General characteristics of annelids. Earthworm Habitat of annelids

CLASS Olychaetae. EARTHWORM

Body structure. Elongated, worm-shaped, segmented, round in cross section. The symmetry is bilateral, the dorsal and ventral sides, the anterior and posterior ends of the body differ. Three-layer animals.

Cover. The skin is covered with a cuticle; each segment has 8 bristles used for movement. The skin has many mucous and poisonous glands. The annular, longitudinal, dorsal and abdominal muscles are attached to it. The skin-muscle sac is stronger than that of other worms.

Body cavity. Secondary, formed by mesoderm. It is lined with epithelium of mesodermal origin - it has its own walls. The epithelium is adjacent to the skin-muscular sac on the inside, and covers the intestines on the outside. The body cavity is filled with fluid, which gives the body elasticity. Cavity fluid communicates the circulatory system with the cells of the body.

Digestive system. It is represented by several sections: mouth, pharynx, esophagus, crop, muscular stomach, midgut, hindgut, anus. The intestines are surrounded by a network of blood capillaries, which ensures the absorption of nutrients into the blood.

Respiratory system. Absent. Absorbs oxygen from the air over the entire surface of the skin.

Circulatory system. Closed type. It is represented by dorsal and abdominal vessels running along the body, and annular vessels in each segment. The largest vessels of the “heart” push blood through. Blood contains hemoglobin - it is reddish. Blood circulates only in the blood vessels, it carries nutrients, oxygen and carbon dioxide, which are transferred to the cells of the body through capillaries and cavity fluid.

Excretory system. It consists of paired tubes in each segment of the body. At the end of each tube there is a funnel through which the final waste products are removed from the blood and cavity fluid.

Nervous system. Nodular type: consists of a peripharyngeal nerve ring and a ventral nerve cord, which has a node in each segment of the body.

Sense organs. Touch and light sensitive cells throughout the skin.

Reproduction. Sexual. Hermaphrodite. Ovaries and testes in different segments. Cross fertilization, internal. The eggs are laid in a cocoon, which forms on the body in the form of a belt and extends from the head end.

Development. Direct: a worm is formed from an egg.

Regeneration. Well expressed.

ECOLOGY OF ANNELED WORMS

Initial level of knowledge:

kingdom, type, cell, tissue, organs, organ systems, heterotroph, predation, saprophyte, detritophage, eukaryotes, aerobes, symmetry, body cavity, larva.

Response Plan:

General characteristics of annelids
Body structure of annelids
Reproduction and development of annelids
Classification of annelids, variety of species
Peculiarities of the structure and development of worms of the class Maloschitaceae using the example of an earthworm
Characteristics of the Polyscutaneous class
Characteristics of the Leech class
Origin of Annelids

General characteristics of annelids

Number of species: about 75 thousand.

Habitat: in salt and fresh waters, found in soil. Aquatic creatures crawl along the bottom and burrow into the mud. Some of them lead a sedentary lifestyle - they build a protective tube and never leave it. There are also planktonic species.

Structure: bilaterally symmetrical worms with a secondary body cavity and a body divided into segments (rings). The body is divided into the head (head lobe), trunk and caudal (anal lobe) sections. The secondary cavity (coelom), unlike the primary cavity, is lined with its own internal epithelium, which separates the coelomic fluid from the muscles and internal organs. The fluid acts as a hydroskeleton and also participates in metabolism. Each segment is a compartment containing external outgrowths of the body, two coelomic sacs, nodes of the nervous system, excretory and genital organs. Annelids have a skin-muscle sac, consisting of one layer of skin epithelium and two layers of muscles: circular and longitudinal. The body may have muscular outgrowths - parapodia, which are organs of locomotion, as well as bristles.

Circulatory system first appeared during evolution in annelids. It is of a closed type: blood moves only through the vessels, without entering the body cavity. There are two main vessels: dorsal (carries blood from back to front) and abdominal (carries blood from front to back). In each segment they are connected by annular vessels. Blood moves due to the pulsation of the spinal vessel or “hearts” - annular vessels of 7-13 segments of the body.

There is no respiratory system. Annelids are aerobes. Gas exchange occurs across the entire surface of the body. Some polychaetes have developed dermal gills - outgrowths of parapodia.

For the first time in the course of evolution, multicellular organisms appeared excretory organs– metanephridia. They consist of a funnel with cilia and an excretory canal located in the next segment. The funnel faces the body cavity, the tubules open on the surface of the body with an excretory pore, through which decay products are removed from the body.

