In nature, there are many classes of different animals. One of them is fish. Many people do not even suspect that these representatives of the animal world have a brain. Read about its structure and features in the article.

History reference

For a long time, almost 70 million years ago, the oceans were inhabited by invertebrates. But fish, the first to acquire a brain, exterminated a significant number of them. Since then, they have dominated the water space. The modern fish brain is very complex. Indeed, it is difficult to follow some kind of behavior without a program. The brain solves this problem using different options. Fish preferred imprinting, when the brain is ready for the behavior that it sets at a certain point in its development.

For example, salmon have an interesting feature: they swim to spawn in the river in which they themselves were born. At the same time, they overcome huge distances, and they have no map. This is possible thanks to this variant of behavior, when certain parts of the brain are like a camera with a timer. The principle of operation of the device is as follows: there comes a moment when the diaphragm works. The images in front of the camera remain on the film. So it is with fish. They are guided in their behavior by images. Imprinting determines the individuality of fish. If given the same conditions, their different breeds will behave differently. In mammals, the mechanism of this mode of behavior, that is, imprinting, has been preserved, but the scope of its important forms has narrowed. In humans, for example, sexual skills have been preserved.

Parts of the brain in fish

This organ in this class is small. Yes, in a shark, for example, its volume is equal to thousandths of a percent of the total body weight, in sturgeon and bony fish - hundredths, in small fish it is about one percent. The brain of fish has a feature: the larger the individuals, the smaller it is.

The family of stickleback fish that live in Lake Mivan, Iceland, has a brain, the size of which depends on the sex of the individuals: the female is smaller, the male is larger.

The fish brain has five sections. These include:

• forebrain consisting of two hemispheres. Each of them is in charge of the sense of smell and schooling behavior of fish.
• midbrain, from which the nerves that respond to stimuli depart, due to which the eyes move. This is the eye of the fish. They regulate the balance of the body and muscle tone.
• Cerebellum- the body responsible for the movement.
• Medulla is the most important department. Performs many functions and is responsible for different reflexes.

The parts of the fish brain do not develop in the same way. This is influenced by the lifestyle of aquatic inhabitants and the state of the environment. So, for example, pelagic species, having excellent skills of movement in water, have a well-developed cerebellum, as well as vision. The structure of the brain of a fish is such that representatives of this class with a developed sense of smell are distinguished by an increased size of the forebrain, predators with good eyesight are medium, and sedentary representatives of the class are oblong.

Intermediate brain

He owes his education to which are also called the thalamus. Their location is the central part of the brain. The thalamus has many formations in the form of nuclei, which transmit the received information to the brain of the fish. There are various sensations associated with smell, sight, and hearing.

The main one is the integration and regulation of the body's sensitivity. It is also involved in the reaction by which fish are able to move around. If the thalamus is damaged, the level of sensitivity decreases, coordination is disturbed, and vision and hearing also decrease.

Brain anterior

It includes a mantle, as well as striatal bodies. The mantle is sometimes called a cloak. The location is the top and sides of the brain. The cloak looks like thin epithelial plates. are located below it. The forebrain of fish is designed to perform such functions as:

• Olfactory. If this organ is removed from fish, they lose the conditioned reflexes developed to stimuli. Physical activity decreases, attraction to the opposite sex disappears.
• Protective and defensive. It manifests itself in the fact that representatives of the Pisces class maintain a flock of life, take care of their offspring.

brain average

It has two departments. One of them is the visual roof, which is called the tectum. It is located horizontally. It looks like swollen visual lobes arranged in pairs. In fish with a high organization, they are better developed than in cave and deep-sea representatives with poor eyesight. Another department is located vertically, it is called the tegmentum. It contains the highest visual center. What are the functions of the midbrain?

• If you remove the visual roof from one eye, the other goes blind. The fish loses its sight when the roof is completely removed, in which the visual grasping reflex is located. Its essence lies in the fact that the head, body, eyes of the fish move in the direction of food objects, which are imprinted on the retina.
• The midbrain of the fish fixes the color. When the upper roof is removed, the body of the fish brightens, and if the eyes are removed, it darkens.
• It has connections with the forebrain and cerebellum. Coordinates the work of a number of systems: somatosensory, visual and olfactory.
• The composition of the middle part of the body includes centers that regulate movement and maintain muscle tone.
• The fish brain makes reflex activity diverse. First of all, this affects the reflexes associated with visual and auditory stimuli.

brain oblongata

He takes part in the formation of the organ trunk. The medulla oblongata of fish is arranged in such a way that substances, gray and white, are distributed without a clear boundary.

