eye, mounted on a
movable stalk (I), which can be turned in any direction :
behind the eyes follow two pairs of feelers ; in one of
these, the feeler ends in two, short, jointed filaments (2) ;
while, in the other, it terminates in a single, many -jointed
filament, like a whip -lash, which is more than half the
length of the body (3). Sometimes turned backwards,
sometimes sweeping forwards, these long feelers con-
tinually explore a considerable area around the body of
the crayfish.

If a number of crayfishes, of about the same size, are
compared together, it will easily be seen that they fall
into two sets ; the jointed tail being much broader,
especially in the middle, in the one set than in the
other (fig. 2). The broad-tailed crayfishes are the

S THE NATURAL HISTORY OF THE COMMON CRAYFISH.

females, the others the males. And the latter may
be still more easily known by the possession of four
curved st}Tles, attached to the under face of the first
two rings of the tail, which are turned forwards between
the hinder legs, on the under side of the body (fig. 3, A;
15, 16). In the female, there are mere soft filaments in
the place of the first pair of styles (fig. 3, B ; 15).

Crayfishes do not inhabit every British river, and even
where they are known to abound, it is not easy to find
them at all times of the year. In granite districts and
others, in which the soil yields little or no calcareous
matter to the waters which flow over it, crayfishes do
not occur. They are intolerant of great heat and of
much sunshine ; they are therefore most active towards
the evening, while they shelter themselves under the
shade of stones and banks during the day. It has been
observed that they frequent those parts of a river which
run north and south, less than those which have an
easterly and westerly direction, inasmuch as the latter
yield more shade from the mid-day sun.

During the depth of winter, crayfishes are rarely to
be seen about in a stream ; but they may be found
in abundance in its banks, in natural crevices and in
burrows which they dig for themselves. The burrows
may be from a few inches to more than a yard deep,
and it has been noticed that, if the waters are liable
to freeze, the burrows are deeper and further from
the surface than otherwise. Where the soil, through

THE FOOD OF THE CRAYFISH.

9

which a stream haunted by crayfishes runs, is soft
and peaty, the crayfishes work their way into it in all
directions, and thousands of them, of all sizes, may be
dug out, even at a considerable distance from the banks.

It does not appear that crayfishes fall into a state of
torpor in the winter, and thus "hybernate" in the strict
sense of the word. At any rate, so long as the weather
is open, the crayfish lies at the mouth of his burrow,
barring the entrance with his great claws, and with pro-
truded feelers keeps careful watch on the passers-by.
Larvae of insects, water-snails, tadpoles, or frogs, which
come within .reach, are suddenly seized and devoured,
and it is averred that the water-rat is liable to the same
fate. Passing too near the fatal den, possibly in search
of a stray crayfish, whose flavour he highly appreciates,
the vole is himself seized and held till he is suffocated,
when his captor easily reverses the conditions of the anti-
cipated meal.

In fact, few things in the way of food are amiss to
the crayfish ; living or dead, fresh or carrion, animal or
vegetable, it is all one. Calcareous plants, such as the
stoneworts (Chara), are highly acceptable; so are any kinds
of succulent roots, such as carrots ; and it is said that
crayfish sometimes make short excursions inland, in
search of vegetable food. Snails are devoured, shells
and all ; the cast coats of other crayfish are turned to
account as supplies of needful calcareous matter ; and
the unprotected or weakly member of the family is

10 THE NATURAL HISTOHY OF THE COMMON CRAYFISH.

not spared. Crayfishes, in fact, are guilty of canni-
balism in its worst form ; and a French observer pa-
thetically remarks, that, under certain circumstances,
the males " meconnaissent les plus saints devoirs; " and,
not content with mutilating or killing their spouses,
after the fashion of animals of higher moral pretensions,
they descend to the lowest depths of utilitarian turpitude,
and finish by eating them.

In the depth of winter, however, the most alert of
crayfish can find little enough food ; and hence, when
they emerge from their hiding-places in the first warm
days of spring, usually about March, the crayfishes are m
poor condition.

At this time, the females are found to be laden with
eggs, of which from one to two hundred are attached be-
neath the tail, and look like a mass of minute berries
(fig. 3, B). In May or June, these eggs are hatched, and
give rise to minute young, which are sometimes to be
found attached beneath the tail of the mother, under
whose protection they spend the first few days of their
existence.

In this country, we do not set much store upon cray-
fishes as an article of food, but on the Continent, and
especially in France, they are in great request. Paris
alone, with its two millions of inhabitants, consumes
annually from five to six millions of crayfishes, and pays
about £16,000 for them. The natural productivity of the
rivers of France has long been inadequate to supply the

THE ORIGIN OF THE WORD CRAYFISH.

11

demand for these delicacies ; and hence, not only are large
quantities imported from Germany, and elsewhere, but
the artificial cultivation of crayfish has been successfully
attempted on a considerable scale.

Crayfishes are caught in various ways ; sometimes the
fisherman simply wades in the water and drags them out
of their burrows ; more commonly, hoop-nets baited with
frogs are let down into the water and rapidly drawn up,
when there is reason to think that crayfish have been
attracted to the bait ; or fires are lighted on the banks at
night, and the crayfish, which are attracted, like moths,
to the unwonted illumination, are scooped out with the
hand or with nets.

Thus far, our information respecting the crayfish is
such as wToulcl be forced upon anyone who dealt in cray-
fishes, or lived in a district in which they were commonly
used for food. It is common knowledge. Let us now
try to push our acquaintance with what is to be learned
about the animal a little further, so as to be able to give
an account of its Natural History, such as might have
been furnished by Buffon if he had dealt with the subject.

There is an inquiry which does not strictly lie
within the province of physical science, and yet suggests
itself naturally enough at the outset of a natural history.

The animal we are considering has two names,
one common, Crayfish, the other technical, Astacus flu-
viatilis. How has it come by these two names, and why,

12 THE NATURAL HISTORY OF THE COMMON CRAYFISH.

having a common English name for it already, should
naturalists call it by another appellation derived from a
foreign tongue ?

The origin of the common name, "crayfish," involves
some curious questions of etymology, and indeed, of his-
tory. It might readily be supposed that the word "cray"
had a meaning of its own, and qualified the substantive
"fish"— as "jelly " and "cod" in "jellyfish" and "codfish."
But this certainly is not the case. The old English
method of writing the word was " crevis " or " crevice,"
and the "cray" is simply a phonetic spelling of the syl-
lable "ere," in which the "e" was formerly pronounced
as all the world, except ourselves, now pronounce that
vowel. While " fish " is the " vis " insensibly modified
to suit our knowledge of the thing as an aquatic
animal.

Now " crevis " is clearly one of two things. Either it
is a modification of the French name " ecrevisse," or of
the Low Dutch name "crevik," by which the crayfish is
known in these languages. The former derivation is that
usually given, and, if it be correct, we must refer "cray-
fish" to the same category as "mutton," "beef," and
"pork," all of which are French equivalents, introduced
by the Normans, for the " sheep's flesh," "ox. flesh," and
" swine's flesh," of their English subjects. In this case,
we should not have called a crayfish, a crayfish, except
for the Norman conquest.

On the other hand, if " crevik" is the source of our

THE TECHNICAL NAME OF THE CRAYFISH.

13

word, it may have come to us straight from the Angle
and Saxon contingent of our mixed ancestry.

As to the origin of the technical name ; cuttclkos, astakos,
was the name by which the Greeks knew the lobster; and
it has been handed down to us in the works of Aristotle,
who does not seem to have taken any special notice of the
crayfish. At the revival of learning, the early naturalists
noted the close general similarity between the lobster and
the crayfish ; but, as the latter lives in fresh water, while
the former is a marine animal, they called the crayfish,
in their Latin, Astacus fluviatilis, or the " river-lobster,"
by way of distinction ; and this nomenclature was re-
tained until, about forty-five years ago, an eminent
French Naturalist, M. Milne-Edwards, pointed out that
there are far more extensive differences between lobsters
and crayfish than had been supposed ; and that it would
be advisable to mark the distinctness of the things by
a corresponding difference in their names. Leaving
Astacus for the crayfishes, he proposed to change the
technical name of the lobster into Homarus, by latin-
ising the old French name " Omar" or " Homar " (now
Homard), for that animal.

At the present time, therefore, while the recognised
technical name of the crayfish is Astacus flaviatilis, that of
the lobster is Homarus vulgaris. And as this nomencla-
ture is generally received, it is desirable that it should not
be altered ; though it is attended by the inconvenience,
that Astacus, as we now employ the name, does not

14 THE NATURAL HISTORY OF THE COMMON CRAYFISH.

denote that which the Greeks, ancient and modern,
signify, by its original, astakos ; and does signify
something quite different.

Finally, as to why it is needful to have two names
for the same thing, one vernacular, and one technical.
Many people imagine that scientific terminology is a
needless burden imposed upon the novice, and ask us
why we cannot be content with plain English. In reply,
I would suggest to such an objector to open a conversation
about his own business with a carpenter, or an engineer,
or, still better, with a sailor, and try how far plain
English will go. The interview will not have lasted long
before he will find himself lost in a maze of unintelligible
technicalities. Every calling has its technical termin-
ology ; and every artisan uses terms of art, which sound
like gibberish to those who know nothing of the art, but
are exceedingly convenient to those who practise it.

In fact, every art is full of conceptions which are
special to itself ; and, as the use of language is to convey
our conceptions to one another, language must supply
signs for those conceptions. There are two ways of
doing this : either existing sigDS may be combined in
loose and cumbrous periphrases; or new signs, having
a well-understood and definite signification, may be in-
vented. The practice of sensible people shows the
advantage of the latter course ; and here, as elsewhere,
science has simply followed and improved upon common
sense.

THE USE OF THE BINOMIAL NOMENCLATURE. 15

Moreover, while English, French, German, and Italian
artisans are under no particular necessity to discuss
the processes and results of their business with one
another, science is cosmopolitan, and the difficulties of
the study of Zoology would be prodigiously increased, if
Zoologists of different nationalities used different tech-
nical terms for the same thing. They need a universal
language ; and it has been found convenient that the lan-
guage shall be the Latin in form, and Latin or Greek in
origin. What in English is Crayfish, is Ecrevisse in
French ; Flusskrebs, in German ; Cammaro, or Gamharo,
or Gammarello, in Italian : but the Zoologist of each
nationality knows that, in the scientific works of all the
rest, he shall find what he wants to read under the head
of Astacus fluviatilis.

But granting the expediency of a technical name for
the Crayfish, why should that name be double ? The
reply is still, practical convenience. If there are ten
children of one family, we do not call them all Smith,
because such a procedure would not help us to dis-
tinguish one from the other ; nor do we call them
simply John, James, Peter, William, and so on, for
that would not help us to identify them as of one family.
So we give them all two names, one indicating their
close relation, and the other their separate individuality
— as John Smith, James Smith, Peter Smith, William
Smith, &c. The same thing is done in Zoology ; only,
in accordance with the genius of the Latin language,

16 THE NATURAL HISTOHY OF THE COMMON CRAYFISH

we put the Christian name, so to speak, after the sur«
name.

There are a number of kinds of Crayfish, so similar
to one another that they bear the common surname of
Astacus. One kind, by way of distinction, is called
jluvlatile, another slender -handed, another Dauric, from
the region in which it lives ; and these double names are
rendered by — Astacus flaviatilis, Astacus leptodactylus,
and Astacus dauricus ; and thus we have a nomenclature
which is exceedingly simple in principle, and free from
confusion in practice. And I may add that, the less
attention is paid to the original meaning of the sub-
stantive and adjective terms of this binomial nomen-
clature, and the sooner they are used as proper names,
the better. Very good reasons for using a term may
exist when it is first invented, which lose their validity
with the progress of knowledge. Thus Astacus fluviatilis
was a significant name so long as we knew of only one
kind of crayfish ; but now that wTe are acquainted with a
number of kinds, all of which inhabit rivers, it is meaning-
less. Nevertheless, as changing it would involve endless
confusion, and the object of nomenclature is simply to
have a definite name for a definite thing, nobody dreams
of proposing to alter it.

Having learned this much about the origin of the
names of the crayfish, we may next proceed to consider
those points which an observant Naturalist, who did not

THE SKELETON EXTERNAL AND CALCIFIED. 17

care to go far beyond the surface of things, would find to
notice in the animal itself.

Probably the most conspicuous peculiarity of the cray-
fish, to any one who is familiar only with the higher
animals, is the fact that the hard parts of the body are
outside and the soft parts inside ; whereas in ourselves,
and in the ordinary domestic animals, the hard parts, or
bones, which constitute the skeleton, are inside, and the
soft parts clothe them. Hence, while our hard framework
is said to be an endoskeleton, or internal skeleton ; that
of the crayfish is termed an exoskeleton, or external
skeleton. It is from the circumstance that the body of
the crayfishes is enveloped in this hard crust, that
the name of Crustacea is applied to them, along with
the crabs, shrimps, and other such animals. Insects,
spiders, and centipedes have also a hard exoskeleton,
but it is usually not so hard and thick as in the
Crustacea.

If a piece of the crayfish's skeleton is placed in strong
vinegar, abundant bubbles of carbonic acid gas are given
off from it, and it rapidly becomes converted into a soft
laminated membrane, while the solution will be found to
contain lime. In fact the exoskeleton is composed of
a peculiar animal matter, so much impregnated with
carbonate and phosphate of lime that it becomes dense
and hard.

It will be observed that the body of the crayfish is
naturally marked out into several distinct regions. There

o

20

Fig. 2. — Astacus fluviatilis. — Dorsal or tergal views (nat. size). A, male;
B, female : — beg, branchio-cardiac groove, which marks the boun-
dary between the pericardial and the branchial cavities ; cg> cervical
groove ; these letters are placed on the carapace ; r, rostrum ; t, t',
the two divisions of the telson ; i, eye-stalks ; 2, antennules ; 3,
antennae ; 20, lateral lobes of tail-fin ; xv-xx, somites of the
abdomen.

THE EXOSKELETON.

19

is a firm and solid front part, covered by a large con-
tinuous shield, which is called the carapace ; and a jointed
hind part, commonly termed the tail (fig. 2). From
the perception of a partially real, and partially fanciful,
analogy with the regions into which the body is divided
in the higher animals, the fore part is termed the cepha-
lo-thorax, or head (cephalon) and chest (thorax) com-
bined, while the hinder part receives the name of
abdomen.

Now the exoskeleton is not of the same constitution
throughout these regions. The abdomen, for example,
is composed of six complete hard rings (fig. 2, xv-xx),
and a terminal flap, on the under side of which the
vent (fig. 3, a) is situated, and which is called the telson
(fig. 2, t, t'). All these are freely moveable upon one
another, inasmuch as the exoskeleton which connects
them is not calcified, but is, for the most part, soft and
flexible, like the hard exoskeleton when the lime salts
have been removed by acid. The mechanism of the joints
will have to be attentively considered by-and-by ; it is
sufficient, at present, to remark that, wherever a joint,
exists, it is produced in the same fashion, by the exo-
skeleton remaining soft in certain regions of the jointed
part.

The carapace is not jointed; but a transverse groove is
observed about the middle of it, the ends of which run
down on the sides and then turn forwards (figs. 1 and 2,
eg). This is called the cervical groove, and it marks off

o 2

20 THE NATURAL HISTORY OF THE COMMON CRAYFISH.

the region of the head, in front, from that of the thorax
behind.

The thorax seems at first not to be jointed at all; but
if its under, or what is better called its sternal, surface is
examined carefully, it will be found to be divided into as
many transverse bands, or segments, as there are pairs of
legs (fig. 3) ; and, moreover, the hindermost of these
segments is not firmly united with the rest, but can be
moved backwards and forwards through a small space
(fig. 3, B ; xiv).

Attached to the sternal side of every ring of the abdomen
of the female there is a pair of limbs, called swimmerets.
In the five anterior rings, these are small and slender
(fig. 3, B; 15, 19); but those of the sixth ring are very
large, and each ends in two broad plates (20). These
two plates on each side, with the telson in the middle,
constitute the flapper of the crayfish, by the aid of which
it executes its retrograde swimming movements. The
small swimmerets move together with a regular swing,
like paddles, and probably aid in propelling the animal
forwards. In the breeding female (B), the eggs are
attached to them ; while, in the male, the two anterior
pairs (A ; 15, 16) are converted into the peculiar styles
which distinguish that sex.

The four pairs of legs which are employed for walking
purposes, are divided into a number of joints, and the
foremost two pairs are terminated by double claws,
arranged so as to form a pincer, whence they are said to

Fig. 3.— Astacusfluviatilis.— Ventral or sternal views (nat. size). A, male ; B, female :—
a, vent; gg} opening of the green gland; lb, labrum; rut, metastoma or lower
lip ; od, opening of the oviduct ; rd, that of the vas deferens. 1, eye-stalk ;
antennule ; 3, antenna; h, mandible; S, second maxillipede ; 9, third or external
maxillipede; 10, forceps ; 11, first leg; lit, fourth leg; 15, in, 19, 20, first, second,
fifth, and sixth abdominal appendages; x., xr., xiv., sterna of the fourth, fifth,
and eighth thoracic somite ; xvi., sternum of the second abdominal somite. In the
male, the 9th to the 14th and the ICth to the 19th appendages are removed on
the animal's left side : in the female, the antenna (with the exception of its basal
joint) and the 5th to the 14th appendages on the animal's right are removed ; the
eggs also are shown attached to the swimmerets of the left side of the body.

22 THE NATURAL HISTORY OF THE COMMON CRAYFISH,

be chelate. The two hindermost pairs, on the other
hand, end in simple claws.

In front of these legs, come the great prehensile
limbs (10), which are chelate, like those which im-
mediately follow them, but vastly larger. They often
receive the special name of chelce ; and the large terminal
joints are called the "hand," We shall escape confusion
if we call these limbs the forceps, and restrict the name
of chela to the two terminal joints.

All the limbs hitherto mentioned subserve locomotion
and prehension in various degrees. The crayfish swims
by the help of its abdomen, and the hinder pairs of ab-
dominal limbs ; walks by means of the four hinder pairs
of thoracic limbs ; lays hold of anything to fix itself, or
to assist in climbing, by the two chelate anterior pairs of
these limbs, which are also employed in tearing the food
seized by the forceps and conveying it to the mouth ;
while it seizes its prey and defends itself with the forceps.
The part which each of these limbs plays is termed its
function, and it is said to be the organ of that function ;
so that all these limbs may be said to be organs of the
functions of locomotion, of offence and defence.

In front of the forceps, there is a pair of limbs which
have a different character, and take a different direction
from any of the foregoing (9). These limbs, in fact, are
turned directly forwards, parallel with one another, and
with the middle line of the body. They are divided into
a number of joints, of which one of those near the base

THE FOOT-JAWS AND THE JAWS.

23

is longer than the rest, and strongly toothed along the
inner edge, or that which is turned towards its fellow.
It is obvious that these two limbs are well adapted to
crush and tear whatever comes between them, and they
are, in fact, jaivs or organs of manducation. At the same
time, it will be noticed that they retain a curiously close
general resemblance to the hinder thoracic legs ; and
hence, for distinction's sake, they are called outer foot-
jaws, or external maxillipedes.

If the head of a stout pin is pushed between these
external maxillipedes, it will be found that it passes
without any difficulty into the interior of the body,
through the mouth. In fact, the mouth is relatively
rather a large aperture ; but it cannot be seen without
forcing aside, not only these external foot-jaws, but a
number of other limbs, which subserve the same function
of manducation, or chewing and crushing the food. We
may pass by the organs of manducation, for the present,
with the remark that there are altogether three pairs of
maxillipedes, followed by two pairs of somewhat differently
formed maxilla, and one pair of very stout and strong
jaws, which are termed the mandibles (4). All these jaws
work from side to side, in contradistinction to the jaws
of vertebrated animals, which move up and down. In
front of, and above the mouth, with the jaws which
cover it, are seen the long feelers, which are called the
antenncB (3) ; above, and in front of them, follow the
small feelers, or antennules (2) ; and over them, again, lie

24 THE NATURAL HISTORY OF THE COMMON CRAYFISH.

the eye stalks (I). The antennae are organs of touch;
the antennules, in addition, contain the organs of hear-
ing ; while, at the ends of the eyestalks, are the organs
of vision.

Thus we see that the crayfish has a jointed and
segmented body, the rings of which it is composed being
very obvious in the abdomen, but more obscurely trace-
able elsewhere ; that it has no fewer than twenty pairs
of what may be called by the general name of ap-
pendages ; and that these appendages are turned to
different uses, or are organs of different functions, in
different parts of the body. The crayfish is obviously
a very complicated piece of living machinery. But we
have not yet come to the end of all the organs that may
be discovered even by cursory inspection. Every one
who has eaten a boiled crayfish, or a lobster, knows
that the great shield, or carapace, is very easily separated
from the thorax and abdomen, the head and the limbs
which belong to that region coming away with the
carapace. The reason of this is not far to seek. The
lower edges of that part of the carapace which belongs to
the thorax approach the bases of the legs pretty closely,
but a cleft-like space is left; and this cleft extends
forwards to the sides of the region of the mouth, and
backwards and upwards, between the hinder margin of
the carapace and the sides of the first ring of the abdo-
men, which are partly overlapped by, and partly overlap,
that margin. If the blade of a pair of scissors is care-

THE u CRABS'-EYES."

29

middle region of that part of the carapace which lies
behind the cervical groove ; or, in other words, in the
dorsal region of the thorax. In front of it, and therefore
in the head, is a large rounded sac, the stomach (fig. 5,
cs ; fig. 6, cs, ps), from which a very delicate intestine
(figs. 5 and 6, lig) passes straight back through the thorax
and abdomen to the vent (fig. 6, a).

Fig. 6. — Astacus fluvlatilis. — A longitudinal vertical section of the ali-
mentary canal, with the outline of the body (nat. size) : — a, vent ; ag,
anterior gastric muscle ; bd, entrance of left bile duct ; eg, cervical
groove ; cce, caecum ; cpv, cardio-pyloric valve ; cs, cardiac portion
of stomach ; the circular area immediately below the end of the
line from cs marks the position of the gastrolith of the left
side; lig, hind-gut; lb, labrum ; , It, lateral tooth of stomach;
m, mouth ; mg, mid-gut ; mt, median tooth; cc, oesophagus ; pc, pro-
cephalic process ; pg, posterior gastric muscle ; ps, pyloric portion of
stomach ; r, annular ridge, marking the commencement of the
hind-gut.

In summer, there are commonly to be found at the sides
of the stomach two lenticular calcareous masses, which
are known as " crabs'-eyes," or gastroliths, and were, in
old times, valued in medicine as sovereign remedies for all
sorts of disorders. These bodies (fig. 7) are smooth and
flattened, or concave, on the side which is turned towards

30 THE NATURAL HISTORY OF THE COMMON CRAYFISH.

the cavity of the stomach; while the opposite side, being
convex and rough with irregular prominences, is some-
thing like a "brain-stone " coral.

Moreover, when the stomach is laid open, three large

Fig. 7. — Astacus fluviatilis. — A gastrolith ; A, from above ; B, from
below ; C, from one side (all x 5) ; D, in vertical section ( x 20).

reddish teeth are seen to project conspicuously into its
interior (fig. 6, It, mt) ; so that, in addition to its six
pairs of jaws, the crayfish has a supplementary crushing
mill in its stomach. On each side of the stomach, there
is a soft yellow or brown mass, commonly known as the

THE GROWTH OF THE CRAYFISH.

31

liver (fig. 5, Lr) ; and, in the breeding season, the
ovaries of the females, or organs in which the eggs are
formed, are very conspicuous from the dark-coloured
eggs which they contain, and which, like the exoskeleton,
turn red when they are boiled. The corresponding part
in a cooked lobster goes by the name of the " coral."

Beside these internal structures, the most noticeable
are the large masses of flesh, or muscle, in the thorax
and abdomen, and in the pincers ; which, instead of
being red, as in most of the higher animals, is white.
It will further be observed that the blood, which flows
readily when a crayfish is wounded, is a clear fluid, and
is either almost colourless, or of a very pale reddish or
neutral tint. Hence the older Naturalists thought that
the crayfish was devoid of blood, and had merely a sort
of ichor in place of it. But the fluid in question is true
blood ; and if it is received into a vessel, it soon forms a
soft, but firm, gelatinous clot.

The crayfish grows rapidly in youth, but enlarges more
and more slowly as age advances. The young animal which
has just left the egg is of a greyish colour, and about
one quarter of an inch long. By the end of the year, it
may have reached nearly an inch and a half in length.
Crayfishes of a year old are, on an average, two inches
long ; at two years, two inches and four-fifths ; at three
years, three inches and a half ; at four years, four inches
and a half nearly ; and at five years, five inches. They

32 THE NATURAL HISTOUY OF THE COMMON CRAYFISH.

go on growing till, in exceptional cases, they may attain
between seven inches and eight inches in length ; but at
what degree of longevity this unusual dimension is reached
is uncertain. It seems probable, however, that the life of
these animals may be prolonged to as much as fifteen or
twenty years. They appear to reach maturity, so far as
the power of reproduction is concerned, in their fifth or,
more usually, their sixth year. However, I have seen
a female, with eggs attached under the abdomen, only
two inches long, and therefore, probably, in her second
year. The males are commonly larger than females of
the same age.

The hard skeleton of a crayfish, once formed, is
incapable of being stretched, nor can it increase by in-
terstitial addition to its substance, as the bone of one
of the higher animals grows. Hence it follows, that the
enlargement of the body, which actually takes place,
involves the shedding and reproduction of its invest-
ment. This might be effected by insensible degrees, and
in different parts of the body at different times, as we
shed our hair ; but, as a matter of fact, it occurs periodi-
cally and universally, somewhat as the feathers of birds
are moulted. The whole of the old coat of the body is
thrown off at once, and suddenly ; and the new coat,
which has, in the meanwhile, been formed beneath
the old one, remains soft for a time, and allows of a
rapid increase in the dimensions of the body before it

THE SHEDDING OF THE SKIN.

33

hardens. This sort of moulting is what is technically
termed ecdysis, or exuviation. It is commonly spoken of
as the " shedding of the skin," and there is no harm in
using this phrase, if we recollect that the shed coat is not
the skin, in the proper sense of the word, but only what
is termed a cuticalar layer, which is secreted upon the
outer surface of the true integument. The cuticular
skeleton of the crayfish, in fact, is not even so much a
part of the skin as the cast of a snake, or as our own nails.
For these are composed of coherent, formed parts of the
epidermis ; while the hard investment of the crayfish con-
tains no such formed parts, and is developed on the out-
side of those structures which answer to the constituents
of the epidermis in the higher animals. Thus the cray-
fish grows, as it were, by starts ; its dimensions remaining
stationary in the intervals of its moults, and then rapidly
increasing for a few days, while the new exoskeleton is
in the course of formation.

The ecdysis of the crayfish was first thoroughly
studied a century and a half ago, by one of the most
accurate observers who ever lived, the famous Reaumur,
and the following account of this very curious process is
given nearly in his words.*

A few hours before the process of exuviation com-

* See Reaumur's two Memoirs, " Sur les diverses reproductions qui
se font dans les ecrevisses, les omars, les crabes, etc.," " Histoire de
TAcademie royale des Sciences," annee 1712 ; and "Additions aux ob-
servations sur la mue des dcrevisses donnees dans les Mdmoires de 1712."
Ibid. 1718.

D

Si THE NATURAL HISTORY OF THE COMMON CRAYFISH.

mences, the crayfish rubs its limbs one against the
other, and, without changing its place, moves each
separately, throws itself on its back, bends its tail,
and then stretches it out again, at the same time vibrat-
ing its antennae. By these movements, it gives the
various parts a little play in their loosened sheaths.
After these preparatory steps, the crayfish appears to
become distended ; in all probability, in consequence of
the commencing retraction of the limbs into the interior
of the exoskeleton of the body. In fact, it has been
remarked, that if, at this period, the extremity of one of
the great claws is broken off, it will be found empty,
the contained soft parts being retracted as far as the
second joint. The soft membranous part of the exo-
skeleton, which connects the hinder end of the carapace
with the first ring of the abdomen, gives way, and the
body, covered with the new soft integument, protrudes ;
its dark brown colour rendering it easily distinguishable
from the greenish-brown old integument.

Having got thus far, the crayfish rests for a while, and
then the agitation of the limbs and body recommences.
The carapace is forced upwards and forwards by the pro-
trusion of the body, and remains attached only in the
region of the mouth. The head is next drawn backwards,
while the eyes and its other appendages are extracted from
their old investment. Next the legs are pulled out, either
one at a time, or those of one, or both, sides together.
Sometimes a limb gives way and is left behind in its sheath.

THE SHEDDING OF THE SKIN.

35

The operation is facilitated by the splitting of the old
integument of the limb along one side longitudinally.

When the legs are disengaged, the animal draws its
head and limbs completely out of their former covering ;
and, with a sudden spring forward, while it extends its
abdomen, it extracts the latter, and leaves its old skele-
ton behind. The carapace falls back into its ordinary
position, and the longitudinal fissures of the sheaths of
the limbs close up so accurately, that the shed integu-
ment has just the appearance the animal had when the
exuviation commenced. The cast exoskeleton is so like
the crayfish itself, when the latter is at rest, that, except
for the brighter colour of the latter, the two cannot be
distinguished.

After exuviation, the owner of the cast skin, ex-
hausted by its violent struggles, which are not unfre-
quently fatal, lies in a prostrate condition. Instead of
being covered by a hard shell, its integument is soft and
flabby, like wet paper ; though Eeaumur remarks, that
if a crayfish is handled immediately after exuviation, its
body feels hard ; and he ascribes this to the violent con-
traction which its muscles have undergone, leaving them
in a state of cramp. In the absence of the hard skeleton,
however, there is nothing to bring the contracted muscles
at once back into position, and it must be some time
before the pressure of the internal fluids is so distributed
as to stretch them out.

When the process of exuviation has proceeded so far

D 2

36 THE NATURAL HISTORY OF THE COMMON CRAYFISH.

that the carapace is raised, nothing stops the crayfish
from continuing its struggles. If taken out of the water
in this condition, they go on moulting in the hand, and
even pressure on their bodies will not arrest their efforts.

The length of time occupied from the first giving way
of the integuments to the final emergence of the animal,
varies with its vigour, and the conditions under which it
is placed, from ten minutes to several hours. The
chitinous lining of the stomach, with its teeth, and the
" crabs'-eyes," are shed along with the rest of the cuti-
cular exoskeleton ; but they are broken up and dissolved
in the stomach.

The new integuments of the crayfish remain soft for
a period which varies from one to three days ; and it is
a curious fact, that the animal appears to be quite aware
of its helplessness, and governs itself accordingly,

An observant naturalist says : "I once had a do-
mesticated crayfish (Astacus fluviatilis) , which I kept in
a glass pan, in water, not more than an inch and a half
deep, previous experiment having shown that in deeper
water, probably from want of sufficient aeration, this
animal would not live long. By degrees my prisoner
became very bold, and when I held my fingers at the
edge of the vessel, he assailed them with promptness and
energy. About a year after I had him, I perceived, as I
thought, a second crayfish with him. On examination,
I found it to be his old coat, which he had left in a most
perfect state. My friend had now lost his heroism, and

THE REPRODUCTION OF LIMBS.

37

fluttered about in the greatest agitation. He was quite
soft ; and every time I entered the room during the next
two days, he exhibited the wildest terror. On the third,
he appeared to gain confidence, and ventured to use his
nippers, though with some timidity, and he was not yet
quite so hard as he had been. In about a week, how-
ever, he became bolder than ever ; his weapons were
sharper, and he appeared stronger, and a nip from him
was no joke. He lived in all about two years, during
which time his food was a very few worms at very uncer-
tain times ; perhaps he did not get fifty altogether/'*

It would appear, from the best observations that have
yet been made, that the young crayfish exuviate two or
three times in the course of the first year ; and that,
afterwards, the process is annual, and takes place usually
about midsummer. There is reason to suppose that very
old crayfish do not exuviate every year.

It has been stated that, in the course of its violent
efforts to extract its limbs from the cast-off exoskeleton,
the crayfish sometimes loses one or other of them ; the
limb giving way, and the greater part, or the whole, of it
remaining in the exuviae. But it is not only in this way
that crayfishes part with their limbs. At all times, if the
animal is held by one of its pincers, so that it cannot
get away, it is apt to solve the difficulty by casting off

* The late Mr. Robert Ball, of Dublin, in Bell's " British Crustacea,"
p. 239.

38 THE NATURAL HISTORY OF THE COMMON CRAYFISH.

the limb, which remains in the hand of the captor, while
the crayfish escapes. This voluntary amputation is always
effected at the same place ; namely, where the limb is
slenderest, just beyond the articulation which unites the
basal joint with the next. The other limbs also readily
part at the joints; and it is very common to meet with
crayfish which have undergone such mutilation. But
the injury thus inflicted is not permanent, as these
animals possess the power of reproducing lost parts to
a marvellous extent, whether the loss has been inflicted
by artificial amputation, or voluntarily.

Crayfishes, like all the Crustacea, bleed very freely when
wounded; and if one of the large joints of a leg is cut
through, or if the animal's body is injured, it is very likely
to die rapidly from the ensuing haemorrhage. A cray-
fish thus wounded, however, commonly throws off the
limb at the next articulation, where the cavity of the
limb is less patent, and its sides more readily fall
together ; and, as we have seen, the pincers are usually
cast off at their narrowest point. When such amputation
has taken place, a crust, probably formed of coagulated
blood, rapidly forms over the surface of the stump ; and,
eventually, it becomes covered with a cuticle. Beneath
this, after a time, a sort of bud grows out from the
centre of the surface of the stump, and gradually takes
on the form of as much of the limb as has been removed.
At the next ecdysis, the covering cuticle is thrown off
along with the rest of the exoskeleton ; while the rudi-

THE REPRODUCTION OF THE SPECIES.

39

mentary limb straightens out, and, though very small,
acquires all the organization appropriate to that limb.
At every moult it grows ; but, it is only after a long time
that it acquires nearly the size of its uninjured and older
fellow. Hence, it not unfrequently happens, that crayfish
are found with pincers and other limbs, which, though
alike useful and anatomically complete, are very unequal
in size.

Injuries inflicted while the crayfish are soft after
moulting, are apt to produce abnormal growths of the
part affected; and these may be perpetuated, and give
rise to various monstrosities, in the pincers and in other
parts of the body.

In the reproduction of their kind by means of eggs the
co-operation of the males with the females is necessary.
On the basal joint of the hindermost pair of legs of the male
a small aperture is to be seen (fig. 3, A; vd). In these, the
ducts of the apparatus in which the fecundating substance
is formed terminate. The fecundating material itself is a
thickish fluid, which sets into a white solid after extru-
sion. The male deposits this substance on the thorax
of the female, between the bases of the hindermost pairs
of thoracic limbs.

The eggs formed in the ovary are conducted to apertures,
which are situated on the bases of the last pair of ambula-
tory legs but two, that is, in the hinder of the two pair
which are provided with chelate extremities (fig. 3, B ; od)9

40 THE NATURAL HISTORY OF THE COMMON CRAYFISH.

After the female has received the deposit of the
spermatic matter of the male, she retires to a burrow,
in the manner already stated, and then the process of
laying the eggs commences. These, as they leave the
apertures of the oviducts, are coated with a viscid matter,
which is readily drawn out into a short thread. The
end of the thread attaches itself to one of the long hairs,
with which the swimmerets are fringed, and as the viscid
matter rapidly hardens, the egg thus becomes attached
to the limb by a stalk. The operation is repeated, until
sometimes a couple of hundred eggs are thus glued on
to the swimmerets. Partaking in the movements of the
swimmerets, they are washed backwards and forwards in
the water, and thus aerated and kept free of impurities ;
while the young crayfish is formed much in the same
way as the chick is formed in a hens egg.

The process of development, however, is very slow,
as it occupies the whole winter. In late spring-time, or
early summer, the young burst the thin shell of the
egg, and, when they are hatched, present a general re-
semblance to their parents. This is very unlike what
takes place in crabs and lobsters, in which the young
leave the egg in a condition very different from the
parent, and undergo a remarkable metamorphosis before
they attain their proper form.

For some time after they are hatched, the young hold
on to the swimmerets of the mother, and are carried
about, protected by her abdomen, as in a kind of nursery.

NEWLY-HATCHED CRAYFISHES. 41

That most careful naturalist, Roesel von Rosenhof,
says of the young, when just hatched : —

"At this time they are quite transparent; and when

Fig. 8. — Astacus fluviatilis. — A, two recently hatched crayfish attached
to one of the swimmerets of the mother ( x 4). pr, protopodite ;
en, endopodite ; and ex> exopodite of the swimmeret ; cc, ruptured
egg-cases. B, chela of a recently hatched crayfish ( x 10).

such a crayfish [a female with young] is brought to
table, it looks quite disgusting to those who do not know

42 THE NATURAL HISTORY OF THE COMMON CRAYFISH.

what the young are ; but if we examine it more closely,
especially with a magnifying-glass, we see with pleasure
that the little crayfish are already perfect, and resemble
the large one in all respects. When the mother of these
little crayfish, after they have begun to be active, is quiet
for a while, they leave her and creep about, a short way
off. But, if they spy the least sign of danger, or there is
any unusual movement in the water, it seems as if the
mother recalled them by a signal ; for they all at once
swiftly return under her tail, and gather into a cluster,
and the mother hies to a place of safety with them, as
quickly as she can. A few days later, however, they
gradually forsake her." *

Fishermen declare that " Hen Lobsters " protect their
young in a similar manner.! Jonston,t who wrote in
the middle of the seventeenth century, says that the little
crayfish are often to be seen adhering to the tail of the
mother. Koesel's observations imply the same thing ;
but he does not describe the exact mode of adherence,
and I can find no observations on the subject in the
works of later writers.

It has been seen that the eggs are attached to the
swimmerets by a viscid substance, which is, as it were,
smeared over them and the hairs with which they are

* " Der Monatfich-herausgegeben Insecten Belustigung." Dritter
Theil, p. 336. 1755.

t Bell's " British Crustacea," p. 249.

J "Joaimis Jonstoni Historic naturalis de Piscibus et Cetis Libri
quinque. Tomus IV. * De Cainmaro seu Astaco nuviatili., "

NEWLY-HATCHED CRAYFISHES.

43

fringed, and is continued by longer or shorter thread-like
pedicles into the coat of the same material which invests
each egg. It very soon hardens, and then becomes very
firm and elastic.

When the young crayfish is ready to be hatched, the egg
case splits into two moieties, which remain attached, like a
pair of watch glasses, to the free end of the pedicle of the
egg (fig. 8, A ; ec). The young animal, though very similar
to the parent, does not quite "resemble it in all respects,"
as Eoesel says. For not only are the first and the last
pairs of abdominal limbs wanting, while the telson is very
different from that of the adult; but the ends of the great
chelae are sharply pointed and bent down into abruptly in-
cuiwed hooks, which overlap when the chelae are shut (fig. 8,
B). Hence, when the chehe have closed upon anything soft
enough to allow of the imbedding of these hooks, it is
very difficult, if not impossible, to open them again.

Immediately the young are set free, they must instinc-
tively bury the ends of their forceps in the hardened
egg-glue which is smeared over the swimmerets, for they
are all found to be holding on in this manner. They
exhibit very little movement, and they bear rough
shaking or handling without becoming detached ; in
consequence, I suppose, of the interlocking of the hooked
ends of the chelae imbedded in the egg-glue.

Even after the female has been plunged into alcohol,
the young remain attached. I have had a female, with
young affixed in this manner, under observation for five

44 THE NATURAL IHSTORY OF THE COMMON CRAYFISH.

days, but none of them showed any signs of detaching
themselves ; and I am inclined to think that they are
set free only at the first moult. After this, it would
appear that the adhesion to the parent is only temporary.

The walking legs are also hooked at their extremities,
but they play a less important part in fixing the young
to the parent, and seem to be always capable of loosing
their hold.

I find the young of a Mexican crayfish (Cambarns) to be
attached in the same manner as those of the English
crayfish; but, according to Mr. Wood-Mason's recent
observations, the young of the New Zealand crayfishes
fix themselves to the swimmerets of the parent by the
hooked ends of their hinder ambulatory limbs.

Crayfishes, in every respect similar to those found
in our English rivers, that is to say, of the species
Astacus fluviatilis, are met with in Ireland, and on the
Continent, as far south as Italy and northern Greece ;
as far east as western Russia ; and as far north as the
shores of the Baltic. They are not known to occur in
Scotland ; in Spain, except about Barcelona, they are
either rare, or have remained unnoticed.

There is, at present, no proof of the occurrence of
Astacus fluviatilis in the fossil state.

Curious myths have gathered about crayfishes, as
about other animals. At one time " crabs'-eyes " were

CRAYFISHES AND PIGS.

45

collected in vast numbers, and sold for medicinal
purposes as a remedy against the stone, among other
diseases. Their real utility, inasmuch as they consist
almost entirely of carbonate of lime, with a little phos-
phate of lime and animal matter, is much the same as
that of chalk, or carbonate of magnesia. It was, for-
merly, a current belief that crayfishes grow poor at the
time of new moon, and fat at that of full moon ; and,
perhaps, there may be some foundation for the notion,
considering the nocturnal habits of the animals. Van
Helmont, a great dealer in wonders, is responsible for
the story that, in Brandenburg, where there is a great
abundance of crayfishes, the dealers were obliged to
transport them to market by night, lest a pig should
run under the cart. For if such a misfortune should
happen, every crayfish would be found dead in the
morning: " Tarn exitialis est porcus cancro." Another
author improves the story, by declaring that the steam
of a pig-stye, or of a herd of swine, is instantaneously
fatal to crayfish. On the other hand, the smell of
putrifying crayfish, which is undoubtedly of the strongest,
was said to drive even moles out of their burrows.

CHAPTEE II.

THE PHYSIOLOGY OF THE CRAYFISH. THE MECHANISM BY
WHICH THE PARTS OF THE LIVING ENGINE ARE SUPPLIED
WITH THE MATERIALS NECESSARY FOR THEIR MAIN-
TENANCE AND GROWTH.

An analysis of such a sketch of the "Natural History
of the Crayfish" as is given in the preceding chapter,
shows that it provides brief and general answers to three
questions. First, what is the form and structure of the
animal, not only when adult, but at different stages of
its growth ? Secondly, what are the various actions of
which it is capable ? Thirdly, where is it found ? If we
carry our investigations further, in such a manner as to
give the fullest attainable answers to these questions,
the knowledge thus acquired, in the case of the first
question, is termed the Morphology of the crayfish ;
in the case of the second question, it constitutes the
Physiology of the animal ; while the answer to the third
question would represent what we know of its Distribu-
tion or Chorology. There remains a fourth problem,
which can hardly be regarded as seriously under dis-
cussion, so long as knowledge has advanced no further
than the Natural History stage ; the question, namely,

TELEOLOGY AND PHYSIOLOGY.

47

how all these facts comprised under Morphology, Physi-
ology, and Chorology have come to be what they are ;
and the attempt to solve this problem leads us to the
crown of Biological effort, JEtiology. When it supplies
answers to all the questions which fall under these four
heads, the Zoology of Crayfish will have said its last
word.

As it matters little in what order we take the first three
questions, in expanding Natural History into Zoology,
we may as well follow that which accords with the history
of science. After men acquired a rough and general
knowledge of the animals about them, the next thing which
engaged their interest was the discovery in these animals
of arrangements by which results, of a kind similar to
those which their own ingenuity effeets through mechanical
contrivances, are brought about. They observed that
animals perform various actions ; and, when they looked
into the disposition and the powers of the parts by which
these actions are performed, they found that these parts
presented the characters of an apparatus, or piece of
mechanism, the action of which could be deduced from
the properties and connections of its constituents, just
as the striking of a clock can be deduced from the
properties and connections of its weights and wheels.

Under one aspect, the result of the search after the
rationale of animal structure thus set afoot is Teleology;
or the doctrine of adaptation to purpose. Under another

48 THE PHYSIOLOGY OF THE COMMON CRAYFISH,

aspect, it is Physiology ; so far as Physiology consists in
the elucidation of complex vital phenomena by deduction
from the established truths of Physics and Chemistry, or
from the elementary properties of living matter.

We have seen that the crayfish is a voracious and
indiscriminate feeder ; and we shall be safe in assuming
that, if duly supplied with nourishment, a full-grown
crayfish will consume several times its own weight of
food in the course of the year. Nevertheless, the increase
of the animal's weight at the end of that time is, at most,
a small fraction of its total weight ; whence it is quite
clear, that a very large proportion of the food taken into
the body must, in some shape or other, leave it again.
In the course of the same period, the crayfish absorbs a
very considerable quantity of oxygen, supplied by the
atmosphere to the water which it inhabits ; while it gives
out, into that water, a large amount of carbonic acid, and
a larger or smaller quantity of nitrogenous and other ex-
crementitious matters. From this point of view, the
crayfish may be regarded as a kind of chemical manu-
factory, supplied with certain alimentary raw materials,
which it works up, transforms, and gives out in other
shapes. And the first physiological problem which offers
itself to us is the mode of operation of the apparatus
contained in this factory, and the extent to which the
products of its activity are to be accounted for by
reasoning from known physical and chemical principles.

THE PROCESS OF FEEDING.

49

We have learned that the food of the crayfish is made
up of very diverse substances, both animal and vegetable ;
but, so far as they are competent to nourish the animal
permanently, these matters all agree in containing a
peculiar nitrogenous body, termed protein, under one of its
many forms, such as albumen, fibrin, and the like. With
this may be associated fatty matters, starchy and sac-
charine bodies, and various earthy salts. And these,
which are the essential constituents of the food, may be,
and usually are, largely mixed up with other substances,
such as wood, in the case of vegetable food, or skeletal
and fibrous parts, in the case of animal prey, which are
of little or no utility to the crayfish.

The first step in the process of feeding, therefore, is
to reduce the food to such a state, that the separation
of its nutritive parts, or those which can be turned to
account, from its innutritious, or useless, constituents,
may be facilitated. And this preliminary operation is
the subdivision of the food into morsels of a convenient
size for introduction into that part of the machinery in
which the extraction of the useful products is performed.

The food may be seized by the pincers, or by the
anterior chelate ambulatory limbs ; and, in the former
case, it is usually, if not always, transferred to the first,
or second, or both of the anterior pairs of ambulatory
limbs. These grasp the food, and, tearing it into
pieces of the proper dimensions, thrust them between
tb-e external maxillipedes, which are, at the same time,

50 THE PHYSIOLOGY OF THE COMMON CRAYFISH.

worked rapidly to and fro sideways, so as to bring their
toothed edges to bear upon the morsel. The other five
pairs of jaws are no less active, and they thus crush and
divide the food brought to them, as it is passed between
their toothed edges to the opening of the mouth.

As the alimentary canal stretches from the mouth,
at one end, to the vent at the other, and, at each of
these limits, is continuous with the wall of the body,
we may conceive the whole crayfish to be a hollow
cylinder, the cavity of which is everywhere closed, though
it is traversed by a tube, open at each end (fig. 6).
The shut cavity between the tube and the walls of the
cylinder may be termed the perivisceral cavity ; and it is
so much filled up by the various organs, which are inter-
posed between the alimentary canal and the body wall,
that all that is left of it is represented by a system of
irregular channels, which are filled with blood, and are
termed blood sinuses. The wall of the cylinder is the
outer wall of the body itself, to which the general name
of integument may be given ; and the outermost layer of
this, again, is the cuticle, which gives rise to the whole
of the exoskeleton. This cuticle, as wTe have seen, is
extensively impregnated with lime salts ; and, moreover,
in consequence of its containing chitin, it is often spoken
of as the chitinous cuticula.

Having arrived at this general conception of the dis*
position of the parts of the factory, we may next proceed
to consider the machinery of alimentation which is con*

THE MACHINERY OF ALIMENTATION. 51

tained within it, and which is represented by th3 various
divisions of the alimentary canal, with its appendages ;
by the apparatus for the distribution of nutriment ; and
by two apparatuses for getting rid of those products
which are the ultimate result of the working of the whole
organism.

And here we must trench somewhat upon the province
of Morphology, as some of these pieces of apparatus are
complicated ; and their action cannot be comprehended
without a certain knowledge of their anatomy.

The mouth of the crayfish is a longitudinally elongated*
parallel-sided opening, in the integument of the ventral
or sternal aspect of the head. Just outside its lateral
boundaries, the strong mandibles project, one on each
side (fig 3, B ; 4) ; their broad crushing surfaces, which
are turned towards one another, are therefore completely
external to the oral cavity. In front, the mouth is over-
lapped by a wide shield-shaped plate termed the upper
lip, or labrum (figs. 3 and 6, lb) ; while, immediately be-
hind the mandibles, there is, on each side, an elongated
fleshy lobe, joined with its fellow by the posterior
boundary of the mouth. These together constitute the
metastoma (fig. 3, B ; mt), which is sometimes called
the lower lip. A short wide gullet, termed the oeso-
phagus (fig. 6, oe), leads directly upwards into a spacious
bag, the stomach, which occupies almost the whole cavity
of the head. It is divided by a constriction into a large
anterior chamber {cs), into the under face of which the

S 2

52 THE PHYSIOLOGY OF THE COMMON CRAYFISH.

gullet opens, and a small posterior chamber (ps)9 from
which the intestine (kg) proceeds.

In a man's stomach, the opening by which the gullet
communicates with the stomach is called the cardia,
while that which places the stomach in communication
with the intestine is named the pylorus ; and these terms
having been transferred from human anatomy to that of
the lower animals, the larger moiety of the crayfish's
stomach is called the cardiac division, while the smaller
is termed the pyloric division of the organ. It must be
recollected, however, that, in the crayfish, the so-called
cardiac division is that which is actually furthest from
the heart, not that which is nearest to it, as in man.

The gullet is lined by a firm coat which resembles thin
parchment. At the margins of the mouth, this strong
lining is easily seen to be continuous with the cuticular
exoskeleton; while, at the cardiac orifice, it spreads out
and forms the inner or cuticular wall of the whole gastric
cavity, as far as the pylorus, where it ends in certain
valvular projections. The chitinous cuticle which forms
the outermost layer of the integument is thus, as it were,
turned in, to constitute the innermost layer of the walls
of the stomach; and it confers upon them so great an
amount of stiffness that they do not collapse when the
organ is removed from the body. Furthermore, just as
the cuticle of the integument is calcified to form the hard
parts of the exoskeleton, so is the cuticle of the stomach
calcified, or otherwise hardened, to give rise, in the first

THE STOMACH OF THE CRAYFISH. 53

place, to the very remarkable and complicated apparatus
which has already been spoken of, as a sort of gastric mill

Fig. 9.— Astacus fluviatilis. — A, the stomach with its outer coat removed, seen from the
left side ; B, the same viewed from the front, after removal of the anterior wall ;
C, the ossicles of the gastric mill separated from one another ; D, the prepy-
loric ossicle and median tooth, seen from the right side ; E, transverse section of
the pyloric region along the line xy in A (all x 2). c, cardiac ossicle ; cpv, cardio-
pyloric valve ; lp, lateral pouch ; It, lateral tooth, seen through the wall of the
stomach in A ; mg, mid-gut ; mt, median tooth, seen through the wall of the
stomach in A ; ces, oesophagus ; p, pyloric ossicle ; pc, pterocardiac ossicle ;
pp, prepyloric ossicle ; itc, uro-cardiac process ; t, convexities on the free surface
of its hinder end ; v1, median pyloric valve ; zc, zygocardiac ossicle.

or food-crusher ; and, secondly, to a filter or strainer,
whereby the nutritive juices are separated from the in-
nutritious hard parts of the food and passed on into the
intestine.

54 THE PHYSIOLOGY OF THE COMMON CRAYFISH.

The gastric mill begins in the hinder half of the cardiac
division. Here, on the upper wall of the stomach, we see
a broad transverse calcined bar (figs. 9-11, c) from
the middle of the hinder part of which another bar (nc),
united to the first by a flexible portion, is continued
backwards in the middle line. The whole has, therefore,
somewhat the shape of a cross-bow. Behind the first-
mentioned piece, the dorsal wall of the stomach is folded
in, in such a manner as to give rise to a kind of pouch ;
and the second piece, or what we may call the handle of
the crossbow, lies in the front wTall of this pouch. The
end of this piece is dense and hard, and its free surface,
which looks into the top of the cardiac chamber, is
raised into two oval, flattened convex surfaces (t). Con-
nected by a transverse joint with the end of the handle
of the crossbow, there is another solid bar, which ascends
obliquely forwards in the back wall of the pouch (pp)*
The end which is articulated with the handle of the cross-
bow is produced into a strong reddish conical tooth (mi),
curved forwards and bifurcated at the summit ; conse-
quently, when the cavity of the stomach is inspected from
the fore part of the cardiac pouch (fig. 9, B), the two-
pointed curved tooth (mi) is seen projecting behind the
convex surfaces (i), in the middle line, into the interior
of that cavity. The joint which connects the handle of the
crossbow with the hinder middle piece is elastic ; hence,
if the two are straightened out, they return to their bent dis-
position as soon as they are released. The upper end of

THE GASTRIC MILL.

