Editing Eye (section)

==Types==
There are ten different eye layouts. 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, [[eye#Superposition eyes|superposition eyes]] can achieve greater sensitivity than [[apposition eye]]s, so are better suited to dark-dwelling creatures.<ref name=Land1992/>

Eyes also fall into two groups on the basis of their photoreceptor's cellular construction, with the photoreceptor cells either being ciliated (as in the vertebrates) or [[rhabdomeric]]. These two groups are not monophyletic; the [[Cnidaria]] also possess ciliated cells,<ref name="Kozmik2008">{{Cite journal| last3=Jonasova | first1=Z.| last2=Ruzickova| last7=Strnad | first2=J. | first3=K.| last6=Kozmikova| last5=Vopalensky | first4=Y.| last4=Matsumoto | first5=P. | first6=I.| last9=Piatigorsky | first7=H. | first9=J. |display-authors=9 <!-- This article has exactly 11 authors -->| title=Assembly of the cnidarian camera-type eye from vertebrate-like components | last1=Kozmik | first8=S.| last8=Kawamura | first11=C.| format=Free full text | journal=Proceedings of the National Academy of Sciences of the United States of America| last10=Paces | volume=105| last11=Vlcek | issue=26 | pages=8989–8993 | date=2008 | pmid=18577593 | doi=10.1073/pnas.0800388105 | pmc=2449352 | first10=V. |bibcode=2008PNAS..105.8989K | doi-access=free}}</ref> and some [[Gastropoda|gastropods]]<ref name="Zhukov2006">{{cite journal | last1=Zhukov|first1=ZH|last2=Borisseko|first2=SL|last3=Zieger|first3=MV|last4=Vakoliuk|first4=IA|last5=Meyer-Rochow|first5=VB|title=The eye of the freshwater prosobranch gastropod Viviparus viviparus: ultrastructure, electrophysiology and behaviour|journal=Acta Zoologica|year=2006|volume=87|pages=13–24|doi=10.1111/j.1463-6395.2006.00216.x}}</ref> and [[annelid]]s possess both.<ref name="Fernald2006">{{Cite journal| author=Fernald, Russell D.| year=2006| title=Casting a Genetic Light on the Evolution of Eyes| journal=Science| volume=313| issue=5795| pages=1914–1918| doi=10.1126/science.1127889| pmid=17008522| bibcode=2006Sci...313.1914F| s2cid=84439732}}</ref>

Some organisms have [[Photosensitivity|photosensitive]] cells that do nothing but detect whether the surroundings are light or [[darkness|dark]], which is sufficient for the [[Entrainment (chronobiology)|entrainment]] of [[circadian rhythm]]s. These are not considered eyes because they lack enough structure to be considered an organ, and do not produce an image.<ref>{{cite web|title=Circadian Rhythms Fact Sheet|work=National Institute of General Medical Sciences (NIGMS) |url=http://www.nigms.nih.gov/Education/Pages/Factsheet_CircadianRhythms.aspx|access-date=3 June 2015|publisher=National Institute of General Medical Sciences|archive-date=13 March 2020|archive-url=https://web.archive.org/web/20200313000520/https://www.nigms.nih.gov/education/pages/factsheet_circadianrhythms.aspx|url-status=live}}</ref>

Every technological method of capturing an optical image that humans commonly use occurs in nature, with the exception of [[Zoom lens|zoom]] and [[Fresnel lens]]es.<ref name=Land1992/>

===Non-compound eyes===
Simple eyes are rather ubiquitous, and lens-bearing eyes have evolved at least seven times in [[vertebrate]]s, [[cephalopod]]s, [[annelid]]s, [[crustacean]]s and [[Cubozoa]].<ref>{{cite journal |author=Nilsson, Dan-E. |year=1989 |title=Vision optics and evolution |journal=[[BioScience]] |volume=39 |issue=5 |pages=298–307 |doi=10.2307/1311112|jstor=1311112 }}</ref>{{Failed verification|date=June 2016|reason="seven times" doesn't appear; arthropods not included}}

====Pit eyes====
Pit eyes, also known as [[Simple eyes in invertebrates#Stemmata|stemmata]], 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&nbsp;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.<ref name=Land1992/>

[[Crotalinae|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 receptor|G-protein coupled receptors]] but TRP are [[ion channel]]s.

