330 COMPARATIVE ZOOLOGY OF CHORDATES 



and arteries. It may be remembered that arteries are vessels 

 leading blood away from the heart, and veins lead blood towards 

 the heart, whatever be the kind of blood which they contain. 

 Further, arterial blood is rich in oxygen, and venous blood poor 

 in oxygen, whatever may be the nature of the vessel which 

 contains it. Actually, the purest arterial blood in the body is 

 in a vein (pulmonary), and the foulest venous blood is in an 

 artery (also pulmonary). 



The Veins. — The description given above applies to the 

 venous system of Amphioxus. In the Craniates, the presence 

 of mesodermal kidneys (pronephros and mesonephros), lying 

 in the track of the posterior cardinal veins, brings about the 

 formation of a renal portal system. The anterior cardinal 

 veins give rise to the jugulars, and in the Gnathostomes there 

 are veins returning blood from the fins or limbs. Those from 

 the anterior limbs are the subclavian veins which run into the 

 ductus Cuvieri ; those from the hind limbs are the pelvic 

 veins which run into the renal portals and into the lateral 

 abdominal veins. The two latter veins often join in the 

 middle line on the ventral side and give rise to the anterior 

 abdominal vein of Ceratodus and higher forms. In the 

 amniotes the lateral abdominal veins receive the blood from the 

 allantois in the embryonic stages of development. Beginning 

 in the Dipnoi, there is another connexion between the circula- 

 tion of the body- wall and that of the gut-wall, apart from the 

 superior venae cavae. This is the inferior vena cava. Pul- 

 monary veins are present in Polyp terus, Dipnoi, and Tetrapods, 

 returning blood from the lungs to the heart. In the amniotes 

 the renal portal veins tend to diminish owing to the fact that 

 the functional kidney of the adult is no longer a mesonephros 

 but a metanephros, and in the birds and mammals there is no 

 renal portal system. 



The Heart. — In Amphioxus there is no specialised heart 

 in which the blood is pumped forwards, but, apart from the 

 specialised bulbils, the whole vascular system is contractile 

 and propels the blood along. Beginning in the Cyclostomes, 

 there is a definite portion of the subintestinal vein in front of 

 the liver and behind the gill-slits which is set apart as a muscular 



