General characteristics of the type Intestinal.
Coelenterates - two-layer animals with radial symmetry.
Symmetry. In the body of the coelenterates there is a main axis, at one end of which there is a mouth opening. Several axes of symmetry pass through the main axis, along which the appendages and internal organs of the animal are located. This type of symmetry is called radial .
life forms. The main life forms of coelenterates are the polyp and the jellyfish.
Body polyp generally cylindrical, at one end there is a mouth opening surrounded by a different number of tentacles, and at the other - a sole. Polyps are usually sedentary or inactive. Polyps mostly form colonies.
Body jellyfish has the form of a regular umbrella or bell, on the lower, concave side of which there is a mouth opening. There are tentacles or lobes along the edge of the umbrella and sometimes around the mouth. Jellyfish lead, as a rule, a mobile lifestyle and do not form colonies.
Different types of coelenterates either exist in the form of only one of these life forms (jellyfish or polyp), or during their life cycle go through both stages.
Systematics. In the type of coelenterates, three classes are distinguished:
hydroid (hydra, obeli, polypodium, siphonophores);
Scyphoid (Aurelia jellyfish, cornerots, cyanides, sea wasps);
coral (black and red corals, acropores, fungi, sea anemones, alcyoniums).
In total, there are 9000 modern types of coelenterates.
Body dimensions coelenterates vary widely. Some types of polyps in adulthood do not exceed a few millimeters, while some sea anemones can reach 1 meter in diameter. In jellyfish, the diameter of the umbrella can be from 2 mm to 2 meters. In addition, the tentacles of some jellyfish can stretch up to 30 m.
Motion. polyps sedentary. They can bend the body, contract, move their tentacles. Hydras can "walk" like ground surveyor caterpillars (larvae of moths). Anemones can crawl slowly on their soles.
Jellyfish actively move by contracting the umbrella. An important role is also played by sea currents that carry jellyfish over long distances.
Body structure. As already mentioned, coelenterates are two-layer animals. Their body wall consists of two cell layers - ectoderm (outdoor) and endoderm (internal). Between them is mesoglea - a layer of structureless gelatinous substance. the only cavity in the body of the coelenterates - intestinal, or gastral .
ectoderm represented by a single-layer flat, cubic or cylindrical epithelium . In addition to ordinary epithelial cells, the ectoderm includes epithelial-muscular cells whose base is elongated longitudinal direction contractile (muscle) fiber. In some corals, muscle fibers separate from the epithelium and lie under it or sink into the mesoglea layer, forming an independent muscular system. Between the epithelial cells are interstitial cells that give rise to various cellular elements of the ectoderm. A characteristic feature of the coelenterates is the presence in the ectoderm of the tentacles of the so-called stinging cells . Each such cell contains a capsule into which a spirally folded long hollow process is inserted - a stinging thread. Outside the cell there is a sensitive hair, upon irritation of which the stinging thread sharply turns out, straightens and pierces the body of the prey or the enemy. At the same time, a poisonous secret is poured out of the capsule, causing paralysis of small animals, as well as a burning sensation in large ones.
Endoderm. The epithelium lining the gastric cavity consists of flagellated cells. Some of these cells are epithelial-muscular , however, the muscular processes are located in the transverse direction, forming together, layers of annular fibers. The cells of the ectodermal epithelium are able to form pseudopodia, with the help of which they capture food particles. There are also glandular cells.
Mesoglea. In polyps, the mesoglea is poorly developed (with the exception of corals), while in jellyfish this layer reaches a considerable thickness. The mesoglea contains a number of ectodermal cells that take part in the formation of the skeleton.
Skeletal formations. Only polyps have a skeleton. In hydroid polyps, the body is covered with a thin chitinous theca - a dense shell that performs a protective function. Most coral species have a calcareous skeleton, sometimes horny. The development of the skeleton can vary from individual spicules scattered in the mesoglea to powerful stone-like formations of various sizes and shapes (in madrepore corals). These skeletons are derived from the ectoderm.
The formation of the skeleton in corals is largely due to the presence of polyps in the body. symbiotic algae . Consider chemical reactions that occur during the formation of a calcareous skeleton. The initial substances - calcium ions and carbon dioxide - are contained in sea water in sufficient quantities.
Carbon dioxide, when dissolved in water, forms a very unstable carbonic acid:
H 2 O + CO 2 ↔ H 2 CO 3, which immediately dissociates into ions:
H 2 CO 3 ↔ H + + HCO 3 -.
When Ca and HCO 3 ions interact, calcium bicarbonate is formed:
Ca ++ + 2 HCO 3 - ↔ Ca (HCO 3) 2. This substance is soluble in water, but it is also not stable and easily turns into insoluble calcium carbonate:
Ca (HCO 3) 2 ↔ Ca CO 3 ↓ + H 2 O + CO 2.
With an excess of CO 2, this reaction shifts to the left and a soluble bicarbonate is formed. With a decrease in the concentration of CO 2, the reaction shifts to the right and lime precipitates.
Algae living in the body of polyps constantly remove carbon dioxide from the tissues of the coelenterates for the process of photosynthesis, constantly creating a reduced concentration of CO 2. Such conditions favor the formation of insoluble calcium carbonate and the construction of a powerful skeleton by polyps.
Digestive system and nutrition. The digestive system is represented by the gastric cavity. Most coelenterates are predators. Prey, killed or stunned by stinging cells, they bring with tentacles to the mouth opening and swallow.
In hydroid polyps, the gastric cavity looks like a simple bag, which communicates with the environment through the oral opening. Various small animals that enter the gastric cavity are most often absorbed by endoderm cells ( intracellular digestion). Larger prey can be digested by enzymes secreted by glandular cells. Undigested residues are ejected through the mouth opening.
In coral polyps, the gastric cavity is divided longitudinally by septa, which increases the area of the endoderm. In addition, the ectodermal pharynx protrudes into the digestive cavity of corals.
As already mentioned, reef-building corals enter into a symbiotic relationship with a certain type of single-celled algae that settle in the endodermal layer. These plants, receiving carbon dioxide and metabolic products from the polyp, supply it with oxygen and a number of organic substances. Algae themselves are not digested by polyps. Under normal conditions, such a symbiosis makes it possible for polyps to do without the intake of organic substances from the environment for a long time.
At jellyfish the gastric cavity is generally formed by the stomach located in the central part of the umbrella, radial canals extending from the stomach and an annular canal running along the edge of the umbrella. Hydromedusas often have 4 radial canals, while scyphomedusas have 16 radial canals. The entire complex of canals forms the so-called gastrovascular system .
Nervous system. At polyps nervous system diffuse type . Separate nerve cells located at the base of the epithelium of the ectoderm and endoderm are connected by their processes into a nervous network. The mouth opening and the sole of the polyps are surrounded by a denser nervous network.
At jellyfish the nervous system is more concentrated than in polyps, which is associated with a mobile lifestyle.
At hydroid jellyfish accumulation of nerve cells is located on the edge of the umbrella. The cells themselves and their processes form a double nerve ring. The outer ring performs sensory functions, while the inner ring performs motor functions.
At scyphoid jellyfish the nerve ring is less pronounced, but at the base of the ropalia (marginal sensory bodies) there are clusters of nerve cells that can be called ganglia.
sense organs. Due to a sedentary lifestyle, polyps special no sense organs . There are only a few sensitive (tactile) cells, which are located mostly near the mouth opening.
At jellyfish there are also sensitive cells, but there are also special sense organs - vision, balance and smell.
Along the edge of the umbrella are located organs of vision - eyes , different in structure. In hydroid jellyfish, the eyes lie singly, while in scyphoid jellyfish, the eyes are on ropalia - sensitive marginal bodies. Moreover, one ropaliy can carry several eyes of varying degrees of complexity at once.
In connection with a mobile lifestyle, jellyfish have appeared organs of balance - statocysts. They are a bubble lined from the inside with sensitive cells. Inside the bubble is a calcareous body - statolith. Depending on the position of the jellyfish in space, the statolith irritates a certain section of the bubble wall. There are other types of structure of statocysts. In addition, statocysts are able to capture the vibrations of water, so they can also be called organs of hearing. In hydroid jellyfish, the balance organs are located along the edge of the umbrella in the amount of 4-80 in different species.
Scyphoid jellyfish also have olfactory pits - organs of chemical sense.
In scyphoids, all the senses are located on 8 ropalia - modified tentacles.
Breath. Gas exchange in coelenterates occurs by diffusion of oxygen and carbon dioxide. Large species (corals) have siphonoglyphs lined with ciliated epithelium on the pharynx. Cells equipped with cilia constantly carry out the flow of fresh water into the intestinal cavity of the animal. Many polyps, as already mentioned, have switched to symbiosis with algae, supplying the coelenterates with oxygen and releasing carbon dioxide.
Sex organs. At polyps there are no special sex organs. Sex cells are laid either in the ectoderm or in the endoderm. In the first case, the gametes exit through the rupture of the ectoderm, in the second, they first enter the gastric cavity, and then out through the mouth. Among the polyps there are hermaphrodites (hydras) and dioecious (corals).
At jellyfish , which are almost always separate sexes, there are sex glands.
At hydromedus they form in the ectoderm of the lower side of the umbrella under the radial canals, less often on the oral proboscis. The number of gonads corresponds to the number of radial canals. Gametes exit through ruptured glands.
At scyphoid jellyfish gonads of endodermal origin. They form in the pockets of the stomach. Gametes first enter the gastric cavity, and then into the environment.
Reproduction. Coelenterates reproduce both asexually and sexually.
asexual reproduction most often proceeds through budding . This path is characteristic of polyps, and is rare in jellyfish. In single polyps, a kidney appears on the body, which gradually forms tentacles and a mouth opening and then breaks away from the mother's body. In colonial hydroids and corals, the daughter individual does not separate from the mother, which leads to the formation of colonies.
Colonial hydroid polyps they are not capable of sexual reproduction, so they bud off and sexual individuals - jellyfish. Jellyfish are formed either on the axis of the colony, or on special outgrowths - blastostyles.
Another method of asexual reproduction is strobilation when the polyp at a certain stage begins to lace up in the transverse direction several times and a small jellyfish is formed from each part. Almost the entire body of the polyp is spent on the formation of jellyfish. This method is typical for scyphoid jellyfish.
Thus, there is a change of polypoid asexual and medusoid sexual generations. At the same time, the polypoid generation predominates in hydroids, and the medusoid generation in scyphoids. Corals do not have a medusoid generation.
In a number of hydroids, jellyfish do not break away from the colony, and in some, the jellyfish is reduced to the state of a "genital sac" - a sporosarca.
Very interesting siphonophores , which are a huge colony consisting of organisms of various structures. Each colony has a pneumatophore, an air bladder that supports the siphonophore on the surface of the water.
sexual reproduction characteristic of all jellyfish, all corals and some hydroid polyps. The sexual process involves haploid cells - gametes that copulate either in the environment or in the body of the intestinal cavity. The egg undergoes complete uniform crushing. Gastrulation of the blastula most often occurs by immigration, less often by invagination. In the future, a two-layer larva is formed - a planula, covered with cilia and leading a mobile lifestyle. For such sedentary animals as corals (which do not have a medusoid generation), the planula is the only settling stage. A polyp is always formed from the planula, later budding from itself either only polyps (corals), or polyps and jellyfish (hydroid), or only jellyfish (scyphoid). Thus, the development of the vast majority of coelenterates goes with metamorphosis. Sometimes a polyp is immediately formed from an egg (for example, in hydra).