Nervous system formed by the peripharyngeal nerve ring, in which the paired suprapharyngeal (cerebral) ganglion is particularly developed, and by the abdominal nerve chain, consisting of pairwise contiguous abdominal nerve ganglia in each segment. From the “brain” ganglion and the nerve chain, nerves extend to the organs and skin.

Sense organs: eyes - organs of vision, palps, tentacles (antennae) and antennae - organs of touch and chemical sense are located on the head lobe of polychaetes. In oligochaetes, due to their underground lifestyle, the sense organs are poorly developed, but the skin contains light-sensitive cells, organs of touch and balance.

Reproduction and development

They reproduce sexually and asexually - by fragmentation (separation) of the body, due to a high degree of regeneration. Budding also occurs in polychaete worms.
Polychaetes are dioecious, while polychaetes and leeches are hermaphrodites. Fertilization is external; in hermaphrodites, it is cross fertilization, i.e. worms exchange seminal fluid. In freshwater and soil worms, development is direct, i.e. Young individuals emerge from the eggs. In marine forms, development is indirect: a larva, a trochophore, emerges from the egg.

Representatives

Type Annelids are divided into three classes: Polychaetes, Oligochaetes, Leeches.

Oligochaete worms (oligochaetes) mainly live in soil, but there are also freshwater forms. A typical representative living in the soil is the earthworm. It has an elongated, cylindrical body. Small forms are about 0.5 mm, the largest representative reaches almost 3 m (giant earthworm from Australia). Each segment has 8 setae, arranged in four pairs on the lateral sides of the segments. Clinging to uneven soil, the worm moves forward with the help of the muscles of the skin-muscular sac. As a result of feeding on rotting plant remains and humus, the digestive system has a number of features. Its anterior section is divided into the muscular pharynx, esophagus, crop and gizzard.

An earthworm breathes over the entire surface of its body due to the presence of a dense subcutaneous network of capillary blood vessels.

Earthworms are hermaphrodites. Cross fertilization. The worms attach themselves to each other with their ventral sides and exchange seminal fluid, which enters the seminal receptacles. After this, the worms disperse. In the anterior third of the body there is a belt that forms a mucous muff in which eggs are laid. As the coupling moves through the segments containing the spermatheca, the eggs are fertilized by sperm belonging to another individual. The muff is shed through the anterior end of the body, becomes compacted and turns into an egg cocoon, where young worms develop. Earthworms are characterized by a high ability to regenerate.

Longitudinal section of the body of an earthworm: 1 - mouth; 2 - pharynx; 3 - esophagus; 4 - goiter; 5 - stomach; 6 - intestine; 7 - peripharyngeal ring; 8 - abdominal nerve chain; 9 - “hearts”; 10 - dorsal blood vessel; 11 - abdominal blood vessel.

The importance of oligochaetes in soil formation. Even Charles Darwin noted their beneficial effect on soil fertility. By dragging the remains of plants into the burrows, they enrich it with humus. By making passages in the soil, they facilitate the penetration of air and water to the roots of plants and loosen the soil.

Polychaetes. Representatives of this class are also called polychaetes. They live mainly in the seas. The segmented body of polychaetes consists of three sections: the head lobe, the segmented body and the posterior anal lobe. The head lobe is armed with appendages - tentacles and carries small eyes. The next segment contains a mouth with a pharynx, which can turn outward and often has chitinous jaws. The body segments have two-branched parapodia, armed with setae and often having gill projections.

Among them there are active predators that can swim quite quickly, bending their bodies in waves (nereids); many of them lead a burrowing lifestyle, making long burrows in the sand or silt (peskozhil).

Fertilization is usually external, the embryo turns into a larva characteristic of polychaetes - a trochophore, which actively swims with the help of cilia.

Class Leeches unites about 400 species. Leeches have an elongated and dorso-ventrally flattened body. At the anterior end there is one oral sucker and at the rear end there is another sucker. They do not have parapodia or bristles; they swim, bending their body in waves, or “walk” along the ground or leaves. The body of leeches is covered with a cuticle. Leeches are hermaphrodites and have direct development. They are used in medicine because... Thanks to their release of the protein hirudin, the development of blood clots that clog blood vessels is prevented.

Origin: Annelids evolved from primitive, flatworm-like, ciliated worms. From polychaetes came oligochaetes, and from them came leeches.

New concepts and terms:, polychaetes, oligochaetes, coelom, segments, parapodia, metanephridia, nephrostomy, closed circulatory system, cutaneous gills, trochophore, hirudin.

