SCALES, HAIRS, AND FHATHERS. 383 
Tn 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. 884.—Vertical section through the skin of an embryonic shark. (, corium or 
dermis ; c,¢, ¢, layers of the corium ; d, uppermost layer ; p, papilla ; #, epidermis ; 
é, its layer of columnar cells; 0, enamel layer.—After Gegenbaur. 
feathers of birds and the hairs of mammals. Most scales 
arise from dermal papille (Fig. 384, 7), and are covered over 
by a layer of enamel (Fig. 384, 0) 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 arises in the same way asa 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 
feathers of birds are epidermal. In the horns of man:mals, 
as of the rhinoceros, and the hoofs of the horse, the epi- 
dermal substance is penetrated by numerous long dermal 
papillee. ' 
