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Diagram of a human eye. Note that not all eyes have the same anatomy as a human eye.

An eye is an organ that detects light. Different kinds of light-sensitive organ are found in a variety of creature. The simplest eyes do nothing but detect whether the surroundings are light or dark. More complex eyes are used to provide the sense of vision.

1 Varieties of eye

In most vertebrates and some molluskCaudofoveata Aplacophora Polyplacophora Chitons Monoplacophora Bivalvia Bivalves Scaphopoda Tusk shells Gastropoda Snails and Slugs Cephalopoda Squids, Octopuses, etc. The mollusks or molluscs are the large and diverse phylum Mollusca which includes a vars (such as octopus14 in two suborders, see text For other meanings of "octopus", see Octopus (disambiguation). The octopus is a cephalopod of the order Octopoda that inhabits many diverse regions of the ocean, especially coral reefs. The term may also refer to only those ces) the eye works by projecting images onto a light-sensitive retinaeye cross-sectional view. Courtesy NIH National Eye Institute. Many animals have eyes different from the human eye. The retina is a thin layer of cells at the back of the eyeball of vertebrates and some cephalopods; it is the part of the eye which transdu, where the light is detected and transmitted to the brainFor other articles about other subjects named brain see brain (disambiguation). In the anatomy of animals, the brain or encephalon is the supervisory center of the nervous system. Although the brain is usually cited as the supervisory center of vertebrate via the optic nerveThe optic nerve is the nerve that transmits visual information from the retina to the brain. Anatomy The nerve is the second of twelve paired cranial nerves but is usually considered to be part of the central nervous system as it is derived from an outpou. The eye is typically roughly spherical, filled with a transparentIn optics, transparency is the property of being transparent or allowing light to pass. The opposite property is opacity . Though transparency usually refers to visible light in common usage, it can actually refer to any type of radiation. For example, fl gel-like substance called the vitreous humourVitreous humour is the clear gel that fills the eyeball, lying between the lens and the retina in the eye. Floaters are small, generally harmless, defects in the vitreous humour. Where the vitreous humour has become cloudy, vitrectomy is surgery to restor, with a focusing lensLight from a single point of a distant object and light from a single point of a near object being brought to a focus by changing the curvature of the lens. The lens or crystalline lens is a component of the eye. In concert with the cornea, it refracts li and often a muscle called the iris that controls how much light enters.

How a complex structure like the eye could have evolved is a difficult question for the theory of evolution, since intermediate forms would presumably have been of little use, and light-sensitive organs are present in a variety of different creatures without any clear evolutionary link. Although they are quite similar in function and appearance once fully developed, vertebrate eyes grow outward from brain cells during embryonic development, while mollusk eyes grow inward from skin cells. These are often claimed as examples of parallel evolution .