ratio of word probabilities predicted from brain for bear and eye

bear	eye
top 10 words in brain distribution (in article): species animal male female breed human hunt wolf live food	top 10 words in brain distribution (in article): cell light animal form produce human muscle body water contain
top 10 words in brain distribution (not in article): bird cat egg wild dog elephant pet water deer ant	top 10 words in brain distribution (not in article): drink lamp wine beer tissue process plant bone bottle structure
times more probable under bear 30 20 10 6 4 2.5 1.25 1 1.25 2.5 4 6 10 20 30 times more probable under eye (words not in the model)
Bears'" are mammals of the family "'Ursidae'". Bears are classified as caniforms, or doglike carnivorans, with the pinnipeds being their closest living relatives. Although there are only eight living species of bear, they are widespread, appearing in a wide variety of habitats throughout the Northern Hemisphere and partially in the Southern Hemisphere. That which pertains to bears is called "ursine". Bears are found in the continents of North America, South America, Europe, and Asia. Common characteristics of modern bears include a large body with stocky legs, a long snout, shaggy hair, plantigrade paws with five nonretractile claws, and a short tail. While the polar bear is mostly carnivorous and the giant panda feeds almost entirely on bamboo, the remaining six species are omnivorous, with largely varied diets including both plants and animals. With the exceptions of courting individuals and mothers with their young, bears are typically solitary animals. They are sometimes diurnal, but are usually active during the night (nocturnal) or twilight (crepuscular). Bears are aided by an excellent sense of smell, and despite their heavy build and awkward gait, they can run quickly and are adept climbers and swimmers. In autumn some bear species forage large amounts of fermented fruits which affects their behaviour.Bears use shelters such as caves and burrows as their dens, which are occupied by most species during the winter for a long period of sleep similar to hibernation. Bears have been hunted since prehistoric times for their meat and fur. To this day, they play a prominent role in the arts, mythology, and other cultural aspects of various human societies. In modern times, the bear's existence has been pressured through the encroachment of their habitats and the illegal trade of bears and bear parts, including the Asian bile bear market. The IUCN lists six bear species as vulnerable or endangered, and even "least concern" species such as the brown bear are at risk of extirpation in certain countries. The poaching and international trade of these most threatened populations is prohibited, but still ongoing. Evolutionary relationships. Fossil of Cave bear ("Ursus spelaeus") The Ursidae family belongs to the order Carnivora and is one of nine families in the suborder Caniformia, or "doglike" carnivorans. Bears' closest living relatives are the pinnipeds, a clade of three families: Odobenidae (the walrus), Otariidae (fur seals and sea lions), and Phocidae (true or earless seals). Bears comprise eight species in three subfamilies: Ailuropodinae (monotypic with the giant panda), Tremarctinae (monotypic with the Spectacled Bear), and Ursinae (containing six species divided into one to three genera, depending upon authority). The origins of Ursidae can be traced back to the very small and graceful "Parictis" that had a skull only 7 cm (3 in) long. Parictis first occur in North America in the Late Eocene (ca. 38 million years ago), but this genus did not appear in Eurasia and Africa until the Miocene. The raccoon-sized, dog-like "Cephalogale", however, is widely regarded as the most primitive ursid and is ideally suited as a representative basal taxon for the family. "Cephalogale" first appeared during the middle Oligocene and early Miocene (approximately 20–30 million years ago) in Europe. "Cephalogale" gave rise to a lineage of early bears of the genus "Ursavus". This genus radiated in Asia and ultimately gave rise to the first true bears (genus "Ursus") in Europe, 5 million years ago. Even among its primitive species, such as "C. minor", it exhibits typical ursid synapomorphic dentition such as posteriorly oriented M2 postprotocrista molars, elongated m2 molars, and a reduction of the premolars. Living members of the ursids are morphologically well defined by their hypocarnivorous (non-strictly meat-eating) dentitions, but fossil ursids include hypercarnivorous (strictly meat-eating) taxa, although they never achieved the extreme hypercarnivory seen in mustelids. Cephalogale was a mesocarnivore (intermediate meat-eater). Other extinct bear genera include "Arctodus", "Agriarctos", "Plionarctos" and "Indarctos". It is uncertain whether ursids were in Asia during the late Eocene, although there is some suggestion that a limited immigration from Asia may have produced "Parictis" in North America due to the major sea level lowstand at ca. 37 Ma, but no "Parictis" fossils have yet to be found in East Asia. Ursids did, however, become very diversified in Asia later during the Oligocene. Four genera representing two subfamilies (Amphicynodontinae and Hemicyoninae) have been discovered in the Oligocene of Asia: "Amphicticeps", "Amphicynodon", "Pachycynodon", and "Cephalogale". "Amphicticeps" is endemic from Asia and the other three genera are