Regeneration. Coelenterates have a high ability to regenerate. Experiments on the study of this phenomenon in hydra were staged as early as 1740 by Tremblay. It turned out that the animal can regenerate from 1/200 of a part.
Origin. Most likely, the ancestors of the coelenterates were free-floating organisms of the type parenchymella , which was described by I.I. Mechnikov. These hypothetical organisms were devoid of a skeleton and therefore could not be preserved as a fossil.
The oldest finds of coelenterates - coral skeletons - date back to the Cambrian period (about 600 million years ago). At the same time, not only individual prints were preserved, but also entire petrified reefs. A few imprints of jellyfish and hydroids are also known. In total, more than 20,000 species of fossil coelenterates are known.
Meaning. In nature, coelenterates, being predators and at the same time food for other animals, participate in complex food chains of marine biocenoses. Corals are of great geochemical importance, forming thick layers of calcareous rocks. Throughout their existence, corals have been involved in the formation of islands. Reefs are unique biocenoses, where a huge number of animal species live.
Practical significance modern coelenterates is small.
Corals (especially red and black) are used as decorations. They are mined in large quantities, mainly by artisanal methods. Coral collection is prohibited on large reefs.
Some jellyfish pose a serious danger to humans. In our seas, these include a small Far Eastern cross jellyfish that lives in thickets of marine plants and a large Black Sea cornerot, often found off the coast. The poison of the cross is sometimes fatal. The most dangerous jellyfish - the sea wasp - lives off the coast of Australia. Touching this animal causes severe pain and shock. Many people died when they met her.
In China and some other countries, specially prepared ropil jellyfish are eaten. There is a special trade there.
The most typical representative of the class is hydra(Fig. 7).
The first person who saw the hydra was the inventor of the microscope and the greatest naturalist of the 17th-18th centuries. A. Leeuwenhoek (1632 - 1723).
Looking at aquatic plants, he saw among the small organisms a strange creature with numerous "horns". He also observed the growth of kidneys on his body, the formation of tentacles in them and the separation of the young animal from the mother's organism.
Hydra is a freshwater single polyp with an oblong sac-like body about 1 cm long. The body consists of two layers of cells: the outer one - ectoderm, and internal - endoderm, lining the intestinal cavity. Two layers of cells are separated by a thin base plate - mesoglea. At the upper end of the hydra's body is a mouth surrounded by a corolla of 6-12 tentacles. With their help, the hydra captures prey and directs it into the mouth. At the lower end of the body is the sole, with which the hydra is attached to underwater objects.
Part ectoderm includes cells of different types: epithelial-muscular, stinging, intermediate, nervous (Fig. 8).
epithelial muscle cells form the basis of the ectoderm. The contractile fibers in the processes of their cells provide the movement of the tentacles and the entire body, which can stretch, contract, and walk like moth caterpillars.
Rice. 7. Schematic longitudinal section of the hydra: 1 - tentacle; 2 - mouth; 3 - ectoderm; 4 - endoderm; 5 - mesoglea; 6 - intestinal cavity; 7 - kidney; 8 - male gonad; 9 - female gonad.
Among the epithelial-muscular cells, stinging cells are located singly or in groups. Especially a lot of them on the tentacles. In the hollow capsule of the cell is a spirally coiled stinging thread. On the outer surface of the cell is sensitive hair, the irritation of which (mechanical or chemical) causes the shooting of a stinging thread. Stinging cells are used only once, after which they die.
Instead of spent stinging, as well as other types of cells, new ones develop in the ectoderm - from numerous small rapidly multiplying undifferentiated intermediate cells. Due to their presence, the hydra has a well-defined ability to regenerate lost or damaged cells and body parts.
Rice. 8. Hydra Body Cells: but- epithelial-muscular cell of the ectoderm; b- nerve cells interconnected by processes; in- two stinging cells (1 - at rest; 2 - discharged).
Nerve cells are located evenly in the depth of the ectoderm; their processes form a reticular plexus - a diffuse nervous system. Irritation from one cell is transmitted to other nerve cells, and from them to skin-muscle cells. The response to external stimulation in hydra is a simple unconditioned reflex.
Thus, ectoderm cells perform protective, motor and sensory functions.
The endoderm is formed by two types of cells: glandular and digestive. glandular cells secrete digestive enzymes into the intestinal tract. Digestive cells similar in structure to the epithelial-muscular cells of the ectoderm, but unlike them, they are equipped with one or two flagella and are able to form pseudopods.
Hence, the cells of the endoderm are specialized in performing a digestive function.
Hydra - predatory animal. With the stinging threads of its tentacles, it strikes small aquatic animals, paralyzing and swallowing them. In the intestinal cavity, food is semi-digested to a mushy state by enzymes secreted by the glandular cells of the endoderm. Then small food particles are captured by the rotational movements of the flagella of the digestive cells and phagocytosed by their pseudopods. Undigested food debris is expelled through the mouth.
Thus, in hydra, like in all coelenterates, digestion mixed.
reproduction hydra occurs in the warm season asexually - by budding. On the body of the hydra, a small tubercle is first formed - a kidney, which is a protrusion outward of two layers of the body. The kidney increases in size, tentacles and a mouth opening are formed on it. Soon the young hydra separates from the mother.
With abundant nutrition, the entire warm period of the year, hydras reproduce by budding. With the onset of autumn cold weather, the hydra begins to sexual reproduction. Hydras of different species can be dioecious and hermaphrodites. Some intermediate cells of the ectoderm differentiate into male and female germ cells, which accumulate in the lower or middle part of the body and are called gonads or gonads. In the developing gonads, a large number of intermediate, undifferentiated cells accumulate, from which both future germ cells and "nourishing" cells are formed, due to which the future egg grows. At the first stages of egg development, these cells turn into mobile amoeboids. Soon one of them begins to absorb the others and increases significantly in size, reaching 1.5 mm in diameter. This large amoeboid, picking up pseudopodia, rounds off and becomes an egg. After it undergoes meiosis, the wall of the gonad bursts and the egg comes out, remaining, however, connected with the body of the hydra by a thin plasma stalk. Each female gonad produces one egg.
By this time, spermatozoa develop in the testes of other hydras, which leave the gonad and swim in the water. One of them penetrates the egg, after which it immediately begins splitting up. The developing embryo is dressed in two shells, the outer of which has dense chitinous walls and is often covered with spines.
Under the protection of a double shell - embryotheca- the embryo overwinters, while adult hydras die with the onset of cold weather. By spring, there is already a formed small hydra inside the embryotheca, which comes out through a break in its wall.
Rice. Fig. 9. Scheme of a longitudinal section of a hydroid jellyfish: On the left - a section in the plane of the radial canal: 1 - oral opening; 2 - stomach; 3 - oral tentacles; 4 - radial channel; 5 - sail; 6 - marginal tentacle; 7 - motor nerve ring; 8 - peephole; 9 - sensitive nerve ring; 10 - gonad; on the right - a section between the radial channels: 11 - ectoderm, 12 - endoderm; 13 - mesoglea; 14 - annular channel.
Hydroid jellyfish are much more complicated (Fig. 9). Externally, the hydrojellyfish has the appearance of a transparent disk, umbrella or bell. An oral proboscis with a mouth at the end hangs from the inner center of the umbrella. The margins of the mouth may be smooth or provided with four more or less fringed oral lobes. The mouth leads to the stomach, which occupies the entire cavity of the oral proboscis; four radial canals extend from the stomach to the periphery of the umbrella. At the edge of the umbrella, they flow into an annular canal. The combination of the stomach and canals is called gastrovascular system. Along the edge of the hydromedus umbrella are tentacles and sensory organs. The tentacles are used for touch and catching prey, they are densely seated with stinging cells.
Some hydromedusas have photosensitive organs - eyes, which are always located at the base of the tentacles and are clearly visible due to their dark coloration. The ocellus consists of two types of cells - photosensitive and pigmented. The eyes look like spots or pits. In the most complex ocelli, the cavity of the fossa is filled with a transparent substance that acts as a lens.
The movement of the jellyfish is carried out due to the contraction of muscle fibers at the edge of the umbrella. Pushing water out of the cavity of the umbrella, the jellyfish receives a jet push and moves the upper side of the umbrella forward. The increase in reactivity is achieved due to the presence on the inside of the umbrella of an annular outgrowth called a sail, which narrows the exit from the cavity of the umbrella.
Jellyfish are dioecious, their gonads are located either in the ectoderm of the oral proboscis, or the ectoderm of the umbrella under the radial canals. Here they are closest to the nutrients necessary for the development of reproductive products. The structure of the cells of the ectoderm and endoderm of jellyfish is the same as that of polyps, but the mesoglea is undoubtedly more developed. It is rich in water and has a gelatinous character, due to which the hydromedusae are very transparent, many, even quite large, jellyfish are difficult to see in the water. The mesoglea is especially strongly developed in the umbrella.
Hydroids are a class of animals belonging to the type Cnidaria (belong to the Intestinal). Hydroids include more than 2.5 thousand species, including freshwater hydra. However, unlike hydra, most hydroids live in the seas and oceans and have two forms in their life cycle - a colonial polyp and a jellyfish. All hydroids have a larval stage from which a polyp develops.
Both the polyp and the jellyfish are predators, capturing prey with tentacles.
Hydroid polyps form colonies as a result of asexual reproduction by budding. Unlike solitary hydras, in marine representatives, daughter individuals do not separate from their mothers. Their intestinal cavities communicate with each other, this allows you to redistribute the food captured by one polyp throughout the colony.
Hydroid jellyfish are small in size (within 1 cm). They form on some polyps of the colony, break away from them and begin to swim.
The jellyfish looks like an inverted polyp. However, her body shape is not cylindrical, but similar to an umbrella or a bell, i.e., the shape of the body is more flattened. A jellyfish can swim by contracting the umbrella and ejecting water from it, but more often in hydroid jellyfish, they swim passively due to the current. At the bottom, the mouth opening is surrounded by tentacles. Unlike polyps, jellyfish have a more powerful layer of mesoglea (gelatinous substance between the ectoderm and endoderm).
With the help of jellyfish, hydroids occur sexual reproduction. Each jellyfish has sex glands from below. In some individuals, they contain eggs, in others, spermatozoa; i.e., hydroid jellyfish are dioecious animals. When the germ cells mature, they enter the water, where fertilization takes place. It happens that the eggs do not leave the jellyfish.
The resulting zygote turns into a larva covered with flagella (moss larva). Such a larva can actively swim. Having sunk to the bottom, it gives rise to a hydroid polyp.
Hydras do not have a medusa stage. Sex cells mature directly on the polyp. There is an opinion that in the process of evolution the medusa stage was reduced in them.
Medusa allows hydroids to settle in larger areas.
Unlike other coelenterates, hydroids do not have any partitions in the intestinal cavity. That is, they can be considered more primitive coelenterates.
Subclass of hydroids (Hydroidea)
The body of a hydroid polyp is cylindrical or ovoid in shape, in its tender part it is usually equipped with a leg, with which single polyps are attached to the soil, aquatic plants, mollusk shells, i.e. generally to any underwater objects.
In colonial forms, the stalk connects the polyp to the colony.
At the upper end of the body of the polyp, there is a mouth opening surrounded by tentacles, which can be arranged in regular corollas or without a strict order.
