The meaning of protozoa in nature and human life. Biology at the Lyceum

Protozoa are a source of food for other animals. In the seas and in fresh waters protozoa, primarily ciliates and flagellates, serve as food for small multicellular animals. Worms, mollusks, small crustaceans, as well as the fry of many fish feed primarily on single-celled organisms. These small multicellular organisms, in turn, feed on other, larger organisms.

The largest animal that has ever lived on Earth, the blue whale, like all other baleen whales, feeds on very small crustaceans that inhabit the oceans. And these crustaceans feed on single-celled organisms. Ultimately, whales depend on single-celled animals and plants for their existence.

Protozoa are participants in the formation of rocks. Examining a crushed piece of ordinary writing chalk under a microscope, you can see that it consists mainly of the smallest shells of some animals. Marine protozoa (rhizopods and radiolarians) play very important role in the formation of marine sedimentary rocks.

Over many tens of millions of years, their microscopically small mineral skeletons settled to the bottom and formed thick deposits. In ancient geological epochs, during the mountain-building process, the seabed became dry land. Limestones, chalk and some others rocks largely consist of the remains of the skeletons of marine protozoa. Limestones have long had a huge practical significance How construction material.

Protozoa, living in the oceans, fresh waters, soil and higher organisms, occupy an important place in the cycle of substances in the biosphere. In the aquatic environment, protozoa are the basis of plankton, used as food by other larger animals. Thick layers of sedimentary rocks are formed from the skeletons of protozoa: foraminifera, radiolarians and armored flagellates - coccolithophores.

Many aquatic protozoa - sedimentators that feed on suspended organic particles and bacteria - play a significant role in the biological purification of water. Soil amoebas, ciliates and flagellates are an important part of the soil fauna: they take part in soil formation. A number of species of protozoa constitute a useful group of symbionts of higher animals that improve digestion and metabolic processes in the body.

More than 200 species of flagellates live in the stomachs of termites, converting fiber into sugar.

Collared flagellates are possible ancestors of multicellular animals.

Slipper ciliates serve as food for the inhabitants of reservoirs.

In the seas and oceans, many shell rhizomes die every hour. Falling to the bottom, they form limestone deposits. The chalk used to write on a blackboard, whiten the walls and ceilings of buildings, and the limestone used to build houses consist mainly of the shells of marine protozoa. Using the remains of accumulations of shells of extinct marine unicellular animals, geologists find places of deposits of oil and other minerals.

Food chains in aquatic ecosystems begin with microscopic algae. The second link in them is usually planktonic protozoa - the first consumers of green products. Then they become the basis of nutrition for the animal-eating inhabitants of aquatic ecosystems: crustaceans, fish fry and all subsequent consumers. When the remains of dead plants and animals sink to the bottom, they are picked up by bottom-dwelling protozoa.

Many protozoa inhabit every millimeter of soil saturated with ground moisture. Together with other inhabitants, they maintain soil fertility.
The irony of fate: herbivorous animals themselves are not able to digest cellulose (fiber - note! Protozoa do this for them, populating their food tract from the first days of life. The intestine of a termite, the cecum of a hare and the stomach of a cow are equipped with special warehouses to accommodate these cohabitants. The owner Assimilates only the result of their digestion, and at the same time the protozoa themselves.

Protozoa are a source of food for other animals. In the seas and fresh waters, protozoa, primarily ciliates and flagellates, serve as food for small multicellular animals. Many worms, mollusks, small crustaceans, as well as the fry of many fish, feed primarily on unicellular organisms; Without protozoa, their existence would be impossible. These multicellular animals, in turn, feed on larger animals, and primarily on growing fish fry. Hence the enormous importance of protozoa in the life of nature and in national economy.

The largest animal that has ever lived on Earth, the blue whale, feeds on very small crustaceans that inhabit the oceans. Other toothless whales also feed on them. And these crustaceans, in turn, feed on small animals. Ultimately, whales depend on single-celled animals and plants for their existence.

Protozoa are participants in the formation of rocks. Examining a crushed piece of ordinary writing chalk under a microscope, you can see that it consists mainly of small shells of some animals. Many calcareous rocks of the Volga region, the Urals, Crimea, and the Caucasus also consist of microscopic shells. Each such shell once contained the body of a simple animal - a foraminifera, which lived in ancient times on the bottom of seas and oceans. Many limestones consist almost entirely of shells of various foraminifera. Limestone has long been of great practical importance as a building material. For example, gigantic ancient structures - the Egyptian pyramids - were built from them.