55

the hinder middle piece (pp) is connected with a second flat
transverse plate which lies in the dorsal wall of the pyloric
chamber (p). The whole arrangement, thus far, may be
therefore compared to a large cross-bow and a small one,
with the ends of their handles fastened together by a
spring joint, in such a manner that the handle of the
one makes an acute angle with the handle of the other ;
while the middle of each bow is united with the middle of
the other by the bent arm formed by the two handles.
But, in addition to this, the outer ends of the two bows
are also connected together. A small, curved, calcified
bar (pc) passes from the outer end of the front crosspiece
downwards and outwards in the wall of the stomach, and
its hinder and lower extremity is articulated with another
larger bar (zc) which runs upwards and backwards to
the hinder or pyloric crosspiece, with which it articulates.
Internally, this piece projects into the cardiac cavity of
the stomach as a stout elongated reddish elevation (It),
the surface of which is produced into a row of strong
sharp, transverse ridges, which diminish in size from
before backwards, and constitute a crushing surface
almost like that of the grinder of an elephant. Thus,
when the front part of the cardiac cavity is cut away,
not only are the median teeth already mentioned seen,
but, on each side of them, there is one of these long
lateral teeth.

There are two small pointed teeth, one under each
of the lateral teeth, and each of these is supported by

•~><; THE PHYSIOLOGY OF THE COMMON CRAYFISH.

a broad plate, hairy on its inner surface, which enters
into the lateral wall of the cardiac chamber. There are
various other smaller skeletal parts, but the most im-

Fig. 10. — AstaeiisJiuviatUis. — Longitudinal section of the stomach ( x 4),
c, cardiac ossicle; cce, caecum ; c.jj.v, cardio-pyloric valve; cs, cushion-
shaped surface ; hind-gut ; lij). aperture of right bile duct ; lp,
lateral pouch ; It, lateral teeth ; mg, mid-gut ; mt, median tooth ; ces,
oesophagus ; pyloric ossicle ; po. pterocardiac ossicles ; pj?, prepy-
loric ossicle ; uc, urocardiac process ; v\ median pyloric valve; v2,
lateral pyloric valve; x, position of gastrolith; zc, zygocardiac ossicle.

portant are those which have been described ; and these,
from what has been said, will be seen to form a sort of
hexagonal frame, with more or less flexible joints at the
angles, and having the anterior and the posterior sides

THE GASTRIC MILL.

57

connected by a bent jointed middle bar. As all these
parts are merely modifications of the hard skeleton, the
apparatus is devoid of any power of moving itself. It
is set in motion, however, by the same substance as that
which gives rise to all the other bodily movements of
the crayfish, namely, muscle. The chief muscles which
move it are four very strong bundles of fibres. Two of
these are attached to the front crosspiece, and proceed
thence, upwards and forwards, to be fixed to the inner face
of the carapace in the front part of the head (figs. 5, 6,
and 12, ag). The two others, which are fixed into the
hinder crosspiece and hinder lateral pieces, pass upwards
and backwards, to be attached to the inner face of the
carapace in the back part of the head [pg)* When these
muscles shorten, or contract, they pull the front and back
crosspieces further away from one another; consequently,
the angle between the handles becomes more open and
the tooth which is borne on their ends travels downwards
and forwards. But, at the same time, the angle between
the side bars becomes more open and the lateral tooth
of each side moves inwards till it crosses in front of the
middle tooth, and strikes against this and the opposite
lateral tooth, which has undergone a corresponding change
of place. The muscles being now relaxed, the elasticity
of the joints suffices to bring the whole apparatus back
to its first position, when a new contraction brings about
a new clashing of the teeth. Thus, by the alternate con-
traction and relaxation of these two pah' of muscles, the

5S THE PHYSIOLOGY OF THE COMMON CRAYFISH.

three teeth are made to stir up and crush whatever ia
contained in the cardiac chamber. "When the stomach is
removed and the front part of the cardiac chamber is cut
away, the front cross-piece may be seized with one pair
of forceps and the hind cross-piece with another. On
slightly pulling the two, so as to imitate the action of the
muscles, the three teeth will be found to come together
sharply, exactly in the manner described.

Works on mechanics are full of contrivances for the
conversion of motion; but it would, perhaps, be difficult to
discover among these a prettier solution of the problem ;
given a straight pull, how to convert it into three simul-
taneous convergent movements of as many points.

What I have called the filter is constructed mainly out
of the chitinous lining of the pyloric chamber. The aper-
ture of communication between this and the cardiac
chamber, already narrow, on account of the constriction of
the walls of the stomach at this point, is bounded at the
sides by two folds ; while, from below, a conical tongue-
shaped process (figs. 6, 10, and 11, cpv), the surface of
which is covered with hairs, further obstructs the opening.
In the posterior half of the pyloric chamber, its side walls
are, as it were, pushed in; and, above, they so nearly meet
in the middle line, that a mere vertical chink is left be-
tween them ; while even this is crossed by hairs set upon
the two surfaces. In its lower half, however, each side
wall curves outwards, and forms a cushion-shaped surface
(fig. 10, cs) which looks downwards and inwards. If the

THE FILTERING APPARATUS.

59

floor of the pyloric chamber were flat, a wide triangular
passage would thus be left open in its lower half. But,
in fact, the floor rises into a ridge in the middle, while, at
the sides, it adapts itself to the shape of the two cushion-
shaped surfaces ; the result of which is that the whole
cavity of the posterior part of the pyloric division of the
stomach is reduced to a narrow three-rayed fissure. In
transverse section, the vertical ray of this fissure is
straight, while the two lateral ones are concave upwards
(fig. 9, E). The cushions of the side walls are covered
with short close-set hairs. The corresponding surfaces
of the floor are raised into longitudinal parallel ridges,
the edge of each of which is fringed with very fine hairs.
As everything which passes from the cardiac sac to the
intestine, must traverse this singular apparatus, only the
most finely divided solid matters can escape stoppage, so
long as its walls are kept together.

Finally, at the opening of the pyloric sac into the
intestine, the chitinous investment terminates in five
symmetrically arranged processes, the disposition of
which is such that they must play the part of valves
in preventing any sudden return of the contents of the
intestine to the stomach, while they readily allow of a
passage the other way. One of these valvular processes
is placed in the middle line above (figs. 10 and 11, vl).
It is longer than the others and concave below. The
lateral processes (i?2,) of which there are two on each side,
are triangular and flat.

60 THE PHYSIOLOGY OF THE COMMON CRAYFISH.

The cuticular lining which gives rise to all the com-
plicated apparatus which has just been described, must

Fig. 11. — Astacm fluviatilis. — View of the roof of the stomach, the
ventral wall of which, and of the mid-gut, is laid open by a longi-
tudinal incision ( x 4). On the right side (the left in the figure),
the lateral tooth is cut away, as well as the floor of the lateral
pouch. The letters have the same signification as in fig. 10.

not be confounded with the proper wall of the stomach,
which invests it, and to which it owes it origin, just as
the cuticle of the integument is produced by the soft

FORE-GUT, MID-GUT, AND HIND-GUT.

61

true skin which lies beneath it. The wall of the stomach
is a soft pale membrane containing variously disposed
muscular fibres ; and, beyond the pylorus, it is continued
into the wall of the intestine.

It has already been mentioned that the intestine is
a slender and thin-walled tube, which passes straight
through the body almost without change, except that it
becomes a little wider and thicker-walled near the vent.
Immediately behind the pyloric valves, its surface is quite
smooth and soft (figs. 9, 10, and 12, mg), and its floor
presents a relatively large"* aperture, the termination of
the bile duct (fig. 12, bd, fig. 10, hp.), on each side. The
roof is, as it were, pushed out into a short median pouch
or ccecum (cce). Behind this, its character suddenly
changes, and six squarish elevations, covered with a
chitinous cuticle, encircle the cavity of the intestine (r).
From each of these, a longitudinal ridge, corresponding
with a fold of the wall of the intestine, takes its rise, and
passes, with a slight spiral twist, to its extremity (hg).
Each of these ridges is beset with small papillae, and the
chitinous lining is continued over the whole to the vent,
where it passes into the general cuticle of the integu-
ment, just as the lining of the stomach is continuous
with the cuticle of the integument at the mouth. The
alimentary canal may, therefore, be distinguished into
a fore and a hind-gat (Jig), which have a thick internal
lining of cuticular membrane ; and a very short mid-
gut {mg), which has no thick cuticular layer. It will be of

5>> <D

THE DIGESTION OF FOOD.

63

importance to recollect this distinction by-and-by, when
the development of the alimentary canal is considered.

If the treatment to which the food is subjected in
the alimentary apparatus were of a purely mechanical
nature, there would be nothing more to describe in this
part of the crayfish's mechanism. But, in order that
the nutritive matters may be turned to account, and
undergo the chemical metamorphoses, which eventually
change them into substances of a totally different cha-
racter, they must pass out of the alimentary canal into
the blood. And they can do this only by making their
way through the walls of the alimentary canal ; to which
end they must either be in a state of extremely fine
division, or they must be reduced to the fluid condition.
In the case of the fatty matters, minute subdivision may
suffice ; but the amylaceous substances and the insoluble
protein compounds, such as the fibrin of flesh, must be
brought into a state of solution. Therefore some sub-
stances must be poured into the alimentary canal, which,
when mixed with the crushed food, will play the part
of a chemical agent, dissolving out the insoluble proteids,
changing the amyloids into soluble sugar, and convert-
ing all the proteids into those diffusible forms of protein
matter, which are known as peptones.

The details of the processes here indicated, which
may be included under the general name of digestion, have
only quite recently been carefully investigated in the
crayfish ; and we have probably still much to learn about

6 t THE PHYSIOLOGY OF THE COMMON CRAYFISH.

them ; but what has been made out is very interesting,
and proves that considerable differences .exist between
crayfishes and the higher animals in this respect.

The physiologist calls those organs, the function of
which is to prepare and discharge substances of a special
character, glands; and the matter which they elaborate
is termed their secretion. On the one side, glands are
in relation with the blood, whence they derive the
materials which they convert into the substances
characteristic of their secretion ; on the other side,
they have access, directly or indirectly, to a free surface,
on to which they pour their secretion as it is formed.

Of such glands, the alimentary canal of the crayfish
is provided with a pair, which are not only of very large
size, but are further extremely conspicuous, on account
of their yellow or brown colour. These two glands (figs.
12 and 13, Ir) are situated beneath, and on each side of, the
stomach and the anterior part of the intestine, and answer
in position to the glands termed liver and pancreas in the
higher animals, inasmuch as they pour their secretion into
the mid-gut. These glands have hitherto always been re
garded as the liver, and the name maybe retained, though
their secretion appears rather to correspond with the
pancreatic fluid than with the bile of the higher animals.

Each liver consists of an immense number of short
tubes, or cceca, which are closed at one end, but open at
the other into a general conduit, which is termed their
duct. The mass of the liver is roughly divided into

THE DIGESTION OF FOOD.

07

the yellow fluid accumulating in the ducts passes into
the mid- gut. The yellow colour is due to the globules of
fat. In the young cells, at the summit of the cseca,
these are either absent, or very small, whence the part
appears colourless. But, lower down, small yellow
granules appear in the cells, and these become bigger
and more numerous in the middle and lower parts. In
fact, few glands are better fitted for the study of the
manner in which secretion is effected than the crayfish's
liver.

We may now consider the alimentary machinery, the
general structure of which has been explained, in
action.

The food, already torn and crushed by the jaws, is
passed through the gullet into the cardiac sac. and there
reduced to a still more pulpy state by the gastric mill.
By degrees, such parts as are sufficiently fluid are
drained off into the intestine, through the pyloric strainer,
while the coarser parts of the useless matters are probably
rejected by the mouth, as a hawk or an owl rejects his
casts. There is reason to believe, though it is not certainly
known, that fluids from the intestine mix with the food
while it is undergoing trituration, and effect the transforma-
tion of the starchy and the insoluble protein compounds
into a soluble state. At any rate, as soon as the strained-oif
fluid passes into the mid-gut it must be mixed with the
secretion of the liver, the action of which rs probably

f 2

68 THE PHYSIOLOGY OF THE COMMON CRAYFISH.

similar to that of the pancreatic juice of the higher
animals.

The mixture thus produced, which answers to the
chyle of the higher animals, passes along the intestine,
and the greater part of it, transuding through the walls of
the alimentary canal, enters the blood, while the rest
accumulates as dark coloured feces in the hind gut, and

Fig. 14. — Astacus fluviatilis. — The corpuscles of the blood (highly mag-
nified). 1-8 show the changes undergone by a single corpuscle
during a quarter of an hour ; 9 and 10 are corpuscles killed by
magenta, and having the nucleus deeply stained by the colouring
matter, n, nucleus.

is eventually passed out of the body by the vent. The
fecal matters are small in amount, and the strainer is
so efficient that they rarely contain solid particles of
sensible size. Sometimes, however, there are a good
many minute fragments of vegetable tissue.

The blood of which the nutritive elements of the food

i

2

3 4

THE BLOOD AND ITS CORPUSCLES. 69

have thus become integral parts, is a clear fluid, either
colourless, or of a pale neutral tint or reddish hue, which,
to the naked eye, appears like so much water. But if
subjected to microscopic examination, it is found to con-
tain innumerable pale, solid particles, or corpuscles,
which, when examined fresh, undergo constant changes
of form (fig. 14). In fact, they correspond very closely
with the colourless corpuscles which exist in our own
blood; and, in its general characters, the crayfish's
blood is such as ours would be if it were somewhat
diluted and deprived of its red corpuscles. In other
words, it resembles our lymph more than it does our
blood. Left to itself it soon coagulates, giving rise to a
pretty firm clot.

The sinuses, or cavities in which the greater part of
the blood is contained, are disposed very irregularly in
the intervals between the internal organs. But there is
one of especially large size on the ventral or sternal side
of the thorax (fig. 15, sc), into which all the blood in the
body sooner or later makes its way. From this sternal
sinus passages (av) lead to the gills, and from these again
six canals (bcv), pass up on the inner side of the inner wall
of each branchial chamber to a cavity situated in the
dorsal region of the thorax, termed the pericardium (p),
into which they open.

The blood of the crayfish is kept in a state of con-
stant circulating motion by a pumping and distributing
machinery, composed of the heart and of the arteries, with

rjn o.

iaa .

Fia 15. — Astacus fluviatilis.—A diagrammatic transverse section of
the thorax through the twelfth somite, showing the course of the
circulation of the blood ( x 3). arb. 12, the anterior or lower, and
arb'. 12, the posterior or upper arthrobranchia of the twelfth
somite ; av, afferent branchial vessel ; bcv, branchio-cardiac vein ;
bg, branchiostegite ; em, extensor muscles of abdomen ; ep, epi-
nieral wall of thoracic cavity ; ev, efferent branchial vessel ; fin,
flexor muscles of abdomen ; fp, floor of pericardium ; gn. 6, fifth
thoracic ganglion ; It, heart ; kg, hind-gut ; iaa, inferior abdominal
artery, in cross section ; la, lateral valvular apertures of heart ; lr,
liver ; mp, indicates the position of the mesophragm by which the
sternal canal is bounded laterally ; p, pericardial sinus ; pdb. 12,
podobranchia, and plb. 12, pleurobranchia of the twelfth somite ;
sa, sternal artery ; saa, superior abdominal artery ; sc9 sternal canal ;
t , testis ; xii., sternum of twelfth somite. The arrows indicate
the direction of the blood flow.

THE HEART AND THE ARTERIES.

71

their larger and smaller branches, which proceed from it
and ramify through the body, to terminate eventually in
the blood sinuses, which represent the veins of the
higher animals.

When the carapace is removed from the middle of the
region which lies behind the cervical groove, that is,
when the dorsal or tergal wall of the thorax is taken
away, a spacious chamber is laid open which is full of
blood. This is the cavity already mentioned as the peri-
cardium (fig. 15, p), though, as it differs in some respects
from that which is so named in the higher animals, it will
be better to term it the pericardial sinus.

The heart (fig. 15, h), lies in the midst of this sinus. It
is a thick muscular body (fig. 16), with an irregularly hexa-
gonal contour when viewed from above, one angle of the
hexagon being anterior and another posterior. The lateral
angles of the hexagon are connected by bands of fibrous tis-
sue (ac) with the walls of the pericardial sinus. Otherwise,
the heart is free, except in so far as it is kept in place by the
arteries which leave it and traverse the walls of the peri-
cardium. One of these arteries (figs. 5, 12, and 16, saa),
starting from the hinder part of the heart, of which it
is a sort of continuation, runs along the middle line of
the abdomen above the intestine, to which it gives off
many branches. A .sacond^-large artery starts from a
dilatation, which is common to it with the foregoing, but
passing directly downwards (figs. 12 and 15, sa, and fig. 16,
st. a), either on the right or on the left side of the intestine,

72 THE PHYSIOLOGY OF THE COMMON CRAYFISH.

traverses the nervous cord (figs. 12 and 15), and divides
into an anterior (fig. 12, sa) and a posterior (iaa) branch,
both of which run beneath and parallel with that cord.

atcu

Fig. 16. — Astacns fluviatilis. — The heart ( x 4). A, from above ; B. from
below ; C, from the left side, aa, antennary artery ; ac, alse cordis,
or fibrous bands connecting the heart with the walls of the peri-
cardial sinus ; bulbous dilatation at the origin of the sternal
artery ; ha, hepatic artery ; la, lateral valvular apertures ; oa, oph-
thalmic artery ; s.a, superior valvular apertures ; s.a.a, superior
abdominal artery ; st.a, sternal artery, in B cut off close to its
origin.

A third artery runs, from the front part of the heart,
forwards in the middle line, over the stomach, to the
eyes and fore part of the head (figs. 5, 12, and 16, oa) ;
and two others diverge one on each side of this, and sweep

THE ACTION OF THE HEART.

73

round the stomach to the antennae (aa). Behind these,
yet two other arteries are given off from the under side of
the heart, and supply the liver (ha). All these arteries
branch out and eventually terminate in fine, so-called
capillary, ramifications.

In the dorsal wall of the heart two small oval aper-
tures are visible, provided with valvular lips (fig. 16,
sa), which open inwards, or towards the internal cavity
of the heart. There is a similar aperture in each of the
lateral faces of the heart (la), and two others in its
inferior face (ia), making six in all. These apertures
readily admit fluid into the heart, but oppose its exit.
On the other hand, at the origins of the arteries, there
are small valvular folds, directed in such a manner as to
permit the exit of fluid from the heart, while they prevent
its entrance.

The walls of the heart are muscular, and, during life,
they contract at intervals with a regular rhythm, in such
a manner as to diminish the capacity of the internal cavity
of the organ. The result is, that the blood which it
contains is driven into the arteries, and necessarily forces
into their smaller ramifications an equivalent amount of
the blood which they already contained ; whence, in the
long run, the same amount of blood passes out of the
ultimate capillaries into the blood sinuses. From the
disposition of the blood sinuses, the impulse thus given
to the blood which they contain is finally conveyed to the
blood in the branchiae, and a proportional quantity of that

74 THE PHYSIOLOGY OF THE COMMON CRAYFISH.

blood leaves the branchiae and passes into the sinuses which
connect them with the pericardial sinus (fig. 15, bev), and
thence into that cavity. At the end of the contraction,
or systole, of the heart, its volume is of course diminished
by the volume of the blood forced out, and the space
between the walls of the heart and those of the pericardial
sinus is increased to the same extent. This space, how-
ever, is at once occupied by the blood from the branchiae,
and perhaps by some blood which has not passed through
the branchiae, though this is doubtful. When the systole
is over, the diastole follows ; that it to say, the elasticity
of the walls of the heart and that of the various parts
which connect it with the walls of the pericardium, bring
it back to its former size, and the blood in the pericardial
sinus flows into its cavity by the six apertures. With a
new systole the same process is repeated, and thus the
blood is driven in a circular course through all parts of
the body.

It will be observed that the branchiae are placed in the
course of the current of blood which is returning to the
heart; which is the exact contrary of what happens in
fishes, in which the blood is sent from the heart to the
branchiae, on its way to the body. It follows, from this
arrangement, that the blood which goes to the branchiae
is blood in which the quantity of oxygen has undergone
a diminution, and that of carbonic acid an increase, as
compared with the blood in the heart itself. For the

THE ORGANS OF RESPIRATION.

75

activity of all the organs, and especially of the muscles,
is inseparably connected with the absorption of oxygen
and the evolution of carbonic acid ; and the only source
from which the one can be derived, and the only recep-
tacle into which the other can be poured, is the blood
which bathes and permeates the whole fabric to which it
is distributed by the arteries.

The blood, therefore, which reaches the branchiae has
lost oxygen and gained carbonic acid ; and these organs
constitute the apparatus for the elimination of the inju-
rious gas from the economy on the one hand, and, on the
other, for the taking in of a new supply of the needful
" vital air," as the old chemists called it. It is thus that
the branchiae subserve the respiratory function.

The crayfish has eighteen perfect and two rudimentary
branchiae in each branchial chamber, the boundaries of
which have been already described.

Of the eighteen perfect branchiae, six (podobranchice) are
attached to the basal joints of the thoracic limbs, from the
last but one to the second (second maxillipede) inclusively
(fig. 4, p. 26, pdb, and fig. 17, A, B) ; and eleven (arthro-
branchice) are fixed to the flexible interarticular mem-
branes, which connect these basal joints with the parts
of the thorax to which they are articulated (fig. 4, arb, arb',
fig. 17, C). Of these eleven branchiae, two are attached
to the interarticular membranes of all the ambulatory
legs but the last, (==6) and to those of the pincers and of
the external maxillipedes, (=4) and one to that of the

Fig. 17. — Astacus fluviatilis.—k, one of the podobranchias from the
outer side ; B, the same from the inner side ; C, one of the arthro-
branchias ; D, a part of one of the coxopoditic setaa ; E, extremity of
the same seta ; F, extremity of a seta from the base of the podo-
branchia ; Gr, hooked seta of the lamina ; (A — C, x 3 ; D — Gr, highly
magnified), b, base of podobranchia ; cs, coxopoditic setse; cxp,
coxopodite ; I, lamina, pi, plume, and st, stem of podobranchia ;
tj tubercle on the coxopodite, to which the setae are attached.

ARTHEOBKANCHIiE AND PODOBRANCHLE. 77

second maxillipede. The first maxillipede and the last
ambulatory limb have none. Moreover, where there are
two arthrobranchise, one is more or less in front of and
external to the other.

These eleven arthrobranchise are all very similar in
structure (fig. 17, C). Each consists of a stem which con-
tains two canals, one external and one internal, separated
by a longitudinal partition. The stem is beset with a great
number of delicate branchial filaments, so that it looks
like a plume tapering from its base to its summit. Each
filament is traversed by large vascular channels, which
break up into a net-work immediately beneath the surface.
The blood driven into the external canals of the stem (fig.
15, av) is eventually poured into the inner canal (ev), which
again communicates with the channels (bcv) which lead to
the pericardial sinus (p). In its course, the blood traverses
the branchial filaments, the outer investment of each of
which is an excessively thin chitinous membrane, so that
the blood contained in them is practically separated by a
mere film from the aerated water in which the gills float.
Hence, an exchange of gaseous constituents readily takes
place, and as much oxygen is taken in as carbonic acid is
given out.

The six podobranchise, or gills which are attached to
the basal joints of the legs, play the same part, but differ
a good deal in the details of their structure from those
which are fixed to the interarticular membranes. Each con-
sists of a broad base (fig. 17, A and B ; b) beset with many

78 THE PHYSIOLOGY OF THiil COMMON CRAYFISH.

fine straight hairs, or setce (F), whence a narrow stem (si)
proceeds. At its upper end this stem divides into two
parts, that in front, the plume (pi), resembling the free
end of one of the gills just described, while that behind,
the lamina (Z), is a broad thin plate, bent upon itself longi-
tudinally in such a manner that its folded edge lies for-
wards, and covered with minute hooked setae (G). The
gill which follows is received into the space included
between the two lobes or halves of the folded lamina
(fig. 4, p. 26). Each lobe is longitudinally plaited into
about a dozen folds. The whole front and outer face of
the stem is beset with branchial filaments ; hence, we may
compare one of these branchiae to one of the preceding
kind, in which the stem has become modified and has
given off a large folded lamina from its inner and
posterior face.

The branchiae now described are arranged in sets of
three for each of the thoracic limbs, from the third
maxillipede to the last but one ambulatory limb, and
two lor the second maxillipede, thus making seventeen
in all (3 X 5 + 2 = 17) ; and, between every two there is
found a bundle of long twisted hairs (fig. 17, A, cx.s; D and
E), which are attached to a small elevation (t) on the basal
joint of each limb. These coxopoditic setce, no doubt, serve
to prevent the intrusion of parasites and other foreign
matters into the branchial chamber. From the mode
of attachment of the six branchiae it is obvious that they
must share in the movements of the basal joints of the

PLEUBOBRA NCHLE, COMPLETE AND RUDIMENTARY. 79

legs ; and that, when the crayfish walks, they must be
more or less agitated in the branchial chamber.

The eighteenth branchia resembles one of the eleven
arthrobranchise in structure ; but it is larger, and it is
attached neither to the basal joint of the hindermost ambu-
latory limb, nor to its interarticular membrane, but to the
sides of the thorax, above the joint. From this mode ot
attachment it is distinguished from the others as &pleuro-
branchia (fig. 4, plb. 14).

Finally, in front of this, fixed also to the walls of the
thorax, above each of the two preceding pairs of ambulatory
limbs, there is a delicate filament, about a sixteenth of
an inch long, which has the structure of a branchial
filament, and is, in fact, a rudimentary pleurobranchia
(fig. 4, plb. 12, plb. 13).

The quantity of water which occupies the space left in
the branchial chamber by the gills is but small, and as
the respiratory surface offered by the gills is relatively
very large, the air contained in this water must be
rapidly exhausted, even when the crayfish is quiescent ;
while, when any muscular exertion takes place, the quan-
tity of carbonic acid formed, and the demand for fresh
oxygen, is at once greatly increased. For the efficient
performance of the function of respiration, therefore, the
water in the branchial chamber must be rapidly renewed,
and there must be some arrangement hy which the
supply of fresh water may be proportioned to the
demand. In many animals, the respiratory surface is

80 THE PHYSIOLOGY OF THE COMMON CRAYFISH.

covered with rapidly vibrating filaments, or cilia, by
means of which a current of water is kept con-
tinually flowing over the gills, but there are none of these
in the crayfish. The same object is attained, however, in
another way. The anterior boundary of the branchial
chamber corresponds with the cervical groove, which, as
has been seen, curves downwards and then forwards,
until it terminates at the sides of the space occupied by
the jaws. If the branchiostegite is cut away along the
groove, it will be found that it is attached to the sides of
the head, which project a little beyond the anterior part
of the thorax, so that there is a depression behind the
sides of the head — just as there is a depression, behind a
man's jaw, at the sides of the neck. Between this
depression in front, the walls of the thorax internally,
the branchiostegite externally, and the bases of the for-
ceps and external foot-jaws below, a curved canal is in-
cluded, by which the branchial cavity opens forwards as by
a funnel. Attached to the base of the second maxilla
there is a wide curved plate (fig. 4, 6) which fits
against the projection of the head, as a shirt collar might
do, to carry out our previous comparison ; and this scoop-
shaped plate (termed the scaphognathite), which is con-
cave forwards and convex backwards, can be readily
moved backwards and forwards.

If a living crayfish is taken out of the water, it will
be found that, as the water drains away from the branchial
cavity, bubbles of air are forced out of its anterior opening.

THE RESPIRATORY CURRENT.

81

Again, if, when a crayfish is resting quietly in the water,
a little coloured fluid is allowed to run down towards
the posterior opening of the branchial chamber, it will
very soon be driven out from the anterior aperture,
with considerable force, in a long stream. In fact, as
the scaphognathite vibrates not less than three or four
times in a second, the water in the funnel-shaped front
passage of the branchial cavity is incessantly baled out ;
and, as fresh water flows in from behind to make up the
loss, a current is kept constantly flowing over the gills.
The rapidity of this current naturally depends on tho
more or less quick repetition of the strokes of the
scaphognathite; and hence, the activity of the respira-
tory function can be accurately adjusted to the wants of
the economy. Slow working of the scaphognathite
answers to ordinary breathing in ourselves, quick working
to panting.

A further self-adjustment of the respiratory apparatus
is gained by the attachment of the six gills to the basal
joints of the legs. For, when the animal exerts its
muscles in walking, these gills are agitated, and thus not
only bring their own surfaces more largely in contact with
the water, but produce the same effect upon the other
gills.

The constant oxidation which goes on in all parts of
the body not only gives rise to carbonic acid ; but, so far
as it affects the proteinaceous constituents, it produces

G

82 THE PHYSIOLOGY OF THE COMMON CRAYFISH.

compounds which contain nitrogen, and these, like other
waste products, must be eliminated. In the higher
animals, such waste products take the form of urea, uric
acid, hippuric acid, and the like ; and are got rid of by
the kidneys. We may, therefore, expect to find some
organ wThich plays the part of a kidney in the crayfish ;
but the position of the structure, which there is much
reason for regarding as the representative of the kidney,
is so singular that very different interpretations have
been put upon it.

On the basal joint of each antenna it is easy to see a
small conical eminence with an opening on the inner side
of its summit (fig. 18). The aperture (x) leads by a
short canal into a spacious sac, with extremely delicate
walls (s), which is lodged in the front part of the head, in
front of and below the cardiac division of the stomach (cs).
Beneath this, in a sort of recess, which corresponds with
the epistoma, and with the base of the antenna, there is a
discoidal body of a dull green colour, in shape somewhat
like one of the fruits of the mallow, which is known as
the green gland (gg). The sac narrows below like a wide
funnel, and the edges of its small end are continuous with
the walls of the green gland ; they surround an aperture
which leads into the interior of the latter structure, and
conveys its products into the sac, whence they are excreted
by the opening in the antennary papilla. The green gland
is said to contain a substance termed guanin (so named
because it is found in the guano which is the accumulated

THE RENAL ORGAN.

83

excrement of birds), a nitrogenous body analogous in
some respects to uric acid, but less highly oxidated ;

Fig. 18. — Astacus ftuviatilis. — A, the anterior part of the body, with
the dorsal portion of the carapace removed to show the position of
the green glands ; B, the same, with the left side of the carapace
removed ; C, the green gland removed from the body (all x 2).
ag, left anterior gastric muscle ; c, circum oesophageal commis-
sures ; cs, cardiac portion of stomach ; gg, green gland, exposed
in A on the left side by the removal of its sac ; ima, inter-
maxillary or cephalic apodeme ; ces, oesophagus seen in transverse
section in A, the stomach being removed ; s, sac of green gland ;
x, bristle passed from the aperture in the basal joint of the antenna
into the sac.

and if this be the case, there can be little doubt that
the green gland represents the kidney, and its secretion

G 2

84 THE TIIYSIOLOGY OF THE COMMON CRAYFISH.

the urinary fluid, while the sac is a sort of urinary
bladder.

Restricting our attention to the phenomena which have
now been described, and to a short period in the life of
the crayfish, the body of the animal may be regarded
as a factory, provided with various pieces of machinery,
by means of which certain nitrogenous and other matters
are extracted from the animal and vegetable substances
which serve for food, are oxidated, and are then delivered
out of the factory in the shape of carbonic acid gas,
guanin, and probably some other products, with which
we are at present unacquainted. And there is no doubt,
that if the total amount of products given out could be
accurately weighed against the total amount of materials
taken in, the weight of the two would be found to be
identical. To put the matter in its most general shape,
the body of the crayfish is a sort of focus to which certain
material particles converge, in which they move for a
time, and from which they are afterwards expelled in new
combinations. The parallel between a whirlpool in a
stream and a living being, which has often been drawn, is
as just as it is striking. The whirlpool is permanent,
but the particles of water which constitute it are in-
cessantly changing. Those which enter it, on the one
side, are whirled around and temporarily constitute a part
of its individuality; and as they leave it on the other
bide, their places are made good by new comers.

THE WHIRLPOOL OF LIFE.

85

Those who have seen the wonderful whirlpool, three
miles below the Falls of Niagara, will not have forgotten
the heaped-up wave which tumbles and tosses, a very
embodiment of restless energy, where the swift stream
hurrying from the Falls is compelled to make a sudden
turn towards Lake Ontario. However changeful in the
contour of its crest, this wave has been visible, approxi-
mately in the same place, and with the same general
form, for centuries past. Seen from a mile off, it would
appear to be a stationary hillock of water. Viewed closely,
it is a typical expression of the conflicting impulses
generated by a swift rush of material particles.

Now, with all our appliances, we cannot get within
a good many miles, so to speak, of the crayfish. If we
could, we should see that it was nothing but the constant
form of a similar turmoil of material molecules which
are constantly flowing into the animal on the one side,
and streaming out on the other.

The chemical changes which take place in the body of
the crayfish, are doubtless, like other chemical changes,
accompanied by the evolution of heat. But the amount
of heat thus generated is so small and, in consequence
of the conditions under which the crayfish lives, it is so
easily carried away, that it is practically insensible. The
crayfish has approximately the temperature of the sur-
rounding medium, and it is, therefore, reckoned among
the cold-blooded animals.

If our investigation of the results of the process of

86 THE PHYSIOLOGY OF THE COMMON CRAYFISH,

alimentation in a well-fed Crayfish were extended over a
longer time, say a year or two, we should find that the
products given out were no longer equal to the materials
taken in, and the balance would be found in the increase
of the animal's weight. If we inquired how the balance
was distributed, we should find it partly in store, chiefly
in the shape of fat ; while, in part, it had been spent in
increasing the plant and in enlarging the factory. That
is to say, it would have supplied the material for the
animal's growth. And this is one of the most remark-
able respects in which the living factory differs from
those which we construct. It not only enlarges itself,
but, as we have seen, it is capable of executing its own
repairs to a very considerable extent.

CHAPTEE III.

THE PHYSIOLOGY OF THE CRAYFISH— THE MECHANISM BY
WHICH THE LIVING ORGANISM ADJUSTS ITSELF TO
SURROUNDING CONDITIONS AND REPRODUCES ITSELF.

If the hand is brought near a vigorous crayfish, free
to move in a large vessel of water, it will generally give
a vigorous flap with its tail, and dart backwards out of
reach ; but if a piece of meat is gently lowered into
the vessel, the crayfish will sooner or later approach and
devour it.

If we ask why the crayfish behaves in this fashion,
every one has an answer ready. In the first case, it is
said that the animal is aware of danger, and therefore
hastens away ; in the second, that it knows that meat is
good to eat, and therefore walks towards it and makes a
meal. And nothing can seem to be simpler or more
satisfactory than these replies, until we attempt to con-
ceive clearly what they mean ; and, then, the explanation,
however simple it may be admitted to be, hardly retains
its satisfactory character.

For example, when we say that the crayfish is " aware
of danger/' or " knows that meat is good to eat," what

8S THE PHYSIOLOGY OF THE COMMON CRAYFISH.

do we mean by " being aware" and "knowing"?
Certainly it cannot be meant that the crayfish says to
himself, as we do, "This is dangerous," "That is nice ; "
for the crayfish, being devoid of language, has nothing to
say either to himself or any one else. And if the cray-
fish has not language enough to construct a proposition,
it is obviously out of the question that his actions should
be guided by a logical reasoning process, such as that
by which a man would justify similar actions. The
crayfish assuredly does not first frame the syllogism,
"Dangerous things are to be avoided; that hand is
dangerous ; therefore it is to be avoided ; " and then act
upon the conclusion thus logically drawn.

But it may be said that children, before they acquire
the use of language, and we ourselves, long after we are
familiar with conscious reasoning, perform a great variety
of perfectly rational acts unconsciously. A child grasps
at a sweetmeat, or cowers before a threatening gesture,
before it can speak ; and any one of us would start back
from a chasm opening at our feet, or stoop to pick up a
jewel from the ground, "without thinking about it."
And, no doubt, if the crayfish has any mind at all, his
mental operations must more or less resemble those which
the human mind performs without giving them a spoken
or unspoken verbal embodiment.

If wTe analyse these, we shall find that, in many cases,
distinctly felt sensations are followed by a distinct desire
to perform some act, which act is accordingly performed ;

THE CRAYFISH MIND.

89

while, in other cases, the act follows the sensation with-
out one being aware of any other mental process ; and,
in yet others, there is no consciousness even of the sensa-
tion. As I wrote these last words, for example, I had
not the slightest consciousness of any sensation of hold-
ing or guiding the pen, although my fingers were caus-
ing that instrument to perform exceedingly complicated
movements. Moreover, experiments upon animals have
proved that consciousness is wholly unnecessary to the
carrying out of many of those combined movements by
which the body is adjusted to varying external conditions.

Under these circumstances, it is really quite an open
question whether a crayfish has a mind or not ; more-
over, the problem is an absolutely insoluble one, inas-
much as nothing short of being a crayfish would give us
positive assurance that such an animal possesses con-
sciousness ; and, finally, supposing the crayfish has a
mind, that fact does not explain its acts, but only shows
that, in the course of their accomplishment, they are
accompanied by phenomena similar to those of which
we are aware in ourselves, under like circumstances.

So we may as well leave this question of the crayfish's
mind on one side for the present, and turn to a more
profitable investigation, namely, that of the order and
connexion of the physical phenomena which intervene
between something which happens in the neighbourhood
of the animal and that other something which responds
to it, as an act of the crayfish.

90 THE PHYSIOLOGY OF THE COMMON CRAYFISH.

Whatever else it may be, this animal, so far as it is
acted upon by bodies around it and reacts on them, is a
piece of mechanism, the internal works of which give rise
to certain movements when it is affected by particular
external conditions ; and they do this in virtue of their
physical properties and connexions.

Every movement of the body, or of any organ of the
body, is an effect of one and the same cause, namely,
muscular contraction. Whether the crayfish swims or
walks, or moves its antennae, or seizes its prey, the imme-
diate cause of the movements of the parts which bring
about, or constitute, these bodily motions is to be sought
in a change which takes place in the flesh, or muscle,
which is attached to them. The change of place which
constitutes any movement is an effect of a previous
change in the disposition of the molecules of one
or more muscles ; while the direction of that move-
ment depends on the connexions of the parts of the
skeleton with one another, and of the muscles with
them.

The muscle of the crayfish is a dense, white substance ;
and if a small portion of it is subjected to examination it
will be found to be very easily broken up into more
or less parallel bundles of fine fibres. Each of these
fibres is generally found to be ensheathed in a fine trans-
parent membrane, which is called the sarcolemma, within
which is contained the proper substance of the muscle.
When quite fresh and living, this substance is soft and

THE STRUCTURE OF MUSCLE. 91

semi-fluid, but it hardens and becomes solid immediately
after death.

Examined, with high magnifying powers, in this

71.

Fig. 19. — Astacus fluviatilis. — A, a single muscular fibre ; transverse
diameter -^th of an inch; B, a portion of the same more highly
magnified ; C, a smaller portion still more highly magnified ;
D and E, the splitting up of a part of fibre into fibrillse ; F, the
connexion of a nervous with a muscular fibre which has been
treated with acetic acid, a, darker, and b, clearer portions of the
fibrilhe ; n, nucleus of sarcolemma ; nt\ nerve fibre; s, sarcolemma;
t, tendon ; 1 — 5, successive dark bands answering to the darker
portions, a, of each fibril la.

92 THE PHYSIOLOGY OF THE COMMON CRAYFISH.

condition, the muscle-substance appears marked by very
regular transverse bands, which are alternately opaque
and transparent ; and it is characteristic of the group of
animals to which the crayfish belongs that their muscle-
substance has this striped character in all parts of the
body.

A greater or less number of these fibres, united into one
or more bundles, constitutes a muscle ; and, except when
these muscles surround a cavity, they are fixed at each
end to the hard parts of the skeleton. The attachment
is frequently effected by the intermediation of a dense,
fibrous, often chitinous, substance, which constitutes the
tendon (fig. 19, A ; t) of the muscle.

The property of the living muscle, which enables it to
be the cause of motion, is this : Every muscular fibre is
capable of suddenly changing its dimensions, in such a
manner that it shortens and becomes proportionately
thicker. Hence the absolute bulk of the fibre remains
practically unchanged. From this circumstance, muscular
contraction, as the change of form of a muscle is called,
is radically different from the process which commonly
goes by the same name in other things, and which
involves a diminution of bulk.

The contraction of muscle takes place with great force,
and, of course, if the parts to which its ends are fixed
are both free to move, they are brought nearer at the
moment of contraction : if one only is free to move that
is approximated to the fixed part ; and if the muscular

MUSCLE AS THE SOURCE OF MOTION.

03

fibre surrounds a cavity, the cavity is lessened when the
muscle contracts. This is the whole source of motor
power in the crayfish machine. The results produced
by the exertion of that power depend upon the manner

, 1

Fig. 20. — Astaciisjlwiatilis.— The chela of the forceps, with one side
cut away to show, in A, the muscles, in B, the tendons (x 2).
cp, carpopodite ; prp, propodite ; dp, dactjdopodite ; m, adductor
muscle ; m\ abductor muscle ; t , tendon of adductor muscle ; t\
tendon of abductor muscle ; hinge.

in which the parts to which the muscles are attached
are connected with one another.

One example of this has already been given in the
curious mechanism of the gastric mill. Another may be
found in the chela which terminates the forceps. If the

94 THE PHYSIOLOGY OF THE COMMON CRAYFISH.

articulation of the last joint (fig. 20, dp) with the one which
precedes it (prp) is examined, it will be found that the
base of the terminal segment (dp) turns on two hinges (x),
formed by the hard exoskeleton and situated at opposite
points of the diameter of the base, on the penultimate
segment ; and these hinges are so disposed that the
last joint can be moved only in one plane, to or from
the produced angle of the penultimate segment (prp),
which forms the fixed claw of the chela. Between the
hinges, on both the inner and the outer sides of the
articulation, the exoskeleton is soft and flexible, and
allows the terminal segment to play easily through a
certain arc. It is by this arrangement that the direction
and the extent of the motion of the free claw of the chela
are determined. The source of the motion lies in the
muscles which occupy the interior of the enlarged penul-
timate segment of the limb. Two muscles, one of very
great size (m), the other smaller (mf), are fastened by
one end to the exoskeleton of this segment. The fibres of
the larger muscle converge to be fixed into the two sides
of a long flat process of the chitinous cuticula, on the
inner side of the base of the terminal segment, which
serves as a tendon (t) ; while those of the smaller muscle
are similarly attached to a like process which proceeds
from the outer side of the base of the terminal seg-
ment (t'). It is obvious that, when the latter muscle
shortens it must move the apex of the terminal seg-
ment (dp) away from the end of the fixed claw ; while,

Motion directed by joints.

95

when the former contracts, the end of the terminal
segment is brought towards that of the fixed claw,

A living crayfish is able to perform very varied move-
ments with its pincers. When it swims backwards, these
limbs are stretched straight out, parallel with one another,
in front of the head; when it walks, they are usually
carried like arms bent at the elbow, the "forearm"
partly resting on the ground ; on being irritated, the
crayfish sweeps the pincers round in any direction to
grasp the offending body ; when prey is seized, it is at
once conveyed, with a circular motion, towards the region
of the mouth. Nevertheless, these very varied actions
are all brought about by a combination of simple flexions
and extensions, each of which is effected in the exact
order, and to the exact extent, needful to bring the chela
into the position required.

The skeleton of the stem of the limb which bears the
chela is, in fact, divided into four moveable segments ;
and each of these is articulated with the segments on
each side of it by a hinge of just the same character as
that which connects the moveable claw of the chela with
the penultimate segment, while the basal segment is
similarly articulated with the thorax.

If the axes of all these articulations * wrere parallel, it is
obvious that, though the limb might be moved as a whole
through a considerable arc, and might be bent in various

* By axis of the articulation is meant a line drawn through the pair
of hinges which constitute it.

96 THE PHYSIOLOGY OF THE COMMON CRAYFISH,

degrees, yet all its movements would be limited to one
plane. But, in fact, the axes of the successive articula-
tions are nearly at right angles to one another ; so that,
if the segments are successively either extended or
flexed, the chela describes a very complicated curve ;
and by varying the extent of flexion or extension of
each segment, this curve is susceptible of endless varia-
tion. It would probably puzzle a good mathematician
to say exactly what position should be given to each
segment, in order to bring the chela from any given
position into any other; but if a lively crayfish is
incautiously seized, the experimenter will find, to his
cost, that the animal solves the problem both rapidly
and accurately.

The mechanism by which the retrograde swimming of
the crayfish is effected, is no less easily analysed. The
apparatus of motion is, as we have seen, the abdomen,
with its terminal five-pointed flapper. The rings of the
abdomen are articulated together by joints (fig. 21, x)
situated a little below the middle of the height of the
rings, at opposite ends of transverse lines, at right
angles to the long axis of the abdomen.

Each ring consists of a dorsal, arched portion, called the
tergum (fig. 21 ; fig. 36, p. 142, t. XIX), and a nearly flat
ventral portion, which is the sternum (fig. 36, st. XIX).
Where these two join, a broad plate is sent down on
each side, which overlaps the bases of the abdominal
appendages, and is known as the pleuron (fig. 86, pi. XIX).

THE JOINTS OF THE ABDOMEN.

97

The sterna are all very narrow, and are connected
together by wide spaces of flexible exoskeleton.

When the abdomen is made straight, it will be found
that these inter sternal membranes are stretched as far
as they will yield. On the other hand, when the abdomen

Fig. 21. — Astacus fluviatills. — Two of the abdominal somites, in vertical
section, seen from the inner side, to show x , x , the hinges by
which they are articulated with one another ( x 3). The anterior
of the two somites is that to the right of the figure.

is bent up as far as it will go, the sterna come close
together, and the intersternal membranes are folded.

The terga are very broad ; so broad, in fact, that each
overlaps its successor, when the abdomen is straightened
or extended, for nearly half its length in the middle
line ; and the overlapped surface is smooth, convex, and

h

98 THE PHYSIOLOGY OF THE COMMON CRAYFISH.

marked off by a transverse groove from the rest of the
tergum as an articular facet The front edge of the
articular facet is continued into a sheet of flexible cuti-
cula, which turns back, and passes as a loose fold to the
hinder edge of the overlapping tergum (fig. 21). This
tergal inter articular membrane allows the terga to move as
far as they can go in flexion ; whilst, in extreme exten-
sion, they are but slightly stretched. But, even if the in-
tersternal membranes presented no obstacle to excessive
extension of the abdomen, the posterior free edge of each
tergum fits into the groove behind the facet in the next
in such a manner, that the abdomen cannot be made more
than very slightly concave upwards without breaking.

Thus the limits of motion of the abdomen, in the
vertical direction, are from the position in which it is
straight, or has even a very slight upward concavity, to
that in which it is completely bent upon itself, the telson
being brought under the bases of the hinder thoracic
limbs. No lateral movement between the somites of the
abdomen is possible in any of its positions. For, when
it is straight, lateral movement is hindered not only by
the extensive overlapping of the terga, but also by the
manner in which the hinder edges of the pleura of each
of the four middle somites overlap the front edges of
their successors. The pleura of the second somite are
much larger than any of the others, and their front edges
overlap the small pleura of the first abdominal somite ;
and when the abdomen is much flexed, these pleura even

THE EXTENSORS AND FLEXORS OF THE ABDOMEN. 99

ride over the posterior edges of the branchiostegites. In
the position of extension, the overlap of the terga is great,
while that of the pleura of the middle somites is small.
As the abdomen passes from extension to flexion, the
overlap of the terga of course diminishes ; but any de-
crease of resistance to lateral strains which may thus
arise, is compensated by the increasing overlap of the
pleura, which reaches its maximum when the abdomen
is completely flexed.

It is obvious that longitudinal muscular fibres fixed
into the exoskeleton, above the axes of the joints, must
bring the centres of the terga of the somites closer
together, when they contract ; while muscular fibres
attached below the axes of the joints must approximate
the sterna. Hence, the former will give rise to extension,
and the latter to flexion, of the abdomen as a whole.

Now there are two pairs of very considerable muscles
disposed in this manner. The dorsal pair, or the exten-
sors of the abdomen (fig. 22, e.m), are attached in front
to the side walls of the thorax, thence pass backwards
into the abdomen, and divide into bundles, which are
fixed to the inner surfaces of the terga of all the somites.
The other pair, or the flexors of the abdomen (f.m) consti-
tute a very much larger mass of muscle, the fibres of
which are curiously twisted, like the strands of a rope.
The front end of this double rope is fixed to a series of
processes of the exoskeleton of the thorax, called apode-
mata, some of which roof over the sternal blood-sinuses

u 2

100 THE PHYSIOLOGY OF THE COMMON CRAYFISH.

and the thoracic part of the nervous system ; while, in the
abdomen, its strands are attached to the sternal exoske-
leton of all the somites and extend, on each side of the
rectum, to the telson.

When the exoskeleton is cleaned by maceration, the

Fig. 22. — Astacus fluviatilis. — A longitudinal section of the body to
show the principal muscles and their relations to the exoskeleton
(nat. size), a, the vent ; add.m, adductor muscle of mandible ;
e.m, extensor, and f.?n, flexor muscle of abdomen ; ces, oesophagus ;
2icp, procephalic process ; t,t', the two segments of the telson ;
xv — xx, the abdominal somites ; 1 — 20, the appendages ; x , x ,
hinges between the successive abdominal somites.

abdomen has a slight curve, dependent upon the form and
the degree of elasticity possessed by its different parts ;
and, in a living crayfish at rest, it will be observed that
the curvature of the abdomen is still more marked.
Hence it is ready either for extension or for flexion.

A sudden contraction of the flexor muscles instantly
increases the ventral curvature of the abdomen, and

THE INFLUENCE OF NERVE ON MUSCLE. 101

throws the tail fin, the two side lobes of which are
spread out, forwards ; while the body is propelled back-
wards by the reaction of the water against the stroke.
Then the flexor muscles being relaxed, the extensor
muscles come into play ; the abdomen is straightened, but
less violently and with a far weaker stroke on the water,
in consequence of the less strength of the extensors and of
the folding up of the lateral plates of the fin, until it
comes into the position requisite to give full force to a
new downward and forward stroke. The tendency of the
extension of the abdomen is to drive the body forward ;
but from the comparative weakness and the obliquity of
its stroke, its practical effect is little more than to check
the backward motion conferred upon the body by the
flexion of the abdomen.

Thus, every action of the crayfish, which involves
motion, means the contraction of one or more muscles.
But what sets muscle contracting ? A muscle freshly
removed from the body may be made to contract in
various ways, as by mechanical or chemical irritation, or
by an electrical shock ; but, under natural conditions,
there is only one cause of muscular contraction, and that
is the activity of a nerve. Every muscle is supplied with
one or more nerves. These are delicate threads which,
on microscopic examination, prove to be bundles of fine
tubular filaments, filled with an apparently structureless
substance of gelatinous consistency, the nerve fibres

102 THE PHYSIOLOGY OF THE COMMON CRAYFISH.

(fig. 23). The nerve bundle which passes to a muscle
breaks up into smaller bundles and, finally, into separate
fibres, each of which ultimately terminates by becoming
continuous with the substance of a muscular fibre fig. 19,
F.) Now the peculiarity of a muscle nerve, or motor
nerve, as it is called, is that irritation of the nerve fibre at
any part of its length, however distant from the muscle,

FlG. 22>.— AstamisfluviaUUs. — Three nerve fibres, with the connective
tissue in which they are imbedded. (Magnified about 250 dia-
meters.) n, nuclei.

brings about muscular contraction, just as if the muscle
itself were irritated. A change is produced in the mole-
cular condition of the nerve at the point of irritation ;
and this change is propagated along the nerve, until it
reaches the muscle, in which it gives rise to that change
in the arrangement of its molecules, the most obvious
effect of which is the sudden alteration of form which we
call muscular contraction.

If we follow the course of the motor nerves in a

NERVE FIBRES AND NERVE CELLS. 103

direction away from the muscles to which they are dis-
tributed, they will be found, sooner or later, to terminate
in ganglia (fig. 24 A. gl.c ; fig. 25, gn. 1 — 13.) A gan-
glion is a body which is in great measure composed of

Fig. 24. — Astacus flnviatilis. — A, one of the (double) abdominal gan-
glia, with the nerves connected with it ( x 25) ; B, a nerve cell or-
gan glionic corpuscle ( x 250). a. sheath of the nerves ; c, sheath
of the ganglion ; co, cor, commissural cords connecting the ganglia
with those in front, and those behind them. gl.c. points to the
ganglionic corpuscles of the ganglia ; nerve fibres.

nerve fibres ; but, interspersed among these, or disposed
around them, there are peculiar structures, which are
termed ganglionic corpuscles, or nerve cells (fig. 24, B.)
These are nucleated cells, not unlike the epithelial cells
which have been already mentioned, but which are larger

Fig. 25.— Astacus fluviatilis. — The central nervous system seen from
above (nat. size), a, vent ; an, antennary nerve ; a'n, antennulary
nerve ; c, circumoesophageal commissures ; gn. 1, supraoesophageal
ganglion ; gn. 2, infracesophageal ganglion ; gn. 6, fifth thoi ac'c
ganglion ; gn. 7, last thoracic ganglion ; gn. 13, last abdominal gang-
lion ; ces, oesophagus in cross section ; on, optic nerve; sa, sternal
artery in cross section ; sgn, stomatogastric nerve.

