CLASSES OF VERTEBRATES. 



5 



in the various classes of Vertebrates, that they are sometimes distin- 

 guished into Cold-blooded and Hot-blooded Vertebrate. 



To the Cold-blooded Vertebrata belong the countless race of 

 Pisces, or fishes. They breathe by gills ; the heart is usually of two 

 chambers ; the limbs when present become fins. The Amphibia have 

 gills and a two-chambered heart when young ; when old, lungs and a 

 heart of three chambers ; the limbs never become fins. The Reptiles 

 have lungs, not gills, a three-chambered heart, and the skin covered with 

 plates or scales. 



The Hot-blooded Vertebrata contain two classes: Ayes, or birds, 

 and Mammalia. In the birds the lungs are connected with air recep- 

 tacles in various parts of the body ; the heart consists of four chambers ; 

 the skin is covered with feathers ; the forearms become wings. Like 

 fishes and reptiles, birds are oviparous or egg-layers. The Mammalia 

 form the subject of this volume. 



The subjoined table will show the method of division or classification 

 that we have adopted in order to narrow our field of view from the 

 world at large down to that portion of its inhabitants to which we our- 

 selves belong. Classes are divided again into Orders; Orders sub- 

 divided into Genera, and Genus into Species. Species may be so 

 modified by accidental external circumstances as to give rise to Varieties, 

 and Varieties embrace more or fewer Individuals. 



