THE FIVE CLASSES OF VERTEBRATES. 125 



tion is incomplete ; the two ventricles of the heart are 

 generally in communication with each other ; the heart 

 has then only three chambers ; the arterial blood and the 

 venous blood are thus mixed in the lower chamber, so 

 that only a portion of the venous blood goes to the lungs 

 for aeration before passing to the arteries, and a portion 

 of the arterial blood returns immediately to the lungs 

 without having traversed the general circulation. Res- 

 piration is aerial and pulmonary, but it is a function 

 always somewhat sluggish. Reptiles are oviparous, or 

 exceptionally ovoviviparous ; their bodies are usually 

 covered with scales. 



Examples of reptiles : alligator, lizard, tortoise, snakes. 



IV. Batrachians are related to reptiles by their gen- 

 eral characters, but they are distinguished from these 

 by the fact that during the early part, and sometimes 

 during the whole course, of their lives respiration is 

 aquatic and is accomplished by gills. The skin of 

 batrachiaris is naked, having neither hair, feathers, nor 

 scales. 



Examples of batrachians : frog, toad, salamander. 



V. Fish, like reptiles and batrachians, are cold-blooded 

 animals. They have complete circulation, that is, there 

 is no mixture of arterial and venous blood ; but the heart 

 is single instead of double, and has only two cavities. 

 These cavities correspond to the right heart of mammals; 

 they are traversed only by the venous blood, which then 

 passes to the respiratory organs. Respiration is aquatic, 

 and takes place by the aid of gills. Fish reproduce their 

 kind by eggs ; several species are ovoviviparous. Their 

 bodies are covered with scales. 



Examples of fishes : pike, carp, bass, eel, sturgeon, ray, 

 trout, salmon, perch, catfish. 



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