Body parts of coelenterates. §6

More than 8 thousand species belong to the coelenterates, leading an exclusively aquatic lifestyle. Among them, there are free-floating forms and sessile organisms attached to the bottom or underwater objects (animals). 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.

The appearance of intestinal cavities in the seas is attributed to the Proterozoic era, when the first representatives of this group of animals appeared.

The emergence of coelenterates was accompanied by large aromorphoses, which significantly expanded the possibilities of their owners in using the habitat:
1) multicellularity;
2) the formation of the first tissues - ecto- and endoderm;
3) ray symmetry;
4) differentiation of cells into a number of specialized cell types;
5) the emergence of a nervous system, a diffuse type, consisting of individual cells interconnected by processes;
6) the appearance of partially intracavitary digestion;
7) the appearance of germ cells specialized for reproduction.

The type of coelenterates includes the classes of hydroid, scyphoid (jellyfish) and coral polyps.

Class Hydroids. freshwater lolip hydra

Body structure. Bilayer aquatic animals. Radial symmetry. The body is saccular, elongated up to 1.5 cm in length. At the front end "of the body there is a mouth surrounded by tentacles that carry a large number of stinging cells. The posterior end is the sole with which the hydra is attached to the substrate (stones, plants, etc.).

Wall body. The outer layer of the body of the hydra is the ectoderm, consisting of stinging, musculoskeletal and nerve cells. Beneath the ectoderm is a non-cellular basement membrane or mesoglea.

Digestive system closed. Represented by the gastric cavity, beginning with the oral opening. The cavity is lined with endoderm, the cells of which are capable of phagocytosis. Digestion is both abdominal and intracellular (digestive vacuoles). Undigested residues are thrown out through the mouth.

Breath hydras. Oxygen dissolved in water is absorbed by the entire surface of the body.

Selection. The end products of dissimilation are excreted through the ectoderm.

Nervous system consists of stellate nerve cells connected by their processes.

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

Reproduction. Asexual predominates - budding. Animals are bisexual (hermaphrodites), eggs and spermatozoa develop in the ectoderm. Cross fertilization.

Development. From the zygote, a two-layer larva is formed with the rudiment of the intestinal cavity - planula, which moves in the water and hibernates at the bottom of the reservoir. Adult hydras die in autumn.

1. Property-assignment and property-alienation: two views on the same problem

2. Property as an attribute of the enslavement of labor by capital and property as a way of freeing the individual from the swarm community

3. The relationship of the concepts of property, property, capital

Interpretation of the new institutional theory. From a legal point of view, the subject of private property is not an individual, but a person, including a legal entity.

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Section 1. Radiant (Radiata) Coelenterata

Radiant are characterized by the following main features of the organization:

the body has a heteropolar (oral-aboral) axis and radial symmetry,

the body is formed by two epithelial layers: ectoderm and endoderm, which come from two germ layers. The ectoderm forms the integument of animals, and the endoderm lines the intestinal cavity.

there is an intestinal (gastric) cavity,

diffuse nervous system

Selection and breath carry out epithelium in direct contact with the environment (epidermis) or with intra-intestinal fluid (gastrodermis)

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Radiata are marine, rarely freshwater animals. This group cannot be considered monophyletic. There are two types:

type Intestinal (Cnidaria)

phylum Ctenophore (Ctenophora)

The intestinal cavities include a variety of jellyfish, polyps with stinging cells on the tentacles, and therefore the second name of the type is stingers. Ctenophores are exclusively swimming marine animals with rows of special comb-shaped plates that are derivatives of flagellar cells. They lack stinging cells, and they are also called non-stinging (Acnidaria). Coelenterates and ctenophores are in many ways similar in their organization, and for a long time they were combined into one type. However, later significant differences in their ontogeny and features of organization were revealed.

Type Intestinal (Cnidaria)

Brief description of the type of intestinal

Coelenterates are predominantly marine, less often freshwater animals, lead a solitary or colonial lifestyle. Two forms of existence are known: polyp (benthic) and jellyfish (planktonic). More than 10 thousand species are known. Bilayer animals. Between the ectoderm and endoderm is the mesoglea. The presence of stinging cells is characteristic. The digestive system is the gastric or gastrovascular cavity. Digestion is abdominal and intracellular. There are real, albeit poorly differentiated tissues. Nervous system of diffuse type. Excretory organs are absent. Breathing with the entire surface of the body. Intestinal animals are dioecious and hermaphrodites. Reproduction is sexual and asexual. The larva planned. Life cycle with or without alternation of polopoid and medusoid stages. The type of coelenterates is divided into three classes.

General characteristics of coelenterates (cnidarians)

1) Type coelenterates includes more than 10 thousand species. These are mainly marine animals, less often freshwater, leading a sedentary or swimming lifestyle. They live alone or in colonies. The organization of the body is rather primitive.

2) The body of intestinal cavities is sac-shaped, the body wall limits gastric or gastrovascular a cavity that opens outward with a mouth surrounded by one or more tentacles. The sense organs, when present, are concentrated at the base of the tentacles. The body is radially or bilaterally symmetrical. There is an oral end of the body, on which the mouth is located, and an opposite aboral end.

Radial symmetry, as a rule, is characteristic of sedentary or immobile animals. Radial symmetry is useful if abundant but spatially dispersed resources (light, plankton) or danger are equally likely to appear from either direction. Consequently, radial symmetry is also useful for most cnidarians: after all, they do not actively search for prey, but feed only on those animals that were brought by the current or that accidentally swam too close to them.

gastric cavity It is a "bag" lined with gastrodermis that opens outward through the mouth. The gastric cavity of large polyps is often divided by partitions that increase the area of the gastrodermis. digestive system of jellyfish gastrovascular system, resulting from the specialization of individual sections of a single gastric cavity, consists of a central stomach and departing from it radial channels, which, in turn, flow into the umbrella passing along the edge annular channel. In most polyps and jellyfish, narrow branches of the digestive system extend into all tentacles.

