Along with plants, untreated soil, water and, most often, live food from a natural reservoir, various animals enter the aquarium, many of which cause significant damage to its inhabitants. These animals do not cause diseases in fish in the classical sense, but are often the cause of their death or the death of their offspring. However, do not rush to classify them as your own enemies - they are dangerous only for the inhabitants of the aquarium, and for a truly inquisitive person they can become objects of observation and even scientific discoveries. And, probably, the first in this series should be called the hydra.
Hydra is a typical representative of coelenterates, standing at the very base of the evolutionary tree of multicellular animals.
It was discovered by the greatest naturalist of the 17th-18th centuries, Antonie van Leeuwenhoek, with the help of his amazing microscopes. But this unique animal did not attract the attention of the animals. And it is unknown how long the hydra would have remained in obscurity if, in 1740, the thirty-year-old Swiss teacher Tremblay had not discovered this amazing creature. To get to know it better, the inquisitive teacher divided it into two parts. From one piece, which he called the “head”, a new body grew, and on the other - a new “head”. In fourteen days, two new living organisms were formed from the two halves.
After this discovery, Tremblay began a deep and serious study of Hydra. He presented the results of his research in the book “Memoirs on the history of a genus of freshwater polyps with arms in the form of horns” (1744).
However, simple observations of the behavior and reproduction (budding) of the animal, of course, could not satisfy the naturalist, and he began conducting experiments to test his assumptions.
One of Tremblay's most famous experiments is that, with the help of a pork bristle, he turned the hydra inside out, that is, its inner side became outer. After this, the animal lived as if nothing had happened, but, as it turned out, not at all because, after turning inside out, the outer side began to perform the functions of the inner one, but because the cells of the inner layer, which had previously been the outer one, leaked through the new outer layer and took their original place.
In his other experiments, Tremblay crushed the hydra more and more, but it was restored each time, and there was no limit to this. It is now known that the hydra is capable of recovering from 1/200 of its body. And then this amazed even the most venerable scientists and prompted them to study such a biological problem as regeneration.
About 250 years have passed since Tremblay's experiments on hydra. Hundreds of articles and books have been written about the hydra, but to this day it occupies the minds of researchers.
It is well known that animals do not react in any way to radioactive rays and, if they get into their zone, they can receive a lethal dose and die. Experiments with the green hydra (Chlorohydra viridissima) showed that it somehow senses mortal danger and strives to get away from the source of radiation.
The death of the hydra is also caused by too high a dose of X-rays; reducing the dose leaves it alive, but suppresses reproduction. But small dosages have a completely unexpected effect on animals; their budding process is enhanced and their ability to self-heal increases.
The results of experiments with painting the wall of an aquarium in all colors of the spectrum were surprising. It turned out that hydras, which do not have any organs of vision, distinguish colors, and each species prefers its own: green hydras, for example, “love” blue-violet color, brown ones (Hydra oligactis) - blue-green.
What is a hydra? Outwardly, it resembles a glove placed vertically, fingers up, only it has from 5 to 12 tentacle fingers. In most species, immediately under the tentacles there is a slight narrowing that separates the “head” from the body. In the head of the hydra there is a mouth opening leading to the gastric cavity. The body walls of hydra, like all coelenterates, are two-layered. Outer layer consists of several types of ec cells: dermal-muscular, which drive the hydra; nervous, giving her the opportunity to feel touch, temperature changes, the presence of impurities in water and other irritants; intermediate, most actively involved in the restoration of damaged or lost parts of the body; and finally, stinging ones, located mostly on the tentacles.
Coelenterates are the only group of animals that have such a weapon as stinging cells. In addition to the protoplasm required for all living cells, the stinging cell contains a bubble-like capsule, inside which the stinging thread is coiled.
Having attached its sole to some substrate, the hydra spreads its tentacles, which are in constant motion. When a victim is detected, the stinging thread of each of the stinging cells quickly straightens and plunges its sharp end into the prey. Through a channel running inside the thread, poison enters the body of the prey from the stinging capsule, causing its death. The stinging capsule can only be used once; The hydra discards the discharged capsule and replaces it with a new one, which is formed from special cells.
