Structure of freshwater hydra drawing. Freshwater polyp hydra (characteristics)

Occur in one cell. In the body of hydra and all other multicellular animals different groups cells have different meaning or, as they also say, various functions.

Structure

The structure of hydra can be different due to the cells that perform different functions. Groups of cells that have the same structure and perform a specific function in the life of an animal are called tissues. The body of the hydra has developed tissues such as integumentary, muscle and nervous. However, these tissues do not form in its body those complex organs that other multicellular animals have. Thus, hydra is the lowest, that is, the most simple in structure, multicellular animal.

In worms and other animals more complex than the freshwater hydra, organs are formed from tissues. From the bodies performing general function in the life of an animal, organ systems are formed in the body of animals (for example, nervous system, circulatory system etc.). Hydra does not have organ systems. Hydra reproduces in two ways: sexual and asexual.

nettle cells

To understand why Daphnia, touching the tentacles freshwater hydra, are paralyzed, it is necessary to examine the structure of the tentacle under a microscope. The entire surface of the tentacle is covered with tiny knotty tubercles. These are special cells that look like bubbles. There are also such cells on the edges of the hydra’s body, but most of them are on the tentacles. The bubbles contain thin threads with points at the ends sticking out. When the prey touches the hydra's body, the thread calm state coiled in a spiral, they are suddenly thrown out of their bubbles and, like arrows, pierce the body of the prey. At the same time, a drop of poison is poured from the bottle into the wound, paralyzing the victim. Hydra cannot attack the relatively thick skin of humans and large animals. But in the seas live animals related to the hydra - sea jellyfish. Large jellyfish can cause severe burns to humans. They burn the skin like nettles. Therefore, these cells are called nettle cells, and the threads are called nettle filaments. Hydra nettle cells are not only an organ of attack on prey, but also an organ of defense.

Muscle cells

Some cells of the outer layer of the hydra's body with inside continued by narrow muscular processes. These processes are located along the body of the hydra. They are capable of contracting. The rapid contraction of the hydra into a small ball in response to irritation occurs precisely due to the contraction of these muscle processes. Cells with such processes are called integumentary muscles. In the life of a hydra, they play the same role as muscles in a person. Thus, the outer cells of the hydra protect it and help it move.

Nerve cells

Hydra perceives irritations by sensitive cells located in the ectoderm (outer layer). These irritations are transmitted through nerve cells located in the integumentary layer, closer to the base of the integumentary muscle cells, on the supporting membrane, connecting with each other. Nerve cells form the nervous network. This network is the rudiment of the nervous system.

From sensitive cells, irritation (for example, from touching with a needle or stick) is transmitted nerve cells and spreads throughout nerve network hydra. From the nervous network, irritation passes to the integumentary muscle cells. Their processes contract, and the entire body of the hydra contracts accordingly. This is how the hydra responds to external irritations. The contraction of the hydra's body when touched has a protective value.

Digestive cells

The cells of the digestive layer are much larger than the cells of the integumentary layer. On their inner part, facing the intestinal cavity, these cells have long flagella. Moving, the flagella mix food particles trapped inside the intestinal cavity. Digestive cells secrete juice that digests food. Digested food is absorbed by the cells of the digestive layer, and from them enters all cells of the body. Undigested food remains are thrown out through the mouth.

Along with plants, untreated soil, water and, most often, live food from a natural reservoir, various animals enter the aquarium, many of which cause significant damage to its inhabitants. These animals do not cause diseases in fish in the classical sense, but are often the cause of their death or the death of their offspring. However, do not rush to classify them as your own enemies - they are dangerous only for the inhabitants of the aquarium, and for a truly inquisitive person they can become objects of observation and even scientific discoveries. And, probably, the first in this series should be called the hydra.

Hydra is a typical representative of coelenterates, standing at the very base of the evolutionary tree of multicellular animals.

It was discovered by the greatest naturalist of the 17th-18th centuries, Antonie van Leeuwenhoek, with the help of his amazing microscopes. But this unique animal did not attract the attention of the animals. And it is unknown how long the hydra would have remained in obscurity if, in 1740, the thirty-year-old Swiss teacher Tremblay had not discovered this amazing creature. To get to know it better, the inquisitive teacher divided it into two parts. From one piece, which he called the “head”, a new body grew, and on the other - a new “head”. In fourteen days, two new living organisms were formed from the two halves.

