COVERING OF THE MAMMALIA. HAIR. 95 



and the other Mammalia of the order Marsupialia ; animals 

 which principally inhabit New Holland. The young complete 

 their development in the interior of this pouch, each suspended 

 by a teat, which penetrates deeply into its mouth, and from which 

 the milk is conveyed into the throat, by the contraction of the 

 muscles of the mammary glands. When arrived at a certain age, 

 they detach themselves : but they still continue to derive support 

 from their mother ; and even for a long period after having left 

 the pouch which has until then been their home, they seek it as a 

 refuge against cold or the dangers by which they are threatened. 

 75. Integuments. The skin, as we have already stated, pre- 

 sents amongst the Mammalia many remarkable peculiarities. In 

 a small number of these animals it is bare ; but in the principal 

 part it is covered with hairs, which serve to protect it, and to 

 preserve the heat developed within the body. The existence of 

 these appendages of the skin is so characteristic of this class, that 

 one of the best modern Zoologists, M. de Blainville, has pro- 

 posed to replace the name of Mammalia by that of Pilifers (hair- 

 bearing) ; which would contrast with the word Pennifers (feather- 

 bearing) and Squamifers (scale-bearing) which he would wish 

 to be adopted to designate Birds and Fishes. The hairs are pro- 

 duced by small secreting organs in the thickness of the true skin, 

 or immediately under it. Each hair is formed in a small egg- 

 shaped bag, with white and resisting walls ; which opens at the 

 surface by a narrow orifice, and which is called the capsule. 

 The interior of this cavity is lined by a membrane, sometimes 

 reddish, sometimes differently coloured, which appears to be a con- 

 tinuation of the outer layer of the skin ; and at its lower part is 

 found a conical papilla or bulb, which receives a nerve and blood- 

 vessels, and which produces the hair. The substance of which 

 these appendages are principally composed, is more analogous to 

 horn than any other. In examining them with a microscope, we 

 can generally distinguish a sheath of horny consistence, like that 

 which forms the barrel of a quill, enveloping a softer medullary 

 or pith-like substance, the tubular character commonly attri- 

 buted to them being an optical illusion. Amongst the greatest 

 number of Mammals the hairs are cylindrical, or rather larger at 



