50 



PHYSIOLOGY 



higher animals the tube is more highly developed. 

 The enlarged stomach is sharply defined; the tube is 



Fig. 34.— Digestive tract of the clam. (See Fig. 36, C.) 



very long in proportion to the length of the body, and 

 is consequently very much twisted on itself (Fig. 35, 



Fig. 35.— Digestive tract of a vertebrate. (See Fig. 36, C.) 



Fig. 36) ; and special glands are developed from it 

 which manufacture special digestive fluids. 



Diet of Animals. — The diet of animals varies greatly. 

 Some animals live entirely upon plants, others eat only 

 the flesh of other animals, still others eat a mixture of 

 plant and animal tissues. Gradually animals have be- 

 come adapted to the diet which in the beginning was 

 adopted either through choice or through necessity and 



