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

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 important organs for life: oral tentacles, ocelli, statocysts, etc. Radial symmetry is inherent only in aquatic organisms. In coelenterates, it indicates their relatively simple organization. Such symmetry developed in the process of evolution in ancient, primarily sessile forms of coelenterates, from which free-swimming forms (jellyfish) subsequently evolved, preserving radial symmetry.

The biological significance of radial symmetry is that it provides the animal with contact with its 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 significance of radial symmetry for radiolarians and colonials leading a pelagic lifestyle.

Coelenterates belong to two-layered invertebrates, since their body is formed from only two germ layers - ectoderm and endoderm, between which a gelatinous layer is formed - mesoglea. Coelenterates not only have organ rudiments. So, for example, in hydra, sea anemone 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.

Coelenterate organism

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

The ectoderm cells also retained some features of protozoa. For example, in the larvae of hydra, sea anemone, 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 of the origin of multicellular animals from unicellular animals.

Although coelenterates have already developed tissues, their cells are functionally ambiguous. For example, 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 carry a secretory function, the latter transmit excitation from one cell to another.

Stinging (nettle) cells located in the ectoderm, the so-called nematocysts, are distinguished by significant autonomy. They function independently, independently of other cells. Touching a sensitive hair protruding outwards - the cnidocil - causes a lightning-fast reaction: the entire cytoplasm of the stinging cell becomes excited, which results in the rapid ejection of the thread from the stinging capsule. It should be remembered that this thread is actually a tube through which a 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 animal. 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 once they enter a foreign body. For example, sometimes coelenterates are eaten by some marine ciliated worms and opisthobranchs. 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, which are part of the epithelium and can turn into any specialized cells: stinging, skin-muscular, reproductive, nervous. The presence of reserve cells determines a high ability to regenerate, which is especially well expressed in hydra. In addition to the indicated features of primitiveness, it should be noted that coelenterates lack respiratory, excretory and circulatory systems. However, in other respects, in comparison with protozoa, coelenterates in the process of evolution have moved further along the path of improving organization.

Nervous system of coelenterates

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

The Hydra nervous network also communicates between sensory and skin-muscle cells. Consequently, 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 streamlining of the relationship of the organism with the environment, making them more diverse and stable.

It is known that for a true reflex the presence of three links is necessary: ​​sensitive (receptor), transmitting excitation (nervous apparatus) and motor (effector). All these links are present in coelenterates, but they are still at a low stage of development, so the reflexes are elementary in 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 irritation of the cnidocil).

Most reactions of coelenterates are associated with nutrition, movement and protection from harmful influences. They may develop temporary connections, or rather summation reflexes, formed as a result of increased excitability of the nervous system during prolonged exposure to the same stimulus. The activity of the nervous system is limited, consisting of the perception of external stimuli and the regulation of coordinated responses to them from individual parts of the body (for example, the feeding reaction of the tentacles in contact with prey).

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

The motor reactions of coelenterates vary depending on the strength of stimulation, the internal state of the body, the nature of the physical or chemical effect, and the biological significance of the stimulus. Since the cellular structures of the coelenterate organism 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, which lead an attached, sessile lifestyle, and jellyfish, which move freely in the water column. For both forms, radial symmetry is vitally important to the same extent and therefore it was fixed in them by the action of natural selection as a useful adaptation. In many species of coelenterates, alternating generations of polyps and jellyfish are observed. Moreover, in some the main life form is asexual polyps, and the generation of jellyfish serves only for sexual reproduction and dispersal of the species (for example, in marine hydroid polyps); in others, on the contrary, the generation of sexual jellyfish represents the main form, and polyps ensure an increase in their numbers through asexual reproduction, leading to the emergence of a new generation of medusoid individuals (for example, hydroid and scyphoid jellyfish).

However, there are also coelenterates in which the jellyfish stage is absent in their individual development, so they exist throughout 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 essentially 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, the differentiation of cellular structures and sensory organs.

Both polyps and jellyfish are represented in nature in solitary and colonial forms. Single polyps include, for example, hydra and sea anemones, and colonial polyps include red noble coral and sea feathers. Examples of solitary jellyfish include Craspedacusta, Krestovichok, Cyanea, 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 coelenterates, large variations in body size are observed. So, for example, a microhydra polyp (in a freshwater jellyfish) barely reaches a height of 1 mm, while a single marine deep-sea polyp branchiocerianthus has a height of more than 2 m. Similarly, among jellyfish there are dwarfs and giants. The diameter of the umbrella of the freshwater jellyfish craspedakusta does not exceed 2 cm, and in the jellyfish cyanea it is often equal to 2 m.

The role of coelenterates

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

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 provide shelter for a diverse fauna and flora that are part of various biocenoses.

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

Table 1: Type classes of coelenterates.

Classification of coelenterates

In modern taxonomy, the type of coelenterates is divided into three main classes, which are characterized by many distinctive features, but for a basic acquaintance with them we can limit ourselves to pointing out the differences in the structure of the intestinal cavity of polypoid individuals, for example:

Of the 9,000 existing species of coelenterates, the majority are corals (over 6,000 species), the second place is occupied by hydroids (2,800 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 the original ones for other, more complex organisms (scyphoid and coral polyps).

The most ancient and primitive multicellular animals. They evolved from primitive primordial multicellular organisms. All coelenterates are aquatic animals, most of which live in the seas and oceans. They inhabit seas from the surface to extreme depths, from tropical waters to polar regions. A small number of species live in fresh waters. About 9,000 species of coelenterates are now known. Among them there are solitary and colonial animals.