After this discovery, Tremblay began a deep and serious study of Hydra. He presented the results of his research in the book “Memoirs on the history of a genus of freshwater polyps with arms in the form of horns” (1744).

However, simple observations of the behavior and reproduction (budding) of the animal, of course, could not satisfy the naturalist, and he began conducting experiments to test his assumptions.

One of Tremblay's most famous experiments is that, with the help of a pork bristle, he turned the hydra inside out, that is, its inner side became outer. After this, the animal lived as if nothing had happened, but, as it turned out, not at all because, after turning inside out, the outer side began to perform the functions of the inner one, but because the cells of the inner layer, which had previously been outer, leaked through the new outer layer and took their original place.

In his other experiments, Tremblay crushed the hydra more and more, but it was restored each time, and there was no limit to this. It is now known that the hydra is capable of recovering from 1/200 of its body. And then this amazed even the most venerable scientists and prompted them to study such a biological problem as regeneration.

About 250 years have passed since Tremblay's experiments on hydra. Hundreds of articles and books have been written about the hydra, but to this day it occupies the minds of researchers.

It is well known that animals do not react in any way to radioactive rays and, if they get into their zone, they can receive a lethal dose and die. Experiments with the green hydra (Chlorohydra viridissima) showed that it somehow senses mortal danger and seeks to move away from the source of radiation.

The death of the hydra is also caused by too high a dose of X-rays; reducing the dose leaves it alive, but suppresses reproduction. But small dosages have a completely unexpected effect on animals; their budding process is enhanced and their ability to self-heal increases.

The results of experiments with painting the wall of an aquarium in all colors of the spectrum were surprising. It turned out that hydras, which do not have any organs of vision, distinguish colors, and each species prefers its own: green hydras, for example, “love” blue-violet color, brown ones (Hydra oligactis) - blue-green.

What is a hydra? Outwardly, it resembles a glove placed vertically, fingers up, only it has from 5 to 12 tentacle fingers. In most species, immediately under the tentacles there is a slight narrowing that separates the “head” from the body. In the head of the hydra there is a mouth opening leading to the gastric cavity. The body walls of hydra, like all coelenterates, are two-layered. The outer layer consists of several types of ec cells: dermal-muscular, causing the hydra to move; nervous, giving her the opportunity to feel touch, temperature changes, the presence of impurities in water and other irritants; intermediate, most actively involved in the restoration of damaged or lost parts of the body; and finally, stinging ones, located mostly on the tentacles.

Coelenterates are the only group of animals that have such a weapon as stinging cells. In addition to the protoplasm required for all living cells, the stinging cell contains a bubble-like capsule, inside which the stinging thread is coiled.

Having attached its sole to some substrate, the hydra spreads its tentacles, which are in constant motion. When a victim is detected, the stinging thread of each of the stinging cells quickly straightens and plunges its sharp end into the prey. Through a channel running inside the thread, poison enters the body of the prey from the stinging capsule, causing its death. The stinging capsule can only be used once; The hydra discards the discharged capsule and replaces it with a new one, which is formed from special cells.

Digestion of food is carried out by the inner layer of cells: they secrete digestive juice into the gastric cavity, under the influence of which the hydra’s prey softens and disintegrates into small particles. The end of the cell of the inner layer, facing the gastric cavity, is equipped, like flagellated protozoans, with several long flagella, which are in constant motion and rake particles to the cells. Like an amoeba, the cells of the inner layer are able to release pseudopods and capture food with them. Further digestion occurs , like protozoa, inside the cell, in digestive vacuoles.

Those scientists who believed that, as a true predator, hydra feeds only on animals turned out to be right. Detailed studies have established that hydra digests fats, proteins and carbohydrates only of animal origin.

Hydras reproduce in two ways - vegetative and sexual. Vegetative propagation occurs by budding. Having separated from the mother's body, young hydras begin to live independently.

After abundant budding, the hydra is exhausted, and for some time no buds are formed on it. But when good nutrition it quickly restores its resources and begins to bud again. Over the five summer months, it is capable of producing thirty generations of twenty-five young hydras each. Reproduction by budding occurs when favorable situation.

