216 AMPHIOXUS. 



systems are practically as in section B, except that 

 the ventral muscles are absent. 



2. The intestine is circular in section. Its walls are thick, 



and, like those of the liver, consist of a single layer 

 of very long and slender epithelial cells, resting on 

 a thin connective-tissue basement membrane. 



3. The atrial cavity surrounds the sides and the ventral 



surface of the intestine, and opens to the exterior 



Fig. 46. 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 

 v of notochord. |, ventral fin. K, anus. K', intestine, o, coeloni. Q, dorsal 



aorta. 



at the atrial pore in the mid-ventral line between a 

 pair of thickened and prominent lips, which are 

 prolonged backwards like a spout. 



4. The coelom is reduced to a narrow space surrounding 



the intestine, between its proper wall and the atrial 

 membrane. 



5. The metapleural canals are very greatly reduced in size. 



