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Eyes!

The stigma or eyespot of euglenoids is a simple photoreceptor that allows the protist to orient to light.

Simple eyespots of the jellyfish, Aurelia. The multifunction lappets are also used as a sense of touch, smell, and gravity.

Planarian (flatworm) eye structure and function. Planarian eyes with few photoreceptors (light-sensitive cells). Planarians do not form an image but is very good at determining the direction of light. Normally, planarians will avoid light by positioning themselves so that the light is shaded by the pigment cells (the posterior of the worm is at the bottom of the diagram). Despite the contention of those that believe in Intelligent Design "Theory", simple eyes such as these explain the more complex eyes found in vertebrates, mollusks, and other creatures. See a webpage on the evolution of the eye here. Be sure to play the movie.

Simple mollusk eyes. (A) scallop, (B) close-up of scallop eye, (C) nautilus.

Scallops have multiple eyes that produce a crude image. Nautili have a pin-hole camera eye.

Octopus Eye. The octopus eye has many features that are similar in structure to that of the vertebrate eye, but these structures evolved independently and are therefore analogous structures, not homologous structures. Any similarities are examples of convergent evolution. The lens, for example does all the focusing (rather than just adjusting the focus to near and far, which is the case in the vertebrate eye). Vertebrate and mollusk eyes focus through different mechanisms (SEE AN ANIMATION HERE). The eyes also differ in retinal structure and in the area where visual processing is performed (HERE). The regular structure of the photoreceptors in the octopus eye has been used as a model for an artificial retina (click on the artificial retina for a larger image).

Octopus Brain with five Colored Lobes

Vertebrate Eye Structure.

Section through a vertebrate eye. Click HERE for the functions of selected structures.
Nerve cells below the rods and cones are responsible for partially decoding images before they are sent to the brain for final processing. An image is not sent, but rather a description of the image (a vertical line here, more or less contrast there, some movement in another portion of the visual field, etc.).

SEM of rod and cone cells. Rods are used for night vision and are sensitive only to one color (deep violet). Cones come in three flavors (red, green, and blue).

A third class of cells (intrinsically photosensitive retinal ganglion cells; ipRGC) were recently discovered (Jan 2006). These cells regulate circadian rhythms. More information is here.