192 



AMPHIOXUS 



3. The atrial cavity surrounds the sides and the ventral 



surface of the intestme, and opens to the exterior in 

 the mid-ventral line between a pair of thickened and 

 prominent lips. 



4. The coelom is reduced to — (1) a narrow space surrounding 



the intestine, between its proper wall and the atrial 

 membrane ; and (2) the metapleural canals, which 

 are very greatly reduced in size. 



FiQ. 35. — AmphioxttS laneeolatus. Transverse seetion of an adult 

 specimen passing through the anus. (a. m. m.) 



A, dorsal fin. B, spinal cord. C, notoohord. D, connective-tissue sheath 

 of notochord. I, ventral fin. K, intestine. K'> anus. O, coslom. Q,, dorsal 

 aorta. 



5. 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 surroimd- 

 ing it. 



