154 ELEMENTARY ZOOLOGY 



live. The spider-crabs are especially strange-looking crea- 

 XTHtures with unusually long and slender legs and a com- 

 paratively small body-trunk. They include the Macro- 

 cJieira of Japan, the largest of the crustaceans. Specimens 

 of this crab are known measuring twelve to sixteen feet 

 from tip to tip of extended legs ; the carapace is only as 

 many inches in width or length. The soft-shelled crab 

 is a species common along our Atlantic coast. It is 

 "soft-shelled " only at the time of molting, and has to 

 be caught in the few days intervening between the shed- 

 ding of the old hard shell and the hardening of the new 

 body-wall. The little oyster-crabs (Pinnotheres] which 

 live with the live oyster in the cavity enclosed by the 

 oyster shell are well-known and interesting crabs. They 

 are not parasites preying on the body of the oyster, but 

 are simply messmates feeding on particles of food brought 

 into the shell by the currents of water created by the 

 oysters. 



^~ Among the most interesting crabs are the hermit crabs 

 (fig. 37), familiar to all who know the seashore. There 

 are numerous species of these crabs, all of which have the 

 habit of carrying about with them, as a protective covering 

 into which to withdraw, the spiral shell of some gastropod 

 mollusc. The abdomen of the crab remains always in 

 the cavity of the shell ; the head and thorax and legs 

 project from the opening of the shell, to be withdrawn 

 into it when the animal is alarmed or at rest. The 

 abdomen being always in the shell and thus protected 

 loses the hard body- wall, and is soft, often curiously 

 shaped and twisted to correspond to the cavity of the 

 shell. It has on it no legs or appendages except a pair 

 for the hindmost segment which are modified into hooks 

 for holding fast to the interior of the shell. As the 

 hermit crab grows it takes up its abode in larger and 

 larger shells, sometimes killing and removing piece-meal 



