Examples of hydroid coelenterates. In freshwater hydra, jellyfish and coral polyp. Origin of freshwater coelenterates

The hydroid jellyfish belongs to the class of hydroids and coelenterates. The habitat is water. They are close relatives of polyps, but are a little more complicated. This type of jellyfish differs from others in that it can live forever, since the hydroid can regenerate from an adult to a child’s organism.

Jellyfish do not have a mouth, but they do have an oral proboscis. She can always trigger the revival mechanism. Fernando Boero reported about the degeneration of the jellyfish; while studying 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 discovered that the jellyfish did not die, but only cast off their tentacles, transforming into larvae.

Nutritional resources and eating process

Plankton, Artemia

The main resource in the food of hydroid jellyfish is plankton. For them, the basis of nutrition is Artemia, such jellyfish are considered predators. The tools for obtaining food are the tentacles, which are located on the edge of the umbrella body. The digestive system of these jellyfish is called gastrovascular. Jellyfish catch prey by passively moving their tentacles in the water, into which plankton falls, after which it begins active swimming. In such jellyfish, the nervous system consists of cellular 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.

One of the hydroid jellyfish have light-sensitive eyes, which are located in the center of the tentacle. Hydra, by its nature, is a predator for food; it chooses ciliates, planktonic crustaceans, and also fry. They wait for prey by clinging to aquatic plant and at the same time open their tentacles wide. When at least one tentacle reaches the prey, then all the other tentacles completely envelop the victim. And it quickly swallows its prey whole; when the hydra is satiated, its tentacles contract.

Reproduction

Reproduction of hydroid jellyfish is more often external than internal. Mature germ cells move outward, after which blastula is formed and some of the cells end up inside, forming endoderm. After some time, several cells degenerate to form a cavity. After this, the egg turns into larvae - a planula, and then into a hydropolyp, which buds into other polyps, as well as small jellyfish. After which the little ones grow up over time and begin to develop independently.

Hydra is one of the most convenient objects for conducting experiments, with the help of which scientists studying regeneration in animals. When the hydra is cut in half, after some time it itself restores the missing parts. Also, this type of surgery is easy to perform without anesthesia and there is no need to use special instruments. Hydra has the property of restoring not only from half, but even from the smallest pieces many polyps are revived.

Hydra habitats

Hydroid jellyfish are not always found, but in large concentrations carried by the current. The benthic class includes stages of polyps that lead a sedentary life, the exception to which is class of planktonic hydroid polyps. Hydroid species are also capable of grouping with the help of the wind into huge groups, but hydroid polyps, when clustered, seem to be one whole. If the jellyfish and the polyp are hungry, their movement will be aimed only at obtaining food, but when the body is saturated, their tentacles will begin to contract and be pulled towards the body.

Habitat zones

Jellyfish move depending on the presence or absence of hunger. In general, all species occupy a specific habitat, this can be either a lake or an ocean. They do not deliberately seize new territories for themselves. Alone prefer to live in warmth, while others, on the contrary, are in the cold. They can also be located both below at depth and on the surface of the water. Hydroid jellyfish can be found in the littoral zone, and they do not have a fear of the surf. Most of these jellyfish have a polyp, which is protected from impact by a skeletal cup (theca). The structure of the theca is thicker than that of other species that live deeper, where the perceptibility of the wave is much less.

At greater depths, a special type of hydroids lives, which is unlike littoral hydroids. At this depth there are colonies, having the form such as:

• tree,
• Christmas tree,
• feather,
• and there are also types of colonies that look like ruff.

Such species grow from 15 to 20 cm and cover the entire seabed with dense forest. Some species, for example, like the sea spider, live in these forests and eat hydropolyps.

