TEGUMENTARY ORGANS. 655 



rapidly and frequently, that of the frog and triton being 

 apparently shed every month. Their copious exterior secre- 

 tions, perhaps, also demand a higher cutaneous oxygenation 

 and a thinner epidermic covering. The epidermis of the 

 triton is shed in an entire piece, as in serpents. On the 

 surface of the tough, thick, fibrous, papillated cutis of ophi- 

 dian and saurian reptiles, the soft rete mucosum, composed 

 of newly-formed cytoblasts, generally presents the most in- 

 tensely and lively coloured pigment-cells, which fade or die 

 before they are shed with the concrete outer layer of epider- 

 mis. As the apparent scales and scuta are only elevated 

 papillae or tubercles of the vascular secreting cutis, some- 

 times partially imbricated, the epidermis passes continuously 

 over them, and is thus cast from the entire body without 

 perforations or scales, and even from the united transparent 

 eye-lids or conjunctiva of serpents, as from the compound 

 eyes of articulata. The tortoise-shells, or epidermic plates, 

 covering the osseous elements of the carapace and plastron 

 of chelonia, are permanent accumulations of cytoblasts, 

 formed in successive and increasing layers from the subjacent 

 vascular periosteum, like the nasal horns of the rhinoceros, or 

 the permanent vaginiform horns of ruminantia; and the 

 limits of the successive layers of growth, are here commonly 

 indicated by peripheral ridges on the exterior of the plates, 

 as on the shells of conchifera and of gasteropods. 



The sparkling and resplendent surface of most fishes, ac- 

 cords with the liquid element and its pebbled bed ; the dull 

 and sombre surface of most amphibia and serpents, accords 

 with their concealed habitats; and the more lively colours 

 of many climbing ophidia and lacertine sauria, are adapted to 

 their arboreal life. The mutable colours of the cham3elion, 

 conceal it from its insect prey ; the dark rough surface of 

 crocodilian sauria, conceal them on the muddy banks of 

 rivers, or among the decayed trunks of fallen trees ; the dark 

 dull surface of most terrestrial chelonia corresponds with their 

 lurking and burrowing habits ; the lustre, transparency, and 

 mottled brown colours of marine turtles, pervading the whole 

 substance of their large, permanent, epidermic plates, resemble 

 those of the dark fuci, on which they rest and feed. The 

 parasitic pigment-cells of epidermic cytoblasts, are little de- 

 veloped amid the snows and darkness of arctic regions, where 



