Hydra. Obelia. The structure of the hydra. Hydroid polyps
They live in marine and rarely in fresh water bodies. Hydroids are the most simply organized coelenterates: a gastric cavity without septa, nervous system without ganglia, the gonads develop in the ectoderm. Often form colonies. Many have a change of generations in their life cycle: sexual (hydroid jellyfish) and asexual (polyps) (see. Coelenterates).
Hydra sp.(Fig. 1) - a single freshwater polyp. The length of the hydra's body is about 1 cm, its lower part - the sole - serves to attach to the substrate; on the opposite side there is a mouth opening, around which 6-12 tentacles are located.
Like all coelenterates, hydra cells are arranged in two layers. Outer layer called ectoderm, internal - endoderm. Between these layers is the basal plate. In the ectoderm there are the following types cells: epithelial-muscular, stinging, nervous, intermediate (interstitial). Any other ectoderm cells can be formed from small undifferentiated interstitial cells, including germ cells during the reproductive period. At the base of the epithelial-muscle cells are muscle fibers located along the axis of the body. When they contract, the hydra's body shortens. Nerve cells are stellate in shape and located on the basement membrane. Connecting with our long shoots, they form a primitive nervous system diffuse type. The response to irritation is reflexive in nature.
rice. 1.
1 - mouth, 2 - sole, 3 - gastric cavity, 4 - ectoderm,
5 - endoderm, 6 - stinging cells, 7 - interstitial
cells, 8 - epithelial-muscular ectoderm cell,
9 - nerve cell, 10 - epithelial-muscular
endoderm cell, 11 - glandular cell.
The ectoderm contains three types of stinging cells: penetrants, volventes and glutinants. The penetrant cell is pear-shaped, has a sensitive hair - cnidocil, inside the cell there is a stinging capsule, which contains a spirally twisted stinging thread. The capsule cavity is filled with toxic liquid. At the end of the stinging thread there are three spines. Touching the cnidocil causes the release of a stinging thread. In this case, the spines are first pierced into the victim’s body, then the venom of the stinging capsule is injected through the thread channel. The poison has a painful and paralyzing effect.
Stinging cells the other two types are performed additional function retention of prey. Volvents shoot trapping threads that entangle the victim's body. Glutinants release sticky threads. After the threads shoot out, the stinging cells die. New cells are formed from interstitial ones.
Hydra feeds on small animals: crustaceans, insect larvae, fish fry, etc. The prey, paralyzed and immobilized with the help of stinging cells, is sent to the gastric cavity. Digestion of food is cavity and intracellular, undigested residues are excreted through the mouth.
The gastric cavity is lined with endoderm cells: epithelial-muscular and glandular. At the base of the epithelial-muscular cells of the endoderm there are muscle fibers located in the transverse direction relative to the axis of the body; when they contract, the body of the hydra narrows. The area of the epithelial-muscle cell facing the gastric cavity carries from 1 to 3 flagella and is capable of forming pseudopods to capture food particles. In addition to epithelial-muscular cells, there are glandular cells that secrete digestive enzymes into the intestinal cavity.
rice. 2.
1 - maternal individual,
2 - daughter individual (bud).
Hydra reproduces asexually (budding) and sexually. Asexual reproduction occurs in the spring-summer season. The buds are usually formed in the middle areas of the body (Fig. 2). After some time, young hydras separate from the mother’s body and begin to lead an independent life.
Sexual reproduction occurs in autumn. During sexual reproduction, germ cells develop in the ectoderm. Sperm are formed in areas of the body close to the mouth, eggs - closer to the sole. Hydras can be either dioecious or hermaphroditic.
After fertilization, the zygote is covered with dense membranes, and an egg is formed. The hydra dies, and a new hydra develops from the egg the following spring. Direct development without larvae.
Hydra has a high ability to regenerate. This animal is able to recover even from a small severed part of the body. Interstitial cells are responsible for regeneration processes. The vital activity and regeneration of hydra were first studied by R. Tremblay.
Obelia sp.- a colony of marine hydroid polyps (Fig. 3). The colony has the appearance of a bush and consists of individuals of two types: hydranthus and blastostyles. The ectoderm of the members of the colony secretes a skeletal organic shell - the periderm, which performs the functions of support and protection.
