Physics Project Essay Example for Free

Physics Project Essay nerve atomic number 18 organs that principal(prenominal)tain go down, and switch over it to electro-chemical impulses in neurons. The elementaryst photoreceptors in conscious mental imagery connect escaped to movement. In towering beings the nub is a complex optical system which collects light from the surrounding environment regulates its in xsity through a diaphragm focuses it through an adjustable assembly of lens systemes to frame an image converts this image into a set of electric signals and transmits these signals to the brain, through complex neural pathways that connect the heart, via the optic nerve, to the visual cortex and different argonas of the brain. Eyes with resolving power bring on come in ten fundamentally variant forms, and 96% of animal species take in a complex optical system.1 Image-resolving mid functions atomic number 18 present in molluscs, chordates and arthropods.2 The simplest look, such(prenominal) as tho se in microorganisms, do nonhing scarcely retrieve whether the surroundings are light or dark, which is sufficient for the entrainment of circadian rhythms. From to a greater extent than complex look, retinal photosensitive ganglion cells delight signals along the retinohypothalamic tr enactment to the suprachiasmatic nuclei to effect circadian adjustment. Contentshide * 1 Overview * 2 Evolution * 3 Types of center of attention * 3.1 Normal eyes * 3.2 Pit eyes * 3.2.1 orbiculate lensed eye * 3.2.2 Multiple lenses * 3.2.3 Refractive cornea * 3.2.4 Reflector eyes * 3.3 Compound eyes * 3.3.1 Apposition eyes * 3.3.2 Superposition eyes * 3.3.3 Parabolic superposition * 3.3.4 Other * 3.3.5 Nutrients of the eye * 4 Relationship to life takements * 5 Visual sharpness * 6 Perception of contorts * 7 Rods and c stars * 8 Pigmentation * 9 See uniformly * 10 References * 10.1 Notes * 10.2 Bibliography * 11 External links edit OverviewEye of the wisent, the European bison Complex e yes understructure distinguish shapes and colors. The visual playing 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 maximize the field of view, such as in rabbits and horses, which engender monocular vision. The first proto-eyes evolved among animals 600 jillion years ago, ab turn up the time of the Cambrian explosion.3 The last common ancestor of animals possessed the biochemical toolkit necessary for vision, and more advanced eyes defend evolved in 96% of animal species in six of the thirty-plus4 main phyla.1 In more or less(prenominal) vertebrates and some molluscs, the eye works by allowing light to enter and project onto a photosensitive 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 ar e 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 focalisation lens and often an iris the relaxing or tightening of the muscles around the iris change the coat of the pupil, thereby regulating the amount of light that enters the eye,5 and reducing aberrations when there is enough light.6 The eyes of nigh cephalopods, fish, amphibians and snakes hand fixed lens shapes, and focusing vision is achieved by telescoping the lenssimilar to how a camera focuses.7 Compound eyes are found among the arthropods and are composed of many simple facets which, depending on the details of anatomy, whitethorn give either a single pixelated image or multiple images, per eye. Each sensor has its own lens and photosensitive cell(s). a few(prenominal) eyes project up to 28,000 such sensors, which are arranged hexagonally, and which can give a full 360-degree field of visio n.Compound eyes are very sensitive to exploit. Some arthropods, including many Strepsiptera, have intricate eyes of only if 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 worlds more or less complex color vision system.8 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 morsel 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 quality 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 can non 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 blur the infra-red light produced by the hot ventsin this way the bearers can spot hot springs and avoid being boiled alive.9 editEvolutionMain article Evolution of the eyeEvolution of the eyePhotoreception is phylogenetically very old, with various theories of phylogenesis.10 The common origin (monophyly) of all animal eyes is now tolerantly acce pted as fact. This is establish upon the shared anatomical and genetic features of all eyes that is, all modern eyes, varied as they are, have their origins in a proto-eye believed to have evolved some 540 million years ago.111213 The majority of the advancements in early eyes are believed to have taken only a few million years to develop, since the first predator to gain true visualize would have touched off an arms race.14 Prey animals and competing predators alike would be at a distinct disadvantage without such capabilities and would be less likely to survive and reproduce. Hence multiple eye typesetters cases and subtypes developed in parallel. Eyes in various animals show adaption to their requirements. For example, birds of fertilize have often greater visual acuity than humans, and some can see ultraviolet light. The different forms of eye in, for example, vertebrates and mollusks are often cited as examples of parallel evolution, patronage their distant common ancestry. The very earliest eyes, called eyespots, were simple patches of photoreceptor protein in unicellular animals.In multicellular beings, multicellular eyespots evolved, physically similar to the receptor patches for taste and smell. These eyespots could only sense ambient brightness they could distinguish light and dark, but not the direction of the lightsource.15 Through gradual change, as the eyespot depressed into a shallow cup shape, the ability to slightly discriminate directional brightness was achieved by using the angle at which the light hit certain cells to identify the source. The equal deepened over time, the opening diminished in size, and the number of photoreceptor cells increased, forming an effective pinhole camera that was capable of dimly distinguishing shapes.16 The thin overgrowth of transparent cells over the eyes aperture, before formed to prevent damage to the eyespot, allowed the segregated contents of the eye chamber to specialize into a transparent humou r that optimized color filtering, blocked harmful radiation, change the eyes refractive index, and allowed functionality out slope of water.The transparent protective cells eventually split into two stages, with circulatory