Biology at the Lyceum. Type annelids The nervous system of annelids includes

TO ringworms belong primary ringlets, polychaete and oligochaete worms, leeches and echiurids. In the phylum of annelids there are about 8 thousand species. These are the most highly organized representatives of the group of worms. The sizes of the rings range from fractions of a millimeter to 2.5 m. These are predominantly free-living forms. The body of the ringlets is divided into three parts: the head, the body, consisting of rings, and the anal lobe. Animals that are lower in their organization do not have such a clear division of the body into sections.

The ringlet's head is equipped with various sensory organs. Many ringlets have well-developed eyes. Some have particularly acute vision, and their lens is capable of accommodation. True, eyes can be located not only on the head, but also on the tentacles, on the body and on the tail. Ringworms also have developed senses of taste. On the head and tentacles, many of them have special olfactory cells and ciliary fossae, which perceive various odors and the effects of many chemical irritants. The ringed birds have well-developed hearing organs, arranged like locators. Recently, hearing organs have been discovered in sea ringed echiurids, very similar to the lateral line organs of fish. With the help of these organs, the animal subtly distinguishes the slightest rustles and sounds, which are heard much better in water than in air.

The body of the ringlets consists of rings, or segments. The number of rings can reach several hundred. Other ringlets consist of only a few segments. Each segment to some extent represents an independent unit of the whole organism. Each segment includes parts of vital organ systems.

Special organs of movement are very characteristic of ringlets. They are located on the sides of each segment and are called parapodia. The word "parapodia" means "foot-like". Parapodia are lobe-shaped outgrowths of the body from which tufts of bristles protrude outward. In some pelagic polychaetes, the length of the parapodia is equal to the diameter of the body. Parapodia are not developed in all ringlets. They are found in primary ringworms and polychaete worms. In oligochaetes only the setae remain. Primitive leech acanthobdella has bristles. Other leeches move without parapodia and setae. U ehiurid there is no parapodia, and setae are present only at the posterior end of the body.

Parapodia, nodes of the nervous system, excretory organs, gonads and, in some polychaetes, paired intestinal pouches are systematically repeated in each segment. This internal segmentation coincides with the external annulation. The repeated repetition of body segments is called the Greek word “metamerism”. Metamerism arose in the process of evolution in connection with the elongation of the body of the ancestors of ringlets. Lengthening the body necessitated repeated repetition, first of the organs of movement with their muscles and nervous system, and then of the internal organs.

Extremely characteristic of ringlets is the segmented secondary body cavity, or coelom. This cavity is located between the intestines and the body wall. The body cavity is lined with a continuous layer of epithelial cells, or coelothelium. These cells form a layer covering the intestines, muscles and all other internal organs. The body cavity is divided into segments by transverse partitions - dissepiments. A longitudinal septum, the mesenterium, runs along the midline of the body, dividing each compartment of the cavity into right and left parts.

The body cavity is filled with liquid, which in its chemical composition is very close to sea water. The fluid filling the body cavity is in continuous motion. The body cavity and abdominal fluid perform important functions. Cavity fluid (like any fluid in general) does not compress and therefore serves as a good “hydraulic skeleton”. The movement of the cavity fluid can transport various nutritional products, secretions of the endocrine glands, as well as oxygen and carbon dioxide involved in the respiration process inside the body of the ringlets.

Internal partitions protect the body in case of severe injuries and ruptures of the body wall. For example, an earthworm cut in half does not die. The septa prevent cavity fluid from flowing out of the body. The internal partitions of the rings thus protect them from death. Sea ships and submarines also have internal hermetic partitions. If the side is broken, then the water pouring into the hole fills only one damaged compartment. The remaining compartments, not flooded with water, maintain the buoyancy of the damaged ship. Likewise, in ringworms, disruption of one segment of their body does not entail the death of the entire animal. But not all annelids have well-developed septa in the body cavity. For example, in echiurids the body cavity does not have partitions. A puncture in the body wall of an echiurid can lead to its death. In addition to the respiratory and protective role, the secondary cavity acts as a container for reproductive products that mature there before being excreted.