Questions for consolidation:

• Why did annelids get this name?
• Why are annelids also called secondary cavities?
• What structural features of annelids indicate their higher organization compared to flat and round worms? What organs and organ systems first appear in annelids?
• What is characteristic of the structure of each body segment?
• What is the significance of annelids in nature and human life?
• What are the structural features of annelids in connection with their lifestyle and habitat?

Literature:

1. Bilich G.L., Kryzhanovsky V.A. Biology. Full course. In 3 volumes - M.: LLC Publishing House "Onyx 21st century", 2002
2. Biology: A guide for applicants to universities. Volume 1. - M.: Novaya Volna Publishing House LLC: ONICS Publishing House CJSC, 2000.
3. Kamensky, A. A. Biology. Reference manual / A. A. Kamensky, A. S. Maklakova, N. Yu. Sarycheva // Complete course of preparation for exams, tests, testing. - M.: JSC "ROSMEN-PRESS", 2005. - 399 p.
4. Konstantinov V.M., Babenko V.G., Kuchmenko V.S. Biology: Animals: Textbook for 7th grade students of secondary schools / Ed. V.M.Konstantinova, I.N. Ponoma-roar. – M.: Ventana-Graf, 2001.
5. Konstantinov, V. M. Biology: animals. Textbook for 7th grade general education schools /V. M. Konstantinov, V. G. Babenko, V. S. Kuchmenko. - M.: Ventana-Graf, 2001. - 304 p.
6. Latyushin, V.V. Biology. Animals: textbook. for 7th grade general education institutions / V.V. Laktyushin, V.A. Shapkin. - 5th ed., stereotype. - M.: Bustard, 2004. - 304 p.
7. Pimenov A.V., Goncharov O.V. Biology manual for applicants to universities: Electronic textbook. Scientific editor Gorokhovskaya E.A.
8. Pimenov A.V., Pimenova I.N. Zoology of invertebrates. Theory. Assignments. Answers: Saratov, OJSC publishing house "Lyceum", 2005.
9. Taylor D. Biology / D. Taylor, N. Green, W. Stout. - M.:Mir, 2004. - T.1. - 454s.
10. Chebyshev N.V., Kuznetsov S.V., Zaichikova S.G. Biology: a guide for applicants to universities. T.2. – M.: Novaya Volna Publishing House LLC, 1998.
11. www.collegemicrob.narod.ru
12. www.deta-elis.prom.ua

Annelids have the highest level of organization compared to other types of worms; For the first time, they have a secondary body cavity, a circulatory system, and a more highly organized nervous system. In annelids, inside the primary cavity, another, secondary cavity has formed with its own elastic walls made of mesoderm cells. It can be compared to airbags, one pair in each segment of the body. They “swell”, fill the space between the organs and support them. Now each segment received its own support from the bags of the secondary cavity filled with liquid, and the primary cavity lost this function.

They live in soil, fresh and sea water.

External structure

The earthworm has an almost round body in cross section, up to 30 cm long; have 100-180 segments, or segments. In the anterior third of the body there is a thickening - the girdle (its cells function during the period of sexual reproduction and egg laying). On the sides of each segment there are two pairs of short elastic setae, which help the animal when moving in the soil. The body is reddish-brown in color, lighter on the flat ventral side and darker on the convex dorsal side.

Internal structure

A characteristic feature of the internal structure is that earthworms have developed real tissues. The outside of the body is covered with a layer of ectoderm, the cells of which form the integumentary tissue. The skin epithelium is rich in mucous glandular cells.

Muscles

Under the cells of the skin epithelium there is a well-developed muscle, consisting of a layer of circular muscles and a more powerful layer of longitudinal muscles located under it. Powerful longitudinal and circular muscles change the shape of each segment separately.

The earthworm alternately compresses and lengthens them, then expands and shortens them. Wave-like contractions of the body allow not only crawling through the burrow, but also pushing the soil apart, expanding the movement.

Digestive system

The digestive system begins at the front end of the body with the mouth opening, from which food enters sequentially into the pharynx and esophagus (in earthworms, three pairs of calcareous glands flow into it, the lime coming from them into the esophagus serves to neutralize the acids of rotting leaves on which the animals feed). Then the food passes into the enlarged crop and a small muscular stomach (the muscles in its walls help grind the food).

The midgut stretches from the stomach almost to the posterior end of the body, in which, under the action of enzymes, food is digested and absorbed. Undigested remains enter the short hindgut and are thrown out through the anus. Earthworms feed on half-rotted plant remains, which they swallow along with the soil. As it passes through the intestines, the soil mixes well with organic matter. Earthworm excrement contains five times more nitrogen, seven times more phosphorus and eleven times more potassium than regular soil.

