Annelids have a secondary body cavity. The circulatory system of annelids. Digestive and excretory systems

Circulatory system . The vessels of the circulatory system carry red blood. The spinal vessel has the ability to pulsate, i.e., contractile movements of the walls and usually drives blood from back to front.

Special vessels covering the intestinal tube and located metamerically in each segment (rings of the body) transfer blood to the abdominal vessel, which is not capable of independent pulsation. Blood moves in it from front to back. In addition to these directions of blood currents, the vessels that carry blood from the spinal vessel to the parapodia are important. These are parapodial vessels. In the parapodia, the blood vessels acquire the character of capillaries, where the oxidation of blood occurs, which is in close contact with the oxygen of the external environment. The dorsal vessel reaches the prostomiamind, the abdominal vessel ends at the level of the pharynx, that is, it is somewhat shorter. The skin is also intensively supplied with capillary blood vessels. However, the intestinal tube and all internal organs, as well as dissipations are also abundantly irrigated with blood.

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1 - nerves to the palps. 2—nerves to the antennae (pyrruses), 3—supraglottic ganglion, 4, 5—circumopharyngeal nerve ring, 6—nerves of the ventral nerve cord, 7.—beginning of the ventral nerve cord

. I - the pharynx and buccal section are retracted; II - the buccal section is turned out, the pharynx is pushed forward:

1 - buccal: department, 2 - pharynx

. Throat (1) with big jaws pushed out

Respiratory system. The capillaries of the circulatory system of parapodia and skin are very great importance in the respiratory processes of Nereis, while there are no special gill outgrowths in species of this genus.

excretory system. In Nereis it consists of paired metanephridia. They are absent only in five anterior metameres and three or four posterior ones. The metanephridia of Nereis are very typical. Each metanephridium consists of a glandular metanephridial body, shaped like a sac, pierced by a convoluted nephridial canal. This channel begins even outside the body of metanephridium in the cavity of the corresponding coelomic sac with a funnel, or nephrostomy. The nephrostomy rapidly narrows into a canal that pierces the wall of the dissipation lying in front of it and enters the cavity of the next coelomic sac (anterior to the previous one), where the body of the metanephridium itself lies. Inside the metanephridial body, the anterior part of the nephridial canal(closest to the nephrostomy) carries thin cilia that work in concert and drive the fluid in the tube of the nephridial canal forward to the outer outlet, i.e. into external environment. This posterior part of the nephridial canal does not bear cilia. The outer opening of the nephridial canal is called the nephropore. Thus, the liquid content of metanephridium, penetrating into the nephridial canal from the coelom, is brought out through the nephropore. Such is the anatomy of metanephridium. As for its function, it consists in removing liquid metabolic products through the nephridial canal, which accumulate partly in the form of grains in the coelomic cavity.

Annelids are characterized by the appearance of the following organizational features:

1. secondary body cavity (or coelom);
2. circulatory system;
3. metamerism (body segmentation).

In addition to the above features that play important role in the evolution of animals, annelids are also characterized by:

The presence of special organs of movement (parapodia);
significant supraesophageal ganglion and ventral nerve cord with nerve ganglions;
the presence of a closed circulatory system;
metanephridial structure of the excretory system.

Low-bristle class

The oligochaete class includes about 2500 species. Among the oligochaetes, they are found as soil (eg. earthworms), and water forms (eg. Tubifex). Animals belonging to this class have an elongated worm-shaped body. The latter is clearly segmented and consists of a large number of rings. Hence the name of the type. ringed worms. In cross section, the body is rounded, but at the same time, the ventral side is somewhat flattened compared to the dorsal side. The body is covered with a dense skin, consisting of a cuticle, under which lies a layer of epithelium.

In the skin of worms there are numerous glands that secrete mucus (such glands are especially developed in soil worms, for example, earthworms, since the secret secreted by them facilitates the movement of animals between soil particles). There are no special organs of locomotion (parapodia) in oligochaete worms, and their body is covered on the outside with a few short setae, for example, in earthworms, 8 setae are located on each ring. Closer to the rear, more thickened, end of the body there is a special formation called the girdle. It secretes a mucus that hardens in the air and forms a cocoon.

Under the epithelium is the muscular system of the worm, consisting of layers of circular muscles and longitudinal muscles. Directly to the longitudinal muscles from the side of the body cavity adjoins a layer of coelothelium - a special tissue lining the secondary body cavity (whole) and covering inner surface muscular system and the surface of internal organs. Its presence is an important anatomical feature. The cuticle layer, epithelium, annular and longitudinal muscles and coelothelium together form skin-muscle sac. Along with the outer segmentation of annelid worms, internal segmentation is also noted. This is expressed in the presence of special partitions between the segments (dissepiments), as well as in the segmentation of internal organs: in each ring there is a nerve node, lateral protrusions of the intestine, annular blood vessels of metanephridia and sex glands.

