Fish and other aquatic creatures

What is a hydra? Freshwater hydra: structure, breeding

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A polyp (meaning multifoot) hydra is a tiny translucent creature that lives in the clear, clear waters of slow-moving rivers, lakes, and ponds. This intestinal animal leads to a sedentary or attached lifestyle. The external structure of the freshwater hydra is very simple. The body has almost regular cylindrical shape. At one of its ends 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 is the sole, with which the animal is able to attach to various objects under water. The body length of a freshwater hydra is up to 7 mm, but the tentacles can be strongly stretched and reach a length of several centimeters.

Ray symmetry

Let us consider in more detail the external structure of the hydra. The table will help to remember the parts of the body and their purpose.

The body of the hydra, like many other animals leading an attached lifestyle, is inherent in ray symmetry. What it is? If we imagine a hydra and hold an imaginary axis along the body, the tentacles of the animal will diverge from the axis in all directions, like the rays of the sun.

The structure of the body of the hydra is dictated by its way of life. It attaches to the underwater object with a sole, hangs down and begins to sway, exploring the surrounding space with the help of tentacles. The animal is hunting. Since the hydra traps the prey, which can appear from either side, the symmetrical radial arrangement of the tentacles is optimal.

Intestinal cavity

The internal structure of the hydra consider in more detail. The body of the hydra is like an oblong sac. Its walls consist of two layers of cells, between which there is an intercellular substance (mesoglea). Thus, inside the body there is an intestinal (gastral) cavity. Food penetrates it through the mouth opening. Interestingly, the hydra, which is not currently eating, has practically no mouth. The ectoderm cells join and grow together just like on the rest of the body surface. Therefore, each time before eating, the hydra has to re-pierce its mouth.

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 - aboral pore. Through it, a liquid and a small bubble of gas can be released from the intestinal cavity. With this mechanism, the hydra is able to detach from the substrate and float to the surface of the water. In such a simple way, with the help of currents, it is settled around the reservoir.

The internal structure of the hydra is represented by the ectoderm and endoderm. Ectoderm is the outer layer of cells that form the body of the hydra. If you look at an animal through a microscope, you can see that several types of cells belong to the ectoderm: stinging, intermediate and epithelial-muscular.

The largest group is the skin-muscle cells. They come into contact with each other by the sides and form the surface of the body of the animal. Each such cell has a base - contractile muscular fibril. This mechanism provides the ability to move.

With the reduction of all fibrous body of the animal is compressed, lengthened, bent. And if the contraction occurred only on one side of the body, then the hydra bends down. Thanks to this work of the cells, the animal can move in two ways - “tumbling” and “walking”.

Also in the outer layer are the star-shaped nerve cells. They have long processes with which they adjoin with each other, forming a single network - the nerve plexus, intertwining the whole body of the hydra. Connect the nerve cells and the skin-muscle.

Between the epithelial-muscular 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, then the intermediate cells begin to grow and divide. They can transform into any type of cell.

Stinging cells

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

A sensitive hair comes out of the cage. If the prey or the enemy touches this hair, then the stinging thread is sharply expanded, and it is thrown out. The sharp tip pierces the body of the victim, and poison passes through the channel passing through the thread, which can kill a small animal.

As a rule, many stinging cells work. Hydra captures prey with tentacles, attracts to the mouth and swallows. The poison secreted by stinging cells serves also for protection. Larger predators do not touch the painfully stinging hydras. The poison of the hydra in its action resembles the poison of nettle.

Strange cells can also be divided into several types. Some threads inject poison, others - are upholstered around the victim, and others are glued to it. After activation, the stinging cell dies, and a new one forms from the intermediate one.

The structure of the hydra implies the presence of such a structure as the inner layer of cells, the endoderm. These cells also have muscular contractile filaments. Their main purpose - the digestion of food. The endoderm cells secrete digestive juice directly into the intestinal cavity. Under his influence, the prey is split into particles. Some endoderm cells have long flagella that are constantly in motion. Their role is to pull up food particles to the cells, which, in turn, release the false-pied paws and capture food.