Circulatory system

The circulatory system is closed and consists of blood vessels. The dorsal vessel stretches along the entire body above the intestines, and below it is the abdominal vessel.

In each segment they are united by a ring vessel. In the anterior segments, some annular vessels are thickened, their walls contract and pulsate rhythmically, thanks to which blood is driven from the dorsal vessel to the abdominal one.

The red color of blood is due to the presence of hemoglobin in the plasma. It plays the same role as in humans - nutrients dissolved in the blood are distributed throughout the body.

Breath

Most annelids, including earthworms, are characterized by cutaneous respiration; almost all gas exchange is provided by the surface of the body, therefore the worms are very sensitive to moist soil and are not found in dry sandy soils, where their skin quickly dries out, and after rains, when in the soil a lot of water, crawling to the surface.

Nervous system

In the anterior segment of the worm there is a peripharyngeal ring - the largest accumulation of nerve cells. The abdominal nerve cord with nodes of nerve cells in each segment begins with it.

This nodular type nervous system was formed by the fusion of nerve cords on the right and left sides of the body. It ensures the independence of the joints and the coordinated functioning of all organs.

Excretory organs

The excretory organs look like thin, loop-shaped, curved tubes, which open at one end into the body cavity and at the other outside. New, simpler funnel-shaped excretory organs - metanephridia - remove harmful substances into the external environment as they accumulate.

Reproduction and development

Reproduction occurs only sexually. Earthworms are hermaphrodites. Their reproductive system is located in several segments of the anterior part. The testes lie in front of the ovaries. During mating, the sperm of each of the two worms is transferred to the seminal receptacles (special cavities) of the other. Cross fertilization of worms.

During copulation (mating) and oviposition, girdle cells on the 32-37 segment secrete mucus, which serves to form an egg cocoon, and a protein liquid to nourish the developing embryo. The secretions of the girdle form a kind of mucous coupling (1).

The worm crawls out of it with its back end first, laying eggs in the mucus. The edges of the coupling stick together and a cocoon is formed, which remains in the earthen hole (2). Embryonic development of eggs occurs in a cocoon, from which young worms emerge (3).

Sense organs

The sense organs are very poorly developed. The earthworm does not have real organs of vision; their role is played by individual light-sensitive cells located in the skin. The receptors for touch, taste, and smell are also located there. Earthworms are capable of regeneration (easily restore the back part).

Germ layers

The germ layers are the basis of all organs. In annelids, the ectoderm (outer layer of cells), endoderm (inner layer of cells) and mesoderm (intermediate layer of cells) appear early in development as three germ layers. They give rise to all major organ systems, including the secondary cavity and the circulatory system.

These same organ systems are subsequently preserved in all higher animals, and they are formed from the same three germ layers. Thus, higher animals in their development repeat the evolutionary development of their ancestors.

Annelids– bilaterally symmetrical segmented animals.

Taxonomy. The phylum includes 5 classes, of which the most famous classes are Polychaeta - 13,000 species, Olygochaeta - 3,500 species and Leeches (Hirudinea) - about 400 species.

Body shape and size. The body of ringlets is overwhelmingly worm-shaped, round or oval in cross section. The body has pronounced both external and internal segmentation. In this case they talk about true metamerism. In this case, metamerism also extends to the internal structure of worms. In leeches, external segmentation does not correspond to internal segmentation.

The sizes of annelids range from a few millimeters to 2 m (terrestrial forms) and even up to 3 m (marine species).

External body structure. Polychaetes have a well-defined head section, bearing organs for various purposes: tentacles, ocelli, palps. In some species, the palps grow into a complex trapping apparatus. The last segment contains one or more pairs of sensory antennae. Each body segment bears parapodia on the sides - complex outgrowths of the body. The main function of these outgrowths is the movement of the worm. Each parapodia consists of two lobes, inside of which there are numerous setae. Of these, several are larger, they are called aciculi. A pair of sensitive antennae are attached to the blades. The parapodia often includes the gill apparatus. Parapodia have a fairly diverse structure.

In oligochaete worms, the head section is weakly expressed, and there are no lateral projections (parapodia). There are only relatively few setae. A “belt” consisting of thickened segments is clearly visible on the body.

Leeches have powerful suckers at the front and rear ends of their bodies. Few species have gill projections on the sides.

Skin-muscle bag. On the outside, the body of annelids is covered with a thin cuticle, under which lie skin epithelial cells. The skin of worms is rich in glandular cells. The secretion of these cells has a protective value. In a number of species, skin secretions are used to build unique houses. Worm bristles are derivatives of the epithelium. Under the skin lies a layer of circular muscles, which allows the animal to change the transverse size of the body. Below are the longitudinal muscles, which serve to change the length of the body. In leeches, between the layers of circular and longitudinal muscles there is a layer of diagonal muscles. The ringlets have special muscles that move parapodia, palps, suckers, etc.