====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.<ref name=Land1992/>

So-called under-focused lens eyes, found in gastropods and polychaete worms, have eyes that are intermediate between lens-less cup eyes and real camera eyes. Also [[box jellyfish]] have eyes with a spherical lens, cornea and retina, but the vision is blurry.<ref>[https://books.google.com/books?id=vQgWDAAAQBAJ&dq=Under-focused+lens+eyes+intermediate+cup+box+jellyfish&pg=PA76 Animal Eyes]</ref><ref>[https://books.google.com/books?id=A26JAgAAQBAJ&dq=Box+jellyfish+rhopalium+cornea+lens+pigment+retina&pg=PA306 Perceiving in Depth, Volume 1: Basic Mechanisms]</ref>

Heterogeneous eyes have evolved at least nine times: four or more times in [[Sensory organs of gastropods|gastropods]], once in the [[copepod]]s, once in the [[annelid]]s, once in the [[cephalopod]]s,<ref name=Land1992/> and once in the [[chiton]]s, which have [[aragonite]] lenses.<ref name="Speiser2011">{{Cite journal | last1=Speiser | first1=D.I. | last2=Eernisse | first2=D.J. | last3=Johnsen | first3=S.N. | doi=10.1016/j.cub.2011.03.033 | title=A Chiton Uses Aragonite Lenses to Form Images | journal=Current Biology | volume=21 | issue=8 | pages=665–670 | year=2011 | pmid= 21497091| s2cid=10261602 | doi-access=free | bibcode=2011CBio...21..665S }}</ref> 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".<ref name=Land1992/>

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.<ref name=Land1992/>

The [[ocellus|ocelli]] of insects bear a simple lens, but their focal point usually lies behind the retina; consequently, those can not 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).<ref name="Wilson1978">{{Cite journal |last=Wilson |first=M. |year=1978 |title=The functional organisation of locust ocelli |journal=Journal of Comparative Physiology |volume=124 |issue=4 |pages=297–316 |doi=10.1007/BF00661380 |s2cid=572458}}</ref>

====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.<ref name=Land1992/> 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.<ref name=Land1992/>

====Refractive cornea====
[[File:Human eye, lateral view.jpg|thumb|A refractive cornea type eye of a human. The cornea is the clear domed part covering the [[Anterior chamber of eyeball|anterior chamber of the eye]].]]
In the [[Mammalian eye|eyes of most mammals]], [[Bird vision#Anatomy of the eye|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.<ref name=Land1992/>

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.<ref name=Land1992/>
[[File:Closed human eye, superior view.jpg|thumb|[[Eyelid|Eyelids]] and [[Eyelash|eyelashes]] are a unique characteristic of most mammalian eyes, both of which are evolutionary features to protect the eye.]]
A unique feature of most mammal eyes is the presence of [[Eyelid|eyelids]] which wipe the eye and spread [[tears]] across the cornea to prevent dehydration. These eyelids are also supplemented by the presence of [[Eyelash|eyelashes]], multiple rows of highly innervated and sensitive hairs which grow from the eyelid margins to protect the eye from fine particles and small irritants such as insects.

====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.<ref name=Land1992/>

Many small organisms such as [[rotifer]]s, copepods and [[flatworm]]s use such organs, but these are too small to produce usable images. Some larger organisms, such as [[scallop]]s, also use reflector eyes. The scallop ''[[Pecten (bivalve)|Pecten]]'' has up to 100 millimetre-scale reflector eyes fringing the edge of its shell. It detects moving objects as they pass successive lenses.<ref name=Land1992/>

There is at least one vertebrate, the [[Brownsnout spookfish|spookfish]], whose eyes include reflective optics for focusing of light. Each of the two eyes of a spookfish collects light from both above and below; the light coming from above is focused by a lens, while that coming from below, by a curved mirror composed of many layers of small reflective plates made of [[guanine]] [[crystal]]s.<ref name="wagner et al">{{cite journal |author1=Wagner, H.J. |author2=Douglas, R.H. |author3=Frank, T.M. |author4=Roberts, N.W. |author5=Partridge, J.C. |name-list-style=amp|title=A Novel Vertebrate Eye Using Both Refractive and Reflective Optics |journal=Current Biology |volume=19 |pages=108–114 |date=Jan 27, 2009 | pmid=19110427 | doi=10.1016/j.cub.2008.11.061 |issue=2
|s2cid=18680315 |doi-access=free |bibcode=2009CBio...19..108W }}</ref>