Hydra can very rarely live in less saline waters, such as in the Gulf of Finland for such species, the salinity of the inhabited space should not exceed 0.5%. The hydroid jellyfish often lives close to the shore and in brighter places. This type of jellyfish does not have a tendency to be mobile; they most often attached to a plant branch or rock. One of the most favorite states of the hydroid jellyfish is to be upside down and have some tentacles hanging down.

Dangerous types of jellyfish for humans

But not all can be safe for human life. One of the most beautiful species called "Portuguese man-of-war" may cause harm to humans. The bell, which is present in it and has a beautiful appearance, attracting attention, can cause harm.

Physalia, which is found in Australia, as well as on the coasts of the Indian and Pacific Oceans and even the Mediterranean, is one of the huge hydroid species. Physalia's bubble can reach a length of 15 to 20 cm. But Physalia's tentacles can be much scarier, since their length and depth can extend to thirty meters. Physalia can leave burns on the victim's body. An encounter with a Portuguese man-of-war is especially harmful for people with weakened immune systems and people prone to allergies.

But most hydroid jellyfish will not harm humans, unlike scyphoids. There is a so-called white algae from the genus polyps, which was previously used as decorative jewelry. Some of the hydroid species act as laboratory animals - these are polyps from the Hydra class, which are used even in school different countries peace.

- a class of cnidarians whose life cycle includes a jellyfish with a characteristic feature - velum, and a polyp, which, unlike other cnidarians, never has internal partitions (septa) and a pronounced pharynx.

general characteristics

The life cycle may not have a polyp or jellyfish stage, but necessarily includes a planula larva. The lifestyle can be solitary (hydra) or colonial (obelia); in most species, colonies are formed in the polyp stages; There are colonies in which both polyps and jellyfish are integrated simultaneously (series Siphonophora).

Evolution

Fossil remains of hydroids have been known since the Precambrian; however, given the small number of solid skeletal structures, these remains are rather few in number and fragmentary. Recent studies of the structure of the medusar nodule (a special structure on a polyp that forms young jellyfish through specific budding) have given results indicating the presence of three germ layers in hydroids (i.e., hydroids are three-layered). The hydromedusa's pidparasol cavity and the layer of striatal muscle that lines it are formed from a morphological structure very similar to the schizocoel: in this case, a third layer (analogous to mesoderm) is formed between the ectoderm and endoderm, which turns into a cavity. Thus, the parasolic cavity is actually a coelom, which subsequently, after the formation of the velar foramen, becomes open to the outside.

There is also now available molecular biological data that shows that the genes that encode the formation of mesoderm structures in bilaterally symmetrical animals (Bilateria) are also present in hydroids. Thus, the polyp stage of hydroids has two germ layers (that is, two-layered), and the medusa stage has three germ layers (that is, three-layered). If these data are confirmed by evidence from other sources, this will mean that the transition from two-layered to three-layered organisms occurs every time a jellyfish buds from a polyp, and thus one of the biggest questions of animal evolution will be solved - how did it happen? transition from Diploblasta (animals with two germ layers) to Tryploblasta (animals with three germ leaves).

Taxonomy

Hydroids are known taxonomists from the very beginning of the existence of zoology as such; a large number of species were described by Carl Linnaeus back in the 18th century.

The taxonomy of hydroids is quite complex, which is caused by the lack of paleontological information on the basis of which it is possible to identify related relationships within the taxon. There are now several general classification options; in this article, the classification is based on the principles generally set out on The Hydrozoa Directory website, and is largely based on the results of molecular biological research recent years. According to the results of the mentioned studies, the hydroid class is clearly divided into two groups of series that have received the status of subclasses: Trachylinae and Leptolinae (the latter is also called Hydroidolina and Hydoidomedusae in other sources).

Hydroids is a cosmopolitan taxon, that is, one that is distributed throughout the world. They are found in both fresh and salt water.

