Question

What are the 4 mechanisms of image-forming animal eyes? Diagram and/or explain each one and describe an animal with an example of each one.

Answer 1

Ray Diagram: (Using a simple outine of the eye) Cornea Lens Retina Rays shown in pink represent light from the upper point of the object WRose o.uk Rays shown in greern represent light from the lower point of the object Object Image (on the retina) Diagram: Copyright IvyRose Ltd., 2009. Above: Basic Ray Diagram of image formation within the Human Eye.

The eye is an optical image-forming system. Many parts of the eye shown and described on the page about the anatomy of the eye play important roles in the formation of an image on the retina (which is the back surface of the eye that consists of layers of cells whose function is to transmit to the brain information corresponding to the the image formed on it).
Those parts of the eye that do not take an active part in the formation of the image on the retina have other important functions, such as providing mechanical support to the structures of the eye, or supplying the tissues with fluids, nutrients, etc..
A ray-diagram can be used to show how light passes from a point on a real object (located somewhere in space outside the body) to the corresponding position on the image of the object on the retina at the back of the eye.
The following example is explained below:
Representation of an object:
First consider the object - which is represented by a simple red arrow pointing upwards (left-hand-side of diagram).
Most real objects have complicated shapes, textures, and so on. This arrow is used to represent a very simple object for which just two clearly defined points on the object are traced through the eye to the retina.
Light leaves the object - propagating in all directions:
It is assumed for simplicity that this is a scattering object, meaning that after light in the area (which may be called "ambient light") reaches the object, it leaves the surface of the object traveling in a wide range of directions.
Light leaving the object in all directions is represented by the small arrows pointing upwards, up-left, up-right (small pink arrows), and downwards, down-left and down-right (small green arrows).
Note that a very similar but slightly simpler case would be to consider a light source instead of a (solid, light scattering) object, and to say that the light source radiated light in all directions. That would result in the same diagram but would be less realistic because most of the light received by the eye is reflected or scattered from solid objects rather than coming directly from a source of illumination e.g. a lamp. Further, one should never stare directly at bright light sources such as the sun, lasers, and other powerful light sources because doing so can cause permanent eye-damage.
Some of the light leaving the object reaches the eye:
Although the object is scattering light in all directions, only a small proportion of the light scattered from it reaches the eye.

The longer strong pink and green lines with the arrows marked along them are called "rays".
They represent the direction of travel of light.
The pink rays indicate paths taken by light leaving the top point of the object (that eventually reaches the retina), while the green rays indicate paths taken by light leaving the lower point of the object (that eventually reaches the retina).
Only two rays are shown leaving each point on the object. This simplification is to keep the diagram clear.
The two rays drawn in each case are the extreme rays, that is those that only just get through the optical system called the eye. These generally represent a cone of light that propagates all the way through the system from the object to the image.
The idea of this cone of light is represented on the diagram by the area between the pink (upper) rays being shaded pale orange. This shaded area is a reminder that light leaving the top of the object along any ray that could be drawn between the two (extreme) pink rays should reach exactly the same position in the image at the back of the eye
