SCALES, HAIRS, AND FEATHERS. 



383 



In the fishes, Amphibians and reptiles, the worn-out teeth 

 are replaced by a succession of new ones ; in mammals (ex- 

 cept cetaceans, where there is no change) there is but a single 

 change, the first (milk) teeth being replaced by a second set 

 ■of permanent teeth. The teeth of the lower Vertebrates 

 are shed while swallowing the food. In the boa (Python) 

 the teeth thus shed are found scattered along the intestinal 

 canal and are discharged with the remnants of the food 

 (Wyman). 



The dermal or exoshelefon consists of the scales of fishes, 

 Teptiles and certain mammals, such as the armadillo, the 



Fig. 384. — "Vertical section through the skin of an embryonic shark. C, corium or 

 ■dermis ; c, c, c, layers of the corium ; d, uppermost layer ; p, papilla ; E, epidermis ; 

 -e, its layer of columnar cells ; o, enamel layer. — After Gegenbaur. 



feathers of birds and the hairs of mammals. Most scales 

 arise from dermal papillfe (Fig. 384, p), and are covered over 

 by a layer of enamel (Fig. 384, o) developed from the epider- 

 mis ; so that the scales of sharks and rays, and turtles, 

 arise from both the dermis and epidermis. 



A hair or feather is a modification of a scale ; the papilla 

 is sunken in a pit of the dermis, the conical cap of epi- 

 dermis arising from it ultimately forming the hair or feather. 

 The plates of turtles, the scales of snakes and lizards, and 

 leathers of birds are epidermal. In the horns of mammals, 

 as of the rhinoceros, and the hoofs of the horse, the epi- 

 dermal substance is penetrated by numerous long dermal 

 papillae. 



