THE HIGHER ORGANISMS 



139 



the systemic blood is much the larger of the two. In 

 both these lung-breathing fishes the blood undergoes a 

 partial separation for that which goes to the lungs returns 

 to the heart before it makes the systemic circuit. So 

 that when the lungs are functional and the gills inactive 

 the blood returning from the systemic circuit is in part 



r. 



Fro. 52. Diagram of the heart and branchial arches in the Frog, 

 carotid gland; I, lungs; l.a, left auricle; r.a, right auricle. (Galloway.) 



c.g, 



passed through the lungs before being again returned 

 to the systemic circuit. 



Among the batrachians the separation of the auricles 

 becomes distinct. With a very few exceptions, these 

 animals are gill-breathers in the larval stage, and lung 

 breathers in adult life. This change necessitates a 

 transformation of the vascular arrangements as the 



