230 GENERAL ZOOLOGY 



tiles it is three-chambered and pumps " half-pure" blood. 

 Birds and mammals show a four-chambered condition; 

 the blood on one side has been aerated and that on the 

 other is charged with waste products. 



There are three kinds of kidneys present in vertebrate 

 animals. These are believed to represent parts of a con- 

 tinuous series of nephridia, somewhat like those of annelids, 

 but in no vertebrate is more than one of the three functional 

 at a time. The most primitive vertebrates (Cyclostomes) 

 have only one kidney, the pronephros, or head-kidney. 

 Fishes and amphibians have a pronephros during early 

 development, but it soon degenerates and is replaced by 

 another kidney, the mesonephros. Reptiles, birds, and 

 mammals have first a pronephros, then a mesonephros, 

 but their functional kidney during adult life is the meta- 

 nephros, which develops after the other two have 

 degenerated. 



Other lines of vertebrate specialization might be described, 

 but enough has been said to show that it is possible to 

 speak in more positive terms concerning this group of ani- 

 mals than of any of those which have been discussed in 

 the preceding pages. Perhaps the most interesting thing 

 to keep in mind as we pass from primitive to specialized 

 vertebrates, is that in general evolution has been from 

 aquatic modes of existence to life on land. Structures and 

 functions have been accordingly modified. The fin, for 

 example, was good enough as an organ of locomotion in 

 the water, but cannot compare with a jointed leg ending 

 in toes for progress on land. 



There are seven classes of vertebrates which may be 

 briefly outlined as follows: 



Class 1. Cyclostomata. Eel-like vertebrates with a chorda and 

 median fins, but without paired fins or true jaws. Lampreys and 

 hagfishes. 



Class 2. Elasmobranchii. Fish-like vertebrates with cartilaginous 

 skeletons, jaws, placoid scales, and a persistent chorda. Sharks, 

 skates, rays. 



