What sensory organs do annelids have. The structure of annelids

Annelids belong to the subsection of coelomic animals Coelomata), a group (supertype) of protostomes (Protostomia). It is characteristic for the primordials:

The primary mouth (blastopore) of the embryo (gastrula) passes into from an adult animal or the definitive mouth is formed in place
primary mouth.
The mesoderm is usually formed teloblastically.
Covers are single layered.
The skeleton is external.
The following types of animals are protostomes: annelids (Annelida), molluscs (Mollusca), arthropods (Arthropoda), onychophora (Onychophora).
Annelids are a vast group of animals, about 12 thousand species are known. They are inhabitants of the seas, fresh water bodies, inhabit the land.

Polychaete annelids Polychaetes

The main features of the type:

The body consists of a head lobe (prostomium), a segmented trunk, and an anal lobe (pygidium). The metamerism of the external and internal structure.
The body cavity is secondary, in most animals it is well developed. The blades are devoid of coelom.
The skin-muscular sac is developed, represented by the epithelium and muscles, circular and longitudinal.
The intestine consists of three sections, the salivary glands are developed.
Excretory system of nephridial type.
The circulatory system is of a closed type, in some groups it is absent.
The respiratory system is either absent, animals breathe with the entire surface of the body, some representatives have gills.
The nervous system consists of a paired brain and a ventral nerve chain or ladder.
Annelids are dioecious or hermaphrodites.
Crushing of eggs on a spiral type, deterministic.
Development with metamorphosis or direct.

Annelids General characteristics

Latin name Annelida

Type of annelids, or rings, is a very important group for understanding the evolution of higher invertebrates. It includes about 8700 species. Compared with the flat and roundworms considered, and even with nemerteans, annelids are much more highly organized animals.

main feature external structure rings is metamerism, or segmentation of the body. The body consists of a more or less significant number of segments, or metameres. The metamerism of rings is expressed not only in the external, but also in the internal organization, in the repetition of many internal organs.

They have a secondary body cavity - generally absent in lower worms. The body cavity of the annulus is also segmented, that is, it is divided by partitions more or less in accordance with the external segmentation.

At rings there is a well-developed closed circulatory system. Excretory organs - metanephridia - are located segmentally, and therefore are called segmental organs.

Nervous system consists of a paired supraoesophageal ganglion, called the cerebrum, connected by circumoesophageal connectives to the ventral nerve cord. The latter consists of a pair of longitudinally approximated trunks in each segment, forming ganglia, or nerve nodes.

Internal structure

musculature

Under the epithelium is a muscular sac. It consists of external circular and internal longitudinal muscles. Longitudinal muscles in the form of a continuous layer or divided into ribbons.
Leeches have a layer of diagonal muscles, which are located between the annular and longitudinal. Dorso-abdominal muscles are well developed in leeches. Wandering polychaetes have developed flexors and extensors of the parapodia, derivatives of the annular muscles. The annular musculature of oligochaetes is more developed in the anterior eight segments, which is associated with lifestyle.

body cavity

Secondary or general. The body cavity is lined with coelomic or perineal epithelium, which separates the cavity fluid from tissues and organs. Each body segment of polychaetes and oligochaetes has two coelomic sacs. On the one hand, the walls of the sacs adjoin the muscles, forming a somatopleura, on the other hand, to the intestines and to each other, a splanchnopleura (intestinal leaf) is formed. The splanchnopleura of the right and left sacs forms the mesentery (mesenterium) - a two-layer longitudinal septum. Either two or one septum is developed. The walls of the sacs facing the neighboring segments form dissipations. Dissepiments disappear in some polychaetes. Generally absent in prostomium and pygidium. In almost all leeches (with the exception of bristle-bearing ones), the parenchyma between the organs is generally preserved in the form of lacunae.

Functions of the coelom: supporting, distributive, excretory, and in polychaetes - sexual.

The origin of the whole. Four hypotheses are known: myocoel, gonocoel, enterocoel and schisocoel.

Digestive system

Represented by three departments. Digestion is abdominal. The pharynx of predatory polychaetes is armed with chitinous jaws. The ducts of the salivary glands open into the throat of annelids. Leech glands contain the anticoagulant hirudin. In earthworms, ducts of calcareous (morren) glands flow into the esophagus. The composition of the foregut of earthworms includes, in addition to the pharynx and esophagus, goiter and muscular stomach. The absorption surface of the middle intestine increases due to outgrowths - diverticulum (leeches, part of polychaetes) or typhlosol (oligochaetes).

excretory system

Nefridial type. As a rule, each segment has two excretory canals, they begin in one segment and open with an excretory pore in the next segment of the body. The excretory organs of polychaetes are the most diverse. Polychaete worms have the following types of excretory system: protonephridia, metanephridia, nephromixia, and myxonefridia. Protonephridia are developed in larvae, they begin with terminal club-shaped cells with a flagellum (solenocytes), then the nephridial canal. Metanephridia begin with a funnel with a nephrostomy, inside
funnels are located cilia, followed by a duct and nephropore. Protonephridia and metanephridia are ectodermal in origin. Nephromyxia and mixonephridia are the fusion of the ducts of the protonephridium or metanephridium with the coelomoduct - the genital infundibulum. Coeloducts of mesodermal origin. The excretory organs of oligochaetes and leeches are metanephridia. In leeches, their number is much less than that of body segments (the medical leech has 17 pairs), the separation of the funnel from the canal is characteristic. In the excretory canals of nephridia, ammonia is converted into macromolecular compounds, and water is absorbed as a whole. Annelids also have “kidneys” of accumulation: chloragogenic tissue (polychaetes, oligochaetes) and botryoid tissue (leeches). They accumulate guanine, uric acid salts, which are removed from the coelom through nephridia.

