Editing Eye (section)

===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>