Cells are unique to cnidarians. Type Cnidaria. Cnidaria V-Q. Features of the external and internal structure

(gr. сnidos- a thread)

Cnidaria or coelenterates ( Coelenterata), these are exclusively aquatic animals (marine and freshwater), which include hydroid and coral polyps, jellyfish, and others. output - anal. The mouth opening is surrounded by tentacles that carry stinging capsules, each of which has a coiled thread with a poisonous liquid inside. When defending and attacking, the thread straightens with lightning speed, paralyzes the victim and pushes it into the throat with tentacles. In addition to the digestive system, cnidarians have muscular, nervous, and skeletal systems; reproduce by budding or division. Three classes are distinguished in the type: hydroid (V-Q), scyphoid (V-Q), coral polyps V-Q. Consider below the class of coral polyps.

(a nthos- flower, Zoa- animal), i.e. animals that look like flowers were multi-colored in life.

Exclusively marine organisms , stenohaline, attached and sessile benthos, extinct and modern, calcareous skeleton. A single organism is called a coral polyp, and its skeleton is corallite.

There are 6 subclasses, extinct among them: Tabulatoidea, Tetracoralla, Heliolitoidea and the Chaetetoidea group (Table 6).

Subclass Tabulatoidea. Tabulatoidea C 2 -P(lat. tabula- board; Greek oides- kind, form)

These are exclusively colonial animals, led a motionless lifestyle. Colonies are massive (walls of one corallite closely adjoin to another), branched, chain-like. In cross section, corallites can be rounded, elliptical, polygonal, reaching up to 10 mm in diameter, and the entire colony up to 1.5 m. In the internal cavity of corallites there are horizontal partitions - bottoms, ceilings (taboules) and vertical (septa) - small, spike-like .

Subclass Tetracoralla. 4-beam; Rugosa. Rugoza O-P(gr. tetra- four; corallion- coral or lat. ruga- wrinkled)

Paleozoic solitary and colonial animals with a calcareous skeleton. The shape of single corals is horn-shaped, cylindrical, prismatic. Not more than 25 cm long and 6 cm across. Massive-type colonies consisted of prismatic corallites, up to 4 cm in diameter, and the colonies themselves up to 1.5 m. The skeleton consisted of bottoms, septa, bubble-like formations, and columns.

The septa were laid regularly. First, one septa was formed, which broke up into one short and one long septa on the opposite edge. Then four side ones appeared. New septa were established in four of the six sectors received.

The cross section of single corals is round, polygonal, quadrangular. Some forms have lids (genus Calceola). Single four-beam corals have a well-developed integumentary wrinkled layer - epithecus. Its presence led to the second name of the subclass - rugosa.

Subclass Heliolithoidea. Heliolithoids O 2 -D 2(gr. helios- sun; lites- distorted from lithos- a rock)

Heliolithoids are colonial animals. The forms of colonies are varied, corallites are cylindrical, with twelve or six septa, reminiscent of the sun.

Chaetetoidea group. Chaetoids O-N(gr. Chaite- hair)

Chaetoids are the subject of constant debate. Chaetetoids are most often referred to as the Cnidaria phylum, Anthozoa class. Some researchers consider the Chaetetoid among bryozoans, algae, or sponges.

Chaetoids are colonial animals. The colonies are massive, represented by calcareous thin, hair-like (0.15-1 mm) tubules (corallites). The cross sections of the tubules are rounded.

