1. Annelids first have a circulatory system. 2. Circulatory system serves to carry oxygen and nutrients to all organs of the animal. 3. Annelids have two main blood vessels. Through the abdominal vessel, blood moves from the anterior end of the body to the posterior. 4. Through the dorsal vessel, blood moves from the posterior end of the body to the anterior. 5. The dorsal vessel passes over the intestine, the abdominal - under it. In each segment, the dorsal and abdominal vessels are interconnected by annular vessels.

Circulatory system 6. Annelids do not have a heart. Several thick annular vessels have muscular walls, due to the contraction of which blood moves. Thinner vessels depart from the main vessels, branching then into the thinnest capillaries. The capillaries receive oxygen from the skin epithelium and nutrients from the intestines. And from other similar capillaries branching in the muscles, there is a return of "waste". Thus, the blood moves all the time through the vessels and does not mix with the cavity fluid. Such a circulatory system is called a closed system. 7. In the blood, iron-containing protein, close to hemoglobin.

The circulatory system of annelids 1. Annelids first have a circulatory system. 2. The circulatory system is closed 3. two main blood vessels: abdominal and dorsal. They are connected in each segment by an annular vessel 4. No real heart

The circulatory system of mollusks: Open (blood from the vessels enters the body cavity) A heart appeared, which increased the rate of blood circulation, which significantly increased the intensity of metabolic processes. Three-chamber or two-chamber heart (1 or 2 atria and ventricle) the aorta departs from the heart, it branches into arteries Colorless blood is saturated with oxygen in the lung (gills) and returns to the heart through the veins Functions: blood carries oxygen and takes carbon dioxide

Unlike other mollusks, cephalopods have a nearly closed circulatory system. In many places (skin, muscles) there are capillaries through which the arteries pass directly into the veins. A highly developed circulatory system enables cephalopods to reach gigantic sizes. Only in the presence of a system of capillaries is the existence of very large animals possible, since only in this case is a full supply of oxygen and nutrients to massive organs ensured. Blood is driven by three hearts. 1. The main one, consisting of the ventricle and two atria (the nautilus has four atria). The main heart drives blood through the body. 2. And two gills. 3. Rhythmic contractions of gill hearts push through venous blood through the gills, from where it, enriched with oxygen, enters the atrium of the main heart. The heart rate depends on the temperature of the water. For example, in an octopus at a water temperature of 22 ° C, the heart rate is 40-50 beats per 1 minute. 4. There are special vessels for supplying blood to the head. The blood of cephalopods has a blue color due to the presence in it of the respiratory pigment hemocyanin, which contains copper. Hemocyanin is produced in special gill glands.

The circulatory system in arthropods is not closed and is represented by the heart and large vessels, from which the hemolymph (a liquid, in many respects similar to the blood of vertebrates) pours into the body cavity, washes the internal organs and returns to the heart again. 1. The heart is capable of rhythmic contractions. The hemolymph enters it from the body cavity through the lateral openings, ostia, and washes the internal organs, supplying them with nutrients. 2. In crustaceans, the hemolymph also performs a respiratory function. It contains oxygen-carrying substances - red hemoglobin or blue hemocyanin. For this, there are special gill vessels.

Circulatory system 1. When the heart contracts, the ostia valves close. 2. And the blood, moving through the arteries, enters the body cavity. Here it gives oxygen and nutrients to the internal organs. 3. Saturated with carbon dioxide and metabolic products. 4. Then the blood enters the gills. 5. Gas exchange takes place there, and the blood, freed from carbon dioxide, is again saturated with oxygen. 6. After that, the blood enters the relaxed heart through the open ostia.

The circulatory system is an open circulatory system. In insects, blood practically does not participate in the transfer of oxygen. the long, tubular heart of insects is located on the dorsal side of the abdomen, divided into several chambers, each chamber has openings with valves - ostia. Through them, blood from the body cavity enters the heart. adjacent chambers are connected to each other by valves that open only forward. Sequential contraction of the chambers of the heart from the back to the front ensures the movement of blood.

Lancelet Circulatory system: closed; no heart; walls of the abdominal aorta contract; Function: blood carries oxygen and nutrients throughout the body, takes away decay products

The circulatory system of fish The circulatory system is closed, one circle of blood circulation, the heart is two-chambered (it consists of a thin-walled atrium and a muscular ventricle) Venous blood is first collected in venous sinus- an expansion that collects blood from the venous vessels, then enters the atrium and is pushed out of the ventricle. From the heart, venous blood enters the abdominal aorta to the gills, arterial blood is collected in the dorsal aorta. From all organs, venous blood through the vessels enters the common venous sinus.