Digestion of food is carried out inner layer cells: they secrete digestive juice into the gastric cavity, under the influence of which the hydra’s prey softens and disintegrates into small particles. The end of the cell of the inner layer, facing the gastric cavity, is equipped, like in flagellated protozoa, with several long flagella, which are in constant motion and rake particles to the cells. Like an amoeba, the cells of the inner layer are able to release pseudopods and capture food with them. Further digestion occurs , like in protozoa, inside the cell, in digestive vacuoles.
Those scientists who believed that, as a true predator, hydra feeds only on animals turned out to be right. Detailed studies have established that hydra digests fats, proteins and carbohydrates only of animal origin.
Hydras reproduce in two ways - vegetative and sexual. Vegetative propagation occurs by budding. Having separated from the mother's body, young hydras begin to live independently.
After abundant budding, the hydra becomes exhausted, and for some time no buds are formed on it. But when good nutrition it quickly restores its resources and begins to bud again. Over the five summer months, it is capable of producing thirty generations of twenty-five young hydras each. Reproduction by budding occurs under favorable conditions.
With the onset of unfavorable conditions - autumn cold, drought, waterlogging, excess carbon dioxide - the hydra switches to sexual reproduction. Most species are dioecious, but there are species in which both male and female gonads are formed in the body.
The gonads are found in the outer layer of cells. In females, they look like spherical bodies, each of which contains one egg, similar to an amoeba; it grows quickly, eating the intermediate cells surrounding it, and reaches a diameter of one and a half millimeters. The grown egg is rounded and divided into two unequal parts, as a result of which the number of chromosomes in the nucleus of the egg is halved. The mature egg emerges from the gonad through a gap in its wall, but remains connected to the body of the hydra with the help of a thin stalk.
At the same time, sperm are formed in the male gonads of other hydras, which in appearance resemble flagellated protozoa. Leaving the gonads. they swim with the help of a long rope and, finally, one of the sperm, having found the egg, penetrates it. Immediately after this, crushing begins.
The hydra embryo is covered on the outside with two shells, the outer of which is quite thick and permeated with chitin. Under such protection, he successfully endures unfavorable conditions. With the onset of spring warming, the rainy season, etc., the young hydra breaks the wall of the protective shell and begins an independent life.
If you want to watch a hydra, place it in an aquarium where there are no other inhabitants, otherwise small animals that serve as food for fish will be eaten, and most importantly, the larvae and fry will be destroyed. Once in a spawning tank or nursery aquarium, the hydra, quickly multiplying by budding, will immediately deal with the young fish.
But it is not advisable to use these animals to fight hydra in an aquarium: trichodins and planaria are also enemies of fish. and getting hydramoebas and anchistropus crustaceans is not easy. Hydras have another enemy - the freshwater mollusk pond snail. but it is also not suitable, since it is a carrier of some fish diseases and also likes to feast on delicate aquatic plants.
Some hobbyists put hungry young gourami in an aquarium where the hydra has entered. Others fight it using the peculiarities of its behavior. Thus, hydras like to settle in the most illuminated areas of the aquarium. It is enough to shade the aquarium on all sides except one, and lean glass against the only illuminated wall, and in two or three days almost all the hydras will gather on it. Then the glass must be removed and cleaned.
Hydras are very sensitive to the presence of copper in water. One of the methods of combating is based on placing a ball of copper wire without insulation over the sprayer. After all the hydras have died, the wire is removed from the aquarium.
Some have successfully used chemical substances:
ammonium sulfate at the rate of 5 grams per 100 liters of water, once,
ammonium nitrate - 6 grams per 100 liters of water, three times, with an interval of three days;
hydrogen peroxide (in an aquarium without plants with sufficient artificial aeration) at the rate of two teaspoons per 10 liters of water. The required amount of 3% solution is first diluted in 200-300 milliliters of water, and then slowly poured into the aquarium over a working sprayer.
To make the fight against hydra more effective, you need to use not one, but two or even three methods simultaneously.
Bibliography
S. Sharaburin. Hydra.
The structure of coelenterates
using the example of freshwater hydra
Appearance of the hydra; Hydra body wall; gastrovascular cavity; hydra cellular elements; hydra reproduction
Freshwater hydra as a laboratory object in the study of coelenterates has the following advantages: wide distribution, accessibility of cultivation and most importantly - clearly expressed features of the Coelenterate type and the Cnidarians subtype. However, it is not suitable for study life cycle coelenterates (see pp. 72-76).
