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■J / - 







T. H. HUXLEY, F. R. S. 






" Alo Set fxtj SutrxepaiVetv TraiSt/cw? ttjv irepi twv aTijotOTepwr ^towv e-iTiaKe\}/i.v' ev ira<Tt 
yap Tois (^vo-iKots eveo-Ti Ti 0avjiAao-Te»'."— Abistotle, J)e Partihus, I. 5. 

" Qui enim Aiitorum verba legentes, renim ipsarum imagines (eorum verbis com- 
prehensa) seiisibus propriis non abstrahimt, hi non veras Ideas, sed falsa Idola et 
phantasmata inania mente concipiunt 

" Insusurro itaque in aurem tibi (amice Lector I) ut qusecunque a nobis in hisce .... 
exercitationibus tractabuntur, ad exactam experientise tnitinam pensites : fidemque 
lis non aliter adhibeas, nisi quatemis eadem indubitato sensuum testimonio firmissime 
stab liri deprehenderis."— Harvey. Exercitationes de Generatione. Frcefatio. 

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

" Le seul et le vrai moycn d'avancer la science est de travailler a la description et 
k I'histoire des differentes choses qui en font I'objet." — Buffon. Discours dc la 
riianiere d'etudier et de trailer VHistoire Naturelle. 

" Ebenso hat mich auch 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 
Bchwer fallen soUte solehes alles deutlich zu beschreiben."— Roesel v, Rosenhof. 
Insecten Belustigungen. — " Der Flxtsskrebs Jnesiges Landes mit seinen merkwurdigen 


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. I^or 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, Bo j anus, 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 


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

It is for tliis 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 I'art 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 


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 fm^nish 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. 

uniiecessary to burden the text ; and, under the 
head of Bibliography, I have given some references 
to the literature of the subject Avhich 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, for 
such excellent specimens of xylographic art as 
the figures of the Crab, Lobster, Eock Lobster, 
and Norway Lobster. 

T. H. H. 


Noveinber, 1879. 






The Natural History of the Common Crayfish . 


The Physiology of the Common Crayfish. The Mechanism 

AND GROWTH . . ' 46 


The Physiology of the Common Crayfish. The Mechanism 
BY WHICH the Living Organism adjusts itself to sur- 


The Morphology of the Common Crayfish. The structure 

AND the development OF THE INDIVIDUAL . ... 137 




The Comparative Morphology of the Crayfish, The struc- 


The Distribution and the Etiology of the Crayfishes . . 288 

NOTES 347 


INDEX. 363 



Frontispiece. The Common Crayfish, Astacus fluviatilis, (male) 
Fig. 1. AstaciLs fluviatilis. Side view of the male. . . 6 

,, 2. ,, ,, DoPvSAL views of MALE AND FEMALE 18 


>> "• >» »» 

,, 4. ,, ,, The gills 

,,5. ,, „ Dissection feom the doesal side 

(male) .... 
Longitudinal veetical section of 


A gasteolith oe " grab's eye" 
Attachment of young to swim 

meeet of mother 
Steuctuee of the stomach .' 
Longitudinal section of the sto 


eoof of the stomach, from within 
Dissection from the side (male) 
Alimentary canal feom above 
Blood corpuscles . 
Tr'ansveese section of thoeax , 
The heaet 

Steuctuee of the gills 
The geeen gland . 

• » 















































> J 
























































































Muscular tissue 

Muscles of chela ... 

Articulation of abdominal so 


Muscular system . 

Xerve fibres .... 

Nerve ganglia 

Nervous system 

Olfactory and auditory organs 

Auditory sac .... 

Structure of eye 

Diagram of eye 

Female reproductive organs . 

Male reproductive organs . 

Structure of ovary . 

Structure of testis 

Spermatozoa .... 

The last thoracic sternum in the 

male and female 
Transverse section of abdomen 
Abdominal appendages 
Connection between thorax and 

abdomen .... 
Cephalothoracic sterna and en 

dophragmal system 
Ophthalmic and antennulary so 


The rostrum .... 
A Segment of the endophragmal 

system .... 
Longitudinal section of cephalo 


The third maxillipede . 












Fig. 45. 

„ 62. 














Astaciis Jluviatilis. The first and second maxilli- 

PEDES 166 

The second ambulatory leg . .169 

The mandible and maxilla . . 171 
The eye-stalk, antennule, and 


Blood corpuscles . . . . 176 

Epithelium 178 

Connective tissue . . . . 179 
Muscular tissue . . . .181 

Muscular tissue . . . . 182 
Nerve ganglia . . . .188 

Nerve fibres ... . . . 189 

CUTICULAR tissue .... 191 

Sections of embryos . ... 208 

Earlier stages of development 210 

Later stages of development . 216 

Newly hatched young . . . 220 
Comparative views of the cara- 
pace, third abdominal somite, 

AND telson .... 233 
' torrentium. \ Comparative views of the first 
nohilis. - AND second abdominal appen- 

^ nigrescens. ] dages of the male . . . 245 
Caraharus Clarkii . . . . . . . .248 

Farastacus brasilietis-is 250 

Astacoides mudagascarensis ...... 251 

Diagram of the morphological relations of the 

Astacidcc ......... 253 

How.arus vulgaris ........ 258 

'' Farastacus. \ 

Nephrops. [-PoDOBRANCHiiB 259 




















> J 




































> J 


J > 








, nigresceiis. 




Fig. 69. Nephrops norvegicus . 

70. Palinurits vulgaris . 

71. Falcemon jaraaicensis 

72. Cancer pagurus . , , 

73. Pcnceus ..... 

74. Cancer 'pagurus. Development 

75. Astacus leptodactylis . 

76. Australian Craxjfish 

77. Map of the distribution of Ceayfishes 

78. Carribarus. Walking leg. 

79. Palasmon jamaicensis . , 
Pscudastacus pastulosus i 
Eryma modestiformis ) 

81. Iloploparia longimana 










(^Astacus JluviatiUs.) 

Many persons seem to beHeve that what is termed 
Science is of a widely different natm^e from ordinary 
knowledge, and that the methods by which scientific 
truths are ascertained involve mental o^^erations 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 Kfe. 

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 


for the commonest x^urposes of everydaj^ 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 wa}^ ; 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 

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 


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 finduig out of 
the causes of things becomes a source of jo}^, 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 w^hich 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 
bej'Ond, 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. 


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, Rondoletius, 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. 


My purpose, in the present work, is to exemplify the 
general truths respectinof the development of zoological 
science which have just been stated by the study of a 
special case ; and, to this end, I have selected an animal, 
the Common Crayfish, which, taking it altogether, is 
better fitted for my purpose than any other. 

It is readily obtained,* and all the most important 
points of its construction are easily deciphered ; hence, 
those who read what follows will have no difficulty in 
ascertaining whether the statements correspond with facts 
or not. And unless my readers are prepared to take this 
much trouble, they may almost as well shut the book ; 
for nothing is truer than Harvey's dictum, that those 
who read without acquiring distinct images of the things 
about which they read, by the help of their own senses, 
gather no real knowledge, but conceive mere phantoms 
and idola. 

It is a matter of common information that a number of 
our streams and rivulets harbour small animals, rarely 
more than three or four inches long, which are ver}'' similar 
to little lobsters, except that they are usually of a dull, 
greenish or brownish colour, generally diversified with 
pale yellow on the under side of the bod}", and some- 
times with red on the limbs. In rare cases, their 

• If crayfish are not to be had, a lobster will be found to answer to 
the description of the former, in almost all points ; but the g-ills and 
the abdominal appendages present differences ; and the last thoracic 
Bomite is united with the rest in the lobster. {See Chap. V.) 



general hue may be red or blue. These are " cra}'- 
fishes," and they cannot possibly be mistaken for any 
other inhabitants of our fresh waters. 

Fig. 1. — Astaciis fluvialilis. — Side view of a male specimen Cnat. size) : — 
hg, brancliiostegite ; eg, cervical groove ; r, rostrum ; t, telson. — 
1, ej^e-stalk ; 2, antennule ; 3, antenna ; 9, external maxillipede ; 
10, forceps; 14, last ambulatory leg; 17, third abdominal ap- 
pendage ; 20, lateral lobe of the tail-fin, or sixth abdominal 
appendage ; xv, the first ; and XX, the last abdominal somite. 
In this and in succeeding figures the numbers of the somites are 
given in Roman, those of the appendages in ordinary numerals. 

The animals may be seen walking along the bottom 
of the shallow waters which they prefer, by means of four 
pairs of jointed legs (fig. 1) ; but, if alarmed, the}" swim 


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 four pairs 
of legs, which are used in walking, there is a pair of 
limbs of a much more massive character, each of which 
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 
of 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 energ}'-. 
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, manj^-jointed 
filament, like a whip -lash, which is more than half the 
length of the body (5). 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 


females, the others the males. And the latter may 
be still more easily known by the possession of four 
curved styles, 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, cra3^fishes 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 
easterty 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 


wliich 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 ai)pear 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 tm-ned to 

account as supplies of needful calcareous matter ; and 

the unprotected or weakly member of the family is 


not spared. Craj^fishes, 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 in 
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 j)rotection they spend the first few days of their 

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 


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 would be forced upon anyone who dealt in cray- 
fishes, or lived in a district in which they were commonl}^ 
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 pro\ince 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, 


having a common English name for it ah'eady, should 
naturalists call it hy another appellation derived from a 
foreign tongue ? 

The origin of the common name, "crayfish," involves 
some curious questions of etymolog}^ and indeed, of his- 
tory. It might readily he 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 ** ere vis "or " crevice," 
and the "cray" is simply a phonetic siDelHng 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 

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 


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 ; aaraKos, astakos, 
was the name by which the Greeks knew the lobster ; and 
it has been handed down to us in the w^orks 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 fliwiatilis, 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 


denote that which the Greeks, ancient and modern, 
signify, by its original, astaJws ; 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 V70uld 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 signs 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 


Moreover, while English, French, German, and Italian 
artisans are under no particular necessity to discuss 
the processes and results of their husiness with one 
another, science is cosmopolitan, and the difficulties of 
the study of Zoology would he prodigiously increased, if 
Zoologists of different nationalities used different tech- 
nical terms for the same thing. They need a universal 
language ; and it has heen found convenient that the lan- 
guage shall he the Latin in form, and Latin or Greek in 
origin. What in English is Crayfish, is Ecrevisse in 
French ; Flusskrebs, in German ; Cammaro, or GambarOy 
or Gammarello, in Italian : hut 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 A stacus fluviatilis. 

But granting the expediency of a technical name for 
the Crayfish, y^hj should that name he 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 w^ould 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, 


we put tlie Christian name, so to speak, after the sur- 

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 
fluviatile, another slender-handed, another Dauric, from 
the region in which it lives ; and these double names are 
rendered by — Astacus fluviatilis, 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 fl^iviatilis 
was a significant name so long as we knew of only one 
kind of crayfish ; but now that we 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 



care to go far bej'ond 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 -pavts inside ; whereas in oui'selves, 
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 craj^fishes 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 

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 
natm-ally marked out into several distinct regions. There 

Fig. 2. — Astacns fuviatiUs. — Dorsal or tergal views (nat. size). A, male; 
B, female : — hcg, branchio-cardiac groove, which marks the boun- 
dary between the pericardial and the branchial cavities ; eg, 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, 
antennse ; 20, lateral lobes of tail-fin ; xv-xx, somites of the 


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 bod}^ is divided 
in the higher animals, the fore part is termed the cepha- 
lo-thorax, or head (cephalon) and chest (thoraoc) com- 
bined, while the hinder part receives the name of 

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 

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 


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

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 sivimmerets. 
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 {30). 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 ; 'svhile, 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. — Astaciisfluviatilis. — Ventral or sternal views (nat. size). A, male ; B, female : — 
a, vent ; gg, opening of the green gland ; lb, labrum ; mt, metastonia or lower 
jp ; od, opening of the oviduct : vd, that of the vas deferens. 1, eye-stalk ; 3, 
antfriinule ; 3, antenna : U, mandible ; S, second maxillipede ; 9, third or external 
maxillipede ; 10, forceps ; 11, first leg ; U, fourth leg ; 15, 16, 19, 20, first, second, 
fifth, and sixth abdominal appendages; x., xt., 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 16th to the 19th appendages are removed on 
the animal's left side : in the female, the antenna (with tlie 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. 


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) f 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 ofi'ence and defence. 

In front of the forceps, there is a piiir of limbs which 
have a different character, and take a different direction 
from any of the foregoing (P). 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 


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, jaws 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 maxiUipedes. 

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 maxillce, 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 
antennce (5) ; above, and in front of them, follow the 
small feelers, or anteiinules {2) ; and over them, again, lie 


the eye stalks (1). The antennse are organs of touch ; 
the antennules, in addition, contain the organs of hear- 
ing ; while, at the ends of the ej^estalks, 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 ai^- 
pendages ; and that these appendages are turned to 
different uses, or are organs of different functions, in 
difterent 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 cursor}^ 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- 


fully introduced into the cleft from behind, as high up 
as it will go without tearing anything, and a cut is then 
made, parallel with the middle line, as far as the cervical 
groove, and thence following the cervical groove to the 
base of the outer foot-jaws, a large flap will be removed. 
This flap of the carapace is called the branchiostegite 
(fig. 1, hg), because it covers the gills or hranchice 
(fig. 4), which are now exposed. They have the appear- 
ance of a number of delicate plumes, which take a direc- 
tion from the bases of the legs upwards and forwards 
behind, upwards and backwards in front, their summits 
converging towards the upper end of the cavity in which 
they are placed, and which is called the branchial 
chamber. These branchiae are the respiratory organs ; 
and they perform the same functions as the gills of a 
fish, to which they present some similarit3% 

If the gills are cleared away, it is seen that the branchial 
cavity is bounded, on the inner side, by a sloping wall, 
formed by a delicate, but more or less calcified layer of 
the exo skeleton, which constitutes the proper outer wall 
of the thorax. At the upper limit of the branchial cavity, 
the layer of exoskeleton is very thin, and turning out- 
wards, is continued into the inner wall or lining of the 
branchiostegite, which is also very thin {see fig. 15, p. 70). 

Thus the branchial chamber is altogether outside the 
bod}^ to which it stands in somewhat the same relation 
as the space between the flaps of a man's coat and his 
waistcoat would do to the part of the body enclosed by the 



arl.S arh.f> plh.\2 arh.\2 p 



Fig. 4. — Astacus flnvuttilis.—In A, the gills, exposed by the removal of the branchio- 
stegite, are seen in their natural position ; inB, the podobranchise (see p. 75) are re- 
moved, and the anterior set of arthrobranchise turned doAvnwards ( x 2): 1, eye-stalk ; 
3, antennule ; 3, antenna ; U, mandible : 6, scaphognathite ; 7, first niaxillipede, inB 
the epipodite. to which the line points, is partly removed ; 8, second niaxillipede : 
9, third maxillipede ; 10, forceps ; lU, foiirth ambulatory leg ; 15, first abdominal 
appendage; xv., first, and xvi., second abdominal somite; arh. 8, arh. 9, arh. 13, 
the posterior arthrobranchise of the second and third maxillipedes and of the third 
ambulatory leg ; arb'. 9, arh'. IS, the anterior arthrobranchise of the third maxillipede 
and of the third ambulatoiy leg ; phd. 8, podobranchise of the second maxillipede ; 
phd. 13, that of the third ambulatory leg ; -plh. 12, jilb. 13, the two rudimentary 
pleurobranchiw ; plh. lU, the functional pleurobranchia ; r, rostrum. 


waistcoat, if we suppose the lining of the flaps to be made 
in one piece with the sides of the waistcoat. Or a closer 
parallel still would be brought about, if the skin of a 
man's back were loose enough to be pulled out, on each 
side, into two broad flaps covering the flanks. 

It will be observed that the branchial chamber is open 
behind, below, and in front ; and, therefore, that the water 
in which the crayfish habitually lives has free ingress 
and egress. Thus the air dissolved in the water enables 
breathing to go on, just as it does in fishes. As is the 
case with many fishes, the crayfish breathes very well 
out of the water, if kept in a situation sufficiently cool 
and moist to prevent the gills from drying up ; and 
thus there is no reason why, in cool and damp weather, 
the crayfish should not be able to live verj^ well on land, 
at an}^ rate among moist herbage, though whether 
our common crayfishes do make such terrestrial excur- 
sions is perhaps doubtful. We shall see, by-and-by, that 
there are some exotic crayfish which habitually live on 
land, and perish if they are long submerged in water. 