Performs the following functions:

• reflex. The centers of all reflexes are located in the brain, whose activity ensures the regulation of breathing, the work of the heart and blood vessels, digestion, and the movement of fins. Thanks to this function, the activity of the organs of taste is carried out.
• Conductor. It lies in the fact that the spinal cord and other parts of the brain conduct nerve impulses. The medulla oblongata is the site of the ascending tracts from the dorsal to the cephalic, which lead to the descending tracts that connect them.

Cerebellum

This formation, which has an unpaired structure, is located in the posterior part and partially covers the medulla oblongata. It consists of the middle part (body) and two ears (lateral sections).

Performs a number of functions:

• Coordinates movements and maintains normal muscle tone. If the cerebellum is removed, these functions are impaired, the fish begin to swim in circles.
• Provides the implementation of motor activity. When the body of the cerebellum of the fish is removed, it begins to swing in different directions. If you also remove the damper, the movements are completely disturbed.
• The cerebellum regulates metabolism. This organ influences other parts of the brain through the nucleoli located in the spinal cord and medulla oblongata.

Spinal cord

Its location is the nerve arcs (more precisely, their channels) of the fish spine, which consists of segments. The spinal cord in fish is a continuation of the medulla oblongata. Nerves extend from it to the right and left sides between pairs of vertebrae. Through them, irritating signals enter the spinal cord. They innervate the surface of the body, the muscles of the trunk and internal organs. What is the brain of a fish? Head and dorsal. The gray matter of the latter is inside it, the white is outside.

The brain of fish is very small, making up thousandths of % of body weight in sharks, hundredths of % in teleosts and sturgeons. In small fish, the mass of the brain reaches about 1%.

The brain of fish consists of 5 sections: anterior, intermediate, middle, cerebellum and medulla oblongata. The development of individual parts of the brain depends on the way of life of fish and their ecology. So, in good swimmers (mainly pelagic fish), the cerebellum and visual lobes are well developed. In fish with a well-developed sense of smell, the forebrain is enlarged. In fish with well-developed vision (predators) - the midbrain. Sedentary fish have a well-developed medulla oblongata.

The medulla oblongata is a continuation of the spinal cord. Together with the midbrain and diencephalon, it forms the brainstem. In the medulla oblongata, compared with the spinal cord, there is no clear distribution of gray and white matter. The medulla oblongata performs the following functions: conduction and reflex.

The conduction function is to conduct nerve impulses between the spinal cord and other parts of the brain. Through the medulla oblongata pass ascending paths from the spinal cord to the brain and descending paths connecting the brain with the spinal cord.

Reflex function of the medulla oblongata. In the medulla oblongata there are centers of both relatively simple and complex reflexes. Due to the activity of the medulla oblongata, the following reflex reactions are carried out:

1) regulation of breathing;

2) regulation of cardiac activity and blood vessels;

3) regulation of digestion;

4) regulation of the work of taste organs;

5) regulation of the work of chromatophores;

6) regulation of the work of electrical organs;

7) regulation of the centers of movement of the fins;

8) regulation of the spinal cord.

The medulla oblongata contains the nuclei of six pairs of cranial nerves (V-X).

V pair - the trigeminal nerve is divided into 3 branches: the ophthalmic nerve innervates the anterior part of the head, the maxillary nerve innervates the skin of the anterior part of the head and palate, and the mandibular nerve innervates the mucous membrane of the oral cavity and mandibular muscles.

VI pair - the opening nerve innervates the muscles of the eyes.

VII pair - the facial nerve is divided into 2 lines: the first innervates the lateral line of the head, the second - the mucous membrane of the palate, the hyoid region, the taste buds of the oral cavity and the muscles of the gill cover.

VIII pair - auditory or sensory nerve - innervates the inner ear and labyrinth.

IX pair - glossopharyngeal nerve - innervates the mucous membrane of the palate and the muscles of the first branchial arch.

X pair - the vagus nerve is divided into two branching branches: the lateral nerve innervates the organs of the lateral line in the trunk, the nerve of the operculum innervates the gill apparatus and other internal organs.

The midbrain of fish is represented by two sections: the visual roof (tectum) - located horizontally and the tegmentum - located vertically.

The tectum or visual roof of the midbrain is swollen in the form of paired visual lobes, which are well developed in fish with a high degree of development of the organs of vision and poorly developed in blind deep-sea and cave fish. On the inner side of the tectum there is a longitudinal torus. It is associated with vision. In the tegmentum of the midbrain, the highest visual center of fish is located. The fibers of the second pair of optic nerves terminate in the tectum.

The midbrain performs the following functions:

1) The function of the visual analyzer, as evidenced by the following experiments. After the removal of the textum on one side of the eye of the fish, the one lying on the opposite side becomes blind. When the entire tectum is removed, complete blindness occurs. The tectum also houses the center of the visual grasping reflex, which consists in the fact that the movements of the eyes, head, and torso are directed in such a way as to maximize the fixation of the food object in the region of greatest visual acuity, i.e. in the center of the retina. In the tectum there are centers of the III and IV pairs of nerves that innervate the muscles of the eyes, as well as muscles that change the width of the pupil, i.e. performing accommodation, allowing you to clearly see objects at different distances due to the movement of the lens.