There are several known species of freshwater hydras, united in one family Hydra - Hydridae; the medusoid stage dropped out of their life cycle. Among them, the most widespread is Hydra oligactis.
Work 1. Appearance of the hydra. It is not difficult to distinguish four sections in the body of the hydra - the head, trunk, stalk and sole (Fig. 24). Elongated and pointed protrusion of the body -
Rice. 24. Hydra stalked. A- appearance (slightly enlarged); B- hydra with a developing kidney, male and female gonads:
1
- sole and place of attachment of the hydra to the substrate; 2
- stalk; 3
- trunk section; 4 -
opening of the digestive cavity; 5
- tentacles; 6
- oral end: 7
- abolic end; 8
- hypostome
the oral cone (or hypostome) bears an oral opening at the apex, and is surrounded by radially arranged tentacles at its base. The hypostome and tentacles form the head section of the body, or head. The end of the body bearing the hypostome is called oral, the opposite end is called aboral. Most of the body is represented by a swollen, expanded trunk, immediately following the head section. Posterior to it is a narrowed part of the body - the stalk passes into
flattened area - sole; its cells secrete a sticky secretion, with the help of which the hydra attaches to the substrate. Such a structure of the body allows several or many planes of symmetry to be drawn through it; each will divide the body of the beer into homogeneous halves (one of them will present a mirror image of the other). In Hydra, these planes run along radii (or diameters) cross section the bodies of the hydra, and intersect in longitudinal axis bodies. This symmetry is called radial (see Fig. 23).
Using living material, you can trace the movement of the hydra. Having attached its sole to the substrate, the hydra remains in one place for a long time. She turns her oral end in different directions and “catches” the space surrounding her with tentacles. The hydra moves using the so-called “stepping” method. Extending the body along the surface of the substrate, it attaches with the oral end, separates the sole and pulls up the aboral end, attaching it close to the oral; This is how one “step” is carried out, which is then repeated many times. Sometimes the free end of the body is thrown to the opposite side of the reinforced head end, and then the “stepping” is complicated by somersaulting over the head.
Progress. 1. Consider a living hydra. To do this, prepare a temporary microrelarate from living hydras; equip the cover glass with tall plasticine legs. Observations are made under a microscope at low magnification (or under a tripod magnifying glass). Draw the contours of the hydra’s body and indicate in the drawing all the elements of it described above external structure. 2. Monitor the contraction and extension of the animal’s body: when pushed, shaken or otherwise stimulated, the hydra’s body will shrink into a ball; in a few minutes, after the hydra has calmed down, its body will take on an oblong, almost cylindrical shape (up to 3 cm).
Work 2. Hydra body wall. The cells in the hydra's body are arranged in two layers: the outer, or ectoderm, and the inner, or endoderm. Throughout, from the hypostome to the sole inclusive, the cell layers are clearly visible, since they are separated, or rather connected, by a special non-cellular gelatinous substance, which also forms a continuous intermediate layer, or base plate(Fig. 25).. Thanks to this, all the cells are connected into a single integral system, and the elasticity of the supporting plate gives and maintains the body shape characteristic of the hydra.
The overwhelming majority of ectodermal cells are more or less homogeneous, flattened, closely adjacent to each other and directly connected with the external environment.
Rice. 25. Diagram of the body structure of the hydra. A- longitudinal section of the body with the intersection (longitudinal) of the tentacles; B- transverse section through the trunk; IN- topography of cellular and other structural elements in the section of the cross section through the wall of the hydra body; G- nervous apparatus; diffusely distributed nerve cells in the ectoderm:
1
- sole; 2
-stalk; 3
- torso; 4
- gastric cavity; 5 - tentacle (wall and cavity); 6
- hypostome and oral opening in it; 7
- ectoderm; 8 -
endoderm; 9 -
support plate; 10
- place of transition of ectoderm into endoderm; 11 - 16 -
hydra cells (11
- stinging, 12
- sensitive, 13
- intermediate (interstitial), 14
- digestive, 15
- glandular, 16
- nervous)
Primitive cover tissue, which they form, isolates the internal parts of the animal’s body from external environment and protects them from the effects of the latter. Endodermal cells are also for the most part homogeneous, although they appear outwardly different due to the formation of temporary protoplasmic processes called pseudolodia. These cells are elongated across the body, with one end facing the ectoderm and the other inside the body; each of them is equipped with one or two flagella (not visible on the preparation). This digestive cells that carry out food digestion and absorption; lumps of food are captured by pseudopodia, and indigestible remains are thrown out by each cell independently. Process intracellular Digestion in hydra is primitive and resembles a similar process in protozoa. Since the ectoderm and endoderm are formed by two groups of specialized cells, hydra serves as an example of the initial differentiation of cellular elements in a multicellular organism and the formation of primitive tissues (Fig. 25).
