Figure: Structure freshwater hydra. Radial symmetry of Hydra
Habitat, structural features and vital functions of the freshwater hydra polyp
In lakes, rivers or ponds with clean, clear 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. The body of an ordinary hydra has an almost regular 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 are best suited to 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, which plays an important role in the movement of the animal. 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 secrete digestive juice into the intestinal cavity, under the influence of which the hydra’s prey softens and breaks down into small particles. Some of the cells of the inner layer are equipped with several long flagella (as in flagellated protozoa). The flagella are in constant motion and sweep particles towards 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.
Figure: Hydra sexual reproduction
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 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.
The first person to see and describe the hydra was the inventor of the microscope and the greatest naturalist of the 17th-18th centuries, A. Levenguk.
Looking at aquatic plants under his primitive microscope, he saw strange creature with “hands in the form of horns.” Leeuwenhoek even managed to observe the budding of a hydra and see its stinging cells.
The structure of freshwater hydra
Hydra is a typical representative of coelenterates. The shape of its body is tube-shaped, at the anterior end there is a mouth opening surrounded by a corolla of 5-12 tentacles. Immediately below the tentacles, the hydra has a small narrowing - the neck, which separates the head from the body. The posterior end of the hydra is narrowed to more or less long leg, or stalk, with a sole at the end. A well-fed hydra has a length of no more than 5-8 millimeters, a hungry one is much longer.
The body of the hydra, like that of all coelenterates, consists of two layers of cells. In the outer layer, the cells are diverse: some of them act as organs that kill prey (stinging cells), others secrete mucus, and others have contractility. Nerve cells are also scattered in the outer layer, the processes of which form a network covering the entire body of the hydra.
Hydra is one of the few representatives of freshwater coelenterates, the bulk of which are inhabitants of the sea. In nature, hydras are found in various bodies of water: in ponds and lakes among aquatic plants, on the roots of duckweed, with a green carpet covering ditches and pits with water, small ponds and river backwaters. In reservoirs with clean water hydras can be found on bare rocks near the shore, where they sometimes form a velvety carpet. Hydras are light-loving, so they usually stay in shallow places near the shores. They are able to discern the direction of light flow and move towards its source. When kept in an aquarium, they always move to a lighted wall.
If you put more aquatic plants into a vessel with water, you can observe hydras crawling along the walls of the vessel and the leaves of the plants. The sole of the hydra secretes a sticky substance, due to which it is firmly attached to stones, plants or the walls of the aquarium, and it is not easy to separate it. Occasionally, the hydra moves in search of food. In the aquarium, you can mark the place of its attachment daily with a dot on the glass. This experience shows that in a few days the movement of the hydra does not exceed 2-3 centimeters. To change place, the hydra temporarily sticks to the glass with its tentacles, separates the sole and pulls it towards the front end. Having attached itself with its sole, the hydra straightens and again leans its tentacles one step forward. This method of movement is similar to the way the moth butterfly caterpillar, colloquially called a “surveyor,” walks. Only the caterpillar pulls the rear end towards the front, and then moves the head end forward again. When walking this way, the hydra constantly turns over its head and thus moves relatively quickly. There is another, much slower way of moving - sliding on the sole. With the force of the muscles of the sole, the hydra barely noticeably moves from its place. Hydras can swim in water for some time: having detached themselves from the substrate, spreading their tentacles, they slowly fall to the bottom. A gas bubble may form on the sole, which carries the animal upward.
How do freshwater hydras feed?
Hydra is a predator; its food is ciliates, small crustaceans - daphnia, cyclops and others; sometimes it comes across larger prey in the form of a mosquito larva or a small worm. Hydras can even cause harm to fish ponds by eating fish fry that hatch from the eggs.
Hydra hunting is easy to observe in an aquarium. Having spread its tentacles wide so that they form a trapping net, the hydra hangs with its tentacles down. If you watch a sitting hydra for a long time, you can see that its body is slowly swaying all the time, describing a circle with its front end. A cyclops swimming past touches the tentacles and begins to fight to free itself, but soon, struck by stinging cells, it calms down. The paralyzed prey is pulled up to the mouth by the tentacle and devoured. During a successful hunt, the small predator swells with swallowed crustaceans, whose dark eyes shine through the walls of the body. Hydra can swallow prey larger than itself. At the same time, the predator’s mouth opens wide, and the walls of the body stretch. Sometimes part of the out-of-place prey sticks out of the hydra's mouth.
Reproduction of freshwater hydra
With good nutrition, the hydra quickly begins to bud. The growth of a bud from a small tubercle to a fully formed hydra, but still sitting on the body of the mother, takes several days. Often, while the young hydra has not yet separated from the old individual, the second and third buds are already formed on the body of the latter. This is how asexual reproduction occurs; sexual reproduction is observed more often in the fall when the water temperature drops. Swellings appear on the hydra's body - gonads, some of which contain egg cells, and others - male reproductive cells, which, floating freely in the water, penetrate the body cavity of other hydras and fertilize the immobile eggs.