THE CHAIN OF GANGLIA.

105

and often give off one or more processes. These pro-
cesses, under favourable circumstances, can be traced
into continuity with nerve fibres.

The chief ganglia of the crayfish are disposed in a
longitudinal series in the middle line of the ventral
aspect of the body close to the integument (fig. 25).
In the abdomen, for example, six ganglionic masses are
readily observed, one lying over the sternum of each
somite, connected by longitudinal bands of nerve fibres,
and giving off branches to the muscles. On careful ex-
amination, the longitudinal connecting bands, or com-
missures (fig. 24, co), are seen to be double, and each
mass appears slightly bilobed. In the thorax, there are
six, larger, double ganglionic masses, likewise connected
by double commissures ; and the most anterior of these,
which is the largest (fig. 25, gn. 2), is marked at the
sides by notches, as if it were made up of several pairs
of ganglia, run together into one continuous whole.
In front of this, two commissures (c) pass forwards,
separating widely, to give room for the gullet (oes), which
passes between them ; while in front of the gullet, just
behind the eyes, they unite with a transversely elongated
mass of ganglionic substance (gn. 1), termed the brain, or
cerebral ganglion.

All the motor nerves, as has been said, are traceable,
directly or indirectly, to one or other of these thirteen
sets of ganglia ; but other nerves are given off from the
ganglia, which cannot be followed into any muscle. In

10G THE PHYSIOLOGY OF THE COMMON CRAYFISH.

fact, these nerves go either to the integument or to the
organs of sense, and they are termed sensory nerves.

When a muscle is connected by its motor nerve with
a ganglion, irritation of that ganglion will bring about
the contraction of the muscle, as well as if the motor
nerve itself were irritated. Not only so ; but if a sensory
nerve, which is in connexion with the ganglion, is irritated,
the same effect is produced ; moreover, the sensory nerve
itself need not be excited, but the same result will
take place, if the organ to which it is distributed is
stimulated. Thus the nervous system is fundamentally
an apparatus by which two separate, and it may be dis-
tant, parts of the body, are brought into relation with
one another ; and this relation is of such a nature, that
a change of state arising in the one part is followed by
the propagation of changes along the sensory nerve to the
ganglion, and from the ganglion to the other part ; where,
if that part happens to be muscle, it produces contraction.
If one end of a rod of wood, twenty feet long, is applied
to a sounding-board, the sound of a tuning-fork held
against the opposite extremity will be very plainly heard.
Nothing can be seen to happen in the wood, and yet
its molecules are certainly set vibrating, at the same
rate as the tuning-fork vibrates ; and when, after
travelling rapidly along the wood, these vibrations
affect the sounding-board, they give rise to vibrations
of the molecules of the air, which reaching the ear, are
converted into an audible note. So in the nerve tract :

THE CO-ORDINATION OF MOVEMENTS. lOY

no apparent change is effected in it by the irritation at
one end ; but the rate at which the molecular change
produced travels can be measured ; and, when it reaches
the muscle, its effect becomes visible in the change of
form of the muscle. The molecular change would take
place just as much if there were no muscle connected
with the nerve, but it would be no more apparent to
ordinary observation than the sound of the tuning-fork
is audible in the absence of the sounding-board.

If the nervous system were a mere bundle of nerve
fibres extending between sensory organs and muscles,
every muscular contraction would require the stimulation
of that special point of the surface on which the appro-
priate sensory nerve ended. The contraction of several
muscles at the same time, that is, the combination of
movements towards one end, would be possible only if the
appropriate nerves were severally stimulated in the proper
order, and every movement would be the direct result of ex-
ternal changes. The organism would be like a piano, which
may be made to give out the most complicated harmonies,
but is dependent for their production on the depression
of a separate key for every note that is sounded. But it
is obvious that the crayfish needs no such separate
impulses for the performance of highly complicated
actions. The simple impression made on the organs of
sensation in the two examples with which we started,
gives rise to a train of complicated and accurately co-
ordinated muscular contractions. To carry the analogy

10S THE PHYSIOLOGY OF THE COMMON CRAYFISH.

of the musical instrument further, striking a single key
gives rise, not to a single note, but to a more or less
elaborate tune; as if the hammer struck not a single
string, but pressed down the stop of a musical box.

It is in the ganglia that we must look for the analogue
of the musical box. A single impulse conveyed by a
sensory nerve to a ganglion, may give rise to a single
muscular contraction, but more commonly it originates a
series of such, combined to a definite end.

The effect which results from the propagation of an
impulse along a nerve fibre to a ganglionic centre, whence
it is, as it wTere, reflected along another nerve fibre to a
muscle, is what is termed a reflex action. As it is by no
means necessary that sensation should be a concomitant
of the first impulse, it is better to term the nerve fibre
which carries it afferent rather than sensory ; and, as
other phenomena besides those of molar motion may be
the ultimate result of the reflex action, it is better to
term the nerve fibre which transmits the reflected im-
pulse efferent rather than motor.

If the nervous commissures between the last thoracic
and the first abdominal ganglia are cut, or if the thoracic
ganglia are destro}red, the crayfish is no longer able to
control the movements of the abdomen. If the forepart
of the body is irritated, for example, the animal makes
no effort to escape by swimming backwards. Never-
theless, the abdomen is not paralysed, for, if it be irri-
tated, it will flap vigorously. This is a case of pure

INVOLUNTARY RHYTHMICAL MOVEMENTS. 109

reflex action. The stimulus is conveyed to the abdo-
minal ganglia through afferent nerves, and is reflected
from them, by efferent nerves, to the abdominal muscles.

But this is not all. Under these circumstances it will
be seen that the abdominal limbs all swing backwards
and forwards, simultaneously, with an even stroke ; while
the vent opens and shuts wTith a regular rhythm. Of
course, these movements imply correspondingly regular
alternate contractions and relaxations of certain sets of
muscles; and these, again, imply regularly recurring
efferent impulses from the abdominal ganglia. The fact
that these impulses proceed from the abdominal ganglia,
may be shown in two ways : first, by destroying these
ganglia in one somite after another, when the move-
ments in each somite at once permanently cease ; and,
secondly, by irritating the surface of the abdomen, when
the movements are temporarily inhibited by the stimula-
tion of the afferent nerves. Whether these movements are
properly reflex, that is, arise from incessant new afferent
impulses of unknown origin, or whether they depend on the
periodical accumulation and discharge of nervous energy in
the ganglia themselves, or upon periodical exhaustion and
restoration of the irritability of the muscles, is unknown.
It is sufficient for the present purpose to use the facts as
evidence of the peculiar co-ordinative function of ganglia.

The crayfish, as we have seen, avoids light; and the
slightest touch of one of its antennae gives rise to active
motions of the whole body. In fact, the animal's posi-

110 THE PHYSIOLOGY OF THE COMMON CRAYFISH.

tion and movements are largely determined by the in-
fluences received through the feelers and the eyes. These
receive their nerves from the cerebral ganglia ; and, as
might be expected, when these ganglia are extirpated,
the crayfish exhibits no tendency to get away from the
light, and the feelers may not only be touched, but
sharply pinched, without effect. Clearly, therefore, the
cerebral ganglia serve as a ganglionic centre, by which
the afferent impulses derived from the feelers and the
eyes are transmuted into efferent impulses. Another
very curious result follows upon the extirpation of the
cerebral ganglia. If an uninjured crayfish is placed upon
its back, it makes unceasing and well-directed efforts to
turn over ; and if everything else fails, it will give a
powerful flap with the abdomen, and trust to the chapter
of accidents to turn over as it darts back. But the
brainless crayfish behaves in a very different way. Its
limbs are in incessant motion, but th^y are " all abroad ; "
and if it turns over on one side, it does not seem able
to steady itself, but rolls on to its back again.

If anything is put between the chelae of an uninjured
crayfish, while on its back, it either rejects the object at
once, or tries to make use of it for leverage to turn over.
In the brainless crayfish a similar operation gives rise to
a very curious spectacle.* If the object, whatever it be

* My attention was first drawn to these phenomena by my friend
Dr. M. Foster, F.R.S., to whom I had suggested the desirableness of
an experimental study of the nerve physiology of the crayfish,

THE ACTIONS OF BRAINLESS CRAYFISHES. Ill

— a bit of metal, or wood, or paper, or one of the ani-
mal's own antennae — is placed between the chelae of the
forceps, it is at once seized by them, and carried back-
wards ; the chelate ambulatory limbs are at the same
time advanced, the object seized is transferred to them,
and they at once tuck it between the external maxilli-
pedes, which, with the other jaws, begin vigorously to
masticate it. Sometimes the morsel is swallowed;
sometimes it passes out between the anterior jaws, as ii
deglutition were difficult. It is very singular to observe
that, if the morsel which is being conveyed to the mouth
by one of the forceps is pulled back, the forceps and the
chelate ambulatory limbs of the other side are at once
brought forward to secure it. The movements of the
limbs are, in short, adjusted to meet the increased
resistance.

All these phenomena cease at once, if the thoracic
ganglia are destroyed. It is in these, therefore, that the
simple stimulus set up by the contact of a body with, for
example, one of the forceps, is translated into all the sur-
prisingly complex and accurately co-ordinated movements,
which have been described. Thus the nervous system
of the crayfish may be regarded as a system of co-ordi-
nating mechanisms, each of which produces a certain
action, or set of actions, on the receipt of an appropriate
stimulus.

When the crayfish comes into the world, it possesses
in its neuro-muscular apparatus certain innate poten-

112 THE PHYSIOLOGY OF THE COMMON CRAYFISH.

tialities of action, and will exhibit the corresponding
acts, under the influence of the appropriate stimuli.
A large proportion of these stimuli come from without
through the organs of the senses. The greater or less
readiness of each sense organ to receive impulses, of
the nerves to transmit them, and of the ganglia to
give rise to combined impulses, is dependent at any
moment upon the physical condition of these parts ; and
this, again, is largely modified by the amount and the
condition of the blood supplied. On the other hand, a
certain number of these stimuli are doubtless originated
by changes within the various organs which compose the
body, including the nerve centres themselves.

When an action arises from conditions developed in
the interior of an animal's body, inasmuch as we cannot
perceive the antecedent phenomena, we call such an
action " spontaneous ; " or, when in ourselves we are
aware that it is accompanied by the idea of the action,
and the desire to perform it, we term the act " volun-
tary." But, by the use of this language, no rational
person intends to express the belief that such acts are
uncaused or cause themselves. " Self-causation " is a
contradiction in terms ; and the notion that any pheno-
menon comes into existence without a cause, is equivalent
to a belief in chance, which one may hope is, by this
time, finally exploded.

In the crayfish, at any rate, there is not the slightest
reason to doubt that every action has its definite physical

SENSORY ORGANS.

113

cause, and that what it does at any moment would be
as clearly intelligible, if we only knew all the internal
and external conditions of the case, as the striking of a
clock is to any one who understands clockwork.

The adjustment of the body to varying external con-
ditions, which is one of the chief results of the working
of the nervous mechanism, would be far less important
from a physiological point of view than it is, if only
those external bodies which come into direct contact
with the organism * could affect it ; though very delicate
influences of this kind take effect on the nervous apparatus
through the integument.

It is probable that the setce, or hairs, which are so
generally scattered over the body and the appendages,
are delicate tactile organs. They are hollow processes of
the chitinous cuticle, and their cavities are continuous
with narrow canals, which traverse the whole thick-
ness of the cuticle, and are filled by a prolongation of
the subjacent proper integument. As this is supplied
with nerves, it is likely that fine nerve fibres reach the
bases of the hairs, and are affected by anything which
stirs these delicately poised levers.

* It may be said that, strictly speaking", only those external bodies
which are in direct contact with the organism do affect it — as the
vibrating ether, in the case of luminous bodies ; the vibrating air or
Water, in the case of sonorous bodies ; odorous particles, in the case of
odorous bodies : but I have preferred the ordinary phraseology to a
pedantically accurate periphrasis.

I

114 THE FHYSIOLOGY OF THE COMMON CRAYFISH.

There is much reason to believe that odorous bodies
affect crayfish ; but it is very difficult to obtain experi-

a a a

Fig. 26. — Astacus fluviatilis. — A, the right antennule seen from the
inner side ( x 5) ; B, a portion of the exopodite enlarged ; C, olfactory
appendage of the exopodite ; a, front view ; b, side view ( x 300) ;
a, olfactory appendages ; au, auditory sac, supposed to be seen through
the wall of the basal joint of the antennule ; b, setas ; en, endopo-
dite ; ex, exopodite ; sp, spine of the basal joint.

mental evidence of the fact. However, there is a good
deal of analogical ground for the supposition that some
peculiar structures, which are evidently of a sensory

THE OLFACTORY ORGANS.

115

nature, developed on the under side of the outer branch
of the antennule, play the part of an olfactory apparatus.

Both the outer (fig. 26 A. ex) and the inner (en)
branches of the antennule are made up of a number of
delicate ring-like segments, which bear fine setse (b) of
the ordinary character.

The inner branch, which is the shorter of the two, pos-
sesses only these setse ; but the under surface of each of
the joints of the outer branch, from about the seventh or
eighth to the last but one, is provided with two bundles
of very curious appendages (fig. 27, A, B, C, a), one in
fiont and one behind. These are rather more than
1 -200th of an inch long, very delicate, and shaped like a
spatula, with a rounded handle and a flattened somewhat
curved blade, the end of which is sometimes truncated,
sometimes has the form of a prominent papilla. There
is a sort of joint between the handle and the blade, such
as is found between the basal and the terminal parts of
the ordinary setae, with which, in fact, these processes
entirely correspond in their essential structure. A soft
granular tissue fills the interior of each of these pro-
blematical structures, to which Leydig, their discoverer,
ascribes an olfactory function.

It is probable that the crayfish possesses something
analogous to taste, and a very likely seat for the organ
of this function is in the upper lip and the metastoma ;
but if the organ exists it possesses no structural pecu-
liarities by which it can be identified.

I 2

116 THE PHYSIOLOGY OF THE COMMON CRAYFISH.

There is no doubt, however, as to the special recipients
of sonorous and luminous vibrations; and these are of
particular importance, as they enable the nervous ma-
chinery to be affected by bodies indefinitely remote
from it, and to change the place of the organism in
relation to such bodies.

Sonorous vibrations are enabled to act as the stimulants
of a special nerve (fig. 25, an) connected with the brain,
by means of the very curious auditory sacs (fig. 26, A, au)
which are lodged in the basal joints of the antennules.

Each of these joints is trihedral, the outer face being con-
vex ; the inner, applied to its fellow, flat ; and the upper,
on which the eyestalk rests, concave. On this upper face
there is a narrow elongated oval aperture, the outer lip of
which is beset with a flat brush of long close-set setae,
which lie horizontally over the aperture, and effectually
close it. The aperture leads into a small sac (au) with
delicate walls formed by a chitinous continuation of the
general cuticula. The inferior and posterior wall of the
sac is raised up along a curved line into a ridge which
projects into its interior (fig. 27, A, r). Each side of this
ridge is beset with a series of delicate setae (as), the
longest of which measures about Tl¥th of an inch ; they
thus form a longitudinal band bent upon itself. These
auditory setce project into the fluid contents of the sac,
and their apices are for the most part imbedded in a
gelatinous mass, which contains irregular particles of sand

THE EAR OF THE CRAYFISH.

117

and sometimes of other foreign matter, A nerve (n n',) is
distributed to the sac, and its fibres enter the bases of
the hairs, and may be traced to their apices, where they
end in peculiar elongated rod-like bodies (fig. 27, C).
Here is an auditory organ of the simplest description.

a.

Fig. 27. — Astaeus fluviatilis. A, the auditory sac detached and seen
from the outside ( x 15) ; B, auditory hair ( x 100) ; C, the distal ex-
tremity of the same more highly magnified, a, aperture of sac ; as,
auditory seta? ; by its inner or posterior extremity ; n n\ nerves ;
r, ridge.

It retains, in fact, throughout life, the condition of a
simple sac or involution of the integument, such as is
that of the vertebrate ear in its earliest stage.

118 THE PHYSIOLOGY OF THE COMMON CRAYFISH.

The sonorous vibrations transmitted through the
water in which the crayfish lives to the fluid and solid
contents of the auditory sac are taken up by the delicate
hairs of the ridge, and give rise to molecular changes
which traverse the auditory nerves and reach the cerebral
ganglia.

The vibrations of the luminiferous ether are brought
to bear upon the free ends of two large bundles of nerve
fibres, termed the optic nerves (fig. 25, on), which proceed
directly from the brain, by means of a highly complex eye.
This is an apparatus, which, in part, sorts out the rays of
light into as many very small pencils as there are separate
endings of the fibres of the optic nerve, and, in part,
serves as the medium by which the luminous vibrations
are converted into molecular nerve changes.

The free extremity of the eyestalk presents a convex,
soft, and transparent surface, limited by an oval contour.
The cuticle in this region, which is termed the cornea,
(fig. 28, a), is, in fact, somewhat thinner and less dis-
tinctly laminated than in the rest of the eyestalk, and it
contains no calcareous matter. But it is directly con-
tinuous with the rest of the exoskeleton of the eyestalk,
to which it stands in somewhat the same relation as the
soft integument of an articulation does to the adjacent
hard parts.

The cornea is divided into a great number of minute,
usually square facets, by faint lines, which cross it from side

THE EYE OF THE CRAYFISH.

119

to side nearly at right angles with one another. A longi-
tudinal section shows that both the horizontal and the
vertical contours of the cornea are very nearly semicir-
cular, and that the lines which mark off the facets merely
arise from a slight modification of its substance between
the facets. The outer .contour of each facet forms part

Fig. 28. — Astacus flvviatilis. — A, a vertical section of the eye-stalk
( x 6) ; B, a small portion of the same, showing the visual ap-
paratus more highly magnified ; a, cornea ; b, outer dark zone ; r,
outer white zone ; d, middle dark zone ; e, inner white zone ;
/, inner dark zone ; cr, crystalline cones ; g, optic ganglion ; oj>,
optic nerve ; 8p9 striated spindles.

of the general curvature of the outer face of the cornea ;
the inner contour sometimes exhibits a slight deviation

120 THE PHYSIOLOGY OF THE COMMON CRAYFISH.

from the general curvature of the inner face, but usually
nearly coincides with it.

When a longitudinal or a transverse section is taken
through the whole eyestalk, the optic nerve (fig. 28,
A, op) is seen to traverse its centre. At first narrow
and cylindrical, it expands towards its extremity into
a sort of bulb (B, g), the outer surface of which is curved
in correspondence with the inner surface of the cornea.
The terminal half of the bulb contains a great quantity
of dark colouring matter or pigment, and, in section,
appears as what may be termed the inner dark zone (/).
Outside this, and in connection with it, follows a white
line, the inner white zone (e)9 then comes a middle dark
zone (d) ; outside this an outer pale band, which may
be called the outer white zone (c), and between this and
the cornea (a) is another broad band of dark pigment, the
outer dark zone (6).

When viewed under a low power, by reflected light, this
outer dark zone is seen to be traversed by nearly parallel
straight lines, each of which starts from the boundary
between two facets, and can be followed inwards through
the outer white zone to the middle dark zone. Thus the
whole substance of the eye between the outer surface of
the bulb of the optic nerve and the inner surface of the
cornea is marked out into as many segments as the
cornea has facets ; and each segment has the form
of a wedge or slender pyramid, the base of which is
four-sided, and is applied against the inner surface of

THE VISUAL PYRAMIDS.

121

one of the facets of the cornea, while its summit lies in
the middle dark zone. Each of these visual pyramids
consists of an axial structure, the visual rod, invested by
a sheath. The latter extends inwards from the margin
of each facet of the cornea, and contains pigment in
two regions of its length, the intermediate space being
devoid of pigment. As the position of the pigmented
regions in relation to the length of the pyramid is always
the same, the pigmented regions necessarily take the form
of two consecutive zones when the pyramids are in their
natural position.

The visual rod consists of two parts, an external
crystalline cone (fig. 28, B, cr), and an internal striated
spindle (sp). The crystalline cone consists of a trans-
parent glassy-looking substance, which may be made to
split up longitudinally into four segments. Its inner end
narrows into a filament which traverses the outer white
zone, and, in the middle dark zone, thickens into a four-
sided spindle-shaped transparent body, which appears
transversely striated. The inner end of this striated
spindle narrows again, and becomes continuous with
nerve fibres which proceed from the surface of the optic
bulb.

The exact mode of connection of the nerve-fibres with
the visual rods is not certainly made out, but it is pro-
bable that there is direct continuity of substance, and that
each rod is really the termination of a nerve fibre.

Eyes having essentially the same structure as that of

122 THE PHYSIOLOGY OF THE COMMON CRAYFISH,

the crayfish are very widely met with among Crustacea
and Insecta, and are commonly known as compound eyes.
In many of these animals, in fact, when the cornea is re-
moved, each facet is found to act as a separate lens ; and
when proper arrangements are made, as many distinct
pictures of external objects are found behind it as there
are facets. Hence the notion suggested itself that each
visual pyramid is a separate eye, similar in principle of
construction to the human eye, and forming a picture of
so much of the external world as comes within the range
of its lens, upon a retina supposed to be spread out on
the surface of the crystalline cone, as the human retina is
spread over the surface of the vitreous humour.

But, in the first place, there is no evidence, nor any
probability, that there is anything corresponding to a
retina on the outer face of the crystalline cone ; and
secondly, if there were, it is incredible that, with such an
arrangement of the refractive media as exists in the
cornea and crystalline cones, rays proceeding from points
in the external world should be brought to a focus in cor-
respondingly related points of the surface of the supposed
retina. But without this no picture could be formed, and
no distinct vision could take place. It is very probable,
therefore, that the visual pyramids do not play the part
of the simple eyes of the Vertebrata, and the only alterna-
tive appears to be the adoption of a modification of the
theory of mosaic vision, propounded many years by
Johannes Mullen

THE THEORY OF MOSAIC VISION. 123

Each visual pyramid, isolated from its fellows by its coat
of pigment, may be supposed, in fact, to play the part of a
very narrow straight tube, with blackened walls, one end
of which is turned towards the external world, while the
other incloses the extremity of one of the nerve fibres. The
only light which can reach the latter, under these circum-
stances, is such as proceeds from points which lie in the

Fig. 29. — Diagram showing" the course of rays of light from three
points x, y, zf through the nine visual rods (supposed to be empty
tubes) A — I of a compound eye ; a— i, the nerve fibres connected
with the visual rods.

direction of a straight line represented by the produced
axis of the tubes.

Suppose A — I to be nine such tubes, a — i the corre-
sponding nerve fibres, and x y z three points from which
light proceeds. Then it will be obvious that the only light

12i THE PHYSIOLOGY OF THE COMMON CRAYFISH.

from x which will excite sensation, will be the ray which
traverses b and reaches the nerve-fibre b, while that from
y will affect only e, and that from x only h. The result,
translated into sensation, will be three points of light on a
dark ground, each of which answers to one of the luminous
points, and indicates its direction in reference to the eye
and its angular distance from the other two.*

The only modification needed in the original form of
the theory of mosaic vision, is the supposition that part,
or the whole, of the visual rod, is not merely a passive
transmitter of light to the nerve-fibre, but is, itself, in
someway concerned in transmuting the mode of motion,
light, into that other mode of motion which we call
nervous energy. The visual rod is, in fact, to be re-
garded as the physiological end of the nerve, and the
instrument by which the conversion of the one form of
motion into the other takes place ; just as the auditory
hairs are instruments by which the sonorous waves are
converted into molecular movements of the substance of
the auditory nerves.

It is wonderfully interesting to observe that, when the
so-called compound eye is interpreted in this manner,

* Since the visual rods are strongly refracting solids, and not empty
tubes, trie diagram given in fig. 29 does not represent the true course of
the rays, indicated by dotted lines, which fall obliquely on any cornea
of a crayfish's eye. Such rays will be more or less bent towards the
axis of the visual rod of that cornea ; but whether they reach its apex
and so affect the nerve or not will depend on the curvature of the cornea ;
its refractive index and that of the crystalline cone ; and the relation
hetween the length and the thickness of the latter.

DO CRAYFISHES HEAR AND SEE ?

125

the apparent wide difference between it and the verte-
brate eye gives place to a fundamental resemblance. The
rods and cones of the retina of the vertebrate eye are
extraordinarily similar in their form and their relations
to the fibres of the optic nerve, to the visual rods of the
arthropod eye. And the morphological discrepancy*
which is at first so striking, and which arises from the
fact that the free ends of the visual rods are turned
towards the light, while those of the rods and cones
of the vertebrate eye are turned from it, becomes a confir-
mation of the parallel between the two when the develop-
ment of the vertebrate eye is taken into account. For it
is demonstrable that the deep surface of the retina in
which the rods and cones lie, is really a part of the outer
surface of the body turned inwards, in the course of the
singular developmental changes which give rise to the
brain and the eye of vertebrate animals.

Thus the crayfish has, at any rate, two of the higher
sense organs, the ear and the eye, which we possess our-
selves; and it may seem a superfluous, not to say a
frivolous, question, if any one should ask whether it can
hear and see.

But, in truth, the inquiry, if properly limited, is a very
pertinent one. That the crayfish is led by the use of its
eyes and ears to approach some objects and avoid others,
is beyond all doubt ; and, in this sense, most indubit-
ably it can both hear and see. But it the question

126 THE PHYSIOLOGY OF THE COMMON CRAYFISH.

means, do luminous vibrations give it the sensations of
light and darkness, of colour and form and distance, which
they give to us ? and do sonorous vibrations produce the
feelings of noise and tone, of melody and of harmony, as
in us ? — it is by no means to be answered hastily, perhaps
cannot be answered at all, except in a tentative, probable
way.

The phenomena to which we give the names of sound
and colour are not physical things, but are states of con-
sciousness, dependent, there is every reason to believe,
on the functional activity of certain parts of our brains.
Melody and harmony are names for states of conscious-
ness which arise when at least two sensations of sound
have been produced. All these are manufactured arti-
cles, products of the human brain ; and it wTould be
exceedingly hazardous to affirm that organs capable of
giving rise to the same products exist in the vastly
simpler nervous system of the crustacean. It would be
the height of absurdity to expect from a meat-jack the
sort of work which is performed by a Jacquardloom ; and
it appears to me to be little less preposterous to look for
the production of anything analogous to the more subtle
phenomena of the human mind in something so minute
and rude in comparison to the human brain, as the
insignificant cerebral ganglia of the crayfish.

At the most, one may be justified in supposing the
existence of something approaching dull feeling in our-
selves ; and, to return to the problem stated in the begin-

THE MORTALITY OF CRAYFISHES.

127

ning of this chapter, so far as such obscure consciousness1
accompanies the molecular changes of its nervous sub-
stance, it will be right to speak of the mind of a crayfish.
But it will be obvious that it is merely putting the cart
before the horse, to speak of such a mind as a factor
in the work done by the organism, when it is merely a
dim symbol of a part of such work in the doing.

Whether the crayfish possesses consciousness or not,
however, does not affect the question of its being an
engine, the actions of which at any moment depend, on
the one hand, upon the series of molecular changes excited,
either by internal or by external causes, in its neuro-
muscular machinery ; and, on the other, upon the dispo-
sition and the properties of the parts of that machinery.
And such a self-adjusting machine, containing the im-
mediate conditions of its action within itself, is what is
properly understood by an automaton.

Crayfishes, as we have seen, may attain a considerable
age ; and there is no means of knowing how long they
might live, if protected from the innumerable destructive
influences to which they are at all ages liable.

It is a widely received notion that the energies of living
matter have a natural tendency to decline, and finally
disappear; and that the death of the body, as a whole,
is the necessary correlate of its life. That all living
things sooner or later perish needs no demonstration,
but it would be difficult to find satisfactory grounds

128 THE PHYSIOLOGY OF THE COMMON CRAYFISH.

for the belief that they must needs do so. The analogy of
a machine that, sooner or later, must be brought to a
standstill by the wear and tear of its parts, does not
hold, inasmuch as the animal mechanism is continually
renewed and repaired ; and, though it is true that indi-
vidual components of the body are constantly dying, yet
their places are taken by vigorous successors. A city
remains, notwithstanding the constant death-rate of its
inhabitants ; and such an organism as a crayfish is only
a corporate unity, made up of innumerable partially
independent individualities.

Whatever might be the longevity of crayfishes under
imaginable perfect conditions, the fact that, notwithstand-
ing the great number of eggs they produce, their number
remains pretty much the same in a given district, if
we take the average of a period of years, shows that
about as many die as are born ; and that, without the
process of reproduction, the species would soon come to
an end.

There are many examples among members of the group
of Crustacea to which the crayfish belongs, of animals which
produce young from internally developed germs, as some
plants throw off bulbs which are capable of reproducing
the parent stock ; such is the case, for example, with the
common water flea (Daphnia). But nothing of this kind
has been observed in the crayfish ; in which, as in the
higher animals, the reproduction of the species is de*
pendent upon the combination of two kinds of living

THE OVARY AND THE TESTIS.

129

matter, which are developed in different individuals,
termed males and females.

These two kinds of living matter are ova and sperma-
tozoa, and they are developed in special organs, the ovary
and the testis. The ovary is lodged in the female ; the
testis, in the male.

The ovary (fig. 30, ov) is a body of a trefoil form,
which is situated immediately beneath, or in front of,
the heart, between the floor of the pericardial sinus and
the alimentary canal. From the ventral face of this

od'

Fm. 30.— Antaeus Jim- in til is.— The female reproductive organs ( x 2);
or, ovary ; od, oviduct ; od', aperture of oviduct.

organ two short and wide canals, the oviducts {od), lead
down to the bases of the second pair of walking limbs,
and terminate in the apertures (od') already noticed
there.

The testis (fig. 31, t) is somewhat similar in form to
the ovary, but, the three divisions are much narrower

130

THE PHYSIOLOGY OF THE COMMON CRAYFISH.

and more elongated : the hinder median division lies
under the heart ; the anterior divisions are situated
between the heart behind, and the stomach and the liver
in front (figs. 5 and 12, t). From the point at which the

Fig. 31. — Asiacus fluviatilis. — The male reproductive organs (x 2);
t, testis ; vdy vas deferens ; vd', aperture of vas deferens.

three divisions join, proceed two ducts, which are termed
the vasa deferentia (fig. 31, vd). These are very narrow,
long, and make many coils before they reach the apertures
upon the bases of the hindermost pair of walking limbs, by
which they open externally (fig. 31, vd', and fig. 35, vd).
Both the ovary and the testis are very much larger

THE OVARY AND THE EGGS, 131

during the breeding season than at other times; the large
brownish-yellow eggs become conspicuous in the ovary,

Fig. 32. — Astacus fluviatilis. — A, a two-thirds grown egg contained in
its ovisac ( x 50) ; B, an egg removed from the ovisac ( x 10) ;
C, a portion of the wall of an ovisac with the adjacent portion of
the contained egg, highly magnified ; ep% epithelium of ovisac ;
gs9 germinal spots ; gv, germinal vesicle ; m9 membrana propria ;
v9 vitellus ; vm, vitelline membrane ; w, stalk of ovisac.

and the testis assumes a milk-white colour, at this
period.

The walls of the ovary are lined internally by a layer of

K 2

132 THE PHYSIOLOGY OF THE COMMON CRAYFISH.

nucleated cells, separated from the cavity of the organ by
a delicate structureless membrane. The growth of these
cells gives rise to papillary elevations which project into
the cavity of the ovary, and eventually become globular

FlG. 33. — Astacus fluviatilis. — A, a lobule of the testis, showing acini,
springing from b, the ultimate termination of a duct(x 50). B,
spermatic cells ; a, with an ordinary globular nucleus n ; b, with a
spindle-shaped nucleus ; c, with two similar nuclei ; and d, with
a nucleus undergoing division ( x 600).

bodies attached by short stalks, and invested by the struc-
tureless membrane as a membrana propria (fig, 32, m)e
These are the ovisacs. In the mass of cells wrhich be-
comes the ovisac, one rapidly increases in size and
occupies the centre of the ovisac, while the others

THE OVA AND THE SPERMATOZOA.

133

surround it as a peripheral coat (ep.). This central cell
is the ovum. Its nucleus enlarges, and becomes what is
called the germinal vesicle (g.v.). At the same time
numerous small corpuscles, flattened externally and
convex internally, appear in it and are the germinal
spots (g.s.). The protoplasm of the cell, as it enlarges,
becomes granular and opaque, assumes a deep brownish-
yellow colour, and is thus converted into the yelk or
vitellus (v.). As the egg grows, a structureless vitelline
membrane is formed between the vitellus and the cells
which line the ovisac, and incloses the egg, as in a
bag. Finally, the ovisac bursts, and the egg, falling
into the cavity of the ovary, makes its way down the
oviduct, and sooner or later passes out by its aperture.
When they leave the oviduct, the ova are invested by
a viscous, transparent substance, which attaches them
to the swimmerets of the female, and then sets ; thus
each egg, inclosed in a tough case, is firmly suspended
by a stalk, which, on the one side, is continued into the
substance of the case, while, on the other, it is fixed to
the swimmeret. The swimmerets are kept constantly in
motion, so that the eggs are well supplied with aerated
water.

The testis consists of an immense number of minute
spheroidal vesicles (fig. 33, A, a), attached like grapes to
the ends of short stalks (b), formed by the ultimate
ramifications of the vasa deferentia. The vesicles may,
in fact, be regarded as dilatations of the ends and sides

Fig. 34. — Astacus fluviatilis.—A.—'D, different stages in the development of a sperma-
tozoon from a seminal cell ; E, a mature spermatozoon seen from the side ; F, the
same viewed <w*{ace (all x 850) ; G, a diagrammatic vertical section of the same.

THE PEOCESS OF FERTILIZATION.

135

of the finest branches of the ducts of the testis. The
cavity of each vesicle is filled by the large nucleated cells
which line its walls (fig. 33, B), and, as the breeding
season approaches, these cells multiply by division.
Finally, they undergo some very singular changes of
form and internal structure (fig. 34, A — D), each becom-
ing converted into a flattened spheroidal body, about
TToijtk of an inch in diameter, provided with a number
of slender curved rays, which stand out from its sides
(fig. 34, E — G). These are the spermatozoa.

The spermatozoa accumulate in the testicular vesicles,
and give rise to a milky-looking substance, which traverses
the smaller ducts, and eventually fills the vasa deferentia.
This substance, however, consists, in addition to the
spermatozoa, of a viscid material, secreted by the walls
of the vasa deferentia, which envelopes the spermatozoa,
and gives the secretion of the testis the form and the
consistency of threads of vermicelli.

The ripening and detachment of both the ova and
the spermatozoa take place immediately after the com-
pletion of ecdysis in the early autumn ; and at this
time, which is the breeding season, the males seek
the females with great avidity, in order to deposit the
fertilizing matter contained in the vasa deferentia on the
sterna of their hinder thoracic and anterior abdominal
somites. There it adheres as a whitish, chalky-looking
mass ; but the manner in which the contained sperma-
tozoa reach and enter the ova is unknown. The analogy

136

THE PHYSIOLOGY OF THE COMMON CRAYFISH.

of what occurs in other animals, however, leaves no doubt
that an actual mixture of the male and female ele-
ments takes place and constitutes the essential part of
the process of impregnation.

Ova to which spermatozoa have had no access,
give rise to no progeny ; but, in the impregnated ovum,
the young crayfish takes its origin in a manner to be
described below, when the question of development is
dealt with.

Fig. 35. — Astacus fluviatilis. — The last thoracic sternum, seen from
behind, with the proximal ends of the appendages, A, in the male,
B, in the female, ( x 3). am, articular membrane ; cxp, coxopo-
dite ; st XIV, last thoraoic sternum ; vd, aperture of vas deferens.

CHAPTEE IV.

THE MORPHOLOGY OF THE COMMON CRAYFISH I THE STRUC-
TURE AND THE DEVELOPMENT OF THE INDIVIDUAL.

In the two preceding chapters the crayfish has been
studied from the point of view of the physiologist, who,
regarding an animal as a mechanism, endeavours to dis-
cover how it does that which it does. And, practically, this
way of looking at the matter is the same as that of the
teleologist. For, if all that we know concerning the pur-
pose of a mechanism is derived from observation of the
manner in which it acts, it is all one, whether we say
that the properties and the connexions of its parts
account for its actions, or that its structure is adapted
to the performance of those actions.

Hence it necessarily follows that physiological pheno-
mena can be expressed in the language of teleology.
On the assumption that the preservation of the indi-
vidual, and the continuance of the species, are the
final causes of the organization of an animal, the exist-
ence of that organization is, in a certain sense, explained,
when it is shown that it is fitted for the attainment of
those ends ; although, perhaps, the importance of de-

138 THE MORPHOLOGY OF THE COMMON CRAYFISH.

monstrating the proposition that a thing is fitted to do
that which it does, is not very great.

But whatever may be the value of teleological ex-
planations, there is a large series of facts, which have as
yet been passed over, or touched only incidentally, of
which they take no account. These constitute the sub-
ject matter of Morphology, which is related to physiology
much as, in the not-living world, crystallography is
related to the study of the chemical and physical pro-
perties of minerals.

Carbonate of lime, for example, is a definite compound
of calcium, carbon, and oxygen, and it has a great variety
of physical and chemical properties. But it may be
studied under another aspect, as a substance capable of
assuming crystalline forms, which, though extraordinarily
various, may all be reduced to certain geometrical types.
It is the business of the crystallographer to work out
the relations of these forms ; and, in so doing, he takes no
note of the other properties of carbonate of lime.

In like manner, the morphologist directs his attention
to the relations of form between different parts of the
same animal, and between different animals; and these
relations would be unchanged if animals were mere
dead matter, devoid of all physiological properties — a
kind of mineral capable of a peculiar mode of growth.

A familiar exemplification of the difference between
teleology and morphology may be found in such works
of human art as houses.

TELEOLOGY AND MORPHOLOGY

139

A house is certainly, to a great extent, an illustration
of adaptation to purpose, and its structure is, to that
extent, explicable by teleological reasonings. The roof
and the walls are intended to keep out the weather; the
foundation is meant to afford support and to exclude
damp; one room is contrived for the purpose of a
kitchen; another for that of a coal-cellar; a third for
that of a dining-room ; others are constructed to serve as
sleeping rooms, and so on ; doors, chimneys, windows,
drains, are all more or less elaborate contrivances directed
towards one end, the comfort and health of the dwellers
in the house. What is sometimes called sanitary architec-
ture, now-a-days, is based upon considerations of house
teleology. But though all houses are, to begin with and
essentially, means adapted to the ends of shelter and
comfort, they may be, and too often are, dealt with from
a point of view, in which adaptation to purpose is largely
disregarded, and the chief attention of the architect is
given to the form of the house. A house may be built in
the Gothic, the Italian, or the Queen Anne style ; and a
house in any one of these styles of architecture may be
just as convenient or inconvenient, just as well or as ill
adapted to the wants of the resident therein, as any of
the others. Yet the three are exceedingly different.

To apply all this to the crayfish. It is, in a sense
a house with a great variety of rooms and offices, in
which the work of the indwelling life in feeding, breath-
ing, moving, and reproducing itself, is done. But the

140 THE MOllPHOLOGY OF THE COMMON CRAYFISH.

same may be said of the crayfish's neighbours, the perch
and the water-snail ; and they do all these things neither
better nor worse, in relation to the conditions of their
existence, than the crayfish does. Yet the most cursory
inspection is sufficient to show that the " styles of archi-
tecture " of the three are even more widely different than
are those of the Gothic, Italian, and Queen Anne houses.

That which Architecture, as an art conversant with
pure form, is to buildings, Morphology, as a science
conversant with pure form, is to animals and plants.
And we may now proceed to occupy ourselves exclusively
with the morphological aspect of the crayfish.

As I have already mentioned, when dealing with the
physiology of the crayfish, the entire body of the animal,
when reduced to its simplest morphological expression,
may be represented as a cylinder, closed at each end, ex-
cept so far as it is perforated by the alimentary aper-
tures (fig. 6) ; or we may say that it is a tube, inclosing
another tube, the edges of the two being continuous at
their extremities. The outer tube has a chitinous outer
coat or cuticle, which is continued on to the inner face
of the inner tube. Neglecting this for the present, the
outermost part of the wall of the outer tube, which
answers to the epidermis of the higher animals, and the
innermost part of the wall of the inner tube, which is
an epithelium, are formed by a layer of nucleated cells.
A continuous layer of cells, therefore, is everywhere to

ENDODERM, MESODERM, AND ECTODERM. 141

be found on both the external and the internal free sur-
faces of the body. So far as these cells belong to the
proper external wall of the body, they constitute the
ectoderm, and so far as they belong to its proper internal
wall, they compose the endoderm. Between these two
layers of nucleated cells lie all the other parts of the
body, composed of connective tissue, muscles, vessels,
and nerves ; and all these (with the exception of the
ganglionic chain, which we shall see properly belongs to
the ectoderm) may be regarded as a single thick stratum,
which, as it lies between the ectoderm and the endoderm,
is called the mesoderm.

If the intestine were closed posteriorly instead of
opening by the vent, the crayfish would virtually be an
elongated sac, with one opening, the mouth, affording an
entrance into the alimentary cavity : and, round this
cavity, the three layers just referred to — endoderm,
mesoderm, and ectoderm — would be disposed concen-
trically.

We have seen that the body of the crayfish thus com-
posed is obviously separable into three regions — the
cephalon or head, the thorax, and the abdomen. The
latter is at once distinguished by the size and the
mobility of its segments : while the thoracic region is
marked off from that of the head, outwardly, only by the
cervical groove. But, when the carapace is removed,
the lateral depression already mentioned, in which the

142 THE MORPHOLOGY OF THE COMMON CRAYFISH.

scaphognathite lies, clearly indicates the natural boundary
between the head and the thorax. It has further been
observed that there are, in all, twenty pairs of ap-
pendages, the six hindermost of which are attached to
the abdomen. If the other fourteen pairs are carefully
removed, it will be found that the six anterior belong to
the head, and the eight posterior to the thorax.

The abdominal region may now be studied in further
detail. Each of its seven movable segments, except the
telson, represents a sort of morphological unit, the repe-
tition of which makes up the whole fabric of the body.

If the abdomen is divided transversely between the

Fig. 36.—Astacnsfluviatilis.—A transverse section through the nine-
teenth (fifth abdominal) somite ( x 2). e.m., extensor muscles ; /.«*.,
flexor muscles ; gn. 12, the fifth abdominal ganglion ; li.g., hind-gut ;
i.a.a,, inferior abdominal artery ; s.a.a, superior abdominal artery ;
pi. XIX, pleura of the somite ; st, XIX, its sternum ; t. XIX, its
tergum ; ep. XIX, its epimera ; 19, its appendages.

s a. a.

SOMITES AND APPENDAGES.

143

fourth and fifth, and the fifth and sixth segments, the fifth
will be isolated, and can be studied apart. It constitutes
what is called a metamere ; in which are distinguishable a
central part termed the somite, and two appendages
(fig. 36).

In the exoskeleton of the somites of the abdomen
several regions have already been distinguished; and
although they constitute one continuous whole, it will
be convenient to speak of the sternum (fig. 36, st. XIX),
the tergum (t. XIX), and, the pleura (pi. XIX), as if they
were separate parts, and to distinguish that portion of
the sternal region, which lies between the articulation
of the appendage and the pleuron, on each side, as the
epimeron (ep. XIX). Adopting this nomenclature, it may
be said of the fifth somite of the abdomen, that it
consists of a segment of the exoskeleton, divisible into
tergum, pleura, epimera, and sternum, with which two
appendages are articulated; that it contains a double
ganglion (gn. 12), a section of the flexor (fm) and extensor
(em) muscles, and of the alimentary (hg) and vascular
(s.a.a, i.a.a) systems.

The appendage (fig. 36, 19), which is attached to an
articular cavity situated between the sternum and the
epimeron, is seen to consist of a stalk or stem, which is
made up of a very short basal joint, the coxopodite (fig. 37,
D and E, cx.p), followed by a long cylindrical second
joint, the basipodite (b.p), and receives the name of pro-
topodite. At its free end, it bears two flattened narrow

cxp

Fig. 87. — Astacus fluviatilis. — Appendages of the left side of the abdo-
men ( x 3). A, the posterior face of the first appendage of the male ;
B, the same of the female ; C, posterior, and C, anterior faces of the
second appendage of the male ; D, the third appendage of the male ;
E, the same of the female ; F, the sixth appendage, a, the rolled
plate of the endopoditc * b, the jointed extremity of the same ; bp.,
basipodite ; cx.j?., coxopodite ; eng., endopodite ; ex.p., exopodite.

SOMITES AND APPENDAGES.

145

plates, of which one is attached to the inner side of the
extremity of the protopodite, and is called the endopodite
(en.p), while the other is fixed a little higher up to the
outer side of that extremity, and is the exopodite (ex.p).
The exopodite is shorter than the endopodite. The
endopodite is broad and is undivided for about half its
length, from the attached end ; the other half is narrower
and is divided into a number of small segments, which,
however, are not united by definite articulations, but are
merely marked off from one another by slight constric-
tions of the exoskeleton. The exopodite has a similar
structure, but its undivided portion is shorter and nar-
rower. The edges of both the exopodite and the endo-
podite are fringed with long setse.

In the female crayfish, the appendages of this and of
the fourth and third somites are larger than in the male
(compare D and E, fig. 37).

The fourth and fifth somites, with their appendages,
may be described in the same terms as the third, and
in the sixth there is no difficulty in recognising the
corresponding parts of the somite ; but the appendages
(fig. 37, F), which constitute the lateral portions of
the caudal fin, at first sight appear very different. In
their size, no less than in their appearance, they depart
widely from the appendages of the preceding somites.
Nevertheless, each will be found to consist of a basal
stalk, answering to the protopodite (cx.p), which how-
ever is very broad and thick, and is not divided into two

146 THE MORPHOLOGY OF THE COMMON CRAYFISH.

joints ; and of two terminal oval plates, which represent
the endopodite (en.p) and the exopodite (ex.p). The
latter is divided by a transverse suture into two pieces ;
and the edge of the larger or basal moiety is beset with
short spines, of which two, at the outer end of the series,
are larger than the rest.

The second somite is longer than the first (fig. 1) ; it
has very broad pleura, while those of the first somite are
small and hidden by the overlapping front margins of the
pleura of the second somite.

In the female, the appendages of the second somite of
the abdomen are similar to those of the third, fourth, and
fifth somites ; but in those of the first somite (fig. 37, B)}
there is a considerable variation. Sometimes, in fact,
the appendages of this somite are altogether wanting ;
sometimes one is present, and not the other ; and
sometimes both are found. But, when they exist, these
appendages are always small ; and the protopodite is
followed by only oijb imperfectly jointed filament, which
appears to represent the endopodite of the other ap-
pendages.

In the male, the appendages of the first and second
somites of the abdomen are not only of relatively large
size, but they are widely different from the rest, those of
the first somite departing from the general type further
than those of the second. In the latter (C, C ') there is
a protopodite (ex.p, bp) with the ordinary structure, and
it is followed by an endopodite (en.p) and an exopodite

SOMITES AND APPENDAGES.

147

(ex.p) ; but the former is singularly modified. The un-
divided basal part is large, and is produced on the
inner side into a lamella {a), which extends slightly
beyond the end of the terminal jointed portion (&). The
inner half of this lamella is rolled upon itself, in such a
manner as to give rise to a hollow cone, something like
an extinguisher (C\ a).

The appendage of the first somite (A) is an unjointed
styliform body, which appears to represent the proto-
podite, together with the basal part and the inner pro-
longation of the endopodite of the preceding appendage.
The terminal half of the appendage is really a broad
plate, slightly bifid at the summit, but the sides of the
plate are rolled in, in such a manner that the anterior
half bends round and partially incloses the posterior half.
They thus give rise to a canal, which is open at each end,
and only partially closed behind.

These two pairs of curiously modified appendages are
ordinarily turned forwards and applied against the sterna
of the posterior part of tli3 thorax, in the interval be-
tween the bases of the hinder thoracic limbs (see fig. 3,
A). They serve as conduits by which the spermatic
matter of the male is conveyed from the openings of the
ducts of the testes to its destination.

If we confine our attention to the third, fourth, and
fifth metameres of the abdomen of the crayfish, it is
obvious that the several somites and their appendages,
and the various regions or parts into which they are

l 2

14S THE MORPHOLOGY OF THE COMMON CRAYFISH.

divisible, correspond with one another, not only in form,
but in their relations to the general plan of the whole
abdomen. Or, in other words, a diagrammatic plan of
one somite will serve for all the three somites, with
insignificant variations in detail. The assertion that
these somites are constructed upon the same plan, in-
volves no more hypothesis than the statement of an
architect, that three houses are built upon the same plan,
though the facades and the internal decorations may
differ more or less.

In the language of morphology, such conformity in the
plan of organisation is termed homology. Hence, the
several metameres in question and their appendages, are
homologous with one another ; while the regions of the
somites, and the parts of their appendages, are also
homologies.

When the comparison is extended to the sixth meta-
mere, the homology of the different parts with those of the
other metameres, is undeniable, notwithstanding the great
differences which they present. To recur to a previous
comparison, the ground plan of the building is the same,
though the proportions are varied. So with regard to
the first and second metameres. In the second pair
of appendages of the male, the difference from the
ordinary type of appendage is comparable to that pro-
duced by adding a portico or a turret to the building ;
while, in the first pair of appendages of the female,
it is as if one wing of the edifice were left unbuilt ;

HOMOLOGY AND HOMOLOGUES.

I4y

and, in those of the male, as if all the rooms were run
into one.

It is further to be remarked, that, just as of a row of
houses built upon the same plan, one may be arranged so as
to serve as a dwelling-house, another as a warehouse, and
another as a lecture hall, so the homologous appendages
of the crayfish are made to subserve various functions.
And as the fitness of the dwelling-house, the warehouse,
and the lecture-hall for their several purposes would not
in the least help us to understand why they should all be
built upon the same general plan ; so, the adaptation of
the appendages of the abdomen of the crayfish to the dis-
charge of their several functions does not explain why
those parts are homologous. On the contrary, it would
seem simpler that each part should have been constructed
in such a manner as to perform its allotted function in
the best possible manner, without reference to the rest.
The proceedings of an architect, who insisted on coi\
structing every building in a town on the plan of a
Gothic cathedral, would not be explicable by considera-
tions of fitness or convenience.

In the cephalothorax, the division into somites is not
at first obvious, for, as we have seen, the dorsal or tergal
surface is covered over by a continuous shield, distin-
guished into thoracic and cephalic regions only by the
cervical groove. Even here, however, when a transverse
section of the thorax is compared with that of the abdo-

150 THE MORPHOLOGY OF THE COMMON CRAYFISH.

men (figs. 15 and 36), it will be obvious that the tergal
and the sternal regions of the two answer to one another ;
while the branchiostegites correspond with greatly de-
veloped pleura ; and the inner wall of the branchial
chamber, which extends from the bases of the appendages
to the attachment of the branchiostegite, represents an
immensely enlarged epimeral region.

On examination of the sternal aspect of the cephalo-
thorax the signs of division into somites become plain
(figs. 3 and 39, A). Between the last two ambulatory
limbs there is an easily recognisable sternum (XIV.),
though it is considerably narrower than any of the
sterna of the abdominal somites, and differs from them
in shape.

The deep transverse fold which separates this hinder-
most thoracic sternum from the rest of the sternal wall
of the cephalothorax, is continued upwards on the inner
or epimeral wall of the branchial cavity ; and thus the
sternal and the epimeral portions of the posterior thoracic
somite are naturally marked off from those of the more
anterior somites.

The epimeral region of this somite presents a very
curious structure (fig. 38). Immediately above the ar-
ticular cavities for the appendages there is a shield-
shaped plate, the posterior, convex edge of which is
sharp, prominent, and setose. Close to its upper
boundary the plate exhibits a round perforation (plb.),
to the margins of which the stem of the hindermost

THE CEPHALOTHORAX.

151

pleurobranchia (fig. 4, plb. 14) is attached; and in
front of this, it is connected, by a narrow neck, with
an elongated triangular piece, which takes a vertical
direction, and lies in the fold which separates the posterior
thoracic somite from the next in front. The base of this

Fig. 38. — Astacns fimiatiUs. — The mode of connexion between the last
thoracic and the first abdominal somites ( x S). a, L-shaped bar ;
cj)e, carapace ; cxp. 14, coxopodite of the last ambulatory leg- ; plb.,
place of attachment of the pleurobranchia ; st. XV, sternum, and
t. XVf tergum of the first abdominal somite.

piece unites with the epimeron of the penultimate somite.
Its apex is connected with the anterior end of the horizontal
arm of an L-shaped calcified bar (fig. 38, a), the upper end
of the vertical arm of which is firmly, but moveably, con-
nected with the anterior and lateral edge of the tergum
of the first abdominal somite (f. XV.). The tendon of one

152 THE MORPHOLOGY OF THE COMMON CRAYFISH.

of the large extensor muscles of the abdomen is attached
close to it.

The sternum and the shield-shaped epimeral plates
constitute a solid, continuously calcified, ventral element
of the skeleton, to which the posterior pair of legs is
attached ; and as this structure is united with the
somites in front of and behind it only by soft cuticle,
except where the shield-shaped plate is connected, by
the intermediation of the triangular piece, with the
epimeron which lies in front of it, it is freely movable
backwards and forwards on the imperfect hinge thus
constituted.

