VERTEBRATES. 311 



In all fishes there is but a single ar.ricle and a single ven- 

 tricle, but when lungs appear, as in the Amphibia, the 

 auricle becomes divided, and now one half (the right) 

 receives the blood from the body, while the left auricle 

 takes the blood returning from the lungs. These both 

 pour the blood into the single ventricle. In the reptiles 

 we find the beginning of a division of the ventricle, which 

 becomes complete in the crocodiles and continues in 

 birds and mammals (fig. 118). In these forms the left 

 auricle pours its blood into the left ventricle, while the 

 same relations exist between the auricle and ventricle of 

 the right side. 



In the fishes the blood leaves the ventricle by an arterial 

 trunk, in which, when best developed, we can distinguish 

 a conus with valves inside to prevent the blood flowing 

 back into the ventricle; or a bidbus, without valves, and 

 in front of these the ventral aorta. From this lateral ves- 

 sels (afferent branchial arteries) are given off, and these 

 pass up through the branchial septa. Consequently the 

 number of these arteries depends primarily upon the num- 

 ber of gill-clefts. In the septa the arteries break up into 

 capillaries which pass through the gills, and collect in 

 efferent branchial arteries which pass above the pharynx. 

 Here they unite and give rise to the main trunk, the 

 dorsal aorta, which runs, above the alimentary canal, 

 through the body, giving off vessels to all parts, see fig. 

 117, A. 



From these vessels the blood passes through the capil- 

 laries and is collected in veins which bring it back to the 

 heart to repeat the circuit. In this circulation the blood 

 changes in its character. When it enters the heart it 

 bears nourishment obtained from the alimentarv canal, 



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and waste from all parts of the body. . Its color is a dark 