Nutrients are partially assimilated by the digestive cells of the endoderm and partially transported through the intermediate noncellular layer; ectodermal cells; receive nutrients through the supporting plate, and possibly directly from the digestive ones, through their processes that pierce the supporting plate. Obviously the support plate, although lacking cellular structure, plays a very significant role in the life of the hydra.
Progress. 1. Familiarize yourself with the structure of the hydra body wall. Examine at low microscope magnification the arrangement of layers in the wall of the hydra’s body on a permanent, stained preparation of a median section through the body of the animal. 2. Draw a schematic sketch of the body wall (contour, without depicting the boundaries between cells); mark in the figure the ectoderm, endoderm and supporting plate and indicate their functions,
Work 3. Gastrovecular cavity. It opens at the oral end with the mouth, which serves as the only opening through which the cavity communicates with the external environment (see Fig. 25). Everywhere, including the oral cone, it is surrounded (or lined) by endoderm. Both cell layers border at the oral opening. With both flagella, endodermal cells create water currents in the cavity.
In the endoderm there are special cells - glandular (not visible on the preparation) - which secrete digestive juices into the cavity (see Fig. 25, 26). Food (for example, caught crustaceans) enters the cavity through the mouth, where it is partially digested. Indigestible food remains are removed through the same single hole, which serves
Rice. 26. Isolated Hydra Cells: A- epithelial-muscular ectoderm cell (greatly enlarged). The set of contractile muscle fibers in the process in the drawing is filled with ink, around it there is a layer of transparent protoplasm; B- a group of endodermal cells. Between the digestive cells there is one glandular and one sensory; IN- interstitial cell between two endodermal cells:
1
- 8
- epithelial muscle cell ( 1
- epithelial area, 2
- core, 3
- protoplasm, 4
- inclusions, vacuoles, 5
- outer cuticular layer, 6 -
muscle process, 7
- protoplasmic case, 8
- muscle fibers); 9
- endoder. baby cages; 10 -
their flagella; 11 -
glandular cell; 12 -
supporting plate;.13
- sensitive cell; 14
- interstitial cell
not only with your mouth, but also with powder. The hydra cavity continues into such parts of the body as the stalk and tentacles (see Fig. 24); digested substances penetrate here; Digestion of food does not occur here.
Hydra has dual digestion: intracellular- more primitive (described above) and extracellular, or cavitary, characteristic of multicellular animals and first arose in coelenterates.
Morphologically and functionally, the hydra cavity corresponds to the intestines of higher animals and can be called gastric. Hydra does not have a special system for transporting nutrients; This function is partially performed by the same cavity, which is therefore called gastrovascular.
Progress. 1. On a microscopic specimen of a longitudinal section at low magnification of the microtrench, examine the shape of the gastrovascular cavity and its position in the body of the hydra. Pay attention to the lining of the cavity (along its entire length) with endodermal cells. You need to verify this by examining the hypostome at high magnification under a microscope. 2. Find areas of the gastrovascular cavity that are not involved in food digestion. Draw all observations and label them in the figure.
functions of different parts of the cavity. 3. Examine and draw a cross-section through the body of the hydra at low microscope magnification. Show in the figure the cylindrical shape of the body, the location of the cell layers and the supporting plate, the difference between ectodermal and endodermal cells, the closedness of the cavity (not counting the oral opening).
Work 4. Cellular elements of Hydra. Despite all the morphological and physiological differences, the cells of both layers in Hydra are so similar that they constitute a single type epithelial muscle cells(see Fig. 26). Each of them has a vesicular or cylindrical region with a nucleus in its center; this is the epithelial part that forms the integument in the ectoderm and the digestive layer in the endoderm. At the base of the cell, contractile processes extend - the muscular element of the cell.
The dual nature of the cell structure corresponds to the dual name of this type of cell.
