Respiratory organs of the intestinal cavity. Type Intestinal

Coelenterates are multicellular animals with radial (radial) symmetry. Their body consists of two layers of cells and has a sac-like, so-called intestinal cavity. Coelenterates are characterized by the presence of special stinging cells.

Radial symmetry is a characteristic common feature of sedentary or sedentary animals. In this case, the animal can be equally threatened from any direction, and food also comes from all sides. Therefore, the bodies of these animals are arranged in such a way that the means of protection or catching prey are directed in different directions, like rays (or radii) from a single center.

Coelenterates are the most ancient and primitive multicellular animals. They evolved from primitive primary multicellular organisms.

All coelenterates are aquatic animals, most of which live in the seas and oceans. They inhabit the seas from the surface to the utmost depths, from tropical waters to the polar regions. A small number of species live in fresh waters. Now about 9000 species of intestinal animals are known. There are solitary and colonial animals among them.

A group of individuals that have similar adaptations for living in the same environment is called an animal life form. Modern coelenterates are characterized by two life forms (two generations): an attached form - a polyp and a free-floating form - a jellyfish.

Polyps (from the Greek polyp - "many-legged") - a life form, so named for its numerous tentacles. In rare cases (Fig. 36, A), polyps are solitary (for example, hydra and actinia), more often they form colonies of up to several thousand individuals. In the form of a jellyfish (Fig. 36, B), coelenterates, as a rule, live alone.

Rice. 36. Schemes of the structure of intestinal animals: A - polyp; B - jellyfish

In many coelenterates, both life forms (both generations) replace each other (alternate) during the life cycle - from the birth of the organism to death. Some (hydras, coral polyps) do not have a free-floating form - jellyfish. Others (some scyphoid jellyfish; see next paragraph) have lost their polyp form.

The body of the coelenterates resembles a two-layer bag open at one end. The outer layer of cells is called ectoderm (from the Greek ectos - “outside” and derma - “skin”), and the inner layer is called endoderm (from the Greek entos - “inside” and derma - “skin”). The only body cavity of these animals - the intestinal one - communicates with the external environment through the oral opening (mouth). Through the mouth, food enters the intestinal cavity, and undigested residues are ejected through it.

In intestinal cavities, stinging cells are located on the tentacles. They serve both for catching prey and for defense. Intestinal - predators. They feed on various small animals "floating" in the water column.

Coelenterates reproduce both asexually and sexually.

The value of coelenterates

Coelenterates have great importance in nature. Many fish feed on coral polyps and hide among the calcareous branching "forests" built by these animals. Sea turtles and some fish feed on jellyfish. The intestinal cavities themselves, being predators, affect marine animal communities by eating planktonic organisms, and large sea anemones and jellyfish also eat small fish. The person uses some coelenterates. From the dead calcareous parts of coral reefs in some coastal countries, building material is mined, lime is obtained during firing. Some types of jellyfish are edible. Black and red corals are used to make jewelry.

Some swimming jellyfish, anemones and stinging corals can cause severe burns to fishermen, divers and swimmers. Coral reefs hinder navigation in some places.

Freshwater hydra - solitary polyp

Habitat. External structure. Lifestyle. Movement. Freshwater hydra lives in the rises with clean water (in the backwaters of rivers, lakes and ponds) (Fig. 37). This is a small translucent animal about 1 cm long. The body of the hydra has a cylindrical shape. Its lower end (sole) is attached to the stems and leaves of aquatic plants, snags and stones. At the upper end of the body, around the mouth, there are 6-12 tentacles. Hydra, like other coelenterates, is characterized by radial symmetry. Hydra leads a sedentary lifestyle. Her body and tentacles can lengthen and shorten. In a calm state, the tentacles are extended by several centimeters. The animal slowly moves them from side to side, lying in wait for prey. If necessary, the hydra can move slowly. At the same time, it seems to be walking (somersaulting), attaching to underwater objects alternately with the upper and lower ends of the body.

Rice. 37. Scheme of the structure of freshwater hydra: 1 - ectoderm; 2 - endoderm; 3 - mesogley: 4 - sole; 5 - intestinal cavity; 6 - kidney; 7 - tentacle; 8 - mouth

The body of the hydra has the form of an oblong sac, the walls of which consist of two layers of cells - the ectoderm and endoderm. Between them lies a thin gelatinous non-cellular layer - mesoglea, which serves as a support. The intestinal cavity of the hydra communicates with the external environment only through the mouth.

The ectoderm forms the cover of the animal's body and consists of several types of cells (Fig. 38). The most numerous of them are epithelial-muscular. Due to the muscle fibers that lie at the base of each cell, the body of the hydra can contract, lengthen and bend.

Rice. 38. Section through the body of the hydra - cells of the ectoderm (1-4) and endoderm (5, 6): 1 - epithelial-muscular cells; 2 - intermediate cell; 3 - stinging cell; 4 - nerve cell; 5 - digestive-muscular cell; 6 - glandular cell; 7 - mesoglea

The ectoderm contains star-shaped nerve cells. The processes of neighboring nerve cells are interconnected, forming a nervous network that covers the entire body of the animal and is the most primitive nervous system in animals.

If the hydra is touched or pricked with a needle, the animal will shrink. This is because the signal received by even one cell will spread throughout the entire nervous network. Nerve cells "give command" to epithelial-muscle cells. There is a contraction of muscle fibers, then the entire body of the hydra is shortened (Fig. 39). The response of the hydra organism to such irritation is an example of an unconditioned reflex. Unconditioned reflexes are characteristic of all multicellular animals.

Rice. 39. Conduct irritation nerve cells hydra

In the ectoderm there are also stinging cells that serve for attack and defense. They are mainly located on the tentacles of the hydra. Each stinging cell contains an oval capsule in which the stinging thread is folded. If the prey or the enemy touches the sensitive hair, which is located outside the stinging cell, in response to irritation, the stinging thread is ejected and pierces the victim's body (Fig. 40). Through the channel of the thread, a substance that can paralyze it enters the body of the victim.

Rice. 40. Diagram of the structure of the stinging cell: 1 - nucleus; 2 - stinging capsule; 3 - sensitive hair; 4 - stinging thread: folded (left) and ejected (right)

There are several types of stinging cells. The threads of some pierce skin animals and inject poison into their bodies. The threads of others wrap around prey. The threads of the third ones are very sticky And adhere to the victim. Usually the hydra "shoots" several stinging cells.

Intermediate cells are also found in the ectoderm. They form other types of cells.

The endoderm lines the entire intestinal cavity from the inside. The endoderm consists of digestive-muscular and glandular cells. There are more digestive-muscular cells than others. Their muscle fibers are capable of contraction. When they shorten, the hydra's body becomes thinner. Complex movements, such as the movement of the hydra "tumbling", occur due to contractions of the muscle fibers of the cells of the ectoderm and endoderm.

Food. Each of the digestive-muscular cells of the endoderm has one to three flagella. Oscillating flagella create a flow of water, which feeds food particles to the cells. The glandular cells present in the endoderm secrete digestive juice into the intestinal cavity, which liquefies and partially digests food.

The digestive-muscular cells of the endoderm are able to form pseudopods, capture and digest small food particles in the digestive vacuoles. Thus, digestion in hydra and all coelenterates is intracellular and cavitary.

Nutrients are distributed throughout the hydra's body. Undigested residues are expelled out through the mouth. Hydras feed on small invertebrates (often crustaceans - daphnia and cyclops), which they catch with tentacles.

Respiration and excretion. Hydra breathes oxygen dissolved in water. She does not have respiratory organs, and she absorbs oxygen throughout the surface of the body, releasing carbon dioxide to the outside. In the process of life, harmful substances are formed in the cells, which are released into the water.

Reproduction and development. Hydra reproduces both sexually and asexually. Asexual reproduction is carried out by budding (Fig. 41). A protrusion is formed on the body of the hydra - a kidney. It consists of two layers of cells - ectoderm and endoderm - and communicates with a common cavity with the mother's body. The kidney increases, grows in length. A mouth and small tentacles appear at its top, and a sole appears at the base. After that, the young hydra separates from the mother's body, sinks to the bottom and moves on to an independent lifestyle. Often several buds are formed on the body of the hydra at once. Hydras most often reproduce asexually.

Rice. 41. Asexual reproduction of hydra (budding)

In autumn, with the approach of cold weather, hydras begin to reproduce sexually (Fig. 42). Sex cells are formed in the ectoderm from intermediate cells. Tubercles of two types appear on the body of the hydra. Some produce sperm, while others produce eggs.

Rice. 42. Hydra sexual reproduction

There are hydras in which sperm and eggs are formed on different individuals. These are separate animals. In other types of hydra, both spermatozoa and eggs are formed on the body of one organism. Such animals, combining the characteristics of both female and male, are called hermaphrodites.

Once in the water, the spermatozoon swims with the help of a long flagellum and reaches the immobile eggs. Fertilization (the connection of the sperm with the egg) occurs in the body of the mother's body. After that, a dense shell forms around the fertilized egg. A fertilized egg divides many times - an embryo is formed. In autumn, the hydra dies, and the shelled embryos sink to the bottom. In spring, the development of the embryo continues. After the reservoir warms up, the shells covering the embryos are destroyed and small hydras come out.

Regeneration. A damaged hydra easily restores lost body parts (Fig. 43), not only after it has been cut in half, but even if it has been dissected into many parts. From each part a new small hydra is formed. This is due to the intensive division of intermediate cells, from which other types of cells arise. The ability of animals to restore damaged or lost body parts is called regeneration.

Rice. 43. Hydra Regeneration

Hydra is a multicellular animal with a primitive structure. Her intestinal cavity looks like a solid bag. Nervous system consists of scattered stellate nerve cells that form the nervous network. Asexual reproduction occurs by budding. Hydra also reproduces sexually.

