What function does the forebrain perform in fish? Structure of the brain of bony fish

The brain of fish is very small, and bigger fish, the smaller the relative mass of the brain. In large sharks, the brain mass is only a few thousandths of a percent of the body mass. In sturgeon and bony fish, weighing several kilograms, its mass reaches hundredths of a percent of body weight. With a fish weighing several tens of grams, the brain makes up a fraction of a percent, and in fish weighing less than 1 g, the brain exceeds 1% of the body weight. This shows that brain growth lags behind the growth of the rest of the body. Apparently, most brain development occurs during embryonic-larval development. Of course, there are also interspecies differences in relative mass brain

The brain consists of five main sections: the forebrain, diencephalon, midbrain, cerebellum and medulla oblongata ( SLIDE 6).

The structure of the brain of different species of fish is different and depends to a greater extent not on the systematic position of the fish, but on their ecology. Depending on which receptor apparatus predominates in a given fish, the parts of the brain develop accordingly. With a well-developed sense of smell, it increases forebrain, with well-developed vision - the midbrain, in good swimmers - the cerebellum. In pelagic fish, the optic lobes are well developed, the striatum is relatively poorly developed, and the cerebellum is well developed. In fish leading a sedentary lifestyle, the brain is characterized by weak development of the striatum, a small pineal-shaped cerebellum, and sometimes a well-developed medulla oblongata.

Rice. 14. Structure of the brain of bony fish:

a - schematic representation of a longitudinal section of the brain; b - crucian carp brain, cut back view; c - yellowtail brain, side view; d - yellowtail brain, dorsal view; forebrain; 2- first cerebral ventricle; 3 - pineal gland; 4 - midbrain; 5- cerebellar valve; 6 - cerebellum; 7 - brain canal; 8 - fourth cerebral ventricle; 9 - medulla oblongata; 10 - vascular sac; 11 - pituitary gland; 12 - third cerebral ventricle; 13 - optic nerve nucleus; 14 - diencephalon; 15 - olfactory tract; 16 - optic lobes; 11 - almond-shaped tubercles; 18 - vagal dilia 1U - spinal cord; 20 - roof of the cerebellum; 21 - olfactory lobes; 22 - olfactory bulb; 23 - olfactory tract; 24 - hypothalamus; 25 - cerebellar protrusions

Medulla. The medulla oblongata is a continuation spinal cord. In its anterior part it passes into the posterior part of the midbrain. Its upper part - the rhomboid fossa - is covered with ependyma, on which the posterior choroid plexus. The medulla oblongata performs a series of important functions . Being a continuation of the spinal cord, it plays the role of a conductor of nerve impulses between the spinal cord and various parts of the brain. Nerve impulses are conducted both in a descending manner, i.e. to the spinal cord, and in the ascending directions - to the midbrain, intermediate and forebrain, as well as to the cerebellum.

The medulla oblongata contains the nuclei of six pairs of cranial nerves (V-X). From these nuclei, which are a cluster of nerve cells, the corresponding cranial nerves originate, emerging in pairs from both sides of the brain. The cranial nerves innervate various muscles and receptor organs of the head. Fibers vagus nerve innervate various organs and the lateral line. Cranial nerves can be of three types: sensory, if they contain branches that conduct afferent impulses from the sense organs: motor, carrying only efferent impulses to organs and muscles; mixed containing sensory and motor fibers.

V pair - trigeminal nerve. It begins on the lateral surface of the medulla oblongata and is divided into three branches: the orbital nerve, which innervates the anterior part of the head; maxillary nerve, which passes along the eye under the eye upper jaw and innervating the skin of the anterior part of the head and palate; mandibular nerve, running along the lower jaw, innervating the skin, mucous membrane oral cavity and mandibular muscles. This nerve contains motor and sensory fibers.

VI pair abducens nerve. Originates from the bottom of the medulla oblongata, its midline, and innervates the muscles of the eye,

VII - facial nerve. It is a mixed nerve, extends from the lateral wall of the medulla oblongata, directly behind the trigeminal nerve and is often connected with it, forms a complex ganglion from which two branches arise: the nerve of the lateral line of the head and the branch innervating the mucous membrane of the palate, sublingual area, taste buds of the oral cavity and muscles of the operculum.

