are only able to discriminate between light and darkness.
Primitive eyes appear in animals very low in the scale of life; probably
the most remarkable of these early organs of sight are to be found in
the medusa, or jelly-fish. This creature, with its bell-shaped body and
pendent stem, bears a striking resemblance to an umbrella; noting this
resemblance, naturalists have given the name _manubrium_, "handle," to
the stem. Around the edge of the umbrella, and situated at regular
intervals, are certain round, cell-like organs, which vary considerably
in number. Some species have only eight, while others have sixty,
eighty, and even (in OEquorea) as high as six hundred.[8] These
so-called "marginal bodies" are the eyes of the jelly-fish. By many
biologists these organs are considered to be ears; they contain within
their capsules transparent bodies, which some scientists deem otoliths,
or "hearing-stones." Experimentation and microscopical examinations,
however, have taught me very recently to believe otherwise. In these
marginal bodies there is always a deposit of pigment; this is,
unquestionably, a primitive retina, while the transparent disk is,
indubitably, a primitive lens. That these creatures can tell the
difference between light and darkness is a fact easily demonstrated.
Time and again have I made them follow a bright light around the wall of
the aquarium in which they were confined. On one occasion I made some
medusae tipsy, and their drunken gravity as they rolled and staggered
through the water in pursuit of the light was as sorrowful as it was
instructive; their actions in this respect were those of intoxicated
men. After I had siphoned off the alcoholized water and replaced it with
pure, they rapidly regained their normal status; whether or not any of
them felt any evil effects from their involuntary debauch, I am not
prepared to state.

[8] Lubbock, _Senses, Instincts, and Intelligence of Animals_, p. 84.

The eyes of sea-urchins are rather highly developed, having corneae,
retinae, and lenses. The lens generally lies in a mass of pigment, and,
as Lubbock remarks, looks like a brilliant egg in a scarlet nest.[9] The
eyes are scattered over the dorsal surface of the creature's body, and
are commonly situated just beneath the skin; they are, however,
sometimes elevated on pear-shaped bulbs. The eyes of starfish are
generally quite primitive in character, as far as I have been able to
determine, being simply