INTEGUMENT 



33 



some fishes this layer contains numerous crystals of guanin which gives it a 

 silvery appearance. This guanin forms the base of 'essence of pearl' from 

 which artificial pearls are made. The scales of fishes, although formed in the 

 skin, are considered in connexion with the skeletion. 



AMPHIBIA. — The amphibia are remarkable in that the epider- 

 mis of the larv£e is ciliated in the early stages, and is two cells in 

 thickness from the first. The skin, in the larvae and the aquatic 

 species, contains numerous mucous glands and some for the produc- 

 tion of poison, some of the' latter being prominent like the 'parotid 

 glands' on the neck of the anura and the gland on the back near the 

 base of the tail. 



Fig. 24. — Section of luminous organ (photophori) of Porickthys, after Greene, e, 

 epidermis with mucous cells; gl, glandular layer of photophore; /, lens; r, reflector 

 surrounded by pigment. 



The corium is thin, and in the frogs is separated from the underlying parts by 

 large lymph spaces which render the skinning of these animals so easy. As the 

 amphibia respire largely by the skin (there are several lungless salamanders) the 

 corium is richly supplied with blood-vessels, and at the time of the metamorpho- 

 sis of the anura these penetrate even into the epidermis, as at that time the lungs 

 are not yet functional and the gills are absorbed. The stratum corneum is shed 

 periodically, either as a whole (urodeles) or in patches. The warts of toads' are 

 in part cornifications of the epidermis, and a similar hardening of the skin on 

 the ends of the toes of some results in claws. In the males of an African frog 

 {Aslylosternus) the skin has the granules of the surface developed, at the breed- 

 ing season into hair-like structures, supplied with nerves and apparently sensory 

 in character. 



REPTILES. — All living reptiles are characterized by the exten- 

 sive development of horny scales and frequently of bony plates in 