The gastric cavity performs primarily digestive, as well as circulatory and adsorption functions, and sometimes serves as the body's hydroskeleton and brood chamber for developing embryos; it can accumulate and destroy excretory products.

Internal transport and circulation of fluid occurs through the beating of gastrodermal cilia, muscle contractions, or both. The movement of fluid in the digestive system is carried out along a certain and often very complex path, which is different in representatives of different taxa, in polyps and jellyfish, in solitary and colonial forms.

3) The body wall of cnidarians consists of three layers formed by different tissues: the outer epithelium, or epidermis; inner epithelium, or gastrodermis, lining the gastric cavity and connecting with the epidermis in the mouth area. Between them lies the mesoglea in the form of a basement membrane or gelatinous extracellular matrix. The mesoglea primarily performs a supporting function, also plays an important role in the locomotion of jellyfish, ensures the stability of conditions and the supply of nutrients to the cells.

Almost all functions of the body are carried out by both epithelium (transmission of information, movement, digestion and internal transport), which include cells of different types. Both epidermis and gastrodermis contain stinging, muscular, nervous, glandular, interstitial and ciliary cells, but the specific functions of these cells may differ depending on their belonging to a particular epithelium. Sensitive cells are confined to the epidermis, germ cells - to the gastrodermis.

4) Coelenterates exist in two morpho-ecological forms: benthic attached - polyp and planktonic floating - jellyfish.

sac-shaped or tubular polyp, usually attached to the substrate, organized simply. The body of the polyp is tubular or sac-like, and has three main sections: (1) a proximal (aboral) section, usually forming sole, - pedal disc,(2) cylindrical body (stalk) (3) distal oral field (peristome) with a single body opening (mouth-anus) in the center and tentacles along the periphery. Inside the gastric cavity of polyps there may be longitudinal partitions - septa.

The polyps of the four groups of cnidarians differ in characteristic features. Anthozoa polyps have an ectoderm-lined pharynx with one or two pharyngeal grooves (siphonoglyphs) densely covered with cilia. Their gastric cavity is divided by septa into gastric pockets. The number of septa (and therefore pockets) can be eight, six, or multiples of six (sometimes many hundreds). Septa carry longitudinal muscles (sarcosepta), sex cells, and at the free end - gastral filaments, equipped with stinging cells, glands and cilia. Scyphoid polyps, on the other hand, always have only four septa and four gastric pockets. Inside these septa pass septal funnels - invaginations of the oral disc, lined with the epidermis and muscles. The sac-like polyps of Cubozoa and Hydrozoa are relatively simple in structure, lacking septa and gastric pockets.

Jellyfish have an umbrella-shaped or bell-shaped body. upper body or exumbrella is aboral and the underside or subumbrella- oral, from the center of the subumbrella departs the mouth (aka anus), located at the free end oral stalk (manubrium). The mouth leads to central stomach. From it to the periphery depart radial channels; they connect an annular channel at the edge of the bell. Along the edge of the umbrella, jellyfish are equipped with trapping tentacles and sensory organs. The mesoglea is strongly thickened as a shaping and supporting element, especially in the region of the exumbrella. Because of this, the jellyfish, in comparison with the polyp, has a reduced share of the gastrovascular system in the total volume of the body.

5) Skeletons of cnidarians are varied. The exoskeleton can be in the form of a thin chitinous cuticle (periderm) in solitary polyps and small colonies, madrepore corals have a hard calcareous exoskeleton.

Gorgonians have an endoskeleton in the form of calcareous spicules or horny organic fibers found in the mesoglea.

Anemones and some polyps do not have a solid skeleton; they maintain the shape of the body due to fluid pressure in the gastric cavity - the hydroskeleton. In jellyfish, only the mesoglea performs a supporting function. . It not only determines the overall shape of the body, but, being an elastic gel, returns to its original shape after deformation caused by muscle contraction during swimming.

The basis of the epithelium is almost cylindrical epithelial muscle cells. These cells are epithelial cylindrical cells that have retractable basal processes located parallel to the longitudinal axis of the body. With the reduction of such processes, the body of the polyp and its tentacles are shortened, and when relaxed, they are extended.

The muscles of polyps are represented by two layers of smooth muscle fibers: the epidermal muscle layer is usually formed by longitudinally oriented muscle fibers, and the gastrodermal one is circular. Functionally, the circular and longitudinal muscles act as antagonists. In jellyfish, the musculature is represented by an annular coronal striated muscle located under the surface of the subumbrella. . Its antagonist is the elastic mesoglea.

In representatives of Anthozoa and Scyphozoa, some of these cells left the epithelial layers, plunged into the mesoglea and turned into myocytes, "real" muscle cells .

6) The presence of stinging cells is characteristic - cnidocytes. Cnidocyte , sensory and at the same time effector cell, plays a key role in prey capture and defense. These cells contain stinging capsules (books, cnidocysts), derivatives of the Golgi apparatus, filled with liquid. Knida contains a coiled hollow stinging thread, the wall of which is a continuation of the capsule wall. Stinging cells in Anthozoa are equipped with a normal cilium, Hydrozoa Scyphozoa have an elastic knidocil, consisting of a modified flagellum and a border of microvilli. When the cnidocil is irritated, the stinging process quickly turns outward. After the shot, the capsule finally dies along with the stinging cell.