The number of tentacles varies from species to species.
For example, a giant single hydroid branchiocerianthus polyp (Branchiocerianthus imperator), reaching more than 1 m in height, has up to 380 tentacles, while a small monobrachium (Monobrachium parasiticum) has only one. More often, each polyp is equipped with 10-30 tentacles, which can be simple or equipped with a pin-shaped thickening at the end.
In one species (Cladocoryne), the tentacles even branch.
There are also polyps that lack tentacles. These include freshwater microhydra (Microhydra). Polyps use their tentacles to capture food, and solitary polyps, such as freshwater hydra, use them when moving.
The internal structure of the hydroid polyp is very simple.
It has the appearance of a two-layer bag. Outside, the polyp is covered with a layer of ectoderm cells, and its intestinal (gastric) cavity is lined with endoderm.
Between the layers of ectoderm and endoderm in hydroid polyps is a thin layer of non-cellular substance - mesoglea. The ectoderm is formed by special epithelial-muscular cells. The body of such a cell has the appearance of a multifaceted prism, the cells are closely spaced, like a honeycomb.
The prismatic part performs an integumentary function.
At their inner end, the epithelial cells have a long outgrowth that extends up and down along the body of the polyp. Inside this process passes a thin muscle fiber. The same contractile processes extend from the cells of the endoderm, but they are located perpendicular to the processes of the ectodermal cells.
The collection of muscle fibers of many ectodermal and endodermal cells allows the polyp and its tentacles to stretch and contract.
Between the epithelial-muscular cells are stinging cells.
In the ectoderm there are also special nerve cells, equipped with long thin processes, which together form reticular nerve plexuses, somewhat denser at the ends of the tentacles and around the mouth.
In polyps that can reproduce sexually (for example, in hydra), germ cells also develop in the ectoderm, which accumulate in the lower or middle part of the body.
These cells are called sex glands or gonads. Finally, here, in the epithelium, there are intermediate or reserve cells, from which the epithelial-muscular, stinging, nerve and sex cells of hydroid polyps develop.
The ectoderm of many types of polyps secretes a thin shell from the outside - the theca, consisting of chitin.
This shell plays the role of the external skeleton in hydroid polyps, which serves as a support and protection.
In some species, the theca forms a cup around the polyps - a hydrotheca, into which they are drawn in in case of danger.
Endoderm consists of glandular and epithelial-muscular cells. Glandular cells produce digestive juice (enzymes) that aid in the digestion of food.
Small animals that have fallen into the gastric cavity of the polyp are digested under the influence of digestive juices and fall apart.
hydroid polyps. Structure. Metagenesis
Endoderm cells have 2-5 thin flagella, which wriggle all the time and move the contents of the gastric cavity. Food particles that are near the walls of the gastric cavity are captured by pseudopodia formed on the surface of epithelial-muscular cells. Further assimilation of food takes place already inside the cell, just as it happens in unicellular animals.
The products of digestion are transferred to the cells of the ectoderm, which are separated from the endoderm by a thin but dense layer mesoglea, through direct contacts of endoderm and exoderm cells, the outgrowths of which cross the mesoglea layer, tightly adjoining each other.
In colonies, the gastric cavities of individual polyps communicate with each other, forming a single cavity of the colony.
The lecture was added on 08/24/2012 at 05:17:45
Class Hydroids (Hydrozoa)
Hydra. Obelia. Hydra structure. hydroid polyps
They live in marine, rarely - in fresh water. Hydroid - the most simply organized coelenterates: the gastric cavity without partitions, the nervous system without ganglia, the gonads develop in the ectoderm. They often form colonies. Many in the life cycle have a change of generations: sexual (hydroid jellyfish) and asexual (polyps) (see.
Coelenterates).
Hydra (Hydra sp.)(Fig. 1) - a single freshwater polyp. The body length of the hydra 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 there are 6-12 tentacles.
Like all coelenterates, hydra cells are arranged in two layers.
The outer layer is called the ectoderm, the inner layer is called the endoderm. Between these layers is the basal lamina. In the ectoderm, the following types of cells are distinguished: epithelial-muscular, stinging, nervous, intermediate (interstitial). From small undifferentiated interstitial cells, any other cells of the ectoderm can form, including during the reproduction period and germ cells.
At the base of the epithelial-muscle cells are muscle fibers located along the axis of the body. With their contraction, the body of the hydra is shortened. Nerve cells are stellate and located on the basement membrane. Connecting with their long processes, they form a primitive nervous system of a diffuse type. The response to irritation has a reflex character.
rice. one.
Hydra:
1 - mouth, 2 - sole, 3 - gastric cavity, 4 - ectoderm,
5 - endoderm, 6 - stinging cells, 7 - interstitial
cells, 8 - epithelial-muscular cell of the ectoderm,
9 - nerve cell, 10 - epithelial-muscular
endoderm cell, 11 - glandular cell.
There are three types of stinging cells in the ectoderm: penetrants, volvents, and glutinants.
The penetrant cell is pear-shaped, has a sensitive hair - knidocil, inside the cell there is a stinging capsule, in which there is a spirally twisted stinging thread.
The cavity of the capsule is filled with a toxic liquid. There are three spines at the end of the stinging thread. Touching the cnidocil causes the ejection of the stinging thread. At the same time, spines are first pierced into the body of the victim, then the poison of the stinging capsule is injected through the thread channel.
The poison has a painful and paralyzing effect.
Stinging cells of the other two types perform an additional function of holding prey. Volvents shoot trapping threads that entangle the victim's body. Glutinants throw out sticky threads. After the filaments are fired, the stinging cells die. New cells are formed from interstitial cells.
Hydra feeds on small animals: crustaceans, insect larvae, fish fry, etc.
The prey, paralyzed and immobilized with the help of stinging cells, is directed to the gastric cavity. Digestion of food - abdominal and intracellular, undigested residues are excreted through the mouth opening.
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 with respect to the axis of the body; when they contract, the body of the hydra narrows.
The section of the epithelial-muscular cell facing the gastric cavity carries from 1 to 3 flagella and is able to form 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.
Class Hydroids (Hydrozoa)
Hydra budding:
1 - maternal individual,
2 - daughter individual (kidney).
Hydra reproduces asexually (budding) and sexually.
Asexual reproduction occurs in the spring-summer season. The kidneys are usually laid in the middle parts 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.
Spermatozoa are formed in areas of the body near the mouth opening, eggs - closer to the sole. Hydra can be both dioecious and hermaphroditic.
After fertilization, the zygote is covered with dense membranes, an egg is formed.
The hydra dies, and a new hydra develops from the egg the following spring. Development is direct without larvae.
Hydra has a high ability to regenerate. This animal is able to recover even from a small cut off part of the body.
Interstitial cells are responsible for regeneration processes. The vital activity and regeneration of the hydra were first studied by R. Tremblay.
Obelia (Obelia sp.)- a colony of marine hydroid polyps (Fig. 3). The colony has the appearance of a bush and consists of individuals of two species: hydrants and blastostyles.
The ectoderm of the members of the colony secretes a skeletal organic membrane - the periderm, which performs the functions of support and protection.
Most of individuals of the colony - hydrants. The structure of the hydrant resembles the structure of the 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 the members of one colony through the branched canals of the common digestive cavity.
rice. 3. Obelia life cycle:
1 - colony of polyps, 2 - hydroid jellyfish,
3 - egg, 4 - planula,
5 - a young polyp with a kidney.
Blastostyle looks like a stalk, has no mouth and tentacles.
Jellyfish bud from the blastostyle. Jellyfish break away from the blastostyle, swim in the water column and grow. The shape of a hydroid jellyfish can be compared to the shape of an umbrella. Between the ectoderm and endoderm is a gelatinous layer - the mesoglea. On the concave side of the body, in the center, on the oral stalk is the mouth.
Numerous tentacles hang along the edge of the umbrella, serving to catch 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 depart from the stomach, encircling the edge of the jellyfish umbrella.
The way the jellyfish moves is “reactive”, this is facilitated by a fold of ectoderm along the edge of the umbrella, called the “sail”. The nervous system is diffuse type, but there are accumulations 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.
A parenchymula larva develops from a fertilized egg, corresponding to a similar sponge larva. The parenchymula then transforms into a two-layer planula larva.
Planula, having floated 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 the alternation of asexual and sexual generations. The asexual generation is represented by polyps, the sexual generation is represented by jellyfish.
Description of other classes of type Coelenterates.
Marine, less often freshwater animals, leading an attached lifestyle or swimming in the water. Attached forms are called polyps, floating - jellyfish.
Double layer animals, their body consists of two cell layers: the outer - ectoderm and internal - endoderm. Endoderm forms intestinal or gastric cavity. The gastric cavity communicates with the environment through an opening that functions as oral And anal. Between the ectoderm and endoderm is mesoglea. In polyps, the mesoglea forms a base plate, while in jellyfish, it forms a thick gelatinous layer.
Ectoderm cells perform protective and motor functions. The ectoderm has special stinging cells for defense and attack. Endoderm cells line the gastric cavity and perform mainly a digestive function. Digestion intracellular And cavity.
Breathing occurs through the entire surface of the body.
Nervous system scattered or diffuse, type. Available tactile sensitivity, and in jellyfish, in connection with a floating lifestyle, light-perceiving "eyes" And balance organs.
Coelenterates have radial or radial, symmetry.
asexual reproduction budding. The sex organs are presented gonads. Fertilization is external. Some representatives are characterized by the alternation of asexual (polyp) and sexual (jellyfish) generations in the life cycle.
The type of coelenterates includes the following classes: Hydrozoa, Scyphoid jellyfish, Coral polyps.
Class Hydrozoa
freshwater hydra
A BRIEF DESCRIPTION OF
|
Habitat |
Freshwater bilayer animals. Lead an attached lifestyle |
|
Appearance |
Saccular up to 1.5 cm. Radial symmetry. The mouth at the anterior end of the body is surrounded by tentacles, the sole is the posterior end of the body, for attachment |
|
body cover |
Ectoderm - outer layer, endoderm - inner layer, mesoglea - middle layer |
|
body cavity |
There is no body cavity. There is only an intestinal cavity |
|
Digestive system |
Blindly closed intestinal cavity. The mouth opening for food intake and for the ejection of undigested food debris. Digestion intracavitary and intracellular |
|
excretorysystem |
Ectoderm cells |
|
Nervous system |
Nerve cells of stellate type. diffuse nervous system |
|
sense organs |
not developed |
|
Respiratory system |
None. Breathing through the entire surface of the body |
|
reproduction |
Asexual - by budding. Hermaphrodites. Cross fertilization. |
GENERAL CHARACTERISTICS
This class includes small forms of coelenterates. polyps And jellyfish belonging to this class are called hydroid.
Structure . The hydra's body is oblong double-layer bag, attached by the base, or sole, to the substrate (Fig. 1). outer layer - ectoderm, inner layer - endoderm. Between the layers there is a space - mesoglea.