Most of the colony's individuals are hydrants. The structure of a hydrant resembles that of a hydra. Unlike hydra: 1) the mouth is located on the oral stalk, 2) the oral stalk is surrounded by many tentacles, 3) the gastric cavity continues in the common “stem” of the colony. Food captured by one polyp is distributed among members of one colony through the branched channels of the common digestive cavity.
rice. 3.
1 - colony of polyps, 2 - hydroid jellyfish,
3 - egg, 4 - planula,
5 - young polyp with a kidney.
The blastostyle has the form of a stalk and does not have a mouth or tentacles. Jellyfish bud from the blastostyle. Jellyfish break away from the blastostyle, float in the water column and grow. The shape of the hydroid jellyfish can be compared to the shape of an umbrella. Between the ectoderm and endoderm there is a gelatinous layer - mesoglea. On the concave side of the body, in the center, on the oral stalk there is a mouth. Numerous tentacles hang along the edge of the umbrella, serving for catching prey (small crustaceans, larvae of invertebrates and fish). The number of tentacles is a multiple of four. Food from the mouth enters the stomach; four straight radial canals extend from the stomach, encircling the edge of the jellyfish's umbrella. The method of movement of the jellyfish is “reactive”; this is facilitated by the fold of ectoderm along the edge of the umbrella, called the “sail”. The nervous system is of a diffuse type, but there are clusters of nerve cells along the edge of the umbrella.
Four gonads are formed in the ectoderm on the concave surface of the body under the radial canals. Sex cells form in the gonads.
From the fertilized egg, a parenchymal larva develops, corresponding to a similar sponge larva. The parenchymula then transforms into a two-layer planula larva. The planula, after swimming with the help of cilia, settles to the bottom and turns into a new polyp. This polyp forms a new colony by budding.
For life cycle obelia is characterized by alternation of asexual and sexual generations. The asexual generation is represented by polyps, the sexual generation by jellyfish.
Description of other classes of the type Coelenterates.
- Subphylum: Medusozoa = Jellyfish-producing
- Class: Hydrozoa Owen, 1843 = Hydrozoans, hydroids
- Subclass: Hydroidea = Hydroids
- Genus: Hydra = Hydras
- Genus: Porpita = Porpita
Order: Anthoathecata (=Hydrida) = Hydras
Genus: Hydra = Hydras
Hydras are very widespread and live only in stagnant bodies of water or rivers with a slow flow. By nature, hydras are a single, sedentary polyp, with a body length from 1 to 20 mm. Hydras usually attach to a substrate: aquatic plants, soil or other objects in the water.
Hydra has a cylindrical body and has radial (uniaxial-heteropole) symmetry. At its front end, on a special cone, there is a mouth, which is surrounded by a corolla consisting of 5-12 tentacles. The body of some types of hydras is divided into the body itself and the stalk. At the same time, at the rear end of the body (or stalk) opposite the mouth there is a sole, the organ of movement and attachment of the hydra.
In structure, the hydra's body is a bag with a wall of two layers: a layer of ectoderm cells and a layer of endoderm cells, between which is mesoglea - a thin layer of intercellular substance. The hydra's body cavity, or gastric cavity, forms protrusions or outgrowths that extend inside the tentacles. One main oral opening leads into the gastric cavity of the hydra, and on the sole of the hydra there is also an additional opening in the form of a narrow aboral pore. It is through this that fluid can be released from the intestinal cavity. From here, a gas bubble is also released, and the hydra, along with it, detaches from the substrate and floats to the surface, being held down by its head (front) end in the water column. It is in this way that it can spread throughout a reservoir, covering a considerable distance with the current. The functioning of the mouth opening is also interesting, which is virtually absent in a non-feeding hydra, since the ectoderm cells of the mouth cone tightly close, forming tight contacts that differ little from those in other parts of the body. Therefore, when feeding, each time the hydra needs to break through and open its mouth again.
The bulk of the hydra's body is formed by epithelial-muscular cells of the ectoderm and endoderm, of which there are about 20,000 in the hydra. Epithelial-muscle cells of the ectoderm and endoderm are two independent cell lines. Ectoderm cells are cylindrical in shape, forming a single layer covering epithelium. The contractile processes of these cells are adjacent to the mesoglea, and they form the longitudinal muscles of the hydra. Epithelial-muscular cells of the endoderm bear 2-5 flagella and are directed by the epithelial parts into the intestinal cavity. On the one hand, these cells, thanks to the activity of flagella, mix food, and on the other hand, these cells can form pseudopods, with the help of which they capture food particles inside the cell, where digestive vacuoles are formed.