In the same way, the first somite of the abdomen,
and, consequently, the abdomen as a whole, moves upon
the hinges formed by the union of the L-shaped pieces
with the triangular pieces.

In the rest of the thorax, the sternal and the epimeral
regions of the several somites are all firmly united
together. Nevertheless, shallow grooves answering to
folds of the cuticle, which run from the intervals
between the articular cavities for the limbs towards the
tergal end of the inner wall of the branchial chamber,
mark off the epimeral portions of as many somites as
there are sterna, from one another.

A short distance above the articular cavities a trans-
verse groove separates a nearly square area of the lower
part of the epimeron from the rest. Towards the
anterior and upper angle of this area, in the two somites

THE CEPHALOTHOKAX.

153

which lie immediately in front of the hindermost, there
is a small round aperture for the attachment of the

Fig. ZS.—Astacus jluviatilis. — The cephalothoracic sterna and the endo-
phragmal system ( x 2). A, from beneath ; B, from above, a, a',
arthrophragms or partitions between the articular cavities for the
limbs ; cap, cephalic apodeme ; cf, cervical fold ; epn. 1, epimeron of
the antennulary somite ; h, anterior, and h', posterior horizontal
process of endopleurite ; lb, labrum ; m, mesophragm ; mt] meta-
stoma ; ]>, paraphragm ; 1 — XIV, cephalothoracic sterna ; 1 — 14,
articular cavities of the cephalothoracic appendages. (The anterior
cephalic sterna are bent downwards in A so as to bring them into
the same plane with the remaining cephalothoracic sterna ; in B
these sterna are not shown.)

154 THE MORPHOLOGY OF THE COMMON CRAYFISH.

rudimentary branchia. These areae of the epimera, in
fact, correspond with the shield-shaped plate of the
hindermost somite. In the next most anterior somite
(that which bears the first pair of ambulatory legs) there
is only a small elevation in the place of the rudimentary
branchia ; and in the anterior four thoracic somites no-
thing of the kind is visible.

On the sternal aspect of the thorax (figs. 3 and 39, A) a
triangular space is interposed between the basal joints or
coxopodites of the penultimate and the ante-penultimate
pairs of ambulatory legs, while the coxopodites of the
more anterior limbs are closely approximated. The
triangular area in question is occupied by two sterna
(fig. 39, A, XII, XIII) , the lateral margins of which are
raised into flange-like ridges. The next two sterna {X,
XI) are longer, especially that which lies between the
forceps (X), but they are very narrow ; while the lateral
processes are reduced to mere tubercles at the posterior
ends of the sterna. Between the three pairs of maxil-
lipedes, the sterna (VII, VIII, IX) are yet narrower, and
become gradually shorter ; but traces of the tubercles at
their posterior ends are still discernible. The most
anterior of these sternal rods passes into a transversely
elongated plate, shaped like a broad arrow (V, VI),
which is constituted by the conjoined sterna of the two
posterior somites of the head.

Anteriorly to this, and between it and the posterior
end of the elongated oral aperture, the sternal region is

THE CEPHALIC SOMITES.

155

occupied only by soft or imperfectly calcified cuticle,
which, on each side of the hinder part of the mouth,
passes into one of the lobes of the metastoma (mt). At
the base of each of these lobes there is a calcified plate,
united by an oblique suture with another, which occupies
the whole length of the lobe and gives it firmness. The
soft narrow lip which constitutes the lateral boundary
of the oral aperture, and lies between it and the man-
dible, passes, in front, into the posterior face of the
labrum (lb).

In front of the mouth, the sternal region which apper-
tains, in part, to the antennae, and, in part, to the man-
dibles, is obvious as a broad plate (III), termed the
epistoma. The middle third of the posterior edge of the
epistoma gives rise to a thickened transverse ridge, with
rounded ends, slightly excavated behind, and is then
continued into the labrum (lb), which is strengthened by
three pairs of calcifications, arranged in a longitudinal
series. The sides of the front edge of the epistoma are
excavated, and bound the articular cavities for the basal
joints of the antennse (3) ; but, in the middle line, the
epistoma is continued forwards into a spear-head shaped
process (figs. 39 and 40, II), to which the posterior end of
the antennulary sternum contributes. The antennulary
sternum is very narrow, and its anterior or upper end runs
into a small but distinct conical median spine (fig. 40, t.).
Upon this follows an uncalcified plate, bent into the form of
a half cylinder (/), which lies between the inner ends of

156 THE MORPHOLOGY OF THE COMMON CRAYFISH.

the eye-stalks and is united with adjacent parts only
by flexible cuticle, so that it is freely movable. This
represents the whole of the sternal region, and probably
more, of the ophthalmic somite #

The sterna of fourteen somites are thus identifiable in
the cephalothorax. The corresponding epimera are

Fig. 40. — Astams fluviatilis. — The ophthalmic and antennulary somites
( x 3). J, ophthalmic, and II, antennulary sternum ; 1, articular
surface for eyestalk ; 2, for ant ennui e ; epm, epimeral plate ;
pcjy, procephalic process ; r, base of rostrum ; t, tubercle.

represented, in the thorax, by the thin inner walls of the
branchial chamber ; the pleura, by the branchiostegites ;
and the terga, by so much of the median region of the
carapace as lies behind the cervical groove. That part of
the carapace which is situated in front of this groove occu-
pies the place of the terga of the head ; while the low
ridge, skirting the oral and prse-oral region, in which it
terminates laterally, represents the pleura of the cephalic
somites.

The epimera of the head are, for the most part, very
narrow ; but those of the antennulary somite are broad
plates (fig. 40, epm.), which constitute the posterior

THE ENDOPHRAGMAL SYSTEM.

157

wall of the orbits. I am inclined to think that a trans-
verse ridge, which unites these under the base of the
rostrum, represents the tergum of the antennulary somite,
and that the rostrum itself belongs to the next or
antennary somite.*

The sharp convex ventral edge of the rostrum (fig. 41)
is produced into a single, or sometimes two divergent
spines, which descend, in front of the ophthalmic somite,
towards the conical tubercle mentioned above : it thus
gives rise to an imperfect partition between the orbits.

Fig. 41. — Astacus fluviatilis. — The rostrum, seen from the left side.

The internal face of the sternal wall of the whole of
the thorax and of the post-oral part of the head, presents
a complicated arrangement of hard parts, which is known
as the endophragmal system (figs. 39, B, 42, and 43), and
which performs the office of an internal skeleton by afford-
ing attachment to muscles, and serving to protect im-
portant viscera, while at the same time it ties the somites
together, and unites them into a solid whole. In reality,
however, the curious pillars and bulkheads which enter
into the composition of the endophragmal system are all

* There are some singular marine Crustacea, the Squillidcr, in which
both the ophthalmic and the antennary somites are free and movable,
while the rostrum is articulated with the tergum of the antennary
somite.

158 THE MORPHOLOGY OF THE COMMON CRAYFISH.

mere infoldings of the cuticle, or apodemes ; and, as such,
they are shed along with the other cuticular structures
during the process of ecdysis.

Without entering into unnecessary details, the gene-
ral principle of the construction of the endophragmal
skeleton may be stated as follows. Four apodemes are
developed between every two somites, and as every
apodeme is a fold of the cuticle, it follows that the
anterior wall of each belongs to the somite in front, and
the posterior wall to the somite behind. All four apodemes
lie in the ventral half of the somite and form a single
transverse series ; consequently there are two nearer
the middle line, which are termed the endosternites, and
two further off, which are the endopleurites. The former
he at the inner, and the latter at the outer ends of the
partitions or arihrophragms (fig. 39, A, a, a', fig. 42, apli),
between the articular cavities for the basal joints of the
limbs, and they spring partly from the latter and partly
from the sternum and the epimera respectively.

The endosternite (fig. 42, ens.) ascends vertically, with a
slight inclination forwards, and its summit narrows and
assumes the form of a pillar, with a flat, transversely
elongated capital. The inner prolongation of the capital
is called the mesophragm (mph.), the outer the paraphragm
(pph.). The mesophragms of the two endosternites of a
somite usually unite by a median suture, and thus form
a complete arch over the sternal canal (s.c), which lies
between the endosternites.

THE ENDOPHRAGMAL SYSTEM. 159

The enclopleurites (en0pl.) are also vertical plates, but
they are relatively shorter, and their inner angles give off
two nearly horizontal processes, one of which passes
obliquely forwards (fig. 39, B, h, fig. 42, h.p.) and unites
with the paraphragm of the endosternite of the somite
in front, while the other, passing obliquely backwards
(fig. 39, h'), becomes similarly connected with the endo-
sternite of the somite behind.

s

Fig. 42. — Astacus fluviatilis. — A segment of the endophragmal system
( x 3). aj)h, arthrophragm ; arth, arthrodial or articular cavity ;
cxj?, coxopodite of the ambulatory leg ; enpl, endopleurite ; ens,
endosternite ; ej?m, epimeron ; lip, horizontal process of endo-
pleurite ; mph, mesophragm ; pjph, paraphragm ; s. sternum of
somite ; sc, sternal canal.

The endopleurites of the last thoracic somite are rudi-
mentary, and its endosternites are small. On the other
hand, the mesophragmal processes of the endosternites of
the two posterior somites of the head (fig. 39, B, cap), by
which the endophragmal system terminates in front, are
particularly strong and closely united together. They
thus, with their endopleurites, form a solid partition be-
tween the stomach, which lies upon them, and the mass of

1G0 THE MORPHOLOGY OF THE COMMON CRAYFISH.

coalesced anterior thoracic and posterior cephalic ganglia
situated beneath them. Strong processes are given off
from their anterior and outer angles, which curve round
the tendons of the adductor muscles of the mandibles, and
give attachment to the abductors.

In front of the mouth there is no such endophragmal
system as that which lies behind it. But the anterior gas-
tric muscles are attached to two flat calcined plates, which
appear to lie in the interior of the head (though they are
really situated in its upper and front wall) on each side
of the base of the rostrum, and are called the procephalic
processes (figs. 40, 43, p.cp). Each of these plates con-
stitutes the posterior wall of a narrow cavity which opens
externally into the roof of the orbit, and has been regarded
(though, as it appears to me, without sufficient reason) as
an olfactory organ. I am disposed to think, though I
have not been able to obtain complete evidence of the
fact, that the procephalic processes are the representa-
tives of the " procephalic lobes " which terminate the
anterior end of the body in the embryo crayfish. At
any rate, they occupy the same position relatively to the
eyes and to the carapace ; and the hidden position of
these processes, in the adult, appears to arise from the
extension of the carapace at the base of the rostrum
over the fore part of the originally free sternal surface of
the head. It has thus covered over the procephalic
processes, in which the sternal wall of the body termi-
nated; and the cavities which lie in front of them are

THE THEORY OF THE SKELETON.

161

simply the interspaces left between the inferior or
posterior wall of the prolongation of the carapace and
the originally exposed external faces of these regions of
the cephalic integument.

Fourteen somites having thus been distinguished in
the cephalothorax, and six being obvious in the abdomen,
it is clear that there is a somite for every pair of append-
ages. And, if we suppose the carapace divided into
segments answering to these sterna, the whole body will
be made up of twenty somites, each having a pair of
appendages. As the carapace, however, is not actually
divided into terga in correspondence with the sterna
which it covers, all we can safely conclude from the
anatomical facts is that it represents the tergal region of
the somites, not that it is formed by the coalescence of
primarily distinct terga. In the head, and in the greater
part of the thorax, the somites are, as it were, run
together, but the last thoracic somite is partly free and
to a slight extent moveable, while the abdominal somites
are all free, and moveably articulated together. At the
anterior end of the body, and, apparently, from the an-
tennary somite, the tergal region gives rise to the
rostrum, which projects between and beyond the eyes.
At the opposite extremity, the telson is a corresponding
median outgrowth of the last somite, which has become
moveably articulated therewith. The narrowing of the
Bternal moieties of the anterior thoracic somites, to-

M

162 THE MORPHOLOGY OF THE COMMON CRAYFISH.

gether with the sudden widening of the same parts in
the posterior cephalic somites, gives rise to the lateral
depression (fig, 39, cf) in which the scaphognathite lies.
The limit thus indicated corresponds with that marked
by the cervical groove upon the surface of the carapace,
and separates the head from the thorax. The three pair
of maxillipedes (7, 8, 9), the forceps (1(9), the ambulatory

Fig. 43. — Astacns Jtuviatilis. — Longitudinal section of the anterior part
of the cephalothorax ( x 3). / — iX, sterna of first nine cephalo-
thoracic somites; i, eyestalk ; basal joint of antennule ; 3, basal
joint of antenna ; 4, mandible ; a, inner division of the masticatory
surface of the mandible ; a', apophysis of the mandible for muscular
attachment ; cp9 free edge of carapace ; e, endosternite ; enpl, endo-
pleurite ; epm, epimeral plate ; l} labrum ; m, muscular fibres con-
necting epimera with interior of carapace ; mt, metastoma ; pep.
procephalic process,

THE THEORY OF THE SKELETON,

163

limbs (11 — 14), and the eight somites of which they are
the appendages (VII — XIV), lie behind this boundary
and belong to the thorax. The two pairs of maxillae (5, 6)
the mandibles (4), the antennae (3), the antennules (£),
the eye stalks (I), and the six somites to which they are
attached (I — VI), lie in front of the boundary and com-
pose the head.

Another important point to be noticed is that, in front
of the mouth, the sternum of the antennary somite (fig.
43, III) is inclined at an angle of 60° or 70° to the direc-
tion of the sterna behind the mouth. The sternum of the
antennulary somite (II) is at right angles to the latter ; and
that of the eyes (I) looks upwards as well as forwards.
Hence, the front of the head beneath the rostrum, though
it looks forwards, or even upwards, is homologous with the
sternal aspect of the other somites. It is for this reason
that the feelers and the eyestalks take a direction so dif-
ferent from that of the other appendages. The change
of aspect of the sternal surface in front of the mouth,
thus effected, is what is termed the cephalic flexure.

Since the skeleton which invests the trunk of the cray-
fish is made up of a twenty-fold repetition of somites,
homologous with those of the abdomen, we may expect
to find that the appendages of the thorax and of the head,
however unlike they may seem to be to those of the ab-
domen, are nevertheless reducible to the same funda-
mental plan.

M 2

164 THE MORPHOLOGY OF THE COMMON CRAYFISH.

The third maxillipede is one of the most complete of
these appendages, and may be advantageously made the
starting point of the study of the whole series.

Fig. 44. — Astacus flnviatUis.—The third or external maxillipede of the
left side ( x 3). e, lamina, and b?% branchial filaments of the
podobranchia ; cscp, coxopodite ; czs, coxopoditic setse ; bp, basi-
podite ; ex, exopodite ; ip, ischiopodite ; mp, meropodite ; cp9
carpopodite ; propodite ; dp, dactylopodite.

Neglecting details for the moment, it may be said that
the appendage consists of a basal portion (fig. 44, cxp, bp),

THE MAXILLIPEDES.

165

with two terminal divisions (ip to dp, and ex), which are
directed forwards, below the mouth, and a third, lateral
appendage (e, br), which runs up, beneath the carapace,
into the branchial chamber. The latter is the gill, or podo-
branchia, attached to this limb, and it is something not
represented in the abdominal limbs. But, with regard
to the rest of the maxillipede, it is obvious that the
basal portion (cxp, bp) represents the protopodite, and
the two terminal divisions the endopodite and the exo-
podite respectively. It has been observed that, in the
abdominal appendages, the extent to which segmentation
occurs in homologous parts varies indefinitely ; an endo-
podite, for example, may be a continuous plate, or may
be subdivided into many joints. In the maxillipede, the
basal portion is divided into two joints ; and, as in the
abdominal limb, the first, or that which articulates with
the thorax, is termed the coxopodite (cxp), while the second
is the basipodite (bp). The stout, leg-like endopodite
appears to be the direct continuation of the basipodite ;
while the much more narrow and slender exopodite arti-
culates with its outer side. The exopodite (ex) is by no
means unlike one of the exopodites of the abdominal
limbs, consisting as it does of an undivided base and a
many-jointed terminal filament. The endopodite, on the
contrary, is strong and massive, and is divided into five
joints, named, from that nearest to the base onwards,
ischiopodite (ip), meropodite (mp), carpopodite (cp), propo-
dite (pp), and dactylopodite (dp).

166 THE MORPHOLOGY OF THE COMMON CRAYFISH.

The second maxillipede (fig. 45, B) has essentially the
same composition as the first, but the exopodite (ex) is
relatively larger, the endopodite (ip — dp) smaller and
softer; and, while the ischiopodite (ip) is the longest
joint in the third maxillipede, it is the meropodite (mp)
which is longest in the second. In the first maxillipede

Fig. 45,—Astacus fluviatilis.—A, the first ; B, the second maxillipede
of the left side ( x 3). cxp, coxopodite ; bp, basipodite ; e, br> po-
dobranchia ; ep, epipodite ; en, endopodite ; ex, exopodite ; ip, is-
chiopodite ; nip, meropodite ; cp, carpopodite ; pp, propodite ; dp,
dactylopodite.

(fig. 45, A) a great modification has taken place. The
coxopodite (cxp) and the basipodite (bp) are broad thin
plates with setose cutting edges, while the endopodite
(en) is short and only two-jointed, and the undivided
portion of the exopodite (ex) is very long. The place of

PODOBRANCHLE AND EPIPODITES. 167

the podobranchia is taken by a broad soft membranous
plate entirely devoid of branchial filaments (ep). Thus,
in the series of the thoracic limbs, on passing forwards
from the third maxillipede, we find that though the plan
of the appendages remains the same ; (1) the protopodite
increases in relative size ; (2) the endopodite diminishes ;
(3) the exopodite increases ; (4) the podobranchia finally
takes the form of a broad membranous plate and loses its
branchial filaments.

Writers on descriptive Zoology usually refer to the
parts of the maxillipedes under different names from those
which are employed here. The protopodite and the endo-
podite taken together are commonly called the stem of
the maxillipede, while the exopodite is the palp, and the
metamorphosed podobranchia, the real nature of which
is not recognised, is termed the flagellum.

When the comparison of the maxillipedes with the
abdominal members, however, had shown the funda-
mental uniformity of composition of the two, it became
desirable to invent a nomenclature of the homologous
parts which should be capable of a general application.
The names of protopodite, endopodite, exopodite, which
I have adopted as the equivalents of the " stem" and the
"palp," were proposed by Milne-Edwards, who at the
same time suggested epipodite for the " flagellum." And
the lamellar process of the first maxillipede is now very
generally termed an epipodite ; while the podobranchiae,
which have exactly the same relations to the following

168 THE MORPHOLOGY OF THE COMMON CRAYFISH,

limbs, are spoken of as if they were totally different
structures, under the name of branchiae or gills.

The flagellum or epipodite of the first maxillipede,
however, is nothing but the slightly modified stem of a
podobranchia, which has lost its branchial filaments;
but the term " epipodite" may be conveniently used for
podobranchioe thus modified. Unfortunately, the same
term is applied to certain lamelliform portions of the
branchiae of other Crustacea, which answer to the laminae
of the crayfishes' branchiae ; and this ambiguity must be
borne in mind, though it is of no great moment.

On examining an appendage from that part of the
thorax which lies behind the third maxillipede, say, for
example, the sixth thoracic limb (the second walking leg)
(fig. 46), the two joints of the protopodite and the five
joints of the endopodite are at once identifiable, and so
is the podobranchia; but the exopodite has vanished
altogether. In the eighth, or last, thoracic limb, the
podobranchia has also disappeared. The fifth and
sixth limbs also differ from the seventh and eighth,
in being chelate ; that is to say, one angle of the distal
end of the propodite is prolonged and forms the fixed leg
of the pincer. The produced angle is that which is
turned downwards when the limb is fully extended
(fig. 46). In the forceps, the great chela is formed in
just the same way ; the only important difference lies in
the fact that, as in the external maxillipede, the basipo-
dite and the ischiopodite are immoveably united. Thus,

Fig. 40. — Astacus fluviatilis. — The second ambulatory leg of the left
side ( x 3). cxj), coxopodite ; hp, basipodite ; hr, gill ; cxs, coxo-
poditic seta3 ; c, lamina of gill or epipodite ; ip, ischiopodite ; mp,
meiopodite ; cp, carpopodite ; pp, propodite ; dp, dactylopodite.

170 THE MORPHOLOGY OF THE COMMON CRAYFISH.

the limbs of the thorax are all reducible to the same type
as those of the abdomen, if we suppose that, in the
posterior five pair, the exopodites are suppressed ; and
that, in all but the last, podobranchue are superadded.

Turning to the appendages of the head, the second
maxilla (fig. 47, C) presents a further modification of the
disposition of the parts seen in the first maxillipede.
The coxopoclite (cxp) and the basipodite (bp) are still
thinner and more lamellar, and are subdivided by deep
fissures which extend from their inner edges. The
endopodite (en) is very small and undivided. In the
place of the exopodite and the epipodite there is only
one great plate, the scaphognathite (sg) which either
is such an epipodite as that of the first maxillipede
with its anterior basal process much enlarged, or repre-
sents both the exopodite and the epipodite. In the first
maxilla (B), the exopodite and the epipodite have dis-
appeared, and the endopodite (en) is insignificant and
unjointed. In the mandibles (A), the representative of
the protopodite is strong and transversely elongated. Its
broad inner or oral end presents a semicircular mastica-
tory surface divided by a deep longitudinal groove into
two toothed ridges. The one of these follows the con-
vex anterior or inferior contour of the masticatory surface,
projects far beyond the other, and is provided with a sharp
serrated edge; the other (fig. 43, a) gives rise to the straight
posterior or superior contour of the masticatory surface,
and is more obtusely tuberculated. In front, the inner

THE MANDIBLES AND MAXILLA.

171

ridge is continued into a process by which the mandible
articulates with the epistoma (fig. 47, A, ar). The endo-

FlG. 47. — Astacus fiumatilis — A, mandible ; B, first maxilla ; C, second
maxilla of the left side ( x 3). ar, internal, and ar', external
articular process of the mandible ; bj), basipodite ; cxp, coxopodite ;
en, endopodite ; p, palp of the mandible ; sg, scaphognathite ; x,
internal process of the first maxilla.

podite is represented by the three-jointed palp (p), the
terminal joint of which is oval and beset with numerous
strong setaB, which are especially abundant along its
anterior edge.

172 THE MORPHOLOGY OF THE COMMON CRAYFISH.

In the antenna (fig. 48, C) the protopodite is two-
jointed. The basal segment is small, and its ventral
face presents the conical prominence on the posterior
aspect of which is the aperture of the duct of the renal
gland (gg). The terminal segment is larger and is subdi-
vided by deep longitudinal folds, one upon the dorsal and

Fig. 48. — Astacus fiuviatilis. — A, eye-stalk ; B, antennule ; C, antenna
of the left side ( x 3). a, spine of the basal joint of the antennule ;
c, corneal surface of the eye ; cxp, exopodite or squame of the
antenna ; gg, aperture of the duct of the green gland.

one upon the ventral face, into two moieties which are
more or less moveable upon one another. In front and
externally it bears the broad flat squame (exp) of the an-
tenna, as an exopodite. Internally, the long annulated
" feeler " which represents the endopodite, is connected
with it by two stout basal segments.

THE ANTENNULES AND THE EYESTALKS. 173

The antennule (fig. 48, B) has a three-jointed stem
and two terminal annulated filaments, the outer of which
is thicker and longer than the inner, and lies rather above
as well as external to the latter. The peculiar form of
the basal segment of the stem of the antennule has already
been adverted to (p. 116). It is longer than the other
two segments put together, and near the anterior end
its sternal edge is produced into a single strong spine (a).
The stem of the antennule answers to the protopodite of
the other limbs, though its division into three joints is
unusual ; the two terminal annulated filaments represent
the endopodite and the exopodite.

Finally, the eyestalk (A) has just the same structure
as the protopodite of an abdominal limb, having a short
basal and a long cylindrical terminal joint.

From this brief statement of the characters of the appen-
dages, it is clear that, in whatever sense it is allowable to
say that the appendages of the abdomen are constructed
upon one plan, which is modified in execution by the
excess of development of one part over another, or by the
suppression of parts, or by the coalescence of one part
with another, it is allowable to say that all the appen-
dages are constructed on the same plan, and are modified
on similar principles. Given a general type of appendage
consisting of a protopodite, bearing a podobranchia, an
endopodite and an exopodite, all the actual appendages
are readily derivable from that type.

174 THE MORPHOLOGY OF THE COMMON CRAYFISH.

In addition, therefore, to their adaptation to the pur-
poses which they subserve, the parts of the skeleton
of the crayfish show a unity in diversity, such as, if
the animal were a piece of human workmanship, would
lead us to suppose that the artificer was under an obliga-
tion not merely to make a machine capable of doing cer-
tain kinds of work, but to subordinate the nature and
arrangement of the mechanism to certain fixed architec-
tural conditions.

The lesson thus taught by the skeletal organs is re-
iterated and enforced by the study of the nervous and the
muscular systems. As the skeleton of the whole body is
capable of resolution into the skeletons of twenty separate
metameres, variously modified and combined ; so is the
entire ganglionic chain resolvable into twenty pairs of
ganglia various in size, distant in this region and
approximated in that ; and so is the muscular system
of the trunk conceivable as the sum of twenty
myotomes or segments of the muscular system appro-
priate to a metamere, variously modified according to
the degree of mobility of the different r3gions of the
organism.

The building up of the body by the repetition and
the modification of a few similar parts, which is so ob-
vious from the study of the general form of the somites
and of their appendages, is still more remarkably illus-
trated, if we pursue our investigations further, and trace

HISTOLOGY. TISSUES.

175

out the more intimate structure of these parts. The
tough, outer coat, which has been termed the cuticula,
except so far as it presents different degrees of hardness,
from the presence or absence of calcareous salts, is
obviously everywhere of the same nature ; and, by
macerating a crayfish in caustic alkali, which destroys all
its other components of the body, it will be readily
enough seen that a continuation of the cuticular layer
passes in at the mouth and the vent, and lines the
alimentary canal; furthermore, that processes of the
cuticle covering various parts of the trunk and limbs
extend inwards, and afford surfaces of attachment to the
muscles, as the apodemata and tendons. In technical
language, the cuticular substance which thus enters so
largely into the composition of the bodily fabric of the
crayfish is called a tissue.

The flesh, or muscle, is another kind of tissue, which
is readily enough distinguished from cuticular tissue by
the naked eye ; but, for a complete discrimination of
all the different tissues, recourse must be had to the
microscope, the application of which to the study of
the ultimate optical characters of the morphological
constituents of the body has given rise to that branch
of morphology which is known as Histology.

If we count every formed element of the body, which
is separable from the rest by definite characters, as a
tissue, there are no more than eight kinds of such tissues
in the crayfish ; that is to say, every solid constituent

176 THE MORPHOLOGY OF THE COMMON CRAYFISH.

of the body consists of one or more of the following eight
histological groups : —

1. Blood corpuscles ; 2. Epithelium ; 3. Connective
tissue; 4. Muscle ; 5. Nerve; 6. Ova; 7. Spermatozoa;
8. Cuticle.

1. A drop of freshly-drawn blood of the crayfish con-
tains multitudes of small particles, the blood corpuscles,

Fig. 49.— Astacits flmiatilis.— The corpuscles of the blood, highly
magnified. 1 — S, show the changes undergone by a single cor-
puscle during a quarter of an hour ; the nucleus ; 9 and 10
are corpuscles killed by magenta, and haviDg the nucleus deeply
stained by the colouring matter.

which rarely exceed l-700th, and usually are about
l-1000th, of an inch in diameter (fig. 49). They
are sometimes pale and delicate, but generally more or
less dark, from containing a number of minute strongly
refracting granules, and they are ordinarily exceedingly
irregular in form. If one of them is watched continu-

EPITHELIUM.

177

ously for two or three minutes, its shape will be seen to
undergo the constant but slow changes to which passing
reference has already been made (p. 69). One or other of
the irregular prolongations will be drawn in, and another
thrown out elsewhere. The corpuscle, in fact, has an
inherent contractility, like one of those low organisms,
known as an Amoeba, whence its motions are frequently
called amoebiform. In its interior, an ill-marked oval
contour may be seen, indicating the presence of a sphe-
roidal body, about l-2000th of an inch in diameter, which
is the nucleus of the corpuscle (n). The addition of some
re-agents, such as dilute acetic acid, causes the corpuscles
at once to assume a spherical shape, and renders the nuc-
leus very conspicuous (fig. 49, 9 and 10). The blood
corpuscle is, in fact, a simple nucleated cell, composed
of a contractile protoplasmic mass, investing a nucleus ;
it is suspended freely in the blood ; and, though as
much a part of the crayfish organism as any other of
its histological elements, leads a quasi-independent ex-
istence in that fluid.

2. Under the general name of epithelium, may be in-
cluded a form of tissue, which everywhere underlies the
exoskeleton (where it corresponds with the epidermis of the
higher animals), and the cuticular lining of the alimen-
tary canal, extending thence into the hepatic c£eca. It is
further met with in the generative organs, and in the green
gland. Where it forms the subcuticular layer of the
integument and of the alimentary canal, it is found to

N

178 THE MORPHOLOGY OF THE COMMON CRAYFISH,

consist of a protoplasmic substance (fig. 50), in which close
set nuclei (n) are imbedded. If a number of blood corpus-
cles could be supposed to be closely aggregated together
into a continuous sheet, they would give rise to such a
structure as this; and there can be no doubt that it
really is an aggregate of nucleated cells, though the
limits between the individual cells are rarely visible in the
fresh state. In the liver, however, the cells grow, and
become detached from one another in the wider and lower

Fig. 50. — Astacus fltiviatilis. — Epithelium, from the epidermic layer
subjacent to the cuticle, highly magnified. A, in vertical section ;
B, from the surface, n, nuclei.

parts of the caeca, and their essential nature is thus
obvious.

3. Immediately beneath the epithelial layer follows a
tissue, disposed in bands or sheets, which extend to the
subjacent parts, invest them, and connect one with
another. Hence this is called connective tissue.

The connective tissue presents itself under three forms.
In the first there is a transparent homogeneous-looking
matrix, or ground substance, through which are scattered
many nuclei. In fact, this form of connective tissue

CONNECTIVE TISSUE.

179

very closely resembles the epithelial tissue, except that
the intervals between the nuclei are wider, and that the
substance in which they are imbedded cannot be broken
up into a separate cell-body for each nucleus. In the
second form (fig. 51, A) the matrix exhibits fine wavy
parallel lines, as if it were marked out into imperfect

Fig. 51. — Astacus fluviatilu. — Connective tissue ; A, second form ; B,
third form, a, cavities ; n, nuclei. Highly magnified.

fibres. In this form, as in the next to be described,
more or less spherical cavities, which contain a clear
fluid, are excavated in the matrix ; and the number of

N 2

180 THE MORPHOLOGY OF THE COMMON CRAYFISH.

these is sometimes so great, that the matrix is propor-
tionally very much reduced, and the structure acquires a
close superficial similarity to that of the parenchyma of
plants. This is still more the case with a third form, in
which the matrix itself is marked off into elongated or
rounded masses, each of which has a nucleus in its
interior (fig. 51, jB). Under one form or another, the
connective tissue extends throughout the body, ensheath-
ing the various organs, and forming the walls of the blood
sinuses.

The third form is particularly abundant in the outer
investment of the heart, the arteries, the alimentary
canal, and the nervous centres. About the cerebral and
anterior thoracic ganglia, and on the exterior of the
heart, it usually contains more or less fatty matter. In
these regions, many of the nuclei, in fact, are hidden by
the accumulation round them of granules of various
sizes, some of which are composed of fat, while others
consist of a proteinaceous material. These aggregates
of granules are usually spheroidal ; and, with the matrix in
which they are imbedded and the nucleus which they sur-
round, they are often readily detached when a portion of
the connective tissue is teased out, and are then known as
fat cells. From what has been said respecting the dis-
tribution of the connective tissue, it is obvious that if
all the other tissues could be removed, this tissue would
form a continuous whole, and represent a sort of model,
or cast, of the whole body of the crayfish.

MUSCULAR TISSUE.

181

4. The muscular tissue of the crayfish always has the
form of bands or fibres, of very various thickness, marked,
when viewed by transmitted light, by alternate darker and

Fia. 52.—Astacus Jluviatilis.—A, a single muscular fibre, transverse
diameter ^th of an inch ; B, a portion of the same more highly
magnified ; C, a smaller portion treated with alcohol and acetic
acid still more highly magnified ; D and E, the splitting up of a
part of a fibre, treated with picro-carmine, into fibrillce ; F, the
connection of a nervous with a muscular fibre which has been
treated with alcohol and acetic acid, a, darker, and ?>, clearer portions
of the fibrillflc ; n't nuclei ; nv, nerve fibre ; sy sarcolemma ; t, tendon ;
1 — 5, successive dark granular stria3 answering to the granular
portions, a, of each fibrilhw

182 THE MORPHOLOGY OF THE COMMON CBAYFISH.

lighter striae, transversely to the axis of the fibres
(fig. 52 A). The distance of the transverse striae from one
another varies with the condition of the muscle, from
1 -4,000th of an inch in the quiescent state to as little as
1 -30,000th of an inch in that of extreme contraction.
The more delicate muscular fibres, like those of the
heart and those of the intestine, are imbedded in the
connective tissue of the organ, but have no special sheaths.

Fig. 53. — Astacus fluvlatilis. — A, living muscular fibres very highly
magnified ; B9 a fibrilla treated with solution of sodium chloride ;
C, a fibrilla treated with strong nitric acid, s, septal lines ; sz9
septal zones ; is, interseptal zones ; a9 transverse line in the inter-
septal zone.

The fibres which make up the more conspicuous muscles
of the trunk and limbs, on the other hand, are much
larger, and are invested by a thin, transparent, structure-
less sheath, which is termed the sarcolemma. Nuclei
are scattered, at intervals, through the striated substance
of the muscle ; and, in the larger muscular fibres, a layer
of nucleated protoplasm lies between the sarcolemma and
the striated muscle substance*

MUSCULAB TISSUE.

183

This much is readily seen in a specimen of muscular
fibre taken from any part of the body, and whether alive
or dead. But the results of the ultimate optical analysis
of these appearances, and the conclusions respecting the
normal structure of striped muscle which may be legiti-
mately drawn from them, have been the subjects of much
controversy.

Quiescent muscular fibres from the chela of the forceps
of a crayfish, examined while still living, without the
addition of any extraneous fluid, and with magnifying
powers of not less than seven or eight hundred diameters,
exhibit the following appearance. At intervals of about
1 -4000th of an inch, very delicate but dark and well-
defined transverse lines are visible ; and these, on careful
focussing, appear beaded, as if they were made of a series
of close-set minute granules not more than 1 -20,000th
to 1 -30,000th of an inch in diameter. These may be
termed the septal lines (fig. 52, D and E, a; C, 1 — 5 ;
fig. 53, s). On each side of every septal line there
is a very narrow perfectly transparent band, which may
be distinguished as the septal zone (fig. 53, sz). Upon
this follows a relatively broad band of a substance which
has a semi-transparent aspect, like very finely ground
glass, and hence appears somewhat dark relatively to the
septal zone. Upon this inter-septal zone (i s) follows
another septal zone, then a septal line, another septal
zone, an inter-septal zone, and so on throughout the
whole length of the fibre.

184? THE MORPHOLOGY OF THE COMMON CRAYFISH.

In the perfectly unaltered state of the muscle no other
transverse markings than these are discernible. But it is
always possible to observe certain longitudinal markings ;
and these are of three kinds. In the first place, the nuclei
which, in the perfectly fresh muscle, are delicate trans-
parent oval bodies, are lodged in spaces which taper off at
each end into narrow longitudinal clefts (fig. 52, A, B). Pro-
longations of the protoplasmic sheath of the fibre extend
inwards and fill these clefts. Secondly, there are similai
clefts interposed between these, but narrow and merely
linear throughout. Sometimes these clefts contain fine
granules. Thirdly, even in the perfectly fresh muscle,
extremely faint parallel longitudinal striae 1-7, 000th
of an inch, or thereabouts, apart, traverse the several
zones, so that longer or shorter segments of the
successive septal lines are inclosed between them. A
transverse section of the muscle appears divided into
rounded or polygonal arese of the same diameter, sepa-
rated from one another here and there by minute inter-
stices. Moreover, on examination of perfectly fresh
muscle with high magnifying powers, the septal lines are
hardly ever straight for any distance, but are broken up
into short lengths, which answer to one or more of the
longitudinal divisions, and stand at slightly different
heights.

The only conclusion to be drawn from these appear-
ances seems to me to be that the substance of the muscle
is composed of distinct fibrils ; and that the longitudinal

MUSCULAR TISSUE.

185

striae and the rounded arese of the transverse section are
simply the optical expressions of the boundaries of these
fibrils. In the perfectly unaltered state of the tissue,
however, the fibrils are so closely packed that their
boundaries are scarcely discernible.

Thus each muscular fibre may be regarded as com-
posed of larger and smaller bundles of fibrils im-
bedded in a nucleated protoplasmic framework which
ensheaths the whole and is itself invested by the sar-
colemma.

As the fibre dies, the nuclei acquire hard, dark contours
and their contents become granular, while at the same
time the fibrils acquire sharp and well-defined boundaries.
In fact, the fibre may now be readily teased out with
needles, and the fibrils isolated.

In muscle which has been treated with various reagents,
such as alcohol, nitric acid, or solution of common salt,
the fibrils themselves may be split up into filaments of
extreme tenuity, each of which appears to answer to
one of the granules of the septal lines. Such an
isolated muscle filament looks like a very fine thread
carrying minute beads at regular intervals.

The septal lines resist most reagents, and remain
visible in muscular fibres which have been subjected to
various modes of treatment ; but they may have the
appearance of continuous bars, or be more or less com-
pletely resolved into separate granules, according to cir-
cumstances. On the other hand, what is to be seen in

186 THE MORPHOLOGY OF THE COMMON CRAYFISH.

the interspace between every two septal lines depends
upon the reagent employed. With dilute acids and
strong solutions of salt, the inter-septal substance swells
up and becomes transparent, so that it ceases to be dis-
tinguishable from the septal zone. At the same time a
distinct but faint transverse line may appear in the
middle of its length. Strong nitric acid, on the con-
trary, renders the inter-septal substance more opaque,
and the septal zones consequently appear very well
defined.

In living and recently dead muscle, as well as in
muscles which have been preserved in spirit or hardened
with nitric acid, the inter-septal zones polarize light; and
hence, in the dark field of the polarizing microscope, the
fibre appears crossed by bright bands, which correspond
with the inter-septal zones, or at any rate, with the
middle parts of them. The substance which forms the
septal zones, on the contrary, produces no such effect,
and consequently remains dark ; while the septal lines
again have the same property as the inter-septal sub-
stance, though in a less degree.

In fibres which have been acted upon by solution of
salt, or dilute acids, the inter-septal zones have lost
their polarizing property. As we know that the reagents
in question dissolve the peculiar constituent of muscle,
myosin, it is to be concluded that the inter-septal sub-
stance is chiefly composed of myosin.

Thus a fibril may be considered to be made up of

NERVOUS TISSUE.

187

segments of different material arranged in regular order ;
S — sz — IS — sz — S — sz — IS — sz — S : S representing the
septal line ; sz, the septal zone ; IS, the inter-septal zone.
Of these, IS is the chief if not the only seat of the
myosin; what the composition of sz and of S may be
is uncertain, but the supposition, that, in the living
muscle, sz is a mere fluid, appears to me to be wholly
inadmissible.

When living muscle contracts, the inter-septal zones
become shorter and wider and their margins darker,
while the septal zones and the septal lines tend to
become effaced — as it appears to me simply in conse-
quence of the approximation of the lateral margins of
the inter-septal zones. It is probable that the sub-
stance of the intermediate zone is the chief, if not the
only, seat of the activity of the muscle during con-
traction.

5. The elements of the nervous tissue are of two kinds,
nerve-cells, and nerve fibres ; the former are found in the
ganglia, and they vary very much in size (fig. 54, B). Each
ganglionic corpuscle consists of a cell body produced
into one or more processes which sometimes, if not
always, end in nerve fibres. A large, clear spherical
nucleus is seen in the interior of the nerve-cell ; and
in the centre of this is a well defined, small round
particle, the nucleolus. The corpuscle, when isolated,
is often surrounded by a sort of sheath of small nucle-
ated cells.

188 THE MORPHOLOGY OF THE COMMON CRAYFISH.

The nerve fibres (fig. 55) of the crayfish are remarkable
for the large size which some of them attain. In the
central nervous system a few reach as much as l-200th of
an inch in diameter; and fibres of l-300th or l-400th of

Fig. 54, — Astacus fluviatilis. — A, one of the (double) abdominal gan-
glia, with the nerves connected with it ( x 25) ; B, a nerve cell or
ganglionic corpuscle ( x 250). a, sheath of the nerves ; c, sheath
of the ganglion ; co, co'. commissural cords connecting the ganglia
with those in front, and those behind them. gl.c. points to the
ganglionic corpuscles of the ganglia ; n, nerve fibres.

an inch in diameter are not rare in the main branches.
Each fibre is a tube, formed of a strong and elastic, some-
times fibrillated, sheath, in which nuclei are imbedded
at irregular intervals ; and, when the nerve trunk gives

NERVOUS TISSUE.

189

off a branch, more or fewer of these tubes divide, sending
off a prolongation into each branch.

When quite fresh, the contents of the tubes are per-
fectly pellucid, and without the least indication of struc-
ture ; and, from the manner in which the contents

71

Fig. 55.— Astacus Jluviatilis. — Three nerve fibres, with the £onnective
tissue in which they are imbedded (magnified about 250 diameters) ;
ft, nuclei.

exude from the cut ends of the tubes, it is evident that
they consist of a fluid of gelatinous consistency. As the
fibre dies, and under the influence of water and of many
chemical re-agents, the contents break up into globules
or become turbid and finely granular.

Where motor nerve fibres terminate in the muscles to
which they are distributed, the sheath of each fibre
becomes continuous with the sarcolemma of the muscle,
and the subjacent protoplasm is commonly raised into a
small prominence which contains several nuclei (fig. 52, F).
These are called the terminal or motor plates.

100 THE MORPHOLOGY OF THE COMMON CRAYFISH.

6, 7. The ova and the spermatozoa have already been
described (pp. 132—135).

It will be observed that the blood corpuscles, the
epithelial tissues, the ganglionic corpuscles, the ova
and the spermatozoa, are all demonstrably nucleated
cells, more or less modified. The first form of con-
nective tissue is so similar to epithelial tissue, that it may
obviously be regarded as an aggregate of as many cells as
it presents nuclei, the matrix representing the more or
less modified and confluent bodies of the cells, or products
of these. But if this be so, then the second and third
forms have a similar composition, except so far as the
matrix of the cells has become fibrillated, or vacuolated,
or marked off into masses corresponding with the several
nuclei. By a parity of reasoning, muscular tissue may
also be considered a cell aggregate, in which the inter-
nuclear substance has become converted into striated
muscle; while, in the nerve fibres, a like process of
metamorphosis may have given rise to the pellucid
gelatinous nerve substance. But, if we accept the
conclusions thus suggested by the comparison of the
various tissues with one another, it follows that every
histological element, which has now been mentioned,
is either a simple nucleated cell, a modified nucleated
cell, or a more or less modified cell aggregate. In
other words, every tissue is resolvable into nucleated
cells.

E

Fig. 56. — Astacus fluviatilis. — The structure of the cuticle. A. trans-
verse section of a joint of the forceps ( x 4) ; s, setae ; Z>, a por-
tion of the same ( x 80) ; C, a portion of B more highly magnified.
a, epiostracum ; b, ectostracum ; c, endostracum ; d, canal of seta ;
e, canals filled with air ; s, seta. D, section of an intersternal
membrane of the abdomen, the portion to the right in the natural
condition, the remainder pulled apart with needles ( x 20) ; F,
small portion of the same, highly magnified ; a, intermediate sub'
stance ; b, laminae. F, a seta, highly magnified ; a and b, joints.

192 THE MORPHOLOGY OF THE COMMON CRAYFISH.

A notable exception to this generalisation, however,
obtains in the case of the cuticular structures, in which
no cellular components are discoverable. In its simplest
form, such as that presented by the lining of the in-
testine, the cuticle is a delicate, transparent membrane,
thrown off from the surface of the subjacent cells, either
by a process of exudation, or by the chemical transfor-
mation of their superficial layer. No pores are discern-
ible in this membrane, but scattered over its surface
there are oval patches of extremely minute, sharp conical
processes, which are rarely more than 1-5, 000th of an
inch long. Where the cuticle is thicker, as in the
stomach and in the exoskeleton, it presents a stratified
appearance, as if it were composed of a number of lamina,
of varying thickness, which had been successively thrown
off from the subjacent cells.

Where the cuticular layer of the integument is un-
calcified, for example, between the sterna of the abdo-
minal somites, it presents an external, thin, dense,
wrinkled lamina, the epiostracum, followed by a soft
substance, which, on vertical section, presents numerous
alternately more transparent and more opaque bands,
which run parallel with one another and with the free
surfaces of the slice (fig. 56, D). These bands are very
close-set, often not more than l-5000th of an inch apart
near the outer and the inner surfaces, but in the middle
of the section they are more distant.

If a thin vertical slice of the soft cuticle is geutly

CUTICULAR TISSUE.

193

pulled with needles in the direction of its depth, it
stretches to eight or ten times its previous diameter,
the clear intervals between the dark bands becoming
proportionally enlarged, especially in the middle of the
slice, wThile the dark bands themselves become apparently
thinner, and more sharply defined. The dark bands
may then be readily drawn to a distance of as much as
l-300th of an inch from one another ; but if the slice is
stretched further, it splits along, or close to, one of the
dark lines. The whole of the cuticular layer is stained
by such colouring matters as hematoxylin ; and, as the
dark bands become more deeply coloured than the inter-
mediate transparent substance, the transverse stratifi-
cation is made very manifest by this treatment.

Examined with a high magnifying power, the trans-
parent substance is seen to be traversed by close-set,
faint, vertical lines, while the dark bands are shown to
be produced by the cut edges of delicate laminae, having
a finely striated appearance, as if they were composed
of delicate parallel wavy fibrillae.

In the calcified parts of the exoskeleton a thin, tough,
wrinkled epiostracum (fig. 56, B, a), and, subjacent to
this, a number of alternately lighter and darker strata
are similarly discernible : though all but the innermost
laminae are hardened by a deposit of calcareous salts,
which are generally evenly diffused, but sometimes take
the shape of rounded masses with irregular contours.

Immediately beneath the epiostracum, there is a zone

o

194 THE MORPHOLOGY OF THE COMMON CRAYFISH.

which may occupy a sixth or a seventh of the thickness
of the whole, which is more transparent than the rest,
and often presents hardly any trace of horizontal or
vertical striation. When it appears laminated, the strata
are very thin. This zone may be distinguished as the
ectostracum (6), from the endostracum (c), which makes
up the rest of the exoskeleton. In the outer part of the
endostracum, the strata are distinct, and may be as much
as 1 -500th of an inch thick, but in the inner part they
become very thin, and the lines which separate them
may be not more than 1- 8000th of an inch apart.
Fine, parallel, close-set, vertical striae (e) traverse all the
strata of the endostracum, and may usually be traced
through the ectostracum, though they are always faint,
and sometimes hardly discernible, in this region. When
a high magnifying power is employed, it is seen that
these striae, which are about l-7000th of an inch apart,
are not straight, but that they present regular short un-
dulations, the alternate convexities and concavities of
which correspond with the light and the dark bands
respectively.

If the hard exoskeleton has been allowed to become
partially or wholly dry before the section is made, the
latter will look white by reflected and black by trans-
mitted light, in consequence of the places of the striae
being taken by threads of air of such extreme tenuity,
that they may measure not more than l-30,000th of an
inch in diameter. It is to be concluded, therefore, that

CUTICULAR TISSUE.

195

the strise are the optical indications of parallel undulating
canals which traverse the successive strata of the cuticle,
and are ordinarily occupied by a fluid. When this dries
up, the surrounding air enters, and more or less com-
pletely fills the tubes. And that this is really the case
may be proved by making very thin sections parallel with
the face of the exoskeleton, for these exhibit innumerable
minute perforations, set at regular distances from one
another, which correspond with the intervals between the
striae in the vertical section ; and sometimes the contours
of the arese which separate the apertures are so well
defined as to suggest a pavement of minute angular
blocks, the corners of which do not quite meet.

When a portion of the hard exoskeleton is decalcified,
a chitinous substance remains, which presents the same
structure as that just described, except that the epios-
tracum is more distinct ; while the ectostracum appears
made up of very thin laminae, and the tubes are repre-
sented by delicate striae, which appear coarser in the
region of the dark zones. As in the naturally soft parts
of the exoskeleton, the decalcified cuticle may be split
into flakes, and the pores are then seen to be disposed
in distinct arese circumscribed by clear polygonal borders.
These perforated areae appear to correspond with indi-
vidual cells of the ectoderm, and the canals thus answer
to the so-called " pore-canals,'' which are common in
cuticular structures and in tli3 walls of many cells
which bound free surfaces.

o 2

196 THE MORPHOLOGY OF THE COMMON CRAYFISH.

The whole exoskeleton of the crayfish is, in fact,
produced by the cells which underlie it, either by the
exudation of a chitinous substance, which subsequently
hardens, from them; or, as is more probable, by the
chemical metamorphosis of a superficial zone of the
bodies of the cells into chitin. However this may be,
the cuticular products of adjacent cells at first form a
simple, continuous, thin pellicle. A continuation of the
process by which it was originated increases the thick-
ness of the cuticle ; but the material thus added to the
inner surface of the latter is not always of the same
nature, but is alternately denser and softer. The denser
material gives rise to the tough laminae, the softer to
the intermediate transparent substance. But the quan-
tity of the latter is at first very small, whence the more
external laminae are in close apposition. Subsequently
the quantity of the intermediate substance increases, and
gives rise to the thick stratification of the middle region,
while it remains insignificant in the inner region of the
exoskeleton.

The cuticular structures of the crayfish differ from
the nails, hairs, hoofs, and similar hard parts of the
higher animals, insomuch as the latter consist of aggrega-
tions of cells, the bodies of which have been metamor-
phosed into horny matter. The cuticle, with all its
dependencies, on the contrary, though no less dependent
on cells for its existence, is a derivative product, the
formation of which does not involve the complete meta-

CUTICULAR TISSUE.

197

morphosis and consequent destruction of the cells to
which it owes its origin.

The calcareous salts by which the calcified exoskeleton
is hardened can only be supplied by the infiltration of a
fluid in which they are dissolved from the blood ; while
the distinctive structural characters of the epiostracum,
the ectostracum, and the endostracum, are the results of
a process of metamorphosis which goes on pari passu with
this infiltration. To what extent this metamorphosis is
a properly vital process ; and to what extent it is explic-
able by the ordinary physical and chemical properties of
the animal membrane on the one hand, and the mineral
salts on the other, is a curious, and at present, un-
solved problem.

The outer surface of the cuticle is rarely smooth.
Generally it is more or less obviously ridged or tubercu-
lated ; and, in addition, presents coarser or finer hair-
like processes which exhibit every gradation from a fine
microscopic down to stout spines. As these processes,
though so similar to hairs in general appearance, are
essentially different from the structures known as hairs
in the higher animals, it is better to speak of them as
setce.

These setae (fig. 56, F) are sometimes short, slender,
conical filaments, the surface of which is quite smooth ;
sometimes the surface is produced into minute serra-
tions, or scale-like prominences, disposed in two or more
series ; in other sete, the axis gives off slender lateral

198 THE MORPHOLOGY OF THE COMMON CRAYFISH.

branches ; and in the most complicated form the branches
are ornamented with lateral branchlets. For a certain
distance from the base of the seta, its surface is usually
smooth, even when the rest of its extent is ornamented
with scales or branches. Moreover, the basal part of the
seta is marked off from its apical moiety by a sort of
joint which is indicated by a slight constriction, or by a
peculiarity in the structure of the cuticula at this point.
A seta almost always takes its origin from the bottom of
a depression or pit of the layer of cuticle, from which it is
developed, and at its junction with the latter it is generally
thin and flexible, so that the seta moves easily in its
socket. Each seta contains a cavit}^, the boundaries of
which generally follow the outer contours of the seta. In
a good many of the setae, however, the parietes, near the
base of the seta, are thickened in such a manner as
almost, or completely, to obliterate the central cavity.
However thick the cuticle may be at the point from
which the setae take their origin, it is always traversed
by a funnel-shaped canal (fig. 56, B, d)y which usually
expands beneath the base of the seta. Through this
canal the subjacent ectoderm extends up to the base of
the seta, and can even be traced for some distance into
its interior.

It has already been mentioned that the apodemata and
the tendons of the muscles are infoldings of the cuticle,
embraced and secreted by corresponding involutions of
the ectoderm.

MORPHOLOGICAL SUMMARY.