Class Anthozoa. Coral polyps V-Q

Table 6

Subclass	Genus	Characteristics of the genus
Tabulatoidea. Tabulates C 2 -P	michelinia C	Massive bun-shaped colony. Corallites are large (up to 8 mm), prismatic in shape, vesicular tabulae.
Favorites S-D	The colony is discoid, hemispherical in shape. Corallites are polygonal, honeycomb-shaped, closely adjacent to each other, tabulae are flat, horizontal.
Halysites O 2 -S	chain colony. Corallites are oval in cross section, small (1-2 mm), tabulae are concave.
Syringopora O 3 -C	Bushy colony of isolated cylindrical corallites. Corallites are connected by thin horizontal tubes. Funnel-shaped tabulae.
Tetracoralla. Four beam; Rugosa O-P	caninia C-R 1	Single coral, cylindrical or horn-shaped, with wrinkled epithecus. Long thin septa not from the very edge and do not reach the center. Attached benthos.
Triplasma altaicus D1	Solitary coral, short thick septa located along the margin. Attached benthos.
Lithostrotion C1	colonial coral. The colony is massive, hemispherical. The septa are short and long, which reach the column in the center. Free-lying benthos.
Heliolithoidea O 2 -D 2	Heliolites D 1-2	Colonies of various shapes, consisting of round and prismatic corallites with 12 septa.
Chaetetoidea O-N	Chaetetes D-P (C)	The colony is massive, hemispherical. The corallites are hairlike and closely adjacent to each other. Attached benthos.

Lifestyle and living conditions. Tabulates and tetracorals are inhabitants of warm shallow seas, mainly in the upper part of the subtidal zone. Participated in reef formation. Corals are very whimsical animals - they do not tolerate desalination, or when there are a lot of suspended particles of silt in the water, so they settled far from the coast.

Geological distribution. Tabulates appeared in the Cambrian, and tetracorals and heliolithoids in the Ordovician. Greater diversity is reached in the middle of the Paleozoic. They die out at the end of the Paleozoic era.

Geological significance. Tabulates, tetracorals and heliolithoids are of great biostratigraphic importance for Paleozoic deposits, since these groups are completely extinct, they are the leading forms.

Corals, as stenobiont animals, are used in the reconstruction of paleogeographic conditions of sedimentation. According to the lines of growth of the epitheca rugosa, one can calculate the number of days in a year in past geological epochs. In this case, corals act as a "geological clock".

The role of corals in rock formation is also enormous. The coral reef builds became coral limestones that trap oil and gas.

The phylum Cnidaria has about 9,000 species, united in several classes, among which the most extensive are the Hydrozoa, Scyphozoa and Anthozoa.

The vast majority of cnidarians are marine animals, although there are species that have mastered fresh and brackish waters. These are radially symmetrical animals with an oral-aboral main axis of symmetry and a relatively simple body plan. The body wall is formed by two epithelial layers - the outer, or epidermis, and the inner, gastrodermis. The latter lines the gastrovascular cavity - celepterone, which also performs a digestive function and ensures the circulation of substances throughout the body of the animal. The gastrovascular cavity communicates with the external environment through an opening that simultaneously performs the functions of both the oral opening and the anus.

The composition of the epithelial layers includes a variety of cellular elements. In the epidermal layer there are epithelial-muscular, sensory, nervous, glandular and stinging cells - nematocytes, as well as undifferentiated multipotent interstitial cells (i-cells). The gastrodermis contains epithelial-muscular and glandular cells. Between the epithelial layers is an extracellular matrix - mesoglea, the degree of development of which varies greatly in different species. In the mesoglea, type IV collagen, fibronectin, heparan-sulfate-proteoglycan, laminin, etc., characteristic of basement membranes, are distinguished. In the mesoglea, Scyphozoa has a self-sustaining population of amoebocytes.

Cnidaria are characterized by two types of organization - polypoid and medusoid. In many species, for example, those belonging to the metagenetic Hydrozoa or the Scyphozoa, there is a regular alternation of these forms, or metagenesis. In this case, sexual reproduction is associated with the medusoid generation, while asexual reproduction is characteristic of the polypoid generation. The medusoid phase may be reduced or completely absent (for example, in representatives of the order Hydrida). The medusa stage is also absent in corals, in which both sexual and asexual reproduction is provided by polyps. However, there are forms represented only by jellyfish. Thus, in the life cycle of animals from the order Trachylida there is no polypoid phase.

Polyps often form colonies with a common gastrovascular cavity. There are different types of polyps, or zooids, in a Hydrozoa colony. Most of them are represented by gastprozooids, or feeding polyps; in some species, dashpilozooids are formed, which, due to the abundance cnidocytes(from Greek - nettle) protective function. Reproduction is carried out by gonozooids, or medusoid buds, which produce gametes. Medusoids either separate from the colony and turn into jellyfish, or remain in the colony as gonophores.

