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THE CRAYFISH, AN INTRODUCTION TO THE STUDY OF ZOOLOGY
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The International Scientific Series.
Vol. XXVIII.
[Illustration: THE COMMON CRAYFISH.
(_Astacus fluviatilis_, Male.)
_Frontispiece._]
THE CRAYFISH
An Introduction to the Study of Zoology
by
T. H. HUXLEY, F.R.S.
With Eighty-two Illustrations
Sixth Edition
London
Kegan Paul, Trench,Trübner & Co., Lt^d.
1896
“Διὸ δεῖ μὴ δυσχεραίνειν παιδικῶς τὴν περὶ τῶν ἀτιμοτέρων ζῴων
ἐπίσκεψιν· ἐν πᾶσι γὰρ τοῖς φυσικοῖς ἔνεστί τι θαυμαστὸν.”—ARISTOTLE,
_De Partibus_, I. 5.
* * * * *
“Qui enim Autorum verba legentes, rerum ipsarum imagines (eorum
verbis comprehensa) sensibus propriis non abstrahunt, hi non veras
Ideas, sed falsa Idola et phantasmata inania mente concipiunt
. . . . . . .
“Insusurro itaque in aurem tibi (amice Lector!) ut quæcunque à
nobis in hisce . . . . exercitationibus tractabuntur, ad exactam
experientiæ trutinam pensites: fidemque iis non aliter adhibeas, nisi
quatenus eadem indubitato sensuum testimonio firmissime stabiliri
deprehenderis.”—HARVEY. _Exercitationes de Generatione. Præfatio._
* * * * *
“La seule et vraie Science est la connaissance des faits: l’esprit
ne peut pas y suppléer et les faits sont dans les sciences ce qu’est
l’expérience dans la vie civile.”
“Le seul et le vrai moyen d’avancer la science est de travailler à
la description et à l’histoire des differentes choses qui en font
l’objet.”—BUFFON. _Discours de la manière d’étudier et de traiter
l’Histoire Naturelle._
* * * * *
“Ebenso hat mich auch die genäuere Untersuchung unsers Krebses
gelehret, dass, so gemein und geringschätzig solcher auch den
meisten zu seyn scheinet, sich an selbigem doch so viel Wunderbares
findet, dass es auch den grossten Naturforscher schwer fallen
sollte solches ailes deutlich zu beschreiben.”—ROESEL V. ROSENHOF.
_Insecten Belustigungen._—“_Der Flusskrebs hiesiges Landes mit seinen
merkwurdigen Eigenschaften._”
PREFACE.
In writing this book about Crayfishes it has not been my intention
to compose a zoological monograph on that group of animals. Such a
work, to be worthy of the name, would require the devotion of years
of patient study to a mass of materials collected from many parts of
the world. Nor has it been my ambition to write a treatise upon our
English crayfish, which should in any way provoke comparison with the
memorable labours of Lyonet, Bojanus, or Strauss Durckheim, upon the
willow caterpillar, the tortoise, and the cockchafer. What I have
had in view is a much humbler, though perhaps, in the present state
of science, not less useful 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 this reason that I have termed the book an “Introduction
to Zoology.” For, whoever will follow its pages, crayfish in hand,
and will try to verify for himself the statements which it contains,
will find himself brought face to face with all the great zoological
questions which excite so lively an interest at the present day;
he will understand the method by which alone we can hope to attain
to satisfactory answers of these questions; and, finally, he will
appreciate the justice of Diderot’s remark, “Il faut être profond
dans l’art ou dans la science pour en bien posséder les éléments.”
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 crayfish,”
well says Roesel von Rosenhof, “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 importance;
and, so far as these are concerned, I venture 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 mistake is
almost unavoidable, but that new lights come with new methods of
investigation; and that better modes of statement follow upon the
improvement of our general views introduced by the gradual widening
of our knowledge.
I sincerely hope that such amplifications and rectifications may
speedily abound; and that this sketch may be the means of directing
the attention of observers in all parts of the world to the
crayfishes. Combined efforts will soon furnish the answers to many
questions which a single worker can merely state; and, by completing
the history of one group of animals, secure the foundation of the
whole of biological science.
In the Appendix, I have added a few notes respecting points of
detail with which I thought it unnecessary to burden the text; and,
under the head of Bibliography, I have given some references to the
literature of the subject which may be useful to those who wish to
follow it out more fully.
I am indebted to Mr. T. J. Parker, demonstrator of my biological
class, for several anatomical drawings; 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,
Rock Lobster, and Norway Lobster.
T. H. H.
LONDON,
_November, 1879_.
CONTENTS.
PREFACE • v
LIST OF WOODCUTS • xi
CHAPTER I.
The Natural History of the Common Crayfish • 1
CHAPTER II.
The Physiology of the Common Crayfish. The Mechanism by which the
Parts of the Living Engine are supplied with the materials necessary
for their maintenance and growth • 46
CHAPTER III.
The Physiology of the Common Crayfish. The Mechanism by which
the Living Organism adjusts itself to surrounding conditions and
reproduces itself • 87
CHAPTER IV.
The Morphology of the Common Crayfish. The structure and the
development of the individual • 137
CHAPTER V.
The Comparative Morphology of the Crayfish. The structure and the
development of the Crayfish compared with those of other living
beings • 227
CHAPTER VI.
The Distribution and the Ætiology of the Crayfishes • 288
NOTES • 347
BIBLIOGRAPHY • 357
INDEX • 363
LIST OF WOODCUTS.
