STRUCTURE OF THE VERTEBRATES 203 



and is a reservoir for the feces. The rectum terminates at the 

 anus which is closed by the sphincter ani. 



4. Cloaca. The cloaca develops from the enlarged part of the 

 hind gut from which the allantois outpockets, and is partly lined 

 by inpocketed ectoderm. With a few exceptions (as in some 

 cyclostomes and fish) a cloaca is present in all vertebrate groups 

 except the marsupials and placentals. In the last two sub-classes 

 the cloaca is present in the embryo until a horizontal septum 

 cuts the dorsal from the ventral part and separates the anus from 

 the urinogenital openings. 



E,n,er.ns V^^r, 



Allantois 



Tail gut 



Fig. A 

 Adult Reptile 



Fig. B ^^S. C 



Mammal (embryo) 



Fig. 118. Structure of the Cloaca. The cloaca remains undivided in the 

 majority of vertebrates, (A). In the mammals (B and C) a horizontal 

 constriction separates the dorsal digestive tract from the ventral urinary 



bladder. 



D. The Human Digestive Tract 

 The human mouth cavity and esophagus are typically mam- 

 malian. The stomach is a simple sac slightly divided into two re- 

 gions: the cardiac or larger portion; and the pyloric region 

 which lies toward the right side and empties into the duodenum. 

 The small intestine averages about twenty-two feet in length, 

 although variations of from sixteen to thirty-one feet have been 

 noted. It is divided into the duodenum, about a foot long; the 

 jejunum, the anterior two-fifths of the remainder and the ileum. 



