Hydroid class. Freshwater hydra Where are the intermediate cells located in the body of the hydra

Hydra is a genus of animals belonging to the Coelenterates. Their structure and activity are often considered on the example of a typical representative - freshwater hydra. Further, this particular species will be described, which lives in fresh water bodies with clean water, attaches to aquatic plants.

Usually the size of the 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 (approximately 6-10), which can be extended in length exceeding the length of the body. The hydra leans in the water from side to side and with its tentacles catches small arthropods (daphnia, etc.), after which it sends them into the mouth.

For hydras, as well as for all coelenterates, it is characteristic radial (or radial) symmetry. If you look at not from above, then you can draw a lot of imaginary planes dividing the animal into two equal parts. Hydra does not care which side food swims up to it, since it leads a motionless lifestyle, therefore, radial symmetry is more beneficial for it than bilateral symmetry (characteristic of most mobile animals).

Hydra's mouth opens into intestinal cavity. This is where the digestion of food takes place. The rest of digestion is carried out in cells that absorb partially digested food from the intestinal cavity. Undigested residues are ejected through the mouth, since coelenterates do not have an anus.

The body of the hydra, like all coelenterates, consists of two layers of cells. The outer layer is called ectoderm, and the inner endoderm. Between them there is a small layer mesoglea- non-cellular gelatinous substance, which may contain various types of cells or processes of cells.

Hydra ectoderm

Hydra ectoderm is made up of several types of cells.

skin muscle cells the most numerous. They create the integuments of the animal, and are also responsible for changing the shape of the body (elongation or reduction, bending). Their processes contain muscle fibers that can contract (while their length decreases) and relax (their length increases). Thus, these cells play the role of not only covers, but also muscles. Hydra does not have real muscle cells and, accordingly, real muscle tissue.

The Hydra can move around using somersaults. She leans so hard that she reaches the support with her tentacles and stands on them, lifting the sole up. After that, the sole already leans and becomes on a support. Thus, the hydra makes a somersault and finds itself in a new place.

The hydra has nerve cells. These cells have a body and long processes that connect them to each other. Other processes are in contact with skin-muscle and some other cells. Thus, the whole body is enclosed in a nervous network. Hydra does not have an accumulation of nerve cells (ganglia, brain), however, even such a primitive nervous system allows them to have unconditioned reflexes. Hydras react to touch, the presence of a number of chemicals, temperature changes. So if you touch the 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 of stinging cells. Especially a lot of them on the tentacles. These cells inside contain stinging capsules with stinging filaments. Outside, the cells have a sensitive hair, when touched, the stinging thread shoots out of its capsule and strikes the victim. In this case, poison is injected into a small animal, usually having a paralytic effect. With the help of stinging cells, the 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, new various cells of the ectoderm and endoderm are formed at the site of the wound. The Hydra can regenerate a fairly large portion 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

The endoderm lines the intestinal cavity of the hydra. The main function of endoderm cells is to capture 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 fibrils are directed towards the mesoglea. Flagella are directed towards the intestinal cavity, which scoop up food particles to the cell. The cell captures them the way amoeba do - forming pseudopods. Further, the food is in the digestive vacuoles.

The endoderm secretes a secret into the intestinal cavity - digestive juice. Thanks to him, the animal captured by the hydra breaks up into small particles.

Hydra breeding

The 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 erupts. Subsequently, the daughter individual is separated. Thus, freshwater hydras do not form colonies.

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

Motion. Hydra can move from place to place. This movement occurs in different ways: either the hydra, bending in an arc, is sucked by the tentacles and partly by the glandular cells surrounding the mouth to the substrate and then pulls the sole, or the hydra, as it were, “tumbles”, being attached alternately by the sole, then by the tentacles.