Lesson learned exercises

1. List the main structural features of representatives of the intestinal type.
2. What is the significance of different types hydra cells?
3. Describe the radial symmetry on the example of one of the representatives of the intestinal type.
4. What kind of lifestyle do coelenterates lead?
5. How does a hydra move?
6. Describe the life features of intestinal cavities: nutrition, digestion, reproduction (for example, hydra).
7. Explain, using the picture, the process of regeneration in hydra.
8. What is the importance of intestinal?

Type coelenterates cell classes meaning biology sponge body characteristic structure life symmetry representatives of the system signs of jellyfish general characteristics hydroid groups

Latin name Coelenterata

To type coelenterates include lower multicellular animals, which, however, are superior to sponges in a number of ways. These are predominantly marine animals, and only a few of them live in fresh waters. The coelenterates type includes about 9000 species. The body of the intestinal cavities is formed by two layers of cells: the outer one - the ectoderm and the inner one - the endoderm. Between the ectoderm and endoderm there is a structureless substance, which in some forms (hydra, marine hydroid polyps) forms a thin basement membrane, while in others (hydromedusas, scyphomedusas, coral polyps) it is represented by gelatinous mesoglea.

Coelenterates

General characteristics of intestinal

The intestinal type includes lower multicellular animals, which, however, are superior to sponges in a number of ways. These are predominantly marine animals, and only a few of them live in fresh waters. Type of coelenterates in includes about 9000 species. The body of the intestinal cavities is formed by two layers of cells: the outer one - the ectoderm and the inner one - the endoderm. Between the ectoderm and endoderm there is a structureless substance, which in some forms ( hydra, marine hydroid polyps) forms a thin basement membrane, while in others (hydromedusas, scyphomedusas, coral polyps) it is represented by gelatinous mesoglea.

In most coelenterates, radial, or radial, symmetry is expressed, however, in more highly organized coral polyps, deviations are observed towards two-radial and even bilateral, or bilateral, symmetry.

Coelenterates are characterized by two life forms: a sessile saccular polyp and a floating discoid jellyfish. Both life forms can alternate in the life cycle of the same species, i.e., the generation of polyps that reproduce asexually gives rise to a generation of swimming jellyfish that form reproductive products (marine hydroid polyps, scyphoid). Thus, most coelenterates are characterized by alternation of generations - metagenesis. However, some groups of coelenterates do not have a medusoid generation (hydras, coral polyps) or have lost the life form of a polyp (some species of hydroid and scyphoid).

All coelenterates are characterized by the presence of special stinging cells that serve as a means of defense and attack, which are not found in animals of other types.

The digestive system of coelenterates is very primitive. The mouth is the only opening leading to the blindly closed gastric cavity. Unlike sponges, digestion of food in coelenterates occurs under the action of enzymes in the gastric cavity. Small food particles into which food breaks down are captured by endoderm cells and digested intracellularly. Thus, in addition to extracellular, or cavity, digestion, there is a primitive intracellular digestion of food. Excrement is excreted through the mouth. In polyps, the gastric cavity is saccular, and in jellyfish, due to the powerful development of mesoglea, it breaks up into a system of channels (radial and annular), which is called gastrovascular. The latter ensures the digestion of food and the distribution of nutrients throughout the body of the animal. In addition, the gastrovascular system is involved in gas exchange.

Within the type of coelenterates, a complication of the structure of the digestive system is observed. In more primitive hydroids and scyphoids, the entire gastric cavity is formed by the endoderm. They have the so-called gastral type of structure: the ectoderm and endoderm converge along the edges of the mouth opening, which resembles the stage of the embryo - the gastrula. In more highly organized coral polyps, the anterior intestine, the ectodermic pharynx, is formed for the first time. An increase in the surface of the gastric cavity can also be considered a progressive feature, which is achieved in polyps by the formation of partitions, or septa, and in jellyfish by a complication of the gastrovascular system.

Coelenterates are at the tissue level of organization, that is, they have real, albeit poorly differentiated, tissues. In more primitive hydroids, the ectoderm and endoderm are formed mainly by epithelial-muscular cells. In this case, the ectoderm combines integumentary and motor functions, and the endoderm combines digestive and motor functions. Within the type, a partial isolation of independent muscle tissue occurs.

For the first time, a nervous system of a diffuse type arises, consisting of scattered nerve cells that come into contact with each other by processes and form a nervous network, or plexus. In swimming jellyfish, concentration of bodies of nerve cells and the formation of organs of vision and balance are observed.

Coelenterates reproduce both asexually and sexually. Incomplete asexual reproduction in a number of species leads to the formation of large colonies. Many coelenterates are dioecious, but there are also hermaphrodites. Sexual products develop in more primitive forms (hydroid) in the ectoderm, and in more highly organized forms (scyphoid, coral polyps) in the endoderm, which ensures their better supply of nutrients. Mature gametes are usually released into the water, fertilization is external. Development with a free-swimming larva covered with cilia - planula - or direct.

Classification of coelenterates

The type of coelenterates combines 3 classes: 1. Hydroid (Hydrozoa); 2. Scyphoid (Scyphozoa); 3. Coral polyps (Anthozoa).

Class Hydroids (Hydrozoa)

General characteristics of Hydroids

Hydroids represent a large group, including about 2800 species of the most primitively organized coelenterates. The primitiveness of their organization is expressed primarily in the simplicity of the structure of the digestive system. In polypoid forms, the gastric cavity is saccular and has no partitions. The throat is missing. Both cell layers - ectoderm and endoderm - converge along the edge of the mouth opening (gastric type of structure). The tissues are poorly differentiated: the ectoderm and endoderm are formed mainly by epithelial-muscular cells, as a result of which the functions of integumentary and muscular tissues are combined.

Sex products are formed in the ectoderm. The nervous system is very primitive, diffuse in nature. Nerve cells - neurons - form nerve networks and plexuses.

Hydroids can exist as a single sessile polyp or a single swimming jellyfish, however, most hydroids (marine hydroid polyps) have a regular alternation of generations: an attached polypoid, which reproduces asexually, and a free-swimming medusoid, which reproduces sexually. The life cycle of hydroids can be greatly modified due to the loss or modification of one of the stages (medusoid or polypoid). Development proceeds with the stage of a free-swimming planula larva; in forms that have taken root in fresh water, the larval stage is absent.

Hydroids - overwhelmingly marine animals, often colonial, only a few species live in fresh water.

The class Hydrozoa is divided into two subclasses: 1. Hydroids ( Hydroidea) and 2. Siphonophores ( Siphonophora).

Subclass Hydroids ( hydroidea)

Hydroids are represented by both single polyps and jellyfish, and colonies of polyps that grow on the seabed. The subclass of hydroids includes several orders, the most important of which are the following: 1. Hydras (Hydrida); 2. Marine hydroid polyps (Leptolida); 3. Trachilids, or Trachymedusa (Tracylida).

Hydra Hydrida

Hydra - Appearance: solitary freshwater polyp. Body length about 1 cm, attached to the substrate with the help of the sole, on the opposite side there is a mouth, around which there are 6-12 tentacles. Distribution: Throughout the temperate zone.

Lifestyle: lives at shallow depths. Attaches to various underwater objects. Food: Predator, feeds on ciliates, oligochaete worms, planktonic crustaceans, small fish fry. Prey is paralyzed by stinging cells. Reproduction: in the spring-summer season - asexual (budding) and in autumn - sexually. Features: has a high ability to regenerate. Able to recover even from a small cut off part of the body. Hydra is a convenient model object for biological research.

Order Marine hydroid polyps Leptolida

Some marine hydroid polyps are solitary, like hydras, but the vast majority are colonial. Colonies grow by budding a large number of polyps called hydrants that do not separate from the colony. Often colonies form trunks creeping along the substrate, from which branches carrying hydrants depart. Polyp colonies can consist of a very large number of individuals. Marine hydroid polyps differ from freshwater hydras in that, in addition to polyps, or hydrants, asexual individuals, they also form special sexual individuals, jellyfish, by budding.

ORDER TRACHILIDA, OR TRACHIMEDUSA TRACHYLIDA

Trachylids differ from marine hydroid polyps in the absence of polypoid generation in most species. They usually only have sexually reproducing jellyfish.

Trachymedusa are almost exclusively marine animals. However, several freshwater forms are also known. Trachymedusa Craspedacusta sowerbii is common in fresh waters of North and South America. Sporadically, it appears in the pools of botanical gardens and in aquariums of amateurs in many countries of the world; in Russia it is found in artificial reservoirs near Tula, in the Don River, in Georgia in a reservoir near Tbilisi, in the reservoirs of Bukhara, and is also found in aquariums of fish farmers in Moscow and St. Petersburg. The life cycle of Craspedacusta is very complex. It has, in addition to the medusoid stage, two generations of polyps.

Among the trachymedusa, there is a very poisonous cross jellyfish (Gonionemus vertens), which lives in the Sea of ​​Japan and near the Kuril Islands. Krestovichki breed in shallow water in thickets of sea grass - zosters. Sometimes they appear en masse. The edge of the umbrella of this jellyfish is armed with 80 tentacles, equipped with numerous batteries of stinging cells and suckers. A cross burn causes general weakness, a drop in cardiac activity and respiratory failure. In this case, the victim may die if he is not given medical care. When crosses appear near settlements, swimming in the sea is prohibited and measures are being taken to destroy these dangerous jellyfish.

Subclass Siphonophore Siphonophora

Siphonophores are a very peculiar group of colonial marine hydroids, characterized by a particularly strong polymorphism of the individuals that make up the colony.

Siphonophores lead a floating lifestyle, and they stay either at the very surface of the water, or close to the surface. They are common in warm seas. Their colonies sometimes reach very large sizes. The largest siphonophores are 2–3 m long, while the smallest are 1–2 cm long.

