The habitat is hydroid. Type Intestinal

Hydroid jellyfish is in the class of hydroid and intestinal. The habitat is water. They belong to close relatives of polyps, but are arranged a little more complicated. This type of jellyfish differs from the rest in that it can live forever, since the hydroid can be regenerated from an adult to a child's body.

The jellyfish has no mouth, but they have a mouth proboscis. It can always trigger a revival mechanism. The rebirth of jellyfish was reported by Fernando Boero, during the study of hydroids, he conducted experiments on them. He placed some of them in the aquarium, but, unfortunately, the experiment was disrupted, as a result of which the water dried up and Fernando found that the jellyfish did not die, but only threw off their tentacles, transforming into larvae.

Nutrition and Eating Process

Plankton, Artemia

The main resource in jellyfish hydroid food is plankton. For them, the basis of nutrition is Artemia, such jellyfish are considered predators. The tool for food is the tentacles, which are located on the edge of the umbrella body. The digestive system of these jellyfish is called gastrovascular. Fishing for jellyfish comes from the passive movement of its tentacles in the water into which plankton enters, after which it goes into active swimming. In such jellyfish, the nervous system consists of cell networks that form 2 rings; one of them is the outer one, which is responsible for sensitivity, and the inner one is responsible for movement.

Some of the hydroid jellyfish have photosensitive eyeswhich are located in the center of the tentacle. Hydra, by its nature, is a predator for food; it chooses for itself ciliates, plankton crustaceans, as well as fry. They are waiting for prey clinging to an aquatic plant and at the same time widely open their tentacles. When at least one tentacle reaches the prey, then all the other tentacles completely envelop the victim. And she quickly swallows her prey whole, when the hydra is full, its tentacles shrink.

Breeding

  Propagation of jellyfish hydroid is more often external than internal. Mature germ cells move out, after which blastula is formed and part of the cells is inside, forming the endoderm. After some time, several cells degenerate to form a cavity. After that, the egg turns into larvae - the planula, and then into a hydropolyp, which buds other polyps, as well as small jellyfish. Then the little ones grow over time and move on to independent development.

Hydra is one of the most convenient facilities for conducting experiments with which scientists study animal regeneration. Hydra, when cut in half after a while, it itself restores the missing parts. Also, in this form, it is easy to do operations without anesthesia and you do not need to use special tools. Hydra has the ability to restore not only from half, but even from the smallest pieces there is a revival of many polyps.

Hydra Habitats

Hydroid jellyfish are not always found, but with large clusters carried by the current. The benthic class includes polyp stages that conduct a settled life, the exception to them is class of planktonic hydroid polyps. Hydroid species are also able to group with the help of wind into huge groups, but hydroid polyps, when accumulated, seem to be one. If the jellyfish and the polyp are hungry, their movement will only be aimed at obtaining food, but according to the saturation of the body, their tentacles will begin to contract and pull themselves towards the body.

Habitat areas

Jellyfish move depending on the presence or absence of hunger. In general, all species occupy a defined habitat, it can be either a lake or an ocean. They do not intentionally seize territories that are new to themselves. Alone prefer to live in warmthwhile others, on the contrary, are in the cold. They can also be located below at a depth, and on the surface of the water. Jellyfish hydroids can be in the littoral zone, and they have no fear of surf. Most of these jellyfish have a polypich, which is protected from being hit by a skeletal cup (teka). Theca in its structure is thicker than that of other species that live deeper, where the perceptibility of the wave is much less.

At a greater depth, a special type of hydroid lives, which is unlike littoral ones. At such a depth colonies are foundhaving the form, such as:

• little tree
• christmas tree
• pen,
• and there are also types of colonies that look like ruffs.

Such species grow from 15 to 20 cm and cover the entire bottom of the sea with dense forest. Some species, for example, like a sea "spider", live in these forests and eat hydro-polyps.

Hydra can very rarely live in less saline waters, such as the Gulf of Finland for such species, the salinity of the inhabited space should not exceed 0.5%. Hydroid jellyfish often lives near the coast and in brighter places. This type of jellyfish does not have a tendency to a moving image, they are most often attached to a plant branch or stone. One of the most beloved states of the jellyfish hydroid is to be upside down and holding down some tentacles.

Dangerous types of jellyfish for humans

  But not everyone can be safe for human life. One of the most beautiful species called "Portuguese boat"  may be harmful to humans. A bell that is present in it and has a beautiful appearance, attracting attention, can be harmful.

Physalia, which is found in Australia, as well as on the coasts of the Indian, Pacific oceans and even the Mediterranean, is one of the huge hydroid species. The bubble of Fizalia can reach a length of 15 to 20 cm. But only the tentacles of Fizalia can be much worse, since their length in depth can go up to thirty meters. Physalia can leave burns on the victim’s body. Meeting with a Portuguese boat is especially harmful to people with a weakened immune system and people prone to allergies.

But most hydroid jellyfish will not harm a person, unlike scyphoids. There is the so-called white seaweed from the genus of polyps, which was previously used as decorative jewelry. Some types of hydroids act as laboratory animals - these are polyps from the Hydra class, which are used even in schools around the world.

More than 9,000 types of intestinal are known. Representatives of this type are jellyfish, hydra, sea anemones, coral polyps. Habitat  - seas and oceans, with the exception of some freshwater species (for example, kraspedakusta). Lifestyle  - free-living (free-floating or attached forms). Body sizes vary greatly: from 1 mm to 2 meters.

Intestinal - bilayer multicellular animals with radial symmetry. Development occurs from two layers of cells - ectoderm and endoderm .   The radial symmetry of the body is called because many planes can be drawn around the longitudinal axis of the animal  and each of them will divide the body into two symmetrical halves.

Most cavernous have two life forms: polyp and jellyfish.

Polyp - attached form. It has the appearance of an elongated bag with a hole - a mouth, which is surrounded by tentacles and leads into the intestinal cavity. These animals got their name because their body forms the intestinal cavity in which food is digested. The back end of the body is sole  - fixed to the substrate. Attached forms can be like solitary (hydra) and colonial (coral polyps).

Jellyfish  - floating form. It has the shape of a bell or umbrella, under the arch of which is located the mouth. Tentacles are located on the edge of the dome. Floating forms always solitary. Jellyfish spend their whole lives floating freely in the water column, and are carried by water streams over long distances.

In intestinal for the first time diffuse type nervous system appears, digestive digestion.

The type of intestinal is divided into three classes: Hydroid, Scyphoid and Coral polyps.

Hydroid class  totals about 3 thousand species which lead the attached and floating way of life. The most famous representative of this class is hydra. It was first observed by Anthony van Levenguk, but they were described only in 1744 by a Swiss naturalist Abraham Tramble. He studied in detail their structure and behavior, discovered them ability to regeneration. Tramble cut the hydra in half. After a while, each half recovered the missing parts of the body. Hydras are able to restore the whole organism from its separate part. Currently, 29 species of hydra are known. We will consider the structural and vital features of hydroids using the example of a freshwater hydra polyp.