VIII - auditory, or sensory, nerve. Innervates inner ear

and labyrinth apparatus. Its nuclei are located between the nuclei of the vagus nerve and the base of the cerebellum.

IX – glossopharyngeal nerve. Departs from the lateral wall of the oblong

brain and innervates the mucous membrane of the palate and the muscles of the first branchial arch.

X – vagus nerve. It departs from the lateral wall of the medulla oblongata by numerous branches that form two branches: the lateral nerve, which innervates the lateral line organs in the trunk; nerve of the gill cover, innervating the gill apparatus and some internal organs. On the sides of the rhomboid fossa there are thickenings - the vagal lobes, where the nuclei of the vagus nerve are located.

Sharks have an XI nerve - the terminal one. Its nuclei are located on the anterior or inferior side of the olfactory lobes, and the nerves pass along the dorsolateral surface of the olfactory tracts to the olfactory sacs.

Vital centers are located in the medulla oblongata region. This part of the brain regulates breathing, cardiac activity, the digestive system, etc.

The respiratory center is represented by a group of neurons that regulate breathing movements. You can distinguish the centers of inhalation and exhalation. If half of the medulla oblongata is destroyed, then respiratory movements stop only on the corresponding side. In the region of the medulla oblongata there is also a center that regulates the functioning of the heart and blood vessels. The next important center of the medulla oblongata is the center that regulates the functioning of chromatophores. When this center is irritated electric shock the entire body of the fish becomes lighter. There are also centers that regulate the functioning of the gastrointestinal tract.

In fish that have electrical organs, the motor areas of the medulla oblongata grow, which leads to the formation of large electrical lobes, which are a kind of center for synchronizing the discharges of individual electrical plates innervated by various motor neurons of the spinal cord.

In fish that lead a sedentary lifestyle, the taste analyzer is of great importance, and therefore they develop special taste lobes.

In the medulla oblongata, the centers responsible for the movement of the fins are located in close proximity to the nuclei of the VIII and X pairs of nerves. With electrical stimulation of the medulla oblongata behind the nucleus of the X pair, changes in the frequency and direction of movement of the fins occur.

Of particular importance in the medulla oblongata is a group of ganglion cells in the form of a kind of nervous network called the reticular formation. It begins in the spinal cord and then occurs in the medulla oblongata and midbrain.

In fish, the reticular formation is associated with afferent fibers of the vestibular nerve (VIII) and lateral line nerves (X), as well as with fibers arising from the midbrain and cerebellum. It contains giant Mountner cells, which innervate the swimming movements of fish. The reticular formation of the medulla oblongata, midbrain and diencephalon is a functionally unified formation that plays an important role in the regulation of functions.

The so-called olive medulla oblongata has a regulatory effect on the spinal cord - a nucleus that is well expressed in cartilaginous fish and worse in bony fish. It is connected to the spinal cord, cerebellum, and diencephalon and is involved in the regulation of movements.

In some fish, characterized by high swimming activity, an accessory olive nucleus develops, which is associated with the activity of the trunk and tail muscles. The areas of the nuclei of the VIII and X pairs of nerves are involved in the redistribution of muscle tone and in the implementation of complex coordinated movements.

Midbrain. The midbrain in fish is represented by two sections: the “visual roof” (tectum), located dorsally, and the tegmentum, located ventrally. The visual roof of the midbrain is swollen in the form of paired formations - the optic lobes. The degree of development of the optic lobes is determined by the degree of development of the visual organs. In blind and deep-sea fish they are poorly developed. On inside of the tectum, facing the cavity of the third ventricle, there is a paired thickening - the longitudinal torus. Some authors believe that the longitudinal torus is associated with vision, since the endings of the optic fibers are found in it; this formation is poorly developed in blind fish. The higher, visual center of fish is located in the midbrain. The fibers of the second pair of nerves, the optic ones, coming from the retina of the eyes, end in the tectum.

