THE ALIMENTARY CANAL 170 



In part of the intestine below the spiral Aahe in the Selachians, 

 and over the entire lower portion in those forms not having a spiral 

 valve, the strnctnre resembles that of the small intestine. The 

 mucosa becomes smoother due to loss of mucosal and subnuicosal 

 folds. 



The rectal gland of tlie Selachian is a short cylindrical structure 

 attaching to the colon just posterior to the end of the spiral valve. 

 A study of sections at various levels shows that it is a com])ound 

 tubular gland. The tubular secretory end-pieces are composed of 

 cuboidal cells apparently serous in nature. The function of the 

 gland is not definitely known. 



The Large Intestine of Amphibia.^ The nuicosa is thrown into 

 se\eral longitudinal folds by the submucosa. The epithelial lining is 

 stratified or pseudostratified columnar with numerous goblet cells. 

 Small glands may extend a short distance into the tunica projjria. 

 The nuiscularis has an inner circular coat and an outer longitudinal 

 coat, but the adventitia is reduced to a very thin membrane of 

 connective tissue covered with simple squamous epithelium. In 

 the frog, seromucous glands occur in the tunica propria, which 

 ma}' be separated from the submucosa by a thin circular muscularis 

 mucosa. 



The Large Intestine of Reptiles.— ^The mucosa has simple tubular 

 glands that extend to a muscularis mucosa. The glands are lined 

 with simple columnar cells among which are goblet cells. Diffuse 

 lymphoid tissue is cjuite abundant in the submucosal tissue. The 

 muscularis has a definite inner circular coat and a thin outer longi- 

 tudinal muscular sheath. Extending lengthwise are branching 

 submucosal folds which appear like long villi when the intestine 

 is studied in sections. 



The Mammalian Large I ntestine. ^The same four coats make up 

 the wall here as observed in the case of the small intestine, but 

 certain characteristic modifications of the mucosa distinguish it. 

 (Fig. 115.) There are no villi and generally no plicse circulares 

 present. The mucosa has many tubular glands extending to the 

 muscularis mucosa. These glands are homologous with the glands 

 of Lieberkiihn of the small intestine, but are longer. The cells 

 lining the surface of the lumen are tall columnar and may ha\e a 

 thin striated cuticular border. Passing down into the glands, the 

 cells become shorter and goblet cells become more numerous. 

 Goblet cells are most numerous in the mid-region of the 

 glands. The basal cells are less differentiated and are supposed at 