After the eggs are formed, the old hydra usually dies, and young hydras emerge from the eggs under favorable conditions.
Regeneration in freshwater hydra
Hydras have an extraordinary ability to regenerate. A hydra cut into two parts very quickly grows tentacles on the lower part and a sole on the upper part. In the history of zoology, remarkable experiments with hydra, carried out in the middle of the 17th century, are famous. Dutch teacher Tremblay. He not only managed to obtain whole hydras from small pieces, but even fused halves of different hydras with each other, turned their body inside out, and obtained a seven-headed polyp, similar to the Lernaean hydra from myths Ancient Greece. Since then, this polyp began to be called hydra.
In the reservoirs of our country there are 4 types of hydras, which differ little from each other. One of the species is characterized by a bright green color, which is due to the presence in the body of hydra of symbiotic algae - zoochlorella. Of our hydras, the most famous are the stemmed or brown hydra (Hydra oligactis) and the stemless or ordinary hydra (H. vulgaris).
To the class hydroid include invertebrate aquatic cnidarians. In their life cycle two forms are often present, replacing each other: polyp and jellyfish. Hydroids can gather in colonies, but solitary individuals are also not uncommon. Traces of hydroids are found even in Precambrian layers, but due to the extreme fragility of their bodies, the search is very difficult.
A bright representative of hydroids - freshwater hydra, single polyp. Its body has a sole, a stalk and long tentacles relative to the stalk. She moves like a rhythmic gymnast - with each step she makes a bridge and somersaults over her “head”. Hydra is widely used in laboratory experiments; its ability to regenerate and high activity of stem cells, providing “eternal youth” to the polyp, prompted German scientists to search and study the “immortality gene.”
Hydra cell types
1. Epithelial-muscular cells form the outer covers, that is, they are the basis ectoderm. The function of these cells is to shorten the hydra's body or make it longer; for this they have muscle fibers.
2. Digestive-muscular cells are located in endoderm. They are adapted to phagocytosis, capture and mix food particles that enter the gastric cavity, for which each cell is equipped with several flagella. In general, flagella and pseudopods help food penetrate from the intestinal cavity into the cytoplasm of hydra cells. Thus, her digestion occurs in two ways: intracavitary (for this there is a set of enzymes) and intracellular.
3. Stinging cells located primarily on the tentacles. They are multifunctional. Firstly, the hydra defends itself with their help - a fish that wants to eat the hydra is burned with poison and throws it away. Secondly, the hydra paralyzes prey captured by its tentacles. The stinging cell contains a capsule with a poisonous stinging thread; on the outside there is a sensitive hair, which, after irritation, gives a signal to “shoot”. The life of a stinging cell is short-lived: after being “shot” by a thread, it dies.
4. Nerve cells, together with shoots similar to stars, lie in ectoderm, under a layer of epithelial-muscle cells. Their greatest concentration is at the sole and tentacles. When exposed to any impact, the hydra reacts, which is an unconditional reflex. The polyp also has such a property as irritability. Let us also remember that the “umbrella” of a jellyfish is bordered by a cluster of nerve cells, and the body contains ganglia.
5. Glandular cells release a sticky substance. They are located in endoderm and promote food digestion.
6. Intermediate cells- round, very small and undifferentiated - lie in ectoderm. These stem cells divide endlessly, are capable of transforming into any other, somatic (except epithelial-muscular) or reproductive cells, and ensure the regeneration of the hydra. There are hydras that do not have intermediate cells (hence, stinging, nerve and reproductive cells), capable of asexual reproduction.
7. Sex cells develop into ectoderm. The egg cell of the freshwater hydra is equipped with pseudopods, with which it captures neighboring cells along with their nutrients. Among the hydras there is hermaphroditism, when eggs and sperm are formed in the same individual, but at different times.
Other features of freshwater hydra
1. Respiratory system Hydras do not have, they breathe over the entire surface of the body.
2. The circulatory system is not formed.
3. Hydras eat larvae of aquatic insects, various small invertebrates, and crustaceans (daphnia, cyclops). Undigested food remains, like other coelenterates, are removed back through the mouth.
4. Hydra is capable of regeneration, for which intermediate cells are responsible. Even when cut into fragments, the hydra completes the necessary organs and turns into several new individuals.
The first to see and describe the hydra was the naturalist A. Levenguk, who invented the microscope. This scientist was the most significant naturalist of the 17th-18th centuries.
While examining aquatic plants with his primitive microscope, Leeuwenhoek noticed a strange creature that had hands “in the form of horns.” The scientist even observed the budding of these creatures and saw their stinging cells.
