On Molecular and Microscopic Science, Volume 2 (of 2)

by Mary Somerville

The compound eyes, which in all the crabs have hexagonal facettes, are on short jointed stems placed in deep and nearly circular orbits like cups, so that the stems are scarcely visible. These orbits, whose edges are sometimes smooth and sometimes notched, are so constructed that the crab can bend the eye-stems horizontally to the right and left, and the front of the carapace either conceals the orbit, or forms the eyebrow.

In all crabs the antennæ appear in front between the eyes. The first or interior pair are short, jointed, and capable of being bent into cavities, which contain their basal joints; these cavities are near the eye orbits, with which they are connected in certain species. Well-developed ears are placed in their basal joints. [Fig. 146] represents a magnified ear seen from behind, and Mr. Gosse mentions that the large eatable crab, whether at rest or feeding, carries these antennæ erect and elevated, always on the watch, and either vibrating them, or incessantly striking the water with them in a peculiar jerking manner.

Fig. 146. Ear of Crab.

The exterior or lower pair of antennæ are always longer than the interior pair; sometimes they are simple and similar to them, as in the flat crabs; and sometimes they have jointed filaments at their extremities. In all the species they are attached to the under-side of the crab, and the organs of smell are openings at the point of junction between their second and third joints. These openings, which lead into the mouth, are covered by a membrane, and closed by a calcareous lid. Each lid is fastened by a little hinge to the side of its cavity, and is opened and shut by muscles fixed at the extremity of a long tendon. Thus the lower antennæ are the organs of smell, while the upper pair are the organs of hearing, and both are probably the organs of touch.

The mouth of the crab is on the under part of the head, its lips are horny plates, and it has a pair of mandibles to cut the food; their action is from side to side. On each side of the mouth there are two pairs of jaws, followed by three pairs of foot-jaws; so called because they are legs modified to serve as jaws, but in some crustaceans they are also instruments of locomotion or prehension, and sometimes of both. The two last pairs have palpi, or feelers, at their base. All the jaws and foot-jaws, when not in use, are folded over the mouth; the joints of the two last are so broad that they completely conceal this complicated apparatus.

Posterior to the mouth and its organs there is a flat broad plate, which forms the ventral side of the body, with a groove in its surface, into which the rudimentary tail is folded back, as in the Carcinus mœnas (D, [fig. 148]), and the feet are fixed by movable joints on each side of this sternal plate. The first pair, which are a little in advance of the others, and bend forwards in a curve towards each other, may be called hand-feet, as they occasionally serve for both. They have very thick short arms and swollen hands, having a curved finger and a thumb with a movable hinge, armed throughout their internal edge with a row of blunt teeth, and terminated by sharp points. The other four pairs, which are the real walking feet, spread out on each side of the animal, and often bend a little backwards; they are rather thin, compressed, and end either in a horny nail, or flattened blade for swimming.

The gills, which are the breathing organs of the crabs and other Decapods, are spindle-shaped bundles of long, slender, four-sided pyramids, fixed by their points on each side of the mid line of the throat, so that they extend in opposite directions, and their spreading bases fit and rest upon the vaulted sides of the carapace, or rather gill chambers, to the right and left. Each of the pyramids is formed of a multitude of parallel membranous cylinders fixed to the axis of the pyramid, and an infinity of capillary bloodvessels form a network in their surfaces.

The crab has nine of these bundles of gills in each gill chamber; a few of them are shown in [fig. 147]. Each gill chamber has two openings; the water is admitted by a slit in the base of the claw feet, and ejected by another into the mouth. But the act of breathing is regulated by a plate on the second pair of jaws, so connected with the exterior pair of foot-jaws that, when the crab applies the latter to its mouth, the plate shuts the slit, the water in the gill chamber is ejected by the mouth, and in order to admit a fresh supply, the crab must open the foot-jaws again, so that they are in constant motion. There are plates called whips on all the appendages of the crab, from the last pair of foot-jaws to the fourth pair of walking feet inclusive, which ascend and descend vertically between the bunches of gills to sweep particles of sand or other foreign matter out of them.