Sex cells are formed from interstitial cells. As studies performed on hydras have shown, among i-cells there is a special population committed as a line of germ cells. In the process of oogenesis, phagocytosis and cell fusion play an important role in supplying the oocyte with nutrients. Representatives of this type are characterized by temporary gonads, although in Scyphozoa permanent gonads are formed.

Fertilization in cnidarians is usually external. Nevertheless, in all classes of cnidarians there are species with internal fertilization, up to the peculiar copulation described in the anemone Sagartia. In the latter case, the pedal disks of the parent individuals form a common chamber into which the gametes are released and in which the fertilized eggs develop to the larval stage.

The first two divisions of crushing are meridional, and the third is equatorial. It is noteworthy that the furrows of cleavage divisions are not circular, but cutting: they begin at one pole of the fertilized egg and gradually spread to the opposite, where the connection between blastomeres is observed for a relatively long time.

Cnidaria are distinguished by a wide variety of types of crushing. With complete and uniform crushing, the radial nature of the location of blastomeres is often observed. In some species, however, the connection between blastomeres is weak, so that they can change their position relative to other cells. If the blastomeres rotate, then figures may appear that resemble spiral fragmentation in appearance, i.e., pseudospirality occurs. In other cases, the crushing embryo loses the definiteness of its geometric forms (anarchic type of crushing). With uneven crushing, the arrangement is disordered and its pattern is changeable. In eggs rich in yolk, cytotomy may be delayed. In some species, the central mass of the yolk does not divide at all. In this case, crushing becomes superficial.

The variety of cleavage forms also affects the structure of the blastula. Several types of blastula have been described in cnidarians: a hollow coeloblastula formed by a single row of cells that surround an extensive blastocoel; dense sterroblastula, also formed by one row of cells, but without a blastocoel, morula, and, finally, periblastula, which is characterized by the location of the outer layer of cells on the surface of the yolk mass. Cells of the coeloblastula are equipped with flagella, which ensure its movement.

At the next stage of development, gastrulation occurs, during which two main layers of the body are formed in cnidarians: the outer layer, or ectoderm, and internal - endoderm.

In cnidarians, a variety of cellular mechanisms for the formation of body layers have been described. Widespread ingression(from lat. ingressus - entry, entry), or immigration of cells. During ingression, some cells of the coeloblastula wall lose flagella, acquire amoeboid mobility and move into the blastula cavity, filling it completely. A distinction is made between unipolar ingression, which occurs in the region of the embryo, where the oral opening is subsequently laid, and multipolar ingression, in which invasion occurs over the entire surface of the embryo.

The colonization of the blastula cavity by individual cells can also occur as a result of oriented cell divisions in the blastula wall. This process is called delamination(from lat. de - separation, lamina - plate, layer). Cells that enter the blastula cavity after division form the endoderm. The immersion of the cells of the blastula wall can also occur as part of the epithelial layer. This type of epithelial morphogenesis is called invagination, or invagination (from lat. invaginatio - invagination).

The formation of the ectoderm and endoderm in the morula occurs as a result of cell rearrangement. The cells occupying the inner region of the embryo give rise to the endoderm, while the cells of the outer layer give rise to the ectoderm. This separation of layers is called morula, or secondary delamination.

Finally, in many species it is described epiboly(from Greek - vestment, cover), or fouling of large macromeres with fissile micromeres. Mixed types of reservoir isolation are also widely represented.

As a result of the gastrulation process, a usually radially symmetrical two-layered larva arises. planula(from Greek - wandering). The outer ectodermal layer of the planula is formed by ciliary cells. Between the ectoderm and endoderm, there is a thin layer of extracellular matrix - mesoglea. At the planula stage, differentiation of cell layers occurs. So, epithelial-muscular, glandular and sensory cells appear in the composition of the ectodermal epithelium. Interstitial cells and their derivatives, including stinging cells, are located between the epithelial cells. The place of formation of interstitial cells is the endoderm, where they are committed. In the endodermal epithelium, digestive and glandular cells are formed. The planula is elongated and slightly expanded at the anterior end, which is the successor to the vegetative region of the crushing embryo. Usually, planulas are lecithotrophic and the nutrition necessary for their life in the form of yolk grains stored during oogenesis is located in their cells. In some Anthozoa, planktotrophic planulae are described, in which, after the completion of invagination, a mouth opening forms in place of the blastopore.

