PRODUCTION OF LIGHT 423 



the epidermis and on the surface of the scales, and in another 

 layer on the inner surface of the derma. The dense white or 

 silvery appearance of a fish is due to a thick and dense layer of 

 iridocytes on the inner surface of the skin ; this layer is called 

 the argenteum. The substance of the iridocytes is quite opaque 

 and has a very remarkable power of reflecting light : in some 

 cases, as on the lower sides of flat-fishes, the surface of the 

 argenteum is merely a dead white, but in a large number of fishes 

 the reflection from the same layer is bright and silvery as in the 

 salmon ; in these cases the reflection is similar in appearance to 

 that from a polished metallic surface or from mercury, although 

 there is no metal in the reflecting substance of the fish. Lastly, 

 in many fishes in addition to the silvery reflection there is a 

 brilliant iridescence or play of colours, which is due to iridocytes 

 scattered singly, not crowded together in a dense layer. The 

 iridocytes consist of a special chemical substance identical with 

 one which was first found in guano and which is therefore 

 called guanin. Guano is the dried excrement of sea-birds which 

 feed principally on fish, and the guanin in it is derived from 

 the skins of the fishes eaten, this substance being so insoluble 

 that it is not affected by the digestive processes, but passes 

 through the birds' intestines unchanged. Owing to its beautiful 

 lustre the reflecting substance of fishes' skins is used for the 

 manufacture of artificial pearls. The species offish from which 

 the supply of the material is usually obtained on the continent 

 is Alburnus lucidus, the bleak, a fresh-water fish of the carp 

 family, which is common also in British rivers. French arti- 

 ficial pearls are made by coating the insides of glass beads with 

 this substance, which is known in the trade as blanc d'ablette or 

 essence d' Orient. Roman pearls on the other hand are made 

 from the reflecting substance of the air-bladder of the Argentine, 

 a marine fish of the salmon family ; and this substance is not 

 placed in glass beads but on the external surface of beads made 

 of wax. 



Among the various organs of fishes known or believed to 

 produce light three different types of structure can be distin- 

 guished : first that which is found in the organs of Myctophidae 

 or Scopelidae ; second that of organs in other families which are 

 not connected with the lateral line; third, that of the lateral 

 line organs in Halosauridae and other families. 



