San Antonio daily light. [volume] (San Antonio, Tex.) 1886-1907, 1897-09-19
NEED NO EYES.
Creatures That See Through Their
Skins
Some Curious Facts Concerning the
Lower Orders of Animal Life
-Snails, slugs and BIZ
valves.

70 w PSRNAoAAf NShoAN~ASN W NC
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 last
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 diff
fers 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.. Nagel, are potential eyes; that
is, they are sensitive to light. In an
imals that have eyes the sensitiveness
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 sensitiveness to light so
slight that it is hardly conscious of 1t.
Yet the skin is sensitive in some de-
gree, as is proved by the, fact that it
sunburns-that is, light may cause
disturbance in the pigment of the skin
just as it does in that of the eye. In
the eye the disturbance is accompanied
by a nervous change which sends
Geographic 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 sun
burn"
But in many eyeless creatures the
lack of eyes is in part made up by in-
creased sensitiyeness of the whole
skin surface to light. Darwin long ago
noticed that the earthworms, although
they have no eyes, will suddenly with
draw into their holes on the approach of
a lighted candle. Some creatures seem
more sensitive to suddenly increased
light, others to sudden diminution. If
a number of oysters, kept in vessel
together, are found to be open, they
will shut all at once if a dark object
comes between them and the light.
Another bivalve called Psammodia.
has long, whitish, transparent tubes
which protrude from the sand in which
it lies buried. If they are suddenly IL
Illuminated, they contract, and the
brighter the light the greater the con-
traction. If number of them be car
ried 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
creatures which respond to sudden
shadows are those that live in strong
shells, while those affected by sud-
den increase of light live in sand or
mud, from which they emerge occasion
ally. In both cases the sensitiyeness of
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 judg-
ment, since even the scientists do not
agree on this point. 1t may be that it
is a universal and documentary prop
erty of all skin, and that animals with
eyes have lost 1t in greater or less de-
gree, because they have no further need
for it. This is one view. Or it may
be that this property has been devel-
oped in eyelcss creatures just because
they arc cyeless and need it. That is
another view. Those who favor the lat
ter opinion point to the fact that some
of the creatures which now have skins
sensitive to the light are probably 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 acquired the
faculty. However this may be, DIR.
Nagel's study of these curious and out
of-the-way facts is certainly interest
ing, and may lead in the future to an adj
vance in cur knowledge of the mechan-
ism of sight.-Boston journal.
How TO Serve Potatoes.