199

Thus the body of the crayfish is resolvable, in the first
place, into a repetition of similar segments, the metameres,
each of which consists of a somite and two appendages ;
the metameres are built up out of a few simple tissues ;
and, finally, the tissues are either aggregates of more or
less modified nucleated cells, or are products of such cells.
Hence, in ultimate morphological analysis, the crayfish
is a multiple of the histological unit, the nucleated cell.

What is true of the crayfish, is certainly true of all
animals, above the very lowest. And it cannot yet be con-
sidered certain that the generalization fails to hold good
even of the simplest manifestations of animal life ; since
recent investigations have demonstrated the presence of
a nucleus in organisms in which it had hitherto appeared
to be absent.

However this may be, there is no doubt that in the
case of man and of all vertebrated animals, in that
of all arthropods, mollusks, echinoderms, worms, and
inferior organisms down to the very lowest sponges, the
process of morphological analysis yields the same result
as in the case of the crayfish. The body is built up of
tissues, and the tissues are either obviously composed of
nucleated cells ; or, from the presence of nuclei, they
may be assumed to be the results of the metamorphosis
of such cells ; or they are cuticular structures.

The essential character of the nucleated cell is that it
consists of a protoplasmic substance, one part of which
differs somewhat in its physical and chemical characters

200 THE MORPHOLOGY OF THE COMMON CRAYFISH.

from the rest, and constitutes the nucleus. What part
the nucleus plays in relation to the functions, or vital
activities, of the cell is as yet unknown ; but that it is
the seat of operations of a different character from those
which go on in the body of the cell is clear enough.
For, as we have seen, however different the several
tissues may be, the nuclei which they contain are very
much alike ; whence it follows, that if all these tissues
were primitively composed of simple nucleated cells, it
must be the bodies of the cells which have undergone
metamorphosis, while the nuclei have remained rela-
tively unchanged.

On the other hand, when cells multiply, as they do
in all growing parts, by the division of one cell into two,
the signs of the process of internal change which ends
in fission are apparent in the nucleus before they are
manifest in the body of the cell ; and, commonly, the
division of the former precedes that of the latter. Thus
a single cell body may possess two nuclei, and may be-
come divided into two cells by the subsequent aggrega-
tion of the two moieties of its protoplasmic substance
round each of them, as a centre.

In some cases, very singular structural changes take
place in the nuclei in the course of the process of cell-
division. The granular or fibrillar contents of the
nucleus, the wall of which becomes less distinct, arrange
themselves in the form of a spindle or double cone,
formed of extremely delicate filaments ; and in the plane

THE DIVISION OF NUCLEI.

201

of the base of the double cone the filaments present knots
or thickenings, just as if they were so many threads with
a bead in the middle of each. When the nuclear spindle
is viewed sideways, these beads or thickenings give rise
to the appearance of a disk traversing the centre of the
spindle. Soon each bead separates into two, and these
move away from one another, but remain connected by a
fine filament. Thus the structure which had the form of
a double cone, with a disk in the middle, assumes that of
a short cylinder, with a disk and a cone at each end. But
as the distance between the two disks increases, the
uniting filaments lose their parallelism, converge in the
middle, and finally separate, so that two separate double
cones are developed in place of the single one. Along
with these changes in the nucleus, others occur in the
protoplasm of the cell body, and its parts commonly dis-
play a tendency to arrange themselves in radii from the
extremities of the cones as a centre; while, as the separa-
tion of the two secondary nuclear spindles becomes com-
plete, the cell body gradually splits from the periphery
inwards, in a direction at right angles to the common
axis of the spindles and between their apices. Thus
two cells are formed, where, previously, only one existed ;
and the nuclear spindles of each soon revert to the
globular form and confused arrangement of the con-
tents, characteristic of nuclei in their ordinary state.
The formation of these nuclear spindles is very beau-
tifully seen in the epithelial cells of the testis of the

202 THE MORPHOLOGY OF THE COMMON CRAYFISH.

crayfish (fig. 33, p. 132) ; but I have not been able to find
distinct evidence of it elsewhere in this animal; and
although the process has now been proved to take place
in all the divisions of the animal kingdom, it would seem
that nuclei may, and largely do, undergo division, without
becoming converted into spindles.

The most cursory examination of any of the higher
plants shows that the vegetable, like the animal body,
is made up of various kinds of tissues, such as pith,
woody fibre, spiral vessels, ducts, and so on. But even
the most modified forms of vegetable tissue depart so
little from the type of the simple cell, that the reduction
of them all to that common type is suggested still more
strongly than in the case of the animal fabric. And
thus the nucleated cell appears to be the morphological
unit of the plant no less than of the animal. Moreover,
recent inquiry has shown that in the course of the
multiplication of vegetable cells by division, the nuclear
spindles may appear and run through all their remark-
able changes by stages precisely similar to those which
occur in animals.

The question of the universal presence of nuclei in
cells may be left open in the case of Plants, as in that
of Animals ; but, speaking generally, it may justly be
affirmed that the nucleated cell is the morphological
foundation of both divisions of the living world; and
the great generalisation of Schleiden and Schwann,
that there is a fundamental agreement in structure and

COMPARATIVE MORPHOLOGY.

203

development between plants and animals, has, in sub-
stance, been merely confirmed and illustrated by the
labours of the half century which has elapsed since its
promulgation.

Not only is it true that the minute structure of the
crayfish is, in principle, the same as that of any other
animal, or of any plant, however different it may be in
detail ; but, in all animals (save some exceptional forms)
above the lowest, the body is similarly composed of
three layers, ectoderm, mesoderm, and endoderm, dis-
posed around a central alimentary cavity. The ectoderm
and the endoderm always retain their epithelial character ;
while the mesoderm, which is insignificant in the lower
organisms, becomes, in the higher, far more complicated
even than it is in the crayfish.

Moreover, in the whole of the Arthropoda, and the
whole of the Vertebrata, to say nothing of other groups
of animals, the body, as in the crayfish, is susceptible
of distinction into a series of more or less numerous
segments, composed of homologous parts. In each
segment these parts are modified according to physio-
logical requirements ; and by the coalescence, segrega-
tion, and change of relative size and position of the
segments, well characterized regions of the body are
marked out. And it is remarkable that precisely the
same principles are illustrated by the morphology of
plants. A flower with its whorls of sepals, petals,
stamens and carpels has the same relation to a stem

204 THE MORPHOLOGY OF THE COMMON CRAYFISH.

with its whorls of leaves, as a crayfish's head has to its
abdomen, or a dog's skull to its thorax.

It may be objected, however, that the morphological
generalisations which have now been reached, are to a
considerable extent of a speculative character ; and that, in
the case of our crayfish, the facts warrant no more than
the assertion that the structure of that animal may be
consistently interpreted, on the supposition that the body
is made up of homologous somites and appendages, and
that the tissues are the result of the modification of
homologous histological elements or cells ; and the ob-
jection is perfectly valid.

There can be no doubt that blood corpuscles, liver
cells, and ova are all nucleated cells ; nor any that the
third, fourth, and fifth somites of the abdomen are con-
structed upon the same plan ; for these propositions are
mere statements of the anatomical facts. But when, from
the presence of nuclei in connective tissue and muscles,
we conclude that these tissues are composed of modified
cells ; or when we say that the ambulatory limbs of the
thorax are of the same type as the abdominal limbs, the
exopodite being suppressed, the statement, as the evi-
dence stands at present, is no more than a convenient
way of interpreting the facts. The question remains,
has the muscle actually been formed out of nucleated
cells ? Has the ambulatory limb ever possessed an
exopodite, and lost it ?

DEVELOPMENT — YELK DIVISION.

205

The answer to these questions is to be sought in the
facts of individual and ancestral development.

An animal not only is, but becomes ; the crayfish is the
product of an egg, in which not a single structure visible
in the adult animal exists : in that egg the different tissues
and organs make their appearance by a gradual process of
evolution ; and the study of this process can alone tell
us whether the unity of composition suggested by the
comparison of adult structures, is borne out by the facts
of their development in the individual or not. The
hypothesis that the body of the crayfish is made up of a
series of homologous somites and appendages, and that
all the tissues are composed of nucleated cells, might be
only a permissible, because a useful, mode of colligating
the facts of anatomy. The investigation of the actual
manner in which the evolution of the body of the crayfish
has been effected, is the only means of ascertaining
whether it is anything more. And, in this sense, deve-
lopment is the criterion of all morphological speculations.

The first obvious change which takes place in an im-
pregnated ovum is the breaking up of the yelk into
smaller portions, each of which is provided with a nucleus,
and is termed a blastomere. In a general morphological
sense, a blastomere is a nucleated cell, and differs from
an ordinary cell only in size, and in the usual, though by
no means invariable, abundance of granular contents ;
and blastomeres insensibly pass into ordinary cells, as

206 THE MORPHOLOGY OF THE COMMON CRAYFISH.

the process of division of the yelk into smaller and
smaller portions goes on.

In a great many animals, the splitting-up into blasto-
meres is effected in such a manner that the yelk is, at
first, divided into equal, or nearly equal, masses ; that
each of these again divides into two ; and that the number
of blastomeres thus increases in geometrical progression
until the entire yelk is converted into a mulberry-like
bod}r, termed a morula, made up of a great number
of small blastomeres or nucleated cells. The whole
organism is subsequently built up by the multiplication,
the change of position, and the metamorphosis of these
products of yelk division.

In such a case as this, yelk division is said to be
complete. An unessential modification of complete yelk
division is seen when, at an early period, the blastomeres
produced by division are of unequal sizes ; or when they
become unequal in consequence of division taking place
much more rapidly in one set than in another.

In many animals, especially those which have large
ova, the inequality of division is pushed so far that only
a portion of the yelk is affected by the process of fission,
while the rest serves merely as food-yelk, for nutriment
to the blastomeres thus produced. Over a greater or
less extent of the surface of the egg, the protoplasmic
substance of the yelk segregates itself from the rest,
and, constituting a germinal layer, breaks up into the
blastomeres, which multiply at the expense of the food-

THE FORMATION OF A BLASTODERM.

207

yelk, and fabricate the body of the embryo. This process
is termed partial or incomplete yelk division.

The crayfish is one of those animals in the egg of
which the yelk undergoes partial division. The first
steps of the process have not yet been thoroughly worked
out, but their result is seen in ova which have been but
a short time laid (fig. 57, A). In such eggs, the great
mass of the substance of the vitellus is destined to play
the part of food-yelk ; and it is disposed in conical
masses, which radiate from a central spheroidal portion
to the periphery of the yelk (v). Corresponding with the
base of each cone, there is a clear protoplasmic plate,
which contains a nucleus ; and as these bodies are all
in contact by their edges, they form a complete, though
thin, investment to the food-yelk. This is termed the
blastoderm (bl).

Each nucleated protoplasmic plate adheres firmly to
the corresponding cone of granular food-yelk, and, in all
probability, the two together represent a blastomere ;
but, as the cones only indirectly subserve the growth of
the embryo, while the nucleated peripheral plates form
an independent spherical sac, out of which the body of
the young crayfish is gradually fashioned, it will be con-
venient to deal with the latter separately.

Thus, at this period, the body of the developing crayfish
is nothing but a spherical bag, the thin walls of which are
composed of a single layer of nucleated cells, while its
cavity is filled with food-yelk. The first modification

Fig. 57. — Astacus fluviatilis. — Diagrammatic sections of embryos ; partly after Reichenbach, partly
original ( x 20). A. An ovum in which the blastoderm is just formed. B. An ovum in which
the invagination of the blastoderm to constitute the hypoblast or rudiment of the mid-gut has
taken place. (This nearly answers to the stage represented in fig. 58, A.) C. A longitudinal
section of an ovum, in which the rudiments of the abdomen, of the hind-gut, and of the fore-
gut have 'appeared. (This nearly answers to the stage re p resented in fig. 58, E.) D. A similar
section of an embryo in nearly the same stage of development as that represented in C, fig. 59.
E. An embryo just hatched, in longitudinal section ; a, anus ; M, blastoderm ; bp, blasto-
pore ; e, eye ; ep.b., epiblast ; fg, fore-gut ; fg\ its oesophageal, and fg2, its gastric portion ;
h, heart ; hg, hind-gut ; m, mouth ; mg> hypoblast, archenteron, or mid-gut ; r, yelk. The
dotted portions in D and E represent the nervous system.

THE AftCHENTERIC INVAGINATION.

209

which is effected in the vesicular blastoderm manifests
itself on that face of it which is turned towards the pedicle
of the egg. Here the layer of cells becomes thickened
throughout an oval area about l-25th of an inch in
diameter. Hence, when the egg is viewed by reflected
light, a whitish patch of corresponding form and size
appears in this region. This may be termed the ger-
minal disk. Its long axis corresponds with that of the
future crayfish.

Next, a depression (fig. 58, A, bp) appears in the hinder
third of the germinal disk, in consequence of this part
of the blastoderm growing inwards, and thus giving rise
to a small wide-mouthed pouch, which projects into the
food-yelk with which the cavity of the blastoderm is
filled (fig. 57, B, mg). As this infolding, or invagination
of the blastoderm, goes on, the pouch thus produced
increases, while its external opening, termed the blasto-
pore (fig. 57, B, and 58, A — E, bp), diminishes in size.
Thus the body of the embryo craj^fish, from being a
simple bag becomes a double bag, such as might be
produced by pushing in the wall of an incompletely
distended india-rubber ball with the finger. And, in
this case, if the interior of the bag contained porridge,
the latter would very fairly represent the food-yelk.

By this invagination a most important step has
been taken in the development of the crayfish. For,
though the pouch is nothing but an ingrowth of part of
the blastoderm, the cells of which its wall is composed

p

Fig. 58.— Astacus fluviat Ms.— Surface views of the earlier stages in the development of
the embryo, from the appearance of the blastopore (A) to the assumption of the
nauplius form (F) (after Reichenbach, x about 23). bp, blastopore ; c, carapace ; fg,
fore-gut involution ; h, heart ; kg, hind-gut involution ; lb, labium ; mg, medullary
groove ; o, optic pit ; p, endodermal plug partly filling up the blastopore ; pc, pro-
cephalic processes; to, abdominal elevation; 2, antennules; St antennae U, man-
dibles.

EPIBLAST, MESOBLAST, AND HYPOBLAST, 21 i

henceforward exhibit different tendencies from those
which are possessed by the rest of the blastoderm. In
fact, it is the primitive alimentary apparatus or archen-
teron, and its wall is termed the hypoblast. The rest of
the blastoderm, on the contrary, is the primitive epider-
mis, and receives the name of epiblast. If the food-
yelk were away, and the archenteron enlarged until the
hypoblast came in contact with the epiblast, the entire
body would be a double-walled sac, containing an ali-
mentary cavity, with a single external aperture. This is
the gastrula condition of the embryo ; and some animals,
such as the common fresh-water polype, are little more
than permanent gastrula.

Although the gastrula has not the slightest resem-
blance to a crayfish, yet, as soon as the hypoblast and
the epiblast are thus differentiated, the foundations of
some of the most important systems of organs of the
future crustacean are laid. The hypoblast will give rise
to the epithelial lining of the mid-gut ; the epiblast
(which answers to the ectoderm in the adult) to the
epithelia of the fore-gut and hind-gut, to the epidermis,
and to the central nervous system.

The mesodermal structures, that is to say the con-
nective tissue, the muscles, the heart and vessels, and
the reproductive organs, which lie between the ectoderm
and the endoderm, aie not derived directly from either
the epiblast or the hypoblast, but have a gwasi-independent
origin, from a mass of cells which first makes its appear-

p 2

212 THE MORPHOLOGY OF THE COMMON CRAYFISH.

ance in the neighbourhood of the blastopore, between the
hypoblast and the epiblast, though they are probably
derived from the former. From this region they gradu-
ally spread, first over the sternal, and then on to the
tergal aspect of the embryo, and constitute the mesoblast.

Epiblast, hypoblast, and mesoblast are at first alike
constituted of nothing but nucleated cells, and they in-
crease in dimensions by the continual fission and growth
of these cells. The several layers become gradually
modelled into the organs which they constitute, before
the cells undergo any notable modification into tissues.
A limb, for example, is, at first, a mere cellular out-
growth, or bud, composed of an outer coat of epiblast
with an inner core of mesoblast ; and it is only subse-
quently that its component cells are metamorphosed into
well-defined epidermic and connective tissues, vessels and
muscles.

The embryo crayfish remains only a short while in
the gastrula stage, as the blastopore soon closes up, and
the archenteron takes the form of a sac, flattened out
between the epiblast and the food-yelk, with which its
cells are in close contact (fig. 57, C and D).* Indeed, as
development proceeds, the cells of the hypoblast actually
feed upon the substance of the food-yelk, and turn it to
account for the general nutrition of the body.

* Whether, as some observers state, the hypoblastic cells grow over
and inclose the food-yelk or not, is a question that may be lelt open. I
have not been able to satisfy myself of this fact.

THE FORE-GUT.

213

The sternal urea of the embryo gradually enlarges
until it occupies one hemisphere of the yelk; in other
words, the thickening of the epiblast gradually extends
outwards#. Just in front of the blastopore, as it closes,
the middle of the epiblast grows out into a rounded
elevation (fig. 58, t a; fig. 59, ab), which rapidly increases
in length, and at the same time turns forwards. This
is the rudiment of the whole abdomen of the crayfish.
Further forwards, two broad and elongated, but flatter
thickenings appear ; one on each side of the middle line
(fig. 58, p c). As the free end of the abdominal papilla
now marks the hinder extremity of the embryo, so do
these two elevations, which are termed the procephalic
lobes, define its anterior termination. The whole sternal
region of the body will be produced by the elongation of
that part of the embryo which lies between these two
limits.

A narrow longitudinal groove-like depression appears
on the surface of the epiblast, in the middle line, between
the procephalic lobes and the base of the abdominal
papilla (fig. 58, C — F, m g). About its centre, this groove
becomes further depressed by the ingrowth of the epi-
blast, which constitutes its floor, and gives rise to a
short tubular sac, which is the rudiment of the whole fore-
gut (fig. 57, C, and fig. 58, E,/ g). At first, this epiblastic
ingrowth does not communicate with the archenteron, but,
after a while, its blind end combines with the front and
lower part of the hypoblast, and an opening is formed by

21 i THE MORPHOLOGY OF THE COMMON CRAYFISH.

which the cavity of the fore-gut communicates with that
of the mid-gut (fig. 57, E). Thus a gullet and stomach,
or rather the parts which will eventually give rise to all
these, are constituted. And it is important to remark
that, in comparison with the mid-gut, they are, at first,
very small.

In the same way, the epiblast covering the sternal face
of the abdominal papilla undergoes invagination and is
converted into a narrow tube which is the origin of the
whole hind-gut (fig. 57, C, and fig. 58, E, Jig). This, like
the fore-gut, is at first blind ; but the shut front end soon
applying itself to the hinder wall of the archenteric sac,
the two coalesce and open into one another (fig. 57, E).
Thus the complete alimentary canal, consisting of a very
narrow, tubular, fore- and hind-gut, derived from the
epiblast, and a wider and more sac-like mid-gut, formed
of the whole hypoblast, is constituted.

The procephalic lobes become more convex ; while,
behind them, the surface of the epiblast rises into six
elevations disposed in pairs, one on each side of the
median groove. The hindermost of these, which lie at
the sides of the mouth, are the rudiments of the
mandibles (fig. 58, E and F, 4 ) ; the other two become
the antennae (3) and the antennules (2), while, at a later
period, processes of the procephalic lobes give rise to the
eyestalks.

A short distance behind the abdomen, the epiblast
rises into a transverse ridge, which is concave forwards,

THE TRANSITORY NAUPLIUS STAGE.

215

while its ends are prolonged on each side nearly as far
as the mouth. This is the commencement of the free
edge of the carapace (fig. 58, E and F, and fig. 59, A, e)
— the lateral parts of which, greatly enlarging, become
the branchiostegites (fig. 59, D, c).

In many animals allied to crayfish, the young, when
it has reached a stage in its development, which answers
to this, undergoes rapid changes of outward form and of
internal structure, without making any essential addition
to the number of the appendages. The appendages which
represent the antennules, the antennae, and the mandibles
elongate and become oar-like locomotive organs ; a
single median eye is developed, and the young leaves the
egg as an active larva, which is known as a Nauplius.
The crayfish, on the other hand, is wholly incapable of
an independent existence at this stage, and continues its
embryonic life within the egg case ; but it is a remark-
able circumstance that the cells of the epiblast secrete
a delicate cuticula, which is subsequently shed. It is
as if the animal symbolized a nauplius condition by
the development of this cuticle, as the foetal whalebone
whale symbolizes a toothed condition by developing teeth
which are subsequently lost and never perform any
function.

In fact, in the crayfish, the nauplius condition is soon
left behind. The sternal disk spreads more and more
over the yelk ; as the region between the mouth and
the root of the abdomen elongates, slight transverse

Fig. 59.— Astacus fluviat ills.— Ventral (A, B, C, F) and lateral (D, E) views of the
embryo in successive stages of development (after Ratlike, x 15). A is a little more
advanced than the embryo represented in fig. 58, F : D, E, and F are views of
the young crayfish when nearly ready to be hatched : in E, the carapace is
removed, and the limbs and abdomen are spread out. 1 — 14, the cephalic and
thoracic appendages ; ah, abdomen ; br, branchiae ; c, carapace ; ep, epipodite
of the first maxillipede ; gg, green gland ; h, heart ; lb, labrum ; Ir, liver ; ?»-,
mandibular muscles.

THE DEVELOPMENT OF THE LIMBS.

217

depressions indicate the boundaries of the posterior
cephalic and the thoracic somites ; and pairs of eleva-
tions, similar to the rudiments of the antennules and
antennae, appear upon them in regular order from before
backwards (fig. 59, C).

In the meanwhile, the extremity of the abdomen
flattens out and takes on the form of an oval plate,
the middle of the posterior margin of which is slightly
truncated or notched ; while, finally, transverse constric-
tions mark off six segments, the somites of the abdomen,
in front ot this. Along with these changes, four pairs
of tubercles grow out from the sternal faces of the four
middle abdominal somites, and constitute the rudiments
of the four middle pairs of abdominal appendages. The
first abdominal somite exhibits only two hardly percept-
ible elevations in place of the appendages of the others,
while the sixth seems, at first, to have none. The ap-
pendages of the sixth somite, however, are already formed,
though, singularly enough, they lie beneath the cuticle
of the telson and are set free only after the first
ecdysis.

The rostrum grows out between the procephalic lobes ;
it remains relatively very short up to the time that the
young crayfish quits the egg, and is directed more down-
wards than forwards. The lateral portions of the cara-
pacial ridge, becoming deeper, are converted into the
branchiostegites, and the cavities which they overarch
are the branchial chambers. The transverse portion of

218 THE MORPHOLOGY OF THE COMMON CRAYFISH.

the ridge, on the other hand, remains relatively short, and
constitutes the free posterior margin of the carapace.

As these changes take place, the abdomen and the
sternal region of the thorax are constantly enlarging in
proportion to the rest of the ovum ; and the food-yelk
which lies in the cephalothorax is, pari passu, being
diminished. Hence the cephalothorax constantly becomes
relatively smaller and the tergal aspect of the carapace
less spherical; although, even when the }^oung crayfish
is ready to be hatched, the difference between it and the
adult in the form of the cephalothoracic region, and in the
size of the latter relatively to the abdomen, is very marked.

The simple bud-like outgrowths of the somites, in
which all the appendages take their origin, are rapidly
metamorphosed. The eyestalks (fig. 59, 1) soon attain
a considerable relative size. The extremities of the
antennules (2) and of the antennae (3) become bifurcated ;
and the two divisions of the antennule remain broad,
thick, and of nearly the same size up to birth. On the
other hand, the inner or endopoditic division of the
antenna becomes immensely lengthened, and at the same
time annulated, wdiile the outer or exopoditic division
remains relatively short, and acquires its characteristic
scale-like form.

The labrum (lb) arises as a prolongation of the middle
sternal region in front of the mouth, while the bilobed
metastoma is an outgrowth of the sternal region be-
hind it.

THE NEWLY-HATCHED CRAYFISH.

219

The posterior cephalic and the thoracic appendages
(5 — 14) elongate and gradually approach the form which
they possess in the adult. I have not been able to
discover, at any period of development, an outer division
or exopodite in any of the five posterior thoracic limbs.
And this is a very remarkable circumstance, inasmuch
as such an exopodite exists in the closely allied lobster
in the larval state ; and, in many of the shrimp and
prawn-like allies of the crayfish, a complete or rudi-
mentary exopodite is found in these limbs, even in the
adult condition.

When the crayfish is hatched (fig. 60) it differs from the
adult in many ways — not only is the cephalothorax more
convex and larger in proportion to the abdomen ; but the
rostrum is short and bent down between the eyes. The
sterna of the thorax are wider relatively, and hence there
is a greater interval between the bases of the legs than in
the adult. The proportion of the limbs to one another
and to the body are nearly the same as in the adult, but
the chelae of the forceps are more slender. The tips of
the chehe are all strongly incurved (fig. 8, B, p. 41), and the
dactylopodites of the two posterior thoracic limbs are hook-
like. The appendages of the first abdominal somite are un-
developed, and those of the last are inclosed within the
telson, which is, as has already been said, of a broad oval
form, usually notched in the middle of its hinder margin,
and devoid of any indication of transverse division. Its
margins are produced into a single series of short conical

220 THE MORPHOLOGY OF THE COMMON CRAYFISH,

processes, and the disposition of the vascular canals in its
interior gives it the appearance of being radially striated.

The setee, so abundant in the adult, are very scanty in
the newly hatched young; and the great majority of those
which exist are simple conical prolongations of the un-

Fig. 60. — Astacus flvviatilis.--Xewly -hatched young ( x G).

calcified cuticle, the bases of which are not sunk in pits
and which are devoid of lateral scales or processes.

The young animals are firmly attached to the ab-
dominal appendages of the parent in the manner already
described. They are very sluggish, though they move
when touched ; and at this period they do not feed, but

THE EVOLUTION OF THE INDIVIDUAL. 221

are nourished by the food-yelk, of which a considerable
store still remains in the cephalothorax.

I imagine that they are set free during the first ecdysis,
and that the appendages of the sixth abdominal somite
are at that time expanded, but nothing is definitely known
at present of these changes.

The foregoing sketch of the general nature of the
changes which take place in the egg of the crayfish
suffice to show that its development is, in the strictest
sense of the word, a process of evolution. The egg is
a relatively homogeneous mass of living protoplasmic
matter, containing much nutritive material ; and the
development of the crayfish means the gradual conver-
sion of this comparatively simple body into an organism
of great complexity. The yelk becomes differentiated
into formative and nutritive portions. The formative
portion is subdivided into histological units : these
arrange themselves into a blastodermic vesicle ; the blas-
toderm becomes differentiated into epiblast, hypoblast,
and mesoblast ; and the simple vesicle assumes the gas-
trula condition. The layers of the gastrula shape them-
selves into the body of the crayfish and its appendages,
while along with this, the cells of which all the parts
are built, become metamorphosed into tissues, each with
its characteristic properties. And all these wonderful
changes are the necessary consequences of the interaction
of the molecular forces resident in the substance of the

222 THE MORPHOLOGY OF THE COMMON CRAYFISH.

impregnated ovum, with the conditions to which it is
exposed; just as the forms evolved from a crystallising
fluid are dependent upon the chemical composition of
the dissolved matter and the influence of surrounding
conditions.

Without entering into details which lie beyond the
scope of the present work, something must be said re-
specting the manner in which the complicated internal
organisation of the crayfish is evolved from the cellular
double sac of the gastrula stage.

It has been seen that the fore-gut is at first an insig-
nificant tubular involution of the epiblast in the region
of the mouth. It is, in fact, a part of the epiblast turned
inwards, and the cells of which it is composed secrete a
thin cuticular layer, as do those of the rest of the epi-
blast, which gives rise to the ectodermal or epidermic
part of the integument. As the embryo grows, the fore-
gut enlarges much faster than the mid-gut, increasing
in height and from before backwards, while its side-walls
remain parallel, and are separated by only a narrow
cavity. At length, it takes on the shape of a triangular
bag (fig. 57, D, fg), attached by its narrow end around
the mouth and immersed in the food-yelk, which it
gradually divides into two lobes, one on the right and one
on the left side. At the same time a vertical plate of
mesoblastic tissue, from which the great anterior and
posterior muscles are eventually developed, connects it
with the roof and with the front wall of the carapace.

ORIGIN OF THE INTERNAL ORGANS.

223

Becoming constricted in the middle, the fore-gut next
appears to consist of two dilatations of about equal size,
connected by a narrower passage (fig. 57, E, fgl, fg2).
The front dilatation becomes the oesophagus and the
cardiac division of the stomach ; the hinder one, the
P3rloric division. At the sides of the front end of the
cardiac division two small pouches are formed shortly
after birth ; in each of these a thick laminated deposit
of chitin takes place, and constitutes a minute crab's-eye
or gastrolith, which has the same structure as in the
adult, and is largely calcified. This fact is the more
remarkable as, at this time, the exoskeleton contains very
little calcareous deposit. In the position of the gastric
teeth, folds of the cellular wall of corresponding shape
arc formed, and the chitinous cuticle of which the teeth
are composed is, as it were, modelled upon them.

The hind-gut occupies the whole length of the abdo-
men, and its cells early arrange themselves into six
ridges, and secrete a cuticular layer.

The mid-gut, or hypoblastic sac, very soon gives off
numerous small prolongations on each side of its hinder
extremity, and these are converted into the creca of the
liver (fig. 57, E, mg). The cells of its tergal wall are in
close contact with the adjacent masses of food-yelk ; and
it is probable that the gradual absorption of the food-
yelk is chiefly effected by these cells. At birth, however,
the lateral lobes of the food-yelk are still large, and
occupy the space left between the stomach and liver

224 THE MORPHOLOGY OF THE COMMON CRAYFISH.

on the one hand, and the cephalic integument on the
other.

The mesoblastic cells give rise to the layer of con-
nective tissue which forms the deeper portion of the
integument, and to that which invests the alimentary
canal ; to all the muscles ; and to the heart, the vessels,
and the corpuscles of the blood. The heart appears
very early as a solid mass of mesoblastic cells in the
tergal region of the thorax, just in front of the origin
of the abdomen (figs. 57, 58, 59, h). It soon be-
comes hollow, and its walls exhibit rhythmical con-
tractions.

The branchiae are, at first, simple papillae of the integu-
ment of the region from which they take their rise.
These papillae elongate into stems, which give off lateral
filaments. The podobranchias are at first similar to the
arthrobranchise, but an outgrowth soon takes place near
the free end of the stem, and becomes the lamina, while
the attached end enlarges into the base.

The renal organ is stated to arise by a tubular involu-
tion of the epiblast, which soon becomes convoluted, and
gives rise to the green gland.

The central nervous system is wholly a product of the
epiblast. The cells which lie at the sides of the longi-
tudinal groove alreadjr mentioned (fig. 58, mg), grow in-
wards, and give rise to two cords which are at first
separate from one another and continuous with the rest
of the epiblast. At the front end of the groove a

DEVELOPMENT OF THE NERVOUS SYSTEM.

225

depression arises, and its cells form a mass which con-
nects these two cords in front of the mouth, and gives
rise to the cerebral ganglia. The epiblastic linings of
two small pits (fig. 58, o) which appear very early on the
surface of the procephalic lobes, are also carried inwards
in the same way, and, uniting with the foregoing,
produce the optic ganglia.

The cells of the longitudinal cords become differ-
entiated into nerve fibres and nerve cells, and the latter,
gathering towards certain points, give rise to the ganglia
which eventually unite in the middle line. By degrees,
the ingrowth of epiblastic cells, from which all these struc-
tures are developed, becomes completely separated from
the rest of the epiblast, and is invested by mesoblastic
cells. The central nervous system, therefore, in a crayfish,
as in a vertebrated animal, is at first, as a part of the
ectoderm, morphologically one with the epidermis; and the
deep and protected position which it occupies in the adult
is only a consequence of the mode in which the nervous
portion of the ectoderm grows inwards and becomes
detached from the epidermic portion.

The visual rods of the eye are merely modified cells of
the ectoderm. The auditory sac is formed by an involu-
tion of the ectoderm of the basal joint of the antennule.
At birth it is a shallow wide-mouthed depression, and
contains no otoliths.

Lastly, the reproductive organs result from the segre-
gation and special modification of cells of the mesoblast

Q

226

THE MORPHOLOGY OF THE CRAYFISH.

behind the liver. Rathke states that the sexual apertures
are not visible until the young crayfish has attained the
length of an inch ; and that the first pair of abdominal
appendages of the male appear still later in the form of
two papillae, which gradually elongate and take on their
characteristic forms.

CHAPTEE V.

THE COMPARATIVE MORPHOLOGY OF THE CRAYFISH. THE

STRUCTURE AND THE DEVELOPMENT OF THE CRAY-
FISH COMPARED WITH THOSE OF OTHER LIVING
BEINGS.

Up to this point, our attention has been directed
almost exclusively to the common English crayfish.
Except in so far as the crayfish is dependent for its
maintenance upon other animals, or upon plants, we
might have ignored the existence of all living things
except crayfishes. But, it is hardly necessary to observe,
that innumerable hosts of other forms of life not only
tenant the waters and the dry land, but throng the air ;
and that all the crayfishes in the world constitute a hardly
appreciable fraction of its total living population.

Common observation leads us to see that these multi-
tudinous living beings differ from not-living things in
many ways ; and when the analysis of these differences
is pushed as far as we are at present able to carry it, it
shews us that all living beings agree with the crayfish
and differ from not-living things in the same particulars.
Like the crayfish, they are constantly wasting away by

q 2

228 THE COMPARATIVE MORPHOLOGY OF THE CRAYFISH.

oxidation, and repairing themselves by taking into their
substance the matters which serve them for food ; like
the crayfish, they shape themselves according to a defi-
nite pattern of external form and internal structure ; like
the crayfish, they give off germs which grow and develope
into the shapes characteristic of the adult. No mi-
neral matter is maintained in this fashion ; nor grows in
the same way ; nor undergoes this kind of development ;
nor multiplies its kind by any such process of reproduc-
tion.

Again, common observation early leads to the discri-
mination of living things into two great divisions. No-
body confounds ordinary animals with ordinary plants,
nor doubts that the crayfish belongs to the former cate-
gory and the waterweed to the latter. If a living thing
moves and possesses a digestive receptacle, it is held to
be an animal ; if it is motionless and draws its nourish-
ment directly from the substances which are in contact
with its outer surface, it is held to be a plant. We need
not inquire, at present, how far this rough definition of
the differences which separate animals from plants holds
good. Accepting it for the moment, it is obvious that
the crayfish is unquestionably an animal, — as much an
animal as the vole, the perch, and the pond-snail, which
inhabit the same waters. Moreover, the crayfish has, in
common with these animals, not merely the motor and
digestive powers characteristic of animality, but they all,
like it, possess a complete alimentary canal; special appa-

COMPARATIVE MORPHOLOGY.

229

ratus for the circulation and the aeration of the blood ;
a nervous system with sense-organs ; muscles and motor
mechanisms ; reproductive organs. Eegarded as pieces
of physiological apparatus, there is a striking similarity
between all three. But, as has already been hinted in
the preceding chapter, if we look at them from a purely
morphological point of view, the differences between the
crayfish, the perch, and the pond- snail, appear at first
sight so great, that it may be difficult to imagine that the
plan of structure of the first can have any relation to
that of either of the last two. On the other hand, if the
crayfish is compared with the water-beetle, notwithstand-
ing wide differences, many points of similarity between
the two will manifest themselves; while, if a small
lobster is set side by side with a crayfish, an unpractised
observer, though he will readily see that the two animals
are somewhat different, may be a long time in making
out the exact nature of the differences.

Thus there are degrees of likeness and unlikeness
among animals, in respect of their outward form and
internal structure, or, in other words, in their morpho-
logy. The lobster is very like a crayfish, the beetle is
remotely like one; the pond-snail and the perch are
extremely unlike crayfishes. Facts of this kind are com-
monly expressed in the language of zoologists, by saying
that the lobster and the crayfish are closely allied
forms; that the beetle and the crayfish present a re-
mote affinity ; and that there is no affinity between the

230 THE COMPARATIVE MORPHOLOGY OF THE CRAYFISH.

crayfish and the pond-snail, or the crayfish and the
perch.

The exact determination of the resemblances and
differences of animal forms by the comparison of the
structure and the development of one with those of
another, is the business of comparative morphology.
Morphological comparison, fully and thoroughly worked
out, furnishes us with the means of estimating the
position of any one animal in relation to all the
rest; while it shews us with what forms that animal
is nearly, and with what it is remotely, allied : ap-
plied to all animals, it furnishes us with a kind of
map, upon which animals are arranged in the order of
their respective affinities ; or a classification, in which
they are grouped in that order. For the purpose of
developing the results of comparative morphology in the
case of the crayfish, it will be convenient to bring toge-
ther, in a summary form, those points of form and struc-
ture, many of which have already been referred to and
which characterise it as a separate kind of animal.

Full-grown English crayfishes usually measure about
three inches and a half from the extremity of the rostrum
in front to that of the telson behind. The largest
specimen I have met with measured four inches.* The

* The dimensions of crayfishes at successive ages given at p. 31,
beginning at the words " By the end of the year," refer to the " ecre-
visse a pieds rouges " of France ; not to the English crayfish, which is

DISTINCTIVE CHARACTERS OF THE CRAYFISH. 231

males are commonly somewhat larger, and they almost
always have longer and stronger forceps than the
females. The general colour of the integument varies
from a light reddish-brown to a dark olive-green ; and
the hue of the tergal surface of the body and limbs is
always deeper than that of the sternal surface, which is
often light yellowish-green, with more or less red at the
extremities of the forceps. The greenish hue of the
sternal surface occasionally passes into yellow in the
thorax and into blue in the abdomen.

The distance from the orbit to the posterior margin of
the carapace is nearly equal to that from the posterior
margin of the carapace to the base of the telson, when
the abdomen is fully extended, but this measurement of
the carapace is commonly greater than that of the abdo-
men in the males and less in the females.

The general contour of the carapace (fig. 61), without
the rostrum, is that of an oval, truncated at the ends :
the anterior end being narrower than the posterior. Its
surface is evenly arched from side to side. The greatest
breadth of the carapace lies midway between the cervical
groove and its posterior edge. Its greatest vertical depth
is on a level with the transverse portion of the cervical
groove.

The length of the rostrum, measured from the orbit

considerably smaller. Doubtless, the proportional rate of increment is
much the same, in the two kinds ; but in the English crayfish it has
not been actually ascertained.

232 THE COMPARATIVE MORPHOLOGY OF THE CRAYFISH.

to its extremity, is greater than half the distance from
the orbit to the cervical groove. It is trihedral in sec-
tion, and its free end is slightly curved upwards (fig. 41).
It gradually becomes narrower for about three-fourths of
its whole length. At this point it has rather less than
half the width which it has at its base (fig. 61, A) ; and its
raised, granular and sometimes distinctly serrated margins
are produced into two obliquefy directed spines, one on
each side. Beyond these, the rostrum rapidly narrows
to a fine point ; and this part of the rostrum is equal in
length to the width between the two spines.

The tergal surface of the rostrum is flattened and
slightly excavated from side to side, except in its an-
terior half, where it presents a granular or finely ser-
rated median ridge, which gradually passes into a low
elevation in the posterior half, and, as such, may gener-
ally be traced on to the cephalic region of the carapace.
The inclined sides of the rostrum meet ventrally in a
sharp edge, convex from before backwards ; the posterior
half of this edge gives rise to a small, usually bifurcated,
spine, which descends between the eye-stalks (fig. 41).
The raised and granulated lateral margins of the rostrum
are continued back on to the carapace for a short distance,
as two linear ridges (fig. 61, A). Parallel with each of
these ridges, and close to it, there is another longitudinal
elevation {a, b), the anterior end of which is raised into a
prominent spine (a), which is situated immediately behind
the orbit, and may, therefore, be termed the post- orbital

DISTINCTIVE CHARACTERS OF THE CRAYFISH. 233

spine. The elevation itself may be distinguished as the
post-orbital ridge. The flattened surface of this ridge is
marked by a longitudinal depression or groove. The

ABC

FlG. 01.— A, D, & G,Astacvs torrentium ; B, E, & II, A. nolilis; C, F, &
I, A. nigrescetis (nat. size). A — C, Dorsal views of carapace ; D — F,
side views of third abdominal somites ; G — I, Dorsal views of telson.
a, b, post-orbital ridge and spines ; cy branchio-cardiac grooves
inclosing the areola.

234 THE COMPARATIVE MORPHOLOGY OF THE CRAYFISH,

posterior end of the ridge passes into a somewhat broader
and less marked elevation, the hinder end of which turns
inwards, and then comes to an end at a point midway
between the orbit and the cervical groove. Generally
this hinder elevation appears like a mere continuation
of the post-orbital ridge ; but, sometimes, the two are
separated by a distinct depression. I have never seen
any prominent spine upon the posterior elevation, though
it is sometimes minutely spinulose. The post-orbital
ridges of each side, viewed together, give rise to a cha-
racteristic lyrate mark upon the cephalic region of the
carapace.

A faintly marked, curved, linear depression runs from
the hinder end of the post-orbital ridge, at first directly
downwards, and then curves backwards to the cervical
groove. It corresponds with the anterior and inferior
boundary of the attachment of the adductor muscle of
the mandible.

Below the level of this, and immediately behind the
cervical groove, there are usually three spines, arranged
in a series, which follow the cervical groove. The points
of all are directed obliquely forwards, and the lowest is
the largest. Sometimes there is only one prominent
spine, with one or two very small ones ; sometimes there
are as many as five of these cervical spines.

The cardiac region is marked out by two grooves which
run backwards from the cervical groove (fig. 61, A, c), and
terminate at a considerable distance from the posterior

DISTINCTIVE CHARACTERS OF THE CRAYFISH. 235

edge of the carapace. Each groove runs, at first, obliquely
inwards, and then takes a straight course parallel with its
fellow. The area thus defined is termed the areola ; its
breadth is equal to about one-third of the total transverse
diameter of the carapace in this region.

No such distinct lines indicate the lateral boundary of
the region in front of the cervical groove which answers
to the stomach. But the middle part of the carapace,
or that which is comprised in the gastric and cardiac
regions, has its surface sculptured in a different way
from the branchiostegites and the lateral regions of the
head. In the former, the surface is excavated by shal-
low pits, separated by relatively broad flat-topped ridges ;
but, in the latter, the ridges become more prominent,
and take the form of tubercles, the apices of which are
directed forwards. Minute setae spring from the depres-
sions between these tubercles.

The branchiostegite has a thickened rim, which is
strongest below and behind (fig. 1). The free edge of
this rim is fringed with close-set setae.

The pleura of the second to the sixth abdominal
somites are broadly lanceolate and obtusely pointed at
their free ends (fig. 61, D) ; the anterior edge is longer
and more convex than the posterior edge. In the females,
the pleura are larger, and are directed more outwards and
less downwards than in the males. The pleura of the
second somite are much larger than the rest, and over-
lap the very small pleura of the first somite (fig. 1). The

230 THE COMPARATIVE MORPHOLOGY OF THE CRAYFISH.

pleura of the sixth somite are narrow, and their posterior
edges are concave.

The pits and setae of the cuticle which clothes the
tergal surfaces of the abdominal somites are so few and
scattered, that the latter appear almost smooth. In the
telson, however, especially in its posterior division, the
markings are coarser and the setae more apparent.

The telson (fig. 61, G) presents an anterior quadrate divi-
sion and a posterior half-oval part, the free curved edge
of which is beset with long setae, and is sometimes slightly
notched in the middle. The posterior division is freely
movable upon the anterior, in consequence of the thin-
ness and pliability of the cuticle along a transverse line
which joins the postero-external angles of the anterior
division, each of which is produced into two strong spines,
of which the outer is the longer. The length of the pos-
terior division of the telson, measured from the middle
of the suture, is equal to, or but very little less than,
that of the anterior division.

On the under side of the head, the basal joints of the
antennules are visible, internal to those of the antennae,
but the attachment of the latter is behind and below
that of the former (fig. 3, A). Behind these, and in
front of the mouth, the epistoma (fig. 39, A, II, III)
presents a broad area of a pentagonal form. The pos-
terior boundary of this area is formed by two thickened
transverse ridges, which meet on the middle line at a
very open angle, the apex of which is turned forwards.

DISTINCTIVE CHARACTERS OF THE CRAYFISH. 237

The posterior edges of these ridges are continuous with
the labrum. The anterior margin is produced in the
middle into a fleur de lys shaped process, the summit
of which terminates between the antennules. At the
sides of this process, the anterior margin of the epis-
toma is deeply excavated to receive the basal joints of
the antennae. Following the contours of these excavated
margins, the surface of the epistoma presents two lateral
convexities. The widest and most prominent part of each
of these lies towards the outer edge of the epistoma,
and is produced into a conical spine. Sometimes
there is a second smaller spine beside the principal one.
Between the two convexities lies a triangular median
depressed area.

The distance from the apex of the anterior median
process to the posterior ridge is equal to a little more
than half the width of the epistoma.

The corneal surface of the eye is transversely elongated
and reniform, and its pigment is black. The eye-stalks
are much broader at their bases than at their corneal
ends (fig. 48, A). The antennules are about twice as long
as the rostrum. The tergal surface of the trihedral
basal joint of the antennule, on which the eye-stalk rests,
is concave ; the outer surface is convex, the inner flat
(figs. 26, A, and 48, B). Near the anterior end of the
sternal edge which separates the two latter faces, there
is a strong curved spine directed forwards (fig. 48, B, a).
When the setse, which proceed from the outer edge of

288 THE COMPARATIVE MORPHOLOGY OF THE CRAYFISH.

the auditory aperture and hide it, are removed, it is
seen, to be a wide, somewhat triangular cleft, which occu-
pies the greater part of the hinder half of the tergal
surface of the basal joint (tig. 26, A).

The exopodites, or squames, of the antennae extend as
far as the apex of the rostrum, or even project beyond it,
when they are turned forwards, while they reach to the
commencement of the filament of the endopodite (Frontis-
jnece). The squame is fully twice as long as it is broad,
with a general convexity of its tergal and concavity of its
sternal surface. The outer edge is straight and thick, the
inner, which is fringed with long setae, is convex and thin
(fig. 48, C). Where these two edges join in front, the
squame is produced into a strong spine. A thick outer
portion of the squame is marked off from the thinner
inner portion by a longitudinal groove on the tergal side,
and by a strong ridge on the sternal side. One or two
small spines generally project from the posterior and
external angle of the squame ; but they may be very
small or absent in individual specimens. Close beneath
these, the outer angle of the next joint is produced into
a strong spine. When the abdomen is straightened out,
if the antennae are turned back as far as they will go
without damage, the ends of their filaments usually reach
the tergum of the third somite of the abdomen (Frontis-
piece). I have not observed any difference between the
sexes in this respect.

The inner edge of the ischiopodite of the third maxilli-

DISTINCTIVE CHARACTERS OF THE CRAYFISH. 239

pecle is strongly serrated and wider in front than behind
(fig. 44) ; the meropodite possesses four or five spines
in the same region ; and there are one or two spines at
the distal end of the carpopodite. When straightened
out, the maxillipedes extend as far as, or even beyond,
the end of the rostrum.

The inner or sternal edge of the ischiopodite of the
forceps is serrated ; that of the meropodite presents two
rows of spines, the inner small and numerous, the outer
large and few. There are several strong spines at the
anterior end of the outer or tergal face of this joint. The
carpopodite has two strong spines on its under or sternal
surface, while its sharp inner edge presents many strong
spines. Its upper surface is marked by a longitudinal de-
pression, and is beset with sharp tubercles. The length
of the propodite, from its base to the extremity of
the fixed claw of the chela, measures rather more than
twice as much as the extreme breadth of its base, the
thickness of which is less than a third of this length
(fig. 20, p. 93). The external angular process, or fixed
claw, is of the same length as the base, or a little shorter.
Its inner edge is sharp and spinose, and the outer more
rounded and simply tuberculated. The apex of the fixed
claw is produced into a slightly incurved spine. Its
inner edge has a sinuous curvature, convex posteriorly,
concave anteriorly, and bears a series of rounded tubercles,
of which one near the summit of the convexity, and one
near the apex of the claw are the most prominent.

240 THE COMPARATIVE MORPHOLOGY OF THE CRAYFISH.

The apex of the dactylopodite, like that of the propo-
dite, is formed by a slightly incurved spine (fig. 20), while
its outer, sharper, edge presents a curvature, the inverse
of that of the edge of the fixed claw against which it is
applied. This edge is beset with rounded tubercles, the
most prominent of which are one at the beginning, and
one at the end of the concave posterior moiety of the edge.
When the dactylopodite is brought up to the fixed claw,
these tubercles lie, one in front of and one behind the
chief tubercle of the convexity of the latter. The whole
surface of the propodite and dactylopodite is covered
with minute elevations, those of the upper surface being
much more prominent than those of the lower surface.

The length of the fully extended forceps generally
equals the distance between the posterior margin of the
orbit and the base of the telson, in well characterized
males ; and, in individual examples, they are even longer ;
while it may not be greater than the distance between
the orbit and the hinder edge of the fourth abdominal
somite, in females ; and, in massiveness and strength, the
difference of the forceps in the two sexes is still more
remarkable (fig. 2). Moreover there is a good deal of
variation in the form and size of the chelae in individual
males. The right and left chelae present no important
differences.

The ischiopodites of the four succeeding thoracic limbs
are devoid of any recurved spines in either sex (Front.,
fig. 46). The first pair are the stoutest3 the second the

DISTINCTIVE CHARACTERS OF THE CRAYFISH. 241

longest : and when the latter are spread out at right
angles to the body, the distance from tip to tip of the dac-
tylopodites is equal to, or rather greater than, the extreme
length of the body from the apex of the rostrum to the
posterior edge of the telson, in both sexes. In both sexes,
the length of the swimmerets hardly exceeds half the trans-
verse diameter of the somites to which they are attached.

The exopodites of the appendages of the sixth abdo-
minal somite (the extreme length of which is rather
greater than that of the telson) are divided into a larger
proximal, and a smaller distal portion (fig. 37, F, p. 144).
The latter is about half as long as the former, and has a
rounded free edge, setose like that of the telson. There
is a complete flexible hinge between the two portions,
and the overlapping free edge of the proximal portion,
which is slightly concave, is beset with conical spines,
the outermost of which are the longest. The endopodite
has a spine at the junction of its outer straight edge
with the terminal setose convex edge. A faintly marked
longitudinal median ridge, or keel, ends close to the
margin in a minute spine. The tergal distal edge of
the protopodite is deeply bilobed, and the inner lobe
ends in two spines, while the outer, shorter and broader
lobe, is minutely serrated.

In addition to the characters distinctive of sex, which
have already been fully detailed (pp. 7, 20, and 145), there
is a marked difference in the form of the sterna of the three
posterior thoracic somites between the males and females*

u

242 THE COMPARATIVE MORPHOLOGY OF THE CRAYFISH.

Comparing a male and a female of the same size, the
triangular area between the bases of the penultimate and
ante-penultimate thoracic limbs is considerably broader
at the base in the female. In both sexes, the hinder
part of the penultimate sternum is a rounded transverse
ridge separated by a groove from the anterior part ; but
this ridge is much larger and more prominent in the
female than in the male, and it is often obscurely divided
into two lobes by a median depression. Moreover, there
are but few setse on this region in the female ; while, in
the male, the setse are long and numerous.

The sternum of the last thoracic somite of the female
is divided by a transverse groove into two parts, of which
the posterior, viewed from the sternal aspect, has the
form of a transverse elongated ridge, which narrows to
each end, is moderately convex in the middle, and is
almost free from setae. In the male, the corresponding
posterior division of the last thoracic sternum is produced
downwards and forwards into a rounded eminence which
gives attachment to a sort of brush of long setae (fig. 35,
p. 136).

The importance of this long enumeration of minute
details* will appear by and by. It is simply a statement of
the more obvious external characters in which all the
full-grown English crayfishes which have come under my

* The student of systematic zoology will find the comparison of a
lobster with a crayfish in all the points mentioned to be an excellent
training of the faculty of observation.

THE GENERAL NAME, SPECIES. 243

notice agree. No one of these individual crayfishes was
exactly like the other; and to give an account of any
single crayfish as it existed in nature, its special peculiari-
ties must be added to the list of characters given above ;
which, considered together with the facts of structure
discussed in previous chapters, constitutes a definition,
or diagnosis, of the English kind, or species, of crayfish.
It follows that the species, regarded as the sum of the
morphological characters in question and nothing else,
does not exist in nature ; but that it is an abstraction,
obtained by separating the structural characters in which
the actual existences — the individual crayfishes — agree,
from those in which they differ, and neglecting the latter.

A diagram, embodying the totality of the structural
characters thus determined by observation to be common
to all our crayfishes, might be constructed; and it
would be a picture of nothing which ever existed in
nature; though it would serve as a very complete
plan of the structure of all the crayfishes which are to
be found in this country. The morphological definition
of a species is, in fact, nothing but a description of the
plan of structure which characterises all the individuals
of that species.