The whole is filled with liquid in which phagocytic cells resembling amoeba are suspended. Apparently, they perform a protective function, capturing foreign particles. The whole, in addition, performs the function of a support (it is a hydroskeleton, like the primary cavity of roundworms), and also participates in the processes of reproduction, excretion of substances and their transport. In the body of worms, the whole is represented by two fluid-filled sacs, delimited by the coelothelium. The axial organs located in the center of the body, in particular the intestines, are enveloped on both sides by the whole body (on the left side, one of the walls of the left sac, and on the right side, the right sac). At the point of contact of the whole body of the left and right sacs, under and above the intestines, a longitudinal two-layer partition appears - the mesenterium, on which the intestines seem to be suspended. In earthworms, the mesentery is partially reduced.

The nervous system is represented by the peripharyngeal nerve ring and the ventral nerve cord. The upper part of the ring is formed by two nerve ganglia, from which annular commissures extend downward (connecting under the digestive tube), when connected, paired subesophageal ganglia (nodes) are formed. From these ganglia, the ventral nerve chain originates, stretching along the entire body. In each segment, it thickens somewhat, as a result of which the ganglia of the corresponding segment are formed. As a result, the abdominal nerve chain has the shape of a rosary. Numerous nerves depart from the ganglions, some of which end in the skin with sensitive receptors that respond to touch, light, temperature, chemical stimuli, etc. Annelids are very sensitive to the action of stimuli.

At the front end of the body is a mouth opening that passes into the pharynx. The latter passes into a narrower esophagus, where three pairs of calcareous glands flow in earthworms, secreting an alkaline, lime-containing secret. It is necessary for the neutralization of humic acids, in in large numbers contained in rotted plant residues and soil humus, which these animals feed on. The esophagus, expanding, passes into the goiter, and behind it begins one or more muscular stomachs. In the stomach, food, as a rule, is only crushed. Digestion takes place in the midgut, which has a metameric organization and an invagination on the dorsal side (tiflozol), which increases the absorption surface. The hindgut does not have such an invagination. From this part of the intestine, undigested residues are excreted into the external environment through the anus.

The circulatory system is closed, there is no real heart, and the movement of blood through the vessels is carried out due to the pulsation of their walls. There are dorsal and abdominal vessels connected by annular bridges (annular vessels). Thus, the circulatory system also has a metameric organization. Lateral vessels depart from the main vessels, which branch out strongly and eventually form a dense network of capillaries. The dense network of blood vessels and capillaries that surrounds the intestines is called the intestinal plexus. In the anterior part of the body, the annular vessels have strongly thickened walls. Such vessels are called "hearts", although, once again, we note that annelids do not have a real heart. Respiration in small-bristle worms is carried out through the skin. They do not have real respiratory organs.

Polychaete class (polychaetes)

Representatives lead a free-floating and attached lifestyle. Movement is carried out by parapodia, equipped with bundles of setae. Parapodia are prototypes of the limbs of arthropods.

Some polychaetes have parapodia gill apparatus, providing gas exchange in aquatic environment. Representatives of the class have a well-separated head section, where the sense organs are located: tentacles, light-sensitive eyes, olfactory fossa. In the structure of the nervous, circulatory, excretory and digestive systems, polychaetes repeat the features of their type. Dioecious, development goes with transformation (there is larval stage).

Polychaetes - a progressive branch of animals from which arthropods descend. They serve as food for marine animals. Nereids are specially acclimatized in the Caspian Sea as food for sturgeons. Palolo living in tropical waters Pacific Ocean, is of commercial importance.

Leech class

The most famous representative of this class is the medicinal leech, which has been used to treat people since ancient times. Leeches are characterized by the presence of two suckers: the front, at the bottom of which the mouth is located, and the back.

The posterior sucker is large, its diameter exceeds half of the maximum width of the body. Leeches bite through the skin with three jaws, seated along the edges with sharp teeth (up to 100 on each jaw). Strong bloodsucker. In medicine, it is used for diseases of the blood vessels (formation of blood clots), hypertension, pre-stroke condition.

