The role of annelids. Type Annelids: characteristics, organ systems, the value of worms in nature

Freshwater oligochaetes play an essential role in fish nutrition. For example, tubifex, which often forms dense settlements at the bottom of reservoirs, is a favorite food for many fish. They are used to feed aquarium fish. Tubifex - soil-eaters that play a significant role in the biological treatment of water bodies. They are red in color because their blood contains hemoglobin. The presence of hemoglobin provides them with normal breathing even in polluted water bodies with a low oxygen content in the water. Swallowing the soil, they digest organic matter and promote their mineralization.

Small whitish annelids less than 10 mm long can live in fresh water, but are more common in the soil. Soil enchytreids include about 400 species. Their density in the soil can reach 150-200 thousand per 1 square meter. They are easily learned to breed in boxes of soil and use as food for aquarium fish, as well as for commercial species in fish hatcheries. Enchytreids feed on organic residues and participate in soil formation along with earthworms.

The family of earthworms (Lumbricidae) includes about 200 species, most of which live in the soil. Arboreal and semi-aquatic inhabitants are less common. The most common type of Lumbricus terrestris is 20 - 30 centimeters long and up to 1 centimeter thick. Large tropical earthworms (up to 1 - 3 meters long) are used by the local population of South America, Africa, Southeast Asia as food in boiled or fried form. Many animals feed on earthworms: moles, shrews, frogs, many birds and some predatory beetles. But the biological significance of earthworms in soil formation is especially great. Ch. Darwin was the first to pay attention to their role in the soil. Later, their significance in the biological cycle was experimentally studied. Earthworms swallow the soil, fallen leaves, plant remains and help to accelerate the humus formation and mineralization of the soil. In addition, earthworms loosen the soil, mix it up, dragging organic residues into the deep layers of the soil and bringing the soil depleted in organic matter from the deep layers to the surface. The soil passed through the intestines of worms has better structure. The improvement of soil fertility is facilitated by the export of manure and peat to the fields, which are important not only as a fertilizer, but also as food for worms. The soil enriched with organic matter contributes to an increase in the number of earthworms, which accelerate the soil-forming process. Experiments were carried out on the acclimatization of earthworms in the regions of Kazakhstan in Central Asia to improve soil fertility in irrigation areas.

The biological and practical importance of polychaete worms in the ocean is very great. The biological significance of polychaetes lies in the fact that they represent an important link in the trophic chains, and are also important as organisms that take part in the purification of sea water and the processing of organic matter.

CLASS LITTLE WORMS

Habitat, structure and lifestyle.

Small-bristle worms live mainly in the soil (earthworms) and in fresh water (tubifex). Earthworms (about 1500 species) have a long body, consisting of 80 or more rings. Setae (usually two bundles) are located on the sides of each ring, except for the mouth ring. There are no sense organs (there are olfactory, tactile, gustatory, photosensitive cells). Earthworms feed mainly on decaying organic residues containing bacteria. Food is captured by the mouth located on the first segment of the body. Earthworms come to the soil surface at dusk and at night. They move by alternately contracting and relaxing the circular and longitudinal muscles. The bristles have a reference value when moving and making passages in the soil. Moving in loose soil, the worm pushes its particles apart, and in dense soil it passes them through the intestines. With the onset of drought or cold weather, earthworms go deep into the soil. Tubifex live at the bottom of water bodies, forming dense settlements. The anterior part of their filamentous body (2/3) is usually located in a tube of mucus and soil particles, the posterior part is free and makes "breathing" movements. Tubifex feed on the organic remains of the soil. Reproduction. Earthworms are hermaphrodites. Before laying eggs, two worms approach each other with the front ends of the body and exchange seminal fluid containing spermatozoa, which enters their seminiferous receptacles. Then, when the eggs mature, each worm on the girdle (this is a glandular thickening of the skin of several specific segments) begins to form a cocoon: the girdle secretes mucus, which forms a muff. By contractions of the body of the worm, the muff is shifted to the front end of the body. Eggs and liquid with spermatozoa get into it. The muff turns into a cocoon, where the fertilization of the eggs takes place. The developed worms break the cocoon and come out of it.

CLASS POLYCHETATE WORMS

Polychaete worms include various types of nereids, sandworms, sickles. Nereids live mainly in the coastal parts of the seas, in muddy soil; sandworms - in the minks dug by them; serpules sit in "houses" built of various materials. At their front end there is a sultan of tentacles with which they filter the water.

The most diverse Nereids in the seas. They have a reddish or green color, cast in all the colors of the rainbow. The anterior segments of the body form a head with a mouth, palps and tentacles (organs of touch), two pairs of eyes and two pits behind them (organs of smell). On the sides of the body of nereids on the segments are short, muscular paired lobe-shaped outgrowths - parapodia with tufts of setae. These are Nereid limbs. Nereids develop special outgrowths of the skin - gills.

Nereids are dioecious animals. The eggs are fertilized in water. The eggs hatch into free-swimming larvae with a band of cilia. Over time, the larvae take on the appearance of adult worms.

ORIGIN AND SIGNIFICANCE OF ANNELS

Origin. Scientists believe that ancient annelid worms evolved from ancient free-living flatworms. Evidence of this is, for example, the presence of cilia in the larvae of marine annelids, excretory organs beginning with stellate cells with a ciliary flame, and the similarity of the nervous system with the nervous system of planarians. Polychaete worms are older than oligochaetes, although they have the most complex structure. The simplification of the structure of oligochaete worms occurred mainly in connection with the transition to life in the soil.

Meaning. Nereids and other marine worms are the main food of many species of fish, crabs and other inhabitants of the seas; many fish and freshwater invertebrates feed on tubifex; earthworms are the main food of moles, hedgehogs, toads, starlings and other land animals. Feeding on silt, various suspensions, annelids free water from excess organic matter. Earthworms and some other soil worms, eating various plant debris and passing soil through their intestines, contribute to the formation of humus. The minks they make are filled with air necessary for the respiration of plant roots and various soil-forming organisms living in the ground.

This genus also includes the Nordenskiöld worm, or Nordenskiold's Eisenia (Eisenia nordenskioldi), named after the famous Swedish traveler, explorer of the polar countries Nordenskiöld, who collected large zoological collections in northern Siberia. It is very variable in color and size; its typical coloration is similar to that of Lumbricus rubellus: rather dark, cherry-red, with the posterior end somewhat lighter. Characterized by two light spots on the sides (on segments 9-11). There are sometimes very weakly colored, almost completely light worms. In some places, especially in Siberia, the Nordenskiöld worm reaches a length of 25-30 cm.

The genus Dendrobaena is characterized by widely spaced bristles. The name of the genus indicates the habitat of its representatives in the forests. The species of this octahedral dendrobena (Dendrobaena octaedra) is a small, very mobile, red-violet worm. This species is so characteristic of the forest zone that it is often called the taiga worm. But not all species of this genus are forest dwellers. Mariupol Dendrobena (Dendrobaena mariupolienis) is common in the Sea of Azov, Crimea and the Caucasus. This is a very large worm, up to 35 cm long, rather thick and strong, dark blackish-brown in color.

Tetrahedral eizeniella (Eiseniella tetraedra) is one of the smallest, only 2.5-6 cm long. The color of tetrahedral eizeniella is grayish-brown, with a yellowish or olive tint; belt from 22-23rd to 26-27th segment; the posterior end of the body is tetrahedral. This is an amphibious species that lives along the banks of reservoirs, sometimes it can also be found in the water near the coast.

Hemaphrodites. Lose the ability to regenerate. Fertilization is internal. Direct development is characteristic.