The circulatory system of annelids

Most annelids have a closed circulatory system. It is represented by two main vessels (dorsal and abdominal) and a network of capillaries. The movement of blood is carried out due to the contraction of the walls of the spinal vessel; in oligochaetes, ring hearts also contract. The direction of blood flow along the dorsal vessel from back to front, abdominal - in the opposite direction. The circulatory system is developed in bristle-bearing and proboscis leeches. In jawed leeches, there are no vessels; the function of the circulatory system is performed by the lacunar system. The process of functional replacement of one organ by another, different in origin, is called organ substitution. The blood of annelids is often colored red due to the presence of hemoglobin. Primitive polychaetes have no circulatory system.

Respiratory system

Most breathe with the entire surface of the body, some polychaetes and some leeches have gills. The respiratory organs are evaginated. The gills of polychaetes by origin are a modified dorsal antennae of parapodia, leeches are skin outgrowths.

Nervous system and sense organs

The structure of the nervous system includes: a paired cerebral (supraglottic) ganglion, connectives, subpharyngeal ganglia and the ventral nerve chain or ladder-type nervous system. The abdominal trunks are connected by commissures. The evolution of the nervous system went in the direction of transforming the ladder-type nervous system into a chain, immersing the system into the body cavity. Nerves extending from the central system make up the peripheral system. There is a different degree of development of the supraesophageal ganglion, the brain is either monolithic or separate departments. For leeches, the fusion of the ganglia of the segments that make up the suckers is characteristic. Sense organs. Polychaetes: epithelial sensory cells, antennae, nuchal organs, parapodial antennae, statocysts, organs of vision (goblet or bubble type eyes). Sense organs of oligochaetes: light-sensitive cells, some inhabitants of the water have eyes, chemical sense organs, tactile cells. Leeches: goblet organs - chemical sense organs, eyes.

Classification

The type of rings is divided into several classes, of which we will consider four:

1. Multi-brush rings (Polychaeta)

2. Echiurida (Echiurida)

Echiurids are an extremely modified group of annulus, the internal organization of which differs from that of polychaetes in an unsegmented coelom, the presence of one pair of metanephria.
The trochophore larva of Echiuridae is of the greatest importance for establishing the unity of the origin of Echiurids with polychaetes.

At the bottom of the sea, among the stones in the silt, sand, there are peculiar animals, but in appearance they very little resemble annelids, primarily due to their lack of segmentation. This includes such forms as Bonellia, Echiurus and some others, in total about 150 species. The body of the female Bonellia, living in the crevices of stones, has the shape of a cucumber and carries a long non-retractable trunk, forked at the end. The length of the trunk can be several times the length of the body. A groove lined with cilia runs along the trunk, and a mouth is located at the base of the trunk. With the flow of water through the groove, small food particles are brought to the mouth. On the ventral side of the anterior part of the body of Bonellia there are two large setae, while in other Echiurids, at the posterior end, there is also a corolla of small setae. The presence of setae brings them closer to the annulus.

3. Small-bristle rings (Oligochaeta)

Small-bristle rings, or oligochaetes, are a large group of rings, including about 3100 species. They undoubtedly descend from polychaetes, but differ from them in many essential features.
The overwhelming majority of oligochaetes live in the soil and at the bottom of fresh water bodies, where they often burrow into silty soil. In almost every fresh water body you can find the Tubifex worm, sometimes in huge numbers. The worm lives in silt, and sits with its head end buried in the ground, and its rear end constantly oscillates.
Soil oligochaetes include a large group of earthworms, an example of which is the common earthworm (Lumbricus terrestris).
Oligochaetes feed mainly on plant foods, mainly on the decaying parts of plants that they find in the soil and in the silt.
Considering the features of oligochaetes, we will have in mind mainly the common earthworm.

4. Leeches (Hirudinea) >> >>

Phylogeny

The problem of the origin of the rings is very controversial, there are various hypotheses on this issue. One of the most common hypotheses to date was put forward by E. Meyer and A. Lang. It is called the turbellar theory, since its authors believed that the polychaete rings originate from turbellarian-like ancestors, that is, they associated the origin of the rings with flatworms. At the same time, supporters of this hypothesis point to the phenomenon of the so-called pseudometamerism observed in some turbellarians and expressed in the repetition of certain organs along the length of the body (intestinal outgrowths, metameric arrangement of the gonads). They also point to the similarity of the trochophore larva of the annulus with the Müllerian larva of turbellaria and to the possible origin of metanephridia by changing the protonephridial system, especially since the larva of the annulus - trochophores - and the lower annulus have typical protonephridia.

However, other zoologists believe that annelids are closer to nemerteans in a number of ways and that they are descended from nemertean ancestors. This point of view is developed by N. A. Livanov.

The third hypothesis is called the trochophore theory. Its supporters produce rings from the hypothetical ancestor of the trochozoon, which has a trochophore-like structure and descends from ctenophores.

As for the phylogenetic relationships within the four classes of annelids considered, they now seem to be fairly clear.

Thus, annelids, which are highly organized protostomes, apparently originate from ancient protostomes.

Undoubtedly, not only modern polychaetes, but also other groups of annelids originated from ancient polychaetes. But it is especially important that polychaetes are a nodal group in the evolution of higher protostomes. Mollusks and arthropods originate from them.

Meaning of annelids

Polychaete worms.

 Food for fish and other animals. Mass species play the greatest role. Introduction of polychaetes of the Azov nereid into the Caspian Sea.
 Human food (palolo and other species).
 Purification of sea water, processing of organic matter.
 Settlement on the bottoms of ships (serpulids) - reduction in speed.

Small-bristle worms.

 Oligochaetes - inhabitants of water bodies are the food of many animals, they are involved in the processing of organic matter.
 Earthworms - animal food and human food. Gallery

The most famous representatives of annelids for each person are leeches (subclass Hirudinea) and earthworms (suborder Lumbricina), which are also called earthworms. But in total there are more than 20 thousand species of these animals.

Systematics

To date, experts refer to the type of annelids from 16 to 22 thousand modern species animals. There is no single approved classification of rings. The Soviet zoologist V.N. Beklemishev proposed a classification based on the division of all representatives of annelids into two superclasses: girdleless, which includes polychaetes and echiurids, and girdle, including oligochaetes and leeches.

