68 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 Watasé, 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 Lendenfeld. 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 cf 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- 