Stinging cells can be of several types: penetrants, volvents, glutinants. Penetrants have nettle properties, contain a large stinging thread. The cavity of the capsule is filled with a caustic liquid, which can also pass into the thread. On the outer surface of the cell there is a sensory hair - cnidocil. Touching the sensory hair of the penetrant causes an instant firing of a stinging thread. In this case, the body of the prey or victim is first pierced stylet: these are three spines, at rest put together and forming a point. They are located at the base of the stinging thread and are screwed inside the capsule before the thread is fired. When the penetrant is fired, the spikes of the stylet push the wound apart, and a stinging thread moistened with a caustic liquid, which can have a painful and paralyzing effect, is pierced into it. Stinging threads, like a harpoon, are fixed with spines in the body of the victim and hold it.

Stinging cells of other types perform the additional function of holding prey. Volvents shoot a short trapping thread that wraps around individual hairs and protrusions of the victim's body. Glutinants throw out sticky threads. After firing, the stinging cells die. Restoration of the composition of stinging cells occurs due to interstitial undifferentiated cells.

The stinging poisons of cnidarians act mainly as neurotoxins on the nervous system.

They prevent the transport of Na + into the cell, which leads to general paralysis. The consequences of high (exceeding physiological) concentrations of stinging poisons are convulsions, cardiac arrest and cardiac circulatory disorders (cardiotoxic effect).

Interstitial cells- these are stem cells that arise in the endoderm of the embryo and later migrate to all tissues of an adult animal. At present, interstitial cells are reliably found only in hydroid polyps and are especially well studied in Hydra. Nevertheless, it is very likely that representatives of other taxa of cnidarians also have them. Hydrozoa interstitial cells have been shown to differentiate into neurons, glandular cells, gametes, and cnidocytes. The entire cellular composition of the hydra is exchanged in 3.5 days. In this case, the number of stem cells should remain constant and exceed the number that have gone into differentiation.

7) Digestive system - gastric or gastrovascular cavity. Digestion is abdominal and intracellular. Undigested food debris is expelled through the mouth.

The main food-producing organs are tentacles polyps and jellyfish. In their epidermis densely located stinging cells. Cnidarians are not characterized by an active search for food; the victim accidentally touches the tentacles or other trapping appendages. The ejected threads of the knidia injure and paralyze the prey, attach it to the tentacles that pull the prey to the mouth. The prey is swallowed whole into the intestinal sac. Manubrium can also capture prey when it is long and agile, like an elephant's trunk. Cnidarians feed on copepods, annelids, nematodes, mollusks, many larvae, and sometimes even fish. Due to the extensibility of the mouth and torso, polyps can swallow even disproportionately large prey.

When the victim is swallowed glandular cells gastrodermis producing enzymes secrete them (mainly proteases) into the digestive cavity, where food is digested extracellularly into a nutrient solution and small particles. Absorption by phagocytosis is carried out by specialized epithelial-muscular cells of the gastrodermis, as well as sex and other cells. In the process of intracellular digestion, lipids and carbohydrates are broken down, and the digestion of proteins is also completed. Spare substances are fats, protein and glycogen. As a rule, the extracellular phase of digestion takes a little time, and the absorption of the nutrient solution by the gastrodermis occurs in 8-12 hours. Completion of the intracellular digestion stage requires several days. Undigested food debris, usually held together by mucus in the fecal mass, is thrown out through the mouth.

Like other invertebrates, cnidarians can absorb organic substances (glucose, amino acids) dissolved in water through the epidermis.

Many cnidarians have symbiotic algae in their gastrodermis cells. Like some freshwater sponges, some types of freshwater hydras, as well as sea anemones (for example, Anthopleura contain green zoochlorella, however, yellow-brown zooxanthellae are characteristic of most marine cnidarians. Zoochlorella and zooxanthella provide hosts with the primary product of photosynthesis, sometimes providing up to 90% of their food needs. What do symbionts get from their hosts? In exchange, symbiotic algae receive nutrients, CO 2 2 and a habitat placed in favorable lighting conditions.

8) Specialized bodies excretion and respiration missing.

Respiration and excretion occur through the epithelium. The tentacles and the wall of the body as a whole are a kind of "gills" through the surface of which gas exchange takes place. Constant water exchange is carried out due to the contractile movements of the body and the flow of water created by the ciliary cells of the epidermis. In some colonial Anthozoa, the inflow and outflow of water occurs due to specialized polyps (siphonozoids), which have a particularly strong and, accordingly, muscular pharynx with a pharyngeal groove (siphonoglyph) bearing a powerful ciliary cover.

Ammonia, a product of cnidarium isolation, is highly soluble in water. It easily diffuses through the body wall and is carried away by the current.

freshwater Hydra concentrates K + in cells and removes Na + . Part of the Na + ions enters the liquid that fills the gastric cavity. Due to this, the osmotic pressure in the latter increases in comparison with the environment. The water flowing along the osmotic gradient increases the mechanical pressure inside the cavity, which in this case more effectively performs the functions of a hydroskeleton. Excess water (and Na +) is excreted through the mouth from time to time.

The circulatory system is absent.

9) Nervous system diffuse. It consists of sensory neurons located superficially, motor neurons (motor neurons), intercalary neurons. The neurons are bipolar or multipolar, connected to each other by processes that pass through the mesoglea and form two networks. One network lies at the base of the epidermis, and the other at the base of the gastrodermis.

Pulses can propagate through the network in any direction. The impulses evoked by the point stimulus usually diverge through the network, like circles in the water from a thrown pebble. In colonial forms, the nervous system continues into stolons and connects individual individuals to each other.