Hydra is a small (1.5 cm long) polyp of bag-shaped elongated shape. They live in stagnant bodies of water and rivers with a slow course, attaching to aquatic plants and soil. Her body is two-layer bag with one hole - the mouth, surrounded by a corolla of 6 - 12 tentacles. Elongated tentacles 3 times the length of the body of the hydra. Following the mouth gastric  (this is the digestive cavity), which goes into a narrower stalkending the soleby which the animal attaches to the substrate. Secrete in the body ectoderm and endodermbetween which is a thin layer structureless mesogleyalso called basement membrane.

Ectoderm consists of  epithelial-muscular, stinging, nerve and intermediate cells.

The most numerous are epithelial muscle cells, which form the integument of the body and participate in movement, as they contain muscle fibers. When these cells contract, the hydra is shortened.

Stinging cells  are a characteristic feature of all intestinal. At hydra, they are scattered throughout the body, but are most numerous on the tentacles.

The stinging cells contain a capsule with poisonous liquid and stinging thread, and on the surface there is a sensitive hair, when touched, the cell is excited and the stinging thread is ejected from the capsule. Function - defense and attack. Thus, hydroids can immobilize or paralyze a rather large prey.

Nerve cells  located under the epithelial-muscular, have a stellate shape and processes. Most often they are found around the mouth and on the sole.

Their totality forms the primitive nervous system of the diffuse (or mesh) typewhich covers the entire body of the animal. If you touch the hydra anywhere in the body, the excitement immediately spreads throughout the network and causes the epithelial-muscle cells to contract - the hydra's body will contract and turn into a lump.

Intermediate cells  are small in size. They give rise to all other types of cells.

Germ cells  - eggs and sperm cells - appear at certain periods of the life cycle from intermediate cells and provide sexual reproduction.

The main function of the endoderm  - digestion of food. It consists of glandular and epithelial-muscular digestive cells.

Glandular cells  digestive enzymes are produced and secreted into the intestinal cavity, which provide partial digestion of food in the digestive cavity.

Digestive cells  have from 2 to 5 flagella and are able to form pseudopods. They line the digestive cavity. Flagella are in constant motion and pick up food particles to the cells, which they capture with pseudopods and the final digestion of food occurs inside the cells.

Thus, hydra is characteristic combined digestion: cavitary and intracellular.

Hydra - Gluttonous predator, it feeds on small aquatic animals, mainly: daphnia, cyclops, and protozoa. Stinging threads paralyze the prey, then the hydra captures it with tentacles and directs it into the mouth opening. Swallowed food enters the digestive cavity.

Hydra has few enemies, and the most dangerous of them is the hydra-eating pond.

The color of the hydra depends on the nature of the food - the animal can be almost colorless, gray, brown or green. The green color is determined by unicellular algae, which live in the body of the hydra as symbionts.

Hydra moves due to the reduction of epithelial-muscle cells. Hydra is able to move slowly with the help of “somersaults”, alternately attaching to the substrate with either the sole or the mouth opening.

Breathing is carried out whole body oxygendissolved in water.

Hydra propagates asexual and sexual. Asexual Reproduction - budding- occurs in the summer in warm weather and plentiful food. On the middle part of the hydra body, tubercles form, which develop in young hydras. At the same time, 2-3 tubercles can develop. Gradually, a mouth breaks out at the top of the tubercle and tentacles form. For some time, the young hydra remains in contact with the mother's body, after which it separates and lives independently.

Animals begin sexual reproduction in the fall with the onset of cold weather. Freshwater hydras are dioeciousbut among other species there are hermaphrodites. In the upper and lower parts of the body of the hydra, tubercles form in which numerous sperm or an egg are formed. Sperm are released into the environment and delivered to the eggs by a stream of water. After fertilization, the egg is covered with a dense shell, turns into an egg and hibernates. The maternal organism dies. In the spring, an egg develops and a new hydra is formed from it.

Most hydroid polyps live in the seas and lead a colonial lifestyle. A colony is formed from one individual as a result of budding. Subsidiary polyps do not come off, but remain forever associated with the parent, and in the future they themselves begin to budding. As a result of this, tree-like colonies arise. For example, Obelium hydroid.

To the class hydroid  include invertebrate aquatic creeping animals. In their life cycle are often present, replacing each other, two forms: polyp and jellyfish. Hydroids can be collected in colonies, but solitary individuals are not uncommon. Traces of hydroids are found even in Precambrian layers, however, due to the extreme fragility of their bodies, the search is very difficult.

Bright representative of hydroid - freshwater hydra, single polyp. Her body has a sole, a stalk and long tentacles relative to the stalk. She moves like an artistic gymnast - at each step she makes a bridge and tumbles over her “head”. Hydra is widely used in laboratory experiments, its ability to regenerate and the high activity of stem cells, which ensures the “eternal youth” of the polyp, prompted German scientists to search and study the “gene of immortality”.

Types of Hydra Cells

1. Epithelial-muscular  cells form the outer integument, that is, they are the basis ectoderm. The function of these cells is to shorten the body of the hydra or make it longer; for this they have muscular fiber.

2. Digestive and muscular  cells are located in endoderm. They are adapted to phagocytosis, capture and mix food particles that fall into the gastric cavity, for which each cell is equipped with several flagella. In general, flagella and pseudopods help food penetrate from the intestinal cavity into the cytoplasm of hydra cells. Thus, her digestion proceeds in two ways: intracavitary (for this there is a set of enzymes) and intracellular.

3. Stinging cells  located primarily on the tentacles. They are multifunctional. Firstly, hydra with their help is protected - a fish who wants to eat hydra is burned with poison and throws it. Secondly, the hydra paralyzes the prey captured by the tentacles. In the stinging cell contains a capsule with poisonous stinging thread, a sensitive hair is located outside, which, after irritation, gives a signal to a “shot”. The life of a stinging cell is fleeting: after a “shot” with a thread, it perishes.

4. Nerve cells, together with processes similar to stars, lie in ectoderm, under a layer of epithelial-muscle cells. The greatest concentration of them is at the sole and tentacles. With any exposure, the hydra reacts, which is an unconditioned reflex. The polyp also has such a property as irritability. Recall also that the “umbrella” of the jellyfish is bordered by an accumulation of nerve cells, and the ganglia are in the body.

5. Glandular cells  emit a sticky substance. They are in endoderm   and contribute to the digestion of food.

6. Intermediate cells  - round, very small and undifferentiated - lie in ectoderm. These stem cells infinitely divide, are able to turn into any other, somatic (except epithelial-muscular) or reproductive and provide hydra regeneration. There are hydras that do not have intermediate cells (therefore, stinging, nerve and genital), capable of asexual reproduction.

7. Germ cells  develop in ectoderm. The ovum of the freshwater hydra is equipped with pseudopods, with which it captures neighboring cells along with their nutrients. Among hydras is found hermaphroditismwhen the eggs and sperm are formed in one individual, but at different times.

Other features of freshwater hydra

1. Hydra have no respiratory system, they breathe the entire surface of the body.

2. The circulatory system is not formed.

3. The food for hydra is the larvae of aquatic insects, a variety of small invertebrates, crustaceans (daphnia, cyclops). Undigested food remnants, like other intestinal cavities, are removed back through the mouth opening.

4. Hydra is capable of regeneration, for which the intermediate cells are responsible. Even cut into fragments, the hydra completes the necessary organs and turns into several new individuals.