2) Participates in the regulation of fish coloration. So, after the removal of the tectum, the body of the fish brightens, while when the eyes are removed, the opposite phenomenon is observed - darkening of the body.

3) In addition, the tectum is closely connected with the cerebellum, hypothalamus, and through them with the forebrain. Therefore, the tectum coordinates the functions of the somatosensory (balance, posture), olfactory, and visual systems.

4) The tectum is connected with the VIII pair of nerves, which perform acoustic and receptor functions, and with the V pair of nerves, i.e. trigeminal nerves.

5) Afferent fibers from the lateral line organs, from the auditory and trigeminal nerves approach the midbrain.

6) In the tectum there are afferent fibers from the olfactory and taste receptors.

7) In the midbrain of fish, there are centers for regulating movement and muscle tone.

8) The midbrain has an inhibitory effect on the centers of the medulla oblongata and spinal cord.

Thus, the midbrain regulates a number of vegetative functions of the body. Due to the midbrain, the reflex activity of the organism becomes diverse (orienting reflexes to sound and visual stimuli appear).

Intermediate brain. The main formation of the diencephalon is the visual tubercles - the thalamus. Under the visual tubercles is the hypothalamic region - the epithalamus, and under the thalamus is the hypothalamic region - the hypothalamus. The diencephalon in fish is partially covered by the roof of the midbrain.

The epithalamus consists of the epiphysis, a rudiment of the parietal eye, which functions as an endocrine gland. The second element of the epithalamus is the frenulum (gabenula), which is located between the forebrain and the roof of the midbrain. The frenulum is a link between the epiphysis and the olfactory fibers of the forebrain, i.e. participates in the performance of the function of light perception and smell. The epithalamus is connected to the midbrain through efferent nerves.

The thalamus (visual tubercles) in fish is located in the central part of the diencephalon. In the visual tubercles, especially in the dorsal part, many nuclear formations were found. The nuclei receive information from receptors, process it and transmit it to certain areas of the brain, where the corresponding sensations arise (visual, auditory, olfactory, etc.). Thus, the thalamus is an organ of integration and regulation of the body's sensitivity, and also takes part in the implementation of the body's motor reactions.

If the visual tubercles are damaged, a decrease in sensitivity, hearing, and vision is observed, which causes impaired coordination.

The hypothalamus consists of an unpaired hollow protrusion - a funnel that forms a vascular sac. The vascular sac responds to pressure changes and is well developed in deep sea pelagic fish. The vascular sac is involved in the regulation of buoyancy, and through its connection with the cerebellum, it is involved in the regulation of balance and muscle tone.

The hypothalamus is the main center for receiving information from the forebrain. The hypothalamus receives afferent fibers from taste endings and from the acoustic system. Efferent nerves from the hypothalamus go to the forebrain, to the dorsal thalamus, tectum, cerebellum and neurohypophysis, i.e. regulates their activities and influences their work.

The cerebellum is an unpaired formation, it is located in the back of the brain and partially covers the medulla oblongata. Distinguish between the body of the cerebellum (middle part) and the ears of the cerebellum (i.e., two lateral sections). The anterior end of the cerebellum forms a flap.

In fish leading a sedentary lifestyle (for example, in bottom ones, such as scorpions, gobies, anglers), the cerebellum is underdeveloped in comparison with fish leading an active lifestyle (pelagic, such as mackerel, herring or predators - pike perch, tuna, pike).

Functions of the cerebellum. With the complete removal of the cerebellum in moving fish, a drop in muscle tone (atony) and impaired coordination of movements are observed. This was expressed in the circular swimming of fish. In addition, the reaction to pain stimuli weakens in fish, sensory disturbances occur, and tactile sensitivity disappears. Approximately, after three to four weeks, the lost functions are restored due to the regulatory processes of other parts of the brain.

After removal of the body of the cerebellum, bony fish show motor disturbances in the form of body swaying from side to side. After removal of the body and the valve of the cerebellum, motor activity is completely disrupted, and trophic disorders develop. This indicates that the cerebellum also regulates metabolism in the brain.

It should be noted that the auricles of the cerebellum reach large sizes in fish with a well-developed lateral line. Thus, the cerebellum is the site of closure of conditioned reflexes coming from the lateral line organs.

Thus, the main functions of the cerebellum are the coordination of movement, the normal distribution of muscle tone and the regulation of autonomic functions. The cerebellum realizes its influence through the nuclear formations of the middle and medulla oblongata, as well as the motor neurons of the spinal cord.