The nervous system of jellyfish includes, in addition to nerve networks, concentrated nerve rings located along the edge of the umbrella ganglia. Ganglion is a cluster of neurons that resembles the brain and, like the latter, performs an integrative function. The ganglion receives information from the senses, combines it with other incoming signals, and generates a response signal that generates a motor response. Jellyfish ganglia are associated with sense organs- statocysts (organs of balance) and eyes, as well as accumulations of mechano- and chemoreceptor cells and muscles that work when swimming.

10) Reproduction. asexual reproduction widespread, most common in polyps. Asexual reproduction can usually occur in the form of longitudinal division, budding, less often transverse division and fragmentation. Cnidarians have an amazing ability to heal wounds and regenerate lost parts of the body. Having lost the oral end of the body, Hydra regenerates the mouth and grows tentacles. Jellyfish (as well as planulas) are also able to restore damaged and lost parts of the body.

Anthozoa exhibit all forms of asexual reproduction, Scyphozoa polyps form buds and divide transversely, while Hydrozoa polyps only bud. Some hydroid jellyfish are also capable of dividing.

Sexual reproduction. Most Cnidaria are dioecious; some of them can change their gender. True hermaphrodites are rare in Hydrozoa and Scyphozoa, but common in Anthozoa.

As a rule, reproduction is sexual with external or internal (in the gastrovascular system) fertilization. Parthenogenesis occurs.

Sex cells are of endodermal origin. Mature sex cells are located in certain regions of the epidermis or between the epidermis and mesoglea. Gametes are released through ruptures in the epithelium.

They usually differentiate and grow in the gastrodermis, and only in some Hydrozoa later migrate to the epidermis. The germ cells of Anthozoa, Cubozoa, and Scyphozoa typically migrate to the mesoglea, where they mainly differentiate.

Most cnidarians lay eggs, but often the offspring are born in the gastrovascular system (viviparous Anthozoa), in the pockets of the oral lobes (Scyphozoa, Semaeostomeae) or in medusoids (Hydrozoa).

Cleavage is complete, a blastula is formed from the zygote. During gastrulation, ecto and endoderm are formed. development with metamorphosis. A planktonic larva develops from the gastrula. planula. It is two-layered, covered with cilia, and floats with the aboral pole forward. After a short planktonic period, it settles to the bottom, attaches itself to the substrate with its aboral end, and turns into a young individual.

Most are characterized by complex life cycles with alternating agamous polypoid and sexual medusoid generations. Life cycles with alternating sexual and asexual reproduction are called metagenetic, and the phenomenon itself is called metagenesis.

Many representatives form colonies, which may consist of polyps, jellyfish, or both. There are the following types of colonies: permanent and temporary. Permanent colonies consist of identical individuals (monomorphic) or individuals that differ in structure and function (polymorphic colonies). Individual polyps are isolated in the colony ( zooids), combined into one coenosarcoma. stolons (outgrowths of the body wall of polyps, the gastric cavity enters them), hydrorhiza(set of stolons). The shape of the colonies are divided into the following types: stolonial (creeping), colonies with cenosarcoma – cortical (tissue structure in the form of a membrane or fleshy mass, from which zooids depart) and tree-like (with monopodial and sympodial types of branching).

Biological and practical significance of coelenterates

The biological significance of coelenterates is great in food chains in the oceans. They are especially important in the absorption of suspended organic matter and the purification of sea water. The role of coral polyps in the calcium cycle in the biosphere and the formation of sedimentary rocks is great. The leading role in the formation of reefs is played by 10-15 leading forms of madrepore corals and 1-2 species of hydrocorals (from the Hydrozoa class). Very rarely, corals of other taxonomic groups predominate, although individual representatives of them are almost always present.

Coelenterates are also objects of fishing. Salted jellyfish are used as food. Their fishery is of local importance, mainly in Japan and China. The main commercial interest is represented by corals, from which jewelry and art objects are made. In addition, collecting coral polyps is now popular. Coral branches are sold as souvenirs. Red and black corals are especially valued, the cost of which is equated to semi-precious stones. They make jewelry. Coral limestone is an excellent building material. In addition, lime is obtained from them. Some hydroid polyps are mined for the purpose of obtaining biologically active substances for medicine.

Phylogeny and ecological radiation of coelenterates

The type of coelenterates is divided into three classes:

Class Hydrozoa- hydrozoa - polypoid and medusoid forms,

Class Scyphozoa- scyphoid jellyfish. Medusoid forms and polypoid forms.

Class Anthozoa- coral polyps. Only polypoid forms.

There are various points of view on the phylogeny of the type. All of them ultimately boil down to several questions: 1) what was primary in the evolution of this group: the medusoid or polypoid stages of ontogenesis, and 2) which of the three main groups of Cnidaria: Hydrozoa, Scyphozoa or Anthozoa lie closest to the base of their phylogenetic tree.

The results of modern molecular and morphological studies of cnidarians suggest evolution in the order: Anthozoa => Scyphozoa => Hydrozoa. The sexually reproducing polyp was the ancestral adult, the planula was its larva, and the medusa stage was absent in the primary life cycle (polyp - "planula -" polyp).

Coral polyps have the most complex structure, which is associated with large sizes and, accordingly, the complexity of the organization of the gastric cavity. Coral polyps in the process of evolution gave a wide range of diversity of polypoid forms: single and colonial, without a skeleton and with a skeleton, and at the same time retained an ancient sign of development - without metagenesis.