Nutrition. Stinging capsules with their threads entangle prey and paralyze it. The prey processed in this way is captured by tentacles and sent to the mouth opening. Hydras can "overpower" very large prey, surpassing them in size, for example, evenfish fry. The extensibility of the mouth opening and the whole body is great. They are very voracious - one hydra can swallow up to half a dozen daphnia in a short time. Swallowed food enters the gastric cavity. Digestion in hydras, apparently, is combined - intra- and extracellular. Food particles are pulled in by endoderm cells with the help of pseudodopodia inside and digested there. As a result of digestion, nutrients accumulate in the cells of the endoderm, and grains of excretion products appear there, thrown from time to time in small portions into the gastric cavity. Excretion products, as well as undigested parts of food, are thrown out through the mouth

I - individual with male gonads; II - individual with female gonads

reproduction. Hydra reproduce asexually and sexually. Etc; asexual reproduction on hydras, buds are formed, gradually breaking away from the mother's body. Budding of hydras under favorable nutritional conditions can be very intense; observations show that in 12 days the number of hydras can increase 8 times. During the summer period, hydras usually reproduce by budding, but with the onset of autumn, sexual reproduction begins, and hydras can be both hermaphroditic and dioecious (stalked hydra).

Sex products are formed in the ectoderm from interstitial cells. In these places, the ectoderm swells in the form of tubercles, in which either numerous spermatozoa or one amoeboid egg are formed. After fertilization, which occurs on the body of the hydra, the egg cell is covered with a shell. Such a shelled egg overwinter, and in the spring a young hydra emerges from it. The larval stage of hydra is absent.

Hydra

Hydras are represented in nature by several species. In our freshwater bodies, they keep on the underside of the leaves of pondweed, white lilies, water lilies, duckweed, etc.

freshwater hydra

Sexually, hydras can be dioecious (for example, brown and thin) or hermaphrodite (for example, ordinary and green). Depending on this, the testes and eggs develop either on the same individual (hermaphrodites) or on different ones (male and female). The number of tentacles in different species varies from 6 to 12 or more. The tentacles of the green hydra are especially numerous.

For educational purposes, it is enough to acquaint students with structural and behavioral features common to all hydras, leaving aside special species characteristics. However, if it turns out to be green among other hydras, one should dwell on the symbiotic relationship of this species with zoochorella and recall a similar symbiosis in. In this case, we are dealing with one of the forms of relationships between the animal and plant worlds that support the circulation of substances in nature. This phenomenon is widespread among animals and occurs in almost every type of invertebrate. It is necessary to explain to the students what the mutual benefit is here. On the one hand, symbiont algae (zoochorella and zooxanthellae) find shelter in the body of their hosts and assimilate the carbon dioxide and phosphorus compounds necessary for synthesis; on the other hand, host animals (in this case, hydras) receive oxygen from algae, get rid of unnecessary substances, and also digest part of the algae, receiving additional nutrition.

You can work with hydras both in summer and winter, keeping them in aquariums with sheer walls, in tea glasses or in bottles with a cut neck (so as to remove the curvature of the walls). In the vessel, the bottom can be covered with a layer of well-washed sand, and it is advisable to lower 2-3 branches of elodea into the water, on which hydras are attached. Do not place other animals with hydras (except for daphnia, cyclops and other food objects). If hydras are kept clean, with room and good nutrition, they can live for about a year, make it possible to conduct long-term observations on them and set up a series of experiments.

Exploring hydras

To examine the hydras in a magnifying glass, they are transferred to a Petri dish or on a watch glass, and during microscopy - on a glass slide, placing pieces of glass hair tubes under the coverslip so as not to crush the object. When the hydras attach themselves to the glass of the vessel or to the branches of plants, one should consider their appearance, note the parts of the body: the mouth end with a corolla of tentacles, the body, the stalk (if any) and the sole. You can count the number of tentacles and note their relative length, which varies depending on the satiety of the hydra. In the hungry, they stretch out strongly in search of food and become thinner. If you touch the body of the hydra with the end of a glass rod or thin wire, you can observe a defensive reaction. In response to a slight irritation, the hydra removes only individual disturbed tentacles, while maintaining the normal appearance of the rest of the body. This is a local reaction. But with strong stimulation, all tentacles shorten, and the body contracts, taking on a barrel shape. In this state, the hydra remains for quite a long time (you can invite students to time the duration of the reaction).