common to both Asia and Europe. This indicates migration of ursids between Asia and Europe during the Oligocene and migration of several taxa from Asia to North America likely occurred later during the late Oligocene or early Miocene. Although "Amphicticeps" is morphologically closely related to "Allocyon", and also to "Kolponomos" of North America, no single genus of the Ursidae from this time period is known to be common to both Eurasia and North America. Cephalogale, however, do appear in North America in the early Miocene. It is interesting to note that rodents, such as "Haplomys" and "Pseudotheridomys" (late Oligocene) and "Plesiosminthus" and "Palaeocastor" (early Miocene), are common to both Asia and North America and this indicates that faunal exchange did occur between Asia and North America during the late Oligocene to early Miocene. Ursid migration from Asia to North America would therefore have also been very likely to occur during this time. In the late Neogene three major carnivoran migrations that definitely included ursids are recognized between Eurasia and North America. The first (probably 21–18 Ma) was waves of intermittent dispersals including "Amphicynodon", "Cephalogale" and "Ursavus". The second migration occurred about 7–8 Ma and included "Agriotherium" – this was unusual among ursoids in that it also colonised sub-Saharan Africa. The third wave took place in the early Pliocene 4 Ma, consisting of "Ursus". The giant panda's taxonomy has long been debated. Its original classification by Armand David in 1869 was within the bear genus Ursus, but in 1870 it was reclassified by Alphonse Milne-Edwards to the raccoon family. In recent studies, the majority of DNA analyses suggest that the giant panda has a much closer relationship to other bears and should be considered a member of the family Ursidae. The status of the red panda remains uncertain, but many experts, including Wilson and Reeder, classify it as a member of the bear family. Others place it with the raccoons in Procyonidae or in its own family, the Ailuridae. Multiple similarities between the two pandas, including the presence of false thumbs, are thought to represent convergent evolution for feeding primarily on bamboo. There is also evidence that, unlike their neighbors elsewhere, the brown bears of Alaska's ABC islands are more closely related to polar bears than they are to other brown bears in the world. Researchers Gerald Shields and Sandra Talbot of the University of Alaska Fairbanks Institute of Arctic Biology studied the DNA of several samples of the species and found that their DNA is different from that of other brown bears. The researchers discovered that their DNA was unique compared to brown bears anywhere else in the world. The discovery has shown that while all other brown bears share a brown bear as their closest relative, those of Alaska's ABC Islands differ and share their closest relation with the polar bear. There is also supposed to be a very rare large bear in China called the blue bear, which presumably is a type of black bear. This animal has never been photographed. Koalas are often referred to as bears due to their appearance; they are not bears, however, but marsupials. Classification. The genera "Melursus" and "Helarctos" are sometimes also included in "Ursus". The Asiatic black bear and the polar bear used to be placed in their own genera, "Selenarctos" and "Thalarctos" which are now placed at subgenus rank. A number of hybrids have been bred between American black, brown, and polar bears (see Ursid hybrids). Biology. Despite being quadrupeds, bears can stand and sit similarly to humans. Bears are generally bulky and robust animals with relatively short legs. Unlike other land carnivorans, bears stand and walk on the soles of their feet rather than on their toes. They distribute their weight toward the hindfeet which makes then look lumbering when they walk. They are still quite fast with the brown bear reaching 30 mph although they are still slower than felines and canines. Bear can stand on their hindfeet and sit up straight with remarkable balance. Bears have non-retractable claws which are used for digging, climbing, tearing and catching prey. Their ears are rounded. Dentition. Unlike most other members of the Carnivora, bears have relatively undeveloped carnassial teeth, and their teeth are adapted for a diet that includes a significant amount of vegetable matter. The canine teeth are large, and the molar teeth flat and crushing. There is considerable variation in dental formula even within a given species. It has been suggested that this indicates bears are still in the process of evolving from a carnivorous to a predominantly herbivorous diet. Polar bears appear to have secondarily re-evolved fully functional carnassials, as their diet has switched back towards carnivory. The dental formula for living bears is: Diet & Interspecific Interactions. Their carnivorous reputation non-withstanding, most bears have adopted to a diet comprised of more plant than animal matter and are completely opportunistic omnivores. One exception is the Polar Bear, who has had to adopt a diet of mainly marine mammals to survive in the Arctic. The other exception is the Giant Panda has adapted a diet comprised mainly of bamboo. The Sloth Bear, though not as specialized as the previous two species, has lost several front teeth usually seen in bears and developed a