Meaning of annelids. Annelids are an important link in the food chain. Many species make up the food base of fish (for example, Nereis). From the Sea of Azov, Nereis was relocated to the Caspian Sea in order to maintain the food base of valuable industrial fish species.

Some species of annelids are eaten by humans (palolo), used as live bait for fishing (sandworms, etc.). The ringed worm is used as food for aquarium fish.

Medical leeches are used for hypertension (high blood pressure), to reduce blood clotting, resorption of blood clots. For this, leeches are caught or specially bred. Get the substance hirudin, which is used in medicine, the perfume industry.

The scale of the activity of earthworms can be judged by the following figures. Studies have shown that the number of worms reaches 2 million/ha, and in some cases even 20 million/ha. Accordingly, live weight (biomass) is measured for territories sparsely populated by them in values from several tens of kilograms to 100–120 kg/ha, and for places where there are a lot of them, up to 2-3 t/ha. Calculations made for various places in the temperate and tropical zones show that the amount of earth thrown out by worms is such that it forms a layer from 1 to 7 m thick in a year.

The influence of the activity of earthworms on the characteristics of the soil is diverse. First of all, as a result of their digging the earth and laying passages, the duty cycle of the soil increases, it becomes looser, its volume increases (by 15-30%). As a result, access to the deep layers of the soil of water and atmospheric air, which are necessary both for the roots of herbaceous plants and trees, and for the activity of beneficial microorganisms, is facilitated. To a depth of about 1 m, the soil is usually permeated with worm passages, and in a number of species their passages go much deeper - up to 4-8 m. The total length of passages per 1 m2 of the earth's surface often exceeds 1 km, in some cases reaches 8 km. The worms constantly mix the surface layers, bringing dilapidated leaves and other organic remains from the surface into the depths of the soil and carrying the earth from the lower horizons to the surface, as well as laying excrement inside their passages.

Under the influence of worms, the chemical characteristics of the soil change. In the processed, mixed with intestinal mucus and thrown out in the form of coprolites, the earth increases the content of calcium, magnesium, ammonia, nitrates, phosphoric acid. Many compounds are converted into another, more accessible form for use by plants. Due to the production by worms in special calcareous glands of the esophagus of intergrowths of calcium carbonate crystals, harmful soil acids are neutralized. Finally, when drying, caprolites gradually disintegrate into small microporous lumps that are not eroded by water for a long time. Due to this, the earth processed by worms acquires a stable, finely cloddy structure. In addition, the number of beneficial soil microorganisms increases in coprolites. All these processes occurring in the soil under the influence of earthworms significantly change and improve its composition, structure and properties, increase its fertility.

Earthworms are not numerous everywhere. For Lumbricidae, the most favorable soil and climatic conditions are in the zone of mixed and broad-leaved forests. In the north, in the tundra and taiga forests, their numbers are usually small. In the dry steppes, where the soil dries up very much in summer, earthworms are quite rare, and in deserts they are completely absent.

Bibliography

1. "Life of animals" in seven volumes. / Ed. Yu.I. Polyansky, M., "Enlightenment", 1987.

2. T.L. Zavedey "Biology: A Handbook for Schoolchildren and Students"./ Rostov-on-Don, 2007.

3. "Biology manual for applicants to universities" / N.A. Lemeza, M.S. Morozik, E.I. Morozov and others; Ed. ON THE. Lemezy, Minsk: Universitetskoe, 1993.

4. Kobyshev N.M., Kubantsev B.S. "Animal geography with the basics of zoology: Proc. Allowance for students ped. in-t on spec. "Geography". / M., "Enlightenment", 1988.

1. The structure of the reproductive system of polychaetes.

2. Embryonic development.

3. Postembryonic development.

4. Reproduction and development of small-bristle worms.

5. Reproduction and development of leeches.

6. Meaning of annelids.

7. Classification and structure of pogonophores.

1. The structure of the reproductive system of polychaetes

Polychaete worms are dioecious animals, few hermaphrodites. Sexual dimorphism is not expressed. Sex glands are formed in all or some segments under the peritoneal epithelium. Sexual products break the epithelium and fall into the body cavity, where they mature. Eggs or spermatozoa are excreted through coelomoducts, nephromixia, in the absence of ducts - through a break in the wall of the animal's body. Fertilization is external. Some species lay their eggs in tubes or burrows, or form gelatinous clutches on the surface of tubes and other objects. Many polychaetes carry eggs.

Reproduction of polychaetes is asexual and sexual. Budding and transverse division (fragmentation) are more common. Among polychaetes, there are species that are characterized by sequential budding - paratomy. During the breeding season, many polychaetes are characterized by epitokia. Epitokia is the formation of a pelagic individual capable of sexual reproduction and an atok benthic form that does not produce sexual products. In the epitonic segments, the intestine disappears, the setae and parapodia are modified, and the size of the segments increases. Epitotic segments break away from atopic ones, atopic segments regenerate posterior segments. Epitokia is an adaptation, it allows you to synchronize puberty and guarantee the meeting of partners.

The development of polychaetes is divided into periods of embryonic and postembryonic development.

2. Embryonic development

It begins with crushing of the egg and ends with gastrulation and the formation of the first larva.

Crushing of the egg is complete. By two successive divisions, the egg is dissected into four blastomeres of approximately the same size: A, B, C, D.

These blastomeres are divided by an inclined furrow into 4 micromeres and 4 macromeres.

Further separation from macromeres towards the animal pole of micromere quartets 2nd, 3rd and 4th quartets. With the separation of each new quartet, the cells of the previously formed quartets also divide. An important feature of the separation of micromeres is the change in the fission spindle. Crushing of spiral type.

Cleavage of the egg is deterministic, i.e., at the early stages of crushing, the "fate" of each blastomere is determined. The D blastomere corresponds to the future dorsal, and the B blastomere to the ventral side of the embryo. Quartets of micromeres go to the formation of ectoderm and its derivatives, macromeres - endoderm, micromeres 4 d - coelomic mesoderm.

As a result of crushing, a blastula is formed. This is a moving stage.

Gastrulation proceeds by invagination - macromeres are immersed

V blastocoel. Epiboly was also noted. At the vegetative pole, the primary mouth, the blastopore, is formed, and at the animal pole, the parietal sultan is formed. The first larva is formed.

3. Post-embryonic development

Trochophore - a planktonic larva of spherical or elliptical shape, has a parietal sultan, represented by long cilia on the parietal plate, prototroch - preoral corolla of cilia, sometimes there is also a postoral corolla. The intestine consists of three sections, ending with the anus. Trochophore has protonephridia, nervous system, primary body cavity. In the region of the anus, on the sides of the intestine, there are teloblasts, descendants of the micromere 4d. The body ends with an anal lobe, in front of which there is a growth zone.

The next stages of development of polychaetes are metatrochophore, nektochaete, juvenile form.

The metatrochophore is formed as follows: the posterior end of the trochophore body is extended, teloblasts multiply, and mesodermal stripes are formed. The body of the larva is simultaneously subdivided into 3, 7, 9–13 segments (body segments), and parapodia develop on the segments. Under the influence of external segmentation, the mesodermal stripes are divided into

LECTURE 12. REPRODUCTION AND DEVELOPMENT OF ANELLITE WORMS. STRUCTURE OF POGONOPHORE

3. Postembryonic development

paired groups of cells. Groups of cells are initially compact, and then a cavity is formed - the germ of the coelom, the cell wall of the cavity becomes the wall of the coelomic sac.

