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 
AAS 
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 
branchia, or gills. In the larger species these branchiz are in the 
form of lamellz, which are traversed by two canals, one of which 