With the coming unfavorable conditions- during autumn cold, drought, waterlogging, excess carbon dioxide- Hydra switches to sexual reproduction. Most species are dioecious, but there are species in which both male and female gonads are formed in the body.

The gonads are found in the outer layer of cells. In females, they look like spherical bodies, each of which contains one egg, similar to an amoeba; it grows quickly, eating the intermediate cells surrounding it, and reaches a diameter of one and a half millimeters. The grown egg is rounded and divided into two unequal parts, as a result of which the number of chromosomes in the nucleus of the egg is halved. The mature egg emerges from the gonad through a gap in its wall, but remains connected to the body of the hydra with the help of a thin stalk.

At the same time, sperm are formed in the male gonads of other hydras, appearance resembling flagellated protozoa. Leaving the gonads. they swim with the help of a long rope and, finally, one of the sperm, having found the egg, penetrates it. Immediately after this, crushing begins.

The hydra embryo is covered on the outside with two shells, the outer of which is quite thick and permeated with chitin. Under such protection, he successfully endures unfavorable conditions. With the onset of spring warming, the rainy season, etc., the young hydra breaks the wall of the protective shell and begins an independent life.

If you want to watch a hydra, place it in an aquarium where there are no other inhabitants, otherwise small animals that serve as food for fish will be eaten, and most importantly, the larvae and fry will be destroyed. Once in a spawning tank or nursery aquarium, the hydra, quickly multiplying by budding, will immediately deal with the young fish.

But it is not advisable to use these animals to fight hydra in an aquarium: trichodins and planaria are also enemies of fish. and getting hydramoebas and anchistropus crustaceans is not easy. Hydras have another enemy - the freshwater mollusk pond snail. but it is also not suitable, since it is a carrier of some fish diseases and also likes to feast on delicate aquatic plants.

Some hobbyists put hungry young gourami in an aquarium where the hydra has entered. Others fight it using the peculiarities of its behavior. Thus, hydras like to settle in the most illuminated areas of the aquarium. It is enough to shade the aquarium on all sides except one, and lean glass against the only illuminated wall, and in two or three days almost all the hydras will gather on it. Then the glass must be removed and cleaned.

Hydras are very sensitive to the presence of copper in water. One of the methods of combating is based on placing a ball of copper wire without insulation over the sprayer. After all the hydras have died, the wire is removed from the aquarium.

Some have successfully used chemicals:

ammonium sulfate at the rate of 5 grams per 100 liters of water, once,

ammonium nitrate - 6 grams per 100 liters of water, three times, with an interval of three days;

hydrogen peroxide (in an aquarium without plants with sufficient artificial aeration) at the rate of two teaspoons per 10 liters of water. The required amount of 3% solution is first diluted in 200-300 milliliters of water, and then slowly poured into the aquarium over a working sprayer.

To make the fight against hydra more effective, you need to use not one, but two or even three methods simultaneously.

References

S. Sharaburin. Hydra.

Hydra biology description internal structure photo lifestyle food reproduction protected from enemies

Latin name Hydrida

To characterize the structure hydroid polyp We can use freshwater hydras as an example, which retain very primitive organizational features.

External and internal structure

Hydras They have an elongated, sac-like body, capable of stretching quite strongly and shrinking almost into a spherical lump. A mouth is placed at one end; this end is called the oral or oral pole. The mouth is located on a small elevation - the oral cone, surrounded by tentacles that can stretch and shorten very strongly. When extended, the tentacles are several times the length of the hydra's body. The number of tentacles varies: there can be from 5 to 8, and some hydras have more. In Hydra, there is a central gastric section, which is somewhat more expanded, turning into a narrowed stalk ending in a sole. With the help of the sole, the hydra attaches to the stems and leaves of aquatic plants. The sole is located at the end of the body, which is called the aboral pole (opposite to the oral, or oral).

The body wall of the hydra consists of two layers of cells - ectoderm and endoderm, separated by a thin basal membrane, and limits a single cavity - the gastric cavity, which opens outwards with the oral opening.

In hydras and other hydroids, the ectoderm is in contact with the endoderm along the very edge of the mouth opening. In freshwater hydras, the gastric cavity continues into the tentacles, which are hollow inside, and their walls are also formed by ectoderm and endoderm.