The epithelial-muscular cells of the ectoderm and endoderm in the upper third of the hydra's body are capable of dividing mitotically. The newly formed cells gradually shift: some towards the hypostome and tentacles, others towards the sole. At the same time, as they move from the place of reproduction, cell differentiation occurs. Thus, those ectoderm cells that are on the tentacles are transformed into stinging battery cells, and on the sole they become glandular cells that secrete mucus, which is so necessary for attaching the hydra to the substrate.
Located in the body cavity of the hydra, the glandular cells of the endoderm, of which there are about 5000, secrete digestive enzymes that break down the food in the intestinal cavity. And glandular cells are formed from intermediate or interstitial cells (i-cells). They are located between the epithelial-muscular cells and have the appearance of small, round cells, of which the hydra has about 15,000. These undifferentiated cells can turn into any type of cell of the hydra body, except epithelial-muscular. They have all the properties of stem cells and are potentially capable of producing both reproductive and somatic cells. Although intermediate stem cells themselves do not migrate, their differentiating descendant cells are capable of fairly rapid migrations.
Hydra biology description internal structure photo lifestyle food reproduction protected from enemies
Latin name Hydrida
To characterize the structure of a hydroid polyp, we can use as an example freshwater hydras, which retain very primitive organizational features.
External and internal structure
Hydras They have an elongated, sac-like body, capable of stretching quite strongly and shrinking almost into a spherical lump. A mouth is placed at one end; this end is called the oral or oral pole. The mouth is located on a small elevation - the oral cone, surrounded by tentacles that can stretch and shorten very strongly. When extended, the tentacles are several times the length of the hydra's body. The number of tentacles varies: there can be from 5 to 8, and some hydras have more. In Hydra, there is a central gastric section, which is somewhat more expanded, turning into a narrowed stalk ending in a sole. With the help of the sole, the hydra attaches to the stems and leaves of aquatic plants. The sole is located at the end of the body, which is called the aboral pole (opposite to the oral, or oral).
The body wall of the hydra consists of two layers of cells - ectoderm and endoderm, separated by a thin basal membrane, and limits a single cavity - the gastric cavity, which opens outwards with the oral opening.
In hydras and other hydroids, the ectoderm is in contact with the endoderm along the very edge of the mouth opening. In freshwater hydras, the gastric cavity continues into the tentacles, which are hollow inside, and their walls are also formed by ectoderm and endoderm.
Hydra ectoderm and endoderm consist of large number cells various types. The main mass of cells of both ectoderm and endoderm are epithelial-muscle cells. Their outer cylindrical part is similar to ordinary epithelial cells, and the base adjacent to the basal membrane is elongated fusiform and consists of two contractile muscular processes. In the ectoderm, the contractile muscular processes of these cells are elongated in the direction of the longitudinal axis of the hydra's body. Their contractions cause shortening of the body and tentacles. In the endoderm, the muscular processes are elongated in a circular direction, across the axis of the body. Their contraction has the opposite effect: the body of the hydra and its tentacles narrow and at the same time lengthen. Thus, the muscle fibers of the epithelial-muscle cells of the ectoderm and endoderm, opposite in their action, make up the entire hydra musculature.
Among the epithelial-muscular cells, various stinging cells are located either singly or, more often, in groups. The same type of hydra, as a rule, has several types of stinging cells that perform different functions.
The most interesting are stinging cells with nettle-like properties, called penetrants. When stimulated, these cells release a long filament that pierces the body of the prey. The stinging cells are usually pear-shaped. A stinging capsule is placed inside the cage, covered with a lid on top. The wall of the capsule continues inward, forming a neck, which then passes into a hollow filament, coiled and closed at the end. At the junction of the neck and the filament, there are three spines inside, folded together and forming a stylet. In addition, the neck and stinging thread are lined with small spines on the inside. On the surface of the stinging cell there is a special sensitive hair - the cnidocil, at the slightest irritation of which the stinging thread is ejected. First, the cap opens, the neck is unscrewed, and the stiletto is pierced into the victim’s cover, and the spikes that make up the stiletto move apart and widen the hole. Through this hole, the twisting thread is pierced into the body. Inside the stinging capsule there are substances that have nettle properties and paralyze or kill prey. Once fired, the stinging thread cannot be used again by the hydroid. Such cells usually die and are replaced by new ones.