long, suctioning tongue in order to feed on the termites and other burrowing insects that they favor. All bears will feed on any food source that becomes available. When taking warm-blooded animals, bears will typically take small or young animals, because of the endurance and potential danger that comes with attacking large prey. Although (besides Polar Bears) both species of black bear and the Brown Bear can sometimes take large prey, such as ungulates. Often, bears will feed on other large animals when they encounter a carcass, whether or not the carcass is claimed by or is the kill of another predator. This competition is the main source of interspecies conflict. Bears are typically the apex predators in their range due to their size and power, and can defend a carcass against nearly all comers. Mother bears also can usually defend their cubs against other predators. The Tiger is the only known predator known to regularly prey on adult bears, including Sloth Bears, Asiatic Black Bears, Giant Pandas, Sun Bears and small Brown Bears. Reproduction. The bear's courtship period is very brief. Bears in northern climates reproduce seasonally, usually after a period of inactivity similar to hibernation, although tropical species breed all year round. Cubs are born toothless, blind, and bald. The cubs of brown bears, usually born in litters of 1–3, will typically stay with the mother for two full seasons. They feed on their mother's milk through the duration of their relationship with their mother, although as the cubs continue to grow, nursing becomes less frequent and learn to begin hunting with the mother. They will remain with the mother for approximately three years, until she enters the next cycle of estrus and drives the cubs off. Bears will reach sexual maturity in five to seven years. Male bears, especially Polar and Brown Bears, will kill and sometimes devour cubs born to another father in order to induce a female to breed again. Female bears are often successful in driving off males in protection of their cubs, despite being rather smaller. Winter dormancy. Many bears of northern regions are assumed to hibernate in the winter. While many bear species do go into a physiological state called hibernation or winter sleep, it is not true hibernation. In true hibernators, body temperatures drop to near ambient and heart rate slows drastically, but the animals periodically rouse themselves to urinate or defecate and to eat from stored food. The body temperature of bears, on the other hand, drops only a few degrees from normal and heart rate slows only slightly. They normally do not wake during this "hibernation", and therefore do not eat, drink, urinate or defecate the entire period. Higher body heat and being easily roused may be adaptations, because females give birth to their cubs during this winter sleep. It can therefore be considered a more efficient form of hibernation because they need not awake through the entire period, but they are more quickly and easily awakened at the end of their hibernation. They have to stay in a den for the whole hibernation. Relationship with humans. Some species, such as the polar bear, American black bear, Sloth Bear and the brown bear, are dangerous to humans, especially in areas where they have become used to people. On the west coast of Canada, the American black bear has become an integral part of the silviculture industries, specifically treeplanting. The bears are coaxed into areas of harvested forest to "flush out" the other wildlife, i.e. moose, which are a far greater threat to planters. All bears are physically powerful and are likely capable of fatally attacking a person, but they, for the most part, are shy, easily frightened and will avoid humans. The danger that bears pose is often vastly exaggerated, in part by the human imagination. However, when a mother feels her cubs are threatened, she will behave ferociously. It is recommended to give all bears a wide berth because they are behaviorally unpredictable. Laws have been passed in many areas of the world to protect bears from hunters habitat destruction. Some populated areas with bear populations have also outlawed the feeding of bears, including allowing them access to garbage or other food waste. Bears in captivity have been trained to dance, box, or ride bicycles; however, this use of the animals became controversial in the late 20th century. Bears were kept for baiting in Europe at least since the 16th century. Bears as food and medicine. Many people enjoy hunting bears and eating them. Their meat is dark and stringy, like a tough cut of beef. In Cantonese cuisine, bear paws are considered a delicacy. The peoples of China, Japan, and Korea use bears' body parts and secretions (notably their gallbladders and bile) as part of traditional Chinese medicine. It is believed more than 12,000 bile bears are kept on farms, farmed for their bile, in China, Vietnam and South Korea. Bear meat must be cooked thoroughly as it can often be infected with trichinellosis. Myth and legend. Some evidence has been brought to light on prehistoric bear worship, see Arctic, Arcturus, Great Bear, Berserker, Kalevala. Anthropologists such as Joseph Campbell have regarded this as a common feature in most of the fishing and hunting-tribes. The prehistoric Finns, along with most Finno-Ugric peoples, considered the bear as the spirit of one's forefathers. This is why the bear was a greatly respected