The entire diversity of the structure of siphonophores can be reduced to two main types. In some, the basis of the colony is a more or less long hollow trunk, the wall of which, like all hydroids, consists of ectoderm, endoderm and mesoglea. On the trunk along its entire length, individuals of the colony are placed, which are connected to each other by a common gastrovascular cavity, which also passes in the trunk. In other siphonophores, the trunk is greatly shortened, and individuals are placed on its lower, very widened part.

The top of the colony in many siphonophores is a special bubble called pneumatophore. The upper part of the bubble contains a cavity filled with gas, and in the lower part there are glandular cells that release gas. In some species of siphonophore, the cavity of the pneumatophore opens to the outside with a pore that can close. When the colony stays near the surface of the water, the pore is closed and the pneumatophore is filled with gas, due to which the density of the colony decreases. If the pneumatophore contracts and gas escapes through the opening pore, the colony sinks deeper into the water. Some siphonophores are constantly near the surface. Their pneumatophore reaches a large size and its cavity is separated by partitions from the mesoglea, there is no pore. The upper part of the pneumatophore is equipped with an S-shaped crest and protrudes above the water surface. Such siphonophores move, driven by the wind, along the surface of the sea. So, in the Portuguese boat (Physalia), the pneumatophore reaches a length of 20-30 cm. It is brightly colored and floats on the surface of the water.

Under the pneumatophore, and if it is absent, at the very top of the colony, many siphonophores have more or less individuals called swimming bells or nektophores. These jellyfish-like individuals are capable, like jellyfish, of rhythmic contractions of the umbrella, but, unlike them, they lack a mouth and a proboscis. The Physalia mentioned above and some other siphonophores, passively carried by the wind and currents, do not have swimming bells.

Below the swimming bells are other individuals of the colony, and they sit on the trunk of the colony in groups that repeat along the length of the trunk. These groups of individuals are called cormidia.

In its most complete form, each cormidium consists of the following individuals: a feeding individual, or gastrozoid, a noose, a cystozoid, a tentacle, an operculum, and a sexual individual, or gonozoid.

Gastrozoids are polyps without tentacles, but with a mouth leading to the gastric cavity, which communicates with the cavity of the colony trunk and continues in the cavity of other individuals without a mouth.

Near the gastrozoid, there is usually a lasso - a more or less long, often branched tentacle, seated with many stinging cells. In the Portuguese boat, the lasso in the extended state reaches 20 m in length and is seated with a large number of stinging cells. Arkanchiki play a protective role and at the same time serve as trapping tentacles. Physal burns are very sensitive and can be dangerous to humans.

Cystozoans differ from gastrozoans in that they lack a mouth. In cystosoid, an unbranched tentacle is usually located. The significance of cystozoids is not entirely clear. It is assumed that they perform an excretory function, and the tentacle is sensitive.
The lid is a flat plate that covers the cormidium from above.

Sexual individuals - gonozoids - are usually built according to the type of medusoids and gonophores of hydroid polyps. Gonozoids are always dioecious, but among the siphonophores there are both species whose colonies form gonozoids of only one sex (either male or female), and species that include hermaphrodite colonies, in which gonozoids of both sexes are placed on one colony.

Thus, pronounced polymorphism is observed in siphonophores. There are a significant number of individuals adapted to perform various functions. Such a peculiar structure of siphonophores has led to disagreements among zoologists on the question of whether to consider siphonophores as individual individuals or as polymorphic colonies. It is currently accepted that the long-term integration of polymorphic siphonophore colonies led to the formation of independent organisms. Individual zooids siphonophores have finally lost the ability to conduct independent image life and actually turned into organs of this whole organism.

Class Scyphoid

Scyphozoa

Scyphoid - A group of marine coelenterates specialized for a floating lifestyle. Most of their life cycle takes place in the form of swimming jellyfish. The polyp phase in the life cycle is short or absent. The general plan of the structure of scyphomedusa coincides with that of hydromedusas. But scyphomedusa have significant distinctive features. As a rule, scyphomedusae are larger than hydromedusae, with a highly developed mesoglea. They do not have a sail, and they move by contracting the walls of the umbrella. Unlike hydroids, scyphomedusas have a more developed nervous system with separate ganglia, more complex sensory organs that form complexes - ropalia. Gonads are formed in the endoderm. The gastrovascular system is complex: with branching and non-branching canals. The stomach is divided into chambers with gastric threads. There is an ectodermal pharynx.

The number of species of scyphomedusa is small, only about 200. However, their number in the seas is very high. So, in the South China Sea, during the period of tropical rains, when a lot of organic matter is carried into the sea and many planktonic organisms develop, coastal waters are overflowing with jellyfish. They are fished in China and Japan and are used for food.

Scyphomedusa are extremely diverse in shape and size. The largest jellyfish, Cyanea arctica, lives in the polar seas and reaches 2 m in diameter, and its tentacles hang down 20-30 m. It is a brightly colored jellyfish with strong stinging properties. The most widespread jellyfish is Aurelia aurita, large specimens of which reach 40 cm in diameter. This species has non-striking properties. This is a flattened jellyfish with a completely transparent body, and only pink or purple horseshoe-shaped gonads stand out in color.

This detachment is small, its representatives usually have a four-sided high umbrella with four ropalia and four simple or branched tentacles. They feed on various planktonic invertebrates, sometimes on juvenile fish. Box jellyfish are found in the shallow waters of warm seas. Some species frequent off the coast of Australia and Indonesia ( Chiropsalmus) can cause severe, sometimes fatal burns to humans.

Class Coral polyps ( Anthozoa)

Latin name Anthozoa- class of marine cnidarians.

coral polyps- Colonial, rarely single polyps; jellyfish do not form. Many have a calcareous or horny skeleton. Dep. individuals are usually cylindrical. forms, with their base fused with the colony or (single, able to crawl slowly) have a sole that attaches them to the ground. At the opposite end of the body is a mouth disk with a crown of tentacles and a mouth in the center. The gastric cavity is divided by radial septa (mesenteries) into the chambers; from the mouth, the ectodermal pharynx descends into it.

Reproduction is sexual and asexual. Sex products develop in the endoderm of the mesenteries. The offspring usually leaves the mother's body at the planula stage, swims for some time, then attaches to the bottom and turns into an adult polyp. Asexual reproduction is by budding. Single non-skeletal K. p. (anemones) can divide longitudinally. Colonies (often large) are formed as a result of budding that does not reach the end. Several subclasses, modern and fossils, including living 6-ray and 8-ray corals, as well as extinct Rugosa, Tabulata, Heliolithoidea. About 6000 modern. species, in the seas of Russia - about 150 species.

As the name of the type itself shows, it includes animals that have only one body cavity - the intestinal one.

Giving a general description of classes such as coelenterates, students should pay attention to the radial symmetry of these animals, which is expressed in the radial arrangement of some organs important for life: oral tentacles, eyes, statocysts, etc. Radial symmetry is inherent only in aquatic organisms. In coelenterates, it indicates a relatively simple organization. Such symmetry developed in the process of evolution in the ancient primarily sessile forms of the coelenterates, from which free-floating forms (jellyfish) subsequently arose, retaining radial symmetry.

The biological significance of radial symmetry lies in the fact that it provides the animal with contact with the surrounding aquatic environment from all sides and allows it to respond in a timely manner to the approach of prey, enemies, or to the influence of other factors (for example, light). It is appropriate to recall the similar meaning of ray symmetry for radiolarians and colonials leading a pelagic way of life.

Coelenterates belong to two-layer invertebrates, since their body is formed only from two germ layers - ectoderm and endoderm, between which a gelatinous layer is formed - mesoglea. Intestinal cavities have not only, but also the rudiments of organs. So, for example, in hydra, sea anemones and corals it is easy to find the front end with a mouth opening, pharynx and tentacles, and in jellyfish - an umbrella, eyes, statocysts, etc. Those leading a sedentary lifestyle have a sole that attaches the animal to the substrate.

The organism of the coelenterates

Along with this, the organism of the coelenterates retained some structural features characteristic of a more primitive one. So, for example, the cells of the endoderm of coelenterates have flagella, the movement of which is similar to the movement of unicellular and colonial flagellates. These flagella in coelenterates create currents in the intestinal cavity, which mix its contents. In addition, endoderm cells, like amoeba, release pseudopods, capturing food particles with them, and carry out intracellular digestion, which is characteristic of unicellular organisms.

Ectoderm cells also retained some of the features of protozoa. So, for example, in the larvae of hydra, sea anemones, scyphoid jellyfish, etc., the body is covered with cilia, which resemble the ciliary apparatus, but have slightly different functions. Finally, the individual development of coelenterates begins with a fertilized egg, that is, from a single cell, which confirms the view that multicellular animals originated from unicellular animals.

Although tissues have already appeared in the coelenterates, their cells are functionally ambiguous. So, for example, the skin-muscle cells of the ectoderm and endoderm perform a mixed function: integumentary and contractile. Glandular and nerve cells are characterized by a narrower specialization: the former have a secretory function, the latter transmit excitation from one cell to another.

Stinging (nettle) cells, the so-called nematocysts, located in the ectoderm, are distinguished by significant autonomy. They function independently, independently of other cells. Touching a sensitive hair protruding outwards - a cnidocil - causes a lightning-fast reaction: the entire cytoplasm of the stinging cell becomes excited, the result of which is the rapid ejection of the thread from the stinging capsule. It should be remembered that this thread is actually a tube through which poisonous liquid is poured from the capsule into the affected part of the body of the prey or enemy.

Interestingly, nettle cells retain their viability and can function even in the body of a dead coelenterate. That is why it is easy to get a burn from contact with an already dead jellyfish thrown ashore by a wave. The autonomy of stinging cells is also confirmed by the fact that they do not lose their ability to act when they enter a foreign organism. So, for example, some marine ciliary worms and posterior gills sometimes eat coelenterates. In this case, part of the nettle cells passes into the body of the predator, enters its outer integument and continues to function here, as before.