Another kind of stinging cells of hydras are volventa. They do not have nettle properties, and the threads they throw out serve to hold prey. They wrap around the hairs and bristles of crustaceans, etc. The third group of stinging cells are glutinants. They throw out sticky threads. These cells are important both in retaining prey and in moving the hydra. Stinging cells are usually located, especially on the tentacles, in groups called “batteries”.
The ectoderm contains small undifferentiated cells, the so-called interstitial, through which many types of cells develop, mainly stinging and reproductive cells. Interstitial cells are often located in groups at the base of epithelial muscle cells.
The perception of irritations in hydra is associated with the presence of sensitive cells in the ectoderm that serve as receptors. These are narrow, tall cells with outside hair. Deeper, in the ectoderm, closer to the base of the skin-muscle cells, there are nerve cells equipped with processes through which they contact each other, as well as with receptor cells and contractile fibers of the skin-muscle cells. Nerve cells are located scatteredly in the depths of the ectoderm, forming with their processes a plexus in the form of a mesh, and this plexus is denser on the perioral cone, at the base of the tentacles and on the sole.
The ectoderm also contains glandular cells that secrete adhesive substances. They concentrate on the sole and on the tentacles, helping the hydra temporarily attach to the substrate.
Thus, in the ectoderm of the hydra there are cells of the following types: epithelial-muscular, stinging, interstitial, nervous, sensory, glandular.
The endoderm has less differentiation of cellular elements. If the main functions of the ectoderm are protective and motor, then the main function of the endoderm is digestive. According to this most of endoderm cells consists of epithelial-muscle cells. These cells are equipped with 2-5 flagella (usually two), and are also capable of forming pseudopodia on the surface, capturing them, and then digesting food particles. In addition to these cells, the endoderm contains special glandular cells that secrete digestive enzymes. The endoderm also contains nerve and sensory cells, but in much smaller quantities than in the ectoderm.
Thus, the endoderm also contains several types of cells: epithelial-muscular, glandular, nervous, sensory.
Hydras do not remain attached to the substrate all the time; they can move from one place to another in a very unique way. Most often, hydras move “walking”, like the caterpillars of moths: the hydra bends with its oral pole towards the object on which it sits, sticks to it with its tentacles, then the sole comes off the substrate, is pulled up to the oral end and is attached again. Sometimes the hydra, having attached itself to the substrate with tentacles, lifts the stalk with the sole upward and immediately carries it to the opposite side, as if “tumbling.”
Hydra Power
Hydras are predators; they sometimes feed on quite large prey: crustaceans, insect larvae, worms, etc. With the help of stinging cells, they capture, paralyze and kill prey. Then the victim is pulled with tentacles to the highly distensible mouth opening and moves into the gastric cavity. In this case, the gastric region of the body becomes greatly inflated.
Digestion of food in hydra, unlike sponges, only partially occurs intracellularly. This is associated with the transition to predation and the capture of fairly large prey. The secretion of glandular cells of the endoderm is secreted into the gastric cavity, under the influence of which the food softens and turns into mush. Small food particles are then captured by the digestive cells of the endoderm, and the digestion process is completed intracellularly. Thus, in hydroids, intracellular or cavity digestion first occurs, which occurs simultaneously with the more primitive intracellular digestion.
Protection from enemies
The nettle cells of the hydra not only infect prey, but also protect the hydra from enemies, causing burns to predators attacking it. And yet there are animals that feed on hydras. These are, for example, some ciliated worms and especially Microstomum lineare, some gastropods (pond worms), Corethra mosquito larvae, etc.
The hydra's ability to regenerate is very high. Experiments carried out by Tremblay back in 1740 showed that pieces of the body of a hydra, cut into several dozen pieces, regenerate into a whole hydra. However, high regenerative ability is characteristic not only of hydras, but also of many other coelenterates.
