Body shape of polychaete worms. Class polychaete annelids. Meaning of annelids

Most famous representatives annelid for each person are leeches (subclass Hirudinea) and earthworms(suborder Lumbricina), which are also called rain. 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 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 ( controversial species)
	Sedentaria	Canalipalpata	Sabellida Spionida Terebellida
		Scolecida (Scolecida)	Capitellida Cossurida Opheliida Orbinida Questida Scolecidaformia
	Palpata		Polygordiida Protodrilida
	Errantia (sometimes called Aciculata)		Amphinomida Eunicida Phyllodocida
Belt class (Clitellata)	Leeches (Hirudinea)	Acanthobdellidea
			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. Fixed number of body segments 33. Body cavity filled 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 oem.

Echiurides are distributed only in marine waters.

Morphology

The main characteristic of representatives of the Annelida type is the division of the body into a number of cylindrical segments, or metameres, total which, depending on the type of worms, 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 in some representatives of this type of animals there may be primitive limbs, called parapodia, on the surface of which bristles 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.

Basic general anatomical features annelids in cross section

Digestive system annelids consists of an unsegmented gut that runs through the middle of the body from 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 excretory and reproductive functions are sometimes served by the same type of canals (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 bodies for gas exchange, and respiration takes place 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 possibly through fragmentation, budding, or fission. 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 The Formation of Vegetable Mold through the Action of Worms (1881), presented the first scientific analysis 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 the decomposition of organic substances, 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 (listed in certain locality people). In glacial regions North America, for example, scientists believe that almost all local earthworms were killed by glaciers and the worms that are currently found in these areas (for example, Amynthas Agrestis) were imported from other areas, primarily from Europe, and in recent times, from Asia. Northern deciduous forests were especially negative impact invasive worms through loss of leaf litter, reduced soil fertility, changes in soil chemistry and loss of ecological diversity.

Marine annelids can make up more than one third of benthic animal species around coral reefs and in intertidal zones. 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 local residents delicacy. 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. AT medical practice In 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 leech used for hirudotherapy, and a valuable remedy is extracted from pharmacies - hirudin

Leeches can attach to human skin 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

Annelida Polychaeta. Photo: Paul.Paquette

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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



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, annelids are connected by a common ancestor with molluscs. 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 divided by septa 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, they accumulate harmful products metabolism, 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. Characteristics various types of worms
Type	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 cutaneous and gill respiration. 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. characteristic feature internal structure is that earthworms have developed true 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 rhythmically pulsate, 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 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 annually 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 to biological method 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 sturgeon fish 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.

Polychaete worms, they are also polychaetes, belong to the class of annelids and live mainly on the bottom of the seas. Only a few species are adapted to life in fresh water. Their role in the ecosystem is significant. Polychaetes filter water, clean the soil from decaying organic residues. In turn, the worms themselves become food for many fish, crustaceans, and echinoderm marine life.

Seta worms live at the bottom of the seas, and are rarely found in freshwater bodies of water.

Description and structure

Outwardly, this representative of the ringed can be characterized as follows:

1. The length of polychaetes can be from 2 mm to 3 m.
2. The body shape of polychaete worms consists of many segments, on the sides of which there are skin-muscular outgrowths that help polychaetes move. These organs of movement are called parapodia. The worm swims near the bottom, bending its body and raking with muscular outgrowths.
3. In addition, the head segment (prostomium) and the caudal lobe (pygidium) are distinguished.
4. Tentacles, palpi and antennae may be present on the head - all of them serve as organs of touch for polychaetes.

Among the polychaete worms, there are sitting subspecies with a reduced number of parapodia, which are preserved only in the anterior part of the body. These polychaetes live inside a protective tube they have built and never leave it.

Internal organs and systems of the representative of the annelids arranged as follows:

Worm larvae lead a planktonic lifestyle in the water column and are carried by the current over long distances, this is how their settlement occurs. Initially, the trochophore consists of two hemispheres, gradually its body is elongated and takes on a worm-like shape due to the growth of larval (larval) segments. The growth zone is more often formed at the posterior end of the larva.

Reproduction of polychaetes

Most polychaete worms reproduces sexually. The females release eggs and the males release sperm. The genital organs of animals are developed in the peritoneal epithelium. Fertilization in most species occurs in the external environment.

As soon as the body segment is overflowing with mature germ cells, the epithelium breaks and they fall out. In other species, there are special funnels for this - coelomoducts. The fertilized larva is called a trochophore. Having settled to the bottom, after a while it turns into an adult.

Marine worms reproduce sexually

Only a few forms have a sophisticated reproductive apparatus that allows them to copulate (for example, Saccocirrus). Many species of polychaete worms reproduce by budding. At the same time, part of the body segments separates and breaks up into separate segments.

