eye eye eyes are organs of the visual system. they provide living organisms with vision, the ability to receive and process visual detail, as well as enabling several photo response functions that are independent of vision. eyes detect light and convert it into electro-chemical impulses in neurons (neurones). in higher organisms, the eye is a complex optical system which collects light from the surrounding environment, regulates its intensity through a diaphragm, focuses it through an adjustable assembly of lenses to form an image, converts this image into a set of electrical signals, and transmits these signals to the brain through complex neural pathways that connect the eye via the optic nerve to the visual cortex and other areas of the brain. eyes with resolving power have come in ten fundamentally different forms, and 96% of animal species possess a complex optical system. image-resolving eyes are present in molluscs, chordates and arthropods. this article is about the organ. for the human eye, see human eye. for the letter, see i. for other uses, see eye (disambiguation). eyea human eyecompound eye of an antarctic krilldetailssystemnervousidentifierslatinoculusta98a15.2.00.001 a01.1.00.007ta2113, 6734anatomical terminology the most simple eyes, pit eyes, are eye-spots which may be set into a pit to reduce the angle of light that enters and affects the eye-spot, to allow the organism to deduce the angle of incoming light. from more complex eyes, retinal photosensitive ganglion cells send signals along the retinohypothalamic tract to the suprachiasmatic nuclei to effect circadian adjustment and to the pretectal area to control the pupillary light reflex. overview eye of european bison human eye complex eyes distinguish shapes and colours. the visual fields of many organisms, especially predators, involve large areas of binocular vision for depth perception. in other organisms, particularly prey animals, eyes are located to maximise the field of view, such as in rabbits and horses, which have monocular vision. the first proto-eyes evolved among animals 600 million years ago about the time of the cambrian explosion. the last common ancestor of animals possessed the biochemical toolkit necessary for vision, and more advanced eyes have evolved in 96% of animal species in six of the ~35 main phyla. in most vertebrates and some molluscs, the eye allows light to enter and project onto a light-sensitive layer of cells known as the retina. the cone cells (for colour) and the rod cells (for low-light contrasts) in the retina detect and convert light into neural signals which are transmitted to the brain via the optic nerve to produce vision. such eyes are typically spheroid, filled with the transparent gel-like vitreous humour, possess a focusing lens, and often an iris. muscles around the iris change the size of the pupil, regulating the amount of light that enters the eye and reducing aberrations when there is enough light. the eyes of most cephalopods, fish, amphibians and snakes have fixed lens shapes, and focusing is achieved by telescoping the lens in a similar manner to that of a camera. the compound eyes of the arthropods are composed of many simple facets which, depending on anatomical detail, may give either a single pixelated image or multiple images per eye. each sensor has its own lens and photosensitive cell(s). some eyes have up to 28,000 such sensors arranged hexagonally, which can give a full 360° field of vision. compound eyes are very sensitive to motion. some arthropods, including many strepsiptera, have compound eyes of only a few facets, each with a retina capable of creating an image. with each eye producing a different image, a fused, high-resolution image is produced in the brain. the eyes of mantis shrimps (here odontodactylus scyllarus) are considered the most complex in the whole animal kingdom. possessing detailed hyperspectral colour vision, the mantis shrimp has the world's most complex colour vision system. trilobites, now extinct, had unique compound eyes. clear calcite crystals formed the lenses of their eyes. they differ in this from most other arthropods, which have soft eyes. the number of lenses in such an eye varied widely; some trilobites had only one while others had thousands of lenses per eye. in contrast to compound eyes, simple eyes have a single lens. jumping spiders have one pair of large simple eyes with a narrow field of view, augmented by an array of smaller eyes for peripheral vision. some insect larvae, like caterpillars, have a type of simple eye (stemmata) which usually provides only a rough image, but (as in sawfly larvae) can possess resolving powers of 4 degrees of arc, be polarization-sensitive, and capable of increasing its absolute sensitivity at night by a factor of 1,000 or more. ocelli, some of the simplest eyes, are found in animals such as some of the snails. they have photosensitive cells but no lens or other means of projecting an image onto those cells. they can distinguish between light and dark but no more, enabling them to avoid direct sunlight. in organisms dwelling near deep-sea vents, compound eyes are adapted to see the infra-red light produced by the hot vents, allowing the creatures to avoid being boiled alive. types there are ten different eye layouts—indeed every technological method of capturing an optical image commonly used by human beings, with the exceptions of zoom and fresnel lenses, occur in nature. eye types can be categorised into "simple eyes", with one concave photoreceptive surface, and "compound eyes", which comprise a number of individual lenses laid out on a convex surface. "simple" does not imply a reduced level of complexity or acuity. indeed, any eye type can be adapted for almost any behaviour or environment. the only limitations specific to eye types are that of resolution—the physics of compound eyes prevents them from achieving a resolution better than 1°. also, superposition eyes can achieve greater sensitivity than apposition eyes, so are better suited to dark-dwelling creatures. eyes also fall into two groups on the basis of their photoreceptor's cellular construction, with the photoreceptor cells either being cilliated (as in the vertebrates) or rhabdomeric. these two groups are not monophyletic; the cnidaria also possess cilliated cells, and some