Reproduction
Hydras reproduce in two ways - asexual and sexual.
Asexual reproduction of hydras occurs by budding. IN natural conditions hydra budding occurs throughout the summer. IN laboratory conditions budding of hydras is observed with sufficiently intense nutrition and a temperature of 16-20 ° C. Small swellings are formed on the body of the hydra - buds, which are protrusions of the ectoderm and endoderm outward. In them, due to the multiplying cells, further growth of the ectoderm and endoderm occurs. The kidney increases in size, its cavity communicates with the gastric cavity of the mother. At the free, outer end of the bud, tentacles and a mouth opening are finally formed.
Soon the newly formed young hydra separates from the mother.
Sexual reproduction of hydras in nature is usually observed in the fall, and in laboratory conditions it can be observed with insufficient nutrition and a drop in temperature below 15-16 ° C. Some hydras are dioecious (Pelmatohydra oligactis), others are hermaphrodites (Chlorohydra viridissima).
Sex glands - gonads - appear in hydras in the form of tubercles in the ectoderm. In hermaphrodite forms, male and female gonads are formed in various places. The testes develop closer to the oral pole, and the ovaries develop closer to the aboral pole. It is formed in the testes a large number of motile sperm. Only one egg matures in the female gonad. In hermaphrodite forms, the maturation of sperm precedes the maturation of eggs, which ensures cross-fertilization and eliminates the possibility of self-fertilization. The eggs are fertilized in the mother's body. The fertilized egg is covered with a shell and spends the winter in this state. Hydras, as a rule, die after the development of sexual products, and in the spring a new generation of hydras emerges from the eggs.
Thus, in freshwater hydras under natural conditions, there is a seasonal change in forms of reproduction: throughout the summer, hydras bud intensively, and in the fall (for middle zone Russia - in the second half of August), with a decrease in temperature in reservoirs and a decrease in the amount of food, they stop reproducing by budding and switch to sexual reproduction. In winter, hydras die, and only fertilized eggs overwinter, from which young hydras emerge in the spring.
The freshwater polyp Polipodium hydriforme also belongs to the order Hydra. Early stages The development of this polyp takes place in the eggs of sterlets and causes great harm to them. Several types of hydra are found in our reservoirs: stalked hydra (Pelmatohydra oligactis), common hydra (Hydra vulgaris), green hydra (Chlorohydra viridissima) and some others.
The common hydra lives in freshwater bodies, attaches one side of its body to aquatic plants and underwater objects, and leads sedentary lifestyle life, feeds on small arthropods (daphnia, cyclops, etc.). Hydra is typical representative coelenterates and has characteristic features their structures.
External structure of the hydra
The hydra's body size is about 1 cm, excluding the length of the tentacles. The body has a cylindrical shape. On one side there is mouth opening surrounded by tentacles. On the other side - sole, they attach the animal to objects.
The number of tentacles can vary (from 4 to 12).
Hydra has a single life form polyp(i.e., it does not form colonies, since during asexual reproduction the daughter individuals are completely separated from the mother; hydra also does not form jellyfish). Asexual reproduction occurs budding. At the same time, a new small hydra grows in the lower half of the hydra’s body.
Hydra is capable of changing its body shape within certain limits. It can bend, bend, shorten and lengthen, and extend its tentacles.
Internal structure of the hydra
Like all coelenterates, in terms of the internal structure of the body, the hydra is a two-layer sac that forms a closed structure (there is only a mouth opening) intestinal cavity. The outer layer of cells is called ectoderm, internal - endoderm. Between them there is a gelatinous substance mesoglea, mainly performing a supporting function. The ectoderm and endoderm contain several types of cells.
Mostly in the ectoderm epithelial muscle cells. At the base of these cells (closer to the mesoglea) there are muscle fibers, the contraction and relaxation of which ensures the movement of the hydra.
Hydra has several varieties stinging cells. Most of them are on the tentacles, where they are located in groups (batteries). The stinging cell contains a capsule with a coiled thread. On the surface of the cell, a sensitive hair “looks” out. When the hydra's victims swim by and touch the hairs, a stinging thread shoots out of the cage. In some stinging cells, the threads pierce the arthropod's cover, in others they inject poison inside, in others they stick to the victim.
