THE HIGHER ORGANISMS 



139 



lungs is demanded. For this reason the gills continue to 

 form the chief aerating organs, and the lungs an auxiliary 

 mechanism to be held in reserve, hence the circulatory 

 arrangements continue more closely to resemble those 

 of gill-breathers than those of lung breathers. The bulk 

 of the blood leaving the ventricle passes into an aorta, 



pre. c 



post c 



ab. 



ab. 



(La. 



FIG. 50. Diagram of the heart and branchial arches in Protopterus (one of 

 the Dipnoi). Position and lettering as in the preceding, pre.c, precaval vein, 

 made up of right and left jugulars, subclavians, etc.; post.c, postcaval, made up 

 of the cardinals, right and left. (Galloway.) 



then through the branchial arteries and is systemic ally 

 distributed, while a small portion passes to the lungs, 

 and then through pulmonary veins into the auricle. 

 The only modification of the heart itself is a partial 

 division of the auricle into two ill-defined chambers 

 right and left auricles. The right auricle which receives 



