The herald., 1897-08-28
EYES NOT ESSENTIAL TO SIGHT
Many Creatures Enabled to See by the
Aid of Sensitive Skins

Eyes are popularity considered to be
quite necessary to sight, but this is an
error if we are to believe DR.. Nagel, a
recent German experimenter. Many
creatures without eyes can see; at least
they can distinguish perfectly well be-
tween light and darkness and even be-
tween different degrees of light. This
is the lowest degree of seeing, to be sure,
but still it is really sight, and differs
scarcely more from the vision of some
insects that possess eyes than this does
from our own clear sight.
Creatures that see without eyes see
by means of their skins. All skins, says
DR.. Nagel, are potential eyes; that is,
they are sensitive to light. In animals
that have eyes the sensitiveness has
been highly localized and greatly in
creased-so that man, for instance, has
a retina very sensitive to light, and an
expanse of ordinary skin which posses
ses a sensitiveness to light so slight that
it is hardly conscious of it. Yet his skin
is sensitive in some degree, as is proved
by the fact that it sunburns-that is.
light may cause disturbance in the pig
ment of the skin just as it does that of
the eye. In the eye the disturbance is
accompanied by nervous change which
sends a telegraphic message along the
optic nerve to the brain. In the skin,
too, there are nerves, and there are mes
sages also, but their tidings imprint no
image on the mind; they simply express
discomfort-cry out, "sunburn."
But in many eyeless creatures the
lack of eyes is in part made up by in
creased sensitiveness Of the whole skin
surface to light Darwin long ago no.
ticed that earthworms, although they
have no eyes, will suddenly withdraw
into their holes at the approach of
lighted candle. Some creatures seen
more sensitive to sudden increase of
light; others to sudden diminution. If
a number Of oysters, kept in vessel to-
gether, are found to be open, they will
shut all at once if a dark object comes
between them and the light. Another BIZ
valve, called Psammodia, has long
whitish, transparent tubes which pro-
trude from the sand in which it lies
buried If these are suddenly illumin-
ated they contract, and the brighter the
light the greater the contraction. If
number of them be carried into direct
sunlight they hasten to bury themselves
in the sand or, if there is no sand, they
move restlessly to and fro in the water
until they are exhausted.
In general DR.. Nagel finds that crea-
lures which respond to sudden shadows
are those that live in strong shells, while
those effected by sudden increase of
light live in sand or mud, from which
they emerge occasionally. In both cases
the sensitiveness of the skin to changes
of light serves to protect the animal.
How does the skin acquire this pecu
liar sensitiveness? 1t will be best fol
the non-expert to suspend Judgment
since even the scientists do not agree or
this point 1t may be it is universal
and rudimentary property of all skin
and that animals with eyes have losi
it in greater or less degree, becausd
they have no further need for it. Thai
is one view Or it may be that this prop
erty has been developed in eyeless crea
lures just because they are eyeless an(
need it. That is anothr view. Thoss
who favor the latter opinion point to tbs
fact that some of the creatures which
now have skins sensitive to the light arc
probably descendants of creatures witt
skins not so sensitive; in these instances
the sensitiveness must have been re-
Gently developed. Snails are sensitive,
but their relatives, the slugs, are not;
this looks as if the former had acquired
the faculty. However this may, be, DR.,
Nagel's study of these curious and out
of-the-way facts is certainly interest
ing and may lead in the future to an ad-
vance in our knowledge of the mechan-
ism of sight.-Boston Journal.