Class Hydroid (Hydrozoa)

The class of hydroids unites the lower representatives of the type of intestinal. These are mainly marine, rarely freshwater, hydroids. Often they form colonies. Many in the life cycle have a generational change: sexual - hydroid jellyfish and asexual - polyps. The primitive structure has a number of organ systems: the gastric cavity (without septum), the nervous system (without ganglia), and sensory organs. Sex glands develop in the ectoderm. In hydroid jellyfish, unlike scyphoid, the radial channels of the gastric system are non-branching.

In total, about 4 thousand species belong to hydroid species. The class is divided into two subclasses: the subclass Hydroids (Hydroidea) and the subclass Siphonophores (Siphonophora).

Fig. 79. The structure of the hydroid polyp and hydroid jellyfish (according to Kholodkovsky): A - polyp, B - jellyfish (longitudinal section); 1 - mouth, 2 - tentacle, 3 - gastric cavity, 4 - mesogley, 5 - radial channel, 6 - sail

Subclass Hydroids (Hydroidea)

The Hydroida subclass combines colonial and solitary forms of polyps, as well as hydroid jellyfish. Colonies of polyps can be monomorphic (of the same type) and dimorphic, less often 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 jellyfish.

General characteristics of the subclass. The structure of a hydroid polyp is most conveniently considered using an example freshwater hydra  (Hydra). This is a single polyp having the form of a stalk attached by the sole to the substrate (Fig. 80). At the upper end of the body (the oral pole) is a mouth surrounded by tentacles, the number of which can range from 5 to 12. Other hydroids can have about 30 tentacles. Hydras usually

Fig. 81. Hydra Hydra olidactis: A - longitudinal section (from Briana), B - transverse section (according to Polyansky), C - section of the section at high magnification (according to Kestner); 1 - ectoderm, 2 - endoderm, 3 - basement membrane, 4 - gastric cavity, 5 - epithelial muscle cell, 6 - interstitial cells, 7 - stinging cells, 8 - sensory cell, 9 - digestive cell, 10 - glandular cell, 11 - mouth, 12 - mouth cone, 13 - daughter kidney, 14 - sole, 15 - female gonad, 16 - male gonad

they sit motionless, sometimes stretching, then contracting their body and tentacles, but occasionally they can also move, walking or tumbling.

The hydra body is bilayer. Between the ectoderm and endoderm there is a basement membrane, or mesoglya. The composition of the ectoderm includes many cells with different functions (Fig. 81). The basis of ectoderm is composed of epithelial-muscle cells belonging to primitive multicellular cells with a double function: integumentary and contractile. These are epithelial cylindrical cells, at the basal end of which there is a contractile process 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. Between the epithelial-muscle cells are small undifferentiated - interstitial cells. Any other ectoderm cells, including genital, can form from them. In the ectoderm, there are stellate nerve cells. They are located under the epithelial-muscle cells. They contact their processes and form the nerve plexus. Such a nervous system is called diffuse and is the most primitive among multicellular ones. The thickening of nerve cells is observed on the sole and near the mouth of the polyp. In response to irritation caused by a polyp, for example

Fig. 82. Types of stinging cells in hydroids (according to Khadorn): a-g penetrant in the process of firing stinging threads, d - glutant, e - volvent; 1 - knidotsil, 2 - stilettos, 3 - stinging thread, 4 - core, 5 - thread base

needle, his body contracts. Thus, the reflex response of the polyp is diffuse, which corresponds to the primitive type of its nervous system.

Hydroids are characterized by the presence of a special group of stinging cells that serve to defend and attack. These cells are mainly concentrated on the tentacles and form convex clusters - a kind of stinging "batteries". Hydroids with strong stinging cells are inedible for many animals. With the help of stinging cells, polyps catch small prey, mainly small crustaceans, larvae of aquatic invertebrates, protozoa.

Stinging cells  can be of several types: penetrants, volvents, glutenants. Of these, only penetrants have nettle properties. Penetrant cell - pear-shaped (Fig. 82). It contains a large stinging capsule with a spirally twisted stinging thread. The cavity of the capsule is filled with caustic fluid, which can pass into the thread. On the outer surface of the cell there is a sensory hair - knidotsil. As shown by electron microscopy, cnidocycle consists of a flagellum surrounded by microvilli - outgrowths of the cytoplasm. Touching the penetrant's sensing hair causes instant stinging

threads. At the same time, a stylet is first pierced into the body of the prey or victim: these are three spines, folded together at rest and forming a tip. They are located at the base of the stitching thread and are screwed inside the capsule until the thread is shot. When shooting penetrant, the stylet spikes push the wound apart, and a stinging thread moistened with caustic liquid, which can have a painful and paralyzing effect, is pierced. Stinging threads, like a harpoon, are fixed with spines in the victim's body and hold it.

Other types of stinging cells have an additional function of retaining prey. Volvents shoot a short hunting thread that wraps around individual hairs and protrusions of the victim’s body. Glutenants throw out sticky threads. After firing, the stinging cells die. The restoration of the composition of stinging cells occurs due to interstitial undifferentiated cells.

The composition of the endoderm includes several types of cells: epithelial-muscular, digestive and glandular (Fig. 81). The endodermal epithelial-muscle cells differ from similar cells in the ectoderm in that they are capable of phagocytosis. The muscle processes of the cells are located in the transverse direction relative to the longitudinal axis of the body. Due to the contraction of the muscle processes, the body of the polyp narrows and expands as it relaxes. Endoderm epithelial-muscle cells have flagella and are able to form pseudopodia to capture food particles that are digested in their cytoplasm. Thus, these cells perform three functions: integumentary, contractile, and digestive. The glandular cells of the endoderm are strongly vacuolated and secrete digestive enzymes into the gastric cavity, where intracavitary digestion occurs. Hydroids have two phases of food digestion. Initially, they swallow a large food lump or an entire animal that undergoes intracavitary digestion. As a result, food breaks up into small particles. Subsequently, intracellular digestion occurs inside the epithelial-muscular digestive cells. Undigested food residues are thrown out through the mouth.

Propagation of hydra occurs asexually and sexually. Asexual reproduction occurs by budding (Fig. 80). Sexual reproduction is usually cross. Male and female germ cells form in the ectoderm of polyps. Male cells form in small tubercles on the upper part of the hydra stalk, and a large egg is located in a bulge at the base of the stalk. Sperm through a rupture of tissue out into the water and penetrate into the egg of another individual. The fertilized egg begins to crush and becomes covered. In this case, an embryo is formed, which can tolerate freezing

and drying of the pond. Under favorable conditions, a young hydra develops in the embryo, which emerges through ruptures of the membrane.

Marine Hydroid Polyps differ in some structural features from freshwater hydras and have a more complex development. In rare cases, they are solitary, and usually form colonies. Colonies are formed by budding new individuals and are similar to brown growths of moss, so they are often called "sea moss". These are brownish, brownish or greenish branching colonies of hydroids. Hydroid colonies are often dimorphic and consist of two types of polyps, for example, in the Obelia polyp (Obelia, Fig. 83). Most of the obeloans are hydrants, similar to hydra. The hydrant differs from hydra in that the mouth is located on the protruding oral stem, around which there are many tentacles without a cavity inside, and its gastric cavity continues into the common stem of the colony. Food captured by polyps alone is distributed among the members of the colony along the branched channels of the common digestive cavity, which is called the gastrovascular.