The transformation of a larva into an adult form is called metamorphosis. During this process, the larva is attached to the substrate by the front end or side surface. Usually, the body of the planula is flattened in the longitudinal direction and turns into a disk, on which a polyp grows, connected to the disk by a stalk. This primary hydrant, the ancestor of the colony, develops tentacles and a mouth opening. In other cases, the planula turns into a hydrorhiza - a filamentous body spread over the substrate, on the surface of which polyps form. Colonial forms result from the budding of primary hydrants.

Sometimes the formation of polyp structures begins very early, even at the stage of a floating larva. In these cases, the larva is compressed along the anterior-posterior axis. In this case, the anterior (future aboral) region flattens, and the posterior (future oral) takes the form of a cone, at the top of which a hole is formed with a surrounding rim of tentacles. A stalk is formed at the aboral pole. Emerging free polyp, or actinula(from Greek - beam) soon settles and attaches to the substrate.

In cnidarians, asexual reproduction is widespread, which can occur in both polyps and jellyfish. As a result of asexual reproduction of the primary polyp, colonial forms arise. With asexual reproduction of hydroid jellyfish, the population of animals capable of sexual reproduction sharply increases.

In Scyphozoa, a single polyp formed after settling is called a scyphistoma, a characteristic feature of which are septa - vertical folds of the endoderm that divide the gastric cavity of the polyp into four pockets. Scyphozoa polyps reproduce asexually by budding and strobilation. Strobilation begins in the oral area of ​​the polyp and spreads aborally. It consists in the sequential formation of disc-shaped elements by transverse divisions of the body. A polyp in the strobilation phase is called strobila(from lat. strobilus - bump). Disks separated from the strobila form ethers, or jellyfish larvae. The formation of the ether involves a radical restructuring associated with the loss of the provisional organs of the scyphistoma and the development of the organs of the emerging jellyfish.

In some Scyphozoa, polyp budding produces podocysts that can remain dormant for a long time. The podocysts then transform into motile larvae. Something similar takes place in Hydrozoa. For example, in representatives of the Leptolid order, fructulation(from lat. frustulum - a piece) - a peculiar form of asexual reproduction by fragmentation, during which planu-shaped frustula larvae arise.

Thus, in representatives of different classes of cnidarians, asexual reproduction occurring at the polypoid or medusoid phase of the life cycle can lead to the formation of a mobile larva characteristic of sexual reproduction. This phenomenon, apparently, can be regarded as evidence of the existence of relatively autonomous modular developmental subroutines that can be initiated both during sexual and asexual reproduction. Verification of this assumption requires a special study.

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Type Intestinal, or Cnidaria. General characteristics of the type

Systematic position of the type

Remark 1

Type Intestinal (Coelenterata) belongs to the sub-kingdom of Animals, its representatives are eumetazoans, or true higher multicellular animals.

Representatives of the Supersection Eumetazoi have a number of common features:

• differentiation of tissues, organs;
• the presence of nerve cells;
• clearly manifested integrity and integration of individuals;
• pronounced bilateral (Section Bilateral) or radial (Section Radiant) symmetry.

Type Intestinal are included in the Section Radiant. They, as representatives of this section, are characterized by:

• beam symmetry;
• two-layer structure;
• the presence of a gastric (intestinal) cavity;
• diffuse nervous system.

Type Celiac includes polyps and jellyfish that have stinging cells, so this type is also called Cnidaria.

This type includes three classes:

• Hydroids (Hydrozoa);
• Scyphozoa (Scyphozoa);
• Coral polyps (Anthozoa).

Features of the external and internal structure

Remark 2

The body of the Coelenterates has a central heteropolar axis, around which morphological structures are located in a certain order. This axis penetrates the oral (oral) and aboral poles of the body.

In relation to the heteropolar axis, the body parts of the coelenterates and individual structures are oriented symmetrically:

• radially;
• asymmetrically, or biradially;
• bilaterally.

