32 



COMPARATIVE MORPHOLOGY OF VERTEBRATES 



and teleosts from the deep seas, where sunlight does not exist. They 

 are apparently modified glands, and the development is known in 

 Porichthys. There is an involution of cells of the Malpighian layer 

 into the corium, where they become cut of! from their point of origin, 

 and are differentiated into a deeper glandular layer and an outer 



Fig. 22. — Section of skin of Protopterus. c, corium; e, epidermis; g, multicellular 

 gland; u, unicellular glands. 



rounded body, the lens (fig. 24). Around this the corium forms a 

 reflecting layer 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. They are 

 really 'dark lanterns.' 



Fig. 23. — A, head of Nolums flavus; B, section of poison gland of Schilbecdes niiurus 

 (after Reed), e, epidermis; p, pore of poison gland, pg; s, spine of pectoral fin. 



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 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 