The ectoderm of a colony of hydroids secretes a skeletal organic membrane - the periderm, which has a supporting and protective value. On the stalks of the colony, this shell forms transverse folds, providing flexibility of the branches. Around the hydrants, the periderm forms a protective bell or hydrotek.

The second group of individuals in the colony - blast  in the form of a stalk without a mouth and tentacles (Fig. 83). On blastostyle, jellyfish bud. Blastostyle with young jellyfish is covered with a periderm, forming a gonoteka. In some polyps, jellyfish do not further break away from blastostyles (medusoids) and gonads form in them. In other cases, the kidneys of attached jellyfish are so modified that they are spherical formations with sex cells (gonophores) on the body of the colony. Marine hydroid polyps are diverse in form of colonies (such as "sea moss", "sea feather", "herringbone", "brush") and the type of individuals. For example, in Korine (Sogupe), jellyfish budding on hydrants. In agalophenia (Agalophenia), each hydrant is protected by three protective - striking polyps, and medusoids are hidden in "baskets" formed by mutated polyps.

Propagation by budding of marine hydroid polyps leads to colony growth. Breakaway branches of the colony can give rise to new colonies. Sexual reproduction of marine hydroids is associated with the emergence of a special sexual generation - hydroid jellyfish, less commonly, sexual products are formed in the medusoid individuals of the polyp colony. On the blastostyles of the colony, jellyfish are budded, which then come off and lead a floating lifestyle. Jellyfish grow, develop, and in

they form the gonads - gonads. Usually jellyfish are dioecious, although sexual dimorphism is not expressed in them.

The structure of the jellyfish is similar to a polyp. It is easy to imagine the morphological transition from the polyp to the jellyfish, if you turn the polyp down with your mouth, mentally shorten the longitudinal axis of the body and increase the layer of intercellular substance - mesogley. There are some floating polyps, and their similarity with jellyfish is great. However, despite a similar plan for organizing jellyfish and polyps, jellyfish have a more complex structure and have adaptations to a floating lifestyle.

Hydromedusa has a more complex gastric cavity compared to polyps, there are primitive sensory organs and adaptations to active movement. Jellyfish has the shape of an umbrella or bell (Fig. 84). The convex side of the body is called exumbrella, and the concave side is called subumbrella. At the edge of the umbrella tentacles hang with stinging cells. On the concave side of the body in the center is the mouth, which is sometimes located on a long mouth stalk. The tentacles of the jellyfish catches prey (small crustaceans, invertebrate larvae), which is picked up by the oral stem and then swallowed. Mouth food

enters the stomach located in the center of the body under the dome. Direct non-branching radial channels departing from it, flowing into the annular channel surrounding the edge of the jellyfish umbrella. Food is digested in the stomach, broken down into small particles, which are transported through the channels of the gastric cavity to different parts of the body, where they are absorbed by the endoderm cells. The complex gastric cavity of the jellyfish is called the gastrovascular system. The jellyfish move "reactively", which is facilitated by the contractile annular fold of the ectoderm along the edge of the umbrella, called the "sail". When the sail relaxes, the water enters under the jellyfish dome, and when it is reduced, the water is pushed out and the jellyfish moves forward with the jerks of the dome.

The nervous system of jellyfish is of a diffuse type, like polyps, but they have accumulations of nerve cells along the edge of the umbrella that innervate the "sail", tentacles, and sensory organs. At the base of the tentacles of hydromedusa, there are often eyes, usually in the form of simple eye holes lined with sensory retinal cells, alternating with pigment cells. In some cases, the eyes can be more complex - vesicular, with a crystalline lens.

Many hydromeduses have balance organs - statocysts. This is a deep invagination of the integument with the formation of a closed vesicle lined with sensory cells with flagella. In one of the cells of the club-shaped form, calcareous nodule is formed - statolite. The sensory hairs of the statocyst cells are directed towards the statolith. Any change in the position of the body of the jellyfish in space is perceived by the sensing cells of the statocyst. The principle of the functions of the statocyst is similar to that of the semicircular canals of the mammalian ear.

In jellyfish, gonads are formed in the ectoderm on the concave surface of the body (subumbrell) under the radial channels of the gastrovascular system, or on the oral stem. Most often, hydromeduza has 4- and 8-beam symmetry. For example, the Obelia hydroid jellyfish has 4-ray symmetry: four radial channels, four gonads and the number of tentacles is a multiple of four.

The most characteristic of marine hydroids is the alternation of sexual and asexual generations in the life cycle. For example, the Obelia hydroid alternates between the polypod generation, reproducing asexually, and the sexual generation, the medusoid generation (Fig. 85). On a polyp colony on blastostyles, jellyfish bud, which then produce germ cells. Of the fertilized eggs by crushing, the blastula stage first appears - a single-layer embryo with ciliary cells. Then, by immigration of blastula cells into the blastocele, a parenchymal larva corresponding to a similar larva in sponges is formed. But in the future, part of the cells inside the parenchymal is destroyed, and at the same time a bilayer larva is formed - the planula with the gastric cavity inside (Fig. 86). The planula floats with the help of cilia, and then settles to the bottom, her mouth breaks out and she turns into a polyp. The polyp by budding forms a colony.

In a number of types of hydroid polyps, the medusoid generation is suppressed and germ cells are formed in modified medusoids: in gonophores or sporosia on the polyp colony. At the same time, the alternation of generations is lost. In some cases, on the contrary, the polypod generation is suppressed and the species exists only in the form of a jellyfish (trachymedus - Trachylida).

The subclass Hydroid (Hydroidea) is divided into several groups.

Order Leptolida (Leptolida) - predominantly marine colonial polyps. Single forms are rare. Among suborder limnomedusa, freshwater species are known. In the colonies there are polypoid and medusoid individuals. Colonies secrete an organic skeleton. Many marine hydroids form dense thickets at the bottom. They relate to fouling organisms that settle on the bottoms of ships, underwater structures. Recently, from colonies of hydroids receive

Fig. 85. Life cycle of the Obelia hydroid (according to Naumov): A - egg, B - planula, C - colony of polyps with developing jellyfish, G - hydromedusa

biologically active substances. Including from polyps of the genus Obelia, which are widely found in the Mediterranean and Black Seas, the substance obelin is obtained, which is used in biodiagnostics in medicine. The suborder Limnomedusae is characterized by a predominance of the medusoid generation. Freshwater jellyfish (Craspedocusta) is found (Fig. 87).

Limnomedusa is a poisonous marine jellyfish - a crucian (Gonionemus), found in the seas of the Far East. In limnomedusa, the polyp phase is short-lived.

Order of Hydrocorallia (Hydrocorallia).  These are marine colonial polyps with a calcareous skeleton. Medusoids are underdeveloped. Their skeletons are known in fossil state from Cambrian and Silurian.

Chondrophora squad (Chondrophora).  Sea floating animals.

Order Sailboats (Velella).  Representative - a marine boat. This is a large floating polyp facing down with tentacles. A triangular hollow sail is formed from its chitinoid hydroteka (Fig. 88), which holds the polyp like a float at the surface of the water. Gonophores or jellyfish bud on the lower surface of the polyp.

