Hydra nutritional features. Hydra - class Hydrozoa: sensory organs, nervous and digestive systems, reproduction

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.
Processes of neighboring nerve cells touch 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: hydra string or nettle cells

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 food remains are 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.

Hydras are a genus of animals belonging to the Coelenterates. Their structure and life activity are often considered using the example of a typical representative - freshwater hydra. Next we will describe exactly this type, which lives in fresh water bodies with clean water, attaches to aquatic plants.

Typically, the size of a hydra is less than 1 cm. The life form is a polyp, which suggests a cylindrical body shape with a sole at the bottom and a mouth opening on the upper side. The mouth is surrounded by tentacles (about 6-10), which can extend to a length exceeding the length of the body. The hydra bends in the water from side to side and with its tentacles catches small arthropods (daphnia, etc.), after which it sends them into its mouth.

Hydras, as well as all coelenterates, are characterized by radial (or ray) symmetry. If you look at it not from above, you can draw many imaginary planes dividing the animal into two equal parts. The hydra does not care from which side the food swims towards it, since it leads a stationary lifestyle, so radial symmetry is more advantageous to it than bilateral symmetry (characteristic of most mobile animals).

The hydra's mouth opens into intestinal cavity. Partial digestion of food occurs here. The rest of the digestion is carried out in the cells, which absorb partially digested food from the intestinal cavity. Undigested remains are expelled through the mouth, since coelenterates do not have an anus.

The body of hydra, like all coelenterates, consists of two layers of cells. The outer layer is called ectoderm, and internal - endoderm. Between them there is a small layer mesoglea- a noncellular gelatinous substance that may contain Various types cells or cell processes.

Hydra ectoderm

Hydra ectoderm consists of several types of cells.

Skin-muscle cells the most numerous. They create the integument of the animal, and are also responsible for changing the shape of the body (lengthening or shortening, bending). Their processes contain muscle fibers that can contract (their length decreases) and relax (their length increases). Thus, these cells play the role of not only the integument, but also the muscles. Hydra does not have real muscle cells and therefore no real muscle tissue.

The hydra can move using somersaults. She bends down so much that her tentacles reach the support and stands on them, lifting her sole up. After this, the sole tilts and rests on the support. Thus, the hydra makes a somersault and ends up in a new place.

Hydra has nerve cells. These cells have a body and long processes with which they connect to each other. Other processes are in contact with skin-muscle and some other cells. Thus the whole body is contained in nerve network. Hydras do not have a cluster of nerve cells (ganglia, brain), but even such a primitive nervous system allows them to have unconditioned reflexes. Hydras react to touch, the presence of a row chemical substances, temperature change. So if you touch a hydra, it shrinks. This means that excitation from one nerve cell spreads to all the others, after which the nerve cells transmit a signal to the skin-muscle cells so that they begin to contract their muscle fibers.

Between the skin-muscle cells, the hydra has a lot stinging cells. There are especially many of them on the tentacles. These cells inside contain stinging capsules with stinging filaments. Outside the cells there is a sensitive hair, when touched, the stinging thread shoots out of its capsule and strikes the victim. In this case, a poison is injected into a small animal, usually having a paralytic effect. With the help of stinging cells, hydra not only catches its prey, but also defends itself from animals attacking it.

Intermediate cells(located in the mesoglea rather than in the ectoderm) provide regeneration. If the hydra is damaged, then thanks to the intermediate cells at the site of the wound, new, different cells of the ectoderm and endoderm are formed. Hydra can restore quite a large part of its body. Hence its name: in honor of the character of ancient Greek mythology, who grew new heads to replace the severed ones.

Hydra endoderm

Endoderm lines the intestinal cavity of the hydra. Main function endoderm cells - this is the capture of food particles (partially digested in the intestinal cavity) and their final digestion. At the same time, endoderm cells also have muscle fibers that can contract. These fibers face the mesoglea. Flagella are directed towards the intestinal cavity, which rake food particles towards the cell. The cell captures them the way amoebas do - forming pseudopods. Next, the food ends up in the digestive vacuoles.

The endoderm secretes a secretion into the intestinal cavity - digestive juice. Thanks to it, the animal captured by the hydra disintegrates into small particles.

Hydra reproduction

Freshwater hydra has both sexual and asexual reproduction.