Hydra ectoderm and endoderm consist of a large number of cells various types. Main mass The cells of both ectoderm and endoderm are epithelial-muscle cells. Their outer cylindrical part is similar to ordinary epithelial cells, and the base adjacent to the basal membrane is elongated fusiform and consists of two contractile muscular processes. In the ectoderm, the contractile muscular processes of these cells are elongated in the direction of the longitudinal axis of the hydra's body. Their contractions cause shortening of the body and tentacles. In the endoderm, the muscular processes are elongated in a circular direction, across the axis of the body. Their contraction has the opposite effect: the body of the hydra and its tentacles narrow and at the same time lengthen. Thus, the muscle fibers of the epithelial-muscle cells of the ectoderm and endoderm, opposite in their action, make up the entire hydra muscle.

Among the epithelial-muscular cells, various stinging cells are located either singly or, more often, in groups. The same type of hydra, as a rule, has several types of stinging cells that perform different functions.

The most interesting are stinging cells with nettle-like properties, called penetrants. When stimulated, these cells release a long filament that pierces the body of the prey. Stinging cells usually pear-shaped. A stinging capsule is placed inside the cage, covered with a lid on top. The wall of the capsule continues inward, forming a neck, which then passes into a hollow filament, coiled and closed at the end. At the junction of the neck and the filament, there are three spines inside, folded together and forming a stylet. In addition, the neck and stinging thread are lined with small spines on the inside. On the surface of the stinging cell there is a special sensitive hair - the cnidocil, at the slightest irritation of which the stinging thread is ejected. First, the cap opens, the neck is unscrewed, and the stiletto is pierced into the victim’s cover, and the spikes that make up the stiletto move apart and widen the hole. Through this hole, the twisting thread is pierced into the body. Inside the stinging capsule there are substances that have nettle properties and paralyze or kill prey. Once fired, the stinging thread cannot be used again by the hydroid. Such cells usually die and are replaced by new ones.

Another kind of stinging cells of hydras are volventa. They do not have nettle properties, and the threads they throw out serve to hold prey. They wrap around the hairs and bristles of crustaceans, etc. The third group of stinging cells are glutinants. They throw out sticky threads. These cells are important both in retaining prey and in moving the hydra. Stinging cells are usually located, especially on the tentacles, in groups called “batteries”.

The ectoderm contains small undifferentiated cells, the so-called interstitial, through which many types of cells develop, mainly stinging and reproductive cells. Interstitial cells are often located in groups at the base of epithelial muscle cells.

The perception of irritations in hydra is associated with the presence of sensitive cells in the ectoderm that serve as receptors. These are narrow, tall cells with outside hair. Deeper, in the ectoderm, closer to the base of the skin-muscle cells, there are nerve cells equipped with processes through which they contact each other, as well as with receptor cells and contractile fibers of the skin-muscle cells. Nerve cells are located scatteredly in the depths of the ectoderm, forming with their processes a plexus in the form of a mesh, and this plexus is denser on the perioral cone, at the base of the tentacles and on the sole.

The ectoderm also contains glandular cells that secrete adhesive substances. They concentrate on the sole and on the tentacles, helping the hydra temporarily attach to the substrate.

Thus, in the ectoderm of the hydra there are cells of the following types: epithelial-muscular, stinging, interstitial, nervous, sensory, glandular.

The endoderm has less differentiation of cellular elements. If the main functions of the ectoderm are protective and motor, then the main function of the endoderm is digestive. In accordance with this, most of the endoderm cells consist of epithelial-muscle cells. These cells are equipped with 2-5 flagella (usually two), and are also capable of forming pseudopodia on the surface, capturing them, and then digesting food particles. In addition to these cells, the endoderm contains special glandular cells that secrete digestive enzymes. The endoderm also contains nerve and sensory cells, but in much smaller quantities than in the ectoderm.

Thus, the endoderm also contains several types of cells: epithelial-muscular, glandular, nervous, and sensory.