_Frontispiece._ THE COMMON CRAYFISH, _Astacus fluviatilis_, (MALE)
1. _Astacus fluviatilis._ SIDE VIEW OF THE MALE • 6
2. — — DORSAL VIEWS OF MALE AND FEMALE • 18
3. — — VENTRAL VIEWS OF MALE AND FEMALE • 21
4. — — THE GILLS • 26
5. — — DISSECTION FROM THE DORSAL SIDE (MALE) • 28
6. — — LONGITUDINAL VERTICAL SECTION OF THE ALIMENTARY CANAL • 29
7. — — A GASTROLITH OR “CRAB’S EYE” • 30
8. — — ATTACHMENT OF YOUNG TO SWIMMERET OF MOTHER • 41
9. — — STRUCTURE OF THE STOMACH • 53
10. — — LONGITUDINAL SECTION OF THE STOMACH • 56
11. — — ROOF OF THE STOMACH, FROM WITHIN • 60
12. — — DISSECTION FROM THE SIDE (MALE) • 62
13. — — ALIMENTARY CANAL FROM ABOVE • 65
14. — — BLOOD CORPUSCLES • 68
15. — — TRANSVERSE SECTION OF THORAX • 70
16. — — THE HEART • 72
17. — — STRUCTURE OF THE GILLS • 76
18. — — THE GREEN GLAND • 83
19. — — MUSCULAR TISSUE • 91
20. — — MUSCLES OF CHELA • 93
21. — — ARTICULATION OF ABDOMINAL SOMITES • 97
22. — — MUSCULAR SYSTEM • 100
23. — — NERVE FIBRES • 102
24. — — NERVE GANGLIA • 103
25. — — NERVOUS SYSTEM • 104
26. — — OLFACTORY AND AUDITORY ORGANS • 114
27. — — AUDITORY SAC • 117
28. — — STRUCTURE OF EYE • 119
29. — — DIAGRAM OF EYE • 123
30. — — FEMALE REPRODUCTIVE ORGANS • 129
31. — — MALE REPRODUCTIVE ORGANS • 130
32. — — STRUCTURE OF OVARY • 131
33. — — STRUCTURE OF TESTIS • 132
34. — — SPERMATOZOA • 134
35. — — THE LAST THORACIC STERNUM IN THE MALE AND FEMALE • 136
36. — — TRANSVERSE SECTION OF ABDOMEN • 142
37. — — ABDOMINAL APPENDAGES • 144
38. — — CONNECTION BETWEEN THORAX AND ABDOMEN • 151
39. — — CEPHALOTHORACIC STERNA AND ENDOPHRAGMAL SYSTEM • 153
40. — — OPHTHALMIC AND ANTENNULARY SOMITES • 156
41. — — THE ROSTRUM • 157
42. — — A SEGMENT OF THE ENDOPHRAGMAL SYSTEM • 159
43. — — LONGITUDINAL SECTION OF CEPHALOTHORAX • 162
44. — — THE THIRD MAXILLIPEDE • 164
45. — — THE FIRST AND SECOND MAXILLIPEDES • 166
46. — — THE SECOND AMBULATORY LEG • 169
47. — — THE MANDIBLE AND MAXILLÆ • 171
48. — — THE EYE-STALK, ANTENNULE, AND ANTENNA • 172
49. — — BLOOD CORPUSCLES • 176
50. — — EPITHELIUM • 178
51. — — CONNECTIVE TISSUE • 179
52. — — MUSCULAR TISSUE • 181
53. — — MUSCULAR TISSUE • 182
54. — — NERVE GANGLIA • 188
55. — — NERVE FIBRES • 189
56. — — CUTICULAR TISSUE • 191
57. — — SECTIONS OF EMBRYOS • 208
58. — — EARLIER STAGES OF DEVELOPMENT • 210
59. — — LATER STAGES OF DEVELOPMENT • 216
60. — — NEWLY HATCHED YOUNG • 220
61. _Astacus torrentium_, _Astacus nobilis_, and _Astacus
nigrescens._ COMPARATIVE VIEWS OF THE CARAPACE, THIRD ABDOMINAL
SOMITE, AND TELSON • 233
62. — —, — —, and — — COMPARATIVE VIEWS OF THE FIRST AND SECOND
ABDOMINAL APPENDAGES OF THE MALE • 245
63. _Cambarus Clarkii_ • 248
64. _Parastacus brasiliensis_ • 250
65. _Astacoides madagascarensis_ • 251
66. DIAGRAM OF THE MORPHOLOGICAL RELATIONS OF THE _Astacidæ_ • 253
67. _Homarus vulgaris_ • 258
68. _Parastacus_, _Nephrops_, and _Palæmon._ PODOBRANCHIÆ • 259
69. _Nephrops norvegicus_ • 260
70. _Palinurus vulgaris_ • 262
71. _Palæmon jamaicensis_ • 269
72. _Cancer pagurus_ • 273
73. _Penæus_ • 281
74. _Cancer pagurus._ DEVELOPMENT • 282
75. _Astacus leptodactylis_ • 301
76. _Australian Crayfish_ • 307
77. MAP OF THE DISTRIBUTION OF CRAYFISHES • 309
78. _Cambarus._ WALKING LEG • 312
79. _Palæmon jamaicensis_ • 329
80. _Pseudastacus pustulosus_ and _Eryma modestiformis_ • 340
81. _Hoploparia longimana_ • 342
THE CRAYFISH:
AN INTRODUCTION TO THE STUDY OF ZOOLOGY.
CHAPTER I.
THE NATURAL HISTORY OF THE COMMON CRAYFISH
(_Astacus fluviatilis._)
Many persons seem to believe that what is termed Science is of a
widely different nature from ordinary knowledge, and that the methods
by which scientific truths are ascertained involve mental operations
of a recondite and mysterious nature, comprehensible only by the
initiated, and as distinct in their character as in their subject
matter, from the processes by which we discriminate between fact and
fancy in ordinary life.
But any one who looks into the matter attentively will soon perceive
that there is no solid foundation for the belief that the realm of
science is thus shut off from that of common sense; or that the
mode of investigation which yields such wonderful results to the
scientific investigator, is different in kind from that which is
employed {2} for the commonest purposes of everyday existence. Common
sense is science exactly in so far as it fulfils the ideal of common
sense; that is, sees facts as they are, or, at any rate, without the
distortion of prejudice, and reasons from them in accordance with the
dictates of sound judgment. And science is simply common sense at
its best; that is, rigidly accurate in observation, and merciless to
fallacy in logic.