Asexual reproduction carried out by budding. It occurs during a favorable period of the year (mainly in summer). A protrusion of the wall forms on the body of the hydra. This protrusion increases in size, after which tentacles form on it and a mouth breaks through. Subsequently, the daughter individual separates. Thus, freshwater hydras do not form colonies.

With the onset of cold weather (autumn), the hydra begins to sexual reproduction. After sexual reproduction, hydras die; they cannot live in winter. During sexual reproduction, eggs and sperm are formed in the body of the hydra. The latter leave the body of one hydra, swim up to another and fertilize its eggs there. Zygotes are formed, which are covered with a dense shell, allowing them to survive the winter. In the spring, the zygote begins to divide, and two germ layers are formed - ectoderm and endoderm. When the temperature gets high enough, the young hydra breaks the shell and comes out.

Hydra is a genus of freshwater animals of the class hydroid type coelenterates. Hydra was first described by A. Leeuwenhoek. Common in water bodies of Ukraine and Russia the following types of this genus: Hydra vulgaris, green, thin, long-stemmed. A typical representative of the genus looks like a single attached polyp with a length of 1 mm to 2 cm.

Hydras live in fresh water bodies with standing water or slow currents. They lead an attached lifestyle. The substrate to which the hydra is attached is the bottom of a reservoir or aquatic plants.

External structure of the hydra . The body has a cylindrical shape, on its upper edge there is a mouth opening surrounded by tentacles (from 5 to 12 different types). In some forms, the body can be conditionally divided into a trunk and a stalk. At the rear edge of the stalk there is a sole, thanks to which the organism is attached to the substrate and sometimes moves. Characterized by radial symmetry.

Internal structure of the hydra . The body is a sac consisting of two layers of cells (ectoderm and endoderm). They are separated by a layer connective tissue- mesoglea. There is a single intestinal (gastric) cavity, forming outgrowths extending into each of the tentacles. The oral opening leads into the intestinal cavity.

Nutrition. It feeds on small invertebrate animals (cyclops, cladocerans - daphnia, oligochaetes). The venom of the stinging cells paralyzes the victim, then, by movements of the tentacles, the prey is absorbed through the mouth and enters the body cavity. On initial stage cavity digestion occurs in the intestinal cavity, then intracellular digestion occurs inside digestive vacuoles endoderm cells. There is no excretory system undigested remains food is removed through the mouth. Transport of nutrients from the endoderm to the ectoderm occurs through the formation of special outgrowths in the cells of both layers, tightly connected to each other.

The vast majority of cells in hydra tissues are epithelial-muscular. From them the epithelial cover of the body is formed. The processes of these ectoderm cells make up the longitudinal muscles of the hydra. In the endoderm, cells of this type bear flagella for mixing food in the intestinal cavity, and digestive vacuoles are also formed in them.

Hydra tissues also contain small interstitial precursor cells that can, if necessary, transform into cells of any type. Characterized by specialized glandular cells in the endoderm that secrete digestive enzymes into the gastric cavity. The function of stinging ectoderm cells is to release toxic substances to infect the victim. IN large quantities these cells are concentrated on the tentacles.

The animal's body also has a primitive diffuse nervous system. Nerve cells are scattered throughout the ectoderm; in the endoderm there are single elements. Clusters of nerve cells are noted in the mouth, sole, and tentacles. Hydra can form simple reflexes, in particular, reactions to light, temperature, irritation, exposure to dissolved chemicals, etc. Breathing is carried out through the entire surface of the body.

Reproduction . Hydra reproduces both asexually (by budding) and sexually. Most species of hydra are dioecious, rare forms are hermaphrodites. When germ cells fuse in the body of hydras, zygotes are formed. Then the adults die, and the embryos overwinter at the gastrula stage. In spring, the embryo turns into a young individual. Thus, the development of hydra is direct.

Hydras play an essential role in natural food chains. In science, in recent years, hydra has been a model object for studying the processes of regeneration and morphogenesis.

From this article you will learn everything about the structure of freshwater hydra, its lifestyle, nutrition, and reproduction.