Through the body of the coelenterates, 2, 4, 6, 8, etc. can be passed. planes of symmetry. Representatives of the type, as a rule, lead a fixed or sedentary lifestyle. In the process of ontogenesis, two germ layers are formed. From the ectoderm (outer leaf) integuments are subsequently formed, and the endoderm (inner leaf) lines the intestinal cavity.

The tissues and organs of the coelenterates are formed by the epidermis and gastrodermis and the mesoglea between them - the intercellular matrix. The epidermis exhibits a high degree of differentiation of cells, tissues, and organs.

Type specific features:

• four-beam symmetry - tetramerism;
• development with metamorphosis, the presence of a two-layer planula larva is characteristic;
• stinging cells that perform the functions of attack and defense;
• the main part of the nervous system is the diffuse plexus.

Direct development is rare. The body of all coelenterates is a bag consisting of two layers with a gastric cavity. The cavity of the sac is lined with endoderm, where food is digested. The function of the mouth is performed by the opening of the “bag”, undigested food residues are also removed through it. The simplest representatives of the coelenterates in structure can be compared with a typical gastrula. This group of animals has a high ability to regenerate.

Morpho-ecological forms of coelenterates

There are two morpho-ecological forms of Coelenterates:

• polyp (benthic attached form);
• jellyfish (planktonic form).

Type Coelenterates is characterized by the presence of floating forms of animals with tentacles. Colonies are sometimes formed from medusoid and polypoid individuals. Often you can find a symbiosis of Cnidaria and unicellular algae. For most representatives of the type, a life cycle with alternating sexual and asexual reproduction is characteristic, the so-called metagenesis between a jellyfish and a polyp. As a rule, a jellyfish is formed from a polyp as a result of:

• formation of special transverse constrictions;
• metamorphosis;
• strobilation (terminal department);
• lateral budding.

The formation of a polyp occurs as a result of sexual reproduction of the jellyfish through the stage of planula formation.

update: Previously, coelenterates and cnidarians were synonymous names for this type of animal. But now the supertype is called coelenterates, which includes the type of cnidarians and the type of ctenophores. So far, I have corrected only the title of the article, but I will soon redo it completely, but for now, you will meet in the text coelenterates as a synonym for cnidarians. Be patient.

Among the cnidarians there are creatures that look like flowers, bushes, trees several meters high, cauliflower and even grassy lawns. The ancient Romans and Greeks believed that corals are sea flowers that immediately petrify in the air (see Ovid's "Metamorphoses"). But many modern Jews, residents of Eilat, even after repeated visits to the Underwater Observatory, by no means believe that these are animals. This is such a cruel people.

Cnidaria isolated in a separate type of animal, including about 9000 species. The type is divided into three classes: hydroids ( Hydrozoa , about 3000 species), scyphoid jellyfish ( Scyphozoa , 200 species) and coral polyps ( Anthozoa , 6000 species). Each class has plant-like sessile animals and active swimmers/crawlers.

For zoologists, coelenterates are remarkable in that for the first time in the evolution of the animal world, real tissues appeared in them. This group got its name because the body of the coelenterates has the appearance of a bag open at one end. Digestion takes place in the cavity of the bag, and the hole serves both as an entrance (that is, a mouth, in our understanding) and an exit (undigested food residues are removed through it). If the animal is attached to the substrate and the mouth is at the top, then it is called a "polyp".

Free-swimming intestinal cavities, whose mouth is directed downwards, are called jellyfish. The division into jellyfish and polyps is not systematic, but purely morphological - the same type of coelenterates at different stages of the life cycle can consistently look like either a polyp or a jellyfish.

Coelenterates are radially symmetrical creatures, which makes them look like flowers. These animals are two-layered, they have only two layers of cells - external and internal. Between them is a non-cellular substance, sometimes in the form of a thin layer, sometimes, for example, in large jellyfish, it is a thick gelatinous layer. Another characteristic feature of the coelenterates is the presence stinging cells .

The most simply arranged coelenterates are hydroids, hydroid polyps and hydromedusae. Colonies of hydroid polyps are usually small and can be seen in aquariums rather than swimming underwater.
Many hydroids look like openwork twigs.