Rings, with few exceptions, have a circulatory system. However, they have no heart. The walls of large vessels themselves contract and push blood through the thinnest capillaries. In leeches, the functions of the circulatory system and the secondary cavity are so identical that these two systems are combined into a single network of lacunae through which blood flows. In some rings the blood is colorless, in others it is colored green by a pigment called chlorocruorin. Often ringlets have red blood, similar in composition to the blood of vertebrates. Red blood contains iron, which is part of the hemoglobin pigment. Some ringlets, burrowing into the ground, experience an acute oxygen deficiency. Therefore, their blood is adapted to bind oxygen especially intensively. For example, the polychaete Magelona papillicornis has a pigment called hemerythrin, which contains five times more iron than hemoglobin.

In ringlets, compared to lower invertebrates, metabolism and respiration are much more intense. Some polychaete ringlets develop special respiratory organs - gills. A network of blood vessels branches out in the gills, and through their wall oxygen penetrates into the blood and is then distributed throughout the body. Gills can be located on the head, parapodia and tail.

The through intestine of ringlets consists of several sections. Each section of the intestine performs its own special function. The mouth leads into the throat. Some ringlets have strong horny jaws and teeth in their throats, which help them grasp live prey more firmly. In many predatory ringlets, the pharynx serves as a powerful weapon of attack and defense. The pharynx is followed by the esophagus. This section is often supplied with a muscular wall. Peristaltic movements of the muscles slowly push food into the next sections. In the wall of the esophagus there are glands, the enzyme of which serves for the primary processing of food. Following the esophagus is the midgut. In some cases, goiter and stomach are developed. The wall of the midgut is formed by epithelium, very rich in glandular cells that produce digestive enzymes. Other cells in the midgut absorb digested food. Some ringlets have a midgut in the form of a straight tube, in others it is curved in loops, and still others have metameric outgrowths on the sides of the intestine. The hindgut ends at the anus.

Special organs - metanephridia - serve to secrete liquid metabolic products. Often they serve to bring out germ cells - sperm and eggs. Metanephridia begins as a funnel in the body cavity; from the funnel there is a convoluted channel, which opens outward in the next segment. Each segment contains two metanephridia.

Rings reproduce asexually and sexually. Asexual reproduction is common in aquatic ringworms. At the same time, their long body breaks up into several parts. After some time, each part restores its head and tail. Sometimes a head with eyes, tentacles and a brain forms in the middle of the worm's body before it splits into parts. In this case, the separated parts already have a head with all the necessary sensory organs. Polychaetes and oligochaetes are relatively good at restoring lost body parts. Leeches and echiurids do not have this ability. These ringlets have lost their segmented body cavity. This is partly why, apparently, they lack the ability to reproduce asexually and restore lost parts.

Fertilization of eggs in ringed fish most often occurs outside the body of the mother's body. In this case, males and females simultaneously release reproductive cells into the water, where fertilization occurs.

In marine polychaetes and echiurids, the crushing of fertilized eggs leads to the development of a larva, which is not at all similar to adult animals and is called a trochophore. The trochophore lives in the surface layers of water for a short time, and then settles to the bottom and gradually turns into an adult organism.

Freshwater and terrestrial ringworms are most often hermaphrodites and have direct development. Freshwater and terrestrial ringworms do not have a free larva. This is due to the fact that fresh water has a salt composition of a completely different nature than sea water. Sea water is more favorable for the development of life. Fresh water even contains some toxic compounds (for example, magnesium) and is less suitable for the development of organisms. Therefore, the development of freshwater animals almost always occurs under the cover of special low-permeable shells. Even more dense shells - shells - are formed in the eggs of ground rings. Dense shells here protect the eggs from mechanical damage and from drying out under the scorching rays of the sun.

The practical importance of annelids is increasingly increasing due to the development of the intensity of biological research.

