298 ELEMENTARY ZOOLOGY. 



impure blood go into separate auricles, but there is only 

 one ventricle and the two kinds get mixed more or less, 

 as both auricles empty into it. On the whole, the circu- 

 lation and aeration are more perfect than in the fishes, 

 but the animals are not yet warm-blooded. The skin in 

 all these animals is important as a respiratory organ. 



FIG. 117. Diagram of the heart and arterial arches of the frog, a, aorta; c, 

 carotid; d.a., dorsal artery; /, lung; La., left auricle; p.a., pulmonary artery; p.v., 

 pulmonary vein; pre.c., precaval vein; post.c., postcaval vein; r.a., right auricle; r.j., 

 right jugular vein; v, ventricle. 1-4 the branchial arches of the aorta corresponding 

 to gill arches in the fishes. 



Questions on .the Figure. How does the heart compare with that 

 of the fish? What is the course of the blood in the parts figured? 

 What kind of blood comes to the left auricle? To the right? Is 

 there anything to suggest that the impurest blood will go to the 

 lungs? 