California is separated from these islands by a third of the
circumference of the globe, one-half of the interval being
occupied by the broad North Atlantic ocean. The fresh
waters of California, however, contain crayfishes which are

B 2

244 THE COMPARATIVE MORPHOLOGY OF THE CRAYFISH.

so like our own, that it is necessary to compare the two in
every point mentioned in the foregoing description in
order to estimate the value of the differences which they
present. Thus, to take one of the kinds of crayfishes found
in California, which has been called Astacus nigrescens ;
the general structure of the animal may be described in
precisely the same terms as those used for the English
crayfish. Even the branchiae present no important
difference, except that the rudimentary pleurobranchise
are rather more conspicuous ; and that there is a third
small one, in front of the two which correspond with those
possessed by the English crayfish.

The Californian crayfish is larger and somewhat diffe-
rently coloured, the undersides of the forceps particularly
presenting a red hue. The limbs, and especially the
forceps of the males, are relatively longer ; the chelae of
the forceps have more slender proportions ; the areola is
narrower relatively to the transverse diameter of the
carapace (fig. 61, C). More definite distinctions are to be
found in the rostrum, which is almost parallel-sided for
two-thirds of its length, then gives off two strong lateral
spines and suddenly narrows to its apex. Behind these
spines, the raised lateral edges of the rostrum present five
or six other spines which diminish in size from before
backwards. The postorbital spine is very prominent,
but the ridge is represented, in front, by the base of this
spine, which is slightly grooved ; and behind, by a distinct
spine which is not so strong as the postorbital spine.

ASTACUS NIGRESCENS.

245

There are no cervical spines, and the middle part of
the cervical groove is angulated backwards instead of
being transverse.

i) R p

Fig. 62. A & D, Astacus torrentiuni; B & E, A. nooilis ; C & F,
A. nigrescens. A — C, 1st abdominal appendage of the male ; D — F,
endopodite of second appendage ( x 3). anterior, and b, posterior rolled
edge ; <?, d, e, corresponding parts of the appendages in each species ;
/, rolled plate of endopodite ; g, terminal division of endopodite.

The abdominal pleura are narrow, equal-sided, and
acutely pointed in the males (fig. 61, F) — slightly
broader, more obtuse, and with the anterior edges

246 THE COMPARATIVE MORPHOLOGY OF THE CRAYFISH.

rather more convex than the posterior, in the females.
The tergal surface of the telson is not divided into two
parts by a suture (fig. 61, I). The anterior process of
the epistoma is of a broad rhomboidal shape, and there
are no distinct lateral spines.

The squame of the antenna is not so broad relatively
to its length ; its inner edge is less convex, and its outer
edge is slightly concave ; the outer basal angle is sharp
but not produced into a spine. The opposed edges of
the fixed and movable claws of the chelae of the forceps
are almost straight and present no conspicuous tubercles.
In the males, the forceps are vastly larger than in the
females, and the two claws of the chelae are bowed out, so
that a wide interval is left when their apices are applied
together ; in the females, the claws are straight and the
edges fit together, leaving no interval. Both the upper and
the under surfaces of the claws are almost smooth. The
median ridge of the endopodite of the sixth abdominal
appendage is more marked, and ends close to the margin
in a small prominent spine.

In the females, the posterior division of the sternum of
the penultimate thoracic somite is prominent and deeply
bilobed ; and there are some small differences in form in
the abdominal appendages of the males. More especially,
the rolled inner process of the endopodite of the second
appendage (fig. 62 F, /) is disposed very obliquely, and
its open mouth is on a level with the base of the jointed
part of the endopodite (g) instead of reaching almost to

THE GENERAL NAME, GENUS. 247

the free end of the latter and being nearly parallel with it.
In the first appendage (C), the anterior rolled edge (a)
more closely embraces the posterior (b), and the groove
is more completely converted into a tube.

It will be observed that the differences between the
English and the Californian crayfishes amount to ex-
ceedingly little ; but, on the assumption that these differ-
ences are constant, and that no transitional forms between
the English and the Californian crayfishes are to be
met with, the individuals which present the characteristic
peculiarities of the latter are said to form a distinct species,
Astacus nigrescens ; and the definition of that species is,
like that of the English species, a morphological abstrac-
tion, embodying an account of the plan of that species,
so far as it is distinct from that of other crayfishes.

We shall see by and by that there are sundry other
kinds of crayfishes, which differ no more from the English
or the Californian kinds, than these do from one an-
other ; and, therefore, they are all grouped as species of
the one genus, Astacus.

If, leaving California, we cross the Rocky Mountains
and enter the eastern States of the North American
Union, many sorts of crayfishes, which would at once be
recognised as such by any English visitor, will be found
to be abundant. But on careful examination it will be
discovered that all of these differ, both from the English
crayfish, and from Astacus nigrescens, to a much greatei'

248 THE COMPARATIVE MORPHOLOGY OF THE CRAYFISH.

extent than those do from one another. The gills are,
in fact, reduced to seventeen on each side, in consequence

Fro. G3. Cambarus Clarhii, male (J nat. size), after Hagen.
of the absence of the pleuro-branchia of the last thoracic
somite; and there are some other differences to which
it is not needful to refer at present. It is convenient to

THE ABSTRACTIONS, SPECIES AND GENUS. 249

distinguish these seventeen-gilled crayfishes, as a whole,
from the eighteen-gilled species ; and this is effected by
changing the generic name. They are no longer called
Astacns, but Cambarus (fig. 63).

All the individual crayfish referred to thus far, there-
fore, have been sorted out, first into the groups termed
species ; and then these species have been further sorted
into two divisions, termed genera. Each genus is an
abstraction, formed by summing up the common char-
acters of the species which it includes, just as each
species is an abstraction, composed of the common
characters of the individuals which belong to it ; and
the one has no more existence in nature than the other.
The definition of the genus is simply a statement of
the plan of structure which is common to all the species
included under that genus; just as the definition of the
species is a statement of the common plan of structure
which runs throughout the individuals which compose
the species.

Again, crayfishes are found in the fresh waters of the
Southern hemisphere ; and almost the whole of what
has been said respecting the structure of the English cray-
fish applies to these ; in other words, their general plan is
the same. But, in these southern crayfishes, the podo-
branchiee have no distinct lamina, and the first somite of
the abdomen is devoid of appendages in both sexes. The
southern crayfishes, like those of the Northern hemi-
sphere, are divisible into many species ; and these species

250 THE COMPARATIVE MORPHOLOGY OF THE CRAYFISH.

are susceptible of being grouped into six genera — Asta-
coides (fig. 65), Astacopsis, Ch<$raps, Parastactis (fig. 64),

Fig. 64.— Parasiticus hrasUiensis (J nat. size). From southern Brazil.

EngcexiSy and Parancphrops — on the same principle as
that which has led to the grouping of the Northern forms
into two genera. But the same convenience which has

252 THE COMPARATIVE MORPHOLOGY OF THE CRAYFISH.

led to the association of groups of similar species into
genera, lias given rise to the combination of allied genera
into higher groups, which are termed Families. It is
obvious that the definition of a family, as a statement of
the characters in which a certain number of genera agree,
is another morphological abstraction, which stands in the
same relation to generic, as generic do to specific abstrac-
tions. Moreover, the definition of the family is a statement
of the plan of all the genera comprised in that family.

The family of the Northern crayfishes is termed
Potamobiidcz ; that of the Southern crayfishes, Par-
astacidce. But these two families have in common all
those structural characters which are special to neither ;
and, carrying out the metaphorical nomenclature of the
zoologist a stage further, we may say that the two form
a Tribe — the definition of which describes the plan which
is common to both families.

It may conduce to intelligibility if these results are put
into a graphic form. In fig. 66, A. is a diagram represent-
ing the plan of an animal in which all the externally
visible parts which are found, more or less modified, in
the natural objects which we call individual crayfishes
are roughly sketched. It represents the plan of the
tribe. B. is a diagram exhibiting such a modification
of A. as converts it into the plan common to the whole
family of the Parastacidce. C. stands in the same re-
lation to the Potamobiidce. If the scheme were thoroughly
worked out, diagrams representing the peculiarities of

254 THE COMPARATIVE MORPHOLOGY OF THE CRAYFISH.

form which characterize each of the genera and species,
would appear in the place of the names of the former, or
of the circles which represent the latter. All these
figures would represent abstractions — mental images
which have no existence outside the mind. Actual facts
would begin with drawings of individual animals, which
we may suppose to occupy the place of the dots above
the upper line in the diagram.

That all crayfishes may be regarded as modifications of
the common plan A, is not an hypothesis, but a generali-
zation obtained by comparing together the observations
made upon the structure of individual crayfishes. It is
simply a graphic method of representing the facts which
are commonly stated in the form of a definition of the
tribe of crayfishes, or Astacina.

This definition runs as follows : —

Multicellular animals provided with an alimentary
canal and with a chitinous cuticular exoskeleton; with
a ganglionated central nervous system traversed by the
oesophagus ; possessing a heart and branchial respiratory
organs.

The body is bilaterally symmetrical, and consists of
twenty metameres (or somites and their appendages), of
which six are associated into a head, eight into a thorax,
and six into an abdomen. A telson is attached to the
last abdominal somite.

The somites of the abdominal region are all free, those
of the head and thorax, except the hindermost, which is

DEFINITION OF THE GROUP ASTACINA. 255

partially free, are united into a cephalothorax, the tergal
wall of which has the form of a continuous carapace.
The carapace is produced in front into a rostrum, at the
sides into branchiostegites.

The eyes are placed at the ends of movable stalks.
The antennules are terminated by two filaments. The
exopodite of the antenna has the form of a mobile scale.
The mandible has a palp. The first and second maxillse
are foliaceous ; the second being provided with a large
scaphognathite. There are three pairs of maxillipedes,
and the endopodites of the third pair are narrow and
elongated. The next pair of thoracic appendages is much
larger than the rest, and is chelate, as are the two fol-
lowing pairs, which are slender ambulatory limbs. The
hindmost two pairs of thoracic appendages are ambu-
latory limbs, like the foregoing, but not chelate. The
abdominal appendages are small swimmerets, except the
sixth pair, which are very large, and have the exopodite
divided by a transverse joint.

All the crayfishes have a complex gastric armature.
The seven anterior thoracic limbs are provided with
podobranchiae, but the first of these is always more or
less completely reduced to an epipodite. More or fewer
arthrobranchise always exist. Pleurobranchise may be
present or absent.

In this tribe of Astacina there are two families, the
Potamobiiclce and the Parastacidce ; and the definition of
each of these families is formed by superadding to the

256 THE COMPARATIVE MORPHOLOGY OF THE CRAYFISH.

definition of the tribe the statement of the special pecu-
liarities of the family.

Thus, the Potamobiiche are those Astacina in which
the podobranchise of the second, fourth, fifth, and sixth
thoracic appendages are always provided with a plaited
lamina, and that of the first is an epipodite devoid of
branchial filaments. The first abdominal somite invari-
ably bears appendages in the males, and usually in both
sexes. In the males these appendages are styliform, and
those of the second somite are always peculiarly modified.
The appendages of the four following somites are rela-
tively small. The telson is very generally divided by a
transverse incomplete hinge. None of the branchial fila-
ments are terminated by hooks ; nor are any of the
coxopoditic setae, or the longer setae of the podobranchiae
hooked, though hooked tubercles occur on the stem and
on the laminae of the latter. The coxopoditic setae are
always long and tortuous.

In the Parastacidce, on the other hand, the podo-
branchiae are devoid of more than a rudiment of a
lamina, though the stem may be alate. The podo-
branchia of the first maxillipede has the form of an
epipodite ; but, in almost all cases, it bears a certain
number of well developed branchial filaments. The first
abdominal somite possesses no appendages in either sex :
and the appendages of the four following somites are
large. The telson is never divided by a transverse hinge.
More or fewer of the branchial filaments of the podo-

ALLIES OF THE CRAYFISH.

257

branchiae are terminated by short hooked spines ; and the
coxopoditic setae, as well as those which beset the stems
of the podobranchiae, have hooked apices.

The definitions of the genera would in like manner be
given by adding the distinctive characters of each genus
to the definitions of the family ; and those of the species
by adding its character to those of the genus. But at
present it is unnecessary to pursue this topic further.

There are no other inhabitants of the fresh waters, or
of the land, which could be mistaken for crayfishes ; but
certain marine animals, familiar to every one, are so
strikingly similar to them, that one of these was formerly
included in the same genus, Astacus ; while another is
very often known as the " Sea-crayfish/ ' These are the
" Common Lobster," the " Norway Lobster," and the
" Eock Lobster " or " Spiny Lobster."

The common lobster (Homarus vulgaris, fig. 67)
presents the following distinctive characters. The last
thoracic somite is firmly adherent to the rest ; the exo-
podite of the antenna is so small as to appear like a mere
movable scale ; all the abdominal appendages are well
developed in both sexes ; and, in the males, the two an-
terior pairs are somewhat like those of the male Astacus,
but less modified.

The principal difference from the Astacina is exhibited
by the gills, of which there are twenty on each side ;
namely, six podobranchiae, ten arthrobranchue, and four

Fig. 67. Homarus vulgaris (J nat. size).

HOMARUS AND NEPHROPS.

259

fully developed pleurobranchise. Moreover, the bran-
chial filaments of these gills are much stiffer and more
closely set than in most crayfishes. But the most im-
portant distinction is presented by the podobranchise, in
which the stem is, as it were, completely split into two
parts longitudinally (as in fig. 68, B) ; one half (ep)

Fig. 68. Podobranchise of A, Parastacw ; B, JVephrops; C, Palcemon.
A', C, transverse sections of A and C respectively, a, point of attach-
ment ; alt wing-like expansion of the stem ; b, base ; br% branchial
filaments ; ep, epipodiie ; /, branchial laminae ; plume ; st, stem.

corresponding with the lamina of the crayfish gill, and the
other (pi) with its plume. Hence the base (b) of the
podobranchia bears the gill in front; while, behind, it
is continued into a broad epipoditic plate (ep) slightly
folded upon itself longitudinally but not plaited, as in the
crayfish.

The Norway Lobster (Nephrops norregicus, fig. 69)

s 2

Fig. 69. Nephrons norvcgicus (| nat. size).

THE ROCK LOBSTER (PALINURUS).

261

resembles the lobster in those respects in which the latter
differs from the crayfishes : but the antennary squame is
large; and, in addition, the branchial plume of the podo-
branchia of the second maxillipede is very small or absent,
so that the total number of functional branchiae is reduced
to nineteen on each side.

These two genera, Homarus and Nephrops, therefore,
represent a family, Homarina, constructed upon the
same common plan as the crayfishes, but differing so
far from the Astacina in the structure of the branchiae
and in some other points, that the distinction must be
expressed by putting them into a different tribe. It is
obvious that the special characteristics of the plan of the
Homarina give it much more likeness to that of the
Potamobiidcz than to that of the Parastacidce.

The Rock Lobster (Palinurus, fig. 70) differs much more
from the crayfishes than either the common lobster or
the Norway lobster does. Thus, to refer only to the more
important distinctions, the antennae are enormous ; none
of the five posterior pairs of thoracic limbs are chelate,
and the first pair are not so large in proportion to the
rest as in the crayfishes and lobsters. The posterior
thoracic sterna are very broad, not comparatively narrow,
as in the foregoing genera. There are no appendages
to the first somite of the abdomen in either sex. In
this respect, it is curious to observe that, in contradis-
tinction from the Homarina, the Rock Lobsters are more
closely allied to the Parastacidce than to the Potamobiidce.

Fig. 70. Palinvrus vulgaris (about \ nat. size).

TRICHOBRANCHLE.

263

The gills are similar to those of the lobsters, but reach
the number of twenty-one on each side.

In their fundamental structure the rock lobsters agree
with the crayfishes ; hence the plans of the two may be
regarded as modifications of a plan common to both.
To this end, the only considerable changes needful in
the tribal plan of the crayfishes, are the substitution of
simple for chelate terminations to the middle thoracic
limbs and the suppression of the appendages of the first
somite of the abdomen.

Thus not only all the crayfishes, but all the lobsters
and rock lobsters, different as they are in appearance,
size, and habits of life, reveal to the morphologist un-
mistakable signs of a fundamental unity of organization ;
each is a comparatively simple variation of the general
theme — the common plan.

Even the branchise, which vary so much in number in
different members of these groups, are constructed upon
a uniform principle, and the differences which they
present are readily intelligible as the result of various
modifications of one and the same primitive arrange-
ment.

In all, the gills are trichobranchice ; that is, each gill
is somewhat like a bottle-brush, and presents a stem
beset, more or less closely, with many series of bran-
chial filaments. The largest number of complete bran-
chiae possessed by any of the Potamobiidce, Parastacidce,
Homaridce, or Palinuridce, is twenty-one on each side ;

264 THE COMPARATIVE MORPHOLOGY OF THE CRAYFISH.

and when this number is present, the total is made up
of the same numbers of podobranchiae, arthrobran-
chiae, and pleurobranchiae attached to corresponding
somites. In Palinurus and in the genus Astacopsis
(which is one of the Parastacidce), for example, there are
six podobranchiae attached to the thoracic limbs from
the second to the seventh inclusively; five pairs of arthro-
branchise are attached to the interarticular membranes
of the thoracic limbs from the third to the seventh
inclusively, and one to that of the second, making eleven
in all ; while four pleurobranchiae are fixed to the
epimera of the four hindmost thoracic somites. More-
over, in Astacopsis, the epipodite of the first thoracic
appendage (the first maxillipede) bears branchial fila-
ments, and is a sort of reduced gill.

These facts may be stated in a tabular form as
follows : —

The branchial formula of Astacopsis.

Somites and

their
Appendages,

VII.

VIII.

IX.

X.

XI.

XII.

XIII.

XIV.

Podo-
branchise.

0 (ep. r.)

Arthrobranchise.
Anterior. Posterior.

Pleuro-
branchise.

0 =

0 -

0 =

0 =

1

1

1

1

0 (cp. r.)
2

3
3

A.

4
4
1

6 + ep. r. + G +

21 + ep. r.

BRANCHIAL FORMULAE.

265

This tabular " branchial formula " exhibits at a glance
not only the total number of branchiae, but that of each
kind of branchia ; and that of all kinds connected with
each somite; and it further indicates that the podo-
branchia of the first thoracic somite has become so far
modified, that it is represented only by an epipodite, with
branchial filaments scattered upon its surface.

In Palinurus, these branchial filaments are absent and
the branchial formula therefore becomes —

Somites and

their
Appendages.

Podo-
braneliise.

Arthrobrancliise.
Anterior. Posterior.

Pleuro-
branchise.

VII. ...

0 (ep.)

0

... 0 ...

r\

\j —

o

VIII. ...

1

1

... 0 ...

0

2

IX. ...

1

1

... 1

0 =

3

X. ...

1

1

... 1

0 =

3

XI. ...

1

1

1

1

4

XII. ...

1

1

I

1

4

XIII. ...

1

1

1 ...

1 =

4

XIV. ...

0

0

... 0 ...

1

1

6 + cp.

+ 6

+ 5 +

4 =

21

In the lobster, the solitary arthrobranchia of the eighth
somite disappears, and the branchiae are reduced to twenty
on each side.

In Astacas, this branchia remains ; but, in the English
crayfish, the most anterior of the pleurobranchiae has
vanished, and mere rudiments of the two next remain.
It has been mentioned that other Astaci present a
rudiment of the first pleurobranchia.

2GG THE COMPARATIVE MORPHOLOGY OF THE CRAYFISH.

The branchial formula of Astacus.

Somites and

Arthrobranchiae

their

Podo-

PI euro-

Appendages, branchiae. Anterior. Posterior.

branchiae.

VII. ...

0 (ep.)

.. 0

... 0 ..

. 0

=

0 (ep.)

VIII. ...

.. 1

... 0 ..

0

2

IX. ...

.. 1

... 1 ..

. 0

3

X. ...

.. 1

... 1 ..

. 0

3

XI. ...

.. 1

... 1 ..

0 or r

3 or 3 +

XII. ...

.. 1

... 1 ..

r

3 + r

XIII. ...

.. 1

... 1 ..

r

3 + r

XIV. ...

0

.. 0

... 0 ..

1

1

6 + ep.

+ 6

+ 5 +

1 + 2or3r

18 + ep.

In Cambarus, the number of the branchiae is reduced
to seventeen by the disappearance of the last pleuro-
branchia; while, in Astacoides, the process of reduction
is carried so far, that only twelve complete branchiae are
left, the rest being either represented by mere rudiments,
or disappearing altogether.

Somites and

their
Appendages.
VII. ...
VIII. ...
IX. ...
X. ...
XI. ...
XII. ...

XIII. ...

XIV. ...

The branchial formula of Astacoides.

Arthrobran chise.

Podo-

branchia?.

Anterior.

0 (ep. r.) 0
r
1
1
1
1
1

0

Posterior.
0
0
0
r

Pleuro-
branchise.
.. 0
.. 0
.. 0
.. 0
.. 0

.. o

.. 0

1

0 (ep. r.)

1 + r
2

2 + r
2 + r
2 + r

6 + cp.r

+ r + 0 + 4 r + 1 — 12 + ep. r. + 5 r.

THE BRANCHIAE OF PEN^EUS.

267

As these formulae show, those trichobranchiate Crus-
tacea, which possess fewer than twenty-one complete
branchiae on each side, commonly present traces of the
missing ones, either in the shape of epipodites, as in the
case of the podobranchiae, or of minute rudiments, in the
case of the arthrobranchiae and the pleurobranchiae.

In the marine, prawn-like, genus Penceus (fig. 73,
Chap. VI.), the gills are curiously modified trichobran-
chiae. The number of functional branchiae is, as in the
lobster, twenty ; but the study of their disposition shows
that the total is made up in a very different way.

The branchial formula of Penceus.

Somites and

Arthrobranchiae.

their

Podo-

Pleuro-

Appendages.

branchiae.

Anterior.

Posterior.

branchiae.

VII. ...

0 (ep.) ,

... 1

0 ...

0 =

1

+ ep.

VIII. ...

0 (ep.) .

1

1

1

3

+ ep.

IX. ...

0 (ep.) .

1

1

1

3

+ ep.

X. ...

0 (ep.) .

1

1

1

3

+ ep.

XI. ...

0 (ep.) .

1

1

1

3

+ ep.

XII. ...

0 (ep.) .

.. 1 ...

1

1

3

+ ep.

XIII. ...

0

1

1 ...

1

o

XIV. ...

0

.. 0 ...

0 ...

1

1

0 + 0 ep.

+ 7 +

6 +

7 =

20

+ G€

This case is very interesting; for it shows that the
whole of the podobranchiae may lose their branchial charac-
ter, and be reduced to epipodites, as is the case with the
first in the crayfish and lobster, and indeed in most of
the forms under consideration. And since all but one of
the somites bear both arthrobranchiae and pleurobranchige,

268 THE COMPARATIVE MORPHOLOGY OF THE CRAYFISH.

the suggestion arises that each hypothetically complete
thoracic somite should possess four gills on each side,
giving the following

Hypothetically complete branchial formula.

Somites and Arthrobranchiae.

their Podo- , — ■ * Pleuro-

Appendages. branchiae. Anterior. Posterior. branchiae.

vii. ... 1 ... 1 ... 1 ... 1 = 4

viii. ... 1 ... 1 ... 1 ... 1 = 4

ix. ... 1 ... 1 ... 1 ... 1 = 4

x. ... 1 ... 1 ... 1 ... 1 = 4

xi. ... 1 ... 1 ... 1 ... 1 = 4
xii. .. 1 ... 1 ... 1 ... 1 = 4

xin. ... 1 ... 1 ... 1 ... 1 = 4

xiv. ... 1 ... 1 ... 1 ... 1 = 4

8 + 8 + 8 + 8 = 32

Starting from this hypothetically complete branchial
formula, we may regard all the actual formulae as pro-
duced from it by the more or less complete suppression
of the most anterior, or of the most posterior branchiae,
or of both, in each series. In the case of the podo
branchiae, the branchiae are converted into epipodites ; in
that of the other branchiae, they become rudimentary, or
disappear.

In general appearance a common prawn (Palcemon,
fig. 71) is very similar to a miniature lobster or crayfish.
Nor does a closer examination fail to reveal a complete
fundamental likeness. The number of the somites, and
of the appendages, and their general character and dispo-

THE PRAWNS.

269

sition, are in fact the same. But, in the prawn, the abdomen
is much larger in proportion to the cephalothorax ; the

Fig. 71. Palamon jamaicensis (about nat. size). A. female ;
B, fifth thoracic appendage of male.

basal scale, or expodite of the antenna, is much larger ;
the external maxillipedes are longer, and differless from the
succeeding thoracic appendages. The first pair of these,
which answers to the forceps of the crayfish, is chelate,
but it is very slender ; the second pair, also chelate, is
always larger than the first, and is sometimes exceedingly

270 THE COMPARATIVE MORPHOLOGY OF THE CRAYFISH.

long and strong (fig. 71, B) ; the remaining thoracic
limbs are terminated by simple claws. The five anterior
abdominal somites are all provided with large swimmerets,
which are used like paddles, when the animal swims
quietly ; and, in the males, the first pair is only slightly
different from the rest. The rostrum is very large, and
strongly serrated.

None of these differences from the crayfish, however,
is so great, as to prepare us for the remarkable change
observable in the respiratory organs. The total number
of the gills is only eight. Of these, five are large pleuro-
branchise, attached to the epimera of the five hinder
thoracic somites ; two are arthrobranchise, fixed to the
interarticular membrane of the external maxillipede ; and
one, which is the only complete podobranchia, belongs
to the second maxillipede. The podobranchise of the
first and third maxillipedes are represented only by small

epipodites.

The branchial formula therefore is

Somites and

Arthrobran ch ise.

their

Podo-

Pleuro-

Appendages.

branehiae.

Anterior.

Posterior.

branchite.

VII. ...

0 (ep.)

0 .

.. 0 ..

. 0

0 (ep.)

VIII. ...

1

0

.. 0 ..

. 0

1

IX. ...

0 (ep.)

1

1

. 0 =

2 (ep.)

X. ...

0

0

.. 0 ..

XI. ...

0

0

0

xn. ...

0

0

0

XIII. ...

0

0

... 0 .

XIV. ...

0

0

0

1 + 2 ep. + 1 + 1 + 5

= 8 + 2 ep.

PHYLLOBKANCHLE.

271

The prawn, in fact, presents us with an extreme case
of that kind of modification of the branchial system, of
which Penceus has furnished a less complete example.
The series of the podobranchise is reduced almost to
nothing, while the large pleurobranchise are the chief
organs of respiration.

But this is not the only difference. The prawn's
gills are not brush-like, but are foliaceous. They are
not trichobranchice, but phyllobranchice ; that is to say,
che central stem of the branchia, instead of being beset
with numerous series of slender filaments, bears only two
rows of broad flat lamellae (fig. 68, C, C, i), which are
attached to opposite sides of the stem (C, s), and gradu-
ally diminish in size from the region of the stem by which
it is fixed, upwards and downwards. These lamellae are
superimposed closely upon one another, like the leaves of
a book ; and the blood traversing the numerous passages
by which their substance is excavated, comes into close
relation with the currents of aerated water, which are
driven between the branchial leaflets by a respiratory
mechanism of the same nature as that of the crayfish.

Different as these phyllobranchise of the prawns are in
appearance from the trichobranchise of the preceding
Crustacea, they are easily reduced to the same type. For in
the genus Axins, which is closely allied to the lobsters,
each branchial stem bears a single series of filaments on its
opposite sides ; and if these biserial filaments are sup-
posed to widen out into broad leaflets, the transition from

272 THE COMPARATIVE MORPHOLOGY OF THE CRAYFISH.

the trichobranchia to the phyllobranchia will be very
easily effected.

The shrimp (Crangon) also possesses pltyllobranchise,
and differs from the prawn chiefly in the character of its
locomotive and prehensile thoracic limbs.

There are yet other very well-known marine animals,
which, in common appreciation, are alwaj^s associated with
the lobsters and crayfishes, although the difference of
general appearance is vastly greater than in any of the
cases which have yet been considered. These are the
Crabs.

In all the forms we have hitherto been considering,
the abdomen is as long as, or longer than, the cephalo-
thorax, while its width is the same, or but little less.
The sixth somite has very large appendages, which,
together with the telson, make up a powerful tail-fin ;
and the large abdomen is thus fitted for playing an
important part in locomotion.

Again, the length of the cephalothorax is much greater
than its width, and it is produced in front into a long
rostrum. The bases of the antennae are freely movable,
and they are provided with a movable exopodite. More-
over, the eye-stalks are not inclosed in a cavity or orbit,
and the eyes themselves appear above and in front of
the antennules. The external maxillipedes are narrow,
and their endopodites are more or less leg-like.

None of these statements apply to the crabs. In these

THE CRABS.

273

animals the abdomen is short, flattened, and apt to escape
immediate notice, as it is habitually kept closely applied
against the under surface of the cephalothorax. It is

Fig. 72. Cancer pagvrv89male (i nat. size). A, dorsal view, with the
abdomen extended ; B, front view of "face." as, antennary sternum ;
or, orbit : r, rostrum ; 1. eyestalk ; 2. antennule ; 3. base of antenna ;
3\ free portion of antenna.

T

274 THE COMPARATIVE MORPHOLOGY OF THE CRAYFISH.

not used as a swimming organ ; and the sixth somite
possesses no appendages whatever. The breadth of the
cephalothorax is often greater than its length, and there
is no prominent rostrum. In its place there is a trun-
cated process (fig. 72, B, r), which sends down a vertical
partition, and divides from one another two cavities, in
which the swollen basal joints of the small antennules (2)
are lodged. The outer boundary of each of these cavities
is formed by the basal part of the antenna (3), which is
firmly fixed to the edge of the carapace. There is no exo-
poditic scale ; and the free part of the antenna (3') is very
small. The convex corneal surface of the eye appears
outside the base of the antenna, lodged in a sort of orbit
(or), the inner margin of which is formed by the base of
the antenna, while the upper and outer boundaries are
constituted by the carapace. Thus, while in all the pre-
ceding forms, the eye is situated nearest the middle line,
and is most forward, while the antennule lies outside
and behind it, and the antenna comes next ; in the crab,
the antennule occupies the innermost place, the antenna
comes next, and the eye appears to be external to and
behind the other two. But there is no real change in
the attachments of the eye-stalks. For if the antennule
and the basal joint of the antenna are removed, it will be
seen that the base of the eye-stalk is attached, as in the
crayfish, close to the middle line, on the inner side,
and in front of the antennule. But it is very long and
extends outwards, behind the antennule and the antenna ;

THE CKABS.

275

its corneal surface alone being visible, as it projects into
the orbit.

Again, the ischiopodites of the external maxillipedes
are expanded into broad quadrate plates, which meet in
the middle line, and close over the other manducatory
organs, like two folding-doors set in a square doorway.
Behind these there are great chelate forceps, as in the
crayfish; but the succeeding four pairs of ambulatory
limbs are terminated by simple claws.

When the abdomen is forcibly turned back, its sternal
surface is seen to be soft and membranous. There are no
swimmerets ; but, in the female, the four anterior pairs
of abdominal limbs are represented by singular appen-
dages, which give attachment to the eggs ; while in the
males there are two pairs of styliform organs attached
to the first and second somites of the abdomen, which
correspond with those of the male crayfishes.

The ventral portions of the branchiostegites are
sharply bent inwards, and their edges are so closely
applied throughout the greater part of their length to
the bases of the ambulatory limbs, that no branchial
cleft is left. In front of the bases of the forceps, how-
ever, there is an elongated aperture, which can be shut
or opened by a sort of valve, connected with the external
maxillipede, which serves for the entrance of water into
the branchial cavity. The water employed in respiration,
and kept in constant motion by the action of the sea-
phognathite, is baled out through two apertures, which

T 2

276 THE COMPARATIVE MORPHOLOGY OF THE CRAYFISH.

are separated from the foregoing by the external maxilli-
pedes, and lie at the sides of the quadrate space in
which these organs are set.

There are only nine gills on each side, and these,
as in the prawn and shrimp, are phyllobranchise.
Seven of the branchiae are pyramidal in shape, and for
the most part of large size. When the branchiostegite
is removed, they are seen lying close against its inner
walls, their apices converging towards its summit. The
two hindermost of these gills are pleurobranchiae, the
other five are arthrobranchise. The two remaining gills
are podobranchise, and belong to the second and the
third maxillipedes respectively. Each is divided into a
branchial and an epipoditic portion, the latter having the
form of a long curved blade. The " branchial portion of
the podobranchia of the second maxillipede is long, and
lies horizontally under the bases of the four anterior
arthrobranchise ; while the gill of the podobranchia of
the third maxillipede is short and triangular, and fits in
between the bases of the second and the third arthro-
branchise. The epipodite of the third maxillipede is very
long, and its base furnishes the valve of the afferent
aperture of the branchial cavity, which has been men-
tioned above. The podobranchia of the first maxillipede
is represented only by a long curved epipoditic blade,
wThich can sweep over the outer surface of the gills, and
doubtless serves to keep them clear of foreign bodies.

THE BRANCHIAE OF THE CRABS.

277

The branchial formula of Cancer pagurus.

Somites and

their
Appendages.
VII.
VIII.
IX.
X.
XI.
XII.
XIII.
XIV.

Podo-
branchise.

0 (ep.)

1

1

0
0
0
0
0

Arthro branchne ,

Anterior.
0
1
1
1
0
0
0
0

Posterior.

0 ...

0 ...

1 ...
1

0 ...

0 ...

0 ...

0 ...

Pleuro-

branchise.

0 =

0 =

0 =

0 =

1
1

0 =
0

2 + ep. + 3

0

2
3
2
1
1
0
0

9 + ep.

It will be observed that the suppression of branchiae
has here taken place in all the series, and at both the
anterior and the posterior ends of each. But the defect
in total number is made up by the increase of size, not of
the pleurobranchise alone, as in the case of the prawns,
but of the arthrobranchise as well. At the same time
the whole apparatus has become more specialized and
perfected as a breathing organ. The close fitting of the
edges of the carapace, and the possibility of closing the
inhalent and exhalent apertures, render the crabs much
more independent of actual immersion in water than most
of their congeners ; and some of them habitually live on
dry land and breathe by means of the atmospheric air
which they take into and expel from their branchial cavities.

Notwithstanding all these wide departures from tha
structure and habits of the crayfishes, however, attentive
examination shows that the plan of construction of the

278 THE COMPARATIVE MORPHOLOGY OF THE CRAYFISH.

crab is, in all fundamental respects, the same as that of the
crayfish. The body is made up of the same number of
somites. The appendages of the head and of the thorax
are identical in number, in function, and even in the
general pattern of their structure. But two pairs of
abdominal appendages in the female, and four pairs in
the male, have disappeared. The exopodites of the
antennae have vanished, and not even epipodites re-
main to represent the podobranchise of the posterior five
pairs of thoracic limbs. The exceedingly elongated eye-
stalks are turned backwards and outwards, above the
bases of the antennules and the antennae, and the bases
of the latter have become united with the edges of the
carapace in front of them. In this manner the extra-
ordinary face, or metope (fig. 72, B) of the crab results
from a simple modification of the arrangement of parts,
every one of which exists in the crayfish. The same
common plan serves for both.

The foregoing illustrations are taken from a few of our
commonest and most easily obtainable Crustacea ; but they
amply suffice to exemplify the manner in which the con-
ception of a plan of organization, common to a multitude
of animals of extremely diverse outward forms and habits,
is forced upon us by mere comparative anatomy.

Nothing would be easier, were the occasion fitting, than
to extend this method of comparison to the whole of the
several thousand species of crab-like, crayfish-like, or

THE CRUSTACEA.

279

prawn-like animals, which, from the fact that they all
have their eyes set upon movable stalks, are termed the
Podophthalmia, or stalk-eyed Crustacea ; and by argu-
ments of similar force to prove that they are all modifica-
tions of the same common plan. Not only so, but the
sand-hoppers of the sea-shore, the wood-lice of the land,
and the water-fleas or the monoculi of the ponds, nay,
even such remote forms as the barnacles which adhere to
floating wood, and the acorn shells wThich crowd every inch
of rock on many of our coasts, reveal the same funda-
mental organization. Further than this, the spiders
and the scorpions, the millipedes and the centipedes, and
the multitudinous legions of the insect world, show us,
amid infinite diversity of detail, nothing which is new in
principle to any one who has mastered the morphology
of the crayfish.

Given a body divided into somites, each with a pair
of appendages : and given the power to modify those
somites and their appendages in strict accordance with
the principles by which the common plan of the Podoph-
thalmia is modified in the actually existing members of
that order ; and the whole of the Arthropoda, which
probably make up two-thirds of the animal world, might
readily be educed from one primitive form.

And this conclusion is not merely speculative. As a
matter of observation, though the Arthropoda are not all
evolved from one primitive form, in one sense of the
words, yet they are in another. For each can be traced

'ISO THE COMPARATIVE MORPHOLOGY OF THE CRAYFISH.

back in the course of its development to an ovum, and
that ovum gives rise to a blastoderm, from which the
parts of the embryo arise in a manner essentially similar
to that in which the young cra}^fish is developed.

Moreover, in a large proportion of the Crustacea, the
embryo leaves the egg under the form of a small oval
body, termed a Nauplius (fig. 73, D), provided with
(usually) three pairs of appendages, which play the part
of swimming limbs, and with a median eye. Changes of
form accompanied by sheddings of the cuticle take place,
in virtue of which the larva passes into a new stage, when
it is termed a Zocea (C). In this, the three pairs of loco-
motive appendages of the Nauplius are metamorphosed
into rudimentary antennules, antennae, and mandibles,
while two or more pairs of anterior thoracic appendages
provided with exopodites and hence appearing bifurcated,
subserve locomotion. The abdomen has grown out and
become a notable feature of the Zosea, but it has no
appendages.

In some Podophtlialmia, as in Penceus (fig. 73), the
young leaves the egg as a Nauplius, and the Nauplius
becomes a Zosea. The hinder thoracic appendages, each
provided with an epipodite, appear ; the stalked eyes and
the abdominal members are developed, and the larva passes
into what is sometimes called the Mysis or Schizopod
stage. The adult state differs from this chiefly in the
presence of branchiae and the rudimentary character of
the exopodites of the five posterior thoracic limbs.

METAMORPHOSES OF THE CRUSTACEA.

281

In the Opossum-shrimps (Mysis) the young does
not leave the pouch of the mother until it is fully

A

Fig. 73. Penceus semixulcatns. A, adult (after deHaan. h nat. size) ;
B, Zoaea, and C, less advanced Zoaea of a species of Ptnceus. D,
Nauplius. (B, C, and D, after Fritz Muller.)

282 THE COMPARATIVE MORPHOLOGY OF THE CRAYFISH.

developed ; and, in this case, the Nauplius state is
passed through so rapidly and in so early and imperfect a
condition of the embryo, that it would not be recognized

Fig. 74. Cancer pagur us. A, newly hatched Zosea ; B, more advanced
Zoasa ; C, dorsal, and D, side view of Megalopa (after Spence Bate).
The figures A and B are more magnified than C and D.)

except for the cuticle which is developed and is subse-
quently shed.

METAMORPHOSES OF THE CRUSTACEA. 283

In the great majority of the Podophthalmia, the Nauplius
stage seems to be passed over without any such clear
evidence of its occurrence, and the young is set free as ,a
Zosea. In the lobsters, which have, throughout life, a
large abdomen provided with swimmerets, the Zosea,
after going through a My sis or Schizopod stage, passes
into the adult form.

In the crab, the young leaves the egg as a Zosea
(fig. 74, A and B). But this is not followed by a
Schizopod stage, inasmuch as the five hinder pair of
thoracic limbs are apparently, from the first, devoid of
exopodites. But the Zosea, after it has acquired stalked
eyes and a complete set of thoracic and abdominal
members, and has passed into what is called the Mega-
lopa stage (fig. 74, C and D), suffers a more complete
metamorphosis. The carapace widens, the fore part of
the head is modified so as to bring about the formation
of the characteristic metope : and the abdomen, losing
more or fewer of its posterior appendages, takes up its
final position under the thorax.

In the Zosea state, those thoracic limbs which give rise
to the maxillipedes are provided with well-developed
exopodites, and in the free Mysis state all these limbs
have exopodites. In the Opossum-shrimps these persist
throughout life ; in Penceus, the rudiments of them only
remain ; in the lobster, they disappear altogether.

Thus, in these animals, there is no difficulty in demon-
strating that embryolo^ical uniformity of type of all the

284 THE COMPARATIVE MORPHOLOGY OF THE CRAYFISH.

limbs, complete evidence of which was not furnished by the
development of the crayfish. In this crustacean, in fact,
it would appear that the process of development has
undergone its maximum of abbreviation. The embryo
presents no distinct and independent Nauplius or Zosea
stages, and, as in the crab, there is no Schizopod or
Mysis stage. The abdominal appendages are developed
very early, and the new born young, which resembles the
Megalopa stage of the crab, differs only in a few points
from the adult animal.

Guided by comparative morphology, we are thus led
to admit that the whole of the Arthropoda are connected
by closer or more remote degrees of affinity with the
crayfish. If we were to study the perch and the pond-
snail with similar care, we should be led to analogous
conclusions. For the perch is related by similar grada-
tions, in the first place, with other fishes ; then more
remotely, with frogs and newts, reptiles, birds, and
mammals ; or, in other words, with the whole of the
great division of the Vertebrata. The pond-snail, by
like reasoning upon analogous data, is connected with
the Mollusca, in all their innumerable kinds of slugs,
shellfish, squids, and cuttlefish. And, in each case, the
study of development takes us back to an egg as the
primary condition of the animal, and to the process of
yelk division, the formation of a blastoderm, and the con-
version of that blastoderm into a more or less modified

THE COMMON PLAN OF ANIMALS.

285

gastrula, as the early stages of development. The like
is true of all the worms, sea-urchins, starfishes, jellyfishes,
polypes, and sponges ; and it is only in the minutest and
simplest forms of animal life that the germ, or repre-
sentative of the ovum becomes metamorphosed into the
adult form without the preliminary process of division.

In the majority even of these Protozoa, the typical
structure of the nucleated cell is retained, and the whole
animal is the equivalent of a histological unit of one of
the higher organisms. An Amoeba is strictly comparable,
morphologically, to one of the corpuscles of the blood of
the crayfish.

Thus, to exactly the same extent as it is legitimate
to represent all the crayfishes as modifications of the
common astacine plan, it is legitimate to represent all
the multicellular animals as modifications of the gastrula,
and the gastrula itself as a peculiarly disposed aggregate
of cells ; while the Protozoa are such cells either isolated,
or otherwise aggregated.

It is easy to demonstrate that all plants are either
cell aggregates, or simple cells ; and as it is impossible
to draw any precise line of demarcation, either physio-
logical or morphological, between the simplest plants,
and the simplest of the Protozoa, it follows that all forms
of life are morphologically related to one another ; and
that in whatever sense we say that the English and the
Californian crayfish are allied, in the same sense, though
not to the same degree, must we admit that all living things

286 THE COMPARATIVE MORPHOLOGY OF THE CRAYFISH.

are allied. Given one of those protoplasmic bodies, of
which we are unable to say certainly whether it is animal
or plant, and endow it with such inherent capacities of
self-modification as are manifested daily under our eyes
by developing ova, and we have a sufficient reason for
the existence of any plant, or of any animal.

This is the great result of comparative morphology;
and it is carefully to be noted that this result is not a
speculation, but a generalisation. The truths of anatomy
and of embryology are generalised statements of facts
of experience ; the question whether an animal is more
or less like another in its structure and in its develop-
ment, or not, is capable of being tested by observation ;
the doctrine of the unity of organisation of plants and
animals is simply a mode of stating the conclusions
drawn from experience. But, if it is a just mode of
stating these conclusions, then it is undoubtedly con-
ceivable that all plants and all animals may have been
evolved from a common physical basis of life, by pro-
cesses similar to those which we every day see at work
in the evolution of individual animals and plants from
that foundation.

That which is conceivable, however, is by no means
necessarily true ; and no amount of purely morpho-
logical evidence can suffice to prove that the forms
of life have come into existence in one way rather
than another.

There is a common plan among churches, no less than

THE MORPHOLOGICAL UNITY OF LIVING THINGS. 287

among crayfishes ; nevertheless the churches have cer-
tainly not been developed from a common ancestor, but
have been built separately. Whether the different kinds
of crayfishes have been built separately, is a problem we
shall not be in a position to grapple with, until we have
considered a series of facts connected with them, which
have not yet been touched upon.

CHAPTER VI.

THE DISTRIBUTION AND THE AETIOLOGY OF THE
CRAYFISHES.

So far as I have been able to discover, all the cray-
fishes which inhabit the British islands agree in every
point with the full description given above, at p. 230.
They are abundant in some of our rivers, such as the
Isis, and other affluents of the Thames ; and they have
been observed in those of Devon ; * but they appear to
be absent from many others. I cannot hear of any, for
example, in the Cam or the Ouse, on the east, or in
the rivers of Lancashire and Cheshire, on the west.
It is still more remarkable that, according to the best
information I can obtain, they are absent in the Severn,
though they are plentiful in the Thames and Severn canal.
Dr. M'Intosh, who has paid particular attention to the
fauna of Scotland, assures me that crayfish are unknown
north of the Tweed. In Ireland, on the other hand,
they occur in many localities ; t but the question whether
their diffusion, and even their introduction into this

* Moore. Magazine of Natural History. New Series, III., 1839.
t Thompson. Annals and Magazine of Natural History, XL, 1843.

THE NAME ASTACUS FLUVIATILIS.

289

island, has or has not been effected by artificial means, is
involved in some obscurity,

English zoologists have always termed our crayfish
Astaeus fluviatilis ; and, up to a recent period, the
majority of Continental naturalists have included a
corresponding form of Astaeus under that specific name.

Thus M. Milne Edwards, in his classical work on the
Crustacea* published in 1837, observes under the head of
"Ecrevisse commune. Astaeus fluviatilis " There are
two varieties of this crayfish ; in the one, the rostrum
gradually becomes narrower from its base onwards, and
the lateral spines are situated close to its extremity;
in the other, the lateral edges of the rostrum are parallel
in their posterior half and the lateral spines are stronger
and more remote from the end."

The " first variety," here mentioned, is known under
the name of " Ecrevisse a pieds blancs" f in France,
by way of distinction from the " second variety," which
is termed "Ecrevisse a pieds rouges," on account of
the more or less extensive red coloration of the forceps
and ambulatory limbs. This second variety is the larger,
commonly attaining five inches in length, and sometimes
reaching much larger dimensions ; and it is more highly
esteemed for the market, on account of its better flavour.

In Germany, the two forms have long been popularly
distinguished, the former by the name of " Steinkrebs,"

* " Histoire Naturelle des Crustacea."
f Carbonnier. " I/iEcrevisse," p. 8.

V

2<J0 DISTRIBUTION AND iETIOLOGY OF THE CRAYFISHES.

or " stone crayfish," and the latter by that of u Edel-
krebs," or " noble crayfish."

Milne Edwards, it will be observed, speaks of these
two forms of crayfish as " varieties " of the species
Astacus fluviatilis ; but, even as far back as the year
1803 some zoologists began to regard the " stone cray-
fish " as a distinct species, to which Schrank applied the
name of Astacus torrentium, while the " noble crayfish"
remained in possession of the old denomination, Astacus
fluviatilis ; and, subsequently, various forms of " stone-
crayfishes " have been further distinguished as the species
Astacus saxatilis, A. tristis, A. pallipes, A. fontinalis,
&c. On the other hand, Dr. Gerstfeldt,* who has devoted
especial attention to the question, denies that these
are anything more than varieties of one species ; but he
holds this and Milne Edwards's " second variety " to be
specifically distinct from one another.

We thus find ourselves in the presence of three views
respecting the English and French crayfishes.

1. They are all varieties of one species — A. fluviatilis.

2. There are two species — A. fluviatilis, and A. tor-
rentium, of which last there are several varieties.

3. There are, at fewest, five or six distinct species.
Before adopting the one or the other of these

views, it is necessary to form a definite conception of
the meaning of the terms " species " and " variety."

* " Ueber die Flusskrebse Europas." Mem, de l'Acad. de St. Peters-
burg, 1859,

THE MEANING OF THE WORD SPECIES.

291

The word " species " in Biology has two significations ;
the one based upon morphological, the other upon
physiological considerations.

A species, in the strictly morphological sense, is simply
an assemblage of individuals which agree with one another,
and differ from the rest of the living world in the sum
of their morphological characters ; that is to say, in
the structure and in the development of both sexes.
If the sum of these characters in one group is repre-
sented by A, and that in another by A + n ; the two
are morphological species, whether n represents an
important or an unimportant difference.

The great majority of species described in works on
Systematic Zoology are merely morphological species.
That is to say, one or more specimens of a kind of animal
having been obtained, these specimens have been found
to differ from any previously known by the character or
characters n ; and this difference constitutes the defi-
nition of the new species, and is all we realty know
about its distinctness.

But, in practice, the formation of specific groups is
more or less qualified by considerations based upon what
is known respecting variation. It is a matter of obser-
vation that progeny are never exactly like their parents,
but present small and inconstant differences from them.
Hence, when specific identity is predicated of a group of
individuals, the meaning conveyed is not that they are
all exactly alike, but only that their differences are so

u 2

292 DISTRIBUTION AND ETIOLOGY OF THE CRAYFISHES.

small, and so inconstant, that they lie within the
probable limits of individual variation.

Observation further acquaints us with the fact, that,
sometimes, an individual member of a species may
exhibit a more or less marked variation, which is pro-
pagated through all the offspring of that individual,
and may even become intensified in them. And, in
this manner, a variety, or race, is generated within the
species ; which variety, or race, if nothing were known
respecting its origin, might have every claim to be
regarded as a separate morphological species. The
distinctive characters, of a race, however, are rarely
equally well marked in all the members of the race.
Thus suppose the species A to develope the race A + x ;
then the difference x is apt to be much less in some
individuals than in others; so that, in a large suite of
specimens, the interval between A + x and A will be
filled up by a series of forms in which x gradually
diminishes.

Finally, it is a matter of observation that modification
of the physical conditions under which a species lives
favours the development of varieties and races.

Hence, in the case of two specimens having respec-
tively the characters A and A + n, although, prima facie,
they are of distinct species ; yet if a large collection
shows us that the interval between A and A + n is filled
up by forms of A having traces of n, and forms of A + n
in which n becomes less and less, then it will be con-

VARIETIES AND TRANSITIONAL FORMS. 293

eluded that A and A + n are races of one species and
not separate species. And this conclusion will be fortified
if A and A + n occupy different stations in the same
geographical area.

Even when no transitional forms between A and A + n
are discoverable, if n is a small and unimportant differ-
ence, such as of average size, colour, or ornamenta-
tion, it may be fairly held that A and A + n are mere
varieties ; inasmuch as experience proves that such
variations may take place comparatively suddenly; or
the intermediate forms may have died out and thus the
evidence of variation may have been effaced.

From what has been said it follows that the groups
termed morphological species are provisional arrange-
ments, expressive simply of the present state of our
knowledge.

We call two groups species, if we know of no tran-
sitional forms between them, and if there is no reason to
believe that the differences which they present are such
as may arise in the ordinary course of variation. But
it is impossible to say whether the progress of in-
quiry into the characters of any group of individuals
may prove that what have hitherto been taken for mere
varieties are distinct morphological species ; or whether,
on the contrary, it may prove that what have hitherto
been regarded as distinct morphological species are mere
varieties.

What has happened in the case of the crayfish is this :

294 DISTRIBUTION AND AETIOLOGY OF THE CRAYFISHES,

the older observers lumped all the Western European
forms which came under their notice under one species,
Astacus fluviatilis ; noting, more or less distinctly, the
stone crayfish and the noble crayfish as races or varieties
of that species. Later zoologists, comparing crayfishes
together more critically, and finding that the stone
crayfish is ordinarily markedly different from the noble
crayfish, concluded that there were no transitional forms,
and made the former into a distinct species, tacitly as-
suming that the differential characters are not such as
could be produced by variation.

It is at present an open question whether further
investigation will or will not bear out either of these
assumptions. If large series of specimens of both stone
crayfishes and noble crayfishes from different localities
are carefully examined, they will be found to present
great variations in size and colour, in the tuberculation
of the carapace and limbs, and in the absolute and
relative sizes of the forceps.

The most constant characters of the stone crayfish
are : —

1. The tapering form of the rostrum and the approxi-
mation of the lateral spines to its point ; the distance
between these spines being about equal to their distance
from the apex of the rostrum (fig. 61, A).

2. The development of one or two spines from the
ventral margin of the rostrum.

3. The gradual subsidence of the posterior part of

THE STONE CRAYFISH AND THE NOBLE CRAYFISH. 295

the post-orbital ridge, and the absence of spines on its
surface.

4. The large relative size of the posterior division of
the telson (G).

On the contrary, in the noble crayfish : —

1. The sides of the posterior two - thirds of the
rostrum are nearly parallel, and the lateral spines are
fully a third of the length of the rostrum from its point ;
the distance between them being much less than their
distance from the apex of the rostrum (B).

2. No spine is developed from the ventral margin of
the rostrum.

3. The posterior part of the post-orbital ridge is a
more or less distinct, sometimes spinous elevation.

4. The posterior division of the telson is smaller
relatively to the anterior division (H).

I may add that I have found three rudimentary pleuro-
branchise in the noble crayfish, and never more than two
in the stone crayfish.

