THE DIGESXrV'E SYSTEM 319 



withstanding differences in ontogenetic development in different groups. 

 The post-anal gut may be interpreted as a special modification correlated 

 with the elongation of the tail, and not as a primitive trait. The assump- 

 tion of a partial homology of the vertebrate anus with the blastoporic 

 mouth of invertebrates seems to be in harmony with all known facts. 



The uncertainty of pre-chordate homologies will explain why most 

 vertebrate morphologists take the intestine of amphioxus as the starting 

 point for intestinal evolution. Yet it is obvious that the intestine of 

 amphioxus is an inheritance from some prechordate ancestor of whatso- 

 ever sort. The intestine of amphioxus extends as a straight tube from 

 the region of the liver directly to the left-sided anus. The intestine of 

 cyclostomes is almost as simple. A spiral fold projecting into the cyclo- 

 stome intestine, however, suggests the beginning of a small intestine, since 

 the small intestine of elasmobranchs contains a spiral valve. Intestinal 

 elongation has its inception in the sigmoid flexure of elasmobranchs. A 

 similar functional result is effected in elasmobranchs and ganoids through 

 the development of a spiral valve in their small intestine. A finger-like 

 rectal gland makes its appearance in elasmobranchs near the anus. The 

 rectal gland is sometimes homologized with the cecum of higher verte- 

 brates, and its position consequently is held to mark the boundary between 

 large and small intestines. A cloaca also makes its first appearance in 

 this group. A further step in advance is seen in the teleosts, which have a 

 convoluted small intestine, intestinal ceca, and a somewhat enlarged 

 colon. Most amphibia except gymnophiona have small and large intes- 

 tines. All have a cloaca. Some have in their small intestine intestinal 

 glands, valvulae conniventes, and villi. 



The intestine of reptiles is relatively short. Their large intestine is 

 short, and they retain a cloaca. 



In mammals, the small intestine becomes greatly elongated and differ- 

 entiated into duodenum, jejunum, and ileum. Valvulae conniventes^ 

 viUi, and intestinal glands become very numerous. Duodenal glands 

 make their appearance. Colon and rectum are differentiated. In many 

 mammals, especially herbivorous forms, the cecum becomes much elon- 

 gated and forms an important organ of absorption. In others, as in man, 

 it degenerates in size and serves as an adenoid organ. 



Mesenteries and Omenta 



The peritoneum lining the abdominal cavity is a serous membrane 

 formed from the embryonic hypomere. It not only lines the body wall, 

 but is reflected over the viscera, so that parietal and visceral portions are 

 distinguishable. The complex relations of the peritoneum are due 

 chiefly to the complications of the alimentary canal with which it is 

 connected. These are best understood by tracing their development in 