Whoso will question the validity of the conclusions of sound science,
must be prepared to carry his scepticism a long way; for it may be
safely affirmed, that there is hardly any of those decisions of
common sense on which men stake their all in practical life, which
can justify itself so thoroughly on common sense principles, as the
broad truths of science can be justified.
The conclusion drawn from due consideration of the nature of the case
is verified by historical inquiry; and the historian of every science
traces back its roots to the primary stock of common information
possessed by all mankind.
In its earliest development knowledge is self-sown. Impressions force
themselves upon men’s senses whether they will or not, and often
against their will. The amount of interest which these impressions
awaken is determined by the coarser pains and pleasures which they
carry in their train, or by mere curiosity; and reason deals with the
materials supplied to it as far as that interest carries it, and no
farther. Such common {3} knowledge is rather brought than sought;
and such ratiocination is little more than the working of a blind
intellectual instinct.
It is only when the mind passes beyond this condition that it begins
to evolve science. When simple curiosity passes into the love of
knowledge as such, and the gratification of the æsthetic sense of
the beauty of completeness and accuracy seems more desirable than
the easy indolence of ignorance; when the finding out of the causes
of things becomes a source of joy, and he is counted happy who is
successful in the search; common knowledge of nature passes into what
our forefathers called Natural History, from whence there is but a
step to that which used to be termed Natural Philosophy, and now
passes by the name of Physical Science.
In this final stage of knowledge, the phenomena of nature are
regarded as one continuous series of causes and effects; and the
ultimate object of science is to trace out that series, from the
term which is nearest to us, to that which is at the furthest limit
accessible to our means of investigation.
The course of nature as it is, as it has been, and as it will be, is
the object of scientific inquiry; whatever lies beyond, above, or
below this, is outside science. But the philosopher need not despair
at the limitation of his field of labour: in relation to the human
mind Nature is boundless; and, though nowhere inaccessible, she is
everywhere unfathomable. {4}
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 understood 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 attempt
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.
{5}
My purpose, in the present work, is to exemplify the general truths
respecting 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,[1] 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.
[1] 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 gills and the abdominal appendages present differences;
and the last thoracic somite is united with the rest in the
lobster. (_See_ Chap. V.)
* * * * *
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 very 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 body, and sometimes with
red on the limbs. In rare cases, their {6} general hue may be red or
blue. These are “crayfishes,” and they cannot possibly be mistaken
for any other inhabitants of our fresh waters.
[Illustration: FIG. 1.—_Astacus fluviatilis._—Side view of a male
specimen (nat. size):—_bg_, branchiostegite; _cg_, cervical groove;
_r_, rostrum; _t_, telson.—1, eye-stalk; 2, antennule; 3, antenna;
9, external maxillipede; 10, forceps; 14, last ambulatory leg; 17,
third abdominal appendage; 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, they swim {7} backwards with rapid jerks,
propelled by the strokes of a broad, fan-shaped flipper, which
terminates the hinder end of the body (fig. 1, _t_, _20_). In front
of the 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 despised, and indicates
a good deal of disposable energy. A sort of shield covers the front
part of the body, and ends in a sharp projecting spine in the middle
line (_r_). On each side of this is an eye, mounted on a movable
stalk (_1_), which can be turned in any direction: behind the eyes
follow two pairs of feelers; in one of these, the feeler ends in two,
short, jointed filaments (_2_); while, in the other, it terminates
in a single, many-jointed filament, like a whip-lash, which is more
than half the length of the body (_3_). Sometimes turned backwards,
sometimes sweeping forwards, these long feelers continually 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
{8} 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,
crayfishes do not occur. They are intolerant of great heat and of
much sunshine; they are therefore most active towards the evening,
while they shelter themselves under the shade of stones and banks
during the day. It has been observed that they frequent those parts
of a river which run north and south, less than those which have an
easterly and westerly direction, inasmuch as the latter yield more
shade from the mid-day sun.
During the depth of winter, crayfishes are rarely to be seen about
in a stream; but they may be found in abundance in its banks, in
natural crevices and in burrows which they dig for themselves. The
burrows may be from a few inches to more than a yard deep, and it has
been noticed that, if the waters are liable to freeze, the burrows
are deeper and further from the surface than otherwise. Where the
soil, through {9} which a stream haunted by crayfishes runs, is soft
and peaty, the crayfishes work their way into it in all directions,
and thousands of them, of all sizes, may be dug out, even at a
considerable distance from the banks.
It does not appear that crayfishes fall into a state of torpor in
the winter, and thus “hybernate” in the strict sense of the word.
At any rate, so long as the weather is open, the crayfish lies at
the mouth of his burrow, barring the entrance with his great claws,
and with protruded feelers keeps careful watch on the passers-by.
Larvæ 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
anticipated meal.
In fact, few things in the way of food are amiss to the crayfish;
living or dead, fresh or carrion, animal or vegetable, it is all
one. Calcareous plants, such as the stoneworts (_Chara_), are highly
acceptable; so are any kinds of succulent roots, such as carrots;
and it is said that crayfish sometimes make short excursions inland,
in search of vegetable food. Snails are devoured, shells and all;
the cast coats of other crayfish are turned to account as supplies
of needful calcareous matter; and the unprotected or weakly member
of the family is {10} not spared. Crayfishes, in fact, are guilty
of cannibalism in its worst form; and a French observer pathetically
remarks, that, under certain circumstances, the males “_méconnaissent
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 beneath the tail, and look like
a mass of minute berries (fig. 3, B). In May or June, these eggs are
hatched, and give rise to minute young, which are sometimes to be
found attached beneath the tail of the mother, under whose protection
they spend the first few days of their existence.
In this country, we do not set much store upon crayfishes 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 {11}
demand for these delicacies; and hence, not only are large quantities
imported from Germany, and elsewhere, but the artificial cultivation
of crayfish has been successfully attempted on a considerable scale.
