194 



AMPHIOXUS 



The portal or sub-intestinal veins are three or four 

 vessels lying side by side on the ventral surface 

 of the intestine, in the coelomio space surround- 

 ing it. 



Pig. 43. — Amphioxus lanceolatus. Transverse section of an adult 

 specimen, passing through the atrial pore. The boundary of the 

 atrial cavity is indicated by a thick black line. (a. m. m.) 



A, skeleton of dorsal fin. B, spinal cord. C, notochord. D, connective- 

 tissue sbeath of notocliord. H, atrial cavity. K, intestine. O, coelomie 

 space surrounding intestine. P, metapleural canal. Q, dorsal aorta. U, 

 portal veins. "V", atrial pore. "W, metapleural folds. 



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. 



