THE ARTHROPODS 



371 



FIG. 121. Hermit-crab in snail shell, 

 abdomen entirely concealed. (After 

 Emerton.) 



crab makes this shell too 



snug for comfort, the crab 



goes in search of a larger 



shell, and having found 



one, it quickly withdraws 



its abdomen from the old 



shell and inserts it in the 



new one, thus exchanging 



s 1 ells. 



Since the abdomen and 



*' s appendages are not 



used as in ordinary long-tailed decapods, degeneration occurs. 



Compare the appearances of the abdomen and its appendages 



in the hermit-crab and crayfish (consult the figures, and ex- 

 amine specimens). The abdominal 

 appendages remaining on the hermit- 

 crab are those which help to hold the 

 animal in the snail-shell. Moreover, 

 the abdomen is covered with a soft 

 skin instead of the hard exo-skeleton 

 of other crustaceans. The append- 

 ages of the head-thorax are not very 

 different from those of ordinary 

 decapods, and are used in locomotion 

 and feeding. The large claws are 

 used to close the mouth of the snail- 

 shell when the crab has withdrawn 

 within, thus protecting the animal. 

 By this peculiar mode of life, the 

 hermit-crab is able to protect itself 

 against the attacks of fishes and 



other enemies. 



A Case of Mutual Advantage: Com- 

 '^- -The snail-shells inhab- 

 its body. (After Leunis.) ited by hermit-crabs are frequently 



FIG. 122. Hermit-crab re- 



moved from snail shell, 



