ALIMENTATION 



101 



and pancreatic duct. In the higher vertebrates the small 

 intestine also produces digestive agents. This apparently is 

 not true of the frog. 



The mucosa of the small intestine of the frog is arranged in 

 irregular folds, but there are no villi or tubular glands such as are 

 found in mammals. The epithelium of 

 the mucosa consists of columnar cells, 

 some of which are goblet cells. The 

 structure of the large intestine is much 

 the same. In the rectum the folds of 

 the mucosa are longitudinal (Fig. 67). 



In man the mucosa of both the large 

 and small intestine is provided with 

 numerous tubular glands. In the small 

 intestine the mucosa between the face of mucosa of small intes- 

 mouths of the glands is raised in thin *?* , of man " G \ , intest > nal 



° gland opening on the surface 



folds Or finger-shaped processes Called between the bases of the villi; 



villi, which are important in absorbing v ' vlIlus " 

 the products of digestion (Fig. 68). A curious example of a sim- 

 plification of a villus structure is seen in the spiral valve of the 

 intestine of the dogfish (Fig. 69). This valve consists of a 

 rather wide, thin, spiral fold of the mucosa which slows the 

 movement of the contents of the intestine and also increases 



Fig. 69.- — Spiral valve of Raia. The arrow indicates the direction in which the 

 food passes. {Modified after Mayer.) 



the absorbing surface of the relatively short intestine. In the 

 higher vertebrates an increase in digestive capacity and absorb- 

 ing area is achieved by an increase in the length of the intes- 

 tine as well as by villiform elevations of the mucosa. Also, 

 a correlation exists between the character of the food and the 

 length of the intestine. The frog tadpole feeds on plants grow- 

 ing in the water and has an alimentary canal relatively and 



