422 FISHES 



that in his specimen the light continued for some hours after 

 death. Nearly all the specimens have been taken at the 

 surface both in the Atlantic and Pacific, but the large eyes and 

 dark or black colour suggest that the fish lives in the dark, 

 perhaps coming to the surface only at night. Spinax niger, 

 which is also black and lives in deep water in the Mediterra- 

 nean, has been observed to be phosphorescent at the Zoological 

 Station of Naples, and the production of light is due to 

 numerous organs of simple structure. 



We have now to consider the principal points in the structure 

 of these light-producing organs and the question how the light 

 is produced. The structure of the organs in different fishes so 

 far as it has been investigated shows a great diversity, so that it 

 is difficult at present to explain them as modifications of a 

 common type. We will first describe the principal kinds and 

 then consider whether any general conclusions can be drawn as 

 to how the light is produced. All the organs are special 

 modifications of the tissues of the skin, and therefore to under- 

 stand them it is necessary to have some knowledge of the 

 ordinary structures present in the skin. The skin consists of 

 two layers, an outer called the epidermis consisting of several 

 layers of cells without blood-vessels and an inner portion, called 

 the derma which is thicker and is composed of felted fibres. 

 The epidermis is almost transparent and soft so that it is easily 

 removed by friction and in preserved specimens may be lost 

 altogether. The scales are thin calcified plates, that is horny 

 plates containing a certain amount of compounds of lime. The 

 scales usually overlap one another, the outer edges being 

 directed towards the tail of the fish, and the scale in front cover- 

 ing about three-fourths of the one behind it. It is a mistake to 

 suppose that the scales are on the surface of the skin, they are 

 in the derma beneath the epidermis, but the posterior edge of 

 the scale may project to some extent through the epidermis. 

 The derma is supplied with blood-vessels and nerves; it contains 

 living cells in the meshes of the network of fibres ; it also con- 

 tains two kinds of structures which contribute to the coloration 

 of the fish. These two kinds are first the pigment cells, some 

 of which are black and others coloured, and second the reflect- 

 ing elements which are called iridocytes. These elements of 

 coloration are situated chiefly in the outer part of the skin below 



