156 FIRST LESSONS IN ZOOLOGY 



teen 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 moulting, 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 

 bod\'-\\all. The little oyster-crabs (Pinnotheres) which 

 live with the live o)'ster in the cavity inclosed by the 

 oyster-shell are well-known and interesting creatures. 

 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 members of this family are 

 the hermit-crabs (fig. 113), familiar to all who know the sea- 

 shore. There are numerous species of these, all of which 

 have the habit of carr\-ing about with them, as a protective 

 co\'ering into which to withdraw, the spiral shell of some 

 gastropod mollusc. The abdomen of the crab remains 

 alwa}'s in the cavity of the shell; the head, thorax, and 

 legs projecting from the opening, to be withdrawn into it 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 spiral cavity of the shell. It 

 has on it no legs or appendages except a pair for the hind- 

 most segment, \\'hich 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, some- 

 times killing and removing piecemeal the original inhab- 

 itant. Certain hermit-crabs spend much of their time on 

 land, traveling far inland, and making burrows in the 

 ground. These " land-crabs " are common in the South 

 Pacific islands. Some hermit-crabs always have attached 

 to the shell certain kinds of sea-anemones. It is believed 