The circulatory system of an amphibian The circulatory system. Two circles of blood circulation (large and small). Since the lungs appeared, a pulmonary (small) circulation occurs. The amphibian heart becomes three-chambered (formed by two atria and one ventricle), three pairs of arterial arches depart from it. The metabolism is not yet very intense, amphibians are poikilothermic (cold-blooded) animals.

The circulatory system of an amphibian Arterial blood enters the left atrium from the lungs through the pulmonary veins, and mixed blood enters the right atrium, since venous blood enters the vena cava from the internal organs, and the skin veins bring arterial blood. In the ventricle, the blood mixes only partially, due to the presence of special dividing mechanisms (various outgrowths and a spiral valve of the arterial cone).

Circulatory system big circle circulation. From the ventricle, blood flows into three pairs of arterial vessels. When the ventricle contracts, venous blood is first pushed out, which fills the first two pairs of arteries. Blood with a maximum oxygen content enters the third pair of arteries, from which the carotid arteries depart, supplying blood to the brain. Then venous blood (from the internal organs through the vena cava) and arterial (through the skin veins) enter the right atrium.

Circulatory system Small circle of blood circulation. The pulmonary arteries carry oxygen-depleted blood to the lungs, where gas exchange occurs, then arterial blood enters the left atrium through the pulmonary veins. Large branches depart from each pulmonary artery - cutaneous arteries that carry blood to the skin, where it is oxidized, and then enters the right atrium. Erythrocytes in amphibians are large, biconvex, have a nucleus. Metabolism is higher than that of fish, but not high enough to maintain a constant body temperature

Circulatory system There is a further separation of arterial and venous blood flow due to the appearance of an incomplete septum in the ventricle of the heart. The septum partially prevents the mixing of arterial and venous blood. Three vessels independently depart from the ventricle: the pulmonary artery, which carries venous blood to the lungs, the right and left aortic arches.

The circulatory system The systemic circulation begins with the aortic arches. The right aortic arch emerges from the left side of the ventricle and carries arterial, oxygenated blood. The carotid arteries, which carry blood to the brain, and the subclavian arteries, which supply blood to the forelimbs, depart from it. The left aortic arch originates from the middle part of the ventricle and carries mixed blood. Both arcs merge into the dorsal aorta, which supplies blood to the rest of the organs.

The circulatory system The small circle begins with the pulmonary artery extending from the right side of the ventricle. Venous blood is delivered to the lungs, gas exchange occurs there, and arterial blood returns through the pulmonary veins to the left atrium. Although the circulatory system is more perfect than that of amphibians, the metabolism is insufficient to maintain a constant body temperature, so reptiles do not have a constant body temperature, they are poikilothermic.

Circulatory system. The heart becomes four-chambered, the septum divides the heart into two parts - right and left. Each part of the heart consists of an atrium and a ventricle. Venous blood returns to the right half of the heart through the vena cava (upper and lower) from the systemic circulation. Small circle of blood circulation. When the right ventricle contracts, venous blood flows through pulmonary arteries to the lungs, where gas exchange occurs, and arterial blood returns through the pulmonary veins from the pulmonary circulation to the left atrium.

Circulatory system Great circle. Blood exits the left ventricle through the right aortic arch. The carotid arteries are separated from it, carrying blood to the head, subclavian - to the upper limbs. The right aortic arch passes into the dorsal aorta, providing blood to the internal organs. Then venous blood is collected in the vena cava and enters the right atrium. Unlike the circulatory system of reptiles, in birds, blood from the heart to the organs in a large circle flows not through two arteries (left and right aortic arches), but only through the right one. The oxygen capacity of blood in birds is 2 times higher than in reptiles. The average body temperature in birds is about 42 degrees.

The circulatory system in the right half of the heart is venous blood, in the left half it is arterial, that is, there is no mixing of blood. The pulmonary circulation begins in the right ventricle, venous blood is brought to the lungs through the pulmonary arteries, gas exchange occurs there, and arterial blood enters the left atrium through the pulmonary veins. The systemic circulation begins in the left ventricle, blood is ejected into the left aortic arch. Arteries supply blood to all internal organs. Venous blood flows through the superior and inferior vena cava into the right atrium.

