ght is directly fatal to these fishes; they soon die
when taken from the river and placed in aquaria where there is an
abundance of light.

These fish, although they have rudimentary eyes, never have the
slightest remaining trace of nerve-cells in the wasted optic nerve (that
is, I have never been able to discover any), thus showing that their
appreciation of light is not derived through the agency of their eyes.
An eyeless spider (_Anthrobia_) taken from the same cavern showed a like
distaste for light, and yet, in this insect, there is absolutely no
vestige of an eye or its nerves.

Finally, a friend of mine, a youth of eighteen, totally blind since
birth, can differentiate between daylight and darkness. On one occasion
I carefully blindfolded him and led him into the well-lighted office of
a brewery (he had never been in a brewery before), and asked him if it
were light or dark. He answered that it was almost as light as day. I
then conducted him into the dark beer vaults, and as soon as he passed
the door he exclaimed, "How cold and dark it is here!" Thinking that he
might possibly associate darkness with coldness, I asked him if this
were the case. "No," he replied, "I _see_ the darkness and I _feel_ the
cold; they are not the same."

In these animals--and I include man--continuous darkness has modified
sensibility (sense of touch) to such an extent that it has partially
taken on the functions of the useless organs--the eyes; these creatures
_see_ with their skins.

I do not believe that there is a creature in existence to-day, whether
it has eyes or not, which cannot tell the difference between night and
day. Professor Semper says that in the Pelew Islands he found a small
fresh-water creature, whose generic name is _Cymothoe_, in pools where
daylight penetrated, that was absolutely blind.[6] We have fresh-water
Cymothoe living in our own waters that are close kin to the Pelew
islander mentioned by Semper, and which are not blind. Along the middle
of their backs, over the edge of each segment, there is an oblong dark
spot. This little collection of coloring-matter is covered by a
transparent membrane, the cornea, and has a special nerve leading to the
brain, if I may use the word. These spots are primitive eyes, the
analogues of which are preserved by many of the true worms. I am
inclined to believe that Semper would find primitive eyes of some form
or other in the Cymothoe he mentions, if he were again to exa