506 



THE OCEAN WORLD. 



Among the Crustacea which have no neck the head gradually 

 mingles with the thorax (Cephalothorax), but the abdomen remains 

 distinct; the middle of the body is compressed. Among some 

 Crustaceans there are neither thorax nor abdomen, nor head, but all 



Fig. 339. Zoea Taurus. 



three form only one mass, often short and squat, as in Pisa tetraodon 

 (Fig. 340), the four-horned spider crab. 



Many of these animals have the abdomen developed into a power- 

 ful tail, consisting of a certain number of altered segments, which it 

 uses in swimming to force it through the water. 



The Crustaceans, so far as they are aquatic, respire by means of 

 branchicE, or gills. In the larger species these branchiae are in the 

 form of lamellae, which are traversed by two canals, one of which 