The following is a classification from the World Register of Marine Species website.

Table of biological taxonomy of annelids

Class*	Subclass	Infraclass	Detachment
Polychaete worms, or polychaetes (lat. Polychaeta)
			Amphinomida Eunicida Phyllodocida
	Polychaeta incertae sedis (disputed species)
	Sedentaria	Canalipalpata	Sabellida Spionida Terebellida
	Sedentaria	Scolecida (Scolecida)	Capitellida Cossurida Opheliida Orbinida Questida Scolecidaformia
	Palpata		Polygordiida Protodrilida
	Errantia (sometimes called Aciculata)		Amphinomida Eunicida Phyllodocida
Belt class (Clitellata)	Leeches (Hirudinea)	Acanthobdellidea
	Leeches (Hirudinea)		Jawed or yueskhobotkovye leeches (Arhynchobdellida) Proboscis leeches (Rhynchobdellida)
	Small-bristle worms (Oligochaeta)		Capilloventrida crassiclitellata Enchytraeida Haplotaxida (this includes the order Earthworms) Lumbriculida Oligochaeta incertae SEDIS (species uncertain)
Echiuridae (Echiura)			Echiura incertae sedis (disputed species) Unreviewed

There is also a superclass Annelida incertae sedis, which includes controversial species. There, according to the World Register of Marine Species, such a controversial group as Myzostomidae (Myzostomida), which other classifications refer to polychaete worms or even separate into a separate class, also entered as a detachment.

Class Polychaete(Polychaetes). Representatives of the class have connected lateral appendages (parapodia) bearing chitinous setae; the name of the group is determined by the presence of a large number of setae per segment. Head with or without appendages. In most cases - dioecious; gametes are dumped directly into the water, where fertilization and development take place; floating freely and are called trochophores. Sometimes they reproduce by budding or fragmentation. The class includes more than 6000 species, which are divided into free-living and sessile forms.
Class Poyaskovye (Clitellata). Representatives of the class on the body have a small number or no bristles at all. Parapodia are absent. They are characterized by the presence of a unique reproductive organ - a girdle, which is formed from the remains of a cocoon and performs a protective function for fertilized eggs. The class has about 10,000 representatives.
- Subclass Small-bristle(Oligochetes). They live primarily in fresh water. They have setae that arise directly from the walls of the body, due to the small number of which (usually 4 on each segment), the subclass was called low-setae. Appendages on the body, as a rule, do not have. Hermaphrodites. Development is direct, there is no larval stage. There are about 3250 species.
- Subclass Leeches. They inhabit mainly freshwater reservoirs, but there are also terrestrial and marine forms. There is a small sucker at the anterior end of the body and a large sucker at the posterior end. The fixed number of body segments is 33. The body cavity is filled with connective tissue. Hermaphrodites. Fertilized eggs are laid in a cocoon. Development is direct, there is no larval stage. There are about 300 types of representatives.

Class Echiuridae (Echiura). This is a small group with only about 170 known species, all of which are exclusively marine life. Echiurids were recently classified as annelids after DNA examinations, but earlier it was a separate type. The reason is that their body is different - it does not have segmentation, like annelids. In some sources, the Echiurids are considered not as a separate class, but as a subclass of Polychaetes.

Spreading

Annelids, depending on the species, live on land, in fresh and salt water.

Polychaete worms, as a rule, live in sea water (with the exception of some species that can also be found in freshwater bodies). They are food for fish, crayfish, as well as birds and mammals.

Small-bristle worms, to a subclass of which the earthworm belongs, live in soil fertilized with humus or fresh water.

Echiurides are distributed only in marine waters.

Morphology

The main characteristic of representatives of the Annelida type is considered to be the division of the body into a number of cylindrical segments, or metameres, the total number of which, depending on the type of worm, varies widely. Each metamere consists of a section of the body wall and a section of the body cavity with its internal organs. The number of outer rings of worms corresponds to the number of inner segments. The body of annelids consists of the region of the head (prostomium); a body consisting of metameres; and a segmented posterior lobe called the pygidium. In some primitive representatives of this type, the metameres are identical, or very similar to each other, each containing the same structures; in more advanced forms, there is a tendency to consolidate some segments and restrict certain organs to certain segments.

The outer shell of the body of annelids (skin-muscular sac) includes the epidermis surrounded by the cuticle, as well as well-developed, segmentally located muscles - annular and longitudinal. Most annelids have external short setae composed of chitin. In addition, on each metamere, some representatives of this type of animals may have primitive limbs called parapodia, on the surface of which setae and sometimes gills are located. The spatial movement of the worms is carried out either through muscle contraction or movements of the parapodia.

The body length of annelids ranges from 0.2 mm to 5 m.

The main general anatomical features of annelids in cross section

Digestive system Annelids consists of an unsegmented intestine that runs through the middle of the body from the oral cavity, located on the underside of the head, to the anus, located on the anal lobe. The intestine is separated from the body wall by a cavity called the whole. The segmented compartments of the coelom are usually separated from each other by thin sheets of tissue called septa that perforate the gut and blood vessels. With the exception of leeches, in general, representatives of annelids are filled with liquid and function as a skeleton, providing muscle movement, as well as transport, sexual, and excretory functions of the body. When the integrity of the body of the worm is damaged, it loses the ability to move properly, since the functioning of the muscles of the body depends on maintaining the volume of coelomic fluid in the body cavity. In primitive annelids, each compartment of the coelom is connected to the outside by means of channels for the release of germ cells and paired excretory organs (nephridia). In more complex species, both the excretory and reproductive functions are sometimes served by one type of canal (the canals may be absent in certain segments).

Circulatory system. In annelids, for the first time in the process of evolution, a circulatory system appeared. Blood usually contains hemoglobin, a red respiratory pigment; however, some annelids contain chlorocruorin, a green respiratory pigment that gives blood its color.