Body cavity. The space between the body wall and the internal organs of the rings represents the coelom - the secondary body cavity. It differs from the primary one by the presence of its own epithelial walls, called coelomic epithelium (coelothelium). The coelothelium covers the longitudinal muscles of the body wall, intestines, muscle cords and other internal organs. On the walls of the intestine, the coelothelium is transformed into chloragogenic cells that perform an excretory function. In this case, the coelomic sac of each body segment is isolated from neighboring ones by partitions - dessepiments. Inside, the coelomic sac is filled with fluid containing various cellular elements. In general, it performs different functions - supporting, trophic, excretory, protective and others. In leeches, the coelom has undergone a strong reduction and the space between the body wall and the internal organs is filled with a special tissue - mesenchyme, in which the coelom is preserved only in the form of narrow canals.

The midgut is shaped like a simple tube that can become more complex. Thus, in leeches and some polychaetes the intestine has lateral projections. In oligochaetes, on the dorsal side of the intestine there is a longitudinal fold that protrudes deeply into the intestinal cavity - typhlosol. These devices significantly increase the internal surface of the midgut, which allows for the most complete absorption of digested substances. The midgut is of endodermic origin. In oligochaete worms, at the border of the foregut and midgut there is an extension - the stomach. It can be either ectodermal or endodermal.

The hindgut, which is a derivative of the ectoderm, is usually short and opens into the anus.

The circulatory system of annelids is closed, that is, blood moves everywhere through the vessels. The main vessels are longitudinal - dorsal and abdominal, connected by circular ones. The spinal vessel has the ability to pulsate and performs the function of the heart. In oligochaetes, this function is also performed by the annular vessels of the anterior part of the body. Blood moves from back to front through the spinal vessel. Through the annular vessels located in each segment, the blood passes into the abdominal vessel and moves in it from front to back. Smaller vessels depart from the main vessels, and they in turn branch into tiny capillaries that carry blood to all the tissues of the worms. In leeches, the blood vessel system is significantly reduced. Blood moves through the system of sinuses - remnants of the coelom.

The blood of most annelids contains hemoglobin. This allows them to exist in conditions with little oxygen.

There are usually no special respiratory organs, so gas exchange occurs through the skin by diffusion. Polychaete worms and some leeches have well-developed gills.

The excretory system is most often represented by metanephridia, which are located metamerically, that is, in pairs in each segment. A typical metanephridium is represented by a long convoluted tube. This tube begins as a funnel, which opens into the whole (secondary body cavity) of the segment, then it penetrates the septum between the segments (dissepiment) and enters the glandular metanephridial body located in the next segment. In this gland, the tube twists strongly and then opens with an excretory pore on the lateral surface of the body. The funnel and tube are covered with cilia, with the help of which the cavity fluid is driven into the metanephridium. As it moves through the tube through the gland, water and various salts are absorbed from the liquid, and only products that need to be removed from the body (urine) remain in the cavity of the tube. These products are excreted through the excretory pore. In many species, in the posterior part of the metanephridial tube there is an extension - the bladder, in which urine temporarily accumulates.

In primitive annelids, the excretory organs, like flatworms, are structured like protonephridia.

The nervous system consists of the peripharyngeal ring and the ventral nerve cord. Above the pharynx lies a powerfully developed paired complex of ganglia, representing a kind of brain. A pair of ganglia also lies under the pharynx. The brain is connected to the subpharyngeal ganglia by nerve cords that cover the pharynx from the sides. This entire formation is called the peripharyngeal ring. Further, in each segment under the intestine there is a pair of nerve ganglia that are connected both to each other and to the ganglia of neighboring segments. This system is called the ventral nerve cord. Nerves extend from all ganglia to various organs.

Sense organs. On the head section of polychaete worms there are well-developed sense organs: antennae and palps (organs of touch), eyes (sometimes quite complex), olfactory pits. Some forms have developed balance organs - statocysts. On the lateral outgrowths of the body (parapodia) there are antennae that perform a tactile function.

In polychaete worms, the sensory organs are much less developed than in polychaete worms. There are chemical sense organs, sometimes tentacles, statocysts, and poorly developed eyes. The skin contains a large number of light-sensitive and tactile cells. Some tactile cells have a pin.

Leeches have many sensitive cells scattered throughout their skin; they also always have eyes and chemical sense organs (taste buds).