Annelids are the most highly organized type of worms. It includes from 12 thousand (according to old sources) to 18 thousand (according to new) species. According to the traditional classification, annelids include three classes: polychaete worms, oligochaete worms, and leeches. However, according to another classification, polychaetes are considered at the rank of class, and oligochaetes and leeches are included in the rank of subclasses in the class Poyaskovye; in addition to these groups, other classes and subclasses are also distinguished.

The body length of annelids, depending on the species, varies from a few millimeters to more than 5-6 meters.

In the process of embryonic development, the ectoderm, mesoderm and endoderm are laid. Therefore, they are classified as three-layer animals.

In annelids, in the process of evolution, a secondary body cavity appeared, that is, they are secondary cavities. The secondary cavity is called in general. It is formed inside the primary cavity, which remains in the form of lumens of blood vessels.

The whole develops from the mesoderm. Unlike the primary cavity, the secondary cavity is lined with its own epithelium. In annelids, the whole body is filled with fluid, which, among other things, performs the function of a hydroskeleton (shape support and support during movement). Also, the coelomic fluid carries nutrients, metabolic products and germ cells are excreted through it.

The body of annelids consists of repeating segments (rings, segments). In other words, their body is segmented. There may be several or hundreds of segments. The body cavity is not single, but is divided into segments by transverse partitions (septa) of the epithelial lining of the coelom. In addition, two coelomic sacs (right and left) are formed in each ring. Their walls touch above and below the intestine and support the intestine. Between the walls are also blood vessels and nerve cord. Each segment has its own nodes nervous system(on the paired abdominal nerve trunk), excretory organs, sex glands, external outgrowths.

The head lobe is called the prostomium. The back of the worm's body is the anal lobe, or pygidium. The segmented body is called the trunk.

The segmented body allows the annelids to grow easily by forming new rings (this occurs posteriorly in front of the anal lobe).

The appearance of a segmented body is an evolutionary progress. However, annelids are characterized by homonomic segmentation, when all segments are approximately the same. In more highly organized animals, segmentation is heteronomous, when the segments and their functions are different. At the same time, in annelids, the formation of the head section of the body is observed by fusion of the anterior segments with a simultaneous increase in the cerebral ganglion. This is called cephalization.

The walls of the body, like those of lower worms, form a skin-muscular sac. It consists of the skin epithelium, a layer of circular and a layer of longitudinal muscles. Muscles achieve more powerful development.

Paired organs of movement arose - parapodia. They are only found in polychaete annelids. They are outgrowths of the skin-muscular sac with bundles of bristles. In the more evolutionarily advanced group of oligochaetes, parapodia disappear, leaving only setae.

Digestive system consists of an anterior, middle and hindgut. The walls of the intestine are formed by several layers of cells, they have muscle cells, thanks to which food moves. The foregut is usually divided into the pharynx, esophagus, crop, and gizzard. The mouth is on the ventral side of the first body segment. The anal opening is located on the caudal lobe. The process of absorption of nutrients into the blood occurs in the middle intestine, which has a fold on top to increase the absorption surface.

Characterized by a closed circulatory system. Previous types of worms (flat, round) did not have a circulatory system at all. As already mentioned, the lumen of the vessels is the former primary cavity of the body, whose cavity fluid began to perform the functions of blood. The circulatory system of roundworms consists of a dorsal vessel (in which blood moves from the caudal lobe to the head), from the abdominal vessel (blood moves from the head lobe to the tail), half rings connecting the dorsal and abdominal vessels, small vessels extending to various organs and tissues . Each segment contains two half rings (left and right). A closed circulatory system means that blood flows only through the vessels.

Blood moves due to the pulsation of the walls of the spinal vessel. In some oligochaete worms, in addition to the dorsal, some annular vessels are reduced.

The blood carries the nutrients of their intestines and the oxygen that has entered through the integument of the body. The respiratory pigment, which reversibly binds oxygen, is found in the blood plasma, and is not contained in special cells, as, for example, in vertebrates, the hemoglobin pigment is found in erythrocytes. Pigments in annelids can be different (hemoglobin, chlorocruarine, etc.), so the color of the blood is not always red.

There are representatives of annelids that do not have a circulatory system (leeches), but in them it was reduced, and a respiratory pigment is present in the tissue fluid.

Although annelids do not have a respiratory system and usually breathe through the entire surface of the body, the transport of gases is carried out by the circulatory system, and not by diffusion through the interstitial fluid. In some marine species, primitive gills are formed on the parapodia, in which there are many small blood vessels located close to the surface.