The structure of freshwater hydra
Hydra belongs to the coelenterate animals. Its body is tube-shaped; in the front part there is a mouth opening, which is surrounded by a corolla consisting of 5-12 tentacles.
Under the tentacles, the hydra's body narrows and a neck is formed, which separates the body from the head. The back of the body is tapered into a stalk or stalk, with a sole at the end. When the hydra is well-fed, its body length does not exceed 8 millimeters, and if the hydra is hungry, the body is much longer.
Like all representatives of the coelenterates, the body of the hydra is formed by two layers of cells.
The outer layer consists of a variety of cells: some cells are used to kill prey, other cells have contractility, and others secrete mucus. And in the outer layer there are nerve cells that form a network covering the body of the guide.
Hydra is one of the few representatives of coelenterates that lives in fresh water, and most of these creatures live in the seas. The habitat of hydras is a variety of bodies of water: lakes, ponds, ditches, river backwaters. They settle on aquatic plants and the roots of duckweed, which covers the entire bottom of the reservoir with a carpet. If the water is clean and transparent, then hydras settle on the rocks near the shore, sometimes forming a velvet carpet. Hydras love light, so they prefer shallow places near the banks. These creatures can discern the direction of light and move towards its source. If hydras live in an aquarium, they always move to the illuminated part of it.
If you place aquatic plants in a vessel with water, you can see hydras crawling along their leaves and the walls of the vessel. There is an adhesive substance on the sole of the hydra, which helps it firmly attach to aquatic plants, stones and the walls of the aquarium; it is quite difficult to tear the hydra from its place. Occasionally, the hydra moves in search of food; this can be observed in aquariums, when a mark remains on the stack in the place where the hydra was sitting. In a few days, these creatures move no more than 2-3 centimeters. While moving, the hydra attaches itself to the glass with a tentacle, tears off the sole and drags it to a new place. When the sole is attached to the surface, the hydra levels out and rests on its tentacle again, taking a step forward.
This method of movement is similar to the movement of moth butterfly caterpillars, which are often called “land surveyors.” But the track pulls the back end towards the front and then moves the front end again. And the hydra turns over its head every time it moves. This is how the hydra moves quite quickly, but there is another, slower way of moving - when the hydra slides on its sole. Some individuals can detach from the substrate and swim in the water. They straighten their tentacles and sink to the bottom. And the hydras rise upward with the help of a gas bubble that forms on the sole.
How do freshwater hydras feed?
Hydras are predatory creatures; they feed on ciliates, cyclops, small crustaceans - daphnia and other small living creatures. They sometimes eat larger prey, such as small worms or mosquito larvae. Hydras can even cause damage to fish ponds as they eat newly hatched fish.
How hydra hunts can be easily observed in an aquarium. She spreads her tentacles widely, which form a net, while she hangs with her tentacles down. If you observe a hydra, you will notice that its body, slowly swaying, describes a circle with its front part. A prey swimming past is touched by the tentacles, tries to free itself, but becomes silent as the stinging cells paralyze it. The hydra pulls the prey to its mouth and begins to eat.
If the hunt is successful, the hydra swells from the number of crustaceans eaten, and their eyes are visible through its body. Hydra can eat prey that is larger than itself. The hydra's mouth can open wide and its body can stretch significantly. Sometimes a part of the victim sticks out of the hydra’s mouth, which did not fit inside.
Reproduction of freshwater hydra
If there is enough food, hydras multiply quickly. Reproduction occurs by budding. The process of growth of a bud from a tiny tubercle to a fully formed individual takes several days. Often several buds form on the hydra’s body until the young individual separates from the mother hydra. Thus, asexual reproduction occurs in hydras.
In autumn, when the water temperature drops, hydras can also reproduce sexually. On the body of the hydra, gonads form in the form of swellings. In some swellings, male reproductive cells are formed, and in others, egg cells. Male reproductive cells float freely in water and penetrate into the body cavity of hydras, fertilizing immobile eggs. When eggs are formed, the hydra usually dies. Under favorable conditions, young individuals emerge from the eggs.
Regeneration in freshwater hydra
Hydras exhibit an amazing ability to regenerate. If a hydra is cut in half, new tentacles will quickly grow in the lower part, and a sole will grow on the upper part.
In the 17th century, the Dutch scientist Tremblay conducted interesting experiments with hydras, as a result of which he was not only able to grow new hydras from pieces, but also to fuse different halves of hydras, obtain seven-headed polyps and turn their bodies inside out. When a seven-headed polyp similar to the hydra from Ancient Greece was obtained, these polyps began to be called hydra.
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 "many-legged") hydra is a tiny translucent creature that lives in clear clear waters slow-flowing rivers, lakes, 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 subjects under the 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 external structure 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 an 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 in this moment does not eat, there is 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. There is a narrow opening on the sole of the animal - 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 large group- 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 any
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 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. Under favorable conditions in summer time 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 into different sides 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 and 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.