GROUPS OF ANIMALS 



255 



such as worms and insects, do not have a back bone and 

 are known as the invertebrates. To the vertebrates 

 belong most of the animals with which you are already 



Fig. 110. The backbone of a snake is composed of many vertebrae. 



familiar from personal observations. You, perhaps, have 

 never attempted to classify them. 



Mammals. Mammals are considered the highest ver- 

 tebrates because of their 

 complicated structure 

 and because of their 

 greater mental develop- 

 ment. The animals in 

 this group are so called 

 because they produce 

 milk, a secretion of the 

 mammary glands, which 

 is used as food for the 

 young. The young of 



almost all mammals are born fully formed and then for 

 some time are cared for by the parents. The skin of 

 mammals is partly or completely covered by hairs. 



Fig:. 111. Two mammals. 



