INTEGUMENT. 29 
is discharged upon stimulation, the threads forming a network in which the mucus 
secreted by the ordinary gland cells is entangled. . 
The corium is.thin and consists of horizontal bands of fibrous tissue, crossed at 
intervals by vertical strands. Fat is common in the tela subcutanea, and in 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 con- 
sidered in connection with the skeleton. 
Fic. 21.—Section of luminous organ (photophore) of Porichthys, after Greene. e, 
epidermis with mucous cells; gi, glandular layer of photophore; /, lens; r, reflector sur- 
rounded by pigment. 
AMPHIBIA.—The amphibia are remarkable in that the epidermis 
of the larvee is ciliated in the early stages, and is two cells in thickness 
from the first. The skin, in the larve and the aquatic species, con- 
tains numerous mucus glands and some for the production 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. 
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 metamorphosis 
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 
(Astylosternus) the skin has the granules of the surface developed, at the breed- 