The internal and external structure of the hydra

To show that the hydra's reactions to external stimuli are not stereotyped and can be individualized, it is enough to knock on the wall of the vessel and cause a slight shaking in it. Observation of the behavior of the hydras will show that some of them will have a typical defensive reaction (the body and tentacles will shrink), others will only slightly shorten the tentacles, and still others will remain in the same state. Consequently, the threshold of irritation was not the same in different individuals. A hydra can become addicted to a certain stimulus, to which it will stop responding. So, for example, if you often repeat a prick with a needle, causing a contraction of the hydra's body, then after repeated use of this stimulus, it will stop responding to it.

In hydras, it is possible to develop a short-term connection between the direction of extension of the tentacles and the obstacle that restricts these movements. If the hydra is attached to the edge of the aquarium so that the extension of the tentacles can be carried out only in one direction, and kept in such conditions for some time, and then given the opportunity to act freely, then after the restriction is removed, it will extend the tentacles mainly to the side, which in the experiment was free. This behavior persists for about an hour after the obstacles are cleared. However, after 3-4 hours, this connection is destroyed, and the hydra again begins searching movements with its tentacles evenly in all directions. Consequently, in this case we are not dealing with a conditioned reflex, but only with its likeness.

Hydras well distinguish not only mechanical, but also chemical stimuli. They reject inedible substances and seize food objects that act on the sensitive cells of the tentacles precisely by chemical means. If, for example, a hydra is offered a small piece of filter paper, it will reject it as inedible, but as soon as the paper is soaked in meat broth or moistened with saliva, the hydra will swallow it and begin to digest it (chemotaxis!).

Hydra nutrition

It is usually believed that hydras feed on small daphnia and cyclops. In fact, hydra food is quite diverse. They can ingest nematode roundworms, coretra larvae and some other insects, small snails, newt larvae, and juvenile fish. In addition, they gradually absorb algae and even silt.

Considering that hydras still prefer daphnia and are very reluctant to eat cyclops, an experiment should be carried out to determine the relationship of hydra to these crustaceans. If you put an equal number of daphnia and cyclops in a glass of hydras, and then after a while count how many of them are left, it turns out that most of the daphnia will be eaten, and many of the cyclops will survive. Since hydras are more likely to eat daphnia, which are difficult to harvest in winter, they began to replace this food with more accessible and easily obtained, namely bloodworms. Moths can be kept all winter in an aquarium along with the silt captured in the fall. In addition to bloodworms, hydras are fed with pieces of meat and earthworms cut into pieces. However, they prefer bloodworms to everything else, and they eat earthworms worse than pieces of meat.

Feeding hydras with various substances should be organized and students should be introduced to the feeding behavior of these intestinal cavities. As soon as the hydra's tentacles touch the prey, they grab a food piece and simultaneously shoot stinging cells. Then they bring the affected victim to the mouth opening, the mouth opens, and food is drawn in. After that, the body of the hydra swells (if the swallowed prey was large), and the victim inside is gradually digested. Depending on the size and quality of the ingested food, it takes from 30 minutes to several hours to break down and assimilate it. The undigested particles are then thrown out through the mouth opening.

Hydra Cell Functions

Regarding nettle cells, it must be borne in mind that these are only one of the types of stinging cells that have a toxic substance. In general, on the tentacles of the hydra there are groups of stinging cells of three types, the biological significance of which is not the same. Firstly, some of her stinging cells do not serve for defense or attack, but are additional organs for attachment and movement. These are the so-called glutinants. They throw out special sticky threads with which hydras attach to the substrate when they move from place to place with the help of tentacles (by the method of walking or turning over). Secondly, there are stinging cells - volvents, which shoot a thread that wraps around the body of the victim, holding it near the tentacles. Finally, the actual nettle cells - penetrants - throw out a thread armed with a stylet that pierces the prey. The poison located in the capsule of the stinging cell penetrates through the thread channel into the wound of the victim (or enemy) and paralyzes its movements. With the combined action of many penetrants, the affected animal dies. According to the latest data, in hydra, some of the nettle cells react only to substances that enter the water from the body of animals harmful to it, and function as a defense weapon. Thus, hydras are able to distinguish food objects and enemies among the organisms around them; attack the former and defend against the latter. Consequently, her neuromotor responses act selectively.