Crayfishes are caught in various ways; sometimes the fisherman
simply wades in the water and drags them out of their burrows; more
commonly, hoop-nets baited with frogs are let down into the water
and rapidly drawn up, when there is reason to think that crayfish
have been attracted to the bait; or fires are lighted on the banks
at night, and the crayfish, which are attracted, like moths, to the
unwonted illumination, are scooped out with the hand or with nets.
* * * * *
Thus far, our information respecting the crayfish is such as would be
forced upon anyone who dealt in crayfishes, or lived in a district in
which they were commonly used for food. It is common knowledge. Let
us now try to push our acquaintance with what is to be learned about
the animal a little further, so as to be able to give an account of
its Natural History, such as might have been furnished by Buffon if
he had dealt with the subject.
There is an inquiry which does not strictly lie within the province
of physical science, and yet suggests itself naturally enough at the
outset of a natural history.
The animal we are considering has two names, one common, _Crayfish_,
the other technical, _Astacus fluviatilis_. How has it come by
these two names, and why, {12} having a common English name for it
already, should naturalists call it by another appellation derived
from a foreign tongue?
The origin of the common name, “crayfish,” involves some curious
questions of etymology, and indeed, of history. It might readily
be supposed that the word “cray” had a meaning of its own, and
qualified the substantive “fish”—as “jelly” and “cod” in “jellyfish”
and “codfish.” But this certainly is not the case. The old English
method of writing the word was “crevis” or “crevice,” and the “cray”
is simply a phonetic spelling of the syllable “cre,” in which the
“e” was formerly pronounced as all the world, except ourselves, now
pronounce that vowel. While “fish” is the “vis” insensibly modified
to suit our knowledge of the thing as an aquatic animal.
Now “crevis” is clearly one of two things. Either it is a
modification of the French name “écrevisse,” 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 “crayfish” 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 {13} word, it
may have come to us straight from the Angle and Saxon contingent of
our mixed ancestry.
As to the origin of the technical name; ἀστακός, _astakos_, was the
name by which the Greeks knew the lobster; and it has been handed
down to us in the works of Aristotle, who does not seem to have taken
any special notice of the crayfish. At the revival of learning, the
early naturalists noted the close general similarity between the
lobster and the crayfish; but, as the latter lives in fresh water,
while the former is a marine animal, they called the crayfish, in
their Latin, _Astacus fluviatilis_, or the “river-lobster,” by way
of distinction; and this nomenclature was retained 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 latinising 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 fluviatilis_, that of the lobster is
_Homarus vulgaris_. And as this nomenclature is generally received,
it is desirable that it should not be altered; though it is attended
by the inconvenience, that _Astacus_, as we now employ the name,
does not {14} denote that which the Greeks, ancient and modern,
signify, by its original, _astakos_; and does signify something quite
different.
Finally, as to why it is needful to have two names for the same
thing, one vernacular, and one technical. Many people imagine that
scientific terminology is a needless burden imposed upon the novice,
and ask us why we cannot be content with plain English. In reply, I
would suggest to such an objector to open a conversation about his
own business with a carpenter, or an engineer, or, still better,
with a sailor, and try how far plain English will go. The interview
will not have lasted long before he will find himself lost in a maze
of unintelligible technicalities. Every calling has its technical
terminology; 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 invented. The
practice of sensible people shows the advantage of the latter course;
and here, as elsewhere, science has simply followed and improved upon
common sense. {15}
Moreover, while English, French, German, and Italian artisans are
under no particular necessity to discuss the processes and results
of their business with one another, science is cosmopolitan, and the
difficulties of the study of Zoology would be prodigiously increased,
if Zoologists of different nationalities used different technical
terms for the same thing. They need a universal language; and it
has been found convenient that the language shall be the Latin in
form, and Latin or Greek in origin. What in English is Crayfish,
is _Écrevisse_ in French; _Flusskrebs_, in German; _Cammaro_, or
_Gambaro_, or _Gammarello_, in Italian: but the Zoologist of each
nationality knows that, in the scientific works of all the rest,
he shall find what he wants to read under the head of _Astacus
fluviatilis_.
But granting the expediency of a technical name for the Crayfish,
why should that name be double? The reply is still, practical
convenience. If there are ten children of one family, we do not
call them all Smith, because such a procedure would not help us to
distinguish one from the other; nor do we call them simply John,
James, Peter, William, and so on, for that would not help us to
identify them as of one family. So we give them all two names,
one indicating their close relation, and the other their separate
individuality—as John Smith, James Smith, Peter Smith, William Smith,
&c. The same thing is done in Zoology; only, in accordance with the
genius of the Latin language, {16} we put the Christian name, so to
speak, after the surname.
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 substantive and adjective terms of this binomial
nomenclature, and the sooner they are used as proper names, the
better. Very good reasons for using a term may exist when it is first
invented, which lose their validity with the progress of knowledge.
Thus _Astacus fluviatilis_ was a significant name so long as we knew
of only one kind of crayfish; but now that we are acquainted with
a number of kinds, all of which inhabit rivers, it is meaningless.
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 {17} care to go far beyond the
surface of things, would find to notice in the animal itself.
Probably the most conspicuous peculiarity of the crayfish, to any one
who is familiar only with the higher animals, is the fact that the
hard parts of the body are outside and the soft parts inside; whereas
in ourselves, and in the ordinary domestic animals, the hard parts,
or bones, which constitute the skeleton, are inside, and the soft
parts clothe them. Hence, while our hard framework is said to be an
_endoskeleton_, or internal skeleton; that of the crayfish is termed
an _exoskeleton_, or external skeleton. It is from the circumstance
that the body of the crayfishes is enveloped in this hard crust, that
the name of _Crustacea_ is applied to them, along with the crabs,
shrimps, and other such animals. Insects, spiders, and centipedes
have also a hard exoskeleton, but it is usually not so hard and thick
as in the _Crustacea_.
If a piece of the crayfish’s skeleton is placed in strong vinegar,
abundant bubbles of carbonic acid gas are given off from it, and
it rapidly becomes converted into a soft laminated membrane, while
the solution will be found to contain lime. In fact the exoskeleton
is composed of a peculiar animal matter, so much impregnated with
carbonate and phosphate of lime that it becomes dense and hard.