Lifestyle

The jellyfish stage of the hydroid life cycle, as well as the polyp stage in the siphonophore, are mostly planktonic organisms. They occur seasonally, often in large aggregations that are carried by currents. Some jellyfish and siphonophores, however, are benthic. Stage polyps usually belong to the benthic, and lead a sedentary lifestyle, but there are exceptions: several planktonic hydroid polyps are known. In particular, the so-called swallowtail jellyfish is a planktonic free-swimming polyp. (Velella velella). The well-known Portuguese man-of-war is also a free-swimming colony formed from specialized hydroid polyps.

Most hydroids are predators and use the peculiarities of their lifestyle to catch prey. Planktonic stages transported by the current are often also capable of active movement in search of food. The location of the attached forms is determined by where the planula settles. Polyp colonies usually occur in areas where there is a constant flow of water, increasing the supply of potential food.

Behavior

Jellyfish lead a strictly individual lifestyle; they can be driven by the second and current into large accumulations, but so far no forms have been recorded from them social behavior. Colonies of hydroid polyps, especially polymorphic ones, can be comparable to a single organism in terms of the level of specialization of individual polyps and the coordination of their actions. Polyps in a colony are usually descendants of a single planula, and are thus combined clones of identical genotype. However, in some species, colonies can mix their tissues or descendants of several planulae to form a single colony. In these cases, different individuals of polyps are in such a close relationship, forming (at the functional, but not at the genetic level) a single organism, which is probably one of the closest forms of social organization.

Most hydroids are dioecious. Fertilization is usually internal, without copulation. Males release sperm into the water by actively swimming past eggs attached to the mother's body (jellyfish or polyp) or thrown into the water by a female. Hydroids are the first organisms in which the presence of sperm attractors (substances that lure sperm during their free movement) was demonstrated, which provide species-specific attraction of sperm to eggs.

Members of the same colony of polyps (zooids) resort to coordinated behavior, requiring some communication between them. In such species as, for example, Thecocodium brieni, dactylozooids catch prey with their tentacles, while gastrozooids, after capturing prey, stretch to dactylozooids, remove the prey from their tentacles and swallow it. This division of labor, which includes advanced coordination, is quite common for polymorphic colonies.

Obviously, planktonic organisms cannot exhibit strong territorial behavior; but, as a number of studies have shown, free-swimming stages in the life cycle of hydroids actively avoid too dense aggregation of individuals of their species when feeding. Territorial behavior is pronounced among benthic organisms, where competition for suitable habitats is usually high. Thus, the high concentration of stinging dactylozooids on the periphery of the colony (in colonial species) is a protective adaptation aimed at limiting the growth of surrounding animals. In the same colonies, gastrozooids are capable of eating settling planulae of other species, and during development they can compete.

Both jellyfish and polyps, in a hungry state, continuously move in search of food; when the digestive cavity (coelenteron) is filled, the tentacles naturally contract and are pulled towards the body, which provides a certain degree of control over the rational expenditure of stinging cells (cnidocitives). The feeding behavior of many species of jellyfish leads to their periodic vertical migrations.

Nutrition

The main food resource of hydroids is plankton - in particular, small crustaceans. In laboratory conditions, the basis of hydroid nutrition is, of course, artemia. Hydroid jellyfish are, for the most part, strict predators, and, in the case of feeding on fish eggs and larvae, can be considered the top of the food pyramid.

The diet of polyps is varied; Some species have symbiotic unicellular algae, and for a time they feed exclusively on the nutrients they supply during photosynthesis. Thus, these species can be considered functionally photosynthetic animals.

Forms of catching prey by jellyfish vary from passive hovering in the water column with motionless tentacles, which can be encountered by edible plankton, to active swimming in search of food objects. Polyps are capable of extending their tentacles and moving them to catch passing prey, but they can also resort to targeted hunting, which is provided by the sensory organs available (not in all species) that signal the approach of prey.

The main hunting weapon in hydroids is cnidocytes. Hydroids share a wide range of types of these stinging cells in all cnidarians.

