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An eye is an organ of vision that detects light. Different kinds of light-sensitive organs are found in a variety of organisms. The simplest eyes do nothing but detect whether the surroundings are light or dark, while more complex eyes can distinguish shapes and colors. more...

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Many animals, including some mammals, birds, reptiles and fish, have two eyes whose fields of vision largely overlap, to allow better depth perception (binocular vision), as in humans; and others are placed so as to minimize the overlap, such as in rabbits and chameleons.

Varieties of eyes

In most vertebrates and some mollusks, the eye works by allowing light to enter it and project onto a light-sensitive panel of cells known as the retina at the rear of the eye, where the light is detected and converted into electrical signals. These are then transmitted to the brain via the optic nerve. Such eyes are typically roughly spherical, filled with a transparent gel-like substance called the vitreous humour, with a focusing lens and often an iris which regulates the intensity of the light that enters the eye. The eyes of cephalopods, fish, amphibians, and snakes usually have fixed lens shapes, and focusing vision is achieved by telescoping the lens â€” similar to how a camera focuses.

Compound eyes are found among the arthropods and are composed of many simple facets which give a pixelated image (not multiple images, as is often believed). Each sensor has its own lens and photosensitive cell(s). Some eyes have up to 28,000 such sensors, which are arranged hexagonally, and which can give a full 360 degree field of vision. Compound eyes are very sensitive to motion. Some arthropods, including many Strepsiptera, have compound eye composed of a few facets each, with a retina capable of creating an image, which does provide multiple-image vision. With each eye viewing a different angle, a fused image from all the eyes is produced in the brain, providing a very wide-angle, high-resolution image.

Possessing detailed hyperspectral color vision, the Mantis shrimp has been reported to have the world's most complex color vision system. Trilobites, which are now extinct, had unique compound eyes. They used clear calcite crystals to form the lenses of their eyes. In this, they differ from most other arthropods, which have soft eyes. The number of lenses in such an eye varied, however: some trilobites had only one, and some had thousands of lenses in one eye.

Some of the simplest eyes, called ocelli, can be found in animals like snails, who cannot actually "see" in the normal sense. They do have photosensitive cells, but no lens and no other means of projecting an image onto these cells. They can distinguish between light and dark, but no more. This enables snails to keep out of direct sunlight. Jumping spiders have simple eyes that are so large, supported by an array of other, smaller eyes, that they can get enough visual input to hunt and pounce on their prey. Some insect larvae, like caterpillars, have a different type of single eye (stemmata) which gives a rough image.