Hydras do not remain attached to the substrate all the time; they can move from one place to another in a very unique way. Most often, hydras move “walking”, like the caterpillars of moths: the hydra bends with its oral pole towards the object on which it sits, sticks to it with its tentacles, then the sole comes off the substrate, is pulled up to the oral end and is attached again. Sometimes the hydra, having attached itself to the substrate with tentacles, lifts the stalk with the sole upward and immediately carries it to the opposite side, as if “tumbling.”

Hydra Power

Hydras are predators; they sometimes feed on quite large prey: crustaceans, insect larvae, worms, etc. With the help of stinging cells, they capture, paralyze and kill prey. Then the victim is pulled with tentacles to the highly distensible mouth opening and moves into the gastric cavity. At the same time gastric section the body is greatly inflated.

Digestion of food in hydra, unlike sponges, only partially occurs intracellularly. This is associated with the transition to predation and the capture of fairly large prey. The secretion of glandular cells of the endoderm is secreted into the gastric cavity, under the influence of which the food softens and turns into mush. Small food particles are then captured digestive cells endoderm, and the digestion process is completed intracellularly. Thus, in hydroids, intracellular or cavity digestion first occurs, which occurs simultaneously with the more primitive intracellular digestion.

Protection from enemies

The nettle cells of the hydra not only infect prey, but also protect the hydra from enemies, causing burns to predators attacking it. And yet there are animals that feed on hydras. These are, for example, some eyelash worms and especially Microstomum lineare, some gastropods (pond snails), Corethra mosquito larvae, etc.

The hydra's ability to regenerate is very high. Experiments carried out by Tremblay back in 1740 showed that pieces of the body of a hydra, cut into several dozen pieces, regenerate into a whole hydra. However, high regenerative ability is characteristic not only of hydras, but also of many other coelenterates.

Reproduction

Hydras reproduce in two ways - asexual and sexual.

Asexual reproduction of hydras occurs by budding. IN natural conditions hydra budding occurs throughout the summer. In laboratory conditions, budding of hydras is observed with sufficiently intense nutrition and a temperature of 16-20 ° C. Small swellings are formed on the body of the hydra - buds, which are protrusions of the ectoderm and endoderm outward. In them, due to the multiplying cells, further growth of the ectoderm and endoderm occurs. The kidney increases in size, its cavity communicates with the gastric cavity of the mother. At the free, outer end of the bud, tentacles and a mouth opening are finally formed.

Soon the matured young hydra separates from the mother.

Sexual reproduction of hydras in nature is usually observed in the fall, and in laboratory conditions it can be observed with insufficient nutrition and a drop in temperature below 15-16 ° C. Some hydras are dioecious (Pelmatohydra oligactis), others are hermaphrodites (Chlorohydra viridissima).

Sex glands - gonads - appear in hydras in the form of tubercles in the ectoderm. In hermaphrodite forms, male and female gonads are formed in different places. The testes develop closer to the oral pole, and the ovaries develop closer to the aboral pole. A large number of motile sperm are formed in the testes. Only one egg matures in the female gonad. In hermaphrodite forms, the maturation of sperm precedes the maturation of eggs, which ensures cross-fertilization and eliminates the possibility of self-fertilization. The eggs are fertilized in the mother's body. The fertilized egg is covered with a shell and spends the winter in this state. Hydras, as a rule, die after the development of sexual products, and in the spring a new generation of hydras emerges from the eggs.

Thus, in freshwater hydras, under natural conditions, there is a seasonal change in forms of reproduction: throughout the summer, hydras bud intensively, and in the fall (for central Russia - in the second half of August), with a decrease in temperature in reservoirs and a decrease in the amount of food, they stop reproducing budding and proceed to sexual reproduction. In winter, hydras die, and only fertilized eggs overwinter, from which young hydras emerge in the spring.

The freshwater polyp Polipodium hydriforme also belongs to the hydra order. Early stages The development of this polyp takes place in the eggs of sterlets and causes great harm to them. Several types of hydra are found in our reservoirs: stalked hydra (Pelmatohydra oligactis), common hydra (Hydra vulgaris), green hydra (Chlorohydra viridissima) and some others.