Cellular structure of hydra

By organizing long-term observations of the life of hydras in an aquarium, the teacher has the opportunity to introduce students to the various movements of these interesting animals. First of all, the so-called spontaneous movements (for no apparent reason) are striking, when the body of the hydra slowly sways, and the tentacles change their position. In a hungry hydra, search movements can be observed when its body is stretched into a thin tube, and the tentacles are greatly elongated and become like spider webs that wander from side to side, making circular movements. If there are planktonic organisms in the water, this eventually leads to the contact of one of the tentacles with the prey, and then a series of quick and energetic actions occur aimed at grasping, holding and killing the victim, pulling it to the mouth, etc. If the hydra is deprived of food , after an unsuccessful search for prey, it separates from the substrate and moves to another place.

The external structure of the hydra

The question arises: how does the hydra attach and detach from the surface on which it was located? Students should be told that the sole of the hydra has glandular cells in the ectoderm that secrete a sticky substance. In addition, there is a hole in the sole - the aboral pore, which is part of the attachment apparatus. This is a kind of suction cup that acts in conjunction with the adhesive and firmly presses the sole to the substrate. At the same time, the pore also promotes detachment, when a gas bubble is squeezed out of the body cavity by the pressure of water through it. Detachment of hydras by releasing a gas bubble through the aboral pore and subsequent floating to the surface can occur not only with insufficient nutrition, but also with an increase in population density. The detached hydras, after swimming for some time in the water column, descend to a new place.

Some researchers consider surfacing as a mechanism that controls the population, as a means of bringing the population to an optimal level. This fact can be used by the teacher in working with older students in the course of general biology.

It is interesting to note that some hydras, falling into the water column, sometimes use a surface tension film for attachment and thereby temporarily become part of the neuston, where they find food for themselves. In some cases, they put their foot out of the water, and then hang their soles on the film, and in other cases, they attach themselves to the film with a wide open mouth with tentacles spread out on the surface of the water. Of course, such behavior can only be noticed with long-term observations. When moving the hydra to another place without leaving the substrate, three ways of movement can be observed:

slip sole;
walking by pulling the body with the help of tentacles (like moth caterpillars);
flip over the head.

Hydras are light-loving organisms, as can be seen by observing their movement to the illuminated side of the vessel. Despite the absence of special photosensitive organs, hydras can distinguish the direction of light and strive for it. This is a positive phototaxis that they developed in the process of evolution as a useful feature that helps to find the place where food objects are concentrated. The planktonic crustaceans that the hydra feeds on are usually found in large clusters in areas of the reservoir with well-lit and sun-warmed water. However, not every light intensity causes a positive reaction in the hydra. Empirically, you can set the optimum lighting and make sure that a weak light has no effect, and a very strong one entails a negative reaction. Hydras, depending on the color of their body, prefer different rays of the solar spectrum. With regard to temperature, it is easy to show how the hydra extends its tentacles towards the heated water. Positive thermotaxis is explained by the same reason as the positive phototaxis noted above.

Hydra Regeneration

Hydras are characterized by a high degree of regeneration. At one time, Peebles established that the smallest part of the hydra's body capable of restoring the whole organism is 1/200. This, obviously, is the minimum at which the possibility of organizing the living body of the hydra in its entirety still remains. It is not difficult to acquaint students with the phenomena of regeneration. To do this, it is necessary to set up several experiments with a hydra cut into pieces and organize observations over the course of recovery processes. If you put the hydra on a glass slide and wait for it to stretch out its tentacles, at this moment it is convenient to cut off 1-2 tentacles for it. You can cut with thin dissecting scissors or the so-called spear. Then, after amputation of the tentacles, the hydra must be placed in a clean crystallizer, covered with glass and protected from direct sunlight. If the hydra is cut across into two parts, then the front part relatively quickly restores the back, which in this case turns out to be somewhat shorter than normal. The rear part slowly builds up the front end, but still forms tentacles, a mouth opening and becomes a full-fledged hydra. Regenerative processes go on in the hydra's body throughout its life, as tissue cells wear out and are continuously replaced by intermediate (reserve) cells.