From an ecological point of view, hydroid jellyfish, which feed on fish eggs, are the most dangerous predators for them; and the ability of polyps to feed on almost any larvae of fish and crustaceans includes them as an important link in the life cycle of a huge number of species. Thus, the ecological significance of the food specialization of hydroids is very great.

Reproduction

In hydroids, no signs of special mating behavior were found.

The eggs are stored in the gonads (gonophores) of females. According to the species, the eggs can be small and numerous, or large and few, up to one large egg per Gonophora.

The hydroid planula is, in fact, an embryo, not a larva, due to its extremely simple structure (in fact, it is a gastrula). Hydroid planula can be hollow (i.e. coeloblastula) or without an internal cavity (i.e. stereogastrula) of course, species that have a jellyfish in their life cycle are inherent in the hollow planula, which spends part of its life in the water column, swimming with the help of epithelial cilia. Species in which the medusoid stage is absent in the life cycle, of course, produce planulae without an internal cavity and immediately settle to the bottom next to the maternal organism (or colony). If there is a jellyfish in the life cycle, it is this generation that is “sexual,” that is, capable of sexual reproduction. The polyp generation is thus a specialized and perennial larva that produces a large number of sexual individuals during its existence. In many species, however, the medusoid stage can be partially or even completely reduced, and in this case the larva (Polyp stage), thanks to paedomorphosis, becomes a sexually mature individual. Almost half of the species of the subclass Leptolinae are characterized by a reduced or absent medusoid stage; thus, this group is a taxon with a common paedomorphosis of all animals.

Some jellyfish (for example, the genus Eleutheria) They have special brood pockets where they contain small young jellyfish. Also, some hydroids are characterized by gonothecae with brood chambers, which contain planulae for some time.

Many hydroid species are strictly seasonal, being active only for a certain period of time. Jellyfish can be observed for weeks or months, after which they completely disappear from the water column, and the species is represented by corresponding polyps in the benthos throughout the rest of the year. The polyps of the colony, in turn, can regress after waiting out a long period of hydrorhiza, reactivating when favorable living conditions return. Planula can encyst and wait out unfavorable conditions, similar to hydrorhiza, being covered with a protective chitinous shell.

Security status

There is not a single species of the hydroid class on the IUCN Red List. For most species, the exact boundaries of their range and abundance are unknown. A large number of species are considered endemic simply because they have not been specifically searched for outside the area of ​​their initial discovery.

In the regional and national Red Data Books there are such representatives of hydroids as calcificans, coral-like families Milliporidae and Stylasteridae, which are also listed as CITES species. These families are traded, along with some other hydroids (known in the North Sea as "white algae"). The decline in their numbers is mainly due to habitat destruction.

Two species of this class are listed in the Red Book of Ukraine: olindias unexpected (Olindias inexpectata) and Merizia Azov (Moerisia maeotica).

Meaning for humans

Tremblay's famous treatise, which describes the transformations of hydroids of the genus Hydra, inspired Mary Shelley to write the novel Frankenstein; modern composer Frank Zappa wrote a song about hydromedusas, named by zoologists in his honor - Phialella zappai. But, of course, hydroids do not pay much attention to people.

"White algae" (colonies of polyps of the genera Hydrallmania And Sertullaria) were previously used as decorative ornaments until the populations of these hydroids began to decline catastrophically. Some hydroids are used as laboratory animals: classic example is a polyp genus Hydra, that, along with scientific research, are used in schooling in many countries of the world; but hydra is not the only example of such use: also in scientific work widely used Aequorea victoria(to obtain the marker protein aequorin), and species from the genera Hydractinia, Laomedea And Tubularia.

Some species of jellyfish can cause serious burns to humans; This danger also exists when coming into contact with polyp colonies of species such as fire corals (Millepora). When moving in large herds, even small jellyfish, such as members of the genus Cytia, can cause significant burns to swimmers.

