312 VERTEBRATE ANIMALS. 
feature in the two lowest orders of the Mammalia. The senses, 
as a rule, attain great perfection in the Mammals ; and the only 
sense which can ever be said to be entirely wanting is that of 
sight. Eyes, however, are always present, though they may be 
rudimentary ; and in those Mammals which are said to be 
“blind,” it is not generally that the eyes are wanting, but that 
the skin passes unbrokenly over the eyeball. Even in these 
cases, however, it is not impossible that there may be some per- 
ception of light through the skin. An external ear for collecting 
sounds is usually present ; but it is wanting in the Whales and 
Dolphins, and in some of the Seals. 
The s#z7 is invariably furnished over a greater or less part of 
its surface with the epidermic appendages known as faz7s, 
which differ from feathers chiefly in not splitting up as they 
are produced. In the scaly Ant-eater (JZanzs), the hairs are 
ageregated together so as to form horny scales; and in the 
Hedgehog, Porcupine, and other animals, many of the hairs are 
developed into long spines or prickles. In other cases again, as 
in the Armadilloes, the skin is more or less covered by an armour 
of bony plates. The only apparent exception to the universal 
presence of hair on some part or other of the integument 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 
neighbourhood of the mouth, even when adult. And the Dol- 
phins, 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 mamme 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 abdomen, and not 
by distinct nipples. In ordinary Mammals the milk is obtained 
by voluntary suction on the part of the young, but in the Mar- 
supials (Kangaroos, Opossums, &c.) the milk is forced into the 
mouth of the young animal by the action of a special muscle. 
