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The pit deepened over time, the opening diminished in size, and the number of photoreceptor cells increased, forming an effective pinhole camera that was capable of dimly distinguishing shapes.
This would have led to a somewhat different evolutionary trajectory for the vertebrate eye than for other animal eyes.
The thin overgrowth of transparent cells over the eye's aperture, originally formed to prevent damage to the eyespot, allowed the segregated contents of the eye chamber to specialise into a transparent humour that optimised colour filtering, blocked harmful radiation, improved the eye's refractive index , and allowed functionality outside of water.
The transparent protective cells eventually split into two layers, with circulatory fluid in between that allowed wider viewing angles and greater imaging resolution, and the thickness of the transparent layer gradually increased, in most species with the transparent crystallin protein.
The gap between tissue layers naturally formed a bioconvex shape, an optimally ideal structure for a normal refractive index. Independently, a transparent layer and a nontransparent layer split forward from the lens: Separation of the forward layer again formed a humour, the aqueous humour.
This increased refractive power and again eased circulatory problems. Formation of a nontransparent ring allowed more blood vessels, more circulation, and larger eye sizes.
Eyes are generally adapted to the environment and life requirements of the organism which bears them. For instance, the distribution of photoreceptors tends to match the area in which the highest acuity is required, with horizon-scanning organisms, such as those that live on the African plains, having a horizontal line of high-density ganglia, while tree-dwelling creatures which require good all-round vision tend to have a symmetrical distribution of ganglia, with acuity decreasing outwards from the centre.
Of course, for most eye types, it is impossible to diverge from a spherical form, so only the density of optical receptors can be altered.
In organisms with compound eyes, it is the number of ommatidia rather than ganglia that reflects the region of highest data acquisition.
An extension of this concept is that the eyes of predators typically have a zone of very acute vision at their centre, to assist in the identification of prey.
The hyperiid amphipods are deep water animals that feed on organisms above them. Their eyes are almost divided into two, with the upper region thought to be involved in detecting the silhouettes of potential prey—or predators—against the faint light of the sky above.
Accordingly, deeper water hyperiids, where the light against which the silhouettes must be compared is dimmer, have larger "upper-eyes", and may lose the lower portion of their eyes altogether.
Acuity is higher among male organisms that mate in mid-air, as they need to be able to spot and assess potential mates against a very large backdrop.
It is not only the shape of the eye that may be affected by lifestyle. Eyes can be the most visible parts of organisms, and this can act as a pressure on organisms to have more transparent eyes at the cost of function.
Eyes may be mounted on stalks to provide better all-round vision, by lifting them above an organism's carapace; this also allows them to track predators or prey without moving the head.
Visual acuity , or resolving power, is "the ability to distinguish fine detail" and is the property of cone cells. For example, if each pattern is 1.
The highest such number that the eye can resolve as stripes, or distinguish from a grey block, is then the measurement of visual acuity of the eye.
For a human eye with excellent acuity, the maximum theoretical resolution is 50 CPD  1. A rat can resolve only about 1 to 2 CPD.
However, in the compound eye, the resolution is related to the size of individual ommatidia and the distance between neighbouring ommatidia.
Physically these cannot be reduced in size to achieve the acuity seen with single lensed eyes as in mammals. Compound eyes have a much lower acuity than vertebrate eyes.
In primates, geckos, and other organisms, these take the form of cone cells , from which the more sensitive rod cells evolved. Most organisms with colour vision are able to detect ultraviolet light.
This high energy light can be damaging to receptor cells. With a few exceptions snakes, placental mammals , most organisms avoid these effects by having absorbent oil droplets around their cone cells.
The alternative, developed by organisms that had lost these oil droplets in the course of evolution, is to make the lens impervious to UV light — this precludes the possibility of any UV light being detected, as it does not even reach the retina.
The retina contains two major types of light-sensitive photoreceptor cells used for vision: Rods cannot distinguish colours, but are responsible for low-light scotopic monochrome black-and-white vision; they work well in dim light as they contain a pigment, rhodopsin visual purple , which is sensitive at low light intensity, but saturates at higher photopic intensities.
Rods are distributed throughout the retina but there are none at the fovea and none at the blind spot. Rod density is greater in the peripheral retina than in the central retina.
Cones are responsible for colour vision. They require brighter light to function than rods require. In humans, there are three types of cones, maximally sensitive to long-wavelength, medium-wavelength, and short-wavelength light often referred to as red, green, and blue, respectively, though the sensitivity peaks are not actually at these colours.
The colour seen is the combined effect of stimuli to, and responses from, these three types of cone cells.
Cones are mostly concentrated in and near the fovea. Only a few are present at the sides of the retina. Objects are seen most sharply in focus when their images fall on the fovea, as when one looks at an object directly.
