MORPHOLOGY OF ANIMALS 



A 



de \ 





Fig. 6. Sections of the Integument of A, young salamander ; ep, epidermis only two cells 

 thick, de, dermis consisting of branched cells and a felt-work of fibers ; B, embryo shark 

 showing formation of denticle (scale) ; its core of dentine d, resting on a plate of bone b, 

 both from the dermis de, and its outer surface covered by enamel e from the columnar 

 epidermal cells c ; C, developing tooth, ep, epidermis of mouth infolded to form enamel cells 

 ec which secrete enamel (black line) ; dp, dental papilla from dermis, covered on outer side 

 by odontoblasts o which secrete dentine dt. (B and C after Wiedersheim). 



at the margins of the shell gland (mantle edges) the secre- 

 tion of these layers continues throughout life. In reptiles, 

 birds, and mammals the superficial epithelium (epider- 

 mis) becomes many layers thick and the outer layers of 

 cells die and are transformed into horny or cuticular sub- 

 stance, an adaptation to life out of water (Fig. 7). In 

 these three classes of vertebrates there are also a number 

 of characteristic epidermal outgrowths: in reptiles these 

 take the form of horny scales or plates; in birds 

 they appear as feathers which are only modified scales; 

 and in mammals as hair (Fig. 7), while nails or claws 

 are formed from the epidermis in all of these classes. In 

 the mammals there are also epidermal ingrowths which 



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