With respect to the internal structure of the crayfish, 
there are some points which cannot escape notice, how- 
ever rough the process of examination may be. 

Thus, when the carapace is removed in a crayfish 
which has been just killed, the heart is seen still 
pulsating. It is an organ of considerable relative size 
(fig. 5, h), which is situated immediately beneath the 



Fig. 5. — Astaciis fluviatiUs. — A male specimen, with the roof of the 
carapace and the terga of the abdominal somites removed to show 
the viscera (nat. size) : — aa, antennary artery ; ag, anterior gastric 
muscles ; amni, adductor muscles of the mandibles; cs, cardiac 
portion of the stomach ; gg, green glands ; h, heart ; hg, hind gut, 
or large intestine ; Lr, liver ; oa, ophthalmic artery ; -pg posterior 
gastric muscles ; saa, superior abdominal artery ; f, testis ; vd, vas 




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, lis), 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. — Astaciis jlvviatilis. — A longitudinal vertical section of the ali- 
mentary canal, witli the outline of the body (nat. size) : — a, vent ; ag, 
anterior gastric muscle ; hd, entrance of left bile duct ; eg, cervical 
groove ; ccs, 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 ; hg, hind-gut; Ih, labrum ; It, lateral tooth of stomach ; 
in, mouth ; mg, mid-gut ; mt, median tooth ; oe, oesophagus ; ^><?, pro- 
cephalic process ; pg, posterior gastric muscle ; 7;-?, pyloric portion of 
stomach ; ?•, annular ridge, marking the commencement of the 

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 


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 ojDen, three large 


Fia. 7.—Asfacus flwiatiUs.—K 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 conspicuousl}^ into its 
interior (fig. 6, It, mt) ; so that, in addition to its six 
pairs of jaws, the craj^fish 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 


liver (Jn^. 5, Lr) ; and, in the breeding season, the 
ovaries of the females, or organs in which the eggs are 
formed, are very consiDicuous 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 nearl}^ ; and at five years, five inches. They 


go on growing till, in exceptional cases, they may attain 
between seven inclies 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 bod}^, 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 


hardens. This sort of moulting is what is technicallj' 
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 onl}^ what 
is termed a cuticular layeVy 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 rapidty 
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 Eeaumur, 
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 
I'Academie royale des Sciences," annee 1712 ; and "Additions aux ob- 
servations sur la mue des dcrevisses donnees dans les Memoires de 1712." 
Ibid. 1718. 



mences, the crayfish rubs its limbs one against the 
other, and, without changing its j^lace, moves each 
sex^arately, 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 plaj^ in their loosened sheaths. 
After these preparatory steps, the crayfish appears to 
become distended ; in all probabilit}^, 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 empt}^ 
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 bod}^, 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 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, exce]3t 
for the brighter colour of the latter, the two cannot be 

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 
flabb}^, like wet paper ; though Reaumur 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 


that tlie carapace is raised, notliing stops the cra^^fish 
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 



fluttered about in the greatest agitation. He was quite 
soft ; and every time I entered the room during the next 
two dajs, he exhibited the wildest terror. On the tliird, 
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 cra^^fish do not exuviate every j^ear. 

It has been stated that, in the course of its violent 
eff'orts 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 exuvige. 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 ofi: 

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


the limb, which remains in the hand of the captor, while 
the crayfish escapes. This volmitary amputation is alwa3^s 
effected at the same place ; namely, where the limb is 
slenderest, just bej^ond 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 injur}^ 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 ver}^ 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- 


mentaiy 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 ver}'- unequal 
in size. 

Injuries inflicted while the cra3' are soft after 
moulting, are apt to produce abnormal growths of the 
part affected; and these ma}^ be j)erpetuated, and give 
rise to various monstrosities, in the pincers and in other 
parts of the hody. 

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 hindeniiost i)air 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- 
tor}^ legs but two, that is, in the hinder of the two pair 
which are provided with chelate extremities (fig. 3, B ; od). 


After the female has received the deposit of the 
spermatic matter of the male, she retires to a buiTow, 
in the manner already stated, and then the process of 
lajang the eggs commences. These, as they leave the 
apertm-es 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 oj^eration 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 hen's 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 difi'erent 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. 




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

'' At this time they are quite trans^Darent ; and when 

Fig. 8. — Astacns fluviatUis. — A, two recently hatched crayfish attached 
to one of the swimmerets of the mother ( x 4). pr, protopodite ; 
eti, endopodite ; and ex, exopodite of the swimmeret ; ec, 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 disgustmg to those who do not know 


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 safetj^ 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. Roesel'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 Monat'ich-herausgegeben Insecten Belustigung." Dritter 
Theil, p. 336. 1755. 

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

X "Joannis Jonstoni Historiae naturalis de Piscibus et Cetis Libri 
quinque. Tomus IV. * De Cammaro seu Astaco fluviatili."* 


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 Roesel 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- 
curved hooks, which overlap when the chelae are shut (fig. 8, 
B). Hence, when the chel£e 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 


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 Avould 
a]3pear 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 (Cambarus) 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 craj^fishes, as 
about other animals. At one time " crabs'-ej^es " were 


collected in vast numbers, and sold for medicinal 
pm-poses 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 noctm-nal 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: " Tam 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. 



An analysis of such a sketch of the "Natural History 
of the Crayfish" as is given in the preceding chapter, 
shows that it provides hrief 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, namel}'-, 


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 

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 effects 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 j)erformed, 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 purj)ose. Under another 


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 cravfish 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 j^ear. 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 craj^fish absorbs a 
very considerable quantity of oxj^gen, 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- 
factorj^, 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. 


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 _p?'oiei«, 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 innutritions, 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 ambulator}' 
limbs. These grasp the food, and, tearing it into 
pieces of the proper dimensions, thrust them between 
the external maxillipedes, which are, at the same time, 


worked rapidly to and fro sidewa3's, 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 bod}^ 
we may conceive the whole cra^'fish 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 cjdinder is the 
outer W'all of the body itself, to which the general nanje 
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 we 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- 


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

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 craj^fish is a longitudinall}' 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 cavit}^ In front, the mouth is over- 
lapped by a wdde shield- shaped plate termed the upper 
lip, or lahnim (figs. 3 and 6, Ih) ; 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 ; vit), which is sometimes called 
the lower lip. A short wide gullet, termed the oeso- 
phagus (fig. 6, oe), leads directly ujDwards into a spacious 
bag, the stomach, w^iich 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 w^hich the 


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

In a man's stomach, the ox^ening 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 dicision, 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 margms 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 
cavit}^, as far as the pylorus, where it ends in certain 
valvular projections. The chitinous cuticle which forms 
the outermost laj^er 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 bod}-. 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 



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

Fig. 9. — Antaeus fltiviatilis. — A, the stomach with its outer coat removed, seen Trom 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 ; Ip, 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 ; as, oesophagus ; p, pyloric ossicle ; pc, pterocardiac ossicle ; 
PP, prepyloric ossicle ; iic, uro-cardiac process ; t, convexities on the free surface 
of its hinder end ; v^, median pyloric valve ; zc, zygocardiac ossicle. 

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


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 calcified bar (figs. 9-11, c) from 
the middle of the hinder j)art of which another bar (iic), 
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 w^all 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 (j'^p). 
The end which is articulated with the handle of the cross- 
bow is produced into a strong reddish conical tooth (mt), 
curved forwards and bifurcated at the summit ; conse- 
quently, wdien the cavity of the stomach is inspected from 
the fore part of the cardiac pouch (fig. 9, B), the two- 
pointed curved tooth (mt) is seen projecting behind the 
convex surfaces (t), 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 hinder middle piece (pjy) is connected with a second flat 
transverse plate which lies in the dorsal wall of the pjdoric 
chamher (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 (^c) passes from the outer end of the front cross^Diece 
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 w^hich it articulates. 
Internally, this piece j^rojects into the cardiac cavity of 
the stomach as a stout elongated reddish elevation {It), 
the sm-face 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 



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. — Astacns fluviatiris. — Long-itudinal section of the stomach ( x 4), 
c, cardiac ossicle; ccp, cajciim ; c.p.v, cardio-pyloric valve; cs, cushion- 
shaped surface ; //(/, hind-gut ; Jqy, aperture of rig-ht bile duct ; Ijy, 
lateral pouch ; It, lateral teeth ; mg, mid-gut ; mt, median tooth ; ces, 
oesophagus ; p, pyloric ossicle ; jjc. pterocardiac ossicles ; p}^^ prepy- 
loric ossicle ; ?/c, urocardiac process ; v^, median pyloric valve; v^, 
lateral pyloric valve; a?, position of gastrolith; zc, zygocardiac ossicle. 

j)ortant 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 


connected by a bent jointed middle bar. As all these 
parts are merely modifications of tlie 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 pair of muscles, the 


three teeth are made to stir up and crush whatever is 
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 j^yloric chamber, its side walls 
are, as it were, pushed in; and, above, they so nearly meet 
in the middle Hue, 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 



floor of the x^j^loric cliamber 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 ever^'thing which passes from the cardiac sac to the 
intestine must traverse this singular apparatus, only the 
most finely divided solid matters can escape stopj)age, 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 
symmetricall}^ 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 ihej readily allow of a 
passage the other wa3^ One of these valvular processes 
is placed in the middle line above (figs. 10 and 11, v^). 
It is longer than the others and concave below. The 
lateral processes {v^,) of which there are two on each side, 
are triangular and flat. 


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

Fia. ll.-A^acns Jiuriatms.-y\^ of the roof of the stomach the 
ventral wall of which, and of the mid-gut, is laid open '^l^^2rl^ 
tudinal incision ( x 4). On the right side (the left in the » 
the lateral tooth is cut away, as well as the floor of the lateral 
pouch. The letters have the same signification as m fag. lU. 

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 


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

It has alread}'^ 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 caecum {cm). Behind this, its character suddenly 
changes, and six squarish elevations, covered with a 
chitinous cuticle, encircle the cavity of the intestine (?•). 
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 Qig). 
Each of these ridges is beset with small papillae, and the 
chitinous lining is continued over the whole to the vent, 
wdiere it passes into the general cuticle of the integu- 
ment, jur^t as the lining of the stomach is continuous 
with the cuticle of the integument at the mouth. The 
alimentary canal ma}^, therefore, be distinguished into 
a fore and a hind-gut [hg), which have a thick internal 
lining of cuticular membrane ; and a ver}' short mid- 
gut {mg), w^hich has no thick cuticular layer. It will be of 




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importance to recollect this distinction bj^-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 purel}^ mechanical 
nature, there would be nothing more to describe in this 
part of the craj^fish'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 
w^ay through the walls of the alimentary canal ; to which 
end they • must either be in a state of extremel}^ fine 
division, or they must be reduced to the fluid condition. 
In the case of the fatty matters, minute subdivision may 
suffice ; but the am3daceous 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 j)rocesses 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 


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 thej^ 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 may be 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 ccsca, 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 

Fig. 13. — Astacus flnviaf\ll<i.—ThQ alimentary canal and livers seen from 
above (nat. size), hd, bile-duct ; cce, csecum ; cs, cardiac portion of 
stomach, the line pointing to the cardiac ossicle ; hg, hind- gut ; mg, 
mid-gut ; 2JC, pterocardiac ossicle ; ps, pyloric portion of stomach, 
the line pointing to the pyloric ossicle ; r, ridge separating mid-gut 
from hind-gut ; zc, zygocardiac ossicle. 


three lobes, one anterior, one lateral, and one posterior ; 
and each lobe has its main duct, into which all the tubes 
composing it open. The three ducts unite together into 
a wide common duct (bd), w^hich opens, just behind the py- 
loric valves, into the floor of the mid-gut. Hence the aper- 
tures of the two hepatic ducts are seen, one on each side, 
in this part of the alimentary canal when it is laid open 
from above. Every caecum of the liver has a thin outer 
wall, lined internally by a layer of cells, constituting what 
is termed an epithelium ; and, at the openings of the 
hepatic ducts, this epithelium passes into a layer of some- 
what similar structure, which lines the mid-gut, and is 
continued through the rest of the alimentar}^ canal, 
beneath the cuticula. Hence the liver may be regarded 
as a much divided side pouch of the mid-gut. 

The epithelium is made up of nucleated cells, which are 
particles of simple living matter, or protoplasm, in the 
midst of each of which is a rounded body, which is termed 
the nucleus. It is these cells which are the seat of the 
manufacturing process which results in the formation of 
the secretion ; it is, as it were, their special business to 
form that secretion. To this end they are constantly being 
newly formed at the summits of the cseca. As they grow, 
they pass down towards the duct and, at the same time, 
separate into their interior certain s^^ecial products, 
among which globules of yellow fatty matter are very 
conspicuous. When these products are fully formed, what 
remains of the substance of the cells dissolves away, and 


the yellow fluid accumulating in the ducts passes into 
the mid- gut. The yellow colour is due to the globules of 
fat. In the 3'oung cells, at the summit of the caeca, 
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 

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

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 
wdiile 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-oft' 
fluid passes into the mid-gut it must be mixed with the 
secretion of the liver, the action of which is probably 



similar to that of the pancreatic juice of the higher 

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 faeces in the hind gut, and 




Fig. 14. — Astacus flnviatills. — 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 
faecal matters are small in amount, and the strainer is 
so efficient that they rarel}^ 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 


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 
Avith 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 ver}^ irregularl}'^ 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 jycricarditim {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 


la.V- }' 



Fig, 15. — Astacus fltiviatilis, — A diagraminatic transverse section of 
the thorax through the twelfth somite, showing the course of the 
circulation of the blood ( x 3). arh. 12, the anterior or lower, and 
arV. 12, the posterior or upper arthrobranchia of the twelfth 
somite ; av, afferent branchial vessel ; hcv^ branchio-cardiac vein ; 
tg, branchiostegite ; em^ extensor muscles of abdomen ; eiy, epi- 
meral wall of thoracic cavity ; cy, efferent branchial vessel ; fm, 
flexor muscles of abdomen ; fp^ floor of pericardium ; gn. 6, fifth 
thoracic ganglion ; h, heart ; kg, hind-gut ; iaa, inferior abdominal 
artery, in cross section ; la, lateral valvular apertures of heart ; li; 
liver ; mp, indicates the position of the mesophragm by which the 
sternal canal is bounded laterally ; 7;, pericardial sinus ; pdb. 13, 
podobranchia, and 2^^^. 12, pleurobranchia of the twelfth somite ; 
4'<2,sternal artery ; saa^ superior abdominal artery ; sc, sternal canal ; 
t, testis ; xii., sternum of twelfth somite. The arrows indicate 
the direction of the blood flow. 


their larger and smaller branches, which proceed from it 
and ramify through the body, to terminate eventually in 
the bljod 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 cavit}^ already mentioned as the peri- 
cardium (fig. 15, ^), 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 b}^ 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 I6,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 second 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, 


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. 




S. (t.€t 






Fig 16. — Astacns jluviatllis. — The heart ( x 4). A, from above ; B, from 
below ; C, from the left side, aa, antennary artery ; ac, alee cordis, 
or fibrous bands connecting- the heart with the walls of the peri- 
cardial sinus ; l, bulbous dilatation at the origin of the sternal 
artery ; 1m, 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 ofE close to its 


A third arter}^ 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 


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, 
so), 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 Avails 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 


blood leaves the branchiae and passes into the sinuses which 
connect them with the pericardial sinus (fig. 15, hcv), 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 


activity of all the organs, and especiall}^ 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 oxj^gen 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 (iwdohranchice) 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, pdbf 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. — Astacns flui'iatiUs.—X, one of the podobrancliiEe from the 
outer side ; B, the same from the inner side ; C, one of the arthro- 
branchitfi ; D, a part of one of the coxopoditic setse ; E, extremity of 
the same seta ; F, extremity of a seta from the base of the podo- 
branchia ; G, hooked seta of the lamina ; (A — C, x 3 ; D — G, highly 
magnified), h, base of podobranchia ; cs, coxopoditic setae ; cxp, 
coxopodite : I, lamina, pi, plume, and st, stem of podobranchia ; 
t, tubercle on the coxopodite, to which the setas are attached. 


