236 CHORDATE ANATOMY 



flatworms to man. An anal aperture makes its first appearance in flat- 

 worms. Vertebrate morphologists generally regard the anus of verte- 

 brates as homologous throughout the group notwithstanding 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 assumption 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. 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 cyclostome intestine, however, suggests the beginning of intestinal 

 differentiation. The intestine of elasmobranchs contains a more elaborate 

 spiral valve. Intestinal elongation has its inception in the sigmoid flexure 

 of elasmobranchs. Increase of intestinal surface is effected in elasmo- 

 branchs and ganoids mainly by the development of a spiral valve. A 

 finger-hke rectal gland makes its appearance in elasmobranchs near the 

 anus. 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 amphib- 

 ians except Gymnophiona differentiate smaU and large intestines. 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, 

 vflh, 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 ahmentary canal with which it is 

 connected. These are best understood by tracing their development in 

 the embryo. In the region of the pharynx the splanchnic layers of meso- 



