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PREFACE o v

LIST OF WOODCUTS o xi

The Natural History of the Common Crayfish o 1

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 o 46

The Physiology of the Common Crayfish. The Mechanism by which the Living Organism adjusts itself to surrounding conditions and reproduces itself o 87

The Morphology of the Common Crayfish. The structure and the development of the individual o 137

The Comparative Morphology of the Crayfish. The structure and the development of the Crayfish compared with those of other living beings o 227

The Distribution and the AEtiology of the Crayfishes o 288

NOTES o 347

BIBLIOGRAPHY o 357

INDEX o 363

LIST OF WOODCUTS.

?2. -- -- DORSAL VIEWS OF MALE AND FEMALE o 18

?3. -- -- VENTRAL VIEWS OF MALE AND FEMALE o 21

?4. -- -- THE GILLS o 26

?5. -- -- DISSECTION FROM THE DORSAL SIDE o 28

?6. -- -- LONGITUDINAL VERTICAL SECTION OF THE ALIMENTARY CANAL o 29

?7. -- -- A GASTROLITH OR "CRAB'S EYE" o 30

?8. -- -- ATTACHMENT OF YOUNG TO SWIMMERET OF MOTHER o 41

?9. -- -- STRUCTURE OF THE STOMACH o 53

THE CRAYFISH:

AN INTRODUCTION TO THE STUDY OF ZOOLOGY.

THE NATURAL HISTORY OF THE COMMON CRAYFISH

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 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 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 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.

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.

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, 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.

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.

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

The animals may be seen walking along the bottom of the shallow waters which they prefer, by means of four pairs of jointed legs ; but, if alarmed, they swim backwards with rapid jerks, propelled by the strokes of a broad, fan-shaped flipper, which terminates the hinder end of the body . 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 . 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 . On each side of this is an eye, mounted on a movable stalk , 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 ; 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 . 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 . 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 . In the female, there are mere soft filaments in the place of the first pair of styles .

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 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. 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 anticipated meal.

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 . 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 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 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 "?crevisse," 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.

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