animal, with several euphemistic names. The bear is the national animal of Finland. This kind of attitude is reflected in the traditional Russian fairy tale "Morozko", whose arrogant protagonist Ivan tries to kill a mother bear and her cubs — and is punished and humbled by having his own head turned magically into a bear's head and being subsequently shunned by human society. "The Brown Bear of Norway" is a Scottish fairy tale telling the adventures of a girl who married a prince magically turned into a bear, and who managed to get him back into a human form by the force of her love and after many trials and difficulties. There has been evidence about early bear worship in China and among the Ainu culture as well (see Iomante). Korean people in their mythology identify the bear as their ancestor and symbolic animal. According to the Korean legend, a god imposed a difficult test on a she-bear, and when she passed it the god turned her into a woman and married her. In addition, the Proto-Indo-European word for bear, "*h₂ŕ̥tḱos" (ancestral to the Greek "arktos", Latin "ursus", Welsh "arth" (cf. Arthur), Albanian ari, Armenian arj, Sanskrit "ṛkṣa", Hittite "ḫartagga") seems to have been subject to taboo deformation or replacement (as was the word for wolf, "wlkwos"), resulting in the use of numerous unrelated words with meanings like "brown one" (English "bruin") and "honey-eater" (Slavic "medved"). Thus four Indo-European language groups do not share the same PIE root. The theory of the bear taboo is taught to almost all beginning students of Indo-European and historical linguistics; the putative original PIE word for bear is itself descriptive, because a cognate word in Sanskrit is "rakṣas", meaning "harm, injury". Legends of saints taming bears are common in the Alpine zone. In the arms of the bishopric of Freising ("see illustration") the bear is the dangerous totem animal tamed by St. Corbinian and made to carry his civilised baggage over the mountains. A bear also features prominently in the legend of St. Romedius, who is also said to have tamed one of these animals and had the same bear carry him from his hermitage in the mountains to the city of Trento. Similar stories are told of Saint Gall and Saint Columbanus. This recurrent motif was used by the Church as a symbol of the victory of Christianity over Paganism, represented by the fiery. Imaginary bears are a popular feature of many children's stories including Goldilocks and the Three Bears, the Berenstein Bears, and Winnie the Pooh. The constellations Ursa Major and Ursa Minor represent bears. Symbolic use. The Russian bear is a common National personification for Russia (as well as the Soviet Union) and even Germany. The brown bear is Finland's national animal. In the United States, the black bear is the state animal of Louisiana, New Mexico, and West Virginia; the grizzly bear is the state animal of both Montana and California. Bears appear in the canting arms of Berne and Berlin. Also, "bear", "bruin", or specific types of bears are popular nicknames or mascots, e.g. for sports teams (Chicago Bears, Boston Bruins); and a bear cub called Misha was mascot of the 1980 Summer Olympics in Moscow, USSR. Smokey Bear has become a part of American culture since his introduction in 1944. Known to almost all Americans, he and his message, "Only You Can Prevent Forest Fires" (updated in 2001 to "Only You Can Prevent Wildfires") has been a symbol of preserving woodlands. Smokey wears a hat similar to one worn by many U.S. state police officers, giving rise to the CB slang "bear" or "Smokey" for the highway patrol. Figures of speech. The physical attributes and behaviours of bears are commonly used in figures of speech in English. Teddy bears. Around the world, many children have stuffed animals in the form of bears. Names. In Scandinavia the word for bear is "Björn" (or "Bjørn"), and is a relatively common given name for males. The use of this name is ancient and has been found mentioned in several runestone inscriptions. The name was also used by J.R.R. Tolkien in his book "The Hobbit", where a bear-like character is named Beorn. The female first name "Ursula", originally derived from a Christian saint's name and common in English- and German-speaking countries, means "Little she-bear" (dimunitive of Latin "ursa"). In Switzerland the male first name "Urs" is especially popular. In Russian and other Slavic languages, the word for bear, "Medved" (медведь), and variants or derivatives such as Medvedev are common surnames. The Irish family name "McMahon" means "Son of Bear" in Irish. One of widely held etymological explanations for the common name "Arthur" is that it originally meant "bear-like". In East European Jewish communities, the name "Ber" (בער) — Yiddish cognate of "Bear" — has been attested as a common male first name, at least since the 18th century, and was among others the name of several prominent Rabbis. The Yiddish "Ber" is still in use among Orthodox Jewish communities in Israel, the US and other countries. With the transition from Yiddish to Hebrew under the influence of Zionism, the Hebrew word for "bear", "Dov" (דב), was taken up in contemporary Israel and is at present among the commonly used male first names in that country. "Ten Bears" (Paruasemana) was the name of a well-known 19th Century chieftain among the Comanche. Also among other Native American tribes, bear-related names are attested.	