PHYLUM ARTHROPODA 



589 



of life. They are always found inhabiting the empty shells of 

 Gastropods (Whelks, Periwinkles, &c.), the abdomen, which has 

 become spirally twisted, 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 (t) on the dorsal side, and its appendages are more or less 

 atrophied except the sixth pair (up), which take the form 

 of a pair of hooks, and are used to hold on to the columella of 

 the shell. The fifth pair of legs (l.o) are much reduced, and in 



FIG. 406. Scyllarus arctus. 



ant.l, antenmile ; ant.2, antenna ; 

 E, eye. (After Cuvier.) 



FIG. 4(iT.' Pagurus bernhardus. ch. chela of 

 first right leg; Lit, 1.5, fourth and fifth legs; 

 t, abdominal terga ; up. uropods. (After Bell.) 



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 and removing piecemeal the original inhabitant. 



Other Anomura, such as the Cocoa-nut Crab (Birgus), Hippa, &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. 



In the Brachyiwa, or true Crabs, we reach the highest degree of 



