STRUCTURE OF THE VERTEBRATES 



201 



carnivorous ones have a simpler stomach than those with herbiv- 

 orous habits. Man is omnivorous in habit, but has a simple, 

 pouch-like stomach, the anterior border being much shorter than 

 the posterior. The cud chewing herbivores have a stomach di- 

 vided into four distinct regions, the anterior acting as a reservoir 

 for the unchewed herbs which are later regurgitated and chewed 

 while the animal is at rest. In most vertebrates the stomach is 

 terminated by the strong pyloric sphincter. 



3. Intestine. The cyclostomes have a straight intestinal tube, 

 not divided into anatomical regions. The dogfish intestine con- 



B. 



Fig. 115. Digestive Tract of a Dogfish. (A) Shows the entire tract. (B) 

 Shows the structure of the spiral vahe which greatly increases the absorp- 

 tive surface of the small intestine. 



sists of a short duodenum, a greatly enlarged spiral valve, and a 

 short rectal region. The valve arises as a spiral infolding of the 

 gut wall, strongly supported by connective tissues and covered 

 with the digestive and absorbing epithelium. The structure gives 

 a greatly increased functional surface. The spiral valve was car- 

 ried over in modified form to the more primitive bony fish, in- 

 cluding the Crossopterygii and a few of the Teleostei. In the 

 higher vertebrates a coiling of the intestine gives the same func- 

 tional result as the spiral value. 



The mammalian intestine is distinctly divided into a thin- 