The important role of the midbrain of fish in relation to the functions of the visual analyzer can be judged by the development of conditioned reflexes to light. These reflexes in fish can be developed by removing the forebrain, but preserving the midbrain. When the midbrain is removed, conditioned reflexes to light disappear, but previously developed reflexes to sound do not disappear. After one-sided removal of the tectum from a minnow, the eye of the fish lying on the opposite side of the body becomes blind, and when the tectum is removed from both sides, complete blindness occurs. The center of the visual grasping reflex is located here. This reflex consists in the fact that the movements of the eyes, head, and entire body, caused from the midbrain region, are pressed to maximize the fixation of an object in the area of ​​greatest visual acuity - the central fovea of ​​the retina. When electrically stimulating certain areas of the trout tectum, coordinated movements of both eyes, fins, and body muscles appear.

The midbrain plays an important role in regulating the coloration of fish. When the eyes are removed from the fish, a sharp darkening of the body is observed, and after bilateral removal of the tectum, the body of the fish becomes lighter.

In the region of the tegmentum there are nuclei of the III and IV pairs of nerves, which innervate the muscles of the eyes, as well as the autonomic nuclei, from which nerve fibers extend, innervating the muscles that change the width of the pupil.

The tectum is closely connected with the cerebellum, hypothalamus and, through them, with the forebrain. The tectum in fish is one of the most important integration systems; it coordinates the functions of the somatosensory, olfactory and visual systems. The tegmentum is connected with the VIII pair of nerves (acoustic) and with the receptor apparatus of the labyrinths, as well as with the V pair of nerves (trigeminal). Afferent fibers from the lateral line organs, from the auditory and trigeminal nerves approach the nuclei of the midbrain. All these connections of the midbrain provide the exclusive role of this part of the central nervous system in fish in neuro-reflex activity, which has adaptive significance. The tectum in fish is apparently the main organ for closing temporary connections.

The role of the midbrain is not limited to its connection with visual analyzer. The endings of afferent fibers from the olfactory and taste buds. The midbrain of fish is the leading center for the regulation of movement. In the region of the tegmentum in fish there is a homologue of the red nucleus of mammals, the function of which is to regulate muscle tone.

When the optic lobes are damaged, the tone of the fins decreases. When the tectum is removed from one side, the tone of the extensors on the opposite side and the flexors on the side of the operation increases - the fish bends towards the operation, and manege movements (movements in a circle) begin. This indicates the importance of the midbrain in the redistribution of the tone of antagonistic muscles. When the midbrain and medulla oblongata are separated, increased spontaneous activity of the fins appears. It follows from this that the midbrain has an inhibitory effect on the centers of the medulla oblongata and spinal cord.

Diencephalon. The diencephalon consists of three formations: the epithalamus - the uppermost supratubercular region; the thalamus - the middle part containing the visual hillocks and the hypothalamus - the subtubercular region. This part of the brain in fish is partially covered by the roof of the midbrain.

Epithalamus consists of the epiphysis or pineal organ and habenular nuclei.

Pineal gland- a vestige of the parietal eye, it functions mainly as an endocrine gland. The epithalamus also includes the frenulum (habenula), located between the forebrain and the roof of the midbrain. It is represented by two habenular nuclei, connected by a special ligament, to which fibers from the pineal gland and olfactory fibers of the forebrain approach. Thus, these nuclei are related to light perception and smell.

Efferent fibers go to the midbrain and to the lower centers. The visual tuberosities are located in the central part of the diencephalon; with their inner lateral walls they limit the third ventricle.

IN thalamus distinguish between dorsal and ventral regions. In the dorsal thalamus of sharks, a number of nuclei are distinguished: the external geniculate body, the anterior, internal and medial nuclei.