Coelenterates have reserve (intermediate) cells that are part of the epithelium and can turn into any specialized cells: stinging, skin-muscular, reproductive, nervous. The presence of reserve cells determines the high ability to regenerate, which is especially well expressed in hydra. In addition to the indicated features of primitiveness, it should be noted the absence of respiratory, excretory and circulatory systems. However, in other respects, in comparison with the simplest coelenterates, in the process of evolution, they have moved further along the path of improving organization.

Nervous system of the coelenterates

In addition to weakly differentiated tissues, they have already formed a primitive nervous system of a diffuse nature, the vital importance of which is very great. She does not have a central section, and nerve cells are dispersed in different parts of the body. In contact with each other with their processes, they form nerve plexuses, which in some intestinal cavities (for example, in hydra) look like a network, in others (for example, in jellyfish) they are clusters in the form of two rings, of which one is connected with the sense organs, and the other - with muscle cells.

The nervous network of the hydra also communicates between sensitive and skin-muscle cells. Consequently, the coelenterates already have an anatomical and morphological basis for reflex activity. This fact should be considered as one of the important stages in the progressive evolution of the animal world, leading to the ordering of the relationship of the organism with the environment, making them more diverse and stable.

It is known that for a genuine reflex, the presence of three links is necessary: ​​sensitive (receptor), transmitting excitation (nervous apparatus) and motor (effector). All these links are present in the coelenterates, but they are still at a low stage of development, so the reflexes are of an elementary nature. It is interesting to note that, along with typical unconditioned reflexes (for example, contraction of the tentacles in response to touching them), they are characterized by taxis-type reactions (for example, throwing out a stinging thread in response to stimulation of the cnidocil).

Most intestinal reactions are associated with nutrition, movement and protection from harmful influences. They may develop temporary connections, or rather summation reflexes, resulting from an increase in the excitability of the nervous system during prolonged exposure to the same stimulus. The activity of the nervous system is limited, it consists of the perception of external stimuli and the regulation of coordinated responses to them from individual parts of the body (for example, the food reaction of tentacles in contact with prey).

The tentacles have the greatest sensitivity in intestinal cavities, where, to a greater extent than in other parts of the body, cells are concentrated that perceive various environmental influences, including stinging cells (nematocysts).

The motor reactions of the intestinal cavity vary depending on the strength of the irritation, the internal state of the organism, the nature of the physical or chemical effect, and the biological significance of the stimulus. Since the cellular structures of the organism of the intestinal cavities are repeated in different parts of the body, its fragments react in the same way as the animal as a whole.

Forms of coelenterates

Coelenterates are represented by two main forms: polyps, leading an attached, sedentary lifestyle, and jellyfish, which move freely in the water column. For both forms, ray symmetry is equally vital, and therefore it has been fixed in them by the action of natural selection as a useful adaptation. In many species of coelenterates, alternation of generations of polyps and jellyfish is observed. Moreover, in some, asexual polyps are the main life form, and the generation of jellyfish serves only for sexual reproduction and distribution of the species (for example, in marine hydroid polyps); in others, on the contrary, the generation of sexual jellyfish is the main form, and polyps provide an increase in their numbers through asexual reproduction, leading to the appearance of a new generation of jellyfish individuals (for example, hydroid and scyphoid jellyfish).

However, there are also such coelenterates in which the medusa stage is absent in individual development, therefore they exist all their lives in the form of polyps (for example, hydra, sea anemones, corals). In this case, polyps reproduce both asexually and sexually. The structure of polyps is simpler than the structure of jellyfish, but in essence there is no fundamental difference between them, and their differences from each other are determined mainly by the degree of development of the mesoglea, the position and shape of the ectodermal and endodermal layers, and the differentiation of cellular structures and sensory organs.

Both polyps and jellyfish are represented in nature by single and colonial forms. Solitary polyps include, for example, hydras and sea anemones, and colonial polyps include noble red coral and sea feathers. An example of a single jellyfish is kraspedakusta, cross, cyanide, aurelia, cornerot. As for the colonial forms, in jellyfish they are of a mixed nature, representing a combination of medusoid and polypoid individuals with a division of functions between them, as, for example, in siphonophores.

Among the coelenterates, there are large fluctuations in body size. So, for example, the microhydra polyp (of a freshwater jellyfish) barely reaches a height of 1 mm, while the marine single deep-water branchiocerianthus polyp has a height of more than 2 m. In the same way, there are dwarfs and giants among jellyfish. The diameter of the umbrella of the freshwater jellyfish kraspedakusta does not exceed 2 cm, while in the cyanide jellyfish it is often 2 m.

The role of coelenterates

Coelenterates are part of many aquatic biocenoses, playing a significant role in their life. They mainly inhabit the ocean at all its depths and at all latitudes, having adapted to the most diverse living conditions: in the near-surface layer of water, in the surf zone of the coasts and on the bottom of the sea, in cold Arctic waters and in the tropical zone of the ocean, in illuminated zones of the hydrosphere and in the absence sunlight at great depths, on rocky substrate and on silty soils. Much fewer of them live in fresh water.

Being the most ancient animals of all existing multicellular organisms, coelenterates participated in the formation of geological deposits (Cambrian, Silurian, Cretaceous, etc.). In the Quaternary period, they left a noticeable mark in the form of coral reefs and atoll islands, which shelter a variety of fauna and flora that are part of various biocenoses.

The practical significance of coelenterates is not very great, except for the use of madrepore corals as a raw material for the production of building materials (lime blocks, tiles, burnt lime). Some scyphoid jellyfish are eaten in Japan and China. Red, or noble, coral and stony corals are used to make various jewelry and small crafts. Some coelenterates are of interest for bionics (for example, physalia, disk jellyfish), as discussed below.

Classification of coelenterates

In modern systematics, the type of coelenterates is divided into three main classes, which are characterized by many distinctive features, but for an elementary acquaintance with them, one can limit oneself to pointing out the differences in the structure of the intestinal cavity of polypoid individuals, for example:

Of the 9000 existing species of coelenterates, the majority are corals (over 6000 species), the second place is occupied by hydroids (2800 species), the remaining 200 species are represented by scyphoid coelenterates. We observe features of primitiveness in representatives of the more ancient class of hydroids, which are considered to be the source for other, more complex organisms (scyphoid and coral polyps).

Intestinal - the first two-layered ancient animals with radial symmetry, intestinal (gastric) cavity and mouth opening. They live in water. There are sessile forms (benthos) and floating (plankton), which is especially pronounced in jellyfish. Predators feeding on small crustaceans, fish fry, aquatic insects.

A significant role in the biology of the southern seas is played by coral polyps, which form reefs and atolls, which serve as shelters and spawning grounds for fish; at the same time they pose a danger to ships.

Large jellyfish are eaten by people, but they also cause serious burns to swimmers. Reef limestone is used for decoration and as a building material. However, destroying reefs, a person reduces fish wealth. The most famous reefs in the southern seas are along the coast of Australia, near the Sunda Islands, in Polynesia.

Intestinal - the oldest type of primitive two-layer multicellular animals. Lacking true organs. Their study is of exceptional importance for understanding the evolution of the animal world: the ancient species of this type were the progenitors of all higher multicellular animals.

Intestinal - predominantly marine, less often freshwater animals. Many of them are attached to underwater objects, others slowly swim in the water. Attached forms are usually goblet-shaped and are called polyps. They are attached to the substrate with the lower end of the body, at the opposite end there is a mouth surrounded by a rim of tentacles. The floating forms are usually bell-shaped or umbrella-shaped and are called jellyfish.

The body of the coelenterates has radial (radial) symmetry. Two or more (2, 4, 6, 8 or more) planes can be drawn through it, dividing the body into symmetrical halves. In the body, which can be compared with a two-layer bag, only one cavity is developed - the gastric cavity, which acts as a primitive intestine (hence the name of the type). It communicates with the external environment through a single opening that functions as a mouth and anus. The sac wall consists of two cell layers: the outer or ectoderm and the inner or endoderm. A structureless substance lies between the cell layers. It forms either a thin supporting plate or a wide layer of gelatinous mesoglea. In many coelenterates (for example, jellyfish), channels depart from the gastric cavity, which together with the gastric cavity form a complex gastrovascular (gastrointestinal) system.

The cells of the body of the coelenterates are differentiated.

• Ectoderm cells represented by several types:
  • integumentary (epithelial) cells - form the cover of the body, perform a protective function

    Epithelial muscle cells - in the lower forms (hydroid) integumentary cells have a long process extended parallel to the surface of the body, in the cytoplasm of which contractile fibrils are developed. The totality of such processes forms a layer of muscular formations. Epithelial muscle cells combine the functions of a protective cover and a motor apparatus. Due to the contraction or relaxation of muscle formations, the hydra can shrink, thicken or narrow, stretch, bend to the side, attach to other parts of the stems and thus move slowly. In higher intestinal cavities, muscle tissue is isolated. Jellyfish have powerful bundles of muscle fibers.

  • stellate nerve cells. The processes of nerve cells communicate with each other, forming the nerve plexus, or diffuse nervous system.
  • intermediate (interstitial) cells - restore damaged areas of the body. Intermediate cells can form integumentary-muscular, nervous, sex and other cells.
  • stinging (nettle) cells - located among the integumentary cells singly or in groups. They have a special capsule in which lies a spirally twisted stinging thread. The cavity of the capsule is filled with liquid. On the outer surface of the stinging cell, a thin sensitive hair is developed - the cnidocil. When touched by a small animal, the hair is deflected, and the stinging thread is thrown outward and straightened, through which a paralyzing poison enters the body of the prey. After the thread is ejected, the stinging cell dies. Stinging cells are renewed due to undifferentiated interstitial cells lying in the ectoderm.
• Endoderm cells line the gastric (intestinal) cavity and perform mainly the function of digestion. These include
  • glandular cells that secrete digestive enzymes into the gastric cavity
  • digestive cells with phagocytic function. Digestive cells (in lower forms) also have processes in which contractile fibers are developed, oriented perpendicular to similar formations of integumentary muscle cells. Flagella are directed from the epithelial-muscular cells towards the intestinal cavity (1-3 from each cell) and outgrowths can form, resembling false legs, which capture small food particles and digest them intracellularly in digestive vacuoles. Thus, in intestinal cavities, intracellular digestion characteristic of protozoa is combined with intestinal digestion characteristic of higher animals.