Light changes direction when it passes from the air into the eye:
When light traveling away from the object, towards the eye, arrives at the eye, the first surface it reaches is the cornea.
The ray-diagram shows the rays changing direction when they pass through the cornea.
This change in direction is due to refraction (i.e. the re-direction of light as it passes from one medium into another, different, medium). For further detail see the page about refraction. Just to describe this ray-diagram it is sufficient to say that several structures in the eye contribute to image formation by re-directing the light passing through them in such a way as to improve the quality of the image formed on the retina. The parts of the eye responsible for most of the refraction of light passing through the eye are the cornea and the lens.
Most of the refraction (bending, or "re-directing" of the light) occurs at the interface between the air outside the eye and the cornea. The lens is important for accommodation, or "focusing", which is also described later.
Location of Focused Image:
Ray-diagrams generally consist of many rays representing light paths through a series of optical components.
These typically indicate light:
* Leaving an object (often drawn on the left-hand-side of the diagram),
* Passing through a series of optical elements (such as the cornea in this example), then eventually
* Forming an image of the object (often on the right-hand-side of the diagram).
How is the location of the image found or defined ?
When rays coming from a specific single location on the object (for example, consider the rays coming from the top of the object in this case), pass through the same position as each other in the area in which the image is formed, the point at which they intersect corresponds to the same location (in the image) as the rays left (on the object).
Complicated ray diagrams such as those used to design optical systems (e.g. telescopes) generally include more than two rays from each position on the object. The accuracy with which many rays from the same point on the object pass through the same position in the image space has important implications for focus and the quality of the image.
In this case the two (pink) rays shown coming from the top of the object meet again on the retina, at the back of the eye. The two (green) rays shown coming from the lower point of the object also meet again on the retina, at the back of the eye.
Therefore (in the ray-diagram shown above) the image is formed on the retina.
Recall that the retina is light-sensitive structure containing photosensitive cells (called rods and cones) that convert the light they receive into nerve impulses sent to the brain along the optic nerve: Images formed anywhere other than on the retina are not transmitted effectively to the brain - hence visual impairment(s).
Image-formation on the retina is essential for good eyesight / vision.
The image formed on the retina is inverted:
Notice the orientation of the image:
The object is an upright arrow, whereas the image is of an arrow pointing downwards.
That is, the human eye forms an inverted image on the retina.
This simple example using an arrow does not look very dramatic. Some textbooks include equivalent sketches of scenes including real objects such as people, buildings, or trees, being imaged upside-down onto the retina.
Take a moment to appreciate that pictures of the scene in front of you are formed upside-down at the back of your eyes !
Although the image formed on the retina is inverted (upside-down), the next stage of the visual process is processing by the brain - which also receives other sources of information about orientation. The brain ensures that we understand the information received by all our senses, including which way is "up".