Fig. 87. Life cycle of a freshwater hydroid jellyfish Craspedocusta (according to Naumov): 1 - an egg, 2 - a frustula larva, 3 - tentacleless polyps, 4 - polyps with tentacles, 5 - budding of a jellyfish

Hydra Squad  - single freshwater polyps that develop without alternation of generations. Representative - freshwater hydra (Hydra vulgaris).

This squad includes exclusively freshwater species of polyps. Hydras are solitary, primitive polyps in structure. There are few of them (15–20 species), but they are widespread throughout the world. Freshwater hydras are small polyps (an average of a few millimeters to 3 cm in length) that attach to freshwater plants. They can usually be found attached to the underside of underwater or floating leaves. The first sketches of hydra were made by the inventor of the microscope A. Levenguk in the XVII century. But these animals became widely known only after the publication of the works of the Swiss teacher and naturalist R. Tramble (1710-1784). He discovered a green hydra, subsequently called Chlorohydra. P. Tremblay published a book on the structure and activity of hydra, in which he proved its animal nature. He conducted observations on the hydra diet, which were actively captured by the tentacles of small crustaceans. Another merit of R. Tramble was conducting classical experiments on the regeneration of hydra. It was first proven that such low-organized multicellular organisms,

like hydra, they can regenerate even from small cut off parts of the body. For the ability to restore the cut off front ("head") part of the body, these animals were named by C. Linnaeus the hydra (Hydra) in honor of the mythical creature - a multi-headed hydra that can re-grow lost heads.

The development is direct, without the formation of larvae.

Subclass Siphonophora (Siphonophora)

Siphonophores are polymorphic colonial hydroids. Siphonophores differ from polymorphic marine hydroid polyps (Leptolida) in that their diversity of individuals in the colony is associated with the functional differentiation of not only polypoid individuals, but also medusoid ones. Siphonophores are exclusively marine floating colonial hydroids. They are diverse in shape and size. The largest of them reach 2-3 m in length, and the smallest - about 1 cm.

Structure and function. Each siphonophore colony consists of a trunk on which individual individuals are located that perform different functions (Fig. 89). The colony trunk is hollow and connects the gastric cavities of all individuals into one gastrovascular system. At the top of the colony is a pneumatic air bubble. This is a modified jellyfish that performs the function of a float, sail and hydrostatic apparatus. Special gas cells inside the pneumatophore are able to secrete gas filling its gastric cavity. The gas composition inside the pneumatophore is close to air, but it contains a higher content of nitrogen, carbon dioxide and lower oxygen content. When the pneumatophore is filled with gas, the colony is held near the surface of the water. During a storm, the walls of the pneumatophore contract, and gas is released outward through the pore. In this case, the pneumatophore decreases, the specific gravity of the colony increases, and it plunges into the depth. Under the pneumatophore is a group of swimming bells - nectophores. These are medusoids without a mouth stalk, tentacles and sense organs. Their function is motor. Reducing the sail, the umbrellas of some nectophores are either filled with water, or sometimes they throw out portions of water to the outside, which provides a "reactive" movement of the colony with the pneumatic torch forward.

On the rest of the trunk are complexes of individuals with different functions - cormidia. The composition of the cormidia may include the following individuals: cap, gastrosoid, palpon, cystozoid, gonofor. Cap  - a modified flattened polyp covering the cormidia. A gastrosoid is a nursing polyp with a mouth. He is accompanied by a polyp, modified into an arcanoch, seated with stinging cells. Food captured by gastrosoids is then distributed throughout the gastro-vascular system between all members of the colony. Palms  present

Fig. 89. Diagram of the structure of the siphonophores (according to Kholodkovsky): 1 - pneumatophore, 2 - nectophore, 3 - gonophore, 4 - gastrosoid, 5 - arcanchik, 6 - cap, 7 - palpon, 8 - trunk of the colony

Fig. 90. Siphonophores: A - Portuguese ship Physalia physalis, B - Physophora hydrostatica (according to Kestner)

a modified polyps without a mouth opening. Recently it turned out that they perform the function of intracellular digestion. Food particles enter the palpons from the cavity of the colony trunk, where they are absorbed by the endoderm cells. Another derivative of polyps is cystozoids with excretory pores instead of the mouth. These are individuals with excretory function. Finally, sexual individuals are permanent members of the cormidium - gonophors. These are modified geniuses with genital products. Colonies can be heterosexual and bisexual. In some siphonophores, jellyfish bud, and then the alternation of generations of a "polymorphic colony and jellyfish appears. Fertilization is external. Sex cells enter the water. Fertilized eggs develop planes, which are first transformed into a single individual, and then into a colony.

The spectacular representative of the siphonophore is the Portuguese ship Physalia (Physaha, Fig. 90). This is a large view of the warm seas.

with a pneumophore up to 30 cm and long tentacles up to 2–3 m. Physalium is a poisonous intestinal cavity. Physalis stinging cells paralyze even such large prey as fish. Physical burns are also dangerous for humans. Pneumatophores of a physalis of pink or blue color. They are thin, but very durable, as they consist of two layers of ectoderm, endoderm and mesoglya as a result of the formation of a double wall, and on top are still covered with a chitinoid membrane secreted by the ectoderm. A comb having a curved S-shape is located on the pneumophore. This is a kind of colony sail. Under the influence of the wind, Portuguese boats drift on the surface of the sea.

The origin of the siphonophore. Such complex polymorphic colonies as siphonophores, in which individual individuals are similar to organs in other multicellular organisms, some scientists consider as a single organism. However, most researchers consider the siphonophore as a complex and perfect multicellular colony. Proof of this is the smooth transition in the class of hydroids from single polyps to colonial, from monomorphic colonies to dimorphic and polymorphic. Siphonophore-like forms are already in the subclass of hydroids (Velella). Here there are evolutionary phenomena of polymerization and oligomerization according to V. A. Dogel (1882-1955). The evolutionary transition of hydroids to colonialism with the formation of many individuals in the colony is a manifestation of the principle of polymerization. And the functional specialization of individuals in the colony with a decrease in the number of functions, complicating the structure, increasing integration of individuals is the result of the oligomerization process.

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

A significant role in the biology of the southern seas is played by coral polyps that 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 jewelry 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, off the Sunda Islands, in Polynesia.

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

Intestinal - mainly marine, rarely freshwater animals. Many of them attach to underwater objects, others slowly swim in the water. Attached forms usually have a goblet shape and are called polyps. The lower end of the body they are attached to the substrate, at the opposite end is a mouth surrounded by a corolla of tentacles. Floating forms are usually in the form of a bell or umbrella and are called jellyfish.

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

The cells of the body of the intestinal cavity are differentiated.