===Compound eyes===
{{main|Compound eye}}
{{further|Arthropod eye}}
[[File:FLY EYE.jpg|thumb|right|An image of a house fly compound eye surface by using [[scanning electron microscope]]]]
[[File:Insect compound eye diagram.svg|thumb|upright=0.9|Anatomy of the compound eye of an insect]]
[[File:Calliphora vomitoria Portrait.jpg|thumb|Arthropods such as this [[Calliphora vomitoria|blue bottle fly]] have compound eyes.]]

A compound eye may consist of thousands of individual photoreceptor units or ommatidia ([[ommatidium]], singular). The image perceived is a combination of inputs from the numerous ommatidia (individual "eye units"), which are located on a convex surface, thus pointing in slightly different directions. Compared with simple eyes, compound eyes possess a very large view angle, and can detect fast movement and, in some cases, the [[Polarization (waves)|polarisation]] of light.<ref>{{cite journal
 |url=http://www.suss-microoptics.com/downloads/Publications/Miniaturized_Imaging_Systems.pdf
 |doi=10.1016/S0167-9317(03)00102-3
 |title=Miniaturized imaging systems
 |date=June 2003
 |journal=Microelectronic Engineering
 |volume=67–68
 |issue=1
 |pages=461–472
 |author1=Völkel, R
 |author2=Eisner, M
 |author3=Weible, KJ
 |url-status=usurped
 |archive-url=https://web.archive.org/web/20081001225326/http://www.suss-microoptics.com/downloads/Publications/Miniaturized_Imaging_Systems.pdf
 |archive-date=2008-10-01
}}</ref> Because the individual lenses are so small, the effects of [[diffraction]] impose a limit on the possible resolution that can be obtained (assuming that they do not function as [[phased array]]s). This can only be countered by increasing lens size and number. To see with a resolution comparable to our simple eyes, humans would require very large compound eyes, around {{convert|11|m}} in radius.<ref>{{cite journal|last=Land|first=Michael|title=Visual Acuity in Insects|journal=Annual Review of Entomology|year=1997|volume=42|pages=147–177|url=http://web.neurobio.arizona.edu/gronenberg/nrsc581/eyedesign/visualacuity.pdf|access-date=27 May 2013|doi=10.1146/annurev.ento.42.1.147|pmid=15012311|url-status=dead|archive-url=https://web.archive.org/web/20041123010008/http://web.neurobio.arizona.edu/gronenberg/nrsc581/eyedesign/visualacuity.pdf|archive-date=23 November 2004}}</ref>

Compound eyes fall into two groups: apposition eyes, which form multiple inverted images, and superposition eyes, which form a single erect image.<ref>{{cite journal
| last=Gaten
| first=Edward
| title=Optics and phylogeny: is there an insight? The evolution of superposition eyes in the Decapoda (Crustacea)
| year=1998
| journal=Contributions to Zoology
| volume=67
| issue=4
| pages=223–236
| doi=10.1163/18759866-06704001
| doi-access=free
}}</ref> Compound eyes are common in arthropods, annelids and some bivalved molluscs.<ref>{{Cite journal
| last=Ritchie
| first=Alexander
| title=''Ainiktozoon loganense'' Scourfield, a protochordate from the Silurian of Scotland
| year=1985
| journal=Alcheringa
| volume=9
| page=137
| doi=10.1080/03115518508618961
| issue=2
| bibcode=1985Alch....9..117R
}}</ref> Compound eyes in arthropods grow at their margins by the addition of new ommatidia.<ref name=Mayer2006>
{{Cite journal
| last=Mayer
| first=G.
| year=2006
| title=Structure and development of onychophoran eyes: What is the ancestral visual organ in arthropods?
| journal=Arthropod Structure and Development
| volume=35
| issue=4
| pages=231–245
| doi=10.1016/j.asd.2006.06.003
| pmid=18089073
| bibcode=2006ArtSD..35..231M
}}</ref>

====Apposition eyes====
Apposition eyes are the most common form of eyes and are presumably the ancestral form of compound eyes. They are found in all [[arthropod]] groups, although they may have evolved more than once within this phylum. Some [[annelids]] and [[bivalves]] also have apposition eyes. They are also possessed by ''[[Limulus]]'', the horseshoe crab, and there are suggestions that other chelicerates developed their simple eyes by reduction from a compound starting point.<ref name=Land1992/> (Some caterpillars appear to have evolved compound eyes from simple eyes in the opposite fashion.)