And currently, a significant part of the ocean floor is covered with silt consisting of foraminifera shells.

Foraminifera are the simplest animals; they are closest to amoebas. Their different types differ in the structure of the calcareous shell, inside which protoplasm with nuclei is placed. Often the shell is spiral and multi-chambered inside. In the partitions between the chambers there are openings through which the protoplasm located in adjacent chambers communicates. Latin word"foramen" means "hole", hence the name "foraminifera" ("hole-bearing").

Remains of foraminifera in rocks are of great importance in geological exploration: the discovery of certain types of foraminifera in limestone indicates the proximity of oil-bearing layers.

It must be borne in mind, however, that not all limestones consist of protozoan shells. Many limestones are formed by the remains of coral skeletons, mollusk shells, etc.

Protozoa are an indicator of the degree of pollution of fresh water bodies. The fight against water pollution is the most important state task. Each type of protozoan animal requires certain conditions to exist. Some protozoa can only live in clean water containing a lot of dissolved air and not polluted by waste from factories and factories; others are adapted to life in water bodies of moderate pollution. Finally, there are protozoa that can live in very polluted wastewater. Being in a body of water certain type protozoa makes it possible to judge the degree of its contamination, after which measures can be prescribed to clean it.

The cause of malaria is that germs of the malarial plasmodium enter the human blood. They invade red blood cells (erythrocytes), feed on them and, as a result, destroy them. When multiplying in human blood, plasmodia infect a huge number of red blood cells, which leads to severe anemia.

Although malaria is a contagious disease, a healthy person cannot become infected directly from a malaria patient. It is transmitted from a sick person to a healthy person special types mosquitoes - the so-called malaria mosquitoes (Anopheles).

Plasmodium malaria goes through a complex development cycle in the human blood and in the mosquito body. If a malaria mosquito sucks the blood of a person with malaria, malaria plasmodia will enter the mosquito's intestines. They multiply in the mosquito's body and a large number of Plasmodium embryos are collected in salivary glands mosquito Such an infected mosquito is dangerous to humans. Sucking the blood, he, with his saliva, introduces the germs of malarial plasmodium into the blood of a healthy person. Currently, in the USSR, thanks to the measures taken, the incidence of malaria has decreased significantly (see article "").

Of the animal diseases caused by trypanosomes, the most dangerous disease within the USSR is suauru disease, which kills camels and horses in the Lower Volga region and Central Asia. The causative agent of suauru is transmitted by horseflies and some blood-sucking flies.

So, protozoa are of great importance in the life of nature, in human life and in the national economy. Some of them are not only useful, but also necessary; others, on the contrary, cause very dangerous diseases.

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1. A source of food for other animals. (They form the first link in the food chain).

2. They act as orderlies, cleaning water bodies from bacteria and rotting substances.

3. Serve as indicators of water purity.

4. Promote geological exploration and serve as guidelines for oil and gas exploration.

5. Participate in the formation of limestone deposits.

6. Participate in the cycle of substances.

7. They influence soil-forming processes.

Pathogens of diseases in domestic animals and humans.

Protozoa - the creators of sushi

Word protozoa We usually associate it with tiny, invisible lumps of protoplasm. They live, feed, reproduce, but what do we care about them - they are so tiny? Few people know that it is to the simplest that we owe the emergence of entire layers of geological rocks, and often mountain ranges!

Freshwater testate amoebae protect their body with a shell of silicate or calcareous plates secreted by the cytoplasm onto the cell surface. In Arcella, the shell has the shape of a saucer, in the center of which there is an orifice - a hole through which the pseudopods of the amoeba protrude out. Difflugia uses microscopic grains of sand or fragments of the skeleton of diatoms to build shells. The construction of the difflugia house can be observed (of course, only under a microscope) during its reproduction. Before dividing, the protozoan cell gains a lot of water and protrudes from the mouth of the shell. It can be seen how diffusion collects grains of sand and fragments of algae shells with its pseudopods. Solid particles are collected on the surface of the cytoplasm and glued together into a shell for the daughter cell using a special solidifying liquid.