The circulatory system is usually closed, i.e. enclosed in well-developed blood vessels; in some species of polychaetes and leeches, an open-type circulatory system appears (blood and abdominal fluid mix directly in the sinuses of the body cavity). The main vessels - the abdominal and dorsal - are interconnected by a network of annular vessels. Blood is distributed in each segment of the body along the lateral vessels. Some of them contain contractile elements and serve as a heart, i.e. play the role of pumping organs that move the blood.

Respiratory system. Some aquatic annelids have thin-walled, feathery gills through which gases are exchanged between the blood and the environment. However, most representatives of this type of invertebrates do not have any special organs for gas exchange, and breathing occurs directly through the surface of the body.

Nervous system, as a rule, consists of a primitive brain, or ganglion, located in the head region, connected by a ring of nerves to the ventral nerve cord. In all metameres of the body there is a separate nerve node.

The sense organs of annelids typically include eyes, taste buds, tactile tentacles, and statocysts, organs responsible for balance.

reproduction annelides occur either sexually or asexually. Asexual reproduction is possible through fragmentation, budding, or division. Among worms that reproduce sexually, there are hermaphrodites, but most species are dioecious. The fertilized eggs of marine annelids usually develop into free-swimming larvae. The eggs of terrestrial forms are encased in cocoons and larvae, like miniature versions of the adults.

The ability to restore lost body parts is highly developed in many annelids with many and few bristles.

Ecological significance

The earthworm is very important for maintaining the condition of the soil

Charles Darwin in book The Formation of Vegetable Mold through the Action of Worms (1881) presented the first scientific analysis of the influence of earthworms on soil fertility. Some of the worms burrow in the soil, while others live exclusively on the surface, usually in wet leaf litter. In the first case, the animal is able to loosen the soil so that oxygen and water can penetrate into it. Both surface and burrowing worms help improve soil in several ways:

by mixing organic and mineral substances;
by accelerating decomposition organic matter, which in turn makes them more accessible to other organisms;
by concentrating minerals and converting them into forms that are more easily absorbed by plants.

Earthworms are also important prey for birds ranging in size from robins to storks, and for mammals ranging from shrews to badgers, in some cases.

Terrestrial annelids in some cases can be invasive (brought into a certain area by people). In glacial regions North America, for example, scientists believe that almost all native earthworms were killed by glaciers and worms that are currently found in these areas (for example, Amynthas Agrestis) were imported from other areas, primarily from Europe, and more recently , from Asia. Northern deciduous forests were particularly affected by invasive worms through the loss of leaf litter, reduced soil fertility, changes in chemical composition soils and loss of ecological diversity.

Marine annelids can make up over one-third of benthic animal species around coral reefs and in intertidal areas. Burrowing annelids increase the infiltration of water and oxygen into the seabed sediment, which promotes the growth of populations of aerobic bacteria and small animals.

Human interaction

Anglers believe that worms are more effective baits for fish than artificial fly baits. In this case, the worms can be stored for several days in a tin can filled with wet moss.

Scientists study aquatic annelids to monitor oxygen levels, salinity and pollution environment in fresh and sea water.

The jaws of polychaetes are very strong. These advantages have attracted the attention of engineers. Research has shown that the jaws of this genus of worms are made up of unusual proteins that bind strongly to zinc.

On the island of Samoa, catching and eating one of the representatives of annelids - the Palolo worm - is a national holiday, and the worm itself is considered a delicacy by the locals. In Korea and Japan, Urechis unicinctus worms from the Echiuridae class are eaten.

Representatives of annelids, which are eaten

Cases of using leeches for medical purposes were known as early as China around 30 AD, India around 200 AD, ancient rome around 50 AD and then throughout Europe. In the medical practice of the 19th century, the use of leeches was so widespread that their stocks in some parts of the world were depleted, and some regions imposed restrictions or bans on their export (while the medicinal leeches themselves were considered an endangered species). More recently, leeches have been used in microsurgery for transplantation of organs and their parts, skin areas. In addition, scientists argue that the saliva of medical leeches has an anti-inflammatory effect, and some anticoagulants contained in it prevent the growth of malignant tumors.

About 17 species of leeches are dangerous for humans.

Medical leeches are used for hirudotherapy, and a valuable remedy is extracted from pharmacies - hirudin

Leeches can attach to the skin of a person from the outside, or penetrate into internal organs (for example, the respiratory or gastrointestinal tract). In this regard, there are two types of this disease - internal and external hirudinosis. With external hirudinosis, leeches are most often attached to human skin in the armpits, neck, shoulders, and calves.

Misostomida on sea lily

The type of annelids, uniting about 12,000 species, is, as it were, a node of the genealogical tree of the animal world. According to existing theories, annelids originate from ancient ciliary worms (turbellar theory) or from forms close to ctenophores (trochophore theory). In turn, arthropods arose from annelids in the process of progressive evolution. Finally, in their origin, the annelids are connected common ancestor with shellfish. All this shows that great importance, which has the type under consideration for understanding the phylogeny of the animal world. Medically, annelides are of limited value. Only leeches are of some interest.

General characteristics of the type

The body of annelids consists of a head lobe, a segmented body, and a posterior lobe. Segments of the trunk throughout almost the entire body have external appendages similar to each other and a similar internal structure. Thus, the organization of annelids is characterized by structural repeatability, or metamerism.

On the sides of the body, each segment usually has external appendages in the form of muscular outgrowths equipped with bristles - parapodia - or in the form of setae. These appendages are important in the movement of the worm. Parapodia in the process of phylogenesis gave rise to the limbs of arthropods. At the head end of the body there are special appendages - tentacles and palygs.

A skin-muscular sac is developed, which consists of a cuticle, one layer of skin cells underlying it and several layers of muscles (see Table 1) and a secondary body cavity, or coelom, in which internal organs are located. The whole is lined with peritoneal epithelium and is divided by partitions into separate chambers. At the same time, each segment of the body has a pair of coelomic sacs (only the head and posterior lobes are devoid of the coelom).

The coelomic sacs in each segment are placed between the intestine and the body wall and are filled with a watery fluid in which the amoeboid cells float.

