Eye

: This article refers to the sight organ. See Eye (disambiguation) for other usages.

Varieties of eyes

In most vertebrates and some mollusks the eye works by allowing light enter it and project onto a light-sensitive panel of cells retina at the rear of the eye, where the light is detected and signals are 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 eye (similar to how a camera focuses).

Related Topics:
Vertebrate - Mollusk - Retina - Brain - Optic nerve - Transparent - Vitreous humour - Lens - Iris - Cephalopod - Fish - Amphibian - Snake - Camera

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Eyes of widely varied species may have evolved differently, but they tend to be similar in function and appearance once fully developed. Mollusks' eyes seem to have evolved from different organs than vertebrate eyes, and may be an example of convergent evolution. Vertebrate eyes grow outward from brain cells during embryonic development, while mollusk eyes grow inward from skin cells. Vertebrate retina are layered with neurons in front of the photosensitive cells, mollusk retina are reverse with photosensitive cells in front of the neurons, this provides mollusk with no blind spot and may provide clearer vision, but may also provide slower retinal recycling and thus slower vision refeshing and motion detection. Some cephalopods have no physical lense and use a pinhole lens. Some mollusks such as scallops use concave mirror to focus light in conjunction with a lense.

Related Topics:
Mollusk - Convergent evolution - Photosensitive - Blind spot - Retinal - Pinhole lens - Scallop - Concave

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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 arthropodes (many Strepsiptera) have compound eye composed of a few facets each with a retina capable of creating an image, which does provide muliple 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.

Related Topics:
Compound eye - Arthropod - Strepsiptera

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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 varies, however: some trilobites had only one, and some had thousands of lenses in one eye.

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Some of the simplest eyes, called ocelli, can be found in animals like snails, who can not actually 'see' in the common 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 (day and night), but no more. This enables snails to keep out of direct sunlight.

Related Topics:
Ocelli - Snail - Photosensitive

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Jumping spiders have simple eyes that are so large, supported by an array of other smaller eyes, that they can get enough visual inputs 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.

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