Common features of this type:

1. The body is sac-shaped, formed by two layers of cells: the outer - ectoderm, and the inner - endoderm, between which there is a structureless substance - mesoglea.
2. Radial, or radial, symmetry of the body, formed in connection with an attached or sedentary lifestyle.
3. Two life forms are characteristic: a sessile sac-like polyp and a free-swimming discoid jellyfish. Both forms can alternate in the life cycle of the same species.

4. Absence of tissue in most species (except coral polyps). The outer and inner layers of the body include several types of cells, different in structure and functions. Many vital processes in coelenterates occur at the cellular level.
5. The digestive system is primitive and consists of a blindly closed intestinal cavity and an oral opening. Digestion of food begins in the intestinal cavity under the action of enzymes, and ends in specialized cells of the endoderm, i.e. the digestion process is mixed. Undigested food remains are removed through the mouth.
6. The diffuse type nervous system that first appeared consists of nerve cells evenly distributed throughout the body, connected by processes and forming a nervous network.
7. Reproduction occurs both asexually and sexually. Incomplete asexual reproduction - budding - leads to the formation of colonies in a number of species. Many coelenterates are dioecious, but there are also hermaphrodites. Fertilization takes place in water, i.e. external. The vast majority of species develop with free-swimming larvae that have cilia.

Classification of Coelenterates

These are lower, predominantly marine, multicellular animals attached to the substrate or floating in the water column. Type Coelenterates combines three classes: Hydroid, Scyphoid and Coral polyps.

Hydroid class

• They live in fresh water bodies and the bottom of seas.
• The intestinal cavity is devoid of partitions.
• Lifestyle - attached; move slowly.
• Representatives: common hydra, brown hydra, green hydra

Class Scyphoid

• They live in deep sea water.
• The lifestyle is floating.
• Representatives: aurelia jellyfish, cyanea jellyfish, cornerotus jellyfish

Class Coral

• They live at the bottom of the sea.
• The intestinal cavity is divided into chambers.
• Lifestyle - attached; have an exoskeleton
• Representatives: sea anemone, red coral, black coral

The importance of coelenterates

The meaning of coelenterates:

• an important link in ecological food chains
• biological treatment of sea water
• participation in the calcium cycle and in the formation of sedimentary rocks
• raw materials for making jewelry and art objects
• raw materials for the production of biologically active substances
• danger to humans (some types of jellyfish)

Table “Coelenterates” (briefly)

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

Coral polyps play a significant role in the biology of the southern seas, forming reefs and atolls that serve as shelters and spawning grounds for fish; at the same time they create a danger for 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, by destroying reefs, people reduce fish resources. The most famous reefs in the southern seas are along the coast of Australia, off the Sunda Islands, and in Polynesia.

Coelenterates are the oldest type of primitive two-layer multicellular animals. Deprived of real organs. Their study is of exceptional importance for understanding the epochulation of the animal world: ancient species of this type were the progenitors of all higher multicellular animals.

Coelenterates are predominantly marine, less often freshwater animals. Many of them attach to underwater objects, while others float slowly in the water. The attached forms are usually goblet-shaped and are called polyps. With the lower end of the body they are attached to the substrate; at the opposite end there is a mouth surrounded by a corolla of tentacles. The floating forms are usually bell- or umbrella-shaped and are called jellyfish.

The body of coelenterates has ray (radial) symmetry. Through it you can draw two or more (2, 4, 6, 8 or more) planes dividing the body into symmetrical halves. In the body, which can be compared to a two-layer sac, only one cavity is developed - the gastric cavity, which plays the role of a primitive intestine (hence the name of the type). It communicates with the external environment through a single opening, which functions as the oral and anal. The wall of the sac consists of two cell layers: the outer, or ectoderm, and the inner, or endoderm. Between the cell layers lies a structureless substance. It forms either a thin supporting plate or a wide layer of gelatinous mesoglea. In many coelenterates (for example, jellyfish), canals extend from the gastric cavity, forming, together with the gastric cavity, a complex gastrovascular (gastrovascular) system.

The cells of the body of coelenterates are differentiated.

• Ectoderm cells are presented in several types:
  • integumentary (epithelial) cells - form the covering of the body, perform a protective function

    Epithelial muscle cells - in lower forms (hydroid) integumentary cells have a long process elongated parallel to the surface of the body, in the cytoplasm of which contractile fibers are developed. The combination of such processes forms a layer of muscular formations. Epithelial muscle cells combine the functions of a protective covering and a motor apparatus. Thanks 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 coelenterates, muscle tissue is separated. Jellyfish have powerful bundles of muscle fibers.

  • star-shaped nerve cells. The processes of nerve cells communicate with each other, forming a nerve plexus, or diffuse nervous system.
  • intermediate (interstitial) cells - restore damaged areas of the body. Intermediate cells can form integumentary muscle, nerve, reproductive and other cells.
  • stinging (nettle) cells - located among the integumentary cells, singly or in groups. They have a special capsule containing a spirally twisted stinging thread. The capsule cavity is filled with liquid. A thin sensitive hair, the cnidocil, is developed on the outer surface of the stinging cell. When a small animal touches, the hair is deflected, and the stinging thread is thrown out and straightened, through which paralyzing poison enters the body of the prey. After the thread is thrown out, 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 (1-3 from each cell) are directed from epithelial-muscular cells towards the intestinal cavity and outgrowths resembling false legs can form, which capture small food particles and digest them intracellularly in digestive vacuoles. Thus, coelenterates combine intracellular digestion characteristic of protozoa 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 extends from their basal end, along which the nerve impulse reaches multi-process (multipolar) nerve cells. The latter are located singly and do not form nerve nodes, but are connected to each other by their processes and form a 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). Many coelenterates are characterized by alternation of generations: polyps, reproducing by budding, give rise to both new polyps and jellyfish. The latter, reproducing sexually, produce 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 sperm 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 (strobilation of the polyp) into a number of individuals that look like stacked saucers. These individuals separate from the polyp and develop into jellyfish that reproduce sexually.