In general, it performs a supporting function. In addition, nutrients from the intestines enter the coelomic fluid, which are then distributed throughout the body. In general, harmful metabolic products accumulate, which are removed by the excretory organs. Male and female gonads develop in the walls of the coelom.

The central nervous system is represented by the supraesophageal ganglion and the ventral nerve cord. Nerves from the sense organs pass to the supraglottic node: eyes, balance organs, tentacles and palps. The abdominal nerve cord consists of nodes (one pair in each segment of the body) and trunks that connect the nodes to each other. Each node innervates all the organs of this segment.

The digestive system consists of the anterior, middle and hindgut. The foregut is usually divided into a number of sections: the pharynx, esophagus, crop, and gizzard. The mouth is on the ventral side of the first body segment. The hindgut opens with an anus on the posterior lobe. In the wall of the intestine there is a musculature that ensures the movement of food.

The organs of excretion - metanephridia - are paired tubular organs, metamerically repeated in body segments. Unlike protonephridia, they have a through excretory canal. The latter begins with a funnel that opens into the body cavity. The cavity fluid enters the nephridium through the funnel. A tubule of nephridium departs from the funnel, sometimes opening outwards. Passing through the tubule, the liquid changes its composition; it concentrates the end products of dissimilation, which are ejected from the body through the outer pore of the nephridium.

For the first time in the phylogenesis of the animal kingdom, annelids have a circulatory system. The main blood vessels run along the dorsal and ventral sides. In the anterior segments they are connected by transverse vessels. The dorsal and anterior annular vessels are able to contract rhythmically and perform the function of the heart. In most species, the circulatory system is closed: blood circulates through a system of vessels, nowhere interrupted by cavities, lacunae or sinuses. In some species, the blood is colorless, in others it is red due to the presence of hemoglobin.

Most species of annelids breathe through skin rich in blood capillaries. A number of marine forms have specialized respiratory organs - gills. They usually develop on the parapodia or on the palps. Vessels carrying venous blood approach the gills; it is saturated with oxygen and enters the body of the worm in the form of arterial blood. Among annelids there are dioecious and hermaphroditic species. The sex glands are located in the body cavity.

Ringed worms have the most high organization compared to other types of worms (see Table 1); for the first time they have a secondary body cavity, a circulatory system, respiratory organs, and a more highly organized nervous system.

Table 1. Characteristic features various types worms
Type of	Skin-muscular sac	Digestive system	Circulatory system	reproductive system	Nervous system	body cavity
flatworms	Includes layers of longitudinal and circular muscles, as well as bundles of dorso-abdominal and diagonal muscles	From the ectodermal foregut and endodermal midgut	not developed	hermaphroditic	Paired brain ganglion and several pairs of nerve trunks	Absent, filled with parenchyma
roundworms	Only longitudinal muscles	From the ectodermal foregut and hindgut and endodermal midgut	Same	Dioecious	Periopharyngeal nerve ring and 6 longitudinal trunks	Primary
	From external circular and internal longitudinal muscles	From the ectodermal foregut and hindgut and endodermal midgut	Well developed, closed	Dioecious or hermaphrodites	Paired brain ganglion, peripharyngeal nerve ring, ventral nerve cord	Secondary

Animals belonging to the type of annelids, or annelids, are characterized by:

three-layer, i.e., the development of ecto-, ento- and mesoderm in embryos;
secondary (coelomic) body cavity;
skin-muscular sac;
two-sided symmetry;
external and internal homonomous (equivalent) metamerism or segmentation of the body;
the presence of the main organ systems: digestive, respiratory, excretory, circulatory, nervous, sexual;
closed circulatory system;
excretory system in the form of metanephridia;
the nervous system, consisting of the supraesophageal ganglion, peripharyngeal commissures and a paired or unpaired ventral nerve cord;
the presence of primitive organs of locomotion (parapodia)

Ringed worms live in fresh and sea waters, as well as in the soil. Several species live in the air. The main classes of the type of annelids are:

polychaetes (Polychaeta)
oligochaeta (Oligochaeta)
leeches (Hirudinea)

Class polychaetal rings

From the point of view of the phylogenesis of the animal world, polychaetes are the most important group of annelids, since the emergence of higher groups of invertebrates is associated with their progressive development. The body of polychaetes is segmented. There are parapodia, consisting of dorsal and ventral branches, each of which bears a tendril. The muscular wall of the parapodia has thick supporting setae, and tufts of thin setae protrude from the apex of both branches. The function of the parapodia is different. Usually these are locomotor organs involved in the movement of the worm. Sometimes the dorsal barnacle grows and turns into a gill. The circulatory system of polychaetes is well developed and always closed. There are species with dermal and gill breathing. Polychaetes are dioecious worms. They live in the seas, mainly in the coastal zone.

Nereid (Nereis pelagica) can serve as a characteristic representative of the class. It is found in abundance in the seas of our country; leads a bottom way of life, being a predator, captures prey with its jaws. Another representative - sandworm (Arenicola marina) - lives in the seas, digs holes. It feeds by passing sea silt through its digestive tract. Breathe with gills.

Class low-bristle rings

The oligochaetes are descended from polychaetes. The external appendages of the body are setae, which sit directly in the wall of the body; no parapodia. The circulatory system is closed; skin breathing. Small-bristle rings are hermaphrodites. The vast majority of species are inhabitants of fresh water and soil.

An earthworm (Lumbricus terrestris) can serve as a characteristic representative of the class. Earthworms live in the soil; during the day they sit in holes, and in the evening they often crawl out. Rummaging in the soil, they pass it through their intestines and feed on the plant residues contained in it. Earthworms play an important role in soil-forming processes; they loosen the soil and contribute to its aeration; leaves are dragged into holes, enriching the soil with organic substances; they extract deep layers of soil to the surface, and superficial ones carry them deeper.

