Eye:

The eye is an organ for sensing light. About 97 percent of the animals known have eyes.^[1] Image-resolving eyes are present in cnidaria, molluscs, chordates, annelids and arthropods.^[2] The eyes found in different animals are different from each other. Some are simpler, others are more complex. Today, ten different kinds of eyes are known.^[1]

The simplest "eyes" are similar to those found in unicellular organisms. They do nothing but detect if the surroundings are light or dark. Most animals have a "clock" inside. These simple "eyes" are used to adjust this inner clock, which is called circadian rhythm. Some snails, for example, see no image (picture) at all, but they "see" light, which helps them stay out of bright sunlight. More complex eyes have not lost this function. A special type of cells in the eye senses light for a different purpose than seeing. These cells are called ganglion cells. They are located in the retina. They send their information about light to the brain along a different path (the retinohypothalamic tract). This information adjusts (synchronizes) the animal's circadian rhythm to nature's light/dark cycle of 24 hours. The system also works for some blind people who cannot see light at all.

Eyes that are a little bit better are shaped like cups, which lets the animal know which way the light is coming from.

More complex eyes give the full sense of vision, including color, motion, and texture. These eyes have a round shape that makes light rays focus on the back part of the eye, called the retina. In mammals, there are (at least) three types of light-sensitive cells (cells that notice light) in the retina. Two of them, rods and cones, allow sight (or seeing) by sending signals through the optic nerve to the brain. The third type senses light for a different purpose than seeing. Some special ganglion cells in the retina send their information about light to the brain along a different path (the retinohypothalamic tract). This information adjusts the animal's circadian rhythm to nature's light/dark cycle of 24 hours. This system also works for some blind people who cannot see light at all.

Some animals can see light that humans can't see. They can see ultraviolet or infrared light.

The lens on the front part of the eye is curved and acts like a camera lens. It can be pulled flatter or rounder by muscles inside the eye. As some people get older, they might not be as able to do this. Many people are born with other small problems or get them later in life, and they may need eyeglasses (or contact lenses) to fix the problem.

Types of eye

Today, ten different layouts of eyes are known. Most ways of capturing an image have evolved at least once. The exception to this are zoom and Fresnel lenses. One way to categorize eyes is to look at the number of "chamber"s. Simple eyes are made of only one concave chamber, perhaps with a lens. Compound eyes have many such chambers with their lenses on a convex surface.^[1] Here, simple does not mean that the eye itself is not a complex design. Eyes can be adapted to their environment, or the behaviour of the animal that has them. The limitations of the eye types are that of resolution. Compound eyes cannot have a resolution better than 1°. Also, superposition eyes can achieve greater sensitivity than apposition eyes; they are therefore better suited to dark-dwelling creatures.^[1]

Eyes also can be grouped according to how the photoreceptor is made. Photoreceptors are either cillated, or rhabdomic. These two groups are not monophyletic; the cnidaira also possess cilliated cells, ^[3] and some annelids possess both.^[4]

Simple eyes

Pit eyes

Pit eyes are also known as Stemma. Very often, they are set in a pit. This is done to reduce the angles at which light can enter. This allows the organism to say where the light is coming from.^[1] Such eyes can be found in about 85% of phyla. They probably came before the development of more complex "simple eyes". Pit eyes are small. They are made of up to about hundred cells, covering about 100 µm.^[1] The directionality can be improved by reducing the size of the aperture, by putting a reflective layer behind the receptor cells, or by filling the pit with a refractile material.^[1]

Pinhole eye

The pinhole eye is an "advanced" form of pit eye. It has several improvements, most notably a small aperture and deep pit. Sometimes, the aperture can be changed. It is only found in the nautiloids.^[1] Without a lens to focus the image, it produces a blurry image, and will blur out a point to the size of the aperture. Consequently, nautiloids can't discriminate between objects with an angular separation of less than 11°.^[1] Shrinking the aperture would produce a sharper image, but let in less light.^[1]

Spherical lensed eye

The resolution of pit eyes can be improved a lot, by adding a material with a higher refractive index to make a lens. This will reduce the radius of the blurring, and increase the resolution that can be achieved.^[1] The most basic form can still be seen in some gastropods and annelids. These eyes have a lens of one refractive index. It is possible to get a better image with materials that have a high refractive index which decreases towards the edges. This decreases the focal length and allows a sharp image to form on the retina.^[1] This also allows a larger aperture for a given sharpness of image - more light will enter the lens; and a flatter lens, reducing spherical aberration.^[1] Such an inhomogeneous lens is necessary in order for the focal length to drop from about 4 times the lens radius, to 2.5 times.^[1]

