266 



CHORDATE ANATOMY 



body-wall. These two vessels parallel one another and both connect 

 anteriorly with the heart. Venous blood from the body-wall is returned 

 to the heart by the cardinal veins, anterior and posterior. 



> !ti 



Amphioxus. In most details the vascular system of Amphioxus 

 resembles the hypothetical ancestral system just described. But Amphi- 

 oxus is heartless, and the circulation of its blood is dependent upon the 

 contractility of the walls of its main blood trunks. Blood is carried for- 

 ward beneath the pharynx by a contractile ventral blood-vessel, the 