Digestion continues inside the cell, therefore, is called intracellular. Food is processed in vacuoles, and undigested residues are ejected through the mouth opening. Breathing and excretion occurs across the entire surface of the body. Consider again the cellular structure of the hydra. The table will help to visually do it.

The structure of the hydra is such that it is able to sense a change in temperature, chemical composition of water, as well as touches and other stimuli. Nerve cells of the animal are able to be excited. For example, if you touch it with the needle tip, then the signal from the nerve cells that feel the touch will be transferred to the rest, and from the nerve cells to the epithelial-muscular one. The skin and muscle cells will react and contract, the hydra will contract into a lump.

Such a reaction is a vivid example of the reflex. This is a complex phenomenon consisting of successive stages - the perception of the stimulus, the transmission of excitation and the 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 be transformed into any other type.

In case of any damage to the body, the intermediate cells begin to very quickly divide, grow, and replace the missing parts. The wound is overgrown. Hydra's regenerative abilities are so high that if you cut it in half, one part grows new tentacles and mouth, and the other part grows the stem and sole.

Asexual reproduction

Breeding hydra can both asexual and sexual way. Under favorable conditions, in summertime a small bump appears on the body of the animal, the wall bulges out. Over time, the bump grows, stretches. Tentacles appear at its end, a mouth breaks through.

Thus, a young hydra appears, connected to the maternal organism of the stalk. This process is called budding, as it is similar to the development of a new shoot in plants. When a young hydra is ready to live independently, she buds. The child and maternal organisms attach themselves to the substrate with tentacles and stretch in different directions until they separate.

Sexual reproduction

When it starts to get cold and adverse conditions are created, it is the turn of sexual reproduction. In the autumn, in the hydras, from the intermediate, germ cells begin to form, male and female, that is, egg cells and spermatozoa. Egg cells hydr similar to the amoebae. They are large, covered with pseudopods. Spermatozoa 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 spermatozoon penetrates the egg cell, their nuclei coalesce and fertilization takes place. The pedicle of the fertilized egg cell is retracted, it is rounded, and the shell becomes thicker. An egg is formed.

All hydras in the fall, with the onset of cold weather, die. The maternal organism decays, but the egg remains alive and winters. In the spring, it begins to actively divide, the cells are arranged in two layers. With the onset of warm weather, a small hydra breaks through the egg shell and begins an independent life.

The story of the discovery of the animal

First of all, a scientific definition should be given. Freshwater hydra is a genus of sedentary (by way of life) intestinal cavities belonging to the hydroid class. Representatives of this genus inhabit rivers with relatively slow currents or standing bodies of water. They are attached to the ground (bottom) or plants. This is a sedentary single polyp.

The first data on what the hydra is, was given by the Dutch scientist, the designer of the microscope, Antoni van Leeuwenhoek. He was also the founder of scientific microscopy.

A more detailed description, as well as the processes of feeding, movement, reproduction and regeneration of the hydra, was revealed by the Swiss scientist Abraham Trembble. He described his results in the book “Memoirs to the history of one kind of freshwater polyps”.

These discoveries, which became the subject of talk, brought great fame to the scientist. At present, it is believed that the experiments on the study of the regeneration of the genus were the impetus for the emergence of experimental zoology.

Later, Carl Linnaeus gave the genus a scientific name that came from the ancient Greek myths about the Hydra of Lernea. Perhaps the scientist connected the genus name with the mythical creature due to its regenerative abilities: when the head was cut off, another one grew up in its place.

Body structure

Expanding the theme “What is a hydra?”, One should also give an external description of the genus.

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

The structure of the hydra is simple. In other words, the body is a bag, the walls of which 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. Cell selection vacuoles play an important role, their main function being osmoregulatory. Its essence lies in the fact that vacuoles remove the remaining water that enters the cells as a result of processes of one-sided diffusion.

Due to the presence of a nervous system with a reticular structure, freshwater hydra performs simple reflexes: the animal reacts to temperature, mechanical irritation, light exposure, the presence of chemicals in the aquatic environment and other environmental factors.

