PRODUCTION OF LIGHT 429 



mentioned above, of more doubtful function than those we have 

 already described, it is possible that the chief facts of structure 

 and development established for them will be found to be true 

 of the majority of others. The skin of Porichthys is destitute of 

 scales, the epidermis is glandular, the derma thick. The phos- 

 phorescent organs are situated in the deeper part of the derma 

 near its internal surface. Each organ (Fig. 31, C.) consists of lens, 

 gland, reflector, and pigment, succeeding each other in the order 

 mentioned from without inwards. The reflector is thick and is 

 shaped like a conical cup with its aperture towards the surface 

 of the skin : lining the cavity of the cup is a layer of large 

 nucleated granular cells which have the character of gland-cells, 

 though of course in this case, as in other luminous organs, there 

 is no duct and the secretion does not escape to the exterior. 

 The rest of the cavity of the cone is filled with a mass of smaller 

 cells which are dense, homogeneous, transparent, and highly re- 

 fractive ; this mass is the lens, but it has no smooth regular 

 surfaces like the lens of an optical instrument or like that of the 

 eye, its density and transparency cause it to refract and con- 

 dense the light, but there is no need for a definite focus or for the 

 formation of an image as in an organ of sight. The glandular 

 layer is well supplied with blood-vessels. On the outside of 

 the reflector there are some pigment cells. No nerve bundles 

 or conspicuous nerves pass to the phosphorescent organs, they 

 are merely supplied by small nerve fibres from the ordinary 

 nerves of the skin. The development of these organs was 

 completely traced out in young fishes ; the lens-cells and gland- 

 cells arise as a thickening of the deeper or internal layers of the 

 epidermis and the thickening forms a little mass of cells 

 which grows inwards into the derma and then becomes separ- 

 ated from it, sinking into the substance of the derma where, on 

 the base and sides of the mass, are developed the reflector and 

 pigment layer, which are merely local developments of the 

 iridocytes and pigment cells of the derma. The mass of cells 

 derived from the epidermis differentiates into the lens-cells and 

 gland-cells. Although the luminous organs are in many parts of 

 the fish in proximity to the sense-organs of the skin, they have 

 no connection with these structures, which also arise as special 

 developments of epidermic cells, but they have quite a different 

 structure and grow outwards to the surface of the epidermis not 



