461 



Comparative Morphology of Chordates 



ated into scales, each of which is merely a local thickening of the 

 stratum corneum, joined to one another by intervening thin bands of 

 the horny layer (Fig. 357). This arrangement greatly increases the 

 flexibility of the protective covering. In some reptiles the horny scale 

 is reinforced by an underlying bony plate produced in the dermis. 

 Horny scales appear locally in birds and in some mammals, but it is 

 only in reptiles that they have become so highly elaborated as to be one 

 of the most distinctive characteristics of the Class. 



In contrast to the highly glandular skin of amphibians, reptilian 

 skin is totally devoid of glands except that in some cases there are 

 localized and highly specialized glands, such as the pair of scent 

 ("musk") glands situated on the under side of the head in alligators 

 and some turtles (the "stinkpot" or "skunk turtle"), and similar 

 glands in the cloacal region of crocodilians and some snakes. In all 

 modern reptiles the general surface of the skin is hard and dry — the 

 surest external means of distinguishing a lizard from a newt, which 

 resemble one another except in the soft, moist skin of the latter. 



Comparing amphibian and reptilian skins, it is evident that nothing 

 new is to be found in the latter. Each has its stratum corneum, in 

 amphibians exceedingly thin, in reptiles tremendously thicker and 

 locally formed into scales. Mucous glands in the one are absent from 

 the other. 



Respiratory interchange of gases can take place through the 

 delicate fdm of horny material on an amphibian skin, but it is quite 



Chromatopi'iores 



Epidermis 





Fig. 357. Bony scales and horny scales. Diagrammatic sections perpendiculai 

 to surface of skin. (A) Dermal bony scales of a fish. (B, C) Epidermal horny scaler 

 of reptiles: (B) flat; (C) overlapping. 