The small Hydrozoa polyps, which have a more favorable surface area-volume ratio, are spared the need to form body wall folds, and, accordingly, their morphological evolution could go in the direction of simplification. Hydroids evolved along the path of colonization and metagenesis with the formation of the medusa-id generation. Some marine colonial hydroids have developed a skeleton (Hydrocorallia, Tecaphora). Others, in connection with the transition to living in fresh waters, have become simpler in organization and have changed their life cycle. Thus, hydras (Hydrida) lost the medusoid generation, and freshwater trachymedusae (Trachymedusae) reduced or lost the polypoid phase of development. Floating polymorphic colonies of siphonophores may also have originated from marine colonial hydroids.

Scyphoid, possibly, evolved from single scyphoid polyps, developing without metagenesis, to metagenetic polyps, forming a floating generation - jellyfish. Then many scyphoids lost the polypoid generation in the life cycle and began to reproduce only sexually.

Class Hydroids (Hydrozoa)

The hydroid class unites about 4 thousand species. These are mainly marine, less often freshwater, hydroids. Often they form colonies, the colonies of which can include both polyp zooids and jellyfish zooids. In the life cycle of hydroids, either polyps or jellyfish can be represented, but very often the polypoid and medusoid generations alternate. When there is both a polyp and a free floating jellyfish, the latter is the sexual generation, and the polyp is asexual. Sex glands develop in the ectoderm. In hydroid jellyfish, unlike scyphoid jellyfish, the radial canals of the gastric system are non-branching. In representatives of Hydrozoa, unlike other cnidarians, nematocysts occur only in the epidermis. The class is divided into two subclasses: the subclass Hydroids (Hydroidea) and the subclass Siphonophora (Siphonophora).

Subclass Hydroids (Hydroidea)

Subclass Hydroids (Hydroidea) combines colonial and single forms of polyps, as well as hydroid jellyfish. Polyp colonies can be monomorphic (of the same type) and dimorphic, rarely polymorphic, but without the specialization of medusoid individuals observed in the siphonophore class. The life cycle of hydroids is most often with alternating sexual and asexual generations (jellyfish - polyp). But there are species that exist only in the form of a polyp or medusa.

Characteristics of the structure and life of a single polyp.

As a rule, polyps are very small. Often they do not exceed 1 mm in height and fractions of a millimeter in diameter. A single polyp looks like a stalk attached to the substrate with its sole. At the upper end of the body (oral pole) is a mouth surrounded by tentacles. The number of tentacles in hydroid polyps can also be very different: usually 10-30, but sometimes their number decreases to four, two, and even one, or increases to 380.

Polyps usually sit motionless, sometimes stretching, sometimes contracting their body and tentacles, but occasionally they can also move, stepping or somersaulting.

The mesoglea layer is thin in the form of a basement membrane, contains a few amoebocytes that serve to transport nutrients. The stinging cells are mainly concentrated on the tentacles. With the help of stinging cells, polyps catch small prey, mainly small crustaceans, larvae of aquatic invertebrates, and protozoa.

Reproduction occurs asexually and sexually. Asexual reproduction occurs by budding. Sexual reproduction is usually cross. In the ectoderm of polyps, male and female germ cells are formed. Male cells form in small tubercles on the top of the hydra stalk; and a large egg is located in a bulge at the base of the stalk. Spermatozoa enter the water through a tissue rupture and penetrate the egg of another individual. The fertilized egg begins to split and becomes covered with a shell. In this case, an embryotheca is formed, which can tolerate freezing and drying out of the reservoir. Under favorable conditions, a young hydra develops in the embryotheca, which exits through the ruptures of the shell.

General characteristics of the type Intestinal.

Coelenterates - two-layer animals with radial symmetry.

Symmetry. In the body of the coelenterates there is a main axis, at one end of which there is a mouth opening. Several axes of symmetry pass through the main axis, along which the appendages and internal organs of the animal are located. This type of symmetry is called radial .

life forms. The main life forms of coelenterates are the polyp and the jellyfish.

Body polyp generally cylindrical, at one end there is a mouth opening surrounded by a different number of tentacles, and at the other - a sole. Polyps are usually sedentary or inactive. Polyps mostly form colonies.

Body jellyfish has the form of a regular umbrella or bell, on the lower, concave side of which there is a mouth opening. There are tentacles or lobes along the edge of the umbrella and sometimes around the mouth. Jellyfish lead, as a rule, a mobile lifestyle and do not form colonies.

Different types of coelenterates either exist in the form of only one of these life forms (jellyfish or polyp), or go through both stages throughout their life cycle.

Systematics. In the type of coelenterates, three classes are distinguished:

hydroid (hydra, obeli, polypodium, siphonophores);

Scyphoid (Aurelia jellyfish, cornerots, cyanides, sea wasps);

coral (black and red corals, acropores, fungi, sea anemones, alcyoniums).

In total, there are 9000 modern types of coelenterates.

Body dimensions coelenterates vary widely. Some types of polyps in adulthood do not exceed a few millimeters, while some sea anemones can reach 1 meter in diameter. In jellyfish, the diameter of the umbrella can be from 2 mm to 2 meters. In addition, the tentacles of some jellyfish can stretch up to 30 m.

Traffic. polyps sedentary. They can bend the body, contract, move their tentacles. Hydras can "walk" like ground surveyor caterpillars (larvae of moths). Anemones can crawl slowly on their soles.

Jellyfish actively move by contracting the umbrella. An important role is also played by sea currents that carry jellyfish over long distances.