The muscular processes of epithelial muscle cells are adjacent to the supporting plate. In the ectoderm they are located along the body (this is not visible on the preparation), and by contracting them the body of the hydra is shortened; in the endoderm, on the contrary, they are directed across the body and when they contract, the body of the hydra decreases in size cross section and stretches out in length. Thus, by the alternating action of the muscular processes of the ectoderm and endoderm cells, the hydra contracts and stretches in length.
Epithelial areas look different depending on where the cell is located: in the outer or inner layer, in the trunk or in the sole.
The dual nature of the structure of the epithelial-muscle cell corresponds to a dual function.
Very small cellular elements - stinging cells ( nettle cells, cnidoblasts) - are located in groups in the ectoderm of the tentacle (Fig. 27). The center of such a group, called stinging battery, is occupied by a relatively large cell, the penetrant, and several smaller ones, the involutes. Less numerous stinging batteries are also present in the ectoderm of the trunk region. The most common features of the cnidae of flippers are as follows: a protoplasmic body, a special cellular organelle - the stinging capsule (cnida) and a hardly visible thin spine or short hair sticking out, called the cnidocil (Fig. 27).
Upon closer examination of nettle cells, three forms can be distinguished. Penetrants (Fig. 27)
Rice. 27. Hydra stinging cells: A- penetranta - the first type of stinging cells; the cnidoblast is shown at rest (on the left) and with a discarded filament (on the right); B- Volventa; IN- a section of a hydra tentacle with batteries of stinging cells of different types:
1
- penetrants; 2
- volvents; 3
- glutinants; 4 - 13 -
stinging cell elements (4
- cap; 5-cnidoblast, protoplasm and nucleus, 6
- capsule, 7
- capsule wall, 8
- a thread, 9
- neck, 10
- cone, 11
- stilettos, 12
- spines, 13
- cnidocil)
have a large pear-shaped capsule; its wall is strong and elastic. In the capsule lies a coiled long thin cylindrical tube - stinging thread, connected to the capsule wall through a neck -
extensions of the thread, on the inner wall of which there are three pointed stylets and several spines.
At rest, the capsule is closed by a cap, above which the cnidocil protrudes; its specific irritation (mechanical and possibly chemical) activates the cnidoblast (see Fig. 27). The lid opens and the neck extends from the opening of the cnida; stilettos, pointed with their pointed end forward, are pierced into the body of the victim and, turning around, widen the wound; a stinging thread penetrates the latter, which is turned inside out; the poisonous liquid introduced by the thread into the wound paralyzes or kills the victim. The action of the penetrant (from irritation of the nail to the penetration of poison) occurs instantly.
Volvents are somewhat simpler. Their cnidia are devoid of poisonous liquid and have a neck with stylets and spines. The stinging filaments, released during irritation, spirally wrap around the swimming bristles (on the legs or antennae of the crustacean) and thereby create a mechanical obstacle to the movement of prey. The role of glutinants (large and small) is less clear.
Nettle cells serve as an adaptation for hydra to defend and attack. On elongated and slowly moving tentacles, when irritated, numerous stinging batteries are simultaneously activated. The cnidoblast acts once; the one that has failed is replaced by a new one, formed from spare undifferentiated cells.
In addition to the specialized groups of cells studied in practical classes (epithelial-muscular, glandular and nettle), hydra also has other cells that are difficult to study in a laboratory lesson. Nevertheless, for completeness of description, the most important features of these cells are given below.
Interstitial cells, or abbreviated “i-cells” - numerous small cells located in groups in the spaces between the epithelial-muscle cells at their bases; this corresponds to their name as intermediate (see Fig. 26). From them, through transformation, stinging cells (see above) and some other cellular elements are formed. That's why they are also called storage cells. They are in an undifferentiated state and specialize into cells of one type or another as a result of a complex developmental process.
Sensitive cells are concentrated mainly in the ectoderm (see Fig. 26); they are distinguished by their elongated shape; with their pointed end they go out, and with the opposite end they go towards the supporting plate along which their processes extend. At their base, sensory cells apparently come into contact with nerve elements.