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

SUBTYPE STINGING

Characteristics of the subtype

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

Class hydroids (Hydrozoa)

An individual has the form of either a polyp or a jellyfish. The intestinal cavity of polyps is devoid of radial septa. The gonads develop in the ectoderm. About 2,800 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 float at the surface of the water. Within each species, all individuals of the medusoid structure are the same.
  • Order Leptolida - there are individuals of both polypoid and medusoid origin. Mostly marine, very rarely freshwater organisms.
  • Order Hydrocorallia (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 buried deep in the skeleton. Exclusively marine organisms.
  • Order Chondrophora - a colony consists of a floating polyp and medusoid individuals attached to it. Exclusively marine animals. Previously they were classified as a subclass of siphonophores.
  • Order Tachylida (Trachylida) - exclusively marine hydroids, jellyfish-shaped, without polyps.
  • Order Hydra (Hydrida) - solitary freshwater polyps; they do not form jellyfish.
• Subclass Siphonophora - floating colonies, which include polypoid and medusoid individuals of various structures. They live exclusively in the sea.

Freshwater polyp Hydra- a typical representative of hydroids, and at the same time of all cnidarians. Several species of these polyps are widespread in ponds, lakes and small rivers.

Hydra is a small, about 1 cm long, brownish-green animal with a cylindrical body shape. At one end there is a mouth, surrounded by a corolla of very mobile tentacles, of which in different species there are from 6 to 12. At the opposite end there is a stem with a sole, which serves for attachment to underwater objects. The pole on which the mouth is located is called oral, the opposite pole is called aboral.

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

Digestion. Under the action of enzymes in the glandular cells of the endoderm lining the gastric cavity, the body of the captured prey disintegrates into small particles, which 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. Digestion of food is completed in these cells. Consequently, in coelenterates there are two methods of digestion: along with the more ancient, intracellular one, an extracellular, more progressive method 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 debris. The gastric cavity performs the function of blood vessels (moving nutrients throughout the body). The distribution of substances in it is ensured by the movement of flagella, which many endodermal cells are equipped with. Contractions throughout the body serve the same purpose.

Breathing and elimination 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 sole. Thus, in coelenterates, the simplest coordination of functions appears.

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

Hydra movement carried out due to 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 restores lost body parts not only after being cut in half, but even if it is divided into a huge number of parts. A new animal can 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 its body there is a budding belt, on which tubercles (buds) are formed. The bud grows, a mouth and a tentacle are formed at its apex, after which the bud laces at the base, separates from the mother’s body and begins to live independently.

With the approach of cold weather in the fall, germ cells - eggs and sperm - are formed in the ectoderm of the hydra from intermediate cells. The eggs are located closer to the base of the hydra, sperm develop in tubercles (male gonads) located closer to the mouth. Each sperm has a long flagellum, with which it swims in 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 overwinters 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. An individual colony, or the so-called hydrant, is similar in structure to a hydra. Its body wall, like that of 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 which there are individual polyps, interconnected by outgrowths of the intestinal cavity into a single digestive system, which allows the distribution of food captured by one polyp among members of the colony. The outside of the coenosarcus is covered with a hard shell, the perisarcoma. Near each hydrant, this shell forms an expansion in the form of a glass - a hydroflow. The corolla of the tentacles can be drawn into the expansion when irritated. The mouth opening of each hydrant is located on a growth around which the corolla of tentacles is located.

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

Gonozoids are elongated rod-shaped formations with an extension at the top, without a mouth opening or 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 blastostyle. The skeletal membrane gives a bottle-shaped extension around the blastostyle - gonotheca. This entire formation as a whole is called gonangia. In the gongangium, on the blastostyle, jellyfish are formed by budding. They bud off from the blastostyle, emerge from the gonangium, and begin to lead a free lifestyle. As the jellyfish grows, germ cells are formed in its gonads, which are released into the external environment, where fertilization occurs.

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

Hydroid jellyfish have the shape of a bell or 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 there are tentacles with stinging cells and adhesive pads (suckers) used for catching prey (small crustaceans, larvae of invertebrates and fish). The number of tentacles is a multiple of four. Food from the mouth enters the stomach, from which four straight radial canals extend, encircling the edge of the jellyfish umbrella (intestinal ring canal). 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 method of movement of the jellyfish is “reactive”; this is facilitated by the fold of ectoderm along the edge of the umbrella, called the “sail”.

Due to their free lifestyle, the nervous system of jellyfish is better developed than that of polyps, and, in addition to the diffuse nervous network, it has clusters of nerve cells along the edge of the umbrella in the form of a ring: external - sensitive and internal - motor. The sensory organs, represented by light-sensitive eyes and statocysts (equilibrium organs), are also located here. Each statocyst consists of a vesicle with a calcareous body - a 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 are dioecious. Their gonads are located under the ectoderm, on the concave surface of the body under the radial canals or in the area of ​​the oral proboscis. In the gonads, germ cells are formed, which, when mature, are excreted through a rupture in the body wall. The biological significance of mobile jellyfish is that thanks to them, hydroids disperse.