Apposition eyes work by gathering a number of images, one from each eye, and combining them in the brain, with each eye typically contributing a single point of information. The typical apposition eye has a lens focusing light from one direction on the rhabdom, while light from other directions is absorbed by the dark wall of the [[ommatidium]].

====Superposition eyes====
The second type is named the superposition eye. The superposition eye is divided into three types:
* refracting,
* reflecting and
* parabolic superposition

The refracting superposition eye has a gap between the lens and the rhabdom, and no side wall. Each lens takes light at an angle to its axis and reflects it to the same angle on the other side. The result is an image at half the radius of the eye, which is where the tips of the rhabdoms are. This type of compound eye, for which a minimal size exists below which effective superposition cannot occur,<ref name="Meyer-Rochow 2004">{{cite journal|last1=Meyer-Rochow|first1=VB|last2=Gal|first2=J|title=Dimensional limits for arthropod eyes with superposition optics|journal=Vision Research|date=2004|volume=44|issue=19|pages=2213–2223|doi=10.1016/j.visres.2004.04.009|pmid=15208008|doi-access=free}}</ref> is normally found in nocturnal insects, because it can create images up to 1000 times brighter than equivalent apposition eyes, though at the cost of reduced resolution.<ref>{{cite thesis|type=PhD |last=Greiner |first=Birgit |title=Adaptations for nocturnal vision in insect apposition eyes |publisher=Lund University |date=16 December 2005 |url=http://www4.lu.se/upload/GreinerThesis.pdf |access-date=13 November 2014 |url-status=dead |archive-url=https://web.archive.org/web/20130209164014/http://www4.lu.se/upload/GreinerThesis.pdf |archive-date=9 February 2013 }}</ref> In the parabolic superposition compound eye type, seen in arthropods such as [[mayfly|mayflies]], the parabolic surfaces of the inside of each facet focus light from a reflector to a sensor array. Long-bodied [[Decapoda|decapod crustaceans]] such as [[shrimp]], [[prawn]]s, [[crayfish]] and [[lobster]]s are alone in having reflecting superposition eyes, which also have a transparent gap but use corner [[mirror]]s instead of lenses.

====Parabolic superposition====
This eye type functions by refracting light, then using a parabolic mirror to focus the image; it combines features of superposition and apposition eyes.<ref name=Cronin2008/>

====Other====
Another kind of compound eye, found in males of Order [[Strepsiptera]], employs a series of simple eyes—eyes having one opening that provides light for an entire image-forming retina. Several of these ''eyelets'' together form the strepsipteran compound eye, which is similar to the 'schizochroal' compound eyes of some [[trilobites]].<ref>{{Cite journal|last1=Horváth|first1=Gábor|last2=Clarkson|first2=Euan N.K.|year=1997|title=Survey of modern counterparts of schizochroal trilobite eyes: Structural and functional similarities and differences|journal=Historical Biology|volume=12|issue=3–4|doi=10.1080/08912969709386565|pages=229–263|bibcode=1997HBio...12..229H }}</ref> Because each eyelet is a simple eye, it produces an inverted image; those images are combined in the brain to form one unified image. Because the aperture of an eyelet is larger than the facets of a compound eye, this arrangement allows vision under low light levels.<ref name=Land1992/>