[Illustration: FIG. 2.—_Astacus fluviatilis._—Dorsal or tergal views
(nat. size). A, male; B, female:—_bcg_, branchio-cardiac groove,
which marks the boundary between the pericardial and the branchial
cavities; _cg_, cervical groove; these letters are placed on the
carapace; _r_, rostrum; _t, t′_, the two divisions of the telson;
_1_, eye-stalks; _2_, antennules; _3_, antennæ; _20_, lateral lobes
of tail-fin; XV–XX, somites of the abdomen.]
It will be observed that the body of the crayfish is naturally marked
out into several distinct regions. There {19} is a firm and solid
front part, covered by a large continuous shield, which is called the
_carapace_; and a jointed hind part, commonly termed the tail (fig.
2). From the perception of a partially real, and partially fanciful,
analogy with the regions into which the body is divided in the higher
animals, the fore part is termed the _cephalo-thorax_, or head
(_cephalon_) and chest (_thorax_) combined, while the hinder part
receives the name of _abdomen_.
Now the exoskeleton is not of the same constitution throughout these
regions. The abdomen, for example, is composed of six complete hard
rings (fig. 2, XV–XX), and a terminal flap, on the under side of
which the vent (fig. 3, _a_) is situated, and which is called the
_telson_ (fig. 2, _t, t′_). All these are freely moveable upon one
another, inasmuch as the exoskeleton which connects them is not
calcified, but is, for the most part, soft and flexible, like the
hard exoskeleton when the lime salts have been removed by acid.
The mechanism of the joints will have to be attentively considered
by-and-by; it is sufficient, at present, to remark that, wherever a
joint, exists, it is produced in the same fashion, by the exoskeleton
remaining soft in certain regions of the jointed part.
The carapace is not jointed; but a transverse groove is observed
about the middle of it, the ends of which run down on the sides
and then turn forwards (figs. 1 and 2, _cg_). This is called the
_cervical groove_, and it marks off {20} the region of the head, in
front, from that of the thorax behind.
The thorax seems at first not to be jointed at all; but if its
under, or what is better called its _sternal_, surface is examined
carefully, it will be found to be divided into as many transverse
bands, or segments, as there are pairs of legs (fig. 3); and,
moreover, the hindermost of these segments is not firmly united with
the rest, but can be moved backwards and forwards through a small
space (fig. 3, B; xiv).
Attached to the sternal side of every ring of the abdomen of the
female there is a pair of limbs, called _swimmerets_. In the five
anterior rings, these are small and slender (fig. 3, B; _15, 19_);
but those of the sixth ring are very large, and each ends in two
broad plates (_20_). These two plates on each side, with the telson
in the middle, constitute the flapper of the crayfish, by the aid
of which it executes its retrograde swimming movements. The small
swimmerets move together with a regular swing, like paddles, and
probably aid in propelling the animal forwards. In the breeding
female (B), the eggs are attached to them; while, in the male, the
two anterior pairs (A; _15, 16_) are converted into the peculiar
styles which distinguish that sex.
[Illustration: FIG. 3.—_Astacus fluviatilis._—Ventral or sternal
views (nat. size). A, male; B, female:—_a_, vent; _gg_, opening
of the green gland; _lb_, labrum; _mt_, metastoma or lower lip;
_od_, opening of the oviduct; _vd_, that of the vas deferens. _1_,
eye-stalk; _2_, antennule; _3_, antenna; _4_, mandible; _8_, second
maxillipede; _9_, third or external maxillipede; _10_, forceps; _11_,
first leg; _14_, fourth leg; _15, 16, 19, 20_, first, second, fifth,
and sixth abdominal appendages; X., XI., 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 the exception of its basal joint) and the
5th to the 14th appendages on the animal’s right are removed; the
eggs also are shown attached to the swimmerets of the left side of
the body.]
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 {22} be _chelate_. The two hindermost pairs, on
the other hand, end in simple claws.
In front of these legs, come the great prehensile limbs (_10_), which
are chelate, like those which immediately follow them, but vastly
larger. They often receive the special name of _chelæ_; 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 abdominal limbs; walks by means of the four
hinder pairs of thoracic limbs; lays hold of anything to fix itself,
or to assist in climbing, by the two chelate anterior pairs of these
limbs, which are also employed in tearing the food seized by the
forceps and conveying it to the mouth; while it seizes its prey and
defends itself with the forceps. The part which each of these limbs
plays is termed its _function_ and it is said to be the _organ_ of
that function; so that all these limbs may be said to be organs of
the functions of locomotion, of offence and defence.
In front of the forceps, there is a pair of limbs which have a
different character, and take a different direction from any of the
foregoing (_9_). These limbs, in fact, are turned directly forwards,
parallel with one another, and with the middle line of the body. They
are divided into a number of joints, of which one of those near the
base {23} is longer than the rest, and strongly toothed along the
inner edge, or that which is turned towards its fellow. It is obvious
that these two limbs are well adapted to crush and tear whatever
comes between them, and they are, in fact, _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 _maxillipedes_.
If the head of a stout pin is pushed between these external
maxillipedes, it will be found that it passes without any difficulty
into the interior of the body, through the mouth. In fact, the
mouth is relatively rather a large aperture; but it cannot be
seen without forcing aside, not only these external foot-jaws,
but a number of other limbs, which subserve the same function of
manducation, or chewing and crushing the food. We may pass by the
organs of manducation, for the present, with the remark that there
are altogether three pairs of maxillipedes, followed by two pairs of
somewhat differently formed _maxillæ_, 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 _antennæ_ (_3_); above, and in front of them,
follow the small feelers, or _antennules_ (_2_); and over them,
again, lie {24} the _eye stalks_ (_1_). The antennæ are organs of
touch; the antennules, in addition, contain the organs of hearing;
while, at the ends of the eyestalks, are the organs of vision.
