"The simplest organ which can be called an eye consists of an optic nerve,
surrounded by pigment-cells, and covered by translucent skin, but
without any lens or other refractive body. We may, however, according
to M. Jourdain, descend even a step lower and find aggregates of
pigment-cells, apparently serving as organs of vision, without any nerves,
and resting merely on sarcodic tissue. Eyes of the above simple nature
are not capable of distinct vision, and serve only to distinguish light from
darkness. In certain star-fishes, small depressions in the layer of pigment
which surrounds the nerve are filled, as described by the author just
quoted, with transparent gelatinous matter, projecting with a convex
surface, like the cornea in the higher animals. He suggests that this serves
not to form an image, but only to concentrate the luminous rays and
render their perception more easy. In this concentration of the rays we
gain the first and by far the most important step towards the formation of
a true, picture-forming eye; for we have only to place the naked
extremity of the optic nerve, which in some of the lower animals lies
deeply buried in the body, and in some near the surface, at the right
distance from the concentrating apparatus, and an image will be formed
on it. "
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"Since then, biologists have vindicated Darwin by discovering many examples of primitive eyes among various species, ranging from the simplest eye spots of a few light-sensitive cells through progressively more complex forms to the complete, highly sophisticated mammalian eye."