192 



AMPHIOXUS 



3. The atrial cavity surrounds the sides and the ventral 



surface of the intestine, and opens to the exterior in 

 the mid-ventral line between a pair of thickened and 

 prominent lips. 



4. The ccelom 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. 



- 



pic,. 35. Amphioxus lanceolatus. Transverse section of an adult 



specimen passing through the anus. (A. M. M.) 



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

 ofuotochord. I, ventral fin. K, intestine. K', anus. O, ccelom. 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 coalomic space surround- 

 ing it. 