The nervous system is primitive. In both cell layers there are special sensitive (receptor) cells that perceive external stimuli. A long nerve process departs from their basal end, along which the nerve impulse reaches multi-processed (multipolar) nerve cells. The latter are located one by one, do not form nerve nodes, but are connected with each other by their processes and make up the nervous network. Such a nervous system is called diffuse.

The reproductive organs are represented only by the sex glands (gonads). Reproduction occurs sexually and asexually (budding). For many coelenterates, alternation of generations is characteristic: polyps, multiplying by budding, give both new polyps and jellyfish. The latter, reproducing sexually, give a generation of polyps. This alternation of sexual reproduction with vegetative reproduction is called metagenesis. [show] .

Metagenesis occurs in many coelenterates. For example, the well-known Black Sea jellyfish - Aurelia - reproduces sexually. The spermatozoa and eggs that arise in her body are released into the water. From fertilized eggs, individuals of the asexual generation develop - Aurelia polyps. The polyp grows, its body lengthens, and then is divided by transverse constrictions (strobilization of the polyp) into a number of individuals that look like stacked saucers. These individuals separate from the polyp and turn into sexually reproducing jellyfish.

Systematically, the type is divided into two subtypes: cnidarians (Cnidaria) and non-cnidators (Acnidaria). There are about 9,000 known species of cnidarians, and only 84 species of non-cnidaria.

SUBTYPE CIDING

Subtype characteristic

Intestinal, called stingers, have stinging cells. These include the classes: hydroid (Hydrozoa), scyphoid (Scyphozoa) and coral polyps (Anthozoa).

Class hydroid (Hydrozoa)

A single individual has the form of either a polyp or a jellyfish. The intestinal cavity of polyps is devoid of radial partitions. Sex glands develop in the ectoderm. About 2800 species live in the sea, but there are several freshwater forms.

• Subclass Hydroids (Hydroidea) - bottom colonies, adherent. In some non-colonial species, polyps are able to swim near the surface of the water. Within each species, all individuals of the medusoid structure are the same.
  • Order Leptolida (Leptolida) - there are individuals of both polypoid and medusoid origin. Mostly marine, very rarely freshwater organisms.
  • Hydrocoral detachment (Hydrocorallia) - the trunk and branches of the colony are calcareous, often painted in a beautiful yellowish, pink or red color. Medusoid individuals are underdeveloped and immersed deep in the skeleton. Exclusively marine organisms.
  • Detachment Chondrophora (Chondrophora) - the colony consists of a floating polyp and medusoid individuals attached to it. Exclusively marine animals. Previously classified as a subclass of siphonophores.
  • Detachment Tachilida (Trachylida) - exclusively marine hydroid, shaped like a jellyfish, no polyps.
  • Order Hydra (Hydrida) - single freshwater polyps, do not form jellyfish.
• Subclass Siphonophora (Siphonophora) - floating colonies, which include polypoid and medusoid individuals of various structures. They live exclusively in the sea.

Hydra freshwater polyp- a typical representative of hydroids, and at the same time all cnidaria. Several species of these polyps are widely distributed in ponds, lakes and small rivers.

Hydra is a small, about 1 cm long, brownish-green animal with a cylindrical body. At one end is a mouth, surrounded by a rim of very mobile tentacles, which various kinds sometimes from 6 to 12. At the opposite end is a stem with a sole that serves to attach to underwater objects. The pole on which the mouth is located is called oral, the opposite is called aboral.

Hydra leads a sedentary lifestyle. Attached to underwater plants and hanging into the water with its mouth end, it paralyzes passing prey with stinging threads, captures it with tentacles and sucks it into the gastric cavity, where digestion occurs under the action of glandular cell enzymes. Hydras feed mainly on small crustaceans (daphnia, cyclops), as well as ciliates, oligochaete worms and fish fry.

Digestion. Under the action of the enzymes of the glandular cells of the endoderm lining the gastric cavity, the body of the captured prey breaks up into small particles that are captured by cells that have pseudopodia. Some of these cells are in their permanent place in the endoderm, others (amoeboid) are mobile and move. These cells complete the digestion of food. Consequently, there are two ways of digestion in coelenterates: along with the more ancient, intracellular, an extracellular, more progressive way of processing food appears. Subsequently, in connection with the evolution of the organic world and the digestive system, intracellular digestion lost its significance in the act of nutrition and assimilation of food, but the ability for it was preserved in individual cells in animals at all stages of development up to the highest, and in humans. These cells, discovered by I. I. Mechnikov, were called phagocytes.

Due to the fact that the gastric cavity ends blindly and the anus is absent, the mouth serves not only for eating, but also for removing undigested food residues. The gastric cavity performs the function of blood vessels (moving nutrients through the body). The distribution of substances in it is ensured by the movement of flagella, which are provided with many endodermal cells. The contractions of the whole body serve the same purpose.

Respiration and excretion carried out by diffusion by both ectodermal and endodermal cells.

Nervous system. Nerve cells form a network throughout the hydra's body. This network is called the primary diffuse nervous system. There are especially many nerve cells around the mouth, on the tentacles and soles. Thus, the simplest coordination of functions appears in the coelenterates.

sense organs. Not developed. Touch with the entire surface, the tentacles (sensitive hairs) are especially sensitive, throwing out stinging threads that kill prey.

Hydra locomotion carried out by transverse and longitudinal muscle fibers included in epithelial cells.

Hydra Regeneration- restoration of the integrity of the hydra body after its damage or loss of part of it. A damaged hydra regenerates lost body parts not only after it has been cut in half, but even if it has been divided into a huge number of parts. A new animal is able to grow from 1/200 of a hydra, in fact, a whole organism is restored from a grain. Therefore, hydra regeneration is often called an additional method of reproduction.

reproduction. Hydra reproduces asexually and sexually.

During the summer, hydra reproduces asexually - by budding. In the middle part of her body is a budding belt, on which tubercles (buds) form. The kidney grows, a mouth and a tentacle are formed on its top, after which the kidney is laced at the base, separated from the mother's body and begins to live on its own.

With the approach of cold weather in autumn, germ cells - eggs and spermatozoa - are formed from intermediate cells in the hydra ectoderm. The eggs are closer to the base of the hydra, the spermatozoa develop in the tubercles (male gonads) located closer to the mouth. Each spermatozoon has a long flagellum, with the help of which it swims in the water, reaches the egg and fertilizes it in the mother's body. The fertilized egg begins to divide, becomes covered with a dense double shell, sinks to the bottom of the reservoir and overwinter there. In late autumn, adult hydras die. In the spring, a new generation develops from overwintered eggs.

Colonial polyps(for example, the colonial hydroid polyp Obelia geniculata) live in the seas. A single individual of the colony, or the so-called hydrant, is similar in structure to the hydra. The wall of its body, like the hydra, consists of two layers: endoderm and ectoderm separated by a jelly-like structureless mass called mesoglea. The body of the colony is a branched coenosarc, inside of which there are separate polyps, interconnected by outgrowths of the intestinal cavity into a single digestive system, which makes it possible to distribute the food captured by one polyp among the members of the colony. Outside, the coenosarc is covered with a hard shell - the perisarc. Near each hydrant, this shell forms an extension in the form of a goblet - a hydrotech. The corolla of tentacles can be drawn into the extension when stimulated. The mouth opening of each hydrant is located on an outgrowth, around which there is a corolla of tentacles.

Colonial polyps reproduce asexually by budding. At the same time, the individuals that developed on the polyp do not come off, like in the hydra, but remain associated with the maternal organism. An adult colony looks like a bush and consists mainly of two types of polyps: gastrozoids (hydrants), which provide food and protect the colony with stinging cells on tentacles, and gonozoids, which are responsible for reproduction. There are also polyps specialized to perform a protective function.

A gonozoid is a rod-shaped formation elongated in length with an extension at the top, without a mouth opening and tentacles. Such an individual cannot feed on its own; it receives food from hydrants through the gastric system of the colony. This formation is called a blastostyle. The skeletal membrane gives a bottle-shaped extension around the blastostyle - the gonotheca. All this formation as a whole is called gonangia. In the gonangium, on the blastostyle, jellyfish are formed by budding. They bud from the blastostyle, emerge from the gonangium, and begin to lead a free lifestyle. As the jellyfish grows, germ cells form in its gonads, which are released into the external environment, where fertilization occurs.

A blastula is formed from a fertilized egg (zygote), with the further development of which a two-layer larva, planula, freely floating in water, covered with cilia, is formed. Planula settles to the bottom, attaches to underwater objects and continues to grow and gives rise to a new polyp. This polyp forms a new colony by budding.

Hydroid jellyfish have the shape of a bell or an umbrella, from the middle of the ventral surface of which hangs a trunk (oral stalk) with a mouth opening at the end. Along the edge of the umbrella are tentacles with stinging cells and sticky pads (suckers) that serve to catch prey (small crustaceans, larvae of invertebrates and fish). The number of tentacles is a multiple of four. Food from the mouth enters the stomach, from which four straight radial canals extend, encircling the edge of the jellyfish umbrella (the annular canal of the intestine). The mesoglea is much better developed than that of the polyp, and makes up the bulk of the body. This is due to the greater transparency of the body. The way the jellyfish moves is "reactive", this is facilitated by a fold of ectoderm along the edge of the umbrella, called the "sail".