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

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

  • stellate nerve cells. The processes of nerve cells communicate with each other, forming the nerve plexus, or diffuse nervous system.
  • intermediate (interstitial) cells - restore damaged areas of the body. From intermediate cells integumentary-muscular, nerve, reproductive and other cells can be formed.
  • stinging (nettle) cells - are located among the integumentary cells individually or in groups. They have a special capsule in which lies a twisted spirally sting thread. The cavity of the capsule is filled with liquid. On the outer surface of the stinging cell, a thin sensitive hair is developed - knidotsil. When touching a small animal, the hair deviates, and the stinging thread is thrown out and straightens, through it the paralyzing poison enters the body of the prey. After the thread is thrown, the stinging cell dies. Stinging cells are resumed due to undifferentiated interstitial cells lying in the ectoderm.
• Endoderm cells   lining the gastric (intestinal) cavity and perform mainly the function of digestion. These include
  • glandular cells secreting digestive enzymes into the gastric cavity
  • digestive cells with phagocytic function. Digestive cells (in lower forms) also have processes in which contractile fibrils are developed, oriented perpendicularly to similar formations of integumentary-muscular cells. Flagella are directed from the epithelial-muscle cells towards the intestinal cavity (1-3 from each cell) and outgrowths can form that resemble false legs, which capture small food particles and digest them intracellularly in digestive vacuoles. Thus, in the intestinal cavity, intracellular digestion characteristic of the simplest is combined with intestinal digestion characteristic of higher animals.

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

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

Metagenesis is found in many intestinal cavities. For example, the well-known Black Sea jellyfish - aurelia - reproduces sexually. The sperm and eggs that appear in her body are released into the water. Of the fertilized eggs, individuals of asexual generation develop - aurelia polyps. The polyp grows, its body lengthens, and then is divided by transverse constrictions (strobilization of the polyp) into a number of individuals that look like folded saucers. These individuals are separated from the polyp and turn into jellyfish that reproduce sexually.

In a systematic sense, the type is divided into two subtypes: creeping (Cnidaria) and non-creeping (Acnidaria). About 9,000 species are known, non-sticky - a total of 84 species.

SUBTYPE ARROW

Subtype Characteristic

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

Class Hydroid (Hydrozoa)

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

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

Freshwater Hydra Polyp (Hydra)  - a typical representative of hydroid, and at the same time, all bowers. 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, which in different species can be from 6 to 12. At the opposite end there is a stem with a sole serving for attachment to underwater objects. The pole on which the mouth is located is called oral, the opposite - aboral.

Hydra leads a sedentary lifestyle. Attached to underwater plants and hanging in the mouth with its mouth, it paralyzes the prey passing by with stinging threads, grabs it with tentacles and sucks it into the gastric cavity, where digestion occurs under the action of fermenting glandular cells. Hydras feed mainly on small crustaceans (daphnia, cyclops), as well as ciliates, low-bristle worms and fish fry.

Digestion. Under the action of enzymes of the glandular cells of the endoderm lining the gastric cavity, the body of the captured prey breaks up into small particles that are captured by cells having pseudopodia. Some of these cells are in their permanent place in the endoderm, while others (amoeboid) are mobile and move. In these cells, digestion is completed. Therefore, in the intestinal cavity, there are two ways of digestion: along with the more ancient, intracellular, an extracellular, more progressive way of processing food appears. Subsequently, in connection with the evolution of the organic world and the digestive system, intracellular digestion lost its importance in the act of nutrition and assimilation of food, but its ability to survive was preserved in separate cells in animals at all stages of development, up to the highest, and 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 serves as a vessel (the movement of nutrients through the body). The distribution of substances in it is ensured by the movement of the flagella, which many endoderm cells are equipped with. Contraction of the whole body serves the same purpose.

Breath and discharge  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. Especially a lot of nerve cells around the mouth, on the tentacles and the sole. Thus, in the intestinal cavity, the simplest coordination of functions appears.

Sensory organs. Not developed. Touching the entire surface, tentacles (sensitive hairs) that throw stinging threads that kill prey are especially sensitive.

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

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

Breeding. Hydra reproduces asexually and sexually.

During the summer, hydra propagates asexually - by budding. In the middle part of her body is a bud belt, on which tubercles (kidneys) form. The kidney grows, a mouth and tentacle forms on its top, after which the kidney laces at the base, is separated from the mother’s body and begins to live independently.

With the approach of colds in the fall in the ectoderm of the hydra, germ cells are formed from the intermediate cells - eggs and spermatozoa. The eggs are located closer to the base of the hydra, sperm develop in the tubercles (male gonads), located closer to the mouth opening. Each sperm has a long flagellum, with which it floats 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 overwinter there. In late autumn, adult hydras die. In the spring of overwintered eggs, a new generation develops.

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. The wall of his body, like hydra, consists of two layers: the endoderm and ectoderm separated by a jelly-like structureless mass called mesoglay. The body of the colony is a branching coenosarc, inside which there are separate polyps connected by outgrowths of the intestinal cavity into a single digestive system, which allows you to distribute food captured by one polyp between the members of the colony. Outside, the cenosarc is covered with a hard shell - perisarcum. Near each hydrant, this shell forms an expansion in the form of a glass - a hydrotek. In the extension, the corolla of the tentacles can be drawn in with irritation. The mouth opening of each hydrant is located on the outgrowth around which the corolla of the tentacles is located.

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

The gonozoid is an elongated rod-shaped formation with an extension at the top, without a mouth opening and tentacles. Such an individual cannot eat on its own, it receives food from hydrants through the colony's gastric system. This formation is called blastostyle. The skeletal membrane gives a bottle-shaped expansion around blastostyles - gonoteka. All this education as a whole is called gonangia. In gonangia, on blastostyle, jellyfish are formed by budding. They budding from blastostyles, get out of gonangia and begin to lead a free lifestyle. As the jellyfish grows, sex cells are formed in its gonads, which are secreted into the external environment, where fertilization takes place.

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

Hydroid jellyfish have the form of a bell or umbrella, from the middle of the ventral surface of which hangs a trunk (oral stem) with a mouth opening at the end. Along the edge of the umbrella are tentacles that have stinging cells and sticky pads (suckers) that serve to catch prey (small crustaceans, invertebrate larvae and fish). The number of tentacles is a multiple of four. Food from the mouth enters the stomach, from which four direct radial channels depart, encircling the edge of the jellyfish umbrella (the annular intestinal canal). Mesogley 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 the ectoderm along the edge of the umbrella, called the "sail".

In connection with a free lifestyle, the nervous system in jellyfish is better developed than in polyps, and, in addition to the diffuse nervous network, 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 photosensitive eyes and statocysts (balance organs) are also located here. Each statocyst consists of a vesicle with a calcareous body - a statolite located on elastic fibers coming from sensitive cells of the vesicle. If the position of the body of the jellyfish in space changes, a shift of the statolite occurs, 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 channels or in the area of \u200b\u200bthe oral proboscis. Sex cells are formed in the gonads, which, when ripe, are excreted through the rupture of the body wall. The biological significance of mobile jellyfish lies in the fact that due to them there is a resettlement of hydroids.

Scyphoid Class (Scyphozoa)

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

• Coronataus squad (Coronata) - mainly deep-sea jellyfish, whose umbrella is divided by a constriction into the central disk and the crown. The polyp forms a protective chitinoid tube around itself.
• Detachment of Discomedusa (Discomedusae) - a continuous jellyfish umbrella, there are radial channels. Polyps lack a protective tube.
• The Cubomedusae detachment is a continuous jellyfish umbrella, but devoid of radial channels whose function is performed by far-reaching pockets of the stomach. Polyp without protective tube.
• Squad Stavromedusae (Stauromedusae) - a kind of bottom organisms that combine in their structure the signs of jellyfish and polyp.