External structure of the hydra

Polyp (meaning "multipede") hydra is a tiny translucent creature that lives in the clean, transparent waters of slow-moving rivers, lakes, and ponds. This coelenterate animal leads a sedentary or sedentary lifestyle. The external structure of freshwater hydra is very simple. The body has an almost regular cylindrical shape. At one of its ends there is a mouth, which is surrounded by a crown of many long thin tentacles (from five to twelve). At the other end of the body there is a sole, with the help of which the animal is able to attach to various objects under water. The body length of freshwater hydra is up to 7 mm, but the tentacles can greatly stretch and reach a length of several centimeters.

Radiation symmetry

Let's take a closer look at the external structure of the hydra. The table will help you remember their purpose.

The body of the hydra, like many other animals leading an attached lifestyle, is characterized by What is it? If you imagine a hydra and draw an imaginary axis along its body, then the animal’s tentacles will diverge from the axis in all directions, like the rays of the sun.

The structure of the hydra's body is dictated by its lifestyle. It attaches itself to an underwater object with its sole, hangs down and begins to sway, exploring the surrounding space with the help of tentacles. The animal is hunting. Since the hydra lies in wait for prey, which can appear from any direction, the symmetrical radial arrangement of the tentacles is optimal.

Intestinal cavity

Let's look at the internal structure of the hydra in more detail. The hydra's body looks like an oblong sac. Its walls consist of two layers of cells, between which there is intercellular substance(mesoglea). Thus, there is an intestinal (gastric) cavity inside the body. Food enters it through the mouth opening. It is interesting that the hydra, which is not currently eating, has practically no mouth. The ectoderm cells close and grow together in the same way as on the rest of the body surface. Therefore, every time before eating, the hydra has to break through its mouth again.

The structure of the freshwater hydra allows it to change its place of residence. On the sole of the animal there is a narrow opening - the aboral pore. Through it, liquid and a small bubble of gas can be released from the intestinal cavity. With the help of this mechanism, the hydra is able to detach from the substrate and float to the surface of the water. In this simple way, with the help of currents, it spreads throughout the reservoir.

Ectoderm

The internal structure of the hydra is represented by ectoderm and endoderm. The ectoderm is called the body-forming hydra. If you look at an animal under a microscope, you can see that the ectoderm includes several types of cells: stinging, intermediate and epithelial-muscular.

The most numerous group is skin-muscle cells. They touch each other with their sides and form the surface of the animal’s body. Each such cell has a base - a contractile muscle fiber. This mechanism provides the ability to move.

When all fibers contract, the animal’s body contracts, lengthens, and bends. And if the contraction occurs on only one side of the body, then the hydra bends. Thanks to this work of cells, the animal can move in two ways - “tumbling” and “stepping”.

Also in the outer layer are star-shaped nerve cells. They have long processes, with the help of which they come into contact with each other, forming a single network - a nerve plexus that entwines the entire body of the hydra. Nerve cells also connect with skin and muscle cells.

Between the epithelial-muscle cells there are groups of small, round-shaped intermediate cells with large nuclei and a small amount of cytoplasm. If the hydra's body is damaged, the intermediate cells begin to grow and divide. They can turn into anything

Stinging cells

The structure of hydra cells is very interesting; the stinging (nettle) cells with which the entire body of the animal, especially the tentacles, are strewn deserve special mention. have a complex structure. In addition to the nucleus and cytoplasm, the cell contains a bubble-shaped stinging chamber, inside of which there is a thin stinging thread rolled into a tube.

A sensitive hair emerges from the cell. If prey or an enemy touches this hair, the stinging thread sharply straightens and is thrown out. The sharp tip pierces the victim’s body, and poison flows through the channel running inside the thread, which can kill a small animal.

Typically, many stinging cells are triggered. The hydra grabs prey with its tentacles, pulls it to its mouth and swallows it. The poison secreted by the stinging cells also serves for protection. Larger predators do not touch the painfully stinging hydras. The venom of the hydra is similar in effect to the poison of nettles.

Stinging cells can also be divided into several types. Some threads inject poison, others wrap around the victim, and others stick to it. After triggering, the stinging cell dies, and a new one is formed from the intermediate one.

Endoderm

The structure of hydra also implies the presence of such a structure as the inner layer of cells, endoderm. These cells also have muscle contractile fibers. Their main purpose is to digest food. Endoderm cells secrete digestive juices directly into the intestinal cavity. Under its influence, the prey is split into particles. Some endoderm cells have long flagella that are constantly in motion. Their role is to pull food particles towards the cells, which in turn release pseudopods and capture food.

