CIRCULATORY SYSTEM 



803 



pulmonary arches. It is of interest to note that in all lung-breathing 

 fishes, the lungs are supplied from the branch of the sixth branchial arch. 

 In most amphibia, a branch of the sixth arch becomes cutaneous, for the 

 skin respiration is almost as important as the pulmonary. The heart is 

 carried back into the trunk and consists of a sinus venosus, right and left 

 auricle, ventricle, and conus arteriosus. The auricle has divided into a 

 systemic half and a pulmonary half which lie in front of the ventricle. 

 The single ventricle receives both arterial and venous blood, but there is 

 very little mixture of the two. 



The postcava is well developed and the lateral abdominal veins (also 

 called epigastric) unite to form an anterior abdominal vein. This latter 

 vein permits the return of blood from the hind legs to the heart either 

 through the anterior abdominal and the hepatic portal system or the 

 renal portal system and the postcava. 



REPTILIA (Figs. 447, 448, 452, 455) 



Fig. 455. 



Embryonic circulation of a Snapping Turtle (Chelydra) to show the relations 

 of allantois. a, right auricle ; al, allantois ; av, allantoic vessels ; c, caudal vein ; 

 da, dorsal aorta ; h, hypogastric artery ; j, jugular, I, liver ; oa, ov, omphalo- 

 mesenteric artery and vein ; pc, post-cardinal ; sc, subcardinal vein ; uv, umbilical 

 vein; w, Wolffian body; y, yolk-sac; 3-6, aortic arches. (From Kingsley after 

 Agassiz and Clarke.) 



The heart is very broad laterally and consists of a sinus venosus 

 (although only distinguishable in Sphenodon externally), two quite dis- 

 tinct auricles (the right receiving venous blood from the body, and the 

 left aerated blood from the lungs), and a ventricle always more or less 

 completely divided into right and left portions. (In the crocodile the 

 partition is complete.) 