Class Scyphozoa

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

• The order Coronomedusae (Coronata) are predominantly 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.
• Order Discomedusae - the umbrella of jellyfish is solid, there are radial canals. Polyps lack a protective tube.
• The order Cubomedusae - the umbrella of the jellyfish is solid, but lacks radial canals, the function of which is performed by the far protruding stomach pouches. Polyp without a protective tube.
• The order Stauromedusae are unique benthic organisms that combine in their structure the characteristics 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 hydroids.

Unlike hydroid jellyfish, scyphoid jellyfish are larger in size, 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 gastric cavity is divided into chambers. Channels extend radially from it, united by a ring channel located along the edge of the body. The collection of channels forms the gastrovascular system.

The method of movement is “jet”, but since scyphoids do not have a “sail”, movement is achieved by contracting the walls of the umbrella. Along the edge of the umbrella there are complex sensory organs - rhopalia. Each rhopalium contains an “olfactory fossa”, an organ of balance and stimulation of the movement of the umbrella - a statocyst, a light-sensitive ocellus. 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. The gametes are removed through the mouth and the fertilized eggs develop into a planula. Further development proceeds with alternation of generations, with the jellyfish generation predominating. 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 serious consequences can be caused by the 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 - by gonionemus vertens.

Representatives of the class of scyphoid jellyfish include:

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

  Eared jellyfish Aurelia aurita

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

  It gets its name from its mouth lobes, which are shaped like donkey ears. The umbrella of the eared jellyfish sometimes reaches 40 cm in diameter. It is easily recognized by its pinkish or slightly purple color and four dark ridges in the middle part of the umbrella - the gonads.

  In summer, in calm, calm weather, during low or high tide, you can see a large number of these beautiful jellyfish, slowly transported by the current. Their bodies sway calmly 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, plunge into the depths.

  At the edge of the aurelia umbrella there are 8 rhopalia bearing ocelli and statocysts. These sense organs allow the jellyfish to stay at a certain distance from the surface of the sea, where its delicate body will quickly be torn apart by the 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 goes into the pharynx and then into the stomach. This is where 8 straight radial canals and the same number of branching ones originate. If you use a pipette to introduce a solution of ink into the stomach of a jellyfish, you can see how the flagellar epithelium of the endoderm drives food particles through the channels of the gastric system. First, the mascara 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.

  The gonads of the aurelia, having the shape of four open or complete rings, are located in the pouches 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 scyphojellyfish, Aurelia exhibits a kind of care for its offspring. The oral lobes of this jellyfish carry along their inner side a deep longitudinal groove, starting from the mouth opening and passing to the very end of the blade. On both sides of the gutter there are numerous small holes that lead into small pocket cavities. In a swimming jellyfish, its oral lobes are lowered down, so that the eggs emerging from the mouth opening inevitably fall into the gutters and, moving along them, are retained in the pockets. This is where fertilization and egg development occurs. From the pockets, fully formed planulae come out. If you place a large female Aurelia in an aquarium, then within a few minutes you will notice a lot of light dots in the water. These are planulae that have left their pockets and float with the help of cilia.

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

  Soon the planulas have a tendency to sink to the bottom, but always in bright places. Here they continue to swim briskly. The period of freely 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 - scyphistoma, 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 planulas of the same species by scyphistomas. Scyphistomas can reproduce by budding, forming similar polyps. Scyphistoma overwinters, and next spring, with the onset of warming, serious changes occur in it. The tentacles of the scyphistoma are shortened, 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 mid-summer, a new generation of eared jellyfish develops from the ether.

• Cyanea jellyfish (Suapea) [show] .

  

  The scyphoid jellyfish cyanea is the largest jellyfish. These giants among coelenterates live only in cold waters. The diameter of the cyanea umbrella can reach 2 m, the length of the tentacles is 30 m. Externally, cyanea is very beautiful. The umbrella is usually yellowish in the center, dark red towards the edges. The oral lobes look like wide crimson-red curtains, the tentacles are colored light pink. Young jellyfish are especially brightly colored. The venom of stinging capsules is dangerous to humans.

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

  

  The 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 into a bucket. The color of the jellyfish is whitish, but along the edge of the umbrella there is a very bright blue or purple border. This jellyfish has no tentacles, but its oral lobes branch in two, and their 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 cornets is overgrown, and its role is played by numerous small holes in the folds of the oral lobes. Digestion also occurs here, in the oral lobes. In the upper part of the mouth lobes of the cornerotus there are additional folds, the so-called epaulettes, which enhance the digestive function. Cornerotes feed on the smallest planktonic organisms, sucking them along with water into the gastric cavity.

  Cornerots are pretty good swimmers. The streamlined shape of the body and the strong muscles of the umbrella allow them to move forward with quick, frequent thrusts. It is interesting to note that, unlike most jellyfish, the cornerot can change its movement in any direction, including downward. Bathers are not very happy to meet a cornet: if you touch it, you can get a rather severe painful “burn”. Cornermouths usually live at shallow depths near the shores, and are often found in large numbers in the Black Sea estuaries.

• edible rhopilema (Rhopilema esculenta) [show] .

  Edible rhopilema (Rhopilema esculenta) lives in warm coastal waters, accumulating in masses near river mouths. It has been noticed that these jellyfish grow most intensively after the onset of the summer tropical rainy season. During the rainy season, rivers carry large amounts of organic matter into the sea, promoting the development of plankton, which jellyfish feed on. Along with Aurelia, Rhopilema is eaten in China and Japan. Externally, Rhopilema resembles the Black Sea Cornerot, differing 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.