Body structure. As already mentioned, coelenterates are two-layer animals. Their body wall consists of two cell layers - ectoderm (outdoor) and endoderm (internal). Between them is mesoglea - a layer of structureless gelatinous substance. the only cavity in the body of the coelenterates - intestinal, or gastral .

ectoderm represented by a single-layer flat, cubic or cylindrical epithelium . In addition to ordinary epithelial cells, the ectoderm includes epithelial-muscular cells whose base is elongated longitudinal direction contractile (muscle) fiber. In some corals, muscle fibers separate from the epithelium and lie under it or sink into the mesoglea layer, forming an independent muscular system. Between the epithelial cells are interstitial cells that give rise to various cellular elements of the ectoderm. A characteristic feature of the coelenterates is the presence in the ectoderm of the so-called tentacles. stinging cells . Each such cell contains a capsule into which a spirally folded long hollow process is inserted - a stinging thread. Outside the cell there is a sensitive hair, upon irritation of which the stinging thread sharply turns out, straightens and pierces the body of the prey or the enemy. At the same time, a poisonous secret is poured out of the capsule, causing paralysis of small animals, as well as a burning sensation in large ones.

Endoderm. The epithelium lining the gastric cavity consists of flagellated cells. Some of these cells are epithelial-muscular , however, the muscular processes are located in the transverse direction, forming together, layers of annular fibers. The cells of the ectodermal epithelium are able to form pseudopodia, with the help of which they capture food particles. There are also glandular cells.

Mesoglea. In polyps, the mesoglea is poorly developed (with the exception of corals), while in jellyfish this layer reaches a considerable thickness. The mesoglea contains a number of ectodermal cells that take part in the formation of the skeleton.

Skeletal formations. Only polyps have a skeleton. In hydroid polyps, the body is covered with a thin chitinous theca - a dense shell that performs a protective function. Most coral species have a calcareous skeleton, sometimes horny. The development of the skeleton can vary from individual spicules scattered in the mesoglea to powerful stone-like formations of various sizes and shapes (in madrepore corals). These skeletons are derived from the ectoderm.

The formation of the skeleton in corals is largely due to the presence of polyps in the body. symbiotic algae . Consider the chemical reactions that take place during the formation of a calcareous skeleton. The initial substances - calcium ions and carbon dioxide - are contained in sea water in sufficient quantities.

Carbon dioxide, when dissolved in water, forms a very unstable carbonic acid:

H 2 O + CO 2 ↔ H 2 CO 3, which immediately dissociates into ions:

H 2 CO 3 ↔ H + + HCO 3 -.

When Ca and HCO 3 ions interact, calcium bicarbonate is formed:

Ca ++ + 2 HCO 3 - ↔ Ca (HCO 3) 2. This substance is soluble in water, but it is also not stable and easily turns into insoluble calcium carbonate:

Ca (HCO 3) 2 ↔ Ca CO 3 ↓ + H 2 O + CO 2.

With an excess of CO 2, this reaction shifts to the left and a soluble bicarbonate is formed. With a decrease in the concentration of CO 2, the reaction shifts to the right and lime precipitates.

Algae living in the body of polyps constantly remove carbon dioxide from the tissues of the intestinal cavities for the process of photosynthesis, constantly creating a reduced concentration of CO 2. Such conditions favor the formation of insoluble calcium carbonate and the construction of a powerful skeleton by polyps.

Digestive system and nutrition. The digestive system is represented by the gastric cavity. Most coelenterates are predators. Prey, killed or stunned by stinging cells, they bring with tentacles to the mouth opening and swallow.

In hydroid polyps, the gastric cavity looks like a simple bag, which communicates with the environment through the oral opening. Various small animals that enter the gastric cavity are most often absorbed by endoderm cells ( intracellular digestion). Larger prey can be digested by enzymes secreted by glandular cells. Undigested residues are ejected through the mouth opening.

In coral polyps, the gastric cavity is divided longitudinally by septa, which increases the area of the endoderm. In addition, the ectodermal pharynx protrudes into the digestive cavity of corals.

As already mentioned, reef-building corals enter into a symbiotic relationship with a certain type of unicellular algae that settle in the endodermal layer. These plants, receiving carbon dioxide and metabolic products from the polyp, supply it with oxygen and a number of organic substances. Algae themselves are not digested by polyps. Under normal conditions, such a symbiosis makes it possible for polyps to do without the intake of organic substances from the environment for a long time.

At jellyfish the gastric cavity is generally formed by the stomach located in the central part of the umbrella, radial canals extending from the stomach and an annular canal running along the edge of the umbrella. Hydromedusas often have 4 radial canals, while scyphomedusas have 16 radial canals. The entire complex of canals forms the so-called gastrovascular system .

Nervous system. At polyps nervous system diffuse type . Separate nerve cells located at the base of the epithelium of the ectoderm and endoderm are connected by their processes into a nervous network. The mouth opening and the sole of the polyps are surrounded by a denser nervous network.

At jellyfish the nervous system is more concentrated than in polyps, which is associated with a mobile lifestyle.

At hydroid jellyfish accumulation of nerve cells is located on the edge of the umbrella. The cells themselves and their processes form a double nerve ring. The outer ring performs sensory functions, while the inner ring performs motor functions.

At scyphoid jellyfish the nerve ring is less pronounced, but at the base of the ropalia (marginal sensory bodies) there are clusters of nerve cells that can be called ganglia.

sense organs. Due to a sedentary lifestyle, polyps special no sense organs . There are only a few sensitive (tactile) cells, which are located mostly near the mouth opening.

At jellyfish there are also sensitive cells, but there are also special sense organs - vision, balance and smell.

Along the edge of the umbrella are located organs of vision - eyes , different in structure. In hydroid jellyfish, the eyes lie singly, while in scyphoid jellyfish, the eyes are on ropalia - sensitive marginal bodies. Moreover, one ropaliy can carry several eyes of varying degrees of complexity at once.

