Fig. 94.—Hermit Crab, using shell of sea snail for a house.
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 the lobsters are of considerable importance because of use as food. Small boys sometimes catch crayfish, 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]).
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 crayfish sheds its covering, or moults three times, and once each year thereafter. It grows very fast for a few days just after moulting, while the covering is soft and extensible. Since it is at the mercy of birds, fish, and other enemies while in this soft and defenceless condition, it stays hidden until the covering hardens. Hence it cannot eat much, but probably by the absorption of water the tissues grow; that is, enlarge. In the intervening periods, when growth is impossible, it develops; that is, the tissues and organs change in structure and become stronger. “Soft-shelled crab” is a popular dish, but there is no species by that name, this being only a crab just after moulting which has been found by fishermen in spite of its hiding.
General Questions.—How do crayfish choose their food? How long can they live out of water? Why do their gills remain moist out of water longer than a fish? How do they breathe out of water? Are they courageous or cowardly animals? When they lose appendages in fighting or moulting these are readily reproduced, but an organ moults several times in regaining its size. Have you seen crayfish with one claw smaller than the other? Compare the crayfish and crab (Figs. [81], [93], and [95]) in the following particulars: shape, body, eyes, legs, abdomen, habitat, movement.
| KEY TO THE FOUR CLASSES IN BRANCH ARTHROPODS | |
|---|---|
| 1. Insects | 3 body divisions, 6 legs |
| 2. Arachnids | 2 body divisions, 8 legs |
| 3. Myriapods | many body divisions, many legs |
| 4. Crustaceans | gill breathers, skeleton (external) limy |
By the aid of the key and of figures [96]–105, classify the following Arthropods: tick, thousand-leg centipede, king crab, pill bug, spider, scorpion, beetle.
