2017-09-01 Coral Magazine

CORAL 43

In the course of evolution, nature has developed nu-

merous very different light-sensing organs. The basic

types are summarized below. Some animal species have

multiple eyes on different parts of the body, such as on

the edge of a mussel’s mantle or the tips of a starfish’s

arms. Others have two or more eyes at the front of the

body. However, all animal classes with visual organs also

have representatives that have lost eyesight due to living

in the dark—for example, cave dwellers or internal para-

sites, such as intestinal worms.

TWO-CELLED EYE PROTOTYPE

The simplest eye known in the animal kingdom is found

in the marine flatworm Polycelis auricularis. It consists of

a pigment cell and a photoreceptor cell. This eye, though

primitive, is still controlled by PAX6.

EYE PATCH

Where photoreceptors occur close together in large num-

bers, we call this the retina. The simplest form of such a

sensing-cell cluster, which can only differentiate light

from dark, is in the form of eye patches in jellyfishes (for

example, Medusa aurelia aurita), starfishes, and ring-

worms (Annelida).

PIGMENT-CUP EYE

In this type of eye, which is somewhat more developed

than the eye patch, the retina is cup-shaped. It is also

called a pigment-cup ocellus, cup eye, or pit eye. The lat-

eral surfaces of the cup-shaped tissue recess are screened

by pigments. As a result, light can only reach the retina

from one direction, which reduces the field of view but

makes it possible to determine the direction from which

Schematic representation of the eye patch.

This 3-mm Cladonema radiatum medusa
already has tiny eye patches (arrows).

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from dark, is in the form of eye patches in jellyfishes (for

example,Medusa aurelia aurita), starfishes, and ring-

worms (Annelida).

PIGMENT-CUP EYE

In this type of eye, which is somewhat more developed

than the eye patch, the retina is cup-shaped. It is also

calledd a ppigigmement-cup ocellus, cup eye, or pit eye. The lat-

eral surfacces of theh ccupup-shaped tissue recess are screened

by pigmennts.s As a result, lilghg t tcan only reach the retina

frrom one ddirection, whichh reduucees the field of view but

mamakeks it posossible to determimine the ddirecection from which

Schematic representation of ththe eye patch.

Copepods have the typical central
“nauplius eye,” a pigment-cup ocellus.

Schematic representation of the pigment-cup ocellus.