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 xery 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 
tlie branchial filaments, the outer investment of each of 
which is an excessivel}' thin chitinous membrane, so that 
the blood contained in them is practically separated by a 
mere film from the aerated water in w^hich the gills float. 
Hence, an exchange of gaseous constituents readily takes 
place, and as much oxj^gen 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 difi'er 
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 


fine straight hairs, or set(^ (F), whence a narrow stem (st) 
proceeds. At its iipx^er 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 (Q, 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 setse (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 oif 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 for the second maxillipede, thus making seventeen 
in all (3 X 5 -h 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 


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 of 
attachment it is distinguished from the others as s^pleiiro- 
hranchia (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, plh. IS). 

The quantity of water which occupies the sj)ace left in 
the branchial chamber by the gills is but small, and as 
the respiratory surface offered by the gills is relativel}^ 
very large, the air contained in this w^ater 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 greatl}" 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 by which the 
supply of fresh water may be proportioned to the 
demand. In many animals, the respiratory surface is 


covered with rapidly vibrating filaments, or cilia, by 
means of which a current of water is kept con- 
tinualty 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 scaj^hognathite) , 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. 


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 the 
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 

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 


compounds wliicli 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 maj^, therefore, expect to find some 
organ which plaj^s 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 dehcate 
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 



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


Fig. 18. — Astacm JiuviatiUs. — A, the anterior part of the body, with 
the dorsal portion of the carapace removed to show the position of 
the g-reen 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 ; <?, circumoesophageal commis- 
sures ; <?.?, 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 ; oes, oesophagus seen in transverse 
section in A, the stomach being removed ; s, sac of green gland ; 
a*, 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 kidnej^, and its secretion 


tlie urinary fluid, while the sac is a sort of urinary 

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 
side, their places are made good by new comers. 



Those who have seen the wonderful whirlpool, three 
miles helow the Falls of Niagara, will not have forgotten 
the heaped-up wave which turahles and tosses, a very 
embodiment of restless energy, where the swift stream 
hm-rying 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. Yiewed closel}'', 
it is a typical expression of the conflicting impulses 
generated by a swift rush of material particles. 

Now, with all our appliances, w^e 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 aw^ay, that it is practicalh^ 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 


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. 




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 


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 ob^dously out of the question that his actions should 
be guided by a logical reasoning process, such as that 
b}^ which a man would justify similar actions. The 
crayfish assuredl}- does not first frame the S3''llogism, 
" 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 we anatyse 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 ; 


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 varjdng external conditions. 

Under these circumstances, it is really quite an open 
question whether a craj^fish has a mind or not ; more- 
over, the problem is an absolutely insoluble one, inas- 
much as nothing short of being a crayfish w^ould 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, namelj^, that of the order and 
connexion of the physical phenomena which intervene 
between something Avhich happens in the neighbourhood 
of the animal and that other something which responds 
to it, as an act of the 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 the}'' do this in virtue of theu' 
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 slyij 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 

The m'uscle 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 generall}^ found to be ensheathed in a fine trans- 
parent membrane, which is called the sarcole^nma, within 
which is contained the proper substance of the muscle. 
When quite fresh and living, this substance is soft and 



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

Examined, with high magnifying powers, in this 



Fig. 19. — A.tfacus fluviatHis.—A, a single muscular fibre ; transverse 
diameter -jiytli 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 
fibril] £6 ; w, nucleus of sarcolerama ; «f, nerve fibre; s, sarcolerama; 
t, tendon; 1 — 5, successive dark bands answering to the darker 
portions, a, of each fibril la. 


condition, the muscle -substance a^Dpears marked by veiy 
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 

A greater or less number of these fibres, united into one 
or more bundles, constitutes a muscle ; and, except when 
these muscles surround a cavit}^, they are fixed at each 
end to the hard parts of the skeleton. The attachment 
is frequently efi'ected 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 Hving 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 



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


Fig. 20. — AstacNs Jluviaf His.— The chela of the forceps, with one side 
cut away to show, in A, the muscles, in B, the tendons (x 2). 
cjy, carpopodite ; j^?7?, propodite ; dp, dactylopodite ; m, adductor 
muscle ; m', abductor muscle ; t, tendon of adductor muscle ; f, 
tendon of abductor muscle ; x, 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 forcej)S. If the 


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 (m'), 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 {f). It is obvious that, when the latter muscle 
shortens it must move the apex of the terminal seg- 
ment (dp) aw^ay from the end of the fixed claw ; while, 



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

A living craj^fish 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 * were 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. 


degrees, j-et 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 verj'- 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 b}^ which the retrograde swimming of 
the craj^fish is effected, is no less easily analysed. The 
apparatus of motion is, as we have seen, the abdomen, 
with its termhial five-pointed flapper. The rings of the 
abdomen are articulated together by joints (tig. 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 ih.Q ijleuron (fig. 36, pi. XIX). 



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 intersternal membranes are stretched as far 
as they will yield. On the other hand, when the abdomen 

Fig. 21. — Astacns JiuviatUls. — 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 rig-ht 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 


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 interarticular membrane allows the terga to move as 
far as they can go in flexion ; wdiilst, 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 concavit}^, 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 


ride over the posterior edges of the hranchiostegites. 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 ahdomen passes from extension to flexion, the 
overlap of the terga of course diminishes; hut an}^ de- 
crease of resistance to lateral strains which may thus 
arise, is compensated hy the increasing overlap of the 
pleura, which reaches its maximum when the ahdomen 
is completely flexed. 

It is ohvious that longitudinal muscular fihres 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 ( consti- 
tute a Yery much larger mass of muscle, the fibres of 
Avhich 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 


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 JfnviatUis. — A longitudinal section of the body to 
show the principal muscles and their relations to the exoskeleton 
(nat. size), a, the vent ; add.jn, adductor muscle of mandible ; 
e.m, extensor, and, flexor muscle of abdomen ; ces, oesophagus ; 
j)cp, procephalic process ; t,f, 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 



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, ever}^ 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 waj^s, 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 


(fig. 23). The nerve bundle which passes to a muscle 
breaks up into smaller bundles and, finall3% 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. 

Fig. 2Z.—Astacus fluviatilis. — 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 u'ritation; 
and this change is propagated along the nerve, until it 
reaches the muscle, in w^hich 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 Ave follow the course of the motor nerves in a 



direction away from the muscles to which they are dis- 
tributed, the}^ 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 bod}^ which is in great measure composed of 


Fig. 2 Jr. — Asfacus fluviatUis. — 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. 

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. — Astacns flnv'mtilis. — The central nervous system seen from 
above (nat. size), a, vent ; «w, antennary nerve ; a! 11, antennulary 
nerve ; e, circum oesophageal commissures ; gn. 1, supracesophageal 
ganglion ; gn, 2, infraoesophageal ganglion ; gn. 6, fifth thoracic 
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. 


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 
readil}' 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 thes^ 
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 (ces), 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 



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 onl}^ 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 sensor}^ 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 rea.ching the ear, are 
converted into an audible note. So in the nerve tract : 


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 sensor}^ nerve ended. The contraction of several 
muscles at the same time, that is, the combination of 
movements towards one end, would be possible onl}^ 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 highl}^ 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 


of tlie 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 were, reflected along another nerve fibre to a 
muscle, is wdiat is termed a reflex action. As it is by no 
means necessar}^ that sensation should be a concomitant 
of the first impulse, it is better to term the nerve fibre 
which carries it afferent rather than sensorj^ ; 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 destroyed, 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 


reflex action. The stimulus is convej^ed to the ahdo- 
minal ganglia through afferent nerves, and is reflected 
from them, by efl'erent 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, simultaneousl}", with an even stroke ; while 
the vent opens and shuts with a regular rhythm. Of 
course, these movements impl}^ correspondingly regular 
alternate contractions and relaxations of certain sets of 
muscles ; and these, again, imply regularl}^ recurring 
efl'erent impulses from the abdominal ganglia. The fact 
that these impulses proceed from the abdominal ganglia, 
may be showTi in two wa}' s : first, by destro3dng 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 afi'erent nerves. Whether these movements are 
properly reflex, that is, arise from incessant new afi'erent 
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 sufiicient 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- 


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 they 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. 


— a bit of metal, or wood, or paper, or one of the aiii- 
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 if 
deglutition were difiicult. 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 

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 

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


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 phj^sical 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 stimuU are doubtless originated 
by changes wdthin 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 bod}^ inasmuch as w^e 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 an}' pheno- 
menon comes into existence without a cause, is equivalent 
to a belief in chance, which one may hope is, hy 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 


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 phj^siological 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 hody 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 fiUed by a prolongation of 
the subjacent proper integument. As this is supplied 
with nerves, it is likelv 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. 


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

FiCt. 26. — Astacns fltiviatilis. — 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 ; h, side view ( x 300) ; 
«,o]factory appendages ; au, auditory sac, supposed to be seen through 
the wall of the basal joint of the antennule ; Z>, setae ; en, endopo- 
dite ; ex, exopodite ; sj). 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 evidentl}^ of a sensory 


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 setae {h) of 
the ordinary character. 

The inner branch, which is the shorter of the two, pos- 
sesses only these setae ; 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 
front and one behind. These are rather more than 
l-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. 


There is no doubt, however, as to the special recipients 
of sonorous and luminous vibrations ; and these are of 
particular importance, as the}^ 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 ajoerture, the outer lip of 
which is beset with a flat brush of long close-set setae, 
which lie horizontallj^ over the aperture, and effectually 
close it. The aperture leads into a small sac (au) with 
delicate walls formed b}^ 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 3^'^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 



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. 


Fig. 27. — Astacus fluvlatUis. 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 setae ; b, 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. 


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 

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 



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. — Astacns flm-iatUis. — 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 ; h, outer dark zone ; c, 
outer white zone ; d, middle dark zone ; e, inner white zone ; 
/, inner dark zone ; cr, crystalline cones ; g, optic ganglion ; oj>, 
optic nerve ; sj). striated spindles. 

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


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, ^), 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 Z07ie (/). 
Outside this, and in connection with it, follows a white 
line, the inner ivhite zone (e), then comes a middle dark 
zone (d) ; outside this an outer pale band, which may 
be called the outer ivhite zone (c), and between this and 
the cornea {a) is another broad band of dark pigment, the 
outer dark zone (b). 

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 


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, c?*), and an internal striated 
spindle (sp). The crystalline cone consists of a trans- 
parent glassj^-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 trans]3arent 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 

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 


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 yerj probable, 
therefore, that the visual pyramids do not play the part 
of the simple eyes of the Vertehrata, and the only alterna- 
tive appears to be the adoption of a modification of the 
theory of mosaic vision, propounded many years by 
Johannes Miiller. 



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 tow^ards 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, z, 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 b}' 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 
hght proceeds. Then it will be obvious that the only light 


from X which will excite sensation, will he the ray which 
traverses b and reaches the nerve-fibre &, 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 mdicates 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 merel}^ 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, the 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 
between the length and the thickness of the latter. 


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 e3'e. 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 e3^e 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 reallj' 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 properl}" 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 


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 

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 ever}^ 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 would be 
exceedingly hazardous to affirm that organs capable of 
giving rise to the same products exist in the vastly 
simpler nervous sj^stem 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- 


ning of this chapter, so far as such obscure consciousness 
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 b}^ 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 disi)o- 
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 b}^ 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 widel}^ 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 


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 djing, 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 prett}^ 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 
oi Crustacea to which the craj^fish 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 



matter, whicli are developed in differeDt individuals, 
termed males B.iid 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 

Fig. ^0.— Asia ens fluvmtilis. — The female reproductive organs ( x 2); 
oi\ ovary ; od, oviduct ; ocV, 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 

The testis (fig. 31, t) is somewhat similar in form to 

the ovary, but, the three divisions are much narrower 




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. — Astacus fluvlatlUs. — The male reproductive organs ( x 2) ; 
t, testis ; vd, 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 the}^ 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 



during the breeding season than at other times; the large 
brownish-yellow eggs become conspicuous in the ovar}^ 

Fig. o2. — Astacu-sJlifviat'dis. — 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 ; ej^, epithelium of ovisac ; 
gs, germinal spots ; gv, germinal vesicle ; m, membrana propria ; 
V, vitellus ; ivn, vitelline membrane ; w, stalk of ovisac. 

and the testis assumes a milk-wdiite colour, at this 

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



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 

Fig. 33. — Astacus fluviatdis. — A, a lobule of the testis, showing a^ acini, 
springing- from h, the ultimate termination of a duct (x 50). B, 
spermatic cells ; a, with an ordinary globular nucleus n ; h, 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 memhrana propria (fig. 32, m). 
These are the ovisacs. In the mass of cells which be- 
comes the ovisac, one rapidly increases in size and 
occupies the centre of the ovisac, while the others 


suiTOimd it as a peripheral coat (ep,). This central cell 
is the ovum. Its nucleus enlarges, and becomes what is 
called the germinal vesicle (cj.v.). At the same time 
numerous small corpuscles, flattened externally and 
convex internally, appear in it and are the germinal 
spots ig.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 (t\). 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. Finall}^ the ovisac bursts, and the egg, falling 
into the cavity of the ovary, makes its wa}^ 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 eggy 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 suj)plied with aerated 

The testis consists of an immense number of minute 
spheroidal vesicles (fig. 33, A, a), attached like grapes to 
the ends of short stalks (6), 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. — Astaais Jluviatilis.—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 en face (all x 850) ; G, a diagrammatic vertical section of the same. 


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 
T-yo^th 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, w^hich 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 ecdj'sis in the early autumn ; and at this 
time, w^hich is the breeding season, the males seek 
the females with great aviditj^, 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 wdiich the contained sperma- 
tozoa reach and enter the ova is unknown. The analogy 



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

Ova to which spermatozoa have had no access, 
give rise to no j)rogeny; 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 fluviatUls. — 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 ; cxj), coxopo- 
dite ; st XIV, last thoracic sternum ; vd, aperture of vas deferens. 



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 teleolog}^ 
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- 


monstrating the proposition that a thing is fitted to do 


that which it does, is not very great. 

But whatever may he 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 MorpJwlogy, 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 crj^stallographer 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 difi'erent 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. 


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 
teleolog}^ 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 stj^le ; 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 


same ma}^ 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 " st^des of archi- 
tecture " of the three are even more widely difi'erent 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, 
maybe 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 laj^er of nucleated cells. 
A continuous layer of cells, therefore, is everywhere to 



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 bod}", 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- 

We have seen that the body of the crayfish thus com- 
posed is obviously separable into three regions — the 
cephalon or h^ad, 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 


scaphognatliite lies, clearl}^ 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 

a a, a. 



Fig. ^Q.—Astacvs fluviatUls.—K transverse section through the nine- 
teenth (fifth abdominal) somite ( x 2). e.m., extensor muscles ; /.w., 
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 ; sf. XIX, its sternum ; t, XIX, its 
tergum ; c}}. XIX, its epimera ; 19, its appendages. 


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 aijpendages 
(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 Qig) and vascular 
{s.a.a, i.a.a) sj^stems. 

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 hasipodite {h.p), and receives the name of pro- 
toiiodite. At its free end, it bears two flattened narrow 






Fig. o7. — AsfacttsJluvlatilis.—Aii-pendagefi of the left side of the ahdo- 
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 endopodite ; h, the jointed extremity of the same ; hp., 
basipodite ; cx.ja., coxopodite ;, endopodite ; cx.2>., exopodite. 



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 setae. 