At the free end of the body is oral cone, surrounded by a halo of 6-12 tentacles. Located on the mouth cone mouth, employee and anus. The entire surface of the body is covered ectoderm, consisting mainly of cylindrical or cuboidal epithelial cells. Their base is elongated upwards and downwards, along the longitudinal axis of the body, into a long process. The cytoplasm of the process differentiates into contractile fibers, in connection with this, the offshoot plays muscular role. Cylindrical parts of cells form single layer epithelium. Thus, cells perform a dual function - coverslip And motor and are called epithelial-muscular. With the simultaneous contraction of all muscular processes, the body of the hydra is shortened. Between the epithelial-muscular cells are small intermediate cells that are involved in the formation stinging And germ cells, as well as in the process regeneration- restoration of lost body parts or organs. Located directly below the epithelium stellate nerve cells. Connecting with their processes, nerve cells form the nervous system scattered, or diffuse, type. Of particular importance in the ectoderm are stinging cells, or capsules used for attack and defense.
Endoderm lines the whole gastric, or digestive cavity. The basis of the cells of the endoderm are epithelial-muscular digestive cells. The muscular processes of these cells, in contrast to the ectodermal ones, are located transversely with respect to the longitudinal axis of the body. When they contract, the body of the hydra narrows and becomes thinner. The endodermal cells are glandular cells that secrete digestive enzymes into the gastric cavity, and cells with phagocytic activity. The latter are able to capture food particles with the help of the movement of 1-3 flagella and the formation of pseudopodia. Thus, the hydra combines two types of digestion: intracellular And abdominal.
Rice. one.The structure of freshwater hydra: a - longitudinal section; b - cross section; c - two-layer body; d - epithelial muscle cell; e - tentacle with ejected stinging filaments; f, g - stinging cells; 1 - tentacles; 2 - testis; 3 - spermatozoa; 4 - gastric cavity; 5 - budding young hydra; 6 - base plate; 7 - endoderm; 8 - ectoderm; 9 - egg at different stages of development; 10 - stinging cells; 11 - mouth opening; 12 - sole
Mesoglea presented in the form of a thin structureless plate - basement membrane.
Asexual reproduction. Approximately at the level of the middle of the body of the hydra there is a so-called budding belt, where from time to time it is formed bud, from which a new individual is subsequently formed. After the formation of the mouth and tentacles, the kidney at the base unfastens, falls to the bottom and begins an independent existence. This type of asexual reproduction is called budding.
sexual reproduction . With the approach of cold weather, hydras begin to reproduce sexually. Intermediate cells of the ectoderm can transform directly into eggs or multiple division - in spermatozoa. Intermediate cells that form eggs located closer to the base of the hydra, and those that form spermatozoa - to the mouth. The eggs are fertilized in the mother's body in autumn and are surrounded by a dense shell, then the mother individual dies, and the eggs remain dormant until spring. In the spring, a new individual develops from them. Hydra separate sexes but meet and hermaphroditic kinds.
Marine hydroid polyps
Most marine hydroid polyps form colonies. Colonies most often have the form of a tree or shrub. The trunk branches, the branches form separate colonies - hydrants. The gastric cavities of all hydrants communicate with each other, so food captured by one hydrant is distributed throughout the colony. In marine hydroid polyps, the ectodermal epithelium forms a special membrane - flow, which gives the entire colony greater stability.
Marine hydroid polyps multiply only asexual way- budding. sexual reproduction carry out sex individuals- jellyfish, which are formed on the polyp by budding and pass to a free-floating way of life. Jellyfish have the same structure as polyps, although
there are also differences (Fig. 2, 3). The body of the jellyfish is characterized strong development of mesoglea which contains a large amount of water. The nervous system is also much more complex. In jellyfish, along the edge of the umbrella is formed continuous nerve ring. There are sense organs: eyes And statocysts (organs of balance). Jellyfish separate sexes. gonads located on the underside of the umbrella between the ectoderm and mesoglea. Fertilization and development of eggs in the external environment. The eggs develop into larvae parenchymula, then the second larva - planula, which floats freely for some time, then sinks to the bottom and gives rise to a polyp. A new colony is subsequently formed from the polyp, and the cycle repeats. Thus, the life of hydroid polyps consists of two generations. One generation- polyps lead a sedentary lifestyle and reproduce asexually. Second generation - jellyfish, lead a free-swimming lifestyle and reproduce sexually. That is, in hydroid polyps, alternation of generations.
Rice. 2.The structure of a hydroid polyp (A) and a hydroid jellyfish (B), turned upside down with its mouth opening: 1 - mouth; 2 - tentacles; 3 - gastric cavity; 4 - mesoglea; 5 - radial channel; 6 - sail
Rice. 3Diagram of the structure of a hydroid jellyfish: 1 - mouth; 2 - oral stalk with gonad (3); 4 - radial channels; 5 - annular channel; 6 - tentacles; 7 - eyes; 8 - sail
Class Scyphoid jellyfish
This class includes jellyfish living only in the seas. They are larger than hydroid jellyfish, and their structure is more complex (Fig. 4). The mouth ends in a pharynx, the gastric cavity is divided into chambers. The annular canal, running along the edge of the body, unites the canals extending from the stomach, forming gastrovascular system. Clusters of nerve cells appear in the form ganglia. Sex cells are formed in gonads- sex glands located in the endoderm. Development proceeds with the alternation of generations (Fig. 5).
Rice. 4.Scheme of the structure of the scyphoid jellyfish: 1 - oral lobes; 2 - mouth opening; 3 - tentacles; 4 - annular channel; 5 - radial channel; 6 - gonad; 7 - gastric threads; 8 - stomach; 9 - ectoderm; 10 - mesoglea; 11 - endoderm
Rice. five.Development of the scyphoid jellyfish: 1 - egg; 2 - planula; 3 - scyphistoma; 4 - budding scyphistoma; 5 - strobilation; 6 - ether; 7 - adult jellyfish
Class Coral polyps
coral polyps have only one life form polyp. They have no alternation of generations. Marine, solitary, mostly colonial animals. Coral polyps differ from other classes in the presence of a hard calcareous skeleton, as well as muscle fibers in the ectoderm and endoderm, which allow them to change the shape of the body.
Wandering along the seashore, we often see ridges of greenish, brown or brown tangled lumps of hard threads thrown out by the waves. Very few people know that a significant part of this "sea grass" is not of vegetable, but of animal origin. Anyone who has been to the sea, of course, has seen that all the stones, piles and other underwater objects are overgrown with some kind of tender bushes wriggling in the waves. If you collect such bushes and look at them under a microscope, then along with real algae, you can see something very special. Here we have a brown jointed twig with pink lumps at the ends. At first, pink lumps are motionless, but after they stand still for a few minutes, they begin to move, stretch out in length, taking the form of a small jug with a rim of tentacles at the upper end of the body. These are hydroid polyps. eudendrium(Eudendrium), living in our northern seas, in the Black Sea and in the seas on Far East. Nearby is another, also jointed, but lighter branch. The polyps on it are also pink, but shaped like a spindle. The tentacles sit on the body of the polyp without any order, and each is equipped at the end with a small head - an accumulation of stinging cells. The movements of polyps are slow, they either bend their body, or slowly sway from side to side, but more often they sit motionless, spreading their tentacles wide - they lie in wait for prey. On some polyps, buds or young developing jellyfish can be seen. The grown-up jellyfish vigorously squeeze and unclench their umbrella, the thin thread connecting the jellyfish with the polyp breaks, and the jellyfish swims away in jerks. These are polyps Korine(Coryne) and their jellyfish. They also live in arctic and temperate seas.
And here is another bush, polyps on it sit inside transparent bells. Outwardly, they are very similar to eudendrium polyps, but behave completely differently. It is worth touching the polyp lightly with the end of the needle, as it is rapidly drawn into the depths of its protective shell, the bell. Jellyfish can also be found on the same bush: they, like polyps, are hidden inside a transparent protective shell. Jellyfish sit tightly on a thin, tentacleless polyp. It's a hydroid colony obelia(Obelia).
Now that we can distinguish hydroids from algae, we should pay attention to the feather-like colony. aglaophenia(Aglaophenia). In this species, which is very common in our Black Sea, feeding polyps sit on a branch in one row. Each is enclosed in a cup - gidroteka and surrounded by three protective polyps.
Aglaophenia does not form free-floating jellyfish, and underdeveloped individuals of the medusoid generation are hidden inside a very complex formation - a basket (a modified branch of the colony).
Colonies of hydroids settle most often at shallow depths - from the littoral to 200-250 m and prefer rocky soil or are attached to various wooden and metal objects. Often they grow very densely on the underwater parts of ships, covering them with a shaggy coat. In these cases, hydroids bring significant harm to navigation, since such a "fur coat" sharply 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 gap and prevented the supply of water. It is rather difficult to fight hydroids, since these animals are unpretentious and develop quite well, it would seem, in adverse 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 a dry dock. Here the ship is cleared of overgrown hydroids, polychaetes, bryozoans, sea acorns and other fouling animals.
IN Lately began to use special poisonous paints - the underwater parts of the ship covered with them are subject to fouling to a much lesser extent.
Hydroids that settle in the littoral zone are not at all afraid of the surf. In many of them, polyps are protected from blows by a skeletal cup - theca; on colonies growing in the surf zone itself, the theca is always much thicker than in the same species living deeper, where surf waves are not felt (Fig. 159).
In other hydroids from the surf zone, the colonies have a long, very flexible trunk and branches, or they are divided into segments. Such colonies meander along with the waves and therefore do not break or tear.
At great depths, special hydroids live, not similar to littoral species. Herringbone or feather-shaped colonies predominate here, many look like trees, and there are species resembling a brush. They reach a height of 15-20 cm and cover the seabed with dense forest. In the thickets of hydroids live worms, mollusks, crustaceans, echinoderms. Many of them, such as sea goat crustaceans, find refuge among hydroids, others, such as sea "spiders" (multi-legged), not only hide in their thickets, but also feed on hydropolyps.
If you move around the settlements of hydroids with a small-meshed net or, even better, use a special, so-called plankton net for this, then among the mass of small crustaceans and larvae of various other invertebrates, hydroid jellyfish will come across. Most types of hydrojellyfish are not very large animals, rarely they reach more than 10 cm in diameter of the umbrella, usually the size of hydrojellyfish is 2-3 cm, and often only 1-2 mm. Hydroid jellyfish are very transparent. You won’t even notice the jellyfish caught and placed in a glass dish right away: only whitish strings of channels and a mouth proboscis are visible. Only if you look closely, you can see the contours of the umbrella.
Looking at a hydroid colony Korine(Sogupe), we have already seen just budding small jellyfish of this species. A fully formed jellyfish has a bell-shaped umbrella 1-8 cm tall, four tentacles and a long, worm-like oral proboscis. With sharp contractions of the umbrella, the jellyfish quickly moves in a horizontal plane or rises up. Down it slowly sinks under the influence of gravity, frozen in the water with spread tentacles. Marine planktonic crustaceans, which make up the main food of the jellyfish, constantly make vertical movements: during the day they sink into the depths, and by night they rise to the surface. They descend into deeper, calmer layers of water also during waves. Jellyfish constantly move after them, two senses help them to pursue their prey - touch and sight. In calm water, the jellyfish's umbrella contracts rhythmically all the time, raising the animal to the surface. As soon as the jellyfish begins to feel the movement of water caused by the waves, its umbrella stops contracting and it slowly sinks into the depths. She distinguishes light with the help of eyes located at the base of the tentacles. Too bright light acts on it like excitement - the umbrella stops contracting and the animal plunges into a darker depth. These simple reflexes help the jellyfish to pursue prey and escape from the excitement that is fatal to it.