Among the ectoderm cells, Hydra has nerve cells. Each cell has many processes. Connecting with their help, nerve cells form the hydra nervous system. Such a nervous system is called diffuse. Signals from one cell are transmitted across the network to others. Some processes of nerve cells contact epithelial muscle cells and cause them to contract when necessary.
Hydras have intermediate cells. They give rise to other types of cells, except epithelial-muscular and digestive-muscular. All these cells provide the hydra with a high ability to regenerate, that is, restore lost parts of the body.
In the body of the hydra in the fall they are formed germ cells. Either sperm or eggs develop in the tubercles on her body.
The endoderm consists of digestive muscle and glandular cells.
U digestive muscle cell on the side facing the mesoglea there is a muscle fiber, like epithelial muscle cells. On the other side, facing the intestinal cavity, the cell has flagella (like euglena) and forms pseudopods (like amoeba). The digestive cell scoops up food particles with flagella and captures them with pseudopods. After this, a digestive vacuole is formed inside the cell. Obtained after digestion nutrients are used not only by the cell itself, but are also transported to other types of cells through special tubules.
Glandular cells secrete a digestive secretion into the intestinal cavity, which ensures the breakdown of prey and its partial digestion. In coelenterates, cavity and intracellular digestion are combined.
Figure: Structure freshwater hydra. Radial symmetry of Hydra
Habitat, structural features and vital functions of the freshwater hydra polyp
In lakes, rivers or ponds with clean, clear water a small translucent animal is found on the stems of aquatic plants - polyp hydra(“polyp” means “multi-legged”). This is an attached or sedentary coelenterate animal with numerous tentacles. The body of an ordinary hydra has an almost regular cylindrical shape. At one end is mouth, surrounded by a corolla of 5-12 thin long tentacles, the other end is elongated in the form of a stalk with sole at the end. Using the sole, the hydra is attached to various underwater objects. The body of the hydra, together with the stalk, is usually up to 7 mm long, but the tentacles can extend several centimeters.
Radial symmetry of Hydra
If you draw an imaginary axis along the body of the hydra, then its tentacles will diverge from this axis in all directions, like rays from a light source. Hanging down from some aquatic plant, the hydra constantly sways and slowly moves its tentacles, lying in wait for prey. Since the prey can appear from any direction, the tentacles arranged in a radial manner are best suited to this method of hunting.
Radiation symmetry is characteristic, as a rule, of animals leading an attached lifestyle.
Hydra intestinal cavity
The body of the hydra has the form of a sac, the walls of which consist of two layers of cells - the outer (ectoderm) and the inner (endoderm). Inside the body of the hydra there is intestinal cavity(hence the name of the type - coelenterates).
The outer layer of hydra cells is the ectoderm.
Figure: structure of the outer layer of cells - hydra ectoderm
The outer layer of hydra cells is called - ectoderm. Under a microscope, several types of cells are visible in the outer layer of the hydra - the ectoderm. Most of all here are skin-muscular. By touching their sides, these cells create the cover of the hydra. At the base of each such cell there is a contractile muscle fiber, which plays an important role in the movement of the animal. When everyone's fiber skin-muscular cells contract, the hydra's body contracts. If the fibers contract on only one side of the body, then the hydra bends in that direction. Thanks to the work of muscle fibers, the hydra can slowly move from place to place, alternately “stepping” with its sole and tentacles. This movement can be compared to a slow somersault over your head.
The outer layer contains and nerve cells. They have a star-shaped shape, as they are equipped with long processes.
The processes of neighboring nerve cells come into contact with each other and form nerve plexus, covering the entire body of the hydra. Some of the processes approach the skin-muscle cells.
Hydra irritability and reflexes
Hydra is able to sense touch, temperature changes, the appearance of various dissolved substances in water and other irritations. This causes her nerve cells to become excited. If you touch the hydra with a thin needle, then the excitement from irritation of one of the nerve cells is transmitted along the processes to others nerve cells, and from them - to the skin-muscle cells. This causes muscle fibers to contract, and the hydra shrinks into a ball.
Picture: Hydra's irritability
In this example, we get acquainted with a complex phenomenon in the animal body - reflex. The reflex consists of three successive stages: perception of irritation, transfer of excitation from this irritation along the nerve cells and response body by any action. Due to the simplicity of the hydra's organization, its reflexes are very uniform. In the future we will become familiar with much more complex reflexes in more highly organized animals.