In the future, each of them forms the head and anal parts, becoming an independent individual. This process is called archetomy. With paratomy, everything happens the other way around - a chain consisting of several individuals is separated. Later they separate, becoming individually existing worms.

Practical value

Marine polychaete worms inhabit salt water bodies in large numbers and serve as food for many commercial species fish. Polychaetes make up the main diet of stellate sturgeon and sturgeon. Only with a lack of polychaete worms does the fish switch to other types of food and begin to eat mollusks, shrimps and other crustaceans. The Caspian Sea, where sturgeon fish is fished, long time had only 5 species of polychaetes.

A group of Soviet scientists carried out acclimatization in the Caspian Sea of ​​the Nereis polychaete, which was delivered from Sea of ​​Azov. It is this worm that is distinguished by unpretentiousness and minimal requirements for the level of salinity of the water. In the 40s of the last century, 65,000 Nereis polychaetes were released into the waters of the Caspian Sea, and by the end of the first decade, the worms inhabited an area of ​​30,000 km2. This made it possible to significantly increase the biological value of the Caspian Sea.

(Fig. 30) - sea ​​worms with paired appendages - parapodia on the trunk segments. The body is covered with a thin cuticle, without a nerve plexus in the ectoderm. The head lobe bears eyes and various appendages - palps and antennae, on which the organs of chemical sense are located (see Fig. 29).

Rice. 30. Polychaetes:

1 - Typhloscolex; 2 - Netochaeta; 3 - Eunice viridis

Depending on the number of segments, oligomeric (small-segmented) and polymeric (multi-segmented) worms are distinguished. Segments may be identical (homonomous) or differ from each other according to the shape and nature of the appendages. Homonomy is a sign of primitive organization and is inherent in free-moving forms.

Parapodia - a two-branched outgrowth of the lateral wall of the segment, equipped with bundles of setae and a tentacle - antennae on each or only on one branch. These are primitive organs of movement. In sessile forms, the parapodia are often partially reduced.

Under the skin are layers of circular and longitudinal muscles. The skin-muscular sac is lined from the inside with epithelium, and under it is placed the body cavity - the whole. In each segment, the whole consists of two sacs, the walls of which converge above and below the intestine, creating a longitudinal septum. At the border between the segments, the coelomic epithelium forms a two-layer septum - a septum, or dissipation. In some departments, the septa may be reduced. As a whole, it performs support (due to the liquid filling it), distributive, excretory and sexual functions.

The digestive system begins with the oral cavity, which passes into the muscular pharynx. It is followed by the esophagus. Some polychaetes have a small stomach. The midgut is a straight tube. The hindgut is short, opening with an anus on the dorsal side of the anal lobe.

Respiration in polychaetes is carried out through the surface of the body, but in most there are special areas through which gas exchange occurs. Usually this is the dorsal antennae of the parapodia, which has turned into a gill.

The circulatory system is closed, consisting of longitudinal main vessels (above and below the intestine), which communicate through a system of annular vessels. The movement of blood is determined by the pulsation of the walls of the spinal vessel. When the system is reduced, its functions are taken over by the coelomic fluid.

The excretory system is represented by nephridia of various structures. As a rule, each of them is a tube, one is expanded, the end of which opens as a whole, the other - outwards. Since nephridia are present in every segment of the body, they are also called segmental organs. In some lower forms, these organs are represented by protonephridia, which have a bundle of club-shaped solenocyte cells at the inner end of the tubule. In other polychaetes, this bundle atrophies, instead of it a hole appears, lined with cilia. Such organs are called metanephridia.

The nervous system within a class varies from the ventral scala to the ventral nerve cord. In addition, it leaves the periphery (from the skin epithelium) deep into, sometimes even into the body cavity. There is a concentration of nerve nodes. The sense organs are diverse and better expressed in freely mobile forms. These are the organs of touch, chemical sense (chemoreceptors) and vision. The latter may look like an eye cup or bubble.

The reproductive system is simple. Polychaete rings are separate sexes. Gonads are formed either in all (with the exception of the first and last), or only in special, or fertile, segments. This is a bunch of germ cells lying under the coelomic epithelium. Mature sexual products through the breaks of the epithelium enter as a whole, and then are excreted by the genital funnels with the excretory canal to the outside. In most cases, these funnels fuse with nephridia and then perform sexual and excretory functions.

Fertilization is external. Cleavage of eggs is complete, spiral, deterministic. After crushing, an extremely typical larva is formed - a trochophore, and then dissected into a small number of metatrochophore segments. New segments (after-larval, or post-larval) always appear in one place - in the growth zone, which is located between the pygidium and the penultimate trunk segment.

Polychaetes play an important role in the ecosystems of the seas, as they serve as food for fish, crabs and other animals. In some cases, some species of polychaetes are even relocated to new water bodies for them to improve the diet of commercial fish.