The Chautauquan, Vol. 05, May 1885, No. 8

by Chautauqua Literary and Scientific Circle; Chautauqua Institution

The chief orders of the Crustacea are the Barnacles,[1] the Water-flea, the Fourteen-footed Crustacea,[2] and Ten-footed Crustacea.[3]

The Crayfish may be taken as a type of the structure of the Crustacea. The body has two principal sections. The anterior, called the cephalo-thorax,[4] extends to the first distinctly marked ring, and the shield, thus far, is comparatively smooth, the segments fitting so closely as to be practically one. In front and between the two pairs of antennæ, or feelers, is a small pointed process in the place of the nasal organ, but serving some other purpose. At the base of each of the smaller antennæ, on the under side, is a minute sac, the mouth of which is protected with delicate hairs. These are the organs of hearing, and near them, on the outer side, are the organs of smell. The sense of touch is in the fine cilia that fringe the mouth and the antennæ.

There are numerous appendages. Of the five pairs of legs, the first two are provided with claws, or nippers. The fore-legs, or arms, have, in the place of hands, strong pincers, similar, but not entirely alike; the one with sharp edge and smaller teeth is used for cutting, the other for mashing, or grinding the food. The other legs terminate in feathery points, and are used, in part, for locomotion, and by the female for carrying her eggs. The posterior pair, called swimmerets, together with the expansion of the last segment of the abdomen into a kind of caudal fin, are the main dependence for swimming. The segments are so loosely jointed that the “tail” can be moved freely, and by flapping it the animal moves easily. As there is no neck, in order to see objects in different directions, the eyes are not sunk in the head, but placed at the extremities of little muscular processes, or “eye stalks,” which are movable, making even hind-sight practicable when backward motion is desired.

THE CRAYFISH.

The crayfish breathes through branchiæ, or gills, situated at the sides of the thorax, protected by the carapace,[5] or horny covering, under the edges of which the water and air reach the gills. Here a very curious appendage is attached, called the “gill bailer,” which moves back and forth, creating a current of water through the gills that finds its way out through an opening near the mouth.

Under the welded sheath or cover of the head are the mandibles, or jaws, between which the mouth opens; a short passage, leading to the capacious, gizzard-like stomach, is provided with grinders, to still further masticate the food before it passes into the intestine. The eggs are small, and attached by glutinous threads to the appendages until they are hatched; the young are also attached, until sufficiently developed to live apart from the parent.

This class of animals undergoes periodic changes which are attended with some degree of violence. The crustaceous covering is a kind of epidermis,[6] having beneath it the true skin. It is formed by some process of exudation from the growing body. This sheath, while soft, expands slowly, but when hardened, the growth is retarded, and in time it is found too small for convenience, so it is cast off, and a new and larger one supplied to take its place. In this process of moulting the animal attempts to put off its outer covering, not in fragments or parts, but in one piece, though many delicate attachments have to be sundered, membranes rent, and sometimes even a limb torn off in the resolute effort to undress. This can not be done at all times, or at any time, without special preparation. A period of apparent sickness precedes, and the muscular parts of the limbs become shrunken, so that they are more easily extricated. The loss of a leg is not so serious a matter, since the damage is repaired by a new one with the same form and articulations. As the work of repairing the limb begins at the joint nearest the body, if the member is torn between that and the extremity, the partially mutilated animal has the strange power of throwing off all that remains beyond that joint.

Of other crustaceans, the common lobster is in most respects so similar to that shown in the first diagram as to need no further description than to say the cephalo-thorax is comparatively smaller, while the forearms and claws are larger.

There are also marine crayfish that are very numerous about the coral reefs off the Florida coasts, and have substantially the same characteristics, only their claws are considerably less, and their ciliated antennæ larger.