Digestion continues inside the cell and is therefore called intracellular. Food is processed in vacuoles, and undigested remains are thrown out through the mouth. Breathing and excretion occurs through the entire surface of the body. Let's look again cellular structure hydra. The table will help you do this clearly.

Reflexes

The structure of the hydra is such that it is able to sense temperature changes, chemical composition water, as well as touch and other irritants. The nerve cells of an animal are capable of being excited. For example, if you touch it with the tip of a needle, the signal from the nerve cells that sensed the touch will be transmitted to the rest, and from the nerve cells to the epithelial-muscular cells. The skin-muscle cells will react and contract, the hydra will shrink into a ball.

Such a reaction is bright. It is a complex phenomenon consisting of successive stages - perception of the stimulus, transfer of excitation and response. The structure of the hydra is very simple, therefore the reflexes are monotonous.

Regeneration

The cellular structure of the hydra allows this tiny animal to regenerate. As mentioned above, intermediate cells located on the surface of the body can transform into any other type.

With any damage to the body, the intermediate cells begin to divide, grow very quickly and replace the missing parts. The wound is healing. The regenerative abilities of the hydra are so high that if you cut it in half, one part will grow new tentacles and a mouth, and the other will grow a stem and sole.

Asexual reproduction

Hydra can reproduce both asexually and sexually. At favorable conditions in the summer, a small tubercle appears on the animal’s body, the wall protrudes. Over time, the tubercle grows and stretches. Tentacles appear at its end and a mouth breaks through.

Thus, a young hydra appears, connected to the mother’s body by a stalk. This process is called budding because it is similar to the development of a new shoot in plants. When a young hydra is ready to live on its own, it buds off. The daughter and mother organisms attach to the substrate with tentacles and stretch in different directions until they separate.

Sexual reproduction

When it starts to get cold and creates unfavourable conditions, the turn of sexual reproduction begins. In the fall, hydras begin to form sex cells, male and female, from the intermediate ones, that is, egg cells and sperm. The egg cells of hydras are similar to amoebas. They are large, strewn with pseudopods. Sperm are similar to the simplest flagellates; they are able to swim with the help of a flagellum and leave the body of the hydra.

After the sperm penetrates the egg cell, their nuclei fuse and fertilization occurs. The pseudopods of the fertilized egg retract, it becomes rounded, and the shell becomes thicker. An egg is formed.

All hydras die in the fall, with the onset of cold weather. The mother's body disintegrates, but the egg remains alive and overwinters. In the spring it begins to actively divide, the cells are arranged in two layers. With the onset of warm weather, the small hydra breaks through the shell of the egg and begins an independent life.

The body shape of the hydra is tubular. The mouth opening of these animals is covered with tentacles. Hydras live in water, and with their stinging tentacles they kill and bring prey to their mouths.

   Type - Coelenterates
   Class - Hydroid
   Genus/Species - Hydra vulgaris, H.oligactis, etc.

   Basic data:
DIMENSIONS
Length: 6-15 mm.

REPRODUCTION
Vegetative: has a budding character. A bud appears on the body of the mother, from which the daughter gradually develops.
Sexual: Most species of hydra are dioecious. The gonads contain cells from which eggs develop. Sperm cells develop in the testis.

LIFESTYLE
Habits: live in fresh and brackish waters.
Food: plankton, fish fry, ciliates.
Lifespan: no data.

RELATED SPECIES
The phylum Coelenterata includes more than 9,000 species, some of them (15-20) live only in fresh waters.

   Freshwater hydras- one of the smallest predators. Despite this, they are able to provide themselves with food. Hydras have a tubular body shape. Using their soles, they attach themselves to underwater plants or rocks and move their tentacles in search of prey. Green hydras contain photosynthetic algae.