Here in the USSR, for the first time in the history of world science, the acclimatization of some invertebrates was carried out to strengthen the food supply of the sea. For example, the polychaete Nereis, acclimatized in the Caspian Sea, became the most important food item for sturgeon and other fish.

Earthworms not only serve as fishing bait and food for birds. They bring great benefits to humans by loosening the soil, making it more porous. This facilitates the free penetration of air and water to the roots of plants and increases crop yields. While burrowing in the ground, worms swallow pieces of soil, crush them and throw them to the surface well mixed with organic matter. The amount of soil brought to the surface by worms is amazingly large. If we were to distribute the soil plowed by earthworms every 10 years over the entire surface of the land, we would get a layer of fertile soil 5 cm thick.

Leeches are used in medical practice for hypertension and the threat of hemorrhage. They release the substance hirudin into the blood, which prevents blood clotting and promotes the dilation of blood vessels.

Type of rings includes several classes. The most primitive are the marine primary rings - archiannelids. Polychaetes and echiurids- inhabitants of the sea. Oligochaete ringlets and leeches- mainly inhabitants of fresh water and soil.

Animal life: in 6 volumes. - M.: Enlightenment. Edited by professors N.A. Gladkov, A.V. Mikheev. 1970 .

The type of annelids, uniting about 12,000 species, represents, as it were, a node in the family tree of the animal world. According to existing theories, annelids originate from ancient ciliated 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 related by a common ancestor to mollusks. All this shows the great importance that the type under consideration has for understanding the phylogeny of the animal world. From a medical point of view, annelids are of limited importance. Only leeches are of particular 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 body 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 repeatability of structure, 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 bristles. 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 sticks.

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

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

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

The central nervous system is represented by the suprapharyngeal ganglion and the ventral nerve cord. Nerves from the sensory organs pass to the suprapharyngeal node: eyes, balance organs, tentacles and palps. The abdominal nerve cord consists of nodes (one pair in each body segment) and trunks connecting the nodes to each other. Each node innervates all organs of a given segment.

The digestive system consists of the foregut, middle and hindgut. The foregut is usually divided into a number of sections: the pharynx, esophagus, crop and gizzard. The mouth is located on the ventral side of the first body segment. The hindgut opens with the anus on the posterior lobe. The intestinal wall contains muscles that move food along.

The excretory organs - metanephridia - are paired tubular organs, metamerically repeated in body segments. Unlike protonephridia, they have a through excretory canaliculus. The latter begins with a funnel that opens into the body cavity. Cavity fluid enters the nephridium through the funnel. A tubule of nephridium extends from the funnel, sometimes opening outward. Passing through the tubule, the liquid changes its composition; the final products of dissimilation are concentrated in it, which are released from the body through the external pore of nephridium.

For the first time in the phylogenesis of the animal world, 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 capable of contracting 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 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 gonads are located in the body cavity.

Annelids have the highest 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.

All worms are classified by type. The body of an earthworm is segmented, which consists of many small rings connected or fused together. The worm's body segments are separated by internal divisions called septa, giving independent movement to each of them. An earthworm consists of 100-150 segments. Segmented body parts provide important structural functions. Segmentation can help earthworm movement.

Each segment or section has muscles and bristles called setae. The bristles help anchor and control the worm as it moves through the soil. They hold one part of the worm firmly to the ground, and the other part of the body protrudes forward. An earthworm uses segments to either contract or relax so that the body lengthens in one area or contracts in other areas. Segmentation helps worms be flexible and strong when moving. Generally well-developed muscles (a fluid-filled body cavity) act as a hydrostatic skeleton.

Characteristics of the functions of partitions (segments)

Why do partitions perform a protective function:

• provide a certain body shape (due to its elasticity).
• protect the internal sensitive organ from injury.
• the secret mucus of the septa, which keeps the surface of the body mucous and kills harmful bacteria.
• alternate contraction and relaxation of the circular and internal longitudinal muscles aids in movement.