The excretory organs are represented by metanephridia. These are tubes that have a funnel with cilia at the end located inside the body (in the whole). On the other hand, the tubules open outward through the surface of the body. Each segment of the annelids contains two metanephridia (right and left).

The nervous system is more developed than roundworms. In the head lobe, a pair of merged nodes (ganglia) forms a kind of brain. The ganglia are located on the peripharyngeal ring, from which the paired abdominal chain departs. It contains paired nerve nodes in each segment of the body.

Sense organs of annelids: tactile cells or structures, a number of species have eyes, chemical sense organs (olfactory pits), there is an organ of balance.

Most annelids are dioecious, but there are also hermaphrodites. Development is direct (a small worm emerges from the egg) or with metamorphosis (a floating trochophore larva emerges; typical for polychaetes).

It is believed that annelids are descended from worms with an undivided body, similar to ciliary worms (a type of flatworm). That is, in the process of evolution, two other groups of worms originated from flatworms - round and ringed.

Animals; the most highly organized among all worms. For the first time, they have a set of organ systems that is characteristic of all higher groups of organisms, including mammals. Length from 2-3 mm to 3 m. The cylindrical or flattened body of annelids is usually clearly segmented. At the same time, partitions are formed in the body cavity, dividing it into separate segments. The outer and inner segmentation most often coincide, but sometimes only one inner segment corresponds to several outer segments. Less often segmentation is absent. The first segment of the body is the head lobe, on which the sense organs can be located: antennae, palps, eyes.

The mouth opens on the lower surface of the second body segment. In polychaete worms, oral cavity powerful chitinous jaws are formed, capable of turning outwards. They serve to capture and hold prey. In leeches, the mouth opening is surrounded by a sucker, formed as a result of the fusion of the first four segments. The anus opens on the last segment of the body. On the sides of all segments, except for the first and last, paired outgrowths develop - parapodia, which act as organs of locomotion. In oligochaete worms and some leeches, they are modified into small setae, which can rarely be completely absent.

Annelids are three-layered animals that develop ectoderm, endoderm, and mesoderm. From the latter, a secondary body cavity (whole) is formed, filled with abdominal fluid. Due to the fact that the liquid is under pressure, the annelids maintain a constant body shape. In addition, the whole acts as the internal environment of the body, maintaining a constant biochemical regime. Annelids have a well-developed skin-muscular sac, consisting of skin epithelium and underlying annular and longitudinal muscles. Thanks to him, the worms are able to perform a variety of movements.

The digestive system of annelids is end-to-end and consists of three sections: the anterior, middle and hindgut. The anterior and posterior intestines develop from the ectoderm, while the middle intestine develops from the endoderm. Some species have paired salivary glands. The vast majority of annelids have a closed circulatory system. Only in some leeches it becomes open for the second time, while in sipunculids it is absent. Paired ciliated funnels function as excretory organs in annelids, which are repeated many times in each segment of the body. At the same time, the funnel itself is located in one segment, and the excretory canal, passing through the partition between the segments, opens with an excretory opening on the side of the next body segment. The nervous system is represented by the peripharyngeal ring and the ventral nerve chain extending from it. On it in each segment of the body are paired ganglia. Annelids are generally dioecious, but hermaphroditism has been observed in some species. Development with metamorphosis, or direct.

About 12 thousand species, divided into 6 classes: primary rings,

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 and several layers of muscles underlying it (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.

Annelids have the highest organization in comparison with 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.

general characteristics

Type Annelids - an extensive group (12 thousand species). It includes deuterated animals, whose body consists of repeating segments, or rings. The circulatory system in annelids is closed. Compared with roundworms, annelids have a more advanced nervous system and sensory organs. The main features of this group need to be told in more detail.

The secondary cavity of the body, or the whole (from the Greek koiloma - “deepening”, “cavity”), develops in the embryo from the mesoderm layer. This is the space between the body wall and internal organs. Unlike the primary body cavity, the secondary cavity is lined from the inside with its own internal epithelium. The whole is filled with a liquid that creates a constancy of the internal environment of the body. Due to the pressure of the liquid, the secondary cavity maintains a certain shape of the body of the worm and serves as a support during movement. In other words, the whole serves as a hydroskeleton. Coelomic fluid is involved in metabolism: it carries nutrients, accumulates and removes harmful substances and also removes sexual products.

