196 Adaptations of Fishes 



" Phosphorescence as applied to the production of light by a 

 living animal is, according to our present ideas, a chemical action, 

 an oxidation process. The necessary conditions for producing it 

 are two an oxidizable substance that is luminous on oxida- 

 tion, i.e., a photogenic substance on the one hand, and the pres- 

 ence of free oxygen on the other. Every phosphorescent organ 

 must have a mechanism for producing these two conditions; 

 all other factors are only secondary and accessory. If the 

 gland of a firefly can produce a substance that is oxidizable 

 and luminous on oxidation, as shown as far back as 1828 by 

 Faraday and confirmed and extended recently by Watase, it is 

 conceivable, indeed probable, that phosphorescence in Myctophum 

 and other deep-sea forms is produced in the same direct way, 

 that is, by direct oxidation of the secretion of the gland found 

 in each of at least ten of the twelve groups of organs described 

 by von Lenden*feld. Free oxygen may be supplied directly 

 from the blood in the capillaries distributed to the gland 

 which he describes. The possibility of the regulation of the 

 supply of blood carrying oxygen is analogous to what takes 

 place in the firefly and is wholly adequate to account for any 

 'flashes of light' 'at the will of the fish.' 



" In the phosphorescent organs of Porichthys the only part 

 the function of which cannot be explained on physical grounds 

 is the group of cells called the gland. If the large granular 

 cells of this portion of the structure produce a secretion, as seems 

 probable from the character of the cells and their behavior 

 toward reagents, and this substance be oxidizable and luminous 

 in the presence of free oxygen, i.e., photogenic, then we have 

 the conditions necessary for a light-producing organ. The 

 numerous capillaries distributed to the gland will supply free 

 oxygen sufficient to meet the needs of the case. Light pro- 

 duced in the gland is ultimately all projected to the exterior, 

 either directly from the luminous points in the gland or reflected 

 outward by the reflector, the lens condensing all the rays into 

 a definite pencil or slightly diverging cone. This explanation 

 of the light-producing process rests on the assumption of a 

 secretion product with certain specific characters. But com- 

 paring the organ with structures known to produce such a sub- 

 stance, i.e., the glands of the firefly or the photospheres of Eu- 