Nerve cells are scattered more evenly throughout the body of the hydra, collectively forming a nervous system of a diffuse nature (see Fig. 25); only in the area of the hypostome and sole there is a richer accumulation of them, but nerve center or even nerve ganglia Hydra doesn't have one yet. Nerve cells are interconnected by processes (see Fig. 25), forming something like a network, the nodes of which are represented by nerve cells; for this reason, the nervous system of the hydra is called reticulate. Like sensory cells, nerve cells are concentrated mainly in the ectoderm.
Irritation from the external environment (chemical, mechanical, excluding irritation of cnidoblasts) is perceived by sensitive cells, and the excitation caused by it is transmitted to nerve cells and slowly diffuses throughout the entire system. The hydra's response movements are expressed
in the form of compression of the entire body, i.e. in the form general reaction, despite the local nature of the irritation. All this is evidence of the low level at which the hydra nervous system is located. Nevertheless, it already plays the role of an organ that connects the structural elements B as a single whole (nerve connections in the body), and the body as a whole with the external environment.
Progress, 1. On a microscopic specimen of a longitudinal section (or on a total section), examine a small section of the tentacle under a microscope at high magnification. Study the appearance of stinging cells, their location in the body and the stinging batteries they form. Sketch the studied area of the tentacle with an image of both cell layers, the area of the gastrovascular cavity and the stinging battery, 2. On a microslide prepared in advance from macerated tissue (see page 12), examine and sketch at high magnification different shapes stinging cells and epithelial muscle cells. Mark the details of the structure and indicate their function.
Work 5. Hydra reproduction. Hydras reproduce both vegetatively and sexually.
Vegetative form of reproduction - budding- is carried out as follows. In the lower part of the body of the hydra, a kidney appears as a cone-shaped tubercle. On distal end its (see Fig. 24) several small tubercles appear, turning into tentacles; in the center between them a mouth opening breaks through. A stalk and sole are formed at the proximal end of the bud. Cells of the ectoderm, endoderm and the material of the supporting plate take part in the formation of the kidney. The gastric cavity of the mother's body continues into the kidney cavity. A fully developed bud separates from the parent and begins an independent existence.
The organs of sexual reproduction are represented in hydras by the sex glands, or gonads (see Fig. 24). The ovary is located in the lower part of the trunk; an ovoid cell in the ectoderm, surrounded by special nutrient cells, represents a large egg with numerous outgrowths resembling pseudopodia. Above the egg, the thinned ectoderm breaks through. Testes with numerous spermatozoa are formed in the distal part (closer to the oral end) of the trunk, also in the ectoderm. Through a break in the ectoderm, sperm enter the water and, upon reaching the egg, fertilize it. In hydra dioecious, one individual carries either a male or female gonad; at
hermaphrodite, i.e. bisexual, in the same individual both a testis and an ovary are formed.
Progress. 1. Familiarize yourself with appearance kidneys on a live hydra or on a microslide (total or longitudinal section). Find out the connection between the cell layers and cavity of the kidney with the corresponding structures of the mother’s body. Draw observations at low magnification of the microscope. 2. A longitudinal section of the preparation must be examined and sketched at low microscope magnification. general form Hydra gonads.
Distal, from Latin distar - distant from the center or axis of the body; V in this case distant from the mother's body.
Proximal, from Latin proximus- closest (closest to the body axis or center).
1: Hermaphrodite, from Greek hermaphroditus- an organism with reproductive organs of both sexes.
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 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 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 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).
Differs in more complex life processes compared to the first ones multicellular organisms- sponges. What structural features is this related to? Let's figure it out together.
What is hydra in mythology
The biological species got its name because of its similarities with the mythological hero - the Lernaean Hydra. According to legend, it was a snake-like monster with poisonous breath. The hydra's body had several heads. No one was able to defeat her - in place of the cut-off head, several new ones immediately grew.
The Lernaean Hydra lived in Lake Lerna, where it guarded the entrance to the underground kingdom of Hades. And only Hercules was able to cut off her immortal head. Then he buried her in the ground and covered her with a heavy stone. This is the second labor of Hercules out of twelve.
Hydra: biology
A high ability to restore lost body parts or regenerate is also characteristic of freshwater hydra. This animal is a representative of the coelenterate phylum. So what is a solitary freshwater polyp that leads an exclusively attached lifestyle.
General characteristics of coelenterates
Like all coelenterates, hydra is an aquatic inhabitant. They prefer shallow puddles, lakes or rivers with little current, which allow them to attach to plants or bottom objects.