The nuclei of the visual thalamus are the site of differentiation of perceptions of various types of sensitivity. Afferent influences from various organs feelings, this is where the analysis and synthesis of afferent signaling takes place. Thus, the visual hillocks are an organ of integration and regulation of the body’s sensitivity, and also take part in the implementation of motor reactions. With the destruction of the diencephalon in sharks, the disappearance of spontaneous movements, as well as impaired coordination of movements, were observed.

The hypothalamus includes an unpaired hollow protrusion - the funnel, which forms a special organ entwined with blood vessels - the vascular sac.

On the sides of the vascular sac are its lower lobes. In blind fish they are very small. It is believed that these lobes are associated with vision, although there are suggestions that this part of the brain is associated with taste endings.

The vascular sac is well developed in deep-sea marine fish. Its walls are lined with shimmering cuboidal epithelium, nerve cells called depth receptors are also located here. It is believed that the vascular sac responds to changes in pressure, and its receptors are involved in the regulation of buoyancy; receptor cells of the vascular sac are related to the perception of speed forward movement fish. The vascular sac has nerve connections with the cerebellum, thanks to which the vascular sac is involved in the regulation of balance and muscle tone during active movements and vibrations of the body. In bottom fishes the vascular sac is rudimentary.

Hypothalamus is the main center where information from the forebrain arrives. Afferent influences from taste endings and from the acoustic-lateral system come here. Efferent fibers from the hypothalamus go to the forebrain, to the dorsal thalamus, tectum, cerebellum, and neurohypophysis.

In the hypothalamus of fish there is a preoptic nucleus, the cells of which have the morphological characteristics of nerve cells, but produce neurosecretion.

Cerebellum. It is located in the back of the brain, partially covering the medulla oblongata on top. Distinguish middle part- the body of the cerebellum - and two lateral sections - the cerebellar auricles. The anterior end of the cerebellum projects into the third ventricle, forming the cerebellar valve.

In bottom-dwelling and sedentary fish (anglerfish, scorpionfish), the cerebellum is less developed than in fish with high mobility. The cerebellum in predators (tuna, mackerel, cod), pelagic or planktivorous (harengula). In mormyrids, the cerebellar valve is hypertrophied and sometimes extends over the callosal surface of the forebrain. In cartilaginous fish, an increase in the surface of the cerebellum due to the formation of folds can be observed.

In teleost fish, in the posterior, lower part of the cerebellum there is a cluster of cells called the “lateral cerebellar nucleus”, which plays a large role in maintaining muscle tone.

When deleting in a shark with half of the auricular lobes, its body begins to bend sharply towards the operation (opisthotonus). When the body of the cerebellum is removed while preserving the auricular lobes, a disturbance in muscle tone and fish movement occurs only if the lower part of the cerebellum, where the lateral nucleus is located, is removed or cut. At complete removal cerebellum, a decrease in tone (atony) and impaired coordination of movements occur - the fish swim in a circle, first in one direction, then in the other. After about three weeks, the lost functions are restored due to regulatory processes in other parts of the brain.

Removal of the cerebellum from fish leading active image life (perch, pike, etc.), causes severe incoordination of movements, sensory disturbances, complete disappearance of tactile sensitivity, weak reaction to painful stimuli.

The cerebellum in fish, being connected through afferent and efferent pathways with the tectum, hypothalamus, thalamus, medulla oblongata and spinal cord, can serve as the highest organ of integration nervous activity. After removal of the body of the cerebellum, motor disturbances in the form of body swaying from side to side are observed in transversestomes and teleost fish. If the body and the cerebellar valve are removed at the same time, motor activity is completely disrupted, trophic disorders develop, and after 3-4 weeks the animal dies. This indicates the motor and trophic functions of the cerebellum.

The cerebellar auricles receive fibers from the nuclei of the VIII and X pairs of nerves. The cerebellar ears reach large sizes in fish that have a well-developed tank line. Enlargement of the cerebellar valve is also associated with the development of the lateral line. In goldfish, the developed differentiation reflexes to the circle, triangle and cross disappeared after coagulation of the cerebellar valve and were not subsequently restored. This indicates that the cerebellum of fish is the place where conditioned reflexes coming from the lateral line organs are closed. On the other hand, numerous experiments show that in carp with the cerebellum removed, on the first day after surgery it is possible to develop motor and cardiac conditioned reflexes to light, sound and interoceptive stimulation of the swim bladder.

