Shepherdstown register. [volume] (Shepherdstown, Va. [W. Va.]) 1849-1955, 1897-12-09
They Need No Eyes.

Eyes are popularity CONSIDERED 10 De
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
between 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 visions
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.. N.gel, are potential eyes,
that is, they are sensitive to light.
In animals that have eyes the sensi-
tiveness has been highly localized and
greatly increased-so that man, for
instance, has a retina very sensitive
to light, and an expanse of ordinary
skin which possesses a sensitiveness to
light so light that it is hardly con-
gcious of it. Yet the skin is sensitive
in some degree, as is proved by the
fact that it sunburns-that is, light
NAV cause a disturbance in the pig
ment of the skin just as it does in that
of the eye. In the eye the distur-
bance is accompanied by nervous
change which sends telegraphic mes
sage along the optic nerve to the brain.
In the skin there are nerves, and there
are messages, 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 the earthworms, although
they have no eyes, will suddenly with
draw into their holes on the approach
of lighted candle. Some creatures
seem more sensitive to suddenly in
creased light, others to sudden dimi-
mutton. If a number Of oysters kept
in vessel together, are found to be
open, they will shut at once if a dark
object comes between them and the
light. Another bivalve called Psam-
media, has long, whitish Transparent
tubes which protrude from the sand
in which it lies buried If they are
suddenly Illuminated, they contract,
and the brighter the light the gre later
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 UNI
til they are exhausted.
In general, DR.. Nagel finds that
creatures which respond to sudden
shadows are those that live in strong
shells, while those affected by sud
den increase of light live in the sand
or mud, from which they emerge oc-
casionally. In both cases the sensi-
tiveness of the the skin to changes Of
light serves to protect the animal.
How does the skin acquire this Pc
culiar sensitiveness? 1t will be best
for the non-expert to suspend judy
ment, since even the scientists do not
agree on this point. 1t may be that
it is universal and rudimentary prop
erty of all skin, and that animals with
eyes have lost it in a greater or less
degree, because they have no need for
it. This is one view. Or it may be
that this property has been developed
in eyeless and need it. That is anoth-
er view. Those who favor the latter
opinion point to the fact that some of
the creatures which now have skins
sensitive to the light are probable de-
scendants of creatures with skins not
so sensitive, in these instances the
sensitiveness must have been recently
developed. Snails are sensitive, but
their relatives, the slugs, are not;
this looks as if the former had ac-
quired the faculty. However, this
may be, DR.. Nagel's study Of these
curious and out-of-the-way facts Is
certainly interesting and may lead in
the future to an advance in our knowl-
edge of the mechanism of sight.
~1En