The classes of coelenterates are represented by hydroids, jellyfish and coral polyps. All their representatives are characterized by ray or radial symmetry. This structural feature is associated with a sedentary lifestyle. In this case, an imaginary point can be placed in the center of the animal’s body, from which rays can be drawn in all directions.
All coelenterates are multicellular animals, but they do not form tissues. Their body is represented by two layers of specialized cells. Inside there is an intestinal cavity in which food is digested. Different classes of coelenterates differ in their lifestyle:
- Hydroids are attached to the substrate using the sole and are solitary.
- Coral polyps are also immobile, but form colonies containing hundreds of thousands of individuals.
- Jellyfish actively swim in the water column. At the same time, their bell contracts and the water is pushed out with force. This movement is called reactive.
Body structure
The body of the freshwater hydra has the shape of a stalk. Its base is called the sole. With its help, the animal attaches to underwater objects. At the opposite end of the body there is a mouth opening surrounded by tentacles. It leads into the intestinal cavity.
The walls of the hydra's body consist of two layers of cells. The outer one is called ectoderm. It consists of dermal-muscular, nerve, intermediate and stinging cells. The inner layer, or endoderm, is formed by their other types - digestive and glandular. Between the layers of the body there is a layer intercellular substance, which looks like a plate.
Cell types and life processes
Since no tissues or organs are formed in the hydra’s body, all physiological processes are carried out with the help of specialized cells. Thus, epithelial-muscular ones provide movement. Yes, despite their fixed lifestyle, hydroids are capable of movement. In this case, the epithelial-muscle cells of one side of the body first contract, the animal “bends over”, stands on the tentacles and again falls on the sole. This movement is called walking.
Between the epithelial-muscular cells there are stellate-shaped nerve cells. With their help, the animal perceives irritations from environment and responds to them in a certain way. For example, if you touch the hydra with a needle, it shrinks.
The ectoderm also contains intermediate cells. They are capable of amazing transformations. If necessary, cells of any type are formed from them. They are the ones who determine high level regeneration of these animals. It is known that hydra can be completely restored from 1/200 of its part or mushy state.
Sex cells are also formed from intermediate cells. This happens with the onset of autumn. In this case, the eggs and sperm fuse to form a zygote, and the mother’s body dies. In the spring, young individuals develop from them. In the summer, by budding, a small tubercle is formed on its body, which increases in size, acquiring the features of an adult organism. As it grows, it splits off and begins to exist independently.
Digestive cells are located in the endoderm of coelenterates. They split nutrients. And enzymes are released into the intestinal cavity, under the influence of which food breaks down into pieces. Thus, hydra is characterized by two types of digestion. They are called intracellular and cavity.
Stinging cells
It is impossible to answer the question of what a hydra is if you do not get acquainted with the features. In nature, they are found only in coelenterate animals. With their help, protection, defeat and retention of prey are carried out. Therefore, most of them are located on the tentacles.
The stinging cell consists of a capsule with a spirally twisted thread. On the surface of this structure there is a sensitive hair. It is he who is touched by prey swimming by. As a result, the thread unwinds and digs forcefully into the victim’s body, paralyzing him.
By type of nutrition, coelenterates, hydra in particular, are heterotrophic predators. They feed on small aquatic invertebrates. For example, daphnia, cyclops, oligochaetes, rotifers, fleas, mosquito larvae and fish fry.
The importance of coelenterates
The importance of hydra in nature lies primarily in the fact that it plays the role of a biological filter. It purifies water from suspended particles that it consumes as food. This is an important link in the food chains of fresh water bodies. Hydras feed on some cladocerans, turbellaria and fish whose size exceeds 4 cm. Hydra itself infects fry with the poison of stinging cells.
But scientists, when asked what a hydra is, will probably answer that it is a well-known object laboratory research. These coelenterates are used to study the features of regeneration processes, the physiology of lower multicellular organisms, and budding.
So, the freshwater hydra is a representative of the Hydroid class. This is a multicellular two-layer animal with radial symmetry, the body of which consists of several types of specialized cells.
Hydra is a typical representative of the class Hydrozoa. It has a cylindrical body shape, reaching a length of up to 1-2 cm. At one pole there is a mouth surrounded by tentacles, the number of which is various types there are from 6 to 12. At the opposite pole, hydras have a sole, which serves to attach the animal to the substrate.
