Male reproductive cells in hydra are formed in. The structure of freshwater hydra. The outer layer of hydra cells is the ectoderm.

In the article, readers will be able to find out what hydra is. You will also get acquainted with the history of the discovery, the characteristics of this animal and its habitat.

History of the discovery of the animal

First of all you should give scientific definition. Freshwater hydra is a genus of sessile (in lifestyle) coelenterates belonging to the hydroid class. Representatives of this genus live in rivers with relatively slow flows or stagnant bodies of water. They are attached to the soil (bottom) or plants. This is a sedentary single polyp.

The first information about what a hydra is was given by the Dutch scientist, microscope designer Antonie van Leeuwenhoek. He was also the founder of scientific microscopy.

More detailed description, as well as the processes of nutrition, movement, reproduction and regeneration of the hydra were revealed by the Swiss scientist Abraham Tremblay. He described his results in the book “Memoirs on the history of a genus of freshwater polyps.”

These discoveries, which became the subject of conversation, brought great fame to the scientist. It is currently believed that it was the experiments in studying the regeneration of the genus that served as the impetus for the emergence of experimental zoology.

Later, Carl Linnaeus gave the genus a scientific name, which came from the ancient Greek myths about the Lernaean Hydra. Perhaps the scientist associated the name of the genus with a mythical creature due to its regenerative abilities: when a hydra’s head was cut off, another grew in its place.

Body structure

Expanding the topic “What is a hydra?”, one should also give external description kind.

The length of the body ranges from one millimeter to two centimeters, and sometimes a little more. The body of the hydra has a cylindrical shape, in front there is a mouth surrounded by tentacles (their number can reach twelve). There is a sole at the back, with the help of which the animal can move and attach to something. There is a narrow pore on it, through which liquid and gas bubbles are released from the intestinal cavity. The individual, together with this bubble, detaches from the support and floats up. In this case, the head is in the water column. In this way, the individual disperses throughout the reservoir.

The structure of the hydra is simple. In other words, the body is a bag whose walls consist of two layers.

Life processes

Speaking about the processes of respiration and excretion, it should be said: both processes occur over the entire surface of the body. In selection important role played by cellular vacuoles, the main function of which is osmoregulatory. Its essence lies in the fact that vacuoles remove residual water that enters cells due to one-way diffusion processes.

Thanks to the presence of a nervous system with a mesh structure, the freshwater hydra carries out the simplest reflexes: the animal reacts to temperature, mechanical irritation, illumination, and the presence of chemical substances V aquatic environment and other environmental factors.

Hydra's diet consists of small invertebrates - cyclops, daphnia, oligochaetes. The animal captures prey with the help of tentacles, and the venom of the stinging cell quickly affects it. Then the food is brought by the tentacles to the mouth, which, thanks to body contractions, is, as it were, put on the prey. The hydra throws out the remaining food through its mouth.

Hydra reproduces asexually under favorable conditions. A bud forms on the body of the coelenterate and grows for some time. Later she develops tentacles and also breaks out her mouth. The young individual separates from the mother, attaches to the substrate with tentacles and begins to lead an independent lifestyle.

Hydra sexual reproduction begins in the fall. Gonads are formed on her body, and germ cells are formed in them. Most individuals are dioecious, but hermaphroditism also occurs. Fertilization of the egg occurs in the body of the mother. The formed embryos develop, and in winter the adult dies, and the embryos overwinter at the bottom of the reservoir. During this period they fall into a process of suspended animation. Thus, the development of hydras is direct.

Hydra nervous system

As mentioned above, the hydra has a mesh structure. In one of the layers of the body, nerve cells form the diffuse nervous system. There are not many nerve cells in the other layer. In total, there are about five thousand neurons in the animal’s body. The individual has nerve plexuses on the tentacles, sole and near the mouth. Recent studies have shown that the hydra has a perioral nerve ring very similar to the hydromedusa's nerve ring.

The animal does not have a specific division of neurons into separate groups. One cell perceives irritation and transmits a signal to the muscles. Is in her nervous system chemical and electrical synapses (the point of contact between two neurons).

Opsin proteins were also found in this primitive animal. There is an assumption that human and hydra opsins have a common origin.

Growth and ability to regenerate

Hydra cells are constantly renewed. They divide in the middle part of the body, then move to the sole and tentacles. This is where they die and flake off. If there is an excess of dividing cells, they move to the kidneys in the lower part of the body.

Hydra has the ability to regenerate. Even after a cross-section of the body into several parts, each of them will be restored to its original form. The tentacles and mouth are restored on the side that was closer to the oral end of the body, and the sole is restored on the other side. The individual is able to recover from small pieces.

Body parts store information about the movement of the body axis in the structure of the actin cytoskeleton. A change in this structure leads to disturbances in the regeneration process: several axes can form.

Lifespan

Speaking about what a hydra is, it is important to talk about the duration of the life cycle of individuals.

Back in the nineteenth century, it was hypothesized that the hydra was immortal. Over the course of the next century, some scientists tried to prove it, and some tried to refute it. Only in 1997 was it finally proven by Daniel Martinez through an experiment that lasted four years. There is also an opinion that the immortality of the hydra is associated with high regeneration. And what is in the rivers in winter middle zone adult individuals die, most likely due to a lack of food or exposure to unfavorable factors.

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.

   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 are one of the smallest predators. Despite this, they are able to provide themselves with food. Hydras have tubular shape bodies. 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

LIFESTYLE

• 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 a mythical creature - the Lernaean Hydra, defeated by the hero of ancient Greece - Hercules.
• During constant movements in the water, the hydra performs quite original acrobatic tricks.

CHARACTERISTIC FEATURES OF HYDRA

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 a 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, separating the head from the body. The posterior end of the hydra is narrowed into a more or less long stalk, 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 larger prey is found 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. Spreading its tentacles wide so that they form catching 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. On a successful hunt small predator swells from 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 what happens asexual reproduction, sexual reproduction observed more often in autumn 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 mid-17th century V. 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"polyp, pushed 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. Circulatory system 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.

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, 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. Body common 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 unfavorable conditions, the 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.