PHYLUM ARTHROPODA 



539 



The Hermit-crabs (Pa-gums, &c., Fig. 426) are very strange!} 7 

 lifted in relation with their peculiar mode of life. They are 

 hvays found inhabiting the empty shells of Gastropods (Whelks, 

 Periwinkles, &c.), the abdomen completely enclosed within the 

 shell and only the cephalothorax protruding. In correspondence 

 with this mode of protection, the abdomen is soft, having only 

 vestiges of terga (f) on the dorsal side, and its appendages are 

 more or less atrophied, except the sixth pair (up), which take the 

 form of pincers, and are used to hold on to the columella of the 



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425. Scyllarus arctus. 



ant.l, antennule ; itiif.3, antenna ; 

 eye. (After Cuvier.). 



FIG. 426. Paeurus bernhardus. ch. chela of 

 first right leg ; l.fi, 1.5, fourth and fifth legs ; 

 t, abdominal terga ; v.p. uropods. (After Bell.) 



shell. The fifth pair of legs (15) are much reduced, and in some 

 species one of the chelipeds is greatly enlarged and its chela (ch) 

 acts as an operculum, completely closing the mouth of the shell 

 when the animal is retracted. As the Hermit-Crab grows it 

 takes up its abode in larger and larger shells, sometimes killing 



d removing piecemeal the original inhabitant. 



Other Macrura, such as the Cocoa-nut Crab (Birgus), Hippo, &c., 

 approach the Brachyura in the short, more or less permanently 

 flexed abdomen, but are clearly separated from them by the 

 structure of the head and its appendages. 





