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CLASS V.* MAMMALIA. 



(MAMMALS.) 



THE peculiarity which, is most distinctive of this 

 Class, the most exalted of all living creatures, 

 is that its members bring forth living young, 

 which they suckle and thus nourish in the period 

 of early infancy with a fluid secreted in the body 

 of the mother. 



The blood is warm, circulating in closed vessels ; 

 the whole quantity is sent from the heart to the 

 lungs, a spongy mass of minute cells in the cavity 

 of the body, communicating with the external air. 

 Over the surface of these cells the blood-vessels 

 ramify, their coats becoming so thin as to admit 

 the union of the oxygen with the contained blood, 

 the result of which is an instantaneous change of 

 its colour from dark-red to vivid scarlet. This 

 renewed blood is now returned by another set of 

 vessels to the heart, whence it is ejected into the 

 arteries, which, large at first, but branching with 

 innumerable ramifications, convey it to every part 

 of the body, at length transferring it through 

 tubes (capillaries] of excessive minuteness into 

 the veins. These now carry it back to the heart, 

 whence it is again pumped, as it were, into the 

 lungs again. The heart is thus composed of four 



* Of the intervening Classes, AMPHIBIA, EEPTILIA, and 

 AVES,. EEPTILIA is the only one which includes marine species ; 

 and of these none can properly be considered as British. 