Hydra breeding

Hydras reproduce by budding and sexually (these processes are described in a school textbook - biology grade 7). Some types of hydra overwinter in the egg stage, which in this case can be likened to an amoeba, euglena or ciliate cyst, since it endures the winter cold and remains viable until spring. To study the process of budding, it is necessary to plant a hydra that does not have kidneys in a separate vessel and provide it with enhanced nutrition. Invite students to keep records and observations with fixing the date of jigging, the time of the appearance of the first and subsequent buds, descriptions and sketches of the stages of development; note and record the time of separation of the young hydra from the mother's body. In addition to familiarizing students with the laws of asexual (vegetative) reproduction by budding, one should give a visual representation of the reproductive apparatus in hydras. To do this, in the second half of summer or autumn, several specimens of hydras must be removed from the reservoir and shown to students the location of the testicles and eggs. It is more convenient to deal with hermaphroditic species, in which eggs develop closer to the sole, and testes closer to the tentacles.

Medusa-cross

This small hydroid jellyfish belongs to the trachymedusa order. Large forms from this order live in the seas, and small ones live in fresh waters. But even among marine trachymedusas there are small-sized jellyfish - gonionema, or crosses. The diameter of their umbrella varies from 1.5 to 4 cm. Within Russia, gonionema are common in the coastal zone of Vladivostok, in the Gulf of Olga, off the coast of the Tatar Strait, in the Amur Bay, off the southern part of Sakhalin and the Kuril Islands. Students need to know about them, since these jellyfish are the scourge of swimmers off the coast of the Far East.

The jellyfish got its name "cross" by the position in the form of a cross of dark yellow radial channels emerging from the brown stomach and clearly visible through a transparent greenish bell (umbrella). Up to 80 movable tentacles hang along the edge of the umbrella with groups of stinging filaments located in belts. Each tentacle has one sucker, with which the jellyfish is attached to the zoster and other underwater plants that form coastal thickets.

reproduction

The crossbreeder reproduces sexually. In the gonads located along the four radial canals, sexual products develop. Small polyps are formed from fertilized eggs, and these latter give rise to new jellyfish that lead a predatory lifestyle: they attack fish fry and small crustaceans, hitting them with the poison of highly toxic stinging cells.

Human danger

During heavy rains that desalinate sea water, jellyfish die, but in dry years they become numerous and pose a danger to swimmers. If a person touches the cross with his body, the latter attaches to the skin with a suction cup and sticks numerous threads of nematocysts into it. The poison, penetrating into the wounds, causes a burn, the consequences of which are extremely unpleasant and even dangerous to health. After a few minutes, the skin turns red and blistered. A person experiences weakness, palpitations, back pain, numbness of the limbs, difficulty breathing, sometimes dry cough, intestinal disorders and other ailments. The victim needs urgent medical assistance, after which recovery occurs in 3-5 days.

During the period of the mass appearance of crosses, swimming is not recommended. At this time, preventive measures are organized: mowing underwater thickets, fencing baths with fine mesh nets, and even a complete ban on swimming.

Of the freshwater trachymedusa, the small jellyfish kraspedakusta (up to 2 cm in diameter) deserves mention, which is found in reservoirs, rivers and lakes in some areas, including the Moscow region. The existence of freshwater jellyfish points to the fallacy of students' perception of jellyfish as exclusively marine animals.

Hydra is a genus of freshwater animals of the hydroid class of the intestinal type. Hydra was first described by A. Leeuwenhoek. In the reservoirs of Ukraine and Russia, the following species of this genus are common: common hydra, green, thin, long-stemmed. A typical representative of the genus looks like a single attached polyp 1 mm to 2 cm long.

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

The 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 in different species). In some forms, the body can be conditionally distinguished into a trunk and a stalk. At the posterior 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.

The internal structure of the hydra . The body is a bag consisting of two layers of cells (ectoderm and endoderm). They are separated by a layer of connective tissue - mesoglea. There is a single intestinal (gastric) cavity, which forms outgrowths extending into each of the tentacles. The mouth opens into the intestinal cavity.