The forebrain of fish consists of two parts: the mantle or cloak and the striatum. The mantle, or the so-called cloak, lies dorsally, i.e. from above and from the sides in the form of a thin epithelial plate above the striatum. In the anterior wall of the forebrain are the olfactory lobes, which are often differentiated into the main part, stalk and olfactory bulb. Secondary olfactory fibers from the olfactory bulb enter the mantle.

Functions of the forebrain. The forebrain of fish performs an olfactory function. This, in particular, is evidenced by the following experiments. When the forebrain is removed, fish lose the developed conditioned reflexes to olfactory stimuli. In addition, the removal of the forebrain of fish leads to a decrease in their motor activity and to a decrease in schooling conditioned reflexes. The forebrain also plays an important role in the sexual behavior of fish (when it is removed, sexual desire disappears).

Thus, the forebrain is involved in the protective-defensive reaction, the ability to swim in schools, the ability to take care of offspring, etc. It has a general stimulating effect on other parts of the brain.

7. Principles of the reflex theory I.P. Pavlova

Pavlov's theory is based on the basic principles of the conditioned reflex activity of the brain of animals, including fish:

1. The principle of structure.

2. The principle of determinism.

3. The principle of analysis and synthesis.

The principle of structurality is as follows: each morphological structure corresponds to a specific function. The principle of determinism is that reflex reactions have a strict causality, i.e. they are determined. For the manifestation of any reflex, a reason, a push, an impact from the outside world or the internal environment of the body is necessary. The analytical and synthetic activity of the central nervous system is carried out due to the complex relationship between the processes of excitation and inhibition.

According to Pavlov's theory, the activity of the central nervous system is based on a reflex. A reflex is a causally determined (deterministic) reaction of the body to changes in the external or internal environment, carried out with the obligatory participation of the central nervous system in response to irritation of the receptors. This is how the emergence, change or cessation of any activity of the body occurs.

Pavlov divided all the reflex reactions of the body into two main groups: unconditioned reflexes and conditioned reflexes. Unconditioned reflexes are congenital, inherited reflex reactions. Unconditioned reflexes appear in the presence of a stimulus without special, special conditions (swallowing, breathing, salivation). Unconditioned reflexes have ready-made reflex arcs. Unconditioned reflexes are divided into various groups according to a number of characteristics. On a biological basis, food (search, intake and processing of food), defensive (defensive reaction), sexual (animal behavior), indicative (orientation in space), positional (taking a characteristic posture), locomotor (motor reactions) are distinguished.

Depending on the location of the irritated receptor, exteroceptive reflexes are isolated, i.e. reflexes that occur when the outer surface of the body (skin, mucous membranes) is irritated, interoreceptive reflexes, i.e. reflexes that occur when irritated by internal organs, proprioceptive reflexes that occur when receptors of skeletal muscles, joints, and ligaments are irritated.

Depending on the part of the brain that is involved in the reflex reaction, the following reflexes are distinguished: spinal (spinal) - centers of the spinal cord participate, bulbar - centers of the medulla oblongata, mesencephalic - centers of the midbrain, diencephalic - centers of the diencephalon.

In addition, reactions are divided according to the organ that is involved in the response: motor or motor (muscle participates), secretory (endocrine or external secretion gland participates), vasomotor (vessel participates), etc.

Unconditioned reflexes - specific reactions. They are common to all representatives of this species. Unconditioned reflexes are relatively constant reflex reactions, stereotyped, little changeable, inert. As a result of this, it is impossible to adapt to the changing conditions of existence only due to unconditioned reflexes.

Conditioned reflexes - a temporary nervous connection of the body with some stimulus of the external or internal environment of the body. Conditioned reflexes are acquired during the individual life of the organism. They are not the same in different representatives of this species. Conditioned reflexes do not have ready-made reflex arcs, they are formed under certain conditions. Conditioned reflexes are changeable, easily arise and also easily disappear, depending on the conditions in which the given organism is located. Conditioned reflexes are formed on the basis of unconditioned reflexes under certain conditions.

For the formation of a conditioned reflex, a combination in time of two stimuli is necessary: ​​an indifferent (indifferent) for a given type of activity, which will later become a conditioned signal (knocking on glass) and an unconditioned stimulus that causes a certain unconditioned reflex (food). The conditioned signal always precedes the action of the unconditioned stimulus. Reinforcement of the conditioned signal with an unconditioned stimulus must be repeated. It is necessary that the conditioned and unconditioned stimuli meet the following requirements: the unconditioned stimulus must be biologically strong (food), the conditioned stimulus must have a moderate optimal strength (knock).