Thus we see that the crayfish has a jointed and segmented body, the
rings of which it is composed being very obvious in the abdomen, but
more obscurely traceable elsewhere; that it has no fewer than twenty
pairs of what may be called by the general name of _appendages_; and
that these appendages are turned to different uses, or are organs of
different functions, in different parts of the body. The crayfish is
obviously a very complicated piece of living machinery. But we have
not yet come to the end of all the organs that may be discovered even
by cursory inspection. Every one who has eaten a boiled crayfish, or
a lobster, knows that the great shield, or carapace, is very easily
separated from the thorax and abdomen, the head and the limbs which
belong to that region coming away with the carapace. The reason of
this is not far to seek. The lower edges of that part of the carapace
which belongs to the thorax approach the bases of the legs pretty
closely, but a cleft-like space is left; and this cleft extends
forwards to the sides of the region of the mouth, and backwards and
upwards, between the hinder margin of the carapace and the sides of
the first ring of the abdomen, which are partly overlapped by, and
partly overlap, that margin. If the blade of a pair of scissors is
{25} carefully 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, _bg_), because it covers the gills or
_branchiæ_ (fig. 4), which are now exposed. They have the appearance
of a number of delicate plumes, which take a direction 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 branchiæ are the respiratory organs; and they perform
the same functions as the gills of a fish, to which they present some
similarity.
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 exoskeleton, 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 outwards, is continued into the inner wall or lining of the
branchiostegite, which is also very thin (_see_ fig. 15, p. 70).
[Illustration: FIG. 4.—_Astacus fluviatilis._—In A, the gills,
exposed by the removal of the branchiostegite, are seen in their
natural position; in B, the podobranchiæ (_see_ p. 75) are removed,
and the anterior set of arthrobranchiæ turned downwards (× 2):
_1_, eye-stalk; _2_, antennule; _3_, antenna; _4_, mandible; _6_,
scaphognathite; _7_, first maxillipede, in B the epipodite, to which
the line points, is partly removed; _8_, second maxillipede; _9_,
third maxillipede; _10_, forceps; _14_, fourth ambulatory leg; _15_,
first abdominal appendage; XV., first, and XVI., second abdominal
somite; _arb. 8_, _arb. 9_, _arb. 13_, the posterior arthrobranchiæ
of the second and third maxillipedes and of the third ambulatory
leg; _arb′. 9_, _arb′. 13_, the anterior arthrobranchiæ of the third
maxillipede and of the third ambulatory leg; _pbd. 8_, podobranchiæ
of the second maxillipede; _pbd. 13_, that of the third ambulatory
leg; _plb. 12_, _plb. 13_, the two rudimentary pleurobranchiæ; _plb.
14_, the functional pleurobranchia; _r_, rostrum.]
Thus the branchial chamber is altogether outside the body, 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 {27} 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
very well on land, at any rate among moist herbage, though whether
our common crayfishes do make such terrestrial excursions 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, however rough the process of
examination may be.
[Illustration: FIG. 5.—_Astacus fluviatilis._—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; _amm_, 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; _t_, testis; _vd_, vas deferens.]
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 {29} middle region of that part of the
carapace which lies behind the cervical groove; or, in other words,
in the dorsal region of the thorax. In front of it, and therefore in
the head, is a large rounded sac, the stomach (fig. 5, _cs_; fig. 6,
_cs_, _ps_), from which a very delicate intestine (figs. 5 and 6,
_hg_) passes straight back through the thorax and abdomen to the vent
(fig. 6, _a_).
[Illustration: FIG. 6.—_Astacus fluviatilis._—A longitudinal vertical
section of the alimentary canal, with the outline of the body (nat.
size):—_a_, vent; _ag_, anterior gastric muscle; _bd_, entrance
of left bile duct; _cg_, cervical groove; _cæ_, cæcum; _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; _lb_,
labrum; _lt_, lateral tooth of stomach; _m_, mouth; _mg_, mid-gut;
_mt_, median tooth; _œ_, œsophagus; _pc_, procephalic process; _pg_,
posterior gastric muscle; _ps_, pyloric portion of stomach; _r_,
annular ridge, marking the commencement of the hind-gut.]
In summer, there are commonly to be found at the sides of the stomach
two lenticular calcareous masses, which are known as “crabs’-eyes,”
or _gastroliths_, and were, in old times, valued in medicine as
sovereign remedies for all sorts of disorders. These bodies (fig. 7)
are smooth and flattened, or concave, on the side which is turned
towards {30} the cavity of the stomach; while the opposite side,
being convex and rough with irregular prominences, is something like
a “brain-stone” coral.
[Illustration: FIG. 7.—_Astacus fluviatilis._—A gastrolith; A, from
above; B, from below; C, from one side (all × 5); D, in vertical
section (× 20).]
Moreover, when the stomach is laid open, three large reddish teeth
are seen to project conspicuously into its interior (fig. 6, _lt_,
_mt_); so that, in addition to its six pairs of jaws, the crayfish
has a supplementary crushing mill in its stomach. On each side of
the stomach, there is a soft yellow or brown mass, commonly known
as the {31} liver (fig. 5, _Lr_); and, in the breeding season, the
ovaries of the females, or organs in which the eggs are formed, are
very conspicuous from the dark-coloured eggs which they contain,
and which, like the exoskeleton, turn red when they are boiled.
The corresponding part in a cooked lobster goes by the name of the
“coral.”
Beside these internal structures, the most noticeable are the large
masses of flesh, or muscle, in the thorax and abdomen, and in the
pincers; which, instead of being red, as in most of the higher
animals, is white. It will further be observed that the blood, which
flows readily when a crayfish is wounded, is a clear fluid, and is
either almost colourless, or of a very pale reddish or neutral tint.
Hence the older Naturalists thought that the crayfish was devoid of
blood, and had merely a sort of ichor in place of it. But the fluid
in question is true blood; and if it is received into a vessel, it
soon forms a soft, but firm, gelatinous clot.