But it is the feeding of some jellyfish that causes the greatest harm to humans (for example Aeroquorea Victoria) and free-floating colonies of polyps (such as Cytia gracilis) larvae and eggs of commercial fish.

Hydra. Obelia. The structure of the hydra. Hydroid polyps

They live in marine and rarely in fresh water bodies. Hydroids are the most simply organized coelenterates: a gastric cavity without septa, a nervous system without ganglia, and the gonads develop in the ectoderm. Often form colonies. Many have a change of generations in their life cycle: sexual (hydroid jellyfish) and asexual (polyps) (see. Coelenterates).

Hydra sp.(Fig. 1) - a single freshwater polyp. The length of the hydra's body is about 1 cm, its lower part - the sole - serves to attach to the substrate; on the opposite side there is a mouth opening, around which 6-12 tentacles are located.

Like all coelenterates, hydra cells are arranged in two layers. The outer layer is called ectoderm, the inner layer is called endoderm. Between these layers is the basal plate. The following types of cells are distinguished in the ectoderm: epithelial-muscular, stinging, nervous, intermediate (interstitial). Any other ectoderm cells can be formed from small undifferentiated interstitial cells, including germ cells during the reproductive period. At the base of the epithelial-muscle cells are muscle fibers located along the axis of the body. When they contract, the hydra's body shortens. Nerve cells are stellate in shape and located on the basement membrane. Connected by their long processes, they form a primitive nervous system diffuse type. The response to irritation is reflexive in nature.

rice. 1.
1 - mouth, 2 - sole, 3 - gastric cavity, 4 - ectoderm,
5 - endoderm, 6 - stinging cells, 7 - interstitial
cells, 8 - epithelial-muscular ectoderm cell,
9 - nerve cell, 10 - epithelial-muscular
endoderm cell, 11 - glandular cell.

The ectoderm contains three types of stinging cells: penetrants, volventes and glutinants. The penetrant cell is pear-shaped, has a sensitive hair - cnidocil, inside the cell there is a stinging capsule, which contains a spirally twisted stinging thread. The capsule cavity is filled with toxic liquid. At the end of the stinging thread there are three spines. Touching the cnidocil causes the release of a stinging thread. In this case, the spines are first pierced into the body of the victim, then the venom of the stinging capsule is injected through the thread channel. The poison has a painful and paralyzing effect.

The other two types of stinging cells perform the additional function of retaining prey. Volvents shoot trapping threads that entangle the victim's body. Glutinants release sticky threads. After the threads shoot out, the stinging cells die. New cells are formed from interstitial ones.

Hydra feeds on small animals: crustaceans, insect larvae, fish fry, etc. The prey, paralyzed and immobilized with the help of stinging cells, is sent to the gastric cavity. Digestion of food is cavity and intracellular, undigested residues are excreted through the mouth.

The gastric cavity is lined with endoderm cells: epithelial-muscular and glandular. At the base of the epithelial-muscular cells of the endoderm there are muscle fibers located in the transverse direction relative to the axis of the body; when they contract, the body of the hydra narrows. The area of ​​the epithelial-muscle cell facing the gastric cavity carries from 1 to 3 flagella and is capable of forming pseudopods to capture food particles. In addition to epithelial-muscular cells, there are glandular cells that secrete digestive enzymes into the intestinal cavity.

rice. 2.
1 - maternal individual,
2 - daughter individual (bud).

Hydra reproduces asexually (budding) and sexually. Asexual reproduction occurs in the spring-summer season. The buds are usually formed in the middle areas of the body (Fig. 2). After some time, young hydras separate from the mother’s body and begin to lead an independent life.

Sexual reproduction occurs in autumn. During sexual reproduction, germ cells develop in the ectoderm. Sperm are formed in areas of the body close to the mouth, eggs - closer to the sole. Hydras can be either dioecious or hermaphroditic.

