354 



COMPARATIVE ANATOMY 



vein of Elasmobranchs divides anteriorly into the paired renal portal 

 veins which break up in 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. 294, 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. 



Fig. 296. — 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.") 



Immediately in front of the conus the truncus divides into four pairs of 

 aortic arches, the third to the sixth of the original series. In the dipnoi 

 pulmonary arteries make their first appearance in the vertebrate series as 

 posterior branches of the last pair of aortic arches. As another novelty 

 in fishes, the right postcardinal vein degenerates and a new vein, the 

 postcava, drains most of the posterior part of the body. The caudal vein 

 in this group bifurcates into the left postcardinal and the postcaval veins. 

 The iliac veins as they leave the pelvic fins divide into pelvic and renal 

 portal veins. The two pelvic veins unite to form a median abdominal 

 vein. Venous blood from the fins may thus reach the heart either by 

 way of a capillary network in the mesonephroi or by the abdominal vein. 

 The efferent renal veins drain into the postcava and into the left post- 

 cardinal veins. Thus in the appearance of an atrial septum, of pulmonary 

 arteries and veins, and of a postcaval vein the dipnoi make notable 

 advances towards the circulatory system of the higher vertebrates. The 

 differences between the dipnoan and amphibian circulation are slight. 