Heterogeneous eyes have evolved at least eight times — four or more times in gastropods, once in the copepods, once in the annelids and once in the cephalopods.^[1] No aquatic organisms have homogeneous lenses; presumably the evolutionary pressure for a heterogeneous lens is great enough for this stage to be quickly "outgrown".^[1]

This eye creates an image that is sharp enough that motion of the eye can cause significant blurring. To minimize the effect of eye motion while the animal moves, most such eyes have stabilizing eye muscles.^[1]

The ocelli of insects have a simple lens, but their focal point always lies behind the retina.They can never form a sharp image. This limits the function of the eye. Ocelli (pit-type eyes of arthropods) blur the image across the whole retina. They are very good at responding to rapid changes in light intensity across the whole visual field — this fast response is accelerated even more by the large nerve bundles which rush the information to the brain.^[5] Focusing the image would also cause the sun's image to be focused on a few receptors. These could possibly be damaged by the intense light; shielding the receptors would block out some light and reduce their sensitivity.^[5] 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).^[5]

Problems

This construction of the eye also has certain problems. One of them is that chromatic aberration is still quite high.^[1] This means that the colors seen are wrong. This is of course a small problem for organisms that cannot see colors

Another problem that can be found in the eye of vertebrates is the blind spot at the optic disc. There, the optic nerve is attached to the back of the eye. There are no rods or cones there which could detect light. The eye of the cephalopod has no blind spot, because the retina is in the opposite orientation.

Many lenses

Some animals that live in the sea have eyes with more than one lens. One such animal is the copeopod Pontella, which has eyes with three lenses. The outer lens has a parabolic surface. This acts against spherical aberration and allows a sharp image to form. Copilla's eyes have two lenses, which move in and out like a telescope.^[1]

Such arrangements cannot be found often. They are also poorly understood. They are an interesting alternative construction, though. Animals that hunt their prey, such as eagles or jumping spiders sometimes have several lenses. They have a refractive cornea. This is a negative lens which makes the image bigger, by up to 50%. This increases the resolution of the eye.^[1]

Refractive cornea

The eyes of most land-living vertebrates (as well as those of some spiders, and [[insect larvae) contain a fluid that has a higher refractive index than the air. That way, the lens does not have to reduce the focal length, because this is done by the fluid. That way, the lens can adjust the focus more easily. That way, a very high resolution can be obtained.^[1]

There are two ways to solve the problem of the spherical aberration of the lens:

Flattening the lens is the worse solution of the two, because the quality of the image is worse, away from the main line of focus. Animals that need to see all around have a disadvantage, if the use the flatter lens design. For this reason, such animals often have an inhomogeneous lens.^[1]

A refractive cornea is only useful outside the water. In the water, there is little difference of the refractive index of the water and the fluid in the eye. Animals that have returned to the water, such as penguins or seals have lost their refractive cornea. They use a simpler lens design instead. Other animals, that dive very often have found a different solution: They use a very strong cornea.^[1]

Reflector eyes

Instead of using a lens it is also possible to have cells inside the eye that act like mirrors. The image can then be reflected to focus at a central point. This design also means that someone looking into the such an eye will see the same image than the organism who has them.^[1]

Many small organisms such as rotifers, copeopods and platyhelminths use such this design, but their eyes are too small to produce usable images.^[1] Some larger organisms, such as scallops, also use reflector eyes. The scallop Pecten has up to 100 millimeter-scale reflector eyes fringing the edge of its shell. It detects moving objects as they pass successive lenses.^[1]

There is at least one vertebrate, the 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 the above is focused by a lens. The light from below is reflected by a curved mirror composed of many layers of small plates made of guanine crystals.^[6]

Compound eyes

Compound eyes are different from simple eyes. Instead of having one organ that can sense light, they put together many such organs. Some compound eyes have thousands of them. The resulting image is put together in the brain, based on the signals of the many "eye units". Each such unit is called oomatidium, several are called oomatidia. The oomatidia are located on a convex surface, each of them points in a slighly different direction. Unlike simple eyes, compound eyes have a very large angle of view. They can detect fast movement, and sometimes the he polarization of light.^[7] A single eye unit is very small. This means that the diffraction of light limits the resolution that can be achieved. The only way to solve this problem is to make the eye unit bigger.

Compound eyes grow at their edges. New ommatidia are simply added to the existing compound.^[10]

Apposition eyes

Apposition eyes are the most common form of eye. They are probably the first form of compound eye that developed. They are found in all arthropod groups, although they may have evolved more than once within this phylum.^[1] Some annelids and bivalves also have apposition eyes. Limulus, the horseshoe crab, also has them. Other chelicerates could have developed their simple eyes by reduction from a compound starting point.^[1] Certain caterpillars seem to have evolved their compound eyes from simple ones.