Nektochaete (the stage is not distinguished by all researchers) has the following organization: the brain is formed due to the cells of the parietal plate and the ventral nerve trunks from the ectoderm ridges. Eyes and palps develop from the ectoderm.

juvenile stage. At this stage of polychaete development, new segments are successively formed from the growth zone. Each segment receives the rudiment of coelomic sacs, they grow together above and below the intestine, and the dorsal and abdominal mesentery is formed. Dissepiments are formed at the contact boundaries of the coelomic sacs. The whole displaces the primary body cavity. The circulatory system is formed from its remains.

adult animal. The body of an adult polychaete consists of a head lobe, a few metatrochophore larval segments (larval segments), numerous postlarval segments, and an anal lobe (pygidium). The phenomenon of the dual origin of segments (metamerism) was discovered by P. P. Ivanov.

4. Reproduction and development of oligochaete worms

Small-bristle worms are hermaphrodites. The reproductive system is associated with a few segments. The location of the gonads varies.

The male reproductive system is represented by two pairs of testes in seminal capsules, the testes are covered by three pairs of seminal sacs (protrusions of dissipations). Sexual products ripen in seed bags. For the withdrawal of seminal products, there are funnels and genital ducts associated with capsules, paired ducts of each side merge

V unpaired seed tube.

IN the composition of the female reproductive system includes: one pair of ovaries, a pair of funnel-shaped oviducts, two pairs of seminal receptacles.

Indirectly related to the reproductive system are numerous unicellular glands that form a thickening - a girdle. It secretes mucus and protein fluid. The girdle is developed only in mature individuals.

Reproduction is asexual and sexual. Asexual reproduction is more common in inhabitants of water bodies, noted in earthworms. Two forms of asexual reproduction are known: architomy (division precedes regeneration) and paratomy.

Sexual reproduction. Cross fertilization. The main stages of reproduction:

LECTURE 12. REPRODUCTION AND DEVELOPMENT OF ANELLITE WORMS. STRUCTURE OF POGONOPHORE

4. Reproduction and development of low-bristle worms

The mating worms are located with their head sections towards each other, attached with the help of pubertal ridges (thickenings of the girdle on the ventral side) and genital setae.

Belts secrete mucus, which envelops the adjoining parts of the body of animals.

Initially, both worms secrete seminal fluid, which enters through the grooves into the seminal receptacles of another individual.

After the exchange of male reproductive products, the worms disperse and crawl out of the mucous "couplings".

Each worm forms a new mucous "clutch" around the girdle. Due to the peristaltic movements of the body, the sleeve slides towards the front end of the body.

From the female genital ducts, eggs enter the clutch, and then someone else's seminal fluid is sprayed out of the seminal ducts. The eggs are fertilized.

The sleeve slips off the worm, closes at the ends, thickens and turns into an egg cocoon, inside which development takes place.

Development of oligochaetes without metamorphosis. The eggs of aquatic oligochaetes contain more yolk. The cocoon of terrestrial worms contains a protein liquid, the eggs are poor in yolk. During development, the embryo actively swallows protein.

5. Reproduction

and development of leeches

Leeches are hermaphrodites. The structure of the reproductive system resembles the structure of oligochaete worms. Leeches have a belt, it becomes noticeable only during the breeding season.

The male reproductive apparatus consists of several pairs (4–12 or more) of the testes. The medicinal leech has 9 pairs of testicles. The testicles are located in the seminal sacs, the vas deferens depart from the testes, opening into the longitudinal paired vas deferens. The vas deferens form the appendages of the testicles, in which the seminal fluid accumulates. Upon exiting the balls, the vas deferens merge into an unpaired ejaculatory canal, which is located in the copulatory organ. In many leeches, cirrus is absent, and spermatozoa are found in spermatophores. Spermatophores are either inserted into the female genital opening or stuck into the skin.

The female reproductive system consists of a pair of ovaries in egg sacs. The oviducts depart from them, then the uterus and unpaired vagina.

Fertilization is internal. Cocoons are deposited on the bottom of a reservoir, on algae or on the shore in damp soil.

The development of leeches proceeds similarly to the development of oligochaete worms. Proboscis leeches in embryonic development resemble worms -

Introduction

Even the farmers of ancient Egypt saw earthworms as a guarantee of future crops. Aristotle called them the intestines of the earth. And this is true: by passing earth and plant residues through their intestines, worms enrich the soil. In the 50s of our century, the question arose of breeding worms on purpose, as producers of a very valuable, environmentally friendly fertilizer. The concept of "Vermiculture" arose - the culture of breeding worms. A red California worm was bred, which is used to create vermiculture. Biohumus can be grown both on an industrial scale and in an apartment, on a balcony and in a summer cottage. The "Californian" is a wonderful "pet". It can be placed at home in a box made of wood or plywood, even in a cardboard box, but lined with polyethylene from the inside, in an old glass aquarium, in a plastic box.

Now the topic of annelids is of particular interest, thanks to the latest research by scientists who are revealing more and more amazing abilities of these animals. For example, it has recently become known that annelids are able to distinguish sharp corners. Another amazing ability is that most worms use their "photonic setups" to disorientate opponents. Worms in the food pyramid of the ocean occupy one of the lower steps, serving as food for a wide variety of organisms - cephalopods, crayfish, crabs, fish, and even aggressive relatives of polychaetes.

When a predator attacks a polychaete and begins to tear and torment its body, the tail section of the worm flares up brightly, attracting the attention of the “aggressor”. He grabs the luminous part of the body, and the second (head) hides in the dark. Subsequently, the tail of the worm grows again. It turns out that annelid worms, long before lizards, were the inventors of a clever trick with a discarded tail.

The object of study of this course work is the type of annelids. A brief description of this type of worms and features of the organization of annelids is given. In the practical part of the work, such classes of this type as the class of leeches, the class of polychaetes, the class of oligochaetes, and the class of echiurids were considered. The systems of these worms and their features are described.

The first part of the work gives general information about the type of annelids. In the practical part of the work, information about some classes of worms of this type.

1. General characteristics of the type of annelids

Annelids are an extensive group of animals, including about 12 thousand species, that live mainly in the seas, as well as in fresh waters and on land. This is a group of non-skeletal invertebrates, which for this reason are of particular importance in the nutrition of other animals, as they are digested without residue. At the same time, all of them are actively involved in the destruction of organic matter in biocenoses, contributing to the biogenic cycle. Particularly diverse are marine forms that are found at different depths up to the limit (up to 10 - 11 kilometers) and in all latitudes of the World Ocean. They play a significant role in marine biocenoses and have a high density of settlements: up to 100 thousand specimens per 1 square meter of the bottom surface. Sea rings are a favorite food of fish and occupy an important position in the trophic chains of marine ecosystems. / 10 /

In the soil, the most numerous are earthworms, or, as we call them, earthworms. Their density in forest and meadow soils can reach 600 specimens per 1 square meter. Earthworms are involved in the process of soil formation and contribute to an increase in crop yields and the productivity of natural biocenoses. Blood-sucking rings - leeches live mainly in fresh waters, and in tropical areas they are found in the soil and on trees. They are used in medicine for the treatment of hypertension. / 25 /

Let us consider the main features of the organization of the type of annelids as the first coelomic animals. / 1 /

1. Metamerism of the external and internal structure. Metamerism is the repetition of identical parts or rings along the main axis of the body (from the Latin words meta - repetition, mera - part). The body is worm-shaped, divided into segments, or segments. Many organ systems are repeated in each segment. The body of annelids consists of a head lobe, a segmented body and an anal lobe.