Forebrain. It consists of two parts. Dorsally lies a thin epithelial plate - a mantle or cloak, delimiting the common ventricle from the cranial cavity; at the base of the forebrain lie the striatal bodies, which are connected on both sides by the anterior ligament. The sides and roof of the forebrain, forming the mantle, repeat in general the shape of the underlying striatum, from which the entire forebrain appears to be divided into two hemispheres, but a true division into two hemispheres is not observed in bony fishes.

In the anterior wall of the forebrain, a paired formation develops - the olfactory lobes, which are sometimes located with their entire mass on the anterior wall of the brain, and sometimes extend significantly in length and are often differentiated into the main part (the olfactory lobe itself), the stalk and the olfactory bulb.

In lungfishes, the anterior wall of the brain slides between the striatum in the form of a fold, dividing the forebrain into two separate hemispheres.

The mantle receives secondary olfactory fibers from the olfactory bulb. Since the forebrain in fish is the brain part of the olfactory apparatus, some researchers call it olfactory brain. After removal of the forebrain, the disappearance of developed conditioned reflexes to olfactory stimuli is observed. After the separation of the symmetrical halves of the forebrain in crucian carp and carp, no disturbances in the spatial analysis of visual and sound stimuli are observed, which indicates the primitiveness of the functions of this section.

After removal of the forebrain, fish retain conditioned reflexes to light, sound, magnetic field, swim bladder stimulation, lateral line stimulation, and taste stimuli. Thus, the arcs of conditioned reflexes to these stimuli are closed at other levels of the brain. In addition to the olfactory functions, the forebrain of fish also performs some other functions. Removal of the forebrain leads to a decrease in motor activity in fish.

For the varied and complex behavior of fish in a school, the integrity of the forebrain is necessary. After its removal, the fish swim outside the school. The development of conditioned reflexes, observed in school conditions, is disrupted in fish lacking the forebrain. When the forebrain is removed, fish lose initiative. Thus, normal fish, swimming through a fine grid, choose different paths, but fish lacking a forebrain are limited to one path and bypass the obstacle with great difficulty. Intact marine fish do not change their behavior in the sea after 1-2 days in the aquarium. They return to the pack, occupy the previous hunting area, and if it is occupied, they enter into a fight and drive out the competitor. Operated individuals released into the sea do not join the flock, do not occupy their hunting area and do not secure a new one for themselves, and if they remain in the previously occupied one, they do not protect it from competitors, although they do not lose the ability to defend themselves. If healthy fish when a dangerous situation arises in their area, they skillfully use the features of the terrain, consistently move to the same shelters, then the operated fish seem to forget the system of shelters, using random shelters.

The forebrain also plays an important role in sexual behavior.

Removal of both lobes in hemichromis and the Siamese cockerel leads to a complete loss of sexual behavior, in tilapia the ability to mate is impaired, and in guppies there is a delay in mating. In stickleback when removed various departments the forebrain changes (increase or decrease) various functions - aggressive, parental or sexual behavior. In male crucian carp, when the forebrain is destroyed, sexual desire disappears.

Thus, after removal of the forebrain, fish lose their defensive reaction, the ability to care for offspring, the ability to swim in schools, and some conditioned reflexes, i.e. there is a change in complex forms of conditioned reflex activity and general behavioral unconditional reactions. These facts do not provide exhaustive evidence that the forebrain in fish acquires the significance of an organ of integration, but they suggest that it has a general stimulating (tonic) effect on other parts of the brain.

Representatives of this class exhibit variations in the structure of the brain, but, nevertheless, common characteristic features can be identified. Their brain has a relatively primitive structure and, in general, small sizes.