The structure and reproduction of the earthworm

The earthworm has an almost round body in cross section, up to 30 cm long; have 100-180 segments or segments. In the front third of the body of the earthworm there is a thickening - a girdle (its cells function during the period of sexual reproduction and oviposition). On the sides of each segment, two pairs of short elastic bristles are developed, 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.

A characteristic feature of the internal structure is that earthworms have developed real tissues. Outside, 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. Under the skin there is a well-developed musculature, consisting of a layer of annular and a more powerful layer of longitudinal muscles located under it. With the contraction of the circular muscles, the body of the animal is stretched and becomes thinner; with the contraction of the longitudinal muscles, it thickens and pushes the soil particles apart.

The digestive system begins at the front end of the body with a mouth opening, from which food enters sequentially into the pharynx, 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 that animals feed on). Then the food passes into an enlarged goiter, and a small muscular stomach (the muscles in its walls contribute to the grinding of food). From the stomach almost to the rear end of the body stretches the middle intestine, in which, under the action of enzymes, food is digested and absorbed. Undigested residues enter the short hindgut and are thrown out through the anus. Earthworms feed on half-decayed plant remains, which they swallow along with the earth. When passing 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 ordinary soil.

The circulatory system is closed and consists of blood vessels. The dorsal vessel stretches along the entire body above the intestines, and under it - the abdominal one. In each segment, they are united by an annular vessel. In the anterior segments, some annular vessels are thickened, their walls contract and pulsate rhythmically, due to which blood is distilled from the dorsal vessel to the abdominal one. The red color of blood is due to the presence of hemoglobin in the plasma. For most annelids, including earthworms, skin respiration is characteristic, almost all gas exchange is provided by the body surface, therefore earthworms are very sensitive to soil moisture and are not found in dry sandy soils, where their skin dries out soon, and after rains, when in soil a lot of water, crawl to the surface.

The excretory system is represented by metanephridia. Metanephridium begins in the body cavity with a funnel (nephrostome) from which a duct extends - a thin loop-shaped curved tube that opens outward as an excretory pore in the side wall of the body. Each segment of the worm has a pair of metanephridia - right and left. The funnel and duct are equipped with cilia that cause the movement of excretory fluid.

The nervous system has a structure typical of annelids (see Table 1), two ventral nerve trunks, their nodes are interconnected and form an ventral nerve chain. The sense organs are very poorly developed. The earthworm does not have real organs of vision, their role is performed by individual light-sensitive cells located in the skin. The receptors for touch, taste, and smell are also located there. Like hydra, earthworms are capable of regeneration.

Reproduction occurs only sexually. Earthworms are hermaphrodites. In front of their body are the testes and ovaries. Fertilization of earthworms is cross. During copulation and oviposition, the cells of the girdle on the 32-37th segment secrete mucus, which serves to form the egg cocoon, and a protein liquid to nourish the developing embryo. The secretions of the girdle form a kind of mucous sleeve. The worm crawls out of it with its rear end forward, laying eggs in the mucus. The edges of the muff stick together and a cocoon is formed, which remains in the earthen burrow. Embryonic development of eggs occurs in a cocoon, young worms emerge from it.

The passages of earthworms are mainly in the surface layer of the soil to a depth of 1 m, for the winter they descend to a depth of 2 m. atmospheric air and water, necessary for plant roots and the vital activity of soil microorganisms. Through its intestines, the worm passes as much soil per day as its body weighs (an average of 4-5 g). On each hectare of land, earthworms daily process an average of 0.25 tons of soil, and per year they throw out to the surface in the form of excrement from 10 to 30 tons of the soil they have processed. In Japan, specially bred breeds of fast-reproducing earthworms are bred and their excrement is used for the biological method of tillage. Vegetables and fruits grown on such soil have an increased sugar content. Charles Darwin was the first to point out the important role of earthworms in soil formation processes.

Annelids play a significant role in the nutrition of bottom fish, since in some places worms make up to 50-60% of the biomass of the bottom layers of water bodies. In 1939-1940. The nereis worm was moved from the Sea of Azov to the Caspian Sea, which now forms the basis of the diet of sturgeons of the Caspian Sea.

Leech class

The body is segmented. In addition to true metamerism, there is false ringing - several rings in one segment. Parapodia and setae absent. The secondary body cavity was reduced; instead, there are sinuses and gaps between the organs. The circulatory system is not closed; blood only part of its path passes through the vessels and pours out of them into the sinuses and lacunae. There are no respiratory organs. The reproductive system is hermaphrodite.

Medical leeches are specially bred and then sent to hospitals. They are used, for example, in the treatment of eye diseases associated with an increase in intraocular pressure (glaucoma), with cerebral hemorrhage and hypertension. With thrombosis and thrombophlebitis, hirudin reduces blood clotting and promotes the dissolution of blood clots.

Consider a number of animals that biology studies - the type of annelids. We learn about their species, lifestyle and habitat, internal and external structure.

general characteristics

Annelids (also called simply annelids, or annelids) are one of the largest, which includes, according to various sources, about 18 thousand species. They are non-skeletal vertebrates that not only participate in the destruction of organic matter, but are also an important component of the nutrition of other animals.

Where can you find these animals? The habitat of annelids is very extensive - these are seas, and land, and fresh water. The annelids living in the salty waters of the ocean are very diverse. Kolchetsov can be found in all latitudes and depths of the World Ocean, even at the bottom of the Mariana Trench. Their density is high - up to 100,000 specimens per square meter of bottom surface. Marine annelids are a favorite food for fish and play an important role in the marine ecosystem.

Aquatic species not only crawl along the bottom or burrow into the silt, some of them can build a protective tube and live without leaving it.

The best known are the annelids that live in the soil, they are called earthworms. The density of these animals in meadow and forest soils can reach up to 600 specimens per square meter. These worms are actively involved in soil formation.

Classes of annelids

Respiratory organs and circulatory system of annelids

Small-bristle worms breathe through the entire surface of their body. But polychaetes have respiratory organs - gills. They are bushy, leaf-like or pinnate outgrowths of the parapodia, penetrated by a large number of blood vessels.

