28 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
Possibly the most striking of the epidermal organs are the luminous 
organs or photophores, which are most common in elasmobranchs and 
teleosts from the deep seas, where sunlight does not exist. They are 
apparently modified glands, and the development is known in Porich- 
Fic. 19.—Section of skin of Protopterus. c, corium; e, epidermis; g, multicellular gland; 
u, unicellular gland. 
thys. There is an involution of cells of the Malpighian layer into the 
corium, where they become cut off from their point of origin, and are 
differentiated into a deeper glandular layer and an outer rounded body, 
the lens (fig. 21). Around this the corium forms a reflecting layer 
Fic. 20.—A, head of Noturus flavus; B, section of poison gland of Schilbeodes miurus 
(after Reed). ¢, epidermis; p, pore of poison gland, pg; s, spine of pectoral fin. 
enclosed in a pigment coat. The glandular layer is the seat of light 
production. In other photophores either reflector or pigment may be 
lacking, but in their highest development they so resemble an eye that 
at first they were described as such. 
In the myxinoids the skin contains numerous thread cells in pockets which may 
extend into the underlying muscles. Each thread cell contains a long thread, which 
