THE VASCULAR SYSTEM 



269 



The two posterior cardinals unite into a common cardinal which passes to 

 the left side of the body and unites with the sinus venosus. In the heart 

 atrio-ventricular valves prevent a reverse flow of blood. With the 

 development of a septum transversum the coelom becomes divided into 

 an anterior pericardial cavity and a posterior abdominal cavity. Red 

 blood corpuscles make their appearance in this phylum and the blood is 

 consequently red. A renal portal system is wanting in cyclostomes, the 

 caudal vein draining directly into the postcardinal veins. 



Elasmobranchs. The blood-vascular system of elasmobranchs differs 

 little from that of cyclostomes and with slight changes may easily be 

 derived from the latter. Associated with the appearance of paired fins, 

 the subclavian and ihac arteries and veins are present. The arteries are 

 connected with the dorsal aorta. The subclavian vein is a branch of the 



Fig. 247. — Diagram of vertebrate circulation based on a urodele. Arteries cross- 

 lined; veins black except the pulmonary vein, white, av, abdominal vein; c, celiac 

 artery; ca, cv, caudal artery and vein; d, dorsal aorta; ec, external carotid; g, gonad; h, 

 hepatic vein; ha, hepatic artery; hy, hypogastric artery; ic, internal carotid; il, iliac 

 artery and vein; j, jugular; Iv, liver; m, mv, mesenteric artery and vein; pa, pulmonary 

 artery; pcd, postcardinal; pcv, postcava; pv, hepatic portal vein; r, rectal artery; ra, 

 renal advehent (portal) vein; sc, subclavian artery and vein. (From Kingsley's 

 "Comparative Anatomy of Vertebrates.") 



precardinal, while each iliac vein drains into a lateral abdominal vein. 

 The latter is believed to have been developed from the median ventral 

 abdominal vein of the primitive chordates. Internal jugular veins paral- 

 leHng the precardinal veins are a novelty in this group. The caudal 

 vein of elasmobranchs divides anteriorly into the paired renal portal 

 veins which break up into capillaries within the mesonephroi. The heart 

 resembles that of cyclostomes. The branches of the dorsal aorta are 

 subclavians, intersegmentals, celiac, anterior and posterior mesenteries, 

 spermatics and ovarians, and renals. (Fig. 245, B) 



Dipnoi. In the Dipnoi, with the emergence of lungs, some advances 

 towards the mammalian circulation are seen. In the heart of lung fishes 

 both atrium and sinus venosus become partly divided by incomplete 

 longitudinal septa. Impure blood from the veins enters the right atrium 

 while aerated blood from the lungs flows into the left atrium. Atrio- 

 ventricular valves are lacking but the conus contains a series of valves. 