Figure: Structure of freshwater hydra. Radial symmetry of Hydra

Habitat, structural features and vital functions of the freshwater hydra polyp

In lakes, rivers or ponds with clean, transparent water, a small translucent animal is found on the stems of aquatic plants - polyp hydra(“polyp” means “multi-legged”). This is an attached or sedentary coelenterate animal with numerous tentacles. Body common hydra is almost correct cylindrical shape. At one end is mouth, surrounded by a corolla of 5-12 thin long tentacles, the other end is elongated in the form of a stalk with sole at the end. Using the sole, the hydra is attached to various underwater objects. The body of the hydra, together with the stalk, is usually up to 7 mm long, but the tentacles can extend several centimeters.

Radial symmetry of Hydra

If you draw an imaginary axis along the body of the hydra, then its tentacles will diverge from this axis in all directions, like rays from a light source. Hanging down from some aquatic plant, the hydra constantly sways and slowly moves its tentacles, lying in wait for prey. Since the prey can appear from any direction, the tentacles arranged in a radial manner best suit this method of hunting.
Radiation symmetry is characteristic, as a rule, of animals leading an attached lifestyle.

Hydra intestinal cavity

The body of the hydra has the form of a sac, the walls of which consist of two layers of cells - the outer (ectoderm) and the inner (endoderm). Inside the body of the hydra there is intestinal cavity(hence the name of the type - coelenterates).

The outer layer of hydra cells is the ectoderm.

Figure: structure of the outer layer of cells - hydra ectoderm

The outer layer of hydra cells is called - ectoderm. Under a microscope, several types of cells are visible in the outer layer of the hydra - the ectoderm. Most of all here are skin-muscular. By touching their sides, these cells create the cover of the hydra. At the base of each such cell there is a contractile muscle fiber that plays important role when the animal moves. When everyone's fiber skin-muscular cells contract, the hydra's body contracts. If the fibers contract on only one side of the body, then the hydra bends in that direction. Thanks to the work of muscle fibers, the hydra can slowly move from place to place, alternately “stepping” with its sole and tentacles. This movement can be compared to a slow somersault over your head.
The outer layer contains and nerve cells. They have a star-shaped shape, as they are equipped with long processes.
The processes of neighboring nerve cells come into contact with each other and form nerve plexus, covering the entire body of the hydra. Some of the processes approach the skin-muscle cells.

Hydra irritability and reflexes

Hydra is able to sense touch, temperature changes, the appearance of various dissolved substances in water and other irritations. This causes her nerve cells to become excited. If you touch the hydra with a thin needle, then the excitement from irritation of one of the nerve cells is transmitted along the processes to other nerve cells, and from them to the skin-muscle cells. This causes muscle fibers to contract, and the hydra shrinks into a ball.

Picture: Hydra's irritability

In this example, we get acquainted with a complex phenomenon in the animal body - reflex. The reflex consists of three successive stages: perception of irritation, transfer of excitation from this irritation along the nerve cells and response body by any action. Due to the simplicity of the hydra's organization, its reflexes are very uniform. In the future we will become familiar with much more complex reflexes in more highly organized animals.

Hydra stinging cells

Pattern: Stringing or nettle cells of Hydra

The entire body of the hydra and especially its tentacles are seated with a large number stinging, or nettles cells. Each of these cells has a complex structure. In addition to the cytoplasm and nucleus, it contains a bubble-like stinging capsule, inside which a thin tube is folded - stinging thread. Sticking out of the cage sensitive hair. As soon as a crustacean, small fish or other small animal touches a sensitive hair, the stinging thread quickly straightens, its end is thrown out and pierces the victim. Through a channel passing inside the thread, poison enters the body of the prey from the stinging capsule, causing the death of small animals. As a rule, many stinging cells are fired at once. Then the hydra uses its tentacles to pull the prey to its mouth and swallows it. The stinging cells also serve the hydra for protection. Fish and aquatic insects do not eat hydras, which burn their enemies. The poison from the capsules is reminiscent of nettle poison in its effect on the body of large animals.

The inner layer of cells is the hydra endoderm

Figure: structure of the inner layer of cells - hydra endoderm

Inner layer of cells - endoderm A. The cells of the inner layer - the endoderm - have contractile muscle fibers, but the main role of these cells is to digest food. They highlight in intestinal cavity digestive juice, under the influence of which the hydra’s prey softens and breaks down into small particles. Some of the cells in the inner layer are equipped with several long flagella (as in flagellated protozoa). The flagella are in constant motion and sweep particles toward the cells. The cells of the inner layer are capable of releasing pseudopods (like those of an amoeba) and capturing food with them. Further digestion occurs inside the cell, in vacuoles (like in protozoa). Undigested leftovers food is thrown out through the mouth.
The hydra has no special respiratory organs; oxygen dissolved in water penetrates the hydra through the entire surface of its body.

