160 FOUNDATIONS OF BIOLOGY 



canal, not only to afford greater secretive and absorptive 

 surface and a larger variety and amount of digestive sub- 

 stances, but also to prolong the length of time the food is 

 subjected to treatment. This increase in area has been 

 effected by folds and elevations of the inner surface of the 

 tract; by outpushings of limited areas of the tube to form 

 large glands which in most cases contribute their products to 

 their point of origin through ducts; and by increasing the 

 length of the inner tube as compared with the outer tube, or 

 body wall, which results in throwing the intestine into vari- 

 ous convolutions within the body cavity. Thus is met the 

 increasingly complex nutritional demands of more highly 

 organized animals. 