Eyes'" are organs that detect light, and send signals along the optic nerve to the visual and other areas of the brain. Complex optical systems 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 cnidaria, mollusks, chordates, annelids and arthropods. The simplest "eyes", in even unicellular organisms, do nothing but detect whether the surroundings are light or dark, which is sufficient for the entrainment of circadian rhythms. From more complex eyes, retinal photosensitive ganglion cells send signals along the retinohypothalamic tract to the suprachiasmatic nuclei to effect circadian adjustment. Overview. Complex eyes can distinguish shapes and colors. The visual fields of many organisms, especially predators, involve large areas of binocular vision to improve depth perception; in other organisms, eyes are located so as to maximise the field of view, such as in rabbits and horses. The first proto-eyes evolved among animals 540 million years ago, about the time of the so-called 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 6 of the thirty-something main phyla. In most vertebrates and some mollusks, the eye works by allowing light to enter it and project onto a light-sensitive panel of cells, known as the retina, at the rear of the eye. The cone cells (for color) and the rod cells (for low-light contrasts) in the retina detect and convert light into neural signals for vision. The visual signals are then transmitted to the brain via the optic nerve. Such eyes are typically roughly spherical, filled with a transparent gel-like substance called the vitreous humour, with a focusing lens and often an iris; the relaxing or tightening of the muscles around the iris change the size of the pupil, thereby regulating the amount of light that enters the eye, and reducing aberrations when there is enough light. The eyes of cephalopods, fish, amphibians and snakes usually have fixed lens shapes, and focusing vision is achieved by telescoping the lens — similar to how a camera focuses. Compound eyes are found among the arthropods and are composed of many simple facets which, depending on the details of anatomy, 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, which are arranged hexagonally, and which can give a full 360-degree 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, creating vision. With each eye viewing a different thing, a fused image from all the eyes is produced in the brain, providing very different, high-resolution images. Possessing detailed hyperspectral color vision, the Mantis shrimp has been reported to have the world's most complex color vision system. Trilobites, which are now extinct, had unique compound eyes. They used clear calcite crystals to form the lenses of their eyes. In this, they differ from most other arthropods, which have soft eyes. The number of lenses in such an eye varied, however: some trilobites had only one, and some had thousands of lenses in one eye. In contrast to compound eyes, simple eyes are those that have a single lens. For example, jumping spiders have a large pair of simple eyes with a narrow field of view, supported by an array of other, smaller eyes for peripheral vision. Some insect larvae, like caterpillars, have a different type of simple eye (stemmata) which gives a rough image. Some of the simplest eyes, called ocelli, can be found in animals like some of the snails, which cannot actually "see" in the normal sense. They do have photosensitive cells, but no lens and no other means of projecting an image onto these cells. They can distinguish between light and dark, but no more. This enables snails to keep out of direct sunlight. In organisms dwelling near deep-sea vents, compound eyes have been secondarily simplified and adapted to spot the infra-red light produced by the hot vents in this way the bearers can spot hot springs and avoid being boiled alive. Evolution. Visual pigments appear to have a common ancestor and were probably involved in circadian rhythms or reproductive timing in simple organisms. Complex vision, associated with dedicated visual organs, or eyes, evolved many times in different lineages. Types of eye. Nature has produced ten different eye layouts — indeed every way of capturing an image has evolved at least once in nature, with the exception of zoom and Fresnel lenses. Eye types can be categorized into "simple eyes", with one concave chamber, and "compound eyes", which comprise a number of individual lenses laid out on a convex surface. Note that "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 rhabdomic. These two groups are not monophyletic; the cnidaira also possess cilliated cells, 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 eyespot, 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 eye". 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. Pinhole eye. The pinhole eye is an "advanced" form of pit eye incorporating these improvements, most notably a small aperture (which may be adjustable) and deep pit. It is only found in the nautiloids. 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°. Shrinking the aperture would produce a sharper image, but let in less light. Spherical lensed 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, still 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 an inhomogeneous lens is necessary in order for the focal length to drop from about 4 times the lens radius, to 2.5 radii. 