CHAPTER XXV 

 MAMMALS 



Although by no means the largest numerically, the class of 

 mammals, Mammalia, is by far the most important group of 

 animals. Not only does it include man himself but practically 

 all of the domestic animals besides scores of others that add to 

 his welfare by furnishing him food or clothing. The name 

 Mammalia refers to the mammary glands of the female which 

 furnish milk for the nourishment of the young for some time 

 after its birth. 



In size, the mammals range from the tiny pigmy-shrew of 

 fields and meadows to the great whales which attain a 

 length of eighty to a hundred feet and a weight of many tons. 

 In structure and habits there is also a remarkable range of 

 variation. Most mammals live on land and their legs are 

 usually well fitted for walking, running or jumping, but some 

 live in trees and have their appendages adapted for holding 

 on to the branches or for taking considerable leaps through the 

 air. Others, like the burrowing gophers and moles, live in the 

 earth and have the forelegs fitted for digging. The water- 

 inhabiting forms often have their appendages modified into 

 fins or flippers and in other ways show remarkable adaptations 

 fitting them for their aquatic life. 



Body -form and Structure. Most mammals are clothed 

 with hairs, which are peculiarly modified epidermal proc- 

 esses. Each hair, usually cylindrical, is composed of two 

 parts, a central pith containing air, and an outer more solid 

 cortex; each hair rises from a short papilla sunk at the bottom 

 of a follicle lying in the true skin. In some mammals the hairs 

 assume the form of spines, or "quills," as in the porcupine. 

 The hairy coat is virtually wanting in whales and is very 

 sparse in certain other forms, the elephant, for example, 



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