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Eyes!
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.
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Section through a vertebrate eye. Click
HERE for the functions of selected
structures.
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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
and
here.
Rod cell structure and effect of light on rhodopsin. Rhodopsin function can be
seen here.
SEM of Fly Eye
Ommatidium Structure. Bees and flowers
Insect nervous system and eye
Insect nervous system and eye
Ommatidium Structure
Ommatidium Structure
Factors affecting visual clarity
Brain-eye organization
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