146 ELEMENTS OF COMPARATIVE ZOOLOGY. 



forced out through the artery (ventral aorta) connected 

 with the ventricle. 



In all fishes there is but a single auricle and a single ven- 

 tricle, but when lungs appear, as in the batrachia, 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. 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 bulbus, without valves, and in front 

 of these the ventral aorta. From this lateral vessels 

 (afferent branchial arteries) are given off, and these pass up 

 through the branchial septa. Consequently the number of 

 these arteries primarily depends upon the number of gill- 

 clefts. In the septa the arteries break up into capillaries 

 which pass through the gills, and collect in efferent bran- 

 chial 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. 



From these vessels the blood passes through the capillaries 

 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 nourish- 

 ment obtained from the alimentary canal, and waste from 

 all parts of the body. Its color is a dark purplish red. In 



