186 



THE INDIVIDUAL ORGANISM 



lining by means of which food may be finely ground, an intestine with 

 secreting and absorptive cells, and an anus. An internal ridge, the typhlo- 

 sole, formed by an infolding of the dorsal wall of the intestine, gives 

 increased surface. The exterior surface of the intestine is covered by a 

 layer of brown chloragogen cells, which appears to function as a digestive 

 gland. 



Digestive Systems in the Vertebrates. In the vertebrates, the digestive 

 system reaches its highest development. Here it not only consists of an 

 alimentary canal, subdivided into regions, but also possesses highly de- 



RESPIRATION 



CIRCULATION, 



OXIDATION 

 j 



tJLT 



o ^\0^ -FEEDING- 



SECRETION 



y 



" y 



7/ MtI0N 



Fig. 13.9. Diagram of metabolic processes in Amoeba. Compare with the similar diagram of 

 metabolism in the human body, Fig. 2.5. (Adapted from Wolcott, Animal Biology.) 



veloped glands w T hich produce digestive secretions. This type has already 

 been studied in man. 



Modifications. Although primarily for digestion, certain parts of the 

 enteron may be used for other purposes in some groups of animals. Thus, 

 in the sea cucumbers and in certain groups of aquatic insects, the rectum 

 is secondarily used as a respiratory organ; and in all insects, excretion is 

 carried on by numerous fine tubules opening into the posterior part of the 

 enteron. 



Respiration 



All animals are dependent upon the oxidation of foodstuffs for energy 

 and so have a continual need for oxygen and for the elimination of carbon 

 dioxide. The quantities of oxygen required and of carbon dioxide that 