Hydroids usually reproduce asexually by budding. Some kidneys are developing differently than usual. Of these, not new polyps are formed, but jellyfish. Usually small (a few centimeters maximum) jellyfish, unlike polyps, form germ cells. Actively swimming jellyfish release mature germ cells into the water. The larva that has developed from a fertilized egg also moves in plankton for some time, and then sinks to the bottom and forms a new colony. Thus, two generations alternate in the life cycle of hydroids - benthic polyps, which reproduce by budding, and planktonic jellyfish, "responsible" for sexual reproduction. The meaning of this phenomenon is simple - planktonic jellyfish, unlike attached polyps, allow the species to settle and capture new substrates.

And there are hydroids that form mixed colonies, consisting of polyps and jellyfish that have not budded to the end. Moreover, these colonies can be not only attached, but also free-floating. Planktonic colonial hydroids are distinguished into a separate subclass siphonophore, which some zoologists consider an independent class.

The most dangerous jellyfish and corals for humans also belong to the class of hydroids. Contact with a small Far Eastern cross jellyfish ( Gonionemus vertens ) can cost the bather his life.

This group includes false fire corals (Millepora ), which can seriously injure the skin when touched. Often, after burns, long-term non-healing ulcers form on the skin. Unlike physalia and cross, fire corals can be found off the coast of Eilat.

The second class of the coelenterate type is the scyphoid jellyfish ( Scyphozoa ).
In scyphoid jellyfish, the body looks like an umbrella with long tentacles suspended from below. The word "jellyfish" in most people is associated with this group. Scyphomedusa are much larger than hydromedusas: the diameter of the umbrella of the Arctic jellyfish Cyanea capillata reaches 2 m. The body of jellyfish is always transparent and very tender, gelatinous. With the contraction of the umbrella, jellyfish swim quite quickly. Jellyfish usually stay on the surface, although an expedition on the Challenger vessel caught a jellyfish from a depth of 2000 meters.

There is a site about the life and reproduction of a typical scyphoid jellyfish, since its mass appearance near the Eilat coast caused panic.

But even among the scyphomedusae there are those that can be mistaken for plants. In our shallow waters, you can find rather strange whitish-greenish formations resembling cauliflower. This is a scyphomedusa, which in Latin is called Cassiopea andromeda , named after the legendary mother and daughter from Greek myths, but in Hebrew and English it is called “cauliflower”. There is also an English name upside down jellyfish - an inverted jellyfish. In its normal state, Cassiopeia lies in the sand, well, if not “upside down”, then, in any case, “upside down”, and catches plankton with short thick tentacles. This jellyfish has learned to grow symbiotic algae in its body.

Finally, the third class of intestinal - coral polyps ( Anthozoa ). This is the most numerous group of coelenterates. Among its representatives, the largest number of forms similar to plants. I want to draw your attention to the fact that those corals that we see in the waters of the Gulf of Eilat are hard colonial reef-forming (their scientific name is madreporous, the most famous, but not the only representatives of coral polyps. They also include sea ​​anemones, soft corals, sea feathers, sea fans, sea fingers, jewelry corals (black and red) and many other interesting animals. Coral polyps are both solitary and colonial animals. Only some of them form a calcareous or horny skeleton. These animals do not have a generational change, and they do not form jellyfish. In addition to microscopic larvae that live in plankton for a short time, the entire life of coral polyps passes on the bottom, to which most of them are firmly attached, although some, such as sea anemones, can crawl.
Corals a separate article is devoted to them, they are very important for the sea and for Eilat.

In soft corals, individual polyps do not have a calcareous skeleton, and their colonies of various shapes and colors sway on reefs, piles, stones, like bushes in the wind. In the water column, these colonies do not have weight, therefore, even without a supporting skeleton, they reach large sizes.

Type Intestinal - Coelenterata, or Crackers - - the most ancient and low-organized organisms from real multicellular animals. Cnidarians got their name from the Greek. knide - to burn. Another common name for this type of animal is coelenterata. Radially symmetrical, mostly marine animals armed with tentacles and unique stinging cells (nematocytes) with which they hold and kill prey.

