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CHAPTER XXXI 



MAMMALIA 



The class Mammalia (mam'ma'li a, milk-forming) includes many 

 of the animals most familiar to man. Nearly one-third of the total 

 number of them occur in America. Besides our common domestic ani- 

 mals, such familiar forms as rats, mice, ground hogs, bats, foxes, bears, 

 deers, seals, whales, man and many others belong in this group. 

 The skin of them all is provided with sweat glands and sebaceous 

 (oil) glands and is more or less covered with hair. These animals 

 are tj^pieally quadrupeds with five digits on each limb. The females 

 have well-developed mammary glands which secrete milk for the 

 nourishment of the young. There is usually a distinct division of 

 the body into head, neck, trunk, and tail regions. All mammals have 

 a definite temperature regulation and are said to be homoiothermal, 

 or warm-blooded. The body temperatures of different species vary 

 from 77° F. to 104° F. In mammals, as in birds, the heart is com- 

 pletely divided into four principal chambers. The single systemic 

 arch of the aorta is the left one. Kespiration is carried on by lungs 

 and the anterior end of the trachea is modified to form a larynx 

 capable of sound production. In general. Mammalian blood contains 

 nonnucleated, circular red corpuscles ; however, in the camel they are 

 oval in shape. The thoracic cavity of the mammal is separated from 

 the abdominal cavity by a complete diaphragm. 



Classification 



The entire class includes approximately 4,000 species of living 

 mammals and 3,500 fossil forms. The class is divided into two sub- 

 classes, several divisions and a number of orders. 



Subclass Prototheria. — The primitive, egg-laying mammals. This 

 group includes only a single order. 



Order Monotremata. — The most primitive mammals and the only 

 ones that lay eggs are placed here. They are limited in their dis- 

 tribution to Australia, Tasmania, and New Guinea. The eggs are 

 similar to turtle eggs and are laid either in a pouch on the female 's 

 abdomen, as in the spiny anteater, or in a tunnel in the ground near 



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