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 a typical representative of 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 carried out 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.
Like all coelenterates internal structure The body of the hydra is a two-layer sac, forming a closed (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 shoots 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 high ability hydra to regeneration, i.e. restoration of 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 best suit 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 that plays important role when the animal moves. When everyone's fiber skin-muscular cells contract, the hydra's body contracts. If the fibers contract on only one side of the body, then the hydra bends in that direction. Thanks to the work of muscle fibers, the hydra can slowly move from place to place, alternately “stepping” with its sole and tentacles. This movement can be compared to a slow somersault over your head.
The outer layer contains and nerve cells. They have a star-shaped shape, as they are equipped with long processes.
The processes of neighboring nerve cells come into contact with each other and form nerve plexus, covering the entire body of the hydra. Some of the processes approach the skin-muscle cells.
Hydra irritability and reflexes
Hydra is able to sense touch, temperature changes, the appearance of various dissolved substances in water and other irritations. This causes her nerve cells to become excited. If you touch the hydra with a thin needle, then the excitement from irritation of one of the nerve cells is transmitted along the processes to other nerve cells, and from them to the skin-muscle cells. This causes muscle fibers to contract, and the hydra shrinks into a ball.
Picture: Hydra's irritability
In this example, we get acquainted with a complex phenomenon in the animal body - reflex. The reflex consists of three successive stages: perception of irritation, transfer of excitation from this irritation along the nerve cells and response body by any action. Due to the simplicity of the hydra's organization, its reflexes are very uniform. In the future we will get acquainted with much more complex reflexes in more highly organized animals.
Hydra stinging cells
Pattern: hydra string or nettle cells
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 towards the cells. The cells of the inner layer are capable of releasing pseudopods (like those of an amoeba) and capturing food with them. Further digestion occurs inside the cell, in vacuoles (like in protozoa). Undigested food remains are thrown out through the mouth.
The hydra has no special respiratory organs; oxygen dissolved in water penetrates the hydra through the entire surface of its body.
Hydra regeneration
The outer layer of the hydra's body also contains very small round cells with large nuclei. These cells are called intermediate. They play a very important role in the life of the hydra. With any damage to the body, intermediate cells located near the wounds begin to grow rapidly. From them, skin-muscle, nerve and other cells are formed, and the wounded area quickly heals.
If you cut a hydra crosswise, tentacles grow on one of its halves and a mouth appears, and a stalk appears on the other. You get two hydras.
The process of restoring lost or damaged body parts is called regeneration. Hydra has a highly developed ability to regenerate.
Regeneration, to one degree or another, is also characteristic of other animals and humans. Thus, in earthworms it is possible to regenerate a whole organism from their parts; in amphibians (frogs, newts) entire limbs, different parts of the eye, tail and internal organs. When a person is cut, the skin is restored.
Hydra reproduction
Asexual reproduction of hydra by budding
Figure: Hydra asexual reproduction by budding
Hydra reproduces asexually and sexually. In summer, a small tubercle appears on the hydra’s body - a protrusion of the wall of its body. This tubercle grows and stretches out. Tentacles appear at its end, and a mouth breaks out between them. This is how the young hydra develops, which at first remains connected to the mother with the help of a stalk. Outwardly, all this resembles the development of a plant shoot from a bud (hence the name of this phenomenon - budding). When the little hydra grows up, it separates from the mother’s body and begins to live independently.
Hydra sexual reproduction
By autumn, with the onset 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.
Occur in one cell. In the body of hydra and all other multicellular animals different groups cells have different meaning or, as they also say, various functions.
Structure
The structure of hydra can be different, due to the cells that perform different functions. Groups of cells that have the same structure and perform a specific function in the life of an animal are called tissues. The body of the hydra has developed tissues such as integumentary, muscle and nervous. However, these tissues do not form in its body those complex organs that other multicellular animals have. Thus, hydra is the lowest, that is, the most simple in structure, multicellular animal.
