SECTION THROUGH ANUS. 217 



6. The circulatory system. 



a. The dorsal aorta is a median vessel between the 



notochord and the intestine. 



b. The portal or sub-intestinal veins are three or four 



vessels lying side by side on the ventral surface 

 of the intestine, in the ccelomic space surround- 

 ing it. 



E. Transverse Section passing through the Anus. 



1. General characters. The section is much smaller than 



the preceding ones. It is oval, with median dorsal 

 and ventral fins, and the notochord occupies its 

 centre. 



The section passes behind the atrial cavity 

 which is therefore entirely absent in it. 



The skin and the skeletal, muscular, and nervous 

 systems are practically as in section B, except that 

 the ventral muscles are absent. 



2. The intestine lies below the notochord, and the anus is 



on the left side of the ventral fin. 



3. The coelom is a space surrounding the dorsal and lateral 



surfaces of the intestine. 



4. The dorsal aorta is a median vessel in the connective 



tissue below the notochord. 



