[Illustration: FIG. 129.--Crayfish seen from the side, with that portion of the carapace removed which covers the branchiæ, or gills. The appendages of the left side only shown, _s_, region of stomach; _A_, abdominal appendages; _B_, bases of the four small legs; _C_, base of large claw; _f_, "gill-bailer," or flabellum, attached to the second maxilliped; _e_, eye. (After Morse.)] [Illustration: FIG. 130.--Under surface of the crayfish or fresh-water lobster (_Astacus_): _a_, first pair of antennæ; _b_, second pair; _c_, eyes; _e_, foot jaws; _f_, _g_, first and fifth pair of thoracic legs; _h_, swimmerets; _i_, anus; _k_, caudal fins.] [Illustration: FIG. 131.--A shrimp, showing anatomy: _s_, stomach; _l_, liver; _i_, intestine; _h_, heart; _g_, chain of ganglia; _hg_, head ganglia.] Some idea of the various internal organs of the crustaceans may be obtained in Figure 131. The breathing organs are conspicuous, curled up like plumes on each side of the crayfish and attached to the base of the legs. Water enters the shell under the edge, back of the great claws, and is swept along over them by a little organ called the gill bailer, the gills taking up oxygen from the water, which in turn is absorbed by the colorless blood. The brain is very small, and nerves can be seen passing from it to the various organs. The ears are situated at the base of the small or first antennæ, and are little sacks on the upper side containing a thick fluid in which are floating minute grains of sand. The tail portion is made up of a number of rings or segments, and is provided with small swimmerets. At the extreme end are seen five paddlelike or fanlike organs (Fig. 132), which constitute a most important swimming organ to the lobsters and crayfishes, by the violently flapping of which they dash away backward. In color the crayfish is yellowish brown or greenish. When alive it presents (Fig. 133) an attractive appearance. [Illustration: FIG. 132.--Paddles of lobster.] [Illustration: FIG. 133.--Fresh-water crayfish.] The crustaceans deposit eggs which they carry about with them attached to the swimmerets, and resembling minute bunches of grapes. When first hatched (Fig. 134) the young crustaceans are totally unlike the parent in appearance, passing through several stages before they reach the adult form. When the crustacean grows too large for its shell, what are known as casting hairs appear on the inner side, which push the shell upward. [Illustration: FIG. 134.--Stages of development in a crab.]