SKIN. 695 



the lifetime of the animal : thus most young deer have spots, but only 

 the Fallow and Axis deer retain these when adult. To an excess of 

 pigment is due the variation known as melanism or blackness, e.g. in 

 black wolves and rabbits ; to a dearth of pigment albinism is due, as in 

 white mice and white elephants. In tropical countries the skin is some- 

 times very darkly coloured, as in Indian cattle ; and many monkeys 

 especially males are notable for the bright colours of the bare parts of 

 the body. 



Among other tegumentary structures are the scales which 

 occur along with hairs on the pangolins (Manis] ; the 

 scales on the tails of rats and beavers and some other forms ; 

 the thickened skin-pads or callosities on the ischia of apes, 

 the breast of camels, the legs of horses ; the nails, claws, or 

 hoofs which ensheath the ends of the digits in all Mammals 

 except Cetaceans. Unique is the armature of the armadillos, 

 for it consists of bony plates developed in the dermis, 

 overlaid by epidermic scales. The median solid horns of 

 the rhinoceros are epidermic outgrowths, comparable to 

 exaggerated warts ; the paired horns of the Ruminants con- 

 sist of epidermic sheaths covering outgrowths of the frontal 

 bones, but extending far beyond these ; the antlers of 

 stags are outgrowths of the frontal bones, are cast and re- 

 grown each year, and are possessed by the males only, 

 except in the reindeer. 



The skin of Mammals, unlike that of Birds, is rich in 

 glands. Sebaceous glands are always associated with the 

 hair follicles, and sudorific or sweat glands are scattered 

 over the skin. 



Specialised glands are also very common, especially those which 

 secrete some strongly odoriferous stuff, scenting which the animals 

 recognise their fellows, their foes, or their prey. Often they are most 

 developed in the males, and their activity increases at the pairing season. 



Among the numerous special glands may be noted those which are 

 connected with a perforated spur on the hind-legs of male Monotremes, 

 the sub-orbital glands of antelopes and deer, the anal glands of carnivores, 

 the perineal glands of the civet, the preputial glands of the musk-deer 

 and beaver, the inter-digital glands of the sheep. 



Most characteristic, however, are the mammary glands, 

 functional in female Mammals after parturition. They 

 seem to be specialisations of sebaceous glands, except in 

 Monotremes, in which they are nearer the sudorific type. 

 They consist of branching tubes opening by one or several 