In connection with the free way of life, the nervous system of jellyfish is better developed than that of polyps, and, in addition to the diffuse nervous network, it has accumulations of nerve cells along the edge of the umbrella in the form of a ring: external - sensitive and internal - motor. The sensory organs are also located here, represented by light-sensitive eyes and statocysts (organs of balance). Each statocyst consists of a vesicle with a calcareous body - statolith, located on elastic fibers coming from the sensitive cells of the vesicle. If the position of the jellyfish's body in space changes, the statolith shifts, which is perceived by sensitive cells.

Jellyfish have separate sexes. Their gonads are located under the ectoderm, on the concave surface of the body under the radial canals, or in the region of the oral proboscis. Sex cells are formed in the gonads, which, when mature, are excreted through a gap in the body wall. The biological significance of mobile jellyfish is that thanks to them, hydroids are resettled.

Scyphozoa class

An individual has the appearance of either a small polyp or a large jellyfish, or the animal bears signs of both generations. The intestinal cavity of polyps has 4 incomplete radial septa. Sex glands develop in the endoderm of jellyfish. About 200 species. Exclusively marine organisms.

• Order Coronomedusa (Coronata) - mainly deep-sea jellyfish, the umbrella of which is divided by a constriction into a central disk and a crown. The polyp forms a protective chitinoid tube around itself.
• Detachment Discomedusae (Discomedusae) - the umbrella of jellyfish is solid, there are radial channels. Polyps lack a protective tube.
• Detachment of Cubomedusae (Cubomedusae) - the jellyfish umbrella is solid, but devoid of radial channels, the function of which is performed by far protruding pockets of the stomach. Polyp without protective tube.
• The detachment of Stauromedusae (Stauromedusae) is a kind of benthic organisms that combine in its structure the signs of a jellyfish and a polyp.

Most of the life cycle of coelenterates from this class takes place in the medusoid phase, while the polypoid phase is short-lived or absent. Scyphoid coelenterates have a more complex structure than hydroid ones.

Unlike hydroid jellyfish, scyphoid jellyfish are larger, have a highly developed mesoglea, and a more developed nervous system with clusters of nerve cells in the form of nodules - ganglia, which are located mainly around the circumference of the bell. The stomach cavity is divided into chambers. Canals extend radially from it, united by an annular canal located along the edge of the body. The collection of channels forms the gastrovascular system.

The mode of movement is "reactive", but since the Scyphoid do not have a "sail", movement is achieved by shortening the walls of the umbrella. Along the edge of the umbrella are complex sense organs - ropalia. Each ropalium contains an "olfactory fossa", an organ of balance and stimulation of the movement of the umbrella - statocysts, a light-sensitive eye. Scyphoid jellyfish are predators, but deep-sea species feed on dead organisms.

Sex cells are formed in the sex glands - gonads located in the endoderm. Gametes are shed through the mouth, and planula develops from fertilized eggs. Further development proceeds with alternation of generations, and the generation of jellyfish prevails. The generation of polyps is short-lived.

The tentacles of jellyfish are equipped with a large number of stinging cells. The burns of many jellyfish are sensitive to large animals and humans. Severe burns with severe consequences can be caused by a polar jellyfish of the genus Cyanea, reaching a diameter of 4 m, with tentacles up to 30 m long. Bathers in the Black Sea are sometimes burned by the jellyfish Pilema pulmo, and in the Sea of ​​Japan - gonionemus (Gonionemus vertens).

Representatives of the class of scyphoid jellyfish include:

• aurelia jellyfish (eared jellyfish) (Aurelia aurita) [show] .

  Aurelia long-eared jellyfish (Aurelia aurita)

  It lives in the Baltic, White, Barents, Black, Azov, Japanese and Bering, and is often found in mass quantities.

  It got its name due to the oral lobes, resembling donkey ears in shape. The umbrella of an eared jellyfish sometimes reaches 40 cm in diameter. It is easily recognizable by its pinkish or slightly purple color and four dark horseshoes in the middle part of the umbrella - the gonads.

  In summer, in calm calm weather, at low tide or high tide, you can see a large number of these beautiful jellyfish, slowly carried by the current. Their bodies gently sway in the water. The eared jellyfish is a poor swimmer, thanks to the contractions of the umbrella, it can only slowly rise to the surface, and then, frozen motionless, sink into the depths.

  On the edge of the aurelia umbrella there are 8 ropalia bearing eyes and statocysts. These sense organs allow the jellyfish to keep a certain distance from the surface of the sea, where its delicate body will quickly be torn apart by waves. The eared jellyfish captures food with the help of long and very thin tentacles, which "sweep" small planktonic animals into the jellyfish's mouth. Swallowed food first enters the throat, and then into the stomach. From here, 8 straight radial canals and the same number of branching canals originate. If, using a pipette, a carcass solution is introduced into the stomach of a jellyfish, then one can trace how the flagellar epithelium of the endoderm drives food particles through the channels of the gastric system. First, the ink penetrates into the non-branching canals, then it enters the annular canal and returns back to the stomach through the branching canals. From here, undigested food particles are thrown out through the mouth opening.

  The sex glands of Aurelia, having the shape of four open or complete rings, are located in the pockets of the stomach. When the eggs in them mature, the wall of the gonad ruptures and the eggs are thrown out through the mouth. Unlike most scyphomedusas, Aurelia shows a kind of concern for offspring. The mouth lobes of this jellyfish carry in their own inside a deep longitudinal groove, starting from the mouth opening and passing to the very end of the lobe. On both sides of the gutter are numerous small holes that lead to small cavities-pockets. In a floating jellyfish, its oral lobes are lowered, so that the eggs emerging from the mouth opening inevitably fall into the gutters and, moving along them, linger in pockets. This is where the eggs are fertilized and developed. From pockets, fully formed planulae come out. If you place a large female Aurelia in an aquarium, then after a few minutes you can notice a lot of bright dots in the water. These are planulas that have left their pockets and float with the help of cilia.

  Young planulas show an urge to move towards the light source, soon they accumulate in the upper part of the illuminated side of the aquarium. Probably, this property helps them to get out of the darkened pockets into the wild and stay close to the surface without going into the depths.

  Soon, the planula tends to sink to the bottom, but always in bright places. Here they continue to swim briskly. The period of free-moving life of the planula lasts from 2 to 7 days, after which they settle to the bottom and attach their front end to some solid object.

  After two or three days, the settled planula turns into a small polyp - a scyphist, which has 4 tentacles. Soon, 4 new tentacles appear between the first tentacles, and then 8 more tentacles. Scyphistomas actively feed, capturing ciliates and crustaceans. Cannibalism is also observed - eating planulae of the same species by scyphistomes. Scyphistomas can reproduce by budding, forming similar polyps. The scyphistoma hibernates, and next spring, with the onset of warming, serious changes occur in it. The tentacles of the scyphistoma shorten, and ring-shaped constrictions appear on the body. Soon, the first ether is separated from the upper end of the scyphistoma - a small, completely transparent, star-shaped jellyfish larva. By the middle of summer, a new generation of eared jellyfish develops from the ether.

• jellyfish cyanea (Suapea) [show] .

  

  Scyphoid jellyfish cyanide - is the largest jellyfish. These giants among the intestinal cavities live only in cold waters. The diameter of the cyanide umbrella can reach 2 m, the length of the tentacles is 30 m. Outwardly, the cyanide is very beautiful. The umbel is usually yellowish in the center, dark red towards the edges. The mouth lobes look like wide crimson-red curtains, the tentacles are painted in a light pink color. Young jellyfish are especially bright in color. The poison of stinging capsules is dangerous for humans.

• jellyfish rhizostoma, or cornerot (Rhizostoma pulmo) [show] .

  

  Scyphoid jellyfish cornerot lives in the Black and Azov Seas. The umbrella of this jellyfish is hemispherical or conical in shape with a rounded top. Large specimens of rhizostomy are difficult to fit in a bucket. The color of the jellyfish is whitish, but a very bright blue or purple border runs along the edge of the umbrella. This jellyfish has no tentacles, but its oral lobes branch in two, and their lateral sides form numerous folds and grow together. The ends of the oral lobes do not bear folds and end with eight root-like outgrowths, from which the jellyfish got its name. The mouth of adult Cornerots is overgrown, and its role is played by numerous small holes in the folds of the oral lobes. Here, in the oral lobes, digestion also occurs. In the upper part of the oral lobes of the cornerot there are additional folds, the so-called epaulettes, which enhance the digestive function. Cornerots feed on the smallest planktonic organisms, sucking them together with water into the gastric cavity.

  Cornerots are pretty good swimmers. The streamlined shape of the body and the strong musculature of the umbrella allow them to move forward with quick, frequent jerks. It is interesting to note that, unlike most jellyfish, Cornerot can change its movement in any direction, including down. Bathers are not very happy with the meeting with the cornerot: touching it, you can get a rather strong painful "burn". Cornerots usually keep at a shallow depth near the coast, often found in large numbers in the Black Sea estuaries.

• edible ropilema (Rhopilema esculenta) [show] .

  Edible Ropilema (Rhopilema esculenta) lives in warm coastal waters, accumulating in masses near river mouths. It has been observed that these jellyfish grow most intensively after the onset of the summer tropical rainy season. During the rainy season, rivers carry a large amount of organic matter into the sea, contributing to the development of plankton, which jellyfish feed on. Along with Aurelia, ropilema is eaten in China and Japan. Outwardly, ropilema resembles the Black Sea cornerot, differs from it in the yellowish or reddish color of the oral lobes and the presence of a large number of finger-like outgrowths. The mesoglea of ​​the umbrella is used for food.

  Ropilemas are immobile. Their movements depend mainly on sea currents and winds. Sometimes, under the influence of current and wind, clusters of jellyfish form belts 2.5-3 km long. In some parts of the coast of South China, the sea turns white in summer from the accumulated ropils, which sway near the surface.