* * * * *
The crayfish grows rapidly in youth, but enlarges more and more
slowly as age advances. The young animal which has just left the egg
is of a greyish colour, and about one quarter of an inch long. By
the end of the year, it may have reached nearly an inch and a half
in length. Crayfishes of a year old are, on an average, two inches
long; at two years, two inches and four-fifths; at three years, three
inches and a half; at four years, four inches and a half nearly;
and at five years, five inches. They {32} go on growing till, in
exceptional cases, they may attain between seven inches and eight
inches in length; but at what degree of longevity this unusual
dimension is reached is uncertain. It seems probable, however, that
the life of these animals may be prolonged to as much as fifteen or
twenty years. They appear to reach maturity, so far as the power of
reproduction is concerned, in their fifth or, more usually, their
sixth year. However, I have seen a female, with eggs attached under
the abdomen, only two inches long, and therefore, probably, in her
second year. The males are commonly larger than females of the same
age.
* * * * *
The hard skeleton of a crayfish, once formed, is incapable of being
stretched, nor can it increase by interstitial addition to its
substance, as the bone of one of the higher animals grows. Hence
it follows, that the enlargement of the body, which actually takes
place, involves the shedding and reproduction of its investment. 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 periodically 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 {33} hardens. This sort of moulting is what is technically
termed _ecdysis_, or _exuviation_. It is commonly spoken of as the
“shedding of the skin,” and there is no harm in using this phrase,
if we recollect that the shed coat is not the skin, in the proper
sense of the word, but only what is termed a _cuticular layer_,
which is secreted upon the outer surface of the true integument. The
cuticular skeleton of the crayfish, in fact, is not even so much
a part of the skin as the cast of a snake, or as our own nails.
For these are composed of coherent, formed parts of the epidermis;
while the hard investment of the crayfish contains no such formed
parts, and is developed on the outside of those structures which
answer to the constituents of the epidermis in the higher animals.
Thus the crayfish grows, as it were, by starts; its dimensions
remaining stationary in the intervals of its moults, and then rapidly
increasing for a few days, while the new exoskeleton is in the course
of formation.
The ecdysis of the crayfish was first thoroughly studied a century
and a half ago, by one of the most accurate observers who ever lived,
the famous Réaumur, and the following account of this very curious
process is given nearly in his words.[2]
[2] See Réaumur’s two Memoirs, “Sur les diverses reproductions
qui se font dans les écrevisses, les omars, les crabes, etc.,”
“Histoire de l’Académie royale des Sciences,” année 1712; and
“Additions aux observations sur la mue des écrevisses données
dans les Mémoires de 1712.” Ibid. 1718.
A few hours before the process of exuviation {34} commences, the
crayfish rubs its limbs one against the other, and, without changing
its place, moves each separately, throws itself on its back, bends
its tail, and then stretches it out again, at the same time vibrating
its antennæ. By these movements, it gives the various parts a little
play in their loosened sheaths. After these preparatory steps,
the crayfish appears to become distended; in all probability, in
consequence of the commencing retraction of the limbs into the
interior of the exoskeleton of the body. In fact, it has been
remarked, that if, at this period, the extremity of one of the great
claws is broken off, it will be found empty, the contained soft parts
being retracted as far as the second joint. The soft membranous part
of the exoskeleton, 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 protrusion of the body, and remains
attached only in the region of the mouth. The head is next drawn
backwards, while the eyes and its other appendages are extracted from
their old investment. Next the legs are pulled out, either one at
a time, or those of one, or both, sides together. Sometimes a limb
gives way and is left behind in its sheath. {35} The operation is
facilitated by the splitting of the old integument of the limb along
one side longitudinally.
When the legs are disengaged, the animal draws its head and limbs
completely out of their former covering; and, with a sudden spring
forward, while it extends its abdomen, it extracts the latter, and
leaves its old skeleton 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 integument has just
the appearance the animal had when the exuviation commenced. The
cast exoskeleton is so like the crayfish itself, when the latter is
at rest, that, except for the brighter colour of the latter, the two
cannot be distinguished.
After exuviation, the owner of the cast skin, exhausted by its
violent struggles, which are not unfrequently fatal, lies in a
prostrate condition. Instead of being covered by a hard shell,
its integument is soft and flabby, like wet paper; though Réaumur
remarks, that if a crayfish is handled immediately after exuviation,
its body feels hard; and he ascribes this to the violent contraction
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 {36} that the
carapace is raised, nothing stops the crayfish from continuing its
struggles. If taken out of the water in this condition, they go on
moulting in the hand, and even pressure on their bodies will not
arrest their efforts.
The length of time occupied from the first giving way of the
integuments to the final emergence of the animal, varies with its
vigour, and the conditions under which it is placed, from ten minutes
to several hours. The chitinous lining of the stomach, with its
teeth, and the “crabs’-eyes,” are shed along with the rest of the
cuticular 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 domesticated 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 aëration, 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 {37} fluttered about
in the greatest agitation. He was quite soft; and every time I
entered the room during the next two days, he exhibited the wildest
terror. On the third, he appeared to gain confidence, and ventured to
use his nippers, though with some timidity, and he was not yet quite
so hard as he had been. In about a week, however, 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 uncertain times;
perhaps he did not get fifty altogether.”[3]
[3] The late Mr. Robert Ball, of Dublin, in Bell’s “British
Crustacea,” p. 239.
It would appear, from the best observations that have yet been made,
that the young crayfish exuviate two or three times in the course
of the first year; and that, afterwards, the process is annual, and
takes place usually about midsummer. There is reason to suppose that
very old crayfish do not exuviate every year.
* * * * *
It has been stated that, in the course of its violent efforts to
extract its limbs from the cast-off exoskeleton, the crayfish
sometimes loses one or other of them; the limb giving way, and the
greater part, or the whole, of it remaining in the exuviæ. But it
is not only in this way that crayfishes part with their limbs. At
all times, if the animal is held by one of its pincers, so that it
cannot get away, it is apt to solve the difficulty by casting off
{38} the limb, which remains in the hand of the captor, while the
crayfish escapes. This voluntary amputation is always effected at the
same place; namely, where the limb is slenderest, just beyond the
articulation which unites the basal joint with the next. The other
limbs also readily part at the joints; and it is very common to meet
with crayfish which have undergone such mutilation. But the injury
thus inflicted is not permanent, as these animals possess the power
of reproducing lost parts to a marvellous extent, whether the loss
has been inflicted by artificial amputation, or voluntarily.