  Ropylemas are inactive. Their movements depend mainly on sea currents and winds. Sometimes, under the influence of currents and wind, clusters of jellyfish form belts 2.5-3 km long. In some places on the coast of Southern China in summer, the sea turns white from the accumulated ripples that sway near the surface.

  Jellyfish are caught with nets or special fishing gear that looks like a large bag of fine-mesh net placed on a hoop. During high or low tide, the bag is inflated by the current and jellyfish get into it, which cannot get out due to their inactivity. The oral lobes of harvested jellyfish are separated and the umbrella is washed until the internal organs and mucus are completely removed. Thus, essentially only the mesoglea of ​​the umbrella goes into 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 also eaten boiled and fried, seasoned with pepper, cinnamon and nutmeg. Of course, jellyfish is a low-nutrition product, but salted ropilems still 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 rhopilema, and some closely related species of scyphojellyfish 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 every year.

Class coral polyps (Anthozoa)

Coral polyps are exclusively marine organisms of a colonial or sometimes solitary form. About 6,000 species are known. Coral polyps are larger in size than hydroid polyps. 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, and in single polyps it is equipped with an attachment sole. The tentacles of coral polyps are located in one or several closely spaced corollas.

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

The upper part of the polyp, between the tentacles, is called the oral disc. In its middle there is a slit-like mouth opening. The mouth leads into 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 continuous movement and drive water into the intestinal cavity of the polyp.

The intestinal cavity of a coral polyp is divided into chambers by longitudinal endodermal septa (septa). In the upper part of the body of the polyp, the septa grow with one edge to the body wall and 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. Along each septum, along one of its sides, there is a muscular ridge.

The free edges of the septa are thickened and are called mesenteric filaments. Two of these filaments, located on a pair of adjacent septa opposing 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 ciliated 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 tiny 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 gonads develop in the endoderm of the septa. Coral polyps are characterized only by a polypoid state; there is no alternation of generations, since they do not form jellyfish and, accordingly, there is no medusoid stage.

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

Eight-rayed corals have a skeleton consisting of individual calcareous needles - spicules located in the mesoglea. Sometimes the spicules are connected to each other, merging or being united by an organic horn-like substance.

Among the six-rayed 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 horn-like substance, or external - calcareous.

The skeleton of representatives of the madreporidae group reaches especially great complexity. It is secreted by the ectoderm of the polyps and at first has the appearance of a plate or low cup in which the polyp itself sits. Next, the skeleton begins to grow, radial ribs appear on it, corresponding to the septa of the polyp. Soon the polyp appears as if impaled on a skeletal base, which protrudes deeply into its body from below, although it is delimited throughout by ectoderm. The skeleton of madrepore corals is very strongly developed: soft tissues cover it in the form of a thin film.

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

• Subclass Eight-rayed corals (Octocorallia) - colonial forms, usually attached 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 divided into units:
  • The order Sun corals (Helioporida) has a solid, massive skeleton.
  • Order Alcyonaria - soft corals, skeleton in the form of calcareous needles [show] .

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

  • Order Horn corals (Gorgonaria) - 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 an object of fishing. Red coral skeletons are used to make jewelry.
  • The order Sea feathers (Pennatularia) is a unique 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-rayed 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 without a skeleton. Includes:

SUBTYPE NON-CHARGING

Characteristics of the subtype

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.

Class Ctenophora- unites 90 species of marine animals with a translucent, sac-shaped gelatinous body in which the channels of the gastrovascular system branch. Along the body there are 8 rows of paddle plates, consisting of fused large cilia of ectoderm cells. There are no stinging cells. There is one tentacle on each side of the mouth, due to which a two-ray type of symmetry is created. Ctenophores always swim forward with the oral pole, using the paddle plates as an organ of movement. The oral opening leads to the ectodermal pharynx, which continues 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 the statocysts of jellyfish.

Ctenophores are hermaphrodites. The gonads are located on the processes of the stomach under the paddle plates. Gametes are expelled 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 phylogeny of the animal world, since in addition to the most important progressive feature - the development between the ecto- and endoderm of the rudiment of the third germ layer - mesoderm, due to which in adult forms numerous muscle elements develop in the gelatinous substance of the mesoglea, they have a number of other progressive features , bringing them closer to higher types of multicellular organisms.

The second progressive sign is the presence of elements of bilateral (bilateral) symmetry. It is especially clear in the crawling ctenophore Coeloplana metschnikowi, studied by A.O. Kowalewsky, and Ctenoplana kowalewskyi, discovered by A.A. Korotnev (1851-1915). These ctenophores have a flattened shape and in adulthood lack paddle 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 world, the ventral and dorsal sides of the body first became separated 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. Their common structural features are determined by the general conditions of existence, which lead to purely external, convergent similarity.

The importance 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 cause significant harm to shipping, since such a “fur coat” sharply reduces the speed of the vessel. There are many cases where hydroids, settling inside the pipes of a marine water supply system, almost completely closed their lumen and prevented the supply of water. It is quite difficult to fight hydroids, since these animals are unpretentious and develop quite well, it would seem, in unfavorable 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 dry dock. Here the ship is cleared of overgrown hydroids, polychaetes, bryozoans, sea acorns and other fouling animals. Recently, special toxic paints have begun to be used; the underwater parts of the ship coated with them are subject to fouling to a much lesser extent.