Sense organs
In the ectoderm of hydras there are stinging or nettle cells that serve for defense or attack. In the inner part of the cell there is a capsule with a spirally twisted thread.
Outside this cell there is a sensitive hair. If any small animal touches a hair, the stinging thread quickly shoots out and pierces the victim, who dies from the poison that gets along the thread. Usually many stinging cells are released at the same time. Fish and other animals do not eat hydras.
The tentacles serve not only for touch, but also for capturing food - various small aquatic animals.
Hydras have epithelial-muscle cells in the ectoderm and endoderm. Thanks to the contraction of the muscle fibers of these cells, the hydra moves, “stepping” alternately with its tentacles and its sole.
Nervous system
The nerve cells that form a network throughout the body are located in the mesoglea, and the processes of the cells extend outwards and into the body of the hydra. This type of building nervous system called diffuse. Especially a lot nerve cells located in the hydra around the mouth, on the tentacles and sole. Thus, coelenterates already have the simplest coordination of functions.
Hydrozoans are irritable. When nerve cells are irritated by various stimuli (mechanical, chemical, etc.), the perceived irritation spreads throughout all cells. Thanks to the contraction of muscle fibers, the hydra's body can shrink into a ball.
Thus, for the first time in organic world reflexes appear in coelenterates. In animals of this type, reflexes are still monotonous. In more organized animals they become more complex during the process of evolution.
Digestive system
All hydras are predators. Having captured, paralyzed and killed prey with the help of stinging cells, the hydra with its tentacles pulls it towards the mouth opening, which can stretch very much. Next, food enters the gastric cavity, lined with glandular and epithelial-muscular endoderm cells.
Digestive juice is produced by glandular cells. It contains proteolytic enzymes that promote the absorption of proteins. Food in the gastric cavity is digested by digestive juices and breaks down into small particles. The endoderm cells have 2-5 flagella that mix food in the gastric cavity.
Pseudopodia of epithelial muscle cells capture food particles and subsequently intracellular digestion occurs. Undigested food remains are removed through the mouth. Thus, in hydroids, for the first time, cavity, or extracellular, digestion appears, running in parallel with the more primitive intracellular digestion.
Organ regeneration
In the ectoderm of the hydra there are intermediate cells, from which, when the body is damaged, nerve, epithelial-muscular and other cells are formed. This promotes rapid healing of the wounded area and regeneration.
If a hydra's tentacle is cut off, it will recover. Moreover, if the hydra is cut into several parts (even up to 200), each of them will restore the entire organism. Using the example of hydra and other animals, scientists study the phenomenon of regeneration. The identified patterns are necessary for the development of methods for treating wounds in humans and many vertebrate species.
Hydra reproduction methods
All hydrozoans reproduce in two ways - asexual and sexual. Asexual reproduction is as follows. In the summer, approximately halfway through, the ectoderm and endoderm protrude from the hydra's body. A mound or bud is formed. Due to cell proliferation, the size of the kidney increases.
The gastric cavity of the daughter hydra communicates with the cavity of the mother. A new mouth and tentacles form at the free end of the bud. At the base, the bud is laced, the young hydra is separated from the mother and begins to lead an independent existence.
Sexual reproduction in hydrozoans natural conditions observed in autumn. Some species of hydra are dioecious, while others are hermaphroditic. In freshwater hydra, female and male sex glands, or gonads, are formed from intermediate ectoderm cells, that is, these animals are hermaphrodites. The testes develop closer to the mouth of the hydra, and the ovaries develop closer to the sole. If many motile spermatozoons are formed in the testes, then only one egg matures in the ovaries.
Hermaphroditic individuals
In all hermaphroditic forms of hydrozoans, spermatozoons mature earlier than eggs. Therefore, fertilization occurs cross-fertilization, and therefore self-fertilization cannot occur. Fertilization of eggs occurs in the mother in the autumn. After fertilization, hydras, as a rule, die, and the eggs remain in a dormant state until spring, when new young hydras develop from them.
Budding
Marine hydroid polyps can be, like hydra, solitary, but more often they live in colonies that appear due to the budding of a large number of polyps. Polyp colonies often consist of a huge number of individuals.
In marine hydroid polyps, in addition to asexual individuals, during reproduction through budding, sexual individuals, or jellyfish, are formed.