Hydra stinging cells
Pattern: Stringing or nettle cells of Hydra
The entire body of the hydra and especially its tentacles are seated with a large number stinging, or nettles cells. Each of these cells has complex structure. In addition to the cytoplasm and nucleus, it contains a bubble-like stinging capsule, inside which a thin tube is folded - stinging thread. Sticking out of the cage sensitive hair. As soon as a crustacean, small fish or other small animal touches a sensitive hair, the stinging thread quickly straightens, its end is thrown out and pierces the victim. Through a channel passing inside the thread, poison enters the body of the prey from the stinging capsule, causing the death of small animals. As a rule, many stinging cells are fired at once. Then the hydra uses its tentacles to pull the prey to its mouth and swallows it. The stinging cells also serve the hydra for protection. Fish and aquatic insects do not eat hydras, which burn their enemies. The poison from the capsules is reminiscent of nettle poison in its effect on the body of large animals.
The inner layer of cells is the hydra endoderm
Figure: structure of the inner layer of cells - hydra endoderm
Inner layer of cells - endoderm A. The cells of the inner layer - the endoderm - have contractile muscle fibers, but the main role of these cells is to digest food. They secrete digestive juice into the intestinal cavity, under the influence of which the hydra’s prey softens and breaks down into small particles. Some of the cells of the inner layer are equipped with several long flagella (as in flagellated protozoa). The flagella are in constant motion and sweep particles toward the cells. The cells of the inner layer are capable of releasing pseudopods (like those of an amoeba) and capturing food with them. Further digestion occurs inside the cell, in vacuoles (like in protozoa). Undigested food remains are thrown out through the mouth.
The hydra has no special respiratory organs; oxygen dissolved in water penetrates the hydra through the entire surface of its body.
Hydra regeneration
The outer layer of the hydra's body also contains very small round cells with large nuclei. These cells are called intermediate. They play a very important role in the life of the hydra. With any damage to the body, intermediate cells located near the wounds begin to grow rapidly. From them, skin-muscle, nerve and other cells are formed, and the wounded area quickly heals.
If you cut a hydra crosswise, tentacles grow on one of its halves and a mouth appears, and a stalk appears on the other. You get two hydras.
The process of restoring lost or damaged body parts is called regeneration. Hydra has a highly developed ability to regenerate.
Regeneration, to one degree or another, is also characteristic of other animals and humans. Thus, in earthworms it is possible to regenerate a whole organism from their parts; in amphibians (frogs, newts) entire limbs, different parts of the eye, tail and internal organs. When a person is cut, the skin is restored.
Hydra reproduction
Asexual reproduction of hydra by budding
Drawing: asexual reproduction hydra budding
Hydra reproduces asexually and sexually. In summer, the hydra appears on the body small bump- protrusion of the wall of her body. This tubercle grows and stretches out. Tentacles appear at its end, and a mouth breaks out between them. This is how the young hydra develops, which at first remains connected to the mother with the help of a stalk. Outwardly, all this resembles the development of a plant shoot from a bud (hence the name of this phenomenon - budding). When the little hydra grows up, it separates from the mother’s body and begins to live independently.
Hydra sexual reproduction
By autumn, with the onset of unfavorable conditions, hydras die, but before that, sex cells develop in their body. There are two types of germ cells: ovoid, or female, and spermatozoa, or male reproductive cells. Sperm are similar to flagellated protozoa. They leave the hydra's body and swim using a long flagellum.
Drawing: sexual reproduction hydra
The hydra egg cell is similar to an amoeba and has pseudopods. The sperm swims up to the hydra with the egg cell and penetrates inside it, and the nuclei of both sex cells merge. Happening fertilization. After this, the pseudopods are retracted, the cell is rounded, and a thick shell is formed on its surface - a egg. At the end of autumn, the hydra dies, but the egg remains alive and falls to the bottom. In the spring, the fertilized egg begins to divide, the resulting cells are arranged in two layers. From them a small hydra develops, which, with the onset of warm weather, comes out through a break in the egg shell.
Thus, the multicellular animal hydra at the beginning of its life consists of one cell - an egg.
Loading...