TEGUMENTARY ORGANS-SKIN .STRUCTURES, 

 form horn, 



453 



they form horn, as in hair, nails, claws, hoofs, horns, 

 feathers, scales, whalebone, etc. If they calcify or ossify 

 by deposit of lime car- 

 bonate or phosphate, 

 then they form bony 

 scales, scutes, shell, 

 etc. 



Importance in 

 Classification. — As 

 the skin comes directly 

 in contact with and is 

 modified by the envi- 

 ronment, these struc- 

 tures are very charac- 

 teristic of the various 

 classes, orders, and 

 families of animals, 

 and are therefore very 

 important in classifi- 

 cation. We have al- 

 ready shown that teeth 

 are skin structures, 

 and their importance 

 in classification is uni- 

 versally acknowl- 

 edged. The structures 

 about to be described 

 are hardly less so. We 

 give only the most im- 

 portant. 



Hair. — True hair is entirely characteristic of mammals. 

 It is always formed in a follicle or infolded socket, in 

 which the epithelial cells multiply more rapidly than 

 elsewhere and at the same time harden to the condition 

 of horn (Fig. 308). Hairs, therefore, grow by successive 



Fig. 308. — Section of skin showing hair 

 follicle : pap, papilla ; ml, Malpighian 

 layer ; rsh, root sheath ; d, dermis ; sgl, 

 sebaceous gland ; c, cuticle ; cort, corti- 

 cal, and med, medullary part ot the hair. 