In worms and other animals more complex than the freshwater hydra, organs are formed from tissues. From the bodies performing general function in the life of an animal, organ systems are formed in the body of animals (for example, the nervous system, circulatory system etc.). Hydra does not have organ systems. Hydra reproduces in two ways: sexual and asexual.
nettle cells
To understand why daphnia are paralyzed when they touch the tentacles of a freshwater hydra, it is necessary to examine the structure of the tentacle under a microscope. The entire surface of the tentacle is covered with tiny knotty tubercles. These are special cells that look like bubbles. There are also such cells on the edges of the hydra’s body, but most of them are on the tentacles. The bubbles contain thin threads with points at the ends sticking out. When the prey touches the hydra's body, the thread calm state coiled in the form of a spiral, they are suddenly thrown out of their bubbles and, like arrows, pierce the body of the prey. At the same time, a drop of poison is poured from the bottle into the wound, paralyzing the victim. Hydra cannot attack the relatively thick skin of humans and large animals. But in the seas live animals related to the hydra - sea jellyfish. Large jellyfish can cause severe burns to humans. They burn the skin like nettles. Therefore, these cells are called nettle cells, and the threads are called nettle filaments. Hydra nettle cells are not only an organ of attack on prey, but also an organ of defense.
Muscle cells
Some cells of the outer layer of the hydra's body with inside continued by narrow muscular processes. These processes are located along the body of the hydra. They are capable of contracting. The rapid contraction of the hydra into a small ball in response to irritation occurs precisely due to the contraction of these muscle processes. Cells with such processes are called integumentary muscles. In the life of a hydra, they play the same role as muscles in a person. Thus, the outer cells of the hydra protect it and help it move.
Nerve cells
Hydra perceives irritations by sensitive cells located in the ectoderm (outer layer). These irritations are transmitted through nerve cells located in the integumentary layer, closer to the base of the integumentary muscle cells, on the supporting membrane, connecting with each other. Nerve cells form the nervous network. This network is the beginning nervous system.
From sensitive cells, irritation (for example, from touching with a needle or stick) is transmitted to nerve cells and spreads throughout the hydra’s nervous network. From the nervous network, irritation passes to the integumentary muscle cells. Their processes contract, and the entire body of the hydra contracts accordingly. This is how the hydra responds to external irritations. The contraction of the hydra's body when touched has a protective value.
Digestive cells
The cells of the digestive layer are much larger than the cells of the integumentary layer. On their inner part, facing the intestinal cavity, these cells have long flagella. Moving, the flagella mix food particles trapped inside the intestinal cavity. Digestive cells secrete juice that digests food. Digested food is absorbed by the cells of the digestive layer, and from them enters all cells of the body. Undigested food remains are thrown out through the mouth.
TYPE COELENTERATATYPE COELENTERATA
Hydroid class (Hydrozoa)
Hydra freshwater (Hydra fusca)
A representative of hydroids is freshwater hydra. Hydra lives in ponds, lakes, rivers, and has a cylindrical shape. At one end there is a mouth surrounded by 5 - 12 thin long tentacles, on the other - sole. Using the sole, the hydra attaches to objects. The hydra's body is 1 - 1.5 cm long.
Hydra is characterized by radial symmetry(Fig. 94).
The walls of the hydra's body consist of two layers: the outer - ectoderm and internal - endoderm. Between them there is a structureless mass - mesoglea.
Inside the body of the hydra is gastric cavity.
The mouth opening serves for eating and removing undigested residues (Fig. 95).
Rice. 94.Longitudinal section of a freshwater hydra.
Ectoderm cells are differentiated into epithelial, epithelial-muscular, interstitial (intermediate), stinging, nervous.
Epithelial muscle cells have a body and two contractile processes. These processes are located along the body. When they contract, the body thickens and shortens.
Between the epithelial-muscle cells are small interstitial cells. Due to them, reproductive and stinging cells are formed. The stinging cells contain an oval stinging capsule with dense walls. The capsule is filled with liquid; Inside the capsule there is a spiral thread, on the surface of the cell there is a thin tactile hair. When this hair is irritated, the stinging capsule throws out an elastic thread. The stinging cells serve the hydra for attack and defense (Fig. 96).
Rice. 95. Hydra Hydra fusca.
A- general view hydra; B- longitudinal section: 1 - mouth; 2 - gastric cavity; 3 - stalk; 4 - sole; 5 - egg cell; 6 - spermatozoa: 7 - ectoderm; 8 - endoderm; IN- cross section; G - nerve cells; D - ectodermal epithelial-muscle cell: 1 - core; E - longitudinal section of the hydra body wall: 1 - ectoderm cell; 2 - endoderm cell; 3 - mesoglea; 4 - nerve cell; 5 - epithelial muscle cell; 6 - interstitial cell; 7 - basement membrane; 8 - stinging cell; 9 - glandular cell.