Worms, mollusks, crustaceans, and echinoderms live in thickets of hydroids that live at great depths. Many of them, for example sea goat crustaceans, find refuge among hydroids, others, such as sea “spiders” (multi-articulated), not only hide in their thickets, but also feed on hydropolyps. If you move a fine-mesh net around hydroid settlements or, even better, use a special, so-called planktonic net, then among the mass of small crustaceans and larvae of various other invertebrate animals you will come across hydroid jellyfish. Despite their small size, hydroid jellyfish are very voracious. They eat a lot of crustaceans and are therefore considered harmful animals - competitors of planktivorous fish. Jellyfish need abundant food for the development of reproductive products. While 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. In the Black and Azov Seas in the summer there are very numerous cornerota jellyfish, and if you touch 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 upon contact with it. Local residents call this jellyfish a “cross” for the cross-shaped arrangement of four dark radial canals, along which four also dark-colored gonads stretch. The umbrella of the jellyfish is transparent, faint yellowish-green in color. The size of the jellyfish is small: the umbrella of some specimens reaches 25 mm in diameter, but usually they are much smaller, only 15-18 mm. At the edge of the umbrella of the cross (scientific name - Gonionemus vertens) there are up to 80 tentacles that can strongly stretch and contract. The tentacles are densely seated with stinging cells, which are arranged in belts. In the middle of the length of the tentacle there is a small suction cup, with the help of which the jellyfish attaches to various underwater objects.

Crossfishes live in the Sea of ​​Japan and near the Kuril Islands. They usually stay in shallow water. Their favorite places are thickets of sea grass Zostera. Here they swim and hang on blades of grass, attached with their suckers. Sometimes they are found in clean water, but usually not far from zoster thickets. During rains, when sea water off the coast is significantly desalinated, jellyfish die. In rainy years there are almost no of them, but by the end of the dry summer, crosses appear in droves.

Although crossfishes 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 using 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 tentacle’s contact turns red and becomes blistered. If you feel a “burn”, you need to immediately get out of the water. Within 10-30 minutes, general weakness sets in, pain in the lower back appears, breathing becomes difficult, arms and legs go numb. It’s good if the shore is close, otherwise you might drown. The affected person should be placed comfortably and a doctor should be called immediately. Subcutaneous injections of adrenaline and ephedrine are used for treatment; in the most severe cases, artificial respiration is used. The disease lasts 4-5 days, but even after this period, people affected by the small jellyfish still 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 medically as anaphyloxia.

It is quite difficult to protect yourself from a cross. In places where a lot of people usually swim, to combat the crossworm, they mow down the zoster, fence the bathing areas with fine mesh, and catch the crossfish with special nets.

It is interesting to note that such poisonous properties are possessed by crossfish that live only in the Pacific Ocean. 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 and 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 B1, B2 and nicotinic acid.

Representatives of the animal world that belong to the type of coelenterates are quite ancient inhabitants of our planet. They can be easily characterized by the presence of radial symmetry, an intestinal cavity and an oral opening.

Most representatives of this type choose an aquatic habitat. It is necessary to note that the coelenterate type is divided into:

• sessile forms or benthos;
• floating forms or plankton.

Due to the fact that this type does not have real organs, it allows us to study and analyze the entire process of evolution that has occurred in the animal world. Coelenterates are the progenitors of all higher multicellular animals.

Today, there are approximately 900 species of coelenterates that attach themselves to various objects under water or move slowly in the water. They have a goblet shape (we are talking about polyps). As for swimming animals of this type, they have the shape of a bell or an umbrella (we are talking about jellyfish).

Internal structure of coelenterates

Coelenterates have radial or radial symmetry. Thus, up to eight planes can be drawn through their entire body, which can divide the entire body into completely identical parts. If we compare the body of the above-mentioned animals, it is more like a two-layer bag.

In the internal structure of the body of coelenterates, only the gastric part is developed, which serves as the primary intestine. This so-called intestine has a single opening, which plays the role of both the oral and anal openings.

Most representatives of this type have a fairly large number of special channels, which, moving away from the gastric cavity, form a complex system with the stomach (scientists call such a system gastrovascular or gastrovascular).

What role do coelenterates perform?

Coelenterates play a rather prominent role in shipping, where they reduce the speed of the vessel due to the fact that they are able to attach to various objects under water, creating a shaggy “fur coat”. Sometimes even they became an obstacle to the normal supply of water.

In the previous lesson we took apart the sponges. You already know that they have no real fabric. The affiliation of sponges with multi-cell animals causes controversy among scientists. This year we are starting to study the intestines, which, undoubtedly, are on-a-hundred-I-have-a-lot-of-kle-to-we-live-here-with-us. Their cells form tissues.

The body of the intestines is a bag, the wall of which has two layers. Between them there is a static, non-cellular substance. The cavity of the sac is the intestinal cavity, where food is transferred. It has one opening - the mouth. The outer layer of cells is ek-to-der-ma. It contains a variety of different cells, including nerve and strangulation cells. All nerve cells are connected to each other in a network of nerve plexuses. Nerve, stria-tel-nye and po-lov-y cells form from inter-precise cells.

The inter-precise cells are, as it were, spare cells; they provide a replacement for the old cells from the world. Kam. The second name of the intestines is sting-like, obtained by them for the presence of stinging adhesives -current. Usually, such a cell is supplied with sensitive hair, which is responsible for the ejection of the stray thread. Through it, poison enters the body of the victim or the predator.

