What kind of vision do animals have?

A person learns about the world around him (shape, tone, shades, texture of objects), orients himself in space, in a word, receives the main share (up to 80%) of information from the external environment thanks to vision. Vision is a unique gift, thanks to which a person can enjoy the fullness of the colors of the living world.

It's all poetry...

In prose, vision is a complex chain of biochemical reactions and biophysical transformations, and the human eye is a complex optical system that perceives and converts light rays into a nerve impulse transmitted along the optic nerve to the brain.

Vision is an amazingly complex and still far from understood joint work of the eye and brain. Science has been studying the eye for centuries, and every scientist, discovering its new properties and new secrets, experiences a feeling of excitement and admiration for its perfection.

Almost all living organisms have light-sensitive cells. Even the simplest react to changes in light intensity, usually moving away from its source. Most plants face their leaves and flowers toward the sun, although they do not have any special light-sensitive structures. In most higher animals, sensitivity to light is highly developed and localized in certain cells. The human eye is an excellent example of an extremely sensitive, specialized organ for sensing light.

Evolution of the eye:

Some protozoa have “eye spots” that are more sensitive to light than the rest of the cell, but the most primitive light-sensitive organs on the evolutionary ladder are the “eyes” of flatworms (planarians). These are not eyes yet, but only photoreceptors, since they are not capable of creating an image.
Almost all turbelaria have two or more eyes. They are located near the cerebral ganglion or (if there are more than two eyes) along the edges of the anterior part of the body. Each eye consists of a glass formed by pigment cells, into which receptor (retinal) cells penetrate, perceiving light. In turbelarians, the eyes are of the inverted (reversed) type, since light first passes through the bodies of the receptor cells, after which it hits the light-receiving segment. Retinal cells are nervous in origin, so they have processes (axons), which together form the optic nerve, which is sent to the cerebral ganglion, where the received information is analyzed.
Although earthworms usually do not have eyes, they are able to respond to light due to the presence of numerous light-sensitive cells in their skin.
We find the most highly developed eyes in arthropods (crustaceans and insects), cephalopods (octopus, squid, etc.) and vertebrates. There are two main types of eyes: chamber compound eyes. Arthropods have compound eyes, while cephalopods have chamber eyes.

Crustacean eyes can be complex or simpler. Compound, or faceted, eyes consist of numerous small eyes - ommatidia, each of which functions individually and covers a certain part of the surrounding space. As a result, cancer perceives the environment as a mosaic image. A separate ommatodia consists of light-refracting lenses and sensory cells, from which processes arise that form the optic nerve. The eyes are separated from each other by thin layers of pigment. It has been established that the number of ommatodes in the compound eye of a crayfish increases with the age of the crayfish, and at the same time their size increases.

Compound eyes are paired and usually sit on special outgrowths - eyestalks, which are clearly visible in crayfish or crabs.

The other type of eye is smaller and not so complicated. It consists of a pigment cup flanked by four groups of sensory cells. Above the glass there is a light-refracting lens - a crystalline lens.

The simple eye is always unpaired; it is usually located on the lower surface of the head between the antennules. Since the larva, the nauplius, has it, it is also called the nauplial eye, but copepods and many barnacles retain such eyes into adulthood, and do not develop complex eyes. Many leaf-footed crayfish have both types of eyes as adults, while some higher crayfish have only complex eyes.

In arachnids, the visual organs are simple eyes, the number of which varies from 2 to 12 in different species. In spiders, they are located on the cephalothorax shield in the form of two arcs, and in scorpions, one pair of eyes is located in front and several more pairs on the sides. Despite the significant number of eyes, arachnids have poor vision. At best, they are able to more or less clearly distinguish objects at a distance of no more than 30 cm, and most species - even less (for example, scorpions see only at a distance of several cm). For some vagrant species (for example, jumping spiders), vision is more important, since with its help the spider looks out for prey and distinguishes between individuals of the opposite sex.

Insect eyes can be simple or complex. Simple eyes can perceive light, but cannot distinguish objects. They have a light-refracting lens, under which there are receptor cells surrounded by pigment cells. Both larvae and adult individuals have simple eyes, but in the latter they are formed anew, since larval eyes degenerate during ontogenesis.

Significantly more advanced are compound eyes, formed by a large number of individual ocelli - ommatidia, the structure of which does not correspond to the usual simple ocelli described above. Each ommatodia is surrounded by a transparent cuticle - the cornea, which, together with the underlying crystal cone, makes up a light-refracting lens. Receptor cells are located under the lens, their light-sensitive parts together make up the photosensitive element - the rhabdom. Ommatodia are separated from each other by layers of pigment cells.

Each ommatode within a compound (or compound) eye functions as a separate eye, perceiving a small portion of space. In this case, the insect receives an image of the surrounding world in the form of a complex mosaic of individual tiny pictures. The total number of ommatidia in the compound eye varies widely among species. For example, an ant has 8-9 of them, a housefly has about 4000, and dragonflies have up to 28,000 in each eye. This indicator depends on how important vision is for a given species.

The eyes of insects are not able to tune in to equidistant objects, since they do not have an accommodation system, so the vision of many insects is relatively unclear.