  They catch jellyfish with nets or special fishing gear, which look like a large bag of fine-mesh net, worn on a hoop. During high or low tide, the bag is inflated by the current and jellyfish enter it, which cannot get out due to their inactivity. In the extracted jellyfish, the oral lobes are separated and the umbrella is washed until the internal organs and mucus are completely removed. Thus, in fact, only the mesoglea of ​​the umbrella enters further processing. According to the figurative expression of the Chinese, the meat of jellyfish is "crystal". Jellyfish are salted with table salt mixed with alum. Salted jellyfish are added to various salads, and are also eaten boiled and fried, seasoned with pepper, cinnamon and nutmeg. Of course, a jellyfish is a low-nutrient product, but nevertheless, salted ropilemas contain a certain amount of proteins, fats and carbohydrates, as well as vitamins B 12, B 2 and nicotinic acid.

  The eared jellyfish, the edible ropilema, and some species of scyphomedusa close to it are, in all likelihood, the only coelenterates that are eaten by humans. In Japan and China there is even a special fishery for these jellyfish, and thousands of tons of "crystal meat" are mined there annually.

Class coral polyps (Anthozoa)

Coral polyps are exclusively marine organisms of a colonial or sometimes solitary form. About 6,000 species are known. In size, coral polyps are larger than hydroids. The body has a cylindrical shape and is not divided into a trunk and a leg. In colonial forms, the lower end of the polyp body is attached to the colony, while in single polyps it is provided with an attachment sole. The tentacles of coral polyps are located in one or more closely spaced corollas.

There are two large groups of coral polyps: eight-ray (Octocorallia) and six-ray (Nehasorallia). The former always have 8 tentacles, and they are equipped with small outgrowths at the edges - pinnules, in the latter the number of tentacles is usually quite large and, as a rule, a multiple of six. The tentacles of six-pointed corals are smooth, without pinnules.

The upper part of the polyp, between the tentacles, is called the oral disc. In its middle is a slit-like mouth opening. The mouth leads to the pharynx lined with ectoderm. One of the edges of the oral fissure and the pharynx descending from it is called the siphonoglyph. The ectoderm of the siphonoglyph is covered with epithelial cells with very large cilia, which are in constant motion and drive water into the intestinal cavity of the polyp.

The intestinal cavity of the coral polyp is divided into chambers by longitudinal endodermal septa (septa). In the upper body of the polyp, the septa grow with one edge to the body wall, and with the other to the pharynx. In the lower part of the polyp, below the pharynx, the septa are attached only to the body wall, as a result of which the central part of the gastric cavity - the stomach - remains undivided. The number of septa corresponds to the number of tentacles. On each septum, along one of its sides, there is a muscular roller.

The free edges of the septa are thickened and are called mesenteric filaments. Two of these filaments, located on a pair of adjacent septa that oppose the siphonoglyph, are covered with special cells bearing long cilia. The cilia are in constant motion and drive water out of the gastric cavity. The joint work of the ciliary epithelium of these two mesenteric filaments and the siphonoglyph ensures a constant change of water in the gastric cavity. Thanks to them, fresh, oxygen-rich water constantly enters the intestinal cavity. Species that feed on the smallest planktonic organisms also receive food. The remaining mesenteric filaments play an important role in digestion, as they are formed by glandular endodermal cells that secrete digestive juices.

Reproduction is asexual - by budding, and sexual - with metamorphosis, through the stage of a free-swimming larva - planula. The sex glands develop in the endoderm of the septum. For coral polyps, only the polypoid state is characteristic, there is no alternation of generations, since they do not form jellyfish and, accordingly, the medusoid stage is absent.

The ectoderm cells of coral polyps produce horny matter or secrete carbonic lime, from which the external or internal skeleton is built. In coral polyps, the skeleton plays a very important role.

Eight-ray corals have a skeleton consisting of individual calcareous needles - spicules located in the mesoglea. Sometimes spicules are interconnected, merging or uniting with an organic horn-like substance.

Among six-pointed corals there are non-skeletal forms, such as sea anemones. More often, however, they have a skeleton, and it can be either internal - in the form of a rod of a horn-like substance, or external - calcareous.

The skeleton of representatives of the Madreporaceae group reaches especially great complexity. It is secreted by the ectoderm of polyps and at first looks like a plate or a low cup in which the polyp itself sits. Further, the skeleton begins to grow, radial ribs appear on it, corresponding to the septa of the polyp. Soon the polyp turns out to be, as it were, impaled on a skeletal base, which protrudes deeply from below into its body, although it is delimited everywhere by the ectoderm. The skeleton of stony corals gets a very strong development: soft tissues cover it in a thin film.

The intestinal skeleton plays a role support system, and together with the stinging apparatus represents a powerful defense against enemies, which contributed to their existence during long geological periods.

• Subclass Eight-beam corals (Octocorallia) - colonial forms, as a rule, adherent to the ground. The polyp has 8 tentacles, eight septa in the gastric cavity, and an internal skeleton. On the sides of the tentacles there are outgrowths - pinnules. This subclass is subdivided into units:
  • Order Solar corals (Helioporida) - solid, massive skeleton.
  • Order Alcyonaria - soft corals, skeleton in the form of calcareous needles [show] .

    Most alcyonaria are soft corals that do not have a pronounced skeleton. Only some tubipores have a developed calcareous skeleton. In the mesoglea of ​​these corals, tubules are formed, which are soldered to each other by transverse plates. The skeleton vaguely resembles an organ in shape, so tubipores have another name - organ. Organs are involved in the process of reef formation.

  • Order Horn corals (Gorgonaria) - a skeleton in the form of calcareous needles, usually there is also an axial skeleton of horn-like or calcified organic matter, passing through the trunk and branches of the colony. This order includes red, or noble coral (Corallium rubrum), which is the object of fishing. Jewelry is made from the skeletons of red coral.
  • Order Sea feathers (Rennaturia) - a kind of colony consisting of a large polyp, on the lateral outgrowths of which secondary polyps develop. The base of the colony is embedded in the ground. Some species are able to move.
• Subclass Six-beam corals (Hexacorallia) - colonial and solitary forms. Tentacles without lateral outgrowths, their number is usually equal to or a multiple of six. The gastric cavity is divided by a complex system of partitions, the number of which is also a multiple of six. Most of the representatives have an external calcareous skeleton, there are groups that are devoid of a skeleton. Includes:

SUBTYPE CLEAR

Subtype characteristic

Non-stinging coelenterates instead of stinging ones have special sticky cells on their tentacles that serve to capture prey. This subtype includes a single class - ctenophores.

Ctenophore class (Ctenophora)- unites 90 species of marine animals with a translucent sac-like gelatinous body, in which the channels of the gastrovascular system branch. Along the body there are 8 rows of rowing plates, consisting of fused large cilia of ectoderm cells. There are no stinging cells. On the sides of the mouth there is one tentacle each, due to which a two-beam type of symmetry is created. Ctenophores always swim forward with the oral pole, using rowing plates as an organ of propulsion. The mouth opening leads to the ectodermal pharynx, which passes into the esophagus. Behind it is the endodermal stomach with radial canals extending from it. At the aboral pole there is a special organ of balance called the aboral. It is built on the same principle as jellyfish statocysts.

Ctenophores are hermaphrodites. The sex glands are located on the processes of the stomach under the rowing plates. The gametes are brought out through the mouth. In the larvae of these animals, the formation of the third germ layer, the mesoderm, can be traced. This is an important progressive feature of ctenophores.

Ctenophores are of great interest from the point of view of the phylogenesis of the animal world, because in addition to the most important progressive feature - the development between the ecto- and endoderm of the rudiment of the third germ layer - the mesoderm, due to which numerous muscle elements develop in the gelatinous substance of the mesoglea in adult forms, they have a number of other progressive features , bringing them closer to higher types of multicellular organisms.

The second progressive feature is the presence of elements of bilateral (bilateral) symmetry. It is especially clear in the crawling comb jelly Coeloplana metschnikowi, studied by A.O. Kovalevsky, and Ctenoplana kowalewskyi, discovered by A.A. Korotnev (1851-1915). These ctenophores have a flattened shape and, in adulthood, lack rowing plates, and therefore can only crawl along the bottom of the reservoir. The side of the body of such a ctenophore facing the ground becomes ventral (ventral); the sole develops on it; the opposite, upper side of the body becomes the dorsal or dorsal side.

Thus, in the phylogenesis of the animal kingdom, the ventral and dorsal sides of the body became distinct for the first time in connection with the transition from swimming to crawling. There is no doubt that modern crawling ctenophores have retained in their structure the progressive features of that group of ancient coelenterates that became the ancestors of higher types of animals.

However, in his detailed studies, V.N. Beklemishev (1890-1962) showed that despite the common structural features of ctenophores and some marine flatworms, the assumption about the origin of flatworms from ctenophores is untenable. The common features of their structure are due to the general conditions of existence, which lead to a purely external, convergent similarity.

The value of coelenterates

Colonies of hydroids, attached to various underwater objects, often grow very densely on the underwater parts of ships, covering them with a shaggy "fur coat". In these cases, hydroids bring significant harm to navigation, since such a "fur coat" sharply reduces the speed of the vessel. There are many cases when hydroids, settling inside the pipes of the sea water supply, almost completely closed their gap and prevented the supply of water. It is quite difficult to deal with hydroids, since these animals are unpretentious and develop quite well, it would seem, in adverse conditions. In addition, they are characterized by rapid growth - bushes 5-7 cm tall grow in a month. To clear the bottom of the ship from them, you have to put it in a dry dock. Here the ship is cleared of overgrown hydroids, polychaetes, bryozoans, sea acorns and other fouling animals. Recently, special poisonous paints have been used - the underwater parts of the ship covered with them are subject to fouling to a much lesser extent.