These testate amoebae live in shallow stagnant bodies of water - ponds, ditches, deep puddles. Their numbers are small, and they<постройки>do not create significant bottom sediments. A completely different matter is marine protozoa, which played a colossal role in the creation of the earth's land. Radiolarians build their delicate skeleton from silicon salts absorbed from seawater. Radiolarians are planktonic organisms, their life proceeds in a state of floating in sea ​​water, therefore, the structure of their skeleton must combine lightness and strength, which is achieved by an openwork structure that increases the surface. The variety of radiolarian skeletal shapes is amazing; these creatures are one of the most beautiful and graceful organisms on Earth. Famous German zoologist and evolutionist of the 19th century. E. Haeckel, who was a good artist, dedicated a large section of his atlas of drawings to them<Красота форм в природе>.

The skeletons of other marine shell protozoa - foraminifera - reach great complexity and diversity. In the seas and oceans, foraminifera can be found in all latitudes and at all depths, but their greatest diversity is observed in the bottom layers at depths of up to 200-300 m. The shells of some foraminifera, like those of difflugia, consist of foreign particles - grains of sand. Foraminifera ingest sand grains and then release them onto the cell surface, where they<приклеиваются>to the outer layer of the cytoplasm. Another, most of the foraminifera have calcareous shells. These shells are built from substances from the animals' own bodies, which are capable of concentrating calcium salts contained in seawater in the cell.

At the bottom of the seas and oceans, dead shells of foraminifera of the genus Globigerina form calcareous silt, which is called blue, or globigerina. True, not all shells reach the bottom. It is calculated that with a size of 0.4 mm, foraminiferal shells descend at a speed of 2 cm/s, i.e. in order to dive to a depth of 1000 m, they need 14 hours. During this time, many of them simply dissolve in sea water, so the growth of blue silt is very slow, on average by 0.5-2 cm per 100 years. Nevertheless, such silt covers an area of ​​120 million km2, i.e. about a third of the world's ocean floor. In some places the thickness of the silt reaches several hundred meters. In the thickness of the silt they go chemical processes, which transform it into chalk, limestone and other sedimentary rocks.

Until recently, there was an opinion that the chalk was formed entirely by foraminiferal shells. However, in fact, the silt also includes the shells of unicellular flagellates, and chalk as such consists of 90-98% of the calcareous shells of coccolithophorid flagellates. Each shell, or coccosphere, consists of 10-20 interconnected calcareous scutes. The number of such shields in 1 cm3 of writing chalk is calculated in astronomical figures - 1010-1011. One line drawn with school chalk on a blackboard contains the remains of many millions of fossil protozoa.

Over tens and hundreds of millions of years, as a result of geological processes, a monolithic rock - limestone - was formed from deposits of protozoan shells. As a result of geological uplifts of areas of the seabed, mountains of limestone appeared on the surface of the land. The Libyan massif is made of limestone, from which the ancient Egyptians extracted material for the construction of the pyramids of the pharaohs. The palaces and temples of Vladimir-Suzdal Rus' and white-stone Moscow were also built from such limestones. Limestones are the main rock that makes up the Alps and Pyrenees, the mountains and highlands of North Africa. The belt of limestone mountains stretches from the Himalayas to Central Asia and the Caucasus.

Certain groups of extinct foraminiferal species are associated with oil-bearing formations. By species composition The remains of foraminifera discovered during drilling in sedimentary rocks formed over millions of years by deposits of the shells of these animals can predict whether there are oil-bearing strata in a given place or not.

But the skeletons of dead radiolarians, settling to the bottom, form other sedimentary rocks - radiolarites, which include, for example, jasper, opals, chalcedony, siliceous shales and clays. The jaspers of the Caucasus, siliceous rocks in the Urals, the Far East (Sikhote-Alin) and Central Asia consist entirely of radiolarites.

The fight against these numerous and dangerous protozoal diseases requires a detailed study of the biology of pathogens and their development cycles.
Free-living protozoa are also of some practical interest. Their different types are confined to a specific complex external conditions, in particular to various chemical composition water.

Certain types of protozoa live at varying degrees fresh water pollution organic substances. Therefore, by the species composition of protozoa one can judge the properties of the water of a reservoir. These features of protozoa are used for sanitary and hygienic purposes in the so-called biological analysis of water.

In the general cycle of substances in nature, protozoa play a significant role. In bodies of water, many of them are energetic eaters of bacteria and other microorganisms. At the same time, they themselves serve as food for larger animal organisms. In particular, the fry of many fish species hatching from the eggs on the most initial stages During their lives they feed mainly on protozoa.