The circulatory system of the annelids is closed. It consists of two large vessels - abdominal and dorsal, which are connected by annular vessels in each segment. The movement of blood is carried out due to contractions of certain sections of the spinal or annular vessels.

The circulatory system of the annelids is filled with the same red blood as in humans. This means that it contains iron. However, the element is not part of hemoglobin, but of another pigment - hemerythrin, which captures 5 times more oxygen. This feature allows worms to live in conditions of oxygen deficiency.

Digestive and excretory systems

The digestive system of annelids can be divided into three sections. The anterior intestine (stomodeum) includes the oral opening and oral cavity, sharp jaws, pharynx, salivary glands, and narrow esophagus.

The oral cavity, which is also called the buccal region, is able to turn inside out. Behind this section are the jaws, which are bent inward. This apparatus is used to capture prey.

Then comes the mesodeum, the midgut. The structure of this department is uniform along the entire length of the body. The middle intestine narrows and expands, it is in it that food is digested. The hindgut is short, ending with an anus.

The excretory system is represented by metanephridia located in pairs in each segment. They remove waste products from the abdominal fluid.

Nervous system and sense organs

All classes of annelids have nervous system ganglion type. It consists of a parapharyngeal nerve ring, which is formed by the connected supraesophageal and subpharyngeal ganglia, and of pairs of a chain of abdominal ganglia located in each segment.

The sense organs of the rings are well developed. Worms have sharp eyesight, hearing, smell, touch. Some annelids not only capture light, but can also emit it themselves.

reproduction

The characteristic of the annelids suggests that representatives of this type of animal can reproduce both sexually and can be produced by dividing the body into parts. The worm breaks into halves, each of them becomes a full-fledged individual.

At the same time, the tail of the animal is an independent unit and can grow a new head for itself. In some cases, a second head begins to form in the center of the worm's body before separation.

Budding is less common. Of particular interest are species in which the budding process can cover the entire body, when the posterior ends bud off from each segment. In the process of reproduction, additional mouth openings may also form, which will later separate into independent individuals.

Worms can be dioecious, but some species (mainly leeches and earthworms) have developed hermaphroditism, when both individuals play the role of both female and male at the same time. Fertilization can occur both in the body and in the external environment.

For example, in sexually reproducing, fertilization is external. Animals of different sexes throw their germ cells into the water, where eggs and sperm merge. From fertilized eggs, larvae appear that do not look like adults. Freshwater and terrestrial annelids do not have a larval stage, they are born immediately similar in structure to adults.

Class Polychaete

Marine annelids belonging to this class are quite diverse in form and behavior. Polychaetes are distinguished by a well-defined head region and the presence of parapodia, peculiar limbs. They are predominantly heterosexual, the development of the worm occurs with metamorphosis.

Nereids actively swim, can burrow into the silt. They have a serpentine body and many parapodia; animals make passages with the help of a retractable pharynx. Sandworms look like earthworms and burrow deep into the sand. An interesting characteristic of the annelids is that it moves in the sand in a hydraulic way, pushing the cavity fluid from one segment to another.

Curious and sedentary worms, serpulids, which live in spiral or twisted calcareous tubes. Serpulids protrude from their dwelling only their heads with large fan-shaped gills.

Class Low-bristle

Small-bristle worms mainly live in the soil and fresh waters; they are found singly in the seas. The structure of annelids of this class is distinguished by the absence of parapodia, homonomous segmentation of the body, and the presence of a glandular girdle in mature individuals.

The head section is not pronounced, it may be devoid of eyes and appendages. On the body are setae, rudiments of parapodia. This body structure is due to the fact that the animal leads a burrowing lifestyle.

Very common and familiar to all low-bristle are earthworms that live in the soil. The body of the worm can be from a few centimeters to three meters (such giants live in Australia). Also in the soil are often found small, about a centimeter in size, whitish enchytreid worms.

In fresh water, you can find worms living in entire colonies of vertical tubes. They are filter feeders feeding on suspended organic residues.

Leech class

All leeches are predators, mostly feeding on the blood of warm-blooded animals, worms, molluscs, and fish. The habitat of annelids of the class of leeches is very diverse. Most often, leeches are found in fresh water, wet grass. But there are also marine forms, and even terrestrial leeches live in Ceylon.

Of interest are the digestive organs of leeches. Their mouth is equipped with three chitinous plates that cut through the skin, or proboscis. The oral cavity contains numerous salivary glands that can secrete a poisonous secret, and the pharynx acts as a sucking pump.

Class Echiuridae

One of the rare species of animals that biology studies is the annelids Echiurids. The Echiurid class is small, it has only about 150 species. These are soft, sausage-like marine worms with a proboscis. The mouth is located at the base of a non-retractable proboscis, which the animal can discard and grow back.

The habitat of annelid worms of the Echiurid class is the great depths of the sea, sandy burrows or rock crevices, empty shells and other shelters. Worms are filter feeders.

annelids- bilaterally symmetrical segmented animals.

Systematics. The type includes 5 classes, of which the most famous classes are Polychaeta (Polychaeta) - 13000 species, Olygochaeta - 3500 species and Leeches (Hirudinea) - about 400 species.

Body shape and size. The body of the rings is overwhelmingly worm-shaped, round or oval in cross section. The trunk has a pronounced both external and internal segmentation. In this case one speaks of true metamerism. At the same time, metamerism 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).

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

In oligochaete worms, the head section is weakly expressed, lateral outgrowths (parapodia) are absent. Only relatively few setae are present. On the body, a “belt” is clearly visible, consisting of thickened segments.

Leeches have powerful suckers at the anterior and posterior ends of the body. Few species have gill outgrowths on the sides.

