142 INVERTEBRATE ANIMALS. 
part of the body in some artificial manner, and this it effects 
by appropriating the empty shell of some dead mollusc, such 
as the common periwinkle or whelk. The abdomen is pro- 
vided with special appendages to enable the intruder to retain 
firm hold of his borrowed dwelling, at the same time that 
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 chela, 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. Brachyura.——The decapod Crustaceans included in this 
tribe are familiarly known as crabs, and they derive their name 
Fig. 64.—Brachyura. The Spiny Spider-crab (Zaia sguinado). 
of Brachyura (Gr. brachus, short; and oura, tail) from the 
rudimentary condition of the abdomen. The abdomen, in fact, 