The basis of the hydra's diet is small invertebrates - cyclops, daphnias, oligochaetes. The animal captures prey with the help of tentacles, and the venom of the stinging cell rather quickly affects it. Then the food is brought by tentacles to the mouth, which, due to the contractions of the body, is put on prey, as it were. The remains of food hydra throws through the mouth.

The reproduction of hydra in favorable conditions occurs asexually. A kidney is formed on the body of the intestinal cavity, which grows for a while. Later, she has tentacles, as well as her mouth breaks. The young individual is separated from the maternal one, attached to the substrate by tentacles, and begins to lead an independent lifestyle.

Sexual reproduction of the hydra begins in the fall. On her body, sex glands are formed, and in them are sex cells. Most of the individuals are dioecious, but hermaphroditism is also found. Fertilization of the egg occurs in the body of the maternal individual. The formed embryos develop, and in winter the adult individual dies, and the embryos overwinter at the bottom of the reservoir. For this period, they fall into the process of anabiosis. Thus, the development of hydr direct.

Hydra Nervous System

As mentioned above, it has a net in hydra. In one of the layers of the body, nerve cells form the dispersed nervous system. There are not many nerve cells in the other layer. Only in the body of the animal about five thousand neurons. The individual has plexuses on the tentacles, the sole and near the mouth. Recent studies have shown that the hydra has a nerve ring around the annulus, very similar to the nerve ring of a hydromedus.

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

Also, opsins are found in this primitive animal. There is an assumption that human opsins and hydras have a common origin.

Growth and regeneration ability

Hydra cells are constantly updated. They are divided in the middle part of the body, then move to the base and tentacles. It is here that they die and slough off. If there is an excess of dividing cells, they move to the kidneys in the lower 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. Tentacles and mouth are restored on the side that was closer to the oral end of the body, and the sole - on the other side. The individual is able to recover from small pieces.

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

Lifespan

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

As early as the nineteenth century, it was hypothesized that the hydra is immortal. Some scientists throughout the next century tried to prove it, and some - to refute. It was only in 1997 that it was still proved by Daniel Martinez with the help of an experiment that lasted four years. It is also believed that the immortality of the hydra is associated with high regeneration. And the fact that in the middle of the rivers in winter in the midland, adults die, is most likely due to lack of food or exposure to adverse factors.

Common building

The body of the hydra has the form of an elongated sac, the walls of which consist of two layers of cells - ectoderm and endoderm.

Between them lies a thin gelatinous noncellular layer - mesoglueserving as a support.

The ectoderm forms the integument of the body of the animal and consists of several types of cells: epithelial-muscular, intermediate and stinging.

The most numerous of them are epithelial-muscular.

Due to muscular fibrilslying at the base of each cell, the body of the hydra can contract, lengthen and bend.

Between the epithelial-muscular cells are groups of small, rounded, with large nuclei and a small amount of cytoplasm of cells, called intermediate.

When the hydra's body is damaged, they begin to grow and divide vigorously. They can transform into other types of hydra body cells, except epithelial-muscular.

In the ectoderm are stinging cellsserving for attack and defense. They are mainly located on the tentacles of the hydra. Each stinging cell contains an oval capsule in which the stinging thread is folded.

The structure of the stinging cells with folded stinging thread

If the prey or the enemy touches the sensitive hair, which is located outside the stinging cage, in response to irritation the stinging thread is thrown and pierced into the body of the victim.

The structure of stinging cells with discarded stinging thread

On the thread channel, a substance that can paralyze the victim enters the victim’s body.

There are several types of stinging cells. The threads of some pierce the skin of animals and inject poison into their bodies. The threads of others are wrapped around the prey. Third strands are very sticky and stick to the victim. Usually hydra "shoots" several stinging cells. After the shot, the stinging cell dies. New stinging cells form from intermediate.