As mentioned above, Korine's jellyfish feeds on planktonic organisms, mainly copepods. The eyes of a jellyfish are not so perfect that she can see her prey, she catches it blindly. Its tentacles can stretch very significantly, surpassing the height of an umbrella by dozens of times. The entire surface of the tentacle is dotted with numerous stinging cells. As soon as a crustacean or some other small planktonic animal touches the tentacle, it is immediately affected by stinging cells.
At the same time, the tentacle quickly contracts and pulls the prey to the mouth. The long proboscis extends towards the prey. If a larger crustacean is caught, the jellyfish braids it not with one, but with two, three or all four tentacles.
Jellyfish with a flat umbrella and numerous tentacles catch their prey in a completely different way, for example thiaropsis(Tiaropsis) - a hydrojellyfish the size of a two-penny coin, very common in our northern seas. Along the edges of her umbrella are up to 300 thin tentacles. In a resting jellyfish, the tentacles are widely spaced and cover a significant area. When the umbrella is contracted, the jellyfish, as it were, sweeps away the crustaceans with them, fitting them to the middle of the underside of the umbrella (see Fig. 160). The mouth of the thiaropsis is wide, equipped with four large fringed lobes, with which the jellyfish captures fitted crustaceans.
Despite their small size, hydroid jellyfish are very voracious. They eat a lot of crustaceans and therefore are considered harmful animals - competitors of plankton-eating fish. Plentiful food is necessary for jellyfish for the development of reproductive products. Swimming, they scatter a huge number of eggs into the sea, which subsequently give rise to the polypoid generation of hydroids.
Above, we called the coelenterates typical inhabitants of the sea. This is true for 9000 species belonging to this type, but about one and a half to two dozen species of coelenterates live in fresh waters and are no longer found in the seas. Apparently, their ancestors moved to fresh waters a very long time ago.
It is very characteristic that all these forms of both freshwater and brackish-water basins refer only to hydroid class and even only to one subclass - hydroid(Hydroidea).
Among all other coelenterates, no tendency to low salinity water is observed.
The most typical inhabitants of the fresh waters of the entire globe, often forming very dense populations, include several species hydra, constituting detachment of hydra(Hydrida).
FRESHWATER HYDRA
In each group of the animal kingdom there are representatives beloved by zoologists, whom they use as the main objects in describing the development and structure of animals and on which numerous experiments in physiology are performed. In the type of coelenterates, the hydra serves as such a classic object. This is understandable. Hydra is easy to find in nature and relatively easy to maintain in the laboratory. They multiply rapidly, and therefore in a short time it is possible to obtain mass material. Hydra is a typical representative of intestinal animals standing at the base of the evolutionary tree of multicellular organisms. Therefore, it is used in clarifying all issues related to the study of anatomy, reflexes and the behavior of lower multicellular organisms. This, in turn, helps to understand the origin of higher animals and the evolution of their physiological processes. In addition, the hydra serves as an excellent object in the development of such general biological problems as regeneration, asexual reproduction, digestion, axial physiological gradient, and much more. All this makes it an indispensable animal for both educational process- from high school to senior courses of the university, and in scientific laboratory, where the problems of modern biology and medicine in their various branches are solved.
The first person who saw the hydra was the inventor of the microscope and the greatest naturalist of the 17th-18th centuries. Anton Levenguk.
Looking at aquatic plants, Leeuwenhoek saw among other small organisms a strange animal with numerous "horns". He also observed the growth of kidneys on his body, the formation of tentacles in them and the separation of the young animal from the mother's organism. Leeuwenhoek depicted a hydra with two kidneys, and also drew the tip of its tentacle with stinging capsules, as he saw it under his microscope.
However, Levenguk's discovery almost did not attract the attention of his contemporaries. Only 40 years later, hydra became interested in connection with the extraordinary discovery of the young teacher Trambley. In his spare time, studying little-known then aquatic animals, Tremblay discovered a creature that looked like both an animal and a plant. To establish its nature, Tremblay cut this creature in half. The regenerative abilities of lower animals were still almost unknown at that time, and it was believed that only plants could restore lost parts. To Tremblay's surprise, a whole hydra grew from each half, both of them moving, grabbing prey, which means it was not a plant. The possibility of turning a piece of the body of a hydra into a whole animal was perceived as a significant discovery in the science of life, and Tremblay engaged in a deep and serious study of the hydra. In 1744, he published the book "Memoirs on the History of a Genus of Freshwater Polyps with Horned Arms." The book described in great detail the structure of the hydra, its behavior (movements, catching prey), reproduction by budding, and some aspects of physiology. To test his assumptions, Tremblay did a series of experiments with the hydra, laying the foundation for a new science, experimental zoology.
Despite the imperfection of the then optics and the poor development of zoology, Tremblay's book was written at such a high scientific level that it has not lost its significance to this day, and the drawings from this book can be found in many textbooks on zoology.
Now the scientific literature on the hydra amounts to many hundreds of articles and books, but nevertheless, the hydra still occupies the minds of researchers. A small primitive animal serves as a touchstone for them, on which many issues are resolved. modern science about life.
If you collect aquatic plants in the coastal part of a lake or river and place them in an aquarium with clean water, then soon you can see hydras on them. At first they are almost invisible. Animals that are disturbed shrink strongly, their tentacles contract. But after some time, the body of the hydra begins to stretch, its tentacles lengthen. Now the hydra can be properly seen. The shape of her body is tubular, at the front end there is a mouth opening, surrounded by a corolla of 5-12 tentacles. Immediately below the tentacles, the hydras of most species have a slight narrowing - a neck that separates the "head" from the body. The posterior end of the hydra is narrowed into a more or less long stalk, or stalk, with a sole at the end (in some species, the stalk is not expressed). In the middle of the sole there is a hole, the so-called aboral pore. The gastric cavity of the hydra is solid, there are no partitions in it, the tentacles are hollow, finger-like gloves.
The wall of the body of the hydra, like that of all coelenterates, consists of two layers of cells, their fine structure has already been described above, and therefore here we will focus on only one feature of the cells of the hydra body, which has been fully studied so far only on this object and has not been found in others. coelenterates.
The structure of the ectoderm (and endoderm) in different parts of the body of the hydra is not equivalent. So, at the head end of the ectoderm cells are smaller than on the body, there are fewer stinging and intermediate cells, but a sharp boundary between the integuments of the “head” and the body cannot be drawn, since the change in the ectoderm from the body to the “head” occurs very gradually. The ectoderm of the sole of the hydra consists of large glandular cells; at the point of transition of the sole to the stalk, the glandular character of the integumentary cells is gradually lost. The same can be said about the cells of the endoderm. Digestive processes occur in the middle part of the body of the hydra, here its endoderm has a large number of digestive glandular cells, and the epithelial-muscular cells of the endoderm of the middle part of the body form numerous pseudopodia. Digestion of food does not occur in the head section of the gastric cavity, in the stalk and in the tentacles. In these parts of the body, the ectoderm has the appearance of a lining epithelium, almost devoid of digestive glandular cells. Again, a sharp boundary between the cells of the digestive section of the gastric cavity, on the one hand, and such cells of the "head", stalk and tentacles, on the other hand, cannot be drawn.
Despite the difference in the structure of the cell layers in different parts of the body of the hydra, all of its cells are not in strictly defined permanent places, but are constantly moving, and their movement is strictly regular.
Using the high ability of hydra to heal wounds, you can do this interesting experience. They take two hydras of the same size and one of them is stained with some kind of vital paint, that is, with such a dye that penetrates into the tissues of the hydra without killing it. Usually, a weak aqueous solution of nilblausulfate is used for this, which stains the tissues of the hydra blue. After that, the hydras undergo an operation: each of them is cut into three parts in the transverse direction. Then, the head and lower ends of the unpainted specimen are added to the middle part of the "blue" hydra. The sections quickly fuse with each other, and we get an experimental hydra with a blue belt in the middle of the body. Shortly after the operation, one can observe how the blue belt spreads in two directions - towards the head end and the stalk. At the same time, it is not the paint that moves along the body of the hydra, but the cells themselves. The layers of ectoderm and endoderm seem to “flow” from the middle of the body to its ends, while the nature of their constituent cells gradually changes (see Fig. 162).
In the middle part of the body of the hydra, the cells multiply most intensively, and from here they move in two opposite directions. Thus, the composition of the cells is constantly updated, although outwardly the animal remains almost unchanged. This feature of the hydra is of great importance in solving questions about its regenerative abilities and for assessing data on lifespan.
Hydra is a typical freshwater animal, only in very rare cases, hydras were found in slightly saline water bodies, for example, in the Gulf of Finland Baltic Sea, and in some brackish lakes, if the salt content in them did not exceed 0.5%. Hydras live in lakes, rivers, streams, ponds, and even ditches, as long as the water is clear enough and contains high amounts of dissolved oxygen. Hydras usually keep near the coast, in shallow places, as they are photophilous. When keeping hydras in an aquarium, they always move to its illuminated side.
Hydras are sedentary animals, most of the time they sit in one place, attaching their soles to a branch of an aquatic plant, a stone, etc. Hydra's favorite posture in a calm state is to hang upside down with its tentacles lowered somewhat apart.
The hydra is attached to the substrate due to the sticky secretions of the glandular cells of the ectoderm of the sole, and also using the sole as a sucker. The hydra is held very firmly, it is often easier to break it than to separate it from the substrate. If you watch a sitting hydra for a long time, you can see that its body is slowly swinging all the time, describing a circle with its front end. Hydra can arbitrarily leave the place it sits very quickly. At the same time, apparently, it opens the aboral pore located in the middle of the sole, and the suction action stops. Sometimes you can watch the hydra "walk". First, she bends the body to the substrate and strengthens on it with the help of tentacles, then pulls the posterior end and strengthens it in a new place. After the first “step”, he takes the second, and so on, until he stops at a new place.
Thus, the hydra moves relatively quickly, but there is another, much slower, way of moving - sliding on the sole. By the force of the muscles of the sole, the hydra barely noticeably moves from place to place. It takes a very long time to notice the movement of the animal. Hydras can swim in the water column for some time. Having detached from the substrate and spreading its tentacles widely, the hydra falls to the bottom very slowly, it is able to form a small gas bubble on the sole, which drags the animal upward. However, hydras rarely use these modes of locomotion.
Hydra is a voracious predator, it feeds on ciliates, planktonic crustaceans, low-bristle worms, and also attacks fish fry. Hydras lie in wait for their prey, hanging on some twig or stem of an aquatic plant, and, spreading their tentacles widely, constantly make circular search movements. As soon as one of the tentacles of the hydra touches the victim, the rest of the tentacles rush to it and paralyze the animal with stinging cells. Now there is no trace of the slowness of the hydra, it acts quickly and "decisively". The prey is pulled by tentacles to the mouth and quickly swallowed. Hydra swallows small animals whole. If the victim is somewhat larger than the hydra itself, it can also swallow it. At the same time, the mouth of the predator opens wide, and the walls of the body are strongly stretched. If the prey does not fit into the gastric cavity as a whole, the hydra swallows only one end of it, pushing the victim deeper and deeper as it digests. A well-fed hydra shrinks somewhat, and its tentacles contract.
In the gastric cavity, where digestive processes are just beginning, the reaction of the medium is slightly alkaline, and in the digestive vacuoles of the endoderm, where digestion ends, it is slightly acidic. Hydra can absorb fats, proteins and animal carbohydrates (glycogen). Starch and cellulose, which are of plant origin, are not absorbed by hydra. Undigested food remains are expelled through the mouth.