Crayfishes, like all the _Crustacea_, bleed very freely when wounded;
and if one of the large joints of a leg is cut through, or if the
animal’s body is injured, it is very likely to die rapidly from the
ensuing hæmorrhage. A crayfish 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 {39} rudimentary
limb straightens out, and, though very small, acquires all the
organization appropriate to that limb. At every moult it grows; but,
it is only after a long time that it acquires nearly the size of its
uninjured and older fellow. Hence, it not unfrequently happens, that
crayfish are found with pincers and other limbs, which, though alike
useful and anatomically complete, are very unequal in size.
Injuries inflicted while the crayfish are soft after moulting, are
apt to produce abnormal growths of the part affected; and these
may be perpetuated, and give rise to various monstrosities, in the
pincers and in other parts of the body.
* * * * *
In the reproduction of their kind by means of eggs the co-operation
of the males with the females is necessary. On the basal joint of the
hindermost pair of legs of the male a small aperture is to be seen
(fig. 3, A; _vd_). In these, the ducts of the apparatus in which the
fecundating substance is formed terminate. The fecundating material
itself is a thickish fluid, which sets into a white solid after
extrusion. 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 ambulatory legs but two,
that is, in the hinder of the two pair which are provided with
chelate extremities (fig. 3, B; _od_). {40}
After the female has received the deposit of the spermatic matter
of the male, she retires to a burrow, in the manner already stated,
and then the process of laying the eggs commences. These, as they
leave the apertures of the oviducts, are coated with a viscid matter,
which is readily drawn out into a short thread. The end of the thread
attaches itself to one of the long hairs, with which the swimmerets
are fringed, and as the viscid matter rapidly hardens, the egg thus
becomes attached to the limb by a stalk. The operation is repeated,
until sometimes a couple of hundred eggs are thus glued on to the
swimmerets. Partaking in the movements of the swimmerets, they are
washed backwards and forwards in the water, and thus aërated 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 resemblance to their parents. This is very unlike what takes
place in crabs and lobsters, in which the young leave the egg in a
condition very different from the parent, and undergo a remarkable
metamorphosis before they attain their proper form.
For some time after they are hatched, the young hold on to the
swimmerets of the mother, and are carried about, protected by her
abdomen, as in a kind of nursery. {41}
[Illustration: FIG. 8.—_Astacus fluviatilis._—A, two recently hatched
crayfish attached to one of the swimmerets of the mother (× 4). _pr_,
protopodite; _en_, endopodite; and _ex_, exopodite of the swimmeret;
_ec_, ruptured egg-cases. B, chela of a recently hatched crayfish
(× 10).]
That most careful naturalist, Roesel von Rosenhof, says of the young,
when just hatched:—
“At this time they are quite transparent; and when such a crayfish
[a female with young] is brought to table, it looks quite disgusting
to those who do not know {42} what the young are; but if we examine
it more closely, especially with a magnifying-glass, we see with
pleasure that the little crayfish are already perfect, and resemble
the large one in all respects. When the mother of these little
crayfish, after they have begun to be active, is quiet for a while,
they leave her and creep about, a short way off. But, if they spy the
least sign of danger, or there is any unusual movement in the water,
it seems as if the mother recalled them by a signal; for they all at
once swiftly return under her tail, and gather into a cluster, and
the mother hies to a place of safety with them, as quickly as she
can. A few days later, however, they gradually forsake her.”[4]
Fishermen declare that “Hen Lobsters” protect their young in a
similar manner.[5] Jonston,[6] 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.
[4] “Der Monatlich-herausgegeben Insecten Belustigung.” Dritter
Theil, p. 336. 1755.
[5] Bell’s “British Crustacea,” p. 249.
[6] “Joannis Jonstoni Historiæ naturalis de Piscibus et Cetis
Libri quinque. Tomus IV. ‘De Cammaro seu Astaco fluviatili.’”
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 {43} fringed, and is continued by longer
or shorter thread-like pedicles into the coat of the same material
which invests each egg. It very soon hardens, and then becomes very
firm and elastic.
When the young crayfish is ready to be hatched, the egg case splits
into two moieties, which remain attached, like a pair of watch
glasses, to the free end of the pedicle of the egg (fig. 8, A; _ec_).
The young animal, though very similar to the parent, does not quite
“resemble it in all respects,” as 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
chelæ are sharply pointed and bent down into abruptly incurved hooks,
which overlap when the chelæ are shut (fig. 8, B). Hence, when the
chelæ 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 instinctively 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 chelæ imbedded
in the egg-glue.
Even after the female has been plunged into alcohol, the young remain
attached. I have had a female, with young affixed in this manner,
under observation for five {44} days, but none of them showed any
signs of detaching themselves; and I am inclined to think that they
are set free only at the first moult. After this, it would appear
that the adhesion to the parent is only temporary.
The walking legs are also hooked at their extremities, but they play
a less important part in fixing the young to the parent, and seem to
be always capable of loosing their hold.
I find the young of a Mexican crayfish (_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 crayfishes, as about other animals.
At one time “crabs’-eyes” were {45} collected in vast numbers, and
sold for medicinal purposes as a remedy against the stone, among
other diseases. Their real utility, inasmuch as they consist almost
entirely of carbonate of lime, with a little phosphate of lime and
animal matter, is much the same as that of chalk, or carbonate of
magnesia. It was, formerly, a current belief that crayfishes grow
poor at the time of new moon, and fat at that of full moon; and,
perhaps, there may be some foundation for the notion, considering
the nocturnal habits of the animals. Van Helmont, a great dealer in
wonders, is responsible for the story that, in Brandenburg, where
there is a great abundance of crayfishes, the dealers were obliged to
transport them to market by night, lest a pig should run under the
cart. For if such a misfortune should happen, every crayfish would be
found dead in the morning: “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.
{46}