Reproductive system. Among annelids there are both hermaphroditic and dioecious forms.

Polychaete worms are mostly dioecious. Sometimes sexual dimorphism occurs. The sex glands (gonads) are formed in the coelomic epithelium. This process usually occurs in the posterior segments of the worm.

In oligochaete worms, hermaphroditism is more common. The gonads are usually located in certain segments of the anterior part of the worm. Relatively small male gonads (testes) have excretory ducts, which are either modified metanephridia or canals separated from them. The larger female gonads (ovaries) have ducts that are modified metanephridia. For example, when the ovary is located in the 13th segment, the female genital openings open on the 14th. There are also seminal receptacles, which are filled during mating with the sperm of another worm. Leeches are mostly hermaphrodites. The testes are located metamerically, there is one pair of ovaries. Fertilization in leeches occurs through the exchange of spermatophores between partners.

Reproduction. Annelids have a wide variety of forms of reproduction.

Asexual reproduction is characteristic of some polychaete and oligochaete worms. In this case, either strobilation or lateral budding occurs. This is a rare example of asexual reproduction among highly organized animals in general.

During sexual reproduction of polychaetes, individuals containing mature gonads (epitocenes) switch from a crawling or sessile lifestyle to a swimming one. And in some species, the sexual segments, when the gametes mature, can even tear off from the body of the worm and lead an independent swimming lifestyle. Gametes enter the water through breaks in the body wall. Fertilization occurs either in water or in the epitocine segments of the female.

Reproduction of oligochaetes begins with cross-fertilization. At this time, the two partners touch each other with their ventral sides and exchange sperm, which enters the seminal receptacles. After which the partners separate.

Subsequently, abundant mucus is secreted on the girdle, forming a muff around the girdle. The worm lays eggs in this muff. When the coupling is moved forward, it passes past the openings of the seminal receptacles; At this moment, fertilization of the eggs occurs. When the sleeve with fertilized eggs slides off the head end of the worm, its edges close, and a cocoon is obtained in which further development occurs. An earthworm cocoon usually contains 1-3 eggs.

In leeches, reproduction occurs in approximately the same way as in oligochaete worms. Leech cocoons are large, reaching 2 cm in length in some species. Different species have from 1 to 200 eggs in the cocoon.

Development. The zygote of annelids undergoes complete, usually uneven, fragmentation. Gastrulation occurs by intussusception or epiboly.

In polychaete worms, a larva called a trochophore is subsequently formed from the embryo. She has eyelashes and is quite mobile. From this larva the adult worm develops. Thus, in most polychaete worms, development occurs with metamorphosis. Species with direct development are also known.

Oligochaete worms have direct development without a larval phase. Fully formed young worms emerge from the eggs.

In leeches, the eggs in the cocoon form peculiar larvae that swim in the cocoon liquid using the ciliary apparatus. Thus, an adult leech is formed by metamorphosis.

Regeneration. Many annelids are characterized by a developed ability to regenerate lost body parts. In some species, an entire organism can regenerate from just a few segments. However, in leeches regeneration is very weakly expressed.

Nutrition. Among polychaete worms there are both predators and herbivorous species. There are also known facts of cannibalism. Some species feed on organic debris (detritivores). Oligochaete worms are mainly detritivores, but predators are also found.

Oligochaete worms are mostly soil dwellers. In soils rich in humus, the number of, for example, enchytraeid worms reaches 100-200 thousand per square meter. They also live in fresh, brackish and salt water bodies. Aquatic inhabitants inhabit mainly surface layers of soil and vegetation. Some species are cosmopolitan, but there are also endemics.

Leeches inhabit fresh water bodies. Few species live in the seas. Some switched to a terrestrial lifestyle. These worms either lead an ambush lifestyle or actively seek out their hosts. A single blood sucking provides leeches with food for many months. There are no cosmopolitans among leeches; they are confined to certain geographical areas.

Paleontological finds of annelids are very few. Polychaetes represent greater diversity in this regard. Not only prints have been preserved from them, but also, in many cases, remains of pipes. On this basis, it is assumed that all the main groups of this class were already represented in the Paleozoic. To date, no reliable remains of oligochaete worms and leeches have been found.

Origin. Currently, the most plausible hypothesis is the origin of annelids from parenchymal ancestors (ciliated worms). Polychaetes are considered to be the most primitive group. It is from this group that the oligochaetes most likely originate, and from the latter the group of leeches emerged.

Significance: In nature, annelids are of great importance. Inhabiting various biotopes, these worms are included in numerous food chains, serving as food for a huge number of animals. Land worms play a leading role in soil formation. By processing plant residues, they enrich the soil with minerals and organic substances. Their passages help improve soil gas exchange and drainage.