The basic light-processing unit of eyes is the photoreceptor cell, a specialized cell containing two types of molecules in a membrane: the opsin, a light-sensitive protein, surrounding the chromophore, a pigment that distinguishes colors. Groups of such cells are termed "eyespots", and have evolved independently somewhere between 40 and 65 times. These eyespots permit animals to gain only a very basic sense of the direction and intensity of light, but not enough to discriminate an object from its surroundings.
Developing an optical system that can discriminate the direction of light to within a few degrees is apparently much more difficult, and only six of the thirty-some phyla[note ppossess such a system. However, these phyla account for 96% of living species.
The planarian has "cup" eyespots that can slightly distinguish light direction.
These complex optical systems started out as the multicellular eyepatch gradually depressed into a cup, which first granted the ability to discriminate brightness in directions, then in finer and finer directions as the pit deepened. While flat eyepatches were ineffective at determining the direction of light, as a beam of light would activate exactly the same patch of photo-sensitive cells regardless of its direction, the "cup" shape of the pit eyes allowed limited directional differentiation by changing which cells the lights would hit depending upon the light's angle. Pit eyes, which had arisen by the Cambrian period, were seen in ancient snails,[clarification needed] and are found in some snails and other invertebrates living today, such as planaria. Planaria can slightly differentiate the direction and intensity of light because of their cup-shaped, heavily pigmented retina cells, which shield the light-sensitive cells from exposure in all directions except for the single opening for the light. However, this proto-eye is still much more useful for detecting the absence or presence of light than its direction; this gradually changes as the eye's pit deepens and the number of photoreceptive cells grows, allowing for increasingly precise visual information.
When a photon is absorbed by the chromophore, a chemical reaction causes the photon's energy to be transduced into electrical energy and relayed, in higher animals, to the nervous system. These photoreceptor cells form part of the retina, a thin layer of cells that relays visual information,] including the light and day-length information needed by the circadian rhythm system, to the brain. However, some jellyfish, such as Cladonema, have elaborate eyes but no brain. Their eyes transmit a message directly to the muscles without the intermediate processing provided by a brain.
During the Cambrian explosion, the development of the eye accelerated rapidly, with radical improvements in image-processing and detection of light direction.
The primitive nautilus eye functions similarly to a pinhole camera.
After the photosensitive cell region invaginated, there came a point when reducing the width of the light opening became more efficient at increasing visual resolution than continued deepening of the cup. By reducing the size of the opening, organisms achieved true imaging, allowing for fine directional sensing and even some shape-sensing. Eyes of this nature are currently found in the nautilus. Lacking a cornea or lens, they provide poor resolution and dim imaging, but are still, for the purpose of vision, a major improvement over the early eyepatches.
Overgrowths of transparent cells prevented contamination and parasitic infestation. The chamber contents, now segregated, could slowly specialize into a transparent humour, for optimizations such as colour filtering, higher refractive index, blocking of ultraviolet radiation, or the ability to operate in and out of water. The layer may, in certain classes, be related to the moulting of the organism's shell or skin. An example of this can be observed in Onychophorans where the cuticula of the shell continues to the cornea. The cornea is composed of either one or two cuticular layers depending on how recently the animal has moulted. Along with the lens and two humors, the cornea is responsible for converging light and aiding the focusing of it on the back of the retina. The cornea protects the eyeball while at the same time accounting for approximately 2/3 of the eye’s total refractive power.
It is likely that a key reason eyes specialize in detecting a specific, narrow range of wavelengths on the electromagnetic spectrum—the visible spectrum—is because the earliest species to develop photosensitivity were aquatic, and only two specific wavelength ranges of electromagnetic radiation, blue and green visible light, can travel through water. This same light-filtering property of water also influenced the photosensitivity of plants.
Lens formation and diversification
Light from a distant object and a near object being focused by changing the curvature of the lens
In a lensless eye, the light emanating from a distant point hits the back of the eye with about the same size as the eye's aperture. With the addition of a lens this incoming light is concentrated on a smaller surface area, without reducing the overall intensity of the stimulus. The focal length of an early lobopod with lens-containing simple eyes focused the image behind the retina, so while no part of the image could be brought into focus, the intensity of light allowed the organism to see in deeper (and therefore darker) waters. A subsequent increase of the lens's refractive index probably resulted in an in-focus image being formed.
The development of the lens in camera-type eyes probably followed a different trajectory. The transparent cells over a pinhole eye's aperture split into two layers, with liquid in between.[citation needed] The liquid originally served as a circulatory fluid for oxygen, nutrients, wastes, and immune functions, allowing greater total thickness and higher mechanical protection. In addition, multiple interfaces between solids and liquids increase optical power, allowing wider viewing angles and greater imaging resolution. Again, the division of layers may have originated with the shedding of skin; intracellular fluid may infill naturally depending on layer depth.[citation needed]
Note that this optical layout has not been found, nor is it expected to be found. Fossilization rarely preserves soft tissues, and even if it did, the new humour would almost certainly close as the remains desiccated, or as sediment overburden forced the layers together, making the fossilized eye resemble the previous layout.