Annelids have a segmented body: it is divided into successive sections - segments, or rings (hence the name - annelids). Such segments different types maybe a few or hundreds. The body cavity is divided into segments by transverse partitions. Each segment is an independent compartment: it has its own external outgrowths, nodes of the nervous system, excretory organs and sex glands.

The phylum Annelids includes the Polychaete worms and the Olichaete worms.

Habitat, structure and activity of polychaete worms

About 7000 species of polychaete worms are known. Most of them live in the seas, a few live in fresh waters, in the litter of tropical forests. In the seas, polychaete worms live at the bottom, where they crawl among stones, corals, thickets of marine vegetation, and burrow into the silt. Among them are sessile forms that build a protective tube and never leave it (Fig. 62). There are planktonic species. Polychaete worms are found mainly in the coastal strip, but sometimes at a depth of up to 8000 m. In some places, up to 90 thousand polychaete worms live on 1 m2 of the seabed. They are eaten by crustaceans, fish, echinoderms, coelenterates, birds. Therefore, some polychaete worms were specially bred in the Caspian Sea as food for fish.

Rice. 62. Various polychaete annelids: 1 - sessile marine worm: 2 - nersis; 3 - sea mouse; 4 - sandstone

The body of polychaete worms is elongated, slightly flattened in the dorsal-abdominal direction, or cylindrical, from 2 mm to 3 m. Like all annelids, the body of polychaete consists of segments, the number of which varies from 5 to 800 in different species. there is a head section and an anal lobe.

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 (Fig. 63, A).

Rice. 63. Nersis: A - head department; B - parapodia (transverse section); B - larva; 1 - tentacle; 2 - palp; 3 - antennae; 4 - eye: 5 - bristles

On the sides of each segment of the body, skin-muscular outgrowths are noticeable - organs of movement, which are called parapodia (from the Greek para - “near” and podion - “leg”) (Fig. 63, B). Parapodia have a kind of reinforcement - bundles of bristles that contribute to the rigidity of the organs of movement. The worm rakes its parapodia from front to back, clinging to the irregularities of the substrate, and thus crawls forward.

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.

The body of polychaete worms is covered with a single-layered epithelium. In sessile forms of worms, the secretions of the epithelium can harden, forming a dense protective sheath around the body. The skin-muscular sac consists of a thin cuticle, skin epithelium and muscles (Fig. 64, A). Under the skin epithelium there are two layers of muscles: transverse, or annular, 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.

Rice. 64. Transverse (A) and longitudinal (B) sections through the body of the Nereis (arrows show the movement of blood through the vessels): 1 - parapodim; 2 - longitudinal muscles; 3 - circular muscles: 4 - intestine; 5 - abdominal nerve chain; 6 - dorsal blood vessel; 7 - abdominal blood vessel; 8 - mouth opening; 9 - pharynx; 10 - brain

Digestive system begins with the mouth, which is located on the ventral side of the head lobe. In the section following the mouth, the muscular pharynx, many predatory worms have chitinous teeth that serve to grasp prey. The pharynx is followed by the esophagus and stomach. The intestine consists of three sections: the anterior, middle and hindgut (Fig. 64, B). The midgut looks like a straight tube. It digests and absorbs nutrients. AT hindgut faecal matter is formed. The anal opening is located on the anal lobe. Stray polychaete worms are mainly predators, while sessile ones feed on small organic particles and plankton suspended in water.

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

The circulatory system of annelids is closed: in any part of the body of the worm, blood flows only through the vessels. There are two main vessels - dorsal and abdominal. One vessel passes over the intestine, the other - under it (see Fig. 64). They are connected to each other by numerous semicircular vessels. There is no heart, and the movement of blood is provided by contractions of the walls of the spinal vessel, in which blood is coming from back to front, in the abdominal - from front to back.

excretory system 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 excreted outside.

Nervous system consists of paired supraesophageal, or brain, nodes (ganglia), connected by strands into the peripharyngeal ring, a paired abdominal nerve chain and nerves extending from them.

sense organs most developed in vagrant polychaete worms. Many of them have eyes. The organs of touch and chemical sense are located on the antennae, antennae and parapodia. There are organs of balance. Touch and other stimuli act on sensitive skin cells. The excitation that has arisen in them is transmitted along the nerves to the nerve nodes, from them along other nerves to the muscles, causing their contraction.

Reproduction. Most polychaete worms have 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. A larva develops from the egg (see Fig. 63, B), 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 daughter individuals do not diverge, and as a result, a chain is formed, including up to 30 individuals, but then it breaks up.