Digestive system

Digestive-muscular cells more than others. Muscle Fibers they are capable of contraction. When they are shortened, the body of the hydra becomes thinner. Сложные движения (передвижение «кувырканием»), происходит за счёт сокращений мускульных волоконцев клеток эктодермы и энтодермы.

Каждая из пищеварительно-мускульных клеток энтодермы имеет 1-3 жгутика. Колеблющиеся жгутики создают ток воды, которым пищевые частички подгоняются к клеткам. Digestive muscular endoderm cells are able to form pseudopods, capture and digest small food particles in the digestive vacuoles.

The structure of the digestive-muscular cells

Glandular cells in the endoderm secrete digestive juice into the intestinal cavity, which dilutes and partially digests food.

The structure of the yellow cell

The prey is captured by tentacles with stinging cells, the venom of which quickly paralyzes small victims. With coordinated movements of the tentacles, the prey is brought to the mouth, and then, using body contractions, the hydra is put on the victim. Digestion begins in the intestinal cavity (digestive digestion), ends inside the digestive vacuole of the epithelial-muscular endoderm cells (intracellular digestion). Nutrients are distributed throughout the body of the hydra.

When in the digestive cavity remains the remains of the victim, which can not be digested, and waste of cellular metabolism, it is compressed and emptied.

Hydra breathes oxygen dissolved in water. She has no respiratory organs, and she absorbs oxygen to the entire surface of the body.

Nervous system

Under the skin-muscle cells are star-shaped cells. These are nerve cells (1). They are interconnected and form a neural network (2).

Nervous system and hydra irritability

If you touch the hydra (2), then excitation (electrical impulses) occurs in the nerve cells, which instantly spreads throughout the nervous network (3) and causes contraction of the skin-muscular cells and shortens the entire hydra body (4). The response of the body hydra to such irritation - unconditioned reflex.

Sex cells

As the cold approaches in the fall, in the ectoderm of the hydra, sex cells form from intermediate cells.

There are two types of germ cells: egg, or female germ cells, and sperm, or male sex cells.

Eggs are located closer to the base of the hydra, spermatozoa develop in the hillocks located closer to the oral opening.

Egg cell the hydra is like an amoeba. It is equipped with pseudopods and grows rapidly, absorbing neighboring intermediate cells.

The structure of the egg cell hydra

The structure of the sperm hydra

Sperm in appearance they resemble flagellated protozoa. They leave the body of the hydra and swim with the help of a long flagellum.

Fertilization. Breeding

The sperm cell swims up to the hydra with the egg cell and penetrates it, with the nuclei of both sex cells merging. After this, the pseudopods are retracted, the cell is rounded, on its surface a thick shell stands out - an egg is formed. When the hydra dies and collapses, the egg remains alive and falls to the bottom. With the onset of warm weather, the living cell inside the containment shell begins to divide, the resulting cells are arranged in two layers. Of these, a small hydra develops, which emerges through the rupture of the shell of the egg. Thus, the multicellular animal hydra at the beginning of its life consists of only one cell - the egg. This suggests that the ancestors of the hydra were unicellular animals.

Asexual reproduction of hydra

Under favorable conditions, the hydra reproduces asexually. On the body of the animal (usually in the lower third of the body) a kidney is formed, it grows, then tentacles form and a mouth breaks. The young hydra buds off the maternal organism (with the maternal and daughter polyps attached by tentacles to the substrate and pulled in different directions) and leads an independent lifestyle. In the autumn, the hydra proceeds to sexual reproduction. On the body, in the ectoderm, gonads are laid - the sex glands, and sex cells develop from intermediate cells in them. When a gonad is formed, a medusoidal nodule is formed. This suggests that the gonads of the hydra are strongly simplified spores, the last stage in the series of transformation of the lost medusoid generation into an organ. Most of the hydra species are dichotomy, hermaphroditism is less common. Hydra ovules grow rapidly, phagocyting surrounding cells. Mature eggs reach a diameter of 0.5-1 mm. Fertilization occurs in the body of the hydra: through a special opening in the gonad, the sperm cell penetrates to the egg cell and merges with it. The zygote undergoes a complete even crushing, as a result of which a coblastoma is formed. Then, as a result of mixed delamination (a combination of immigration and delamination), gastrulation is performed. A dense protective shell (embryo) with spines is formed around the embryo. At the stage of the gastrula, the embryos fall into anabiosis. Adult hydra die, and embryos sink to the bottom and hibernate. In the spring, development continues, in the parenchyma of the endoderm, the intestinal cavity is formed by the divergence of the cells, then the rudiments of tentacles form, and a young hydra emerges from under the membrane. Thus, unlike most marine hydroids, the hydra has no free-floating larvae, its development is direct.