Hydras reproduce in two ways: vegetatively and sexually. Vegetative reproduction in hydras is budding. The buds arise in the lower part of the body of the hydra above the stem, the subsequent buds are slightly higher than the previous ones, sometimes they sit on opposite sides of the hydra's body, sometimes they are arranged in a spiral (the order of occurrence and location of the buds depends on the type of hydra). At the same time, 1 - 3, rarely more buds develop on the body of the hydra, however, hydras with 8 or more buds were observed.
At the first stages, the kidney appears as a barely noticeable conical tubercle, then it is pulled out, taking on a more or less cylindrical shape. At the outer end of the kidney, rudiments of tentacles appear, at first they look like short blunt outgrowths, but gradually stretch out, and stinging cells develop on them. Finally, the lower part of the body of the kidney thins into a stalk, and a mouth opening breaks through between the tentacles. The young hydra still remains connected to the mother's organism for some time, sometimes the next generation of buds are even laid on it. The separation of budding hydras occurs in the same sequence in which buds arise. The young hydra is somewhat smaller than the parent hydra and has an incomplete number of tentacles. Missing tentacles appear later.
After abundant budding, the mother hydra is depleted and no buds appear on it for some time.
Some researchers also observed the division of hydras, but this method of reproduction, apparently, should be classified as abnormal (pathological) processes. Hydra division occurs after damage to its body and can be explained by the high regenerative capacity of this animal.
With abundant nutrition, the entire warm period of the year, hydras reproduce by budding, they begin sexual reproduction with the onset of autumn. Most types of hydra are dioecious, but there are also hermaphrodites, that is, those in which both male and female germ cells develop on the same individual.
Gonads are formed in the ectoderm and look like small tubercles, cones or rounded bodies. The order of appearance and the nature of the location of the gonads are the same as those of the kidneys. Each female gonad produces one egg.
In the developing gonads, a large number of intermediate, undifferentiated cells accumulate, from which both future germ cells and "nourishing" cells are formed, due to which the future egg grows. At the first stages of egg development, intermediate cells acquire the character of mobile amoeboids. Soon one of them begins to absorb the others and increases significantly in size, reaching 1.5 mm in diameter. After that, a large amoeboid picks up its pseudopodia and its outlines are rounded. Following this, two divisions of maturation occur, in which the cell is divided into two unequal parts, and on the outer side of the egg there remain two small so-called reduction bodies - cells separated from the egg as a result of division. At the first division of maturation, the number of egg chromosomes is halved. The mature egg emerges from the gonad through a gap in its wall, but remains connected to the body of the hydra with the help of a thin protoplasmic stalk.
By this time, spermatozoa develop in the testes of other hydras, which leave the gonad and swim in the water, one of them penetrates the egg, after which cleavage immediately begins.
At the time when the cells of the developing embryo are dividing, on the outside it is dressed in two shells, the outer of which has rather thick chitinoid walls and is often covered with spines. In this state, the embryo, under the protection of the double shell-embryotheca, overwinters. (Adult hydras die with the onset of cold weather.) By spring, there is already an almost formed small hydra inside the embryotheca, which leaves its winter shell through a break in its wall.
Currently, about a dozen species of hydras are known that inhabit the fresh waters of the continents and many islands. Different types of hydras differ from each other very slightly. One of the species is characterized by a bright green color, which is due to the presence in the body of these animals of symbiotic algae - zoochlorella. Among our hydras, the most famous stalked, or brown, hydra(Hydra oligactis) and stemless, or - ordinary, hydra(Hydra vulgaris).
How does the hydra behave in its environment, how does it perceive irritations and respond to them?
Like most other intestinal cavities, hydra responds to any adverse irritation with a contraction of the body. If the vessel in which the hydras are sitting is slightly shaken, then some of the animals will shrink immediately, such a push will not affect others at all, some of the hydras will only slightly tighten their tentacles. This means that the degree of reaction to irritation in hydras is very individual. Hydra is completely devoid of the ability to "remember": you can prick it for hours with a thin pin, but after each contraction it is again pulled in the same direction. If the injections are very frequent, then the hydra stops responding to them.
Although hydras do not have special organs for the perception of light, they definitely react to light. The anterior end of the hydra is most sensitive to light rays, while its stalk almost does not perceive light rays. If you shade the whole green hydra, then it will shrink in 15-30 seconds, but if you shade the headless hydra or shade only the stalk of the whole hydra, then it will shrink only after 6-12 minutes. Hydras are able to distinguish the direction of the flow of light and move towards its source. The speed of movement of the hydra towards the light source is very low. In one of the experiments, 50 green and the same number of brown hydras were placed in a vessel at a distance of 20 cm from the glass wall through which the light fell. The first to move towards the light were the green hydras; after 4 hours, 8 of them reached the light wall of the aquarium, after 5 hours there were already 21 of them, and after 6 hours - 44. By this time, the first 7 brown hydras also came there. In general, it turned out that brown hydras went into the light worse, only after 10 hours 39 brown hydras gathered at the light wall. The rest of the experimental animals were still on the way by this time.
The ability of hydras to move towards a light source or simply move to lighter areas of the pool is very important for these animals. Hydras feed mainly on planktonic crustaceans - cyclops and daphnia, and these crustaceans always stay in bright and well-warmed places by the sun. Thus, going towards the light, hydras approach their prey.
For a researcher studying the reactions of lower organisms to light, hydras open up the widest field of activity. You can set up experiments to identify how sensitive animals are to weak or, on the contrary, very strong light sources. It turned out that hydras do not react at all to too weak light. Very strong light makes the hydra go to shaded places and can even kill the animal. Experiments were carried out to determine how sensitive the hydra is to changes in light intensity, how it behaves between two light sources, and whether it distinguishes between individual parts of the spectrum. In one of the experiments, the wall of the aquarium was painted in all colors of the spectrum, while green hydras gathered in the blue-violet region, and brown ones in the blue-green region. This means that hydras distinguish color, and their different species have different “tastes” for it.
Hydras (except green) do not need light for normal life. If they are well fed, they live well in the dark. The green hydra, in whose body symbiotic zoochlorella algae live, even with an abundance of food in the dark, feels bad and is greatly reduced.
On hydras, experiments can be carried out on the effects on the body of various kinds of harmful radiation. So, it turned out that brown hydras die after a minute of exposure to ultraviolet rays. The green hydra turned out to be more resistant to these rays - it dies only at the 5-6th minute of exposure.
Experiments on the effect of X-rays on hydras are very interesting. Small doses of X-rays cause increased budding in hydras. Irradiated hydras, compared with non-irradiated ones, give about 2.5 times more offspring in the same period. Increasing the dose of radiation causes suppression of reproduction; if the hydras receive too much X-rays, they die soon after. It is important to note that low doses of radiation increase the regenerative abilities of hydro.
When exposed to radioactive radiation, a completely unusual result was obtained. It is well known that animals do not feel radioactive rays in any way and therefore, once in their zone, they can receive a lethal dose and die. The green hydra, reacting to the radiation of radium, seeks to get away from its source.
From the above examples, it can be seen that such experiments with hydras, such as studying the influence of various environmental factors on them, are not empty fun, not science for the sake of science, but a serious and very important matter, the results of which can give very significant practical conclusions.
Of course, a study was made of the effect on the hydra of temperature, concentration carbon dioxide, oxygen, as well as a number of poisons, drugs, etc.
Hydra proved to be a very convenient object for carrying out a number of experimental studies on the study of the phenomenon of regeneration in animals.
As has been repeatedly mentioned, hydra easily restores lost body parts. The animal, cut in half, soon restores the missing parts. But it becomes unclear: why does a “head” with tentacles always grow at the front end of the segment, and a stalk at the back? What laws govern recovery processes? It is likely that some of these laws may be common to both the hydra and the more highly organized animals. Once you know them, you can important findings applicable even to medicine.
It is very easy to perform operations on hydras, it does not require any anesthetics or complex surgical instruments. All the equipment of the “operating room” consists of a needle inserted into a wooden handle, a sharp eye scalpel, small scissors and thin glass tubes. The first experiments to determine the regenerative abilities of the hydra were carried out more than 200 years ago by Tremblay. This painstaking researcher observed how whole animals arise from the longitudinal and transverse halves of hydras. Then he began to make longitudinal incisions and saw that stems were formed from the shreds in the lower part of the polyp, and “heads” were formed from the shreds in its upper part. Repeatedly operating on one of the experimental polyps, Tremblay received a seven-headed polyp. Having cut off all seven “heads” for him, Tremblay began to wait for the results and soon saw that a new one appeared in place of each cut off “head”. The seven-headed polyp, in which severed “heads” grow again, was like two drops of water similar to a mythical creature - the Lernean hydra, slain by the great hero ancient greece Hercules. Since then, the name hydra has been preserved behind the freshwater polyp.
Along the way, Tremblay found that the hydra is restored not only from halves, but also from very small pieces of the body. Now it has been established that even from 1/200 of the body part of the hydra, a whole polyp can develop. However, later it turned out that the regenerative ability of such small pieces from different parts of the hydra's body is not the same. A section of the sole or stalk is restored into a whole hydra much more slowly than a section from the middle part of the body. However, this fact remained unexplained for a long time.
The internal forces regulating and directing the processes of normal regeneration were discovered much later by the famous American physiologist Child (CM. Child). Child found that in a number of lower animals there is a pronounced physiological polarity in the body. So, under the influence of toxic substances, cells on the body of an animal die and are destroyed in a very definite sequence, namely from the front end to the back (in hydra from the “head” to the “sole”). Therefore, the cells located in different parts of the body are physiologically unequal. The difference between them lies in many other manifestations of their physiology, including the effect on developing young cells at the site of injury.
The gradual change in the physiological activity of cells from one pole to another (along the axis of the body) is called the axial physiological gradient.
Now it becomes clear why the pieces cut from the sole of the hydra are very slowly restored by the hypostome and tentacles - the cells that form them are physiologically very far from the cells that form the "head". The axial gradient plays a very important role in regeneration, but other factors also have a noticeable influence on this process. During regeneration, the presence on the regenerating part of a developing kidney or an artificially transplanted area of tissue from another part of the animal's body, especially from its anterior part, is of great importance. Possessing high physiological activity, the developing kidney or cells of the "head" in a certain way affect the growth of regenerating cells and subordinate their development to their influence. Such groups of cells or organs that make their own adjustments to the action of the axial gradient are called organizers. The elucidation of these features of regeneration helped to understand many obscure questions in the development of the animal organism.
IN largest center physiology - in the institute created by Academician Pavlov in Koltushi there is a monument to a dog. Most of the laws set forth in Pavlov's teachings were discovered during experiments on dogs. Perhaps a small freshwater polyp deserves the same monument.