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 


joints ; and of two terminal oval i^lates, which represent 
the endopodite {en.p) and the exopodite {ex.}?). 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, 5), 
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 one imperfectly jointed filament, which 
appears to represent the endopodite of the other ap- 

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 ((7, C ') there is 
a protopodite {cx.p, tp) with the ordinary structure, and 
it is followed by an endopodite (en.p) and an exopodite 


(ex.p) ; but tlie former is smgularl}^ 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 (h). 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 j)ro- 
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 i^airs of curiously modified appendages are 
ordinarily turned forwards and applied against the sterna 
of the posterior part of the 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 convej^ed 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 


divisible, correspond witli one another, not onl}^ 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 fa9ades 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 

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 tj'pe 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 ; 


aud, in those of tlie 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 ujDon 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 vfhy 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 w^ould 
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 con- 
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- 


men (figs. 15 and 36), it will be obvious that the tergal 
and the sternal regions of the two answ^er 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 oif 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 



pie urobr an cilia (fig. 4, plh. 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. — Asfacm fliiviatilis. — The mode of connexion between the last 
thoracic and the first abdominal somites ( x 3). a, L-shaped bar ; 
cj)e, carapace ; cxp. I4, coxopodite of the last ambulatory leg ; plb-., 
place of attachment of the pleurobranchia ; st. XV, sternum, and 
t. XV, 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 (t. XV.). The tendon of one 


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 

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 



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


Fig. Sd.—Astact/s fuviatiliii. — The cepbalothoracic sterna and the endo- 
phrag-mal system ( x 2). A, from beneath ; B, from above, a, a', 
arthrophragms or partitions between the articular cavities for the 
limbs ; ^.ff/?, cephalic apcdeme ; cf, cervical fold ; ejm. i,epimeronof 
the antennulary somite ; h, anterior, and h', posterior horizontal 
process of endopleurite ; lb, labrum ; m, mesophragm ; mt, meta- 
stoma ; 7;, paraphragm ; I — XIV, cepbalothoracic sterna ; 1 — 14, 
articular cavities of the cepbalothoracic appendages. (The anterior 
cephalic sterna are bent downwards in A so as to bring them into 
the same plane with the remaining cepbalothoracic sterna ; in B 
these sterna are not shown.) 


rudimentary brancliia. These arese of tlie 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, shajDed like a broad arrow (F, 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 


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 antennae (5) ; but, in the middle line, the 
epistoma is continued forwards into a spear-head shaped 
process (figs. 39 and 40, 7J), 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 


the e3'e- stalks and is united with adjacent j^arts 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. — Asiacvs fluviatilis. — The oplifhalmic and antennulary somites 
( X 3). /, ophthalmic, and II, antennulary sternum ; 1, articular 
surface for eyestalk ; S, for antennule ; ejim, epimeral plate ; 
2)0}), procephalic process ; r, base of rostrum ; t, tubercle. 

represented, in tlie 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 

The ej^imera of the head are, for the most x^art, very 
narrow ; but those of the antennulary somite are broad 
plates (fig. 40, eiwi.), which constitute the posterior 


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 sometmies 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 flaviatiUs. — 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 reaht}^, 
however, the curious pillars and bulkheads which enter 
into the composition of the endoj^hragmal system are all 

* There are some sin g'ular marine Crustacea, the SqutlUdcp, in which 
both the ophthalmic and the antennary somites are free and movable, 
while the rostrum is articulated wath the tergum of the antennary 


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 oif, which are the endopleurites. The former 
lie at the inner, and the latter at the outer ends of the 
partitions or arthrophragms (fig. 39, A, a, a, fig. 42, ap/i), 
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 respective^. 

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 mesopliragm {mph.), the outer the paraphragm 
{ppli.). 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 endopleurites (en, pi.) 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')y becomes similarly connected with the endo- 
sternite of the somite behind. 



Fig. 42. — Astacns flnviatiUft. — A segment of the endophragmal system 
(x 3). o^A, arthrophragm ; arth. arthrodial or articular cavity; 
cxp, coxopodite of the ambulatory leg ; enj)!, endopleurite ; ens, 
endosternite ; epm, epimeron ; Jip, horizontal process of endo- 
pleurite ; mjjh, mesophragm ; jt|/;7(, paraphragm ; s. sternum of 
somite ; sc, sternal canal. 

The endopleurites of the last thoracic somite are radi- 
mentar}^, 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 m 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 


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 
sj^stem as that which lies behind it. But the anterior gas- 
tric muscles are attached to two flat calcified 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 occujDy 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 


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 uj) 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 be^^ond the eyes. 
At the opposite extremit}^ the telson is a corresponding 
median outgrowth of the last somite, which has become 
moveably articulated therewith. The narrowing of the 
sternal moieties of the anterior thoracic somites, to- 



getlier with the sudden widening of the same parts in 
the j)osterior 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 (10), the ambulatory 

Fig. 43. — Astacus fluviatilis. — Long-itudinal section of the anterior part 
of the cephalothorax ( x <j). I — IX, sterna of first nine cephalo- 
thoracic somites; i, eyestalk ;2, basal joint of antennule ; 5, basal 
joint of antenna ; 4> mandible ; a, inner division of the masticatory 
surface of the mandible ; a', apophysis of the mandible for muscular 
attachment ; cj), free edge of carapace ; e, endostemite ; eyij)!, endo- 
pleurite ; cpi7i, epimeral plate ; 1, labrixm ; m, muscular fibres con- 
necting- epimera with interior of carapace; mi, metastoma ',2*^1'' 
procephalic process. 


limbs {11 — M), 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 maxillse (5, 6) 
the mandibles (4), the antennge (5), the antennules (-<?), 
the eyestalks (i), and the six somites to which they are 
attached (J — 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 (1) 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. 


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. ii—AstacnsJlm-iatilis. — The third or external maxillipede of the 
left side ( x 3). e, lamina, and br, branchial filaments of the 
XX)dobranchia ; exjj, coxopodite ; cxs, coxopoditic setse ; bjJ, basi- 
podite ; ex, exopodite ; ij), ischiopodite ; w^?, meropodite ; c/7, 
carpopodite ; 2U^) propcdite ; dj), dactylopodite. 

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


with tivo terminal divisions {ip to dp^ and ex)^ which are 
directed forwards, below the mouth, and a third, lateral 
appendage {e, hr), 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, hp) 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, va^j 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 (hp). 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 {i}:)), meropodite (mp), carpopodite (cp), propo- 
dite (pi?), and dactylopodite (dp). 



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. — AstacHs fluviatilis.—K, the first ; B, the second maxillipede 
of the left side ( x 3). cx^y^ coxopodite ; hj), basipodite ; e, hi', po- 
dobranchia ; ej), epipodite; en, endopodite; f;r, exopodite ; ijp, is- 
chiopodite ; tiqj, meropodite ; cjJ, carpopodite ; pp, propodite ; dp^ 

(fig. 45, A) a great modification has taken place. The 
coxopodite {cxp) and the basipodite (hp) are broad thin 
jolates^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 


the podobrancliia 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 j)lan 
of the appendages remains the same ; (1) the protopodite 
increases in relative size ; (2) the endopodite diminishes ; 
(3) the exopodite increases ; (4) the podobrancliia 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 jpaZp, and the 
metamorphosed podobranchia, the real nature of which 
is not recognised, is termed the flag ellum. 

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 eplpodite for the "flagellum." And 
the lamellar process of the first maxillipede is now very 
generally termed an epipodite ; while the podobranchise, 
which have exactty the same relations to the following 


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 
podobranchise thus modified. Unfortunately, the same 
term is applied to certain lamelliform portions of the 
branchise 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. 46. — Asfaous fluviatUis. — The second ambulatory leg" of the l^^ft 
side (^ X 3). cjcp, coxopodite ; hjJ, basipodite ; h', gill ; cvs, coxo- 
poditic setae ; r, lamina of gill or epipodite ; ij), ischiopodite ; mp^ 
meiopodite ; c}), carpopodite ; pp^ propodibe ; djy, dactylopodite. 


the limbs of the thorax are all reducible to the same tj^pe 
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, podobranchiie 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 coxopodite (cxj)) and the basii)odite (bj)) 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 
ylace 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 maxilhpede 
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 



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




Fig, 47. — Astacnsfluviatilis. — A, mandible ; B, first maxilla ; C, second 
maxilla of the left side ( x 3), ar, internal, and ar\ external 
articular process of the mandible ; hj), basipodite ; cxp, coxopodite ; 
en, endopodite ; p, palp of the mandible ; sg, scaphognathite ; .r, 
internal process of the first maxilla. 

podite is represented by the three-jointed^aZp {p), the 
terminal joint of w^hich is oval and beset with numerous 
strong setse, which are especially abundant along its 
anterior edge. 


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. — Astac7(s fluviatilis. — 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 ; e.vjy, 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 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 annulate.d filaments represent 
the endopodite and the exopodite. 

Finall}^ the ej^estalk (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 ajDpendages 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. 


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 diiferent regions 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 


out the more intimate structure of these parts. The 
tough, outer coat, which has been termed the cutlcula, 
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 



of the Doay 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 hlood coiyuscles, 

Fig. 49. — Astac^is Jlnviatilis.— The corpuscles of tlie blood, Lig-lily 
mag-nified. 1 — 8, show the changes undergone by a single cor- 
puscle during a quarter of an hour ; n, the nucleus ; 9 and 10 
are corpuscles killed by magenta, and having the nucleus deeply 
stained by the colouring matter. 

which rarety exceed l-700th, and usually are about 
1- 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 S 
refracting granules, and they are ordinarily exceedingly 
irregular in form. If one of them is watched continu- 


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 corj)uscle, in fact, has an 
inherent contractility, like one of those low organisms, 
known as an Amoeba, whence its motions are frequently 
called amoehiform. 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 (ii). 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 protoj^lasmic 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 caeca. 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 


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 closel}^ aggregated together 
into a continuous sheet, the}^ 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. — Astacvs furintUis.—'E-^iih.eYmm, from the epidermic laj'er 
subjacent to the cuticle, highly magnified. A, in vertical section ; 
B, from the surface. %, nuclei. 

parts of the caeca, and their essential nature is thus 

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 



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-bod}^ for each nucleus. In the 
second form (fig. 51, A) the matrix exhibits fine w^avy 
parallel lines, as if it were marked out into imperfect 


Fig. 51. — Astacns fliiviatilis. — Connective tissue ; A, second form ; B, 
third form, a, cavities ; w, nuclei. H'g-hly magnified. 

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


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, B). Under one form or another, the 
connective tissue extends throughout the hody, ensheath- 
ing the various organs, and forming the walls of the blood 

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. 



4. The muscular tissue of the crayfish alwaj^s has the 
form of bands or fibres, of ver}^ various thickness, marked, 
when viewed by transmitted light, by alternate darker and 



i''-:'''^i^ ■IEJi'-^ LqI' 



Fig. 52. — Astacns flnviat'ilis. — A, a single muscular fibre, transveri-e 
diameter yyo^Ii of an incli ; 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 fibrillse ; F, the 
connection of a nervous with a muscular fibre which has been 
treated with alcohol and acetic acid, a, darker, and h, clearer portions 
of thefibrillae ; n, nuclei ; nv. nerve fibre ; s, sarcolemma ; f, tendon ; 
1 — -5, successive dark granular st.ria3 answering to the granular 
portions, a, of each fil:)rilla. 



lighter striae, transversely to tlie 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. 

is — I 

Fig. 53. — Astacus fliiv'iafUis. — A, living- muscular fibres very highly 
magnified ; B, a fibrilla treated with solution of sodium chloride ; 
C, a fibrilla treated with strong nitric acid, s, septal lines ; sz, 
septal zones ; is, interseptal zones ; a, transve'se 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 la^'er 
of nucleated protoplasm lies between the sarcolemma and 
the striated muscle substance. 



This much is, readily seen in a specimen of muscular 
fibre taken from an}^ 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 

Quiesc«>nt 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 
l-4000tli 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 tlie septal lines (fig. 52, D and E, a; C, 1 — 5 ; 
fig. 53, §), On each side of every septal line there 
is a verj^ 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. 


In the perfectly unaltered stat'i 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 similar 
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 polj^gonal areae 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 

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 fihrils ; and that the longitudinal 


striae and the rounded areee of the transverse section are 
simpl}^ 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 Jibre 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- 

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 ma}^ 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 


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 

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, w^hich 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 contrarj^, 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 o 
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 


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, aj^pears to me to be wholly 

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-sex^tal 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- 

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 sj)herical 
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. 


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 1- 200th of 
an inch in diameter; and fibres of l-300th or l-400th of 

Fig. 54. — A'sfaciis JiuvlatiUs. — 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, go', 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, wlien the nerve trunk gives 



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 

Fig. 55. — Astacns flnriatilis. — Three nerve fibres, with the connective 
tissue in which they are imbedded (magnified about 250 diameters) ; 
n, nuclei. 

exude from the cut ends of the tubes, it is evident that 
they consist of a fluid of gelatinous consistenc3\ 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 j^lates. 


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 de'monstrably 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 parit}^ 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 w^e 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 


Fig. 56. — Astacm fluciatiHs. — The structure of the cuticle. .4. trans- 
verse section of a joint of the forceps ( x -i) ; s, setae ; B, a por- 
tion of the same ( x 30) ; C, a portion of B more highly magnified. 
ff , epiostracum ; Z>, 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) ; -&', 
small portion of the same, highly magnified ; a, intermediate sub- 
stance : h, laminae. F, a seta, highly magnified ; a and J, joints. 


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 laj^er. 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 laminae, 
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 gently 


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, especiall}^ in the middle of the 
slice, while 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 
1- 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 h8ematox3din ; 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 fijiely striated appearance, as if they were composed 
of delicate parallel wavy fibrillse. 

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 difi'used, but sometimes take 
the shape of rounded masses with irregular contours. 

Immediately beneath the epiostracum there is a zone 


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 (b), 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 l-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 

If the hard exoskeleton has been allowed to become 
partially or wholly dry before the section is made, the 
latter will look wdiite 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 


the striae 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- 

1^ pletely fills the tubes. And that this is really the case 
may be proved by making very thin sections parallel with 

t the face of the exoskeleton, for these exhibit mnumerable 

■ 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 

m 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 areae circumscribed by clear potygonal borders. 
These perforated areae appear to correspond with indi- 
vidual cells of the ectoderm, and the canals thuf? answer 
to the so-called " pore-canals," which are common in 
cuticular structures and in the walls of many cells 
which bound free surfaces. 


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 alternatel}^ denser and softer. The denser 
materia] gives rise to the tough laminse, 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 

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 contrar}^, though no less dependent 
on cells for its existence, is a derivative product, the 
formation of which does not involve the complete meta- 


morpliosis 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 ; wdiile 
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 ordinarj^ 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 

These setse (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 setae, the axis gives off slender lateral 


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 wliich 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 orighi 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 cavity, 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), 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. 


Thus the body of the cra3'fish 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, finall}', 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 


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 3'et unknown ; hut 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 he, the nuclei which they contam 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 j)rocess of internal change which ends 
in fission are apparent in the nucleus before they are 
manifest in the body of the cell; and, commonlj^, 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 b}^ 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 


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 sj^indles 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 si)indles is ver}'- beau- 
tifully seen in the epithelial cells of the testis of the 


crayfish (fig. 33, p. 132) ; but I have not been able to find 
distmct 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 
strongty 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 



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 

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 cavit}^ 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 Vertehrata, 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 


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 ? 


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 hlastomere. 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 ordinar}' cells, as 


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 mulberr3^-like 
body, 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, j^elk 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 rapidl}^ 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 afi'ected 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 multipl}^ at the expense of the food- 



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-j^elk ; 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, the}^ form a complete, though 
thin, investment to the food-3'elk. This is termed the 
blastoderm {hi). 

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. bl .—Astacvs flnviatilis. — Diograminatic sections of cnihryos ; partly after Reichenbaoli, parlly 
original ( X 20). A. An ovum in which the blastoderm is .iust formed. B. An ovum in which 
the invagination of the blastoderm to constitute the hypol)last or rudiment of the mid-j;ut has 
taken place. (This nearly answers to the stage represented in fig. 58, A.) C. A longitudinnl 
section of an ovum, in which the rudiments ot the abdomen, of the hind-gut, and of the fore- 
gut have appeared. (This nearly answers to the stage represented 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 ; hi. blastoderm ; bp, blasto- 
pore ; e, eye ; ep.h., epiVjlast ; /f/, fore-gut ; /|7', its oesophageal, and /f/*. its gastric portion; 
h, heart ; hg, hind-gut ; n>, mouth ; mg, hypoblast, archenteron, or mid-gnt ; r, yelk. The 
dotted portions in D and E represent the nervous system. 


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, Avhen the egg is viewed by reflected 
light, a whitish patch of corresponding form and size 
aj^pears in this region. This may be termed the ger- 
7ninal disk. Its long axis corresponds with that of the 
future crayfish. 

Next, a depression (fig. 58, A, hp) ai)pears 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 hlasto- 
poi'e (fig. 57, B, and 58, A — E, bp), diminishes in size. 
Thus the body of the embryo crayfish, from being a 
simple bag becomes a double bag, such as might be 
l)roduced by pushing in the w^all 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 craj^fish. For, 
though the pouch is nothing but an ingrowth of part of 
the blastoderm, the cells, of which its wall is com^^osed 

Fio. 58—^1 /crews fluviatllis. —Surfaca views of the earlier stages in the developninit of 
the embryo, from the appearance of the blastopore (A) to the assumption of the 
iiaiiplius form (F) (after Reichenbach, x about 23). hp, blastojiore , c, carapace ; fg, 
fore-gyt involution ; h, heart ; hg, hind-g\it involution ; Ih, labrum ; mg, medullary 
groove ; o, optic pit ; jt, en<lodermal plug partly filling up the blastopore ; pc, pro- 
cephalic processes ; ta, abdominal elevation; 2, antennules ; 3, antennae >4, man- 


henceforward exhibit difterent 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 gastrulce. 