2. There is a skin-muscular sac, consisting of skin epithelium, annular and longitudinal muscles, which are lined from the inside by coelomic epithelium. / 2 /

3. The secondary body cavity (coelom) is filled with coelomic fluid, which acts as the internal environment of the body. In general, a relatively constant biochemical regime is maintained and many functions of the body are carried out (transport, excretory, sexual, musculoskeletal). / 2 /

4. The intestine consists of three functionally different sections: the anterior, middle and hindgut. Some species have salivary glands. The anterior and posterior sections are ectodermal, and the middle section of the digestive system is of endodermal origin. / 1 /

5. Most rings have a closed circulatory system. This means that blood flows only through the vessels and has a network of capillaries between arteries and veins. / 1 /

6. The main excretory organs are metanephridia of ectodermal origin. Each pair of metanephridia begins in one segment with funnels, open as a whole, from which the excretory canals continue in the next segment and open there outwards with paired holes. metanephridia are not only organs of excretion, but also the regulation of water balance in the body. In the channels of the metanephridia, the excretion products thicken (ammonia is converted into uric acid), and water is absorbed back into the coelomic fluid. Thus, moisture is saved in the body and a certain water-salt regime is maintained in the whole. Saving moisture is especially necessary for ground and soil rings. / 1 /

7. The nervous system consists of paired dorsal cerebral ganglia and the ventral nerve chain with metamerically repeated paired ganglia in each segment. The appearance of the brain, located dorsally above the pharynx, significantly distinguishes annelids from flatworms. The paired dorsal lobes of the annular brain are divided into anterior, middle, and posterior ganglia. This feature of the structure of the brain distinguishes ringworms from roundworms. / 1 /

8. Annelids are usually dioecious, but often there is a simultaneous development of male and female gonads (hermaphroditism). / 1 /

9. Development often proceeds with metamorphosis. A typical larva in sea rings is a trochophore. / 2 /

Thus, in the organization of annelids, progressive features of the organization of coelomic animals can be traced: the presence of a coelom, metamerism of the structure, the appearance of a circulatory system, an excretory system like metanephridia, a more highly organized nervous system and sensory organs. This ring differs from the lower worms flat and round. / 1 /

However, a number of features in the organization of the rings testify to their relationship with the lower worms. Thus, the larvae of annuli - trochophores - have a primary body cavity, protonephridia, an orthogonal nervous system, and, in the early stages, a blind intestine. These features are sometimes found in adult rings from primitive groups./1/

The type of annelids is divided into classes:

Class Primary rings (Archiannelida),

Class Polychaeta (Polychaeta),

Class Small-bristle (Oligochaeta),

Leech class (Hirudinea),

Class Echiurida (Echiurida),

Class Sipunculida (Sipunculida).

1.1 Signs and variety of rings

About 200 years ago, the great French naturalist J. Cuvier, working on the creation of a system of the animal world, identified six types of animals, including the articulated type, into which he combined all creatures whose body is divided into segments: insects, crayfish, spiders, wood lice, earthworms and leeches. modern science has more extensive information about leeches and earthworms, and therefore these worms are distinguished into a special type - ringlets. / 1 /

Annelids are characterized by the appearance of the following organizational features: the presence of a secondary body cavity or celloma, the circulatory system, the presence of metamerism - body segmentation / 1 /

In addition to the above features that play an important role in the evolution of animals, annelids are also characterized by the presence of special organs of movement - parapodia, a significant development of the central nervous system, consisting of the supraesophageal ganglion and the abdominal nerve chain with nerve nodes; the presence of a closed circulatory system, the metanephridial structure of the excretory system. / 1 /

1 Primary rings

2 Polychaetes

3 Low-bristle

5 Echiurids

6 Sipunculids

1.2 External structure of annelids

Annelids are the most highly organized representatives of the group of worms. The sizes of the rings range from fractions of a millimeter to two and a half meters. Mostly these are free-living forms. The body of the annulus is subdivided into three parts: the head, the trunk, consisting of rings, and the anal lobe. Such a clear division of the body into sections is not found in animals that are lower in their organization. / 1 /

The head of the rings is equipped with various sense organs. Many ringlets have well developed eyes. Some species have particularly sharp eyesight, and their lens is capable of accommodation. True, the eyes can be located not only on the head, but also on the tentacles, on the body and on the tail. The rings also have developed taste sensations. On the head and tentacles, many of them have special olfactory cells and ciliary pits that perceive various smells and the actions of many chemical stimuli. The organs of hearing, arranged according to the type of locators, are well developed in the rings. Recently, auditory organs have been opened in Echiruid marine rings, very similar to the organs of the lateral line in fish. With the help of these organs, the animal subtly distinguishes the slightest rustles and sounds that are heard much better than in the air. / 1 /

1.3 The internal structure of the rings

Digestive system consists of three sections: anterior, middle and hindgut. The foregut is strongly differentiated into a number of organs: mouth, pharynx, esophagus, goiter, stomach.

Circulatory system closed. It consists of large longitudinal vessels - dorsal and abdominal, connected in each of the segments by annular vessels. The movement of blood is carried out due to the pumping activity of the contractile sections of the spinal, less often annular vessels. The blood plasma contains respiratory pigments close to hemoglobin, thanks to which the ringlets inhabited habitats with a wide variety of oxygen content. Many annelids have red blood, like humans. It is so colored, of course, due to the presence of iron. But at the same time, iron is part of a completely different pigment, not like hemoglobin - hemerythrin. It is able to capture oxygen 5 times more than hemoglobin. The choice of pigment is due to the peculiarities of the lifestyle of such worms. These are bottom creatures, spending most of their time in the thickness of the soil, where they experience an acute oxygen deficiency. / 1 /

Respiratory system in polychaete worms, gills are thin-walled, leaf-like, pinnate or bushy external outgrowths of a part of the dorsal lobes of the parapodia, pierced by blood vessels. Small-bristle worms breathe the entire surface of the body. / 1 /

excretory organs- metanephridia located in pairs in each segment, removing the end products of vital activity from the cavity fluid. The metanephridial funnel is located in the coelom of one segment, and the short tubule extending from it opens outward in the next segment. / 1 /

Nervous system ganglion type. It consists of paired supraoesophageal and suboesophageal ganglia, connected by nerve trunks into the peripharyngeal nerve ring, and many pairs of ganglia of the ventral nerve cord, one pair in each segment. /1/

sense organs. A number of rings have well-developed sense organs, primarily the eyes. Unlike humans and other warm-blooded animals, worms sometimes have a significant number of eyes, which can be located on the head, at the posterior end of the body, on the sides (including on each segment), and even on the tail. Marine polychaetes are not only sensitive to light, but are also able to independently emit it. / 2 /

Reproduction of rings. The vast majority of ringlets are dioecious animals, less often hermaphrodites. The gonads develop either under the coelomic epithelium in all trunk segments (in polychaete worms) or only in some (in oligochaete worms). In polychaete worms, germ cells through ruptures in the coelomic epithelium enter the coelom fluid, from where they are excreted into the water by special sex funnels or metanephridia. In most water rings, fertilization is external, in soil forms it is internal. Development with metamorphosis (in polychaete worms) or direct (in oligochaete worms, leeches). Some types of rings, in addition to sexual reproduction, also reproduce asexually (by fragmentation of the body, followed by regeneration of the missing parts). Type Annelids are divided into three classes - Polychaetes, Low-bristle and Leeches.

1.4 Features of reproduction of annelids

Annelids can reproduce both sexually and asexually. The first is most typical of aquatic species, especially some marine polychaetes. Asexual reproduction is reduced either to the division of the body into parts, or to budding. When dividing, the body of the worm breaks into halves, each of which subsequently restores the missing end.