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

Unlike hydroids, scyphoid jellyfish have larger sizes, highly developed mesogley, a more developed nervous system with accumulations of nerve cells in the form of nodules - ganglia, which are located mainly around the circumference of the bell. The stomach cavity is divided into chambers. Channels radially radiate from it, united by an annular channel located along the edge of the body. The set of channels forms the gastrovascular system.

The method of movement is "reactive", but since the scyphoid ones do not have a "sail", movement is achieved by reducing the walls of the umbrella. On the edge of the umbrella are complex sensory organs - ropalia. Each ropalia contains an "olfactory fossa", an organ of equilibrium and stimulation of umbrella movement - a statocyst, a photosensitive eye. Scyphoid jellyfish are predators, but deep-sea species feed on dead organisms.

Sex cells form in the sex glands - gonads located in the endoderm. Gametes are removed through the mouth, a planula develops from fertilized eggs. Further development proceeds with alternating generations, with the generation of jellyfish predominating. The generation of polyps is short-lived.

The tentacles of the jellyfish are equipped with a large number of stinging cells. Burns of many jellyfish are sensitive to large animals and humans. Severe burns with severe consequences can be caused by a polar jellyfish of the genus Cyanea, reaching a diameter of 4 m, with tentacles up to 30 m in length. Bathers in the Black Sea sometimes suffer burns from the jellyfish Pilema pulmo, and in the Sea of \u200b\u200bJapan - gononemes (Gonionemus vertens).

Representatives of the class of scyphoid jellyfish include:

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

  Long-eared jellyfish Aurelia (Aurelia aurita)

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

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

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

  At the edge of the aurelia umbrella there are 8 ropalia carrying eyes and statocysts. These senses allow the jellyfish to stay at a certain distance from the surface of the sea, where its delicate body will quickly break the waves. Eared jellyfish captures food with the help of long and very thin tentacles, which "sweep" small plankton animals into the jellyfish's mouth. Swallowed food first enters the throat, and then into the stomach. From here originate 8 direct radial channels and the same number of branching ones. If a carcass solution is introduced into the jellyfish’s stomach with a pipette, it can be seen how the flagellar epithelium of the endoderm drives food particles through the channels of the gastric system. At first, the mascara penetrates the non-branching canals, then it enters the annular channel and returns through the branching channels back to the stomach. From here, undigested food particles are ejected through the mouth opening.

  The genital glands of aurelia, in the form of four open or full rings, are located in the pockets of the stomach. When the eggs in them mature, the wall of the genital gland is torn and the eggs are thrown out through the mouth. Unlike most scyphomeduses, aurelia shows a peculiar concern for the offspring. The oral lobes of this jellyfish carry on their inner side a deep longitudinal trough, starting from the oral opening and extending to the very end of the lobe. On both sides of the gutter there are numerous small holes that lead into small pockets. In a floating jellyfish, its oral lobes are lowered down, so that the eggs emerging from the oral opening inevitably fall into the gutters and, moving along them, are held in pockets. Here the fertilization and development of eggs takes place. Ie pockets come out fully formed planula. If you place a large female Aurelia in the aquarium, then after a few minutes in the water you can notice a lot of bright spots. These are planules that have left pockets and are floating with cilia.

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

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

  After two or three days, the settled planula turns into a small polyp - a scyphoid, which has 4 tentacles. Soon between the first tentacles appears 4 new, and then another 8 tentacles. Scyphistomas actively feed, capturing ciliates and crustaceans. Cannibalism is also observed - eating by planes of the same type by the Scythists. Scyphistomas can multiply by budding, forming polyps like themselves. Scythystoma hibernates, and next spring, with the onset of warming, serious changes occur in it. The tentacles of the scyphistoma are shortened, and annular constrictions appear on the body. Soon from the upper end of the scyphistoma, the first ether separates - a small completely transparent star-shaped jellyfish larva. By the middle of summer, a new generation of eared jellyfish is developing from the ether.

• jellyfish cyanoea (Suapea) [show] .

  

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

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

  

  Scyphoid jellyfish kornerot lives on the Black and Azov Seas. The umbrella of this jellyfish is hemispherical or conical in shape with a rounded top. Large specimens of a rhizostomy are hardly placed in a bucket. The color of the jellyfish is whitish, but a very bright blue or purple border runs along the edge of the umbrella. This jellyfish does not have tentacles, but its mouth lobes branch in two, and their sides form numerous folds and fuse together. The ends of the oral lobes of the folds do not bear and end with eight root-shaped outgrowths, from which the jellyfish got its name. The mouth in adult cornerotas is overgrown, and its role is played by numerous small holes in the folds of the oral lobes. Here, in the oral lobes, digestion also occurs. In the upper part of the oral lobes of the cornerot there are additional folds, the so-called epaulettes, enhancing the digestive function. Cornerota feed on the smallest planktonic organisms, sucking them together with water into the gastric cavity.

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

• edible ropilema (Rhopilema esculenta) [show] .

  Edible ropilema (Rhopilema esculenta) lives in warm coastal waters, accumulating in masses near river mouths. It is noticed that these jellyfish grow most intensively after the onset of the summer season of tropical rain rains. During the rainy season, rivers carry a large amount of organic matter into the sea, contributing to the development of plankton, which feed on jellyfish. Along with aurelia, ropilema is eaten in China and Japan. Outwardly ropilema resembles the Black Sea cornerotus, differs from it in yellowish or reddish coloration of the oral lobes and in the presence of a large number of finger-shaped outgrowths. Umbrella mesogley is used as food.

  Ropylem sedentary. Their movements depend mainly on sea currents and winds. Sometimes, under the influence of the current and the wind, jellyfish clusters form belts 2.5-3 km long. In some places of the coast of South China in the summer, the sea turns white from the ropils accumulated there, which sway at the very surface.

  Jellyfish are caught with netting or special fishing gears that look like a large bag of fine mesh worn on a hoop. During high tide or low tide, the bag is inflated by the current and jellyfish fall into it, which cannot get out due to its inactivity. In extracted jellyfish, the oral lobes are separated and the umbrella is washed until the internal organs and mucus are completely removed. Thus, in the further processing, in fact, only the mesoglea of \u200b\u200bthe umbrella enters. In the figurative expression of the Chinese, jellyfish meat is "crystal". Jellyfish are salted with sodium chloride 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 non-nutritious product, but salted ropilemi still contains a certain amount of proteins, fats and carbohydrates, as well as vitamins B 12, B 2 and nicotinic acid.

  Long-eared jellyfish, edible ropilema, and some closely related species of scyphoid jellyfish, in all likelihood, are the only intestinal cavities that people eat. In Japan and China, there is even a special fishery for these jellyfish, and thousands of tons of "crystal meat" are mined there annually.

Grade Coral Polyps (Anthozoa)

Coral polyps are exclusively marine organisms of a colonial or sometimes solitary form. About 6,000 species are known. In size, coral polyps are larger than hydroid. The body has a cylindrical shape and is not subdivided 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 more closely related corollas.

There are two large groups of coral polyps: eight-beam (Octocorallia) and six-beam (Nehasorallia). The first always has 8 tentacles, and they are equipped with small outgrowths - pinnules along the edges, in the second the number of tentacles is usually quite large and, as a rule, is a multiple of six. The tentacles of the six-pointed coral are smooth, without kicked.