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 sj^stems 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 heai-t and vessels, and 
the reproductive organs, which lie between the ectoderm 
and the endoderm, are not derived directly from either 
the epiblast or the hypoblast, but have a g^itasi-independent 
origin, from a mass of cells which first makes its appear- 


ance in the neiglibourhood of the blastopore, between the 
h3'pobhist and the epiblast, though they are probably 
derived from the former. From this region the}'' gradu- 
ally spread, first over the sternal, and then on to , the 
tergal aspect of the embryo, and constitute the mesohlast. 

Epiblast, h^'poblast, 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 

The embr^^o 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 left open. I 
have not been able to satisfy myself of this fact. 



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, ta; 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 bod}^ will be produced by the elongation of 
that part of the embryo which lies between these two 

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, mg). 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,/a). 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 


which the cavit}^ 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, hg). 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 (5) and the antennules {2), while, at a later 
period, i)rocesses of the procephalic lobes give rise to the 

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



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, c) 
— the lateral parts of which, greatly enlarging, hecome 
the hranchiostegites (fig. 59, D, c). 

In many animals allied to craj^fish, 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 numher of the appendages. The appendages which 
represent the antennules, the antennae, and the mandibles 
elongate and become oar-hke 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 craj'fish, 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 

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.—^stacus flnviatiUa.—YentraX (A, B, C, F) ami lateral (D, E) views of the 
embryo in successive stages of development (after Rathke, 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 ; r/jf, green gland; h, heart; lb, labrum ; Ir, liver; m, 
mandibular muscles. 


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 Avhich is slightly 
truncated or notched ; while, finally, transverse constric- 
tions mark off six segments, the somites of the abdomen, 
in front ol 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 

The rostrum grows out between the procephalic lobes ; 
it remains relativel}" 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 


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 j^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 (5) 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, while the outer or exopoditic division 
remains relatively short, and acquires its characteristic 
scale-like form. 

The labrum (Ih) 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 i^osterior 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 
coQvex 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 ar^ nearly the same as in the adult, but 
the chelse of the forceps are more slender. The tips of 
the chelae 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 



processes, and tlie disi^osition of the vascular canals in its 
interior gives it the appearance of being radially striated. 
Tlie setae, 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 ^?/t"/«f?7/6\— Newly -hatched young ( x fi). 

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 


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

I imagine that the}^ are set free during the first ecdj^sis, 
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 craj^fish 
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 craj^fish means the gradual conver- 
sion of this comparatively simple body into an organism 
of great complexity. The jeYk becomes differentiated 
hito 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 ej)iblast, hj'poblast, 
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 


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 

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 craj^fish 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. 


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, fg^, fg^). 
The front dilatation becomes the oesophagus and the 
cardiac division of the stomach ; the hinder one, the 
pyloric 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 
are 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 hj^poblastic sac, very soon gives off 
numerous small prolongations on each side of its hinder 
extremit}^, and these are converted into the C£Eca of the 
liver (fig. 57, E, mg). The cells of its tergal wall are in 
close contact with the adjacent masses of food-j^ellv ; 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 


on the one hand, and the cephalic integument on the 

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 
yevy earl}^ 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- 

The branchi£e are, at first, simple papillae of the integu- 
ment of the region from which they take their rise. 
These papill£e elongate into stems, which give off lateral 
filaments. The podobranchiEe 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 alread}^ 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 


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 procephaJic lobes, are also carried inwards 
in the same w^ay, 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 


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 papilliB, which gradually elongate and take on their 
characteristic forms. 




XJp to this point, our attention has been directed 
ahnost 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 anal^^sis 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 


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- 

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- 


ratus for the circulation and the aeration of the blood ; 
a nervous system with sense-organs ; muscles and motor 
mechanisms ; reproductive organs. Regarded as pieces 
of ph3^siological 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 afiinit}' ; and that there is no affinity between the 


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

The exact determmation 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 remotelj^, 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 g-iven at p. 31, 
beginning- at the words " By the end of the year," refer to the '' ecre- 
visse a pieds rou^^es " of France ; not to the English crayfish, wh.'ch is 


males are commonly somewhat larger, and they abnost 
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 

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. 


to its extremity, is greater tlian 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 obliquel}^ 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 backwaitls ; 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 («), which is situated immediately behind 
the orbit, and may, therefore, be termed the post-orhltal 



spine. The elevation itself ma}^ be distiiiguislied as the 

post-orhital ridge. The flattened surface of this ridge is 

marked h}' a longitudinal depression or groove. The 


Fig. 61.— A, D, & G,Astactis torrcntium ; B, E, & H, ^l. noMlis; C, F, & 
I, A. nigrescent (nat. size). A— C, Dorsal views of carapace ; D— F, 
side views of third abdominal somites ; G — I, Dorsal views of telson. 
a, h, post-orbital ridge aiid spines ; c, branchio -cardiac grooves 
inclosing the areola. 


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 

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 corresjponds 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 


edge of the carapace. Eacli groove rims, 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 sculptm'ed 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 
dii'ected 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 ver}^ small pleura of the first somite (fig. 1). The 


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 setse 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. 


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 promuient 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 triliedral 
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 setae, which proceed from the outer edge of 


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 (fig. 26, A), 

The exopodites, or squames, of the antennae extend as 
far as the apex of the rostrum, or even project bej'ond it, 
when they are turned forwards, while they reach to the 
commencement of the filament of the endopodite (Frontis- 
piece), 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 usuall}^ 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- 


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

The inner or sternal edge of the ischiopodite of the 
m 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 
roimded and simply tuberculated. The apex of the fixed 
claw is produced into a slightl}' 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. 


The apex of the dactylopodite, like that of the propo- 
dite, is formed hy 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 prox3odite and dact3doj)odite 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 mdividual 
males. The right and left chelae present no important 

The ischiopodites of the four succeeding thoracic limbs 
are devoid of any recurved spines in either sex (Front., 
fig. 40). The first pair are the stoutest, the second the ' 


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 abeady been fully detailed (pp. 7, 20, and 145), there 
is a marked difi'erence in the form of the sterna of the three 
posterior thoracic somites between the males and females. 


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 
X^osterior 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 zoolog-y will find the comparison of a 
lobster with a crayfish in all the points mentioned to be an e::ccllent 
training- of the faculty of observation. 



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 EngHsh kmd, 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 w^hich 
the actual existences — the individual crayfishes — agree, 
from those in which they diff*er, 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 structm'e 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 sj)ecies. 

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 


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. 



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

being transverse. 




Fig, 62, A & D, Astacus torrcnt'mm ; B & E, >1, nohilis ; C & F, 

A. nigrcscens. A — C, 1st abdominal appendage of the male ; D — F, 

endopodite of second appendage ( x 3). a, anterior, and J, posterior rolled 

edge ; c, d, e, corresponding parts of the appendages in each species ; 

/, rolled plate of endopodite ; g, terminal division of endopodite. 

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


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 bowled 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 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 (6), 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- 
ceedingty 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 
let with, the individuals which present the characteristic 
)eculiarities of the latter are said to form a distinct species, 
[stacus nigrescens ; and the definition of that species is, 
Ike that of the English species, a morphological abstrac- 
tion, embod3ing 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 

dnds of crayfishes, w^hich 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 

ind 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 greater 


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


Fig. 63. Camharus.Clarhii, male (^ nat. size), after Ilag-en. 

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 


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 
Astac2(s, but Camharus (fig. 63). 

All the individual crayfish referred to thus far, there- 
fore, have been sorted out, first into the groups termed 

k 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 

fcthe 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- 
branchise 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 


are susceptible of being grouped into six genera — Asta- 
coides (fig. 65), Astacopsis, Chcera})8, Parastacus (fig. 64), 

Fig. 64. — Parasiacus Irasiliensis (^ nat. size). From soutbern Brazil. 

Engaus, and Parancplwops — on the same piinciple as 
that which has led to the grouping of the Northern forms 
into two genera. But the same convenience which has 


Fig. Q5. — Astacoides madagascarensu {5 nat. size). From Madagascar 


led to the association of groups of similar species into 
genera, has 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 
Potamohiidce ; that of the Southern crayfishes, Par- 
astacidce. But these two famihes 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 Potamohiidce. If the scheme were thoroughly 
worked out, diagrams representing the peculiarities of 

r ■ 


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 
figm^es 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 

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 q,bdominal region are all free, those 
of the head and thorax, except the hindermost, which is 


partially free, are united into a ceplialotliorax, the tergal 
wall of whi^h 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 filamonts. The 
exopodite of the antenna has the form of a mobile scale. 
The mandible has a palp. The first and second maxillae 
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 paii'S, 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 
podobranchise, 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 
PotamohiidcB and the Parastacidce ; and the definition of 
each of these families is formed by superadding to the 


definition of tlie tribe the statement of the special pecu- 
liarities of the familj'-. 

Thus, the Potamohiidce are those Astacina in which 
the podobranchige 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 b}^ a 
transverse incomplete hinge. None of the branchial fila- 
ments are terminated by hooks ; nor are any of the 
coxopoditic setse, or the longer setae of the podobranchise 
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- 
branchise 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- 


branchiae are terminated by short hooked spines; and the 
coxopoditic setse, as well as those which beset the stems 
of the podobranchise, 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, wliich 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 ; W'hile 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 {Homanis 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 fi'om the Astacina is exhibited 
by the gills, of which there are twenty on each side ; 
namely, six podobranchiae, ten arthrobranchise, and four 

i^iG. 67. ITomarm vnlgaris (^ nat. size). 



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 (ej)) 

C st 

Fig. 68. Fodohra,nch.i?eot A, Pant sf(7cu.9 ; B, Xrphrojfs; C^ Palcemon, 
A', C, transverse sections of A and C respectively, a, point of attach- 
ment ; rtZ, wing-like expansion of the stem ; h, base ; hr, branchial 
filaments ; ep^ epipodite ; /, branchial laminas ; j.*/, plume ; st, stem. 

corresponding with the lamina of the crayfish gill, and the 
other (j;Q with its plume. Hence the base {h) 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 

The Norway Lobster {Nephrops norvegiais, fig. 69) 

Fig. GO. N^ejjhrojys 7iorvcg,cus (^ nat. size). 


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 famil}^ Homarina, constructed upon the 
same common plan as the craj^fishes, 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 
Potamohiidce than to that of the Parastacida, 

The Eock 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 Eock Lobsters are more 
closely allied to the Parastacidce than to the Potamohiidce, 

Fig. 70. Palimtrus vidgaris (about | nat. size). 



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, difi'erent 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 brancliise, which vary so much in number in 
different members of these groups, are constructed upon 
a uniform principle, and the differences w^hich they 
present are readily intelligible as the result of various 
modifications of one and the same primitive arrange- 

In all, the gills are tricliohranchice ; 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 Potamohiidce, Parastacida, 
Homarid(E, or Palinui'idce, is twenty-one on each side ; 


and when this number is present, the total is made up 
of the same numbers of podobranchise, arthrobran- 
chiffi, and pleurobranchise attached to corresponding 
somites. In Palinurus and in the genus Astacopsis 
(which is one of the Parastacidcs), for example, there are 
six podobranchise attached to the thoracic limbs from 
the second to the seventh inclusively; five pairs of arthro- 
branchiae 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 pleurobranchise 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 hra n chia I formula of A stacopsis. 

Somites and 





(ep. r.) 


*- • • • 






X ... 






G + ep. r. 


Anterior. Posterior. 










(ep. r.) 




















= 21 -f ep. r. 

bhanchial formula. 


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. 










VII. ... 




VIII. ... 



IX. ... 



X. ... 



XI. ... 

. 1 



XII. ... 

. 1 



XIII. ... 




XIV. ... 

. ' 1 



G + ep. + 6 + 

+ 4 = 21 + ep. 

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

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


The hranchial formula of Astacus, 

Somites and 


■flip IT* 




Appendages, branchioc. 

Anterior. Posterior. 


VII. ... 

(ep.)... . 

.. . 




VIII. ... 

.. . 




IX. ... 




X. ... 




XI. ... 

or 7* 


3 or 3 + r 

XII. ... 



3 + r 

XIII. ... 



3 + r 

XIV. ... 

... . 


.. 1 



G + cp. ■»- 6 +5 + 1 h2or3r= 18 + ep. + 2or3r. 

In Camharns, the number of the branchise 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 branchise are 
left, the rest being either represented by mere rudiments, 
or disappearing altogether. 

The branchial formula of Astacoides. 

Somites a 












(ep. r.) 




(cp. r.) 





I + r 










2 + 7* 





2 + r 





2 + r 





2 + r 

XIV. ... 

1 = 

6 + €2^. r 5 + r + + 4 r -f 1 = 12 + ep. r. + 5 r. 



As these formulae show, those trichobrancliiate Crus- 
tacea, which possess fewer than twenty-one complete 
brancliise on each side, commonly present traces of the 
missing ones, either in the shape of epipodites, as in the 
case of the podobranchi?e, or of minute rudiments, in the 
case of the arthrobranchise and the pleurobranchise. 

In the marine, prawn-like, genus Penceus (fig. 73, 
Chap. YI.), the gills are curiously modified trichobran- 
chise. 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 a 







^ X 







• ' • X • ■ • 



1 + ep. 




1 ... 


3 + ep. 



« • • -i • • • 

1 ... 


3 + ep. 



• • • -L ■ • • 



3 + ep 






3 + ep 






3 + ep 


• • • A • • • 

1 ... 




... ... 




+ 6 ep. + 7 

= 20 + 6 ep. 

This case is very interesting; for it shows that the 
whole of the podobranchise may lose theii* branchial charac- 
ter, and be reduced to ei)ipodites, 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 arthrobranchite and pleurobranchise, 


the suggestion arises that each hypothetically complete 
thoracic somite should possess four gills on each side, 
giving the following 

Hypothetically complete hrcmcldal formula. 

Somites and 






ntorior. Posterior. 

VII. ... 

1 ... 1 ... 1 



VIII. ... 



IX. ... 

i. • • • J- i 



X. ... 



XI. ... 



XII. ... 



XIII. ... 



XIV. ... 





= 32 

Starting from this hypothetically complete branchial 
formula, we may regard all the actual formulas 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- 
branchise, the branchiae are converted into epipodites ; in 
that of the other branchiae, they become rudimentary, or 

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- 



sition, are in fact the same. But, in the prawn, the abdomen 
is much larger in proportion to the cephalothorax ; the 

Fig. 71. Palcsmon jamaicensis (about 4 nat. size\ A. female ; 
B, fifth thoracic appendage of male. 

basal scale, or expodite of the antenna, is much larger ; 
the external maxilHpedes are longer, and differ less 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 


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- 
branchiae, attached to the epimera of the five hinder 
thoracic somites ; two are arthrobrandhise, fixed to the 
interarticular membrane of the external maxillipede ; and 
one, which is the only complete podobranchia, belongs 
to the second maxillipede. The podobrancliise of the 
first and third maxillipedes are represented only b}^ small 
epipodites. The branchial formula therefore is : — 

Somites and 


■f VlPlT 









VII. ... 



• • 

» • • 



VIII. ... 


. . 




IX. ... 





2 (ep.) 

X. ... 

, , 


XI. ... 

■ ■ • 


XII. ... 

> • • 


XIII. ... 

• • • 


XIV. ... 



1 + 2 cp. 

+ 1 






8 + 2 


The prawn, in fact, presents us with an extreme case 
of that kind of modification of the branchial S3^stem, of 
w^iich PencEus has furnished a less complete example. 
The series of the podobranchise is reduced almost to 
nothing, w^hile the large pleurobranchi?e are the chief 
organs of respiration. 

But this is not the onl}^ difference. The prawn's 
gills are not brush-like, but are foliaceous. They are 
not trichobranchice, but phyllohranchifs ; that is to say, 
the 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 
diiven between the branchial leaflets by a respiratory 
mechanism of the same nature as that of the crayfish. 

Different as these phyllobranchi£e 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 Axius, 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 


the trichobranchia to tlie pliyllobranchia will be veiy 
easily effected. 