It is curious that the tail end, after separation, is an independent creature and is able to grow a new head for itself. Sometimes this head grows back long before the worm has split in half. In the middle of the body of such a ring, which is preparing to prolong the genus, there is a second head. After some time, the two-headed creature disintegrates to give life to two new worms./24/

The cocoon of a medical leech easily feeds its numerous cubs until they have heads. / 24 /

Fertilization in sexually reproducing marine worms is external. Females and males throw germ cells into the water, where the spermatozoa merge with the eggs. In the future, larvae hatch from the eggs - trochophores, which do not look like adults. Terrestrial and freshwater ringworms, including leeches, have direct development, with juveniles replicating adults almost exactly. Young leeches develop from cocoons containing eggs./24/

An important role in the reproduction of annelids is played by luminescence. The glow of worms is provided by the presence in the body of a special substance called luciferin. Under the action of a special enzyme luciferase, luciferin is oxidized by oxygen to form carbon dioxide. In this case, the released chemical energy goes to the release of excited atoms of light particles - photons. Luciferin is contained in worms in granules that float in the liquid cellular substance, where they are oxidized. Therefore, it seems that the tissues of the body are luminous in polychaetes. / 24 /

Worms are endowed with a relatively efficient oxidation mechanism, with a return of 10 to 20 percent and possibly even more. This means that marine polychaetes have learned to convert more than 10 percent of the chemical energy of luciferin into light, and the rest is useless losses. Compared to incandescent lamps, polychaete cells are extremely economical and compact biological devices. Thus, for every 3 molecules of luciferin, there are 3 oxygen molecules, and as a result of the reaction, 3 molecules of carbon dioxide and 2 photons are released. / 24 /

The biological significance of the glow can be different. There are cases when invertebrates use their illumination in order to communicate with relatives, primarily with representatives of the opposite sex. Once a year, many tropical worms leave their benthic shelters and swim up to the surface of the ocean to swarm. Here females meet males./24/

Polychaetes of the Bermuda Triangle use flashlights just during swarming. Females attract males with an intense glow, causing them to dance, during which the gentlemen must encourage their ladies to release sexual products into the water. If the female does not glow, then she has already performed her “dance of love”. She is not interested in males. Probably, the "moving candles" that X. Columbus observed in the waters of the Caribbean Sea were such swarming worms. / 24 /

2. Classification of annelids

In total, the type includes 9000 species of animals, which is 1/4 of the total species diversity of worms (scolecid). Zoologists distinguish 4 classes in the type of annelids, in addition to leeches: archiannelids, or primary rings, oligochaetes, polychaetes, echiurids. Oligochaetes and polychaetes are often also called oligochaetes and polychaetes.

Rings are the most highly organized of all other scolecids, they represent the pinnacle of the evolution of this vast group of living beings. At the same time, annelids are considered the ancestors of many different organisms, among which arthropods, represented by insects, spiders, crayfish, crabs, etc., are of paramount importance in nature. The annelids themselves originated from flatworms.

In the course of evolutionary development, species of flatworms appeared that increased the length of their body, growing segment by segment. These creatures simply added another muscular ring. But then the changes covered the entire internal structure of the worms. Their organs also began to grow in parts. As a result, animals appeared with external and internal division of the body, in which almost each new segment copies the previous one in general terms. Such a repetition of homogeneous segments of the body has received the name of metamerism in science.

In reality, of course, the development of the metameric structure of the body proceeded much longer and more complicated. Even in primitive microscopic rhizopods of foraminifera, the division of shells into similar segments is observed. Thus, the segmentation of the body underlay the evolution of the organic world. It was a consequence of the adaptation of living beings to the environment and the evolution of body symmetry types, which increases the adaptive capabilities of species. Metamerism itself is considered by scientists as a special type of symmetry.

Jellyfish and their coelenterate relatives, as well as lower worms, lack true metameres (somites). However, many of them are characterized by either polymeric organs or pseudometamerism, i.e. the presence of false segments. Such coelenterates as hydrozoans, scyphozoans, anthozoans, and ctenophores have polymeric organs.

Chemicals related to polyethylene are commonly referred to as polymers. In biology, the same name was given to organs and structures that are repeatedly repeated in the body. Usually these are the organs of the reproductive system. Pseudometamery and polymer formations are typical for a number of groups of lower worms: cestodes, turbellarians, nemertines. In scolecids, body cavities are laid, which is why there are similarities of segments.

Further division of the body of animals was made possible due to the complication of the nervous system and, mainly, the appearance of the circulatory system. It became possible to isolate the branches of nerves (with the formation of neurosomites) and blood vessels (with the formation of angiosomites).

The third and, perhaps, the most essential condition for the progress of metamerism was the growth of muscles. Ringworms have the most developed muscle sac among worms, the volume of which reaches 60-70 percent of the total body volume. Due to the intensive development of muscles, there was a separation of sections of the longitudinal and annular muscles with the formation of myosomites, separated by transverse partitions-dissepiments.

The growth of the muscular sac and the segmentation that accompanies this phenomenon were a single evolutionary breakthrough of the annulus, which allowed them to occupy the most important ecological niches of the sea, fresh water and soil from those that are only occupied by worms. The scolecides have fought for reliable protection and speed throughout their history. Only the descendants of primitive worms were able to solve this problem, which explains the progress of these groups. Snails "invented" for themselves reliable protection in the form of shells. Squids have created a powerful jet "engine" from their muscle bag.

Annelids acquired dense integuments, cut into lobules - segments corresponding to internal metamerism. Segmentation allowed these invertebrates to freely bend their body to move in waves, while developing considerable speed. Subsequently, insects, crayfish and other arthropods were covered with a dense segmented shell. Man in his organization bears the imprint of the metamerism of the ancient rings, but our body does not consist of segments. Their number in vertebrates has sharply decreased, which is why instead of somites, the so-called somites were formed. departments.

2.1 Polychaete class (Polychaeta)

The polychaete class differs from other annuli in a well-separated head region with sensory appendages and the presence of limbs - parapodia with numerous setae. Mostly dioecious. development with metamorphosis.

General morphofunctional characteristics. The body of polychaete worms consists of a head section, a segmented trunk, and an anal lobe. The head is formed by the head lobe (prostomium) and the oral segment (perestomium), which is often complex as a result of fusion with 2-3 trunk segments. The mouth is located ventrally on the perestomium. Many polychaetes have ocelli and sensory appendages on their heads. So, in a Nereid, on the prostomium of the head there are two pairs of eyes, tentacles - tentacules and two-segmented palps, on the bottom of the perestomium there is a mouth, and on the sides there are several pairs of antennae. On the trunk segments there are paired lateral outgrowths with setae - parapodia. these are primitive limbs with which polychaetes swim, crawl or burrow into the ground. Each parapodia consists of a basal part and two lobes - dorsal (notopodium) and ventral (neuropodium). At the base of the parapodia on the dorsal side there is a dorsal, and on the ventral - ventral antennae. These are the sensory organs of polychaetes. Often, the dorsal barbel in some species is turned into feathery gills. Parapodia are armed with tufts of setae, consisting of organic matter close to chitin. Among the setae there are several large aciculous setae, to which muscles are attached from the inside, setting the parapodia and the tuft of setae in motion. The limbs of polychaetes make synchronous movements like oars. In some species leading a burrowing or attached lifestyle, the parapodia are reduced.

Skin-muscle bag. The body of polychaetes is covered with a single layer of skin epithelium, which exposes a thin cuticle to the surface. In some species, some parts of the body may have ciliated epithelium (longitudinal abdominal band or ciliary bands around segments). The glandular cells of the epithelium of sessile polychaetes can secrete a protective horny tube, often impregnated with lime.