After fertilization, the zygote is covered with dense membranes, and an egg is formed. The hydra dies, and a new hydra develops from the egg the following spring. Direct development without larvae.

Hydra has a high ability to regenerate. This animal is able to recover even from a small severed part of the body. Interstitial cells are responsible for regeneration processes. The vital activity and regeneration of hydra were first studied by R. Tremblay.

Obelia sp.- a colony of marine hydroid polyps (Fig. 3). The colony has the appearance of a bush and consists of individuals of two types: hydranthus and blastostyles. The ectoderm of the members of the colony secretes a skeletal organic shell - the periderm, which performs the functions of support and protection.

Most of the colony's individuals are hydrants. The structure of a hydrant resembles that of a hydra. Unlike hydra: 1) the mouth is located on the oral stalk, 2) the oral stalk is surrounded by many tentacles, 3) the gastric cavity continues in the common “stem” of the colony. Food captured by one polyp is distributed among members of one colony through the branched channels of the common digestive cavity.

rice. 3.
1 - colony of polyps, 2 - hydroid jellyfish,
3 - egg, 4 - planula,
5 - young polyp with a kidney.

The blastostyle has the form of a stalk and does not have a mouth or tentacles. Jellyfish bud from the blastostyle. Jellyfish break away from the blastostyle, float in the water column and grow. The shape of the hydroid jellyfish can be compared to the shape of an umbrella. Between the ectoderm and endoderm there is a gelatinous layer - mesoglea. On the concave side of the body, in the center, on the oral stalk there is a mouth. Numerous tentacles hang along the edge of the umbrella, serving 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; four straight radial canals extend from the stomach, encircling the edge of the jellyfish's umbrella. 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”. The nervous system is of a diffuse type, but there are clusters of nerve cells along the edge of the umbrella.

Four gonads are formed in the ectoderm on the concave surface of the body under the radial canals. Sex cells form in the gonads.

From the fertilized egg, a parenchymal larva develops, corresponding to a similar sponge larva. The parenchymula then transforms into a two-layer planula larva. The planula, after swimming with the help of cilia, settles to the bottom and turns into a new polyp. This polyp forms a new colony by budding.

The life cycle of obelia is characterized by alternation of asexual and sexual generations. The asexual generation is represented by polyps, the sexual generation by jellyfish.

Description of other classes of the type Coelenterates.

The variety of species of marine animals is so wide that it will not be long before humanity will be able to study them in their entirety. However, even long-discovered and well-known inhabitants of the waters can surprise with hitherto unprecedented features. For example, it turned out that the most common hydroid (jellyfish) never dies of old age. It seems that this is the only creature known on earth that has immortality.

General morphology

The hydroid jellyfish belongs to the hydroid class. These are the closest relatives of polyps, but they are more complex. Probably everyone has a good idea of ​​what jellyfish look like - transparent discs, umbrellas or bells. They may have ring-shaped constrictions in the middle of the body or even be in the shape of a ball. Jellyfish do not have a mouth, but they do have an oral proboscis. Some individuals even have small pinkish tentacles at the edges.

The digestive system of these jellyfish is called gastrovascular. They have a stomach, from which four radial canals extend to the periphery of the body, flowing into a common annular canal.

Tentacles with stinging cells are also located on the edges of the umbrella body; they serve both as an organ of touch and as a hunting tool. There is no skeleton, but there are muscles that allow the jellyfish to move. In some subspecies, part of the tentacles are transformed into statoliths and statocysts - organs of balance. The method of movement depends on the type to which a particular hydroid (jellyfish) belongs. Their reproduction and structure will also be different.

The nervous system of hydromedusas is a network of cells that form two rings at the edge of the umbrella: the outer one is responsible for sensitivity, the inner one is responsible for movement. Some have light-sensitive eyes located at the base of the tentacles.