The type of protozoa is geologically very ancient. Those species of protozoa that had a mineral skeleton (foraminifera, radiolarians - website note) are well preserved in the fossil state. Their fossil remains are known from the most ancient Lower Cambrian deposits.

Marine protozoa - rhizopods and radiolarians - played and continue to play a very significant role in the formation of marine sedimentary rocks. Over the course of many millions and tens of millions of years, microscopically small mineral skeletons of protozoa, after the death of animals, sank to the bottom, forming thick marine sediments here.

When the relief of the earth's crust changed, during mining processes in past geological eras, the seabed became dry land. Marine sediments turned into sedimentary rocks. Many of them, such as some limestones, chalk deposits, etc., largely consist of the remains of the skeletons of marine protozoa. Because of this, the study of paleontological remains of protozoa plays a large role in determining the age of different layers of the earth's crust and, therefore, is of significant importance in geological exploration, in particular in mineral exploration.

The role of protozoa in human life

1. Pathogens of diseases in humans and animals.
2. Tenants and symbionts in human and animal bodies (help in digesting food).

The study of fossil remains of protozoa plays a large role in determining the age of different layers of the earth's crust and finding oil-bearing layers.

The fight against water pollution is the most important state task. Protozoa are an indicator of the degree of pollution of fresh water bodies. Each type of protozoan animal requires certain conditions to exist. Some protozoa live only in clean water, containing a lot of dissolved air and not polluted by waste from factories and factories; others are adapted to life in water bodies of moderate pollution.

Finally, there are protozoa that can live in very polluted wastewater. Thus, the presence of a certain species of protozoa in a reservoir makes it possible to judge the degree of its pollution.

This group of single-celled organisms plays an important role in nature and human life. Being part of various ecosystems, protozoa take part in biogenic cycles of substances and energy. Many species of them are food for fish fry, aquatic invertebrates, and insect larvae.

Protozoa such as radiolarians and foraminifera form sedimentary rocks of the earth's crust. The deposits they form can be used in construction, metalworking, and indicate the proximity of oil fields. About 3.5 thousand species of protozoa are causative agents of diseases in animals and humans.

GENERAL CHARACTERISTICS OF PROTOZOA.

Protozoa are animals whose body consists of one cell, which is an independent organism. They are characterized by a specific type of metabolism, irritability, reproduction, individual development or life cycle.

The body consists of a nucleus and cytoplasm, which contains organelles that perform certain physiological functions in protozoa. The cytoplasm is separated from the external environment by the plasma membrane and differentiated into two layers - the outer - ectoplasm and the inner - endoplasm. In the cytoplasm, in addition to general cellular organelles - the endoplasmic reticulum, mitochondria, Golgi complex and others, there are also those that perform specific functions: digestive, contractile vacuoles, movement organelles. In sarcodidae they are represented by pseudopods, in flagellates - by flagella, in ciliates - by cilia.

Excretion can occur by diffusion of metabolic products through the plasma membrane or with the help of contractile vacuoles - one or two (ciliates). These same organelles provide osmoregulation.

The simplest aerobes obtain oxygen due to its diffusion through the cell membrane. Carbon dioxide is removed in the same way. Irritability manifests itself in the form of taxis (chemo-, photo-, rheotaxis) positive or negative.

In the preservation and distribution of protozoan species, encystation plays a major role. A cyst is a single-celled organism covered with a protective membrane and capable of surviving unfavorable environmental conditions for a long time. IN unfavorable conditions the cell stops movement, becomes rounded, discards or retracts movement organelles, slows down metabolic processes and secretes a dense protective shell, i.e. encysts. Once in favorable environment unicellular excyst and lead in the form of vegetative forms active image life.

Most protozoa reproduce asexually, dividing longitudinally or transversely into two daughter cells. Some (sporophytes) are characterized by multiple divisions. But in some species, asexual reproduction alternates with sexual reproduction in the form of copulation (sporophytes, colonial flagellates) or conjugation (ciliates).

SUBKINGDOM MULTICELLULAR.

Multicellular organisms are characterized by the fact that their body consists of many cells that are specialized in structure and function. In this regard, they have lost their independence and are only parts of the body. Morphological and functional differentiation of cells is accompanied by their association into more complex structures - tissues. In multicellular organisms, there are four types of tissues: nervous, muscular, connective, and epithelial. The relationships and interdependencies between organs established in the process of evolution lead to the formation of functional systems that form the basis whole organism. Systemic organization of functions is a characteristic feature of multicellular organisms. Important feature multicellular - the presence in them life cycle complex individual development. It is based on the process of implementing the genetic program of a species under certain environmental conditions. As a result of this, from one zygote cell, an organism develops with structural and functional features characteristic of the species.