The shrimp {Crangon) also possesses phyllobranchise, 
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 elwaj^s associated with 
the lobsters and crayfishes, although the difference of 
general appearance is vastly greater than in any of the 
cases which have vet been considered. These are the 

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 e3^es themselves appear above and in front of 
the antennules. The external maxilHpedes are narrow, 
and their endopodites are more or less leg-like. 

None of these statements apply to the crabs. In these 



animals the abdomen is short, flattened, and apt to escape 
immediate notice, as it is habitually kept closely applied 
against the mider surface of the cephalothorax. It is 

Fig. 7'2. Cnnccv pagvrvs,vci2i\e H 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 ; ?>. base of antenna ; 
3 , free portion of antenna. 


not used as a swimming organ ; and the sixth somite 
possesses no appendages whatever. The breadth of the 
cephaiothorax 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 
fnml}^ fixed to the edge of the carapace. There is no exo- 
poditic scale; and the free part of the antenna (5') 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 imier 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 tlie 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 ; 


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, Hke two folding-doors set in a square doorwaj^ 
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 stjdiform 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 
appKed throughout the greater part of their length to 
the bases of the ambulatory Hmbs, 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 
maxillij)ede, 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 sca- 
phognathite, is baled out through two apertures, which 


are separated from the foregoing by the external maxilli- 
pecles, 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 phyllobranchiae. 
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 pleurobranchise, the 
other five are arthrobranchise. The tw^o remaining gills 
are podobranchiae, and belong to the second and the 
third maxillipedes respectiveh'. 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 maxillij^ede is short and triangular, and fits in 
between the bases of the second and the third arthro- 
branchise. The epipodite of the thii'd maxillipede is very 
long, and its base furnishes the valve of the afferent 
aperture of the branchial cavity, wliich has been men- 
tioned above. The podobranchia of the first maxillipede 
is represented only by a long curved epipoditic blade, 
which can sweep over the outer surface of the gills, and 
doubtless serves to keep them clear of foreign bodies. 



The branchial formula of Cancer pagurus. 

Somites and 









VII. ... 
VIII. ... 






IX. ... 






X. ... 





XI. ... 




XII. ... 




XIII. ... 


XTV. ... 



+ ep. 

+ 3 







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 pleurobranchi^e alone, as in the case of the prawns, 
but of the arthrobranchiae as well. At the same time 
the whole apparatus has become more sj)ecialized 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 the 
structure and habits of the crayfishes, however, attentive 
examination shows that the plan of construction of the 



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 63^6- 
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 exemplifj^ 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 


prawn-like animals, whicli, 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 which 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 bodj^ 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 


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 crayfish 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 ap]3endages of the Nauplms 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 

In some Podophthalinia, 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 e3^es and 
the abdominal members are developed, and the larva passes 
into what is sometimes called the My sis 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. 




In the Opossum-shrimps (Mysis) the young does 
not leave the pouch of the mother until it is full)' 

Fig. 73. Penatns semisulcatus. A, adult (after de Haan, ^ nat. size) ; 
B, Zosea, and C, less advanced Zoasa of a species of Penceus. D, 
Nauplius. (B, C, and D, after Fritz Miiller.) 


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 2)(igurus. A, newly hatched Zoaea ; B, more advanced 
Zoaea ; 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. 


In the great majority of the Pocloijhthalmia, the Nauplius 
stage seems to be passed over without any such clear 
evidence of its occurrence, and the young is set free as a 
Zoaea. 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 embryological uniformity of type of all the 


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 b}^ comparative morphology, w^e are thus led 
to admit that the whole of the Arthrojpoda 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, w-ith frogs and newts, reptiles, birds, and 
mammals ; or, in other words, with the whole of the 
great division of the Vertehrata. 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 
primar}^ 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 


gastrula, as the early stages of development. The like 
is true of all the worms, sea-urchins, starfishes, jell^^fishes, 
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 preliminar}^ 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 


are allied. Given one of those protoplasmic bodies, of 
which we are unable to say certainlj^ whether it is animal 
or plant, and endow it with such inherent capacities of 
self-modification as are manifested daily under our e^^es 
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 


among cra3'fishes ; nevertheless the churches have cer- 
tainly not been developed from a common ancestor, but 
have been built separatelj^ 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. 




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. 
Thej^ 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 
examj^le, 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. MTntosh, 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, XI., 1843. 


island, has or has not been effected by artificial means, is 
involved in some obscurity. 

English zoologists have alwavs termed our cravfish 
Astacus fluviatilis ; and, up to a recent period, the 
majority of Continental naturalists have included a 
corresponding form of Astacus 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. Astacus 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"t 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 Crustaces." 
f Carbonnier. "L'Ecrevisse," p. 8. 


or " stone crayfish," and the latter by that of *' 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 I'Acad. de St. Peters- 
burg, 1859. 


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 moi'phological species, whether n represents an 
important or an unimportant difference. 

The great majority of species described in works on 
Systematic Zoology are merel}^ 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 really know 
about its distinctness. 

But, in practice, the formation of specific groups is 
more or less qualified hy 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 


small, and so inconstant, that they lie within the 
probahle 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 -f ^ aind A will be 
filled up by a series of forms in which x gradually 

Finallj', 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 -f- w is filled 
up by forms of A having traces of n, and forms of A -f 7i 
in which n becomes less and less, then it will be con- 


eluded that A and A + w are races of one species and 
not separate si)ecies. And this conclusion will be fortified 
if A and A + ^ occupy different stations in the same 
geographical area. 

Even when no transitional forms between A and A + ^ 
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 w4iat has been said it follows that the groups 
termed morphological species are provisional arrange- 
ments, expressive simply of the present state of our 

We call two grou^^s 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 

What has happened in the case of the crayfish is this : 


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 cra^^fish and the noble cra^'fish as races or varieties 
of that species. Later zoologists, comparing crayfishes 
together more critically, and finding that the stone 
cra^^fish 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 w411 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 wdll 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 post- orbital ridge, and the absence of spines on its 

4. The large relative size of the posterior division of 
the telson (G). 

On the contrar}^ in the noble crawfish : — 

1 . The sides of the posterior two - thirds of the 
rostrum are nearl}^ 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 develojDed 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- 
branchitB 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 


differences between the two kinds of craj^fishes, 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 b}^ variation. 

From a morphological point of view, then, it is reall}^ 
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 nohilis* 

In the physiological sense, a species means, firstlj^ a 
group of animals the members of which are capable of 
completely fertile union wdth one another, but not with 
the members of anj'- 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.flnviattlis (var. torrentiuvi) and A. fluviatilis (var. nobilis) on the 
hypothesis that the stone crayfish and the noble crayfish are varieties ; 
and A. torrentiuvi 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. 


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 rouojes " exhibited a marked 
superiority in size over the "pieds blancs." M. Car- 
bonnier, in fact, ssLjs 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 tliat A. torren- 
tiiim and A. nohilis 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 


specimens of English and Irish crayfishes wliich 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 f 
been enabled, b}^ the kindess of Dr. BoUvar, 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 J 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 Yeglia, at Trieste, 
ill 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 Russian empire. 

* See Bell. " British Stalk-eyed Crustacea," p. 237. 

f Since the statement respecting the occurrence of crayfishes in Spain 
on p. 44 was printed. 

J " Die Crustaceen des Slldlichen Europas," 1863. 

§ " Die Russischen Flusskrebse." Bulletin de la Society Imperiale 
des Naturalistes de Moscow, 1874. 


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, not far from the famous caves of 
Adelsberg. It is not known in Dalmatia, in Turkey, nor 
in 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 crayfish 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 artificiall}' connected with the Volga, 
and in which it is partially replaced by A. leptodactylus." 
It still inhabits the Lakes of Beresai and Bologoe, as 
well as the afiluents of the Msta and the Wolchow ; and 
it is met with in affluents of the Dnieper, as far as 
Mobile w. Astacus nobilis is also found in Denmark and 
Southern Sweden ; but, in the latter countr}^, 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 


be remembered, are much less salt than ordinary sea 

It will be observed that while the two forms, A. torren- 
tium and A. nohiliSf are intermixed over a large part of 
Central Europe, A. torrentiiim 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. nohilis 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 Caspian 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 q-nd Gerstfeldt, 
in their memoirs already cited. 

Fig. 75. — Astacus lc2)tudactylus (after Eatlike, i nat. size). 


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 Yolga with the rivers which flow into the Baltic 
and into the White Sea. In the latter, the invading A. 
leptodactyhis is everywhere overcoming and driving out 
A. nohilis 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 pachypuSf 
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. colcliicns, 
has recently been discovered in the Eion, or Phasis of 
the ancients, which flows into the eastern extremit}^ of 
the Black Sea. 

With respect to the question whether these Ponto- 
caspian crayfishes are specifically distinct from one 
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. 369-70), has an in- 
teresting discussion of this question. 


crayfishes. Gerstfelclt, who has had the opportunity of 
examining large series of specimens, concludes that the 
Pontocaspian crayfishes and A. nohilis are all varieties 
of one species. Kessler, on the contrary, while he 
admits that A, angulosus is, and A, pachi/pus may he, 
a variety of A. leptodactylus, affirms that the latter is 
specificall}^ distinct from A. nohilis. 

Undoubtedly, well marked examples of A. leptodactylus 
are very different from A. nohilis. 

1. Tlie edges of the rostrum are produced into five or 
six sharp spines, instead of being smooth or slightly 
serrated as in A. nohilis. 

2. The fore part of the rostrum has no serrated 
spinous median keel, such as commonly, though not uni- 
versally, exists in A. nohilis. 

3. The posterior end of the post-orbital ridge is still 
more distinct and spiniform than in A. nohilis. 

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. nohilis vary in the direction of A. leptodactylus and 
vice versa; and if A. angidosus and A. pachypus are 
varieties of A. leptodactylus, I cannot see why Gerst- 
feldt's conclusion that A. nohilis is another variety of 


the same form need be questioned on morphological 
grounds. However, Kessler asserts that, in those lo- 
calities in which A. leptodactylus and A. nohilis 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 f , if not more, 
kinds of craj^fishes 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 crayfish 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 Rion. No crayfish has 3'et been disco- 
vered in the whole continent of Africa. I 

* It would be hazardous, however, to assume that none exist, especi- 
ally in the Oxus, which formerly flowed into the Caspian. 

f A. dauricns and A. SclirencMi. 

X Whatever the so-called Astacuti cajjcjcsis of the Cape Colony may 
be, it is certainly not a crayfish. 


Tims, 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 
boundar}^, 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 
Eocky 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 
Camharus (fig. 63, p. 248). Species of this genus also 
occur in Cuba,f but, so far as is at present known, not 
in any of the other West Indian islands. The occurrence 
of a curious dimoi-phism among the male Camhari has 
been described by Dr. Hagen ; and a blind Camharus 

* Dr. Hagen in his " Monograph, of the North American Astacidse," 
enumerates six species ; A. Gavihelii, A. klamathensis, A. leeniscnlus, 
A. nigrescens, A. oregamis, and A. Trowhridgii. 

f Von Martens. Camharus ciiheridis, Archiv. f iir Naturgeschichte, 


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 Parastacidce, and in respect of the number and variety 
of forms and the size which they reach, the head-quarters 
of the Parastacidcs 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 Camhari, live habitually on 
land, in burrows which they excavate in the soil. 
' New Zealand has a peculiar genus of crayfishes, 
Paranephi'ops, a species of which is found in the Fiji 
Islands, but none are known to occur elsewhere in 

Two kmds 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, which is situated in 30° 
S. Latitude, close to the mouth of the Jacuhy, at the 
north end of the great Laguna do Patos, which communi- 

* Siiilbrasilische Suss- und Brackwasser Crustaceen, nach den Samm- 
lungen des Dr. lleinh. Hensel. Archiv. flir Naturgeschichte, xxxv. 

Fig. 76. — Australian Crayfish (^ nat. size).* 

* The nomenclature of the Australian crayfishes requires thoroug-h 
revision. I therefore, for the present, assign no name to this cray- 


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. hrasiliensis, fig. 64) was obtained at Porto 
Alegre, and further inland, in the region of the primitive 
forest at Rodersburg, 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 which they were specially sought foi- by 
Agassiz ; or in the La Plata, on the eastern side of the 
Andes. But, on the west, an "Astacus'' chiknsis 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 
Chihan coast are doubtless to be understood. 

Finally, Madagascar has a genus and species of craj^- 
fish (Astacoides onadagascariensis, fig. Q5) 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.nobllu of Dana and the A. ar- 
mat lis of Von Martens. 



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representation of the broad morphological differences 
which mark off the Potamobiidcs 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 
Potamohiiclce, Astacus torrentium and nohilis 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 Astacus 
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 Camhari of the Eastern or Atlantic water-shed (Y.) 
are separated by the great phj'sical barrier of the Rocky 
Mountain ranges. Finally, with regard to the Par- 
astacidce, the widel}^ 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 


be found that the parallel between the geographical and the 
morphological facts cannot be quite strictly carried out. 

Astaciis torrentmrrij as we have seen, inhabits both 
the British Islands and the continent of Europe ; never- 
theless, there is every reason to believe that twent}'' 
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 craj^fishes 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 Astaciis 
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-lilie process ; while, in the 
females, the hinder edge of the penultimate thoracic 


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. — Camba7'iis (Guatemala) penultimate leg. c.rj?, coxopodite ; 
cr.s, coxopoditic setae ; j^db, podobranehia ; bp, basipodite ; ip, ischiopo- 
dite ; nq), meropodite ; cjf carpopodite ; 2U^- pi'opodite ; dj), 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 Camhari, the pleurobranchise appear to be 
entirel}^ suppressed, and the hindermost podobranehia has 
no lamina ; while the areola is usually extremely narrow. 
The proportional size of the areola in the Amurland 

* Kessler, 1. c. 


crayfishes is not recorded ; in the Jaj^anese 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 approach 
the Canihari. 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 Parastaciclce ; 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. %d) and in the extreme modification of its branchial 
system, which I have described elsewhere, the Madagascar 
genus stands alone. 

The Paranepliro'ps of New Zealand and the Fijis, with 
its wide and short e]3istoma, 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 

If the distribution of the crayfishes is compared 
with that of terrestrial animals in general, the points of 


difference are at least as remarkable as the resem- 

With respect to the latter, the area oocupied by the 
Potamohiidce, corresponds roughly with the Palsearctic 
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 craj^fishes, not that of ter- 
restrial animals in general, but only that of freshwater 


fishes, some very curious points of approximation become 
manifest. The Sahnonidce, 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 thej^ 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 Reti'opinna, 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 HcqAochitonidce and the 
GalaxidcBj which stand in somewhat the same relation to 
the SalmonidiB as the Parastacidce do to the Potamohiidcs, 
take the place of the Sahnonidce in the fresh waters of 
New Zealand, Australia, and South America. There 
are two species of Hajplochiton 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 
Falldand 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 


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 
ver}- difficult to say whether a given crustacean belonged 
to the Astacine, or to the closel}^ 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 existing American crayfishes ; whether they are 
Camhari or Astaci does not appear. But, in the lower 
chalk of Ochti;up, in Westphalia, and therefore in a 
marine deposit, Yon der Marck and Schliiter * have 
obtained a single, somewhat imperfect, specimen of a 
crustacean, which they term Astacus politus, and which, 
singularly enough, has the divided telson found onl}- 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 

Such are the more important facts of Morphology, 
Physiolog}^, and Distribution, which make up 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 w^hen we attack the final 
problem of Biology, which is to find out wh}^ 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 w^e must seek the origin of crayfishes in 
conditions extraneous to the ordinarj^ course of natural 

* Neue Fische und Krebse aus der Kreide von Westphalcn. Pala^on- 
tographica, Bd. XV., p. 302 ; tab. XLIV., figs. 4 and 5. 


operations, by what is commonly termed Creation; or we 
must seek for it in conditions afforded by the usual 
course of nature, Avhen the hypothesis assumes some 
shape of the doctrine of Evolution. And there are two 
forms of the latter hypothesis ; for, it ma}^ be assumed, 
on the one hand, that crayfishes have come into exist- 
ence, independently of any 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 word 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, 
whether 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 


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 onl}^ refuge, therefore, appears to be the hj^po- 
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 hj'pothesis 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 the}^ exhibit are the results 
of the interaction, through past time, of two series of 


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 torrentmm 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 cra3''fish 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 nohilis is said to venture into 
the brackish waters of the Gulf of Finland, and A. lepto- 
dactylus, as we have seen, makes itself at home in the 
more or less salt Caspian, there is no reason to believe 
that Astacus torrentium is capable of existing in sea- 


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 transj)orted 
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 jDresence of Astacus torrentium 
hj reference to the same operation. 