In practical terms, a number of species of earthworms are used as vermicompost producers. The worm - enchytraea is used as food for aquarium fish. Enchitraevs are bred in huge quantities. For the same purposes, the tubifex worm is harvested from nature. Medicinal leeches are currently used to treat certain diseases. In some tropical countries, palolo is eaten as food - the sexual (epitocene) segments of worms that have separated from the front of the animal and floated to the surface of the water.

Annelids, also called annelids, include a huge number of animal species. Their body consists of numerous repeating elements, which is why they got their name. The general characteristics of annelids unite about 18 thousand different species. They live on land in the soil and on the surface in tropical rainforests, in the seawater of the oceans and the fresh water of rivers.

Classification

Annelids are a type of invertebrate animal. Their group is called protostomes. Biologists distinguish 5 classes of annelids:

Belt, or leeches;

Oligochaetes (the most famous representative of this class is the earthworm);

Polychaetes (peskozhil and nereid);

Misostomidae;

Dinophylids.

Considering the general characteristics of annelids, you understand their important biological role in soil processing and aeration. Earthworms loosen the soil, which is beneficial for all surrounding vegetation on the planet. To understand how many of them there are on earth, imagine that in 1 sq. meter of soil is aerated with 50 to 500 annelids. This increases the productivity of agricultural land.

Annelids are one of the main links in the food chains of ecosystems both on land and in the oceans. They feed on fish, turtles, birds and other animals. Even people use them as a supplement when breeding commercial fish species in both fresh and sea waters. Fishermen use worms as bait on a hook when catching fish with a fishing rod.

Everyone knows about the importance of medicinal leeches, which suck blood from sore spots, relieving a person of bruises. People have long understood their medicinal value. Leeches are used for hypertension and increased blood clotting. Leeches have the ability to produce hirudin. This is a substance that reduces blood clotting and dilates the vessels of the human circulatory system.

Origin

Studying the general characteristics of annelids, scientists found that they have been known since the Cambrian period. Considering their structure, biologists came to the conclusion that they originated from a more ancient type of lower flatworms. The similarity is obvious in certain structural features of the body.

Scientists believe that the main group of polychaete worms appeared first. In the process of evolution, when this type of animal moved to life on the surface and in fresh water bodies, oligochaetes, later called leeches, appeared.

Describing the general characteristics of annelids, we note that this is the most progressive type of worms. It was they who first developed the circulatory system and the ring-shaped body. On each segment, paired organs of movement appeared, which later became the prototype of the limbs.

Archaeologists have found extinct annelids that had several rows of calcareous plates on their backs. Scientists believe that there is a certain connection between them and mollusks and brachiopods.

General characteristics

In grade 7, the type of annelids is studied in more detail. All representatives have a fairly characteristic structure. Both from the front and from the back the body looks the same and symmetrical. Conventionally, it is divided into three main sections: the head lobe, numerous segments of the central part of the body and the posterior or anal lobe. The central segmented part, depending on the size of the worm, can include from ten to several hundred rings.

General characteristics of annelids include information that their sizes vary from 0.25 mm to a length of 5 meters. The movement of worms is carried out in two ways, depending on its type. The first way is through contraction of the body muscles, the second is with the help of parapodia. These are the bristles found in polychaete worms. They have lateral bilobed projections on the walls of the segments. In oligochaete worms, organs such as parapodia are absent altogether or have separately growing small bundles.

Structure of the head blade

Annelids have sensory organs located at the front. These are eyes, olfactory cells, which are also present on the tentacles. Ciliary fossae are organs that distinguish between the effects of various odors and chemical irritants. There are also hearing organs that have a structure reminiscent of locators. And, of course, the main organ is the mouth.

Segmented part

This part represents the same general characteristic of the type of annelids. The central region of the body consists of rings, each of which represents a completely independent part of the body. This area is called the coelom. It is divided into segments by partitions. They are noticeable when looking at the appearance. The outer rings of the worm correspond to the internal partitions. It is on this basis that the worms received their main name - annelids, or annular worms.

This division of the body is very important for the life of the worm. If one or more rings are damaged, the rest remain intact, and the animal regenerates in a short period of time. The internal organs are also arranged according to the segmentation of the rings.

Secondary body cavity, or coelom

The structure of annelids has the following general characteristic: the skin-muscle sac has coelomic fluid inside. It consists of the cuticle, dermal epithelium and circular and longitudinal muscles. The fluid contained in the body cavity maintains a constant internal environment. All the main functions of the body are carried out there: transport, excretory, musculoskeletal and sexual. This fluid is involved in the accumulation of nutrients and removes all waste, harmful substances and sexual products.