In the thickets of hydroids living at great depths, live worms, mollusks, crustaceans, echinoderms. Many of them, such as sea goat crustaceans, find refuge among hydroids, others, such as sea "spiders" (multi-legged), not only hide in their thickets, but also feed on hydropolyps. If you move around the settlements of hydroids with a fine-meshed net or, even better, use a special, so-called plankton net for this, then among the mass of small crustaceans and larvae of various other invertebrates, hydroid jellyfish will come across. Despite their small size, hydroid jellyfish are very voracious. They eat a lot of crustaceans and therefore are considered harmful animals - competitors of plankton-eating fish. Plentiful food is necessary for jellyfish for the development of reproductive products. Swimming, they scatter a huge number of eggs into the sea, which subsequently give rise to the polypoid generation of hydroids.

Some jellyfish pose a serious danger to humans. Cornerot jellyfish are very numerous in the Black and Azov Seas in the summer, touching them, you can get a strong and painful "burn". In the fauna of our Far Eastern seas there is also one jellyfish that causes serious diseases when it comes into contact with it. Locals call this jellyfish "cross" for the cruciform arrangement of four dark radial canals, along which four also dark-colored gonads stretch. The umbrella of the jellyfish is transparent, of a faint yellowish-green color. The size of the jellyfish is small: the umbrella of individual specimens reaches 25 mm in diameter, but usually they are much smaller, only 15-18 mm. On the edge of the umbrella of the cross (scientific name - Gonionemus vertens) there are up to 80 tentacles that can be strongly extended and contracted. The tentacles are densely seated with stinging cells, which are arranged in bands. In the middle of the length of the tentacle there is a small suction cup, with which the jellyfish is attached to various underwater objects.

Krestovichki live in the Sea of ​​Japan and near the Kuril Islands. They usually stay in shallow water. Their favorite places are thickets of seagrass zostera. Here they swim and hang on blades of grass, attached with their suckers. Sometimes they come across in clean water, but usually not far from the zoster thickets. During the rains, when the sea water off the coast is significantly desalinated, the jellyfish die. In rainy years, they are almost absent, but by the end of a dry summer, crosses appear in masses.

Although they can swim freely, they usually prefer to lie in wait for prey by attaching themselves to an object. Therefore, when one of the tentacles of the cross accidentally touches the body of a bathing person, the jellyfish rushes in this direction and tries to attach itself with the help of suction cups and stinging capsules. At this moment, the bather feels a strong "burn", after a few minutes the skin at the site of the touch of the tentacle turns red, blistered. Feeling the "burn", you must immediately get out of the water. After 10-30 minutes, general weakness sets in, back pain appears, breathing becomes difficult, arms and legs go numb. Well, if the shore is close, otherwise you can drown. The affected person should be placed comfortably and a doctor should be called immediately. For treatment, subcutaneous injections of adrenaline and ephedrine are used; in the most severe cases, artificial respiration is used. The disease lasts 4-5 days, but even after this period, people affected by a small jellyfish cannot fully recover for a long time.

Repeated burns are especially dangerous. It has been established that the poison of the cross not only does not develop immunity, but, on the contrary, makes the body hypersensitive even to small doses of the same poison. This phenomenon is known in medicine under the name of anaphylaxis.

It is quite difficult to protect yourself from the cross. In places where many people usually bathe, to combat the cross, they mow the zoster, enclose the baths with a fine-mesh net, and catch crosses with special nets.

It is interesting to note that crosses that live only in the Pacific Ocean have such poisonous properties. A very close form belonging to the same species, but to a different subspecies, living on the American and European coasts of the Atlantic Ocean, is completely harmless.

Some tropical jellyfish are eaten in Japan and China, they are called "crystal meat". The body of jellyfish has a jelly-like consistency, almost transparent, contains a lot of water and a small amount of proteins, fats, carbohydrates, vitamins B 1, B 2 and nicotinic acid.

Type Intestinal - these are multicellular individuals, inhabitants of water expanses, mainly seas. Some species have adapted to a sedentary lifestyle (attach to the bottom or substrate), while others actively move, covering long distances.

There are over 10,000 species of coelenterates. The variety of coelenterates is very large: there are small individuals up to a couple of millimeters, and huge representatives are jellyfish cyanoea, about two meters wide, and the tentacles reach 15 meters in length.

Why are intestinal animals given this name? Coelenterates have a two-layer body, so that a cavity is formed between the cells of the layers, which is equipped with one mouth opening. The cavity is called intestinal, and the name intestinal cavity was formed.

For coelenterates, radial symmetry is characteristic, if you draw a line from the lower edge to the upper, then the opposite parts of the body relative to the drawn axis will be identical. The wall of the polyp consists of three layers.

Epidermis

The first layer is the outer ball of epithelial cells (epidermis).

The ectoderm also includes:

• contractile cells(provide movement);
• stinging that perform a protective function. In the capsule of stinging cells there is a paralyzing poison; when danger approaches, poisonous substances enter a special channel, which is located in the stinging thread and goes to the body of the victim. After splashing out the poison, the cell dies, a new one begins to form from the intermediate cells;
• intermediate cells capable of constant division and transformation into specialized ones, this is how the regeneration of the body is carried out;
• sex cells- eggs and spermatozoa are formed in ectodermal tubercles.

Endoderm

The second layer is the inner (endodermis). The cell ball lines the intestinal cavity, consists of two types of cells:

• Digestive- have flagella and pseudopods, with the help of which they capture food particles and carry out intracellular digestion;
• glandular- secrete enzymes for the breakdown of food in the gastric cavity.

Mesoglea

Mesoglea, which is located between the layers and is a jelly-like mass, with collagen fibers, does not contain cells.

Coelenterates lack mesoderm - the middle germ layer.

Coelenterates

All representatives are deprived of specialized respiratory, circulatory, excretory organs. Nervous system coelenterates is represented by nerve cells that are connected to the nerve plexus. Jellyfish have nerve rings near the mouth and dome.

Digestion carried out in the intestinal cavity due to glandular cells, epithelial-muscular cells are responsible for intracellular digestion. Digested residues are excreted through the mouth opening (the digestive system is closed).

reproduction coelenterates goes by budding, this is an asexual mechanism, when the body is divided in longitudinal or transverse directions. During sexual division, sperm and eggs enter the external environment, where they merge. First, a zygote is formed, and then a larva emerges - a planula. After the transformation of the planula, either a polyp or a jellyfish can form from it.

Life cycle of coelenterates

Depending on the life cycle of the coelenterates, two groups are distinguished: asexual generation (polyps) and sexual generation (jellyfish).

polyps- These are single organisms or colonial ones, which unite from tens to thousands of individual individuals. Equipped with a mouth opening with tentacles, which passes into the gastric cavity. The lower part of the polyp is the sole with which it is attached to underwater objects or the bottom.

The internal cavity is divided by septa, the number of which corresponds to the number of tentacles. Cilia depart from the septa, which are in constant motion and provide a regular change of water inside the polyp.

The continuous movement of water ensures high blood pressure in the intestinal cavity, so the polyps straighten out and stay in this position for a long time. When he gets tired, he changes his position by bending over or moving a short distance.

The shape of the body is similar to a bell, the contractile cells of which ensure the active movement of individuals in the water. The mesoglea is 98% water, the rest is connective tissue. Jellyfish, due to their high water content, are easy to keep in the aquatic environment.

On the bottom of the bell is a mouth opening with mouth lobes. With the help of the mouth, food is captured, which enters the intestinal cavity. It consists of many tubules that have departed from the central cavity. In the mouth area there are stinging cells that serve to obtain food and protect against enemies.

Jellyfish have sensory organs, on the surface of the body there are eyes that perceive light rays. If the jellyfish is washed ashore, it will die due to the complete evaporation of water.

What stage of the life cycle of coelenterates promotes their settlement?

The dispersal of animals across the sea is at the larval and medusoid stages. During these periods of life, they are able to move or are carried by the current. A polyp, on the other hand, can only move a couple of meters over the entire period of existence, and most are completely motionless.

Types of coelenterates

The following types of coelenterates are distinguished: hydroid, scyphoid and coral polyps.

hydroid- have a relatively simple structure in comparison with other representatives of the type. They feed on plankton and small animals. In the spring-summer period, it reproduces asexually, buds develop on the body, which, when ripe, leave the mother. In autumn, sexual reproduction takes place, with the formation of an egg, which in the spring will give life to new organisms.

Scyphoid- a class of free-swimming jellyfish, the polyp stage is either absent or poorly developed. Reproduction is sexual, a scyphostomy is formed, from which jellyfish bud (the young form is ether).

coral- organisms with an internal keratinized skeleton. They lead a sedentary lifestyle, reproduce by budding, while not separated from the mother's body, or sexually.

Flatworms have a more complex structure and developed differentiation of tissues and organs. But representatives of the coelenterates have evolved significantly in comparison with the simplest organisms, which is manifested in the structure, way of life, procreation.

Compare the life features of coelenterates and protozoa using the table below.

The role of coelenterates in nature

Participate in the regulation of the number of small fish, crustaceans, as they are food for intestinal organisms.

They are an integral part of the marine biocenosis.

They form coral reefs - a mass accumulation of madreporous corals. They are located near the islands, gradually growing upwards, forming islands (atolls).

They serve as raw material for the extraction of lime.

Coelenterates can live in symbiosis with other animals. Anemones, which lead a sedentary lifestyle, often attach themselves to crayfish and thus move faster. Cohabitation is also beneficial for cancer, as anemone protects it from enemies.

The anemone's tentacles provide hiding places for small shrimp.

The value of intestinal organisms in human life

Widely used in the food industry (edible jellyfish - cornerot). The Japanese catch several thousand tons of Ropilem jellyfish every year, from which various dishes are prepared.

Jewelry is made from the skeleton of a red coral polyp.

Coral reef islands become an obstacle to transport ships.

A poison that is dangerous to human health, which is secreted by stinging cells of the coelenterates, causes severe burns, as well as respiratory failure and cardiac arrhythmia.