CHAPTER XXXVIII 

 THE STRUCTURE AND ACTIVITIES OF MAMMALS 



Mammals are popularly known as " animals " or beasts. 

 We are all familiar with the domesticated species such as the 

 dog, cat, horse, cow, etc. and with many of the wild forms. The 

 term mammals was applied to the group because the young, 

 which are born in a very immature condition, are fed with milk 

 from the mammary glands of the mother. There are about 

 7500 species of living mammals, but only a small proportion 

 of these occur in this country. Mammals range in size from 

 the mouse at one extreme to the whale at the other extreme. 

 Among the simpler species are the egg-laying mammals of Aus- 

 tralia and the opossum and kangaroo which carry their young 

 about with them in a pouch. Other well-known species are 

 the moles, shrews, bats, dogs, cats, seals, rabbits, rats, ant 

 eaters, armadillos, camels, deer, horses, elephants, whales, 

 monkeys, apes, and man. 



Habitats. — There is great diversity among the members of 

 the phylum Mammalia, due chiefly to their various modes of 

 life. Most of them live on the ground, but many are aquatic, 

 others arboreal, and a few aerial in habit. The whales, dolphins, 

 seals, walruses, and sea cows are aquatic, living almost without 

 exception in the sea. They are not aquatic in the same sense 

 that fish are, however, since they cannot take oxygen from the 

 water, but must come to the surface to breathe. 



Among the arboreal mammals are the monkeys, squirrels, 

 and sloths. Some of the squirrels can even " fly " through 

 the air for short distances, but as in the case of the flying 

 dragon flight here is really only sailing through the air on 



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