En-to-der-ma is an internal layer consisting of glandular and epi-te-li-al-no-muscular cells. The gelatinous cells you de-la-yet have a pi-sche-va-ri-tel-ny secret. Epi-those-al-no-muscular cells provide contraction or expansion of the whole body and individual its parts. Cells en-to-der-we have a burning sensation.

The food parts grab hold of and move inside the cells. Almost all the cells of the body of the intestine are connected with water. Water provides them with gas exchange and de-le-tion.

Intestines have a radiant symmetry, which allows them to carry several planes through the body live, dividing it with each plane into two equal parts. The intestinal forms have re-ge-ne-ra-tion, but still not as much as sponges.

One of the most characteristic signs of intestines is the feeling of the fingers. These are long and thin appendages, capable of strongly stretching and contracting. They are used for catching fish and protecting them. Almost all of the intestines are carnivores: they feed on the little creatures that live here.

For the intestinal-but-lost ha-rak-ter-but che-re-do-va-nie in-ko-le-niy - si-dya-che-go (po-li-py) and free-but-pla-va-yu-sche-go (me-du-zy). Externally, the medulla and the polyp bear little resemblance to each other. The multiplication of intestinal-but-lost processes proceeds in a crazy and crazy way.

The typical life cycle looks like this. After fertilization, the egg cell forms a moving li-chin. It moves in the water column, and then, attaching itself to the bottom, turns into a polyp. The polyp multiplies in a crazy way, sometimes forming a colony. In the future, there are a lot of jellyfish, which multiply in the same way. In different intestinal cavities, certain stages of the life cycle can be shortened or completely disappeared.

About 11 thousand species of intestines are known. Most live in the seas and oceans and only a few in fresh waters. Colonized animals can lead a co-lo-ni-al or solitary lifestyle. A number of or-ga-niz-movs, which have skeletal origins, form reefs.

Other co-lo-ni-al-nye intestines can float: for example, the Port-Tu-Gal-sky co-slave-face. Some nocturnal individuals exist in the form of a po-li-pa or a me-du-zy. They are little-moving and often lead a sedentary lifestyle. Honeys usually float freely in the water column.

There are three classes in the intestinal type. We'll talk about them in the next lesson.

Do-pol-ni-tel-ny ma-te-ri-al

Sim-bion-you are intestinal-but-lost

For the intestinal-but-lost har-rak-ter-ny different types of sim-bi-o-za - comm-men-sa-ism, mu-tu-a-ism and pa -ra-zi-tism.

Many intestines lead an attached lifestyle and have a strong skeleton. A good example of this is ko-ral-lov-vye-lip-py. Such intestinal-but-cavities often serve as a sub-stratum for the or-ga-niz-mov-about-ras-ta-te-leys, including for other representatives -vi-te-ley type. Intestines can grow on the surface of the body with acids, sponges, layers of water.

The co-habitation of some hydro-id-po-lips and crayfish-from-shell-ni-kov is a year for both or-ga-niz-mothers. The constant movement of cancer contributes to better gas-flow, hydro-and-yes, and the rest food, eating it with cancer, eating it with it. At the same time, the growth of hydro-i-da mas-ki-ru-et ra-ko-vi-nu.

A striking example of mu-tu-a-liz-ma is the co-habitation of ak-ti-nii adam-sia and ra-ka-ot-shel-ni-ka. Cancer can protect itself only by entering into such a symbiosis. He gets reliable protection for the str-ka-tel-cells of the ak-ti-nii, and the ak-ti-nii gets possi- ble -ability to push through the water and eat the leftover food of the cancer.

Many intestinal-but-lost-living foods are mainly due to single-cell waters, living inside their bodies.

There is a loving sim-bi-oz of ak-ti-niy and clown fish. These fish hide among the tentacles in case of any danger. Ak-ti-niya protects them in this way and, in turn, obtains food from the leftover food of the fish. The mucus covering the body of clown fish prevents the formation of str-ka-tel-cells of the ak-ti-nii about -tiv them.

Among the intestines, there are a small number of one hundred parasi-ts. So, some hydro-id jellyfish are capable of feeding at the expense of other jellyfish, and hydro-id jellyfish Li-po-di-um develops-vi-va-et-sya inside the ik-ri-nok of the sturgeon-rich fish.

Re-cor-dy ki-shech-but-lost

The smallest creatures of this type have a length of about 1 mm, and the largest, such as between the qi-a-neya, have tentacles up to 30 m.

Colonized intestines can live in coastal shallows, at very shallow depths. Once upon a time, some ak-ti-nii live at max-small depths, up to 10 km.

The area of ​​the coral islands and the surrounding reefs is 8 million km2, which is a little more, at -mer, square-di ma-te-ri-ka Australia.

Row-ne-vi-ki

The closest relatives of the ki-shech-but-lost phenomena are comb-not-vi-ki. These are marine, predominantly plank-toned animals. Their name is connected with the nature of the “row-rows” - rows of rowing plates, shaped nykh fused-shi-mi-sya res-nich-ka-mi. Dimensions range from 2-3 mm to 3 meters. About 150 species are known.

Row-ne-vi-ki about-la-da-yut ra-di-al-noy two-lu-che-howl sim-met-ri-ey. The only thing that separates them in this sense from two-way symmetrical animals is the absence of expression. women's dorsal and abdominal sides of the body.

The body of the comb-not-vi-kov sn-ru-zhi is covered with a single-layer epi-te-li-em. There are seven rows of eyelash cells that form combing plates - combs of fused eyelashes . The tentacle-like combs from special recesses have two tentacles. In the blood epi-thelium of the tentacle there are cells equipped with adhesive cap-su-la-mi. They pos-pose to row-not-vi-came to catch small plank-toned or-ga-bottoms.

