174 



THE DIGESTIVE SYSTEM 



The Small Intestine of Fishes. — In the small intestine of the dog- 

 fish (yonng Sqiialns acanthus) several submucosal folds extend into 

 the lumen. (Fig. 111.) The mucosa is composed of columnar 

 epithelial cells which rest upon a tunica propria which, due to the 

 absence of a distinct muscularis mucosa, is continuous with the 

 submucosa. Folds of the mucosa form tubular pockets at the base 

 of the folds; these resemble simple tubular glands but their cells 

 show no special secretory activity differentiating them from the 

 other cells of the mucosa. The muscularis coat has a broad circular 

 region but the longitudinal portion is poorly represented. The 

 connective tissue of the adventitia is covered by cuboidal mesothe- 

 lial cells. 



Fig. 111.— Diagram of a cross-section of the duodenum of a dogfish. The sub- 

 mucosa has ruga-like extensions into the lumen. The mucosa consists of simple 

 tubular glands lined with columnar epithelium and a thin muscularis mucosa. The 

 submucosa is thickest in the region of the ruga-like extensions. The muscularis is 

 represented by a thick circular coat and a thin longitudinal coat which is present in 

 separate strands. The adventitia is a serosa covered with cuboidal epithelium. 



Among the teleosts the structural plan is similar to that in the 

 dogfish })ut the mucosal folds a])i)ear much more elaborate and the 

 epithelial membrane a])pears to be commonly composed of stratified 

 or pseudostratified columnar cells. 



The SniaU fntestinc of Aiuphihia. — Tlwre are wide, longitudinal 

 submucosal ridges in the intestine of urodeles. In Xecturus, the 

 epithelium is simple columnar in type, with maii\- goblet cells 

 present in it. There is diffuse lymphoid tissue in tiie subnuicosa. 

 The circular muscle coat of the muscularis is thicker than 

 the longitudinal coat. Superficially, mesotheliuni (•o\crs the ad- 

 ventitia. In the frog (F'ig. 112), numerous small folds com- 

 posed of mucosa and a core of connective tissue e.xteiul into the 