Types of hydroid jellyfish

Subclasses that have the same equilibrium organs - statocysts - are called trachylides. They move by pushing water out of the umbrella. They also have a sail - a ring-shaped outgrowth on the inside, narrowing the exit from the body cavity. It adds speed to the jellyfish when moving.

Leptolids lack statocysts, or they are transformed into a special vesicle, inside of which there may be one or more statoliths. They move in water far less reactively, because their umbrella cannot contract frequently and intensely.

There are also jellyfish hydrocorals, but they are underdeveloped and bear little resemblance to ordinary jellyfish.

Chondrophores live in large colonies. Some of their polyps bud from jellyfish, which then live independently.

Siphonophore is a hydroid whose unusual and interesting appearance. This is a whole colony, in which everyone plays their role for the functioning of the whole organism. Externally it looks like this: on top there is a large floating bubble in the shape of a boat. It has glands that produce gas that helps it float to the top. If the siphonophore wants to go back deeper, it simply relaxes its muscular organ, the closure. Under the bladder on the trunk there are other jellyfish in the shape of small swimming bells, followed by gastrozoans (or hunters), then gonophores, whose goal is procreation.

Reproduction

The hydroid jellyfish is either male or female. Fertilization often occurs externally rather than inside the female's body. The gonads of jellyfish are located either in the ectoderm of the oral proboscis or in the ectoderm of the umbrella under the radial canals.

Mature germ cells end up outside due to the formation of special breaks. Then they begin to fragment, forming a blastula, some of the cells of which are then drawn inward. The result is endoderm. During further development, some of its cells degenerate to form a cavity. It is at this stage that the fertilized egg becomes a planula larva, then settles to the bottom, where it turns into a hydropolyp. Interestingly, it begins to bud new polyps and small jellyfish. Then they grow and develop as independent organisms. In some species, only jellyfish are formed from planulae.

The variation in egg fertilization depends on what type, species or subspecies the hydroid (jellyfish) belongs to. Physiology and reproduction, as well as structure, differ.

Where do they live?

The vast majority of species live in the sea; they are much less common in freshwater bodies. You can meet them in Europe, America, Africa, Asia, Australia. They can appear in greenhouse aquariums and artificial reservoirs. Where polyps come from and how hydroids spread throughout the world is still unclear to science.

Siphonophores, chondrophores, hydrocorals, and trachylids live exclusively in the sea. Only leptolids can be found in fresh water. But there are much fewer dangerous representatives among them than among the marine ones.

Each occupies its own habitat, for example, a specific sea, lake or bay. It can expand only due to the movement of water; jellyfish do not specifically capture new territories. Some people prefer cold, others prefer warmth. They can live closer to the surface of the water or at depth. The latter are not characterized by migration, while the former do this in order to search for food, going deeper into the water column during the day, and rising up again at night.

Lifestyle

The first generation in the hydroid life cycle is the polyp. The second is a hydroid jellyfish with a transparent body. What makes it so is the strong development of the mesoglea. It is gelatinous and contains water. It is because of this that the jellyfish can be difficult to spot in the water. Hydroids, due to the variability of reproduction and the presence of different generations, can actively spread in the environment.

Jellyfish consume zooplankton as food. The larvae of some species feed on eggs and fish fry. But at the same time, they themselves are part of the food chain.

The hydroid (jellyfish), a lifestyle essentially devoted to feeding, usually grows very quickly, but of course does not reach the same size as the scyphoids. As a rule, the diameter of the hydroid umbrella does not exceed 30 cm. Their main competitors are planktivorous fish.

Of course, they are predators, and some are quite dangerous to humans. All jellyfish have something that they use during hunting.

How do hydroids differ from scyphoids?

By morphological characteristics this is the presence of a sail. Scyphoids do not have it. They are usually much larger and live exclusively in seas and oceans. in diameter reaches 2 m, but the poison of its stinging cells is hardly capable of causing severe harm to humans. The greater number of radial canals of the gastrovascular system helps scyphoids grow to large sizes than hydroids. And some types of such jellyfish are eaten by humans.