The intestinal cavity opens with the only opening - the mouth.

Between the blood epi-te-li-em and the lining of the intestinal tract there is a thick layer of stu-de-no-one-hundred-th between-glue the exact substance.

The nervous system appears as a nerve weave, located under the blood epi-tele-em.

Most of the combs are predators. The difference between the ability of some of them to glow and the rainbow refraction of light on the rowing plates -kah.

source of abstract - http://interneturok.ru/ru/school/biology/7-klass/zhivotnye-kishechnopolostnye/kishechnopolostnye?seconds=0&chapter_id=78

video source - http://www.youtube.com/watch?v=dBP40d0sG8w

video source - http://www.youtube.com/watch?v=Z_HAvMAPOM4

video source - http://www.youtube.com/watch?v=sHqse68IwkU

video source - http://www.youtube.com/watch?v=hHbKB7R3nk8

video source - http://www.youtube.com/watch?v=fObn7iA3OJU

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http://www.animals-wild.ru/964-kishechnopolostnye-zhivotnye.html

General characteristics, variety of types

The type of coelenterates has about 9 thousand species. They originated from colonial protozoa - flagellates and are distributed in all seas and freshwater bodies. The type of coelenterates is divided into three classes: hydroid, scyphoid and coral polyps.

The main aromorphoses that contributed to the appearance of coelenterates:

• the emergence of multicellularity as a result of specialization and association of interacting cells;
• the appearance of a two-layer structure;
• the occurrence of cavity digestion;
• the appearance of body parts differentiated by function;
• the appearance of radial symmetry.

Coelenterates lead an aquatic, free-living or sedentary lifestyle. These are two-layer animals, in ontogenesis forming two germ layers - ecto- and endoderm, between which there is mesoglea - the supporting plate. Their internal cavity is called the gastric cavity. Here food is digested, the remains of which are removed through the mouth, surrounded by tentacles (in hydras).

Hydroid class

A representative of this class is the freshwater hydra.

Hydra is a polyp about 1 cm in size. It lives in freshwater bodies, attaching itself to the substrate with its sole. The front end of the animal's body forms a mouth surrounded by tentacles. The body of the hydra is covered with ectoderm, consisting of several types of cells:

• epithelial-muscular;
• intermediate;
• stinging;
• sexual;
• nervous.

Hydra endoderm consists of epithelial-muscular, digestive cells and glandular cells.

Left - Diagram of the location of nerve cells in the body of the hydra. (according to Hesse). On the right - Stinging cells: A - in a resting state, B - with the stinging thread thrown out (according to Kuhn): 1 - nucleus; 2 - stinging capsule; 3 - cnidocil; 4 - stinging thread with spines; 5 - spikes

Important features of coelenterates:

1. the presence of stinging cells in the outer layer. They develop from intermediate ones and consist of a stinging capsule filled with liquid and a stinging thread placed in the capsule. Stinging cells serve as weapons of attack and defense;
2. cavity digestion with preservation of intracellular digestion.

Hydras are predators that feed on small crustaceans and fish fry.

Breathing and excretion are carried out over the entire surface of their body.

Irritability manifests itself in the form of motor reflexes. The tentacles react most clearly to irritation, since nerve and epithelial-muscle cells are densely concentrated in them.

Hydras reproduce by budding and sexually. The sexual process occurs in the fall. Some intermediate cells of the ectoderm turn into germ cells. Fertilization occurs in water. In the spring, new hydras appear. Among the coelenterates there are hermaphrodites and dioecious animals.

Many coelenterates are characterized by alternation of generations. For example, jellyfish are formed from polyps, larvae - planulae - develop from fertilized jellyfish eggs, and polyps develop from the larvae again.

Hydras are able to restore lost parts of the body due to the reproduction and differentiation of nonspecific cells. This phenomenon is called regeneration.

Class Scyphoid

This class unites large jellyfish (representatives - cornerot, aurelia, cyanea).

Jellyfish live in the seas. In their life cycle, sexual and asexual generations naturally alternate. The body is shaped like an umbrella and consists mainly of gelatinous mesoglea, covered on the outside by one layer of ectoderm, and on the inside by a layer of endoderm. Along the edges of the umbrella there are tentacles surrounding the mouth, located on the underside. The mouth leads into the gastric cavity, from which radial canals extend, which are connected to each other by a ring canal. As a result, the gastric system is formed.

The nervous system of jellyfish is more complex than that of hydras.

Rice. 34. Development of scyphomedusa: 1 - egg; 2 - planula; 3 - single polyp; 4 - budding polyp; 5 - dividing polyp; 6 - young jellyfish; 7 - adult jellyfish

In addition to the general network of nerve cells, along the edge of the umbrella there are clusters of nerve ganglia, forming a continuous nerve ring and special balance organs - statocysts. Some jellyfish develop light-sensitive eyes, sensory and pigment cells corresponding to the retina of higher animals.

Jellyfish are dioecious. Their gonads are located under the radial canals or on the oral stalk. Reproductive products exit through the mouth into the sea. From the zygote, a free-living larva develops - a planula, which in the spring turns into a small polyp.

Class Coral polyps

Includes solitary (anemone) or colonial forms (red coral). They have a calcareous or silicon skeleton formed by needle-shaped crystals, live in tropical seas, reproduce asexually and sexually (there is no jellyfish stage of development). Clusters of coral polyps form coral reefs.