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. No 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 minimize the effect of eye motion while the animal moves, most such eyes have stabilizing eye muscles. The ocelli of insects bear a simple lens, but their focal point always lies behind the retina; consequently they can never form a sharp image. This capitulates the function of the eye. 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. Focussing the image would also cause the sun's image to be focussed 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). Weaknesses. One weakness of this eye construction is that chromatic aberration is still quite high — although for organisms without color vision, this is a very minor concern. A weakness of the vertebrate eye is the blind spot which results from a gap in the retina where the optic nerve exits at the back of the eye; the cephalopod eye has no blind spot as the retina is in the opposite orientation. Multiple lenses. Some marine organisms bear more than one lens; for instance the copeopod "Pontella" has three. The outer has a parabolic surface, countering the effects of spherical aberration while allowing a sharp image to be formed. "Copilla'"s eyes have two lenses, which move in and out like a telescope. Such arrangements are rare and poorly understood, but represent an interesting alternative construction. An interesting use of multiple lenses is seen in some hunters such as eagles and jumping spiders, which have a refractive cornea (discussed next): 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 terrestrial vertebrates (along with spiders and some insect larvae) the vitreous fluid has a higher refractive index than the air, relieving the lens of the function of reducing the focal length. This has freed it up for fine adjustments of focus, allowing a very high resolution to be obtained. As with spherical lenses, the problem of spherical aberration caused by the lens can be countered either by using an inhomogeneous lens material, or by flattening the lens. Flattening the lens has a disadvantage: the quality of vision is diminished away from the main line of focus, meaning that animals requiring all-round vision are detrimented. Such animals often display an inhomogeneous lens instead. As mentioned above, a refractive cornea is only useful out of water; in water, there is no difference in refractive index between the vitreous fluid and the surrounding water. Hence creatures which have returned to the water — penguins and seals, for example — lose their refractive cornea and return to lens-based vision. An alternative solution, borne by some divers, is to have a very strong 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, copeopods and platyhelminths 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 millimeter-scale reflector eyes fringing the edge of its shell. It detects moving objects as they pass successive lenses. Compound eyes. A compound eye may consist of thousands of individual photoreception units. 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 of light. Because the individual lenses are so small, the effects of diffraction impose a limit on the possible resolution that can be obtained. This can only be countered by increasing lens size and number — to see with a resolution comparable to our simple eyes, humans would require compound eyes which would each reach the size of their head. Compound eyes fall into two groups: apposition eyes, which form multiple inverted images, and superposition eyes, which form a single erect image. Compound eyes are common in arthropods, and are also present in annelids and some bivalved molluscs. Compound eyes, in arthropods at least, grow at their margins by the addition of new ommatidia. Apposition eyes. Apposition eyes are the most common form of eye, and are presumably the ancestral form of compound eye. 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. (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. In the other kind of apposition eye, found in the Strepsiptera, lenses are not fused to one another, and each forms an entire image; these images are combined in the brain. This is called the schizochroal compound eye or the neural superposition eye. Because images are combined additively, this arrangement allows vision under lower light levels. Superposition eyes. The second type is named the superposition eye. The superposition eye is divided into three types; the refracting, the reflecting and the parabolic superposition eye. 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. In the parabolic superposition compound eye type, seen in arthropods such as mayflies, 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. 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 acute vision. In the acute zone the eye are flattened and the facets larger. The flattening allows more ommatidia to receive light from a spot and therefore higher resolution. 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 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 numerous 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. Relationship to lifestyle. Eyes are generally adapted to the environment and lifestyle 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. Optical superposition eyes are constrained to a spherical shape, but other forms of compound eyes may deform to a shape where more ommatidia are aligned to, say, the horizon, without altering the size or density of individual ommatidia. Eyes of horizon-scanning organisms have stalks so they can be easily aligned to the horizon when this is inclined, for example if the animal is on a slope. 