Under the skin lies the annular and longitudinal muscles. The longitudinal muscles form four longitudinal bands: two on the dorsal side of the body and two on the ventral side. Longitudinal tapes may be more. On the sides there are bundles of fan-shaped muscles that set in motion the blades of the parapodia. The structure of the skin-muscular sac varies greatly depending on the lifestyle. The inhabitants of the ground surface have the most complex structure of the skin-muscular sac, close to that described above. This group of worms crawls along the surface of the substrate with the help of a serpentine bending of the body and movements of the parapodia. The inhabitants of calcareous or chitinous pipes have limited mobility, as they never leave their shelters. In these polychaetes, strong longitudinal muscle bands provide a sharp lightning-fast contraction of the body and withdrawal into the depth of the tube, which allows them to escape from the attack of predators, mainly fish. In pelagic polychaetes, the muscles are poorly developed, as they are passively carried by ocean currents.

Secondary body cavity- In general, polychaetes have a very diverse structure. In the most primitive case, separate groups of mesenchymal cells cover the inside of the muscle bands and the outer surface of the intestine. Some of these cells are capable of contraction, while others are able to turn into germ cells that mature in a cavity, only conditionally called secondary. In a more complex coelomic epithelium may completely cover the intestines and muscles. The coelom is fully represented in case of development of paired metameric coelomic sacs. When paired coelomic sacs close in each segment above and below the intestine, the dorsal and abdominal mesentery, or mesentery, are formed. Between the coelomic sacs of two adjacent segments, transverse partitions are formed - dissipations. The wall of the coelomic sac lining the inside of the muscles of the body wall is called the parietal mesoderm, and the coelomic epithelium covering the intestines and forming the mesenterium is called the visceral mesoderm. The coelomic septa contain blood vessels.

In general, it performs several functions: musculoskeletal, transport, excretory, sexual and homeostatic. The cavity fluid supports the turgor of the body. With the contraction of the ring muscles, the pressure of the cavity fluid increases, which provides the elasticity of the worm's body, which is necessary when making passages in the ground. Some worms are characterized by a hydraulic mode of movement, in which the abdominal fluid, when the muscles contract under pressure, is distilled to the anterior end of the body, providing vigorous forward movement. In general, there is a transport of nutrients from the intestines and dissimilation products from various organs and tissues. The excretory organs of metanephridia open as a whole with funnels and ensure the removal of metabolic products, excess water. In general, there are mechanisms to maintain the constancy of the biochemical composition of the liquid and water balance. In this favorable environment, gonads are formed on the walls of coelomic sacs, germ cells mature, and in some species, juveniles even develop. Derivatives of the coelom - coelomoducts serve to remove the reproductive products from the body cavity.

Digestive system consists of three departments. The entire anterior section consists of derivatives of the ectoderm. The anterior section begins with a mouth opening located on the peristomium from the ventral side. The oral cavity passes into a muscular pharynx, which serves to capture food objects. In many species of polychaetes, the pharynx can turn outward, like a finger of a glove. In predators, the pharynx consists of several layers of annular and longitudinal muscles, armed with strong chitinous jaws and rows of small chitinous plates or spikes that can firmly hold, injure and crush captured prey. In herbivorous and detritivorous forms, as well as in seston-eating polychaetes, the pharynx is soft, mobile, adapted to swallowing liquid food. The pharynx is followed by the esophagus, into which the ducts of the salivary glands, also of ectodermal origin, open. Some species have a small stomach.

The middle section of the intestine is a derivative of the endoderm and serves for the final digestion and absorption of nutrients. In predators, the middle section of the intestine is relatively shorter, sometimes equipped with paired blind side pockets, while in herbivores, the middle section of the intestine is long, tortuous, and usually filled with undigested food debris.

The posterior intestine is of ectodermal origin and can perform the function of regulating the water balance in the body, since there water is partially absorbed back into the coelom cavity. Fecal masses are formed in the hindgut. The anal opening usually opens on the dorsal side of the anal lobe.

Respiratory system. Polychaetes mainly have cutaneous respiration. But a number of species have dorsal skin gills, which are formed from the parapodial antennae or appendages of the head. They breathe oxygen dissolved in water. Gas exchange occurs in a dense network of capillaries in the skin or gill appendages.

Circulatory system closed and consists of the dorsal and abdominal trunks connected by annular vessels, as well as peripheral vessels. The movement of blood is carried out as follows. Through the dorsal, the largest and pulsating vessel, blood flows to the head end of the body, and through the abdominal - in the opposite direction. Through the annular vessels in the anterior part of the body, blood is distilled from the dorsal vessel to the abdominal one, and vice versa in the posterior part of the body. Arteries depart from the annular vessels to parapodia, gills and other organs, where a capillary network is formed, from which blood is collected into venous vessels that flow into the abdominal bloodstream. In polychaetes, the blood is often red in color due to the presence of the respiratory pigment hemoglobin dissolved in the blood. Longitudinal vessels are suspended on the mesentery (mesenterium), annular vessels pass inside the dissipations. Some primitive polychaetes (Phyllodoce) have no circulatory system, and hemoglobin is dissolved in nerve cells.

excretory system polychaetes are most often represented by metanephridia. This type of nephridia appears for the first time in the type of annelids. Each segment contains a pair of metanephridia. Each metanephridium consists of a funnel lined with cilia and open as a whole. The movement of cilia into the nephridium drives solid and liquid metabolic products. A channel departs from the funnel of nephridium, which penetrates the septum between the segments and in another segment opens outwards with an excretory opening. In convoluted channels, ammonia is converted into macromolecular compounds, and water is absorbed as a whole. In different types of polychaetes, the excretory organs can be of different origin. Thus, some polychaetes have protonephridia of ectodermal origin, similar in structure to those of flatworms and roundworms. Most species are characterized by metanephridia of ectodermal origin. Individual representatives form complex organs - nephromixia - the result of the fusion of protonephridia or metanephridia with genital funnels - coelomoducts of mesodermal origin. An additional function can be performed by chloragogenic cells of the coelomic epithelium. These are peculiar accumulation kidneys in which excreta grains are deposited: guanine, salts of uric acid. Subsequently, chloragogenic cells die and are removed from the coelom through nephridia, and new ones are formed to replace them.

Nervous system. Paired supraesophageal ganglia form the brain, in which three sections are distinguished: proto-, meso- and deutocerebrum. The brain innervates the sense organs on the head. Near-pharyngeal nerve cords depart from the brain - connectives to the abdominal nerve chain, which consists of paired ganglia, repeating segment by segment. Each segment has one pair of ganglia. Longitudinal nerve cords connecting the paired ganglia of two adjacent segments are called connectives. The transverse cords connecting the ganglia of one segment are called commissures. When paired ganglia merge, a neural chain is formed. In some species, the nervous system is complicated by the fusion of the ganglia of several segments.

sense organs most developed in mobile polychaetes. On the head they have eyes (2-4) of a non-inverted type, goblet-shaped or in the form of a complex eye bubble with a lens. Many sessile tube-dwelling polychaetes have numerous eyes on the feathery gills of the head region. In addition, they have developed organs of smell, touch in the form of special sensory cells located on the appendages of the head and parapodia. Some species have balance organs - statocysts.