Apposition eyes work by gathering a number of images, one from each eye, and combining them in the brain. Each eye typically contributes 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. In the other kind of apposition eye, found in the Strepsiptera, lenses are not fused to one another, each forms an entire image. These images are combined in the brain. This is called schizochroal compound eye or neural superposition eye. Images are combined one after another. which allows vision even if there is very little light.^[11]

Superposition eyes

The second type is named the superposition eye. There are three different types of superposition eyes:

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 kind is used mostly by nocturnal insects.

Arthropods such as mayflies have parabolic superposition eyes. In this type, the parabolic surfaces of the inside of each facet focus light from a reflector to a sensor array. Long-bodied decapod crustaceans such as shrimp, prawns, crayfish and lobsters are alone in having reflecting superposition eyes, which also has a transparent gap but uses corner mirrors 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.^[11]

Other

Good fliers like flies or honey bees, or prey-catching insects like praying mantis or dragonflies, have specialized zones of ommatidia organized into a fovea area which gives sharp vision. In this zone the eyes are flattened and the facets are larger. The flattening allows more ommatidia to receive light from a spot. This gives a higher resolution.

There are some exceptions to the types mentioned above. Some insects have a so-called single-lens compound eye. This type is 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, but there is a large facet. This facet has three times the diameter of the others. Behind it is an enlarged crystalline cone. The cone projects an upright image onto a specialized retina.The resulting eye is a mixture of a simple eye within a compound eye.

Another version is the pseudofaceted eye, as seen in Scutigera. This type of eye consists of a cluster of many ocelli on each side of the head, organized 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 chitons.

References

↑ ^1.00 ^1.01 ^1.02 ^1.03 ^1.04 ^1.05 ^1.06 ^1.07 ^1.08 ^1.09 ^1.10 ^1.11 ^1.12 ^1.13 ^1.14 ^1.15 ^1.16 ^1.17 ^1.18 ^1.19 ^1.20 ^1.21 ^1.22 ^1.23 ^1.24 ^1.25 ^1.26 ^1.27 ^1.28 Land, M F (1992). "The Evolution of Eyes". Annual Review of Neuroscience 15: 1–29. DOI:10.1146/annurev.ne.15.030192.000245.
↑ Frentiu, Francesca D. (2008), "A butterfly eye's view of birds", BioEssays 30: 1151, DOI:10.1002/bies.20828
↑ Kozmik, Zbynek; Ruzickova, Jana; Jonasova, Kristyna; Matsumoto, Yoshifumi; Vopalensky, Pavel; Kozmikova, Iryna; Strnad, Hynek; Kawamura, Shoji; Piatigorsky, Joram; Paces, Vaclav; Vlcek, Cestmir (2008), "Assembly of the cnidarian camera-type eye from vertebrate-like components", Proceedings of the National Academy of Sciences 105 (26): 8989–8993, DOI:10.1073/pnas.0800388105
↑ Fernald, Russell D. (2006), "Casting a Genetic Light on the Evolution of Eyes", Science 313 (5795): 1914–1918, DOI:10.1126/science.1127889
↑ ^5.0 ^5.1 ^5.2 Wilson, M. (1978), "The functional organisation of locust ocelli", Journal of Comparative Physiology (no. 4): 297–316
↑ Wagner, H.J., Douglas, R.H., Frank, T.M., Roberts, N.W., and Partridge, J.C. (Jan. 27, 2009). "A Novel Vertebrate Eye Using Both Refractive and Reflective Optics". Current Biology 19: 108–114. DOI:10.1016/j.cub.2008.11.061.
↑ Völkel, R.; Eisner, M.; Weible, K. J. (June 2003). "Miniaturized imaging systems" (PDF). Microelectronic Engineering 67-68 (1): 461–472. DOI:10.1016/S0167-9317(03)00102-3.
↑ Gaten, Edward (1998). "Optics and phylogeny: is there an insight? The evolution of superposition eyes in the Decapoda (Crustacea)". Contributions to Zoology 67 (4): 223–236.
↑ Ritchie, Alexander (1985). "Ainiktozoon loganense Scourfield, a protochordate? from the Silurian of Scotland". Alcheringa 9: 137.
↑ Mayer, G. (2006), "Structure and development of onychophoran eyes: What is the ancestral visual organ in arthropods?", Arthropod Structure and Development 35 (4): 231–245, DOI:10.1016/j.asd.2006.06.003
↑ ^11.0 ^11.1 Template:Cite doi

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