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. In deep water organisms, it may not be the centre of the eye that is enlarged. 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. Depth perception can be enhanced by having eyes which are enlarged in one direction; distorting the eye slightly allows the distance to the object to be estimated with a high degree of accuracy. 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. On the other hand, the eyes of organisms which operate in low light levels, such as around dawn and dusk or in deep water, tend to be larger to increase the amount of light that can be captured. 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. Acuity. Visual acuity is often measured in cycles per degree (CPD), which measures an angular resolution, or how much an eye can differentiate one object from another in terms of visual angles. Resolution in CPD can be measured by bar charts of different numbers of white — black stripe cycles. For example, if each pattern is 1.75 cm wide and is placed at 1 m distance from the eye, it will subtend an angle of 1 degree, so the number of white — black bar pairs on the pattern will be a measure of the cycles per degree of that pattern. The highest such number that the eye can resolve as stripes, or distinguish from a gray block, is then the measurement of visual acuity of the eye. For a human eye with excellent acuity, the maximum theoretical resolution would be 50 CPD (1.2 arcminute per line pair, or a 0.35 mm line pair, at 1 m). A rat can resolve only about 1 to 2 CPD. A horse has higher acuity through most of the visual field of its eyes than a human has, but does not match the high acuity of the human eye's central fovea region. Spherical aberration limits the resolution of a 7 mm pupil to about 3 arcminutes per line pair. At a pupil diameter of 3 mm, the spherical aberration is greatly reduced, resulting in an improved resolution of approximately 1.7 arcminutes per line pair. A resolution of 2 arcminutes per line pair, equivalent to a 1 arcminute gap in an optotype, corresponds to 20 20 (normal vision) in humans. Color. All organisms are restricted to a small range of the electromagnetic spectrum; this varies from creature to creature, but is mainly between 400 and 700 nm. This is a rather small section of the electromagnetic spectrum, probably reflecting the submarine evolution of the organ: water blocks out all but two small windows of the EM spectrum, and there has been no evolutionary pressure among land animals to broaden this range. The most sensitive pigment, rhodopsin, has a peak response at 500 nm. Small changes to the genes coding for this protein can tweak the peak response by a few nm; pigments in the lens can also "filter" incoming light, changing the peak response. Many organisms are unable to discriminate between colors, seeing instead in shades of "grey"; color vision necessitates a range of pigment cells which are primarily sensitive to smaller ranges of the spectrum. In primates, geckos, and other organisms, these take the form of cone cells, from which the more sensitive rod cells evolved. Even if organisms are physically capable of discriminating different colors, this does not necessarily mean that they can perceive the different colors; only with behavioral tests can this be deduced. Most organisms with color 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. Rods and cones. The retina contains two major types of light-sensitive photoreceptor cells used for vision: the rods and the cones. Rods cannot distinguish colors, but are responsible for low-light black-and-white (scotopic) vision; they work well in dim light as they contain a pigment, 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 color vision. They require brighter light to function than rods require. 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 colors). The color 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 this spot, as when one looks at an object directly. Cone cells and rods are connected through intermediate cells in the retina to nerve fibers of the optic nerve. When rods and cones are stimulated by light, the nerves send off impulses through these fibers to the brain. Pigment. 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. The eyes of vertebrates usually contain cilliary cells with c-opsins, and (bilaterian) invertebrates have rhabdomeric cells in the eye with r-opsins. 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. Likewise, c-opsins have been found to be expressed in the "brain" of some invertebrates. They may have been expressed in ciliary cells of larval eyes, which were subsequently resorbed into the brain on metamorphosis to the adult form. C-opsins are also found in some derived bilaterian-invertebrate eyes, such as the pallial eyes of the bivalve molluscs; however, the lateral eyes (which were presumably the ancestral type for this group, if eyes evolved once there) always use r-opsins. Cnidaria, which are an outgroup to the taxa mentioned above, express c-opsins but r-opsins are yet to be found in this group. Incidentally, the melanin produced in the cnidaria is produced in the same fashion as that in vertebrates, suggesting the common descent of this pigment.