Leeches are applied to a certain part of a sick person in order to suck blood, as a result, blood clots dissolve, blood pressure decreases, and the person's condition improves. Besides, salivary glands medicinal leeches produce a valuable substance - hirudin - which prevents blood clotting. Therefore, after leech injections, the wound bleeds for a long time. Being in the stomach of a leech, the blood under the influence of hirudin is stored for months without being subjected to coagulation and decay.

Digestive system leeches are built in such a way that they can accumulate large reserves of blood, preserved with hirudin. The size of a leech that has sucked blood increases significantly. Due to this feature, leeches can starve for a long time (from several months to 1 year - approx. site). The leech lives up to 5 years. Leeches are hermaphrodites. I reach in nature! puberty only in the third year of life and lay cocoons once a year in the summer.

Leeches are characterized by a straight developed. Leeches include a non-blood-sucking predatory leech - a large false horse. Eats worms (including leeches), soft-bodied, larvae of aquatic insects, small vertebrates (tadpoles), which she can overcome.

The phylum annelids are protostomes, the most highly organized of all worms. The type includes, according to various sources, 10-18 thousand species and is divided into three classes: polychaetes (the most numerous, more than 10 thousand species), oligochaetes and leeches. These worms live in water bodies, not excluding the ocean floor and icy waters Atlantic, and in the soil. The exceptions are several species of leeches, which have adapted to terrestrial living during moist forests tropics. Well, the most famous and familiar low-bristle worms in our latitudes are earthworms, so named for the custom, together, in dozens, crawl out of the soil during rain to breathe in oxygen. In addition, their presence is given out by small tubercles-excavations, especially clearly visible in spring - young worms loosen the soil, simultaneously saturating it with oxygen. We will talk about earthworms in more detail, and now we will consider important aromorphoses and structural features of annelids.

Aromorphoses of annelids

1. Overall - secondary body cavity, that is, a closed, fluid-filled space that separates the intestines from the walls of the body. It is important to emphasize that, unlike the primary cavity of roundworms, the whole annelids has an epithelial membrane, a lining. In multi- and low-bristle worms, in general, it occupies a fairly large volume. In the fluid that fills it, cells are free floating, participating in excretory, gas exchange and other processes.

2. Closed circulatory system- its occurrence is directly connected with the appearance of the coelom. As the worm larva develops, the secondary cavity displaces the primary cavity, the remnants of which turn into blood vessels.

3. Metamerism- segmentation, duplication of internal organs, due to which the loss of a body part is not critical for the worm. Each ringlet has its own sex glands, excretory organs, nerve nodes, etc.

4. Parapodia- outgrowths conducive to movement on the sides of the body in polychaetes.

The structure of annelids

1. Body sizes from a quarter of a millimeter to three or even more meters.

2. In cross section, the body is close to a circle or oval. There are three sections: head (head lobe), trunk and anal lobe. Ringworms grow by forming new segments in the region of the anal lobe.

2. The body is segmented by constrictions into many homogeneous rings(segments). It is important that the whole also has partitions, in accordance with the outer division. Upper layer skin-muscular sac- cuticle, next - single-layer epithelium. Muscles of two types, circular on the outside, longitudinal lie deeper.

3. On the head, in addition mouth opening, y different types there may be eyes, organs of touch (various whiskers, palps, etc.).

4. Grow from the cuticle bristles, which can be very numerous along the entire length of the body.

Organ systems

1. Digestive system not closed, it is divided into three sections, represented by the anterior, middle (here nutrients absorbed) and the hindgut. Some types of worms have acquired salivary glands.

2. Ringworms are heartless creatures, in their closed circulatory system there are only different types of vessels through which blood moves. Interestingly, the red color for the blood of worms is not necessary - it all depends on the pigments.

3. Breath can be carried out in two ways - either by the surface of the body (as in oligochaete worms and leeches), or through primitive gills that are located on outgrowths-parapodia (in polychaete worms).

4. Nervous system begins in the head of the worm, where two nerve ganglia, supraglottic and subpharyngeal, connected by strands, form the peripharyngeal nerve ring. From the ganglion under the pharynx, a pair of nerve trunks with ganglia comes out and stretches along the body, connected by jumpers in each segment. This is the so-called ventral nerve cord.

5. sense organs quite well developed in active worms: tactile cells, eyes (not all species have them), chemoreceptors, an organ of balance.

6. excretory system presented in all rings in pairs metanephridia: tubules located in the coelom that open outward on the surface of the body.

Origin of annelids

1. The ancestors of annelids were free-living flatworms. How can you prove it? The larvae of polychaete worms are very similar to planarians. What is it expressed in? Trochophora, a polychaete larva, has cilia, ocelli, tube-shaped metanephridia with stellate cells and a "flickering flame" formed by the beating of cilia. In addition, the nervous system of the trochophore is very similar to nervous system planaria.

