420 J. BARRELL INFLUENCE OE CLIMATES ON VERTEBRATES 



that an animal possessing it could not survive if compelled to abandon 

 wholly the use of its gills. 



This inefficiency is partly avoided by modifications which exist in 

 INTeoceratodus, but to a greater degree in the adult amphibian heart, as 

 shown in figure 1, C, in which the gills have disappeared. In the sala- 

 manders a network of muscular strands with intervening spaces imper- 



A. SELACHIAN 



B. LUNG FISH NEOCERATODUS 



C. SALAMANDER ADULT 



D. MAMMAL 



Figure 1. — Diagrammatic Side View, respiratory-circulatory System of Vertebrates 



In the primitive plan, as shown by the selachian circulation, the blood enters the 

 heart from the sinus venosus ; thence it passes into the single auricle. This opens 

 through a two-lipped valve into the single ventricle. From the ventricle the blood 

 passes Into the conus arteriosus, provided with rows of valves, and thence into the 

 ventral aorta. This divides and then subdivides into five afferent branchial arteries on 

 each side of the pharynx, the afferent vessels to the gills. From the gills the blood 

 passes by means of four efferent branchial arteries on each side. From the first efferent 

 vessel the carotid and hyoidean arteries draw their blood supply for the head. The four 

 efferent branchial arteries unite into the dorsal aorta, whose branches supply the body. 

 This is as much detail as the diagram shows. 



fectly separates the single auricle of the fish-heart into a right and left 

 auricle ; the deoxygenated or impure blood from the body is received into 

 the right side ; the oxygenated or pure blood from the lung is received 

 into the left. In the contraction of the heart the pure and impure blood 

 are not separated by any wall in the ventricle and become more or less 

 intermingled. The head, however, receives fairly pure blood, the body 

 somewhat mixed blood, and the lungs mostly impure blood. In the frog 