8. Behavior of fish

The behavior of fish becomes more complicated in the course of their development, i.e. ontogeny. The simplest reaction of the body of a fish in response to an irritant is kinesis. Kinesis is an increase in motor activity in response to adverse effects. Kinesis is observed already at the last stages of the embryonic development of fish, when the oxygen content in the environment decreases. An increase in the movement of larvae in eggs or in water in this case improves gas exchange. Kinesis promotes the movement of larvae from poor living conditions to better ones. Another example of kinesis is the erratic movement of schooling fish (verkhovka, uklya, etc.) when a predator appears. This confuses him and prevents him from focusing on one fish. This can be considered a defensive reaction of schooling fish.

A more complex form of fish behavior is taxis - this is a directed movement of fish in response to a stimulus. A distinction is made between positive taxis (attraction) and negative taxis (avoidance). An example is phototaxis, i.e. reaction of fish to the light factor. Thus, anchovy and big-eyed kilka have positive phototaxis, i.e. are well attracted to the light, forming clusters, which makes it possible to use this property in the fishery of these fish. In contrast to the Caspian sprat, the mullet exhibits negative phototaxis. Representatives of this species of fish tend to get out of the illuminated background. This property is also used by humans when fishing for this fish.

An example of negative phototaxis is the behavior of salmon larvae. During the day, they hide among stones, in gravel, which allows them to avoid meeting with predators. And in the larvae of cyprinids, positive phototaxis is observed, which allows them to avoid deadly deep-sea areas and find more food.

The direction of taxis may undergo age-related changes. Thus, fry of salmon at the stage of the pestryanka are typical benthic sedentary fish that protect their territory from their own kind. They avoid light, live among stones, easily change color to the color of the environment, and when frightened, they are able to hide. As they grow in front of a slope in the sea, they change color to non-silver, gather in flocks, lose their aggressiveness. When frightened, they quickly swim away, are not afraid of light, and vice versa, stay near the surface of the water. As you can see, the behavior of juveniles of this species changes to the opposite with age.

In fish, unlike higher vertebrates, there is no cerebral cortex, which plays a leading role in the development of conditioned reflexes. However, fish are able to produce them without it, for example, a conditioned reflex to sound (Frolov's experiment). After the action of a sound stimulus, a current was switched on in a few seconds, to which the fish reacted by moving its body. After a certain number of repetitions, the fish, without waiting for the electric current, reacted to the sound, i.e. reacted with body movements. In this case, the conditioned stimulus is the sound, and the unconditioned stimulus is the induction current.

In contrast to higher animals, fish develop reflexes worse, they are unstable and difficult to develop. Fish are less able than higher animals to differentiate, i.e. distinguish between conditioned stimuli or changes in the external environment. It should be noted that in bony fish conditioned reflexes are developed faster and they are more persistent than in others.

There are works in the literature that show rather persistent conditioned reflexes, where the unconditioned stimuli are a triangle, a circle, a square, various letters, etc. If a feeder is placed in a pond that gives a portion of food in response to pressing a lever, pulling a bead or other devices, then the fish master this device quickly enough and receive food.

Those who are engaged in aquarium fish farming, they have observed that when approaching the aquarium, the fish gather at the feeding place in anticipation of food. This is also a conditioned reflex, and in this case, you are the conditioned stimulus, and knocking on the glass of the aquarium can also serve as a conditioned stimulus.

In hatcheries, the fish are usually fed at certain times of the day, so they often gather at certain places at the time to feed. The fish also quickly get used to the type of food, the way food is distributed, etc.

Of great practical importance may be the development of conditioned reflexes to a predator in the conditions of fish hatcheries and NVH in juveniles of commercial fish, which are then released into natural reservoirs. This is due to the fact that in the conditions of fish hatcheries and NVH, juveniles do not have the experience of communicating with enemies and at the first stages become the prey of predators until they get an individual and spectacular experience.

Using conditioned reflexes, various aspects of the biology of various fish are explored, such as the spectral sensitivity of the eye, the ability to distinguish silhouettes, the effect of various toxicants, the hearing of fish by the strength and frequency of sound, the thresholds of taste sensitivity, the role of various parts of the nervous system.

In the natural environment, the behavior of fish depends on the lifestyle. Schooling fish have the ability to coordinate maneuvers when feeding, at the sight of a predator, etc. Thus, the appearance of a predator or food organisms at one edge of the flock causes the entire flock to react accordingly, including individuals that did not see the stimulus. The reaction can be very diverse. So at the sight of a predator, the flock instantly scatters. You can observe this in the spring time in the coastal zone of our reservoirs, fry of many fish concentrate in flocks. This is one kind of imitation. Another example of imitation is following the leader, i.e. for an individual in whose behavior there are no elements of oscillation. The leader is most often individuals who have great individual experience. Sometimes even a fish of a different species can serve as such a leader. So, carps learn to take food on the fly faster if they are planted with trout or carp individuals that can do this.