In order to ascertain whether no crayfish exist in
which the characters of the parts here referred to are
intermediate between those defined, it would be neces-
sary to examine numerous examples of each kind of cray-
fish from all parts of the areas which they respectively
inhabit. This has been done to some extent, but by no
means thoroughly ; and I think that all that can be safely
said, at present, is that the existence of intermediate
forms is not proven. But, whatever the constancy of the

296 DISTRIBUTION AND ^ETIOLOGY OF THE CRAYFISHES.

differences between the two kinds of crayfishes, there can
surely be no doubt as to their insignificance ; and no
question that they are no more than such as, judging by
analogy, might be produced by variation.

From a morphological point of view, then, it is really
impossible to decide the question whether the stone cray-
fish and the noble crayfish should be regarded as species
or as varieties. But, since it will, hereafter, be convenient
to have distinct names for the two kinds, I shall speak
of them as Astacus torrentium and Astacus nobilis.*

In the physiological sense, a species means, firstly, a
group of animals the members of which are capable of
completely fertile union with one another, but not with
the members of any other group ; and, secondly, it
means all the descendants of a primitive ancestor or
ancestors, supposed to have originated otherwise than by
ordinary generation.

It is clear that, even if crayfishes had an unbegotten
ancestor, there is no means of knowing whether the
stone crayfish and the noble crayfish are descendants of
the same, or of different ancestors, so that the second
sense of species hardly concerns us. As to the first
sense, there is no evidence to show whether the two

* According- to strict zoological usage the names should be written
A. fluviattlis (var. torrentium) and A. fluviatilis (var. nobilis) on the
hypothesis that the stone crayfish and the noble crayfish are varieties ;
and A. torrentium and A. fluviatilis on the hypothesis that they are
species ; but as I neither wish to prejudge the species question, nor to
employ cumbrously long names, I take a third course

PHYSIOLOGICAL SPECIES.

297

kinds of crayfish under consideration are capable of fertile
union or whether they are sterile. It is said, however,
that hybrids or mongrels are not met with in the waters
which are inhabited by both kinds, and that the breeding
season of the stone crayfish begins earlier than that of
the noble crayfish.

M. Carbonnier, who practises crayfish culture on a large
scale, gives some interesting facts bearing on this ques-
tion in the work already cited. He says that, in the
streams of France, there are two very distinct kinds of
crayfishes — the red-clawed crayfish (L'Ecrevisse a pieds
rouges), and the white-clawed crayfish (L'Ecrevisse a
pieds blancs), and that the latter inhabit the swifter
streams. In a piece of land converted into a crayfish
farm, in which the white-clawed crayfish existed natur-
ally in great abundance, 300,000 red-clawed crayfish
were introduced in the course of five years ; neverthe-
less, at the end of this time, no intermediate forms were
to be seen, and the " pieds rouges" exhibited a marked
superiority in size over the " pieds blancs." M. Car-
bonnier, in fact, says that they were nearly twice as big.

On the whole, the facts as at present known, seem to
incline rather in favour of the conclusion that A. torren-
tium and A. nobilis are distinct species; in the sense
that transitional forms have not been clearly made out,
and that, possibly, they do not interbreed.

As I have already remarked, the very numerous

298 DISTRIBUTION AND ETIOLOGY OF THE CRAYFISHES.

specimens of English and Irish crayfishes which have
passed through my hands, have all presented the charac-
ter of Astacus torrentium, with which also the description
given in works of recognised authority coincides as far as
it goes.* The same form is found in many parts of
France, as far south as the Pyrenees, and it is met with
as far east as Alsace and Switzerland. I have recently t
been enabled, by the kindess of Dr. Bolivar, of Madrid,
who sent me a number of crayfishes from the neighbour-
hood of that city, to satisfy myself that the Spanish
peninsula contains crayfishes altogether similar to those
of Britain, except that the subrostral spine is less de-
veloped. Further, I have no doubt that Dr. Heller i is
right in his identification of the English crayfish with
a form which he describes under the name of A.
saxatilis. He says that it is especially abundant in
Southern Europe, and that it occurs in Greece, in
Dalmatia, in the islands of Cherso and Veglia, at Trieste,
in the Lago di Garda, and at Genoa. Further, Astacus
torrentium appears to be widely distributed in North
Germany. The eastern limit of this crayfish is uncertain ;
but, according to Kessler,§ it does not occur within the
limits of the Bussian empire.

* See Bell. " British Stalk-eyed Crustacea," p. 237.

+ Since the statement respecting the occurrence of crayfishes in Spain
on p. 44 was printed.

J « Die Crustaceen des Siidlichen Europas," 1863.

§ " Die Russischen Flusskrebse." Bulletin de la Society Imperiale
des Naturalistes de Moscow, 1874.

ASTACUS NOBILIS. 299

Astacus torrentium appears to be particularly addicted
to rapid highland streams and the turbid pools which
they feed.

Astacus nobilis is indigenous to France, Germany, and
the Italian peninsula. It is said to be found at Nice
and at Barcelona, though I cannot hear of it elsewhere
in Spain. Its south-eastern limit appears to be the Lake
of Zirknitz, in Carniola,tnot far from the famous caves of
Adelsberg. It is not known in Dalmatia, in Turkey, nor
hi Greece. In the Russian empire, according to Kessler,
this crayfish chiefly inhabits the watershed of the Baltic.
The northern limit of its distribution lies between Chris-
tianstad, in the Gulf of Bothnia (62° 16' N), and Serdobol,
at the northern end of Lake Ladoga. " Eastward of
Lake Ladoga it is found in the Uslanka, a tributary of
the Swir. It appears to be the only craj^fish which exists
in the waters which flow from the south into the Gulf of
Finland and into the Baltic; except in those streams and
lakes which have been artificial^ connected with the Volga,
and in which it is partially replaced by A. leptodactyliis."
It still inhabits the Lakes of Beresai and Bologoe, as
well as the affluents of the Msta and the Wolchow ; and
it is met with in affluents of the Dnieper, as far as
Mohilew. Astacus nobilis is also found in Denmark and
Southern Sweden ; but, in the latter country, its intro-
duction appears to have been artificial. This crayfish
is said occasionally to be met with on the Livonian coast
in the waters of the Baltic, which, however, it must

300 DISTRIBUTION AND ETIOLOGY OF THE CRAYFISHES.

be remembered, are much less salt than ordinary sea
Mater.

It will be observed that while the two forms, A. torren-
tium and A. nobilis, are intermixed over a large part of
Central Europe, A. torrentium has a wider north-west-
ward, south-westward, and south-eastward extension,
being the sole occupant of Britain, and apparently of
the greater part of Spain and of Greece. On the other
hand, in the northern and eastern parts of Central
Europe, A. nobilis appears to exist alone.

Further to the east, a new form, Astacus leptodactylus
(fig. 75), makes its appearance. Whether A. leptodactylus
exists in the upper waters of the Danube, does not appear,
but in the lower Danube and in the Theiss it is the domi-
nant, if not the exclusive, crayfish. From hence it extends
through all the rivers which flow into the Black, Azov,
and Caspiar Seas, from Bessarabia and Podolia on the
west, to the Ural mountains on the east. In fact, the
natural habitat of this crayfish appears to be the water-
shed of the Pontocaspian area, excluding that part of the
Black Sea which lies southward of the Caucasus on the
one hand, and of the mouths of the Danube on the other.*

It is a remarkable circumstance that this crayfish not
only thrives in the brackish waters of the estuaries of
the rivers which debouche into the Black Sea and the
Sea of Azov, but that it is found even in the Salter

* These statements rest on the authority of Kessler and Gerstfeldt3
in their memoirs already cited.

Fig. 75. — Astacus leptodactylus (after Ratlike, £ nat. size).

302 DISTRIBUTION AND AETIOLOGY OF THE CRAYFISHES.

southern parts of the Caspian, in which it lives at
considerable depths.

In the north, Astacus leptodactylus is met with in the
rivers which flow into the White Sea, as well as in many
streams and lakes about the Gulf of Finland. But it
has probably been introduced into these streams by the
canals which have been constructed to connect the basin
of the Volga with the rivers which flow into the Baltic
and into the White Sea. In the latter, the invading A.
leptodactylus is everywhere overcoming and driving out
A. nobilis in the struggle for existence, apparently in
virtue of its more rapid multiplication.*

In the Caspian and in the brackish waters of the
estuaries of the Dniester and the Bug, a somewhat
different crayfish, which has been called Astacus pachypus,
occurs ; another closely allied form (A . angulosus) is met
with in the mountain streams of the Crimea and of the
northern face of the Caucasus ; and a third, A. colchicus,
has recently been discovered in the Bion, or Phasis of
the ancients, which flows into the eastern extremity of
the Black Sea.

With respect to the question whether these Ponto-
caspian crayfishes are specifically distinct from (me
another, and whether the most widely distributed kind,
A. leptodactylus, is distinct from A. nobilis, exactly the
same difficulties arise as in the case of the west European

* Kessler (Die Russischen Flusskrebse, 1. c. p. 3G9-70), lias an in-
teresting discussion of this question.

ASTACUS LEPTODACTYLUS.

303

crayfishes. Gerstfeldt, who has had the opportunity of
examining large series of specimens, concludes that the
Pontocaspian crayfishes and A. nobilis are all varieties
of one species. Kessler, on the contrary, while he
admits that A. angulosus is, and A. pachypus may be,
a variety of A. leptodactylus, affirms that the latter is
specifically distinct from A. nobilis.

Undoubtedly, well marked examples of A. leptodactylus
are very different from A. nobilis.

1. The edges of the rostrum are produced into five or
six sharp spines, instead of being smooth or slightly
serrated as in A. nobilis.

2. The fore part of the rostrum has no serrated
spinous median keel, such as commonly, though not uni-
versally, exists in A. nobilis.

3. The posterior end of the post-orbital ridge is still
more distinct and spiniform than in A. nobilis.

4. The abdominal pleura of A. leptodactylus are nar-
rower, more equal sided, and triangular in shape.

5. The chelae of the forceps, especially in the males,
are more elongated ; and the moveable and fixed claws
are slenderer and have their opposed edges straighter
and less tuberculated.

But, in all these respects, individual specimens of
A. nobilis vary in the direction of A. leptodactylus and
vice versa; and if A. angulosus and A. pachypus are
varieties of A. leptodactylus, I cannot see why Gerst-
feldt's conclusion that A. nobilis is another variety of

304 DISTRIBUTION AND ^ETIOLOGY OF THE CRAYFISHES.

the same form need be questioned on morphological
grounds. However, Kessler asserts that, in those lo-
calities in which A. leptodactylns and A. nobilis live
together, no intermediate forms occur, which is pre-
sumptive evidence that they do not intermix by breeding.

No crayfishes are known to inhabit the rivers of the
northern Asiatic watershed, such as the Obi, Yenisei,
and Lena. None are known * in the sea of Aral, or the
great rivers Oxus and Jaxartes, which feed that vast
lake ; nor any in the lakes of Balkash and Baikal. If
further exploration verifies this negative fact, it will be
not a little remarkable ; inasmuch as two t, if not more,
kinds of crayfishes are found in the basin of the great
river Amur, which drains a large area of north-eastern
Asia, and debouches into the Gulf of Tartary, in about
the latitude of York.

Japan has one species (A. japonicus), perhaps more;
but no cra3Tfish has as yet been made known in any part
of eastern Asia, south of Amurland. There are cer-
tainly none in Hindostan ; none are known in Persia,
Arabia, or Syria. In Asia Minor the only recorded
locality is the Bion. No crayfish has yet been disco-
vered in the whole continent of Africa. .!

* It would be hazardous, however, to assume that none exist, especi-
ally in the Oxus, which formerly flowed into the Caspian.

f A. dauricus and A. SchrencMi.

X Whatever the so-called Astacns capensis of the Cape Colony may
be, it is certainly not a crayfish.

NORTH AMERICAN CRAYFISHES.

305

Thus, on the continent of the old world, the crayfishes
are restricted to a zone, the southern limit of which
coincides with certain great geographical features ; on
the west, the Mediterranean, with its continuation, the
Black Sea ; then the range of the Caucasus, followed by
the great Asiatic highlands, as far as the Corea on the
east. On the north, though there is no such physical
boundary, the crayfishes appear to be entirely excluded
from the Siberian river basins ; while east and west,
though a sea-barrier exists, the crayfishes extend beyond
it, to reach the British islands and those of Japan.

Crossing the Pacific, we meet with some half-a-dozen
kinds of crayfishes,* different from those of the old
world, but still belonging to the genus Astacus, in
British Columbia, Oregon, and California. Beyond the
Bocky Mountains, from the Great Lakes to Guatemala,
crayfishes abound, as many as thirty-two different species
having been described, but they all belong to the genus
Cambarus (fig. 63, p. 248). Species of this genus also
occur in Cuba,t but, so far as is at present known, not
in any of the other West Indian islands. The occurrence
of a curious dimorphism among the male Cambari has
been described by Dr. Hagen ; and a blind Cambarus

* Dr. Hagen in his " Monograph of the North American Astaeidse/'
enumerates six species ; A. Garribclii, A. Mamathemis, A. leenisculus,
A. nigrescens, A. or eg anus, anal A. Trowbridgii*

f Von Martens. Cambarus cubensis. Archiv. f iir Naturgeschichte,
xxxviii.

yOG DISTRIBUTION AND ETIOLOGY OF THE CRAYFISHES.

is found, along with other blind animals, in the sub-
terranean caves of Kentucky.

All the crayfishes of the northern hemisphere belong
to the Potamobiidce, and no members of this family are
known to exist south of the equator. The crayfishes of the
southern hemisphere, in fact, all belong to the division of
the Parastacidtf, and in respect of the number and variety
of forms and the size which they reach, the head-quarters
of the Parastacidce is the continent of Australia. Some
of the Australian crayfishes (fig. 76) attain a foot or
more in length, and are as large as full-sized lobsters.
The genus Engceus of Tasmania comprises small cray-
fish which, like some of the Cambari, live habitually on
land, in burrows which they excavate in the soil.

New Zealand has a peculiar genus of crayfishes,
Paranephrops, a species of which is found in the Fiji
Islands, but none are known to occur elsewhere in
Polynesia.

Two kinds of crayfish have been obtained in southern
Brazil, and have been described by Dr. v. Martens,* as
A. pilimanus and A. brasiliensis. I have shown that
they belong to a peculiar genus, Parastacus. The former
was procured at Porto Alegre, wThich is situated in 30°
S. Latitude, close to the mouth of the Jacuhy, at the
north end of the great Laguna do Patos, wThich communi-

* Slidbrasilische Suss- und Brackwasser Crustaceen, nach den Samra-
lungen des Dr. Reinh. Hensel. Archiv. fur Naturgeschichte, xxxv.
18G9.

Fig. 76. — Australian Crayfish (J nat. size).*

* The nomenclature of the Australian crayfishes requires thorough
revision. I therefore, for the present, assign no name to this cray-

x 2

308 DISTRIBUTION AND .ETIOLOGY OF THE CRAYFISHES.

cates by a narrow passage with the sea ; and also at Sta.
Cruz in the upper basin of the Rio Pardo, an affluent of
the Jacuhy, " by digging it out of holes in the ground."
The latter (P. brasiliensis, fig. 64) was obtained at Porto
Alegre, and further inland, in the region of the primitive
forest at Eodersburg, in shallow streams.

In addition to these, no crayfish have as yet been
found in any of the great rivers, such as the Orinoko ;
the Amazon, in wThich they were specially sought for by
Agassiz ; or in the La Plata, on the eastern side of the
Andes. But, on the west, an " Astacus" chilensis is
described in the " Histoire Naturelle des Crustacees,"
(vol. ii. p. 333). It is here stated that this crayfish
"habite les cotes du Chili," but the freshwaters of the
Chilian coast are doubtless to be understood.

Finally, Madagascar has a genus and species of cray-
fish (Astacoides madagascariensis, fig. 65) peculiar to itself.

On comparing the results obtained by the study of the
geographical distribution of the crayfishes with those
brought to light by the examination of their morphological
characters, the important fact that there is a broad and
general correspondence between the two becomes ap-
parent. The wide equatorial belt of the earth's surface
which separates the crayfishes of the northern from those
of the southern hemisphere, is a sort of geographical

fish. It is probably identical with the A* nobilis of Dana and the A. ar*
matiis of Von Martens.

310 DISTRIBUTION AND ETIOLOGY OF THE CRAYFISHES.

representation of the broad morphological differences
which mark off the Potamobiidte from the Parastacidce.
Each group occupies a definite area of the earth's surface,
and the two are separated by an extensive border-land
untenanted by crayfishes.

A similar correspondence is exhibited, though less
distinctly, when we consider the distribution of the
genera and species of each group. Thus, among the
Potamobiidce, Astacus torrentium and nobilis belong essen-
tially to the northern, western, and southern watersheds
of the central European highlands, the streams of which
flow respectively into the Baltic and the North Seas, the
Atlantic and the Mediterranean (fig. 77, I.) ; A. leptodac-
tylus, pachypus, angulosus, and colchicus, appertain to the
Pontocaspian watershed, the rivers of which drain into
the Black Sea and the Caspian (I.) ; while Astacas
dauricus and A. Schrenckii are restricted to the widely
separated basin of the Amur, which sheds its waters
into the Pacific (II ). The Astaci of the rivers of
western North America, which flow into the Pacific (IV.),
and the Cambari of the Eastern or Atlantic water-shed (V.)
are separated by the great physical barrier of the Rocky
Mountain ranges. Finally, with regard to the Par-
astacidce, the widely separated geographical regions of
New Zealand (VIII.), Australia (IX.), Madagascar (XII.),
and South America (VI. and VII.), are inhabited by
generically distinct groups.

But when we look more closely into the matter, it will

MORPHOLOGICAL AND GEOGRAPHICAL GROUPS. 311

be found that the parallel between the geographical and the
morphological facts cannot be quite strictly carried out.

Astacus torrentium, as we have seen, inhabits both
the British Islands and the continent of Europe ; never-
theless, there is every reason to believe that twenty
miles of sea water is an insuperable barrier to the
passage of crayfishes from one land to the other. For
though some crayfishes live in brackish water, there is
no evidence that any existing species can maintain them-
selves in the sea. A fact of the same character meets us
at the other side of the Eurasiatic continent, the Japanese
and the Amurland crayfishes being closely allied ; although
it is not clear that there are any identical species on the
two sides of the Sea of Japan.

Another circumstance is still more remarkable. The
West American crayfishes are but little more different from
the Pontocaspian crayfishes, than these are from Astacus
torrentium. On the face of the matter, one might there-
fore expect the Amurland and Japanese crayfishes, which
are intermediate in geographical position, to be also
intermediate, morphologically, between the Pontocaspian
and the West American forms. But this is not the
case. The branchial system of the Amurland Astaci
appears to be the same as that of the rest of the genus ;
but, in the males, the third joint (ischiopodite) of the
second and third pair of ambulatory limbs is provided
with a conical, recurved, hook-like process ; while, in the
females, the hinder edge of the penultimate thoracic

:>1*2 DISTRIBUTION AND ETIOLOGY OF THE CRAYFISHES.

sternum is elevated into a transverse prominence, on the
posterior face of which there is a pit or depression.*

In both these characters, but more especially in the
former, the Amurland and Japanese Astaci depart from
both the Pontocaspian and the West American Astaci,
and approach the Cambari of Eastern North America.

In these crayfishes, in fact, one or both of the same
pairs of legs in the male are provided with similar

Fig. 78. — Cambarus (Guatemala) penultimate leg. exp, coxopodite *,
ex.s, coxopoditic setse ; pdb, podobranchia ; bp, basipodite ; ip, iscliiopo-
dite; mp, meropodite ; op carpopodite ; pp9 propodite ; dp, dactylopodite.

hook-like processes ; while, in the females, the modifi-
cation of the penultimate thoracic sternum is carried
still further and gives rise to the curious structure de-
scribed by Dr. Hagen as the " annulus ventralis."

In all the Cambari, the pleurobranchise appear to be
entirely suppressed, and the hindermost podobranchia has
no lamina ; while the areola is usually extremely narrow.
The proportional size of the areola in the Amurland

Xesslerj 1. c.

MORPHOLOGICAL AND GEOGRAPHICAL GROUPS. 313

crayfishes is not recorded; in the Japanese crayfish,
judging by the figure given by De Haan, it is about the
same as in the western Astaci. On the other hand, in
the West American crayfishes it is distinctly smaller ; so
that, in this respect, they perhaps more nearly approac}
the Cambari. Unfortunately, nothing is known as to
the branchiae of the Amurland crayfishes. According
to De Haan, those of the Japanese species resemble
those of the western Astaci: as those of the West
American Astaci certainly do.

With respect to the Parastacidce ; in the remarkable
length and flatness of the epistoma, the crayfishes of
Australia, Madagascar, and South America, resemble
one another. But in its peculiar truncated rostrum (see
fig. 65) and in the extreme modification of its branchial
system, which I have described elsewhere, the Madagascar
genus stands alone.

The Paranephrops of New Zealand and the Fijis, with
its wide and short epistoma, long rostrum, and large
antennary squames, is much more unlike the Australian
forms than might be expected from its geographical
position. On the other hand, considering their wide
separation by sea, the amount of resemblance be-
tween the New Zealand and the Fiji species is very
remarkable.

If the distribution of the crayfishes is compared
with that of terrestrial animals in general, the points of

314 DISTRIBUTION AND ETIOLOGY OF THE CRAYFISHES.

difference are at least as remarkable as the resem-
blances.

With respect to the latter, the area oocupied by the
Potamobiidce, corresponds roughly with the Pabearctic
and Nearctic divisions of the great Arctogaeal provinces
of distribution indicated by mammals and birds; while
distinct groups of crayfishes occupy a larger or smaller
part of the other, namely, the Austro- Columbian, Aus-
tralian, and Novozelanian primary distributional pro-
vinces of mammals and birds. Again, the peculiar
crayfishes of Madagascar answer to the special features
of the rest of the fauna of that island.

But the North American crayfishes extend much
further South than the limits of the Nearctic fauna in
general ; while the absence of any group of crayfishes
in Africa, or in the rest of the old world, south of the
great Asiatic table-land, forms a strong contrast to the
general resemblance of the North African and Indian
fauna to that of the rest of Arctogsea. Again, there is
no such vast difference between the crayfishes of New
Zealand, Australia, and South America, as there is
between the mammals and the birds of those regions.

It may be concluded, therefore, that the conditions
which have determined the distribution of crayfishes have
been very different from those which have governed the
distribution of mammals and birds. But if we compare
with the distribution of the crayfishes, not that of ter-
restrial animals in general, but only that of freshwater

THE DISTRIBUTION OF FRESHWATER CRAYFISHES. 315

fishes, some very curious points of approximation become
manifest. The Salmonidce, or fishes of the salmon and
trout kind, a few of which are exclusively marine, many
both marine and freshwater, while others are confined
to fresh water, are distributed over the northern hemi-
sphere, in a manner which recalls the distribution of
the Potamobine crayfishes,* though they do not extend
so far to the South in the new world, while they go a
little further, namely, as far as Algeria, Northern Asia
Minor, and Armenia, in the old world. With the excep-
tion of the single genus Retropinna, which inhabits New
Zealand, no true salmonoid fish occurs south of the
equator ; but, as Dr. Giinther has pointed out, two
groups of freshwater fishes, the Haplochitonidce and the
Galaxidce, which stand in somewhat the same relation to
the Salmonidce as the Parastacidce do to the Potamobiidce,
take the place of the Salmonidce in the fresh waters ol
New Zealand, Australia, and South America. There
are two species of Haplochiton in Tierra del Fuego ; and
of the closely allied genus Prototroctes, one species is
found in South Australia, and one in New Zealand ; of
the Galaxidce, the same species, Galaxias attennuatus,
occurs in the streams of New Zealand, Tasmania, the
Falkland Islands, and Peru.

Thus, these fish avoid South Africa, as the crayfishes

* According to Dr. Giinther their southern range is similarly limited by
the Asiatic Highlands. But they abound in the rivers both of the old
and new worlds which flow into the Arctic sea ; and though those on

316 DISTRIBUTION AND ETIOLOGY OF THE CRAYFISHES

do ; but I am not aware that any member of the group is
found in Madagascar, and thus completes the analogy.

The preservation of the soft parts of animals in the
fossil state depends upon favourable conditions of rare
occurrence ; and, in the case of the Crustacea, it is not
often that one can hope to meet with such small hard
parts as the abdominal members, in a good state of
preservation. But without recourse to the branchial
apparatus, and to the abdominal appendages, it might be
very difficult to say whether a given crustacean belonged
to the Astacine, or to the closely allied Homarine group.
Of course, if the accompanying fossils indicated that the
deposit in which the remains occur, was of freshwater
origin, the presumption in favour of their Astacine nature
would be very strong ; but if they were inhabitants of the
sea, the problem whether the crustacean in question was
a marine Astacine, or a true Homarine, might be very
hard to solve.

Undoubted remains of crayfishes have hitherto been
discovered only in freshwater strata of late tertiary age.
In Idaho, North America, Professor Cope * found, in
association with Mastodon mirificus, and Equus excelsus,
several species, which he considers to be distinct from

the western side of the Rocky Mountains are different from the Eastern
American forms, yet there are species common to both the Asiatic and
the American coasts of the North Pacific.

* On three extinct Astaci from the freshwater Tertiary of Idaho. Pro
ceedings of the American Philosophical Society, 1869-70*

THE ^ETIOLOGY OF THE CKAYFISHES. 317

the existing American crayfishes ; whether they are
Cambari or Astaci does not appear. But, in the lower
chalk of Ochtrup, in Westphalia, and therefore in a
marine deposit, Von der Marck and Schliiter* have
obtained a single, somewhat imperfect, specimen of a
crustacean, which they term Astacus politus, and wThich,
singularly enough, has the divided telson found only in
the genus Astacus. It would be very desirable to know
more about this interesting fossil. For the present it
affords a strong presumption that a marine Potamobine
existed as far back as the earlier part of the cretaceous
epoch.

Such are the more important facts of Morphology,
Physiology, and Distribution, which make rip the sum
of our present knowledge of the Biology of Crayfishes.
The imperfection of that knowledge, especially as re-
gards the relations between Morphology and Distribution,
becomes a serious drawback when we attack the final
problem of Biology, which is to find out why animals
of such structure and active powers, and so localized,
exist ?

It would appear difficult to frame more than two
fundamental hypotheses in attempting to solve this pro-
blem. Either we must seek the origin of crayfishes in
conditions extraneous to the ordinary course of natural

* Neue Fische und Krebse aus der Kreide von Westphalen. Pakeon-
tographica, Bd. XV., p. 302 ; tab. XLIV., figs. 4 and 5.

318 DISTRIBUTION AND ETIOLOGY OF THE CRAYFISHES.

operations, by what is commonly termed Creation; or we
must seek for it in conditions afforded by the usual
course of nature, when the hypothesis assumes some
shape of the doctrine of Evolution. And there are two
forms of the latter hypothesis ; for, it may be assumed,
on the one hand, that crayfishes have come into exist-
ence, independently of airy other form of living matter,
which is the hypothesis of spontaneous or equivocal
generation, or abiogenesis ; or, on the other hand,
we may suppose that crayfishes have resulted from the
modification of some other form of living matter ; and
this is what, to borrow a useful wTord from the French
language, is known as transformism.

I do not think that any hypothesis respecting the
origin of crayfishes can be suggested, which is not
referable to one or other of these, or to a combination
of them.

As regards the hypothesis of creation, little need be
said. From a scientific point of view, the adoption of
this speculation is the same thing as an admission that
the problem is not susceptible of solution. Moreover,
the proposition that a given thing has been created,
wThether true or false, is not capable of proof. By
the nature of the case direct evidence of the fact is
not obtainable. The only indirect evidence is such
as amounts to proof that natural agencies are incom-
petent to cause the existence of the thing in question.
But such evidence is out of our reach. The most that

CREATION AND EVOLUTION.

319

can be proved, in any case, is that no known natural
cause is competent to produce a given effect ; and it is
an obvious blunder to confound the demonstration of our
own ignorance with a proof of the impotence of natural
causes. However, apart from the philosophical worth-
lessness of the hypothesis of creation, it would be a waste
of time to discuss a view which no one upholds. And,
unless I am greatly mistaken, at the present day, no
one possessed of knowledge sufficient to give his opinion
importance is prepared to maintain that the ancestors of
the various species of crayfish were fabricated out of in-
organic matter, or brought from nothingness into being,
by a creative fiat.

Our only refuge, therefore, appears to be the hypo-
thesis of evolution. And, with respect to the doctrine
of abiogenesis, we may also, in view of a proper
economy of labour, postpone its discussion until such
time as the smallest fragment of evidence that a crayfish
can be evolved by natural agencies from not living matter,
is brought forward.

In the meanwhile, the hypothesis of transformism
remains in possession of the field ; and the only pro-
fitable inquiry is, how far are the facts susceptible of
interpretation, on the hypothesis that all the existing
kinds of crayfish are the product of the metamorphosis
of other forms of living beings ; and that the bio-
logical phenomena which they exhibit are the results
of the interaction, through past time, of two series of

320 DISTRIBUTION AND AETIOLOGY OF THE CRAYFISHES.

factors : the one, a process of morphological and con-
comitant physiological modification ; the other, a process
of change in the condition of the earth's surface.

If we set aside, as not worth serious consideration, the
assumption that the Astacus torrentium of Britain was
originally created apart from the Astacus torrentium of
the Continent ; it follows, either that this crayfish has
passed across the sea by voluntary or involuntary migra-
tion ; or that the Astacus torrentium existed before the
English Channel, and spread into England while these
islands were still continuous with the European main-
land ; and that the present isolation of the English cray-
fishes from the members of the same species on the
Continent is to be accounted for by those changes in the
physical geography of western Europe which, as there is
abundant evidence to prove, have separated the British
Islands from the mainland.

There is no evidence that our crayfish has been
purposely introduced by human agency into Great
Britain ; and from the mode of life of crayfish and the
manner in which the eggs are carried about by the
parent during their development, transport by birds or
floating timber would seem to be out of the question.
Again, although Astacus nobilis is said to venture into
the brackish waters of the Gulf of Finland, and A. leptdr
dactylus, as we have seen, makes itself at home in the
more or less salt Caspian, there is no reason to believe
that Astacvs torrentium is capable of existing in sea-

THE DISTRIBUTION OF THE CRAYFISHES. 321

water, still less of crossing the many miles of sea which
separate England from even the nearest point of the
Continent. In fact, the existence of the same kind of
crayfish on both sides of the Channel appears to be
only a case of the general truth, that the Fauna of the
British Islands is identical with a part of that of the
Continent ; and as our foxes, badgers, and moles cer-
tainly have neither swum across, nor been transported
by man, but existed in Britain while it was still con-
tinuous with western Europe, and have been isolated
by the subsequent intervention of the sea, so we may
confidently explain the presence of Astacus torrentium
by reference to the same operation.

If we take into account the occurrence of Astacus
nobilis over so large a part of the area occupied by
Astacus torrentium ; its absence in the British Islands,
and in Greece ; and the closer affinity which exists be-
tween A. nobilis and A. leptodactylus, than between A.
nobilis and A. torrentium; it seems not improbable that
Astacus torrentium was the original tenant of the whole
western European area outside the Ponto-Caspian water-
shed ; and that A. nobilis is an invading offshoot of the
Ponto-Caspian or leptodactylus form which has made its
way into the western rivers in the course of the many
changes of level which central Europe has undergone ;
in the same way as A. leptodactylus is now passing into
the rivers of the Baltic provinces of Russia.

The study of the glacial phenomena of central Europe

I

322 DISTRIBUTION AND ETIOLOGY OF THE CRAYFISHES.

lias led Sartorius von Waltershausen * to the conclusion
that at the time when the glaciers of the Alps had a
much greater extension than at present, a vast mass of
freshwater extended from the valley of the Danube to
that of the Rhone, around the northern escarpment of the
Alpine chain, and connected the head-waters of the
Danube with those of the Rhine, the Rhone, and the
northern Italian rivers. As the Danube debouches into
the Black Sea, and this was formerly connected with
the Aralo-Caspian Sea, an easy passage would thus be
opened up by which crayfishes might pass from the Aralo-
Caspian area to western Europe. If they spread by this
road, the Astacus torrentium may represent the first wave
of migration westward, while A. nobilis answers to a
second, and A. leptodactylus, with its varieties, remains
as the representative of the old Aralo-Caspian crayfishes.
And thus the crayfishes would present a curious parallel
with the Iberian, Aryan, and Mongoloid streams of west-
ward movement among mankind.

If we thus suppose the western Eurasiatic crayfishes
to be simply varieties of a primitive Aralo-Caspian stock,
their limitation to the south by the Mediterranean and by
the great Asiatic highlands becomes easily intelligible.

The extremely severe climatal conditions which obtain
in northern Siberia may sufficiently account for the

* * 4 Untersuchungen ueber die Klimate der Gegenwart und der Vorwelt."
Natuurkundige Verhandelingen van de Hoilandsehe Maatschappij der
Wetenschappen te Haarlem, 18(35,

CHANGES IN PHYSICAL GEOGRAPHY.

323

absence of crayfishes (if they are really absent) in the
rivers Obi, Yenisei, and Lena, and in the great lake
Baikal, which lies more than 1,300 feet above the sea,
and is frozen over from November to May. Moreover,
there can be no doubt that, at a comparatively recent
period, the whole of this region, from the Baltic to the
mouth of the Lena, was submerged beneath a southward
extension of the waters of the Arctic ocean to the Aralo-
Caspian Sea and Lake Baikal, and a westward extension
to the Gulf of Finland.

The great lakes and inland seas which stretch, at
intervals, from Baikal, on the east, to Wenner in Sweden,
on the west, are simply pools, isolated partly by the rising
of the ancient sea-bottom and partly by evaporation; and
often completely converted into fresh water by the inflow
of the surrounding land-drainage. But the population
of these pools was originally the same as that of the
Northern Ocean, and a few species of marine crustaceans,
mollusks, and fish, besides seals, remain in them as
living evidences of the great change which has taken place.
The same process which, as wre shall see, has isolated
the Mysis of the Arctic seas in the lakes of Sweden and
Finland, has shut up with it other arctic marine Crustacea,
such as species of Gammarns and Idothea. And the very
same species of Gammarus is imprisoned, along with
arctic seals, in the waters of Lake Baikal.

The distribution of the American crayfishes agrees
equally well with the hypothesis of the northern origin of

Y 2

324 DISTRIBUTION AND ETIOLOGY OF THE CRAYFISHES.

the stock from which they have been evolved. Even
under existing geographical conditions, an affluent of the
Mississippi, the St. Peter's river, communicates directly,
in rainy weather, with the Red river, which flows into
Lake Winnipeg, the southernmost of the long series of
intercommunicating lakes and streams, which occupy the
low and flat water-parting between the southern and the
northern watersheds of the North American Continent.
But the northernmost of these, the Great Slave Lake,
empties itself by the Mackenzie river into the Arctic
Ocean, and thus provides a route by which crayfishes
might spread from the north over all parts of North
America east of the Eocky Mountains.

The so-called Eocky Mountain range is, in reality, an
immense table-land, the edges of which are fringed by two
principal lines of mountainous elevations. The table-
land itself occupies the place of a great north and south
depression which, in the cretaceous epoch, was occupied by
the sea and probably communicated with the ocean at its
northern, as well as at its southern end. During and
since this epoch it became gradually filled up, and it
now contains an immense thickness of deposits of all
ages from the cretaceous to the pliocene — the earlier
marine, the later more and more completely freshwater.
During the tertiary epoch, various portions of this area
have been occupied by vast lakes, the more northern of
which doubtless had outlets into the Northern sea. That
crayfish existed in the vicinity of the Eocky Mountains

CHANGES IN PHYSICAL GEOGRAPHY. 325

in the latter part of the tertiary epoch is testified by the
Idaho fossils. And there is thus no difficulty in under-
standing their presence in the rivers which have now cut
their way to the Pacific coast.

The similarity of the crayfish of the Amurland and of
Japan is a fact of the same order as the identity of the
English crayfish with the Astacus torrentium of the Euro-
pean Continent, and is to be explained in an analogous
fashion. For there can be no doubt that the Asiatic
continent formerly extended much further to the east-
ward than it does at present, and included what are now
the islands of Japan. Even with this alteration of the
geographical conditions, however, it is not easy to see
how crayfishes can have got into the Amur-Japanese
fresh waters. For a north-eastern prolongation of the
Asiatic highlands, which ends to the north in the Sta-
novoi range, shuts in the Amur basin on the west ; while
the Amur debouches into the sea of Okhotsk, and the
Pacific ocean washes the shores of the Japanese islands.

But there are many grounds for the conclusion that, in
the latter half of the tertiary epoch, eastern Asia and
North America were connected, and that the chain of the
Kurile and Aleutian islands may indicate the position of
a great extent of submerged land. In that case, the sea
of Okhotsk and Behring's sea may occupy the site of
inland waters which formerly placed the mouth of the
Amur in direct communication with the Northern Ocean,
just as the Black Sea, at present, brings the basin of the

320 DISTRIBUTION AND AETIOLOGY OF THE CRAYFISHES.

Danube into connection, first with the Mediterranean and
then with the western Atlantic ; and, as in former times,
it gave access from the south to the vast area now
drained by the Volga. When the Black Sea communi-
cated with the Aralo-Caspian sea, and this opened to
the north into the Arctic sea, a chain of great inland
waters must have skirted the eastern frontier of Europe,
just such as would now lie on the eastern frontier of Asia
if the present coast underwent elevation.

Supposing, however, that the ancestral forms of the
PotamobiidcB obtained access to the river basins in which
they are now found, from the north, the hypothesis that
a mass of fresh water once occupied a great part of the
region which is now Siberia and the Arctic Ocean, would
be hardly tenable, and it is, in fact, wholly unnecessary
for our present purpose.

The vast majority of the stalk- eyed crustaceans are, and
always have been, exclusively marine animals ; the cray-
fishes, the Atyidce, and the fluviatile crabs (Thelphusiclce),
being the only considerable groups among them which
habitually confine themselves to fresh waters. But
even in such a genus as Penceus, most of the species
of which are exclusively marine, some, such as Penceus
brasiliensis, ascend rivers for long distances. More-
over, there are cases in which it cannot be doubted
that the descendants of marine Crustacea have gradually
accustomed themselves to fresh water conditions, and
have, at the same time, become more or less modified,

CONVERSION OF MARINE INTO FRESHWATER ANIMALS. 327

so that thoy are no longer absolutely identical with those
descendants of their ancestors which have continued to
live in the sea.*

In several of the lakes of Norway, Sweden and
Finland, and in Lake Ladoga, in Northern Europe ; in
Lake Superior and Lake Michigan, in North America;
a small crustacean, My sis relicta, occurs in such abund-
ance as to furnish a great part of the supply of food to
the fresh water fishes which inhabit these lakes. Now,
this Mysis relicta is hardly distinguishable from the
My sis oculata which inhabits the Arctic seas, and is
certainly nothing but a slight variety of that species.

In the case of the lakes of Norway and Sweden, there
is independent evidence that they formerly communicated
with the Baltic, and were, in fact, fiords or arms of the
sea. The communication of these fiords with the sea
having been gradually cut off, the marine animals they
contained have been imprisoned ; and as the water has
been slowly changed from salt to fresh by the drainage
of the surrounding land, only those which were able to
withstand the altered conditions have survived. Among
these is the Mysis oculata, which has in the meanwhile
undergone the slight variation which has converted it
into Mysis relicta. Whether the same explanation ap-

* See on this interesting subject: Martens, "On the occurrence of
marine animal forms in fresh water." Annals of Natural History, 1858 :
Loven. " Ueber einige im Wetter und Wener See gef undone Crustaceen."
Halle Zeitschrift fur die Gesammten Wissenschaften, xix., 18G2: G. O.
Sars, " Histoire Xaturelle des Crustaces d'eau douce do Norvege," 18G7.

328 DISTRIBUTION AND ETIOLOGY OF THE CRAYFISHES.

plies to Lakes Superior and Michigan, or whether the
Mysis oculata has not passed into these masses of fresh
water by channels of communication with the Arctic
Ocean which no longer exist, is a secondary question.
The fact remains that Mysis relicta is a primitively
marine animal which has become completely adapted to
fresh-water life.

Several species of prawns (Palcemon) abound in our
own seas. Other marine prawns are found on the coasts
of North America, in the Mediterranean, in the South
Atlantic and Indian Oceans, and in the Pacific as far
south as New Zealand. But species of the same genus
(Palcemon) are met with, living altogether in fresh water,
in Lake Erie, in the rivers of Florida, in the Ohio, in
the rivers of the Gulf of Mexico, of the West India
Islands and of eastern South America, as far as southern
Brazil, if not further ; in those of Chili and those of
Costa Rica in western South America ; in the Upper
Nile, in West Africa, in Natal, in the Islands of Johanna,
Mauritius, and Bourbon, in the Ganges, in the Molucca
and Philippine Islands, and probably elsewhere.

Many of these fluviatile prawns differ from the marine
species not only in their great size (some attaining a foot
or more in length), but still more remarkably in the vast
development of the fifth pair of thoracic appendages.
These are always larger than the slender fourth pair
(which answer to the forceps of the crayfishes) ; and, in
the males especially, they are very long and strong, and

FRESHWATER PRAWNS.

329

are terminated by great chelae, not unlike those of the
crayfishes. Hence these fluviatile prawns (known in
many places by the name of " Cammarons ") are not
unfrequently confounded with true crayfishes ; though

Fig. 79. Palccmon jamaicensu (about f nat. size). A, female;
B, fifth thoracic appendage of male,,

the fact that there are only three pair of ordinary legs
behind the largest, forceps-like pair, is sufficient at once
to distinguish them from any of the Astacidce.

Species of these large - clawed prawns live in the

330 DISTRIBUTION AND ETIOLOGY OF THE CRAYFISHES.

brackish water lagoons of the Gulf of Mexico, but I
am not aware that any of them have yet been met with
in the sea itself. The Palcemon lacastris (Anchistia
migratoria, Heller) abounds in fresh-water ditches and
canals between Padua and Venice, and in the Lago di
Garda, as well as in the brooks of Dalmatia; but its
occurrence in the Adriatic or the Mediterranean, which
has been asserted, appears to be doubtful. So the Nile
prawn, though very similar to some Mediterranean
prawns, does not seem to be identical with any at
present known.*

In all these cases, it appears reasonable to apply the
analogy of the Mysis relicta, and to suppose that the
fluviatile prawns are simply the result of the adaptive
modification of species which, like their congeners, were
primitively marine.

But if the existing sea prawns were to die out, or to
be beaten in the struggle for existence, we should have,
scattered over the world in isolated river basins, more
or less distinct species of freshwater prawns,! the areas
inhabited by which might hereafter be indefinitely en-
larged or diminished, by alteration in the elevation of the

* Heller, "Die Crustaceen des sudlichen Europas," p. 259. Klunzinger,
" Ueber eine Susswasser-crustacee im Nil," with the notes by von Mar-
tens and von Siebold: Zeitschrift fiir Wissenschaftliche Zoologie, 186(5.

f This seems actually to have happened in the case of the widely-
spread allies and companions of the fluviatile prawns, A tya and Carl-
dina. I am not aware that truly marine species of these genera are
known.

THE ORIGIN OF CRAYFISHES.

331

land and by other changes in physical geography. And,
indeed, under these circumstances, the freshwater prawns
themselves might become so much modified, that, even if
the descendants of their ancestors remained unchanged
in structure and habits in the sea, the relationship of the
two might no longer be obvious.

These considerations appear to me to indicate the di-
rection in which we must look for a rational explanation
of the origin of crayfishes and their present distribution.

I have no doubt that they are derived from ancestors
which lived altogether in the sea, as the great majority of
the My s idee and many of the prawns do now ; and that, of
these ancestral crayfishes, there were some which, like
Mysis oculata or Penceus hrasiliensis, readily adapted them-
selves to fresh water conditions, ascended rivers, and took
possession of lakes. These, more or less modified, have
given rise to the existing crayfishes, while the primitive
stock would seem to have vanished. At any rate, at the
present time, no marine crustacean with the characters
of the Astacidce is known.

As crayfishes have been found in the later tertiaries
of North America, we shall hardly err in dating the
existence of these marine crayfishes at least as far back
as the miocene epoch ; and I am disposed to think that,
during the earlier tertiary and later mesozoic periods,
these Crustacea not only had as wide a distribution as
the Prawns and Pencei have now, but were differentiated
into two groups, one with the general characters of Ihe

832 DISTRIBUTION AND .ETIOLOGY OF THE CRAYFISHES.

Potamobiidce in the northern hemisphere, and another,
with those of the Parastacidce, in the southern hemisphere.

The ancestral Potamobine form probably presented
the peculiarities of the Potamobiidce in a less marked
degree than any existing species does. Probably the
four pleurobranchiae were all equally well developed ; the
laminae of the podobranchise smaller and less distinct
from the stem ; the first and second abdominal appen-
dages less specialised ; and the telson less distinctly
divided. So far as the type was less specially Pota-
mobine, it must have approached the common form in
which Homarus and Nephrops originated. And it is
to be remarked that these also are exclusively confined
to the northern hemisphere.

The wide range and close affinity of the genera
Astacus and Cambarus appear to me to necessitate the
supposition that they are derived from some one already
specialised Potamobine form ; and I have already men-
tioned the grounds upon which I am disposed to believe
that this ancestral Potamobine existed in the sea which
lay north of the miocene continent in the northern
hemisphere.

In the marine primitive crayfishes south of the equator,
the branchial apparatus appears to have suffered less
modification, while the suppression of the first abdominal
appendages, in both sexes, has its analogue among the
Palinuridce, the headquarters of which are in the
southern hemisphere. That they should have ascended

DISTRIBUTIONAL DIFFICULTIES,

333

the rivers of New Zealand, Australia, Madagascar, and
South America, and become fresh water Parastacidce, is
an assumption which is justified by the analogy of the
fresh-water prawns. It remains to be seen whether
marine Parastacidce still remain in the South Pacific
and Atlantic Oceans, or whether they have become
extinct.

In speculating upon the causes of an effect which is
the pToduct of several co-operating factors, the nature
of each of which has to be divined by reasoning back-
wards from its effects, the probability of falling into
error is very great. And this probability is enhanced
when, as in the present case, the effect in question
consists of a multitude of phenomena of structure and
distribution about which much is yet imperfectly known.
Hence the preceding discussion must rather be regarded
as an illustration of the sort of argumentation by which
a completely satisfactory theory of the aetiology of the
crayfish will some day be established, than as sufficing
to construct such a theory. It must be admitted that
it does not account for the whole of the positive facts
which have been ascertained ; and that it requires sup-
plementing, in order to furnish even a plausible explana-
tion of various negative facts.

The positive fact which presents a difficulty is the
closer resemblance between the Amur-Japanese crayfish
and the East American Cambari, than between the

334 DISTRIBUTION AND .ETIOLOGY OF THE CRAYFISHES,

latter and the West American Astaci; and the closer
resemblance between the latter and the Pontocaspian
crayfish, than either bear to the Amur-Japanese form.
If the facts had been the other way, and the West
American and Amur-Japanese crayfish had changed
places, the case would have been intelligible enough.
The primitive Potamobine stock might then have been
supposed to have differentiated itself into a western
astacoid, and an eastern cambaroid form ; * the latter
would have ascended the American, and the former the
Asiatic rivers. As the matter stands, I do not see that
any plausible explanation can be offered without recourse
to suppositions respecting a former more direct com-
munication between the mouth of the Amur, and that
of the North American rivers, in favour of which no
definite evidence can be offered at present.

The most important negative fact which remains to
be accounted for is the absence of crayfishes in the
rivers of a large moiety of the continental lands, and in
numerous islands. Differences of climatal conditions are
obviously inadequate to account for the absence of cray-
fishes in Jamaica, when they are present in Cuba; for
their absence in Mozambique, and the islands of Johanna
and Mauritius, when they are present in Madagascar ;
and for their absence in the Nile, when they exist in
Guatemala.

* Just as there is an American form of Idothea and an Asiatic form
in the Arctic ocean at the present day.

DISTRIBUTIONAL DIFFICULTIES.

335

At present, I confess that I do not see my way to a
perfectly satisfactory explanation of the absence of cray-
fishes in so many parts of the world in which they
might, d priori, be expected to exist ; and I can only
suggest the directions in which an explanation may be
sought.

The first of these is the existence of physical obstacles
to the spread of crayfishes, at the time at which the
Potamobine and the Parastacine stocks respectively began
to take possession of the rivers, some of which have
now ceased to exist ; and the second is the probability
that, in many rivers which have been accessible to cray-
fishes, the ground was already held by more powerful
competitors.

If the ancestors of the Potamobine crayfishes originated
only among those primitive crayfishes which inhabited the
seas north of the miocene continent, their present limita-
tion to the south, in the old world, is as easily intelligible
as is their extension southward, in the course of the river
basins of Northern America as far as Guatemala, but
no further. For the elevation of the Eurasiatic high-
lands had commenced in the miocene epoch, while the
isthmus of Panama was interrupted by the sea.

With respect to the Southern hemisphere, the absence
of crayfishes in Mauritius and in the islands of the Indian
Ocean, though they occur in Madagascar, may be due
to the fact that the former islands are of comparatively
late volcanic origin ; while Madagascar is the remnant of

336 DISTRIBUTION AND ^ETIOLOGY OF THE CRAYFISHES.

a very ancient continental area, the oldest indigenous
population of which, in all probability, is directly de-
scended from that which occupied it at the beginning
of the tertiary epoch. If Parastacine Crustacea inhabited
the southern hemisphere at this period, and subsequently
became extinct as marine animals, their preservation in
the freshwaters of Australia, New Zealand, and the older
portions of South America may be understood. The
difficulty of the absence of crayfishes in South Africa *
remains ; and all that can be said is, that it is a difficulty
of the same nature as that which confronts us when we
compare the fauna of South Africa in general with that
of Madagascar. The population of the latter region has
a more ancient aspect than that of the former; and it
may be that South Africa, in its present shape, is of very
much later date than Madagascar.

With respect to the second point for consideration, it
is to be remarked that, in the temperate regions of the
world, the craj^fishes are by far the largest and strongest
of any of the inhabitants of freshwater, except the Verte-
brata ; and that while frogs and the like fall an easy prey
to them, they must be formidable enemies and com-
petitors even to fishes, aquatic reptiles, and the smaller
aquatic mammals. In warm climates, however, not only
the large prawns which have been mentioned, but Atyce

* But it must be remembered that we have as yet everything1 to learn
respecting- the fauna of the great inland lakes and river systems of
South Africa.

The distribution of crabs and crayfishes. 337

and tiuviatile crabs (Thelphusa) compete for the posses-
sion of the freshwaters ; and it is not improbable that
under some circumstances, they may be more than a
match for crayfishes; so that the latter might either be
driven out of territory they already occupied, as Astacas
leptodactylus is driving out A. nobilis in the Russiau
rivers; or might be prevented from entering rivers already
tenanted by their rivals.

In connection with this speculation, it is worthy of
remark that the area occupied by the fluviatile crabs is
very nearly the same as that zone of the earth's surface
from which crayfish are excluded, or in which they are
scanty. That is to say, they are found in the hotter
parts of the eastern side of the two Americas, the West
Indies, Africa, Madagascar, Southern Italy, Turkey and
Greece, Hindostan, Burmah, China, Japan, and the
Sandwich Islands. The large -clawed fluviatile prawns
are found in the same regions of America, on both
east and west coasts, in Africa, Southern Asia, the
Moluccas, and the Philippine Islands ; while the Atyiclce
not only cover the same area, but reach Japan, extend
over Polynesia, to the Sandwich Islands, on the north,
and New Zealand, on the south, and are found on both
shores of the Mediterranean; a blind form (Troglocaris
Schmidtii), in the Adelsberg caves, representing the blind
Cambarus of the caves of Kentucky.

The hypothesis respecting the origin of crayfishes

z

338 DISTRIBUTION AND ETIOLOGY OF THE CRAYFISHES

which has been tentatively put forward in the preceding
pages, involves the assumption that marine Crustacea of
the astacine type were in existence during the deposition
of the middle tertiary formations, when the great con-
tinents began to assume their present shape. That
such was the case there can be no doubt, inasmuch as
abundant remains of Crustacea of that type occur still
earlier in the mesozoic rocks. They prove the existence
of ancient crustaceans, from winch the crayfishes may have
been derived, at that period of the earth's history when
the conformation of the land and sea were such as to
admit of their entering the regions in which we now find
them.

The materials which have, up to the present, time been
collected are too scanty to permit of the tracing out of all
the details of the genealogy of the crayfish. Nevertheless,
the evidence which exists is perfectly clear, as far as it
goes, and is in complete accordance with the require-
ments of the doctrine of evolution.

Mention has been made of the close affinity between
the crayfishes and the lobsters — the Astacina and the Ho-
marina ; and it fortunately happens that these two groups,
which may be included under the common name of the
Astacomorpha, are readily distinguishable from all the
other Podophthalmia by peculiarities of their exoskeleton
which are readily seen in all well-preserved fossils.
In all, as in the crayfish, there are large forceps, fol-
lowed by two pairs of chelate ambulatory limbs, while

FOSSIL ASTACOMORPHA.

