158 



ELEMENTS OF BIOLOGY 



of the dorsal wall o£ the digestive canal, extending downward into 

 the cavity of the stomach-intestine throughout its entire length. In 

 the shark the food during digestion in the intestine passes around a 

 SPIRAL VALVE (Fig. 105), which is a modified typhlosole and which 



TYPHLOSOLE 

 BODY WALL 



SEGMENT 



INTESTINE COELOM 

 Fig. 104. — A portion of the intestine of the earthworm diagrammed to show the 



position of the typhlosole. 



thus serves to increase the area for food absorption. In the human 

 digestive canal the lining of the intestine, in which most of the 

 food absorption takes place, is thrown into transverse folds, termed 



Fig. 105. — Diagram of the opened small intestine of a dogfish shark to show the 



relations of the spiral valve. 



PLiCiE (Fig. 106) covered with smaller folds known as villi (Fig. 

 107) ; these also increase the area of the lining of the digestive canal 

 and make for more complete absorption of the products of diges- 

 tion. (3) A third factor is concerned in the completeness of food 



