GUG 
may be regarded eitlier as accessory eyes, or as pliospliorescent 
organs j but, as many fish which possess them have also Avell- 
developed cephalic eyes, as for example, the Ribbon-fish 
Trachijpterus, it is possible that they may be, as it were, bull’s-eye 
lanterns, the lens performing the function of concentrating the light 
given out by the posterior part of the organ. Animals which are 
dredged from great depths often reach the surface dead, and almost 
falling to pieces, in consequence of the expansion of their contained 
gases, owing to the removal of the great pressure of the water, and 
it is possible, therefore, that many deep-sea forms, which have not 
been hitherto supposed to be phosphorescent, are in reality so. At 
great depths phosphorescence is the only light which can be 
supposed to exist ; for Forel, in experiments on the Lake of Geneva, 
found that in no case was sensitized paper affected by light at 
a depth greater than one hundred fathoms. There is a serious 
difficulty in accounting for the existence of the phosphorescent 
animals themselves, as in many cases they belong to groups which 
have no eyes, and in others are blind representatives of families, 
the members of which usually possess eyes. Many of the lower 
animals, however, though not provided with visual organs, are 
perfectly able to distinguish between light and darkness, as, for 
example. Hydra, the freshwater Polype, which has, when kept in a 
jar, the habit of coming to the side nearest to the light. The 
higher animals, as we know by experiment, are insensible to light, 
except through the medium of their eyes, and it is difficult in such 
cases to understand of what use the phosphorescence of blind 
animals can be to them. A fish, Ipnops, living at great depths, 
has completely lost its eyes, their place being taken by two largo 
metallic-looking phosphorescent organs, — one instance out of many 
of highly organised animals, which are blind, equalling in their 
phosphorescent power those which jiossess well-developed eyes. 
Some zoologists suppose that in such cases the phosphorescence is an 
accident, occurring in the normal metabolism of its possessor ; but it 
seems more reasonable to believe that in some, although at present 
unexplained, manner, it is of service, even to forms which are 
quite blind. 
It cannot be doubted that phosphorescence must be very useful 
to animals possessing e}fes. Gunther gives a figure of a fish 
{Chiasmodus) which lives at a depth of fifteen hundred fathoms. 