339

the succeeding two pairs of legs are terminated by simple
claws. The exopodite of the last abdominal appendage
is divided into two parts by a transverse suture. The
pleura of the second abdominal somite are larger than
the others, and overlap those of the first somite,
which are very small. Any fossil crustacean which
presents all these characters, is certainly one of the
Astacomorpha.

The Astacina, again, are distinguished from the Homa-
rina by the mobility of the last thoracic somite, and the
characters of the first and second abdominal appendages,
when they are present ; or by their entire absence.
But it is so difficult to make out anything about either
of these characters in fossils, that, so far as I am aware,
we know nothing about them in any fossil Astacomorph.
And hence, it may be impossible to say to which division
any given form belongs, unless its resemblances to
known types are so minute and so close as to remove
doubt.

For the present purpose, the series of the fossiliferous
rocks may be grouped as follows : — 1. Eecent and
Quaternary. 2. Newer Tertiary (Pliocene and Miocene).
3. Older Tertiary (Eocene). 4. Cretaceous (Chalk,
Greensand and Gault). 5. Wealden. 6. Jurassic (Pur-
beck to Inferior Oolite). 7. Liassic. 8. Triassic. 9.
Permian. 10. Carboniferous. 11. Devonian. 12.
Silurian. 13. Cambrian.

Now the oldest known member of the group of the

z 2

Fig. 80. — A, Psevdastacus pustulosus (nat. size). B, Eryma modcsti-
formis ( x 2). Both figures after Oppel.

THE EXTINCT GENUS ERYMA.

341

decapod Podophthalmia to which the Astacomorpha belong
occurs in the Carboniferous formation. It is the genus
Anthrapalcemon — a small and very curious crustacean,
about which nothing more need be said at present, as it
does not appear to have special affinities with the Astaco-
morpha. In the later formations, up to the top of the
Trias, podophthalmatous Crustacea are very rare ; and,
unless the Triassic genus Pemphlx is an exception, no
Astacomorphs are known to occur in them. The speci-
mens of Pemphix which I have examined are not suffi-
ciently complete to enable me to express any opinion
about them.

The case is altered when we reach the Middle Lias. In
fact this yields several forms of a genus, Eryma (fig. 80, B),
which also occurs in the overlying strata almost up to the
top of the Jurassic series, and presents so many variations
that nearly forty different species have been recognised.
Eryma is, in all respects, an Astacomorph, and so far as
can be seen, it differs from the existing genera only
in such respects as those in which they differ from
one another. Thus it is quite certain that Astacomor-
phous Crustacea have existed since a period so remote
as the older part of the Mesozoic period ; and any hesi-
tation in admitting this singular persistency of type on
the part of the crayfishes, is at once removed by the
consideration of the fact that, along with Eryma, in the
Middle Lias, prawn-like Crustacea, generically iden-
tical with the existing Penceus, flourished in the sea

842 DISTRIBUTION AND AETIOLOGY OF THE CRAYFISHES.

and left their remains in the mud of the ancient sea
bottom.

Eryma is the only crustacean, which can be certainly
ascribed to the Astacomorpha, that has hitherto been
found in ths strata from the Middle Lias to the litho-
graphic slates; which last lie in the upper part of the
Jurassic series. In the freshwater beds of the Wealden,
no Astacomorpha are known, and although no very great

Fig. 81. — IToploparia longimana (§ nat. size). — cj), carapace;
r, rostrum, T, telson ; xv., xvi., first and second abdominal somites ;
10, forceps j 20, last abdominal appendage.

weight is to be attached to a negative fact of this kind, it
is, so far, evidence that the Astacomorpha had not yet
taken to freshwater life. In the marine deposits of the
Cretaceous epoch, however, astacomorphous forms, which

HOPLOPARIA AND PSEUDASTACUS.

843

are known by the generic names of Hoploparia and
Enoploclytia, are abundant.

The differences between these two genera, and between
both and Eryma, are altogether insignificant from a broad
morphological point of view. They appear to me to be
of less importance than those which obtain between the
different existing genera of crayfishes.

Hoploparia is found in the London clay. It therefore
extends beyond the bounds of the Mesozoic epoch into
the older Tertiary. But when this genus is compared
with the existing Homaras and Nephrops, it is found
partly to resemble the one and partly the other. Thus,
on one line, the actual series of forms which have
succeeded one another from the Liassic epoch to the
present day, is such as must have existed if the common
lobster and the Norway lobster are the descendants of
Erymoid crustaceans which inhabited the seas of the
Liassic epoch.

Side by side with Eryma, in the lithographic slates,
there is a genus, Psendastacus (fig. 80, A), which, as its
name implies, has an extraordinarily close resemblance to
the crayfishes of the present day. Indeed there is no point
of any importance in which (in the absence of any know-
ledge of the abdominal appendages in the males) it differs
from them. On the other hand, in some features, as in the
structure of the carapace, it differs from Eryma, much
as the existing crayfishes differ from Nephrops. Thus, in
the latter part of the Jurassic epoch, the Astacine type

344 DISTRIBUTION AND AETIOLOGY OF THE CRAYFISHES.

was already distinct from the Homarine type, though
both were marine ; and, since Eryma begins at least
as early as the Middle Lias, it is possible that Pseudas-
tacus goes back as far, and that the common protas-
tacine form is to be sought in the Trias. Pseudastacus
is found in the marine cretaceous rocks of the Leba-
non, but has not yet been traced into the Tertiary
formations.

I am disposed to think that Pseudastacus is comparable
to such a form as Astacus nigrescens rather than to any
of the Parastacidce, as I doubt the existence of the latter
group at any time in northern latitudes.

In the chalk of Westphalia (also a marine deposit) a
single specimen of another Astacomorph has been dis-
covered, which possesses an especial interest as it is
a true Astacus (A. politus, Von der Marck and Schliiter),
provided with the characteristic transversely divided
telson which is found in the majority of the Pota-
mobiidce.

If wTe arrange the results of palaeontological inquiry
which have now been stated in the form of a table
such as that which is given on the following page,
the significance of the succession of astacomorphous
forms, in time, becomes apparent.

THE GENEALOGY OF THE CRAYFISHES. 345
Successive Forms of the Astacomorphous Type.

i. RftcO'.Jw rotamdbiidce. Homarina. Penceus.

II.

Later Tertiary

A*ta
(Ida

MIS

tio).

III.

Earlier Tertiary.

Hoplc

paria.

IV.

Cretaceous.

Astc

cus.

Pseud astacus.

Enoploclytia.

Hoploparia.

V.

Wealden
(Fresh Water).

VI.

Jurassic.

Pseudastacus

/

Eryma.

1

Pena

VII.

Liassic.

1

Eryma.

Penceus.

viit. Triassic.

IX. Permian.

x. Carboniferous. Anthrapakcmon

xi. Devonian.

xii. Silurian.

xiii. Cambrian.

If an Astacomorplious crustacean, having characters
intermediate between those of Eryma and those of
Pseudastacus, existed in the Triassic epoch or earlier ;
if it gradually diverged into Pseudastacine and Erymoid
forms ; if these again took on Astacine and Homarine

846 DISTRIBUTION AND ^ETIOLOGY OF THE CRAYFISHES.

characters, and finally ended in the existing Potamobiidce
and Homarbia, the fossil forms left in the track of this
process of evolution would be very much what they
actually are. Up to the end of the Mesozoic epoch
the only known Potamobiidce are marine animals. And
we have already seen that the facts of distribution
suggest the hypothesis that they must have been so,
at least up to this time.

Thus, with respect to the ^Etiology of the crayfishes,
all the known facts are in harmony with the requirements
of the hypothesis that they have been gradually evolved
in the course of the Mesozoic and subsequent epochs
of the world's history from a primitive Astacomorphous
form.

And it is well to reflect that the only alternative sup-
position is, that these numerous successive and coexistent
forms of insignificant animals, the differences of which
require careful study for their discrimination, have been
separately and independently fabricated, and put into the
localities in which we find them. By whatever verbal fog
the question at issue may be hidden, this is the real
nature of the dilemma presented to us not only by the
crayfish, but by every animal and by every plant ; from
man to the humblest animalcule ; from the spreading
beech and towering pine to the Micrococci which lie at
the limit of microscopic visibility.

NOTES.

Note 1., Chapter L, p. 17.

THE CHEMICAL COMPOSITION OF THE EXOSKELETON.

The harder parts of the exoskeleton of the crayfish contain rather
more than half their weight of calcareous salts. Of these nearly
seven-eighths consist of carbonate of lime, the rest being phosphate of
lime.

The animal matter consists for the most part of a peculiar substance
termed Chit in, which enters into the composition of the hard parts not
only Of the Artliropoda in general but of many other invertebrated
animals. Chitin is not dissolved even by hot caustic alkalies, whence
the use of solutions of caustic potash and soda in cleaning the skeletons
of crayfishes. It is soluble in cold concentrated hydrochloric acid with-
out change, and may be precipitated from its solution by the addition of
water.

Chitin contains nitrogen, and according to the latest investigations
(Ledderhose, " Ueber Chitin und seine Spaltungs-produkte : " Zeitschrift
fiir Physiologische Chemie, II. 1879) its composition is represented by the
formula C16 H2fl Na 010.

Note II., Chapter I., p. 20.

THE CRAB'S EYES, OR GASTROLITHS.

The " Gastroliths," as the " crab's eyes " may be termed, are found
fully developed only in the latter part of the summer season, just before
ecdysis sets in. They then give rise to rounded prominences, one on

NOTES.

each side of the anterior part of the cardiac division of the stomach. The
proper wall of the stomach is continued over the outer surface of the
prominence ; and, in fact, forms the outer wall of the chamber in which
the gastrolith is contained, the inner wall being formed by the cuti-
cular lining of the stomach. When the outer wall is cut through, it is
readily detached from the convex outer surface of the gastrolith, with
which it is in close contact. The inner surface of the gastrolith is usually
flat or slightly concave. Sometimes it is strongly adherent to the chi-
tonous cuticula ; but when fully formed it is readily detached from the
latter. Thus the proper wall of the stomach invests only the outer face
of the gastrolith, the inner face of which is adherent to, or at any rate in
close contact with, the cuticula. The gastrolith is by no means a mere
concretion, but is a cuticular growth, having a definite structure. Its
inner surface is smooth, but the outer surface is rough, from the projec-
tion of irregular ridges which form a kind of meshwork. A vertical sec-
tion shows that it is composed of thin superimposed layers, of which the
inner are parallel with the flat inner surface, while the outer becomes
gradually concentric with the outer surface. Moreover, the inner layers
are less calcified than the outer, the projections of the outer surface being
particularly dense and hard. In fact, the gastroliths are veiy similar to
other hard parts of the exoskeleton in structure, except that the densest
layers are nearest the epithelial substratum, instead of furthest away
from it.

When ecdysis occurs, the gastroliths are cast off along with the gas-
tric armature in general, into the cavity of the stomach, and are there
dissolved, a new cuticle being formed external to them from the proper
wall of the stomach. The dissolved calcareous matter is probably used
up in the formation of the new exoskeleton.

According to the observations of M. Chantran (Comptes Eendus,
LXXVIII. 1874) the gastroliths begin to be formed about forty days
before ecdysis takes place in crayfish of four years' old ; but the
interval is less in younger crayfish, and is not more than ten days
during the first year after birth. When shed into the stomach during
ecdysis they are ground down, not merely dissolved. The process
of destruction and absorption takes twenty-four to thirty hours
in very young crayfish, seventy to eighty hours in adults. Unless
the gastroliths are normally developed and re-absorbed, ecdysis is
not healthily effected, and the crayfish dies in the course of the
process.

NOTES.

349

According to Dulk ( "Chemische Untersuchung der Krebsteine :" Muller's
Archiv. 1835), the gastroliths have the following composition : —
Animal matter soluble in water . . . .11-43
Animal matter insoluble in water (probably chitin) 4*33

Phosphate of lime 18-60

Carbonate of lime 63*16

Soda reckoned as carbonate 1*41

98-93

The proportion of mineral to animal matter and of phosphate to car-
bonate of lime is therefore greater in the gastroliths than in the exo-
skeleton in general.

Note III., Chapter I., p. 31.
GROWTH OF CRAYFISH.

The statements in the text, after the words " By the end of the year,"
regarding the sizes of the crayfish at different ages, are given on the
authority of M. Carbon nier (L'Ecrevisse. Paris, 1869) ; but they obviously
apply only to the large "Ecrevisse a pieds rouges" of France, and not to
the English crayfish, which appears to be identical with the " Ecrevisse
a pieds blancs," and is of much smaller size. According to M. Carbonnier
(1. c. p. 51), the young crayfish just born is " un centimetre et demi
environ," that is to say, three-fifths of an inch long. The young of the
English crayfish still attached to the mother, which I have seen, rarely
exceeds half this length.

M. Soubeiran ("Sur l'histoire naturelle et l'education des Ecrevisses:"
Comptes Rendus, LX. 1865) gives the result of his study of the growth
of the crayfishes reared at Clairefontaine, near Rambouillet, in the
following table :

Mean length.

Mean weight.

Metres.

Grammes.

Crayfish of the year ,

0-025

0-50

»

1 year old

0-050

1-50

n

2 years old

0-070

3-50

>»

3 years „

0-090

6-50

j>

4 years „

o-no

17*50

}>

5 years .,

0-125

18*50

»»

indeterminate .

0-160

3000

very old

0-190

. 126-00

These observations must also appJy to the " Ecrevisse a pieds rouges."

350

NOTES.

Note IV., Chapter I, p. 37.

THE ECDYSES OF CRAYFISHES.

There is a good deal of discrepancy between different observers as to
the frequency of the process of ecdysis in crayfishes. In the text I have
followed M. Carbonnier, but M. Chantran ( " Observations sur l'histoire
naturelle des Ecrevisses:" Comptes Rendus, LXXI. 1870, and LXXIII.
1871), who appears to have studied the question (on the "ecrevisse
a pieds rouges " apparently) very carefully, declares that the young
crayfish moults no fewer than eight times in the course of the first twelve
months. The first moult takes place ten days after it is hatched ; the
second, third, fourth, and fifth, at intervals of from twenty to twenty-five
days, so that the young animal moults five times in the course of the
ninety to one hundred clays of July, August, and September. From the
latter month to the end of April in the following year, no ecdysis takes
place. The sixth takes place in May, the seventh in June, and the eighth
in July. In the second year of its age, the crayfish moults five times, that
is to say, in August and in September, and in May, June, and July
following. In the third year, the crayfish commonly moults only twice,
namely in July and in September. At a greater age than this, the
females moult only once a year, from August to September ; while the
males moult twice, first in June and July ; afterwards in August and
September.

The details of the process of ecdysis are discussed by Braun, " Ueber
die histologischen Yorgange bei der Hautung von Astacus fluviatilis"
Wurzburg Arbeiten, Bd. II.

Note V., Chapter I., p. 39.

REPRODUCTION IN CRAYFISHES.

The males are said to approach the females in November, December,
and January, in the case of the French crayfishes. In England they
certainly begin as early as the beginning of October, if not earlier.
According to M. Chantran (Comptes Rendus, 1870), and M. Gerbe
(Comptes Rendus, 1858), the male seizes the female with his pincers,
throws her on her back, and deposits the spermatic matter, firstly, on the
external plates of the caudal fin ; secondly, cn the thoracic sterna around
the external openings of the oviducts. During this operation, the
appendages of the two first abdominal somites are carried backwards,

NOTES.

351

the extremities of the posterior pair are inclosed in the groove of the
anterior pair ; and the end of the vas deferens becoming everted and
prominent, the seminal matter is poured out, and runs slowly along the
groove of the anterior appendage to its destination, where it hardens and
assumes a vermicular aspect. The filaments of which it is composed are,
in fact, tubular spermatophores, and consist of a tough case or sheath
filled with seminal matter. The spoon-shaped extremity of the second
abdominal appendage, working backwards and forwards in the groove
of the anterior appendage, clears the seminal matter out of it, and
prevents it from becoming choked.

After an interval which varies from ten to forty-five days, oviposition
takes place. The female, resting on her back, bends the end of the
abdomen forward over the hinder thoracic sterna, so that a chamber is
formed into which the oviducts open. The eggs are passed into the
chamber by one operation, usually during the night, and are plunged
into a viscous greyish mucus with which it is filled. The spermatozoa
pass out of the vermicular spermatophores, and mix with this fluid, in
which the peculiarity of their form renders them readily recognisable.
The spermatozoa are thus brought into close relation with the ova, but
what actually becomes of them is unknown.

The origin of the viscous matter which fills the abdominal chamber
when the eggs are deposited in it, and the manner in which these become
fixed to the abdominal limbs is discussed by Lereboullet (" Kecherches
sur le mode de fixation des ceafs aux faux pattes abdominaux dans les
Ecrevisses." Annales des Sciences Naturelles, 4e Ee. T. XIV. 1860),
and by Braun (Arbeiten aus dem Zoologisch-Zootomischcn Institut in
WUrzburg, II.).

Note VI., Chapter I., p. 42.
ATTACHMENT OF THE YOUNG CRAYFISH TO THE MOTHER.

I observe that I had overlooked a passage in the Report on the
award of the Prix Montyon for 1872, Comptes Rcndus, LXXV. p. 1341,
in which M. Chantran is stated to have ascertained that the young
crayfishes fix themselves "en saisissant avec un de leurs pinccs le
filament qui suspend l'oeuf a une fausse pattc de la mere."

In the paper already cited from the Comptes Rendus for 1870, M. Chan-
tran states that the young remain attached to the mother during ten days
after hatching, that is to say, up to the first moult. Detached before
this period, they die ; but after the first moult, they sometimes leave the

352

NOTES.

mother and return to lier again, up to twenty-eight days, when they
become independent.

In a note appended to M. Chantran's paper, M. Robin states, that " the
young are suspended to the abdomen of the mother by the intermediation
of a chitinous hyaline filament, which extends from a point of the
internal surface of the shell of the egg as far as the four most in-
ternal filaments of each of the lobes of the median membranous plate
of the caudal appendage. The filaments exist when the embryos have
not yet attained three -fourths of their development," Is this a larval
coat ? Rathke does not mention it and I have seen nothing of it in
those receutly hatched young which I have had the opportunity of
examining.

Note VIL, Chapter II., p. 64.

THE " SALIVARY " GLANDS AND THE SO-CALLED "LIVER" OF
THE CRAYFISH.

Braun (Arbeiten aus dem Zoologisch-Zootomischen Institut in
Wiirzburg, Bd. II. and III.) has described " salivary " glands in the
walls of the oesophagus, in the metastoma, and in the first pair of maxillas
of the crayfish.

Hoppe-Seyler (Pfliigers Archiv, Bd. XIV. 1877) finds that the yellow
fluid ordinarily found in the stomachs of crayfishes always contains pep-
tone. It dissolves fibrin readily, without swelling it up, at ordinary tem-
peratures ; more quickly at 40° Centigrade. The action is delayed by even
a trace of hydrochloric acid, and is stopped by the addition of a few drops
of water containing 0.2 per cent, of that acid. By adding alcohol to the
yellow fluid, a precipitate is obtained, which is soluble in water and in
glycerine. The aqueous solution of the precipitate has a strong digestive
action on fibrin, which is arrested by acidulation with hydrochloric acid.
These reactions show that the fluid is very similar to, if not identical
with, the pancreatic fluid of vertebrates.

The secretion of the " liver " taken directly from that gland, has a
more strongly acid reaction than the fluid in the stomach, but has
similar digestive properties. So has an aqueous extract of the gland,
and a watery solution of the alcoholic precipitate. The aqueous extract
also possesses a strong diastatic action on starch, and breaks up olive oil.
There is no more glycogen in the " liver " than is to be found in other
organs, and no constituents of true bile are to be met with.

NOTES.

352

Note VIII., Chapter II., p. 81.

ANAL RESPIRATION IN CRAYFISH.

Lereboullet (" Note sur une respiration anale observee chez plusieurs
Crustaces ; " Memoires de la Societe d'Histoire Naturelle de Strasbourg,
IV. 1850) has drawn attention to what he terms " anal respiration " in
young crayfish, in which he observed water to be alternately taken into
and expelled from the rectum fifteen to seventeen times in a minute.
1 have never been able to observe anything of this kind in the uninjured
adult animal, but if the thoracic ganglia are destroyed, a regular
rhythmical dilatation and closing of the anal end of the rectum at once
sets in, and goes on as long as the hindermost ganglia of the abdomen
retain their integrity. I am much disposed to imagine that the rhyth-
mical movement is inhibited, when the uninjured crayfish is held in such
a position that the vent can be examined.

Note IX. Chapter II., p. 82.
THE GREEN GLAND.

The existence of guanin in the green gland rests on the authority
of Will and Gorup-Besanez (Gelehrte Anzeigen, d. k. Baienzschen
Akademie, No. 233, 1848), who say that in this organ and in the organ of
Bojanus of the freshwater mussel, they found " a substance the reactions
of which with the greatest probability indicate guanin," but that they
had been unable to obtain sufficient material to give decisive results.

Leydig (Lehrbuch der Histologic, p. 467) long ago stated that the
green gland consists of a much convoluted tube containing granular cells
disposed around a central cavity. Wassiliew (" Ueber die Niere des
Flusskrebses : " Zoologischer Anzeiger, I. 1878) supports the same view,
giving a full account of the minute structure of the organ, and com-
paring it with its homologues in the Copcpoda and Phyllopoda,

Note X., Chapter III., p. 105.

THE ANATOMY OF THE NERVOUS SYSTEM OF THE
CRAYFISH.

The details respecting the origin and the distribution of the nerves are
intentionally omitted. See the memoir by Lemoine of which the title is
given in the "Bibliography."

A A

354

NOTES.

Note XI., Chapter III., p. 110.

THE FUNCTIONS OF THE NERVOUS SYSTEM OF THE
CRAYFISH.

Mr. J. Ward, in his " Observations on the Physiology of the Nervous
System of the Crayfish," (Proceedings of the Royal Society, 1879) has
given an account of a number of interesting and important experiments
on this subject.

Note XII., Chapter III. p. 124.

THE THEORY OF MOSAIC VISION.

Oscar Schmidt ("Die Form der Krystalkegel im Arthropoden Auge : "
Zeitschrift fiir Wissenschaftliche Zoologie, XXX. 1878) has pointed out
certain difficulties in the way of the universal application of the theory of
mosaic vision in its present form, which are well worthy of consideration.
I do not think, however, that the substance of the theory is affected by
Schmidt's objections.

Note XIII., Chapter III., p. 135.
THE SPERMATOZOA.
Since the discovery of the spermatozoa of the crayfish in 1835-36 by
Henle and von Siebold. the structure and development of these bodies
have been repeatedly studied. The latest discussion of the subject is
contained in a memoir of Dr. C. Grobben (" Beitrage zur Kenntniss der
mannlichen Geschlechtsorgane der Dekapoden : " Wien, 1878). There
is no doubt that the spermatozoon consists of a flattened or hemi-
spherical body, produced at its circumference into a greater or less
number of long tapering curved processes (fig. 34 F). In the interior
of this are two structures, one of which occupies the greater part
of the body, and, when the latter lies flat, looks like a double ring.
This may be called, for distinctness' sake, the annulate corpuscle. The
other is a much smaller oval corpuscle, which lies on one side of the
first. The annulate corpuscle is dense, and strongly refracting : the oval
corpuscle is soft, and less sharply defined. Dr. Grobben describes the
annulate corpuscle as " napfartig," or cup-shaped ; closed below, open
above, and with the upper edge turned inwards, and applied to the
inner side of the wall of the cup. It appeared to me, on the other
hand, that the annulate corpuscle is really a hollow ring, somewhr.1

NOTES.

355

like one of the ring-shaped air-cushions one sees, on a very small scale.
Dr. Grobben describes the spermatoblastic cells of the testis and their
nuclear spindles ; but his account of the development of the spermatozoa
does not agree with my own observations, which, so far as they have
gone, lead me to infer that the annulate corpuscle of the spermatozoon
is the metamorphosed nucleus of the cell from which the spermatozoon is
developed. For want of material, however, I was unable to bring my
investigations to a satisfactory termination, and I speak with reserve.

Note XIV., Chapter IV., p. 174.
THE MOBPHOLOGY OF THE CRAYFISH.

The founder of the morphology of the Crustacea, M. Milne Edwards,
counts the telson as a somite, and consequently considers that twenty-
one somites enter into the composition of the body in the Podopli-
thalmia. Moreover, he assigns the anterior seven somites to the head, the
middle seven to the thorax, and the hinder seven to the abdomen.
There is a tempting aspect of symmetry about this arrangement ; but as
to the limits of the head, the natural line of demarcation between it and
the thorax seems to me to be so clearly indicated between the somite
which bears the second maxillse and that which carries the first maxilli-
pedes in the Crustacea, and between the homologous somites in Insects, that
I have no hesitation in retaining the grouping which I have for many years
adopted. The exact nature of the telson needs to be elucidated, but I can
find no ground for regarding it as the homologue of a single somite.

It will be observed that these differences of opinion turn upon ques-
tions of grouping and nomenclature. It would make no difference to
the general argument if it were admitted that the whole body consists
of twenty-one somites and the head of seven.

Note XV., Chapter IV., p. 199.

THE HISTOLOGY OF THE CRAYFISH.

In dealing with the histology of the crayfish I have been obliged to
content myself with stating the facts as they appear to me. The discus-
sion of the interpretations put upon these facts by other observers, espe-
cially in the case of those tissues, such as muscle, on which there is as
yet no complete agUfcment even as to matters of observation, would
require a whole treatise to itself.

A A 2

356

NOTES.

Note XVI., Chapter IV., p. 221.

THE DEVELOPMENT OF THE CRAYFISH.

The remark made in the last note applies still more strongly to the
history of the development of the crayfish. Notwithstanding the mas-
terly memoir of Rathke, which constitutes the foundation of all our
knowledge on this subject ; the subsequent investigations of Lereboullet ;
and the still more recent careful and exhaustive works of Reichenbach
and Bobretsky, a great many points require further investigation. In
all its most important features I have reason to believe that the account
of the process of development given in the text, is correct.

Note XVII., Chapter VI., p. 297.

PARASITES OF CRAYFISHES.

In France and Germany crayfishes (apparently, however, only
A. nohilis) are infested by parasites, belonging to the genus Branchio-
hdella. These are minute, flattened, vermiform animals, somewhat like
small leeches, from one-half to one-third of an inch in length, which
attach themselves to the under side of the abdomen (B. parasitica), or
to the gills {B. astaci), and live on the blood and on the eggs of the
crayfish. A full account of this parasite, with reference to the literature
of the subject, is given by Dormer ("Ueber die Gattung Branchio-
bdella : " Zeitschrift fur Wiss. Zoologie, XV. 1865). According to Gay, a
similar parasite is found on the Chilian crayfish. I have never met with
it on the English crayfish. The Lobster has a somewhat similar parasite,
Histriobdella. Girard, in the paper cited in the Bibliography, gives a
curious account of the manner in which the little lamellibranchiate
mollusk, Cyclas fontinalis, shuts the ends of the ambulatory limbs of
crayfishes which inhabit the same waters, between its valves, so that the
crayfish resembles a cat in walnut shells, and the pinched ends of the
limbs become eroded and mutilated.

BIBLIOGRAPHY.

The subjoined list indicates the chief books and memoirs, in addition
to those mentioned in the text and in the Appendix, which may be
advantageously consulted by any one who wishes to study more fully
the biology of the crayfishes.

1 .—NATURAL HISTORY.

Roesel von Rosenhof. Der Monatlich-herausgegeben Insekten

Belustigung. 1755.
Carbonnier. L'Ecrevisse, Paris, 1869.

Brandt and Ratzeburg. Medizinische Zoologie. Bd. II., pp.
58-70.

Bell. British Stalk-eyed Crustacea, 1853.

Soubeiran. Sur l'Histoire naturelle et l'Education des Ecrevisses.

Comptes Rendus, LX., 1865.
Chantran. Observations sur l'Histoire naturelle des Ecrevisses.

Comptes Rendus, LXXL, 1870.

Sur la Fecondation des Ecrevisses. Ibid., LXXIV., 1872.

Experiences sur la Regeneration des Yeux chez les Ecrevisses.

Ibid., LXXVIL, 1873.-
Observations sur la Formation des Pierres chez les Ecrevisses.

Ibid., LXXVIIL, 1874.
Sur le Mecanisme de la Dissolution intrastomacale des

Concretions gastriques des Ecrevisses. Ibid., LXXVIIL, 1874.
Stefpenberg. Bijdrag til kanne domen om flodkraftens natural

historia, 1872. Abstract in Zoological Record, IX.
Vallot. Sur l'Ecrevisse fluviatile et sur son parasite l'Astacobdelle

branchiale. Comptes Rendus Acad. Sciences, Dijon. Memoires,

1843-44. Dijon, 1845.
PUTNAM. On some of the Habits of the Blind Crayfish. Proceedings

Boston Society of Nat. History, XVIII.

358

BIBLIOGRAPHY.

Heller. Ueber einen Flusskrebs- albino. Verhand d. Z. Bot.

Gesellschaft, Wien. Bd. 7, 1857, and Bd. 8, 1858.
Lebeboullet. Sur les variety Rouge et Bleue de l'ficrevisse

fluviatile. Comptes Rendus, XXXIII., 1857.
Girard. Quelques Remarques sur l'Astacus fluviatilis. Ann. Soc.

Entom. France, T. VII. 1859.

II. —ANATOMY AND PHYSIOLOGY.
Brandt and Ratzeburg. Op. cit.

Milne Edwards. Histoire naturelle des Orustaces. 1834.
Rolleston. Forms of Animal Life. 1870.

Huxley. Manual of the Anatomy of Vertebrated Animals. 1877.
Huxley and Martin. Elementary Biology. 1875.
Suckow. . Anatomisch-Physiologische Untersuchungen. 1818.
Krohn. Verdauungsorgane des Krebses. Gefasssystem des

Flusskrebses. Isis, 1834.
Von Baer. Ueber die sogenannte Erneuerung des Magens der

Krebse und die Bedeutung der Krebssteine. Mliller's Archiv,

1835.

Oesterlen. Ueber den Magen des Flusskrebses. Muller's Archiv,
1840.

T. J. Parker. On the Stomach of the Freshwater Crayfish.
Journal of Anatomy and Physiology, 1876.

BARTSCH. Die Ernahrungs- und Verdauungsorgane des Astacus
leptodactylus. Budapester Naturhistor. Hefte II. 1878.

Deszo. Ueber das Herz des Flusskrebses und des Hummers.
Zoologischer Anzeiger, I. 1878.

Lereboullet. Note sur une Respiration anale observee chez
plusieurs Crustacees. Mem. de la Societe d' Histoire Naturelle de
Strasbourg, IV., 1850.

Wassiliew. Ueber die Niere des Flusskrebses. Zoologischer An-
zeiger, I. 1878.

Lemoine. Recherches pour servir a l'histoire des systemes nervenx^

musculaire et glandulaire de l'Ecrevisse. Annales des Sciences

Naturelles, Se. IV. T. 15, 1861.
Dietl. Die Organization des Arthropoden Gehirns. Zeitschrift

fur Wiss. Zoologie, XXVII., 1876.
Krieger. Ueber das centrale Nerven system des Flusskrebses.

Zoologischer Anzeiger, I., 1878.
Leydig. Das Auge der Gliederthiere. lcS64.

BIBLIOGRAPHY.

359

Max Schulze. Die Zusammengesetzten Augen der Krebse unci
Insekten, 1868.

Berger. Untersuchungen iiber den Ban des Gehirns und der

Retina der Arthropoden. 1878.
Grenacher. Untersuchungen iiber das Sehorgan der Arthropoden.

1879.

0. Schmidt. Die Form der Krystalkegel im Arthropoden Ange.

Zeitschrift fiir Wiss. Zoologie, XXX., 1878.
Farre. On the organ of hearing in the Crustacea. Phil. Trans.

1843.

Leydig. Ueber Geruchs- und Gehororgane der Krebse und Insekten.
Muller's Archiv, 1860.

Hensen. Studien iiber das Gehororgan der Decapoden. Zeit-
schrift fiir Wissenschaftliche Zoologie, XIII. 1863.

Grobben. Beitrage zur Kenntniss der mannlichen Geschlechts-
organe der Dekapoden. 1878.

Brocchi. Recherches sur les Organes genitaux males des Crustac£s
decapodes. Annales des Sciences Naturelles, Se. VI. ii.

Leydig. Zur feineren Bau der Arthropoden. Muller's Archiv,
1855.

Handbuch der Histologic. 1857.

Haeckel. Ueber die Gewebe des Flusskrebses. Muller's Archiv,
1857.

1>RAUN. Ueber die histologischen Vorgange bei der Hautung von

Astacus tluviatilis. Wiirzburg Arbeiten, II.
P.aur. Ueber den Bau der Chitinsehne am Kiefer des Flusskrebses

und ihr Verhalten beim Schalenwechsel. Reichert u. Du Bois

Archiv, 1860.

Coste. Faits pour servir a l'Histoire de la Fecondation chez les

Crustaces. Comptes Rendus, XLVI. 1858.
Lereboullet. Recherches sur la mode de Fixation des (Exits aux

fausses pattes abdominales dans les Ecrevisses. Annales des

Sciences Naturelles, Se. IV. T. 11, 1860.

I IT. — DEVELOPMENT.

Uathke. Ueber die Bildung und Entwickelung des Flusskrebses
1829.

Li:ui:boullet. Recherches d'Embryologie comparee sur le deve-
loppement du Brochet, de la Perche et de i'Ecrevisse. 1862.

360

BIBLIOGRAPHY.

Bo bret sky. (A Memoir in Russian, of which an abstract is given
in Hofmann and Schwalbe, Jahresbericht f iir 1873 (1875) ).

Reichenbach. Die Embryonanlage und erste Entwickelung des
Fiusskrebses. Zeitschrift fiir Wiss. Zoologie. 1877.

IV. — TAXONOMY AND DISTRIBUTION OF CRAYFISHES.

A. General.

Milne Edwards. Op. cit.

Erichson. Uebersicht der Arten der Gattung Astacus. Wiede-
mann's Archiv fiir Naturgeschichte, XII. 1846.

Dana. Crustacea of the United States Exploring Expedition. 1852.

De Satjssure. Note carcinologique sur la Famille des Thalassinides
et sur celle des Astacides. Rev. et Magazin de Zoologie, IX.

Huxley. On the Classification and the Distribution of the Cray-
fishes. Proceedings of the Zoological Society. 1878.

B. European and Asiatic.

Rathke. Zur Fauna der Krym. 1836.
Gerstfeldt and Kessler. Cited in the text.
De Haan. Fauna Japonica. 1850.

Lereboullet. Description de deux nouvelles Especes d'^crevisses
{A. longicomis, A.pallipes). Mem. Soc. Science Nat. Strasbourg.

V. 1858.

Heller. Crustaceen des sudlichen Europa. 1863.
Kessler. Ein neuer russischer Flusskrebs, Astacus colchicus. Bul-
letin de la Soc. Imp. des Naturalistes de Moscou, L. 1876.

C. American.

Stimpson. Crustacea and Echinodermata of the Pacific shores of
North America. Journal of Boston Society of Natural History

VI. ; 1857-8.

De Satjssure. Memoire sur divers Crustacees nouveaux des Antilles
et du Mexique. Mem. de la Societe de Physique de Geneve
T. XIV., 1857.

VON Martens. Sudbrasilische Suss- und Brackwasser Crustaceen
(A.pilimanus, A. Irasilicnsis), Wiegmann's Archiv, XXXV., 1869.
— . Ueber Cubansche Crustaceen. Ibid. XXXVIII.
HAGEN. Monograph of the North American Astacidce. 1870.

BIBLIOGRAPHY.

361

D. Madagascar.

Audouin and Milne Edwards. Shi one Espeee nouvelle du genre
Ecrevisse (Astacus). Ecrevisse de Madagascar {A. Madagas-
cariensis). Mem. du Museum d'Hist. naturelle. T. II. 1841.

E. Australia.

Von Martens. On a new Species of Astacus, Annals k Mag. of

Natural History, 1866.
Heller. Eeise der " Novara." Zool. Theil. Bd. II. 1865.

F. New Zealand.

Miers. Notes on the Genera Astacoid.es and Paranephrops. Trans-
actions of the New Zealand Institute, IX., 1876.

Paranephrops. Zoology of " Erebus" and " Terror," 1874. Cata-
logue of New Zealand Crustacea, 1876.

Annals of Natural History, 1876.

Wood-Mason. On the mode in which the Young of the New Zealand
Astacxdcp attach themselves to the Mother. Ann. k Mag.
Natural History. 1876.

G. Fossil Astacomorpha.

Oppel. Palseontologische Mittheilungen. 1862.

Bell. British Fossil Crustacea. Palaeontographical Society.

P. Van Beneden. Sur la Decouverte d'un Homard fossile dans

l'Argile de Rupelraonde. Bulletin del'Acad. Eoyale de Belgique.

XXXIIL. 1-72.

Von dee Marck und Schluteb. Neue Fische und Krebse von der
Kreide von Westphalen. Palreontologica, XV. 1865.

COPE. On three extinct Astaci from the freshwater tertiary of Idaho.
Proceedings of the American Philosophical Society, XL, 1869-70.

INDEX.

A.

Abdomen, 19, 141

development of, 213
Abdominal appendages, 143

development of, 217
Abdominal somite, characters of, 1 42
/Etiology, 47
Agassiz, 308
Alimentary canal, 51

development of, 213, 222
/Ambulatory legs, 168
American Crayfishes, 243, 247
Amoeba, 285

Amurland Crayfishes, 304
Antenna, 23, 172

development of, 214, 218
Antennule, 23, 173

development of, 214, 218
Antlirapalcemon, 341
Anus, 29

Apodeme, 99, 158, 175
Appendage, 24, 143, 161, 173

abdominal, 143

cephalic, 170

thoracic, 164
Archenteron, 211
Arctogaeal province, 314
Areola, 235

Aristotle, referred to, 4
Arteries, 71

Arteries, development of, 224
Arthrobranchia, 75
Arthrophragm, 158
Arthropoda, 279, 284
Articulations, 95
Asiatic Crayfishes, 304
Astacina, 254
Astacoides, 250, 313
Astacomo?p7ia, 338
Astacopsis, 250, 264
Astacus, division into sub-genera,
290

Astacus angxilosua, 302, 310
colcHcus, 302, 310
dawriem, 301, 310
fluvia tills,

anatomy, general account

of, 17—31
attachment of young to

mother, 40, 351
branchial formula, 266
development, 205-226
distribution, geographical

44, 288, 298
distribution, chronologi-
cal, 44
ecdysis, 32, 350
general characters, 6
growth, 31, 349
habits, 8

364

INDEX.

Asta ens J! uma tills — continued
histology, 174
mortality, 127
muscular system, 90
myths concerning, 44
name, origin of, 13
nervous system, 101
newly hatched young, cha-
racters of, 219
nutrition, 48
occurrence, 5, 8
organs of alimentation,
51

circulation, 68
excretion, 82, 353
hearing, 116
reproduction, 128
respiration, 75, 353
sight, 118
smell, 114
taste, 115
touch, 113
prehension of food, 49
putrid, effect of smell of,
45

reproduction of lost limbs,
38

reproduction, sexual, 39,

128, 135, 350
sexual characters, 7, 20,

32, 145, 241
somites and appendages,

143

systematic description,
230

use as food, 10, 289

varieties, 289
Jontinalis, 290
japonicus, 304
lilamathensis, 305
leniustculuSy 305

Astacus leptodactylus, 299, 302
303, 310, 320
nigrescens, 244

nobilis, 290, 295, 296, 299,
310

oreganus, 305

pachypus, 302, 310

pallipes, 290

politust 344

sascatilis, 290

SchrencJdi, 304, 310

torrentium, 290, 294, 298, 310,
311

tristis, 290

Trowbridgii, 305
Atya, Atyidce, 331, 336
Auditory organ, 116

setas, 116
Australian Crayfishes, 306

province, 314
Austrocolumbian province, 314
Axius, 271

B.

Ball, B., quoted. 36
Basipodite, 143
Bell, T., quoted, 37, 42
Bile-duct, 61, 66
Biological sciences, scope of, 4
Blastoderm, 207
Blastomere, 205
Blastopore, 209
Blood, 31, 68, 176

corpuscles, 69, 176

development of, 224

sinuses, 50, 69
Bobretsky, referred to, 356
Bolivar, Dr., 298
Branchise,

Astacoides, 266

Astacopsis, 264

INDEX.

365

Branchiae — continued

Astacus, 25, 75, 265

development of, 224

Cancer, 276

Homarus, 257

Palcemon, 270

Palinurus, 264

Penceus, 267
Branchial chamber, 25

formula,

Astacoides, 266

Astacopsis, 264

Astacus, 266

Cancer, 277

hypothetically complete, 268

Palcemon, 270

Palinurus, 265

Penaus, 267
Branchiobdella, 356
Branchiostegite, 25

development of, 217
Bratjn, quoted, 352
Brazilian Crayfishes, 306

C.

Caecum, 61

Calcification of exoskeleton, 197
Californian Crayfishes, 243
Cambarus, 44, 247, 310, 312
Cancer, 272, 283
Carapace, 19

development of, 214
Carbonnier, M., quoted, 297, 349,

350
Cardia, 52
Caridina, 330
Carpopodite, 165
Cell, 66, 199
Cell-aggregate, 190, 199

division, 200

theory, 202, 204

Cephalic appendages, 170
development of, 217

flexure, 163

somites, 154
Cephalon, 19, 141
Cephalothorax, 19
Cervical groove, 19

spines, 234
Chantran, M., quoted, 348, 350.

351
Chelae, 22

Chilian Crayfishes, 308
Chitin, 50

composition of, 347
Chceraps, 250
Chorology, 46
Circulation, 73

organs of, 68
Common knowledge and science, 3
Connective tissue, 178

development of, 224
Cope, Prof., quoted, 316
Cornea, 118
Coxopodite, 143
Coxopoditic setae, 78
Crab, see Cancer
Crab's-eye, see Gastrolith
Crangon, 272

Crayfish, origin of name, 12

common, see Astacusfl u v iati I is
Crayfishes, Amurland, 304

Asiatic, 304

Australian, 306

Brazilian, 306

Californian, 243

Chilian, 308

definition of, 254

Eastern North American, 217,
305

European, 288. 297
evolution of, 331

366

INDEX.

Crayfishes, Figian, 300, 313
Japanese, 304, 313
Mascarene, 308, 313
northern and southern, com-
pared, 252
Novozelanian, 306, 313
southern, 249
Tasmanian, 306
Western North American, 305,
313

Crustacea, 271, 278
Crystalline cones, 121
Cuticle, 33, 50, 175, 192
Cyclas, 356

D.

Dactylopodite, 165

DaplvnAa, asexual reproduction of

128

Darwin, C, referred to, 4
De Haan, quoted, 313
Development, 205

abdomen, 213

abdominal appendages, 217

alimentary canal, 213, 222

antennas, 214, 2J8

antennules, 214, 218

blood and blood vessels, 224

branchiostegite, 217

carapace, 214

cephalic appendages, 2J7, 219
connective tissue, 224
ear, 225
eye, 225

eyestalk, 214, 218
gills, 224
heart, 224
kidney, 224
labrum, 218
mandibles, 214

Development of muscles, 224
nervous system, 213, 224
reproductive organs, 225
rostrum, 217

thoracic appendages, 217, 219
Digestion, 63
Distribution, 46

chronological, of crayfishes, 44,
316, 339

table of, 345

geographical, of crayfishes, 44>
288

causes of, 335

results of study of, 308, 314
Dormer, quoted, 356
Dulk, quoted, 349

E.

Ear, 116

development of, 225
Ecdysis, 32, 350

Ecrevisse a pieds blancs. 289, 297

a pieds rouges, 289, 297
Ectoderm, 141
Ectostracum, 194
Edelkrebs, 290
Endoderm, 141
Endophragmal system, 157
Endopleurite, 158
Endopodite, 145
Endoskeleton, 17
Endosternite, 158
Endostracum, 194
Engceus, 250, 306
Enojplocytia, 342
Epiblast, 211
Epidermis, 140
Epimeron, 143
Epiostracum, 192
Epipodite, 167

WDJSX.

367

Epistoma, 155

Epithelium, 140, 177

Equus exeelsus, occurring w ith fossil

crayfishes, 316
Eryma, 341

Evolution of crayfishes, 331
Excretion, organs of, 82
Exopodite, 145
Exoskeleton, 17

chemical composition, 347
Eye, 118

compound, 122

development of, 225
Eye-stalk, 24, 173

development of, 214

F.

Family, 252
Fat-cells, 180
Fibre, muscular, 185
Fibril, muscular, 185
Figian Crayfishes, 306
Filament, muscular, 185
Filter of stomach, 58
Flagellum, 167
Food-yelk, 206
Foot-jaws, see maxillipcdes
Forceps, 22
Foregut, 61

development of, 213, 222
Fossil crayfishes, 316
Foster, Dr. M., referred to, 110
France, consumption of crayfish

in, 10
Function, 22

G.

Oalaxidcc, 315
Gammarus, 323
Ganglion, 103, 105
Ganglionic corpuscle, 87, 103

Gastric mill, 53
Gastrolith, 29, 347

chemical composition, 349
Gastrula, 211
Gay, quoted, 356
Genus, 249

Geographical distribution, see Dis-
tribution
Gerbe, M., quoted, 350
Germinal disc, 209

layer, 206

spot, 133

vesicle, 133
Gerstfeldt, Dr., quoted, 290
Gills, see Branchiae
Girard, quoted, 356
GORUP-BESANEZ, quoted, 353
Green-gland, 83, 353

development of, 224
Grobben, Dr., quoted, 354
Growth of crayfish, 31, 349
Guanin, 82, 353
Gullet, see (Esophagus
Gunther, Dr., quoted, 315

H.

HAGEN, Dr., quoted, 305, 312
HaplocMtonidce i 315
Harvey, quoted, 5
Head, see Cephalon
Hearing, organ of, 116
Heart, 27, 71

development of, 224
Heller, Dr., quoted, 298, 330
Hepatic duct, see Bile duel
Hind gut, 61

development of, 214, 223
Histology, 175
Histriohdella,, 356
Homaridce, 263

368

INDEX.

J To ma rum, 261
Homarus, 13, 42, 257, 332
Homology, homologous, homolo-

gue, 148
Hoj?lqparia, 342
Hypoblast, 21 1

I.

Idotliea. 323, 334
Impregnation, 135, 350
Integument, 50
Interseptal zone, 183
Intestine, 29, 61
Ischiopodite, 165

J.

Japanese Crayfishes, 313, 314
Jaws, 23

Johnston, J., quoted, 42

K.

Kessleb, quoted, 298, 304
Kidney, see Green gland
Kltjnzingeb, referred to, 330

L.

Labrum, 51

development of, 218
Lamakck, referred to, 4
Lereboullet, quoted, 353
Legs, ambulatory, 168
Lemoine, referred to, 353
Leydig, referred to, 115, 353
Liver, 30, 64

development of, 223

nature of secretion, 352
Lobster, common, see Homarus

Norway, see JYephrops

Lobster, Rock, see PaMnurus
Loven, referred to, 327

M.

Machine, living, 128

M'Intosh, Dr. W. C, quoted, 288

Mandible, 23, 51, 170

development of, 214
Martens, Von, 306
Mastodon mirificus, occurring with

fossil crayfishes, 316
Maxillse, 23, 170
Maxillipedes, 23, 164
Medullary groove, 213
Megalopa stage of development,

283

Meropodite, 165
Mesoblast, 212
Mesoderm, 141
Mesophragm, 158
Metamere, 143
Metastoma, 51
Metope, 278
Midgut, 61

development of, 211, 214, 223
Milne-Edwards, quoted, 13, 289
Mollusca, 284
Morphology, 46, 138

comparative, 230
Mortality of crayfishes, 128
Morula, 206
Mosaic vision, 122, 354
Motor plates, 189
Mouth, 51

Muller, Johannes, referred to,
122

Muscle, 57, 90, 175, 181

development of, 224

histology of, 90, 181
Muscles of abdomen, 99

INDEX.

Muscles of chela, 93

of stomach, 57
Myosin, 186
Myotome, 174
Mysis, 281, 323

relicta, origin of, from M.
oculata, 327
Mysis stage of development, 280

N.

Natural History, 3

Philosophy, 3
Nauplius stage of development,

215, 280
Nearctic province, 314
Nephrons, 259, 332
Nerve, 101

auditory, 117

optic, 118
Nerve-cells, 103, 187

fibres, 101, 188
Nervous system, 105

development of, 213, 224

functions of, 354
Noble crayfish, see Astaeus nobilis
Nomenclature, binomial, 13, 15
Norway lobster, see Neplivops
Novozelanian province, 314
Nucleated cell, 199
Nucleolus, 187
Nucleus, 177, 200

changes of, in cell-division, 200

O.

(Esophagus, 51
Olfactory organ, 114
Organ, 22

Origin of crayfish, evidence as to,
320, 331

Ovary, 31, 129

structure of, 131
Oviduct, 129
Oviposition, 351
Ovisac, 132
Ovum, 129

structure of, 133

P.

Palsearctic province, 314
Palcemon, 268, 328
Palinuridce, 263
Palinurus, 261, 264
Palp, 171

Para7iephrops, 250, 306, 313
Paraphragm, 158
Parasites of crayfish, 356
Parastacidce, 252, 256, 306, 313
Parastacus, 250, 306
Pempliix, 341
Penczus, 267, 280
Pericardium, 69
Perivisceral cavity, 50
Phyllobranchia, 271
Physiology, 46
Pleurobranchia, 79
Pleuron, 96, 143
Podobranchia, 75, 165
Podovht7ialmia, 279
Pore-canals, 195
Post-orbital ridge, 233

spine, 232
PotamoMidce, 252, 256
Prawn, see Palcemon
Prehension of food. 49
Procephalic lobes, 160

development of, 2 1 3
Propodite, 165
Protopodite, 143
PrototrocteSy 3 1 5

370

INDEX.

Protozoa, 285
Pseudastacus, 343
Pylorus, 52

Race, 292

RATHKE, quoted, 350
Reaumur, quoted, 33
Reflex action, 108
Reichenbach, quoted, 356
Renal organ, see Green-gland
Reproduction of lost limbs, 38

sexual, 39, 128, 135, 350
Reproductive organs, 128

development of, 225
Respiration, anal, 353
Respiratory organs, see Branchire
Iletr optima, 315
Robin, quoted, 352
Rock lebster, see Palinurus

. ROESEL VON ROSENHOF, quoted,

41. 43

Rondoletius, referred to, 4
Rostrum, 157

development of, 217

S.

Salivary glands, 352

Salmonidce, parallel between their

distribution, and that of Asta-

cidoe, 315
Sarcolemma, 90, 182
Saes, G. 0.y referred to, 327
Saetobius von Walterhausen,

quoted, 322
Scaphognathite, 80. 170
Schizopod stage of development,

280

SCHLUTER, 317

Schmidt, O., quoted, 354

SCHRANK, 290

Science, physical, 3
Science and common sense, 1
Segmentation, 174
Self-causation, 112
Sensory organs, 113
Septal line, 183

zone, 183
Setae, 197

Shrimp, see Crangon
Siebold, Von, referred to, 331
Sight, organ of, 118
Sinus, sternal, 69
Smell, organ of, 114
Somite, 143, 161, 355

abdominal, 142

cephalic, 154

thoracic, 150
Soubeiran, M., quoted, 349
Southern Crayfishes, 249
Species, 243, 290

morphological, 291

physiological, 296
Spermatozoa, 129, 135, 354
Spontaneous action, 112
Squame of antenna, 172
Steinkrebs, see Astacus torrent 'mm
Sternum, 96, 143
Stomach, 29, 51

Stone-crayfish, see Astacus torren-
tium

Striated spindle, 121
Swimmeret, 20

T.

Taste, organ of, 115
Teleology, 47, 137
Tendon, 92, 175

INDEX.

Tergum, 90, 143
Terminal plates, 189
Terminology, scientific, 14
Testis, 129

structure of, 133
Thoracic appendages, 164

development of, 217

somites, 150
Thorax, 19, 141
Tissue, 175
Touch, organ of, 113
Transformism, 318
Treviranus, referred to, 4
Tribe, 252

Trichobranchias, 263
Troglocaris, 337

V.

Valves of heart, 73
of stomach, 59
Van Helmont, quoted, 45
Variety, 290, 292
Vas deferens, 130
Vent, see Anus
Vertebrata, 284

TLTE

Vertebrata, eye of, 122, 125
Visual pyramid, 121

rod, 121
Vitelline membrane, 133
Vitellus, 133
Voluntary action, 112
Von der Marck, 317

W.

Ward, J., referred to, 354
WASSILIEW, quoted, 353
Whirlpool of life, 84
Will, quoted, 353
Wood-Mason, quoted, 41

Y.

Yelk, 133
Yelk-division, 205
Young of Astacus, newly hatched,
characters of, 219

Z.

Zoaea stage of development, 280

END.

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