When fish live in groups, a “social” organization can arise with dominant and subordinate fish. So, in a flock of Mozambian tilapia, the most intensely colored male is the main one, the next in the hierarchy are lighter. Males, which do not differ from females in color, are subordinate and do not participate in spawning at all.

The sexual behavior of fish is very diverse, this includes elements of courtship and rivalry, building nests, etc. Complex spawning and parental behavior is typical for fish with low individual fecundity. Some fish take care of eggs, larvae and even fry (protect the nest, aerate the water (zander, smelt, catfish)). Juveniles of some fish species feed near their parents (for example, discus even feed their juveniles with their mucus). Juveniles of some fish species hide with their parents in the oral and gill cavities (tilapia). Thus, the plasticity of fish behavior is very diverse, as can be seen from the above materials.

Questions for self-control:

1. Features of the structure and function of nerves and synapses.

2. Parabiosis as a special kind of localized excitation.

3. Scheme of the structure of the nervous system of fish.

4. Structure and functions of the peripheral nervous system.

5. Features of the structure and function of the brain.

6. Principles and essence of the reflex theory.

7. Features of the behavior of fish.

Bony fish are the largest class of vertebrates, with about 20,000 species. The most ancient representatives of this class originated from cartilaginous fish at the end of the Silurian. At present, 99% of the class belong to the so-called bony fish, which first appeared in the middle of the Triassic, but their evolution was slow for a long time and only at the end of the Cretaceous accelerated sharply and reached an amazing flowering in the Tertiary period. They inhabit a wide variety of water bodies (rivers, seas and oceans down to the greatest depth, found in arctic waters). Thus, bony fish are vertebrates most adapted to living in the aquatic environment. In addition to bony fish, the class also includes several dozen species of ancient bony fish that have retained some features of cartilaginous fish.

general characteristics

Most of the species in this class are adapted for fast swimming, and their body shape is similar to that of sharks. Less fast swimming fish have a higher body (for example, in many species of cyprinid fish). Species that lead a sedentary lifestyle on the bottom (for example, flounders) have the same flattened body shape as skates.

Bony fish:

1 - herring (herring family); 2 - salmon (fam. Salmon); 3 - carp (family Cyprinidae); 4- catfish (fam. Catfish); 5 - pike (fam. Pike); 6- eel (fam. Acne);

7 - pike perch (fam. Perch); 8 - river goby (family Goby); 9 - flounder (flounder family)

Covers. The length of the body of fish is different - from a few centimeters to several meters. In contrast to the cartilaginous and ancient bony fishes, among the bony fishes there are many small species that have mastered small biotopes that are inaccessible to larger species. The skin of the vast majority of bony fish is covered with small bony, relatively thin scales overlapping each other in a tiled manner. They protect the fish well from mechanical damage and provide sufficient body flexibility. There are cycloid scales with a rounded upper edge and ctenoid scales with small teeth on the upper edge. The number of scales in the longitudinal and transverse rows for each species is more or less constant and is taken into account when determining the species of fish. In cold weather, the growth of fish and scales slows down or stops, so annual rings form on the scales, counting which you can determine the age of the fish. In a number of species, the skin is bare, devoid of scales. There are many glands in the skin, the mucus they secrete reduces friction when swimming, protects against bacteria, etc. In the lower layers of the epidermis there are various pigment cells, due to which the fish are hardly noticeable against the background of their environment. In some species, body color may change in accordance with changes in the color of the substrate. Such changes are carried out under the influence of nerve impulses.

Nervous system. The size of the brain in relation to the size of the body is somewhat larger than that of cartilaginous fish. The forebrain is relatively small in comparison with other parts, but its striatal bodies are large and, through their connections with other parts of the central nervous system, influence the implementation of some rather complex forms of behavior. There are no nerve cells in the roof of the forebrain. The diencephalon and the epiphysis and pituitary gland separated from it are well developed. The midbrain is larger than other parts of the brain, in its upper part there are two well-developed visual lobes. The cerebellum in well-swimming fish is large. The size increased and the structure of the medulla oblongata and spinal cord became more complicated. The subordination of the latter to the brain, compared to what is observed in cartilaginous fish, has increased

Perch brain:

1 - olfactory capsule; 2 - olfactory lobes; 3 - forebrain; 4 - midbrain; 5 - cerebellum; 6 - medulla oblongata; 7 - spinal cord; 8 - ophthalmic branch of the trigeminal nerve; 9 - auditory nerve; 10 - vagus nerve

Skeleton. During the evolution of the class under consideration, the skeleton gradually ossified. The notochord was preserved only among the lower representatives of the class, the number of which is insignificant. When studying the skeleton, it must be borne in mind that some bones arise as a result of the replacement of cartilage with bone tissue, while others develop in the connective tissue layer of the skin. The first are called the main, the second - integumentary bones.