Hydra regeneration

The outer layer of the hydra's body also contains very small round cells with large nuclei. These cells are called intermediate. They play a very important role in the life of the hydra. With any damage to the body, intermediate cells located near the wounds begin to grow rapidly. From them, skin-muscle, nerve and other cells are formed, and the wounded area quickly heals.
If you cut a hydra crosswise, tentacles grow on one of its halves and a mouth appears, and a stalk appears on the other. You get two hydras.
The process of restoring lost or damaged body parts is called regeneration. Hydra has a highly developed ability to regenerate.
Regeneration, to one degree or another, is also characteristic of other animals and humans. Thus, in earthworms it is possible to regenerate a whole organism from their parts; in amphibians (frogs, newts) entire limbs, different parts of the eye, tail and internal organs. When a person is cut, the skin is restored.

Hydra reproduction

Asexual reproduction of hydra by budding

Figure: Hydra asexual reproduction by budding

Hydra reproduces asexually and sexually. In summer, a small tubercle appears on the hydra’s body - a protrusion of the wall of its body. This tubercle grows and stretches out. Tentacles appear at its end, and a mouth breaks out between them. This is how the young hydra develops, which at first remains connected to the mother with the help of a stalk. Outwardly, all this resembles the development of a plant shoot from a bud (hence the name of this phenomenon - budding). When the little hydra grows up, it separates from the mother’s body and begins to live independently.

Hydra sexual reproduction

By autumn, with the onset of unfavorable conditions, hydras die, but before that, sex cells develop in their body. There are two types of germ cells: ovoid, or female, and spermatozoa, or male reproductive cells. Sperm are similar to flagellated protozoa. They leave the hydra's body and swim using a long flagellum.

Drawing: sexual reproduction hydra

The hydra egg cell is similar to an amoeba and has pseudopods. The sperm swims up to the hydra with the egg cell and penetrates inside it, and the nuclei of both sex cells merge. Happening fertilization. After this, the pseudopods are retracted, the cell is rounded, and a thick shell is formed on its surface - a egg. At the end of autumn, the hydra dies, but the egg remains alive and falls to the bottom. In the spring, the fertilized egg begins to divide, the resulting cells are arranged in two layers. From them a small hydra develops, which, with the onset of warm weather, comes out through a break in the egg shell.
Thus, the multicellular animal hydra at the beginning of its life consists of one cell - an egg.

In order to study the internal structure of the hydra's body, it is killed, painted, and using special instruments, longitudinal and transverse sections are made through its body, as well as thin sections of individual parts of the animal's body. Examining such sections under a microscope, you can notice that the body of the hydra does not consist of one cell, like that of the common amoeba, green euglena or slipper ciliate, but of many. Animals whose body consists of large quantity cells are called multicellular. This means that hydra is a multicellular animal.

Hydra cells form body walls, which consist of two layers: outer and inner. Between these layers there is a thin transparent supporting membrane separating them. The outer layer, or ectoderm, is also called the cutaneous or integumentary layer. The inner layer, or endoderm, is also called the digestive layer.

External structure

The body of the freshwater hydra has the shape of a long sac. Usually it is attached at one end of its cylindrical body to an aquatic plant, underwater rock or other object. The end of the freshwater hydra's body, with which it attaches to underwater objects, is called the sole. At the opposite, free end of the body there are from 6 to 12 thin, hair-like tentacles. In an extended position, the tentacles can exceed the length of the hydra’s body, reaching 25 cm.

Most invertebrate animals are characterized by a certain body symmetry, that is correct location parts of the body and some organs relative to the body axis. The symmetry of the body of a particular invertebrate animal is closely related to its lifestyle. Freshwater hydra and most other coelenterates are characterized by ray (radial) symmetry of the body. Through the body of such animals, when dividing them into two identical halves, many planes of symmetry can be drawn. Radiation symmetry of the body is possible only in animals living in water.