IISfTRODUCTORY 5 



most of their lives in places which seem to have been sought, at first, 

 for shelter during the moulting period. A species of Porcellana 

 clings to the lower surface of the broad shell of Limulus, and the 

 Pinnixas live in the burrows which annelids make in the floor of the 

 ocean. I have found a species of Pinnixa living on the shoals at 

 Beaufort, N.C., in the parchment-like tubes with which the annelid 

 Chaetopterus lines its burrow, and as the opening of this tube is too 

 small for a Pinnixa to pass, it must enter while small and pass 

 the rest of its life there. 



The period of moulting is dangerous, not only because of 

 enemies, but also because of its critical nature, for many crabs 

 die in the act, while others lose their limbs and their gills. The 

 general constitutional disturbance is so great that it is difficult to 

 carry a full-grown crab safely through it in an aquarium. The 

 power to replace lost parts which is so well developed in crabs 

 is an adjustment to meet and compensate for this danger among 

 others. Most of the direct danger comes from the stony hard- 

 ness and inflexibility of the old shell, and the shells of crabs 

 like the Pinnixa, and the female Pinnotheres which lives within 

 the shell of the oyster, are softer than those of more exposed 

 crabs. 



The hermit-crabs and soldier-crabs live in the spiral shells of 

 gasteropod mollusks, and, as these houses are strong enough to 

 furnish ample protection, all the hinder part of the body of these 

 crabs is covered by a thin flexible shell which may be stripped 

 off without danger, although the claws and other exposed parts 

 are covered by very hard strong shell. When born, the little her- 

 mit-crab is straight and its hind-body carries swimming feet, but 

 when it is about as large as a mosquito, these become converted 

 into knobs for clinging to the inside of the house, and the hind- 

 body becomes twisted into a spiral to fit the inside of the spiral 

 shell. Crabs outgrow the shells of mollusks just as children out- 

 grow their clothes ; and hermit-crabs are always on the watch for 

 new shells, and exhibit what the human observer finds himself 

 disposed to call a lively interest in shells. If half a dozen of 

 them are placed in an aquarium, they soon begin to measure and 

 compare shells, and even to make vacant one that seems eligible, 

 by pulling out its occupant piece by piece and eating him. One 