The body wall consists of two layers surrounding the gastrovascular cavity: the outer (epidermis) of ectodermal origin and the inner (gastrodermis) of endodermal origin. These layers are separated by a gelatinous connective tissue called mesoglea. The gastrovascular cavity serves to digest food and circulate water throughout the body.

Cnidarians for the first time had real nerve cells and a diffuse-type nervous system (in the form of a network). Polymorphism is characteristic, i.e. the presence within the same species of forms that differ sharply in appearance. One typical form is a sessile polyp attached to the substrate and similar to a cylinder, at the free end of which is a mouth surrounded by tentacles; another form is a free-floating jellyfish, resembling an inverted bowl or umbrella with tentacles hanging down the edges. Polyps form jellyfish by budding. Those, in turn, reproduce sexually: a fertilized egg develops into a larva, giving rise to a polyp. Thus, in the life cycle of many cnidarians, there is an alternation of sexual and asexual generations. Species that do not have a medusoid form reproduce sexually or by budding. They may be dioecious or hermaphroditic.

Their body consists of two layers of cells - the outer one, which forms the ectoderm, and the inner one, which is called the endoderm. Between these layers there is a developed non-cellular layer - mesoglea.

The function of support in the coelenterates is performed by the mesoglea. In polyps, it looks like a thin base plate.

The coelenterates have the most primitive type of nervous system among multicellular organisms. In the ectoderm, nerve cells that perceive irritation are relatively evenly distributed. The irritation is transmitted through the contacting processes of the nerve cells to the contractile fibers of the epithelial-muscle cells, and then the response follows - the contraction of the body of the hydra.

Coelenterates are characterized by radial symmetry and a two-layer body structure.
Most coelenterates have pronounced radial or radial symmetry. In coral polyps, there are deviations towards two-beam or even bilateral (bilateral) symmetry.

Coelenterates are characterized by two life forms: a sessile saccular polyp (coral polyps) and a floating discoid jellyfish. The polyp has the following structure. The part of the body that attaches the body to objects is called the sole. On the upper part of the body there is a mouth surrounded by tentacles. All coelenterates are characterized by the presence of special stinging cells, which are designed to protect against enemies, as well as attacks. This has not been found in other animals.

Stinging cells contain capsules with paralyzing poison. It enters the body of the victim through a special channel located in the stinging thread of these cells. When a sensitive hair is irritated, the stinging thread straightens with force and pierces the victim. After the shot, the stinging cell dies, and a new one is formed from the intermediate cell.

In addition to stinging coelenterates, they also have other specialized cells: skin-muscular, glandular, reproductive, and nervous.

The digestive system of coelenterates is very primitive. The mouth leads into the intestinal or gastric cavity.

Digestion of food at the first stage occurs under the action of enzymes in the gastric cavity. This is extracellular or cavity digestion. Small food particles, into which food breaks down, are captured by endoderm cells, i.e. the inner layer of cells, and are digested intracellularly.

Coelenterates reproduce both asexually and sexually.

Simply arranged cnidarians include hydra, reaching 2.5–3 cm in length and leading a solitary lifestyle. Many form large colonies. Approximately 10,000 species have been described, grouped into three classes.

The coelenterates type unites about 9000 species - inhabitants of the seas and oceans and about 20 species of fresh water inhabitants. The type of coelenterates includes three classes:
Hydroids (Hydrozoa) Scyphozoa (Scyphozoa) Coral polyps (Anthozoa)

The value of the coelenterates is great. Calcareous skeletons of reef-building coral polyps form reefs and atolls in tropical seas. Coral reefs and islands are a dangerous obstacle to navigation. Coral polyps play a useful role in the purification of sea water from suspended organic particles. Huge strata of limestone were formed from the skeletons of coral polyps that had died off over many millennia. In many tropical coastal countries, it is used in construction. From the skeletons of some types of corals, for example, red coral, various decorations are made.

Jellyfish sensitively pick up sound vibrations that occur when water rubs against air, and long before a storm approaches, they sail away from the coast. Based on this property, bionics scientists created the Medusa Ear device, which allows you to determine the approach of a storm about 15 hours before its onset.

Some types of jellyfish serve as a refuge for fish fry and hermit crab. Coelenterates are of great importance in the food chain of marine biocenoses.