The forebrain, or telencephalon, in most fish consists of one hemisphere (some sharks leading a bottom-dwelling lifestyle have two) and one ventricle. The roof does not contain nerve elements and is formed by epithelium, and only in sharks do nerve cells rise from the base of the brain to the sides and partly to the roof. The bottom of the brain is represented by two clusters of neurons - these are the striatal bodies (corpora striata).

Anterior to the brain are two olfactory lobes (bulbs), connected by olfactory nerves to the olfactory organ located in the nostrils.

In lower vertebrates, the forebrain is a section of the nervous system that serves only the olfactory analyzer. It is the highest olfactory center.

The diencephalon consists of the epithalamus, thalamus and hypothalamus, which are characteristic of all vertebrates, although the degree of their expression varies. A special role in the evolution of the diencephalon is played by the thalamus, in which the ventral and dorsal parts are distinguished. Subsequently, in vertebrates, during evolution, the size of the ventral part of the thalamus decreases, and the dorsal part increases. Lower vertebrates are characterized by a predominance of the ventral thalamus. Here are the nuclei that act as an integrator between the midbrain and the olfactory system of the forebrain; in addition, in lower vertebrates the thalamus is one of the main motor centers.

Below the ventral thalamus is the hypothalamus. From below it forms a hollow stalk - a funnel, which passes into the neurohypophysis, connected to the adenohypophysis. The hypothalamus plays a major role in the hormonal regulation of the body.

The epithalamus is located in the dorsal part of the diencephalon. It does not contain neurons and is connected to the pineal gland. The epithalamus, together with the pineal gland, constitutes a system of neurohormonal regulation of daily and seasonal activity of animals.

Rice. 6. Perch brain (dorsal view).

1 – nasal capsule.
2 – olfactory nerves.
3 – olfactory lobes.
4 – forebrain.
5 – midbrain.
6 – cerebellum.
7 – medulla oblongata.
8 – spinal cord.
9 – diamond-shaped fossa.

The midbrain of fish is relatively large. It consists of a dorsal part - the roof (thecum), which has the appearance of a colliculus, and a ventral part, which is called the tegment and is a continuation of the motor centers of the brain stem.

The midbrain has developed as the primary visual and seismosensory center. The visual and auditory centers are concentrated in it. In addition, it is the highest integrative and coordinating center of the brain, approaching in its importance cerebral hemispheres forebrain of higher vertebrates. This type of brain, where the midbrain is the highest integrative center, is called ichthyopsid.

The cerebellum is formed from the posterior medullary vesicle and forms a fold. Its size and shape vary significantly. In most fish, it consists of the middle part - the body of the cerebellum and the lateral ears - the auricle. Bony fish are characterized by anterior growth - a valve. The latter in some species takes on such large dimensions that it can hide part of the forebrain. In sharks and bony fishes, the cerebellum has a folded surface, due to which its area can reach significant sizes.

Through ascending and descending nerve fibers The cerebellum connects to the middle cord, medulla oblongata and spinal cord. Its main function is the regulation of coordination of movements, and therefore in fish with high motor activity it is large and can account for up to 15% of the total mass of the brain.

The medulla oblongata is a continuation of the spinal cord and generally repeats its structure. The border between the medulla oblongata and the spinal cord is considered to be the place where the central canal of the spinal cord is cross section takes the form of a circle. In this case, the cavity of the central canal expands, forming a ventricle. Side walls the latter grow strongly to the sides, and the roof is formed by an epithelial plate in which the choroid plexus is located with numerous folds facing the cavity of the ventricle. The lateral walls contain nerve fibers that provide innervation to the visceral apparatus, lateral line organs and hearing. In the dorsal sections of the lateral walls there are nuclei of gray matter, in which the switching of nerve impulses occurs along the ascending pathways from the spinal cord to the cerebellum, midbrain and to the neurons of the striatum of the forebrain. In addition, there is also a switching of nerve impulses to descending pathways connecting the brain with motor neurons of the spinal cord.

The reflex activity of the medulla oblongata is very diverse. It contains: the respiratory center, the regulatory center cardiovascular activity, through the nuclei of the vagus nerve, the digestive organs and other organs are regulated.