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

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

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

Reproduction is asexual - by budding, and sexual - with metamorphosis, through the stage of a freely floating larva - planula. Sex glands develop in the endoderm of septum. 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 jellyfish stage.

Cells of the ectoderm of coral polyps produce horny matter or secrete carbonate, from which the outer or inner 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 mesogley. Sometimes spicules are interconnected, merging or uniting with an organic horn-like substance.

Among six-pointed corals, there are skeletal-free 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.

Especially great difficulty is the skeleton in representatives of the madreporic group. It is secreted by the ectoderm of the polyps and at first it looks like a plate or a low cup in which the polyp itself sits. Then the skeleton begins to grow, on it there are radial ribs corresponding to the septa of the polyp. Soon, the polyp appears to be planted on a skeletal base, which deeply protrudes from below into his body, although it is delimited everywhere 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 the intestinal cavity plays the role of a support system, and together with the stinging apparatus it provides a powerful defense against enemies, which contributed to their existence over long geological periods.

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

    Most alcyonaria are soft corals that do not have a pronounced skeleton. Only some tubipores possess a developed calcareous skeleton. In the mesoglayer of these corals, tubes are formed that are soldered to each other by transverse plates. The skeleton in shape is vaguely reminiscent of an organ, so tubipores have another name - organ organs. Organ bodies are involved in the process of reef formation.

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

SUBTYPE NON-CLOSING

Subtype Characteristic

Rather than stinging intestinal cavitary cells have special sticky cells on the tentacles, which serve to capture prey. The only class belongs to this subtype - ctenophores.

Class Ctenophora (Ctenophora)  - combines 90 species of marine animals with a translucent gelatinous body of a saccular shape, in which the channels of the gastrovascular system branch. 8 rows of rowing plates, consisting of large cilia of ectoderm cells grown together, pass along the body. There are no stinging cells. One tentacle is located on the sides of the mouth, due to which a two-beam type of symmetry is created. The combs always float forward with the oral pole, using rowing plates as an organ of movement. The oral opening leads to the ectoderm pharynx, which passes into the esophagus. Behind it is the endodermal stomach with radial channels extending from it. At the aboral pole is a special organ of equilibrium called aboral. It is built on the same principle as the jellyfish statocysts.

Combs are hermaphrodites. Sexual glands are located on the processes of the stomach under the rowing plates. Gametes are brought out through the mouth. In the larvae of these animals, the formation of the third germinal leaf, the mesoderm, can be traced. This is an important progressive feature of ctenophores.

The combs are of great interest from the point of view of the phylogenesis of the animal world, since in addition to the most important progressive sign - the development between the ecto- and endoderm the embryo of the third embryonic leaf - the mesoderm due to which numerous muscular elements develop in the gelatinous substance of the mesogley, they have a number of other progressive signs bringing them closer to higher types of multicellular.

The second progressive sign is the presence of elements of bilateral (bilateral) symmetry. It is especially clear in the creeping ctenophore Coeloplana metschnikowi, studied by A.O. Kovalevsky, and Ctenoplana kowalewskyi, discovered by A.A. Korotnev (1851-1915). These ctenophores have a flattened shape and in the adult state are devoid of rowing plates, and therefore can only crawl along the bottom of the reservoir. The body side 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 were isolated for the first time in connection with the transition from swimming to crawling. There is no doubt that modern crawling ctenophores retained in their structure the progressive signs of that group of ancient intestinal cavities, which became the ancestors of higher types of animals.

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

The value of the intestinal

Hydroid colonies, 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" dramatically reduces the speed of the vessel. There are many cases when hydroids, settling inside the pipes of the sea water supply, almost completely closed their lumen and prevented the water supply. It is quite difficult to deal with hydroids, as these animals are unpretentious and develop quite well, it would seem, in adverse conditions. In addition, they are characterized by rapid growth - in a month, bushes grow 5-7 cm tall. To clear the bottom of the ship from them, you have to put it in a dry dock. Here, the ship is cleaned of overgrown hydroids, polychaetes, bryozoans, sea acorns and other fouling animals. Recently, special poisonous paints began 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, such as crustaceans, find refuge among hydroids, while others, such as sea "spiders" (multi-legged), not only hide in their thickets, but also feed on hydro-polyps. If you move a small-mesh net around the hydroid settlements or, even better, use a special so-called plankton net for this, then among the mass of small crustaceans and larvae of various other invertebrate animals, hydroid jellyfish can be found. Despite their small size, hydroid jellyfish are very voracious. They eat a lot of crustaceans and therefore are considered harmful animals - competitors of planktonivorous fish. Abundant food is necessary for jellyfish for the development of reproductive products. When swimming, they spread 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, cornerota jellyfish are very numerous in the summer. Touching them, you can get a strong and painful “burn”. In the fauna of our Far Eastern seas, there is also one jellyfish that causes serious illness when it comes in contact with it. Locals call this jellyfish a "cross" for the cross-shaped arrangement of four dark radial channels along which four dark-colored reproductive glands stretch. Umbrella jellyfish transparent, faint yellowish-green. The jellyfish are small in size: the umbrella of individual specimens reaches 25 mm in diameter, but usually they are much smaller, only 15-18 mm. On the edge of the umbrella of the krestovichka (scientific name - Gonionemus vertens) there are up to 80 tentacles that can greatly stretch and contract. The tentacles are densely seated with stinging cells, which are located by the belts. In the middle of the length of the tentacle there is a small suction cup with which the jellyfish is attached to various underwater objects.

Krestovichki live in the Sea of \u200b\u200bJapan and near the Kuril Islands. They usually stay in shallow water. Their favorite places are thickets of sea grass zoster. Here they swim and hang on blades of grass, attaching themselves with their suction cups. Sometimes they come across in clear water, but usually not far from the bushes of zoster. During rains, when sea water off the coast is significantly desalinated, jellyfish die. In rainy years, they are almost absent, but by the end of the dry summer, the little daggers appear in droves.

Although krestovichki can freely swim, they usually prefer to lie in wait for prey by attaching to any object. Therefore, when one of the tentacles of the crosspiece accidentally touches the body of a bathing person, the jellyfish rushes in this direction and tries to attach itself with the help of suction cups and stinging capsules. At this moment, the bather feels a severe “burn”, after a few minutes the skin at the place of touch of the tentacle turns red, covered with blisters. Feeling a "burn", you need to immediately get out of the water. After 10-30 minutes, general weakness occurs, lower back pain appears, breathing is difficult, arms and legs go numb. Well, if the coast is close, otherwise you can drown. The affected person should be conveniently put in and immediately call a doctor. For treatment, subcutaneous injections of adrenaline and ephedrine are used; in the most severe cases, artificial respiration is used. The disease lasts 4-5 days, but even after this period, people affected by a small jellyfish can not fully recover for a long time.

Repeated burns are especially dangerous. It has been established that the poison of the krestovichka not only does not develop immunity, but, on the contrary, makes the body highly sensitive even to small doses of the same poison. This phenomenon is known in medicine as anaphyloxia.

Protecting yourself from the krestovichka is quite difficult. In places where a lot of people usually bathe, zoster is mowed up to fight the krestovchik, the baths are fenced with a fine mesh net, and krestovichiks are caught with special nets.

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

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