Based on the nature of body symmetry, multicellular organisms are divided into radiate and bilaterally symmetrical. If an organism develops from two germ layers (ecto- and endoderm) and retains a two-layer structure, it is classified as a lower multicellular organism, and if it develops from three (ecto-, endo- and mesoderm) - to a higher one.

In higher multicellular organisms, a body cavity may be absent or developed. Due to the peculiarities of its structure, multicellular organisms can be primary or secondary cavity.

Depending on the characteristics of the formation of the mouth opening in ontogenesis, multicellular organisms can be classified as protostomes (all invertebrates except echinoderms) and deuterostomes (echinoderms, chordates).

TYPE COELOCENTARY.

GENERAL CHARACTERISTICS OF THE TYPE AND CLASSIFICATION.

Coelenterates number more than 9 thousand species that lead an exclusively aquatic lifestyle, swimming in the water or attached to the bottom.

The symmetry of the body is radial. Develops in connection with adaptation to a sedentary lifestyle, because organs located along the main axis of the body are in the same conditions. The number of rays corresponds to the number of tentacles.

Coelenterates are lower multicellular organisms, because in ontogenesis they develop from two germ layers - ecto- and endoderm and subsequently retain a two-layer structure. In adult individuals, between the endoderm and ectoderm there is a supporting plate - mesoglea, which is especially strongly developed in jellyfish.

The cells that form the body are differentiated morphologically and functionally into epithelial-muscular, stinging, secretory, nervous, and intermediate. Presence of stinging cells - characteristic feature this type. Inside the body there is an intestinal cavity that communicates with the external environment through the mouth.

The most important aromorphosis in this type is the appearance of a diffuse type nervous system. In this regard, irritability manifests itself in the form of reflexes. Digestion is mixed - cavity and intracellular. Breathing and excretion are carried out over the entire surface of the body. Reproduction is both asexual (budding) and sexual.

Coelenterates are divided into three classes: Hydroid, Scyphoid jellyfish, Coral polyps.

FRESHWATER HYDRA.HABITAT.

EXTERIOR BUILDING.

A representative of the Hydroid class is Hydra. This is a freshwater polyp about 1 cm in size, living in ponds, lakes with clear clear water. The body looks like an oblong sac, consisting of two layers of cells. Its base is blindly closed and forms a sole with which the polyp is attached to the substrate. At the free end of the stalk there is a mouth surrounded by 6-12 tentacles. They perform the functions of organs of touch and food capture.

DOUBLE LAYER. NUTRITION. The outer wall of the body is formed by ectoderm. Most of it consists of epithelial muscle cells. They fit tightly to each other and form the covering of the body. The part of them facing the mesoglea forms long protrusions in which contractile muscle fibers are located, oriented longitudinally relative to the long axis of the body. With the simultaneous contraction of muscle fibers, the body of the hydra is shortened.

Intermediate cells are located between the epithelial-muscular cells, due to which epithelial-muscular, stinging, reproductive, and nerve cells are formed. Intermediate cells play an important role in the processes of hydra regeneration, budding, and sexual reproduction.

Characteristic feature hydroids is the presence of stinging cells in the integument of the body. They perform the functions of attack and defense. Inside these cells there is a stinging capsule with a spirally twisted stinging thread. There is a thin sensitive hair on the outer surface of the cell. When you touch it, the stinging thread is thrown out and strikes the prey with poison, which enters the victim’s body through a channel inside the stinging thread.

Endoderm lines the intestinal cavity. It is based on epithelial muscle cells. Their muscle processes are located transversely relative to the longitudinal axis of the body. When they contract, the body of the polyp narrows and lengthens.

The surface of epithelial cells facing the intestinal cavity bears 1-3 flagella and is capable of forming pseudopods. They serve to capture small food particles.

Between the epithelial-muscular cells of the endoderm are secretory or glandular cells that secrete digestive enzymes into the intestinal cavity.

Hydra is a predator that feeds on small animals. Digestion is mixed - cavity and intracellular. Food (small crustaceans), with the participation of digestive enzymes, is broken down into small particles, which are phagocytosed by epithelial-muscle cells of the endoderm. In the digestive vacuoles of these cells, food particles are hydrolyzed to monomers. Undigested remains are expelled through the mouth.

