132 



INVERTEBRATE ANIMALS. 



he can change it at will when too small or otherwise incon: 

 venient. The first pair of legs are developed into pretty 

 powerful nipping-claws or chelse, and one of them is always 

 much larger than the other, and acts as a kind of plug, block- 

 ing up the entrance of the shell when the animal is retracted 

 within it. 



C. Brachywa. — ^The decapod Crustaceans included in 

 this tribe are familiarly known as crabs, and they derive their 

 name of Brachyura (G. irachus, short ; and oura, tail) from 





I^G. 51. — ^Brachyura. The Spiny Spider-crab {Maia sguiTUXdo). 



the rudimentary condition of the abdomen. The abdomen, in 

 fact, is not only extremely short, but it is always tucked up 

 beneath the greatly-developed cephalo-thorax, so that it is not 

 visible at all, except when the animal is looked at from below 

 (Fig. 51). The crabs are very various in their habits, but they 

 are mostly denizens of the shore, hiding beneath stones or sea- 