10 pairs of cranial nerves depart from the brain stem (midbrain, medulla oblongata and pons) in fish.

Intelligence. How your brain works Sheremetyev Konstantin

Fish brain

Fish brain

Fish were the first to acquire a brain. The fish themselves appeared about 70 million years ago. The habitat of fish is already comparable to the area of ​​the Earth. Salmon (Fig. 9) swim thousands of miles from the ocean to spawn in the river where they hatched. If this does not surprise you, then imagine that without a map you need to get to an unknown river, having traveled at least a thousand kilometers. All this became possible thanks to the brain.

Rice. 9. Salmon

Together with the brain, fish for the first time have a special type of learning – imprinting. A. Hasler in 1960 established that Pacific salmon, at a certain point in their development, remember the smell of the stream in which they were born. They then go down the stream into the river and swim into the Pacific Ocean. They frolic in the ocean for several years, and then return to their homeland. In the ocean, they navigate by the sun and find the mouth of the desired river, and find their native stream by smell.

Unlike invertebrates, fish can travel considerable distances in search of food. There is a known case when a ringed salmon swam 2.5 thousand kilometers in 50 days.

Fish are myopic and see clearly at a distance of only 2–3 meters, but they have well-developed hearing and sense of smell.

It is commonly believed that fish are silent, although in fact they communicate using sounds. Fish make sounds by clenching their swim bladder or grinding their teeth. Typically, fish make cracking, grinding or chirping noises, but some can howl, and the Amazonian Pirarara catfish has learned to scream so that it can be heard at a distance of up to a hundred meters.

The main difference between the nervous system of fish and the nervous system of invertebrates is that the brain has centers responsible for visual and auditory function. As a result, fish can distinguish between simple geometric figures, and, interestingly, fish are also susceptible to visual illusions.

The brain took over the function of general coordination of fish behavior. The fish swims according to rhythmic commands from the brain, which are transmitted through the spinal cord to the fins and tail.

Fish easily develop conditioned reflexes. They can be taught to swim to a certain place when signaled by a light.

In the experiments of Rozin and Mayer, goldfish supported constant temperature water in the aquarium by activating a special valve. They quite accurately kept the water temperature at 34 °C.

Like invertebrates, fish reproduction is based on the principle of large offspring. Herring lay hundreds of thousands of small eggs every year and do not care about them.

But there are fish that take care of the young. Female Tilapia natalensis holds the eggs in his mouth until the fry hatch from them. For some time, the fry stay in a school near their mother and, in case of danger, hide in her mouth.

Caring for fish fry can be quite difficult. For example, a male stickleback builds a nest, and when the female lays eggs in this nest, he uses his fins to drive water into this nest to ventilate the eggs.

A big problem for fry is recognizing their parents. Cichlid fish consider any slowly moving object as their parent. They line up behind him and swim after him.

Some species of fish live in schools. There is no hierarchy or clearly defined leader in the pack. Usually a group of fish is knocked out of the school, and then the whole school follows them. If an individual fish escapes from the school, it immediately returns. The forebrain is responsible for schooling behavior in fish. Erich von Holst removed the forebrain from a river minnow. After this, the minnow swam and fed as usual, except that it had no fear of breaking away from the school. Minnow swam where he wanted, without looking back at his relatives. As a result, he became the leader of the pack. The whole flock considered him very smart and followed him relentlessly.

In addition, the forebrain allows fish to form an imitation reflex. The experiments of E. Sh. Airapetyants and V. V. Gerasimov showed that if one of the fish in a school shows a defensive reaction, then the other fish imitate it. Removal of the forebrain stops the formation of the imitation reflex. Non-schooling fish do not have an imitation reflex.

Pisces begin to sleep. Some fish even lie down on the bottom to take a nap.

In general, although the fish brain demonstrates good innate abilities, it has little ability to learn. The behavior of two fish of the same species is almost identical.

The brains of amphibians and reptiles have undergone minor changes compared to fish. Basically, the differences are related to the improvement of the senses. Significant changes in the brain occurred only in warm-blooded animals.

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