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Fig. 93.—Crab from below.
Decapods.—All crustacea which have ten feet belong in the order called decap'oda (ten-footed). This order includes the crabs, lobsters, shrimp, etc. The crabs and lobsters are of considerable importance because of use as food. Small boys sometimes catch crawfish, and in some instances are known to cook and eat them for amusement, the only part cooked being the muscular tail. The crab's tail is small and flat and held under the body (Fig. 93).
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Fig. 94.—Hermit Crab, using shell of sea snail for a house.
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Fig. 95.—Development of a Crab.
a, nauplius just after hatching; b, c, d, zoëa; e, megalops; f, adult.
Question: Which stage is most like a crayfish? Compare with metamorphoses of insects.
Since the limy covering to serve the purpose of protection is not soft enough to be alive and growing, it is evident that the crustacea are hampered in their growth by their crusty covering. During the first year the crawfish sheds its covering, or molts three times, and once each year thereafter. It grows very fast for a few days just after molting, while the covering is soft and extensible. Since it is at
