CHARACTERS OF VERTEBRATES. 173 



hind-pair — and internally having a skeleton embracing the 

 features already noticed. 



In the iishes the head is continuous with the body, the 

 fore and hind legs are represented by fins, one pair directly 

 back of the head, corresponding with the front-legs, and an- 

 other pair of fins, which represent the hind-legs, variously 

 situated behind, below, or even in front of the first pair. 



155. The animals which possess a vertebral column, and 

 hence called Yertebrates (from the Latin word verto, "I 

 turn "), are the fishes, frogs, toads, salamanders, snakes, liz- 

 ards, turtles, crocodiles, birds, and the warm-blooded, four- 

 legged beasts, such as the cat, dog, sheep, horse, and elephant. 

 These last-mentioned creatures belong to a large class of 

 animals called Mammalia, the leading features of which are, 

 that the young are born alive, and the mother nurses the 

 young. 



The fishes, frogs, toads, salamanders, snakes, lizards, tur- 

 tles, and crocodiles, and others like them, are cold-blooded, 

 while the birds and mammals are warm-blooded, and all of 

 them, except the mammals, lay eggs from which their young 

 hatch. The fishes, snakes, lizards, and other reptiles are, 

 generally speaking, covered with scales. The frogs, toads, 

 and salamanders, are smooth-skinned ; the birds are covered 

 with feathers, while the mammals, with few exceptions, are 

 clothed with fur. In the general grouping of the verte- 

 brates the fishes and amphibians — namely, the toads, frogs, 

 and salamanders — form one group, the reptiles and birds 

 form another group, and the mammals a third group. 