The type of annelids also has common characteristics in the area of ​​body cell structure. The upper (outer) layer is called the ectoderm, followed by the mesoderm with a secondary cavity lined with its cells. This is the space from the body walls to the internal organs of the worm. The fluid contained in the secondary body cavity, thanks to pressure, maintains the constant shape of the worm and plays the role of a hydroskeleton. The last inner layer is called endoderm. Since the body of annelids consists of three shells, they are also called three-layered animals.

Worm food system

General characteristics of annelids in grade 7 briefly describe the structure of the digestive system of these animals. In the front part there is a mouth opening. It is located in the first segment from the peritoneum. The entire digestive tract has a through system of structure. This is the mouth itself, then there is a peripharyngeal ring that separates the worm’s pharynx. The long esophagus ends in the goiter and stomach.

The intestine has a common characteristic for the class of annelids. It consists of three departments with different purposes. These are the foregut, middle and hindgut. The middle compartment consists of endoderm, and the rest are ectodermal.

Circulatory system

The general characteristics of annelids are briefly described in the 7th grade textbook. And the structure of the circulatory system can be seen in the schematic image above. Vessels are indicated in red. The figure clearly shows that the circulatory system of annelids is closed. It consists of two long longitudinal vessels. These are dorsal and ventral. They are connected to each other by the annular vessels present in each segment, which resemble veins and arteries. The circulatory system is closed; blood does not leave the vessels and does not pour into the body cavities.

The color of blood in different types of worms can be different: red, transparent and even green. This depends on the properties of the chemical structure of the respiratory pigment. It is close to hemoglobin and has different oxygen content. Depends on the habitat of the ringed worm.

The movement of blood through the vessels is carried out due to contractions of some sections of the spinal and, less commonly, annular vessels. After all, they don’t. The rings contain special contractile elements in these vessels.

Excretory and respiratory systems

These systems in the type annelids (the general characteristics are briefly described in the 7th grade textbook) are associated with the skin. Respiration occurs through the skin or gills, which in marine polychaete worms are located on the parapodia. The gills are branched, thin-walled projections on the dorsal lobes. They can be of different shapes: leaf-shaped, feathery or bushy. The inside of the gills is permeated with thin blood vessels. If the worms are small-chaete, then respiration occurs through the moist skin of the body.

The excretory system consists of metanephridia, protonephridia and myxonephridia, located in pairs in each segment of the worm. Myxonephridia are the prototype of the kidneys. Metanephridia have the shape of a funnel located in the coelom, from which a thin and short channel leads the excretory products to the outside in each segment.

Nervous system

If we compare the general characteristics of roundworms and annelids, the latter have a more advanced nervous system and sensory organs. They have a cluster of nerve cells above the peripharyngeal ring of the anterior lobe of the body. The nervous system consists of ganglia. These are suprapharyngeal and subpharyngeal formations connected by nerve trunks into a peripharyngeal ring. In each segment you can see a pair of such ganglia of the ventral chain of the nervous system.

You can see them in the figure above. They are indicated in yellow. Large ganglia in the pharynx play the role of the brain, from which impulses diverge along the abdominal chain. The worm's sensory organs also belong to the nervous system. He has a lot of them. These are the eyes, the organs of touch on the skin, and the chemical senses. Sensitive cells are located throughout the body.

Reproduction

Describing the general characteristics of the type of annelids (class 7), one cannot fail to mention the reproduction of these animals. They are mostly heterosexual, but some have developed hermaphroditism. The latter include the well-known leeches and earthworms. In this case, conception occurs in the body itself, without fertilization from the outside.

In many polychaetes, development occurs from the larva, while in other subspecies it is direct. The gonads are located under the coelomal epithelium in each or almost every segment. When a rupture occurs in these cells, the germ cells enter the coelom fluid and are excreted through the organs of the excretory system. For many, fertilization occurs on the outer surface, while in underground soil worms, fertilization occurs inside.

But there is another type of reproduction. In conditions favorable for life, when there is a lot of food, individuals begin to grow individual body parts. For example, several mouths may appear. Subsequently, the rest grows. The worm breaks down into several separate parts. This is an asexual type of reproduction, when a certain part of the body appears, and the rest are regenerated later. An example is the ability of Aulophorus for this type of reproduction.

In the article you learned in detail all the main characteristics of annelids, which are studied in the 7th grade of school. We hope that such a detailed description of these animals will help you learn more easily.