Compound eye of Antarctic krill
Vertebrate lenses are composed of adapted epithelial cells which have high concentrations of the protein crystallin. These crystallins belong to two major families, the α-crystallins and the βγ-crystallins. Both were categories of proteins originally used for other functions in organisms, but eventually were adapted for the sole purpose of vision in animal eyes. In the embryo, the lens is living tissue, but the cellular machinery is not transparent so must be removed before the organism can see. Removing the machinery means the lens is composed of dead cells, packed with crystallins. These crystallins are special because they have the unique characteristics required for transparency and function in the lens such as tight packing, resistance to crystallization, and extreme longevity, as they must survive for the entirety of the organism’s life. The refractive index gradient which makes the lens useful is caused by the radial shift in crystallin concentration in different parts of the lens, rather than by the specific type of protein: it is not the presence of crystallin, but the relative distribution of it, that renders the lens useful.
It is biologically difficult to maintain a transparent layer of cells. Deposition of transparent, nonliving, material eased the need for nutrient supply and waste removal. Trilobites used calcite, a mineral which today is known to be used for vision only in a single species of brittle star. In other compound eyes[verification needed] and camera eyes, the material is crystallin. A gap between tissue layers naturally forms a biconvex shape, which is optically and mechanically ideal for substances of normal[clarification needed] refractive index. A biconvex lens confers not only optical resolution, but aperture and low-light ability, as resolution is now decoupled from hole size – which slowly increases again, free from the circulatory constraints.
Independently, a transparent layer and a nontransparent layer may split forward from the lens: a separate cornea and iris. (These may happen before or after crystal deposition, or not at all.) Separation of the forward layer again forms a humour, the aqueous humour. This increases refractive power and again eases circulatory problems. Formation of a nontransparent ring allows more blood vessels, more circulation, and larger eye sizes. This flap around the perimeter of the lens also masks optical imperfections, which are more common at lens edges. The need to mask lens imperfections gradually increases with lens curvature and power, overall lens and eye size, and the resolution and aperture needs of the organism, driven by hunting or survival requirements. This type is now functionally identical to the eye of most vertebrates, including humans. Indeed, "the basic pattern of all vertebrate eyes is similar."
Other developments  
Color vision
Learn moreThis section needs additional citations for verification.
Five classes of visual opsins are found in vertebrates. All but one of these developed prior to the divergence of Cyclostomata and fish. the five opsin classes are variously adapted depending on the light spectrum encountered. As light travels through water, longer wavelengths, such as reds and yellows, are absorbed more quickly than the shorter wavelengths of the greens and blues. This creates a gradient of light as the depth of water increases. the visual opsins in fish are more sensitive to the range of light in their habitat and depth. However, land environments do not vary in wavelength composition, so that the opsin sensitivities among land vertebrates does not vary much. This directly contributes to the significant presence of communication colors. Color vision gives distinct selective advantages, such as better recognition of predators, food, and mates. Indeed, it is thought[by whom?] that simple sensory-neural mechanisms may selectively control general behavior patterns, such as escape, foraging, and hiding. Many examples of wavelength-specific behaviors have been identified, in two primary groups: Below 450 nm, associated with direct light, and above 450 nm, associated with reflected light. As opsin molecules were tuned to detect different wavelengths of light, at some point color vision developed when the photoreceptor cells used differently tuned opsins. This may have happened at any of the early stages of the eye's evolution, and may have disappeared and reevolved as organisms became predator or prey. Similarly, night and day vision emerged when photoreceptor cells differentiated into rods and cones, respectively.[citation needed]
Further information: Evolution of color vision
Polarization vision  
As discussed earlier, the properties of light under water differ from those in air. One example of this is the polarization of light. Polarization is the organization of originally disordered light, from the sun, into linear arrangements. This occurs when light passes through slit like filters, as well as when passing into a new medium. Sensitivity to polarized light is especially useful for organisms whose habitats are located more than a few meters under water. In this environment, color vision is less dependable, and therefore a weaker selective factor. While most photoreceptors have the ability to distinguish partially polarized light, terrestrial vertebrates’ membranes are orientated perpendicularly, such that they are insensitive to polarized light. However, some fish can discern polarized light, demonstrating that they possess some linear photoreceptors. Additionally, cuttlefish are capable of perceiving the polarization of light with high visual fidelity, although they appear to lack any significant capacity for color differentiation.Like color vision, sensitivity to polarization can aid in an organism's ability to differentiate surrounding objects and individuals. Because of the marginal reflective interference of polarized light, it is often used for orientation and navigation, as well as distinguishing concealed objects, such as disguised prey.
Focusing mechanism  
By utilizing the iris sphincter muscle, some species move the lens back and forth, some stretch the lens flatter. Another mechanism regulates focusing chemically and independently of these two, by controlling growth of the eye and maintaining focal length. In addition, the pupil shape can be used to predict the focal system being utilized. A slit pupil can indicate the common multifocal system, while a circular pupil usually specifies a monofocal system. When using a circular form, the pupil will constrict under bright light, increasing the focal length, and will dilate when dark in order to decrease the depth of focus. Note that a focusing method is not a requirement. As photographers know, focal errors increase as aperture increases. Thus, countless organisms with small eyes are active in direct sunlight and survive with no focus mechanism at all. As a species grows larger, or transitions to dimmer environments, a means of focusing need only appear gradually.
Location
Prey generally have eyes on the sides of their head so to have a larger field of view, from which to avoid predators. Predators, however, have eyes in front of their head in order to have better depth perception. Flatfish are predators which lie on their side on the bottom, and have eyes placed asymmetrically on the same side of the head. A transitional fossil from the common symmetric position is Amphistium.