Nutrition. It feeds on small invertebrates (cyclops, cladocerans - daphnia, oligochaetes). The poison of stinging cells paralyzes the prey, then, with the movements of the tentacles, the prey is absorbed through the mouth opening and enters the body cavity. At the initial stage, cavity digestion occurs in the intestinal cavity, then intracellular - inside the digestive vacuoles of endoderm cells. There is no excretory system, undigested food residues are removed through the mouth. The transport of nutrients from the endoderm to the ectoderm occurs through the formation of special outgrowths in the cells of both layers, tightly interconnected.

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

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

The body of the animal also has a primitive diffuse nervous system. Nerve cells are scattered throughout the ectoderm, in the endoderm - single elements. Accumulations of nerve cells are noted in the area of the mouth, soles, and on the 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 reproduction occurs both asexually (budding) and sexually. Most species of hydras are dioecious, rare forms are hermaphrodites. When the sex cells merge in the body of the hydra, zygotes are formed. Then the adults die, and the embryos hibernate at the gastrula stage. In spring, the embryo turns into a young individual. Thus, the development of the 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.

Type: Cnidaria = coelenterates, cnidarians
Subtype: Medusozoa = Medusoproducing
Class: Hydrozoa Owen, 1843 = Hydrozoa, hydroid
Subclass: Hydroidea = Hydroids
Squad: Hydrida = Hydra
Genus: Hydra = Hydra

Genus: Hydra = Hydra

Hydra is characterized by a primitive diffuse nervous system, formed in the ectoderm by nerve cells in the form of a scattered nerve plexus. In the endoderm there are only individual nerve cells, and in total the hydra has about 5000 neurons. Nerve plexuses are thyme on the sole, around the mouth and on the tentacles. There is evidence that the hydra has a near-mouth nerve ring, similar to that of an umbrella in hydromedusas. Although the hydra does not have a clear division into sensory, intercalary and motor neurons, nevertheless, there are sensory and ganglionic nerve cells. The bodies of sensitive cells are located across the epithelial layer, they have an immobile flagellum surrounded by a collar of microvilli, which sticks out into the external environment and is able to perceive irritation. The processes of ganglion cells are located at the base of the epithelial-muscular cells and do not go into the external environment. Hydra is the most primitive animal, in whose nerve cells opsin proteins, which are sensitive to light, are found, which have a common origin in hydra and humans. In general, the presence of a nervous system in the hydra allows it to carry out simple reflexes. Thus, hydra reacts 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 intermediate cells only in the region of the body. There are about 55,000 stinging cells in hydra and they are the most numerous of all cell types. Each stinging cell has a stinging capsule, which is filled with a poisonous substance, and a stinging thread is screwed into the capsule. Only a sensitive hair rots on the surface of the cell, upon irritation of which the thread is immediately thrown out and strikes the victim. The stinging cell dies after the thread is fired, and in its place new cells form from intermediate cells.

Hydra has four types of stinging cells. Desmonems (volvents) are the first to shoot when hunting hydra: their spiral stinging threads entangle the outgrowths of the victim's body and ensure its retention. When the victim tries to free himself by jerks, stenotels (penetrants), which have a higher threshold of irritation, are triggered from the vibration caused by them. And the spikes at the base of their stinging threads anchor in the body of the prey, and poison is injected into its body through the hollow stinging thread. Large glutinants (their stinging filament has spikes, but does not have, like the volvents, a hole at the top) seem to be mainly used for defense. Small glutinants are used only when moving the hydra to firmly attach the tentacles to the substrate. Their firing is blocked by extracts from the tissues of Hydra victims.

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

Ultra-high-speed filming of the firing of the hydra penetrant showed that the entire firing process takes about 3 ms. Moreover, in the initial phase of firing, the speed reaches 2 m / s, and the acceleration is about 40.000 g; which appears to be one of the fastest cellular processes known in nature. In the early phase of nematocyst firing, the speed of this process is 9-18 m/s, and the acceleration is from 1,000,000 to 5,000,000 g, which allows a nematocyst weighing about 1 ng to develop a pressure of the order of 7 hPa, which is comparable to the pressure of a bullet on a target and allows you to pierce a fairly thick cuticle of the victims ...