When threatened, a worm can release a scent that will disable its attacker. This protection is released at the last minute from the glands around the septa. But there is another aspect of the earthworm that may be a defense device: its ability to regenerate. Not all earthworms have this ability; most can regrow parts of themselves that have been torn apart. Although it is not true that an earthworm cut in half forms two new worms, because the most important internal organs are in one half, the part with these organs can usually replace the missing one through segmented cuts.

Annelids, or annelids (from the Latin annulus - ring) are a class of worms with external and internal segmentation. All of them have annular projections, usually corresponding to the internal division of the body. The phylum has about 18 thousand species.

They belong to protostomes; the body is divided into segments, the number of which in some species reaches several hundred. Let's start studying annelids with classification.

The appearance of annelids (ringworms) was accompanied by large, significant aromorphoses.

Aromorphoses of annelids

We will study the main details of the structure of annelids using the example of a typical representative - an earthworm (in the oligochaete section).

©Belevich Yuri Sergeevich

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Annelids belong to the subsection of coelomic animals Coelomata), a group (superphylum) of protostomes (Protostomia). For primary stomates it is characteristic:

• The primary mouth (blastopore) of the embryo (gastrula) passes into the adult animal or the definitive mouth is formed in place
• primary mouth.
• Mesoderm is formed, as a rule, by a teloblastic method.
• The covers are single-layered.
• External skeleton.
• Protostomes are the following types of animals: annelids (Annelida), mollusks (Mollusca), arthropods (Arthropoda), onychophorans (Onychophora).
• Annelids are a large group of animals, about 12 thousand species are known. They are inhabitants of the seas, fresh water bodies, and inhabit land.

Main characteristics of the type:

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

Annelids General characteristics

Latin name Annelida

Type annelids, or rings, is a very important group for understanding the evolution of higher invertebrate animals. It includes about 8,700 species. Compared with the considered flat and roundworms and even with nemerteans, annelids are significantly more highly organized animals.

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

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

U ringlets there is a well-developed closed circulatory system. The excretory organs - metanephridia - are located segment by segment, and therefore are called segmental organs.

Nervous system consists of a paired suprapharyngeal ganglion, called the brain, connected by peripharyngeal connectives to the ventral nerve cord. The latter consists of a pair of longitudinally contiguous trunks in each segment, forming ganglia, or nerve ganglia.

Internal structure

Musculature

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

Body cavity

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

The functions of the coelom are: supporting, distributive, excretory and, in polychaetes, reproductive.

Origin of the coelom. There are 4 known hypotheses: myocoel, gonocoel, enterocoel and schizocoel.

Digestive system

Represented by three departments. Cavity digestion. The pharynx of predatory polychaetes is armed with chitinous jaws. The ducts of the salivary glands open into the pharynx of annelids. Leech glands contain the anticoagulant hirudin. In earthworms, ducts of calcareous (morrain) glands flow into the esophagus. The foregut of earthworms includes, in addition to the pharynx and esophagus, a crop and a muscular stomach. The absorption surface of the midgut increases due to outgrowths - diverticulum (leeches, part of the polychaetes) or typhlosol (oligochaetes).

Excretory system

Nephridial type. As a rule, each segment has two excretory canals; they begin in one segment and open with an excretory pore in the next segment of the body. The excretory organs of polychaetes are the most diverse. Polychaete worms have the following types of excretory systems: protonephridia, metanephridia, nephromyxia and myxonephridia. Protonephridia are developed in larvae; they begin with club-shaped terminal cells with a flagellum (solenocytes), then the nephridia canal. Metanephridia begins with a funnel with a nephrostomy, inside
the funnels contain the cilia, followed by the duct and nephropore. Protonephridia and metanephridia are ectodermal in origin. Nephromyxia and myxonephridia are the fusion of the ducts of the protonephridia or metanephridia with the coelomoduct - the genital funnel. Coelomoducts of mesodermal origin. The excretory organs of oligochaetes and leeches are metanephridia. In leeches, their number is significantly smaller than the body segments (medicinal leeches have 17 pairs), and the separation of the funnel from the canal is typical. In the excretory canals of the nephridia, ammonia is converted into high molecular weight compounds, and water is absorbed as a whole. Annelids also have storage “buds”: chloragogenous tissue (polychaetes, oligochaetes) and botryodenic tissue (leeches). They accumulate guanine and uric acid salts, which are removed from the coelom through nephridia.