If we take into account the occurrence of Astacua 
nobilis over so large a j)art 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 ^. 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. leptodacti/lus is now passing into 
the rivers of the Baltic provinces of Russia. 

The study of the glacial phenomena of central Europe 


has led Sartorius von Waltershausen * to the conchision 
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 Ehone, around the northern escarpment of the 
Alpine chain, and connected the head-waters of the 
Danube with those of the Ehine, 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 w^ould thus be 
opened up hy 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. nohilis 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, Ar3^an, and Mongoloid streams of west- 
ward movement among mankind. 

If we thus suppose the western Eurasiatic craj^fishes 
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 intelHgible. 

The extremely severe chmatal conditions which obtain 
in northern Siberia may sufficiently account for the 

* " Untersuchungen ueber dieKlimate derGegenwartundderVorwelt." 
Natuurkimdige Verhandelingen van de Hollandsche Maatschappij der 
Wetenschappen te Haarlem, l8Go. 


absence of crayfishes (if tlie}^ are really absent) in the 
rivers Obi, Yenisei, and Lena, and in the great lake 
Baikal, which lies more than 1,800 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 we 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 Gammarus and Iclothea. 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 


the stock from which they have been evolved. Even 
nnder existing geographical conditions, an affluent of the 
Mississippi, the St. Peter's river, communicates directly, 
in rainy weather, w4th the Red river, which flows into 
Lake Winnipeg, the southernmost of the long series of 
intercommunicating lakes and streams, which occuj)y 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 Rocky 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 


in the latter 'pixri 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 cra3'fish 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 formerh^ 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 


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 
Potamohiidce obtained access to the river basins in which 
they are now foimd, 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 {Thelphusidce), 
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 Pencsus 
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, 


SO that they 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 w^ater fishes which inhabit these lakes. Now, 
this Mysis relicta is hardly distinguishable from the 
My sis ocidata which inhabits the Arctic seas, and is 
certainly nothing but a slight variety of that species. 

Li 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 ai*ms 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 undene Crustaceen." 
Halle Zeitschrift fiir die Gesammten Wissenschaften, xix., 1862 : G. O. 
Sars, "Histoire Naturelle des Crustaces d'eau douce deNorvege," 1867. 


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 (Palccmon) 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 Bica in western South America ; in the Uj)per 
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 m lies especially, they are very long and strong, and 



are terminated b}^ great chelae, not unlike those of the 
crayfishes. Hence these fluviatile prawns (known in 
many places b}^ the name of " Cammarons ") are not 
unfrequently confounded with true crayfishes; though 

Fig. 79. Palccmon jamoiceiuis (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 Astaciche. 

Species of these large - clawed prawns live in the 


brackish water lagoons of the Gulf of Mexico, but I 
am not aware that £Lnj of them have yet been met with 
in the sea itself. The Palcemon lacustris {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 My sis 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 Svisswasser-crustacee im Nil," with the notes by von Mar- 
tens and von Siebold: Zeitschrift fur Wissenschaftliche Zoologie, 1866. 

f This seems actually to have happened in the case of the widely- 
spread allies and companions of the fluviatile prawns, A tya and Cai-i- 
dina. 1 am not aware that truly marine species of these genera are 


land and by other changes in ph3^sical 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 majorit}^ of 
the MysidcB and many of the prawns do now ; and that, of 
these ancestral crayfishes, there were some which, like 
Mysis oculata orPenceus hrasiliensis, residily 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 Praw^ns and Pencei have now, but were difierentiated 
into two groups, one with the general characters of the 


PotamohiidcE in the northern hemisphere, and another, 
with those of the Parastacidce, in the southern hemisphere. 

The ancestral Potamohine form probahly presented 
the peculiarities of the Potamohiidce in a less marked 
degree than any existing species does. Probably the 
four pleurobranchise were all equally well developed ; the 
laminae of tiie 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- 
mohine, 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 Camharus appear to me to necessitate the 
supposition that they are derived from some one already 
specialised Potamohine form ; and T have already men- 
tioned the grounds upon which I am disposed to believe 
that this ancestral Potamohine existed in the sea which 
lay north of the miocene continent in the northern 

In the marine j)rimitive 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 
Palinimd(Sf the headquarters of which are in the 
southern hemisphere. That they should have ascended 


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 wdiether 
marine Parastacidce still remain in the South Pacific 
and Atlantic Oceans, or whether they have become 

In speculating upon the causes of an effect which is 
the product 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 Camhcwi, than between the 


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 cra^^fish 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 

* Just as there is an American form of Idothea and an Asiatic form 
in the Arctic ocean at the present day. 


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 priorif be expected to exist ; and I can onl}^ 
suggest the directions in which an explanation may be 

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 

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 


a veiy ancient continental area, the oldest indigenous 
population of which, in all j)i'ob ability, 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 crayfishes are b}^ far the largest and strongest 
of any of the inhabitants of freshwater, except the Verte- 
hrata ; and that Avhile 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 everything^ to learn 
respecting the fauna of the great inland lakes and river systems of 
South Africa. 


Rud Rnyisitile crahs {Theljyhusa) 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 craj'fishes; so that the hitter might either be 
driven out of territory they akeady occupied, as Astacns 
leptodactylus is driving out A. nohilis in the Russian 
rivers ; or might be prevented from entering rivers ah'eady 
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, tlie West 
Indies, Africa, Madagascar, Southern Italy, Turkey and 
Greece, Hindostan, Burmali, 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 Atyidce 
not onl}^ 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 
Schniidtii), in the Adelsberg caves, representing the blind 
Camharus of the caves of Kentucky. 

The hypothesis respecting the origin of 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 tertiarj^ 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 which 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 

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 afiinity 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 ambulatoiy limbs, while 


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 

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 the}^ 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 

For the present purpose, the series of the fossiliferous 
rocks may be grouped as follows : — 1. Recent and 
Quaternary. 2. Newer Tertiary (Pliocene and Miocene). 
3. Older Tertiary (Eocene). 4. Cretaceous (Chalk, 
Greensand and Gault). 5. Wealden. 0. 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 

Fig. 80. — A, Pscudastacus puntHlosus (nat. size). B, Eryma modcsti- 
f 07' mis ( X 2). Both figures after Oppel. 


decapod Podophtludmia to which the Astacomorpha belong 
occurs in the Carboniferous formation. It is the genus 
AntlirapalcEmon — a small and very curious crustacean, 
about which nothing more need be said at present, as it 
does not aj^pear 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 Pemjjhix is an exception, no 
Astacomorphs are known to occur in them. The speci- 
mens of Femphix 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 t3^j)e 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 


and left their remains in the mud of the ancient sea 

Eri/ma is the onh' crustacean, which can be certainly 
ascribed to the Astacomorpha, that has hitherto been 
found in the 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 Astacomorj)]ia are known, and although no verj^ great 

Fig. 81. — TloplojJarla long'xmana (f nat. size). — cp, carapace; 
?% rostrum, T, telson ; xv., xvi., first and second abdominal somites ". 
10, forceps ; 20, last abdominal appendage. 

weight is to be attached to a negative fact of this kind, it 
is, so far, evidence that tlie Astacomorpha had not yet 
taken to freshwater life. In the marine deposits of the 
Cretaceous epoch, however, astacomorphous forms, which 


are known by the generic names of Tloijlojpana 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 Homarus 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 
Eryinoid crustaceans which inhabited the seas of the 
Liassic epoch. 

Side by side with Eryma, in the lithographic slates, 
there is a genus, Pseudastacus (fig. 80, A), which, as its 
name implies, has an extraordinaril}'' 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 


was already distinct from the Homarine tj^pe, 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 

I am disposed to think that Ptieudastacus 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- 

If we arrange the results of palseontological 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. 



Successive Foeiis of the Astacomorphous Type. 

I. Recent 




, . m X- A4acus 

II. Later Tertiary ^j^^j^^) 

III. Earlier Tertiary. 


IV. Cretaceous. Astacus. Psevxlastacus. Enoplodytia. Hoploparia. 


V. Wealfleu 

(Fresh Water). 

VI. Jurassic. 

VII. Liassic. 

Pseudaitacus Eryma. 




VIII. Triassic. 

IX. Permian. 

X. Carboniferous. 

XI. Devonian. 

XXI. Silurian. 

XIII. Cambrian. 


If an Astacomori)hous crustacean, having characters 

intermediate between those of Eryma and those of 

Pseiidastacus, existed in the Triassic epoch or earlier ; 

if it gradually diverged into Pseudastacine and Er3'moid 

forms ; if these again took on Astacine and Homarine 


characters, and finally ended in the existing Potamohiidce 
and Homarina, 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 Potamohiidce 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 iEtiology 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 Astacomoi'phous 

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. 


Note 1., Chapter L, p. 17, 

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 

The animal matter consists for the most part of a peculiar substance 
termed Chitin, which enters into the composition of the hard parts not 
only of the Arthropoda 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 

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 C^ H„g N^j O^q. 

Note IL, Chapter I., p. 29. 


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 

348 ■ 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 fro_a the 
latter. Thus the proijer 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 very 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 Rendus, 
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 fii'st 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 



According to Dulk("Chemische UntersuchungderKrebsteine:" Mullcr's 
Archiv. 1855), the gastroliths have the following composition : — 

Animal matter soluble in water . . . . 
Animal matter insoluble in water (probably chitin) 
Phosphate of lime ... . . . . 

Carbonate of lime 

Soda reckoned as carbonate . , , . . 







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., Chaptee L, p. 31. 

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. Carbonnier (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 
k 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 I'histoire naturelle et I'education des Ecrevisses:" 
Comptes Rendus, LX. 1865) gives the result of his study of the growtli 
of the crayfishes reared at Clairefontaine, near Rambouillet, in the 
following table : 

Mean length. 

Mean weight. 




of the year . , 0-025 


1 year old . . 0-050 


2 years old . . 0-070 


3 years „ . . 0*090 


4 years „ . . 0-110 


5 years „ . , 0-125 , , 


indeterminate . . O'lCO 


very old . . 0-190 


These observations must also apply to the " Ecrevisse 

a pieds rouges." 

350 ^ NOTES. 

Note IV., Chapter I, p. 37. 

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 I'histoire 
naturelledes Ecrevisses :" Comptes Eendus, LXXI. 1870, and LXXIII. 
1871), who appears to have studied the question (on the "6crevisse 
k picds 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 days 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 

The details of the process of ecdysis are discussed by Braun, ** Ueber 
die histologischen Yorgiinge bei der Hautung von Astacus fiuviatilis.'" 
WUrzburg Arbeiten, Bd. II. 

Note V., Chapter I., p. 39. 


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 Rcndus, 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, on 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 LerebouUet ("Eecherches 
sur le mode de fixation des oeafs aux faux pattes abdominaux dans les 
lEcrevisses." Annales des Sciences Naturelles, 4e Ee. T. XIV. 1860), 
and by Braun (Arbeiten aus dem Zoologisch-Zootomischen Institut in 
Wilrzburg, TI.). 

Note VI., Chapter I., p, 42. 

I observe that I had overlooked a passage in the Report on the 
award of the Prix Montyon for 1872, Comptes Rendus, LXXV. p. 1341, 
in which M. Chantran is stated to have ascertained that the young 
crayfishes fix themselves " en saisissaiit avec un de leurs pinces le 
filament qui suspend I'oeuf a une fausse patte 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 

852 NOTES, 

mother and return to her again, up to twenty-eight days, when they 
become independent. 

In a note appended to M. Chantran's paper, M. Eobin 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 mentiqji it and I have seen nothing of it in 
those recently hatched young which I have had the opportunity of 

Note VIL, Chapter II., p. 64. 



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 maxillie 
of the crayfish. 

Hoppe-Seyler (Pflugers 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. 353 

Note VIIE., Chaptee II., p. 81. 


LcrebouUet (" Note sur une respiration anale observee chez plusieurs 
Crustaces ; " Memoires de la Societe d'Histoire Naturelle de Strasbourg, 
IV. 1860) has drawa 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. 
I have never been able to observe anything of this kind in the uninjured 
adult animal, but if the thoracic ganglia are de^rtroyed, 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 existence of guanih 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 Copepoda and Phyllopoda. 

Note X., Chapter III., p. 105. 



The details respecting the origin and the distribution of the nerves are 
intentionally omitted. See the memoir by Leraoine of which the title is 
given in the "Bibliography." 

354 NOTES. 

Note XI., Chapter III., p. 110. 



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. 


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. 
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 tbe 
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," oi 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, somewhat 

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 rae 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 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 PodopU- 
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 maxillae and that which carries the first maxiUi- 
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. 


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 
y^t no complete agreement even as to matters of observation, would 
require a whole treatise to itself. 

856 NOTES. 

Note XVI., Chapter IV., p. 221. 


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 LerebouUet ; 
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 
Df the process of development given in the text, is correct. 

Note XVII., Chapter VL, p. 207. 


In France and Germany crayfishes (apparently, however, only 
A. nohilis) are infested by parasites, belonging to the genus Branchio- 
bdella. 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 fiir 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, 
Histriohdella. 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. 



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. 


ROESEL VON RosENHOF. Der Monatlich-herausgegeben Insekten 

Belustigung. 1755. 
Carbonnier. L'Ecrevisse, Paris, 1869. 
Brandt and Ratzeburg. Medizinische Zoologie. Bd. 11., pp. 

Bell. British Stalk-eyed Crustacea. 1853. 
Soubeiran. Sur I'Histoire naturelle et I'Education des Ecrevisses. 

Comptes Rendus, LX., 1865. 
Chantran. Observations sur I'Histoire naturelle des Ecrevisses. 

Comptes Rendus, LXXI., 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., LXXVIII., 1874. 

Sur le Mecanisme de la Dissolution intrastomacale des 

Concretions gastriques des Ecrevisses. Ibid., LXXVIII., 1874. 

Steffenberg. Bijdrag til kanne domen om flodkraftens natural 

historia, 1872. Abstract in Zoological Record, IX. 
Vallot. Sur I'Ecrevisse fluviatile et sur son parasite I'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. 



Heller. Ueber einen Flusskrebs - albino. Verhand d. Z. Bot. 

Gesellschaft, Wien. Bd. 7, 1857, and Bd. 8, 1858. 
Lebeboullet. Sur les variet^s Rouge et Bleue de I'Ecrevisse 

fluviatile. Comptes Rendus, XXXIII., 1857, 
GiRARD. Quelques Remarques sur I'Astacus fluviatilis. Ann. Soc, 

Entom. France, T. VII. 1859. 


Brandt and Ratzebltrg. Op. cit. 

Milne Edwards. Hisloire naturelle des Crustac^s. 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, 

Oesterlen. Ueber den Magen des Flusskrebses. Mliller's Archiv, 

T. J. Parker. On the Stomach of the Freshwater Crayfish. 

Journal of Anatomy and Physiology, 1876. 
Bartsch. Die Ernahrungs- und Verdauungsorga'ie des Astacus 

leptodactylus, Budapester Naturhistor. Hefte II. 1878. 
Deszo, Ueber das Herz des Flusskrebses und des Hummers. 

Zoologischer Anzeiger, L 1878. 
Lereboullet. Note sur une Respiration anale observee chez 

plusieurs Crustac^es. 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 I'histoire des syst^mes nerveux^ 

musculaire et glandulaire de I'Ecrevisse. Annales des Sciences 

Naturelles, Se. IV. T. 15, 1861. 
Dietl. Die Organization des Arthropoden Gehirns. Zeitschrift 

fur Wiss. Zoologie, XXVIL, 1876. 
Krieger. Ueber das centrale Nervensystem des Flusskrebses. 

Zoologischer Anzeiger, I., 1878. 
Letdig. Das Auge der Gliederthiere. 1864. 


Max Schulze. Die Zusammengesetzten Augen der Krebse und 

Insekten, 1868. 
Bekger. Untersachungen liber den Bau dcs Gehirns und der 

Retina der Arthropoden. 1878. 
Geenacher. Untersuchungen iiber das Sehorgan der Arthropoden. 

O. Schmidt. Die Form der Krystalkegel im Arthropoden Auge. 

Zeitschrift fur Wiss. Zoologie, XXX., 1878. 
FArhe. On the organ of hearing in the Crustacea. Phil. Trans. 

