

356 SYSTEMATIC ZOOLOGY 



entirely absent except in some of the marine species, and even 

 in such cases they are present during development; they con- 

 sist entirely of epidermal cells. Frequently there are two kinds 

 of hairs : the stiff contour hairs, which give the general shape 

 to the body, and the shorter, soft, woolly hairs ; in some instances 

 the contour hairs are veritable bristles and spines, as in the 

 hedgehog and porcupine. Usually the hairs fall out from time 

 to time and are replaced by new ones. 



Other structures also develop from the epidermis, chiefly 

 through cornification, such as claws, hoofs, nails, some horns as 

 in the rhinoceros, ox, and buffalo, the scales on the tails of rats 

 and mice, and the large scales of the pangolins. In some mam- 

 mals bony structures develop in the derma ; these form the 

 shell-like covering of the armadillo and the antlers of the deer 

 and moose. The skin of mammals is generally pigmented ; the 

 outermost layer of the epidermis is cornified and comes away 

 in small pieces. Glands arc generally numerous in the integu- 

 ment, and there are two kinds, the sweat glands, largely excre- 

 tory in function, and the sebaceous glands, which have an oily 

 secretion which keeps the skin soft. The latter always occur in 

 connection with the hairs and also on parts of the skin where 

 hairs are absent. There are various other glands, which are 

 modifications of these, the most Important being the milk or 

 mammary glands, from which the class derives its name. In 

 the lowest mammals the ducts from these glands open directly 

 at the surface of the skin, but in all the others there is an 

 elevation forming a teat or nipple, and of these there may be 

 from one to eleven pairs, arranged in two rows, either along the 

 whole ventral side of the trunk of the body, or confined to either 

 the thorax or the abdomen. The number corresponds in a 

 general way to the maximum number of young that may be pro- 

 duced at a birth. Mammary glands are well developed in the 

 female ; they are present in the male, but rudimentary, and nor- 

 mally are non-functional. All the Mammalia have two pairs of 

 appendages, except two of the marine orders, in which the pelvic 

 limbs are absent; there is naturally very great variation in the 

 form and size of the appendages, corresponding to the different 

 uses to which they are put in different species. 



The general shape of the body varies greatly, but we can gen- 