In connection with a mobile lifestyle, jellyfish have appeared organs of balance - statocysts. They are a bubble lined from the inside with sensitive cells. Inside the bubble is a calcareous body - statolith. Depending on the position of the jellyfish in space, the statolith irritates a certain section of the bubble wall. There are other types of structure of statocysts. In addition, statocysts are able to capture the vibrations of water, so they can also be called organs of hearing. In hydroid jellyfish, the balance organs are located along the edge of the umbrella in the amount of 4-80 in different species.

Scyphoid jellyfish also have olfactory pits - organs of chemical sense.

In scyphoids, all the senses are located on 8 ropalia - modified tentacles.

Breath. Gas exchange in coelenterates occurs by diffusion of oxygen and carbon dioxide. Large species (corals) have siphonoglyphs lined with ciliated epithelium on the pharynx. Cells equipped with cilia constantly carry out the flow of fresh water into the intestinal cavity of the animal. Many polyps, as already mentioned, have switched to symbiosis with algae, supplying the coelenterates with oxygen and releasing carbon dioxide.

Sex organs. At polyps there are no special sex organs. Sex cells are laid either in the ectoderm or in the endoderm. In the first case, the gametes exit through the rupture of the ectoderm, in the second, they first enter the gastric cavity, and then out through the mouth. Among the polyps there are hermaphrodites (hydras) and dioecious (corals).

At jellyfish , which are almost always separate sexes, there are sex glands.

At hydromedusa they form in the ectoderm of the lower side of the umbrella under the radial canals, less often on the oral proboscis. The number of gonads corresponds to the number of radial canals. Gametes exit through ruptured glands.

At scyphoid jellyfish gonads of endodermal origin. They form in the pockets of the stomach. Gametes first enter the gastric cavity, and then into the environment.

Reproduction. Coelenterates reproduce both asexually and sexually.

asexual reproduction most often proceeds through budding . This path is characteristic of polyps, and is rare in jellyfish. In single polyps, a kidney appears on the body, which gradually forms tentacles and a mouth opening and then breaks away from the mother's body. In colonial hydroids and corals, the daughter individual does not separate from the mother, which leads to the formation of colonies.

Colonial hydroid polyps they are not capable of sexual reproduction, so they bud off and sexual individuals - jellyfish. Jellyfish are formed either on the axis of the colony, or on special outgrowths - blastostyles.

Another method of asexual reproduction is strobilation when the polyp at a certain stage begins to lace up in the transverse direction several times and a small jellyfish is formed from each part. Almost the entire body of the polyp is spent on the formation of jellyfish. This method is typical for scyphoid jellyfish.

Thus, there is a change of polypoid asexual and medusoid sexual generations. At the same time, the polypoid generation predominates in hydroids, and the medusoid generation in scyphoids. Corals do not have a medusoid generation.

In a number of hydroids, jellyfish do not break away from the colony, and in some, the jellyfish is reduced to the state of a "genital sac" - a sporosarca.

Very interesting siphonophores , which are a huge colony consisting of organisms of various structures. Each colony has a pneumatophore, an air bladder that supports the siphonophore on the surface of the water.

sexual reproduction characteristic of all jellyfish, all corals and some hydroid polyps. The sexual process involves haploid cells - gametes that copulate either in the environment or in the body of the intestinal cavity. The egg undergoes complete uniform crushing. Gastrulation of the blastula most often occurs by immigration, less often by invagination. In the future, a two-layer larva is formed - a planula, covered with cilia and leading a mobile lifestyle. For such sedentary animals as corals (which do not have a medusoid generation), the planula is the only settling stage. A polyp is always formed from the planula, later budding from itself either only polyps (corals), or polyps and jellyfish (hydroid), or only jellyfish (scyphoid). Thus, the development of the vast majority of coelenterates goes with metamorphosis. Sometimes a polyp is immediately formed from an egg (for example, in hydra).

Regeneration. Coelenterates have a high ability to regenerate. Experiments on the study of this phenomenon in hydra were made as early as 1740 by Tremblay. It turned out that the animal can regenerate from 1/200 of a part.

Origin. Most likely, the ancestors of the coelenterates were free-floating organisms of the type parenchymella , which was described by I.I. Mechnikov. These hypothetical organisms were devoid of a skeleton and therefore could not be preserved as a fossil.

The oldest finds of coelenterates - coral skeletons - date back to the Cambrian period (about 600 million years ago). At the same time, not only individual prints were preserved, but also entire petrified reefs. A few imprints of jellyfish and hydroids are also known. In total, more than 20,000 species of fossil coelenterates are known.

Meaning. In nature, coelenterates, being predators and at the same time food for other animals, participate in complex food chains of marine biocenoses. Corals are of great geochemical importance, forming thick layers of calcareous rocks. Throughout their existence, corals have been involved in the formation of islands. Reefs are unique biocenoses, where a huge number of animal species live.

Practical significance modern coelenterates is small.

Corals (especially red and black) are used as decorations. They are mined in large quantities, mainly by artisanal methods. Coral collection is prohibited on large reefs.

Some jellyfish pose a serious danger to humans. In our seas, these include a small Far Eastern cross jellyfish that lives in thickets of marine plants and a large Black Sea cornerot, often found off the coast. The poison of the cross is sometimes fatal. The most dangerous jellyfish - the sea wasp - lives off the coast of Australia. Touching this animal causes severe pain and shock. Many people died when they met her.

In China and some other countries, specially prepared ropil jellyfish are used for food. There is a special trade there.

To type of coelenterates include lower multicellular organisms, the body of which consists of two layers of cells and has radial symmetry. Coelenterates are characterized by the presence of stinging cells.

Hydra

About 9,000 species are known. The most typical representative is the structure freshwater hydra.

