ARTHROPODS AND MOLLUSCS 153 



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 shedding 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 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. 

 The hermit crabs all 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, 

 thorax, and legs projecting from the opening, to be with- 

 drawn 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, 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 kill- 

 ing and removing piecemeal the original inhabitant. 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. 



Crustaceans which at first glance are hardly recognizable 

 as such are the stalked or sessile barnacles (fig. 62), which 

 live fixed in great numbers on the rocks between tide lines, 

 or on the piles supporting wharves, or on the bottom of 

 ships or even on the bodies of whales. The body of the 

 stalked barnacle is enclosed in a sort of bi-valved shell 

 formed by a fold of the skin and stiffened by five calcareous 

 plates. The legs of which there are usually six pairs are 

 long and feathery and divided nearly to the base. These 