Good fliers such as flies or honey bees, or prey-catching insects such as [[praying mantis]] or [[dragonfly|dragonflies]], have specialised zones of [[ommatidium|ommatidia]] organised into a [[Fovea centralis|fovea]] area which gives acute vision. In the acute zone, the eyes are flattened and the facets larger. The flattening allows more ommatidia to receive light from a spot and therefore higher resolution. The black spot that can be seen on the compound eyes of such insects, which always seems to look directly at the observer, is called a [[pseudopupil]]. This occurs because the [[ommatidia]] which one observes "head-on" (along their [[optical axis|optical axes]]) absorb the [[incident light]], while those to one side reflect it.<ref name="Zeil">{{cite journal |author1=Jochen Zeil |author2=Maha M. Al-Mutairi |year=1996 |title=Variations in the optical properties of the compound eyes of ''Uca lactea annulipes'' |journal=[[The Journal of Experimental Biology]] |volume=199 |issue=7 |pages=1569–1577 |doi=10.1242/jeb.199.7.1569 |url=http://jeb.biologists.org/cgi/reprint/199/7/1569.pdf |pmid=9319471 |access-date=2008-09-15 |archive-date=2009-02-25 |archive-url=https://web.archive.org/web/20090225084203/http://jeb.biologists.org/cgi/reprint/199/7/1569.pdf |url-status=live }}</ref>

There are some exceptions from the types mentioned above. Some insects have a so-called single lens compound eye, a transitional type which is something between a superposition type of the multi-lens compound eye and the single lens eye found in animals with simple eyes. Then there is the [[mysid]] shrimp, ''Dioptromysis paucispinosa''. The shrimp has an eye of the refracting superposition type, in the rear behind this in each eye there is a single large facet that is three times in diameter the others in the eye and behind this is an enlarged crystalline cone. This projects an upright image on a specialised retina. The resulting eye is a mixture of a simple eye within a compound eye.

Another version is a compound eye often referred to as "pseudofaceted", as seen in ''[[Scutigera]]''.<ref name="Müller 2003">{{cite journal|last1=Müller|first1=CHG|last2=Rosenberg|first2=J|last3=Richter|first3=S|last4=Meyer-Rochow|first4=VB|title=The compound eye of Scutigera coleoptrata (Linnaeus, 1758) (Chilopoda; Notostigmophora): an ultrastructural re-investigation that adds support to the Mandibulata concept|journal=Zoomorphology|date=2003|volume=122|issue=4|pages=191–209|doi=10.1007/s00435-003-0085-0|s2cid=6466405}}</ref> This type of eye consists of a cluster of numerous [[ommatidia]] on each side of the head, organised in a way that resembles a true compound eye.

The body of ''[[Ophiocoma wendtii]]'', a type of [[brittle star]], is covered with ommatidia, turning its whole skin into a compound eye. The same is true of many [[chiton]]s. The tube feet of sea urchins contain photoreceptor proteins, which together act as a compound eye; they lack screening pigments, but can detect the directionality of light by the shadow cast by its opaque body.<ref name="Ullrich-Luter2011">{{Cite journal | last1=Ullrich-Luter | first1=E.M. | last2=Dupont | first2=S. | last3=Arboleda | first3=E. | last4=Hausen | first4=H. | last5=Arnone | first5=M.I. | title=Unique system of photoreceptors in sea urchin tube feet | doi=10.1073/pnas.1018495108 | journal=Proceedings of the National Academy of Sciences | volume=108 | issue=20 | pages=8367–8372 | year=2011 | pmid= 21536888| pmc=3100952| bibcode=2011PNAS..108.8367U | doi-access=free }}</ref>

====Nutrients====
The '''ciliary body''' is triangular in horizontal section and is coated by a double layer, the ciliary epithelium. The inner layer is transparent and covers the vitreous body, and is continuous from the neural tissue of the retina. The outer layer is highly pigmented, continuous with the retinal pigment epithelium, and constitutes the cells of the dilator muscle.

The '''vitreous''' is the transparent, colourless, gelatinous mass that fills the space between the lens of the eye and the retina lining the back of the eye.<ref name=Ali&Klyne1985>{{harvnb|Ali|Klyne|1985|page=8}}</ref> It is produced by certain retinal cells. It is of rather similar composition to the cornea, but contains very few cells (mostly phagocytes which remove unwanted cellular debris in the visual field, as well as the hyalocytes of Balazs of the surface of the vitreous, which reprocess the [[hyaluronic acid]]), no blood vessels, and 98–99% of its volume is water (as opposed to 75% in the cornea) with salts, sugars, vitrosin (a type of collagen), a network of collagen type II fibres with the [[mucopolysaccharide]] hyaluronic acid, and also a wide array of proteins in micro amounts. Amazingly, with so little solid matter, it tautly holds the eye.