reproductive system. Most polychaete worms have separate sexes. Their gonads develop in all segments of the body or only in some of them. Sex glands of mesodermal origin and are formed on the wall of the coelom. Sex cells from the gonads fall into the whole, where their final maturation takes place. Some polychaetes do not have reproductive ducts, and the germ cells enter the water through ruptures in the body wall, where fertilization occurs. In this case, the parental generation dies. A number of species have genital funnels with short channels - coelomoducts (of mesodermal origin), through which the reproductive products are brought out into the water. In some cases, germ cells are removed from the coelom through nephromixia, which simultaneously perform the function of the reproductive and excretory ducts.

reproduction polychaetes can be sexual and asexual. In some cases, there is an alternation of these two types of reproduction (metagenesis). Asexual reproduction usually occurs by transverse division of the body of the worm into parts (strobilation) or by budding. This combing is accompanied by the regeneration of the missing parts of the body. Sexual reproduction is often associated with the phenomenon of epitokia. Epitoky is a sharp morphophysiological restructuring of the worm's body with a change in the shape of the body during the period of maturation of the reproductive products: the segments become wide, brightly colored, with swimming parapodia. In worms that develop without epitokia, males and females do not change their shape and reproduce in bottom conditions. Species with epitokia may have several life cycle variants. One of them is observed in Nereids, the other in Palolo. So, in Nereis virens, males and females become epitonic and float to the surface of the sea for reproduction, after which they die or become prey to birds and fish. From eggs fertilized in water, larvae develop, settling to the bottom, from which adults are formed. In the second case, as in the palolo worm (Eunice viridis) from the Pacific Ocean, sexual reproduction is preceded by asexual reproduction, in which the anterior end of the body remains at the bottom, forming an atopic individual, and the posterior end of the body is transformed into an epitonic tail filled with sexual products. The backs of the worms break off and float to the surface of the ocean. Here the reproductive products are released into the water and fertilization takes place. Epitoke individuals of the entire population emerge for reproduction at the same time, as if on a signal. This is the result of the synchronous biorhythm of puberty and biochemical communication of sexually mature individuals of the population. The mass appearance of breeding polychaetes in the surface layers of water is usually associated with the phases of the moon. So, the Pacific palolo rises to the surface in October or November on the day of the new moon. The local population of the Pacific Islands knows these palolo breeding dates, and fishermen en masse catch palolos stuffed with "caviar" and use them for food. At the same time, fish, gulls, sea ducks feast on worms.

Development. The fertilized egg undergoes uneven, spiral crushing. This means that as a result of crushing, quartets of large and small blastomeres are formed: micromeres and macromeres. In this case, the axes of the spindles of cell fragmentation are arranged in a spiral. The inclination of the spindles is reversed with each division. Due to this, the crushing figure has a strictly symmetrical shape. Cleavage of the egg in polychaetes is deterministic. Already at the stage of four blastomeres, determination is expressed. Quartets of micromeres give derivatives of the ectoderm, and quartets of macromeres give derivatives of the endoderm and mesoderm. The first mobile stage is the blastula, a single layered larva with cilia. The macromeres of the blastula at the vegetative pole sink into the embryo and a gastrula is formed. At the vegetative pole, the primary mouth of the animal, the blastopore, is formed, and at the animal pole, an accumulation of nerve cells and a ciliary tuft, the parietal sultan of cilia, are formed. Further, a larva develops - a trochophore with an equatorial ciliary belt - a troch. The trochophore has a spherical shape, a radially symmetrical nervous system, protonephridia, and a primary body cavity. The blastopore at the trochophore is displaced from the vegetative pole closer to the animal along the ventral side, which leads to the formation of bilateral symmetry. The anus erupts later at the vegetative pole, and the intestine becomes through.

2.2 Class low-bristle

Small-bristle worms are inhabitants of fresh waters and soil, which are sporadically found in the seas. More than 5000 species are known. Distinctive features of the external structure of oligochaete worms are homonomous segmentation of the body, the absence of parapodia, and the presence of a glandular belt in the anterior third of the body in sexually mature individuals. Their head section is not expressed. The head lobe, as a rule, is devoid of eyes and appendages. There are also no appendages on the anal lobe (pygidium). Setae are located on the sides of the body, usually four pairs of tufts per segment. These are the rudiments of parapodia. Such a simplification of the external structure is associated with adaptations to a burrowing lifestyle. in oligochaetes, there is a convergent resemblance to burrowing polychaetes. This confirms the reason for their morphoecological similarity in connection with the development of a similar, burrowing lifestyle. The oligochaetes most familiar to us are earthworms living in the soil. Their body reaches several centimeters, the largest among them - up to 3 m (in Australia). In the soil, small whitish annelids - enchytreids (5 - 10 mm) are also common. Earthworms and enchytreids feed on plant residues in the soil and play an important role in soil formation. In freshwater bodies one can often see oligochaetes with long bristles or living in vertical tubes and forming dense settlements at the bottom. They feed on suspended organic residues and are useful filter feeders that play an essential role in water purification.

General morphophysiological characteristics . The body length varies from a few millimeters to 3 m. The body is long, worm-shaped, and segmented. The number of segments varies from 5 - 6 to 600. The mouth is located on the first segment of the body after the head lobe. The anal opening is located on the anal lobe. Oligochaetes move by contracting the muscles of the body. When digging, the worm spreads the soil with the front end of the body, relying on numerous bristles. The bristles rest against the walls of the dug passage, so it is difficult to pull the earthworm out of the mink.

Skin-muscular sac. The body is covered with a layer of skin epithelium, often with a large number of glandular cells. The skin exudes a thin cuticle. Abundant secretion of mucus protects the skin of earthworms from mechanical damage and drying. Under the skin, like in polychaetes, they have circular and longitudinal muscles, lined from the inside with coelomic epithelium.

Digestive system. The intestine runs along the entire length of the body. In the anterior intestine of the earthworm, the oral cavity, muscular pharynx, relatively narrow esophagus, goiter and stomach are separated. In the walls of the esophagus there are three pairs of calcareous glands, the secrets of which neutralize humic acids in the food of earthworms. From the stomach, food enters the midgut, where nutrients are absorbed. Undigested food residues and mineral soil particles enter the short hindgut and are removed through the anus to the outside. In the midgut, an internal longitudinal fold is located dorsally - tiflozol, hanging into the lumen of the intestine and increasing the absorption surface of the intestine.

Circulatory system oligochaetes are similar in structure to the circulatory system of polychaetes. There are dorsal and abdominal pulsating vessels, which are connected by annular vessels. Unlike polychaetes, in oligochaetes, the annular vessels in the esophagus pulsate and are called "ring hearts". The blood contains a respiratory pigment - hemoglobin, which is dissolved in the blood plasma, in contrast to vertebrates, in which hemoglobin is found in red blood cells. The circulatory system performs a transport function in oligochaetes for the transfer of nutrients, oxygen and metabolic products.

excretory system represented by metanephridia. The ability of metanephridia to conserve moisture in the body by reabsorbing water ensures that oligochaetes are adapted to life on land. Solid excretions accumulate in the chloragogenic cells of the coelomic epithelium. Partially these cells filled with excreta are removed through funnels of nephridia or through special pores in the body wall.

