THE SKIN AND SKELETON 



337 



These scales in the higher animals are constantly 

 wearing off in the form of scurf, and as constantly being 

 renewed from below. In lizards and serpents, the old 

 epidermis is cast entire, being stripped off from the 

 head to the tail ; in the toad it comes off in two pieces ; 

 in the frog, in shreds ; in fishes and some mollusks, in 

 the form of slime. However modified the epidermis, 

 or whatever its appendages, the like process of removal 



FIG. 291. Section of Skin from Horse's Nostril (magnified) : E, epidermis; D, dermis; 

 i, horny layer of epidermis; 2, rete mucosum; 3, papillary layer of dermis; 4, ex- 

 cretory duct of a sudoriparous, or sweat, gland; 5, glomerule, or convoluted tube of 

 the same; 6, hair follicle; 7, sebaceous gland; 8, internal sheath of the hair follicle; 

 9, bulb of the hair; 10, mass of adipose tissue. 



goes on. Mammals shed their hair ; birds, their feathers; 

 and crabs, their shells. When the loss is periodical, it 

 is termed molting. 



2. The Skeletons. (i) The Exoskeleton is developed by 

 the hardening of the skin, and, with very few exceptions, 

 is the only kind of skeleton possessed by invertebrate 

 animals. The usual forms are coral, shells, crusts, 

 scales, plates, hairs, and feathers. It is horny or cal- 

 careous ; while the endoskeleton is generally a deposit 

 DODGE'S GEN. ZOOL. 22 