Circulatory system of annelids

Most annelids have a closed circulatory system. It is represented by two main vessels (dorsal and abdominal) and a network of capillaries. Blood movement is carried out due to contraction of the walls of the dorsal vessel; in oligochaetes, the annular hearts also contract. The direction of blood movement through the spinal vessel is from back to front, and in the abdominal vessel - in the opposite direction. The circulatory system is developed in bristle-bearing and proboscis leeches. In jaw leeches there are no vessels; the function of the circulatory system is performed by the lacunar system. The process of functional replacement of one organ with another, different in origin, is called organ substitution. The blood of annelids is often colored red due to the presence of hemoglobin. Primitive polychaetes do not have a circulatory system.

Respiratory system

Most breathe over the entire surface of the body; some polychaetes and some leeches have gills. Respiratory organs are evaginated. The gills of polychaetes are in origin a modified dorsal antennae of parapodia, while those of leeches are skin outgrowths.

Nervous system and sensory organs

The nervous system includes: the paired medullary (suprapharyngeal) ganglion, connectives, subpharyngeal ganglia and the ventral nerve cord or scalene nervous system. The abdominal trunks are connected by commissures. The evolution of the nervous system went in the direction of transforming the ladder-type nervous system into a chain, immersing the system in the body cavity. The nerves that arise from the central system make up the peripheral system. There are varying degrees of development of the suprapharyngeal ganglion; the brain is either monolithic or divided into sections. Leeches are characterized by the fusion of ganglion segments that make up the suckers. Sense organs. Polychaetes: epithelial sensory cells, antennae, nuchal organs, antennae of parapodia, statocysts, organs of vision (goblet or bubble type eyes). Sense organs of oligochaetes: light-sensitive cells, some water inhabitants have eyes, chemical sense organs, tactile cells. Leeches: goblet organs – chemical sense organs, eyes.

Classification

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

1. Polychaeta ringlets

2. Echiurida

Echiurids are a highly modified group of ringlets, the internal organization of which differs from that of polychaetes by an unsegmented coelom and the presence of one pair of metanephrpdia.
The trochophore larva of echiurids is of greatest importance for establishing the unity of origin of echiurids with polychaetes.

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

3. Oligochaeta

The oligochaetes, or oligochaetes, are a large group of annelids, including about 3,100 species. They undoubtedly descend from polychaetes, but differ from them in many significant features.
Oligochaetes overwhelmingly live in the soil and at the bottom of fresh water bodies, where they often burrow into muddy soil. The Tubifex worm can be found in almost every freshwater body, sometimes in huge quantities. The worm lives in silt, and sits with its head end buried in the ground, and its back end constantly makes oscillatory movements.
Soil oligochaetes include a large group of earthworms, an example of which is the common earthworm (Lumbricus terrestris).
Oligochaetes feed mainly on plant foods, mainly on decaying parts of plants, which they find in the soil and silt.
When considering the characteristics of oligochaetes, we will mainly have in mind the common earthworm.

4. Leeches (Hirudinea) >> >>

Phylogeny

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

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

The third hypothesis is called the trochophore theory. Its proponents produce ringlets from a hypothetical ancestor of Trochozoon, which has a trochophore-like structure and originates from ctenophores.

As for the phylogenetic relationships within the four classes of annelids considered, they currently seem quite clear.

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

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

The meaning of annelids

Polychaete worms.

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

Oligochaete worms.

 Oligochaetes, inhabitants of water bodies, provide food for many animals and participate in the processing of organic matter.
 Earthworms are animal food and human food.Gallery