SCALES, UAIBS, AND FEATHERS. 



403 



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 exoskeleton consists of the scales of fishes, 

 reptiles and certain mammals, such as the armadillo, the 



Fig. 384. — Vertical section through the ekin of an embryonic Rharli. C, corium or 

 dei'mis ; c, c, c, layers of the corium ; d, uppermost layer ; p, papilla ; E, epidermis ; 

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



feathers of birds and the hairs of mammals. Most scales 

 arise from dermal papillae (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 sliarks and rays, for example, 

 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 liair or feather. 

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

 the claws 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 

 papillse. 