FRESHWATER MEDUSA
In 1880, jellyfish suddenly appeared in the pool with tropical plants of the London Botanical Society. Immediately two zoologists Lankester (Lankester) and a major connoisseur of coelenterates Olmen (A1man) reported this find in the pages of the journal "Nechur" ("Nature"). The jellyfish were very small, the largest of them barely reaching 2 cm in diameter of an umbrella, but their appearance excited the then zoologists: before that, they had not imagined that freshwater jellyfish could exist. Jellyfish were considered typical inhabitants of the sea. Shortly before this, the magnificent South American aquatic plant Victoria Regia had been planted in the pool, so it has been suggested that the jellyfish were brought to London along with planting material from the Amazon. After a while, the jellyfish disappeared from the pool as mysteriously as they appeared. They were discovered again only five years later, also in London, but in a different pool with the same tropical plant. In 1901, these jellyfish appeared in Lyon (France), also in the greenhouse pool with Victoria Regia. Then they began to be found in Munich, Washington, St. Petersburg, Moscow. Jellyfish were found either in the pools of botanical gardens or in aquariums with tropical fish. To the surprise of amateur aquarists, they suddenly have new pets. Tiny jellyfish (often only 1 - 2 mm in diameter of an umbrella) suddenly appeared in large numbers in an aquarium in which there had not been a single one the day before. For several days it was possible to observe how the jellyfish move in jerks in the water and willingly eat small crustaceans. But one fine day, looking into his aquarium, the owner found only fish in it, there were no jellyfish there.
By this time, the freshwater jellyfish had been described in detail in specialized zoological literature. It turned out that she belonged to hydroid class. They called her kraspedakustoy(Craspedacusta). The smallest jellyfish have a hemispherical umbrella, 4 radial canals and 8 tentacles. As the jellyfish grows, the shape of its umbrella becomes more and more flat, and the number of tentacles increases.
Sexually mature jellyfish reach 2 cm in diameter and carry a wide sail along the edge of the umbrella and about 400 thin tentacles seated with stinging cells. The oral proboscis is tetrahedral, with a cruciform mouth opening, the edges of the mouth are slightly folded. At the point of origin of the radial canals from the oral proboscis, 4 gonads develop. Jellyfish are very transparent, their mesoglea is colorless, and their tentacles, radial canals, oral proboscis and gonads are whitish or cream in color.
This jellyfish made a wish to zoologists difficult riddle. If we agree with the opinion that it gets into greenhouses along with plants from the tropics, then how does it survive during transportation? Victoria-regia was transported from the shores of the Amazon in the form of seeds or rhizomes. Delicate jellyfish, accidentally captured along with rhizomes, must surely die during the long journey across the ocean. But even if we assume that the jellyfish, despite drying out, can survive, then how does it get into small aquariums of exotic fish lovers?
Soon, jellyfish began to be found in natural reservoirs. The first time she was caught in the Yangtze River in China, then in Germany, then in the USA. However, both in natural and artificial reservoirs, the finds were very rare and always unexpected: for example, once a jellyfish was found in the vaults of the Washington water supply.
Observations on the jellyfish made it possible to establish that it buds FROM tiny tentacleless polyps, called microhydras(Microhydra). These polyps were found back in 1884 in the same pools in London where jellyfish were also caught, but then no one suspected a connection between these two such dissimilar creatures. Microhydra polyps are visible to the naked eye as white dots against the background of green leaves of aquatic plants on which they usually settle. Their height usually does not exceed 0.5-1 mm, the shape of the body resembles a skittle: the body is in the form of a bottle, and a spherical “head” with a mouth in the middle sits on a short neck. The head is densely seated with stinging cells, there are no tentacles. Polyps sometimes form primitive colonies of 2-7 individuals. Microhydra reproduces by budding and forms tentacleless polyps similar to itself. From time to time, a group of cells shaped like a small worm separates from one side of the polyp's body. Such groups of cells are called frustules. Frustula is able, wriggling, to crawl along the bottom and climb onto aquatic plants, here it turns into a young microhydra.
Once it was possible to observe how a jellyfish began to develop on the body of a microhydra from a kidney; when she separated from the polyp and began to swim, it was easy to recognize a young craspedacusta in her. It was also possible to follow the development of craspedacusta eggs. Initially, a worm-like larva is formed from the egg, devoid of cilia and very similar to the microhydra frustula. After some period of crawling along the substrate, the larva attaches to it and turns into a tentacleless polyp. Thus, it was established that the jellyfish kraspedakusta and the polyp microhydra belong to the same species of coelenterates, but to its different generations.
The experiments carried out showed that at the change of generations this type of hydroid is extremely big influence provided by environmental conditions. Budding of jellyfish on polyps occurs only at a water temperature not lower than 26-33°C, and budding of polyps and separation of frustules occurs at a temperature of 12-20°C. After that, it became clear that the existence of a species can be maintained for a long time due to the reproduction of polyps. Neither aquarists nor botanists in greenhouses pay attention to small motionless microhydras, since they are almost invisible to the naked eye, it is very difficult to find them in nature. Polyps can live for a long time in an aquarium, and when the temperature rises, all polyps develop jellyfish buds and they separate jellyfish. Kraspedakusta jellyfish are mobile and can be seen in the water with the naked eye. Now it becomes clear why they were almost always found in pools with tropical plants and fish: these pools were artificially heated. Only one thing is unclear: have jellyfish always lived in Europe or were they brought there? (Polyps may be able to tolerate some desiccation and a long journey in adverse conditions.) And where is the homeland of microhydra-craspedacusta?
It is rather difficult to answer this question. Since the first discovery of jellyfish in London, over 100 cases of their presence in various parts of the world have been described. Here short description distribution of the species. In the USSR, their habitat is the Lyubov reservoir near Tula, the Don River, Lake Karayazi near Tbilisi (at an altitude of almost 2000 m above sea level), the Kura River, and artificial reservoirs in Old Bukhara. In addition, jellyfish and polyps have repeatedly appeared in aquariums of amateur fish farmers and at the universities of Moscow and Leningrad. Outside of our country, this species was found in almost all European countries, in India, China and Japan, in Australia, North and South America. It is impossible to indicate now where his homeland is and where he was brought.
More recently, this type of coelenterates again made zoologists think. Now that the distribution, lifestyle, structure of polyps and jellyfish seemed to be well studied, it suddenly became clear that polyps of two genera can develop from the eggs of Kraspedakusta - the tentacleless and tentacled polyps described above. Both genera of polyps form frustules. Tentacle-bearing polyps with the help of budding form similar to themselves and tentacleless polyps, they cannot bud jellyfish. Tentacleless polyps form polyps and jellyfish similar to themselves, but are not able to bud polyps equipped with tentacles. Both forms of polyps are formed from frustules. Tentacle-bearing polyps have so far been discovered only twice: in 1960 in Hungary and in 1964 in the aquarium of Leningrad University. The conditions that cause their appearance are not yet clear. In the rivers of India and the great lakes of Africa, there are two more species of freshwater jellyfish, close relatives of kraspedakusta. A well-known jellyfish from the African Lake Tanganyika, called limnocnida(Limnocnida tanganjice).
ORIGIN OF FRESHWATER CELESTONES
Of these hydroids, first of all it is necessary to say about Cordylophora.
Cordylophora forms small, tender colonies in the form of bushes up to 10 cm high. Polyps sit at the ends of branches and have a spindle shape. Each polyp has 12-15 tentacles, sitting without a strict order in the middle part of the body. There are no free-swimming jellyfish in Cordylophora; individuals of the jellyfish generation are attached to the colony.
This species was first discovered by Academician of the Russian Academy P.S. Pallas in 1771 in the northern part of the Caspian Sea, because cordylophora and is called the Caspian (Cordylophora caspia). However, its distribution is by no means limited to this basin, it lives in the Baltic, Black and Azov seas, and is also found along the entire Atlantic coast of Europe and at the mouths of all major rivers in Asia, America and Australia. This species settles only in heavily desalinated areas of the sea and lives at shallow depths, usually no deeper than 20 m.
The name given by Pallas to Cordylophore - Caspian - has its own meaning. The fact is that the birthplace of Cordylophora is the Caspian Sea. Only in the middle of the last century, Cordylophora penetrated the Baltic Sea along the Volga and the Mariinsky system, where, due to its low salinity (0.8%), it found its second home. Cordylophora is an organism that grows; it settles on all solid underwater objects, both fixed and moving. Further assistance in resettlement was provided by countless ships flowing from all sides into the Baltic Sea. Returning home, they took away from the Baltic Sea on their bottom an intruder, a “violator of borders”.
But how did free-living coelenterates get into fresh water bodies? Can they not use the mouths of the rivers flowing into the sea for this? Of course they can, but in doing so they have to overcome two hurdles. One of them is a decrease in salinity. Only species that can withstand very significant desalination can enter rivers.
Among typical marine life, there are those on which even the slightest decrease in the percentage of salt in sea water has a detrimental effect. These include almost all coral polyps, scyphoid jellyfish and most hydroids. But some hydroids can still exist even with some desalination. Of the coelenterates mentioned in this book, Corine belongs to the euryhaline. This species can live both in water with normal oceanic salinity and in desalinated seas, for example, in the White and Black.
From among the euryhaline species, those whose descendants actively made their way into freshwater reservoirs came out. The process of conquering rivers and lakes proceeded gradually. At first, a group of brackish-water hydroids stood out, which could no longer return to the ocean, since they could not stand the high salinity of its waters. Then already brackish-water came close to the mouths of the rivers. Not all of them were able to overcome this "barrier", most of them remained in the river mouth. Cordylophora is currently following this path.
Once in the river, marine animals met on their way another "barrier" - the current. With the active penetration of marine or brackish coelenterates into fresh waters, they inevitably had to overcome the oncoming flow of water, which carried planktonic jellyfish and attached polyps or their colonies incapable of independent movement back into the sea. Promotion of such attachment polyps towards the current was difficult.
In remote geological epochs, the map of the Earth was different from what we see it now. In many places, the modern land was covered by the sea. When the sea left, closed salt pools remained, and marine animals were preserved in them. Some of these pools gradually desalinated, and the animals either died or adapted to new conditions. The now closed Caspian Sea, which is essentially a huge brackish lake, was formerly connected to the ocean, and many animals of marine origin have been preserved in it. Among them is an interesting coelenterate - Pallas Merizia(Moerisia pallasi). This type of hydroid has two forms of polyps: some live in a colony at the bottom, others lead a planktonic lifestyle. Floating polyps form colonies of two individuals connected to each other by their legs. From time to time, the colony is torn in half, and at the site of the rupture, each polyp forms a new corolla, tentacle and mouth. In addition, polyps also reproduce by budding, separating small free-swimming jellyfish from themselves. One close species of merizia lives in the Black and Azov Seas, the other in the salt lakes of Northeast Africa.
It is quite clear that all three types of merisia originated from one common ancestor that once lived in the ancient Sarmatian Sea. When the Sarmatian Sea left, a number of reservoirs remained in its place, including the closed Caspian Sea and the lakes of Egypt. They developed independent types of merizia.
If you imagine that the desalination of the reservoir goes even further, then you can understand how freshwater jellyfish can arise. Their way of conquering freshwater basins is a long-term adaptation to increasing desalination. At the same time, they do not need to move anywhere, they make their way from the sea to fresh water not in space, but in time.
In 1910 on the Atlantic coast North America several small hydrojellyfish were caught. It turned out that they belong to a previously unknown species. By itself, this fact is of little importance. And now several new species of coelenterates are described annually - there is still a lot of unexplored in the sea. Another thing is interesting. This jellyfish is named blackfordia(Blackfordia) - after 15 years was caught in the Black Sea. Neither in the Mediterranean Sea, whose fauna is very well known, nor on the European coast of the Atlantic Ocean does this species live. How did the American Blackfordia end up in the Black Sea? The second incident happened very recently. One of the types of hydroids that live in the Kiel Canal is bougainvillea- was unexpectedly discovered again in the Black Sea. And blackfordia and the mentioned Baltic hydroid(Bougainvillia megas) - brackish water species; in order to get from one basin with low salinity to another, they must, like Cordylophora, overcome an obstacle - the sea with its high salinity.