The ability to perceive colors in different types of insects also varies. It has been established that insects are able to perceive short-wave light (even ultraviolet), but do not perceive long-wave light.

In gastropods, in the simplest case, the eyes are a pit lined with sensitive cells. More complex eyes have a bubble submerged under the epithelium, in which light rays pass through the lens - the lens and the vitreous body, after which they hit the light-sensitive cells.

In cephalopods, the organs of vision are especially advanced and can have a different structure. In the most primitive four-branched forms (nautiluses), the eye is a large pit, which communicates with the external environment through a preserved opening.

Chordata (invertebrates, class lancelets) do not have real eyes, but they have photosensitive Hessian ocelli, consisting of two cells - a photosensitive one and a pigment cell located underneath it.

In cartilaginous fish, the organs of vision, located on the sides of the head, are equipped with six oculomotor muscles, the contraction of which provides greater mobility of the eyes, which is especially important since the head of the fish is connected to the body motionlessly. The eye has a flat cornea, which functions most optimally in an aquatic environment; spherical lens; Sharks have a nictitating membrane that covers the eye like an eyelid.

In bony fishes, the organ of vision has the usual structure for fish, a flat cornea and an almost spherical lens; they are more effective in the aquatic environment, but allow one to clearly distinguish only nearby objects. A crescent-shaped process extends from the choroid of the eye near the optic nerve, which protrudes into the cavity of the eyeball and attaches to the lens. When this process contracts, it moves the lens deeper into the eye, thereby providing accommodation. In addition, the tunica argentum is characteristic, which is a layer of the choroid rich in deposits of guanine crystals. This membrane is located between the vascular and fibrous (albuginine) membranes; in front it passes onto the iris, forming its outer layer, which is why the eye acquires a greenish-golden shine.

Different species may have their own characteristics in the structure of the eye, for example, mullet and some herrings have a nictitating membrane. The eyes of the four-eyed fish (lives in Central and South America) are divided into two halves - the lower one has a flat cornea, which allows it to navigate well in water, and the upper one has a convex cornea, which provides vision in the air. In some species, vision is secondarily reduced; such species navigate using other senses.

The eyes of amphibians are adapted to terrestrial life: they have a convex cornea and a biconvex lens, which allows them to distinguish objects in the air at a fairly considerable distance. The cornea can flatten when an animal enters water, and in amphibians, the eyes retract when swallowing food, thereby pushing it into the esophagus. Amphibians also develop eyelids that protect the eyes from drying out on land. It has been experimentally proven that amphibians react only to moving objects.

In reptiles, the organ of vision is characterized by the presence of striated muscle fibers in the ciliary muscle, which makes accommodation of the eye more effective, since this muscle is capable of not only moving the lens back and forth, but also changing its curvature (remember that in fish, accommodation is limited only to changing distance from the lens to the retina). In the sclera of the eyeball there is a ring of thin bone plates, and from the back of the wall a ridge, a growth rich in blood vessels, protrudes into the cavity of the eyeball filled with vitreous humor.

Unlike amphibians, the eyeballs of reptiles cannot retract, but are able to rotate with the help of the extraocular muscles. This is especially effective in chameleons, whose eyes are not only very mobile, but also move independently of each other. Most reptiles have eyelids, but geckos and snakes do not have movable eyelids, so geckos simply lick the cornea from time to time, and snakes periodically remove the superficial layer of fused eyelids during molting.

The organ of vision in birds is very well developed, since for most birds vision is the main way of orientation. The eyes are usually large, especially in nocturnal birds. Birds are distinguished by the most advanced double mechanism of eye accommodation among all vertebrates. In this case, the lens not only changes its curvature under the action of the ciliary muscle, but the distance between the lens and the retina also changes. Thus, the lens can change its parameters more widely, while ensuring optimal focusing of the rays hitting the retina. All this provides the bird’s eye with amazing sharpness, especially in birds of prey, for example, the peregrine falcon notices moving prey at a distance of up to 1100 m, while the highest resolution allows you to see the object in detail.. In nocturnal birds, the retina captures very small portions of light, for example, owls see a mouse at night with lighting of only 0.000002 lux.

Most mammals have an organ of vision. The structure of the eye is typical of terrestrial vertebrates (convex cornea and biconvex lens), accommodation is less perfect than in birds, and is carried out only by changing the curvature of the lens under the influence of the ciliary muscle. In small rodents (for example, mice), accommodation is generally absent.

Not all mammals can distinguish colors, for example, the forest ferret and many other animals are deprived of color vision, other species distinguish only certain colors of the spectrum, and only very few mammals (the higher primates of the eastern hemisphere) have full color vision. Most species are able to distinguish only moving objects, leaving stationary ones without attention. This is due to the fact that in mammals other sense organs are predominantly developed, and vision for orientation plays a much smaller role than in birds. Inhabitants of steppes, savannas, prairies and other vast open areas, as well as nocturnal animals, have sharper vision. On the contrary, forest mammals see worse. In some species, mainly underground (mole rats, some moles), vision has completely atrophied, and the reduced eyes are completely covered with membranes.