M, 



CHAPTER 30 



U)L 



OJI 



Class Mammalia. 

 Mammals 



Lan is a mammal, a vertebrate with tne 

 most advanced brain and mental develop- 

 ment. The name is derived from the fact 

 that mammals possess mammary glands 

 which secrete milk for the nourishment of 

 their young. Mammals also possess cover- 

 ings of hair at some time in their existence 

 and are distinguished by this characteristic 

 as certainly as birds are by their feathers. 

 With few exceptions, adult mammals are 

 provided with at least a small number of 

 hairs, and all are warm-blooded. Parental 

 care is generally most highly developed in 

 this group and reaches its climax in humans. 

 The 5000 or more species of living mam- 

 mals are found in all kinds of habitats from 

 the tropics to the polar regions, and from 

 the oceans to the driest deserts. Some are 

 hunted for sport and others for their furs. 



The majority of the mammals are vivip- 

 arous, and their young are nourished before 

 birth through placental tissue, which con- 

 nects the mother with her offspring. The 

 placenta is unique among all animals as an 

 adaptation for the developing young. It 

 makes possible the development of the 

 voung to a more advanced stage before birth. 

 Since detailed studies of mammals are 

 ordinarily reserved for special courses in 

 mammalian biology, only a brief account is 

 presented here to furnish a general knowl- 

 edge of the structure and physiology of a 

 mammalian type. An idea of the variety of 

 forms (Fig. 337) and activities exhibited by 

 mammals is indicated in the section on 

 other mammals. Some of the more interest- 

 ing features of mammals are described here, 

 such as integumentary structures (hair, 

 teeth, nails, claws, cutaneous glands, and 

 others), the development of the young from 

 the egg, hibernation, migration, domestica- 

 tion of species, fossil forms, and the rela- 

 tions of mammals to man. The cat is so 

 much like man in structure and function 

 that a study of it is a good introduction to 

 the human body. In Chapters 31 to 34, the 

 morphology and physiology of man are con- 

 sidered. 



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