fluid in between that allowed wider viewing angles and greater imaging resolution, and the thickness of the transparent layer gradually increased, in most species with the transparent crystallin protein.17 The gap between tissue layers naturally formed a bioconvex shape, an optimally ensample structure for a normal refractive index. Independently, a transparent layer and a nontransparent layer split forward from the lens the cornea and iris. Separation of the forward layer again formed a humour, the aqueous humour. This increased refractive power and again eased circulatory problems. Formation of a nontransparent ring allowed more tune vessels, more circulation, and larger eye sizes.17 editTypes of eyeThere are ten different eye layoutsindeed every way of cap turing an image known to man, with the exceptions of whizz and Fresnel lenses. Eye types can be categorized into simple eyes, with one concave photoreceptive surface, and compound eyes, which comprise a number of individual lenses laid out on a convex surface.1 Note that simple does not imply a reduced level of complexity or acuity. Indeed, any eye type can be adapted for almost any behavior or environment. The only limitations specific to eye types are that of resolutionthe 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.1 Eyes as well as hang into two groups on the basis of their photoreceptors cellular construction, with the photoreceptor cells either being cilliated (as in the vertebrates) or rhabdomeric. These two groups are not monophyletic the cnidaria alike possess cilliated cells, 18 and some annelids possess bo th.19 editNormal eyesHuman eyes are examples of normal eyesSimple eyes are rather ubiquitous, and lens-bearing eyes have evolved at least seven times in vertebrates, cephalopods, annelids, crustacea and cubozoa.20 editPit eyesPit 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.1 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 nose candy cells covering about 100 m.1 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.1 editSpherical lensed eyeThe 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 un iversal gas constant encounteredhence increasing the resolution obtainable.1 The most basic form, withal seen in some gastropods and annelids, consists of a lens of one refractive index. A far hastyer 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.1 This also allows a larger aperture for a given over sharpness of image, allowing more light to enter the lens and a flatter lens, reducing spherical aberration.1 Such an inhomogeneous lens is necessary in roam for the focal length to drop from about 4 times the lens radius, to 2.5 radii.1Heterogeneous eyes have evolved at least eight times quaternary or more times in gastropods, once in the copepods, once in the annelids and once in the cephalopods.1 No aquatic organisms possess homogeneous lenses presumptively 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 term the animal moves, most such eyes have alter eye muscles.1 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.21 Focusing the image would also cause the suns 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.21 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).21 editMultiple lensesSome marine organisms bear more than one lens for instance the copepod Pontella has three. The outer has a parabolic surface, countering the effectuate of spherical aberration while allowing a sharp image to be formed. Another copepod, Copilias eyes have two lenses, arranged like those in a telescope.1 Such arrangements are rare and poorly understood, but represent an interest election 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 forbid lens, enlarging the observed image by up to 50% over the receptor cells, thus increasing their optical resolution.1 editRefractive corneaIn the eyes of most mammals, birds, reptiles, and most other terrestrial ve rtebrates (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.1 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.1 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.1 As mentioned above, a refractive cornea is only usable 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 waterpenguins and seals, for examplelose their refractive cornea and return to lens-base d vision. An alternative solution, borne by some divers, is to have a very strong cornea.1 editReflector eyesAn alternative to a lens is to line the in nerve of the eye with mirrors, and reflect the image to focus at a central point.1 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.1 Many small organisms such as rotifers, copeopods and platyhelminths use such organs, but these 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 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 crystals.22 editCompound eyesAn image of a house fly compound eye surface by using Scanning Electron MicroscopeAnatomy of the compound eye of an insectArthropods such as this carpenter bee have compound eyesA compound eye may consist of thousands of individual photoreceptor units or ommatidia (ommatidium, singular). The image perceived is a cabal 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.23 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.24 Compound eyes are common in arthropods, and are also present in annelids and some bivalvedd molluscs.25 Compound eyes, in arthropods at least, grow at their margins by the addition of new ommatidia.26 expression of the ommatidia of apposition compound eyesedit Apposition eyesApposition 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.1 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 head start point.1 (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.1 editSuperposition eyesThe 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 b etween 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 have a transparent gap but use corner mirrors instead of lenses. editParabolic superpositionThis eye type functions by refracting light, then using a parabolic mirror to focus the image it combines features of superposition and apposition eyes.9 editOtherThe compound eyes of a dragonfly close 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 piercing 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. 