Leydig. Ueber Geruchs- und Gehororgane der Krebse und Insekten. 

Mliller's Archiv, 1860. 
Hensen. Studien liber 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 Crus<:aces 

decapodes. Annales des Sciences Naturelles, Se. VI. ii. 
Leydig. Zur feineren Bau der Arthropoden. JVIliller's Archiv, 


Handbuch der Histologic. 1857. 

Haeckel. Ueber die Gewebe des Flusskrebses. Mliller's Archiv, 

Braun. Ueber die histologischen Vorgange bei der Hautung von 

Astacus flaviatilis. Wiirzburg Arbeiten, II. 
Baur. Ueber den Bau der Chitinsehne am Kiefer des Flusskrebses 

und ihr Verhalten beim Schalenwechsel. Reichert u. Du Bois 

Archiv, 1860. 
Coste. Faits pour servir k I'Histoire de la Fecondation chez les 

Crustaces. Comptes Rendus, XLVI. 1858. 
Lereboullet. Recherches sur la mode de Fixation des (Eufs aux 

fausses pattes abdominales dans les Ecrevisses. Annales des 

Sciences Naturelles, S6. IV. T. 14, 1860. 


Rathke. Ueber die Bildung und Entwickelung des Flusskrebses 

Lereboullet. Recherches d'Embryologie compar^e sur le d6ve- 

loppement du Brochet, de la Perche et de I'Ecrevisse. 1862. 


BoBEETSKY. (A Memoir in Russian, of which an abstract is given 
in Hofmann and Schwalbe, Jahresbericht fiir 1873 (1875) ). 

Reichenbach. Die Embryonanlage und erste Entwickelung des 
Flusskrebses. Zeitschrift fiir Wiss. Zoologie. 1877. 


A. General. 

Milne Edwards. Oij. cit. 

Erichson. Uebersicht der Arten der Gattung Astacus. Wieg- 
mann's Archiv fiir Naturgeschichte, XII. 1846. 

Dana. Crustacea of the United States Exploring Expedition. 1852. 

De Saussure. Note carcinologique sur la Famille des Thalassinides 
et sur cclle 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'Ecrevisses 
{A'longicornis, A.;pallipes). Mem. Soc. Science Nat. Strasbourg. 
V. 1858. 

Heller. Crustaceen des siidlichen Europa. 1863, 

Kessler, Ein neuer russischer Flusskrebs, Astacus colcJiicus. 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 Histoiy, 

VI.; 1857-8. 
De Saussure. Memoire sur divers Crustacees nouveaux des Antilles 

et du Mexique. Mem. de la Societe de Physique de Geneve 

T. XIV., 1857. 
Von Martens. SUdbrasiliscbe Siiss- und Brackwasser Crustaceen 

(A.pilimanus, A. JrasiZiensw),Wiegmann's Archiv, XXXV., 1869. 

. Ueber Cubansche Crustaceen. Ibid. XXXVIII. 

Hagen. Monograph of the North American Astacldce. 1870, 


D. Madagascar. 

AuDOUiN and Milne Edwards. Sur une Esp^ce nouvelle du genre 
Ecrevisse (Astacus). Ecrevisse de Madagascar {A. Madagas- 
cariensis). Mem. du Museum d'Hist. naturelle, T. II. 1841. 

E. AiLstralia. 

Von Martens. On a new Species of Astacus. Annals & Mag. of 

Natural History, 1866. 
Heller. Reise der " Novara." Zcol. Theil. Bd. II. 1865. 

F. New Zealand. 

MiERS. Notes on the Genera Astacoides and ParaneiJhrops. Trans- 
actions of the New Zealand Institute, IX., 1876. 

Parancphrops. 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 
Astacidcs attach themselves to the Mother. Ann. & Mag. 
Natural History, 1876. 

G. Fossil Astacommpha. 

Oppel. PaljBontologische Mittheilungen, 1862. 

Bell. British Fossil Crustacea. Palaeontographical Society. 

P. Van Beneden. Sur la Decouverte d'un Homard fossile dans 

FArgile de Rupelmonde. Bulletin de I'Acad. Royale de Belgique. 

XXXIIL, 1872. 
Von der Marck und Schlijter. Neue Fische und Krebse von der 

Kreide von Westphalen. Palseontologica, XV. 1865. 
Cope, On three extinct Astaci from the freshwater tertiary of Idaho. 

Proceedings of the American Philosophical Society, XL, 1809-70. 



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 

developm3nt of, 214, 218 
Anthi'ajJalcemon, 341 
Anus, 29 

Apodeme, 99, 158, 175 
Appendage, 24, 143, 161, 173 

abdominal, 143 

cephalic, 170 

thoracic, 164 
Archenteron, 211 
Arctogseal province, 314 
Areola, 235 

Aristotle, referred to, 4 
Arteries, 71 

Arteries, development of, 224 

Arthrobranchia, 75 

Arthrophragm, 158 

Arthroj)oda, 279, 284 

Articulations, 95 

Asiatic Crayfishes, 304 

Astacina, 254 

Astacoides, 250, 313 

Astacomorpha, 338 

Astacopsis, 250, 264 

Astacus, division into sub-genera, 

Astacns angiilosiis, 302, 310 
colchicus, 302, 310 
dauricns, 304, 310 

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 



AsiacKS fluvxa tilis — 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, 
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, 

reproduction of lost limbs, 

reproduction, sexual, 39, 

128, 135, 350 
sexual characters, 7, 20, 

32, 145, 241 
somites and appendages, 

systematic description, 

use as food, 10, 289 
varieties, 289 
fontinalis, 290 
jajjonieus, 304 
lilamathensis^ 305 
leniuscnlns, 305 

Astacus Icjytodactylns, 299, 302 
303, 310, 320 

nigrescens, 244 

noUUs, 290, 295, 296, 299, 

oreganns, 305 

pachypus, 302, 310 

pallipes, 290 

2)olitus, 344 

saxatilis, 290 

SchrencUi, 304, 310 

torrentiuvi, 290, 294, 298, 310, 

tristis, 290 

Trowbridgii, 305 
Atya, AtyidcB, 331, 336 
Auditory organ, 116 

setae, 116 
Australian Crayfishes, 306 

province, 314 
Austrocolumbian province, 314 
Axius, 271 


Ball, R., 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 
BOLIVAK, Dr., 298 

Astacoides, 266 

Astacojms, 264 



Branchiae — conthmed 

Astacus, 25, 75, 265 

development of, 224 

Cancer, 276 

Ilomarus, 257 

Palamoii, 270 

Palinurns, 264 

PeticeiLS, 267 
Branchial chamber, 25 


Aatacoidcs, 266 

Astacojjsis, 264 

Astacm, 266 

Cancer, 277 

hypothetically complete, 2G8 

Palcemon, 270 

Palinurns, 265 

Penceus, 267 
Branchiohdella, 356 
Branchiostegite, 25 

development of, 217 
Braun, quoted, 352 
Brazilian Crayfishes, 206 


Caecum, 61 

Calcification of exoskeleton, 197 

Californian Crayfishes, 243 

Caniharns, 44, 247, 310, 312 

Cancer, 272, 283 

Carapace, 19 

development of, 214 
Caebonniee, M,, quoted, 297, 349, 

Cardia, 52 
CaridiTia, 330 
Carpopodite, 165 
Cell, 66, 199 
Cell-aggregate, 190, 193 

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 
Chantkan, M., quoted, 348, 350, 

Chel£e, 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, 2T2i 
Crayfish, origin of name. 12 

common, see Astacus fl nriatUis 
Crayfishes, Amurland, 304 

Asiatic, 304 

Australian, 306 

Brazilian, 306 

Californian, 243 

Chilian, 308 

definition of, 254 

Eastern North American, 247, 

European, 288, 297 

evolution of, 331 



Crawfishes, Figian, 306, 313 

Japanese, 304, 313 

Mascarene, 308, 313 

northern and southern, com- 
pared, 252 

Novozelanian, 306, 313 

southern, 249 

Tasmanian, 306 

Western North American, 305, 
Crustacea, 271, 278 
Crystalline cones, 121 
Cuticle, 33, 50, 175, 192 
Cyelas, 356 


.Dactylopodite, 165 
Daplima, asexual reproduction of 

Darwin, C, referred to, 4 
De Haan, quoted, 313 
Development, 205 

abdomen, 213 

abdominal appendages, 217 

alimentary canal, 213, 222 

antennae, 214, 218 

antennules, 214, 218 

blood and blood vessels, 224 

branchiostegite, 217 

carapace, 214 

cephalic appendages, 217, 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 oi'gans, 225 

rostrum, 217 

thoracic appendages, 217, 219 
Digestion, 63 
Distribution, 46 

chronological, of crayfishes, 44, 
316, 339 

table of, 345 

geographical, of crayfishes, 44, 

causes of, 335 

results of study of, 308, 314 
DOEMEE, quoted, 356 
DuLK, quoted, 349 


Ear, 116 

development of, 225 
Ecdysis, 32, 350 
Ecrevisse h, pieds blancs, 289, 297 

a pieds rouges, 289, 2i;7 
Ectoderm, 141 
Ectostracum, 194 
Edelkrebs, 290 
Endoderm, 141 
Endophragmal system, 1 57 
Endopleurite, 158 
Endopodite, 145 
Endoskeleton, 17 
Endosternite, 158 
Endostracum, 194 
EngcBus, 250, 306 
Enoplocytia, 342 
Epiblast, 211 
Epidermis, 140 
Epimeron, 143 
Epiostracum, 192 
Epipodite, 167 



Epistoma, 155 

Epithelium, 140, 177 

Equus excelsns, occurring with 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, 21 4 


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 maxilli pedes 
Forceps, 22 
Foregut, 61 

development of, 213, 222 
Fossil crayfishes, 316 
FosTEB, Dr. M., referred to, 110 
France, consumption of crayfish 

in, 10 
Function, 22 


(ralaxidcB, 315 
Gammariig, 323 
Ganglion, 103, 105 
Garglionic corpuscle, 87, 103 

Gastric mill, 53 
Gastrolith, 29, 347 

chemical composition, 349 
Gastrula, 211 
Gay, quoted, 356 
Genus, 249 

Geographical distribution, see Dis- 
Gerbe, M., quoted, 350 
Germinal disc, 209 

layer, 206 

spot, 133 

vesicle, 133 
Gerstfeldt, Dr., quoted, 200 
Gills, see Branchiae 
GiEARD, quoted, 356 
Gorup-Besanez, quoted, 353 
Green-gland, 83, 353 

development of, 224 
Geobben, Dr., quoted, 354 
Growth of crayfish, 31, 349 
Guanin, 82, 358 
Gullet, see CEsophagus 
GuNTHER, Dr., quoted, 315 


Hagen, Dr., quoted, 305, 312 
Haplochitonidce , 315 
Harvey, quoted, 5 
Head, see Cephalon 
Hearing, organ of, 116 
Heai-t, 27, 71 

development of, 224 
Heller, Dr., quoted, 298, 330 
Hepatic duct, see Bile duct 
Hind gut, 61 

development of, 214, 223 
Histology, 175 
Histriohdella, 356 
Jlomaridce, 263 



Homarina, 261 
Ilomarus, 13, 42, 257, 332 
Homology, homologous, liomolo- 

gue, 148 
Hoploparia, 342 
Hypoblast, 211 


Jdotliea, 323, 334 
Impregnation, 135, 350 
Integument, 50 
Interseptal zone, 183 
Intestine, 29, 61 
Ischiopodite, 165 


Japanese Crayfishes, 313, 314 

Jaws, 23 

Johnston, J., quo'.cd, 42 


Kesslee, quoted, 298, 304 
Kidney, see Green gland 
Klunzinger, referred to, 330 


Labrum, 51 

development of, 218 
Lamarck, referred to. 4 
Lereboullet, quoted, 353 
Legs, ambulatory, 168 
Lemoine, referred to, 353 
Leydig, referred to, 115, 363 
Liver, 30, 64 

development of, 223 

nature of secretion, 352 
Lobster, common, see Homarus 

Norway, see Neplirops 

Lobster, Rock, see Palinums 
LovfeN, referred to, 327 


Machine, living, 128 

M'iNTOSH, Dr. W. C, quoted, 288 

Mandible, 23, 51, 170 

development of, 214 
Martens, Von, 306 
Mastodon mirijicus, occurring with 

fossil crayfishes, 316 
Maxillae, 23, 170 
Maxillipedes, 23, 164 
Medullary groove, 213 
Megalopa stage of development, 

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, 2S9 
Ilollusca, 284 
Morphology, 46, 138 

comparative, 230 
Mortality of crayfishes, 123 
Morula, 206 
Mosaic vision, 122, 364 
Motor plates, 189 
Mouth, 51 
MtJLLER, Johannes, referred to, 

Muscle, 57, 90, 175, 181 

development of, 224 

histology of, 90, 181 
Muscles of abdomen. 99 



Muscles of chela, 93 

of stomach, 57 
Myosin, 186 
Myotome, 174: 
My sis, 281, 323 

relicta, origin of, from M. 
ocnlata, 327 
Mysis stage of development, 280 

Natural History, 3 

Philosophy, 3 
Nauplius stage of development, 

215, 28a 
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 Astacus noliUs 
Nomenclature, binomial, 13, 15 
Norway lobster, see Neplirops 
Novozelanian province, 314 
Nucleated cell, 199 
Nucleolus, 187 
Nucleus, 177, 200 

changes of, in cell-division, 200 


CEsophagus, 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 


Paltearctic province, 314 
Palmmon, 268, 328 
PaUmiridce 203 
Palinvrns, 261, 264 
Palp, 171 

Paraneplirops, 250, 306, 313 
Paraphragm, 158 
Parasites of crayfish, 356 
Parastacidcs, 252, 256, 306, 313 
Parastacus, 250, 306 
Pemjphix, 341 
Penaus, 267, 280 
Pericardium, 69 
Perivisceral cavity, 50 
Phyllobranchia, 271 
Physiology, 46 
Pleurobranchia, 79 
Pleuron, 96, 143 
Podobranchia, 75, 165 
Podojflithalmia, 279 
Pore-canals, 195 
Post-orbital ridge, 233 

spine, 232 
Potamohiidce, 252, 256 
Prawn, see Palcevwn 
Prehension of food, 49 
Procephalic lobes, 160 

development of, 213 
Propodite, 165 
Protopodite, 143 
Prototroctcs, 315 



Protozoa^ 285 
Pseudastacus, 343 
Pylorus, 52 


Race, 292 

Rathke, quoted, 356 
R^AUMUE, quoted, 33 
Keflex 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 Branchiae 
Retropinna, 315 
Robin, quoted, 352 
Rock lobster, see Palinui'us 


RoNDOLETius, referred to, 4 
Rostrum, 157 

development of, 217 


Salivary glands, 352 

Salmonidce, parallel between their 

distribution, and that of Asta- 

cidcB, 315 
Sarcolemma, 90, 182 
Saes, G. 0., referred to, 327 
Sartorius von Walteehausen, 

quoted, 322 
Scaphognathite, 80, 170 
Schizopod stage of development, 



Schmidt, O., quoted, 354 

Science, physical, 3 
Science and common sense, 1 
Segmentation, 174 
Self-causation, 112 
Sensory organs, 113 
Septal line, 183 

zone, 183 
Setse, 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 Astacns torrentium 
Sternum, 96, 143 
Stomach, 29, 51 

Stone-crayfish, see Astacns torren- 
Striated spindle, 121 
Swimmeret, 20 


Taste, organ of, 115 
Teleology, 47, 137 
Tendon, 92, 175 



Tergum, 96, 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 
Trevieanus, referred to, 4 
Tribe, 252 

Trichobranchite, 263 
Troglocaris, 337 


Valves of heart, 73 
of stomach, 59 
Van Helmont, quoted, 45 
Variety, 290, 292 
Vas deferens, 130 
Vent, see Anus 
Vcrtcbrata, 284 

Vertebrata, eye of, 122, 125 
Visual pyramid, 121 

rod, 12f 
Vitelline membrane, 133 
Vitellus, 133 
Voluntary action, 112 
Von der Marck, 317 

Ward, J., referred to, 354 
Wassiliew, quoted, 353 
Whirlpool of life, 84 
Will, quoted, 353 
Wood-Mason, quoted, 44 


Yelk, 133 
Yelk-division, 205 
Young of Astacns, newly hatched, 
characters of, 219 


Zoaea stage of development, 280 



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