gastropods, as well as some annelids possess both. some organisms have photosensitive cells that do nothing but detect whether the surroundings are light or dark, which is sufficient for the entrainment of circadian rhythms. these are not considered eyes because they lack enough structure to be considered an organ, and do not produce an image. non-compound eyes simple eyes are rather ubiquitous, and lens-bearing eyes have evolved at least seven times in vertebrates, cephalopods, annelids, crustaceans and cubozoa. pit eyes pit eyes, also known as stemma, are eye-spots which may be set into a pit to reduce the angles of light that enters and affects the eye-spot, to allow the organism to deduce the angle of incoming light. found in about 85% of phyla, these basic forms were probably the precursors to more advanced types of "simple eyes". they are small, comprising up to about 100 cells covering about 100 µm. the directionality can be improved by reducing the size of the aperture, by incorporating a reflective layer behind the receptor cells, or by filling the pit with a refractile material. pit vipers have developed pits that function as eyes by sensing thermal infra-red radiation, in addition to their optical wavelength eyes like those of other vertebrates (see infrared sensing in snakes). however, pit organs are fitted with receptors rather different from photoreceptors, namely a specific transient receptor potential channel (trp channels) called trpv1. the main difference is that photoreceptors are g-protein coupled receptors but trp are ion channels. spherical lens eye the resolution of pit eyes can be greatly improved by incorporating a material with a higher refractive index to form a lens, which may greatly reduce the blur radius encountered—hence increasing the resolution obtainable. the most basic form, seen in some gastropods and annelids, consists of a lens of one refractive index. a far sharper image can be obtained using materials with a high refractive index, decreasing to the edges; this decreases the focal length and thus allows a sharp image to form on the retina. this also allows a larger aperture for a given sharpness of image, allowing more light to enter the lens; and a flatter lens, reducing spherical aberration. such a non-homogeneous lens is necessary for the focal length to drop from about 4 times the lens radius, to 2.5 radii. heterogeneous eyes have evolved at least nine times: four or more times in gastropods, once in the copepods, once in the annelids, once in the cephalopods, and once in the chitons, which have aragonite lenses. no extant aquatic organisms possess homogeneous lenses; presumably the evolutionary pressure for a heterogeneous lens is great enough for this stage to be quickly "outgrown". this eye creates an image that is sharp enough that motion of the eye can cause significant blurring. to minimise the effect of eye motion while the animal moves, most such eyes have stabilising eye muscles. the ocelli of insects bear a simple lens, but their focal point usually lies behind the retina; consequently, those can't form a sharp image. ocelli (pit-type eyes of arthropods) blur the image across the whole retina, and are consequently excellent at responding to rapid changes in light intensity across the whole visual field; this fast response is further accelerated by the large nerve bundles which rush the information to the brain. focusing the image would also cause the sun's image to be focused on a few receptors, with the possibility of damage under the intense light; shielding the receptors would block out some light and thus reduce their sensitivity. this fast response has led to suggestions that the ocelli of insects are used mainly in flight, because they can be used to detect sudden changes in which way is up (because light, especially uv light which is absorbed by vegetation, usually comes from above). multiple lenses some marine organisms bear more than one lens; for instance the copepod pontella has three. the outer has a parabolic surface, countering the effects of spherical aberration while allowing a sharp image to be formed. another copepod, copilia, has two lenses in each eye, arranged like those in a telescope. such arrangements are rare and poorly understood, but represent an alternative construction. multiple lenses are seen in some hunters such as eagles and jumping spiders, which have a refractive cornea: these have a negative lens, enlarging the observed image by up to 50% over the receptor cells, thus increasing their optical resolution. refractive cornea in the eyes of most mammals, birds, reptiles, and most other terrestrial vertebrates (along with spiders and some insect larvae) the vitreous fluid has a higher refractive index than the air. in general, the lens is not spherical. spherical lenses produce spherical aberration. in refractive corneas, the lens tissue is corrected with inhomogeneous lens material (see luneburg lens), or with an aspheric shape. flattening the lens has a disadvantage; the quality of vision is diminished away from the main line of focus. thus, animals that have evolved with a wide field-of-view often have eyes that make use of an inhomogeneous lens. as mentioned above, a refractive cornea is only useful out of water. in water, there is little difference in refractive index between the vitreous fluid and the surrounding water. hence creatures that have returned to the water—penguins and seals, for example—lose their highly curved cornea and return to lens-based vision. an alternative solution, borne by some divers, is to have a very strongly focusing cornea. reflector eyes an alternative to a lens is to line the inside of the eye with "mirrors", and reflect the image to focus at a central point. the nature of these eyes means that if one were to peer into the pupil of an eye, one would see the same image that the organism would see, reflected back out. many small organisms such as rotifers, copepods and flatworms use such organs, but these are too small to produce usable images. some larger organisms, such as scallops, also use reflector eyes. the scallop pecten has up to 100 millimetre-scale reflector eyes fringing the edge of its shell. it detects moving objects as they pass successive lenses. there is at least one vertebrate, the spookfish, whose eyes include reflective