In a freshwater hydra polyp, a body up to 1 cm long has the appearance of a sac, the walls of which consist of two layers of cells: the outer ectoderm and internal - endoderm. Inside the body there is intestinal cavity. At one end of the body is mouth surrounded by tentacles. With them, the hydra captures food and directs it into the mouth.

The other end - sole- hydra is attached to underwater objects, leads a constantly motionless lifestyle. Sometimes it can move by bending the body in one direction or another and moving the sole to another object, to which it is attached. The bulk of the ectoderm is made up of skin-muscle cells, at the base of which there are contractile muscle fibers. When they contract, the body of the hydra shrinks into a lump, one-sided contraction leads to a bend in the body. At the base of the skin-muscle cells lie star-shaped nerve cells with long processes (a very primitive nervous system).

On the body of the hydra, especially on the tentacles, are located stinging cells having a capsule stinging thread. Protrudes from the stinging cell stinging hair, upon contact with which a stinging thread is pierced into the body of the prey, its poison kills the animal, which the hydra then swallows with its tentacles.

The main function of the endoderm is the digestion of food. Some of its cells secrete digestive juice, under the influence of which partial digestion of food occurs in the intestinal cavity. Undigested food remains are expelled through the mouth. Endodermal cells also perform an excretory function. Hydra breathes the entire surface.

Hydra is characterized by both asexual and sexual reproduction.

Asexual reproduction is called budding. It happens under favorable conditions. Protrusions form on the walls of the hydra's body - kidneys, at the ends of which tentacles appear, and between them - a horn. Small hydras separate and live on their own.

During sexual reproduction, tubercles are formed on the body of hydras, in which small mobile cells are formed in some individuals - spermatozoa, on others - large - eggs.

Ripe spermatozoa swim up to the hydra with the egg cell and penetrate inside - the nuclei of the germ cells merge. going on fertilization. The ovum turns into egg covered with a thick shell. The hydra dies, and the egg falls to the bottom of the reservoir and is stored there. In the spring, a small hydra develops from it.

The hydra has a highly developed ability to restore lost and damaged parts of the body - regeneration.

Polyps and jellyfish

Among representatives of the intestinal type living in the seas, there are sessile forms - polyps and free-floating jellyfish. Among polyps there are single and colonial forms. Solitary marine polyps include anemone. With the help of a muscular leg, she can slowly move along the bottom. One of the adaptations for movement over long distances in sea anemones is symbiosis- its cohabitation with a hermit crab: a hermit crab is less noticeable at the bottom if there is an anemone on its shell, while an anemone gets the opportunity to move long distances, which increases its ability to get food.

Colonial coral polyps can be varied in shape (spherical, tree-like), have an external or internal skeleton of lime or horn-like organic substances of various colors. They are used to make jewelry.

Colonial madreporous polyps form dense settlements in shallow water - coral reefs and coral islands - atolls, which are often a dangerous obstacle to navigation.

Jellyfish- floating predators. Prey is killed with poison stinging cells. Their translucent body has the shape of a bell or an umbrella with a diameter of 0.3 to 2 m. Their digestive cavity consists of central part and departing from it channels.

The nervous system has a more complex structure than that of the hydra. In addition to the general nerve cluster around umbrella, there is an accumulation of nerve cells that form together with processes nerve ring.

The jellyfish have photosensitive eyes and balance organs. Jellyfish move in a jet way by shortening the bell, pushing water out from under it.

Some jellyfish ( cornerot, cross) are dangerous to humans. Others are of commercial importance, for example ropilema, which is eaten in China and Japan.

Type Intestinal (Coelenterata) includes about 10 thousand species of marine and freshwater animals. Consider the features of the structure and life of the intestinal cavities using the example of freshwater hydra.

Structure

Hydra has the appearance of a transparent stalk (1 cm long) attached to the substrate. Its lower part is called the sole. In the body of the hydra, there is only one hole - a mouth surrounded by tentacles.

The body is made up of two layers of cells:

ectoderm (outer layer);
endoderm (inner layer).

Rice. 1. Freshwater hydra.

In the ectoderm and endoderm there are various cells that do not form organs, but are located diffusely:

epithelial-muscular;
interstitial;
nervous;
sexual;
stinging.

Epithelial-muscular cells are the most numerous. They perform integumentary, digestive and contractile functions. Hydra, although attached to the substrate most of the time, is able to move in steps and somersaults.

Food

Food with water enters through the mouth into the body cavity, which is the intestine.

Endoderm cells capture food and secrete digestive juices for their digestion. After that, nutrients enter the cytoplasm of endoderm cells, and undigested particles are removed through the mouth.

reproduction

Coelenterates have two methods of reproduction:

asexual;
sexual.

In asexual reproduction, the offspring develop as an outgrowth of the body of the parent. Subsequently, they bud off from it. In some species, the daughter individuals do not separate, and then a colony, or community of organisms, is formed.

Rice. 2. Coral polyps.

During sexual reproduction, the male sex cells enter the water and fertilize the female cells located in the ectoderm. A larva grows from the zygote, which in some species becomes a polyp, in others a jellyfish.

For many coelenterates, jellyfish and polyp are two successive developmental stages. A jellyfish is called an umbrella-shaped or bell-shaped floating form, a polyp - attached to the substrate.

classes

In the type of coelenterates, 3 classes are distinguished:

Scyphoid;
hydroid;

Scyphomedusa are scyphoid. These are animals of a more complex structure than the hydra. They have light-sensitive eyes, organs of smell, movement and balance.

Rice. 3. Medusa Nomura.

Class Hydroid contains 4000 species. Among them, there are both hydromedusas with a polypoid generation, and polyps without a jellyfish phase, like freshwater hydra .. Total ratings received: 891.