2. The oligochaetes evolved from the ancient polychaetes as a result of a simplification in their structure caused by life in the thickness of the soil.

3. Leeches are descended from ancient oligochaete worms.

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. 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 biological taxonomy annelids

There is also a superclass Annelida incertae sedis, which includes controversial species. There, according to the World Register 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, numbering 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 usually live in sea ​​water(with the exception of some species that may also be found in freshwater reservoirs). 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 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, 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.

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 invertebrates do not have any special organs for gas exchange, and respiration 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 the annelids usually include the 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

Charles Darwin, in 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 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 (brought into a certain area by 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 Lately, 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 national holiday, and the worm itself is considered local residents delicacy. In Korea and Japan, Urechis unicinctus worms from the Echiuridae class 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 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.

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, calves.

The Polychaete Class includes free-living annelids, in which numerous long bristles are collected in bundles and are located on the sides of each segment. Polychaete worms about \ (7000 \) species. Most of them live in the seas, where they crawl along the bottom, burrow into the silt or swim in the water column.

Like all annelids, the body of Polychaetes consists of segments, the number of which varies from \(5\) to\(800\) in different species. Polychaete worms have a head section and an anal lobe.

On the sides of each segment of the body, skin-muscular outgrowths are noticeable - organs of movement, which are called parapodia. The worm rakes its parapodia from front to back, clinging to the irregularities of the substrate, and thus crawls forward.

Nereid

Sea mouse (Aphrodite)

Nereis

Among the Polychaete worms there are sessile forms that build a protective tube and never leave it.

In sessile forms of worms, a partial reduction (contraction) of the parapodia occurs: often they are preserved only in the anterior part of the body.

Sedentary form of a marine worm

Skin-muscular sac

Skin-muscular sac consists of thin cuticles, skin epithelium And muscles. Located under the skin epithelium two layers of muscles: transverse (ring) and longitudinal. Under the layer of muscles there is a single-layer internal epithelium, which lines the secondary cavity of the body from the inside and forms partitions between the segments.

Transverse and longitudinal sections through the body of Nereis

Digestive system

Digestive system begins mouth, which is located on the ventral side of the head lobe, continues with a muscular throat(for many predatory worms it has chitinous teeth that serve to capture prey). The throat is followed esophagus And stomach.

The intestine is made up of three sections: anterior, middle and hindgut. anus located on the anal lobe.

free-living polychaete worms mainly predators feeding on crustaceans, molluscs, coelenterates and worms. Sessiles feed on small organic particles suspended in water and plankton.

Respiratory system

In polychaete worms, gas exchange (oxygen uptake and carbon dioxide release) is carried out either the entire surface of the body, or areas of parapodia, into which blood vessels enter. In some sessile forms, the corolla of tentacles on the head lobe performs the respiratory function.

Circulatory system

The circulatory system in annelids is closed: in any part of the body of the worm blood is flowingonly for vessels.

There are two main vessels - dorsal and abdominal (one vessel passes over the intestine, the other - under it), which are interconnected by numerous semicircular vessels. no heart, and the movement of blood is provided by contractions of the walls of the spinal vessel, in which blood flows from back to front, in the abdominal - from front to back.

excretory system

The excretory system is represented by paired tubules located in each segment of the body. Each tubule begins with a wide funnel facing the body cavity. The edges of the funnel are covered with shimmering cilia. The opposite end of the tubule opens outward on the lateral side of the body. With the help of a system of excretory tubules, waste products that accumulate in the coelomic fluid are brought out.

Nervous system

The nervous system consists of paired supraoesophageal nodes (ganglia) connected by strands into the peripharyngeal ring, a paired abdominal nerve chain, and nerves extending from them.

The sense organs are most developed in free-living polychaete worms. On the head of these worms are a pair of palps, a pair of tentacles and antennae. These are the organs of touch and chemical sense. Many of the polychaete worms have eyes. There are organs of balance.

Reproduction and development

Mostly polychaete worms separate sexes. Sex glands are present in almost every segment. Mature sex cells (in females - eggs, in males - spermatozoa) first enter the whole, and then through the tubules of the excretory system - into the water.

Fertilization is external. From the egg, a larva develops, which swims with the help of cilia. Then she settles to the bottom and turns into an adult worm.

Some species also reproduce asexually. In some species, the worm divides across, and each half restores the missing part. In others, the children do not diverge, and as a result, a chain is formed, including up to \(30\) individuals, but then it breaks up.