The medulla of the skull is a box that protects the brain and sense organs: smell, vision, balance and hearing.

Diagram of the arrangement of bones in the skull of a bony fish. The visceral skeleton is separated from the cerebral skull. The gill cover is not drawn. The main bones and cartilage are covered with dots, the integumentary bones are white:

/ - angular; 2 - articular; 3 - main occipital; 4 - main wedge-shaped; 5 - copula; 6 - tooth; 7 - lateral olfactory; 8 - external pterygoid; 9 - internal pterygoid; 10 - lateral occipital; 11 - frontal; 12 - pendants; 13 - hyoid; 14 - ossified ligament; 15 - lateral wedge-shaped; 16 - middle olfactory; 17 - posterior pterygoid; 18 - maxillary; 19 - nasal; 20 - eye wedge-shaped; 21 - parietal; 22 - palatine; 23 - premaxillary; 24 - parasphenoid; 25 - square; 26 - upper occipital; 27 - additional; 28 - coulter; 29-33 - ear bones; I-V - gill arches

The roof of the skull is formed by paired nasal, frontal, and parietal bones. The latter are adjacent to the superior occipital bone, which, together with the paired lateral occipital bones and the main occipital bone, forms the back of the skull. The underside of the skull consists (from front to back) of the vomer, parasphenoid (a wide long bone very characteristic of the skull of fish) and the basal bone. The front part of the skull is occupied by a capsule protecting the organs of smell; on the sides are the bones that surround the eyes, and a number of bones that protect the organs of hearing and balance.

The visceral part of the skull consists of a series of bony gill arches, which are the support and protection of the gill apparatus and the anterior part of the digestive system. Each of the mentioned arcs includes several bones. The arcs to which the gills are attached, in most fish (on each side). At the bottom, the gill arches are interconnected, and the anterior one is connected to the hyoid arch, which consists of several bones. The upper of these bones - the hyoid-maxillary (hyomandibular) is attached to the brain region of the skull in the region of the auditory region and is connected through the square bone with the bones surrounding the oral cavity. Thus, the hyoid arch serves to connect the gill arches to the rest of the visceral region, and its upper bone to the brain region of the skull.

The edges of the mouth and the entire oral cavity are reinforced with a series of bones. The maxillary row of bones is represented (on each side) by the premaxillary and maxillary bones. Next comes a series of bones: palatine, several pterygoid and square. The quadrate bone adjoins the suspension (hyomandibular) at the top, and the lower jaw at the bottom. The latter consists of several bones: the dentary (the largest), the angular and the articular, connected to the square bone. In ancient fish (which still had a cartilaginous skeleton), all arches of the visceral part of the skull carried gills, but later the anterior of these arches turned into hyoid arches and jaw rows of bones.

The vertebral column consists of a large number of biconcave (amphicoelous) vertebrae, between which remains of the notochord are preserved. From each vertebra a long spinous process extends upward and somewhat backward. The bases of these processes are divided, and they form a canal through which the spinal cord passes. Two short transverse processes extend from the underside of the vertebral bodies, to which long curved ribs are attached in the trunk region. They freely terminate in the muscles and form the frame of the side walls of the body. In the caudal part of the body, only the lower spinous processes extend downward from the vertebrae.

Bibliography

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127. Draw a diagram of the external structure of the fish. Sign its main parts.

128. List the features of the structure of fish associated with the aquatic lifestyle.
1) A streamlined torpedo-shaped body, flattened in the lateral or dorsal-ventral (in demersal fish) directions. The skull is fixedly connected to the spine, which has only two sections - the trunk and tail.
2) Bony fish have a special hydrostatic organ - the swim bladder. As a result of a change in its volume, the buoyancy of the fish changes.
In cartilaginous fish, buoyancy of the body is achieved by accumulation in the liver, less often in other organs, of fat reserves.
3) The skin is covered with placoid or bone scales, rich in glands that abundantly secrete mucus, which reduces the friction of the body against water and performs a protective function.
4) Respiratory organs - gills.
5) Two-chambered heart (with venous blood), consisting of an atrium and a ventricle; one circle of blood circulation. Organs and tissues are supplied with arterial blood rich in oxygen. The life of fish depends on the temperature of the water.
6) Trunk kidneys.
7) The sense organs of fish are adapted to functioning in the aquatic environment. A flat cornea and an almost spherical lens allow the fish to see only close objects. The sense of smell is well developed, allows you to stay in the flock and detect food. The organ of hearing and balance is represented only by the inner ear. The lateral line organ allows one to navigate in water currents, to perceive the approach or removal of a predator, prey or pack partner, and to avoid collision with underwater objects.
8) Most have external fertilization.

129. Fill in the table.

Fish organ systems.

