These differences are apparent at the first glance, and closer examination reveals even greater contrasts. The three primary divisions of the body into head, thorax, and abdomen, which are so characteristic of insects, are not obvious in the crayfish; for the head and thorax are here fused into one mass, the cephalothorax[36] (Fig. 229, c.th.), which is covered by a shield called the carapace. Moreover, in the crayfish every segment except the last bears a pair of appendages, which vary in form in different regions of the body according to their duties, but which can all be shown, by careful comparison, to be modifications of one primitive form, which is Y-shaped and consists of a basal stalk and two branches. This form of appendage is well seen in the swimmerets (17, Fig. 229) of the abdominal region; further forwards the appendages become walking legs (9-13, Fig. 229); next come three pairs which combine the characters of legs and jaws, and are called maxillipedes (8, Fig. 229); then are the true jaws—two pairs of maxillae, and one pair of biting and crushing mandibles; and lastly, in front of the jaws, two pairs of feelers (ant. 2 and ant. 1). It is believed that the first pair of feelers corresponds to the single pair of feelers of the [Pg 376] cockroach; that the jaws correspond to the jaws; that the maxillipedes of the crayfish are the equivalents of the walking legs of the cockroach; while the remaining appendages of the crayfish have no representatives in the insect. Lastly, the crayfish differs essentially from the insect in its method of respiration (p. 355), for it is an aquatic animal and breathes dissolved oxygen. It therefore possesses neither lungs nor spiracles but gills (Fig. 230). These are situated at the sides of the true body-wall, in gill-chambers formed by the downgrowth of the sides of the carapace. The gills are delicate plumes, containing fine blood-vessels, so that an exchange of gases readily takes place between the blood and the surrounding water. On each side, a scoop (Fig. 230, ep. 5) on the second maxilla is continually baling water out of the front of the gill-chamber, fresh water flowing in from behind to take its place. Certain of the gills are attached to the legs, so that the motion of the legs also is some assistance to respiration. Fig. 231.—Crab; A, from above, B, from below, ant. 1, first, and ant. 2, second feelers; abd. 3, third, and abd. 7, seventh abdominal segments; E, eye; l. 1, pincers; l. 5, last walking leg; mxp, third maxillipede. (× ⅙.)