FOOD

   Hydra is a predatory animal that lives in water. It feeds on small organisms living in water, for example, ciliates, oligochaete worms, planktonic crustaceans, water fleas, insects and their larvae, and fish fry. A hydra that hunts attaches itself to an aquatic plant, branch or leaf and hangs on it. Her tentacles are very wide open. They constantly make circular searching movements. If one of them touches the victim, others rush towards it. Hydra paralyzes prey with stinging cell venom. The hydra uses its tentacles to pull its paralyzed prey towards its mouth. She swallows small animals whole. If the prey is larger than the hydra, the predator opens its mouth wide and the walls of its body stretch. If such prey is so large that it does not fit into the gastric cavity, then the hydra swallows only part of it and, to the extent of digestion, pushes the victim deeper and deeper.

LIFESTYLE

   Hydras live alone. However, in places that are particularly rich in food, several hydras hunt at once. This happens because the water current brings a lot of food to a certain place. Hydras of the Nuiga genus prefer fresh water. These animals were discovered by the researcher who invented the microscope, A. Leeuwenhoek (1632-1723). Another scientist, G. Tremblay, discovered that hydras easily restore lost body parts. A nondescript tubular body topped with tentacles that grow around the mouth opening and a sole at the end of the body are the main features appearance hydra. The gastric cavity of this animal is continuous. The tentacles are hollow. The body walls consist of two layers of cells. There are glandular cells located in the middle part of the hydra's body. Different kinds very similar to each other. They differ mainly in color (and, as a consequence, different colors talk about some structural feature). Bright green hydras have symbiotic algae living in their bodies. Hydras react to light and swim towards it. These animals are sedentary. They spend most of their lives in an attached state, waiting for prey. With the sole, like a suction cup, hydras are firmly attached to plants.

REPRODUCTION

   Hydras reproduce in two ways - sexual and vegetative. Vegetative propagation is represented by budding. Under suitable external conditions, several buds develop on the hydra’s body. At the very beginning, the bud looks like a small mound, later miniature tentacles appear at its outer end. Tentacles grow, appear on them stinging cells. Bottom part the body of the daughter individual becomes thinner, the hydra's mouth opens, the young individual branches off and begins an independent life. These animals reproduce by budding in warm time of the year. With the onset of autumn, hydras begin sexual reproduction. Sex cells are formed in the gonads. The gonad cracks and an egg emerges. Around the same time, sperm are formed in the testes of other hydras. They also leave the gonad and swim in the water. One of them fertilizes the egg. An embryo develops in the egg. Protected by a double shell, it overwinters at the bottom. In the spring, a fully formed hydra emerges from the egg.
  

DID YOU KNOW THAT...

• Hydra does not age, since every cell in its body is renewed after a few weeks. This animal lives only in the warm season. With the beginning of winter, all adult hydras die. Only their eggs, protected by a strong double shell - the embryotheca, can survive the winter.
• Hydras easily restore their lost limbs. The scientist G. Tremblay (1710-1784), as a result of his numerous experiments, obtained a seven-headed polyp, from which severed heads grew back. He looked like mythical creature- Lernaean Hydra defeated by the hero ancient Greece- Hercules.
• During constant movements in the water, the hydra performs quite original acrobatic tricks.

  

CHARACTERISTIC FEATURES OF HYDRA

   Tentacles: the mouth opening is surrounded by a corolla with 5-12 tentacles with stinging cells. With their help, the animal paralyzes its prey and pulls it into its mouth. A hydra that hunts attaches itself to a hard surface and, spreading its tentacles widely, makes circular searching movements with them.
   Body: body shape is tubular. At the anterior end is a mouth opening surrounded by tentacles. The aboral pore is located in the middle of the sole. The hydra wall consists of two layers of cells. Digestive processes occur in the midsection of the body.
   Mouth opening: covered with a corolla of tentacles. With its tentacles, the hydra pulls the animal into its mouth and swallows it.
   Leg: The rear end of the hydra is narrowed - this is a leg that has a sole at the end.
   Gonads: are formed in the ectoderm and have the appearance of tubercles. Sex cells accumulate in them.
   Dome: length about 13 mm. This is for self-defense. The hydra rises and forms a dense dome.
   Bud: The vegetative propagation of hydra has the nature of budding. Several buds may appear on the body at the same time. The buds are growing quickly.

PLACES OF ACCOMMODATION
Freshwater hydras live in fresh and brackish waters. They inhabit rivers, lakes, swamps and other bodies of water. The most common species are the common and brown hydra.
PRESERVATION
Each species of a genus living in a certain territory. These days they are not in danger of extinction.