130. Look at the picture. Write the names of the sections of the fish skeleton, indicated by numbers.

1) skull bones
2) spine
3) tail fin rays
4) ribs
5) rays of the pectoral fin
6) gill cover

131. In the drawing, color the organs of the fish digestive system with colored pencils and sign their names.

132. Sketch and label the parts of the circulatory system of a fish. What is the importance of the circulatory system?

The circulatory system of fish provides the movement of blood, which delivers oxygen and nutrients to the organs and removes metabolic products from them.

133. Study the table “Superclass Pisces. Perch structure. Consider the drawing. Write the names of the internal organs of the fish, indicated by numbers.

1) kidney
2) swim bladder
3) bladder
4) ovary
5) intestines
6) stomach
7) liver
8) heart
9) gills.

134. Look at the picture. Sign the names of the parts of the fish brain and parts of the nervous system, indicated by numbers.

1) brain
2) spinal cord
3) nerve
4) forebrain
5) midbrain
6) cerebellum
7) medulla oblongata

135. Explain how the structure and location of the nervous system of fish differ from the nervous system of hydra and beetle.
In fish, the nervous system is much more developed than in the hydra and the beetle. There is a dorsal and head mogz, consisting of departments. The spinal cord is located in the spine. Hydra has a diffuse nervous system, that is, it consists of cells scattered in the upper layer of the body. The beetle has a ventral nerve cord, with an extended oglo-pharyngeal ring and supra-oesophageal ganglion at the head end of the body, but no brain as such.

136. Complete the laboratory work "The external structure of the fish."
1. Consider the features of the external structure of the fish. Describe the shape of her body, the color of her back and abdomen.
The fish has a streamlined oblong body shape. The color of the abdomen is silver, the back is darker.
2. Make a drawing of the body of the fish, sign its departments.
See question #127.
3. Consider the fins. How are they located? How many? Write the names of the fins on the picture.
The fins of the fish are paired: ventral, anal, pectoral and unpaired: caudal and dorsal.
4. Examine the head of the fish. What sense organs are located on it?
On the head of the fish are eyes, taste buds in the mouth and on the surface of the skin, nostrils. In the head section there are 2 openings of the inner ear, on the border between the head and the body there are gill covers.
5. Look at the fish scales under a magnifying glass. Calculate the lines of annual growth and determine the age of the fish.
Scales bony, translucent, covered with mucus. The number of lines on the scales corresponds to the age of the fish.
6. Write down the features of the external structure of the fish associated with the aquatic lifestyle.
see question #128

Nervous system of fish divided by peripheral and central. central nervous system consists of the brain and spinal cord, and peripheral- from nerve fibers and nerve cells.

The brain of fish.

fish brain consists of three main parts: forebrain, midbrain and hindbrain. forebrain consists of the telencephalon ( telencephalon) and diencephalon - diencephalon. At the anterior end of the telencephalon are bulbs responsible for the sense of smell. They receive signals from olfactory receptors.

Schematic of the olfactory chain in fish can be described as follows: in the olfactory lobes of the brain there are neurons that are part of the olfactory nerve or a pair of nerves. Neurons join the olfactory tracts of the telencephalon, which are also called the olfactory lobes. Olfactory bulbs are particularly prominent in fish that use the senses, such as sharks, which survive on scent.

Diencephalon consists of three parts: epithalamus, thalamus and hypothalamus and performs the functions of a regulator of the internal environment of the fish body. The epithalamus contains the pineal organ, which in turn consists of neurons and photoreceptors. pineal organ located at the end of the epiphysis and in many fish species it can be sensitive to light due to the transparency of the skull bones. Due to this, the pineal organ can act as a regulator of activity cycles and their change.

The midbrain of fish contains visual lobes and tegmentum or a tire - both are used to process optical signals. The optic nerve of fish is very branched and has many fibers extending from the visual lobes. As with the olfactory lobes, enlarged visual lobes can be found in fish that rely on vision to survive.

The tegmentum in fish controls the internal muscles of the eye and thus ensures its focus on the object. Also, the tegmentum can act as a regulator of active control functions - it is here that the locomotor region of the midbrain is located, which is responsible for rhythmic swimming movements.

The hindbrain of fish is made up of cerebellum, elongated brain and bridge. The cerebellum is an unpaired organ that performs the function of maintaining balance and controlling the position of the body of the fish in the environment. The medulla oblongata and the pons together make up brain stem, to which a large number of cranial nerves that carry sensory information stretch. Most of all nerves communicate with and enter the brain through the brainstem and hindbrain.

Spinal cord.

Spinal cord is located inside the neural arches of the vertebrae of the fish spine. The spine has segmentation. In each segment, neurons connect to the spinal cord via dorsal roots, and agile neurons exit them via ventral roots. Within the central nervous system are also interneurons that provide communication between agile and sensory neurons.