Cone cells and rods are connected through intermediate cells in the retina to nerve fibres of the optic nerve. When rods and cones are stimulated by light, they connect through adjoining cells within the retina to send an electrical signal to the optic nerve fibres.
The optic nerves send off impulses through these fibres to the brain. The pigment molecules used in the eye are various, but can be used to define the evolutionary distance between different groups, and can also be an aid in determining which are closely related — although problems of convergence do exist.
Opsins are the pigments involved in photoreception. Other pigments, such as melanin, are used to shield the photoreceptor cells from light leaking in from the sides.
The opsin protein group evolved long before the last common ancestor of animals, and has continued to diversify since.
There are two types of opsin involved in vision; c-opsins, which are associated with ciliary-type photoreceptor cells, and r-opsins, associated with rhabdomeric photoreceptor cells.
However, some ganglion cells of vertebrates express r-opsins, suggesting that their ancestors used this pigment in vision, and that remnants survive in the eyes.
They may have been expressed in ciliary cells of larval eyes, which were subsequently resorbed into the brain on metamorphosis to the adult form.
From Wikipedia, the free encyclopedia. This article is about the organ. For the human eye, see Human eye. For other uses, see Eye disambiguation.
For other uses, see Eyeball disambiguation , Eyes disambiguation , and Ocular disambiguation. Evolution of the eye.
Annual Review of Neuroscience. National Institute of General Medical Sciences. Retrieved 3 June Physiology, Psychology and Ecology.
What animal has a more sophisticated eye, Octopus or Insect? Archived at the Wayback Machine. Journal of Insect Physiology.
Evolution Education and Outreach. Journal of Comparative Physiology. Archived from the original PDF on Annual Review of Entomology. Archived from the original PDF on 23 November Retrieved 27 May The evolution of superposition eyes in the Decapoda Crustacea ".
What is the ancestral visual organ in arthropods? Arthropod Structure and Development. Archived from the original PDF on 9 February Retrieved 13 November Structural and functional similarities and differences".
The Journal of Experimental Biology. Proceedings of the National Academy of Sciences. Handbook of Sensory Physiology. The Crucible of Creation.
Archived from the original on The Evolution of Eyes: Where Do Lenses Come From? Journal of Comparative Physiology A. Proceedings of the Royal Society of London.
The Image Processing Handbook. The upper limit finest detail visible with the human eye is about 50 cycles per degree, Casarett and Doull's Toxicology: The Basic Science of Poisons.
Brain, Behaviour and Evolution. The Quarterly Review of Biology. Anatomy of the globe of the human eye. Episcleral layer Schlemm's canal Trabecular meshwork.
Capillary lamina of choroid Bruch's membrane Sattler's layer. Ciliary processes Ciliary muscle Pars plicata Pars plana. Stroma Pupil Iris dilator muscle Iris sphincter muscle.
Inner limiting membrane Nerve fiber layer Ganglion cell layer Inner plexiform layer Inner nuclear layer Outer plexiform layer Outer nuclear layer External limiting membrane Layer of rods and cones Retinal pigment epithelium.
P cell , M cell , K cell , Muller glia. Vitreous chamber Vitreous body Retina Choroid. Retrieved from " https: Eye Sensory organs Visual system.
Compound eye of Antarctic krill. Anatomical terminology [ edit on Wikidata ]. Wikimedia Commons has media related to Eyes.
Sclera Episcleral layer Schlemm's canal Trabecular meshwork. Choroid Capillary lamina of choroid Bruch's membrane Sattler's layer.
He wears a patch over one eye. I have something in my eye. Only a trained eye can tell the difference between the original painting and a good copy.
For decorating, they rely on her discerning eye. He has an artist's eye for color. He reviewed the proposal with a jaundiced eye.
The biographer cast a cold eye on the artist's life. Verb I saw someone eyeing me from across the street. Recent Examples on the Web: Noun In addition to claiming more natural resources and trading posts, the British also had their eye on a piece of priceless treasure: Verb Meteorologists at the National Hurricane Center are eyeing two storm systems in the Atlantic Ocean, including one with a high chance of forming into a tropical cyclone.
Will he ever play in Philly again? First Known Use of eye Noun before the 12th century, in the meaning defined at sense 1a Verb 15th century, in the meaning defined at transitive sense 1a 1.
Learn More about eye. Resources for eye Time Traveler! Explore the year a word first appeared. From the Editors at Merriam-Webster. Dictionary Entries near eye Eyak eyas Eyck, van eye eyeable eye agate eye appeal.
Statistics for eye Look-up Popularity. Time Traveler for eye The first known use of eye was before the 12th century See more words from the same century.
More Definitions for eye. More from Merriam-Webster on eye Thesaurus: All synonyms and antonyms for eye Spanish Central: Translation of eye Nglish: Translation of eye for Spanish Speakers Britannica English: Translation of eye for Arabic Speakers Britannica.
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