286 VERTEBRATE ANIMALS. 



The skin is invariably furnished over a greater or less part 

 of its surface with the epidermic appendages known as hairs, 

 which differ from feathers chiefly in not splitting up as they 

 are produced. In the scaly Ant-eater (Manis), the hairs are 

 aggregated together so as to form horny scales ; and in the 

 Hedge-hog, Porcupine, and other animals, many of the hairs 

 are developed into long spines or prickles. In other cases, 

 again, as in the Armadillos, the skin is more or less covered 

 by an armor of bony plates. The only apparent exception to 

 the universal presence of hair on some part or other of the in- 

 tegument of all Mammals is constituted by the true Cetaceans 

 (Whales and Dolphins), many of which are without hair when 

 grown up. Some, however, such as the Whales, have a few 

 bristles in the neighborhood of the mouth, even when adult. 

 And the Dolphins, which are totally hairless when grown 

 up, exhibit tufts of hair upon the muzzle before they are 

 born. 



The young Mammal is always born in a helpless condition, 

 and is nourished for a longer or shorter time by means of the 

 milk of the mother. The milk is secreted by special organs, 

 called the mammary glands, which are present in both sexes, 

 but are normally undeveloped in the male. The number and 

 position of the mammae vary a good deal in different cases, 

 but they are always placed on the lower surface of the body, 

 and their ducts almost always open upon a special eminence, 

 called the teat or nipple. In one or two cases, however, the 

 mammary glands open by simple slits in the skin of the abdo- 

 men, and not by distinct nipples. In ordinary Mammals the 

 milk is obtained by voluntary suction on the part of the young, 

 but in the Marsupials (Kangaroos, Opossums, etc.) the milk is 

 forced into the mouth of the young animal by the action of a 

 special muscle. 



So much difference of opinion obtains as to the best foun- 

 dation upon which to establish a division of the Mammalia 

 into great primary sections, that it has been thought advisable 

 to leave this subject wholly out of consideration. For our 

 present purpose it is enough to adopt the old classification of 

 Mammals into the two great divisions of the Placental and 

 Non-placental forms. In the Placental Mammals the young 

 is nourished within the body of the mother by means of a 

 structure called the placenta, or " afteH-birth," through which 

 the nutrient materials of the mother's blood reach the young. 

 In consequence of this, the young of the Placental Mammals 

 can be retained within the body for a considerable period, and, 