Breathing and excretion of metabolic products occurs through the surface of the body.

NERVOUS SYSTEM. IRRITABILITY. Under the ectoderm are stellate-shaped nerve cells. They have many processes that contact each other, forming a nerve plexus - a diffuse nervous system. The largest number of nerve cells is concentrated around the mouth and sole, in the tentacles.

Irritability manifests itself in the form of reflexes - reactions to the action of stimuli through the nervous system. Under the influence of stimuli, an excitation occurs in the nerve cells, which is conducted to the epithelial-muscle cells, causing their response - contraction. Since the nervous system forms a plexus, the nature of the reflexes is diffuse.

REGENERATION. Hydra has a well-developed ability to regenerate, i.e. restoration of lost or damaged parts of the body. It is carried out due to intensive reproduction at the site of damage to the intermediate cells. All types of ecto- and endoderm cells develop from them. If the hydra's body is cut into two halves, then each of them regenerates into an independent organism.

REPRODUCTION. Hydras reproduce asexually and sexually. Asexual reproduction (budding) begins with the formation of a protrusion of the body walls in the region of the budding belt, located at the level of the middle of the body. As it grows, a mouth and tentacles form at the top. Then a constriction forms at the base of the kidney. The daughter individual separates from the mother, falls to the bottom and begins independent life.

With the approach of cold weather it begins sexual reproduction. Most hydras are dioecious, but some are hermaphrodites. Sex cells develop from intermediate cells of the ectoderm. The eggs develop towards the base of the body and the sperm towards the oral end. Having completed development, sperm are released into external environment and penetrate the eggs of the mother's body. The resulting zygote is covered with a dense protective shell and in the fall, after the death of the hydra, it sinks to the bottom of the reservoir, where it overwinters. In the spring, the zygote begins development, ending with the formation new generation hydr.

CLASS SCYPHOID JELLYFISHES. There are about 200 species living in various seas. Representatives are Aurelia, Cornerot, Cyanea.

The body has the shape of an umbrella, formed by ecto- and endoderm, between which lies a thick layer of mesoglea. There are numerous tentacles along the edges of the umbrella. On the lower side of the body, in the center, there is an oral opening, along the edges of which the oral lobes hang. The intestinal cavity forms a system of interconnected channels. They flow into a common ring canal. Jellyfish are predators that feed on planktonic invertebrates and small fish. An active lifestyle led to the concentration of nerve cells into nodes and the formation of organs of vision, in the form of eyespots, and balance, located at the edges of the umbrella. They swim by cutting the edges of the umbrella. Jellyfish are dioecious and reproduce with alternating generations - sexual, jellyfish, and asexual, polyps.

CLASS CORAL POLYPS. There are about 6000 species. Live in warm seas and can be represented by both single organisms and colonial ones, forming extensive colonies - Coral reefs. The body has the shape of a cylinder. Its lower end is blindly closed and forms a wide sole. The upper end bears a mouth opening surrounded by 6-8 tentacles, hollow inside. The mouth leads into a tubular pharynx, which opens into the intestinal cavity, divided by vertical partitions into several chambers. The number of partitions corresponds to the number of tentacles. The mesoglea is well developed; skeletal formations are formed in it from lime salts. Muscular elements are separated from epithelial cells. Nervous system diffuse, with a pronounced tendency to concentrate nerve cells around the oral opening.

Reproduce coral polyps asexually and sexually. Asexual reproduction occurs either by budding or by longitudinal division of the polyp body. If the daughter individuals do not separate from the mother, a colony is formed. Corals are mostly dioecious. The gonads are formed in the vertical partitions of the intestinal cavity between the endoderm and mesoglea. Spermatozoa, after maturation, exit through the mouth into the external environment and through the mouth of the female individual penetrate the eggs and fertilize them. A mobile larva develops from the zygote. It attaches to underwater objects and turns into a polyp.

Reproducing in shallow waters in tropical seas, colonial corals form extensive settlements - coral reefs. There are three types of reefs: coastal, barrier and atoll. Atolls are ring-shaped colonies of coral that rise above sea level. In the center of the atoll there is a lake - a lagoon. Charles Darwin believed that atolls are formed from coastal reefs surrounding islands. As the ocean floor sinks, the island sinks under water, and the coastal reef continues to grow, forming an atoll with a lagoon in place of the island.

TYPE FLAT WORMS.