Rice. 96.Stinging cell. 1 - stinging capsule; 2 - tactile hair; 3 - stinging thread with spines; 4 - spikes; 5 - core.
Rice. 97.The location of nerve cells in the body of the hydra (according to Hesse).
Rice. 98.Hydra irritability.
The ectoderm contains star-shaped nerve cells. They are connected to each other by processes, forming a diffuse nervous system (Fig. 95 (d), 97, 98).
Endodermlines the entire gastric cavity (Fig. 99).
Rice. 99.The structure of the endoderm cell (inner layer) of the hydra body.
Cells endoderm differentiated by epithelial-muscular, digestive, glandular, nervous.
The muscular processes of endodermal epithelial-muscular cells are located transversely with respect to the longitudinal axis of the body. When they contract, the hydra's body narrows and becomes thinner.
The epithelial part of the endodermal cells, directed towards the gastric cavity, bears 1-3 flagella and is capable of forming pseudopodia, which can capture small food particles. This is intracellular digestion.
The glandular cells of the endoderm secrete digestive juices directly into the gastric cavity, where digestion also occurs. Hydra combines intracellular and cavity digestion. Hydra feeds on daphnia and cyclops. Hydra breathes over the entire surface of its body.
Hydra breeds asexual And sexually(Fig. 100).
At asexual reproduction on the body of the hydra are formed kidneys They gradually increase in size, take the shape of a hydra and separate from the mother’s body (Fig. 101).
Rice. 100.Hydra fusca at low magnification.
A- hydra with male gonads; B- hydra with female gonads; IN-
budding hydra (according to Polyansky).
Rice. 101.Hydra budding.
When the temperature drops, hydra reproduces sexually.
Hydra - hermaphrodite. Germ cells are formed from interstitial cells of the ectoderm. Tubercles appear at the sites where germ cells are formed.
Rice. 102.Hydra sexual reproduction.
The eggs are located closer to the base, and the sperm are located closer to the mouth of the hydra. Cross fertilization. In autumn, the egg is fertilized in the mother's body, surrounded by a dense shell, then the hydra dies. The eggs remain dormant until spring, when new hydras develop from them (Fig. 102).
Hydra is capable of regeneration.
Questions for self-control
1. What organisms are multicellular?
2. Who is a representative of the hydroid class?
3. Where does hydra live?
4. What is the structure of hydra?
5. How many layers of cells does the hydra’s body consist of?
6. How are ectoderm cells differentiated?
7. What structure do stinging cells have and what functions do they perform?
8. How are endoderm cells differentiated?
9. How does hydra digestion? 10.How does hydra reproduce?
11.How does asexual reproduction occur in hydra? 12. How does sexual reproduction occur in Hydra?
Key words of the topic “Subkingdom Multicellular. Type Coelenterates"
tubercles spring
intracellular digestion
execution
gastric cavity
hermaphrodite
hydra
hydroid daphnia
differentiation diffuse nervous system flagella
glandular cells animals liquid protection
star shape
coelenterates
cells
interstitial cells
nerve cells
stinging cells
epithelial muscle cells
epithelial cells
maternal organism
mesoglea
multicellular
attack
undigested remains
education
lake
fertilization
organism
organs
autumn
base
tactile hair relation
digestive juices
food particles
surface
sole
sub-kingdom
item
representative
meal
process
pond
pseudopodia work
radial symmetry
irritation
dimensions
asexual reproduction
sexual reproduction
regeneration
result
rivers
genus
mouth opening
system
layers
contractile processes resting state spiral filament body wall stinging capsule body tissue type
elastic thread functions
functional unit
Cyclops
tentacles
endoderm
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 the 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 out and into the body of the hydra. This type of structure of the nervous system is called diffuse. Especially many nerve cells are 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 irritating nerve cells 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 is digested in the gastric cavity 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 whole 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 huge number individuals.
In marine hydroid polyps, in addition to asexual individuals, during reproduction through budding, sexual individuals, or jellyfish, are formed.