Before the canal was built between the Volga and the Don, there were only two types of coelenterates in the Caspian Sea - the Caspian Merizia and the Cordylophora. When the canal was ready and navigation began, three more species moved from the Azov-Black Sea basin to the Caspian Sea. A year after the canal was put into operation, blackfordia moved to the Caspian Sea, a year later the Black Sea merizia, and after it the Baltic hydroid (Bougainvillia megas), which had recently entered the Black Sea from the Kiel Bay. Of course, not only coelenterates travel this way, but also molluscs, and crustaceans, and worms, and other brackish-water organisms.
“SAILING FLEET”
Hydroid class is divided into two subclasses - hydroids And siphonophore. We now turn to the description of these amazing pelagic colonial coelenterates.
A whole world of living beings lives on the verge of two elements - water and air. On floating algae, fragments of wood, pieces of pumice and other objects, you can find a variety of adherent or firmly clinging animals. You should not think that they got here by accident - "in distress." On the contrary, many of them are closely connected with both the water and the air environment, and in other conditions they cannot exist. In addition to such "passive passengers", here you can also see animals actively swimming near the surface itself, equipped with variously arranged organs - floats, or animals that are held using a film of surface tension of water. This whole complex of organisms (pleuston) is especially rich in the subtropics and tropics, where the destructive effect of low temperatures is not felt.
Above, when it came to the action of stinging cells, the “Portuguese warship” was already mentioned - a large siphonophore physalia(Physalia, see color table 8).
Like all siphonophores, physalia is a colony, which includes both polypoid and medusoid individuals. An air bubble rises above the surface of the water, OR a pneumatophore, a modified medusoid individual of the colony. In large specimens, the pneumatophore reaches 30 cm. It usually has a bright blue or reddish color. An air bubble floats on the surface of the sea like a tightly inflated rubber balloon. The gas filling it is close in composition to air, but differs in an increased content of nitrogen and carbon dioxide and a reduced amount of oxygen. This gas is produced by special gas glands located inside the bladder. The walls of the pneumatophore can withstand rather strong gas pressure, as they are formed by two layers of ectoderm, two layers of endoderm and two layers of mesoglea. In addition, the ectoderm secretes a thin chitinoid membrane, due to which the strength of the pneumatophore also increases significantly, although its walls remain very thin. Top part The pneumatophore has an outgrowth in the form of a comb. The crest is located on the pneumatophore somewhat diagonally and has a slightly curved S-shape. All other individuals of the colony are located on the underside of the pneumatophore and are submerged in water.
Feeding polyps, or gastrozoids, sit in one row. They are more or less bottle-shaped and face down with their mouths. Each feeding polyp is equipped with one long tentacle - a noose. Throughout its length, the noose is densely covered with stinging cells. Next to each feeding polyp on the underside of the bubble is attached the base of the gonodendra - an individual of the polypoid generation. On the gonodendra and its lateral outgrowths there are clusters of reduced medusoid individuals - gonophores, in which reproductive products develop. Protective tentacleless polyps - palpons - also sit here. Each gonodendra has one medusoid, called a nektophore or swimming bell. Sex cells in the nektophore are not formed, and its umbrella reaches a significant size and is able to contract, like in free-swimming jellyfish. Before the onset of puberty, the gonophores break away from the colony and float near the sea surface, with the nectophore performing locomotor functions.
Due to the oblique arrangement of the ridge on the swim bladder, the physalia is asymmetric, and two forms of physalia are known - “right” and “left”, which are, as it were, a mirror image of each other. It was noticed that all physalians living in one part of the sea have the same structure, that is, they are all either “right” or “left”. In this regard, it has been suggested that there are two species or two geographical races of Physalia.
However, when they began to study the development of these siphonophores, it was found that among the offspring of one physalia there is always an equal number of both “right” and “left”. This means that there are no special races for physalia. But how do clusters of "left" and "right" siphonophores arise, and why do these two forms not occur together?
The answer to this question was obtained after a detailed study of the structure of the air bladder of physalia. It turned out that the shape and location of the crest at its top are very important for physalia. As mentioned above, the crest of the physalia is slightly curved in the shape of the letter S. The physalia moves along the surface of the sea due to the fact that the wind hits its air bubble. If there were no ridge, the siphonophore would constantly move in a straight line and would eventually be washed ashore. But the presence of a crest makes significant changes to the sailing equipment of the "Portuguese boat". An obliquely set and curved crest makes the animal swim at an acute angle to the wind and from time to time make a turn around its axis against the wind.
If you observe a physalia floating near the shore, towards which the wind is blowing, you can see how it either approaches the shore, then, unexpectedly turning to the observer on the other side, slowly sails away from him. Entire armadas of “Portuguese ships” maneuver in this way, reminiscent of the actions of the sailing fleet of the period of medieval wars. When moving, the "right" and "left" "Portuguese boats" behave differently. Under the influence of the wind blowing in one direction, they diverge in different directions - "right" to the left, and "left" to the right. That's why there are clusters of identical forms of physalia.
Pleistonic organisms also include very peculiar coelenterates - porpita(Porpita) and velella(Velella), which is also called a sailboat.
For a long time, these animals were classified as siphonophores, and their individual appendages were considered specialized individuals of the colony. Now more and more zoologists are inclined to believe that the porpita and the sailboat are not a colony, but a large single floating polyp, and classifies them as order chondrophore(Chondrophora) of hydroid class. Their body is flattened; in porpita it has the shape of a circle, in a sailboat it is oval. The upper side of the disc is covered with a chitinoid membrane, under which is placed a complex air bell - pneumatophore. It consists of a central chamber, a large number of annular chambers surrounding it and thin tubes extending from them to all parts of the body - trachea, which are used for breathing. On the underside of the disk are the organs of the polyp. There is a mouth cone in the center, and numerous tentacles are located along the periphery. Between the mouth cone and the tentacles there are special outgrowths of the body - gonodendria, on which individuals of the medusoid generation bud. The upper side of the coastal porpita disc is smooth; a velella living in the open ocean has a high triangular outgrowth on it - a sail. The sail of the velella has the same meaning as the crest on the air bladder of the physalia. It is located asymmetrically and slightly S-shaped on the oval body of the sailboat. The sail allows the animal to move not in a straight line, but to maneuver, although, of course, not arbitrarily, but more or less randomly.
In the subtropical parts of the ocean, where the temperature does not fall below 15 ° C, sailboats are found in very large numbers. In some places, these large coelenterates (they reach 12 cm along the long axis of the disk) gather in huge flocks several tens of miles long, and for each square meter The surface of the ocean is accounted for by the sailboat. Together with large sailboats, young ones also swim, the size of which is measured in millimeters.
The wind, striking the sail, drives a flock of velles across the sea, and they can travel many hundreds of miles.
Living in the open ocean, sailboats are not afraid of water: they cannot drown, as they have a very perfect pneumatophore, consisting of a large number of independent chambers. If the wave nevertheless overturns the velella, then with the help of movements of the edges of the disk, it assumes a normal position and again exposes the sail to the wind. In addition to sailboats, you can also meet many other animals here, which, however, are almost invisible at first.
It is well known that the high seas of the tropics have an intense blue color. In this regard, sailboats and most of the animals that live with them are also painted in blue or blue tones - this serves as a good protection for them.
Sailboats and other animals living among them create a special, closely connected little world in the open sea - a pleistonic biocenosis, which, by the will of the current and wind, floats all the time on the surface of the ocean.
Velella, like all coelenterates, is a predator; it feeds on plankton, its food includes crustaceans, larvae of various invertebrates, and fish fry. All other animals that are part of the floating biocenosis either feed on sailboats or use them as a permanent or temporary substrate for attachment. Thus, the entire biocenosis exists due to plankton, but only sailboats directly use plankton.
On the upper side of the velella disk, as on the deck of a ship, small blue crabs travel planes(Planes). Here they find protection from enemies, and also get food. A hungry crab quickly moves to the underside of the sailboat disk and takes away the captured planktonic crustaceans from it. Having eaten, the crab again climbs onto the upper side of the disk and settles under the sail, closely clinging to it. Crabs never devour their ship, which is not the case with many other pleustian animals.
On the underside of the sailboat, you can often find the predatory gastropod mollusc Janthina. Yantins eat soft tissues until only a chitinoid skeleton remains from the sailboat. Having lost its support, the yantina does not sink, as it is well adapted to life near the surface of the water. As soon as the eaten sailboat begins to sink, the yantin releases abundant mucus, which forms air-filled bubbles. This mucus hardens very quickly, and a good float is obtained, on which the mollusk can swim independently, moving from one sailboat to another. Swim to new victim, the yantina leaves the now unnecessary float and quickly crawls onto the velella. An abandoned yantina float is soon colonized by hydroids, bryozoans, sea ducks and other attached animals, as well as small crabs; sometimes they settle on the shell of the mollusk itself.
Together with yantinops, another predatory mollusk settles on sailboats - the nudibranch aeolis (Aeolis).
Sometimes, next to the sailboat, you can see the accompanying nudibranch mollusks (Glaucus). The body of this shellless mollusk is elongated, fish-like, on the sides there are three pairs of branched tentacle-like outgrowths, with the help of which the mollusk is attached to the surface film of water. It swims with a dark blue ventral side up, its dorsal side is silvery white. This makes floating glaucus invisible from both air and water. Hungry glaucus, raking tentacle-like outgrowths, swims up to the sailboat and, holding on to it, pulls out and eats large pieces of the edge of the disk.
Eaten by mollusks, sailboats die, but they leave a chitinoid skeleton, in which the system of air chambers is still preserved. Such dead sailboats float for some time on the surface, and larvae of barnacles - sea ducks (Lepas fasciculatus) settle on them. As new settlers grow, the skeleton of the sailboat sinks deeper and deeper, and on the leg, with which the sea duck is attached to the substrate, an additional spherical float develops, which increases the buoyancy of the crustacean.
All free-living barnacles are attached animals, the only exception is the species of sea duck mentioned above. When its spherical float reaches a significant size, it separates from the sailboat, and after that the sea duck can independently float on the surface of the water and even swim, waving its legs. In the rest of the barnacles, the flapping of the legs drives food to the crustacean - small planktonic organisms, but this species of sea duck, unlike all its relatives, leads a predatory lifestyle. Swimming to the sailboat, the sea duck grabs the edge of its disk with its legs and, moving along the edge, quickly eats away a significant part of the velella.
In addition to the animals described here, the velella biocenosis also includes some shrimp, eyelash worms, water strider bugs and a number of other animals, including one species of flying fish, Prognichthys agae, which lays eggs on sailboats. Water strider bugs halobates live in close contact with velella and porpita, using them both as a "pie" and as a "raft".
The little world of velella floating in the open ocean is very limited, but all its inhabitants are closely connected with each other. It is interesting to note that most of the species that make up this biocenosis belong to such groups of animals that usually lead a benthic lifestyle. Based on this, it can be said with certainty that Pleistonic animals originate from benthic (rather than planktonic) organisms that have lost contact with the bottom and began to attach themselves to various floating objects or use the surface film of water as a support.
Animal life: in 6 volumes. - M.: Enlightenment. Edited by professors N.A. Gladkov, A.V. Mikheev. 1970 .
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