The Yale expositor., 1897-08-13
EYES NOT ESSENTIAL TO SIGHT
aany Creatures Enabled to See by che
AID of Sensitive skins.

uhA5 AfghtuIeh 10 STU ny Ahc
Aid OF Sensitive Skins.
From the Boston Journal: Eyes are
popularity considered to be quite nee
sssary to sight, but this is an error,
We are to believe DR.. Nagel, a re-
.ent German experimenter. Many
creatures without eyes can see; at least
hey can distinguish perfectly well be-
.ween light and darkness and even be-
tween different degrees of light. This
IS the lowest degree of seeing, to be
sure, but still 1t is really sight, and
iiffers scarcely more from the vision
Of some insects that possess eyes than
his does from our own clear sight.
Creatures that see without eyes see by
means of their skins. All skins, says
Or. Nagel, are potential eyes; that is,
they are sensitive to light. In animals
that have eyes the sensltiveness has
been highly localized and greatly in
;reased-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
slight that 1t is hardly conscious OF 1t.
Yet his skin is sensitive in some de-
gree, as is proved by the fact that 1t
sunburns-that is, light may cause a
disturbance in the pigment of the skin
just as 1t does in that of the eye. In
the eye the disturbance is accompan-
led 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
If eyes is in part made up by increased
constituencies of the whole skin sur
rec to light. Darwin long ago noticed
that earthworms, although they have
no eyes, will suddenly withdraw into
their holes at the approach of lighted
candle. Some creatures seem most
sensitive to sudden increase Of light;
others to sudden diminution. II num-
Der OF oysters, kept in a vessel togeth-
er, are found to be open, they will
shut all at once If dark object comes
between them and the light. Another
bivalve, called Psammodia, has long,
lwhitish, transparent tubes which pro-
trude from the sand in which 1t lies
buried. II these are suddenly illumi-
mated they contract, and the brighter
the light the greater the contraction.
If number of them be carried into di-
rect sunlight they hasten to bury them
selves 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-
ows are those that live in strong shells,
while those affected by sudden in
crease 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 pro-
tect the animal. How does the skin
acquire this peculiar constituencies? 1t
will be best for the non-expert to sus-
pend judgment, since even the scien-
lists do not agree on this point. 1t
may be that it Is a universal and ru-
dimentary property of all skin, and
that animals with eyes have lost it in
a greater or less degree, because they
have no further need for it. That is
one view. Or it may be that this prop
erty has been developed in eyeless
creatures just because they are eyeless
and need it. That is another view.
Those who favor the latter opinion
point to the fact that some of the
creatures which now have skins SEN
sitive 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, DR..
Nagle's study of these curious and out
of-the-way facts is certainly interest
ing and may lead in the future to an
advance in our knowledge of the me-
ehanism OF sight.