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, a s 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 discase into a compound eye. The same is true of many chitons. editNutrients of the eyeThe cilial body is triangular in prospecttal 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 blushered, continuous with the retinal key epithelium, and constitutes the cells of the dilator muscle. The vitreous is the transparent, colorless, gelatinous mass that fills the space between the lens of the eye and the retina lining the back of the eye.27 It is produced by certain retinal cells.It is of rather similar composition to the cornea, but cops 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 employ 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 fibers 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. editRelationship to life requirementsEyes are generally adapted to the environment and life requirements of the organism which bears them. For instance, the distribution of photoreceptors tends to match the area in which the highest acuity is required, with horizon-scanning organisms, such as those that live on the African plains, having a horizontal line of high-density ganglia, while tree-dwelling creatures which require good all-round vision tend to have a symmetrical distribution of gangl ia, with acuity decreasing outwards from the centre. Of course, for most eye types, it is impossible to variegate 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.123-4 opthalmic 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.28Eyes 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.29 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.28 In deep water organisms, it may not be the centre of the eye that is enlarged. The hyperiid amphipod s are deep water animals that cater 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 preyor predatorsagainst 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.28Depth perception can be enhanced by having eyes which are enlarged in one direction distorting the eye slightly allows the aloofness to the object to be estimated with a high degree of accuracy.9 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.28 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.28 It is not o nly 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.28 Eyes may be mounted on stalks to provide better all-round vision, by lifting them above an organisms carapace this also allows them to track predators or prey without moving the head.9 editVisual acuityA hawks eyeVisual acuity, or resolving power, is the ability to distinguish fine detail and is the property of cones.30 It is often footstepd 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 surmount 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 is 50 CPD31 (1.2 second per line pair, or a 0.35 mm line pair, at 1 m). A rat can resolve only about 1 to 2 CPD.32 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 eyes 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.33 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. editPerception of coloursColour vision is the fa culty of the organism to distinguish lights of different spectral qualities.34 all told organisms are restricted to a small range of electromagnetic spectrum this varies from creature to creature, but is mainly between 400 and 700 nm.35 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.36 The most sensitive pigment, rhodopsin, has a circular response at 500 nm.37 Small changes to the genes coding for this protein can tweak the peak response by a few nm2 pigments in the lens can also filter incoming light, changing the peak response.2 Many organisms are unable to discriminate between colours, 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 othe r organisms, these take the form of cone cells, from which the more sensitive rod cells evolved.37 Even if organisms are physically capable of sharp different colours, this does not necessarily mean that they can perceive the different colours only with behavioural tests can this be deduced.2 Most organisms with colour vision are able to detect ultraviolet light. This high energy light can be damaging to receptor cells. With a few exceptions (snakes, placental mammals), most organisms avoid these effects by having absorbent oil droplets around their cone cells. The alternative, developed by organisms that had lost these oil droplets in the course of evolution, is to make the lens impervious to UV light this precludes the possibility of any UV light being detected, as it does not even reach the retina.37 editRods and conesThe retina contains two major types of light-sensitive photoreceptor cells used for vision the rods and the cones. Rods cannot distinguish colours, but are respon sible for low-light (scotopic) monochrome (black-and-white) vision they work well in dim light as they contain a pigment, rhodopsin (visual purple), which is sensitive at low light intensity, but saturates at higher (photopic) intensities. Rods are distributed throughout the retina but there are no(prenominal) at the fovea and none at the blind spot. Rod density is greater in the peripheral retina than in the central retina. retinal cones are responsible for colour vision.They require brighter light to function than rods require. In humans, there are three types of cones, maximally sensitive to long-wavelength, medium-wavelength, and short-wavelength light (often referred to as red, green, and blue, respectively, though the sensitivity peaks are not actually at these colours). The colour seen is the combined effect of stimuli to, and responses from, these three types of cone cells. Cones are mostly concentrated in and near the fovea. Only a few are present at the sides of the reti na. Objects are seen most sharply in focus when their images fall on the fovea, as when one looks at an object directly. Cone cells and rods are connected through intermediate cells in the retina to nerve fibres of the optic nerve. When rods and cones are stimulated by light, the nerves send off impulses through these fibres to the brain.37 editPigmentationThe 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.37 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.2 There are two types of opsin involved in vision c-opsins, which are associated with ciliary-type photoreceptor ce lls, and r-opsins, associated with rhabdomeric photoreceptor cells.38 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.38Likewise, 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.38 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.38 Cnidaria, which are an outgroup to the taxa mentioned above, express c-opsins but r-opsins are yet to be found in this group .38 Incidentally, the melanin produced in the cnidaria is produced in the same fashion as that in vertebrates, suggesting the common descent of this pigment.38