62 



EVOLUTION AND GENETICS 



of one chamber force the blood through a single cycle about the 

 body and back again to the heart, with a maximum of three and 

 a minimum of two capillary systems interpolated in the various 

 divisions of this cycle. 



Terrestrial Vertebrates. In all vertebrates above the fishes air 

 breathing is the rule, and in the very lowest of these classes, 

 the Amphibia, another cycle is interpolated. Here we find a 

 three-chambered heart, the atrium divided into two auricles, but 



Fig. 41. — Stages in the development of the vertebrate heart, as illustrated by- 

 various classes. A, elasmobranch fishes; B, teleost fishes; C, amphibia; D, 

 lower reptiles; E, alligator; F, birds and mammals, a, atrium; ao, aorta; b, 

 bulbus arteriosus; c, conus; cd, duct of Cuvier or common cardinal vein; 

 h, hepatic veins; pa, pulmonary arteries; pc, precaval and postcaval veins; 

 pv, pulmonary veins; s, sinus venosus; sa, interatrial septum; v, ventricles. 

 (From Kingsley's Comparative Anatomy of Vertebrates, with the permission 

 of P. Blakiston's Son & Co.) 



the ventricle still a single chamber (Fig. 41C). The blood is 

 driven out through the truncus arteriosus into a series of paired 

 vessels, aortic arches, but they are only three in number (Fig. 

 43C). The posterior pair leads to the lungs and skin, both of 

 which serve as respiratory organs in this class. In the Amphibia 

 which have no gills, no capillary system is interpolated in the aortic 

 arches but the blood is aerated by being forced to the lungs or skin. 

 A limited portion of the body is therefore burdened with a function 

 dealing with all of the blood, and hence a second cycle is established 

 so that blood now courses from right auricle to ventricle to lungs 

 and back to the heart, where it enters the left auricle and is then 

 passed into the ventricle and pumped to the bod^^ This results 



