ASSIMILATION 



49 



outside. The shape of the tube, its length, and the way 

 it lies in the body differ in the various groups, but it 

 bears a close relation to the shape of the animal and to 

 the character of its food. 



Variations in Development. — The starfish is a flat, 

 five-rayed animal. The tube passes through the animal 

 from the dorsal, or upper, to the ventral, or under, 

 side, and is very short. It swells out into two 



Fig. 31 .—Digestive tract of the starfish (section). (See Fig. 36, B.) 



enlargements or stomachs, each of which has five 

 pouches corresponding to the five rays (Fig. 31). 



Fig. 32.— Digestive tract of the earthworm. (See Fig. 36, B.) 



The lobster and the earthworm are elongated. The 

 tube is correspondingly long and straight, and the stom- 

 achs comparatively small (Fig. 32, Fig. 33). In 



Fig. 33.— Digestive tract of the lobster. (See Fig. 36, B.) 



clams as the tube is too long to pass directly through 

 the body, it becomes twisted on itself (Fig. 34). In 



