St. Tammany farmer. [volume] (Covington, La.) 1874-current, 1897-09-18
eyes NOT ESSENTIAL 70 sloH7


EYES NOT ESSENTIAL TO ss0H7
Many cp~atsses Enenssa T. see by Ne
ASA % Sensitive Skscs.
Eyes are popularity considered to BE-
cutie necessary to sight, hut this IS
an error. II we are to believe DR.. Naecl. y
recent German experimenter. MANY
creatures without eyes can rec. ut laat
they can distinguish perfectly well :Ie-
tween light and darkness. and even be-
tween different degrees Of light. This
il the lowest degree Of seeing. to be
sure, but still it II really sight. and dif.
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 SEQ
by means of their skins. All skins,
says DR.. Nagel, are potential eyes; that
il, 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.
las . retina very sensitive to light.
and IN expansc Of ordinary skin which
possesses q sensitiveness to light %
alight that il il hardly conscious Of il.
Yet the skin il sensitive in some de-
gree, - il proved by the fact that it
sunburns-that II, light may cause =
disturbance in the pigment Of the skin
just AZ it does in that Of the eye. In
the eye the disturbance II accompanied
by . nervous change which sends Z
telegraphic nessage 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 oat "sun-
burn"
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
noticed that the earthworms, although
they have NO eyes, win 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. II
. number Of oysters, kept in . vesse!
together, are found to be open, they
will shut all at once il = 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. II they are suddenly IL
Illuminated, they contract, and the
brighter the light the greater the con-
traction. II q number Of them be car.
ried into direct sunlight, they hasten to
bury themselves In the sand: or. il
there IL 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 stron:
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 sensitiveness OF
the skin to changes Of light serves tc
protect the animal.
How does the skin acquire this pc.
culiar sensitiveness? It will be best
for the non-expert to suspend judg'
ment, since even the scientists do NO:
agree ON this point. 1t may Be that il
Is . universal and rudimentary- prop,
erty Of all skin. and that animals with
eyes have lost 1t in . greater or lesede'
gree. because they have NO further need
for II. This II one view. Or Il may
be that this property has been devei-
oped in eyeless creatures just because
they are eyeless and need 1t. That 10
another view. Those who favor the lat
ter opinion point to the fact that some
Of the creatures which now have skins
sentative to the light are probably de-
scendants Of creatures with skins not
so sensitive; in these instances the
scnsitiveness must have been recently
developed. Snails are sensitive. but
their relatives. the slugs, are not; this
looks a. IL the former had acquired the
faculty. However this may be. DE.
Nagel's study Of these curious and out.
of-the-way facts II certainly interest.
ing. and may lead in the future to an adj
vance in our knowledge Of the mechaD
ism Of sight.-Boston Journal