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

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

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.*

A few hours before the process of exuviation com

* 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,

mences, the crayfish rubs its limbs one against the other, and, without changing its place, moves each separately, throws itself on its back, bends its tail, and then stretches it out again, at the same time 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,

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