Skin-muscle bag. Outside, the body of annelids is covered with a thin cuticle, under which lie the cells of the skin epithelium. The skin of worms is rich in glandular cells. The secret of these cells has a protective value. In a number of species, skin secretions are used to build peculiar houses. The bristles of the worms 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 that serve to change the length of the body. In leeches, between the layers of the annular and longitudinal muscles, there is a layer of diagonal muscles. Rings have special muscles that set in motion parapodia, palps, suckers, etc.

body cavity. The space between the wall of the body and the internal organs of the annulus represents the whole - the secondary cavity of the body. It differs from the primary one by the presence of its own epithelial walls, which are called the coelomic epithelium (the whole body). The coelothelium covers the longitudinal muscles of the body wall, intestines, muscle cords and other internal organs. On the walls of the intestine, the whole body is transformed into chloragogenic cells that perform an excretory function. At the same time, the coelomic sac of each body segment is isolated from the neighboring ones by partitions - dessepiments. Inside the coelomic sac is filled with a liquid containing various cellular elements. As a whole, it performs various functions - supporting, trophic, excretory, protective and others. In leeches, the whole 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 whole is preserved only in the form of narrow channels.

The midgut is shaped like a simple tube that can become more complex. So, in leeches and some polychaetes, the intestine has lateral outgrowths. The oligochaetes have a longitudinal fold on the dorsal side of the intestine, which protrudes deeply into the intestinal cavity - typhlosol. These devices significantly increase the inner surface of the midgut, which allows the most complete assimilation of digested substances. The midgut is endodermic in origin. In small-bristle worms, on the border of the anterior and middle intestines, 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 with an anus.

The circulatory system of annelids is closed, that is, blood moves everywhere through the vessels. The main vessels - longitudinal - dorsal and abdominal, connected by annular. 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 along the dorsal 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 the smallest capillaries that carry blood to all the tissues of the worms. In leeches, the system of blood vessels is significantly reduced. Blood moves through the system of sinuses - the remnants of the coelom.

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

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 coiled tube. This tube begins with a funnel that opens as a 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 winds 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 forced into the metanephridium. When moving through the tube through the gland, water and various salts are absorbed from the liquid, and only products to be removed from the body (urine) remain in the tube cavity. These products are excreted through the excretory pore. In many species, there is an extension in the posterior part of the metanephridial tube - the bladder, in which urine temporarily accumulates.

In primitive annelids, the excretory organs, like flatworms, are arranged according to the type of 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 lie under the pharynx. The brain is connected to the subpharyngeal ganglia by nerve cords covering the pharynx from the sides. All this formation is called the peripharyngeal ring. Further, in each segment under the intestine there is a pair of nerve ganglia, which are connected both to each other and to the ganglia of neighboring segments. This system is called the ventral nerve cord. From all ganglia, nerves depart 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 organs of balance - statocysts. On the lateral outgrowths of the body (parapodia) there are antennae that perform a tactile function.

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

In leeches, many sensitive cells are scattered in the skin, there are always eyes and chemical sense organs (taste buds).

Sexual system. Among annelids, there are both hermaphroditic and dioecious forms.

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

In small-bristle worms, hermaphroditism is more common. The sex glands 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 isolated from them. Larger female sex glands (ovaries) have ducts, which are altered 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 spermatozoa of another worm. Leeches are mostly hermaphrodites. The testes are located metamerically, the ovaries are one pair. Fertilization in leeches occurs by the exchange of spermatophores between partners.

Reproduction. Ringed worms are characterized by 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, polychaete individuals containing mature gonads (epitocal) move from a crawling or sedentary lifestyle to a swimming one. And in some species, the sexual segments during the maturation of gametes can even break away from the body of the worm and lead an independent floating lifestyle. Gametes enter the water through breaks in the body wall. Fertilization takes place either in water or in the epitonic segments of the female.

Reproduction of oligochaetes begins with cross-fertilization. At this time, two partners are applied to each other by the abdominal sides and exchange sperm, which enters the seminiferous receptacles. After that, the partners disperse.

Subsequently, abundant mucus is secreted on the girdle, forming a sleeve around the girdle. The worm lays its eggs in this clutch. When the clutch is moved forward, it passes by the holes of the seed receptacles; at this point, fertilization of the eggs occurs. When the clutch with fertilized eggs slides off the head end of the worm, its edges close, and a cocoon is obtained in which further development takes place. The cocoon of earthworms usually contains 1-3 eggs.

In leeches, reproduction occurs in much the same way as in oligochaete worms. Leech cocoons are large, reaching 2 cm in length in some species. Located in a cocoon different types from 1 to 200 eggs.

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

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

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

In leeches, peculiar larvae form from eggs in a cocoon, which swim in the cocoon fluid with the help of a 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 weak.

Nutrition. Among the polychaete worms, there are both predators and herbivorous species. There are also known cases of cannibalism. Some species feed on organic remains (detritivores). Small-bristle worms are mainly detritivores, but there are also predators.

Small-bristle worms are mostly soil inhabitants. In humus-rich soils, the number of, for example, enchitreid worms reaches 100-200 thousand per square meter. They also live in fresh, brackish and salty water bodies. Aquatic inhabitants inhabit mainly the surface layers of the soil and vegetation. Some of the species are cosmopolitan, and some are endemic.

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

Paleontological finds of annelids are very few. Polychaetes are more diverse in this respect. Not only prints have been preserved from them, but also in many cases the remains of pipes. On this basis, it is assumed that all the main groups of this class were represented already in the Paleozoic. Reliable remains of oligochaete worms and leeches have not been found to date.

Origin. Currently, the most plausible hypothesis is the origin of annelids from parenchymal ancestors (ciliary worms). The most primitive group is considered to be polychaetes. It is from this group that the oligochaetes most likely originate, and from the latter a 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. Terrestrial worms play a leading role in soil formation. By processing plant residues, they enrich the soil with mineral and organic substances. Their moves contribute to the improvement of soil gas exchange and its drainage.

In practical terms, a number of earthworm species are used as vermicompost producers. The worm - enchitreus is used as food for aquarium fish. Enchitreev breed in huge quantities. For the same purpose, the tubifex worm is mined in nature. Medicinal leeches are currently used to treat certain diseases. In some tropical countries, palolo are eaten - sexual (epitocal) segments of worms that have separated from the front of the animals and floated to the surface of the water.