INTERNAL ANATOMY. 



53 



complicated by the occurrence of a further flexure in a 

 different direction, in consequence of which the hinder 

 Hmb of the U has been raised, so to speak, to nearly the 

 same plane as the anterior limb. The shape of the heart 

 at this stage bears a characteristic resemblance to the 

 Greek letter sigma. The permanent condition of the 

 heart in Amphioxus therefore corresponds to an early 

 stage of its development in the higher Vertebrates. 



Again, in the craniate embryo the dorsal aorta arises as 

 a pair of vessels on either side of the notochord, which 

 later fuse together into one median dorsal vessel. (Cf. 

 Fig. 24.) In Amphioxus, throughout a great portion of 

 its extent, — namely, in the region of the pharynx, — the two 

 halves of the dorsal aorta remain permanently separated 

 from one another by the dorsal groove of the pharynx. 

 (Cf. Figs. 2 and 28.) 



One of the most striking peculiarities of the vascular 

 system of Amphioxus is the presence of the sub-intestiiial 

 vein, in its capacity as the main venous trunk of the body. 

 It collects the blood from the capillaries of the intestinal 

 wall, and conducts it to the base of the liver, where it again 

 breaks up into capillaries.* It acts, therefore, physiologi- 

 cally, as a portal vein, while morphologically it is the 

 siib-intestinal vein. Curiously enough, it is much larger in 

 its posterior than in its anterior moiety, ^nd in transverse 

 sections through the hinder region of the intestine there 

 appear to be several separate vessels lying side by side, 

 sometimes as many as six. These, however, if traced 

 backwards or forwards, are found to anastomose with one 



* In the larva of Amphioxus the sub-intestinal vein and branchial artery 

 form one continuous blood-vessel. Later, when the hepatic ccecum (liver) 

 grows out from the ventral wall of the alimentary canal, an interruption occurs 

 in the continuity of the vessel, through the insertion of a capillary portal system 

 in its course. 