There is also a difference in the type of movement - hydroids contract the annular fold at the base of the umbrella, and scyphoids contract the entire bell. The latter have more tentacles and sensory organs. Their structure is also different, since scyphoids have muscular and nerve tissue. They are always dioecious, they do not have vegetative reproduction and colonies. These are loners.

Scyphoid jellyfish can be surprisingly beautiful - they can be of different colors, have fringe around the edges and a bizarre bell shape. It is these inhabitants of the waters who become the heroines of television programs about sea and ocean animals.

Jellyfish hydroid is immortal

Not long ago, scientists discovered that the hydroid jellyfish Turitopsis nutricular has amazing ability to rejuvenation. This species never dies by natural causes! She can trigger the regeneration mechanism as many times as she wants. It would seem that everything is very simple - having reached old age, the jellyfish again turns into a polyp and goes through all the stages of growing up again. And so on in a circle.

Nutricula lives in the Caribbean and is very small in size - the diameter of its umbrella is only 5 mm.

The fact that the hydroid jellyfish is immortal became known by accident. Scientist Fernando Boero from Italy studied hydroids and conducted experiments with them. Several individuals of Turitopsis Nutricula were placed in the aquarium, but for some reason the experiment itself was postponed for such a long period that the water dried up. Boero, having discovered this, decided to study the dried remains, and realized that they did not die, but simply cast off their tentacles and became larvae. Thus, the jellyfish adapted to unfavorable environmental conditions and pupated in anticipation of better times. After placing the larvae in water, they turned into polyps and the life cycle began.

Dangerous representatives of hydroid jellyfish

The most beautiful species is called (siphonophora physalia) and is one of the most dangerous marine inhabitants. Its bell shimmers in different colors, as if luring you to it, but it is not recommended to approach it. Physalia can be found on the coasts of Australia, the Indian and Pacific oceans, and even in the Mediterranean. Perhaps this is one of the largest types of hydroids - the length of the bubble can be 15-20 cm. But the worst thing is the tentacles, which can go 30 m deep. Physalia attacks its prey with poisonous stinging cells that leave severe burns. It is especially dangerous to encounter the Portuguese man-of-war for people who have weakened immune systems and are prone to allergic reactions.

In general, hydroid jellyfish are harmless, unlike their scyphoid sisters. But in general it is better to avoid contact with any representatives of this species. All of them have stinging cells. For some, their poison will not turn into a problem, but for others it will cause more serious harm. It all depends on individual characteristics.

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 acts as 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. On the outer surface of the stinging cell, a thin sensitive hair is developed - the cnidocil. 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

Subtype characteristics

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, which various types 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 food processing 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 it has been cut in half, but even if it has been 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 the 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 of which there are individual polyps, interconnected by outgrowths of the intestinal cavity into a single digestive system, which allows food captured by one polyp to be distributed 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 and tentacles. Such an individual cannot feed on its own; it receives food from hydrants through the gastric system of the colony. This formation is called 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 form in its gonads, which are released into external environment, where fertilization occurs.

A blastula is formed from a fertilized egg (zygote), when further development which forms a free-floating two-layer larva covered with cilia - the planula. 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. Biological significance mobile jellyfish is that thanks to them the settlement of hydroids occurs.

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 have more large sizes, highly developed mesoglea, 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 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 big amount 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 got its name due to the oral lobes, reminiscent in shape 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 shows a peculiar 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 lobe. 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 on the Black and Seas of Azov. 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.

  Cornermouths 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 current 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 captured jellyfish are separated and the umbrella is washed until completely removed. internal organs and mucus. 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 ropilem still contains 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 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. Coral polyps have very big role skeleton plays.

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 a role 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 made 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

Subtype characteristics

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. On each side of the mouth there is one tentacle, 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, as adults, 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. Common features their structures 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 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. 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.