222 STRUCTURE OF THE VERTEBRATES 



Anterior mesenteric 



Coeliac Renal portal 



Post mesenteric,^^ j 



Fig. 129. Generalized Vascular System of the Elasmobranch. Only the 

 larger vessels are shown, and the connections between arteries and veins 



are omitted. 



lism, and collecting food materials from the intestine. Veins are 

 formed from the capillaries and in Amphioxus there are two 

 venous systems: (1) the subintestinal vein, which is homologous 

 with the lateral body veins and the veins entering the liver 

 (hepatic portal) of the higher vertebrates; and (2) the cardinal 

 veins, a small group of vessels which drain the gonads and 

 nephridia. The cardinal veins enter the subintestinal vessel an- 

 terior to its passage around and through the hepatic caecum. 

 The combined vessel is continuous with the pulsating portion of 

 the ventral aorta, completing the vascular system of the animal. 

 (See the diagram of the blood system on page 19.) 



A. Vascular System of the Dogfish 



The heart of the dogfish is a muscular organ consisting of 

 four chambers in a series, lying along the antero-posterior axis 

 of the body. It is enclosed in a pericardial sac, a division of the 

 primitive coelomic cavity. As the most anterior chamber and the 

 most posterior are found only in the embryos of the higher 

 classes of vertebrates, the heart is usuallv considered as consist- 