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2) Seasonal changes in wildlife are studied using the method:

2. Conducting experiments

3) The organic substances that make up the cell include:

Genus: Hydra = Hydra

The hydr is characterized by a primitive diffuse nervous system formed in the ectoderm by nerve cells in the form of a diffuse nerve plexus. In the endoderm there are only separate nerve cells, and in total the hydra has about 5000 neurons. Nerve plexuses time on the sole, around the mouth and on the tentacles. There is evidence that the hydra has a peri-annular nerve ring, similar to that of an umbrella in hydromedus. Although the hydra does not have a clear division into sensitive, intercalary and motor neurons, but nevertheless, there are sensitive and ganglionic nerve cells. The bodies of sensitive cells are located across the epithelial layer, they have a fixed flagellum, surrounded by a microvillus collar, which sticks out to the external environment and is able to perceive irritation. The processes of the ganglion cells are located at the base of the epithelial-muscular and do not extend into the external environment. The hydra is the most primitive animal, in the nerve cells of which there are light-sensitive proteins opsins, which in hydra and humans have a common origin. In general, the presence of the nervous system of the hydra allows it to exercise simple reflexes. Thus, the hydra responds to mechanical irritation, temperature, light, the presence of certain chemicals in the water and a number of other environmental factors.

Stinging cells are formed from the intermediate only in the trunk. The stinging cells of a hydra are about 55.000 and they are the most numerous of all cell types. Each stinging cell has a stinging capsule that is filled with a poisonous substance, and a stinging thread is screwed into the capsule. On the surface of the cell, only a sensitive hair rts, with the stimulation of which a thread is immediately thrown out and hits the victim. The stinging cell dies after the shooting of the filament, and in its place new cells are formed from the intermediate cells.

The hydra has four types of stinging cells. The desmona (volunte) is the first to shoot when the hydra is hunting: their spiral stinging threads entangle the outgrowths of the victim's body and ensure its retention. When the victim tries to free himself with jerks, walling (penetrants) with a higher threshold of irritation trigger from the vibration caused by them. And the thorns, which are at the base of their stinging threads, are anchored in the body of the prey, and poison is injected into its body through a hollow stinging thread. Large glutenant (their stinging thread has spikes, but does not have, like the Volvent, holes on the top), apparently, are mainly used for protection. Small glutenants are used only when the hydra is moving to firmly attach its tentacles to the substrate. Their shooting is blocked by extracts from the tissues of victims of the hydra.

On the tentacles of the hydra is the largest number of stinging cells, which form here stinging batteries. The composition of stinging batteries usually includes one large epithelial-muscular cell, in which stinging cells are immersed. In the center of the battery is a large penetrant, around it - smaller volvents and glutenants. Cnidocytes are connected by desmosomes to the muscular fibers of the epithelial-muscular cell.

Ultra-high-speed filming of shooting Hydrant's penetrant showed that the entire shooting process takes about 3 ms. Moreover, in the initial phase of shooting, the speed reaches 2 m / s, and the acceleration is about 40,000 g, which apparently is one of the fastest cellular processes known in nature. In the early phase of nematocysts firing, the speed of this process is 9–18 m / s, and acceleration ranges from 1,000,000 to 5,000,000 g, which allows a nematocyst weighing about 1 ng to develop a pressure on the tips of spikes (diameter about 15 nm) 7 hPa, which is comparable to the pressure of a bullet on a target and allows you to pierce a fairly thick cuticle of victims.

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