468 Vertebrata. 



Like the feathers of BirdSj hairs are shed at regular intervals 

 and replaced by new ones ; each is detached from the papilla, 

 and a new one arises from the base of the follicle. In some 

 Mammalia (Man and the Apes) this moulting goes on 

 gradually throughout the whole year, now one hair, now another 

 being thrown off and replaced by a new one. In others, however, 

 it is confined to a short period recurring annually ; and usually, for 

 northern forms, in the spring,* when a total ecdysis takes place, both 

 wool and hair being thrown off. Simultaneously new hairs arise, and 

 the tips of the wool hairs bud out, but complete their development 

 later in the year. In some animals which are dark in summer, white 

 in winter, there is a colour changet of the summer coat in 

 late autumn, in the Alpine Hares, for instance. In others, in which 

 the summer and winter coats are different, a change of hair occurs in 

 the autumn, as well as in the spring, e.g., in Stags. 



In many Mammals some portion of tlie body is covered by scales or plates 

 like those of the Reptiles (Manis, Dasypus, tails of Mice). Sometimes, e.g., in 

 Dasypus, the dermal part of each scale contains an ossification; and 

 besides this there are in several Mammalia, smaller or larger independent 

 ossifications in the dermis. 



Small glands in connection with the skin are usually distributed over 

 almost the whole surface ; two principal kinds may be distinguished, 

 sebaceous and sudoriparous. The sebaceous glands are 

 small and racemose, and usually open into the hair follicles ; rarely 

 directly on to the surface ; they are therefore generally absent from 

 hairless tracts, they secrete a fatty substance. The sudoriparous 

 glands are simple and tubular, with the lower portion, usuallj' 

 lying in the loose layer below the skin, coiled into a knot. They also 

 often discharge their secretion into the hair follicles, but closer to 

 the surface than the sebaceous glands ; many open quite independently, 

 in great numbers, for instance, in certain naked tracks. Like the 

 sebaceous glands, too, they occur on various regions, in various 

 numbers, and are of various sizes. Most of them secrete sweat ; the 

 secretion may, however, be more fatty ; for instance, the wax-glands 

 of the ear are peculiar sweat-glands. 



The mammary glands, common to all Mammalia, are 

 peculiarly modified sudoriparous glands. Those of the M o n o - 

 tr ernes are of the most primitive type; here, there is, on either 

 side of the abdomen, a small hairy pit into which a number of large 

 branched sudoriparous glands open ; they secrete milk. In other 

 Mammalia they open u]3on somewhat projecting papillae, the 

 mammillae, or nipples, usually several upon each (about twenty in 

 Man, five to eight in the Dog, two in the Horse), seldom there is only 



* But a change of some hairs may also occur at other tunes. 

 t Cf., change of coloiir in feathers, p. 434. 