Nervous system it is represented, as in all rings, by a pair of supraesophageal ganglia (brain) and an abdominal nerve cord.

sense organs in oligochaetes, they are less developed than in most polychaetes, due to a burrowing lifestyle. The eyes are usually absent. In the skin of oligochaetes there are numerous sensory cells: light-sensitive, tactile, and other earthworms sensitively react to factors of light, humidity, and temperature. This explains their vertical migrations in the soil during the day and in seasons.

reproductive system oligochaete hermaphroditic. Hermaphroditic individuals of oligochaetes are of the same type, in contrast to sexually mature individuals of polychaetes with sexual dimorphism. Hermaphroditism in the animal kingdom is an adaptation to increase fertility, as all 100% of individuals in a population can lay eggs. Consider the structure of the reproductive system using the example of an earthworm. The gonads in oligochaetes are concentrated in the anterior segments of the body. The testicles (two pairs) are located in the 10th and 11th segments of the body and are covered by three pairs of seed sacs. In the seminal sacs accumulates sperm flowing from the testicles. This is where spermatozoa mature. Sperm enter the ciliated funnels of the vas deferens. The vas deferens merge in pairs on the left and right sides of the body, and two longitudinal canals are formed, opening with paired male genital openings on the 15th segment of the body. The female reproductive system is represented by a pair of ovaries located on the 13th segment, a pair of oviducts with funnels opening with genital openings on the 14th segment. In the 13th segment, dissipiments form egg sacs that cover the ovaries and funnels of the oviducts. The female reproductive system also includes special skin protrusions on the 9th and 10th segments - two pairs of seed receptacles with holes on the ventral side of the body.

Reproduction and development. In sexually mature earthworms, a glandular belt develops on the 32nd - 37th segments. During the breeding season, at first all individuals become, as it were, males, since only the testes are developed in them. The worms are connected by their head ends towards each other, while the girdle of each worm is located at the level of the seminal receptacles of the other worm. The girdle secretes a mucous "clutch" that connects the two worms. Thus, mating worms are united by two bands of mucous muffs in the region of their girdle. From the male openings of both worms, sperm is released, which, through special grooves on the ventral side of the body, enters the seminal receptacles of another individual. After exchanging male reproductive products, the worms disperse. After some time, the ovaries mature in the worms and all individuals become, as it were, females. The "clutch" from the girdle area slides to the anterior end of the body due to the peristaltic movements of the worm's body. At the level of the 14th segment, eggs from the female genital openings enter the clutch, and at the level of the 9th - 10th segments, "foreign" seminal fluid is injected. This is how cross fertilization occurs. Then the muff slips from the head end of the body and closes. An egg cocoon is formed with developing eggs. The cocoon of earthworms is shaped like a yellow-brown lemon; its dimensions are 4 - 5 mm in diameter.

Development in oligochaetes proceeds without metamorphosis, i.e. without larval stages. Small worms that look like adults hatch from the egg cocoon. Such direct development without metamorphosis arose in oligochaetes in connection with the transition to life on land or to living in fresh water bodies, which often dry up. The embryonic development of the oligochaete embryo proceeds, as in most polychaetes, according to the spiral type of crushing and with the teloblastic anlage of the mesoderm.

Asexual reproduction is known in some families of freshwater oligochaetes. In this case, a transverse division of the worm into several fragments occurs, from which whole individuals then develop, or by differentiation of the worm into a chain of short daughter individuals. Later, this chain breaks down. In earthworms, asexual reproduction is extremely rare, but the ability to regenerate is well expressed. The cut worm, as a rule, does not die, and each part of it restores the missing ends. The worm most easily restores the posterior end of the body. The head end of the body is restored rarely and with difficulty.

2.3 Leech class

Leeches (Hirudinei) - detachment of the class of annelids.

Body elongated or oval, more or less flattened in the dorsal-abdominal direction, clearly divided into small rings, which in number 3 - 5 correspond to one segment of the body; numerous glands in the skin that secrete mucus; at the posterior end of the body there is usually a large sucker, often at the anterior end there is a well-developed sucker, in the center of which the mouth is placed; more often, the mouth is used for suction. At the anterior end of the body there are 1-5 pairs of eyes arranged in an arc or in pairs one after the other.

powder on the dorsal side above the posterior sucker.

Nervous system consists of a two-lobed supraoesophageal ganglion or brain, connected to it by short commissures under the pharyngeal node (derived from several merged nodes of the abdominal chain) and the abdominal chain itself, located in the abdominal blood sinus and having about 20 nodes. The head node innervates the sense organs and the pharynx, and 2 pairs of nerves depart from each node of the abdominal chain, innervating the body segments corresponding to them; the lower wall of the intestine is equipped with a special longitudinal nerve that gives branches to the blind sacs of the intestine.

Digestive organs they begin with a mouth armed with either three chitinous toothed plates (jaw leeches - Gnathobdellidae), which serves to cut through the skin when sucking blood in animals, or a proboscis capable of protruding (in proboscis leeches - Rhynchobdellidae); numerous salivary glands open into the oral cavity, sometimes secreting a poisonous secret; the pharynx, which plays the role of a pump during sucking, is followed by an extensive, highly extensible stomach, equipped with lateral sacs (up to 11 pairs), of which the posterior ones are the longest; the hindgut is thin and short.

Circulatory system partly consists of real, pulsating, vessels, partly from cavities - sinuses, representing the remainder of the cavity (secondary) of the body and interconnected by annular channels; blood in proboscis leeches is colorless, in jaw leeches it is red, due to hemoglobin dissolved in the lymph.

Special respiratory organs are available only at the river. Branchellion, in the form of leaf-like appendages on the sides of the body.

excretory organs arranged according to the type of metanephridia or segmental organs of annelids, and most leeches have a pair in each of the middle segments of the body.

Reproductive system and reproduction Leeches are hermaphrodites: The male genital organs consist of most of the vesicles (testes), in pairs in 6 - 12 middle segments of the body, connected on each side of the body by a common excretory duct; these ducts open outwards with one opening lying on the ventral side of one of the anterior rings of the body; the female genital opening lies one segment behind the male and leads into two separate oviducts with saccular ovaries. Two individuals copulate, each simultaneously playing the role of a female and a male. During the laying of eggs, leeches secrete thick mucus, which surrounds the middle part of the body of the leech in the form of a cover, into this cover, after which the leech crawls out of it, and the edges of its holes come together, stick together and form such in the form of a capsule with eggs inside, usually attached to the lower surface of the algae leaf; the embryos, leaving the egg shell, sometimes (Clepsine) keep for some time on the underside of the mother's body.

Variety of leeches. All leeches are predators, feeding on the blood of mostly warm-blooded animals, or mollusks, worms, and the like; they live mainly in fresh waters or in wet grass; but there are marine forms (Pontobdella) as well as terrestrial forms (in Ceylon). Hirudo medicinalis - medicinal leech up to 10 cm long and 2 cm wide, black-brown, black-green, with a longitudinal patterned reddish pattern on the back; the belly is light grey, with 5 pairs of eyes on the 3rd, 5th and 8th rings and strong jaws; distributed in the marshes of the south. Europe, southern Russia and the Caucasus. In Mexico, Haementaria officinalis is used in medicine; another species, the mexicana leech, is poisonous; in tropical Asia, Hirudo ceylonica and other related species living in humid forests and in the grass are common, causing painful bleeding bites to humans and animals. Aulostomum gulo - horse leech, black-green in color, with a lighter bottom, has a weaker mouth armament and therefore unsuitable for therapeutic purposes; the most common species in the north. and central Russia. Nephelis vulgaris - a small leech with a thin narrow body, gray in color, sometimes with a brown pattern on the back; equipped with 8 eyes located in an arc at the head end of the body; related to her original Archaeobdella Esmonti, pink, without posterior sucker; lives on the silt bottom in the Caspian and Azov seas. Clepsiue tesselata - Tatar leech, with a wide oval body, greenish brown in color, with several rows of warts on the back and 6 pairs of triangular eyes, arranged one after the other; lives in the Caucasus and Crimea, where it is used by the Tatars for medicinal purposes; the transitional place to the order of bristle-legged (Chaetopoda Oligochaeta) worms is occupied by Acanthobdella peledina, found in Lake Onega.

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